Write a Partial Decay Series for Rn-220 Undergoing the Following Sequential Decays: Þâ±, Þâ±, Þâ², Þâ±.
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| A simultaneous combination of porosity and tunable optoelectronic properties, common in covalent organic frameworks, is rare in shape-persistent organic cages. Yet, organic cages offer important molecular advantages such as solubility and modularity. Herein, we report the synthesis of a series of chiral imine organic cages with three built-in rylene units by means of dynamic imine chemistry and we investigate their textural and optoelectronic properties. Thereby we demonstrate that the synthesized rylene cages can be reversibly reduced at accessible potentials, absorb from UV up to green light, are porous, and preferentially adsorb CO2 over N2 and CH4 with a good selectivity. In addition, we discovered that the cage incorporating three perylene-3,4:9,10-bis(dicarboximide) units displays an efficient delayed fluorescence. Time-correlated single photon counting and transient absorption spectroscopy measurements suggest that the delayed fluorescence is likely a consequence of a reversible intracage charge-separation event. Rylene cages thus offer a promising platform that allows combining the porosity of processable materials and photochemical phenomena useful in diverse applications such as photocatalysis or energy storage. | ||||||||
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| The nature of the electronic excited state of many symmetric multibranched donor–acceptor molecules varies from delocalized/multipolar to localized/dipolar depending on the environment. Solvent-driven localization breaks the symmetry and traps the exciton in one branch. Using a combination of ultrafast spectroscopies, we investigate how such excited-state symmetry breaking affects the photochemical reactivity of quadrupolar and octupolar A(-π-D)2,3 molecules with photoisomerizable A-π-D branches. Excited-state symmetry breaking is identified by monitoring several spectroscopic signatures of the multipolar delocalized exciton, including the S2�S1 electronic transition, whose energy reflects interbranch coupling. It occurs in all but nonpolar solvents. In polar media, it is rapidly followed by an alkyne-allene isomerization of the excited branch. In nonpolar solvents, slow and reversible isomerization corresponding to chemically-driven symmetry breaking, is observed. These findings reveal that the photoreactivity of large conjugated molecules can be tuned by controlling the localization of the excitation. | ||||||||
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| The formation of a halogen-bond (XB) complex in the excited state was recently reported with a quadrupolar acceptorâ€"donorâ€"acceptor dye in two iodine-based liquids (J. Phys. Chem. Lett. 2017, 8, 3927â€"3932). The ultrafast decay of this excited complex to the ground state was ascribed to an electron transfer quenching by the XB donors. We examined the mechanism of this process by investigating the quenching dynamics of the dye in the S1 state using the same two iodo-compounds diluted in inert solvents. The results were compared with those obtained with a non-halogenated electron acceptor, fumaronitrile. Whereas quenching by fumaronitrile was found to be diffusion controlled, that by the two XB compounds is slower, despite a larger driving force for electron transfer. A Smoluchowskiâ€"Collinsâ€"Kimball analysis of the excited-state population decays reveals that both the intrinsic quenching rate constant and the quenching radius are significantly smaller with the XB compounds. These results point to much stronger orientational constraint for quenching with the XB compounds, indicating that electron transfer occurs upon formation of the halogen bond. | ||||||||
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| The influence of torsional disorder around the ethynyl pi-bridges of a linear D-pi-A-pi-D molecule on the nature of its S1 excited state was investigated using ultrafast time-resolved infrared spectroscopy. By tuning the pump wavelength throughout the S1<- �S0 absorption band, subpopulations with different extents of asymmetry could be excited. In non-polar solvents, the equilibrated S1 state is symmetric and quadrupolar independently of the initial degree of distortion. Photoexcitation of distorted molecules is followed by planarization and symmetrization of the S1 state. Excited-state symmetry breaking is only observed in polar environments, where the equilibrated S1 state has a strong dipolar character. However, neither the extent nor the rate of symmetry breaking are enhanced in an initially distorted molecule. They are only determined by the polarity and the dynamic properties of the solvent. | ||||||||
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| Intermolecular H‐bonding dynamics around a photoexcited quadrupolar dye is directly observed using transient 2D‐IR spectroscopy. Upon solvent‐induced symmetry breaking, the H‐bond accepting abilities of the two nitrile end‐groups change drastically, and in extremely protic (superprotic) solvents, a tight H‐bond complex forms at one end. The time evolution of the 2D C≡N lineshape in methanol points to rapid, 2–3 ps, spectral diffusion due to fluctuations of the H‐bonding network. Similar behavior is observed in a superprotic solvent shortly after photoexcitation of the dye. However, at later times, the completely inhomogeneous band does not exhibit spectral diffusion for at least 5 ps, pointing to a glass‐like environment around one side of the dye. About half of the excited dyes show this behavior attributed to the tight H‐bond complex, whereas the others are loosely bound. A weak cross peak indicates partial exchange between these excited state subpopulations. | ||||||||
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| Time-resolved photoluminescence is one of the most standard techniques to understand and systematically optimize the performance of optical materials and optoelectronic devices. Here, we present a machine learning code to analyze time-resolved photoluminescence data and determine the decay rate distribution of an arbitrary emitter without any a priori assumptions. To demonstrate and validate our approach, we analyze computer-generated time-resolved photoluminescence data sets and show its benefits for studying the photoluminescence of novel semiconductor nanocrystals (quantum dots), where it quickly provides insight into the possible physical mechanisms of luminescence without the need for educated guessing and fitting. | ||||||||
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| The excited-state dynamics of a T-shaped bichromophoric molecule, consisting of two strong fluorophores, diphenyloxazole and diphenylpyrazoline, directly linked in an orthogonal geometry, was investigated. Despite the weak coupling ensured by this geometry and confirmed by the electronic absorption spectra, this dyad exhibits only weak fluorescence in both apolar and polar solvents, with fluorescence lifetimes ranging from 200 ps in CHX to 10 ps in ACN. Ultrafast spectroscopic measurements reveal that the fluorescence quenching in polar solvents is due to the population of a charge-separated state. In non-polar solvents, this process is energetically not feasible, and a quenching due to an efficient intersystem crossing (ISC) to the triplet manifold is proposed, based on quantum-chemical calculations. This process occurs via the spin–orbit charge-transfer (SOCT) ISC mechanism, which is enabled by the charge-transfer character acquired by the S 1 state of the dyad upon structural relaxation and by the orthogonal arrangement of the molecular orbitals involved in the transition. The same mechanism is proposed to explain why the recombination of the charge-separated state is faster in medium than in highly polar solvents, as well as to account for the fast decay of the lowest triplet state to the ground state. | ||||||||
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| Previously, we have shown that the use of a cyclopeptidic carrier could be of great interest for the design of fully characterized prodrugs for further use in photodynamic therapy. In order to further optimize the design, we decided to modify the highly quenched conjugate uPA-cPPP 4/5 by co-loading a long-distance fluorescence quencher. For this purpose we tethered two black hole quenchers (BHQ3) together with two pheophorbide A moities onto the same PEGylated backbone and assessed the modified photophysical properties. In addition, to prove the reliability of our concept, we designed two analogues, uPA-cPPQ 2+2/5 and CathB-cPPQ 2+2/5 , by using two different peptidic linkers as substrates for uPA and cathepsin B, respectively. These two conjugates proved to be much more water-soluble than their analogues bearing only Phas. These conjugates are not only highly quenched in their native state with regard to their fluorescence emission (up to 850 ± 287 times less fluorescent for CathB-cPPQ 2+2/5 as compared to the unquenched monosubstituted reference uPA-cPPP 1/5 ), but also prevent singlet oxygen production (with a total quenching of the emission when the quenchers are co-loaded with photosensitizers) when the photosentistizers are excited. After proteolytic activation, these conjugates recover their photophysical properties in the same way as occurred for uPA-cPPP 4/5 , with up to a 120-fold increase in fluorescence emission for uPA-cPPQ 2+2/5 after two hours of incubation with uPA. | ||||||||
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| Herein, we report the synthesis of a new prodrug system consisting of regioselectively addressable functionalized templates bearing multiple pheophorbide A moieties for use in photodynamic therapy. These coupling reactions were achieved using copper-free "click" chemistry, namely a strain-promoted azide–alkyne cycloaddition. This new design was used to obtain well-defined quenched photosensitizer prodrugs with perfect knowledge of the number and position of loaded photosensitizers, providing structures bearing up to six photosentitizers and two PEG chains. These conjugates are ideally quenched in their native state regarding their fluorescence emission (up to 155 ± 28 times less fluorescent for an hexasubstituted conjugate than a monosubstituted non-quenched reference compound) or singlet oxygen production (decreased 8.7-fold in the best case) when excited. After 2 h of proteolytic activation, the fluorescence emission of a tetrasubstituted conjugate was increased 17-fold compared with the initial fluorescence emission. | ||||||||
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| The annulation of two redox-active molecules into a compact and planar structure paves the way toward a new class of electronically versatile materials whose physical properties can be tuned via a substitution of one of the constituting moieties. Specifically, we present tetrathiafulvalene–benzothiadiazole donor–acceptor molecules. The critical role played by the dielectric properties of these molecules is evident by the large spectral shifts of the ground-state absorption spectra in a range of solvents. Stark spectroscopy is performed to determine experimentally dipole and polarizability change over transitions in the visible range with particular attention to the transition from the highest-occupied molecular orbital (HOMO) to the lowest-unoccupied molecular orbital (LUMO). The experimental results are compared to the results of time-dependent density functional theory calculations, and we reciprocally validate results from calculation and experiment. This allows us to filter out effective models and reveal important insights. The calculations are initially performed in the gas phase and subsequently a polarizable continuum model is adopted to probe the influence of the solvent on the molecular dielectric properties. The results show a large charge displacement from the HOMO to the LUMO and confirm the intramolecular charge transfer nature of the lowest-energy transition. Substitution of the acceptor moiety with electron-withdrawing groups results in changes to the experimentally determined molecular properties consistent with the effects predicted by computational results. The dominant contribution to the electroabsorption signal is due to the change in dipole moment, which is measured to be roughly 20 D for all samples and forms a small angle with the transition dipole moment in a toluene solvent environment. | ||||||||
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| The fast metal exchange reaction between Au 38 and Ag x Au 38−x nanoclusters in solution at −20 °C has been studied by in situ X-ray absorption spectroscopy (time resolved quick XAFS) in transmission mode. A cell was designed for this purpose consisting of a cooling system, remote injection and mixing devices. The capability of the set-up is demonstrated for second and minute time scale measurements of the metal exchange reaction upon mixing Au 38 /toluene and Ag x Au 38−x /toluene solutions at both Ag K-edge and Au L 3 -edge. It has been proposed that the exchange of gold and silver atoms between the clusters occurs via the SR(-M-SR) n (n = 1, 2; M = Au, Ag) staple units in the surface of the reacting clusters during their collision. However, at no point during the reaction (before, during, after) evidence is found for cationic silver atoms within the staples. This means that either the exchange occurs directly between the cores of the involved clusters or the residence time of the silver atoms in the staples is very short in a mechanism involving the metal exchange within the staples. | ||||||||
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| The excited-state dynamics of the push–pull azobenzene Methyl Orange (MO) were investigated in several solvents and water/glycerol mixtures using a combination of ultrafast time-resolved fluorescence and transient absorption in both the UV-visible and the IR regions, as well as quantum chemical calculations. Optical excitation of MO in its trans form results in the population of the S 2 ππ* state and is followed by internal conversion to the S 1 nπ* state in ∼50 fs. The population of this state decays on the sub-picosecond timescale by both internal conversion to the trans ground state and isomerisation to the cis ground state. Finally, the cis form converts thermally to the trans form on a timescale ranging from less than 50 ms to several minutes. Significant differences depending on the hydrogen-bond donor strength of the solvents, quantified by the Kamlet Taft parameter α, were observed: compared to the other solvents, in highly protic solvents (α > 1), (i) the viscosity dependence of the S 1 state lifetime is less pronounced, (ii) the S 1 state lifetime is shorter by a factor of ≈1.5 for the same viscosity, (iii) the trans-to-cis photoisomerisation efficiency is smaller, and (iv) the thermal cis-to-trans isomerisation is faster by a factor of ≥10 3 . These differences are explained in terms of hydrogen-bond interactions between the solvent and the azo nitrogen atoms of MO, which not only change the nature of the S 1 state but also have an impact on the shape of ground- and excited-state potentials, and, thus, affect the deactivation pathways from the excited state. | ||||||||
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| A series of bis(pyreneamide) macrocycles, synthesized in two steps from THF, THP, oxepane and 1,4-dioxane, are tested as chemosensors for a large range of mono-, di- and trivalent cations. In their native states, these macrocycles exhibit a strong excimer fluorescence that is quenched upon the addition of the metal ions (alkaline, alkaline earth, p-, d-, and f-block metals). UV-Vis spectrophotometric titrations, cyclic voltammetry, excimer fluorescence quenching and transient absorption spectroscopy experiments helped characterize the On-Off changes occurring upon binding and demonstrate that the highest stability constants are obtained with divalent cations Ca2+ and Ba2+ specifically. | ||||||||
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| The influence of the length of the push–pull branches of quadrupolar molecules on their excited-state symmetry breaking was investigated using ultrafast time-resolved IR spectroscopy. For this, the excited-state dynamics of an A-π-D-π-A molecule was compared with those of an ADA analogue, where the same electron donor (D) and acceptor (A) subunits are directly linked without a phenylethynyl π-spacer. The spatial distribution of the excitation was visualized in real time by monitoring C≡C and C≡N vibrational modes localized in the spacer and acceptor units, respectively. In nonpolar solvents, the excited state is quadrupolar and the excitation is localized on the π-D-π center. In medium polarity solvents, the excitation spreads over the entire molecule but is no longer symmetric. Finally, in the most polar solvents, the excitation localizes on a single D-π-A branch, contrary to the ADA analogue where symmetry breaking is only partial. | ||||||||
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| The excited-state dynamics of an anilineâ€"triazine electron donorâ€"acceptor dyad with an alkyne spacer has been investigated using a combination of ultrafast broadband mid-IR and visible transient absorption and fluorescence spectroscopies. The transient IR data reveal the occurrence of an efficient alkyne to allene isomerization of the spacer with a time constant increasing from a few hundreds of femtoseconds to a few picoseconds with solvent viscosity. This process is faster than the vibrational cooling of the Franckâ€"Condon excited state, indicative of nonequilibrium dynamics. The transient electronic absorption and fluorescence data evidence that this transformation is accompanied by a charge separation between the donor and the acceptor subunits. The allene character of the spacer implies an orthogonal orientation of the donor and acceptor moieties, similar to that proposed for twisted intramolecular charge-transfer states. Such states are often invoked in the excited-state dynamics of donorâ€"acceptor dyads, but their involvement could never be unambiguously evidenced spectroscopically. The alkyneâ€"allene isomerization involves not only a torsional motion but also a bending of the molecule due to the sp to sp2 rehybridization of one of the alkyne carbon atoms. This twisted and rehybridized intramolecular charge transfer (“TRICTâ€�) state decays back to the planar and linear alkyne ground state on a time scale decreasing from a few hundred to ten picoseconds upon going from weakly to highly polar solvents. The different solvent dependencies reveal that the dynamics of the allene buildup are controlled by the structural changes, whereas the decay is limited by the charge recombination step. | ||||||||
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| Fluoride substitution in LiBH 4 is studied by investigation of LiBH 4 -LiBF 4 mixtures (9:1 and 3:1). Decomposition was followed by in-situ synchrotron radiation X-ray diffraction (in-situ SR-PXD), thermogravimetric analysis and differential scanning calorimetry with gas analysis (TGA/DSC-MS) and in-situ infrared spectroscopy (in-situ FTIR). Upon heating, fluoride substituted LiBH 4 forms (LiBH 4-x F x ) and decomposition occurs, releasing diborane and solid decomposition products. The decomposition temperature is reduced more than fourfold relative to the individual constituents, with decomposition commencing at T / °C = 80 °C. The degree of fluoride substitution is quantified by sequential Rietveld refinement and shows a selective manner of substitution. In-situ FTIR experiments reveal formation of bands originating from LiBH 4-x F x . Formation of LiF and observation of diborane release implies that the decomposing materials have a composition that facilitates formation of diborane and LiF, i.e. LiBH 4-x F x (LiBH 3 F). An alternative approach for fluoride substitution was performed, by addition of Et 3 N∙3HF to LiBH 4 , yielding extremely unstable products. Spontaneous decomposition indicates fluoride substitution to have occurred. From our point of view, this is the most significant destabilization effect seen for borohydride materials so far. | ||||||||
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| The nucleation and growth properties of domains of molecules of the same state in open boundary three-dimensional (3D) spin-crossover systems of various shapes are discussed within the framework of the mechanoelastic model. The molecules are situated on face-centered-cubic lattices and are linked by springs through which they interact. Monte Carlo simulations imply that clusters nucleate from corners in the case of systems having well-developed faces and from kinks in the case of spherical samples, in accordance with available experimental data. In addition, a method to characterize the cooperativity in these systems is proposed, which by scanning the fluctuations in the 3D samples can be related directly to powder x-ray-diffraction experiments. | ||||||||
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| 1,4-Dioxepines result from the decomposition of α-diazo-β-keto esters in the presence of oxetanes using the catalytic combination of the (cyclopentadienyl)ruthenium complex [CpRu(CH3CN)3][BArF] and 1,10-phenanthroline. The regioselective [4+3] insertions follow an SN1-like mechanism and occur yet enantiospecifically (es 74%). The retention of configuration was ascertained by vibrational circular dichroism (VCD) and solid state analyses. Furans, products of [4+1] insertions, are only observed as traces in the above protocol. To promote their formation under CpRu catalysis, it is necessary to use a two-step process with γ-halogenated alcohols as substrates. | ||||||||
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| Implementation, benchmarking, and representative applications of the new FDE-ADC(3) method for describing environmental effects on excited states as a combination of frozen density embedding (FDE) and the algebraic-diagrammatic construction scheme for the polarization propagator of third order (ADC(3)) are presented. Results of FDE-ADC(3) calculations are validated with respect to supersystem calculations on test systems with varying molecule–environment interaction strengths from dispersion up to multiple hydrogen bonds. The overall deviation compared to the supersystem calculations is as small as 0.029 eV for excitation energies, which is even smaller than the intrinsic error of ADC(3). The dependence of the accuracy on the choice of method and functional for the calculation of the environment and the nonelectrostatic part of the system–environment interaction is evaluated. In three representative examples, the FDE-ADC method is applied to investigate larger systems and to analyze excited state properties using visualization of embedded densities and orbitals. | ||||||||
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| The kinetic lability of hexadentate gallium-based tripods is sufficient to ensure thermodynamic self-assembly of luminescent heterodimetallic [GaLn(L3)3]6+ helicates on the hour time scale, where Ln is a trivalent 4f-block cation. The inertness is however large enough for preserving the triple-helical structure when [GaLn(L3)3]6+ is exposed to lanthanide exchange. The connection of a second gallium-based tripod further slows down the exchange processes to such an extent that spectroscopically active [CrErCr(L4)3]9+ can be diluted into closed-shell [GaYGa(L4)3]9+ matrices without metal scrambling. This feature is exploited for pushing molecular-based energy transfer upconversion (ETU) at room temperature. | ||||||||
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| A thorough understanding of the microscopic mechanism of excited-state proton transfer (ESPT) and the influence of the solvent environment on its dynamics are of great fundamental interest. We present here a detailed investigation of an ESPT to solvent (DMSO) using time-resolved broadband fluorescence and transient absorption spectroscopies. All excited-state species are resolved spectrally and kinetically using a global target analysis based on the two-step Eigen-Weller model. Reversibility of the initial short-range proton transfer producing excited contact ion pairs (CIP*) is observed unambiguously in fluorescence and must be explicitly considered to obtain the individual rate constants. Close inspection of the early dynamics suggests that the relative populations of the protonated form (ROH*) and CIP* are governed by solvent relaxation that influences the relative energies of the excited states. This constitutes a breakdown of the Eigen-Weller model, although the overall agreement between the data and the analysis using classical rate equations is excellent. | ||||||||
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| Symmetry breaking of the excited state of a linear quadrupolar acceptor-donor-acceptor molecule was investigated using time-resolved infrared spectroscopy in 55 solvents allowing the influence of several solute–solvent interactions to be examined separately. No symmetry breaking was found in nonpolar solvents irrespective of their refractive index, indicating that differences in dispersion interactions between the two arms of the molecule do not suffice to induce an asymmetric distribution of the excitation. However, symmetry breaking was observed in nondipolar but quadrupolar solvents like benzene to an extent that can be as large as that found in medium dipolar solvents like THF. Whereas larger symmetry breaking occurs in the most dipolar solvents, the strongest are observed in protic solvents due to hydrogen bonding. Strong evidence of the formation of halogen bonds in the excited state is also presented, confirming the idea of symmetry-breaking-induced asymmetrical photochemistry. | ||||||||
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| Lithium amide–borohydrides Li[BH4]1–x [NH2] x possess liquid-like Li superionic conductivity at nearly ambient temperature. The fast Li+ diffusion facilitated by the localized motions of the anions is proposed to occur through a network of vacant tetrahedral sites, acting as conduction channels. To study the reorientational dynamics of the anions, we have performed quasielastic neutron scattering experiments on samples with different compositions (x = 2/3, 0.722, 0.737, 3/4) over a broad temperature and time range. To unambiguously disentangle the contributions of the two species, [BH4]− and [NH2]−, we took advantage of deuterium labeling and could clearly demonstrate that the quasielastic broadening is mainly determined by the [BH4]− reorientations. With the help of a newly developed model, supported by ab initio molecular dynamics calculations, we have identified three relaxation components, which account for generally anisotropic C 3-rotations of the [BH4]− tetrahedra including jumps by a small angle from the equilibrium position. | ||||||||
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| Chiral metal dithiolene complexes represent a family of chiral precursors, which can give rise to molecular materials with properties resulting from the interplay of chirality with conductivity, magnetism, and photophysics. We describe herein the first examples of chiral metal diimine dithiolene complexes, by the use of a platinum(II) centre coordinated by 2,2'-bipyridine and helicene-dithiolene ligands. Straightforward synthesis of racemic and enantiopure complexes allows the preparation of luminescent Pt(bipy) [4] and [6]helicene compounds for which the solid-state structure was determined as well. TD-DFT calculations support the assignment of the low energy bands observed in the UV-vis absorption spectra as mixed metal-ligand-to-ligand charge transfer transitions and confirm that the emission band results from the T1 excited state. Interestingly the enantiopure [6]helicene complexes show CPL activity at room temperature in acetonitrile solutions with anisotropy factors of 3×10-4. | ||||||||
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| Compared with divalent ruthenium coordination complexes, which are widely exploited as parts of multi-component photonic devices, optically active trivalent chromium complexes are under-represented in multi-metallic supramolecular architectures performing energy conversion because of the tricky preparation of stable heteroleptic CrIII building blocks. We herein propose a kind of remedy with the synthesis of a novel family of kinetically inert hetereloptic bis-terdentate mononuclear complexes, which can be incorporated into dinuclear rod-like diads as a proof-of-concept. The mechanism and magnitude of intermetallic Cr · ··Cr communications have been unraveled by a combination of magnetic, photophysical and thermodynamic investigations. Alternated aromatic/alkyne connectors provided by Sonogashira coupling reactions emerge as the most efficient wires for long-distance communication between two chromium centres bridged by Janus-type back-to-back bis-terdentate receptors. | ||||||||
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| High ionic conductivity of up to 6.4 ×10−3S cm−1 near room temperature (40 °C) in lithium amide-borohydrides is reported, comparable to values of liquid organic electrolytes commonly employed in lithium-ion batteries. Density functional theory is applied coupled with X-ray diffraction, calorimetry, and nuclear magnetic resonance experiments to shed light on the conduction mechanism. A Li4Ti5O12 half-cell battery incorporating the lithium amide-borohydride electrolyte exhibits good rate performance up to 3.5 mA cm−2 (5 C) and stable cycling over 400 cycles at 1 C at 40 °C, indicating high bulk and interfacial stability. The results demonstrate the potential of lithium amide-borohydrides as solid-state electrolytes for high-power lithium-ion batteries. | ||||||||
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| The fluorescence quenching of organic dyes via H-bonding interactions is a well-known phenomenon. However, the mechanism of this Hydrogen-Bond Induced Nonradiative Deactivation (HBIND) is not understood. Insight into this process is obtained by probing in the infrared the O H stretching vibration of the solvent after electronic excitation of a dye with H-bond accepting cyano groups. The fluorescence lifetime of this dye was previously found to decrease from 1.5 ns to 110 ps when going from an aprotic solvent to the strongly protic hexafluoroisopropanol (HFP). Prompt strengthening of the H-bond with the dye was identified by the presence of a broad positive O-H band of HFP, located at lower frequency than the O-H band of the pure solvent. Further strengthening occurs within a few picoseconds before the excited H-bonded complex decays to the ground state in 110 ps. The latter process is accompanied by the dissipation of energy from the dye to the solvent and the rise of a characteristic hot solvent band in the transient spectrum. Polarization-resolved measurements evidence a collinear alignment of the nitrile and hydroxyl groups in the H-bonded complex, which persists during the whole excited-state lifetime. Measurements in other fluorinated alcohols and in chloroform/HFP mixtures reveal that the HBIND efficiency depends not only on the strength of the H-bond interactions between the dye and the solvent but also on the ability of the solvent to form an extended H-bond network. The HBIND process can be viewed as an enhanced internal conversion of an excited complex consisting of the dye molecule connected to a large H-bond network. | ||||||||
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| K3Fe(CN)6 reacts with the viologen 1,1′-bis(2,4-dinitrophenyl)-4,4′-bipyridinium dication, (DNP)2+, to form a supramolecular complex, (DNP)3[Fe(CN)6]2·10H2O (1). The crystal structure of 1 reveals that there are two [Fe(CN)6]3– anions within an organic framework of three (DNP)2+ cations with the shortest Fe(III)···Fe(III) distances of ca. 9.8 Å, distances that minimize extensive long-range magnetic exchange coupling interactions between the [Fe(CN)6]3– anions, and, thus, 1 is paramagnetic above ca. 17 K and exhibits weak ferromagnetic coupling between 17 and 3 K and antiferromagnetic coupling between 3 and 1.8 K. The long Fe(III)···Fe(III) distances permit slow spin–spin and slow spin–lattice paramagnetic relaxation, relative to the iron-57 Larmor precession frequency, as is evidenced by the Mössbauer spectra measured between 3 and 60 K; between 85 and 295 K, rapid paramagnetic relaxation is observed. Both the slow spin–spin and slow spin–lattice relaxation are mediated by the organic, π-conjugated viologen cations. The Fe–C distances, the Mössbauer isomer shifts, the temperature dependence of the magnetic susceptibility, and the 3 K magnetization results all indicate the presence of low-spin Fe(III) ions in the [Fe(CN)6]3– anions in 1. There is no unequivocal indication of the presence of any formal electron delocalization or transfer from the [Fe(CN)6]3– anion to the (DNP)2+ cations in the results obtained from X-ray crystallography, magnetic measurements, and Mössbauer spectra. Because of enhancement of the spin–orbit coupling by the heavy-atom or -ion effect, the Fe(III) ions in the [Fe(CN)6]3– anions interact with the (DNP)2+ cations, causing them to fluoresce with increasing intensity upon cooling from 90 to 25 K when excited at 300 nm. The resulting luminescence of the viologen (DNP)2+ cation induced by the [Fe(CN)6]3– anions indicates the presence of significant mixing of the molecular orbitals derived from the [Fe(CN)6]3– anions and the molecular orbitals associated with the (DNP)2+ cations to yield bonding supramolecular orbitals in 1, a mixing that is also observed between 50 and 3 K in the temperature dependence of the isomer shift of 1. | ||||||||
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| Enhancement of infrared signals from polyelectrolyte adsorbed on gold nanoparticles (GNPs) was studied in situ by attenuated total reflection infrared spectroscopy. Nanoparticles and polyelectrolytes were deposited using layer-by-layer techniques, and the infrared signal was studied as a function of particle size, particle density, and distance from particle surface. It was observed that enhancement is more pronounced for larger nanoparticles and it decreases with increasing distance from the particle surface. Furthermore, at high GNP coverage, the signal from the first polyelectrolyte layer is particularly enhanced, and the signal increases slowly with time, in contrast to subsequent layers. We assign this to polyelectrolyte adsorption within narrow gaps between nanoparticles, where the electric field is enhanced. Furthermore, enhanced absorption was observed in the gap between the GNPs and the germanium internal reflection element, which was confirmed by polarized measurements. This enhancement is more pronounced for silver particles, and it represents a promising route for analysis of surfaces by infrared spectroscopy. | ||||||||
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| The ligand exchange reaction between heteroatom doped (Pd, Pt) Au25(2-PET)18 (2-PET = 2-phenylethylthiolate) clusters and enantiopure 1,1′-binaphthyl-2,2′-dithiol (BINAS) was monitored in situ using chiral high-performance liquid chromatography (HPLC). During the ligand exchange reactions, replacement of two protecting thiols (2-PET) with one new entering BINAS ligand on the cluster surface occurs. The rigid dithiol BINAS adsorbs in a specific mode that bridges the apex and one core site of two adjacent S(R)–Au–S(R)–Au–S(R) units. This is the most favorable binding mode and theoretically preserves the original structure. A kinetic investigation on these in situ ligand exchange reactions revealed a decrease in reactivity after multiple exchange. A comparison of relative rate constants demonstrates a similar exchange rate toward BINAS for both (Pd, Pt) systems. The possible structural deformation after incorporation of BINAS was investigated by X-ray absorption spectroscopy (XAS) at the S K-edge and Au L3-edge. First, a thorough assignment of all sulfur contributions to the XANES spectrum was performed, distinguishing for the first time long and short staple motifs. Following that, a structural comparison of doped systems using XANES and EXAFS confirmed the unaltered Au25 structure, except for some slight influence on the Au–S bonds. Additionally, an intact staple motif was confirmed after incorporation of rigid dithiol BINAS by both XANES and EXAFS. This finding agrees with a BINAS interstaple binding mode predicted by calculation, which does not perturb the cluster structure. | ||||||||
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| We propose a simple method for predicting the spin state of homoleptic complexes of the Fe(II) d6 ion with chelating diimine ligands. The approach is based on the analysis of a single metric parameter within a free (noncoordinated) ligand: the interatomic separation between the N-donor metal-binding sites. An extensive analysis of existing complexes allows the determination of critical N···N distances that dictate the regions of stability for the high-spin and low-spin complexes, as well as the intermediate range in which the magnetic bistability (spin crossover) can be observed. The prediction has been tested on several complexes that demonstrate the validity of our method. | ||||||||
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| A simple model has been developed to describe the symmetry-breaking of the electronic distribution of A L –D–A R type molecules in the excited state, where D is an electron donor and A L and A R are identical acceptors. The origin of this process is usually associated with the interaction between the molecule and the solvent polarization that stabilizes an asymmetric and dipolar state, with a larger charge transfer on one side than on the other. An additional symmetry-breaking mechanism involving the direct Coulomb interaction of the charges on the acceptors is proposed. At the same time, the electronic coupling between the two degenerate states, which correspond to the transferred charge being localised either on A L or A R , favours a quadrupolar excited state with equal amount of charge-transfer on both sides. Because of these counteracting effects, symmetry breaking is only feasible when the electronic coupling remains below a threshold value, which depends on the solvation energy and the Coulomb repulsion energy between the charges located on A L and A R . This model allows reproducing the solvent polarity dependence of the symmetry-breaking reported recently using time-resolved infrared spectroscopy. | ||||||||
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| Na2(B12H12)0.5(B10H10)0.5, a new solid-state sodium electrolyte is shown to offer high Na+ conductivity of 0.9 mS cm-1 at 20 °C, excellent thermal stability up to 300 °C, and a large electrochemical stability window of 3 V including stability towards sodium metal anodes, all essential prerequisites for a stable room-temperature 3 V all-solid-state sodium-ion battery. | ||||||||
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| Because of its crucial role in many areas of science and technology, photoinduced electron transfer is the most investigated photochemical reaction. Despite this, several important questions remain open. We present recent efforts to answer some of them, which concern both inter- and intramolecular processes. The decisive factor that allowed these issues to be successfully addressed was the use of time-resolved infrared (TRIR) spectroscopy. Many different transient species, such as tight and loose ion pairs (TIPs and LIPs) and exciplexes, have been invoked to explain the dynamics of intermolecular photoinduced charge separation reactions (i.e., electron transfer between two neutral species) and the production of free ions. However, their structures are essentially unknown, and their exact roles in the reaction mechanism are unclear. Indeed, the commonly used transient electronic absorption spectroscopy does not give much structural insight and cannot clearly distinguish ion pairs from free ions, at least in the visible region. Unambiguous spectral signatures of TIPs, LIPs, and exciplexes could be observed in the IR using electron donor/acceptor (D/A) pairs with adequate vibrational marker modes. The ability to spectrally distinguish these intermediates allowed their dynamics to be disentangled and their roles to be determined. Structural information could be obtained using polarization-resolved TRIR spectroscopy. Our investigations reveal that moderately to highly exergonic reactions result in the formation of both TIPs and LIPs. TIPs are not only generated upon direct charge-transfer excitation of DA complexes, as usually assumed, but are also formed upon static quenching with reactant pairs at distances and orientations enabling charge separation without diffusion. On the other hand, dynamic quenching produces primarily LIPs. In the case of highly exergonic reactions, strong indirect evidence for the generation of ion pairs in an electronic excited state was found, accounting for the absence of an inverted region. Finally, weakly exergonic reactions produce predominantly exciplexes, which can evolve further into ion pairs or recombine to the neutral ground state. The high sensitivity of specific vibrational modes to the local electronic density was exploited to visualize the photoinduced charge flow in symmetric A–(π–D)2- and D–(π–A)2-type molecules developed for their two-photon absorption properties. The electronic ground state and Franck–Condon S1 state of these molecules are purely quadrupolar, but the strong solvatochromism of their fluorescence points to a highly dipolar relaxed S1 state. This has been explained in terms of excited-state symmetry breaking induced by solvent and/or structural fluctuations. However, real-time observation of this process was missing. Direct visualization of symmetry-breaking charge transfer was achieved using TRIR spectroscopy by monitoring vibrations localized in the two arms of these molecules. A transition from a purely quadrupolar state to a symmetry-broken state on the timescale of solvent relaxation could be clearly observed in polar solvents, indicating that symmetry breaking occurs primarily via solvent fluctuations. In the case of the D–(π–A)2 molecule, this breaking results in different basicities at the two A ends and consequently in different affinities for H-bonds, which in turn leads to the formation of an asymmetric tight H-bonded complex in highly protic solvents. | ||||||||
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| This work deals with the synthesis and crystal structure study of new bismuth oxo-arsenates and their oxo-phosphates homologous: Bi6ZnO7(AsO4)2 vs Bi6ZnO7(PO4)2 and Bi3.667Cd3O4(AsO4)3 vs Bi3Cd4O4(PO4)3. Their crystal structures were solved using single crystal X-Ray Diffraction. These are two other examples of crystal structures built on ribbon-like polycations formed of the linkage of oxo-centered O(Bi,M)4 tetrahedra sharing edges and surrounded by isolated XO4 groups (X= As or P), where the O(Bi,M)4 units are derived from the fluorite topology structure. Dealing with Bi6ZnO7(PO4), its acentric space group was confirmed by preliminary second harmonic generation (SHG). The P/As substitution led to a centrosymmetric space group due to local reorientation of oxo-anions. This is strongly related to steric effects between AsO4 (d As-O= 1.6-1.7Å) and PO4 (d P-O= 1.4-1.5Å). Concerning Bi3.667Cd3O4(AsO4)3 and Bi3Cd4O4(PO4)3, they show a second example of the reorientation of the XO4 groups depending of the X chemical nature. Finally, we present an original topology of oxo-centered units obtained with Bi5KO5(AsO4). The photoluminescence properties of Bi5KO5(AsO4) and Bi6ZnO7(AsO4)2 were also investigated. The first one emits at room temperature in the reddish-orange range (single band peak at 615nm assigned to the Bi3+: 3P 1 →1S 0 transition whereas the second exhibits a weak emission in the green range (peak at 530nm). Its intriguing temperature dependence is discussed in the paper. | ||||||||
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| The possibility of the direct encapsulation of living cells via two-photon induced photopolymerization enables the microfabrication of hydrogel scaffolds with high initial cell loadings and intimate matrix-cell contact. While highly efficient water-soluble two-photon initiators based on benzylidene ketone dyes have been developed, they exhibit considerable cyto- and phototoxicity. To address the problem of photoinitiator migration from the extracellular matrix into the cytosol, a two-photon initiator bound to a polymeric hyaluronan backbone (HAPI) was synthesized in this work. HAPI exhibited a distinct improvement of cytocompatibility compared to a reference two-photon initiator. Basic photophysical investigations were performed to characterize the absorption and fluorescence behavior of HAPI. Laser scanning microscopy was used to visualize and confirm the hindered transmembrane migration behavior of HAPI. The performance of HAPI was tested in two-photon polymerization at exceedingly high printing speeds of 100 mm s-1 producing gelatin-based complex 3D hydrogel scaffolds with a water content of 85%. The photodamage of the structuring process was low and viable MC3T3 cells embedded in the gel were monitored for several days after structuring. | ||||||||
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| Introduction of heterocycles in the helical skeleton of helicenes allows modulation of their redox, chiroptical and photophysical properties. Herein, we describe the straightforward preparation and structural characterization by single crystal X-ray diffraction of thiadiazole-[7]helicene, which has been resolved into (M) and (P) enantiomers by chiral HPLC, together with its S-shaped double [4]helicene isomer, as well as the smaller congeners thiadiazole-[5]helicene and benzothiadiazole-anthracene. A copper(II) complex with two thiadiazole-[5]helicene ligands has been structurally characterized and it shows the presence of both (M) and (P) isomers coordinated to the metal centre. The emission properties of the unprecedented heterohelicenes are highly dependent on the helical turn, as the [7]- and [5]helicene are poorly emissive, whereas their isomers, that is, the S-shaped double [4]helicene and thiadiazole-benzanthracene, are luminescent, with quantum efficiencies of 5.4% and 6.5%, respectively. DFT calculations suggest a quenching of the luminescence of enantiopure [7]helicenes through an intersystem crossing mechanism arising from the relaxed excited S1 state. | ||||||||
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| Ultrafast time-resolved infrared spectroscopy of [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine), [Ru(mbpy)3]2+ (mbpy = 6-methyl-2,2'-bipyridine), and [Ru(mphen)3]2+ (mphen = 2-methyl-1,10'-phenanthroline) in deuterated acetonitrile serves to elucidate the evolution of the system following pulsed excitation into the 1MLCT band at 400 nm. Whereas for [Ru(bpy)3]2+ no intermediate state can be evidenced for the relaxation of the corresponding 3MLCT state back to the ground state, for [Ru(mbpy)3]2+ and [Ru(mphen)3]2+ an intermediate state with a lifetime of about 400 ps is observed. The species associated IR difference spectra of this state are in good agreement with the calculated difference spectra of the lowest energy 3dd state using DFT. The calculated potential energy curves for all the complexes in the triplet manifold along the metal-ligand distance show that for [Ru(bpy)3]2+ the 3dd state is at higher energy than the 3MLCT state and that there is a substantial barrier between the two minima. For [Ru(mbpy)3]2+ and [Ru(mphen)3]2+, the 3dd state is at lower energy than the 3MLCT state. | ||||||||
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| Sum frequency mixing of fluorescence and ∼1300 nm gate pulses, in a thin β-barium borate crystal and non-collinear type II geometry, is quantified as part of a femtosecond fluorimeter [X.-X. Zhang et al., Rev. Sci. Instrum. 82, 063108 (2011)]. For a series of fixed phasematching angles, the upconversion efficiency is measured depending on fluorescence wavelength. Two useful orientations of the crystal are related by rotation around the surface normal. Orientation A has higher efficiency (factor ∼3) compared to B at the cost of some loss of spectral coverage for a given crystal angle. It should be used when subtle changes of an otherwise stationary emission band are to be monitored. With orientation B, the fluorescence range λF > 420-750 nm is covered with a single setting of the crystal and less gate scatter around time zero. The accuracy of determining an instantaneous emission band shape is demonstrated by comparing results from two laboratories. | ||||||||
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| We demonstrate and characterize an optical control of the plasmonic heat delivered by a monolayer substrate of gold nanoparticles, obtained by modulating the effective refractive index of the neighboring dielectric medium. The effect, which exploits the dependence of the nematic liquid crystal (NLC) refractive index on the molecular director orientation, is realized by using a polarization dependent, light-induced molecular reorientation of a thin film of photo-alignment layer that the NLC is in contact with. For a suitable alignment, plasmonic pumping intensity values ranging from 0.25 W/cm2 to 6.30 W/cm2 can induce up to 17.4 °C temperature variations in time intervals of the order of seconds. The reversibility of the optically induced NLC molecular director orientation enables an active control of the plasmonic photo-induced heat. | ||||||||
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| The manganese-nitronyl-nitroxide two dimensional coordination polymer {[Mn2(NITIm)3]ClO4} n (1) (NITImH = 2-(2-imidazolyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxide-1-oxyl) undergoes an unusual hysteretic thermoinduced valence tautomeric transition near room temperature, during which the manganese(II) ions are oxidized to manganese(III) and two of the three deprotonated radicals (NITIm-) are reduced to their diamagnetic aminoxyl form (denoted NITImRed 2-). Upon cooling, the high-temperature species {[MnII 2(NITIm)3]ClO4} n (1HT ) turns into the low-temperature species {[MnIII 2(NITImRed)2(NITIm)]ClO4} n (1LT ) around 274 K, while on heating the process is reversed at about 287 K. This valence tautomeric phenomenon is supported by temperature-dependent magnetic susceptibility measurements, differential scanning calorimetry (DSC), crystal structure determination, UV-vis absorption, X-ray absorption (XAS) an emission (XES) and Electron Paramagnetic Resonance (EPR) spectroscopies in the solid-state. | ||||||||
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| The physicochemical properties of cationic dioxa (1), azaoxa (2), and diaza (3) [6]helicenes demonstrate a much higher chemical stability of the diaza adduct 3 (pK R+=20.4, E red 1/2 =−0.72 V) compared to its azaoxa 2 (pK R+=15.2, E red 1/2 =−0.45 V) and dioxa 1 (pK R+=8.8, E red 1/2 =−0.12 V) analogues. The fluorescence of these cationic chromophores is established, and ranges from the orange to the far-red regions. From 1 to 3, a bathochromic shift of the lowest energy transitions (up to 614 nm in acetonitrile) and an enhancement of the fluorescence quantum yields and lifetimes (up to 31 % and 9.8 ns, respectively, at 658 nm) are observed. The triplet quantum yields and circularly polarized luminescence are also reported. Finally, fine tuning of the optical properties of the diaza [6]helicene core is achieved through selective and orthogonal post-functionalization reactions (12 examples, compounds 4–15). The electronic absorption is modulated from the orange to the far-red spectral range (560–731 nm), and fluorescence is observed from 591 to 755 nm with enhanced quantum efficiency up to 70 % (619 nm). The influence of the peripheral auxochrome substituents is rationalized by first-principles calculations. | ||||||||
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| Symmetry-breaking charge transfer upon photoexcitation of a linear A-π-D-π-A molecule (D and A being electron donating and accepting groups) could be visualized using ultrafast time-resolved infrared spectroscopy by monitoring the CN stretching modes on the A units. Whereas in apolar solvents, the S1 state remains symmetric and quadrupolar, symmetry breaking occurs within ca. 100 fs in polar solvents as shown by the presence of two CN bands, instead of one in apolar solvents, with a splitting that increases with polarity. In protic solvents, symmetry breaking is significantly amplified by H-bonding interactions, which are the strongest at the CN group with the highest basicity. In strongly protic solvents, the two CN bands transform in about 20 ps into new bands with a larger splitting, and the lifetime of the S1 state is substantially reduced. This is attributed to the formation of an excited asymmetric tight H-bond complex. | ||||||||
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| Germanium – polyelectrolyte – gold nanoparticle composites were prepared and characterized using FTIR-ATR spectroscopy and scanning electron microscopy. The germanium (Ge) element served as internal reflection element and the buildup of the layered system was followed in situ. Positively charged polyelectrolyte poly (allylamine hydrochloride) (PAH) adsorbs spontaneously on negatively charged Ge. Citrate-stabilized gold nanoparticles can then be adsorbed onto the PAH layer. Upon illumination of the device with visible light a prominent absorption over the entire mid infrared region is observed which is due to intervalence band transitions in Ge. The strong infrared signals are evidence for holes in the valence band of the Ge semiconductor, which arise due to electron transfer to the gold nanoparticles (GNP). The electron transfer, as evidenced by the holes in Ge, is affected by the nature of the gap between the Ge and the GNP. Increasing the gap by adsorbing polyelectrolyte multilayers hinders the electron transfer. Also heating and vacuum have a pronounced effect. The device is proposed as a sensor, where the sensing event is transduced into an optical signal in the infrared, as demonstrated for a thiol molecule. The thiol has a large affinity for the gold and therefore affects the germanium – gold nanoparticle gap. This reduces the electron transfer and therefore the absorption in the infrared upon illumination with visible light. Removal of the thiol from the solution leads to a recovering of the signal. | ||||||||
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| Here we reproduce the static and dynamical properties of spin-crossover complexes in the framework of the mechanoelastic model applied to triangular lattices. The switching processes between the high-spin and low-spin states are studied by combining the Monte Carlo method with the elastic lattice relaxation. The transition probabilities between the two states take into account intrinsic parameters, the values of which are approximated from experimental quantities (e.g., the energy gap, and the degeneracy ratio from the thermodynamic enthalpy and the entropy difference between the states), and the elastic force or elastic energy stored in the springs connecting the spin-changing centres. The value of the corresponding spring constant is estimated from the experimentally determined variation of the ligand-field strengths in the two spin states due to the cooperativity and the bulk modulus. Both simulated hysteresis loops and relaxation curves are in agreement with experimental data. Cooperativity related phenomena such as like-spin domain formation and the evolution of the interaction distribution with the HS fraction are also analysed. | ||||||||
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| Organic molecules imprinting can be used for introducing specific properties and functionalities such as chirality to mesoporous materials. Particularly organic self-assemblies can work as a scaffold for templating inorganic materials such as silica. During recent years chiral imprinting of anionic surfactant for fabrication of twisted rod-like silica structures assisted by co-structuring directing agent were thoroughly investigated. The organic self-assemblies of anionic surfactants can also be used for introducing other shapes in rod-like silica structures. Here we report the formation of amphiphilic N-miristoyl-L-alanine self-assemblies in aqueous solution upon stirring and at presence of L-arginine. These anionic surfactant self-assemblies form fibers that grow by increasing the stirring duration. The fibers were studied using transmission electron microscopy, infra-red spectroscopy and vibrational circular dichroism. Addition of silica precursor 1,2-bis(triethoxysilyl)ethylene and co-structuring directing agent N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride at different stages of fibers' growth leads to formation of different silica structures. By controlling stirring duration, we obtained twisted tubular silica structures as a result of fibers encapsulation. We decorated these structures with gold nanoparticles by different methods and measured their optical activity. | ||||||||
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| The linear and nonlinear optical properties of a series of oligomeric squaraine dyes were investigated by one-photon absorption spectroscopy (1PA) and two-photon absorption (2PA) induced fluorescence spectroscopy. The superchromophores are based on two indolenine squaraine dyes with transoid (SQA) and cisoid configuration (SQB). Using these monomers, linear dimers and trimers as well as star-shaped trimers and hexamers with benzene or triphenylamine cores were synthesised and investigated. The red-shifted and intensified 1PA spectra of all superchromophores could well be explained by exciton coupling theory. In the linear chromophore arrangements we also found superradiance of fluorescence but not in the branched systems. Furthermore, the 2PA showed enhanced cross sections for the linear oligomers but only additivity for the branched systems. This emphasizes that the enhancement of the 2PA cross section in the linear arrangements is probably caused by orbital interactions of higher excited configurations. | ||||||||
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| The combination of Frozen Density Embedding Theory (FDET) and the Algebraic Diagrammatic Construction (ADC) scheme for the polarization propagator for describing environmental effects on electronically excited states is presented. Two different ways of interfacing and expressing the so-called embedding operator are introduced. The resulting excited states are compared with supermolecular calculations of the total system at the ADC(2) level of theory. Molecular test systems were chosen to investigate molecule–environment interactions of varying strength from dispersion interaction up to multiple hydrogen bonds. The overall difference between the supermolecular and the FDE-ADC calculations in excitation energies is lower than 0.09 eV (max) and 0.032 eV in average, which is well below the intrinsic error of the ADC(2) method itself. | ||||||||
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| Considered at the beginning of the 21th century as being incompatible with the presence of closely bound high-energy oscillators, lanthanide-centered superexcitation, which is the raising of an already excited electron to an even higher level by excited-state energy absorption, is therefore a very active topic strictly limited to the statistical doping of low-phonon bulk solids and nanoparticles. We show here that molecular lanthanide-containing coordination complexes may be judiciously tuned to overcome these limitations and to induce near-infrared (NIR)-to-visible (VIS)-light upconversion via the successive absorption of two low-energy photons using linear-optical responses. Whereas single-ion-centered excited-state absorption mechanisms remain difficult to implement in lanthanide complexes, the skillful design of intramolecular intermetallic energy-transfer processes operating in multimetallic architectures is at the origin of the recent programming of erbium-centered molecular upconversion. | ||||||||
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| The cooperativity index in a metallopolymer can be thought of as an estimation of the level of available water in nature. In a river, the water molecules can only feed immediate surroundings. Those in an aqueduct possess sufficient potential energy to extend this beneficial effect to wider areas. Twenty centuries ago, the Romans exploited this concept for extending their settlement in Western Europe. A reliable access to allosteric cooperativity indexes might push host–guest chemistry toward unexplored areas. More information can be found in the Full Paper by C. Piguet et al. (DOI: 10.1002/chem.201600857). The basic concept of allosteric cooperativity used in biology, chemistry and physics states that any change in the intermolecular host–guest interactions operating in multisite receptors can be assigned to intersite interactions. Using lanthanide metals as guests and linear multi-tridentate linear oligomers of variable lengths and geometries as hosts, this work shows that the quantitative modeling of metal loadings requires the consideration of a novel phenomenon originating from solvation processes. It stepwise modulates the intrinsic affinity of each isolated site in multisite receptors, and this without resorting to allosteric cooperativity. An easy-to-handle additive model predicts a negative power law dependence of the intrinsic affinity on the length of the linear metallopolymer. Applied to lanthanidopolymers, the latter common analysis overestimates cooperativity factors by more than two orders of magnitude. | ||||||||
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| Unprecendented regioselective post-functionalization of racemic and enantiopure cationic diaza [4]helicenes is afforded. The peripheral auxochrome substituents allow a general tuning of the electrochemical, photophysical and chiroptical properties of the helical dyes (26 examples). For instance, electronic absorption and circular dichroism are modulated from the orange to near-infrared spectral range (575-750 nm), fluorescence quantum efficiency is enhanced up to 0.55 (631 nm) and circularly polarized luminescence is recorded in the red (│glum│ ~ 10-3) | ||||||||
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| Two reactive hydride composite systems, Ca(BH4)2–NaNH2 and Mg(BH4)2–NaNH2, were systematically studied by in situ synchrotron radiation powder diffraction, in situ Fourier transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry. Metathesis reactions between the amides and borohydrides take place in both systems between 100°C and 150°C yielding amorphous materials with the proposed composition M(BH4)(NH2). Simultaneously, a fraction of NaNH2 decomposes to Na3N and ammonia via a complex pathway. The main gas released under 300°C is ammonia for both systems, while significant amounts of hydrogen are released only above 350°C. | ||||||||
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| Polydiacetylene (PDA) is a conductive polymer that has a mechanochromism. When the polymer is exposed to mechanical stresses, change in temperature (thermochromism), pH (ionochromism) etc., the structural perturbation can be seen by the change in its color. Although it presents interesting electrical and optical properties, the relationship between these signals has rarely been investigated. We studied the correlation between the electrical conductivity and the absorption spectra of PDA. Upon UV irradiation, PDA absorption spectra presented a blue shift, which coincided with the decrease in the electrical conductivity. | ||||||||
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| Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S1 electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S1 state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S1 state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S1 state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations. | ||||||||
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| Optical spectroscopy of transition metal complexes plays an important role in establishing excited state electronic and nuclear structures and thus in the elucidation of the multitude of photophysical and photochemical relaxation processes. The most important advances in this area of research over the past decade are due to the development of new experimental techniques such as ultrafast spectroscopy as well as structure determination in conjunction with other methods such as high pressure and variable temperature techniques. In this contribution, several paradigmatic systems, namely of complexes if chromium(III), iron(II), ruthenium(II), nickel(II), platinum(II) and palladium(II), are discussed with regard to their excited electronic and nuclear structures and photophysical relaxation processes. | ||||||||
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| Besides molecular electron densities obtained within the Born–Oppenheimer approximation (ρ B (r)) to represent the environment, the ensemble averaged density (〈ρ B 〉(r)) is also admissible in frozen-density embedding theory (FDET) [Wesolowski,Phys. Rev. A, 2008,77, 12054]. This makes it possible to introduce an approximation in the evaluation of the solvent effect on quantum mechanical observables consisting of replacing the ensemble averaged observable by the observable evaluated at ensemble averagedρ B (r). This approximation is shown to affect negligibly the solvatochromic shift in the absorption of hydrated acetone. The proposed model provides a continuum type of representation of the solvent, which reflects nevertheless its local structure, and it is to be applied as a post-simulation analysis tool in atomistic level simulations | ||||||||
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| The properties of binary mixtures of dimethylsulfoxide and glycerol, measured by several techniques, are reported. Special attention is given to those properties contributing or affecting chemical reactions. In this respect the investigated mixture behaves as a relatively simple solvent and it is especially well suited for studies on the influence of viscosity in chemical reactivity. This is due to the relative invariance of the dielectric properties of the mixture. However, special caution must be taken with specific solvation, as the hydrogen-bonding properties of the solvent changes with the molar fraction of glycerol. | ||||||||
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| Late endosomes are a major trafficking hub in the cell at the crossroads between endocytosis, autophagy, and degradation in lysosomes. Herein is disclosed the first small molecule allowing their selective imaging and monitoring in the form of a diazaoxatriangulene fluorophore, 1a (hexadecyl side chain). The compound is prepared in three steps from a simple carbenium precursor. In nanospheres, this pH-sensitive (pK a = 7.3), photochemically stable dye fluoresces in the red part of visible light (601 and 578 nm, acid and basic forms, respectively) with a quantum yield between 14 and 16% and an excited-state lifetime of 7.7–7.8 ns. Importantly, the protonated form 1a·H+ provokes a specific staining of late endosome compartments (pH 5.0–5.5) after 5 h of incubation with HeLa cells. Not surprisingly, this late endosome marking depends on the intra-organelle pH, and changing the nature of the lipophilic chain provokes a loss of selectivity. Interestingly, fixation of the fluorophore is readily achieved with paraformaldehyde, giving the possibility to image both live and fixed cells. | ||||||||
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| The properties of a series of oxazole yellow dyes, including the dicationic YOPRO-1 and its homodimeric parent YOYO-1 and two monocationic dyes (YOSAC-1 and YOSAC-3), have been investigated at the dodecane/water interface using stationary and time-resolved surface second harmonic generation (SSHG) combined with quantum chemical calculations. Whereas YOYO-1 exists predominantly as a H-dimer in aqueous solution, the stationary SSHG spectra reveal that such dimers are not formed at the interface. No significant H-aggregation was observed with YOPRO-1, neither in solution nor at the interface. In the case of the monocationic YOSAC dyes, a distinct SSHG band due to H-aggregates was measured at the interface, whereas only weak aggregation was found in solution. These distinct aggregation behaviors can be explained by the different orientations of the dyes at the interface, as revealed from the analysis of polarization-resolved experiments, the doubly-charged dyes lying more flat on the interface than the singly charged ones. Although YOYO-1 and YOPRO-1 do not form H-dimer/aggregates at the interface, time-resolved SSHG measurements point to the occurrence of intra- and intermolecular interactions, respectively, which inhibit the ultrafast non-radiative decay of the excited dyes via large amplitude motion, and lead to a nanosecond excited-state lifetime. The distinct behavior evidenced here for YOYO-1 and YOSAC dyes points to their potential use as fluorescent or SHG interfacial probes. | ||||||||
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| Size-controlled nanocrystals (140 nm and 670 nm) and microcrystals (2.5 mm) of the three-dimensional oxalate network [Ru(bpy)3][NaCr(ox)3], ox = oxalate, bpy = 2,2'-bipyridine, were prepared by the reverse micelle technique. The photo-physical properties of the [Cr(ox)3]3- chromophores in the nanocrystals at low temperatures are significantly different from those of the same chromophore in 4 mm crystallites prepared by fast precipitation. For the latter, the absorption in the region of the R lines of the 4A2 → 2E transition is characterized by a sharp doublet. For the nanocrystals the inhomogeneous broadening of the two lines is considerably larger with tails on the low-energy side. Whereas the 4 mm crystallites at low temperatures just show equally sharp emission from the R1 line, the emission intensity from the nanocrystallites is shifted into the low-energy tail. Time resolved fluorescence line narrowing spectra and luminescence decay curves demonstrate that this is due to efficient directional energy migration from the center of the nanocrystals towards the surface | ||||||||
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| In this chapter, the mechanism of plasmonic coupling of the near-fields that takes place when nanostructure are very close to each other is described. After a theoretical introduction illustrating the main physical principles governing plasmonic coupling, several experimental systems are considered. Interestingly, experimnetal results show that both periodic and rigid, random, and flexible systems of gold nanoparticles exhibit a universal scaling behaviour and verify the plamonic ruler equation. | ||||||||
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| We have carried out an experiment on a flexible polymeric substrate, coated with a monolayer of gold nanoparticles, which demonstrates how the combined effect of nanoparticle growth and stretching influences the average normalized gap between particles, thus modifying the extinction spectra of the sample. The study paves the way for the realization of a plasmonic strain sensor based on the plasmonic coupling of gold nanoparticles deposited onto elastomeric films: application of a mechanical stretching induces a change of colour of the device and a fine control of the applied strain allows a continuous tuning of the colour. | ||||||||
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| Elongated plasmonic nanoparticles show superior optical properties when compared to spherical ones. Facile, versatile and cost-effective bottom-up approaches for fabrication of anisotropic nanoparticles in solution have been developed. However, fabrication of 2-D plasmonic templates from elongated nanoparticles with spatial arrangement at the surface is still a challenge. We used controlled seed-mediated growth in the presence of porous and functionalized surface of flexible polydimethylsiloxane (PDMS) templates to provide directional growth and formation of elongated gold nanoparticles (AuNPs). Atomic force microscopy (AFM) and spectroscopy revealed embedding of the particles within the functionalized porous surface of PDMS. Nanoparticles shapes were observed with transmission electron microscope (TEM), UV–Vis spectroscopy, and X-ray powder diffraction (XRPD) measurements, which revealed an overall orientation of particles at the surface. Anisotropic and oriented particles on a flexible substrate are of interest for sensing applications. | ||||||||
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| The ionophore (salph)Co(III)OAc is used here for the first time as a recognition molecule for nitrite in lipophilic membranes. The nature of the recognition process is evaluated by UV-Vis, ATR-IR and potentiometry, while the purity of the compound is confirmed by LC-MS and cyclic voltammetry. Experimental evidence suggests a replacement of the acetate ligand by nitrite. The formation constant between (salph)Co(III)OAc and NO2 −, β∼108, is estimated potentiometrically with the sandwich membrane method. The analytical parameters, LOD∼3 μM, linear range of four orders of magnitude, stability over four days, and the selectivity over chloride (log KNO2,Cl POT∼-4.1) are similar to previous ionophores for the same ion. Undiluted urine samples were spiked with incremental concentrations of nitrite to characterize the matrix effect. Beyond the observed analytical performance, we aim here to offer a rational study to understand an unconventional charged ionophore recognition process. | ||||||||
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| The thermodynamically disfavored isomerization of α,β-unsaturated esters to deconjugated β,γ-unsaturated analogues occurs readily when coupled to an amidation. Within the framework of macrocyclic derivatives, it is shown that 15, 16, and 18 membered macrocycles react with tBuOK and anilines to generate in one-pot the β,γ-unsaturated amides (yields up to 88%). Importantly, single (chiral) diastereomers are isolated (d.r. > 49:1, 1H NMR) irrespective of the size and nature of the rings showing an effective transmission of remote stereochemistry during the isomerization process. CSP-chromatographic resolution and absolute configuration determination by VCD are achieved. | ||||||||
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| The ligand 3-chloro-6-dipicolylamino-1,2,4,5-tetrazine (Cl-TTZ-dipica) 1, prepared by the direct reaction between 3,6-dichloro-1,2,4,5-tetrazine and di(2-picolyl)-amine, afforded a series of four neutral transition metal complexes formulated as [Cl-TTZ-dipica-MCl2]2, with M = Zn(II)2a, Cd(II) 2b, Mn(II) 2c and Co(II) 2d, when reacted with the corresponding metal chlorides. The dinuclear structure of the isostructural complexes was disclosed by single crystal X-ray analysis, clearly indicating the formation of [MII-(m-Cl)2MII] motifs and the involvement of the amino nitrogen atom in semi-coordination with the metal centers, thus leading to distorted octahedral coordination geometries. Moreover, the chlorine atoms, either coordinated to the metal or as substituent on the tetrazine ring, engage respectively in specific anion-p intramolecular and intermolecular interactions with the electron poor tetrazine units in the solid state, thus controlling the supramolecular architecture. Modulation of the emission properties is observed in the case of the Zn(II) and Cd(II) complexes when compared to the free ligand. A striking difference is observed in the magnetic properties of the Mn(II) and Co(II) complexes. An antiferromagnetic coupling takes place in the dimanganese(II) compound (J = -1.25 cm-1) while the Co(II) centers are ferromagnetically coupled in the corresponding complex (J = +0.55 cm-1), the spin Hamiltonian being defined as H = -JS A . S B. | ||||||||
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| The recently obtained spin-crossover nanoparticles are possible candidates for applications in the recording media industry as materials for data storage, or as pressure and temperature sensors. For these applications, the intermolecular interactions and interactions between spin-crossover nanoparticles are extremely important, as they may be essential factors in triggering the transition between the two stable phases: the high-spin and low-spin ones. In order to find correlations between the distributions in size and interactions and the transition temperatures distribution, we apply the FORC (First Order Reversal Curves) method, using simulations based on a mechanoelastic model applied to 2D triangular lattices composed of molecules linked by springs and embedded in a surfactant. We consider two Gaussian distributions: one is the size of the nanoparticles and another is the elastic interactions between edge spin-crossover molecules and the surfactant molecules. In order to disentangle the kinetic and non-kinetic parts of the FORC distributions, we compare the results obtained for different temperature sweeping rates. We also show that the presence of few larger particles in a distribution centered around much smaller particles dramatically increases the hysteresis width. | ||||||||
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| The synthesis, crystal structure and photophysical properties of the new compound [Mn4(ThiaSO2)2F][K(18-crown-6)], ThiaSO2 = p-tertbutylsulphonylcalix[4]arene, are presented and compared to the ones of [Mn4(ThiaSO2)2F]K. The strong orange luminescence is attributed to the Mn2+ centred 4T1 → 6A1 transition. Its temperature and pressure dependence and quenching by molecular dioxygen are reported. The latter is attributed to energy transfer from the 4T1 state exciting dioxygen to its 1Σ+ g state. In the solid state, the quenching is much more efficient in [Mn4(ThiaSO2)2F][K(18-crown-6)] than in [Mn4(ThiaSO2)2F]K. This is attributed to the open pore structure of the former allowing fast diffusion of dioxygen into the crystal lattice. | ||||||||
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| Single crystals of tetragonal RCuGa3 (R=La, Pr, Nd and Gd), with BaNiSn3 type structure (space group I4 mm), have been grown by high temperature solution growth technique using Ga as flux. Their magnetic properties were determined by heat capacity and the measurement of magnetization and electrical resistivity along [100] and [001] directions. Except LaCuGa3, the compounds order magnetically. PrCuGa3 undergoes a ferromagnetic transition with Curie temperature of 4.6 K. NdCuGa3 shows a bulk magnetic transition at 3.3 K. The data on GdCuGa3 indicate combined characteristics of spin glass and antiferromagnetic behavior at low temperatures. From the Schottky heat capacity data the crystal electric field level energy spectra have been determined. Further we have performed temperature dependent measurements of electron spin resonance (ESR) on GdCuGa3 between 11 K and room temperature. The ESR data indicate an enhancement of magnetic fluctuations associated with spin reorientation and both homogeneous and inhomogeneous thermal broadening of the linewidth. | ||||||||
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| In this report, "fluorescent flippers" are introduced to create planarizable push–pull probes with the mechanosensitivity and fluorescence lifetime needed for practical use in biology. Twisted push–pull scaffolds with large and bright dithienothiophenes and theirS,S-dioxides as the first "fluorescent flippers" are shown to report on the lateral organization of lipid bilayers with quantum yields above 80% and lifetimes above 4 ns. Their planarization in liquid-ordered (Lo) and solid-ordered (So) membranes results in red shifts in excitation of up to +80 nm that can be transcribed into red shifts in emission of up to +140 nm by Förster resonance energy transfer (FRET). These unique properties are compatible with multidomain imaging in giant unilamellar vesicles (GUVs) and cells by confocal laser scanning or fluorescence lifetime imaging microscopy. Controls indicate that strong push–pull macrodipoles are important, operational probes do not relocate in response to lateral membrane reorganization, and two flippers are indeed needed to "really swim," i.e., achieve high mechanosensitivity. | ||||||||
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| Size-controlled micro- and nanocrystals of a [Ru(bpy)3][NaCr(ox)3] coordination network are prepared using reverse micelles. Compared with the bulk reference, the 2E emission of the Cr(III) ions indicates an efficient directional energy migration toward the surface of the nanocrystals. | ||||||||
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| Establishing a tractable yet complete reaction coordinate for the spin-state interconversion in d4–d7 transition metal complexes is an integral aspect of controlling the dynamics that govern their functionality. For spin crossover phenomena, the limitations of a single-mode approximation that solely accounts for an isotropic increase in the metal–ligand bond length have long been recognized for all but the simple octahedral monodentate FeII compounds. However, identifying the coupled deformations that also impact on the unimolecular rate constants remains experimentally and theoretically challenging, especially for samples that do not display long-range order or when crystallization profoundly alters the dynamics. Owing to the rapid progress in ultrafast X-ray absorption spectroscopy (XAS), it is now possible to obtain transient structural information in any physical phase with unprecedented details. Using picosecond XAS and DFT modeling, the structure adopted by the photoinduced high-spin state of solvated [Fe(terpy)2]2+ (terpy: 2,2′:6′,2″-terpyridine) has been recently established. Based on these results, the methodology of the continuous shape measure is applied to classify and quantify the short-lived distortion of the first coordination shell. The reaction coordinate of the spin-state interconversion is clearly identified as a double axial bending. This finding sets a benchmark for gauging the influence of first-sphere and second-sphere interactions in the family of FeII complexes that incorporate terpy derivatives. Some implications for the optimization of related photoactive FeII complexes are also outlined. | ||||||||
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| Characterizing structural distortions in the metastable spin states of d4–d7 transition metal ion complexes is crucial to understand the nature of their bistability and eventually control their switching dynamics. In particular, the impact of the Jahn–Teller effect needs to be assessed for any electronic configuration that could be effectively degenerate, as in e.g. the high-spin (HS) manifold of highly symmetric homoleptic FeII complexes. However, capturing its manifestations remains challenging since crystallization generally alters the molecular conformations and their interconversion. With the rapid progress of ultrafast X-ray absorption spectroscopy, it is now possible to collect data with unprecedented signal-to-noise ratio, opening up for detailed structural characterization of transient species in the homogeneous solution phase. By combining the analysis of picosecond X-ray absorption spectra with DFT simulations, the structure of the photoinduced HS state is elucidated for solvated [Fe(terpy)2]2+ (terpy = 2,2′:6′,2″-terpyridine). This species can be viewed as the average5B structure inD 2 symmetry that originates from a dynamic Jahn–Teller effect in the HS manifold. These results evidence the active role played by this particular instance of vibronic coupling in the formation of the HS state for this benchmark molecule. Ultimately, correlating the interplay between intramolecular and intermolecular degrees of freedom to conformational strain and distortions in real time should contribute to the development of advanced functionalities in transition metal ion complexes. | ||||||||
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| A series of long-tail alkyl ethanolamine analogs containing amide-, urea-, and thiourea moieties was synthesized and the behavior of the corresponding monolayers was assessed on the Langmuir–Pockels trough combined with grazing incidence X-ray diffraction experiments and complemented by computer simulations. All compounds form stable monolayers at the soft air/water interface. The phase behavior is dominated by strong intermolecular headgroup hydrogen bond networks. While the amide analog forms well-defined monolayer structures, the stronger hydrogen bonds in the urea analogs lead to the formation of small three-dimensional crystallites already during spreading due to concentration fluctuations. The hydrogen bonds in the thiourea case form a two-dimensional network, which ruptures temporarily during compression and is recovered in a self-healing process, while in the urea clusters the hydrogen bonds form a more planar framework with gliding planes keeping the structure intact during compression. Because the thiourea analogs are able to self-heal after rupture, such compounds could have interesting properties as tight, ordered, and self-healing monolayers. | ||||||||
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| Bis(thiomethyl)- and bis(thiohexyl)-tetrathiafulvalene-bromo-benzothiadiazoles, containing electron donor tetrathiafulvalene (TTF) and electron acceptor benzothiadiazole (BTD) units, have been prepared by Stille coupling reactions between the TTF-SnMe3 precursors and BTD-Br2. In another series of experiments, TTF-acetylene-BTD compounds have been synthesized by Sonogashira coupling between either TTF-acetylenes and BTD-Br2 in low yields, or TTF-iodine and BTD-acetylene in moderate yields. In the compound TTF-C≡C-BTD the TTF and BTD units are coplanar in the solid state, as shown by the single crystal X-ray structure, and there is segregation in the packing between the donor and acceptor units. All the derivatives have good electron donor properties, as determined by cyclic voltammetry measurements, and they can also be reversibly reduced thanks to the presence of the BTD moiety. UV-visible spectroscopy and photophysical investigations show the presence of an intramolecular charge transfer (ICT) band and an emission band originating from the charge transfer. Both the absorption and the emission are modulated by the substitution scheme and the insertion of the acetylenic bridge. | ||||||||
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| The Hoffman-type coordination compound [Fe(pz)Pt(CN)4]⋅2.6 H2O (pz=pyrazine) shows a cooperative thermal spin transition at around 270 K. Synchrotron powder X-Ray diffraction studies reveal that a quantitative photoinduced conversion from the low-spin (LS) state into the high-spin (HS) state, based on the light-induced excited spin-state trapping effect, can be achieved at 10 K in a microcrystalline powder. Time-resolved measurements evidence that the HS→LS relaxation proceeds by a two-step mechanism: a random HS→LS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HS→LS transformation has been reached. | ||||||||
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| Europium doped crystalline Ba7F12Cl2 phosphors have been prepared at temperatures between 650 and 900 °C using alkali chloride fluxes, yielding both disordered (with the incorporation of small amounts of Na) and ordered crystal modifications. The white emission spectrum excited in the near UV consists roughly of two broad emission bands at ca 450 and 590 nm, as well as weak sharp Eu2+ 4f-4f emission bands around 360 nm. The incorporation of Eu2+ is further studied using EPR spectroscopy on single crystals, and reveals a significant zero field splitting. The emission spectrum can be significantly tuned by varying the excitation wavelength between 300 and 390 nm. Fine tuning may also be achieved by chemical substitutions to form Ba7-xMyF12Cl2-zBrz (M = Na, Ca,Eu). Quantitative measurements of the light produced using commercial near UV LEDs show that the color temperature ranges between 4000 and 9700 K with CIE chromaticity coordinates close to the ideal values of x=y=0.333. The best color rendering index (CRI) found was 0.83, and the highest light to light conversion yield was 171 lumen/W. These results show that the title compound is a very promising candidate for white light generation using near UV LED excitation. | ||||||||
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| This work shows that the operation of near-infrared to visible light-upconversion in a discrete molecule is not limited to non-linear optical processes, but may result from superexcitation processes using linear optics. The design of nine-coordinate metallic sites made up of neutral N-heterocyclic donor atoms in kinetically inert dinuclear [GaEr(L1)3]6+ and trinuclear [GaErGa(L2)3]9+ helicates leads to [ErN9] chromophores displaying unprecedented dual visible nanosecond Er(4S3/2→4I15/2) and near-infrared microsecond Er(4I13/2→4I15/2) emissive components. Attempts to induce one ion excited-state absorption (ESA) upconversion upon near-infrared excitation of these complexes failed because of the too-faint Er-centred absorption cross sections. The replacement of the trivalent gallium cation with a photophysically-tailored pseudo-octahedral [CrN6] chromophore working as a sensitizer for trivalent erbium in [CrEr(L1)3]6+ improves the near-infrared excitation efficiency, leading to the observation of a weak energy transfer upconversion (ETU). The connection of a second sensitizer in [CrErCr(L2)3]9+ generates a novel mechanism for upconversion, in which the superexcitation process is based on the CrIII-sensitizers. Two successive Cr→Er energy transfer processes (concerted-ETU) compete with a standard Er-centred ETU, and a gain in upconverted luminescence by a factor larger than statistical values is predicted and observed. | ||||||||
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| The electronic absorption spectrum of 3-hydroxyflavone (3HF) in various solvents exhibits a long-wavelength (LW) band, whose origin has been debated. The excited-state dynamics of neutral and basic solutions of 3HF in alcohols upon excitation in this LW band has been investigated using a combination of fluorescence up-conversion and transient electronic and vibrational absorption spectroscopies. The ensemble of results reveals that, in neutral solutions, LW excitation results in the population of two excited species with similar fluorescence spectra but very different lifetimes, namely 40–100 ps and 2–3 ns, depending on the solvent. In basic solutions, the relative concentrations of these species change considerably in favor of that with the short-lived excited state. On the basis of the spectroscopic data and quantum chemistry calculations, the short lifetime is attributed to the excited state of 3HF anion, whereas the long one is tentatively assigned to an excited hydrogen-bonded complex with the solvent. Excited-state intermolecular proton transfer from the solvent to the anion yielding the tautomeric form of 3HF is not operative, as the excited anion decays to the ground state via an efficient nonradiative transition. | ||||||||
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| The role of ligand-field states for the photophysical properties of d6 systems has been discussed in a large number of publications over the past decades. Since the seminal paper by Houten and Watts, for instance, the quenching of the 3MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to the presence of the first excited ligand-field state, namely a component of the 3 T 1(t 2g 5 e g 1) state, at similar energies. If this state lies above the 3MLCT state, the luminescence is quenched via thermal population at elevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenic temperatures. In this contribution we present transient absorption spectra on non-luminescent ruthenium polypyridyl complexes such as [Ru(m-bpy)3]2+, m-bpy = 6-methyl-2,2'-bipyridine, in acetonitrile at room temperature, which reveal an ultra-rapid depopulation of the 3MLCT state but a much slower ground state recovery. We propose that in this and related complexes the methyl groups force longer metal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the 3dd state drops to below the 3MLCT state, and that furthermore the population of this state from the 3MLCT state occurs faster than its decay to the ground state. In addition we demonstrate that in this complex the luminescence can be switched on by external pressure, which we attribute to a destabilisation of the ligand-field state by the pressure due to its larger molecular volume compared to the ground state as well as the 3MLCT state. | ||||||||
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| The Au102(p-MBA)44 cluster (p-MBA: para-mercaptobenzoic acid) is observed as a chiral compound comprised of achiral components in its single-crystal structure. So far the enantiomers observed in the crystal structure are not isolated, nor is the circular dichroism spectrum known. A chiral phase transfer method is presented which allows partial resolution of the enantiomers by the use of a chiral ammonium bromide, (−)-1R,2S-N-dodecyl-N-methylephedrinium bromide ((−)-DMEBr). At sufficiently low concentration of (−)-DMEBr, the phase transfer from water to chloroform is incomplete. Both the aqueous and organic phases show optical activity of near mirror image relationship. Differences in the spectra are ascribed to the formation of diastereomeric salts. At high concentrations of (−)-DMEBr, full phase transfer is observed. The organic phase, however, still displays optical activity. We assume that one of the diastereomers has very strong optical activity, which overrules the cancelation of the spectra with opposite sign. Comparison with computations further corroborates the experimental data and allows a provisional assignment of handedness of each fraction. | ||||||||
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| Surface-enhanced Raman spectroscopy takes advantage of plasmonic substrates that sustain resonances at tunable frequencies with a reproducibly extraordinary field enhancement. Low-cost and large-scale fabrication of these substrates is further required. Here, we present stacked large-scale arrays of strongly coupled gold nanoparticles as promising candidates for such substrates. These arrays are fabricated by bottom-up techniques that fulfill the aforementioned requirements. The distance between adjacent arrays in the stack is controlled with high precision using a discrete number of monolayers of molecules that enable the spectral position of the plasmonic resonances to be tuned. Although the nanoparticles are randomly arranged in each array, the spatial proximity of the stacked arrays enables a strong coupling among nanoparticles to be achieved in adjacent arrays. The huge field enhancements due to these strongly coupled gold nanoparticles are shown to enhance the Raman signal. We show that effectively the optical response from these stacked arrays and the Raman signals can be understood in a simplifying picture where only an individual nanoparticle dimer is considered. The possibility to tune the plasmonic resonances of the substrate across the visible spectrum makes our material a plasmonic substrate of choice for many applications where light–matter interactions need to be intensified. | ||||||||
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| A simple method is presented to control and trigger the coupling between plasmonic particles using both a growing process of gold nanoparticles (GNPs) and a mechanical strain applied to the elastomeric template where these GNPs are anchored. The large scale samples are prepared by first depositing and then further growing gold nanoparticles on a flexible PDMS tape. Upon stretching the tape the particles move further apart in the direction of the stretching and closer together in the direction perpendicular to it. The synergy between the controlled growth of GNPs and the mechanical strain, leads to a drastic shift of the plasmon band and a color change of the sample. Furthermore, the stretching by only a few percent of the amorphous and initially isotropic sample results in a strong polarization-dependent plasmon shift. At smaller gap sizes between neighboring particles, induced by stretching the PDMS tape, the plasmon shift strongly deviates from the behaviour expected considering the plasmon ruler equation. This shows that multipolar coupling effects significantly contribute to the observed shift. Overall, these results indicate that a macroscopic mechanical strain allows one to control the coupling and therefore the electromagnetic field at the nanoscale. | ||||||||
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| The most abundant of the modified nucleosides, and once considered as the "fifth" nucleotide in RNA, is pseudouridine, which results from the action of pseudouridine synthases. Recently, the mammalian pseudouridine synthase 1 (hPus1p) has been reported to modulate class I and class II nuclear receptor responses through its ability to modify the Steroid receptor RNA Activator (SRA). These findings highlight a new level of regulation in nuclear receptor (NR)-mediated transcriptional responses. We have characterised the RNA association and activity of the human Pus1p enzyme with its unusual SRA substrate. We validate that the minimal RNA fragment within SRA, named H7, is necessary for both the association and modification by hPus1p. Furthermore, we have determined the crystal structure of the catalytic domain of hPus1p at 2.0 Å resolution, alone and in a complex with several molecules present during crystallisation. This model shows an extended C-terminal helix specifically found in the eukaryotic protein, which may prevent the enzyme from forming a homodimer, both in the crystal lattice and in solution. Our biochemical and structural data help to understand the hPus1p active site architecture, and detail its particular requirements with regard to one of its nuclear substrates, the non-coding RNA SRA. | ||||||||
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| Cationic azaoxa[4]helicenes can be prepared in a single step from a common xanthenium precursor by addition of nucleophilic amines under monitored conditions (160 °C, 2 min, MW). The (−)-(M) and (+)-(P) enantiomers can be separated by chiral stationary-phase chromatography. Determination of the absolute configuration and racemization barrier (ΔG⧧ (433 K) 33.3 ± 1.3 kcal·mol–1) was achieved by VCD and ECD spectroscopy, respectively. | ||||||||
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| To celebrate the International Year of Crystallography among the general public, a consortium of chemists, physicists and crystallographers of the University of Geneva organised in Spring 2014 an incentive crystal growth contest for Geneva scholars aged 4 to 19. Starting from a kit containing a salt and user instructions, classes had to prepare a crystal that met specific criteria according to their category of age. The composition of the salt – potassium dihydrogen phosphate (KDP) – was only disclosed to the participants during the final Awards Ceremony. This contest positively exceeded our expectations with almost 100 participating classes (ca. 1800 participants) and 54 specimens received over all categories. | ||||||||
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| The second-order nonlinear response of two dyes adsorbed at the dodecane/water interface was investigated by surface second harmonic generation (SSHG). These dyes consist of the same chromophoric unit, 2-pyridinyl-5-phenyloxazole, with an alkyl chain located at the two opposite ends. The analysis of the polarization dependence of the SSHG intensity as usually performed points to similar tilt angles of the two dyes with respect to the interface but does not give information on the absolute direction. Molecular dynamics (MD) simulations reveal that both dyes lie almost flat at the interface but have opposite orientations. A refined SSHG data analysis with the width of the orientational distribution yields tilt angles that are in very satisfactory agreement with the MD simulations. | ||||||||
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| An easy and versatile Cu-catalyzed propargylic substitution process is presented. Using easily prepared prochiral dichloro substrates, readily available Grignard reagents together with catalytic amount of copper salt and chiral ligand, we accessed a range of synthetically interesting trisubstituted chloroallenes. Substrate scope and nucleophile scope are broad, providing generally high enantioselectivity for the desired 1,3-substitution products. The enantioenriched chloroallenes could be further transformed into the corresponding trisubstituted allenes or terminal alkynes bearing all-carbon quaternary stereogenic centers, through the copper-catalyzed enantiospecific 1,1/1,3-substitutions. The two successive copper-catalyzed reactions could be eventually combined into a one-pot procedure and different desired allenes or alkynes were obtained respectively with high enantiomeric excesses. | ||||||||
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| An efficient and highly stereoselective fluorinative aza-semipinacol rearrangement is described. The catalytic reaction requires use of Selectfluor in combination with the chiral, enantiopure phosphate anion derived from acid L3. Under optimized conditions, cyclopropylamines A were transformed into β-fluoro cyclobutylimines B in good yields and high levels of diastereo- and enantiocontrol. Furthermore, the optically active cyclobutylimines were reduced diastereoselectively with L-Selectride in the corresponding fluorinated amines C, compounds of significant interest in the pharmacological industry. | ||||||||
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| Configurationally stable diaza[4]helicenes have been prepared in two steps by using a particularly facile N–N bond-cleavage reaction. The synthetic procedure uses hydrazine (NH2NH2) for the introduction of a single nitrogen atom. The strategy is general, modular and highly tolerant to functional groups. A mechanistic rationale is proposed for the spontaneous N–N bond-cleavage reaction. The resulting helical quinacridines are dyes that present absorption and emission properties that can be modulated as a function of pH; the pink quinacridine and green protonated forms (pK a ≈ 9.0) display distinct optical features in the near-IR region. Single enantiomers were obtained by chiral stationary phase HPLC resolution. The absolute configurations were assigned by comparison of the ECD spectra of the conjugated acids with those of known dialkylquinacridinium derivatives. A rather high racemization barrier was measured by means of variable-temperature ECD experiments (ΔG‡ = 30.7 ± 4.0 kcal mol–1 at 140 °C). | ||||||||
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| Herein, we address the question whether anion–π and cation–π interactions can take place simultaneously on the same aromatic surface. Covalently positioned carboxylate–guanidinium pairs on the surface of 4-amino-1,8-naphthalimides are used as an example to explore push–pull chromophores as privileged platforms for such "ion pair–π" interactions. In antiparallel orientation with respect to the push–pull dipole, a bathochromic effect is observed. A red shift of 41 nm found in the least polar solvent is in good agreement with the 70 nm expected from theoretical calculations of ground and excited states. Decreasing shifts with solvent polarity, protonation, aggregation, and parallel carboxylate–guanidinium pairs imply that the intramolecular Stark effect from antiparallel ion pair–π interactions exceeds solvatochromic effects by far. Theoretical studies indicate that carboxylate–guanidinium pairs can also interact with the surfaces of π-acidic naphthalenediimides and π-basic pyrenes. | ||||||||
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| A C-C bond forming reaction occurs spontaneously between tris-(2,6-dimethoxyphenyl)carbenium ion and indoles / anilines. The carbocation acts both as electrophile and oxidant. Effective cationic π-conjugated dyes are formed resulting in a strong hyper- and bathochromism | ||||||||
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| The excited-state dynamics of a diketopyrrolopyrrole (DPP) derivative with push–pull substituents has been investigated in a variety of solvents and at the dodecane/water and dodecane/heavy-water interfaces using a combination of ultrafast spectroscopic techniques, including transient electronic absorption and time-resolved surface second-harmonic generation. Whereas the photophysics of a nonpolar DPP analogue is mostly independent of the solvent, the fluorescence decay of the push–pull DPP accelerates strongly by going from aprotic to protic solvents. As this effect increases with the polarity and the hydrogen-bond-donating ability of the solvent, it is attributed to the occurrence of H-bond-assisted nonradiative deactivation induced by the charge-transfer character of the excited state that favors the coupling of the molecule to the H-bond network of the solvent. At the dodecane/water interface, the excited-state lifetime is longer by a factor of ca. 20 than that estimated in pure water and increases further by a factor of about 3 when going to the dodecane/heavy-water interface. This isotope effect, that is more than twice as strong as that measured in bulk solutions, and molecular dynamic simulations indicate that the slowing down of the dynamics at the interface cannot be solely ascribed to a reduced accessibility of the DPP molecule to the aqueous phase. The slower excited-state decay is rather assigned to the conjunction of several effects, such as the strengthening of the H-bond network formed by the interfacial water molecules and the lower local polarity of the interfacial region. | ||||||||
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| By using a polyelectrolyte layer gold nanoparticles have been assembled onto a Ge internal reflection element. Upon illumination with visible and near infrared light a strong infrared absorption has been observed, which can be traced to intervalence band transitions in Ge. This reveals the existence of holes in the Ge near its valence band edge. The switching between bright and dark states is faster than 160 μs and the device acts as an infrared modulator. The effect develops with a peculiar kinetics, which may indicate the development of an interfacial layer between germanium and gold that allows efficient electron transfer upon illumination. | ||||||||
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| Depending on the iron(II) concentration, the mixed crystals of {[Zn1-xFex(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, 0.01 ≤ x ≤ 1, show macroscopic light-induced bistability between the high-spin and the low-spin state. In the highly diluted system with x = 0.01 and up to x = 0.31, the photoinduced low-spin state always relaxes back to the high-spin state independent of the initial light-induced low-spin fraction. In the highly concentrated mixed crystals with x = 0.67, 0.87 and 1, the strong cooperative effects coupled to a crystallographic phase transition result in light-induced bistability with decreasing critical light-induced low-spin fraction and increasing hysteresis width for increasing iron(II) concentrations. The lower limit for the light-induced bistability is estimated at x ≈ 0.5. | ||||||||
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| Rather than lead to the usual deoxygenation pathway, metal carbenes derived from α-diazo-β-ketoesters undergo three-atom insertions into epoxides using a combination of 1,10-phenanthroline and [CpRu(CH3CN)3][BArF] as the catalyst. Original 1,4-dioxene motifs are obtained as single regio- and stereoisomers. A perfect syn stereochemistry (retention, e.r. up to 97:3) is observed for the ring opening, which behaves as an SN1-like transformation. | ||||||||
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| We report on a comprehensive study of the magnetization, resistivity and heat capacity on the single crystals of Ce2RhGa12 synthesized using Ga flux. Single crystal X-ray diffraction data confirm the tetragonal Pb/nbm structure of Ce2RhGa12, which is isostructural to Ce2PdGa12. Ce2RhGa12 orders antiferromagnetically at TN = 3.5 K and exhibits anisotropic magnetic behavior, inferred from the magnetization and resistivity data, taken along the two principal crystallographic directions of the crystal, viz., along [100] and [001]. The anisotropic magnetic response of Ce2RhGa12 establishes [001] as the easy axis of magnetization, and a weak meta-magnetic transition is also observed in the magnetic isotherm at 2K along the same axis. A sharp peak in specific heat signals the bulk antiferromagnetic transition at TN = 3.5 K, which shifts to lower temperatures in low applied fields. The electrical resistivity along the two directions shows metallic behavior from 300K down to 1.8K and establishes Ce2RhGa12 as a normal, trivalent cerium compound. | ||||||||
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| Light-induced excited spin state trapping (LIESST) in iron(II) spin-crossover compounds, i.e., the light-induced population of the high-spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal-ligand charge transfer (MLCT) bands of the low-spin (S=0) species the acknowledged sequence takes the system from the initially excited 1MLCT to the high-spin state via the 3MLCT state within ~150 fs, thereby bypassing low-lying ligand-field (LF) states. Nevertheless, these play role, as borne out by the observation of LIESST and reverse-LIESST on irradiation directly into the LF bands for systems with only high-energy MLCT states. Herein we elucidate the ultrafast reverse-LIESST pathway by identifying the lowest energy S=1 LF state as intermediate state with a lifetime of 39 ps for the light-induced high-spin to low-spin conversion on irradiation into the spin-allowed LF transition of the high-spin species in the NIR. | ||||||||
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| For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/lowspin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specifi c deformation in the photoinduced high-spin state of solvated [Fe(terpy)2 ]2+ , a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal− ligand bonds. The correlated changes in Fe−NAxial , Fe− NDistal , and bite angle NDistal − Fe− NAxial extracted from the measurements are in very good agreement with those predicted by DFT calculations in D2 d symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order. | ||||||||
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| The vapor pressure and molecular weight of effusing vapors of α, β, and amorphous Mg(BH4)2 were determined by Torsion-effusion gravimetric method, under dynamic vacuum. A Cahn balance in the system yielded the rate of the weight loss. Molecular weights measured revealed if the effusion was congruent or there was disproportionation. The vaporization behavior of crystalline Mg(BH4)2, was measured up to 533 K at pressures of ∼10−5 torr. It was found that Mg(BH4)2 disproportionates to form predominantly H2 gas (∼95%) with a small amount of Mg(BH4)2 (∼5%) in the gas phase. The combined average molecular weight measured is 4.16 g/mol. The equations for vapor pressures for crystalline Mg(BH4)2 are given by: logP Total (bar) = 9.2303 − 7286.2/T, log P Mg(BH4)2 (bar) = 8.2515 - 7286.2 /T , and log P H2 (bar) = 9.1821 - 7286.2 / T . The partial pressures of the gaseous species were determined asPMg2(4BH)(g)/PT=0.105 andPH2(g)/PT=0.895. Enthalpies of vaporization for the effusing gases were calculated to be ΔH = +558.0 kJ/mol H2 and ΔH = +135 kJ/mol Mg(BH4)2. The standard Gibbs free energy changes, ΔG°(kJ/mol), for the complete decomposition reaction (Mg(BH4)2(s) → Mg(s) + 2B(s) + 4H2(g)), sublimation reaction (Mg(BH4)2(s) → Mg(BH4)2(g)) and the disproportionation reaction for Mg(BH4)2 are reported in this paper. The decomposition pathway of amorphous Mg(BH4)2 was also carried out between 388.2 K and 712.8 K showing multistep decomposition of a-Mg(BH4)2 Different reaction products were obtained depending on the method used in the vaporization experiment. The behavior of the amorphous Mg(BH4)2(s) is very different from those for the two crystalline phases (α and β). The vapor pressure behavior and thermodynamics of vaporization of different phases of Mg(BH4)2 are presented. | ||||||||
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| A large pi-conjugated chromophore composed of two dipyrido[3,2-a:2',3'-c]phenazine (dppz) units directly fused to the central tetrathiafulvalene (TTF) core, has been prepared as a bridging ligand, and its strong binding ability to Ru2+ forming a new dinuclear complex is presented. The electronic absorption and luminescence and the electrochemical behaviour of the free ligand as well as the Ru2+ complex have been investigated in detail. The free ligand shows a very strong band in the UV region consistent with ligand centred π-π* transitions and an intense broad band in the visible region corresponding to an intramolecular charge transfer (ILCT) transition. Upon coordination, a metal-to-ligand charge transfer (MLCT) appears at 22520 cm-1 while the ILCT band is bathochromically shifted by 1620 cm-1. These electrochemically amphoteric chromophores have also been characterized by spectroelectrochemical methods. The oxidized radical species of the free ligand show a strong tendency to undergo aggregation, in which long-distance attractive interactions overcome the electrostatic repulsion. Moreover, these two new chromophores reveal an ILCT fluorescence with large solvent-dependent Stokes shifts and quantum efficiencies of 0.052 for the free ligand and 0.016 for its dinuclear Ru2+ complex in CH2Cl2. | ||||||||
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| Attenuated total reflection infrared (ATR-IR) spectroscopy is used to study the adsorption of gold and silver nanoparticles and the layer-by-layer (LBL) growth of polyelectrolyte multilayers on a Ge ATR crystal. The Ge ATR crystal is first functionalized using positively charged polyelectrolyte poly(allylamine hydrochloride) (PAH). Then citrate-stabilized gold or silver nanoparticles are adsorbed onto the modified Ge ATR crystal. When gold or silver nanoparticles are adsorbed, a drastic increase of the water signal is observed which is attributed to an enhanced absorption of IR radiation near the nanoparticles. This enhancement was much larger for the silver nanoparticles (SNP). On top of the nanoparticles multilayers of oppositely charged polyelectrolytes PAH and poly(sodium 4-styrenesulfonate) (PSS) were deposited, which allowed to study the enhancement of the IR signals as a function of the distance from the nanoparticles. Furthermore, adsorption of a thiol, N-acetyl-l-cysteine, on the nanoparticles confirmed the enhancement. In the case of SNP an absorbance signal of about 15% was observed, which is a factor of about 40 times larger compared to typical signals measure without nanoparticles. | ||||||||
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| Several nanoporous platforms were functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes using surface-initiated atom transfer radical polymerization (SI-ATRP). The growth of the PMAA brush and its pH-responsive behavior from the nanoporous platforms were confirmed by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and atomic force microscopy (AFM). The swelling behavior of the pH-responsive PMAA brushes grafted only from the nanopore walls was investigated by AFM in aqueous liquid environment with pH values of 4 and 8. AFM images displayed open nanopores at pH 4 and closed ones at pH 8, which rationalizes their use as gating platforms. Ion conductivity across the nanopores was investigated with current–voltage measurements at various pH values. Enhanced higher resistance across the nanopores was observed in a neutral polymer brush state (lower pH values) and lower resistance when the brush was charged (higher pH values). By adding a fluorescent dye in an environment of pH 4 or pH 8 at one side of the PMAA-brush functionalized nanopore array chips, diffusion across the nanopores was followed. These experiments displayed faster diffusion rates of the fluorescent molecules at pH 4 (PMAA neutral state, open pores) and slower diffusion at pH 8 (PMAA charged state, closed pores) showing the potential of this technology toward nanoscale valve applications. | ||||||||
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| Light-upconversion via stepwise energy transfer from a sensitizer to an activator exploits linear optics for converting low-energy infrared or near-infrared incident photons to higher energy emission occurring in the part of the electromagnetic spectrum ranging from visible to ultraviolet. Stepwise excitation is restricted to activators possessing intermediate long-lived excited states such as those found for trivalent lanthanide cations dispersed in solid-state matrices. When the activator is embedded in a molecular complex, efficient non-radiative relaxation processes usually reduce excited state lifetimes to such an extent that upconversion becomes too inefficient to be detected under practical excitation intensities. Theoretical considerations suggest that the combination of millisecond timescale sensitizers with a central lanthanide activator located in supramolecular complexes circumvents this bottleneck by creating a novel pathway reminiscent of the energy transfer upconversion mechanism observed in doped solids. Application of this novel concept to chromium/erbium pairs in discrete triple-stranded helicates demonstrates that strong-field trivalent chromium chromophores irradiated with near-infrared photons produce upconverted green erbium-centered emission both in the solid state and in solution. | ||||||||
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| The reaction of 4,5-bis(2'-cyano-ethylsulfanyl)-4',5'-dipropylthiotetrathiafulvalene with [Pt(phen)Cl2] (phen = 1,10-phenanthroline) with CsOH as base in CH3OH–THF affords the target complex 1 in 44% yield. This complex crystallizes in the monoclinic space group P21/c, M = 790.01, a = 12.1732(12), b = 15.851(2), c = 14.5371(16) Å, b = 107.693(12)˚, V = 2672.4(5) Å3 and Z = 4. It undergoes two reversible single-electron oxidation and two irreversible reduction processes. An intense electronic absorption band at 15200 cm-1 (658 nm) in CH2Cl2 is assigned to the intramolecular mixed metal/ligand-to-ligand charge transfer (LLCT) from a tetrathiafulvalene-extended dithiolate-based HOMO to a phenanthroline-based LUMO. This band shifts hypsochromically with increasing solvent polarity. Systematic changes in the optical spectra upon oxidation allow precise tuning of the oxidation states of 1 and reversible control over its optical properties. Irradiation of 1 at 15625 cm-1 (640 nm) in glassy solution below 150 K results in emission from the 3LLCT excited state. | ||||||||
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| The excited-state dynamics of two donor–acceptor biaryls that differ by the strength of the acceptor, a pyridinium or a pyrylium moiety, have been investigated using a combination of steady-state solvatochromic absorption, ultrafast fluorescence, as well as visible and infrared transient absorption spectroscopies. The negative solvatochromic behavior of pyridinium phenolate indicates that the permanent electric dipole moment experiences a decrease upon S1 ← S0 excitation, implying that the ground state possesses more zwitterionic character than the excited state. In contrast, pyrylium phenolate exhibits a weakly positive solvatochromic behavior corresponding to a small increase in the dipole moment upon excitation, implying more zwitterionic character in the excited than the ground state. Both compounds are therefore situated at different sides of the cyanine-limit structure, which has equally polar ground and excited states. Despite these differences, both molecules exhibit qualitatively similar excited-state properties. They are characterized by a very short fluorescence lifetime, increasing from about 1 to 20 ps, when varying solvent viscosity from 0.4 to 11 cP. There are, however, characteristic differences between the two compounds: The excited-state lifetimes of the pyrylium dye are shorter and also depend somewhat on polarity. The ensemble of spectroscopic data can be explained with a model where the emitting Franck–Condon excited state relaxes upon twisting around the single bond between the aryl units to a point where the excited- and ground-state surfaces are very close or intersect. After internal conversion to the ground state, the distorted molecule relaxes back to its equilibrium planar configuration, again largely dependent upon solvent viscosity. However, in this case, the kinetics for the pyrylium dye are slower than for the pyridinium dye and the polar solvent-induced acceleration is significantly stronger than in the excited state. This difference of kinetic behavior between the two compounds is a direct consequence of the change of the electronic structure from anormal to anovercritical merocyanine evidenced by steady-state spectroscopy. | ||||||||
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| Acylgermanes have been shown to act as efficient photoinitiators. In this investigation we show how dibenzoyldiethylgermane1 reacts upon photoexcitation. Our real-time investigation utilizes femto- and nanosecond transient absorption, time-resolved EPR (50 ns), photo-chemically induced dynamic nuclear polarization, DFT calculations, and GC-MS analysis. The benzoyldiethylgermyl radicalG• is formed via the triplet state of parent1. On the nanosecond time scale this radical can recombine or undergo hydrogen-transfer reactions. RadicalG• reacts with butyl acrylate at a rate of 1.2 ± 0.1 × 108 and 3.2 ± 0.2 × 108 M–1 s–1, in toluene and acetonitrile, respectively. This is˜1 order of magnitude faster than related phosphorus-based radicals. The initial germyl and benzoyl radicals undergo follow-up reactions leading to oligomers comprising Ge–O bonds. LC-NMR analysis of photocured mixtures containing1 and the sterically hindered acrylate 3,3-dimethyl-2-methylenebutanoate reveals that the products formed in the course of a polymerization are consistent with the intermediates established at short time scales. | ||||||||
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| Modern spectroscopic techniques such as time-resolved second-harmonic-generation spectroscopy allow molecules to be examined selectively directly at phase interfaces. Two-phase systems formed by glycerol/water and alkane layers have previously been studied by time-resolved second-harmonic-generation spectroscopic measurements. In this molecular dynamics study, a triphenylmethane dye was inserted at the glycerol/water–alkane interface and was used as a probe for local properties such as viscosity. We now show how extensive simulations over a wide range of concentrations can be used to obtain a detailed view of the molecular structure at the glycerol/water–alkane interface. Glycerol is accumulated in a double layer adjacent to the alkane interface, which results in increased viscosity of the glycerol/water phase in the direct vicinity of the interface. We also show that conformational ensembles created by classical molecular-dynamics simulations can serve as input for QM/MM calculations, yielding further information such as transition dipoles, which can be compared with spectroscopic measurements. | ||||||||
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| The structure and optical properties of a set of R-1,1´-binaphthyl-2,2´-dithiol (R-BINAS) monosubstituted A-Au38(SCH3)24 clusters are studied by means of time dependent density functional theory (TD-DFT). While it was proposed earlier that BINAS selectively binds to monomer motifs (SR-Au-SR) covering the Au23 core, our calculations suggest a binding mode that bridges two dimer (SR-Au-SR-Au-RS) motifs. The more stable isomers show a negligible distortion induced by BINAS adsorption on the Au38(SCH3)24 cluster which is reflected by similar optical and Circular Dichroism (CD) spectra to those found for the parent cluster. The results furthermore show that BINAS adsorption does not enhance the CD signals of the Au38(SCH3)24 cluster. | ||||||||
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| Ultrafast transient absorption spectroscopy serves to identify the3dd state as intermediate quencher state of the3MLCT luminescence in the non-luminescent ruthenium complexes [Ru(m-bpy)3]2+ (m-bpy = 6-methyl-2,2′-bipyridine) and [Ru(tm-bpy)3]2+ (tm-bpy = 4,4′,6,6′-tetramethyl-2′,2′-bipyridine). For [Ru(m-bpy)3]2+, the population of the3dd state from the 3MLCT state occurs within 1.6 ps, while the return to the ground state takes 450 ps. For [Ru(tm-bpy)3]2+, the corresponding values are 0.16 and 7.5 ps, respectively. According to DFT calculations, methyl groups added in the 6 and 6′ positions of bipyridine stabilize the3dd state by ∼4000 cm–1 each, compared to [Ru(bpy)3]2+. | ||||||||
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| Ru(II) complexes with chelating ligands, 4′,5′-ethylenedithiotetrathiafulvenyl[4,5-f][1,10]phenanthroline (L1), 1,3-dithiole-2-thiono[4,5-f][1,10]phenanthroline (L2), and 1,3-dithiole-2-ono[4,5-f][1,10]phenanthroline (L3), have been prepared and their structural, electrochemical, and photophysical properties investigated. Density functional theory (DFT) calculations indicate that the highest occupied molecular orbital of [Ru(bpy)2(L1)](PF6)2 (1) is located on the tetrathiafulvalene (TTF) subunit and appears ≈0.6 eV above the three Ru-centered d orbitals. In agreement with this finding, 1 exhibits three reversible oxidations: the two at lower potentials take place on the TTF subunit, and the one at higher potential is due to the Ru3+/Ru2+ redox couple. Complexes [Ru(bpy)2(L2)](PF6)2 (2) and [Ru(bpy)2(L3)](PF6)2 (3) exhibit only the Ru3+/Ru2+-related oxidation. The optical absorption spectra of all complexes reveal a characteristic metal-to-ligand charge transfer (MLCT) band centered around 450 nm. In addition, in the spectrum of 1 the MLCT band is augmented by a low-energy tail that extends beyond 500 nm and is attributed to the intraligand charge transfer (ILCT) transition of L1, according to time-dependent DFT calculations. The substantial decrease in the luminescence quantum yield of 1 compared to those of 2 and 3 is attributed to the reductive quenching of the emissive state via electron transfer from the TTF subunit to the Ru3+ center, thus allowing nonradiative relaxation to the ground state through the lower-lying ILCT state. In the presence of O2, complex 1 undergoes a photoinduced oxidative cleavage of the central C═C bond of the TTF fragment, resulting in complete transformation to 3. This photodegradation process was studied with 13C NMR and optical absorption spectroscopy. | ||||||||
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| Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)3](ClO4)2}∞ (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)3](BF4)2}∞, Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time and intensity. Crystallographic studies reveal a photo-induced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation. | ||||||||
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| In the spin-crossover compound [Fe(6-mepy)3tren](PF6)2, (6-mepy)3tren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-butenyl}amine, the high-spin state can be populated as metastable state below the thermal transition temperature via irradiation into the metal to ligand charge transfer absorption band of the low-spin species. At 10 K, the lifetime of this metastable state is only 1 s. Despite this, it is possible to determine an accurate excited state structure by following the evolution of relevant structural parameters by synchrotron X-ray diffraction under continuous irradiation with increasing intensity. The difference in metal-ligand bond length between the high-spin and the low-spin state is found to be 0.192 Å obtained from an analysis of the experimental data using the mean-field approximation to model cooperative effects. | ||||||||
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| Unambiguous evidence for the formation of excited ions upon ultrafast bimolecular photoinduced charge separation is found using a combination of femtosecond time-resolved fluorescence up-conversion, infrared and visible transient absorption spectroscopy. The reaction pathways are tracked by monitoring the vibrational energy redistribution in the product after charge separation and subsequent charge recombination. For moderately exergonic reactions, both donor and acceptor are found to be vibrationally hot, pointing to an even redistribution of the energy dissipated upon charge separation and recombination in both reaction partners. For highly exergonic reactions, the donor is very hot, whereas the acceptor is mostly cold. The asymmetric energy redistribution is due to the formation of the donor cation in an electronic excited state upon charge separation, confirming one of the hypotheses for the absence of the Marcus inverted region in photoinduced bimolecular charge separation processes | ||||||||
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| A new cyclen derivative L, bearing a methyl-chromeno-pyridinylidene hydrazone moiety, was synthesized and studied in MeOH, as potential fluorescent "OFF-on-ON" sensors for Zn(II). Photocphysical properties of this ligand being PET regulated, L was only weakly emissive in the absence of metal ions (OFF). L fluorescence was increased modestly upon addition of one equivalent of Zn(II), and further increased upon addition of a second equivalent. Therefore, Zn:L behaved as a highly sensitive ON sensor for zinc. This efficiency was correlated to Zn(II) coordination via the hydrazone moiety of the fluorophore, producing an efficient CHelation-Enhanced Fluorescence (CHEF) effect. A complementary theoretical study carried out with DFT calculations further elucidated of the optical properties. | ||||||||
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| A switch in time: A fast precipitation technique was used to prepare 75 nm FeII spin-crossover nanocrystals. Their photoswitching dynamics, based on the light-induced excited spin-state trapping effect, has been investigated by means of optical spectroscopy. A significant variation of the switching proprieties is observed compared to similar but amorphous nanoparticles. | ||||||||
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| The insertion of a [Fe(sal2-trien)]+ complex cation into a 2D oxalate network in the presence of different solvents results in a family of hybrid magnets with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compounds [FeIII(sal2-trien)][MnIICrIII(ox)3]·CHCl3 (1·CHCl 3 ), [FeIII(sal2-trien)][MnIICrIII(ox)3]·CHBr3 (1·CHBr 3 ), and [FeIII(sal2-trien)][MnIICrIII(ox)3]·CH2Br2 (1·CH 2 Br 2 ). The three compounds crystallize in a 2D honeycomb anionic layer formed by MnII and CrIII ions linked through oxalate ligands and a layer of [Fe(sal2-trien)]+ complexes and solvent molecules (CHCl3, CHBr3, or CH2Br2) intercalated between the 2D oxalate network. The magnetic properties and Mössbauer spectroscopy indicate that they undergo long-range ferromagnetic ordering at 5.6 K and a spin crossover of the intercalated [Fe(sal2-trien)]+ complexes at different temperatures T 1/2. The three compounds present a LIESST effect with a relaxation temperature T LIESST inversely proportional to T 1/2. The isostructural paramagnetic compound, [FeIII(sal2-trien)][ZnIICrIII(ox)3]·CH2Cl2 (2·CH 2 Cl 2 ) was also prepared. This compound presents a partial spin crossover of the inserted FeIII complex as well as a LIESST effect. Finally, spectroscopic characterization of the FeIII doped compound [Ga0.99Fe0.01(sal2trien)][MnIICrIII(ox)3]·CH2Cl2 (3·CH 2 Cl 2 ) shows a gradual and complete thermal spin crossover and a LIESST effect on the isolated FeIII complexes. This result confirms that cooperativity is not a necessary condition to observe the LIESST effect in an FeIII compound. | ||||||||
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| The one-electron equation for orbitals embedded in frozen electron density (Eqs. 20-21 in [Wesolowski and Warshel, J. Phys. Chem, 97 (1993) 8050]) in its exact and approximated version is solved for an analytically solvable model system. The system is used to discuss the role of the embedding potential in preventing the collapse of a variationally obtained electron density onto the nucleus in the case when the frozen density is chosen to be that of the innermost shell. The approximated potential obtained from the second-order gradient expansion for the kinetic energy prevents such a collapse almost perfectly but this results from partial compensation of flaws of its components. It is also shown that that the quality of a semi-local approximation to the kinetic-energy functional, a quantity needed in orbital-free methods, is not related to the quality of the non-additive kinetic energy potential - a key component of the effective embedding potential in one-electron equations for embedded orbitals. | ||||||||
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| Four-dimensional (4D) electron microscopy (EM) uniquely combines the high spatial resolution to pinpoint individual nano-objects, with the high temporal resolution necessary to address the dynamics of their laser-induced transformation. Here, using 4D-EM, we demonstrate thein situ irreversible transformation of individual nanoparticles of the molecular framework Fe(pyrazine)Pt(CN) 4 . The newly formed material exhibits an unusually large negative thermal expansion (i.e. contraction), which is revealed by time-resolved imaging and diffraction. Negative thermal expansion is a unique property exhibited by only few materials. Here we show that the increased flexibility of the metal–cyanide framework after the removal of the bridging pyrazine ligands is responsible for the negative thermal expansion behavior of the new material. Thisin situ visualization of single nanostructures during reactions should be extendable to other classes of reactive systems. | ||||||||
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| The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structural dynamics in real and reciprocal space. Here, we demonstrate the selectivity and sensitivity of the technique by visualizing the spin crossover dynamics of single, isolated metal–organic framework nanocrystals. By introducing a small aperture in the microscope, it was possible to follow the phase transition and the associated structural dynamics within a single particle. Its behaviour was observed to be distinct from that imaged by averaging over ensembles of heterogeneous nanoparticles. The approach reported here has potential applications in other nanosystems and those that undergo (bio)chemical transformations. | ||||||||
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| Palladium-catalyzed cross-coupling reactions between chlorinated 1,3,5-triazines (TZ) and tetrathiafulvalene (TTF) trimethyltin derivatives afford mono- andC 3 symmetric tris(TTF)-triazines as donor–acceptor compounds in which the intramolecular charge transfer (ICT) is modulated by the substitution scheme on TTF and TZ and by chemical or electrochemical oxidation. The TTF-TZ-Cl2 and (SMe)2TTF-TZ-Cl2 derivatives show fully planar structures in the solid state as a consequence of the conjugation between the two units. Electrochemical and photophysical investigations, supported by theoretical calculations, clearly demonstrate that the lowest excited state can be ascribed to the intramolecular charge transfer (ICT) π(TTF)→π*(TZ) transition. The tris(TTF) compound [(SMe)2TTF]3-TZ shows fluorescence when excited in the ICT band, and the emission is quenched upon oxidation. The radical cations TTF+• are easily observed in all of the cases through chemical and electrochemical oxidation by steady-state absorption experiments. In the case of [(SMe)2TTF]3-TZ, a low energy band at 5000 cm–1, corresponding to a coupling between TTF+• and TTF units, is observed. A crystalline radical cation salt with the TTF-TZ-Cl2 donor and PF6 – anion, prepared by electrocrystallization, is described. | ||||||||
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| The interaction of a series of chiral cationic [4]helicene derivatives, which differ by their substituents, with double-stranded DNA has been investigated by using a combination of spectroscopic techniques, including time-resolved fluorescence, fluorescence anisotropy, and linear dichroism. Addition of DNA to helicene solutions results to a hypochromic shift of the visible absorption bands, an increase of fluorescence quantum yield and lifetime, a slowing down of fluorescence anisotropy decay, and a linear dichroism in flow-oriented DNA, which unambiguously points to the binding of these dyes to DNA. Both helicene monomers and dimeric aggregates, which form at higher concentration, bind to DNA, the former most probably upon intercalation and the latter upon groove binding. The binding constant depends substantially on the dye substituents and is, in all cases, larger with theM than theP enantiomer, by factors ranging from 1.2 to 2.3, depending on the dye. | ||||||||
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| The melting behavior, the solubility, and the influence of hydrogen bonds were analyzed for a series of single- and double-tailed surfactant alcohols. Various effects such as the presence of free amides or the intermolecular spacing were found to be important factors for increasing or decreasing the melting temperature of a surfactant. Furthermore, we present a model for the packing of diamido-lipids and study the temperature-dependence of the IR signals. | ||||||||
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| Two benzodifuran (BDF)-coupled spiropyran (SP) systems and their BDF reference compounds were obtained in good yields through Huisgen–Meldal–Sharpless "click" chemistry and then subjected to investigation of their electrochemical and photophysical properties. In both SP and merocyanine (MC) forms of the coupled molecules, the BDF-based emission is quenched to around 1 % of the quantum yield of emission from the BDF reference compounds. Based on electrochemical data, this quenching is attributed to oxidative electron-transfer quenching. Irradiation at 366 nm results in ring opening to the MC forms of the BDF-coupled SP compounds and the SP reference compound with a quantum efficiency of about 50 %. The rate constants for the thermal ring closing are approximately 3.4×10−3 s−1. However, in the photostationary states the MC fractions of the coupled molecules are substantially lower than that of the reference SP compound, attributed to the observed acceleration of the ring-closing reaction upon irradiation. As irradiation at 366 nm invariably also excites higher-energy transitions of the BDF units in the coupled compounds, the ring-opening reaction is accelerated relative to the SP reference, which results in lower MC fractions in the photostationary state. Reversible photochromism of these BDF-coupled SP compounds renders them promising in the field of molecular switches. | ||||||||
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| The structurally characterized tetrathiafulvalene-1,2,4,5-tetrazine donor–acceptor system shows redox tuneable intramolecular charge transfer, solvatochromic and electrochromic behaviour. Attachment of a dipicolyl-amine chelating unit affords a multifunctional ligand, which allows the preparation of the ZnCl2 complex in which an anion-π interaction is seen. | ||||||||
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| Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal-to-ligand charge-transfer excited state of the [Re(CO)3(phen)(py)]+ complex (phen=1,10-phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited-state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton-coupled electron-transfer process. | ||||||||
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| We report a detailed DFT study of the energetic and structural properties of the spin-crossover Co(II) complex [Co(tpy) 2 ] 2+ (tpy = 2,2′:6′,2′′-terpyridine) in the low-spin (LS) and the high-spin (HS) states, using several generalized gradient approximation and hybrid functionals. In either spin-state, the results obtained with the functionals are consistent with one another and in good agreement with available experimental data. Although the different functionals correctly predict the LS state as the electronic ground state of [Co(tpy) 2 ] 2+ , they give estimates of the HS–LS zero-point energy difference ΔE 0 HL (tpy) which strongly depend on the functional used. This dependency on the functional was also reported for the DFT estimates of the zero-point energy difference ΔE 0 HL (bpy) in the HS complex [Co(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine) [A. Vargas, A. Hauser and L. M. Lawson Daku,J. Chem. Theory Comput., 2009,5, 97]. The comparison of the ΔE 0 HL (tpy) and ΔE 0 HL (bpy) estimates showed that all functionals correctly predict an increase of the zero-point energy difference upon the bpy → tpy ligand substitution, which furthermore weakly depends on the functionals, amounting to (ΔE 0 HL)bpy->tpy ≈ +2670 cm-1 . From these results and basic thermodynamic considerations, we establish that, despite their limitations, current DFT methods can be applied to the accurate determination of the spin-state energetics of complexes of a transition metal ion, or of these complexes in different environments, provided that the spin-state energetics is accurately known in one case. Thus, making use of the availability of a highly accurateab initio estimate of the HS–LS energy difference in the complex [Co(NCH) 6 ] 2+ [L. M. Lawson Daku, F. Aquilante, T. W. Robinson and A. Hauser,J. Chem. Theory Comput., 2012,8, 4216], we obtain for [Co(tpy) 2 ] 2+ and [Co(bpy) 3 ] 2+ best estimates of ΔE 0 HL (bpy) ≈ -2800 cm-1 and ΔE 0 HL (tpy) ≈ 0 cm-1 , in good agreement with the known magnetic behaviour of the two complexes. | ||||||||
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| Electrochemical and photophysical analysis of new donor–acceptor systems2 and3, in which a benzothiadiazole (BTD) unit is covalently linked to a tetrathiafulvalene (TTF) core, have verified that the lowest excited state can be ascribed to an intramolecular-charge-transfer (ICT) π(TTF)→π*(benzothiadiazole) transition. Owing to better overlap of the HOMO and LUMO in the fused scaffold of compound3, the intensity of the1ICT band is substantially higher compared to that in compound2. The corresponding CT fluorescence is also observed in both cases. The radical cation TTF+ . is easily observed through chemical and electrochemical oxidation by performing steady-state absorption experiments. Interestingly, compound2 is photo-oxidized under aerobic conditions. | ||||||||
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| The mechanoelastic model is applied to reproduce the experimental relaxation and thermal transition curves as determined for crystals of pure and diluted {[Fe x Zn1–x (bbtr)3](ClO4)2}∞ [bbtr = 1,4-di(1,2,3-triazol-1-yl)butane] spin-crossover systems. In the mechanoelastic model, the spin-crossover complexes are situated in a hexagonal planar lattice, which is similar to the 2D coordination polymer with (3,6) network topology of [Fe(bbtr)3](ClO4)2. These complexes are linked by springs, which simulate the elastic interactions between them. Owing to the change in volume of the complexes during the spin transition, an elastic force accompanies the switch of every complex. This force propagates through the entire lattice and causes a shift of all molecules in the system and thus results in a new nuclear configuration. First, the ability of the model to reproduce various shapes of thermal transition and relaxation curves in pure compounds is analyzed; these range from gradual to very steep and include hysteresis behavior for the former and from single exponential to sigmoidal or with several steps for the latter. A structural phase transition can also be accounted for by changing the shape of the sample at a fixed temperature from a regular to an elongated hexagon. Furthermore, the effect of adding Zn as a dopant in a mixed crystal series is discussed. The role of dopants on the cluster evolution is also analyzed directly and by using the correlation factor. | ||||||||
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| The synthesis and photophysical properties of the complex [Fe(phen)2(TTF-dppz)]2+ (TTF-dppz = 4′,5′-bis-(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2′,3′-c]phenazine, phen = 1,10-phenanthroline) are described. In this complex, excitation into the metal–ligand charge transfer bands results in the population of a high-spin state of iron(II), with a decay lifetime of approximately 1.5 ns, in dichloromethane, at room temperature. An intraligand charge transfer state can also be obtained and has a lifetime of 38 ps. A mechanism for the different states reached is proposed based on transient absorption spectroscopy. | ||||||||
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| The synthesis, X-ray structures and photophysical properties of several new Ln(III) complexes with pyrazine-2,6-dicarboxylic acid (H2PYZ) that demonstrate excellent stability and solubility in non-aqueous solution are reported, and compared to structurally analogous complexes with pyridine-2,6-dicarboxylic acid (H2DPA). The Eu(III) and Yb(III) complexes demonstrate efficient metal centered luminescence in the visible and Near Infra-Red (NIR) regions respectively. Low temperature (77 K) phosphorescence measurements using the corresponding Gd(III) complex allowed the photophysical properties of the sensitizer to be rationalized, together with corresponding TD-DFT studies for a model complex. Lastly, we have evaluated the sensitization efficiencies for these complexes, and have undertaken femtosecond transient absorption (TA) measurements in order to evaluate the relative importance of the intersystem crossing and energy transfer processes involved with sensitized Ln(III) emission via the antennae effect. | ||||||||
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| The absorption spectrum of fluorenone in zeolite L is calculated from first-principles simulations. The broadening of each band is obtained from the explicit treatment of the interactions between the chromophore and its environment in the statistical ensemble. The comparison between the simulated and measured spectra reveals the main factors affecting the spectrum of the chromophore in hydrated zeolite L. Whereas each distinguishable band is found to originate from a single electronic transition, the bandwidth is determined by the statistical nature of the environment of the fluorenone molecule. The K+...O=C motif is retained in all conformations. Although the interactions between K+ and the fluorenone carbonyl group result in an average lengthening of the C=O bond and in a redshift of the lowest energy absorption band compared to gas phase or non-polar solvents, the magnitude of this shift is noticeably smaller than the total shift. An important factor affecting the shape of the band is fluorenone's orientation, which is strongly affected by the presence of water. The effect of direct interactions between fluorenone and water is, however, negligible. | ||||||||
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| Chromium(III)-trisoxalate,[Cr(ox)3]3- (ox = C2O4 2-), incorporated into polymeric networks of composition [NaCr(ox)3][MII(bpy)3] and [NaCr(ox)3][MIII(bpy)3]ClO4 (bpy= 2,2'-bipyridine, MII =Zn, Fe, Ru; MIII =Rh, Cr), results in interesting features ranging from phonon-assisted and resonant energy migration within the R1 line the 2E state to persistent spectral side-hole burning via the latter, and manifestations of specific nearest-neighbour π–π interactions between bipyridine and oxalate. | ||||||||
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| Conventional lipid-tube formation is based on either a tube phase of certain lipids or the shape transformation of lamellar structures by applying a point load. In the present study, lipid blocks in inverted hexagonal phase made of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) were shown to protrude lipid nanotubes upon a fluid-dynamic flow on polyelectrolyte-functionalized surfaces in physiological buffer solution. The outer diameter of the tubes is 19.1 ± 4.5 nm and their lengths are up to several hundred micrometers. The method described enables the alignment and patterning of lipid nanotubes into various (including curvy) shapes with a microfluidic system. | ||||||||
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| The enormous progress of nanotechnology during the last decade has made it possible to fabricate a great variety of nanostructures. On the nanoscale, metals exhibit special electrical and optical properties, which can be utilized for novel applications. In particular, plasmonic sensors including both the established technique of surface plasmon resonance and more recent nanoplasmonic sensors, have recently attracted much attention. However, some of the simplest and most successful sensors, such as the glucose biosensor, are based on electrical readout. In this review we describe the implementation of electrochemistry with plasmonic nanostructures for combined electrical and optical signal transduction. We highlight results from different types of metallic nanostructures such as nanoparticles, nanowires, nanoholes or simply films of nanoscale thickness. We briefly give an overview of their optical properties and discuss implementation of electrochemical methods. In particular, we review studies on how electrochemical potentials influence the plasmon resonances in different nanostructures, as this type of fundamental understanding is necessary for successful combination of the methods. Although several combined platforms exist, many are not yet in use as sensors partly because of the complicated effects from electrochemical potentials on plasmon resonances. Yet, there are clearly promising aspects of these sensor combinations and we conclude this review by discussing the advantages of synchronized electrical and optical readout, illustrating the versatility of these technologies. | ||||||||
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| Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO4)2⋅0.5H2O (1), [Fe(TPMA)(XBIM)](ClO4)2 (2), and [Fe(TPMA)(XBBIM)](ClO4)2 ⋅0.75CH3OH (3), were prepared by reactions of FeII perchlorate and the corresponding ligands (TPMA=tris(2-pyridylmethyl)amine, BIM=2,2′-biimidazole, XBIM=1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, XBBIM=1,1′-(α,α′-o-xylyl)-2,2′-bibenzimidazole). The compounds were investigated by a combination of X-ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex1 exhibits a gradual spin crossover (SCO) withT 1/2=190 K, whereas2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T 1/2=196 K on cooling and 203 K on heating). Complex3 is in the high-spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter- and intramolecular interactions in1 and2. The crystal packing of2features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In3, steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the FeII coordination environment, which stabilizes the high-spin state of the complex. Both1 and2 exhibit a photoinduced low-spin to high-spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of1 and2 also manifests in the kinetics of the decay of the photoinduced high-spin state. For1, the decay rate constant follows the single-exponential law, whereas for2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high-spin state at 50 K are similar to those determined for the high-spin state at 295 K. This study shows thatN-alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes. | ||||||||
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| A two-fold active control of the plasmonic resonance of randomly distributed gold nanoparticles (GNPs) has been achieved. GNPs have been immobilized on an Indium Tin Oxide (ITO) coated glass substrate and then covered with a liquid crystalline compound. The system has been investigated by means of atomic force and scanning electron microscopy, revealing the presence of isolated and well distributed GNPs. The application of an external electric field to the sample has a two-fold consequence: the re-orientation of the hybrid-aligned liquid crystal layer and the formation of a carrier accumulation layer in the proximity of the ITO substrate. The refractive indices of both liquid crystal and accumulation layers are influenced by the applied field in a competitive way and produce a "dancing behavior" of the GNP's plasmonic resonance spectral position. | ||||||||
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| Highly accurate estimates of the high-spin/low-spin energy difference ΔEHL el in the high-spin complexes [Fe(NCH)6]2+ and [Co(NCH)6]2+ have been obtained from the results of CCSD(T) calculations extrapolated to the complete basis set limit. These estimates are shown to be strongly influenced by scalar relativistic effects. They have been used to assess the performances of the CASPT2 method and of 30 density functionals of the GGA, meta-GGA, global hybrid, RSH and double-hybrid types. For the CASPT2 method, the results of the assessment support the proposal [Kepenekian, M.; Robert, V.; Le Guennic, B.J. Chem. Phys. 2009,131, 114702] that the ionization potential–electron affinity (IPEA) shift defining the zeroth-order Hamiltonian be raised from its standard value of 0.25 au to 0.50–0.70 au for the determination of ΔEHL el in Fe(II) complexes with a [FeN6] core. At the DFT level, some of the assessed functionals proved to perform within chemical accuracy (±350 cm-1) for the spin-state energetics of [Fe(NCH)6]2+, others for that of [Co(NCH)6]2+, but none of them simultaneously for both complexes. As demonstrated through a reparametrization of the CAM-PBE0 range-separated hybrid, which led to a functional that performs within chemical accuracy for the spin-state energetics of both complexes, performing density functionals of broad applicability may be devised by including in their training sets highly accurate data like those reported here for [Fe(NCH)6]2+ and [Co(NCH)6]2+. | ||||||||
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| The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn1-x Fe x (bbtr)3](ClO4)2} ∞ (x = 0.02–1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H.Chem. Eur. J. 2011,17, 6807). In contrast, the diluted crystals withx ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already atx = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase transition begins to influence the spin transition. The kinetic behavior of the thermal spin transition is reproduced within the framework of a dynamic mean-field model. | ||||||||
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| A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)3(η6-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η6-enediyne complex is the lowest-energy path, forming a metal-boundp-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state. | ||||||||
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| This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligandL2 with stoichiometric amounts of M(CF3SO3)2 (M = Cr, Zn) and Ln(CF3SO3)3 (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield theD 3-symmetrical trinuclear [MLnM(L2)3](CF3SO3)n complexes (M = Zn,n = 7; M = Cr,n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)3]9+ induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)3]6+, the connection of a second strong-field [CrN6] sensitizer in [CrLnCr(L2)3]9+ significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)3]9+ under reasonable pumping powers. | ||||||||
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| Colloidal Mn2+-doped semiconductor nanocrystals such as Mn2+:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn2+ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn2+ ion and an exciton within the same nanocrystal. A lower limit of k = 2 × 1010 s–1 is established for the fundamental rate constant of the Mn2+(4T1)-exciton cross relaxation. | ||||||||
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| This contribution investigates Ln III complexes formed with a small ditopic ligand, L1, and their structural, thermodynamic and photophysical properties. The spectrophotometric and NMR titrations evidence the triangular assemblies [Ln 3 (L1-H) 3 ] 6+ at stoichiometric conditions and their properties are discussed in relation to L2-containing analogues. In addition, the dinuclear species, [Ln 2 (L1-H)] 5+ , is observed with an excess of metal. | ||||||||
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| Due to its extreme kinetic inertness, trivalent chromium, Cr(III), has been rarely combined with labile trivalent lanthanides, Ln(III), to give discrete self-assembled (supra)molecular polynuclear complexes. However, the plethora of accessible metal-centered excited states possessing variable lifetimes and emissive properties, combined with the design of efficient intramolecular Cr(III)↔ Ln(III) energy transfer processes open attractive perspectives for programming directional light-conversion within these heterometallic molecules. Efforts made to address this exciting challenge for both light-sensitization and light-upconversion are discussed in this article. | ||||||||
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| In the covalently linked 2D coordination network {[Fe(bbtr)3](BF4)2}∞, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, the iron(II) centers stay in the high-spin (HS) state down to 10 K. They can, however, be quantitatively converted to the low-spin (LS) state by irradiating into the near-IR spin allowed5dd band and back again by irradiating into the visible1dd band. The compound shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures. | ||||||||
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| The electronic structures of two uranium compounds supported by redox-active α-diimine ligands, (MesDABMe)2U(THF) (1) and Cp2U(MesDABMe) (2) (MesDABMe = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), have been investigated using both density functional theory and multiconfigurational self-consistent field methods. Results from these studies have established that both uranium centers are tetravalent, that the ligands are reduced by two electrons, and that the ground states of these molecules are triplets. Energetically low-lying singlet states are accessible, and some transitions to these states are visible in the electronic absorption spectrum. | ||||||||
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| Dicyanoaurate reacts with the organic acceptor molecule, 1,1′-bis-(2,4-dinitrophenyl)-4,4′-bipyridinium, DNP, to form a supramolecular complex with the general formula {[Au(CN)2]2 DNP}·4H2O. The complex was characterized by X-ray crystallography, and its photophysical properties were investigated in the solid-state. Although the initial (DNP)Cl2 compound does not show photoluminescence behavior and the dicyanoaurate shows photoluminescence only in the UV range, the resulting supramolecular complex displays two simultaneous, essentially independent, photoluminescence bands in the visible range originating from individual contributions of the DNP unit and the dicyanoaurate dimers. This unusual simultaneous photoluminescence behavior displayed by both the dicyanoaurate donor units and the redox-active 4,4′-bipyridinium acceptor have lifetimes of 0.5 μs and several hundred μs, respectively. | ||||||||
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| We investigate the optical properties of a true three-dimensional metamaterial that was fabricated using a self-assembly bottom-up technology. The metamaterial consists of closely packed spherical clusters being formed by a large number of non-touching gold nanoparticles. After presenting experimental results, we apply a generalized Mie theory to analyze its spectral response revealing that it is dominated by a magnetic dipole contribution. By using an effective medium theory we show that the fabricated metamaterial exhibits a dispersive effective permeability, i.e. artificial magnetism. Although this metamaterial is not yet left-handed it might serve as a starting point for achieving bulk metamaterials by using bottom-up approaches. | ||||||||
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| Single crystals of Ce2RhGa12 have been synthesized using Ga flux and their structure was determined by single-crystal X-ray diffraction. Ce2RhGa12 crystallizes in the tetragonal space groupP4/nbm (No. 125), and is isostructural to Ce2PdGa12, withZ = 2 and lattice parametersa = 6.0405 Å andc = 15.706 Å. Data were collected at the Swiss Norwegian Beam Line at the European Synchrotron Facility, Grenoble, France. Laue diffraction was carried out to confirm the quality of the single crystal and showed well-defined spots and tetragonal symmetry. | ||||||||
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| The green-fluorescent protein of the jellyfish operates with the most powerful phenolate donors in the push–pull fluorophore. To nevertheless achieve red fluorescence with the same architecture, sea anemone and corals apply oxidative imination, a process that accounts for the chemistry of vision as well. The objective of this study was to apply these lessons from nature to one of the most compact family of panchromatic fluorophores,i.e. core-substituted naphthalenediimides (cNDIs). We report straightforward synthetic access to hydroxylated cNDI and cPDI cores by palladium-catalyzed cleavage of allyloxy substituents. With hydroxylated cNDIs but not cPDIs in water-containing media, excited-state intramolecular proton transfer yields a second bathochromic emission. Deprotonation of hydroquinone, catechol and boronic ester cores provides access to an impressive panchromism up to the NIR frontier at 640 nm. With cNDIs, oxidative imination gives red shifts up to 638 nm, whereas the expanded cPDIs already absorb at 754 nm upon deprotonation of hydroquinone cores. The practical usefulness of hydroquinone cNDIs is exemplified by ratiometric sensing of the purity of DMF with the "naked eye" at a sensitivity far beyond the "naked nose". We conclude that the panchromatic hypersensitivity toward the environment of the new cNDIs is ideal for pattern generation in differential sensing arrays. | ||||||||
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| The title compound, an achiral flexible molecule containing a 1,2,3-triazole structure as the acceptor subunit, crystallizes as a single enantiomorph in the space group P212121. The material exhibits nonlinear optical properties and is capable of second harmonic generation. Thus, the developed molecular scaffold represents an interesting novel type of NLO chromophore. | ||||||||
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| Two molecules containing two phenylphosphaalkene moieties linked by an anthracene (1) or by a naphthalene (2) ring have been synthesized and their crystal structures have been determined. While electrochemical measurements show that these two systems are easily reduced, EPR spectra indicate that, at room temperature, the electronic structures of the two reduction compounds 1˙ − and 2˙ − are quite different. In 1˙ − , in good accordance with DFT predictions, the unpaired electron is delocalized on the full molecule while in 2˙ − it is confined on a single phosphaalkene moiety. This difference is attributed to the short distance between the two phenylphosphaalkene groups in 2˙ − which hinders their reorientation after addition of an electron. The role of this motion is consistent with the fact that two additional paramagnetic species are detected at 145 K: the dianion 2 2− characterized by a rather small exchange coupling constant and the radical monoanion 2*˙ − resulting from the formation of a one electron P–P bond. These two species are probably reaction intermediates which can lead to the formation of biphosphane. | ||||||||
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| An efficient synthetic approach to a symmetrically functionalized tetrathiafulvalene (TTF) derivative with two diamine moieties, 2-[5,6-diamino-4,7-bis(4-pentylphenoxy)-1,3-benzodithiol-2-ylidene]-4,7-bis(4-pentylphenoxy)-1,3-benzodithiole-5,6-diamine (2), is reported. The subsequent Schiff-base reactions of 2 afford large π-conjugated multiple D–A arrays, for example the triad 2-[4,9-bis(4-pentylphenoxy)-1,3-dithiolo[4,5-g]quinoxalin-2-ylidene]-4,9-bis(4-pentylphenoxy)-1,3-dithiolo[4,5-g]quinoxaline (8) and the corresponding tetrabenz[bc,ef,hi,uv]ovalene-fused pentad 1, in good yields and high purity. The novel redox-active nanographene 1 is so far the largest known TTF-functionalized polycyclic aromatic hydrocarbon with a well-resolved 1H NMR spectrum. The electrochemically highly amphoteric pentad 1 and triad 8 exhibit various electronically excited charge-transfer states in different oxidation states leading to intense optical intramolecular charge transfer (ICT) absorbances over a wide spectral range. The chemical and electrochemical oxidations of 1 result in an unprecedented TTF•+ radical cation dimerization, leading to the formation of [1 •+]2 at room temperature in solution due to the stabilizing effect arising from strong π–π interactions. Moreover, ICT fluorescence is observed with large solvent-dependent Stokes shifts and quantum efficiencies of 0.05 for 1 and 0.035 for 8 in CH2Cl2. | ||||||||
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| We model here the behavior of spin transition compounds, considering molecules arranged in a 2D hexagonal lattice and interacting via springs. The role of impurities in the clustering and nucleation phenomena is analyzed, as well as the manner in which the impurities affect the relaxation curves. The switching of the individual molecules is checked using a Monte Carlo procedure. When a molecule changes its state, it also modifies its volume, and the new equilibrium positions of all the molecules are calculated. As in previously reported experiments, it is found here that bigger impurities slow down the relaxation from the metastable high-spin state to the low-spin state, while smaller impurities act in an opposite way. It is shown that if the concentration of the impurities is higher than a certain threshold, then they act as a barrier, trammeling the fast evolution of domains developing from the edges. | ||||||||
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| PURPOSE. To compare the photochemical properties of UV filter molecules present in the human lens (kynurenine, KN; 3-hy- droxykynurenine, 3OHKN; 3-hydroxykynurenine O-gluco- side, 3OHKG; 4-(2-aminophenyl)-4-oxobutanoic acid, AHA; and glutathionyl-kynurenine, GSH-KN) with the use of the following parameters: excited singlet lifetime, fluorescence quantum yield, triplet quantum yield, photodecomposition quantum yield. METHODS. The excited singlet lifetimes were measured with the use of fluorescence upconversion (time resolution, 210 fs) and pump-probe transient absorption (time resolution, 200 fs) methods. The fluorescence quantum yields were determined relative to an aqueous solution of quinine bisulfate. The triplet quantum yields were measured with the use of nanosecond laser flash photolysis. The photodecomposition quantum yields were determined by steady state photolysis followed by the high-performance liquid chromatography analysis. RESULTS. The secondary UV filters—AHA and GSH-KN are better photosensitizers than the primary ones -KN, 3OHKN and 3OHKG: the singlet state lifetimes of the secondary UV filters are longer, and the quantum yields of fluorescence and triplet state formation are higher. CONCLUSIONS. With aging, the ratio primary/secondary UV filters in the human lens decreases from approximately 10:1 to 2:1. The obtained results demonstrate that the quality of the secondary UV filters is inferior compared to the primary ones, which may result in a higher susceptibility of old lenses to UV light. That might be an important factor for the development of the age-related cataract. | ||||||||
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| Facile access to complex systems is crucial to generate the functional materials of the future. Herein, we report self-organizing surface-initiated polymerization (SOSIP) as a user-friendly method to create ordered as well as oriented functional systems on transparent oxide surfaces. In SOSIP, self-organization of monomers and ring-opening disulfide exchange polymerization are combined to ensure the controlled growth of the polymer from the surface. This approach provides rapid access to thick films with smooth, reactivatable surfaces and long-range order with few defects and high precision, including panchromatic photosystems with oriented four-component redox gradients. The activity of SOSIP architectures is clearly better than that of disordered controls. | ||||||||
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| In this article, the synthesis of a novel high-conjugated ligand and its corresponding Ru(II) complex PTFTF:Ru is reported, along with the linear and nonlinear optical characterizations. Two-photon absorption based optical power limiting properties (OPL), especially in the near infrared, are described and compared to those of the analogous complexes previously published. Combined with a preliminary theoretical approach, this allows us to highlight several key parameters for OPL optimization in such molecular systems and more particularly the spectral overlap between TPA and excited-state absorption. | ||||||||
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| This work illustrates a simple approach for optimizing the lanthanide luminescence in molecular dinuclear lanthanide complexes and identifies a particular multidentate europium complex as the best candidate for further incorporation into polymeric materials. The central phenyl ring in the bis-tridentate model ligands L3–L5, which are substituted with neutral (X = H, L3), electron-withdrawing (X = F, L4), or electron-donating (X = OCH3, L5) groups, separates the 2,6-bis(benzimidazol-2-yl)pyridine binding units of linear oligomeric multi-tridentate ligand strands that are designed for the complexation of luminescent trivalent lanthanides, Ln(III). Reactions of L3–L5 with [Ln(hfac)3(diglyme)] (hfac– is the hexafluoroacetylacetonate anion) produce saturated single-stranded dumbbell-shaped complexes [Ln2(L k )(hfac)6] (k = 3–5), in which the lanthanide ions of the two nine-coordinate neutral [N3Ln(hfac)3] units are separated by 12–14 Å. The thermodynamic affinities of [Ln(hfac)3] for the tridentate binding sites in L3–L5 are average (6.6 ≤ log(β2,1 Y,L k ) ≤ 8.4) but still result in 15–30% dissociation at millimolar concentrations in acetonitrile. In addition to the empirical solubility trend found in organic solvents (L4 > L3 >>L5), which suggests that the 1,4-difluorophenyl spacer in L4 is preferable, we have developed a novel tool for deciphering the photophysical sensitization processes operating in [Eu2(L k )(hfac)6]. A simple interpretation of the complete set of rate constants characterizing the energy migration mechanisms provides straightforward objective criteria for the selection of [Eu2(L4)(hfac)6] as the most promising building block. | ||||||||
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| Self-assembly processes between a tripodal ligand and LnIII cations have been investigated by means of supramolecular analytical methods. At an equimolar ratio of components, tetranuclear tetrahedral complexes are readily formed in acetonitrile. The structural analysis of the crystallographic data shows a helical wrapping of binding strands around metallic cations. The properties of this series of highly charged 3D compounds were examined by using NMR spectroscopy and optical methods in solution and in the solid state. In the presence of excess metal, a new trinuclear complex was identified. The X-ray crystal structure elucidated the coordination of metallic cations with two ligands of different conformations. By varying the metal/ligand ratio, a global speciation of this supramolecular system has been evidenced with different spectroscopic methods. In addition, these rather complicated equilibria were successfully characterised with the thermodynamic stability constants. A rational analysis of the self-assembly processes was attempted by using the thermodynamic free energy model and the impact of the ligand structure on the effective concentration is discussed. | ||||||||
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| The structure and thermodynamic properties of lanthanide complexes with a new tripodal ligandL2 have been elucidated using different physicochemical methods. At stoichiometric ratios, the tetrahedral three-dimensional complexes with lanthanide cations are formed in acetonitrile with good stabilities. Despite minor structural changes comparing to previously investigated tripodal ligands, the resulting assembly exhibits different features revealed with the crystal structure of [Eu4 L2 4](OH)(ClO4)11 (orthorhombic,Pbcn). Interestingly, the highly charged edifice contains an inner cage encapsulating a perchlorate anion. Such lanthanide mediated cage-like assemblies are rare, and may be of interest for different sensing applications. Indeed, the anionic guest can be exchanged with different anions. The related host–guest equilibria were investigated with NMR techniques. Various aspects of these reactions are qualitatively discussed. | ||||||||
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| The p53 tumour suppressor gene, the most frequently mutated gene in human cancer, encodes a transcription factor that contains sequence-specific DNA binding and homo-tetramerization domains. Interestingly, the affinities of p53 for specific and non-specific DNA sites differ by only one order of magnitude, making it hard to understand how this protein recognizes its specific DNA targets in vivo. We describe here the structure of a p53 polypeptide containing both the DNA binding and oligomerization domains in complex with DNA. The structure reveals that sequence- specific DNA binding proceeds via an induced fit mechan- ism that involves a conformational switch in loop L1 of the p53 DNA binding domain. Analysis of loop L1 mutants demonstrated that the conformational switch allows DNA binding off-rates to be regulated independently of affinities. These results may explain the universal prevalence of conformational switching in sequence-specific DNA binding proteins and suggest that proteins like p53 rely more on differences in binding off-rates, than on differences in affinities, to recognize their specific DNA sites. | ||||||||
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| We theoretically analyze, fabricate, and characterize a three-dimensional plasmonic nanostructure that exhibits a strong and isotropic magnetic response in the visible spectral domain. Using two different bottom-up approaches that rely on self-organization and colloidal nanochemistry, we fabricate clusters consisting of dielectric core spheres, which are smaller than the wavelength of the incident radiation and are decorated by a large number of metallic nanospheres. Hence, despite having a complicated inner geometry, such a core–shell particle is sufficiently small to be perceived as an individual object in the far field. The optical properties of such complex plasmonic core–shell particles are discussed for two different core diameters. | ||||||||
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| The synthesis of a novel alkali-metal aluminium borohydride NaAl(BH 4 ) x Cl 4−x from NaBH 4 and AlCl 3 using a solid state metathesis reaction is described. Structure determination was carried out using synchrotron powder diffraction data and vibrational spectroscopy. An orthorhombic structure (space groupPmn2 1 ) is formed which contains Na + cations and complex [Al(BH 4 ,Cl) 4 ] − anions. Due to the high chlorine content (1 ≤x ≤ 1.43) the hydrogen density of the borohydride is only between 2.3 and 3.5 wt.% H 2 in contrast to the expected 14.6 wt.% for chlorine free NaAl(BH 4 ) 4 . The decomposition of NaAl(BH 4 ) x Cl 4−x is observed in the target range for desorption at about 90 °C by differential scanning calorimetry (DSC),in situ Raman spectroscopy and synchrotron powder X-ray diffraction. Thermogravimetric analysis (TG) shows extensive mass loss indicating the loss of H 2 and B 2 H 6 at about 90 °C followed by extensive weight loss in the form of chloride evaporation. | ||||||||
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| The connection of two CrIII sensitizers around a central ErIII acceptor in a self-assembled cation provides high local metal concentrations that favor efficient nonlinear energy transfer upconversion luminescence (see picture). Upon selective low-energy near-infrared irradiation of CrIII-centered transitions, 1 displays an unprecedented molecular two-photon upconverted green ErIII-centered emission. | ||||||||
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| We investigated by optical microscopy the thermal spin transition in single crystals of [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl) butane). The growth of the low-spin phase was observed for different crystal orientations and sizes. The process always started from a corner of the crystal but its further development depended on the size, shape and thermal history of the crystal. In crystals of smaller size, under isothermal conditions, the low-spin phase developed in a continuous way, through the propagation of a rather well defined transformation front, with a higher propagation velocity inside the planes perpendicular to the c axis. In larger crystals the spontaneous occurrence of inhomogeneous stresses led to a stepwise propagation process. | ||||||||
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| The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)2(Aqphen)]2+(TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2′,3′-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal−ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy. | ||||||||
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| Whereas the neat polymeric iron(II) compound [Fe(bbtr)3](ClO4)2, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [Fe x Zn1−x (bbtr)3](ClO4)2,x = 0.02 and 0.1, stay predominantly in the5T2 high-spin state down to cryogenic temperatures. However, the1A1 low-spin state can be populated as metastable state via irradiation into the spin-allowed5T2 → 5E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spin→high-spin relaxation is 1840(20) cm−1. | ||||||||
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| The structural and vibrational properties of the isostructural compounds Ca2FeH6 and Sr2RuH6 are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A2TH6 (A = Mg,Ca,Sr; T = Fe, Ru) in order to identify correlations between selected frequencies and the T-H bond length. The bulk moduli of Ca2FeH6 and Sr2RuH6 have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg2FeH6 and several other compounds of this structure. | ||||||||
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| The high energy of hydrogen vibrations in solids is the origin of their strong impact on thermodynamic properties. The peculiar structure of complex hydrides amplifies this impact. We shed light on the vibrational properties of three allotropes of Ca(BH4)2 using density-functional theory calculations, infrared spectroscopy, and inelastic neutron scattering. We show that the vibrational properties of Ca(BH4)2 depend on the specific phase and are hitherto the origin of their differences in stability. | ||||||||
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| The recently developed second-order perturbation theory restricted active space (RASPT2) method has been benchmarked versus the well-established complete active space (CASPT2) approach. Vertical excitation energies for valence and Rydberg excited states of different groups of organic (polyenes, acenes, heterocycles, azabenzenes, nucleobases, and free base porphin) and inorganic (nickel atom and copper tetrachloride dianion) molecules have been computed at the RASPT2 and multistate (MS) RASPT2 levels using different reference spaces and compared with CASPT2, CCSD, and experimental data in order to set the accuracy of the approach, which extends the applicability of multiconfigurational perturbation theory to much larger and complex systems than previously. Relevant aspects in multiconfigurational excited state quantum chemistry such as the valence−Rydberg mixing problem in organic molecules or the double d-shell effect for first-row transition metals have also been addressed. | ||||||||
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| The effective embedding potential introduced by Wesolowski and Warshel [J. Phys. Chem., 97 (1993) 8050] depends on two electron densities: that of the environment (nB ) and that of the investigated embedded subsystem (nA ). In this work, we analyze this potential for pairs nA and nB , for which it can be obtained analytically. The obtained potentials are used to illustrate the challenges in taking into account the Pauli exclusion principle. | ||||||||
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| The photoreactivity of two iron(II)−styrylpyridine frameworks Fe(stpy)4(NCSe)2 (stpy = 4-styrylpyridine) has been investigated for the very first time in a crystalline solid. A quantitative cis-to-trans isomerization of stilbenoids is shown to occur in the confined environment of the inorganic solid. The photochromic reaction was driven by a visible excitation into the metal-to-ligand charge transfer absorption of the high-spin all-cis complex. The solid-state transformation is accompanied by a unit-cell volume increase and an amorphization. Interestingly, the photoproduct formed by irradiating the high-spin all-cis reactant undergoes a spin conversion when the temperature is decreased. This observation is related to the "ligand-driven light-induced spin change" effect in a constrained environment. | ||||||||
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| The thiolate-for-thiolate ligand exchange reaction between the stable Au38(2-PET)24 and Au40(2-PET)24 (2-PET: 2-phenylethanethiol) clusters and enantiopure BINAS (BINAS: 1,1′-binaphthyl-2,2′-dithiol) was investigated by circular dichroism (CD) spectroscopy in the UV/vis and MALDI mass spectrometry (MS). The ligand exchange reaction is incomplete, although a strong optical activity is induced to the resulting clusters. The clusters are found to be relatively stable, in contrast to similar reactions on [Au25(2-PET)18]− clusters. Maximum anisotropy factors of 6.6 × 10−4 are found after 150 h of reaction time. During the reaction, a varying ratio between Au38 and Au40 clusters is found, which significantly differs from the starting material. As compared to Au38, Au40 is more favorable to incorporate BINAS into its ligand shell. After 150 h of reaction time, an average of 1.5 and 4.5 BINAS ligands is found for Au38 and Au40 clusters, respectively. This corresponds to exchange of 3 and 9 monodentate 2-PET ligands. To show that the limited exchange with BINAS is due to the bidentate nature of the ligand, exchange with thiophenol was performed. The monodentate thiophenol exchange was found to be faster, and more ligands were exchanged when compared to BINAS. | ||||||||
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| The emission spectra of the solids [n-Bu 4 N] 2 Tc 2 X 8 (X = Cl, Br) have been investigated at room temperature and 77 K. In each case, the emission originates in the 1 δ–δ* excited state, as with the rhenium homologues, but has a shorter lifetime. | ||||||||
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| In the dilute mixed-crystal system [Zn1−x Fe x (bbtr)3](ClO4)2, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K | ||||||||
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| The only operating mechanism in the oxidation of water to dioxygen catalyzed by the mononuclear cis-[RuII(bpy)2(H2O)2]2+ complex when treated with excess CeIV was unambiguously established. Theoretical calculations together with 18O-labeling experiments (see plot) revealed that it is the nucleophilic attack of water on a Ru=O group. | ||||||||
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| Resonant excitation energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ in the doped 3D oxalate networks [Rh1-xCrx(bpy)3][NaMIII 1-yCry(ox)3]ClO4 (ox=C2O4 -, bpy=2,2'-bipyridine, M=Al,Rh) is due to two types of interaction, namely super exchange coupling and electric dipole–dipole interaction. The energy transfer probability for both mechanisms is proportional to the spectral overlap of the 2E→4A2 emission of the [Cr(ox)3]3- donor and the 4A2→2T1 absorption of the [Cr(bpy)3]3+ acceptor.The spin-flip transitions of (pseudo-)octahedral Cr3+ are known to shift to lower energy with increasing pressure. Because the shift rates of the two transitions in question differ, the spectral overlap between the donor emission and the acceptor absorption is a function of applied pressure. For [Rh1-xCrx(bpy)3][Na-M1-yCry(ox)3]ClO4 the spectral overlap is thus substantially reduced on increasing pressure from 0 to 2.5 GPa. As a result, the energy transfer probability decreases with increasing pressure as evidenced by a decrease in the relative emission intensity from the [Cr(bpy)3]3+ acceptor. | ||||||||
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| This article describes the morphological and chemical characterization of stimuli-responsive functionalized silicon surfaces provided in parallel by atomic force spectroscopy (AFM) and Fourier transform infrared spectroscopy (FT-IR) enhanced by the single-beam sample reference attenuated total reflection method (SBSR-ATR). The stimuli-responsive behavior of the surfaces was obtained by grafting-to in melt carboxyl-terminated poly-N-isopropylacryl amides (PNIPAAM) with different degree of polymerization (DP) on epoxide-functionalized silicon substrates. The unprecedented real time and in situ physicochemical insight into the temperature-triggered response of the densely packed superficial brushes allowed for the selection of a PNIPAAM with a specific DP as a suitable polymer for the fabrication of silicon membranes exhibiting switchable nanopores. The fabrication process combines the manufacture of nanoporous silicon surfaces and their subsequent chemical functionalization by the grafting-to in melt of the selected polymer. Then, relevant information was obtained in what concerns the chemical modifications behind the topographical changes that drive the functioning of PNIPAAM-based hybrid nanovalves as well as the timescale on which the opening and closing of the nanopores occur. | ||||||||
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| The thermal conductivity of concentrated colloids in fluid, glass, and gel states was analyzed. SiO2 colloids at 10−31 vol % and Al2O3 colloids at 4.8 vol % in the fluid, the gel, and the glassy states were studied by dynamic light scattering, rheology, and transmission electron microscopy. Thermal conductivity of the three states was measured as a function of volume fraction. For the fluid and gel states the thermal conductivity increases almost linearly with concentration, reaching roughly 18% enhancement for silica at a volume fraction of 31 vol %. In contrast, in the glass state thermal conductivity strongly decreases with increasing volume fraction. | ||||||||
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| A series of bis(TTF) donors containing aromatic linkers between the two TTF units has been synthesized in order to investigate on the electronic structure of the oxidized species from an experimental and theoretical point of view. A mono(TTF)-pyridine compound has been also prepared and characterized by single-crystal X-ray diffraction analysis. Oxidation of a solution of 2,6-bis(TTF)-pyridine (TTF-Pyr-TTF) or of 1,3-bis(TTF)-benzene (TTF-Bz-TTF) in CH 2 Cl 2 with less than 0.1 equivalent of [Cp 2 Fe][PF 6 ] gives rise to a seven-line EPR spectrum consistent with the hyperfine structure calculated by DFT for the corresponding radical monocation. Increasing the proportion of oxidant leads to a four-line hyperfine structure, similar to the quartet pattern observed after oxidation of mono(TTF)-pyridine (Pyr-TTF) or mono(TTF)-benzene (Bz-TTF). In good accordance with the very weak value ofJ calculated by DFT for the dicationic biradicals these four-line spectra are attributed to [2,6-bis(TTF)-pyridine] 2+ and [1,3-bis(TTF)-benzene] 2+ . Similar experimental results are obtained for 1,4-bis(TTF)-benzene. In this case, however, electrochemical oxidation leads to the monoradical at low potential and to the diradical at higher potential, while only the diradical could be observed by electrochemical oxidation of 2,6-bis(TTF)-pyridine or of 1,3-bis(TTF)-benzene. | ||||||||
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| The objective of this study was to synthesize multichromophoric donor-acceptor systems with non-halogenated red (RO) naphthalenediimides (NDIs) attached along p-oligophenyl (POP) and oligophenylethynyl (OPE) scaffolds, and to evaluate their usefulness for zipper assembly of artificial photosystems. Compared to halogenated red NDIs (RCl, RBr), the HOMO of RO is 0.2 eV higher and the HOMO/LUMO gap 0.1 eV smaller, the latter introducing a shade of pink. Consistent with higher HOMO levels, RO zippers generate less photocurrent than RBr zippers in their respective action spectra. RO zippers are less sensitive to topological mismatch than RBr zippers and thus more robust and broadly applicable. Transient absorption measurements reveal efficient electron transfer from excited OPE donors to RO acceptors and less efficient hole injection from excited RO donors into OPE acceptors. Both processes demonstrate compatibility with OMARG-SHJ photosystems (supramolecular n/p-heterojunctions with oriented multicolored antiparallel redox gradients). Decreasing hole transfer with decreasing HOMO energy differences further demonstrates that SHJ-type hole injection disappears gradually (rather than abruptly). Losses in photonic energy during this process can thus be minimized by optoelectronic finetuning, but eventual gains in open circuit voltages risk coming with complementary losses in short circuit current. | ||||||||
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| Thisfeature article reviews research of core-substituted naphthalenediimides (cNDIs) in a comprehensive yet easily readable manner. Their synthesis, electrochemistry and spectroscopy are covered first with emphasis on the ability of cNDIs with electron donating substituents to absorb and fluoresce in all colors without global structural changes and cNDIs with electron withdrawing substituents to reach unprecedented extents of π-acidity. The section on supramolecular chemistry covers face-to-face π-stacks and peripheral hydrogen bonds, that on molecular recognition moves from pH and fluoride sensors to the binding to telomeric DNAin vivo and intercalation into π-stacks and sticky tweezers. cNDIs can recognize and transport anions by functional anion–π interactions. The section on electron transport describes cNDIs as air-stable n-semiconductors with high charge mobility and use as OFETs. Photoinduced electron transport by rainbow cNDIs has been used for the creation of artificial photosystems in solution, in bilayer membranes and on solid substrates. Examples include multicolor light harvesting architectures, organic solar cells, photosystems that can open up into ion channels, and supramolecular n/p-heterojunctions with antiparallel redox gradients. The review is highly interdisciplinary but should appeal most to organic, biosupramolecular and physical chemists. | ||||||||
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| The compounds Tc2Cl4(PMe3)4 and Tc2Br4(PMe3)4 were formed from the reaction between (n-Bu4N)2Tc2X8 (X = Cl, Br) and trimethylphosphine. The Tc(II) dinuclear species were characterized by single-crystal XRD, UV−visible spectroscopy, and cyclic voltammetry techniques, and the results are compared to those obtained from density functional theory and multiconfigurational (CASSCF/CASPT2) quantum chemical studies. The compound Tc2Cl4(PMe3)4 crystallizes in the monoclinic space group C2/c [a = 17.9995(9) Å, b = 9.1821(5) Å, c = 17.0090(9) Å, β = 115.4530(10)°] and is isostructural to M2Cl4(PMe3)4 (M = Re, Mo, W) and to Tc2Br4(PMe3)4. The metal−metal distance (2.1318(2) Å) is similar to the one found in Tc2Br4(PMe3)4 (2.1316(5) Å). The calculated molecular structures of the ground states are in excellent agreement with the structures determined experimentally. Calculations of effective bond orders for Tc2X8 2− and Tc2X4(PMe3)4 (X = Cl, Br) indicate stronger π bonds in the Tc2 4+ core than in Tc2 6+ core. The electronic spectra were recorded in benzene and show a series of low intensity bands in the range 10000−26000 cm−1. Assignment of the bands as well as computing their excitation energies and intensities were performed at both TD-DFT and CASSCF/CASPT2 levels of theory. Calculations predict that the lowest energy band corresponds to the δ* → σ* transition, the difference between calculated and experimental values being 228 cm−1 for X = Cl and 866 cm−1 for X = Br. The next bands are attributed to δ* → π*, δ → σ*, and δ → π* transitions. The cyclic voltammograms exhibit two reversible waves and indicate that Tc2Br4(PMe3)4 exhibits more positive oxidation potentials than Tc2Cl4(PMe3)4. This phenomenon is discussed and ascribed to stronger metal (d) to halide (d) back bonding in the bromo complex. Further analysis indicates that Tc(II) dinuclear species containing π-acidic phosphines are more difficult to oxidize, and a correlation between oxidation potential and phosphine acidity was established. | ||||||||
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| The new double-cation Al-Li-borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (~70 °C) combined with a high hydrogen density (17.2 wt %). It was synthesised by high-energy ball milling of AlCl3 and LiBH4. The structure of the compound was determined from image-plate synchrotron powder diffraction supported by DFT calculations. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) Å). The unexpected composition Al3Li4(BH4)13 can be rationalized on the basis of a complex cation [(BH4)Li4]3+ and a complex anion [Al(BH4)4]-. The refinements from synchrotron powder diffraction of different samples revealed the presence of limited amounts of chloride ions replacing the borohydride on one site. In situ Raman spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TG) and thermal desorption measurements were used to study the decomposition pathway of the compound. Al-Li-borohydride decomposes at ~70 °C, forming LiBH4. The high mass loss of about 20 % during the decomposition indicates the release of not only hydrogen but also diborane. | ||||||||
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| The mechanism of the photoinduced low-spin → high-spin spin crossover is actively being investigated in Fe(II) complexes in solution using ultrafast spectroscopies. These studies accurately inform on the reaction coordinate of the Fe(II) chromophore upon photoexcitation. However, they leave open questions regarding the role of the solvent. Here, we report the description from a fully ab initio molecular dynamics study of the structure of [Fe(bpy)3]2+ in water and of the organization of its solvation shell in the low-spin and the high-spin states. In particular, the low-spin → high-spin change of states is shown to be accompanied (i) by a 0.191 Å lengthening of the Fe−N bond, in agreement with experiment, and (ii) by an increased thermal fluctuation of the molecular edifice, which both result from the weakening of the Fe−N bond. Furthermore, our results suggest that about two water molecules are expelled from the first solvation shell of [Fe(bpy)3]2+, which consists of water molecules intercalated between the bpy ligands. | ||||||||
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| We report a thorough investigation of the absorption spectra of the cis and trans isomers of the 4-styrylpyridine photoswitch based on TDDFT calculations. The spectra of both isomers were analysed first from the results of excitation calculations performed on their optimised geometries. The main absorption band of the cis isomer is thus predicted to be due to the S 0 → S 1 and S 0 → S 2 transitions, while the main absorption band of the trans isomer is predicted to originate exclusively from the S 0 → S 1 transition. The convolution of the calculated oscillator strengths with Gaussians helped mimic the broadening of the electronic transitions. However, it proved necessary to use Gaussians with a large full width at half maximum of 5000 cm -1 ; and, compared to experiment, the calculated main absorption bands of the two isomers are significantly red-shifted and far too symmetric. Consequently, as required for the detailed analysis of the finite-temperature absorption spectrum of a molecule as flexible as 4-styrylpyridine, the influence of the thermal fluctuations has been taken into account by calculating the spectra as time averages over Car–Parrinello molecular dynamics trajectories. For both isomers, this led to a noticeable improvement in the relative positions of the calculated and experimental main absorption bands, and the asymmetry of the calculated bands brings them in better agreement with the experimental ones. Furthermore, these last results show that, actually, the S 0 → S 1 and S 0 → S 2 transitions both contribute significantly to the finite-temperature main absorption bands of the two isomers. Finally, in order to also take the vibrational broadening into account, the Franck–Condon factors of the relevant vibrations were calculated within the displaced harmonic oscillator approximation. By thus taking both the thermal and the vibrational broadening into account for the calculation of the absorption bands, the agreement between experiment and theory could be further improved. | ||||||||
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| The results of a computational study with multiconfigurational quantum chemical methods on actinide monoxides (AnO) and dioxides (AnO2) for An = Th, Pa, U, Np, Pu, Am, and Cm, are presented. First and second ionization energies were determined and compared with experimental values, when available. The trend along the series is analyzed in terms of the electronic configurations of the various species. The agreement with experiment is excellent in most cases. Of particular interest is the first ionization of PuO2. We applied cutting-edge theoretical methods to refine the ionization energy, but our computed data fall in the range of ~6 eV and not in the ~7 eV region as the experiment dictates. Such a system requires further computational and experimental attention. | ||||||||
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| Playing with a full deck: Single-crystal X-ray and neutron diffraction data show that the Th center in the title complex 1 (see structure; Th orange, B beige, N purple, C black, H blue) forms bonds with 15 H atoms, thus making 1 the first crystallographically characterized example of a complex with a Werner coordination number of fifteen. DFT calculations suggest that 1 adopts the fully symmetric 16-coordinate structure in the gas phase. | ||||||||
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| In the 3D network [Rh(bpy)3][NaCr(ox)3]ClO4 (ox = oxalate, bpy = 2,2'-bipyridine) phonon-assisted as well as resonant energy migration within the R1 line of the 4A2→2E transition of Cr3+ has been identified. The latter is dominant below 4.2 K, and in a fluorescence line narrowing spectrum, it manifests itself in a multiline pattern across the inhomogeneous line width with spacings corresponding to the zero-field splitting of the 4A2 ground state (Milos, M.; Kairouani, S.; Rabaste, S.; Hauser, A. Coord. Chem. Rev. 2008, 252, 2540). H. Riesen demonstrated efficient spectral hole burning within the R1 line of Cr3+ doped at low concentrations into partially deuterated NaMg[Al(ox)3]·9H2O (Riesen, H. Coord. Chem. Rev. 2006 250, 1737). Here we show that at higher Cr3+ concentrations in the same host, both phenomena can be observed simultaneously, the resonant energy migration thus creating an additional series of persistent side holes. | ||||||||
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| In the mixed crystal series of the cubic three-dimensional networks of composition [Zn1−x Ru x (bpy)3][NaCr(ox)3] (0 ≤ x ≤1, ox = C2O4 2−, bpy = 2,2′-bipyridine), high-resolution absorption spectroscopy in the region of the 4A2 → 2E transition (R-lines) reveals the creation of five specific spectroscopic sites for the [Cr(ox)3]3− complex. The concentration of these spectroscopic sites follows a binomial distribution of [Zn(bpy)3]2+ and [Ru(bpy)3]2+ among the four nearest neighbors of a given [Cr(ox)3]3− complex within the network. The tris-bipyridine complexes occupying those positions have an optimal π−π interaction with the oxalate ligands of the tris-oxalate chromophore. The energy of each spectroscopic [Cr(ox)3]3− site depends on the total concentration of [Ru(bpy)3]2+ in the mixed crystal and on its specific distribution among the four nearest neighbors. Single crystal X-ray diffraction indicates a reduction of the unit cell volume when [Zn(bpy)3]2+ (a = 15.6365(18) Å) is substituted by [Ru(bpy)3]2+ (a = 15.5098(6) Å). This alone would lead to a red-shift of the R lines in analogy to the red-shift of 25.2 cm−1/GPa due to the decrease of the metal ligand Cr−O bond length as observed in high-pressure luminescence experiments. However, specific π−π interactions with the nearest neighbors have the opposite effect and shift the transition in discrete jumps to higher energies with increasing [Ru(bpy)3]2+ mole fraction. | ||||||||
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| The electronic structure and the photophysical properties of the vanadium(III)ion in pseudo-octahedral oxygen coordination is reviewed. V3+ has received much interest from spectroscopists in recent years due to the advancement of state-of-the-art experimental techniques such as inelastic neutron scattering and high-field electron paramagnetic resonance spectroscopy that directly interrogate its large ground state zero-field splittings (ZFSs) and to rational parameterization of the ligand fieldp arameters using the angular overlap model. However, for V3+ these ZFSs can be large enough to also be probed directly by high-resolution electronic absorption spectroscopy of intra-configurational (t2 2g → t2 2g) spin-forbidden transitions in the near-IR and visible regions. The luminescent properties of V3+ with hexa-oxo and tris-bidentate di-oxo-coordination are quite disappointing compared to its neighbor in the periodic table, Cr3+, in similar environments. The efficient non-radiative pathways in these compounds are reviewed and compared to recent work on V3+ doped into NaMgAl(ox)3⋅9H2O. The poor luminescence quantum efficiencies of V3+ oxo complexes is explained by strong coupling of multi-phonon processes with a dynamic Jahn-Teller distortion originating from the 3E trigonal component of the 3T1g ground state. | ||||||||
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| The experimental ultrafast photophysics of thioxanthone in several aprotic organic solvents at room temperature is presented, measured using femtosecond transient absorption together with high-level ab initio CASPT2 calculations of the singlet- and triplet-state manifolds in the gas phase, including computed state minima and conical intersections, transition energies, oscillator strengths, and spin-orbit coupling terms. The initially populated singlet ππ* state is shown to decay through internal conversion and intersystem crossing processes via intermediate nπ* singlet and triplet states, respectively. Two easily accessible conical intersections explain the favorable internal conversion rates and low fluorescence quantum yields in nonpolar media. The presence of a singlet-triplet crossing near the singlet ππ* minimum and the large spin-orbit coupling terms also rationalize the high intersystem crossing rates. A phenomenological kinetic scheme is proposed that accounts for the decrease in internal conversion and intersystem crossing (i.e. the very large experimental crescendo of the fluorescence quantum yield) with the increase of solvent polarity. | ||||||||
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| The excited-state dynamics of five derivatives of the GFP-chromophore, which differ by the position and nature of their substituents, has been investigated in solvents of various viscosity and polarity and in rigid media using femtosecond-resolved spectroscopy. In polar solvents of low viscosity, like acetonitrile or methanol, the fluorescence decays of all compounds are multiexponential, with average lifetimes of the order of a few picoseconds, whereas in rigid matrices (polymer films and low temperature glasses), they are single exponential with lifetimes of the order of a few nanoseconds and fluorescence quantum yields close to unity. Global analysis of the fluorescence decays recorded at several wavelengths and of the transient absorption spectra reveals the presence of several excited-state populations with slightly different fluorescence and absorption spectra and with distinct lifetimes. These populations are attributed to the existence of multiple ground-state conformers. From the analysis of the dependence of the excited-state dynamics on the solvent and on the nature of the substituents, it follows that the nonradiative deactivation of all these excited chromophores involves an intramolecular coordinate with large amplitude motion. However, depending on the solvent and substituent, additional channels, namely, inter- and intramolecular hydrogen bond assisted nonradiative deactivation, are operative. This allows tuning of the excited-state lifetime of the chromophore. Finally, an ultrafast photoinduced intramolecular charge transfer is observed in polar solvents with one derivative bearing a dimethylaminophenyl substituent. | ||||||||
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| A molecular donor-acceptor dyad comprising a hexarhenium cluster core, [Re6(μ3-Se)8]2+, and a fullerene moiety which are covalently linked through a pyridine ligand was synthesized and fully characterized. The electrochemical and photophysical properties are reported. The detailed study includes cyclic voltammetry, steady-state absorption and fluorescence spectroscopy, radiation chemistry and transient absorption spectroscopy. A light-induced electron transfer between the inorganic cluster moiety and the fullerene can be excluded. However, a light-induced energy transfer from the rhenium cluster to the fullerene is proposed. | ||||||||
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| The introduction of pump-probe techniques to the field of x-ray absorption spectroscopy (XAS) has allowed the monitoring of both structural and electronic dynamics of disordered systems in the condensed phase with unprecedented accuracy, both in time and in space. We present results on the electronically excited high-spin state structure of an Fe(II) molecular species, [FeII(bpy)3]2+, in aqueous solution, resolving the Fe-N bond distance elongation as 0.2 Å. In addition an analysis technique using the reduced χ2 goodness of fit between FEFF EXAFS simulations and the experimental transient absorption signal in energy space has been successfully tested as a function of excited state population and chemical shift, demonstrating its applicability in situations where the fractional excited state population cannot be determined through other measurements. Finally by using a novel ultrafast hard x-ray 'slicing' source the question of how the molecule relaxes after optical excitation has been successfully resolved using femtosecond XANES. | ||||||||
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| We have developed a simple method for the preparation of nearly mono-dispersed stable gold colloids with a fairly high concentration using a two step procedure. First we synthesize citrate capped gold nanoparticles and then exchange the citrate ions with triethyleneglycolmono-11-mercaptoundecylether (EGMUDE). This leads to the immediate precipitation and formation of composite assemblies. The gold nanoparticles were successfully self-redispersed after a few days. The prepared gold colloid can be easily concentrated up to 20 times by separation of the flocculated part. UV-visible spectra, transmission electron microscopy (TEM), and dynamic light scattering (DLS) were used to characterize the products thus formed. | ||||||||
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| Pd(II)-coordinated phosphinous acids catalyzed the formal enantioselective [2+1] cycloaddition of norbornene derivatives with terminal alkynes. The absolute configuration of (+)-3aa was assigned using VCD. | ||||||||
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| Surface-initiated ATRP was applied for the step-by-step growth of the biomimetic amphiphilic block copolymer membrane on a gold support. Different thicknesses of membranes were achieved through the variation of the polymerization conditions. The OH-groups of the hydrophilic polymer blocks can be further functionalized to tune the membrane properties. Synthesis, characterization, and solvent-responsive properties of the amphiphilic triblock copolymer membrane are presented. | ||||||||
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| The historical background and the current activities of the Chemical Society of Geneva are described. Founded 88 years ago from the merger of three student associations, the Society today connects some 190 chemists and biochemists from all professional horizons. The Society's main goal is to promote molecular sciences studies and applications in Geneva. In addition to regularly organizing scientific lectures and visits, it sponsors major scientific seminars and grants annual awards to secondary school pupils and bachelor students. | ||||||||
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| A brief historical overview of physical chemistry at the University of Geneva as well as a description of the present research activities at the department of physical chemistry are presented. | ||||||||
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| Specifically labeled NaBD3H has been synthesized and characterized using X-ray diffraction, NMR, and vibrational spectroscopy. The isotopic purity of the compound, as estimated from NMR spectra, was found to be about 85% with the compound NaBD2H2 as the second product. IR spectra confirm the relatively strong intensity of the single B–H stretching mode predicted from DFT calculations. Anharmonic DFT calculations show that for the BD3H- ion Fermi resonances with the single B–H stretching mode are very limited, making this mode a promising structural probe for complex borohydrides which can be prepared by metathetical reactions. | ||||||||
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| Palladium carbene complexes, CX2=PdX2, are prepared along with the insertion products, CX3–PdX, in reactions of laser-ablated Pd atoms with tetrahalomethanes and identified from matrix infrared spectra and density functional frequency calculations. The carbon–metal bonds of the CCl2=PdCl2 and CClF=PdCl2 complexes are essentially double bonds with effective bond orders of 1.9, near those for the Pt and Ni analogues, as calculated by CASPT2 methods. On the other hand, only insertion complexes are generated from mono-, di-, and trihalomethane precursors. While the carbenes have staggered allene-type structures, many insertion complexes containing C–Cl bonds reveal distinct bridged structures, which indicate effective coordination of Cl to the metal center. | ||||||||
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| Multiconfigurational second-order perturbation theory based on either a complete active space reference wave function (CASSCF/CASPT2) or a restricted active space reference wave function (RASSCF/RASPT2) has been applied to compute one-electron ionization potentials and vertical electronic energy differences of oligomers of length n formed from ethylene (n = 1-10), acetylene (n = 1-5), and phenylene (n = 1-3) subunits. The RASSCF/RASPT2 approach offers an accuracy similar to CASSCF/CASPT2 at significantly reduced computational expense (both methods show good agreement with experimental data where available). It is shown that RASPT2 extends the range of CASPT2-like approaches by permitting the use of larger active spaces. | ||||||||
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| Matching matters when building supramolecular n/p-heterojunction photosystems on solid supports that excel with efficient photocurrent generation, important critical thickness, smooth surfaces, and flawless responsiveness to functional probes for the existence of operational intra- and interlayer recognition motifs. | ||||||||
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| In this study, we describe synthesis, characterization, and zipper assembly of yellow p-oligophenyl naphthalenediimide (POP-NDI) donor−acceptor hybrids. Moreover, we disclose, for the first time, results from the functional comparison of zipper and layer-by-layer (LBL) assembly as well as quartz crystal microbalance (QCM), atomic force microscopy (AFM), and molecular modeling data on zipper assembly. Compared to the previously reported blue and red NDIs, yellow NDIs are more π-acidic, easier to reduce, and harder to oxidize. The optoelectronic matching achieved in yellow POP-NDIs is reflected in quantitative and long-lived photoinduced charge separation, comparable to their red and much better than their blue counterparts. The direct comparison of zipper and LBL assemblies reveals that yellow zippers generate more photocurrent than blue zippers as well as LBL photosystems. Continuing linear growth found in QCM measurements demonstrates that photocurrent saturation at the critical assembly thickness occurs because more charges start to recombine before reaching the electrodes and not because of discontinued assembly. The found characteristics, such as significant critical thickness, strong photocurrents, large fill factors, and, according to AFM images, smooth surfaces, are important for optoelectronic performance and support the existence of highly ordered architectures. | ||||||||
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| Nickel carbene complexes, CX2 = NiX2, are prepared along with the insertion products, CX3 – NiX, in reactions of laser-ablated Ni atoms with tetrahalomethanes. These reaction products are identified from matrix infrared spectra and density functional frequency calculations. In agreement with the previously studied Pt cases, the carbon – nickel bonds of the Ni carbene complexes are essentially double bonds with CASPT2-computed effective bond orders of 1.8 – 1.9. On the other hand, only insertion complexes are generated from dihalomethane and trihalomethane precursors. The nickel carbenes have staggered structures, and several insertion complexes containing C – Cl bonds reveal distinct bridged structures similar to those observed in the corresponding Fe products, which indicate effective coordination of Cl to the metal center. The unique F-bridged CH2F – NiCl structure is also observed. | ||||||||
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| Multiconfigurational second-order perturbation theory calculations based on a complete active space reference wave function (CASPT2), employing active spaces of increasing size, are well converged at the level of 12 electrons in 12 orbitals for the singlet−triplet state−energy splittings of three supported copper−dioxygen and two supported copper−oxo complexes. Corresponding calculations using the restricted active space approach (RASPT2) offer similar accuracy with a significantly reduced computational overhead provided an inner (2,2) complete active space is included in the overall RAS space in order to account for strong biradical character in most of the compounds. The effects of the different active space choices and the outer RAS space excitations are examined, and conclusions are drawn with respect to the general applicability of the RASPT2 protocol. | ||||||||
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| CoCo loco! Ligand-bridged dimers (see picture) with the shortest known Co-Co interactions are the first amidinato and guanidinato cobalt(I) complexes. The nature of the interactions has been probed by magnetic and theoretical investigations, and has been shown to be multiconfigurational. Preliminary reactivity studies of the complexes have also been carried out. | ||||||||
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| The ground-state electronic structures of K3V(ox)3·3H2O, Na3V(ox)3·5H2O, and NaMgAl1–xVx(ox)3·9H2O (0 < x <= 1, ox = C2O4 2–) have been studied by Fourier–transform electronic absorption and inelastic neutron scattering spectroscopies. High-resolution absorption spectra of the 3Γ(t2g 2) → 1Γ(t2g 2) spin-forbidden electronic origins and inelastic neutron scattering measurements of the pseudo-octahedral [V(ox)3]3– complex anion below 30 K exhibit both axial and rhombic components to the zero-field-splittings (ZFSs). Analysis of the ground-state ZFS using the conventional S = 1 spin Hamiltonian reveals that the axial ZFS component changes sign from positive values for K3V(ox)3·3H2O (D ≈ +5.3 cm–1) and Na3V(ox)3·5H2O (D ≈ +7.2 cm–1) to negative values for NaMgAl1–xVx(ox)3·9H2O (D ≈ –9.8 cm–1 for x = 0.013, and D ≈ –12.7 cm–1 for x = 1) with an additional rhombic component, |E|, that varies between 0.8 and 2 cm–1. On the basis of existing crystallographic data, this phenomenon can be identified as due to variations in the axial and rhombic ligand fields resulting from outer-sphere H-bonding between crystalline water molecules and the oxalate ligands. Spectroscopic evidence of a crystallographic phase change is also observed for K3V(ox)3·3Y2O (Y = H or D) with three distinct lattice sites below 30 K, each with a unique ground-state electronic structure. | ||||||||
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| The photophysical properties of two hybrid multichromophoric systems consisting of an oligophenylethynyl (OPE) scaffold decorated by 10 red or blue naphthalene diimides (NDIs) have been investigated using femtosecond spectroscopy. Ultrafast charge separation was observed with both red and blue systems. However, the nature of the charge-separated state and its lifetime were found to differ substantially. For the red system, electron transfer occurs from the OPE scaffold to an NDI unit, independently of whether the OPE or an NDI is initially excited. However, charge separation upon OPE excitation is about 10 times faster, and takes place with a 100 fs time constant. The average lifetime of the ensuing charge-separated state amounts to about 650 ps. Charge separation in the blue system depends on which of the OPE scaffold or an NDI is excited. In the first case, an electron is transferred from the OPE to an NDI and the hole subsequently shifts to another NDI unit, whereas in the second case symmetry-breaking charge separation between two NDI units occurs. Although the charges are located on two NDIs in both cases, different recombination dynamics are observed. This is explained by the location of the ionic NDI moieties that depends on the charge separation pathway, hence on the excitation wavelength. The very different dynamics observed with red and blue systems can be accounted for by the oxidation potentials of the respective NDIs that are higher and lower than that of the OPE scaffold. Because of this, the relative energies of the two charge-separated states (hole on the OPE or an NDI) are inverted. | ||||||||
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| The excited-state dynamics of biotin–spacer–Lucifer-Yellow (LY)constructs bound to avidin (Avi) and streptavidin (Sav) was investigatedusing femtosecond spectroscopy. Two different locations in the proteins,identified by molecular dynamics simulations of Sav, namely the entrance of the binding pocket andthe protein surface, were probed by varying the length of thespacer. A reduction of the excited-state lifetime, stronger inSav than in Avi, was observed with the long spacer construct.Transient absorption measurements show that this effect originatesfrom an electron transfer quenching of LY, most probablyby a nearby tryptophan residue. The local environment of theLY chromophore could be probed by measuring the time-dependent polarisation anisotropy and Stokes shift of the fluorescence. Substantial differences in both dynamics were observed.The fluorescence anisotropy decays analysed by using thewobbling-in-a-cone model reveal a much more constrained environment of the chromophore with the short spacer. Moreover, the dynamic Stokes shift is multiphasic in all cases, with a~ 1 ps component that can be ascribed to diffusive motion ofbulk-like water molecules, and with slower components withtime constants varying not only with the spacer, but with theprotein as well. These slow components, which depend strongly on the local environment of the probe, are ascribed to themotion of the hydration layer coupled to the conformationaldynamics of the protein. | ||||||||
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| Laser-ablated late lanthanide metal atoms were condensed with pure hydrogen at 4 K, and new infraredabsorptions are assigned to binary metal hydrides on the basis of deuterium substitution and density functionaltheory frequency calculations. The dominant absorptions in the 1330-1400 cm-1 region are identified asLnH3 complexes with very weak ligand bands near 3900 cm-1. With ytterbium, YbH and YbH2 were themajor initial products, but YbH3 increased at their expense upon sample irradiation. Evidence is also presentedfor the LuH and ErH molecules and the tetrahydride anions in solid hydrogen. | ||||||||
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| The synthesis and structural characterization of a tetrathiafulvalene-fused perylenediimide molecular dyad is presented. Its largely extendedπ-conjugation provides intense optical absorption bands over a wide spectral range. The planar functional molecule exhibits a short-livednonluminescent excited state attributed to intramolecular charge separation. | ||||||||
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| The relaxation in a spin transition compound is modeled on the basis of molecules interacting by theway of connecting springs and situated in a bidimensional open boundary hexagonal lattice. The switch ofindividual molecules is randomly checked using a standard Monte Carlo procedure. The switchingprobability depends on the energy gap between the two states in the absence of interactions and on theelongations of the nearest springs. The main characteristics of the experimental relaxation curves arereproduced and clustering and nucleation phenomena are detected. | ||||||||
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| The synthesis and structural characterization of the λ5-bis(phosphine sulfide) and the bimetalliccomplexes bis[phosphino-M(CO)5] (M = Mo, W) of the 3,4-dimethyltetrathiafulvalene (ortho-DMTTF)-based rigid dimer (PPh)2(o-DMTTF)2, containing a central 1,4-dihydro-1,4-diphosphi-nine ring, are described. Single-crystal X-ray analyses have been performed for the trans isomers(PhPX)2(o-DMTTF)2 (X = S, Mo(CO)5, and W(CO)5) and for the cis isomer [PhPW(CO)5]2-(o-DMTTF)2. Planar or slightly folded boat-type conformations are observed for the central six-membered ring, together with different packings characterized by short intermolecular S · · · Scontacts. The optical signature of the oxidized species in the case of the free ligand (PPh)2-(o-DMTTF)2 has been evidenced by UV-vis spectroelectrochemistry measurements. SolutionEPR measurements on the radical cation species of (PPh)2(o-DMTTF)2 definitely assess the fulldelocalization of the unpaired electron over both electroactive TTF units, with an associatedcoupling of 0.48 G with 12 equivalent protons. The EPR signal of the dication proves the radicalnature of this species, in favor of a triplet ground state. The radical cation of the cis-[PhPW(CO)5]2-(o-DMTTF)2 isomer was also investigated by EPR, for which the observed hyperfine structuredemonstrates the extended delocalization of the electron, together with a larger coupling constantwith the phosphorus nuclei. DFT calculations for the radical cation of (PPh)2(o-DMTTF)2 afford aboat-type conformation for the central ring and a SOMO consistent with a full delocalization of theelectron over both TTF units. Moreover, the calculations indicate that in the case of the dication of(PPh)2(o-DMTTF)2 the triplet state is more stable by 11.7 kcal mol-1 than the singlet state. | ||||||||
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| In the 3D oxalate networks [NaCr(ox)3][Rh(bpy)3]ClO4 and [NaCr(ox)3][Ru(bpy)3] (ox=oxalate, bpy=2,2′-bipyridine) three different types of energy migration within the 4A2→2E transition can be identified. One is a resonant process between spectral members spaced by the ground-state zero-field splitting (ZFS). This leads to the sequential appearance of additional sharp lines spaced by the ground-state ZFS in the fluorescence line narrowing spectrum across the inhomogeneous line. The second one is a quasi-resonant process between spectral neighbours and manifests itself by rapid spectral diffusion. The third one is the well-known phonon-assisted process setting in at higher temperature. | ||||||||
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| High-resolution Fourier transform absorption and luminescence spectroscopy reveal axial and rhombic zero-field splittings of the spin-forbidden electronic origins of V3+ in NaMgAl(ox)3·9H2O (ox=oxalate) single crystals below 25 K. The temperature dependence of the integrated absorption of the split features display behavior consistent with a Boltzmann distribution within the zero-field split 3Â2 ground state of V3+. Weak luminescence is observed in the near-IR from the lowest energy spin-forbidden transition with a luminescence lifetime of less than 0.5 μs at 11 K and an estimated quantum efficiency of the order of 10-5 | ||||||||
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| A radical species characterized by a large g-anisotropy and a clearly resolved hyperfine structure with 95/97Mo and 31P nuclei is formed, at 77 K, by radiolysis of a single crystal of Mo(CO)5PPh3. The corresponding EPR signals disappear irreversibly with increasing temperature and the angular dependence of the various coupling constants imply a spin delocalization of not, vert, ∼60% and not, vert, ∼4% on the molybdenum and the phosphorus atoms, respectively and are, a priori, consistent with the trapping of a one-electron deficient centre. The ability of DFT to predict the EPR tensors for a 17-electron Mo(I) species is verified by calculating the g-tensor and the various 14N and 13C coupling tensors previously reported by Hayes for [Mo(CN)5NO]3-. Calculations at the B3LYP/ZORA/SOMF level of theory show that, in contrast to Mo(CO)5PH3, one-electron oxidation of Mo(CO)5PPh3 causes an appreciable change in the geometry of the complex. The g-tensor and the 95/97Mo and 31P isotropic and anisotropic coupling constants calculated for [Mo(CO)5PPh3]+· confirm the trapping of this species in the irradiated crystal of Mo(CO)5PPh3; they also show that the conformational modifications induced by the electron release are probably hindered by the nearby complexes. | ||||||||
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| The effective embedding potential introduced by Wesolowski and Warshel [J. Phys. Chem., 97 (1993) 8050] depends on two electron densities: that of the environment (n B ) and that of the investigated embedded subsystem (n A ). In this work, we analyze this potential for pairs n A and n B , for which it can be obtained analytically. The obtained potentials are used to illustrate the challenges in taking into account the Pauli exclusion principle. | ||||||||
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| The technetium(III) compound (n-Bu4N)2[Tc2Br8] was prepared by metathesis of (n-Bu4N)2[Tc2Cl8] with concentrated aqueous HBr in acetone and recrystallized from acetone–diethyl ether solution (2 : 1 v/v). The acetone solvate obtained, (n-Bu4N)2[Tc2Br8]·4[(CH3)2CO] ( 1), crystallizes in the monoclinic space group P21/n with a = 13.8959(8) Å, b = 15.2597(9) Å, c = 15.5741(9) Å, β = 109.107(1)°, R1 = 0.028, and Z = 4. The Tc–Tc distance (2.1625(9) Å) and the average Tc–Br distances (2.4734(7) Å) are in excellent agreement with those previously determined by EXAFS spectroscopy. These and other experimental data on quadruply metal–metal bonded group 7 [M2X8]2- dimers (M = Tc, Re; X = Cl, Br) are compared to the results of a set of multi-configurational quantum chemical studies. The calculated molecular structures of the ground states are in very good agreement with the structures determined experimentally. The theory overestimates the * transition energies by some 1000 cm-1, but mimics the trends in δ – δ* energies across the series. | ||||||||
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| In order to study the electronic interactions in donor-acceptor ensembles as a function of pH, an efficient synthetic route to three imidazole-annulated tetrathiafulvalene (TTF) derivatives 1-3 is reported. Their electronic absorption spectra, in view of photoinduced intramolecular charge transfer, and their electrochemical behavior were investigated, and pKa values for the two protonation processes on the acceptor unit were determined in organic solvents by photometric titration. The influence of the TTF moiety on these values is discussed. | ||||||||
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| Triangular luminescent box: Self-assembly of a new multidentate receptor with europium cations results in the formation of trinuclear discrete complexes. X-ray crystallography shows that nine-coordinate cations are linked by ligands to provide a triangular complex in the solid state and in solution. Despite the coordinated solvent molecules, this topologically unusual complex exhibits remarkable luminescent properties. | ||||||||
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| Electronic absorption spectrum of 1 in DMF solution at room temperature, together with the calculated oscillator strengths. | ||||||||
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| The kinetics of the dual fluorescence of several derivatives of dimethylaminobenzonitrile (DMABN) has been compared using fs-fluorescence upconversion experiments. Variation of the size and twist angle of the donor (dialkylamino group) suggest a large amplitude solvent-viscosity controlled diffusional twisting motion towards larger twist angles as the rate limiting step. Large rate differences were observed for an ester group as acceptor. Temperature dependent studies indicate that these differences are not connected with different activation barriers but with changes in the Arrhenius preexponential factor. It is argued that conical intersections along the reaction path can bring about these entropy changes. | ||||||||
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| The results of a combined spectroscopic and computational study of lanthanide hydrides with the general formula MH x (H2) y , where M = La, Ce, Pr, Nd, Sm, Eu, and Gd, x = 1−4, and y = 0−6 are reported. To understand the nature of the dihydrogen complexes formed with lanthanide metal hydride molecules, we have first identified the binary MH x species formed in the ablation/deposition process and then analyzed the dihydrogen supercomplexes, MH x (H2) y . Our investigation shows that the trihydrides bind dihydrogen more weakly than the dihydrides and that the interaction between the central lanthanide and the H2 molecules occurs via a 6s electron transfer from the lanthanide to the H2 molecules. Evidence is also presented for the SmH and EuH diatomic molecules and the tetrahydride anions in solid hydrogen. | ||||||||
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| Reaction of 2,4,6-trichloro-1,3,5-triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono- and bis(TTF)-triazines as new covalently linked (multi)donor-acceptor systems. Single-crystal X-ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)-triazine compound, while mixed TTF-triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time-dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)-triazine is paramagnetic, but no spin-spin exchange interaction could be detected. | ||||||||
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| Density functional theory is applied within a supramolecular approach to the study of the guest−host interactions in [Fe(bpy)3]2+@Y and their influence on the structural, energetic, and 57Fe Mössbauer spectroscopy properties of the encapsulated [Fe(bpy)3]2+ complex in the low- and high-spin states. The structures of the isolated complex and the supramolecular model used for [Fe(bpy)3]2+@Y were optimized in both spin-states using different generalized gradient approximation (PBE, HCTH, OLYP) and hybrid (B3LYP*, O3LYP) functionals. The results obtained are consistent with one another and show that, in either spin-state, the structure of [Fe(bpy)3]2+ shrinks and distorts upon encapsulation. Still, the structural changes experienced by the complex in a given spin-state remain limited, especially in that they do not lead to a substantial variation of the 57Fe quadrupole splitting, whose calculated values are in very good agreement with avalaible experimental data. The decomposition of the guest−host interaction energy into its electrostatic, Pauli and orbital contributions shows that the bonding between the complex and the supercage is more electrostatic than covalent. The ability of modern functionals to accurately describe the interactions explains the remarkable consistency of the results obtained with the various functionals. In particular, although the functionals perform very differently for the determination of the high-spin/low-spin energy difference ΔE HL el in [Fe(bpy)3]2+ and [Fe(bpy)3]2+@Y, they consistently predict that the encapsulation entails a destabilization of the high-spin state with regard to the low-spin state of Δ(ΔE HL el) = 2500 cm−1. Using for [Fe(bpy)3]2+ the CASPT2 value of ΔE HL el = 3700 cm−1 [Pierloot, K.; Vancoillie, S. J. Chem. Phys.2006, 125, 124303; Pierloot, K.; Vancoillie, S. J. Chem. Phys.2008, 128, 034104], we obtain for the high-spin/low-spin energy difference in [Fe(bpy)3]2+@Y, a best ab initio estimate of ΔE HL el = 6200 cm−1. | ||||||||
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| The spin-transition (1A1↔5T2) behaviour of a new mononuclear iron(II) compound [FeII(L)3][PF6]2[L = 2-[3-(2′-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by 57Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom. | ||||||||
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| The objective with synthetic multifunctional nanoarchitecture is to create large suprastructures with interesting functions. For this purpose, lipid bilayer membranes or conducting surfaces have been used as platforms and rigid-rod molecules as shape-persistent scaffolds. Examples for functions obtained by this approach include pores that can act as multicomponent sensors in complex matrices or rigid-rod π-stack architecture for artificial photosynthesis and photovoltaics. | ||||||||
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| Using simple organic synthetic transformations, a novel diazaoxatricornan derivative, the 12c-methyl-12-phenyl-8-propyl-12,12c-dihydro-8H-4-oxa-8,12-diazadibenzo[cd,mn]pyrene (6a), was prepared. This novel chiral cup-shaped molecule was isolated in racemic form and in excellent yield after the addition of methyl lithium to the BF4 salt of a novel unsymmetrical diazaoxatriangulenium cation. Compound 6a was found to be stable under classical laboratory conditions—something not obvious considering the extreme stability of the carbenium ion precursor, the electron-rich nature of the core, and the strain induced by the pyramidalization of the central carbon. The enantiomers were readily separated by chiral stationary phase chromatography, and the absolute configuration of (−)-(S)-6a was determined by a comparison of the experimental and theoretical vibrational circular dichroism (VCD) spectra. This isolation of (−)-(S)-6a and (+)-(R)-6a constitutes thus the first report of a nonracemic closed-capped chiral bowl molecule for which the chirality is due to the intrinsic dissymmetry of the central core of the structure only. | ||||||||
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| Liquid-crystalline dendrons carrying either a thiol or disulfide function which display nematic, smectic A, columnar, or chiral nematic phases have been synthesized. Their mesomorphic properties are in agreement with the nature of the mesogenic units and structure of the dendrons. The first-generation poly(aryl ester) dendron containing two cyanobiphenyl mesogenic units was used to functionalize gold nanoparticles. For full coverage, a smectic-like supramolecular organization on the nanometer scale is observed, when the gold nanoparticles are spread onto carbon-coated copper grids. This result indicates that the dendritic ligands reported here act as self-organization promoters. | ||||||||
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| The trigonal planar geometry of the nitrogen atom in commonly used phosphoramidite ligands is not in line with the traditional valence shell electron pair repulsion (VSEPR) model. In this work, the effects governing nitrogen configuration in several substituted aminophosphines, A2PNB2 (A or B = H, F, Cl, Br, Me, OMe, BINOP), are examined using modern computational analytic tools. The electron delocalization descriptions provided by both electron localization function (ELF) and block localized wavefunction analysis support the proposed relationships between conformation and negative hyperconjugative interactions. In the parent H2PNH2, the pyramidal nitrogen configuration results from nitrogen lone pair electron donation into the σ* P — H orbital. While enhanced effects are seen for F2PNMe2, placing highly electronegative fluorine substituents on nitrogen (i.e., Me2PNF2) eliminates delocalization of the nitrogen lone pair. Understanding and quantifying these effects can lead to greater flexibility in designing new catalysts. | ||||||||
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| Polarization-sensitive ultrafast infrared measurements on photoinduced electron transfer in donor-acceptor pairs in polar acetonitrile show distinct contributions from loose and tight ion pairs. Highly anisotropic signals from tight ion pairs reveal the importance of mutual orientation of the reactants (see picture) and thus the need to refine theoretical models based on spherical species that solely involve reaction distances. | ||||||||
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| A π-extended, redox-active bridging ligand 4′,5′-bis(propylthio)tetrathiafulvenyl[i]dipyrido[2,3-a:3′,2′-c]phenazine (L) was prepared via direct Schiff-base condensation of the corresponding diamine−tetrathiafulvalene (TTF) precursor with 4,7-phenanthroline-5,6-dione. Reactions of L with [Ru(bpy)2Cl2] afforded its stable mono- and dinuclear ruthenium(II) complexes 1 and 2. They have been fully characterized, and their photophysical and electrochemical properties are reported together with those of [Ru(bpy)2(ppb)]2+ and [Ru(bpy)2(μ-ppb)Ru(bpy)2]4+ (ppb = dipyrido[2,3-a:3′,2′-c]phenazine) for comparison. In all cases, the first excited state corresponds to an intramolecular TTF → ppb charge-transfer state. Both ruthenium(II) complexes show two strong and well-separated metal-to-ligand charge-transfer (MLCT) absorption bands, whereas the 3MLCT luminescence is strongly quenched via electron transfer from the TTF subunit. Clearly, the transient absorption spectra illustrate the role of the TTF fragment as an electron donor, which induces a triplet intraligand charge-transfer state (3ILCT) with lifetimes of approximately 200 and 50 ns for mono- and dinuclear ruthenium(II) complexes, respectively. | ||||||||
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| The photophysical properties of multichromophoric systems consisting of eight red or blue naphthalene diimides (NDIs) covalently attached to a p-octiphenyl scaffold, as well as a blue bichromophoric system with a biphenyl scaffold, have been investigated in detail using femtosecond time-resolved spectroscopy. The blue octachromophoric systems have been recently shown to self-assemble as supramolecular tetramers in lipid bilayer membranes and to enable generation of a transmembrane proton gradient upon photoexcitation (Bhosale, S.; Sisson, A. L.; Talukdar, P.; Fürstenberg, A.; Banerji, N.; Vauthey, E.; Bollot, G.; Mareda, J.; Röger, C.; Würthner, F.; Sakai, N.; Matile, S. Science2006, 313, 84). A strong reduction of the fluorescence quantum yield was observed when going from the single NDI units to the multichromophoric systems in methanol, the effect being even stronger in a vesicular lipid membrane. Fluorescence up-conversion measurements reveal ultrafast self-quenching in the multichromophoric systems, whereas the formation of the NDI radical anion, evidenced by transient absorption measurements, points to the occurrence of photoinduced charge separation. The location of the positive charge could not be established unambiguously from the transient absorption measurements, but energetic considerations indicate that charge separation should occur between two NDI units in the blue systems, whereas both an NDI unit and the p-octiphenyl scaffold could act as electron donor in the red system. The lifetime of the charge-separated state was found to increase from 22 to 45 ps by going from the bi- to the octachromophoric blue systems in methanol, while a 400 ps decay component was observed in the lipid membrane. This lifetime lengthening is explained in terms of charge migration that is most efficient when the octachromophoric systems are assembled as supramolecular tetramers in the lipid membrane. Furthermore, the average charge-separated state lifetime of the red system in methanol is even larger and amounts to 750 ps. This effect cannot be simply explained in terms of Marcus inverted regime as the driving force for charge recombination in the red system is only slightly larger than in the blue one. A better spatial separation of the charges in the red system stemming from the localization of the hole on the p-octiphenyl scaffold could additionally contribute to the slowing down of charge recombination. | ||||||||
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| The mononuclear Os II complex [Os( L1) 3 ](PF 6 ) 2 ( L1 = 5-methyl(1-methylbenzimidazol-2-yl)pyridine) is an obvious candidate for the design of an inert d-block-based tripodal receptor capable of binding and photosensitizing trivalent lanthanides (Ln III ). It has thus been prepared and its two enantiomeric meridional (Δ-mer and Λ-mer) and facial (rac-fac) isomers have been separated by ion-exchange chromatography. The optical isomers have been characterized by CD spectroscopy and assignments of absolute configuration confirmed by an X-ray crystallographic study of Λ-mer-[Os( L1) 3 ](PF 6 ) 2 ·1.5MeCN (monoclinic, P2 1 , Z = 4). Comparison of the latter structure with that of racemic fac-[Os( L1) 3 ](PF 6 ) 2 (monoclinic, C2/c, Z = 8) and [Os(bipy) 3 ](PF 6 ) 2 (where bipy = 2,2' -bipyridine) shows minimal structural variations, but differences are observed in the photophysical and electrochemical properties of the respective compounds. Luminescence emissions from Os II complexes of L1 are typically lower in energy, with shorter lifetimes and lower quantum yields than their bipy analogues, whilst metal-centred oxidation processes are more facile due to the enhanced π-donor ability of L1. The key relationships between these parameters are discussed. Finally, though challenged by (i) the low reactivity of many osmium precursors and (ii) the irreversible formation of competing side products, the synthesis and purification of the heterobimetallic triple-stranded helicate HHH-[OsLu( L2) 3 ](CF 3 SO 3 ) 5 has been realised, in which L2 is a segmental ligand containing the same bidentate unit as that found in L1 further connected to a tridentate binding site adapted for complexing Ln III . Its solid-state structure has been established by X-ray crystallography (triclinic, P1-, Z = 2). | ||||||||
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| We report a comprehensive THz, infrared and optical study of Nb-doped SrTiO3 as well as dc conductivity and Hall effect measurements. Our THz spectra at 7 K show the presence of an unusually narrow (<2meV) Drude peak. For all carrier concentrations the Drude spectral weight shows a factor of three mass enhancement relative to the effective mass in the local density approximation, whereas the spectral weight contained in the incoherent midinfrared response indicates that the mass enhancement is at least a factor two. We find no evidence of a particularly large electron-phonon coupling that would result in small polaron formation. | ||||||||
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| We describe the preparation of a helicate containing four closely spaced, linearly arrayed copper(I) ions. This product may be prepared either directly by mixing copper(I) with a set of precursor amine and aldehyde subcomponents, or indirectly through the dimerization of a dicopper(I) helicate upon addition of 1,2-phenylenediamine. A notable feature of this helicate is that its length is not limited by the lengths of its precursor subcomponents: each of the two ligands wrapped around the four copper(I) centers contains one diamine, two dialdehyde, and two monoamine residues. This work thus paves the way for the preparation of longer oligo- and polymeric structures. DFT calculations and electrochemical measurements indicate a high degree of electronic delocalization among the metal ions forming the cores of the structures described herein, which may therefore be described as "molecular wires". | ||||||||
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| 77Se-enriched CpNi(bds) (bds = 1,2-benzenediselenolate), has been synthesized and its g tensor and 77Se hyperfine tensors have been obtained from its frozen solution electron paramagnetic resonance (EPR) spectrum. These parameters are consistent with those calculated by density functional theory (DFT); it is shown that 10% of the spin is localized on each selenium and that the direction associated to the maximum 77Se couplings is aligned along the g min direction, perpendicular to the Ni(bds) plane. EPR measurements and DFT calculations are also carried out on the 77Se enriched complex CpNi(dsit) as well on the two dithiolene analogues CpNi(bdt) and CpNi(dmit). The optimized structures of the isolated CpNi(bds) and CpNi(bdt) complexes have been used to generate the idealized dimers (bds)NiCp···CpNi(bds) and (bdt)NiCp···CpNi(bdt) characterized by Cp···Cp overlap. The exchange parameters J calculated at the DFT level for these systems are in reasonable accord with the experimental values. The influence of the geometry of the dimer on its magnetic properties is assessed by calculating the variation of J as a function of the relative orientation of the two Ni(diselenolene) or Ni(dithiolene) planes. | ||||||||
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| A large set of electronic states of scandium dimer has been calculated using high-level theoretical methods such as quantum diffusion Monte Carlo (DMC), complete active space perturbation theory as implemented in GAMESS-US, coupled-cluster singles, doubles, and triples, and density functional theory (DFT). The3Σu and 5Σu states are calculated to be close in energy in all cases, but whereas DFT predicts the 5Σu state to be the ground state by 0.08 eV, DMC and CASPT2 calculations predict the 3Σu to be more stable by 0.17 and 0.16 eV, respectively. The experimental data available are in agreement with the calculated frequencies and dissociation energies of both states, and therefore we conclude that the correct ground state of scandium dimer is the 3Σu state, which breaks with the assumption of a 5Σu ground state for scandium dimer, believed throughout the past decades. | ||||||||
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| Excitation energy migration is an important phenomenon at high concentration of luminescent chromophores. In crystalline solids it results in a quenching of the intrinsic luminescence of the chromophore as the excitation energy migrates to impurity centres or other forms of trap sites. As concluded from the extensively studied systems where Cr3+ is doped as the active chromophore into inert host lattices, energy migration in crystalline solids is usually a phonon-assisted process, in which the simultaneous creation or annihilation of phonons helps to bridge the energy miss-match in the energy levels of two neighbouring chromophores within a inhomogeneously broadened absorption band. However, in the three-dimensional network systems [Ru(bpy)3][NaCr(ox)3] and [Rh(bpy)3][NaCr(ox)3]ClO4, it proved possible to unambiguously identify three different mechanisms for energy migration within the R1 line of the 4A2 →2E transition of Cr3+. In addition to the common temperature dependant phonon-assisted process, a resonant process between the zero-field split components of the 4A2 ground state leading to a multi-line pattern in a fluorescence line narrowing spectrum and a quasi-resonant process within the same component leading to fast spectral diffusion can be identified at very low temperature. The parameters governing these processes are discussed and the behaviour of the model systems is compared to more conventional doped oxides and related systems. | ||||||||
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| The thermal and the light-induced spin transition in [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)) as well as the high-spin → low-spin relaxation following the light-induced population of the high-spin state below the thermal transition temperature are discussed in relation to the accompanying crystallographic phase transition. The experimental data have exclusively been obtained using optical single crystal absorption spectroscopy. | ||||||||
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| The spin-crossover compound [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin→low-spin transition on cooling as well as the relaxation kinetics just below T c ↓ depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process. | ||||||||
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| Supramolecular 3D organization on gold with interdigitating intra- and interlayer recognition motifs (see picure, blackp-oligophenyl rods; red, blue naphthalenediimide (NDI) stacks) is designed to access supramolecular cascade n/p-heterojunctions or the adaptable directionality needed to control fill factors in current-voltage curves. | ||||||||
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| A multireference second-order perturbation theory using a restricted active space self-consistent field wave function as reference (RASPT2/RASSCF) is described. This model is particularly effective for cases where a chemical system requires a balanced orbital active space that is too large to be addressed by the complete active space self-consistent field model with or without second-order perturbation theory (CASPT2 or CASSCF, respectively). Rather than permitting all possible electronic configurations of the electrons in the active space to appear in the reference wave function, certain orbitals are sequestered into two subspaces that permit a maximum number of occupations or holes, respectively, in any given configuration, thereby reducing the total number of possible configurations. Subsequent second-order perturbation theory captures additional dynamical correlation effects. Applications of the theory to the electronic structure of complexes involved in the activation of molecular oxygen by mono- and binuclear copper complexes are presented. In the mononuclear case, RASPT2 and CASPT2 provide very similar results. In the binuclear cases, however, only RASPT2 proves quantitatively useful, owing to the very large size of the necessary active space. | ||||||||
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| The relative energies of side-on versus end-on binding of molecular oxygen to a supported Cu(I) species, and the singlet versus triplet nature of the ground electronic state, are sensitive to the nature of the supporting ligands and, in particular, depend upon their geometric arrangement relative to the O2 binding site. Highly correlated ab initio and density functional theory electronic structure calculations demonstrate that optimal overlap (and oxidative charge transfer) occurs for the side-on geometry, and this is promoted by ligands that raise the energy, thereby enhancing resonance, of the filled Cu d xz orbital that hybridizes with the in-plane π* orbital of O2. Conversely, ligands that raise the energy of the filled Cu d z 2 orbital foster a preference for end-on binding as this is the only mode that permits good overlap with the in-plane O2 π*. Because the overlap of Cu d z 2 with O2 π* is reduced as compared to the overlap of Cu d xz with the same O2 orbital, the resonance is also reduced, leading to generally more stable triplet states relative to singlets in the end-on geometry as compared to the side-on geometry, where singlet ground states become more easily accessible once ligands are stronger donors. Biradical Cu(II)-O2 superoxide character in the electronic structure of the supported complexes leads to significant challenges for accurate quantum chemical calculations that are best addressed by exploiting the spin-purified M06L local density functional, single-reference completely renormalized coupled-cluster theory, or multireference second-order perturbation theory, all of which provide predictions that are qualitatively and quantitatively consistent with one another. | ||||||||
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| The new bis(pentalene) complex Cr2(η5:η5-C8H4 1,4-Sii Pr3 )2 has been synthesized and characterized; it is found to exhibit paramagnetism at room temperature, and solid-state magnetic studies show that the dimer is best modeled as containing a pair of antiferromagnetically interacting S = ½ centers with the separation between the singlet ground state and triplet excited state being 2.23 kJ mol−1. Structural data show a Cr−Cr distance of 2.2514(15) Å, consistent with a strong metal−metal interaction. The bonding has been further investigated by density functional, hybrid, and CASPT2 methods. The metal−metal interaction is best described by a double bond with each metal having an 18-electron count. Theory predicts the singlet and triplet states to lie close in energy but puts the triplet state at a slightly lower energy than the singlet. The energy difference predicted by CASPT2 is closest to the experimental value. | ||||||||
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| A new type of stable radical ligand featuring a 1,1-bis-phosphinosulfide alkene backbone has been prepared and characterized on the basis of X-ray diffraction, EPR and DFT studies. | ||||||||
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| Laser-ablated Th atoms react with molecular hydrogen to give thorium hydrides and their dihydrogen complexes during condensation in excess neon and hydrogen for characterization by matrix infrared spectroscopy. The ThH2, ThH4, and ThH4(H2) x (x = 1−4) product molecules have been identified through isotopic substitution (HD, D2) and comparison to frequencies calculated by density functional theory and the coupled-cluster, singles, doubles (CCSD) method and those observed previously in solid argon. Theoretical calculations show that the Th−H bond in ThH4 is the most polarized of group 4 and uranium metal tetrahydrides, and as a result, a strong attractive "dihydrogen" interaction was found between the oppositely charged hydride and H2 ligands ThH4(H2) x . This bridge-bonded dihydrogen complex structure is different from that recently computed for tungsten and uranium hydride super dihydrogen complexes but is similar to that recently called the "dihydrogen bond" (Crabtree, R. H. Science 1998, 282, 2000). Natural electron configurations show small charge flow from the Th center to the dihydrogen ligands. | ||||||||
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| The codeposition of laser-ablated tungsten atoms with neat hydrogen at 4 K forms a single major product with a broad 2500 cm- 1 and sharp 1860, 1830, 1782, 1008, 551, and 437 cm- 1 absorptions, which are assigned to the WH4(H2)4 complex on the basis of isotopic shifts and agreement with isotopic frequencies calculated by density functional theory. This D 2 d structured complex was computed earlier to form exothermically from W atoms and hydrogen molecules. Annealing the matrix allows hydrogen to evaporate and the complex to aggregate and ultimately to decompose. Comparison of the H−H stretching mode at 2500 cm- 1 and the W−H2 stretching mode at 1782 cm- 1 with 2690 and 1570 cm- 1 values for the Kubas complex W(CO)3(PR3)2(H2) suggests that the present physically stable WH4(H2)4 complex has more strongly bound dihydrogen ligands. Our CASPT2 calculations suggest a 15 kcal/mol average binding energy per dihydrogen molecule in the WH4(H2)4 complex. | ||||||||
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| Two new ethynylbipyridine-linked mono- and bis-tetrathiafulvalene (TTF) derivatives, together with a Ru(II) complex, were synthesized using Sonogashira coupling reactions and characterized by UV/vis spectroscopy and cyclic voltammetry. They display a clear electrochemically amphoteric behavior consisting of two reversible single-electron oxidation waves (typical for TTF derivatives) and one reversible single-electron reduction wave (bpy) and act as donor–acceptor (D–A) systems. Furthermore, for the Ru(II) complex, a quite intense fluorescence originating from the 3MLCT state is observed. | ||||||||
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| Values ofσ and σ +, for use in linear free energy relationships, are determined for para hydrogen atoms having nuclear charges other than 1 (nucleomers). Hammett ρ values for a variety of free energies of activation, reaction, and other extrathermodynamic properties (e.g., vibrational frequencies) are computed therefrom and compared to those computed using typical para functional groups. The nucleomer correlations show excellent qualitative agreement with standard correlations but the quantitative agreement is less good, typically underestimating the standard ρ-value by 10-60%. | ||||||||
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| Macrophage infectivity potentiator (MIP) was originally reported to be a chlamydial lipoprotein from experiments showing incorporation of radiolabeled palmitic acid into native and recombinant MIP; inhibition of posttranslational processing of recombinant MIP by globomycin, known to inhibit signal peptidase II; and solubility of native MIP in Triton X-114. However, the detailed structural characterization of the lipid moiety on MIP has never been fully elucidated. In this study, bioinformatics and mass spectrometry analysis, as well as radiolabeling and immunochemical experiments, were conducted to further characterize MIP structure and subcellular localization. In silico analysis showed that the amino acid sequence of MIP is conserved across chlamydial species. A potential signal sequence with a contained lipobox was identified, and a recombinant C20A variant was prepared by replacing the probable lipobox cysteine with an alanine. Both incorporation of U-14C-esterified glycerol and [U-14C]palmitic acid and posttranslational processing that was inhibitable by globomycin were observed for recombinant wild-type MIP but not for the recombinant C20A MIP variant. The fatty acid contents of native and recombinant MIP were analyzed by gas chromatography-mass spectrometry, and the presence of amide-linked fatty acids in recombinant MIP was investigated by alkaline methanolysis. These results demonstrated a lipid modification in MIP similar to that of other prokaryotic lipoproteins. In addition, MIP was detected in an outer membrane preparation of Chlamydia trachomatis elementary bodies and was shown to be present at the surfaces of elementary bodies by surface biotinylation and surface immunoprecipitation experiments. | ||||||||
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| A new barium silico-aluminate phase with the stoichiometry Ba13.35(1) Al30.7 Si5.3 O70 has been found and characterized. The compound crystallizes in the space group P63 /m (No. 176) with a = 15.1683(17) Å, c = 8.8708(6) Å, V = 1767.5(4) Å3 , Z = 1, Rw = 0.026, 32 refined parameters. A 3-dimensional matrix of Al/SiO4 tetrahedra with Ba(II) ions located in channels along the c axis builds up the structure. One of these channels is partially filled with Ba(II) ions (CN 6+3) in Wyckoff position 2a, leaving ∼ 1/3 of the positions empty. The second and third type of Ba(II) ions occupy channels orientated along the c axis with CN 4+2+2 and 4+3+1, respectively. The structure shows a rare clustered arrangement of six tetrahedra filled exclusively by Al(III) and therefore is an exception to Loewenstein's rule. The other tetrahedral positions show an Al to Si ratio of ∼ 4 : 1. The Al/Si–O bond lengths in the tetrahedral Al/Si positions drawn vs. site occupation show linear behavior similar to the prediction by Vegard's rule for solid solutions. After doping with Eu(II) the compound shows bright orange-yellow luminescence with an unusual large shift of the Eu(II) emission band. | ||||||||
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| The creation of long-lived charge-separated states in donor-acceptor assemblies has been the goal of many studies aimed at mimicking the primary processes in photosynthesis. Here we present such assemblies based on tetrathiafulvalene (TTF) as electron donor and a dipyridophenazine (dppz) unit as electron acceptor in the form of a fused ligand (TTF-dppz) coordinated to ruthenium(II) via the dipyrido coordination site and with 2,2′-bipyridine (bpy) as auxiliary ligand, namely [Ru(bpy)3−x(TTF-dppz)x]2+ (x = 1−3). For x = 2, irradiation into the metal to dppz charge transfer transition results in electron transfer from TTF to ruthenium, thus creating a charge-separated state best described by [(TTF+-dppz)Ru(dppz−-TTF)(bpy)]2+ with a lifetime of 2.5 μs in dichloromethane. | ||||||||
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| We discuss and illustrate by several examples how the ultrafast excited-state dynamics of a chromophore can be altered when changing its environment from a homogenous solution to a biological molecule such as proteins or nucleic acids. | ||||||||
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| The excited-state dynamics of the DNA intercalator YO-PRO-1 and of three derivatives has been investigated in water and in DNA using ultrafast fluorescence spectroscopy. In the free form, the singly charged dyes exist both as monomers and as H-dimers, while the doubly charged dyes exist predominantly as monomers. Both forms are very weakly fluorescent: the monomers because of ultrafast nonradiative deactivation, with a time constant on the order of 3−4 ps, associated with large amplitude motion around the methine bridge, and the H-dimers because of excitonic interaction. Upon intercalation into DNA, large amplitude motion is inhibited, H-dimers are disrupted, and the molecules become highly fluorescent. The early fluorescence dynamics of these dyes in DNA exhibits substantial differences compared with that measured with their homodimeric YOYO analogues, which are ascribed to dissimilarities in their local environment. Finally, the decay of the fluorescence polarization anisotropy reveals ultrafast hopping of the excitation energy between the intercalated dyes. In one case, a marked change of the depolarization dynamics upon increasing the dye concentration is observed and explained in terms of a different binding mode. | ||||||||
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| The topology of the ground-state potential energy surface of M(CN)6 with orbitally degenerate 2T2g (M = TiIII (t2g 1), FeIII and MnII (both low-spin t2g 5)) and 3T1g ground states (M = VIII (t2g 2), MnIII and CrII (both low-spin t2g 4)) has been studied with linear and quadratic Jahn−Teller coupling models in the five-dimensional space of the εg and τ2g octahedral vibrations (Tg⊗(εg+τ2g) Jahn−Teller coupling problem (Tg = 2T2g, 3T1g)). A procedure is proposed to give access to all vibronic coupling parameters from geometry optimization with density functional theory (DFT) and the energies of a restricted number of Slater determinants, derived from electron replacements within the t2g 1,5 or t2g 2,4 ground-state electronic configurations. The results show that coupling to the τ2g bending mode is dominant and leads to a stabilization of D 3 d structures (absolute minima on the ground-state potential energy surface) for all complexes considered, except for [Ti(CN)6]3-, where the minimum is of D 4 h symmetry. The Jahn−Teller stabilization energies for the D 3 d minima are found to increase in the order of increasing CN−M π back-donation (TiIII < VIII < MnIII < FeIII < MnII < CrII). With the angular overlap model and bonding parameters derived from angular distortions, which correspond to the stable D 3 d minima, the effect of configuration interaction and spin−orbit coupling on the ground-state potential energy surface is explored. This approach is used to correlate Jahn−Teller distortion parameters with structures from X-ray diffraction data. Jahn−Teller coupling to trigonal modes is also used to reinterpret the anisotropy of magnetic susceptibilities and g tensors of [Fe(CN)6]3-, and the 3T1g ground-state splitting of [Mn(CN)6]3-, deduced from near-IR spectra. The implications of the pseudo Jahn−Teller coupling due to t2g−eg orbital mixing via the trigonal modes (τ2g) and the effect of the dynamic Jahn−Teller coupling on the magnetic susceptibilities and g tensors of [Fe(CN)6]3- are also addressed. | ||||||||
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| The spin transition of the [Co(terpy)2]2+ complex (terpy = 2,2′:6′,2″-terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy)2](ClO4)2 shows an asymmetric absorption band at 14 400 cm−1 with an intensity typical for a spin-allowed d–d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the high-spin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, high-spin → low-spin relaxation curves following pulsed laser excitation for [Co(terpy)2](ClO4)2 dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone)2](ClO4)2 spin-crossover compound are given. | ||||||||
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| An unsymmetric, peripherally octasubstituted phthalocyanine (Pc) 1, which contains a combination of dipyrido[3,2-f:2',3'-h] quinoxaline and 3,5-di-tert-butylphenoxy substituents, has been obtained via a statistical condensation reaction of two corresponding phthalonitriles. Synthetic procedures for the selective metalation of the macrocyclic cavity and the periphery of 1 were developed, leading to the preparation of the key precursor metallophthalocyanines 3 − 5 in good yields. Two different strategies were applied to the synthesis of compact dyads MPc−Ru(II) 6 − 8 (M = Mg(II), Co(II), Zn(II)). Intramolecular electronic interactions in these dyads were studied by absorption, emission, and transient absorption spectroscopy. Upon photoexcitation, these dyads exhibit efficient intramolecular energy transfer from the Ru(II) chromophore to the MPc moiety. | ||||||||
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| The vibrational spectra of UBz and ThBz have been measured in solid argon. Complementary quantum chemical calculations have allowed the assignments of the vibrational spectra. According to the calculations, AcBz are stable molecules, as well as other species like BzAcBz and BzAc2Bz. Experimentally, there is no evidence for the sandwich compounds BzAcBz and BzAc2Bz due to the limitations in the reagent concentrations. | ||||||||
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| Rearrangement of cholesta-2,4,6-triene in the presence of p-toluenesulfonic acid in acetic acid at 70 °C leads to 4-methyl-19-nor-cholesta-1,3,5(10)-triene and 1(10 → 6)-abeo-14β-cholesta-5,7,9(10)-triene in less than 2 h. Postulated mechanisms of formation of these products are supported by molecular mechanics calculations of the relative stabilities of reaction intermediates. The results suggest that Δ5,7-sterols, the most common natural precursors of triunsaturated steroidal hydrocarbons in contemporary sediments, constitute another major source for monoaromatic A and B steroids in addition to Δ5-sterols. | ||||||||
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| Three ruthenium(II) polypyridine complexes of general formula [Ru(bpy)3- n (TTF-dppz) n ](PF6)2 (n=1-3, bpy=2,2'-bipyridine), with one, two or three redox-active TTF-dppz (4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2',3'-c]phenazine) ligands, were synthesised and fully characterised. Their electrochemical and photophysical properties are reported together with those of the reference compounds [Ru(bpy)3](PF6)2, [Ru(dppz)3](PF6)2 and [Ru(bpy)2(dppz)](PF6)2 and the free TTF-dppz ligand. All three complexes show intraligand charge-transfer (ILCT) fluorescence of the TTF-dppz ligand. Remarkably, the complex with n=1 exhibits luminescence from the Ru2+dppz metal-to-ligand charge-transfer (3MLCT) state, whereas for the other two complexes, a radiationless pathway via electron transfer from a second TTF-dppz ligand quenches the 3MLCT luminescence. The TTF fragments as electron donors thus induce a ligand-to-ligand charge-separated (LLCS) state of the form TTF-dppz--Ru2+→ -dppz-TTF+. The lifetime of this LLCS state is approximately 2.3 μs, which is four orders of magnitude longer than that of 0.4 ns for the ILCT state, because recombination of charges on two different ligands is substantially slower. | ||||||||
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| Design, synthesis and evaluation of advanced rigid-rod π-stack photosystems with asymmetric scaffolds are reported. The influence of push–pull rods on self-organization, photoinduced charge separation and photosynthetic activity is investigated and turns out to be surprisingly small overall. | ||||||||
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| The fluorescence enhancement mechanisms of a series of DNA stains of the oxazole yellow (YO) family have been investigated in detail using steady-state and ultrafast time-resolved fluorescence spectroscopy. The strong increase in the fluorescence quantum yield of these dyes upon DNA binding is shown to originate from the inhibition of two distinct processes: 1) isomerisation through large-amplitude motion that non-radiatively deactivates the excited state within a few picoseconds and 2) formation of weakly emitting H-dimers. As the H-dimers are not totally non-fluorescent, their formation is less efficient than isomerisation as a fluorescent contrast mechanism. The propensity of the dyes to form H-dimers and thus to reduce their fluorescence contrast upon DNA binding is shown to depend on several of their structural parameters, such as their monomeric (YO) or homodimeric (YOYO) nature, their substitution and their electric charge. Moreover, these parameters also have a substantial influence on the affinity of the dyes for DNA and on the ensuing sensitivity for DNA detection. The results give new insight into the development and optimisation of fluorescent DNA probes with the highest contrast. | ||||||||
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| The synthesis and characterization of two ortho-dimethyltetrathiafulvalene (o-DMTTF)-based rigid dimers containing dimethylsilicon (Me2Si) or dimethylgermanium (Me2Ge) linkers are described. Single-crystal X-ray analysis reveals planar geometry for the central 1,4-disilicon or 1,4-digermanium six-membered rings. DFT calculations provide optimized conformations in agreement with the experimental ones, and also emphasize the role of the heteroatomic linkers in the conjugation between the two redox active units. Cyclic voltammetry measurements show sequential oxidation into radical cation, and then dication species. Solution EPR measurements on the radical-cation species indicate full delocalization of the unpaired electron over both electroactive TTF units, with an associated coupling of 0.42 G with twelve equivalent protons. DFT calculations afford fully planar geometry for the radical-cation species and confirm the experimental isotropic coupling constant. Single-crystal X-ray analyses of two charge-transfer compounds obtained upon chemical oxidation, formulated as [(Me2Si)2(o-DMTTF)2]-1/2[TCNQ]·1/2[TCNQF4] and [(Me2Ge)2(o-DMTTF)2]·[TCNQ], demonstrate the occurrence of genuine mixed-valence radical-cation species, as well as a three-dimensional network of short S···S intermolecular contacts. Temperature-dependent conductivity measurements demonstrate semiconducting behavior for both charge-transfer compounds, with an increase of the absolute value of the conductivity upon applying external pressure. Band structure calculations reveal peculiar pseudo-two-dimensional electronic structures, also confirming electronic interactions through SiMe2 and GeMe2 bridges. | ||||||||
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| The incorporation of enantiopure 1-amino-2,3-propanediol as a subcomponent into a dicopper double helicate resulted in perfect chiral induction of the helicate's twist. DFT calculations allowed the determination of the helicity of the complex in solution. The same helical induction, in which S amines induced a Λ helical twist, was observed in the solid state by X-ray crystallography. Electronic structure calculations also revealed that the unusual deep green color of this class of complexes was due to a metal-to-ligand charge transfer excitation, in which the excited state possesses a valence delocalized Cu2 3+ core. The use of a racemic amine subcomponent resulted in the formation of a dynamic library of six diastereomeric pairs of enantiomers. Surprisingly, this library converted into a single pair of enantiomers during crystallization. We were able to observe this process reverse upon redissolution, as initial ligand exchange was followed by covalent imine metathesis. | ||||||||
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| We describe an advanced setup for time-resolved x-ray absorption fine structure (XAFS) Spectroscopy with picosecond temporal resolution. It combines an intense femtosecond laser source synchronized to the x-ray pulses delivered into the microXAS beamline of the Swiss Light Source (SLS). The setup is applied to measure the short-lived high-spin geometric structure of photoexcited aqueous Fe(bpy)3 at room temperature. | ||||||||
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| To study the electronic interactions in donor-acceptor (D-A) ensembles, D and A fragments are coupled in a single molecule. Specifically, a tetrathiafulvalene (TTF)-fused dipyrido[3,2-a:2',3'-c]phenazine (dppz) compound having inherent redox centers has been synthesized and structurally characterized. Its electronic absorption, fluorescence emission, photoinduced intramolecular charge transfer, and electrochemical behavior have been investigated. The observed electronic properties are explained on the basis of density functional theory. | ||||||||
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| The fluorine superhyperfine (shf) tensor measured in aFCl:La2+has been found to be practically isotropic, a result which is certainly anomalous when compared to that for em>d9centers with one unpaired electron in a em>x2−y 2orbital. This puzzling fact has been explored by means of density functional calculations. Obtained results confirm that in the em>C4v equilibrium geometry the unpaired electron lies in a em>b1(∼x 2−y 2)orbital which overlaps with the sorbitals of four −ligands. For explaining the origin of the near isotropy, which is well reproduced by the present calculations, the simple em>D4h and i>C4v aF4 2− F4 2− and gF4 2−centers have also been investigated. Although the obtained results stress the high dependence of the isotropic shf constant i>A s on the metal-ligand distance i>R a near isotropy of the shf tensor is only reached for aF4 2−(but not for F4 2− under i>C4v symmetry which corresponds to the actual symmetry of the a2+center in the BaFCl lattice. The origin of this peculiar situation is shown to come from the mixing between dand forbitals of a2+allowed in i>C4v symmetry thus stressing the role played by forbitals in bonding properties. Writing i>A s =C R −n s it is shown that for the i>D4h aF4 2−and F4 2−complexes the exponent i>n s is around 20, while it is only equal to 4 for gF4 2− This huge difference is shown to stem from the quite distinct slope of the radial i>dwave function at the equilibrium distance for the two i>d1centers and the i>d9gF4 2−unit. Finally, the present calculations strongly support that the intense band peaked at 7 890cm −1recorded in the optical absorption spectrum of aFCl:La2+is indeed a d→4ftransition. | ||||||||
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| Several monouranium and diuranium polyhydride molecules were investigated using quantum chemical methods. The infrared spectra of uranium and hydrogen reaction products in condensed neon and pure hydrogen were measured and compared with previous argon matrix frequencies. The calculated molecular structures and vibrational frequencies were used to identify the species present in the matrix. Major new absorptions were observed and compared with the previous argon matrix study. Spectroscopic evidence was obtained for the novel complex, UH4(H2)6, which has potential interest as a metal hydride with a large number of hydrogen atoms bound to uranium. Our calculations show that the series of complexes UH4(H2)1,2,4,6 are stable. | ||||||||
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| The opposite orientation of the ester spacers in the rodlike ligands L 4 C12 (benzimidazole-OOC-phenyl) and L 5 C12 (benzimidazole-COO-phenyl) drastically changes the electronic structure of the aromatic systems, without affecting their meridional tricoordination to trivalent lanthanides, LnIII, and their thermotropic liquid crystalline (i.e., mesomorphic) behaviors. However, the rich mesomorphism exhibited by the complexes [Ln(L 4 C12)(NO3)3] (Ln=La-Lu) vanishes in [Ln(L 5 C12)(NO3)3], despite superimposable molecular structures and comparable photophysical properties. Density functional theory (DFT) and time-dependant DFT calculations performed in the gas phase show that the inversion of the ester spacers has considerable effects on the electronic structure and polarization of the aromatic groups along the strands, which control residual intermolecular interactions responsible for the formation of thermotropic liquid-crystalline phases. As a rule of thumb, an alternation of electron-poor and electron-rich aromatic rings favors intermolecular interactions between the rigid cores and consequently mesomorphism, a situation encountered for L 4 C12, L 5 C12, [Ln(L 4 C12)(NO3)3], but not for [Ln(L 5 C12)(NO3)3]. The intercalation of an additional electron-rich diphenol ring on going from [Ln(L 5 C12)(NO3)3] to [Ln(L 6 C12)(NO3)3] restores mesomorphism despite an unfavorable orientation of the ester spacers, in agreement with our simple predictive model. | ||||||||
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| We report an in-depth theoretical study of 4-styrylpyridine in its singlet S0 ground state. The geometries and the relative stabilities of the trans and cis isomers were investigated within density functional theory (DFT) as well as within Hartree-Fock (HF), second-order Møller-Plesset (MP2), and coupled cluster (CC) theories. The DFT calculations were performed using the B3LYP and PBE functionals, with basis sets of different qualities, and gave results that are very consistent with each other. The molecular structure is thus predicted to be planar at the energy minimum, which is associated with the trans conformation, and to become markedly twisted at the minimum of higher energy, which is associated with the cis conformation. The results of the calculations performed with the post-HF methods approach those obtained with the DFT methods, provided that the level of treatment of the electronic correlation is high enough and that sufficiently flexible basis sets are used. Calculations carried out within DFT also allowed the determination of the geometry and the energy of the molecule at the biradicaloid transition state associated with the thermal cis ↔trans isomerization and at the transition states associated with the enantiomerization of the cis isomer and with the rotations of the pyridinyl and phenyl groups in the trans and cis isomers. Car-Parrinello molecular dynamics simulations were also performed at 50, 150, and 300 K using the PBE functional. The studies allowed us to evidence the highly flexible nature of the molecule in both conformations. In particular, the trans isomer was found to exist mainly in a nonplanar form at finite temperatures, while the rotation of the pyridinyl ring in the cis isomer was incidentally observed to take place within ≈1 ps during the simulation carried out at 150 K on this isomer. | ||||||||
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| The ground and excited electronic state properties of calicene (triapentafulvalene or 5-(cycloprop-2-en-1-ylidene)cyclopenta-1,3-diene) have been studied with a variety of density functional models (mPWPW91, PBE, TPSS, TPSh, B3LYP) and post-Hartree−Fock models based on single (MP2 and CCSD(T)) and multideterminantal (CASPT2) reference wave functions. All methods agree well on the properties of ground-state calicene, which is described as a conjugated double bond system with substantial zwitterionic character deriving from a charge-separated mesomer in which the three- and five-membered rings are both aromatic. Although the two rings are joined by a formal double bond, contributions from the aromatic mesomer reduce its bond order substantially. A rotational barrier of 40−41 kcal mol-1 is predicted in the gas phase and solvation effects reduce the barrier to 37 and 33 kcal mol-1 in benzene and water, respectively, because of increased zwitterionic character in the twisted transition-state structure. Multi-state CASPT2 (MS-CASPT2) is used to characterize the first few excited singlet and triplet states and indicates that the most important transition occurs at 4.93 eV (251 nm). A cis − trans photoisomerization about the inter-ring double bond is found to be inefficient. | ||||||||
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| Structural changes of the iron(II)-tris-bipyridine ([FeII(bpy)3]2+) complex induced by ultrashort pulse excitation and population of its short-lived (≤0.6ns) quintet high spin state have been detected by picosecond x-ray absorption spectroscopy. The structural relaxation from the high spin to the low spin state was followed over the entire lifetime of the excited state. A combined analysis of the x-ray-absorption near-edge structure and extended x-ray-absorption fine structure spectroscopy features delivers an Fe-N bond elongation of 0.2 Å in the quintet state compared to the singlet ground state. | ||||||||
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| We present novel insight on like-spin domains (LSD) in cooperative spin transition solids by following the photo-transformation and the subsequent relaxation of a [Fe(ptz)6](BF4)2 single crystal in the vicinity of the light-induced instability. Self-organization under light is observed, accompanied by Barkhausen-like noise and jumps which reveal the presence of elastic interactions between LSDs. The light-induced phase separation process is discussed in terms of a dynamic potential providing spinodal instability in the corresponding temperature range. This useful concept is applicable to all types of switchable molecular solids. | ||||||||
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| The endohedral fullerene CH4@C84 has been studied using density functional theory (DFT) and second-order Møller-Plesset perturbation theory (MP2). In addition to the structure with a C— bond of CH4 in a tetrahedral pocket conformation, we find an alternative minimum, very close in energy (6.3-9.5 kJ/mol higher according to the level of theory), with the methane inverted, which we call the antipocket conformation. Computed IR spectra are reported for CH4@C84 and also for the reference system CH4@C60. The calculated vibrational levels, in a harmonic approximation, reveal close-lying translational, librational, and shell-vibrational modes. The results are also presented for the isoelectronic species NH4 +@C60. | ||||||||
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| Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy. | ||||||||
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| Optically active liquid-crystalline side-chain polysiloxanes have been prepared by grafting planar chiral ferrocene-based vinyl monomers onto commercially available polyhydrosiloxane. Two ferrocene monomers have been synthesized: a linear-type monomer, which displays a monotropic chiral smectic C (SC*) phase and enantiotropic smectic A (SA) and chiral N (N*) phases, and a laterally branched monomer, which shows an enantiotropic N* phase. X-ray diffraction analysis indicates a monomolecular organization of the monomeric units within the smectic layers. The polymers retain the liquid-crystalline phases of their corresponding monomers. The UV-vis and circular dichroism (CD) spectra are in agreement with the structure of the monomers and polymers. The molar absorption coefficient (ϵ) and molar circular dichroic absorption coefficient (Δϵ) values of the polymers are proportional to the number of monomeric units grafted onto them. The absolute configuration of the ferrocene carboxylic acid intermediate, used to synthesize the monomers, has been determined on the basis of CD spectra. The helical twisting power (HTP) of the nematogenic monomer and polymer have been determined in E7, and indicate that such materials could be used as chiral dopants. Finally, this study demonstrates that the nature of chiral phases can be controlled by structural engineering of the organic groups only, with ferrocene acting as the source of chirality. | ||||||||
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| The enantiomers of tert-butyl(dimethylamino)phenylphosphine−borane complex 2 have been separated by HPLC using cellulose tris-p-methylbenzoate as chiral stationary phase. The borane protection could be removed without racemization and the P-configuration of the free aminophosphine 1 has shown to be stable in solution. Infrared (IR) and vibrational circular dichroism (VCD) spectra have been measured in CD2Cl2 solution for both enantiomers. B3LYP/6-31+G(d) DFT calculations allowed a prediction that complex (S)-2 exists as three conformers in equilibrium and computed population-weighted IR and VCD spectra. Predicted and experimental IR and VCD spectra compared very well and indicate that enantiomer (+)-2 has the S absolute configuration. This assignment has been confirmed by an X-ray diffraction study on a single crystal of (+)-2. The crystal structure of enantiomerically pure 2 appears to be very close to the most stable computed conformer which proved to be predominant in solution. | ||||||||
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| Circular dichroism (CD) spectra and density functional theory (DFT) calculations are reported for a series of conformationally bistable chiroporphyrins with 8-methylene bridles MBCP-8, which can display either an αααα or an αβαβ orientation of their meso substituents. From DFT geometry optimizations, the most stable form of ZnBCP-8 is found to be the αααα conformer. By passing to NiBCP-8, there is a strong stabilization of the αβαβ conformation with respect to the αααα conformation, consistent with the X-ray structures of αααα-ZnBCP-8 and αβαβ-NiBCP-8. A correlation between the sign of the CD signal in the Soret region and the conformation of the BCP-8 compounds is reported: the αααα conformers H2BCP-8 and ZnBCP-8 show a positive CD signal, whereas the αβαβ conformers NiBCP-8 and CuBCP-8 exhibit a negative signal. The possible contributions to the rotational strengths of αβαβ-NiBCP-8 and αααα-ZnBCP-8, calculated on the basis of their crystal structures, have been analyzed. The CD signals are found to result from a combination of both the inherent chirality of the porphyrin and of extrinsic contributions due to the chiral bridles. These results may have a broad significance for understanding the chiroptical properties of chiral porphyrins and hemoproteins and for monitoring stimuli-responsive, conformationally bistable chiroporphyrin compounds. | ||||||||
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| About 3 µm thick tungsten trioxide film electrodes consisting of partly sintered, 40-80 nm in diameter, particles deposited on conducting glass substrates exhibit high photon-to-current conversion efficiencies for the photooxidation of water, exceeding 70% at 400 nm. This is facilitated by a ca. 40% film porosity resulting in high contact area with the electrolyte. It is shown that the activity of the WO3 electrodes towards photooxidation of water is enhanced by addition of even small amounts of halide (Cl-, Br-) ions to the acidic electrolyte. Photoelectrolysis experiments performed either in acidic electrolytes containing chloride or bromide anions or in a 0.5 M NaCl solution, under simulated 1.5 AM solar illumination, demonstrated long term stability of the photocurrents. Oxygen remains the main product of the photoanodic reaction even in a 0.5 M NaCl solution, a composition close to the sea water, with chlorine accounting for ca. 20% of current efficiency. | ||||||||
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| One-electron reduction of a diphosphafulvenium dication gives the first stable diphosphafulvenium monoradical cation (see scheme). An X-ray crystal structure analysis, EPR measurements, and DFT calculations clearly show that reduction takes place at the exocyclic double bond and that the excess of electron density is stabilized by the two electron-withdrawing phosphonium groups (see SOMO; P orange, C dark gray, H light gray). | ||||||||
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| A series of molybdenum and tungsten nitrido, [M(N)(X)(diphos)2], and imido complexes, [M(NH)(X)(diphos)2)]Y, (M = Mo, W) with diphosphine coligands (diphos = dppe/depe), various trans ligands (X = N3 -, Cl-, NCCH3) and different counterions (Y- = Cl-, BPh4 -) is investigated. These compounds are studied by infrared and Raman spectroscopies; they are also studied with isotope-substitution and optical-absorption, as well as emission, spectroscopies. In the nitrido complexes with trans-azido and -chloro coligands, the metal−N stretch is found at about 980 cm- 1; upon protonation, it is lowered to about 920 cm- 1. The 1A1 → 1E (n → π*) electronic transition is observed for [Mo(N)(N3)(depe)2] at 398 nm and shows a progression in the metal−N stretch of 810 cm- 1. The corresponding 3E → 1A (π* → n) emission band is observed at 542 nm, exhibiting a progression in the metal−N stretch of 980 cm- 1. In the imido system [Mo(NH)(N3)(depe)2]BPh4, the n → π* transition is shifted to lower energy (518 nm) and markedly decreases in intensity. In the trans-nitrile complex [Mo(N)(NCCH3)(dppe)2]BPh4, the metal−N(nitrido) stretching frequency increases to 1016 cm- 1. The n → π* transition now is found at 450 nm, shifting to 525 nm upon protonation. Most importantly, the reduction of this nitrido trans-nitrile complex is drastically facilitated compared to its counterparts with anionic trans-ligands (E p red = −1.5 V vs Fc+/Fc). On the other hand, the basicity of the nitrido group is decreased (pK a{[Mo(NH)(NCCH3)(dppe)2](BPh4)2} = 5). The implications of these findings with respect to the Chatt cycle are discussed. | ||||||||
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| State-of-the-art generalized gradient approximation (GGA) (PBE, OPBE, RPBE, OLYP, and HCTH), meta-GGA (VSXC and TPSS), and hybrid (B3LYP, B3LYP*, O3LYP, and PBE0) functionals are compared for the determination of the structure and the energetics of the D 3 [Co(bpy)3]2+ complex in the 4A2 and 4E trigonal components of the high-spin 4T1g( t5 2g e2 g ) state and in the low-spin 2E state of octahedral 2Eg( t6 2g e1 g) parentage. Their comparison extends also to the investigation of the Jahn−Teller instability of the 2E state through the characterization of the extrema of C 2 symmetry of this spin state's potential energy surface. The results obtained for [Co(bpy)3]2+ in either spin manifold are very consistent among the functionals used and are in good agreement with available experimental data. The functionals, however, perform very differently with respect to the spin-state energetics because the calculated values of the high-spin/low-spin energy difference Δ E el HL vary between −3212 and 3919 cm- 1. Semilocal functionals tend to give too large Δ E el HL values and thus fail to correctly predict the high-spin state as the ground state of the isolated complex, while hybrid functionals tend to overestimate the stability of the high-spin state with respect to the low-spin state. Reliable results are, however, obtained with the OLYP, HCTH, B3LYP*, and O3LYP functionals which perform best for the description of the isolated complex. The optical properties of [Co(bpy)3]2+ in the two spin states are also analyzed on the basis of electronic excitation calculations performed within time-dependent density functional response theory. The calculated absorption and circular dichroism spectra agree well with experimental results. | ||||||||
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| A new 1,3-dithiol-2-ylidene substituted naphthopyranone 2 has been synthesized and characterized. UV–vis spectroscopic and cyclic voltammetry results, interpreted on the basis of density functional theory, show that 2 displays an intramolecular charge-transfer transition and acts like a donor–acceptor (D–A) system. Furthermore, a weak fluorescence originating from the excited charge-transfer state is observed. | ||||||||
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| Vibrational spectra of BH4 - and its isotopic analogues in a crystalline environment of alkali metals cations (K+, Rb+, Cs+) have been investigated beyond the harmonic approximation using a variational approach supported by computations of B3LYP type anharmonic force fields. From the comparison of the observed and simulated IR spectra, the influence of the anharmonic couplings on the band position and on the relative intensity of the allowed vibrational transitions is discussed. Here, the effect of the crystalline environment induces a blue shift of about 50 and 100 cm-1 respectively for the bending and stretching modes of BH4 -. Furthermore, anharmonic effects, which are exclusively well reproduced by a variational approach, are needed to yield reliable positions and relative amplitudes of IR allowed combination and overtone transitions. This leads to theoretical results fitting their experimental counterpart between 6 and 30 cm-1 in the investigated series. | ||||||||
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| A series of 6-styryl-2,4-diphenylpyrylium salts exhibiting dual fluorescence has been investigated by fluorescence up-conversion in conjunction with quantum chemical calculations. The short-wavelength emission is due to an excited state localized on the pyrylium fragment and the long-wavelength emission arises from a charge-transfer state delocalized over the whole molecule. The two fluorescing states do not exhibit a precursor−successor relationship. The rise time of the short-wavelength fluorescence is smaller than 200 fs, and that of the long-wavelength emission depends on the electron-donating property of the styryl group substituent. The rise is almost prompt with the weaker donors but is slower, wavelength and viscosity dependent with the strongest electron-donating group. A model involving a S2/S1 conical intersection is proposed to account for these observations. | ||||||||
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| Rigid p-octiphenyl rods were used to create helical tetrameric π-stacks of blue, red-fluorescent naphthalene diimides that can span lipid bilayer membranes. In lipid vesicles containing quinone as electron acceptors and surrounded by ethylenediaminetetraacetic acid as hole acceptors, transmembrane proton gradients arose through quinone reduction upon excitation with visible light. Quantitative ultrafast and relatively long-lived charge separation was confirmed as the origin of photosynthetic activity by femtosecond fluorescence and transient absorption spectroscopy. Supramolecular self-organization was essential in that photoactivity was lost upon rod shortening (from p-octiphenyl to biphenyl) and chromophore expansion (from naphthalene diimide to perylene diimide). Ligand intercalation transformed the photoactive scaffolds into ion channels. | ||||||||
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| The excited-state dynamics of the DNA bisintercalator YOYO-1 and of two derivatives has been investigated using ultrafast fluorescence up-conversion and time-correlated single photon counting. The free dyes in water exist in two forms: nonaggregated dyes and intramolecular H-type aggregates, the latter form being only very weakly fluorescent because of excitonic interaction. The excited-state dynamics of the nonaggregated dyes is dominated by a nonradiative decay with a time constant of the order of 5 ps associated with large amplitude motion around the monomethine bridge of the cyanine chromophores. The strong fluorescence enhancement observed upon binding of the dyes to DNA is due to both the inhibition of this nonradiative deactivation of the nonaggregated dyes and the dissociation of the aggregates and thus to the disruption of the excitonic interaction. However, the interaction between the two chromophoric moieties in DNA is sufficient to enable ultrafast hopping of the excitation energy as revealed by the decay of the fluorescence anisotropy. Finally, these dyes act as solvation probes since a dynamic fluorescence Stokes shift was observed both in bulk water and in DNA. Very similar time scales were found in bulk water and in DNA. | ||||||||
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| The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF6 -, BPh4 -; R-Cat=catecholate dianion substituted by R=NO2, Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below ≈40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, k HL(T→0), vary in the range from 0.58 to 8.84 s-1. These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry. | ||||||||
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| The high-spin → low-spin relaxation in spin-crossover compounds can be described as non-adiabatic multi-phonon process in the strong coupling limit, in which the low-temperature tunnelling rate increases exponentially with the zero-point energy difference between the two states. Based on the hypothesis that the experimental bond length difference between the high-spin and the low-spin state of ~0.2 Å is also valid for low-spin iron(II) complexes, extrapolation of the single configurational coordinate model allows an estimate of the zero-point energy difference for low-spin complexes from kinetic data. DFT calculations on low-spin [Fe(bpy)3]2+ support the structural assumption. However, for low-spin [Fe(terpy)2]2+ the relaxation rate constant shows an anomalous behaviour in so far as it is more in line with spin-crossover systems. This is attributed to very anisotropic bond length changes associated with the spin state change, and the subsequent breakdown of the single mode model. | ||||||||
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| A planar π-conjugated heteroaromatic molecule 1 has been synthesized and fully characterized; it combines two characteristics, a charge-transfer transition originating from its inherent donor–acceptor nature in its neutral state and an intervalence charge-transfer transition in its 1 2+ mixed-valence state. | ||||||||
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| The local structure and optical and vibrational properties associated with Mn2+-doped cubic AMF3 (A = K, Rb; M = Mg, Zn, Cd) fluoroperovskites are studied by means of embedding calculations using Kohn–Sham equations with constrained electron density. It is shown that while an electronic parameter like 10Dq essentially depends on the Mn2+–F− distance, the local vibration frequencies ω i (i = a1g, eg modes) are dominated by the interaction between F− ligands and nearest M2+ ions lying along bonding directions. The high ωa values observed for KMgF3:Mn2+ and KZnF3:Mn2+, the huge variations of ωe and ωa frequencies when the host lattice is changed, as well as the increase of Huang–Rhys factors and the Stokes shift following the host lattice parameter, are shown to be related to this elastic coupling of the MnF6 4− complex to the rest of the host lattice. The present results support the conclusion that the Stokes shift is determined by the interaction of the excited 4T1g state with a1g and eg local modes while the coupling with the t2g shear mode is not relevant. The variations of local vibrational frequencies and the Stokes shift induced by a hydrostatic pressure on a given system are shown to be rather different to those produced by the chemical pressure associated with distinct host lattices. | ||||||||
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| In supramolecular systems, the interaction between different units modulates their photophysical properties. a) For platinum(II) complexes with ligands that have extended π systems, π-stacking and direct metal-metal interactions result in the formation of excimers with characteristically red-shifted luminescence. Time-resolved emission spectra show clear evidence of dual luminescence. b) In phthalocyanines to which electron-donating tetrathiafulvalene (TTF) groups have been fused, the luminescence is strongly quenched by intramolecular electron transfer. The luminescence can be switched on by oxidation of the TTF groups. c) The luminescence of ruthenium tris-bipyridyl derivatives is strongly influenced by the environment. Linked to biotin, the luminescence quantum yield of such a complex is enhanced by 30 % upon binding to avidin. Furthermore, the binding to avidin induces a circular-dichroism signal from the π-π* transition of the initially racemic ruthenium tris-bipyridyl derivative. | ||||||||
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| The scope of the asymmetric hydrogenation of functionalized ketones over cinchona-modified platinum was extended to achiral α-hydroxyketones. Cinchonidine showed by far the best catalytic performance affording an enantiomeric excess between 57 and 82% depending on the substrate. O-methoxy-cinchonidine showed poor enantioselection. O-phenoxy-cinchonidine favoured the opposite enantiomer compared to cinchonidine. Solvents with empirical solvent parameters E T N – ranging from 0.10 to 0.65 were tested. Tert-butylmethylether proved to be the most suitable. The highest ratio of substrate/cinchonidine where no loss in e.e. was observed was at around 540, independent of the structure of the α-hydroxyketone. The oxygen in α-position to the ketone seems to play an important role in the enantioselection as well as a phenyl ring or a rigid cis-conformation. The dependence of the enantiomeric excess on the modifier structure and the inversion of the sense of enantiodifferentiation is interpreted in terms of repulsive interactions, which become more evident as the steric demand of the functional group (OH, O-Me, O-Ph) of the modifier increases. The findings indicate that a hydrogen bond in the modifier reactant complex involving the hydroxyl functionality of cinchonidine is not crucial in order to achieve high enantioselectivity. | ||||||||
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| Alumina-supported rhodium modified with cinchonidine has been investigated with regard to its applicability in the enantioselective hydrogenation of various aromatic ketones possessing an α-hydroxy or α-methoxy group. The study revealed that depending on the substrate, rhodium can outperform the catalytic behavior of platinum. With one of the substrates, 2-hydroxy-1-(4-methoxy-phenyl)-ethanone (4), an enantiomeric excess (ee) of 80% at 89% conversion was reached, which is the highest ee reported so far for chirally modified rhodium. However, completely different conditions are required to achieve optimal catalytic performance with rhodium, compared with platinum. Rhodium requires a much higher modifier concentration, and high hydrogen pressure is favorable. The higher modifier concentration required is traced to the much higher activity of rhodium for the hydrogenation of the quinoline ring, which is assumed to be the anchoring moiety of the cinchona modifiers on the platinum group metals. Changing the modifier from cinchonidine to O-phenoxy-cinchonidine resulted in a switch of the major enantiomer of the product, as exemplified for 2-hydroxyacetophenone (1), which showed a switch from 73% ee in favor of the (R)-product to 68% ee for the (S)-product when the modifier was changed from cinchonidine to O-phenoxy-cinchonidine. | ||||||||
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| To bring evidence for or against the hypothesis of catalytic hydrogenation by intact trinuclear arene ruthenium clusters containing an oxo cap, cationic Ru3O clusters with three different arene ligands (intrinsically chiral tetrahedra) have been synthesized as racemic mixtures. By introduction of a chiral auxiliary substituent at one of the three different arene ligands, the separation of the two diastereomers was possible. The chiral Ru3O framework was evidenced by X-ray crystallography, by circular dichroism in the UV and IR regions, and by chiral shift reagents in the NMR spectra. The catalytic hydrogenation of the prochiral substrate methyl 2-acetamidoacrylate using a chiral Ru3O cluster showed no asymmetric induction, suggesting that the catalytically active species is not the intact Ru3O cluster. | ||||||||
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| The optical properties of a thin film of the [Ru(bpy)3][NaCr(ox)3] network structure obtained by pulsed laser deposition are described. The luminescence shows the characteristic doublet of R lines at 14 400 cm−1 of the spin-forbidden ligand field transition 2E (t2g3)→ 4A2 (t2g3) of the [Cr(ox)3]3− chromophore. The resonant energy migration within the R 1 line shows that the three-dimensional crystallographic structure is preserved during the coating process. The observation of the R lines of [Cr(bpy)3]3+ at 13 710 cm−1 indicates that a small fraction of Cr3+ ions migrate from the oxalate network to the tris-bipyridine cation site in the cavities of the network. | ||||||||
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| UNIL088 is a water-soluble prodrug of cyclosporine A (CsA) developed for topical eye delivery. Such a prodrug has to fulfil two paradoxical requirements as it must be rapidly hydrolysed under physiological conditions but also retain a long shelf-life in aqueous media. This study has been conducted to explore the stability of UNIL088 formulated as an eyedrop solution. The stability study of the prodrug was performed over a pH range of 5–7 at 20 °C and at various ionic strengths. The molecule was more stable at pH 5 than at pH 7 with conversion rate constant of 3.2 × 10−3 and 26.0 × 10−3 days−1, respectively. The effect of temperature was studied at four different temperatures and activation energy was determined. Conversion of UNIL088 followed a pseudo-first-order kinetic with an activation energy of 79.4 kJ mol−1. Due to its low solubility, CsA generated precipitated in the solution. The average size of CsA precipitates, determined by photon spectroscopy, was 0.22 and 1.08 μm at 7 and 14 days, respectively. The hydrolysis mechanism was partially elucidated by identification of the intermediate pSer-Sar-CsA. | ||||||||
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| Four compounds containing metal−metal quadruple bonds, the [M2(CH3)8] n - ions (M = Cr, Mo, W, Re and n = 4, 4, 4, 2, respectively), have been studied theoretically using multiconfigurational quantum-chemical methods. The molecular structure of the ground state of these compounds has been determined and the energy of the δ → δ* transition has been calculated and compared with previous experimental measurements. The high negative charges on the Cr, Mo, and W complexes lead to difficulties in the successful modeling of the ground-state structures, a problem that has been addressed by the explicit inclusion of four Li+ ions in these calculations. The ground-state geometries of the complexes and the δ → δ* transition have been modeled with either excellent agreement with experiment (Re) or satisfactory agreement (Cr, Mo, and W). | ||||||||
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| The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraazacyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly, it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group, [ZnL2]2+ (Chart 1). By contrast, Stern−Volmer analysis of intermolecular quenching of [ZnL2]2+ by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge in influencing intramolecular PET is highlighted. | ||||||||
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| Mixed matlokite hosts of composition BaFBrxI1−x(0≤x≤1) (pure and doped with Sm2+, Eu2+) were studied with x-ray crystallography, luminescence, Raman, and electron paramagnetic resonance (EPR) spectroscopy. Results are presented for BaFBr0.5I0.5 which demonstrate that a ferrielectric domain structure is formed due to the fact that the heavy halogen ions form separate sublattices with randomly distributed domain walls. The space group of a domain is P4m m (No. 99). The EPR data from Eu2+ allowed to determine the volume fraction of domains. | ||||||||
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| In this study, we investigate the performance of the frozen-density embedding scheme within density-functional theory [ J. Phys. Chem. 97, 8050 (1993) ] to model the solvent effects on the electron-spin-resonance hyperfine coupling constants (hfcc's) of the H2NO molecule. The hfcc's for this molecule depend critically on the out-of-plane bending angle of the NO bond from the molecular plane. Therefore, solvent effects can have an influence on both the electronic structure for a given configuration of solute and solvent molecules and on the probability for different solute (plus solvent) structures compared to the gas phase. For an accurate modeling of dynamic effects in solution, we employ the Car-Parrinello molecular-dynamics (CPMD) approach. A first-principles-based Monte Carlo scheme is used for the gas-phase simulation, in order to avoid problems in the thermal equilibration for this small molecule. Calculations of small H2NO-water clusters show that microsolvation effects of water molecules due to hydrogen bonding can be reproduced by frozen-density embedding calculations. Even simple sum-of-molecular-densities approaches for the frozen density lead to good results. This allows us to include also bulk solvent effects by performing frozen-density calculations with many explicit water molecules for snapshots from the CPMD simulation. The electronic effect of the solvent at a given structure is reproduced by the frozen-density embedding. Dynamic structural effects in solution are found to be similar to the gas phase. But the small differences in the average structures still induce significant changes in the computed shifts due to the strong dependence of the hyperfine coupling constants on the out-of-plane bending angle. | ||||||||
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| The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. The computer time used in this new implementation scales approximately linearly (with a low prefactor) with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved. | ||||||||
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| The cubic Prussian blue analogue Mn3[Mn(CN)6]2 · 15 H2O, which has the advantage of being transparent and magnetic (T N = 35 K) at the same time, has been investigated by density functional theory (DFT) calculations. The three-dimensional structure is built of MnII ions linked to MnIII ions by μ-bridging cyanides, to form a crystal structure, which is related to the NaCl type. In a first step, the relative stabilities of the mononuclear complexes [Mn(CN)6] z- (z = 2 to 4) have been studied as a function of the oxidation state, spin configuration, and the linkage isomerism of the cyanide ligand. The results we have obtained by this investigation are in good agreement with our chemical expertise. In addition, the calculations have been extended to the dinuclear [Mn2(CN)11] z- (z = 5 and 6) clusters. Furthermore, we used DFT to model the magnetic properties as well as the 3 T 1 → 1 T 2 transition, which has been observed by single-crystal near-IR spectra of Mn3[Mn(CN)6]2 · 15 H2O. | ||||||||
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| The compound {Fe(pmd)[Ag(CN)2][Ag2(CN)3]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) ↔ low-spin (LS) transition. The critical temperatures are T c1=185 and T c2=148 K. Each step involves ~50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are ΔH 1=3.6±0.4 kJ mol-1 and ΔS 1=19.5±3 J K-1 mol-1; ΔH 2=4.8±0.4 kJ mol-1 and ΔS 2=33.5±3 J K-1 mol-1. Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green-red light (550-650 nm). HS-to-LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two-step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry (P21/c, Z=16) at all temperatures. There are five [FeN6] pseudo-octahedral sites linked by pmd, [Ag(CN)2]- and [Ag2(CN)3]- bridging ligands to form an unprecedented three-dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two-step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the Ag…Ag distances. Correlations between the variation of the unit-cell volume and the change of Ag…Ag interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred. | ||||||||
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| The ability of different density functionals to describe the structural and energy differences between the high- [5 T 2g :(t 2g )4(e g )2] and low- [1 A 1g :(t 2g )6(e g )0] spin states of small octahedral ferrous compounds is studied. This work is an extension of our previous study of the hexaquoferrous cation, [Fe(H2O)6]2+, [J. Chem. Phys. 120, 9473 (2004)] to include a second compound—namely, the hexaminoferrous cation, [Fe(NH3)6]2+—and several additional functionals. In particular, the present study includes the highly parametrized generalized gradient approximations (GGAs) known as HCTH and the meta-GGA VSXC [which together we refer to as highly parametrized density functionals (HPDFs)], now readily available in the GAUSSIAN03 program, as well as the hybrid functional PBE0. Since there are very few experimental results for these molecules with which to compare, comparison is made with best estimates obtained from second-order perturbation theory-corrected complete active space self-consistent field (CASPT2) calculations, with spectroscopy oriented configuration interaction (SORCI) calculations, and with ligand field theory (LFT) estimations. While CASPT2 and SORCI are among the most reliable ab initio methods available for this type of problem, LFT embodies many decades of empirical experience. These three methods are found to give coherent results and provide best estimates of the adiabatic low-spin–high-spin energy difference, ΔE L Hadia, of 12 000–13 000 cm−1 for [Fe(H2O)6]2+ and 9 000–11 000 cm−1 for [Fe(NH3)6]2+. All functionals beyond the purely local approximation produce reasonably good geometries, so long as adequate basis sets are used. In contrast, the energy splitting, ΔE L Hadia, is much more sensitive to the choice of functional. The local density approximation severely over stabilizes the low-spin state with respect to the high-spin state. This "density functional theory (DFT) spin pairing-energy problem" persists, but is reduced, for traditional GGAs. In contrast the hybrid functional B3LYP underestimates ΔE L Hadia by a few thousands of wave numbers. The RPBE GGA of Hammer, Hansen, and Nørskov gives good results for ΔE L Hadia as do the HPDFs, especially the VSXC functional. Surprisingly the HCTH functionals actually over correct the DFT spin pairing-energy problem, destabilizing the low-spin state relative to the high-spin state. Best agreement is found for the hybrid functional PBE0. | ||||||||
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| Inert and optically active pseudo-octahedral CrIIIN6 and RuIIN6 chromophores have been incorporated by self-assembly into heterobimetallic triple-stranded helicates HHH-[CrLnL3]6+ and HHH-[RuLnL3]5+. The crystal structures of [CrLnL3](CF3SO3)6 (Ln=Nd, Eu, Yb, Lu) and [RuLnL3](CF3SO3)5 (Ln=Eu, Lu) demonstrate that the helical structure can accommodate metal ions of different sizes, without sizeable change in the intermetallic M…Ln distances. These systems are ideally suited for unravelling the molecular factors affecting the intermetallic nd→4f communication. Visible irradiation of the CrIIIN6 and RuIIN6 chromophores in HHH-[MLnL3]5/6+ (Ln=Nd, Yb, Er; M=Cr, Ru) eventually produces lanthanide-based near infrared (NIR) emission, after directional energy migration within the complexes. Depending on the kinetic regime associated with each specific d-f pair, the NIR luminescence decay times can be tuned from micro- to milliseconds. The origin of this effect, together with its rational control for programming optical functions in discrete heterobimetallic entities, are discussed. | ||||||||
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| In the iron(II) low-spin complex [Fe(bpy)3]2+, the zero-point energy difference between the 5T2g(t4 2ge2 g) high-spin and the 1A1g(t6 2g) low-spin states, ΔE0 HL, is estimated to lie in the range of 2500-5000 cm-1. This estimate is based on the low-temperature dynamics of the high-spin→low-spin relaxation following the light-induced population of the high-spin state and on the assumption that the bond-length difference between the two states ΔrHL is equal to the average value of ≈0.2 Å, as found experimentally for the spin-crossover system. Calculations based on density functional theory (DFT) validate the structural assumption insofar as the low-spin-state optimised geometries are found to be in very good agreement with the experimental X-ray structure of the complex and the predicted high-spin geometries are all very close to one another for a whole series of common GGA (PB86, PW91, PBE, RPBE) and hybrid (B3LYP, B3LYP*, PBE1PBE) functionals. This confirmation of the structural assumption underlying the estimation of ΔE0 HL from experimental relaxation rate constants permits us to use this value to assess the ability of the density functionals for the calculation of the energy difference between the HS and LS states. Since the different functionals give values from -1000 to 12000 cm-1, the comparison of the calculated values with the experimental estimate thus provides a stringent criterion for the performance of a given functional. Based on this comparison the RPBE and B3LYP* functionals give the best agreement with experiment. | ||||||||
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| The synthesis of tetrakis(tetrathiafulvalene)-annulated metal-free and metallophthalocyanines 5−8 via the tetramerization of the phthalonitrile derivative 4 is reported. All of them have been fully characterized by electronic absorption spectroscopy, thin-layer cyclic voltammetry, mass spectrometry, and elemental analysis. Their solution electrochemical data show two reversible four-electron oxidation waves, indicating that these fused systems are strong π-electron donors, which give rise to tetra- or octaradical cation species. For the metal-free phthalocyanine 5, additionally a reversible one-electron wave was found in the negative direction arising from the reduction of the macrocycle. Moreover, the tetrathiafulvalene unit acts as an efficient reductive electron-transfer quencher for the phthalocyanine emission, but upon its oxidation, an intense luminescence is switched on. | ||||||||
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| Visible pump−probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [LCoIIINCMII(CN)5]-, where L is a pentadentate macrocyclic pentaamine (L14) or triamine-dithiaether (L14S) and M is Fe or Ru. Nanosecond pump−probe spectroscopy on frozen solutions of [L14CoIIINCFeII(CN)5]- and [L14SCoIIINCFeII(CN)5]- at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350−400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L14CoIIINCRuII(CN)5]- was too short-lived to allow its detection. Femtosecond pump−probe spectroscopy on aqueous solutions of [L14CoIIINCFeII(CN)5]- and [L14SCoIIINCFeII(CN)5]- at room temperature enabled the lifetimes of their CoII−FeIII MMCT excited states to be determined as 0.8 and 1.3 ps, respectively. | ||||||||
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| The emission from two photoactive 14-membered macrocyclic ligands, 6-((naphthalen-1-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L1) and 6-((anthracen-9-ylmethyl)-amino)-trans-6,13-dimethyl-13-amino-1,4,8,11-tetraaza-cyclotetradecane (L2) is strongly quenched by a photoinduced electron transfer (PET) mechanism involving amine lone pairs as electron donors. Time-correlated single photon counting (TCSPC), multiplex transient grating (TG), and fluorescence upconversion (FU) measurements were performed to characterize this quenching mechanism. Upon complexation with the redox inactive metal ion, Zn(II), the emission of the ligands is dramatically altered, with a significant increase in the fluorescence quantum yields due to coordination-induced deactivation of the macrocyclic amine lone pair electron donors. For [ZnL2]2+, the substituted exocyclic amine nitrogen, which is not coordinated to the metal ion, does not quench the fluorescence due to an inductive effect of the proximal divalent metal ion that raises the ionization potential. However, for [ZnL1]2+, the naphthalene chromophore is a sufficiently strong excited-state oxidant for PET quenching to occur. | ||||||||
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| The EPR spectrum of the novel radical Mes*(CH3)P—PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P—P bond. | ||||||||
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| A new four-wave-mixing technique with evanescent optical fields generated by total internal reflection at a liquid-liquid interface is described. Several applications of this method to measure thermoacoustic and dynamic properties near liquid-liquid interfaces are presented. | ||||||||
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| An optical investigation of the properties of europium (II) ions introduced in Ba2Mg3F10 single crystals is presented. The spectra and time-dependence of both the inter-configurational (f–d) and the intra-configurational (f–f) transitions are described. The emission spectrum consists of two emitting Eu2+ centres, each showing one broad f–d band and one 6P7/2 quartet. The thermal equilibrium between the f–f and f–d emitting states is investigated and found to take place on a microsecond timescale for one of the two Eu2+ centres. The spectroscopic results combined with numerical modelling of the influence of the host crystal on the 6P7/2 energy scheme allow the assignment of each f–f and f–d emission to its corresponding Eu2+ centre. | ||||||||
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| Transient grating experiments performed with evanescent fields resulting from total internal reflection at an interface between a polar absorbing solution and an apolar transparent solvent are described. The time evolution of the diffracted intensity was monitored from picosecond to millisecond time scales. The diffracted signal originates essentially from two density phase gratings: one in the absorbing phase induced by thermal expansion and one in the transparent solvent due to electrostriction. A few nanoseconds after excitation, the latter grating is replaced by a thermal grating due to thermal diffusion from the absorbing phase. The speed of sound and the acoustic attenuation measured near the interface are found to be essentially the same as in the bulk solutions. However, after addition of a surfactant in the polar phase, the speed of sound near the interface differs substantially from that in the bulk with the same surfactant concentration. This effect is interpreted in terms of adsorption at the liquid/liquid interface. Other phenomena, which are not observed in bulk experiments, such as acoustic echoes and a fast oscillation of the signal intensity, are also described. | ||||||||
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| We investigate the usefulness of a frozen-density embedding scheme within density-functional theory [ J. Phys. Chem. 97, 8050 (1993) ] for the calculation of solvatochromic shifts. The frozen-density calculations, particularly of excitation energies have two clear advantages over the standard supermolecule calculations: (i) calculations for much larger systems are feasible, since the time-consuming time-dependent density functional theory (TDDFT) part is carried out in a limited molecular orbital space, while the effect of the surroundings is still included at a quantum mechanical level. This allows a large number of solvent molecules to be included and thus affords both specific and nonspecific solvent effects to be modeled. (ii) Only excitations of the system of interest, i.e., the selected embedded system, are calculated. This allows an easy analysis and interpretation of the results. In TDDFT calculations, it avoids unphysical results introduced by spurious mixings with the artificially too low charge-transfer excitations which are an artifact of the adiabatic local-density approximation or generalized gradient approximation exchange-correlation kernels currently used. The performance of the frozen-density embedding method is tested for the well-studied solvatochromic properties of the n→π * excitation of acetone. Further enhancement of the efficiency is studied by constructing approximate solvent densities, e.g., from a superposition of densities of individual solvent molecules. This is demonstrated for systems with up to 802 atoms. To obtain a realistic modeling of the absorption spectra of solvated molecules, including the effect of the solvent motions, we combine the embedding scheme with classical molecular dynamics (MD) and Car-Parrinello MD simulations to obtain snapshots of the solute and its solvent environment, for which then excitation energies are calculated. The frozen-density embedding yields estimated solvent shifts in the range of 0.20–0.26 eV, in good agreement with experimental values of between 0.19 and 0.21 eV. | ||||||||
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| The photophysics of the dye Lucifer Yellow ethylenediamine (LYen) has been investigated in various polar solvents. The main deactivation pathways of its first singlet excited state are the fluorescence and the intersystem crossing. In water, non-radiative decay by intermolecular proton transfer becomes a significant deactivation channel. The early fluorescence dynamics, which was investigated in liquids and in reverse micelles, was found to depend substantially on the environment. An important static quenching of LYen by tryptophan and indole occurring in the subpicosecond timescale was observed. The use of the fluorescence dynamics of LYen as a local probe is illustrated by preliminary results obtained with a biotinylated Lucifer Yellow derivative complexed with avidin. | ||||||||
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| The effects of milling and doping NaAlH4 with TiCl3, TiF3 and Ti(OBu n )4, and of cycling doped NaAlH4 have been investigated by infrared (IR) and Raman spectroscopy and X-ray powder diffraction. Milling and doping produce similar effects. Both decrease the crystal domain size (~900 Å for milled and ~700 Å for doped, as compared to ~1600 Å for unmilled and undoped NaAlH4) and increase anisotropic strain (by a factor >2.5, mainly along c). They also influence structure parameters such as the axial ratio c/a, cell volume and atomic displacement amplitudes. They show IR line shifts by ~15 cm−1 to higher frequencies for the Al–H asymmetric stretching mode ν 3, and by ~20 cm−1 to lower frequencies for one part of the H–Al–H asymmetric bending mode ν 4, thus suggesting structural changes in the local environment of the [AlH4]− units. The broad ν 3 bands become sharpened which suggests a more homogeneous local environment of the [AlH4]− units, and there appears a new vibration at 710 cm−1. The Raman data show no such effects. Cycling leads to an increase in domain size (1200–1600 Å), IR line shifts similar to doped samples (except for TiF3: downward shift by ~10 cm−1) and a general broadening of the ν 3 mode that depend on the nature of the dopants. These observations support the idea that some Ti diffusion and substitution into the alanate lattice does occur, in particular during cycling, and that this provides the mechanism through which Ti-doping enhances kinetics during re-crystallisation. | ||||||||
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| The radical cation of the redox active ligand 3,4-dimethyl-3',4'-bis-(diphenylphosphino)-tetrathiafulvalene ( P2) has been chemically and electrochemically generated and studied by EPR spectroscopy. Consistent with DFT calculations, the observed hyperfine structure (septet due to the two methyl groups) indicates a strong delocalization of the unpaired electron on the central S 2 C=CS 2 part of the tetrathiafulvalene (TTF) moiety and zero spin densities on the phosphine groups. In contrast with the ruthenium(0) carbonyl complexes of P2 whose one-electron oxidation directly leads to decomplexation and produces P2 • + , one-electron oxidation of [Fe( P2)(CO) 3 ] gives rise to the metal-centered oxidation species [Fe (I) ( P2)(CO) 3 ], characterized by a coupling with two 31 P nuclei and a rather large g-anisotropy. The stability of this complex is however modest and, after some minutes, the species resulting from the scission of a P–Fe bond is detected. Moreover, in presence of free ligand, [Fe (I) ( P2)(CO) 3 ] reacts to give the complex [Fe (I) ( P2) 2 (CO)] containing two TTF fragments. The two-electron oxidation of [Fe( P2)(CO) 3 ] leads to decomplexation and to the P2 • + spectrum. Besides EPR spectroscopy, cyclic voltammetry as well as FTIR spectroelectrochemistry are used in order to explain the behaviour of [Fe( P2)(CO) 3 ] upon oxidation. This behaviour notably differs from that of the Ru(0) counterpart. This difference is tentatively rationalized on the basis of structural arguments. | ||||||||
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| The enantioselective hydrogenation of ring-substituted acetophenones that possess no functional group in the α-position to the keto group represents the latest extension of the application range of the Pt–cinchona system. The influence of the type of solvent, pressure, temperature, and modifier/substrate/Pt molar ratios was investigated in the hydrogenation of 3,5-di(trifluoromethyl)acetophenone. Modification of a 5 wt% Pt/Al2O3 catalyst by cinchonidine (CD) afforded the corresponding (S)-1-phenylethanol (69.5% ee). Working in strongly polar solvents, addition of trifluoroacetic acid in a weakly polar solvent, and replacing CD by its ether derivatives resulted in the inversion of enantioselectivity. Addition of CD or any of its derivatives always led to a lower reaction rate, contrary to the generally observed rate acceleration in the hydrogenation of α-functionalized activated ketones over the same catalyst system. Another fundamental difference to the hydrogenation of α-functionalized activated ketones is that both the quinuclidine N and the OH functions of CD influence the stereochemical outcome of the reaction, as clarified by using O- and N-substituted derivatives of CD. Ab initio calculations confirmed these remarkable mechanistic differences. Inversion of enantioselectivity in the presence of strongly polar and acidic solvents is attributed to special interactions with the OH function of CD, and to the formation of a CD–acid ion pair, respectively. A possible explanation for the moderate ee's in the hydrogenation of ring-substituted acetophenones is that a reaction pathway without involvement of the OH function of CD is also feasible. This competing pathway is even faster and provides low ee to the opposite enantiomer. | ||||||||
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| Epoxidation of cyclohex-2-en-1-ol and cyclooct-2-en-1-ol on titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) as oxidant was studied by in situ attenuated total reflection (ATR) Fourier transform infrared spectroscopy. Probing of the catalytic liquid–solid interface revealed different adsorption behaviors for the two allylic alcohols on the aerogel. Cyclohexenol was found to adsorb stronger and less reversible on the catalyst surface and Ti sites than cyclooctenol. The spectroscopic measurements under working conditions support the previously proposed hydroxy-assisted mechanism for the formation of cyclohexenol oxide and the silanol-assisted mechanism for cyclooctenol epoxidation. The evidence of the former is traced to the occurrence of a framework vibration upon adsorption of cyclohexenol, whereas the latter is supported by large negative bands of the silanol groups at 3700 and 980 cm−1 in the case of cyclooctenol epoxidation. | ||||||||
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| The enantioselective hydrogenation of several isatine derivatives over cinchonidine modified Pt/Al2O3 was investigated. A maximum enantiomeric excess (e.e.) of 45% was found for (R)-5,7-dimethylisatin. The enantiomeric excess was limited by racemization catalyzed by the basic cinchonidine in solution, leading to low enantiomeric excess at high cinchonidine concentration. The modifier in solution also catalyzed the formation of the corresponding isatide. High cinchonidine concentration favored isatide formation, whereas low cinchonidine concentration and high hydrogen pressure favored alcohol formation. The isatide, formed from the isatin reactant and the alcohol, underwent disproportionation. Though both hydrogenation and isatide formation are fast reactions, isatide formation was considerably faster at least at the beginning of the reaction. Substitution of the isatin reactant had relatively little effect on enantiomeric excess but affected considerably the rate of racemization. | ||||||||
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| Excitation energy transfer processes play an important role in many areas of physics, chemistry and biology. The three-dimensional oxalate networks of composition [MIII(bpy)3][MIMIII(ox)3]ClO4 (bpy=2,2-bipyridine, ox=oxalate, MI=alkali ion) allow for a variety of combinations of different transition metal ions. The combination with chromium(III) on both the tris-bipyridine as well as the tris-oxalate site constitutes a model system in which it is possible to differentiate unambiguously between energy transfer from [Cr(ox)3]3– to [Cr(bpy)3]3+ due to dipole-dipole interaction on the one hand and exchange interaction on the other hand. Furthermore it is possible to just as unambiguously differentiate between the common temperature dependent phonon-assisted energy migration within the 2E state of [Cr(ox)3]3–, and a unique resonant process. | ||||||||
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| The compound of stoichiometry Mn(II)3[Mn(III)(CN)6]2·zH2O (z = 12−16) (1) forms air-stable, transparent red crystals. Low-temperature single crystal optical spectroscopy and single crystal X-ray diffraction provide compelling evidence for N-bonded high-spin manganese(II), and C-bonded low-spin manganese(III) ions arranged in a disordered, face-centered cubic lattice analogous to that of Prussian Blue. X-ray and neutron diffraction show structured diffuse scattering indicative of partially correlated (rather than random) substitutions of [Mn(III)(CN)6] ions by (H2O)6 clusters. Magnetic susceptibility measurements and elastic neutron scattering experiments indicate a ferrimagnetic structure below the critical temperature T c = 35.5 K. | ||||||||
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| The lessons learned from p-octiphenyl β-barrel pores are applied to the rational design of synthetic multifunctional pore 1 that is unstable but inert, two characteristics proposed to be ideal for practical applications. Nonlinear dependence on monomer concentration provided direct evidence that pore 1 is tetrameric (n = 4.0), unstable, and "invisible," i.e., incompatible with structural studies by conventional methods. The long lifetime of high-conductance single pores in planar bilayers demonstrated that rigid-rod β-barrel 1 is inert and large (d ≈ 12 Å). Multifunctionality of rigid-rod β-barrel 1 was confirmed by adaptable blockage of pore host 1 with representative guests in planar (8-hydroxy-1,3,6-pyrenetrisulfonate, K D = 190 μM, n = 4.9) and spherical bilayers (poly-l-glutamate, K D ≤ 105 nM, n = 1.0; adenosine triphosphate, K D = 240 μM, n = 2.0) and saturation kinetics for the esterolysis of a representative substrate (8-acetoxy-1,3,6-pyrenetrisulfonate, K M = 0.6 μM). The thermodynamic instability of rigid-rod β-barrel 1 provided unprecedented access to experimental evidence for supramolecular catalysis (n = 3.7). Comparison of the obtained k cat = 0.03 min- 1 with the k cat ≈ 0.18 min- 1 for stable analogues gave a global K D ≈ 39 μM3 for supramolecular catalyst 1 with a monomer/barrel ratio ≈ 20 under experimental conditions. The demonstrated "invisibility" of supramolecular multifunctionality identified molecular modeling as an attractive method to secure otherwise elusive insights into structure. The first molecular mechanics modeling (MacroModel, MMFF94) of multifunctional rigid-rod β-barrel pore hosts 1 with internal 1,3,6-pyrenetrisulfonate guests is reported. | ||||||||
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| The unsymmetrical tridentate benzimidazole–pyridine–carboxamide units in ligands L1–L4 react with trivalent lanthanides, Ln III , to give the nine-co-ordinate triple-helical complexes [Ln(L i ) 3 ] 3+ (i=1–4) existing as mixtures of C 3 -symmetrical facial and C 1 -symmetrical meridional isomers. Although the β 13 formation constants are 3–4 orders of magnitude smaller for these complexes than those found for the D 3 -symmetrical analogues [Ln(L i ) 3 ] 3+ (i=5–6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2) 3 ] 3+ is significantly larger. The fac-[Ln(L i ) 3 ] 3+ ↔mer-[Ln(L i ) 3 ] 3+ (i =1–4) isomerisation process in acetonitrile is slow enough for Ln=Lu III to be quantified by 1 H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand -stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 13 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1) 2 (CF 3 SO 3 ) 2 (H 2 O)](CF 3 SO 3 )(C 3 H 5 N) 2 (H 2 O) ( 6, C 45 H 54 EuF 9 N 10 O 13 S 3 , monoclinic, P2 1 /c, Z=4) and [Eu(L4) 2 (CF 3 SO 3 ) 2 (H 2 O)](CF 3 SO 3 )(C 4 H 4 O) 1.5 ( 7, C 51 H 66 EuF 9 N 8 O 15.5 S 3 , triclinic, P, Z=2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands. | ||||||||
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| The electron transfer quenching dynamics of excited perylene and cyanoperylene in various donating solvents has been investigated by using ultrafast fluorescence up-conversion and multiplex transient grating. The strongly nonexponential fluorescence decays have been analyzed by using the orientational model described in the first article of this series (J. Phys. Chem. A 2003, 107, 5375). It appears that the solvent dependence of the quenching dynamics is strongly connected to the number of surrounding donor molecules enabling ultrafast electron transfer. This number depends mainly on the driving force for electron transfer, on steric interactions, and on the occurrence of dipole−dipole interactions with the acceptor. The quenching product is an exciplex with a strong charge-transfer character. The complicated wavelength dependence of the fluorescence dynamics in the exciplex region, as well as the spectral dynamics observed in the transient grating data, is attributed to dipolar solvation, which leads to an increase of the charge-transfer character of the exciplex. The strong donor dependence of the exciplex lifetime is very similar to that reported earlier for the charge recombination time of geminate ion pairs in acetonitrile, and can be rationalized in terms of different intramolecular reorganization energies and electronic coupling constants. | ||||||||
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| EPR spectra show that one-electron reduction of bis(3-phenyl-6,6-(trimethylsilyl)phosphinine-2-yl)dimethylsilane (1) on an alkali mirror leads to a radical anion that is localized on a single phosphinine ring, whereas the radical anion formed from the same reaction in the presence of cryptand or from an electron transfer with sodium naphthalenide is delocalized on the two phosphinine rings. Density functional theory (DFT) calculations show that in the last species the unpaired electron is mainly confined in a loose P — P bond (3.479 Å), which results from the overlap of two phosphorus p orbitals. In contrast, as attested by X-ray spectroscopy, the P — P distance in neutral 1 is large (5.8 Å). As shown by crystal structure analysis, addition of a second electron leads to the formation of a classical P — P single bond (P — P 2.389 Å). Spectral modifications induced by the presence of cryptand or by a change in the reaction temperature are consistent with the formation of a tight ion pair that stabilizes the radical structure localized on a single phosphinine ring. It is suggested that the structure of this pair hinders internal rotation around the C — Si bonds and prevents 1 from adopting a conformation that shortens the intramolecular P — P distance. The ability of the phosphinine radical anion to reversibly form weak P — P bonds with neutral phosphinines in the absence of steric hindrance is confirmed by EPR spectra obtained for 2,6-bis(trimethylsilyl)-3-phenylphosphinine (2). Moreover, as shown by NMR spectroscopy, in this system, which contains only one phosphinine ring, further reduction leads to an intermolecular reaction with the formation of a classical P — P bond. | ||||||||
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| The phenomenon of the thermal spin transition, as observed for octahedral transition metal complexes having a d 4 to d 7 electronic configuration, can be fully rationalised on the basis of ligand field theory. In order to arrive at a self-consistent description of the vibronic structure of spin crossover compounds, it is essential to take into account the fact that the population of anti-bonding orbitals in the high-spin state results in a substantially larger metal-ligand bond length than for the low-spin state. Whereas the electron-electron repulsion is not affected to any great extent by such a bond length difference, the ligand field strength for iron(II) spin crossover compounds can be estimated to be almost twice as large in the low-spin state as compared to the one for the high-spin state. In fact, the dependence of the ligand field strength on the metal-ligand distance may be considered the quantum mechanical driving force for the spin crossover phenomenon. | ||||||||
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| The discovery of a light-induced spin transition at cryogenic temperatures in a series of iron(II) spin-crossover compounds in 1984 has had an enormous impact on spin-crossover research. Apart from being an interesting photophysical phenomenon in its own right, it provided the means of studying the dynamics of the intersystem crossing process between the high-spin and the low-spin state in a series of compounds and over a large temperature range. It could thus be firmly established that intersystem crossing in spin-crossover compounds is a tunnelling process, with a limiting low-temperature lifetime below 50 K and a thermally activated region above 100 K. This review begins with an elucidation of the mechanism of the light-induced spin transition, followed by an in depth discussion of the chemical and physical factors, including cooperative effects, governing the lifetimes of the light-induced metastable states. | ||||||||
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| A comparison of density functionals is made for the calculation of energy and geometry differences for the high- [5 T 2g : (t 2g )4(e g )2] and low- [1 A 1g : (t 2g )6(e g )0] spin states of the hexaquoferrous cation [Fe(H2O)6]2+. Since very little experimental results are available (except for crystal structures involving the cation in its high-spin state), the primary comparison is with our own complete active-space self-consistent field (CASSCF), second-order perturbation theory-corrected complete active-space self-consistent field (CASPT2), and spectroscopy-oriented configuration interaction (SORCI) calculations. We find that generalized gradient approximations (GGAs) and the B3LYP hybrid functional provide geometries in good agreement with experiment and with our CASSCF calculations provided sufficiently extended basis sets are used (i.e., polarization functions on the iron and polarization and diffuse functions on the water molecules). In contrast, CASPT2 calculations of the low-spin–high-spin energy difference ΔE L H = E L S −E H S appear to be significantly overestimated due to basis set limitations in the sense that the energy difference of the atomic asymptotes (5 D→1 I excitation of Fe2+) are overestimated by about 3000 cm−1. An empirical shift of the molecular ΔE L H based upon atomic calculations provides a best estimate of 12 000–13 000 cm−1. Our unshifted SORCI result is 13 300 cm−1, consistent with previous comparisons between SORCI and experimental excitation energies which suggest that no such empirical shift is needed in conjunction with this method. In contrast, after estimation of incomplete basis set effects, GGAs with one exception underestimate this value by 3000–4000 cm−1 while the B3LYP functional underestimates it by only about 1000 cm−1. The exception is the GGA functional RPBE which appears to perform as well as or better than the B3LYP functional for the properties studied here. In order to obtain a best estimate of the molecular ΔE L H within the context of density functional theory (DFT) calculations we have also performed atomic excitation energy calculations using the multiplet sum method. These atomic DFT calculations suggest that no empirical correction is needed for the DFT calculations. | ||||||||
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| In the three-dimensional oxalate network structures of composition [Co x MII 1-x (bpy)3][MICr(ox)3], the spin state of the [Co x (bpy)3]2+ complex can be tuned by means of chemical pressure. With MI=Na it is a classic high-spin complex. Substitution of Na by Li stabilises the complex and it becomes a spin-crossover complex. Dilution with MII=Fe reinforces this effect, and MII=Zn reverses it. | ||||||||
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| Unsymmetrical substituted bidentate benzimidazol-2-ylpyridine ligands L2 and L3 react with [Ru(dmso)4Cl2] in ethanol to give statistical 1:3 mixtures of fac-[Ru(Li)3]2+ and mer-[Ru(Li)3]2+ (i=2, 3; ΔGΘisomerisation=-2.7 kJ mol-1). In more polar solvents (acetonitrile, methanol), the free energy of the facial ↔ meridional isomerisation process favours mer-[Ru(Li)3]2+, which is the only isomer observed in solution at the equilibrium (ΔGΘisomerisation≤-11.4 kJ mol-1). Since the latter process takes several days for [Ru(L2)3]2+, fac-[Ru(L2)3]2+ and mer-[Ru(L2)3]2+ have been separated by chromatography, but the 28-fold increase in velocity observed for [Ru(L3)3]2+ provides only mer-[Ru(L3)3](ClO4)2 after chromatography (RuC60H51N9O8Cl2, monoclinic, P21/n, Z=4). The facial isomer can be stabilised when an appended tridentate binding unit, connected at the 5-position of the benzimidazol-2-ylpyridine unit in ligand L1, interacts with nine-coordinate lanthanides(III). The free energy of the facial↔meridional isomerisation is reversed (ΔGΘisomerisation≥11.4 kJ mol-1), and the Ru — N bonds are labile enough to allow the quantitative thermodynamic self-assembly of HHH-[RuLu(L1)3]5+ within hours ([RuLu(L1)3](CF3SO3)4.5Cl0.5(CH3OH)2.5: RuLuC106H109Cl0.5N21O19S4.5F13.5, triclinic, P, Z=2). Electrochemical and photophysical studies show that the benzimidazol-2-ylpyridine units in L1-L3 display similar π-acceptor properties to, but stronger π-donor properties than, those found in 2,2'-bipyridine. This shifts the intraligand π→π* and the MLCT transitions toward lower energies in the pseudo-octahedral [Ru(Li)3]2+ (i=2, 3) chromophores. The concomitant short lifetime of the 3MLCT excited state points to efficient, thermally activated quenching via low-energy Ru-centred d-d states, a limitation which is partially overcome by mechanical coupling in HHH-[RuLu(L1)3]5+. | ||||||||
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| Alkali borohydrides MBH4 and their deuterides have been investigated by X-ray and neutron powder diffraction (M=K,Rb,Cs) and by infrared and Raman spectroscopy (M=Na,K,Rb,Cs). At room temperature the compounds crystallize with a cubic high temperature (HT) structure having Fm3m symmetry in which the [BH4]− complexes are disordered. At low temperature (LT) the potassium compound transforms into a tetragonal low temperature structure having P42/n mc symmetry in which the [BH4]− complexes are ordered such as in the isotypic sodium congener. The B---H distances within the complex as measured on the deuteride at 1.5 K are 1.205(3) Å. Indications for a partial ordering in the rubidium and cesium compounds exist but are not sufficient for a full structural characterization. Infrared and Raman spectra at room temperature are fully assigned for both hydrides and deuterides, including the overtones and combination bands, the Fermi resonance type interactions and the 10B to 11B splitting due to the presence of natural boron in the samples. | ||||||||
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| The C 2-symmetric electron-poor ligand (R)-BINOP-F (4) was prepared by reaction of (R)-BINOL with bis(pentafluorophenyl)-phosphorus bromide in the presence of triethylamine. The iodo complex [CpRu((R)-BINOP-F)(I)] ((R)-6) was obtained by substitution of two carbonyl ligands by (R)-4 in the in situ-prepared [CpRu(CO)2H] complex followed by reaction with iodoform. Complex 6 was reacted with [Ag(SbF6)] in acetone to yield [CpRu((R)-BINOP-F)(acetone)][SbF6] ((R)-7). X-ray structures were obtained for both (R)-6 and (R)-7. The chiral one-point binding Lewis acid [CpRu((R)-BINOP-F)][SbF6] derived from either (R)-7 or the corresponding aquo complex (R)-8 activates methacrolein and catalyzes the Diels−Alder reaction with cyclopentadiene to give the [4 + 2] cycloadduct with an exo/endo ratio of 99:1 and an ee of 92% of the exo product. Addition occurs predominantly to the methacrolein Cα-Re face. In solution, water in (R)-8 exchanges readily. Moreover, a second exchange process renders the diastereotopic BINOP-F phosphorus atoms equivalent. These processes were studied by the application of variable-temperature 1H, 31P, and 17O NMR spectroscopy, variable-pressure 31P and17O NMR spectroscopy, and, using a simpler model complex, density functional theory (DFT) calculations. The results point to a dissociative mechanism of the aquo ligand and a pendular motion of the BINOP-F ligand. NMR experiments show an energy barrier of 50.7 kJ mol- 1 (12.2 kcal mol- 1) for the inversion of the pseudo-chirality at the ruthenium center. | ||||||||
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| Raman spectra of the alkali borohydride series MBH4 (M=Li, Na, K, Rb, Cs) have been measured as a function of temperature in the range 300–540 K. For the cubic modification of M=Na, K, Rb and Cs, the analysis of the Raman line widths suggests that the energy barrier of reorientation of the [BH4]− anions decreases as a function of cation size in the sequence Na: 12.1(5), K: 9.2(4), Rb: 8.8(3) and Cs: 8.2(4) kJ/mol. For the hexagonal high temperature modification of LiBH4, the data suggest two energy barriers of reorientation at ~5 and ~ 60 kJ/mol, respectively. | ||||||||
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| The asymmetric hydrogenation of cyclohexane-1,2-dione over cinchonidine-modified platinum was investigated. Despite the fact that the first hydrogenation step is close to nonenantioselective, a high enantiomeric excess is obtained for the (R)-α-hydroxyketone due to kinetic resolution. In the second hydrogenation step one out of the four reactions of the network is substantially accelerated with respect to the others and with respect to the reaction in the absence of modifier, leading to an enantiomeric excess of (1R,2R)-trans-cyclohexane-1,2-diol of over 80%. Comparison with recently reported asymmetric hydrogenation of α-hydroxyethers indicates striking similarities, which hint at similar reactant–modifier interaction in both cases. The importance of cis versus trans conformation of the reactant for the reactant–modifier interaction emerges from a comparison of suggested reaction intermediates for cyclohexane-1,2-dione and butane-2,3-dione hydrogenation, respectively. | ||||||||
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| The epoxidation of cyclohexene over titania–silica aerogel catalysts using t-butylhydroperoxide (TBHP) was investigated by in situ attenuated total reflection (ATR) infrared spectroscopy. In order to distinguish between relevant and spectator species and to increase sensitivity, ATR was combined with modulation spectroscopy. In the latter technique the catalyst system is periodically perturbed by a forced concentration change. The interaction of cyclohexene with the aerogels is found to be weak. In contrast, TBHP adsorbs strongly on the catalysts on two different sites. Modulation experiments reveal that TBHP adsorbed on Si-OH groups is a spectator, whereas the one adsorbed on the Ti-sites is involved in the catalytic cycle. The latter species is stronger adsorbed and the associated signals increase with the Ti content of the catalyst. Adsorption of the TBHP on the Ti sites results in a strong shift of the C-O stretching vibration and changes in the Ti-O-Si modes of the catalyst. The study furthermore reveals vastly different pore diffusion for cyclohexene and TBHP due to their different interaction with the polar catalyst surface. In the modulation experiments the reaction product appears retarded with respect to the admittance of the reactants, which indicates that pore diffusion and kinetics of adsorption and desorption are important factors for the catalysis. Methylation of the aerogel has a beneficial effect on the catalysis, which can be traced to the different pore size distribution and polarity with respect to the unmodified catalyst. When the flow-through ATR cell is slowly heated, a change in the framework vibrations of the catalyst occurs simultaneously with the onset of reaction, indicating reaction induced structural changes. | ||||||||
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| Individual molecular orbital (MO) contributions to the magnetic shielding of atoms as well as to the nucleus-independent chemical shifts (NICS) of aromatic compounds can be computed by the widely used gauge-including atomic orbital (GIAO) method. Detailed analyses of magnetic shielding MO-NICS contributions provide interpretive insights that complement and extend those given by the localized MO ("dissected NICS", LMO-NICS) method. Applications to (4n + 2) π-electron systems, ranging from [n] annulenes to D nh S3, S5, and N6H6 2+ rings as well as to D 2 h cyclobutadiene, show the extent to which their diatropic character results from the σ framework and from the π orbitals. The diatropicity of both these contributions decreases with the number of nodes of the wave function around the ring. The highest-energy orbitals can become paratropic. This is generally the case with the σ orbitals, but is found only for "electron-rich" π systems such as sulfur rings. MO-NICS contributions, which can be interpreted using London−Hückel theory, correlate with inverse ring size. | ||||||||
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| Copper(II) complexes of the pentadentate bispidine ligands exist in two isomeric forms (see structure) with bond-length differences up to 0.5 Å. The stabilization of either isomer may be achieved by a variation of the substituent at N7. | ||||||||
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| In this paper we discuss on the quantum efficiency in spin crossover compounds. Spin crossover solids are text-book examples of photo switchable materials that present a thermal spin transition from the diamagnetic low-spin state, thermodynamically stable at low temperatures, to the paramagnetic high-spin state becoming the thermodynamically stable state at elevated temperature. By irradiating them with an appropriate wavelength, they can pass from the stable low spin state to the metastable high spin state at temperatures below the thermal transition temperature. For the compound [Fe(pic)3]Cl2·EtOH, the question regarding the quantum efficiency of the photo-conversion process that is the number of molecules converted by one single photon and its possible dependency on irradiation intensity gave rise to a controversy. The experimental results presented in this paper demonstrate that the quantum efficiency of the photo-conversion at 11 K is on the order of unity, with no noticeable dependency of the quantum efficiency on light intensity. It does, however, depend to a small extent on the fraction of complexes already converted to the high-spin state. | ||||||||
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| The relation between the internal pressure during spin-crossover is compared to the chemical pressure induced by dilution with zinc. Further, the light of a specific LIESST (Light Induced Excited Spin State Trapping) wavelength is used to induce partial stabilisation of high-spin state and thus shift temperature of the spin-crossover towards lower values. The de-coupling of the spin-crossover and structural phase transition is discussed. | ||||||||
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| Electronic energy transfer (EET) rate constants between a naphthalene donor and anthracene acceptor in [ZnL4a](ClO4)2 and [ZnL4b](ClO4)2 were determined by time-resolved fluorescence where L4a and L4b are the trans and cis isomers of 6-((anthracen-9-yl-methyl)amino)-6,13-dimethyl-13-((naphthalen-1-yl-methyl)amino)-1,4,8,11-tetraazacyclotetradecane, respectively. These isomers differ in the relative disposition of the appended chromophores with respect to the macrocyclic plane. The trans isomer has an energy transfer rate constant (k EET) of 8.7 × 108 s-1, whereas that of the cis isomer is significantly faster (2.3 × 109 s-1). Molecular modeling was used to determine the likely distribution of conformations in CH3CN solution for these complexes in an attempt to identify any distance or orientation dependency that may account for the differing rate constants observed. The calculated conformational distributions together with analysis by 1H NMR for the [ZnL4a]2+ trans complex in the common trans-III N-based isomer gave a calculated Förster rate constant close to that observed experimentally. For the [ZnL4b]2+ cis complex, the experimentally determined rate constant may be attributed to a combination of trans-III and trans-I N-based isomeric forms of the complex in solution. | ||||||||
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| The electronic structure of La2CuO4 has been investigated using first-principles cluster calculations and the chemical shieldings at the copper nucleus have been determined with several state-of-the-art quantum chemical methods. We have also calculated the copper shieldings for CuCl, which is often used as a reference substance for copper nuclear magnetic resonance shifts measurements, and found an appreciable paramagnetic contribution in agreement with precise measurements. The calculated chemical shift at the copper nucleus in La2CuO4 for an applied field parallel to the CuO2 planes is generally smaller than, but still in reasonable agreement with, the values derived from experiment with the assumption that the spin susceptibility vanishes at zero temperature. For the field perpendicular to the planes, the quantum chemical result is substantially smaller than the experimental data but in accord with a perturbation theoretical estimate. Inconsistencies in previous representations and interpretations of the copper magnetic shift data are pointed out and corrected. | ||||||||
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| Density functional theory generalized gradient approximation calculations, which were tested in our previous detailed study of [RhCl(PF3)2]2 (Seuret et al., 2003, Phys. Chem. chem. Phys., 5, 268-274), were applied for a series of homologous organometallic compounds of the [RhXL2]2 (X = Cl, Br, or I; L = CO, PH3, or PF3) type. Various properties of the studied compounds were obtained. Optimized geometries of [RhCl(PH3)2]2 and [RhCl(CO)2]2 are in very good agreement with available experimental data. Geometries of other compounds as well as other properties (thermochemistry of selected fragmentation channels, barriers to structural changes, frontier orbitals) which are not available experimentally were predicted. All the considered compounds are not planar. Enforcing planarity of the central [RhX]2 moiety requires only a small energetic cost ranging from 2.2 to 3.9 kcal mol-1. The analysis of frontier orbitals indicates that the metals provide the most favourable site for the electrophilic attack in all considered compounds. The analysis of the shape of the lowest unoccupied molecular orbitals indicates that the halogens and ligands provide the most favourable site for the nucleophilic attack for [RhCl(CO)2]2 or [RhCl(PF3)2]. For [RhBr(PF3)2]2, [RhI(PF3)2]2 and [RhCl(PH3)2]2, the nucleophilic attack on the halogen is less probable. Except for [RhCl(CO)2]2, the least energetically expensive decomposition channel involves initial separation of ligands. For [RhCl(CO)2]2, its decomposition into the RhCl(CO)2 fragments was found to be the least energetically expensive fragmentation reaction which is probably one of the reasons for the known catalytic activity of this compound. | ||||||||
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| The hydrolysis of terminal tbutyl-ester groups provides the novel nonadentate podand tris{2-[N-methylcarbamoyl-(6-carboxypyridine-2)-ethyl]amine} (L13) which exists as a mixture of slowly interconverting conformers in solution. At pH = 8.0 in water, its deprotonated form [L13 − 3H]3 - reacts with Ln(ClO4)3 to give the poorly soluble and stable podates [Ln(L13 − 3H)] (log(β110) = 6.7−7.0, Ln = La−Lu). The isolated complexes [Ln(L13 − 3H)](H2O)7 (Ln = Eu, 8; Tb, 9; Lu, 10) are isostructural, and their crystal structures show Ln(III) to be nine-coordinate in a pseudotricapped trigonal prismatic site defined by the donor atoms of the three helically wrapped tridentate binding units of L13. The Ln−O(carboxamide) bonds are only marginally longer than the Ln−O(carboxylate) bonds in [Ln(L13 − 3H)], thus producing a regular triple helix around Ln(III) which reverses its screw direction within the covalent Me−TREN tripod. High-resolution emission spectroscopy demonstrates that (i) the replacement of terminal carboxamides with carboxylates induces only minor electronic changes for the metallic site, (ii) the solid-state structure is maintained in water, and (iii) the metal in the podate is efficiently protected from interactions with solvent molecules. The absolute quantum yields obtained for [Eu(L13 − 3H)] ( Φtot Eu = 1.8 × 10- 3) and [Tb(L13 − 3H)] ( Φtot Eu = 8.9 × 10- 3) in water remain modest and strongly contrast with that obtained for the lanthanide luminescence step (ΦEu = 0.28). Detailed photophysical studies assign this discrepancy to the small energy gap between the ligand-centered singlet (1ππ*) and triplet (3ππ*) states which limits the efficiency of the intersystem crossing process. Theoretical TDDFT calculations suggest that the connection of a carboxylate group to the central pyridine ring prevents the sizable stabilization of the triplet state required for an efficient sensitization process. The thermodynamic and electronic origins of the advantages (stability, lanthanide quantum yield) and drawbacks (solubility, sensitization) brought by the "carboxylate effect" in lanthanide complexes are evaluated for programming predetermined properties in functional devices. | ||||||||
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| A [4](hetero)helicenium cation was resolved using the hexacoordinated phosphorus-containing binphat anion (see picture: N, blue; O, red; C, gray). Its absolute configuration was determined by vibrational circular dichroism spectroscopy. The barrier of interconversion of its enantiomers is higher than that of [6]helicene. | ||||||||
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| The electron transfer (ET) quenching dynamics of excited perylene (Pe), cyanoperylene (PeCN), methanolperylene (PeOH), and methylperylene (PeMe) in N,N-dimethylaniline (DMA) has been investigated using ultrafast fluorescence up-conversion. Measurements of the rotational dynamics of PeCN and PeMe in nonpolar and polar inert solvents using optically heterodyned polarization spectroscopy are also presented. The fluorescence decay in DMA is strongly nonexponential and about 10 times faster with PeCN than with the other electron acceptors. The quenching dynamics has been analyzed with a model distinguishing three types of donor molecules surrounding the acceptor: those with optimal orientation for ET and those requiring orientational or translational diffusion prior to ET. According to this model, which can account for the whole fluorescence decay, the faster quenching dynamics of PeCN is not due to a larger ET rate constant, but to a larger number of donor molecules, typically three to four, with an optimal orientation. This is explained by the effect of dipole−dipole interaction between PeCN and the donor molecules, which favors mutual orientations with a large electronic coupling. With the other acceptors, this interaction is either not present or does not lead to ET active geometries. The occurrence of this interaction is substantiated by the rotational dynamics measurements. | ||||||||
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| Reduction of a solution of octamethylcyclo-di(m-silylphenylenedisiloxane) 4 in THF on a potassium mirror leads to EPR/ENDOR spectra characterized by a large coupling (~20 MHz) with two protons, similar to the spectra obtained after reduction of the m-disilylbenzene derivative 5, consistent with a localization of the extra electron on a single ring of 4. The spectra recorded after reduction of 4 at low temperature in the presence of an equimolar amount of 18-crown-6 exhibit couplings of ~10 MHz with four protons and indicate that embedding the counterion in crown-ether provokes the delocalization of the unpaired electron on the two phenyl rings of 4. The measured hyperfine interactions agree with those calculated by DFT for the optimized structure of 4 • -. Direct information on the structure of this anion is obtained from the X-ray diffraction of crystals grown at -18 °C in reduced solutions containing 4, potassium, and crown ether in a THF/hexane mixture. Both DFT and crystal structures clearly indicate the geometry changes caused by the addition of an electron to 4: the interphenyl distance drastically decreases, leading to a partial overlap of the two rings. The structure of 4 • - is a model for an electron transfer (ET) transition state between the two aromatic rings. The principal reason for the adoption of this structure lies in the bonding interaction between the LUMO (π* orbitals) of these two fragments; moreover, the constraints of the macrocycle probably contribute to the stabilization of this structure. | ||||||||
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| Experimental and theoretical techniques have been applied to study the decomposition of the [RhCl(PF 3 ) 2 ] 2 molecule which is known as a precursor in electron beam induced deposition (EBID) of Rh. Mass spectrometry (MS) has been carried out to study the electron ionisation and fragmentation of isolated molecules. Auger electron spectroscopy has been used to characterize the EBID deposit. The MS data indicate the presence of free phosphorus and rhodium ions. This is in agreement with the analysis of the composition of the EBID deposit containing: 60% Rh, 12–25% P, 2–13% Cl, no F, 3–20% O and N. Theoretical calculations (density functional theory) has been used to characterize the precursor molecule and to derive the enthalpies of several simple decomposition reactions. The calculated geometries are in a good agreement with the available X-ray crystallographic data. The [RhCl(PF 3 ) 2 ] 2 appears not to be rigid: the PF 3 groups can rotate with a relatively low barrier (0.6 kcal mol –1 ) whereas the barrier for the butterfly-like motion of (RhCl) 2 moiety is only 3.5 kcal mol –1 . According to the theoretical results, the lowest energy pathway of the decomposition corresponds to a consecutive loss of PF 3 ligands, resulting in a (RhCl) 2 moiety (without phosphorus). The same conclusion is also valid for the ionised precursor. Experimental data combined with the theoretical results concerning the energetics of the considered various simple decomposition processes indicate that the electron induced dissociation of the precursor cannot be seen as a simple one-step decomposition process. | ||||||||
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| The relation between the electronic structure of α-substituted ketones and their reactivity in the racemic and enantioselective platinum-catalyzed hydrogenation has been investigated using a combined theoretical and experimental approach. A correlation between the keto carbonyl orbital energy and the hydrogenation rate has been found, which rationalizes the effect of the substituent on the rate of hydrogenation. The uncovered relationship between the keto carbonyl orbital energy and the hydrogenation rate provides a rational explanation for the often observed rate acceleration that occurs when cinchona-modified platinum is used as a enantioselective hydrogenation catalyst. The previously suggested model for enantiodiscrimination based on the different stability of the diastereomeric complexes formed between the reactant and the cinchona modifier is discussed in the light of the new kinetic findings. | ||||||||
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| The mechanism of alcohol oxidation was investigated using the conversion of cinnamyl alcohol (1) over Pd-based catalysts as a sensitive test reaction. Studies in a slurry reactor revealed that dehydrogenation and oxidative dehydrogenation of 1 follow the same reaction pathways independent of the presence or absence of oxygen and reaction conditions. Hydrogenation and hydrogenolysis side reactions indicated the presence of hydrogen on the metal surface during reactions. Catalyst deactivation in Ar is attributed to decarbonylation reactions and site blocking by CO. Introduction of molecular oxygen induced a dramatic enhancement of alcohol conversion rate by a factor of up to 285 due to oxidative removal of CO. Strong adsorption of CO on Pd/Al2O3 and its rapid removal by oxygen were corroborated by in situ ATR-IR spectroscopy. All these observations conform to a model according to which oxidation of 1 follows the classical dehydrogenation mechanism, and the key role of oxygen is the continuous oxidative removal of CO and other degradation products from the active sites. This oxidative cleaning of the metal surface allows a high rate of alcohol dehydrogenation even when the oxidation of the co-product hydrogen is slow and incomplete. It is likely that the observed effects are not limited to the oxidation of 1 on Pd, and regeneration of the active sites by oxygen generally plays an important role during aerobic oxidation of alcohols on platinum metals. | ||||||||
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| A novel ruthenium complex with a 6,7-dicyanosubstituted dppz ligand has been synthesised: its crystal structure and physico-chemical studies are reported. | ||||||||
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| We use variable-cell first principles molecular dynamics as an optimization tool to investigate the structural and electronic properties of Mg-based anhydrous hydrotalcite-like compounds. The formation energy as a function of the ratioR between di- and trivalent cations shows a minimum atR~ 3, in good agreement with experimental stability ranges for these materials. At the same valueR~ 3, a maximum is found in the calculated interlayer distance, suggesting a correlation between energetic stability and structure. The energies and character of the electronic states of hydrotalcites containing different interlayer anions and trivalent cations have been compared. The nature of the anions is found to have a major influence on the electronic properties. In particular, OH- anions, rather than, e.g., Cl-, lead to a significantly smaller HOMO−LUMO gap, with a LUMO spatially more localized in the interlayer region. These features are related to the observed differences in the catalytic properties of hydrotalcites containing OH- vs Cl- anions. | ||||||||
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| The absorption and photostimulated spectra of single crystals of the new substance Ba12F19Cl5 doped with Europium ions were studied. Creation of color-center-type absorption bands was observed under C band UV irradiation of the doped crystals. These samples show photostimulated luminescence when subsequently excited with the 20,492 cm−1 line of an Ar ion laser. Our experiments support the assignment that the PSL signal is from the Eu2+ ions. This system may be of interest as an UV storage phosphor. | ||||||||
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| The physical and photophysical properties of three classic transition metal complexes, namely [Fe(bpy)3]2+, [Ru(bpy)3]2+, and [Co(bpy)3]2+, can be tuned by doping them into a variety of inert crystalline host lattices. The underlying guest-host interactions are discussed in terms of a chemical pressure. | ||||||||
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| The application of statistical simulations to the estimation of transfer free energies of pharmacologically relevant organic molecules is reported. Large-scale molecular dynamics simulations have been carried out on a series of four solutes, viz. antipyrine, caffeine, ganciclovir, and α-d-glucose, at the water−dodecane interface as a model of a biological water−membrane interfacial system. Agreement with experimentally determined partition coefficients is remarkable, demonstrating that free energy calculations, when executed with appropriate protocols, have reached the maturity to predict thermodynamic quantities of interest to the pharmaceutical world. The computational effort that warrants accurate, converged free energies remains, however, in large measure, incompatible with the high-throughput exploration of large sets of pharmacologically active drugs sought by industrial settings. Compared to the cost-effective, fast estimation of simple partition coefficients, the present free energy calculations, nevertheless, offer a far more detailed information about the underlying energetics of the system when the solute is translocated across the water−dodecane interface, which can be valuable in the context of de novo drug design. | ||||||||
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| The [Ru(bpy)3][LiCr(ox)3] system (bpy = 2,2'-bipyridine, ox = oxalate) has two crystallographically non-equivalent [Cr(ox)3]3- sites. In steady-state resonant and nonresonant fluorescence line narrowing (FLN) experiments on the R1 lines of the two non-equivalent [Cr(ox)3]3- chromophores, multiline spectra are observed at 1.6 K. Such multiline spectra are clear evidence for resonant energy transfer processes within the inhomogeneously broadened R1 lines. In addition, time-resolved experiments show that also site-to-site energy transfer occurs, which turns out to be resonant, too, however with a non-negligible phonon-assisted contribution even at 1.5 K. | ||||||||
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| Efficient resonant energy transfer occurs within the R1 line of the4A2 →2E transition of the [Cr(ox)3]3- chromophore in mixed crystal [Rh(bpy)3][NaAl1 - x Cr x (ox)3]ClO4 (x = 0.05−0.9, ox = oxalate, bpy = 2,2'-bipyridine). This manifests itself in the form of multiline patterns in resonant fluorescence line narrowing (FLN) experiments at 1.5 K. The conditions for such a resonant process to occur are that the inhomogeneous line width of the R1 line is larger than the zero-field splitting of the ground state, which, in turn, is larger than the homogeneous line width of the transition. The number of lines and their relative intensities depend critically upon the [Cr(ox)3]3- concentration and the excitation wavelength within the inhomogeneous distribution. The basic model for resonant energy transfer as presented by von Arx et al. (Phys. Rev B 1996,54, 15800) is extended to include the effects of diluting the chromophores in an inert host lattice and of nonresonant R2 excitation. In addition, Monte Carlo simulations serve to explain the temporal evolution of the multiline pattern following pulsed excitation. | ||||||||
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| Variable-temperature1H and13C NMR measurements of theD 3-symmetrical triple-helical complexes [Ln(L1-2H)3]3 - (L1 = pyridine-2,6-dicarboxylic acid; Ln = La−Lu) show evidence of dynamic intermolecular ligand-exchange processes whose activation energies depend on the size of the metal ion. At 298 K, the use of diastereotopic probes in [Ln(L3-2H)3]3 - (L3 = 4-ethyl-pyridine-2,6-dicarboxylic acid) shows that fast intramolecularP ↔M interconversion between the helical enantiomers occurs on the NMR time scale. Detailed analyses of the paramagnetic NMR hyperfine shifts according to crystal-field independent techniques demonstrate the existence of two different helical structures, one for large lanthanides (Ln = La−Eu) and one for small lanthanides (Ln = Tb−Lu), in complete contrast with the isostructurality proposed 25 years ago. A careful reconsideration of the original crystal-field-dependent analysis shows that an abrupt variation of the axial crystal-field parameter A2 0 2>parallels the structural change leading to some accidental compensation effects that prevent the detection of structural variations according to the classical one-nucleus method. Crystal structures in the solid state and density functional theory calculations in the gas phase provide structural models that rationalize the paramagnetic NMR data. A regular triple-helical structure is found for small lanthanides (Ln = Tb−Lu) in which the terdentate chelating ligands are rigidly tricoordinated to the metals. A flexible and distorted structure is evidenced for Ln = La−Eu in which the central pyridine rings interact poorly with the metal ion. The origin of the simultaneous variation of structural parameters and crystal-field and hyperfine constants near the middle of the lanthanide series is discussed together with the use of crystal-field-independent techniques for the interpretation of paramagnetic NMR spectra in axial lanthanide complexes. | ||||||||
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| Electrochemical and chemical reductions of Rh(I) complexes ofL P4 (a macrocycle containing four phosphinine rings) and ofL P2S2 (a macrocycle containing two phosphinine rings and two thiophene rings) lead, in liquid solution, to EPR spectra exhibiting large hyperfine couplings with31P nuclei. An additional coupling (27 MHz) with103Rh is detected, in the liquid state, for the spectrum obtained with[L P2S2 Rh (0) ]; moreover, resolved31P hyperfine structure is observed in the frozen solution spectrum of this latter complex. DFT calculations performed on Rh(I) complexes of model macrocycles L' P4 andL' P2S2 indicate that, in these systems, the metal coordination is planar and that one-electron reduction induces a small tetrahedral distortion. The calculated couplings, especially the dipolar tensors predicted for[L' P2S2 Rh (0) ], are consistent with the experimental results. Although the unpaired electron is mostly delocalized on the ligands, the replacement of two phosphinines by two thiophenes tends to increase the rhodium spin density (ρRh =0.35 for [L' P2S2 Rh (0) ]). It is shown that coordination to Rh as well as one-electron reduction of the resulting complex provoke appreciable changes in the geometry of the macrocycle. | ||||||||
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| The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent 31 P nuclei. Additional couplings with two equivalent 13 C are observed with Ar–P= 13 C= 13 C=P–Ar. The 31 P anisotropic coupling constants could be obtained from spectra recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT andab initioSCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]• – and [H–P=CH–CH=P–H]• – . Although the optimized geometries of these two species are clearly different, neither of them leads to 13 C/ 31 P hyperfine tensors in conflict with the experimental results. The absence of any 1 H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar]• – more probable. | ||||||||
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| Nondestructive immobilization of cobalt and copper Schiff base complexes in silica aero- and xerogels was achieved via the sol−gel method using a precursor N,N'-ethylenebis(salicylidenaminato) (salen) ligand modified with pendant silyl ethoxy groups. Aerogels were obtained by semicontinuous extraction of the wet gels with supercritical CO2 and xerogels by conventional drying. Cobalt and copper(salen) containing silica gels were characterized by FTIR, UV−vis, and XPS spectroscopy, laser ablation-ICP-MS, and EPR studies. Aero- and xerogel incorporated salen compounds exhibited similar spectroscopic properties to cobalt/copper(salen) precursors and known metal(salen) compounds. BET measurements confirmed the importance of supercritical CO2 drying in maintaining the mesoporous structure of the aerogel. Laser ablation-ICP-MS and EPR studies of the aerogels showed that a uniform distribution of the isolated metal(salen) complex was achieved via molecular mixing using the sol−gel method. Stability of these materials was demonstrated by thermogravimetric analyses in air and leaching studies conducted under typical liquid-phase oxidation conditions. XPS analyses showed surface relative atomic concentrations in the modified gels to be similar before and following leaching studies. | ||||||||
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| The results of a study on the ground states of tricarbonato complexes of dioxouranate using multiconfigurational second-order perturbation theory (CASSCF/CASPT2) are presented. The equilibrium geometries of the complexes corresponding to uranium in the formal oxidation states VI and V, [UO2(CO3)3]4 - and [UO2(CO3)3],5 - have been fully optimized inD 3 h symmetry at second-order perturbation theory (MBPT2) level of theory in the presence of an aqueous environment modeled by a reaction field Hamiltonian with a spherical cavity. The uranyl fragment has also been optimized at CASSCF/CASPT2, to obtain an estimate of the MBPT2 error. Finally, the effect of distorting theD 3 h symmetry toC 3 has been investigated. This study shows that only minor geometrical rearrangements occur in the one-electron reduction of [UO2(CO3)3]4 - to [UO2(CO3)3],5 - confirming the reversibility of this reduction. | ||||||||
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| An implementation of spin–orbit coupling within a two-component generalization of the density functional code MAGIC is described. The spin–orbit operator is represented in the effective one-electron mean-field approximation and included into the Fock matrix within an iterative self-consistent scheme. First tests have been carried out for the spin–orbit splitting of several atoms. The spin–orbit effect on the bond distance and harmonic frequency of some diatomics has also been determined. This scheme allows to include spin–orbit in a simple way and can be efficiently used to treat large systems. | ||||||||
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| The1H and15N resonances of the carbon monoxide complex of ferrocytochromec ' ofRhodobacter capsulatus, a ferrous diamagnetic heme protein, have been extensively assigned by TOCSY−HSQC, NOESY−HSQC, and HSQC−NOESY−HSQC 3D heteronuclear experiments performed on a 7 mM sample labeled with15N. Based on short-range and medium-range NOEs and HN exchange rates, the secondary structure consists of four helices: helix 1 (3−29), helix 2 (33−48), helix 3 (78−101), and helix 4 (103−125). The15N,1HN, and1Hαchemical shifts of the CO complex form are compared to those of the previously assigned oxidized (or ferric) state. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. The χ-tensor is axial, and the orientation of thez-axis is approximately perpendicular to the heme plane. The paramagnetic chemical shifts of the protons of the heme ligand have been determined and decomposed into the Fermi shift and dipolar shift contributions. Magnetic susceptibility studies in frozen solutions have been performed. Fits of the susceptibility data using the model of Maltempo (Maltempo, M. M.J. Chem. Phys.1974,61, 2540−2547) are consistent with a rather low contribution of theS =3/2 spin state over the range of temperatures and confirm the value of the axial anisotropy. Values in the range 10.4−12.5 cm- 1 have been inferred for the axial zero-field splitting parameter (D). Analysis of the contact shift and the susceptibility data suggests that cytochromec ' ofRb. capsulatus exhibits a predominant high-spin character of the iron in the oxidized state at room temperature. | ||||||||
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| The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)6](BF4)2, by means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spin→low-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given. | ||||||||
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| The excited-state dynamics of the radical cations of perylene (PE•+), tetracene (TE•+), and thianthrene (TH•+), as well as the radical anions of anthraquinone (AQ•-) and tetracenequinone (TQ•-), formed by γ irradiation in low-temperature matrices (PE•+, TH•+, AQ•-, and TQ•-) or by oxidation in sulfuric acid (PE•+, TE•+, and TH•+) have been investigated using ultrafast pump−probe spectroscopy. The longest ground-state recovery time measured was 100 ps. The excited-state lifetime of PE•+ is substantially longer in low-temperature matrices than in H2SO4, where the effects of perdeuteration and of temperature on the ground-state recovery dynamics indicate that internal conversion is not the major decay channel of PE•+*. The data suggest that both PE•+* and TE•+* decay mainly through an intermolecular quenching process, most probably a reversible charge transfer reaction. Contrarily to AQ•-*, TQ•-* exhibits an emission in the visible which, according to theoretical calculations, occurs from an upper excited state. | ||||||||
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| Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The31P and13C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of2 in THF. Structures of the model molecules HP=C=Cp (where Cp is a cyclopentadienyl ring), [HP=C=Cp]√− and [HP---CH=Cp]√ have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]√− is readily protonated and that the EPR spectra are due to the phosphaallylic radical. | ||||||||
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| Stable for at least one week below -30°C: crystals of1, the first highly persistent diphosphanyl radical, have been isolated and characterized. This phosphorus-centered radical exhibits hyperfine coupling whose anisotropy is considerably larger than that for well-established nitrogen radicals (hydrazyls, nitroxides). This feature is of potential interest for studies of fast molecular movements. Mes*=2,4,6-tBu3C6H2. | ||||||||
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| Ternary V2O5–WO3/TiO2 catalysts were prepared by sequential or simultaneous grafting steps of vanadia and tungsta onto titania and were compared with similarly prepared binary catalysts. Different grafting sequences including alternating grafting of vanadia and tungsta were compared. During the grafting process the metal oxide precursor reacted with hydroxyl groups of the other grafted metal oxide. The interaction of vanadia with tungsta species resulted in a different reduction behavior of both metal oxides compared to the binary catalysts as indicated by temperature-programmed reduction with hydrogen. The strength of interaction of the grafted species depended on preparation sequence and metal loading. At low coverage (‹monolayer) catalyst properties were found to depend strongly on loading, but relatively little on the grafting mode, as indicated by vibrational spectroscopy. Laser Raman experiments at different laser power revealed reversible effects due to temperature induced structural changes of surface vanadia species. For all catalysts, even at a loading of more than one and a half monolayers, no evidence of crystalline vanadia or tungsta could be found. After calcination of WO3/TiO2 catalyst at 1023 K instead of 573 K and subsequent grafting with vanadia, new species with hydroxyl groups showing a vibrational frequency below 3600 cm−1 were formed. The increase of the calcination temperature had no significant influence on the reduction of vanadia by hydrogen. | ||||||||
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| The total rate constantk 1 has been determined atP = 1 Torr nominal pressure (He) and atT = 298 K for the vinyl-methyl cross-radical reaction: (1) CH3 + C2H3→ Products. The measurements were performed in a discharge flow system coupled with collision-free sampling to a mass spectrometer operated at low electron energies. Vinyl and methyl radicals were generated by the reactions of F with C2H4 and CH4, respectively. The kinetic studies were performed by monitoring the decay of C2H3 with methyl in excess, 6 < [CH3]0/ [C2H3]0 < 21. The overall rate coefficient was determined to bek 1(298 K) = (1.02 ± 0.53) × 10-10cm3 molecule-1 s-1 with the quoted uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C2H3 + H and C2H3 + C2H3. The present result fork 1 atT = 298 K is compared to two previous studies at high pressure (100-300 Torr He) and to a very recent study at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH3 + C2H5 and with a value fork 1 estimated by the geometric mean rule employing values fork(CH3 + CH3) andk(C2H3 + C2H3). Qualitative product studies atT = 298 K and 200 K indicated formation of C3H6, C2H2, and C3H5 as products of the combination-stabilization, disproportionation, and combination-decomposition channels, respectively, of the CH3 + C2H3 reaction. We also observed the secondary C4H8 product of the subsequent reaction of C3H5 with excess CH3; this observation provides convincing evidence for the combination-decomposition channel yielding C3H5 + H. RRKM calculations with helium as the deactivator support the present and very recent experimental observations that allylic C-H bond rupture is an important path in the combination reaction. The pressure and temperature dependencies of the branching fractions are also predicted. | ||||||||
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| Present paper is an overview of our efforts during the past few years to understand complicated corelations of physical phenomena related to pressure in Fe(I1) solid state spin transition systems. Some principal results concerningp, T, λ-experiments are extracted. In the context of correlation of the crystallographic phase transition with simultaneous HS → LS relaxation and LS → HS photopopulation, we show the latest results: Brillouin and magnetic measurements on the crystal [Fe(pt6](BF6)2. | ||||||||
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| Two distinct methods were used to investigate the role of Trp residues during Mg-ADP binding to cytosolic creatine kinase (CK) from rabbit muscle: (1) Raman spectroscopy, which is very sensitive to the environment of aromatic side-chain residues, and (2) reaction-induced infrared difference spectroscopy (RIDS) and photolabile substrate (ADP[Et(PhNO2)]), combined with site-directed mutagenesis on the four Trp residues of CK. Our Raman results indicated that the environment of Trp and of Tyr were not affected during Mg-ADP binding to CK. Analysis of RIDS of wild-type CK, inactive W227Y, and active W210,217,272Y mutants suggested that Trp227 was not involved in the stacking interactions. Results are consistent with Trp227 being essential to prevent water molecules from entering in the active site [as suggested by Gross, M., Furter-Graves, E. M., Wallimann, T., Eppenberger, H. M., and Furter, R. (1994)Protein Sci.3, 1058−1068] and that another Trp could in addition help to steer the nucleotide in the binding site, although it is not essential for the activity of CK. Raman and infrared spectra indicated that Mg-ADP binding does not involve large secondary structure changes. Only 3−4 residues absorbing in the amide I region are directly implicated in the Mg-ADP binding (corresponding to secondary structure changes less than 1%), suggesting that movement of protein domains due to Mg-nucleotide binding do not promote large secondary structure changes. | ||||||||
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| The use of hybridab initio QM/MM methods in studies of metalloenzymes and related systems presents a major challenge to computational chemists. Methods that include the metal ion in the quantum mechanical region should also include the ligands of the metal in this region. Such a treatment, however, should be very demanding if one is interested in performing the configurational averaging needed for proper calculations of activation free energies. In the present work we examine the ability of the frozen DFT (FDFT) and the constrained DFT (CDFT) approaches to be used inab initio studies of metal-catalyzed reactions, while allowing for an effective QM (rather than a QM/MM) treatment of the reacting complex. These approaches allow one to treat the entire enzyme byab initio DFT methods, while confining the SCF calculations to a relatively small subsystem and keeping the electron density of the rest of the system frozen (or constrained). It is found that the FDFT and CDFT models can reproduce the trend obtained by a full DFT calculation of a proton transfer between two water molecules in a (Im)3Zn2+(H2O)2 system. This and related test cases indicate that our approximated models should be capable of providing a reliable representation of the energetics of metalloenzymes. The reasons for the special efficiency of the FDFT approach are clarified, and the strategies that can be used in FDFT studies of metalloenzymes are outlined. | ||||||||
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| Incorporation of [Co(bpy)3]2+ into the cavities of the three-dimensional oxalate network structure in [Co(bpy)3][LiCr(ox)3] produces chemical pressure that destabilises the normal high-spin ground state4T1 to such an extent that the [Co(bpy)3]2+ complex becomes a spin-crossover complex. It shows a temperature-dependent equilibrium between the2E low-spin and the4T1 high-spin states. | ||||||||
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| The radical anion (tmbp) • -, where tmbp = 4,4',5,5'-tetramethyl-2,2'-biphosphinine, was generated by reduction of tmbp on a potassium mirror. EPR/ENDOR spectra and DFT calculations show that, in contrast to the neutral species, this anion is planar and that the unpaired electron is mainly delocalized on the PCCP fragment with a large participation of the phosphorus pπ orbitals. This planar structure was confirmed by the first crystal structure of an anionic biphosphinine: [tmbp][Li(2.2.1)]. Reduction of [Ni(tmbp)2] led to the 19-electron complex whoseg and31P hyperfine tensors were obtained from EPR in liquid and frozen solutions. These results, together with DFT calculations on [Ni(bp)2] and [Ni(bp)2] • -, indicate that, by accepting an extra electron, the neutral nickel complex distorts toward a more planar geometry and that the dihedral angle between the two phosphinine rings of each ligand slightly increases. In the reduced Ni complex, the unpaired electron is mainly delocalized on the ligands, in a molecular orbital which retains the characteristics of the SOMO found for the reduced isolated ligand. A charge decomposition analysis (CDA) shows that, in [Ni(bp)2], metal−ligand back-donation strongly contributes to the metal−ligand bonding. | ||||||||
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| In contrast to high-spin ferrous paramagnetic heme proteins, the chemical shifts of the heme protons are very unusual in the ferrocytochromesc'. Magnetic susceptibility studies ofRhodobactercapsulatus ferrocytochromec' in frozen solutions have been performed and indicate anS = 2 spin state and a large negative axial (D) zero-field splitting parameter (−18.3 cm- 1) as well as a significant rhombic (E) value (−4.9 cm- 1). The1H and15N resonances have been extensively assigned by TOCSY−HSQC, NOESY−HSQC, and HSQC−NOESY−HSQC 3-D heteronuclear experiments performed on a 8 mM sample labeled with15N. Based on short-range and medium-range NOEs and HN exchange rates, the secondary structure consists of four helices: helix-1 (3−30), helix-2 (34−49), helix-3 (78−97), and helix-4 (103−117). The15N, HN, and Hα chemical shifts of the reduced (or ferro) state are compared to those previously assigned for the diamagnetic carbon monoxide complex form. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. Values of −23 and −3 cm- 1 have been inferred forD andE, and thez-axis of the tensor is tilted approximately 30° from the normal to the heme. The paramagnetic chemical shifts of the heme protons have been determined and split up into Fermi shift and the dipolar shift contributions. The pattern of the contact shifts is very unusual, exhibiting a 2-fold symmetry, and is discussed in terms of molecular orbital interactions between the porphyrin macrocycle and the imidazole ring. | ||||||||
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| The propagation of the high-spin (HS) → low-spin (LS) relaxation at 53 K in a single crystal of the iron (II) spin-crossover compound [Fe(ptz)6](BF4)2 was followed by photography, after inducing the local photoexcitation to the metastable HS state at 20 K using the single wavelength (457 nm Ar± ion laser) irradiation. The photoinduced formation of the HS—LS patterns with a characteristic diameter of some 0.1 mm was observed to occur inhomogeneously at a macroscopic scale already during photoexcitation. The contrast between the HS (transparent) and the LS (purple) regions was amplified during relaxation. The effect is described in terms of a transient instability, for which a microscopic model in the mean-field approximation is proposed. The mechanism for the development of patterns at the macroscopic scale is discussed. | ||||||||
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| Persistent spectral hole burning was performed on the 7 F 0–5 D 1 transition of Sm2+ in thin films of SrFCl. Depending on the substrate and the growth conditions, a total hole depth between 47% and 70% was reached. The holes were Lorentzians of width 4(±0.3) cm−1. The time evolution of the hole depth was studied. It is described by two exponentials: a short time decay (t 1 = 0.37 days) and a long time decay (t 2 = 20.4 days) with a 20% infinite time limit. One- and two-photon burning mechanisms act. | ||||||||
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| Electronic energy transfer from [Cr(ox)3]3- (ox = oxalate) in three-dimensional (3D) anionic oxalate networks to encapsulated [Cr(bpy)3]3+ (bpy = 2,2'-bipyridine) cations at 1.5 K was investigated by time-resolved luminescence spectroscopy. Two series of mixed crystals of nominal compositions [NaAl1 - x Cr x (ox)3][Rh0.99Cr0.01(bpy)3]ClO4 (x = 0, 0.01, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1) and [NaAl0.99Cr0.01(ox)3][Rh1 - y Cr y (bpy)3]ClO4 (y = 0, 0.01, 0.02, 0.03, 0.04, and 0.05) were utilized. Energy transfer from [Cr(ox)3]3- to [Cr(bpy)3]3+ occurs by two mechanisms. Rapid, short-range transfer (k et > 106 s-1) is attributed to superexchange coupling between the Cr3+ ions via π overlap of the oxalate and bipyridine ligands. In addition, at low [Cr(ox)3]3- concentrations (nominallyx = 0.01) a very much slower process with a maximumk et ≈ 200 s-1 is identified in the time-resolved spectra and attributed to a dipole−dipole mechanism. Furthermore, the resonant [Cr(ox)3]3- to [Cr(ox)3]3- energy migration previously reported by von Arx et al. (Phys. Rev.(1996),B54, 15800) assists [Cr(ox)3]3- to [Cr(bpy)3]3+ transfer as the [Cr(ox)3]3- concentration increases. | ||||||||
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| In iron(II) spin-crossover compounds, the transition from the1A1 low-spin state at low temperatures to the5T2 high-spin state at elevated temperatures is accompanied by a large increase in metal-ligand bond lengths. The resulting elastic interactions may be pictured as an internal pressure which is proportional to the concentration of the low-spin species. Because pressure stabilises the low-spin state relative to the high-spin state this results in a positive feedback. Thermal transition curves in neat iron(II) spin-crossover compounds are thus invariable much steeper than in diluted mixed crystals, and the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is self-accelerating. Strong interactions give rise to a thermal hysteresis, and light-induced bistabilities may be observed for compounds with initially a high-spin ground state and the potential for a light-induced population of the low-spin state. For such compounds, the increasing internal pressure may stabilise the low-spin state sufficiently so that it becomes the molecular ground state above some critical light-induced low-spin fraction. Secondary effects of the elastic interactions include crystallographic phase transitions, inhomogeneous distributions of sites, and anomalies such as steps in the transition curve. | ||||||||
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| The adsorption of methanol on the (110) surface of γ-alumina was investigated using both ab initio and density functional theory quantum chemical methods. A [Al3O9H10]+ cluster model comprising one tetrahedral and two octahedral aluminum cations were used to describe the surface and the mechanism of adsorption of methanol. This has allowed us to rationalize the stable structures of adsorbate and the mode of bonding. The IR frequency shifts between the gas phase and the adsorbed species were also calculated and they exhibit good agreement with experiment. | ||||||||
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| A "CO-like matrix", showing coordination analogous to that of carbonyl groups, is provided by silacalix[4]phosphinine macrocycles. Reaction with AuI leads to the first gold(I) complexes of macrocycles, which can be reduced with sodium or potassium to the paramagnetic gold(0) complexes (an example is shown), as evidenced by cyclic voltammetry and EPR spectroscopy. | ||||||||
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| Single crystals of the new host Ba12F19Cl5 doped with Eu2+ were grown and studied by electron paramagnetic resonance (EPR) and luminescence emission spectroscopy. Three different Eu2+ sites were observed. Two of them had orthorhombic point symmetry while the last one was monoclinic. Physico-chemical and symmetry arguments allowed us to establish correspondence between the different Eu2+centres and the host cation lattice sites. All three centres presented in their ground state important crystal field splitting. The 80 K luminescence emission spectrum consisted of one broad unsymmetrical f-d band peaking at 22 700 cm-1. No 4f-4f transitions of the Eu2+ ion were observed between room temperature and 80 K. | ||||||||
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| The 4d10→4d95s1 transitions of the Ag+ ion in single crystals of NaF were investigated by one- and two-photon spectroscopy and luminescence experiments. The one-photon absorption spectrum extends from 238 to below 180 nm. At low temperature, several bands show resolved fine structure, but compared to NaF:Cu+, there is much less, and the lines are broader. The Jahn–Teller effect is much larger in Ag+ than in Cu+ and has been identified in three of the excited states. Trapping in Jahn–Teller minima is shown to change the emission kinetics radically compared to Cu+. The off-center force from the d9p states has much less effect on Ag+ than on Cu+. All of the evidence shows that Ag+ is more strongly coupled to the lattice than is Cu+. | ||||||||
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| Multicomponent thin films with spectral hole burning capacity at room temperature were synthesized by using molecular beam and pulsed laser deposition techniques All materials were activated by Sm2+ in low-symmetry alkaline earth sites, the synthesis involved the control of ionic diffiision rate during multilayer growth and special reduction of Samarium. Enhancement of hole burning rate by 1-2 orders is obtained in nanocrystalline films as compared to bulk and microcrystalline materials New hypothetic mechanism involving the creation of Sm-defect (photochromic) centers is proposed for reversible photoburning. | ||||||||
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| An implementation of the Douglas�Kroll (DK) transformation is described within a new relativistic quantum chemistry code, MAGIC, which performs calculations on systems containing heavy atoms. This method reduces the computational cost in terms of memory requirements that are associated with completeness identities in the DK implementation by factorizing the one-electron matrices into smaller ones that depend only on two atoms at a time. Examples are presented. | ||||||||
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| A study on the UF6 monomer and dimer was carried out within the density functional method. The U−F distance in the UF6 monomer was optimized at different levels of theory, pointwise, assuming octahedral geometry, (1) by using an all-electron basis for both U and F in a nonrelativistic calculation; (2) by using a relativistic effective core potential (RECP) on U and nonrelativistic effective core potential (ECP) on the fluorines; and (3) by using RECP on the U atom and an all-electron basis on the F atoms. Atomization energies of 23.11, 33.92, and 35.66 eV were obtained at the three levels, respectively. Relativistic effects lead to about a 50% increase in the atomization energy. For the UF6 dimer, the potential energy curve, as a function of the intermolecular U−U distance, was computed at level 2, and the rotational barrier between the two monomers was determined. Similar calculations were performed on the corresponding PuF6 species. Comparisons are made with experiment and other theoretical studies, where available. | ||||||||
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| An efficient approach for evaluating effective core potential integrals which involve projection operators has been implemented in the MAGIC quantum chemistry program. The methodology is presented and its performance is examined through illustrative calculations on transition metal and actinide compounds. | ||||||||
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| A series of compounds containing the [Mo3-μ3S-(μS2)3-(dtc)3]+ complex (dtc = diethyldithiocarbamate) with the anions I- (1), I- and Br- (2), S2- (3), ClO4- (4), NO3- (5), and SO42- (6) was prepared and characterized by elemental analysis, NMR, IR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH2Cl2 in the crystal having the composition [Mo3S7(dtc)3]I�0.5CH2Cl2 (1a), which was in contradiction to the previous protocol. The corresponding ClO4- compound (4a) is isotypic. Crystal data: C15.5H31Cl2Mo3N3O4S13, orthorhombic space group Aba2, a = 25.816(5) �, b = 17.761(4) �, c = 16.250(3) �, Z = 8. For 1a, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms Sax of the disulfido bridges. The Raman data showed a significant decrease of the Seq−Sax stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the Seq−Sax bond length and with a significant shortening of the X���Sax distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO3- and ClO4- the Sax���X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I- and S2- significant covalency is observed. In addition, the general reaction of [Mo3S7]4+ complexes with a nucleophile was modeled. With regard to the side-on bonding of the μ-S2 groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to μ3-S, are less pronounced. | ||||||||
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| The migrative insertion of CO into the Ni�CH=CH2 bond has been investigated by both static and dynamic density functional methods. The stationary points of the potential surface for the migrative insertion of CO into the Ni�CH=CH2 bond have been characterized using Cl(CO)2Ni�CH=CH2 as a model compound. Such a reaction has been found exothermic by 16 kJ mol−1, with an energy barrier of 9 kJ mol−1. Dynamic simulations have also been performed on Cl(CO)2Ni�CH=CH2 and show that the migrative insertion begins from the cis isomer and occurs via a simultaneous detachment of the vinyl group from the metal and formation of the vinyl�carbonyl bond. | ||||||||
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| Bicyclonucleosides bearing a 5-deoxy-5-N-hydroxyamino-3,N5-(1,1-ethano)-β-o-furanosyl sugar moiety (15-18) have been prepared by glycosidation of the corresponding bicyclosugars obtained via an intramolecular reverse Cope elimination. The configuration of the asymmetric carbon of the 1,1-ethano bridge is the most important factor directing the conformation of the N-hydroxypyrrolidine ring and its invertomers ratio as shown by variable temperature H NMR experiments. | ||||||||
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| Cyclic voltammetry shows that monophosphaallene ArPCC(C6H5)2 (where Ar = C6H2tBu3-2,4,6), 1a, undergoes irreversible reduction at 2266 mV in THF. The EPR spectra of the reduction products are obtained in liquid and frozen solutions after specific 13C enrichment of the allenic carbon atoms. The resulting hyperfine tensors are compared with those obtained from ab initio MP2, MCSCF, CI, and DFT calculations for the radical anion (HPCCH2)-� and for the monophosphaallylic radical (HP�−CHCH2) ↔ (HPCH−�CH2). The most elaborate treatments of the hyperfine structure (CI and DFT) indicate that the species observed by EPR is the monophosphaallylic radical. | ||||||||
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| Liquid phase EPR spectra of a diphosphaallenic radical anion have been Recorded after electrochemical reduction of a solution of ArPCPAr in THF at 293 K (Ar = 2,4,6-But3C6H2). The hyperfine coupling interactions of two 31P and one 13C nuclei (in the case of Ar13CPAr) are discussed in the light of AM1 calculations carried out on (ArPCPAr)�, of ab initio calculations performed on the model radical anion (HPCPH)� at the MP2 and MCSCF levels of theory and of DFT calculations on (HPCPH)�. The structure of the radical anion is compared with that of the neutral molecule. | ||||||||
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| A recent investigation of the (BaF2–MgF2) phase diagram produced several new compounds which are suitable hosts for Rare Earth impurities. We present results on single crystals of Ba2Mg3F10 doped with Eu2 +. The local structure and optical properties of this system were investigated by luminescence emission and by EPR. We observed two different Eu2 + sites. Both show Cs point symmetry and an important ground state splitting. Correlating our EPR and optical results with the new Ba2Mg3F10 structure data allowed the assignment of each of them to a specific barium lattice site. The luminescence emission of both the 4f7–4f65d and the 4f7–4f7 transitions is observed. The relative importance of the two emissions is strongly temperature dependent. The emission intensities of the intra f-shell6P7/2→8S7/2transitions increase strongly on going from 295 K to 77 K. Thus, the lowest levels of the 4f65d configuration are approximately degenerate with the6P7/2 manifold. | ||||||||
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| Eu2+ was introduced into pure and oxygen codoped BaMgF4 single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is Cs. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba0.8Eu0.2MgF4 single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu2+EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm−1. The results due to the Eu2+ impurity are given and discussed qualitatively within the 4f7↔ 4f65d1 scheme. | ||||||||
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| Molecular modelling and site-directed mutagenesis were used to identify eleven amino acid residues which may be involved in antagonist binding of the human tachykinin NK1 receptor. Recombinant receptors were expressed in mammalian cells using the Semliki Forest virus system. Wild type and mutant receptors showed similar expression levels in BHK and CHO cells, verified by metabolic labelling. Binding affinities were determined for a variety of tachykinin NK1 receptor antagonists in SFV-infected CHO cells. The binding affinity for GR203040, CP 99,994 and CP 96,345 was significantly reduced by mutant Q165A. The mutant F268A significantly reduced the affinity for GR203040 and CP 99,994 and the mutant H197A had reduced affinity for CP 96,345. All antagonists seemed to bind in a similar region of the receptor, but do not all rely on the same binding site interactions. Functional coupling to G-proteins was assayed by intracellular Ca2+ release in SFV-infected CHO cells. The wild type receptor and all mutants except A162L and F268A responded to substance P stimulation. | ||||||||
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| Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems. | ||||||||
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| A helium gas pressure cell for pressures up to 1 kbar (0.1 GPa) has been developed in conjunction with a closed-cycle He refrigerator allowing variable temperatures between 15 and 300 K. Both cell and refrigerator are equipped with optical windows suitable for photophysical measurements, such as temperature- and pressure-dependent absorption spectroscopy or laser flash photolysis. Examples of measurements on iron(II) spin-crossover systems are given. In these compounds, comparatively small external pressures induce significant changes in the thermodynamic equilibrium as well as in the relaxation dynamics. | ||||||||
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| Temperature-dependent Raman measurements between 190 and 358 K yield conformational enthalpy differences between 550 and 690 cal mol−1 for racemic liquid 2-butanol. The conformational properties of 2-butanol were also studied using MM2 and MM3 calculations. The conformer 1, where C1 and C4 are in anti position, was found to be the most stable comformer with the MM3 calculations. Conformer 2, which has C1 and O5 in anti arrangement, has ca. 500 cal mol−1 higher energy than 1. Comparisons of the calculated MM3 vibrational frequencies with the Raman spectra suggest that the most stable conformer in the liquid phase also adopts a Cl,C4 anti conformation. | ||||||||
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| Laser flash photolysis experiments were performed on the mixed crystal [Zn1−x Fe x (6-mepy)3tren](PF6)2 ( x =0.00025) at 10 K in the pressure range between 1 bar and 20 kbar. An external pressure of 20 kbar accelerates the low-temperature tunneling process by almost eight orders of magnitude. | ||||||||
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| The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H2O-ax)(H2O-eq)4L]2+ general formula. We have studied a series of complexes, where L = H2O, MeCN, Me2SO, H2CCH2, CO, and F2CCH2. We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E��(+2/+3), increases with the π-accepting capabilities of the ligands. For L = N2, the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)−N2 bond. The water exchange reactions have been followed by 17O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H2O)5(H2CCH2)]2+ (ΔVax and ΔVeq = +6.5 � 0.5 and +6.1 � 0.2 cm3 mol-1) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F2CCH2 CO < Me2SO < N2 < H2CCH2 < MeCN < H2O. The increase of the lability is accompanied by a decrease of the E�� values, but no change was found in the calculated Ru−H2Oeq bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N2 MeCN < H2O < CO < Me2SO < H2CCH2 < F2CCH2. A variation of the Ru−H2Oax bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru−H2Oax bond energies. It appears, also, that a decrease of the electronic density along the Ru−Oax bond and the increase of the lability can be related to an increase of the π-accepting capability of the ligand. For L = N2, the calculations have shown that the Ru(II)−N2 bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N2 ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H2O)5N2]2+ complex. | ||||||||
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| The isotropic hyperfine coupling constants of the diphosphaalkene radical cation have been measured by EPR spectroscopy after electrochemical oxidation of ArP]] C]] PAr (and ArP]] 13C]] PAr) in tetrahydrofuran (THF). The two 31P constants as well as the 13C coupling are close to 90 MHz. Taking HPCPH as a model compound, the structure has been assessed, by extensive ab initio calculations including correlation effects at the MP2 and MCSCF levels of theory. It is found that oxidation of the allenic ]P]] C]] P] structure leads to the formation of two rotamers with HPPH dihedral angles of 458 and 1358. These two structures are compatible with the Jahn�Teller distortion of allene. The calculated hyperfine constants support the EPR results. | ||||||||
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| The results of a detailed optical and paramagnetic-resonance study performed on copper in CaF2 and silver in BaF2 are presented. Two different Ag+ centers were identified in BaF2. One is associated with an interstitial F- ion whereas the other one has a cubic surrounding. The Cu2+ ion in CaF2 was shown to reorient at 4.2 K between 6 equivalent minima of D2h symetry. This fact is interpreted with the aid of a T2gx(T2g+Eg) type Jahn-Teller effect. The nonlinear mixed coupling terms are shown to play an important role. The Cu+ impurity in CaF2 is presumably off-center in the F- sublattice without associated defect or impurity. | ||||||||
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| A detailed analysis of Zeeman splittings of highly resolved spin-forbidden transitions in [Cr(bpy)3](PF6)3 is presented. Assignments of vibronic bands are made based on low-temperature absorption, emission, and infrared spectra. The pattern of doublet states, obtained for H = 0 and H = 5 T, is consistent with angular overlap model (AOM) calculations, which allow one to consider σ- and π-interactions between the metal-d and relevant ligand orbitals and the particular angular geometry of the chromophore simultaneously. The observed level splittings are found to result from the combined effect of trigonal distortion and contributions of the symmetry adapted dπ-orbitals involved due to coupling with corresponding counterparts from the bidentate ligand (phase coupling). The larger splitting of the lowest excited state2Eg(O h ) in the analogous ClO4 - salt is due to the more distorted geometry of the [CrN6] moiety. Related properties of the bipyridine ligand, which turn out to show donor behavior in the present compounds, and the acetylacetonate ligand are discussed, and AOM parameters for the metal−ligand π-interaction are correlated with results of MO calculations. | ||||||||
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| Fresh fracture surfaces of the martian meteorite ALH84001 contain abundant polycyclic aromatic hydrocarbons (PAHs). These fresh fracture surfaces also display carbonate globules. Contamination studies suggest that the PAHs are indigenous to the meteorite. High-resolution scanning and transmission electron microscopy study of surface textures and internal structures of selected carbonate globules show that the globules contain fine-grained, secondary phases of single-domain magnetite and iron sulfides. The carbonate globules are similar in texture and size to some terrestrial bacterially induced carbonate precipitates. Although inorganic formation is possible, formation of the globules by biogenic processes could explain many of the observed features, including the PAHs. The PAHs, the carbonate globules, and their associated secondary mineral phases and textures could thus be fossil remains of a past martian biota. | ||||||||
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| Resonant fluorescence line narrowing of the R1 line of the [Cr(ox)3]3− chromophore in [Rh(bpy)3][NaCr(ox)3]ClO4 at 1.6 K neither gives rise to the usual three-line pattern nor to spectral diffusion. Instead multi-line spectra with spacings equal to the zero-field splitting of the ground state are observed. This phenomenon is attributed to efficient non-radiativeresonant energy transfer within the R1 line. | ||||||||
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| Hydroxyurea (HU), a ribonucleotide reductase inhibitor has been used in the treatment of some malignant and viral diseases and seems now to be promising, in association with 2,3dideoxynucleosides, for the management of AIDS. In an attempt to increase the specificity of action of this radical scavenger, or at least, to study the topological aspects of its reactivity, we introduced the N-hydroxyureido group into nucleosides by using Mitsunobu reaction or by reacting a nucleoside nitrogen nucleophile with a carbonyl electrophile. From the currently available antiviral testing results, concerning the nucleoside analogues it appears that the most noticable activity exert against Varicella Zoster virus (VZV). One acyclonucleoside derivative was found to be very active against the virus HIV-1, its therapeutic index is better than 100.000. We prepared peptid-like dinucleotide analogues33,36 also in which the internucleosidic bridge consists of a spacer of approximately the same length as in the natural compounds. These compounds could be tested as inhibitors of nucleotide-protein interactions, we supposed that they are able to disrupt zinc finger parts of nucleocapsid. Antiviral activity of these dinucleotides were tested in vitro against HIV-1, HIV-2, HSV-1, HSV-2, CMV, VZV and EBV but in no case EC50 values inferior to 10 �M was found. | ||||||||
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| Methane concentrations were measured in the water column of the northern basin of Lake Lugano in 1993 and 1994. Methane profiles show three distinct zones: (1) low concentrations generally below 0.1 mmol m−3 from the surface to 80�90-m depth; (2) a. sharp rise in concentration up to 50 mmol m−3 from 80�90- to not, vert, similar 150-m depth; and (3) from not, vert, similar 150-m depth to the bottom with a lower concentration gradient and with the highest methane concentrations in near-bottom water was not, vert, similar 80 mmol m−3. These profiles result from a combination of several factors such as fluxes from sediments, spatially variable vertical mixing, and aerobic and anaerobic bacterial oxidation in the water column. δ13C values indicate a biogenic origin for the methane. The methane inventory in the anoxic hypolimnion is not, vert, similar 2000 metric tonnes, yet its transfer to the surface water is almost completely cancelled by oxidation at the permanent redox interface situated at not, vert, similar 90-m water depth. Increased methane concentrations in near-surface water are not related to the deep-water methane reservoir but probably result from bacterial production of methane in the photic zone. Surface water is over saturated with methane in respect to the atmosphere. Anthropogenic eutrophication in temperate zone lakes has affected the carbon budget and methane production and storage. The potential impact of these changes on global emissions to the atmosphere should be evaluated. | ||||||||
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| The liquid Mg - Bi system exhibits strong compound formation at the `octet' composition . We present results of first-principles molecular dynamics simulations of this alloy system at different compositions: the pure Mg and Bi liquid components, the stoichiometric liquid, and a Mg-rich composition . For the pure liquids, our results are in excellent agreement with experimental diffraction data. For , a significant modification of the characteristics of the local ordering is found w.r.t. the crystalline -phase: the ordering in the liquid is much more ionic. This structural modification is consistent with the structure of the superionic -phase, that was reported recently by Barnes et al 1994 J. Phys.: Condens. Matter 6 L467. Our simulations cannot reproduce the `reverse' metal - nonmetal transition observed upon melting, the computed conductivity being much larger than found in experiments. Instead, for the Mg-rich alloy, the calculated conductivity approaches closely to the experimental value. | ||||||||
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| The electroaffinity of the O2 molecule is revisited using density functional theory (DFT) and perturbation treatments built on a MCSCF wavefunction that includes most of the non-dynamic correlation effects (MC/P approach). Using a standard 6�31 + G* basis set, DFT treatments based on BLYP or B3LYP functionals provide electroaffinities of the order of +0.6 eV that compare favorably to experiment. Coupled MCSCF/perturbation treatments using an Epstein-Nesbet partition of the molecular Hamiltonian give a more accurate value of +0.492 eV in excellent agreement with the most recent experimental data (+0.431 eV) as well as with highest-level purely variational ab initio treatments which are far less tractable for larger systems. The analysis of the results in terms of differential correlation effects made it possible to identify the failure of the previous MCSCF-limited treatments as arising from the dynamic correlation of the electron pair describing the σO---:O bond. | ||||||||
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| From our combined experimental and computer modeling study we found a structurally and kinetically well-defined second coordination shell around chromium(III) ions in aqueous solution. Strong hydrogen binding due to polarization of first coordination sphere water molecules leads to a mean coordination number of 12.94 water molecules in the second shell and to short first shell hydrogen−second shell oxygen distances of about 1.4 �. The experimentally measured exchange rate constant of = (7.8 � 0.2) � 109 s-1 (ΔH = 21.3 � 1.1 kJ mol-1, ΔS = +16.2 � 3.7 J K-1 mol-1) corresponds to a lifetime of 128 ps for one water molecule in the second coordination shell and compares very well with a lifetime of 144 ps as observed from molecular dynamics simulation of a [Cr(H2O)6]3+ complex in aqueous solution. The geometry and the partial atomic charges of [Cr(H2O)6]3+ were determined by density functional theory (DFT) calculations. Water exchange from the second coordination shell to the bulk of the solution proceeds between a H2O sitting in the second shell and an adjacent one which just entered this shell from the bulk. By a small rotation of the first coordination shell water molecule, one of its two hydrogen bonds jumps to the entered water molecule and the one which lost its hydrogen bond leaves the second shell of the [Cr(H2O)6]3+. This associative reaction mode is a model for water exchange between water molecules which are bound by strong hydrogen bonds, as in the case for strongly polarizing 3+ ions such as Al3+ or Rh3+. Furthermore, the exchange phenomenon between second sphere and bulk water involving only two adjacent water molecules is strongly localized and independent of other water molecules of the second shell. In this respect it may be considered as a starting point for a study of water exchange on a protonated metal oxide surface. | ||||||||
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| We present molecular dynamidfree energy calculations on the molecules acetamide, N-methylacetamide, N,N-dimethylacetamide, ammonia, methylamine, dimethylamine, and trimethylamine. Unlike the experimental data, which suggest a very non-additive solvation free energy (N-methylacetamide and methylamine having the most negative free energy of solvation), the calculations all find that the free energy of solvation monotonically increases as a function of methyl addition. The disagreement with experiment is surprising, given the very good agreement (within 0.5 kcal/mol) with experiment for calculation of the solvation free energy of methane, ethane, propane, water, methanol, and dimethyl ether. | ||||||||
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| A series of 2,3-O-cyclopentylidene C-glycoside analogs in which the furanose ring has been replaced with a N-hydroxypyrrolidine have been prepared. A structural study of these tricyclic compounds and the aminoxyl radicals thereof has been carried out using variable temperature 1H NMR, X-ray diffraction, molecular dynamics and EPR spectroscopy. Both types of compounds, N-hydroxypyrrolidines and pyrrolidine N-oxyls, fundamentally prefer - in solutions- N-endo conformations over the alternate, N-exo forms found by X-ray diffraction studies and computed to be more stable by molecular dynamics. | ||||||||
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| The fundamental aspects of N2 and 0 2 adsorption in zeolites have been investigated by density functional calculations on different models. Simple systems where these molecules interact with a positive point charge or with isolated Li+ and Na+ cations have led to a qualitative explanation for the N2/02 separation process. A classical description involving electrostatic and induction energies is adequate to explain the basic reason for a stronger N2 adsorption. At short distances (bonding interaction), the electronic structure of the cation has to be taken into account. The presence of core electrons in large cations limits the stabilizing contribution of the electrostatic and induction terms to the total energy, implying that Li+ is more efficient than Na+ in the adsorption process. The presence of zeolite clusters decreases the binding energies for both N2 and 0 2 , but the main trends remain valid. Moreover, due to a larger screening of 0 2 adsorption, it improves the efficiency of Li+ with respect to Na+ for the N2/02 separation. | ||||||||
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| The63Cu NQR spectra of five dicoordinated complex cations of Cu(I) with substituted imidazoles as ligands and six analogous complexes with substituted pyrazoles as ligands are reported. The structures of four of these complexes have been previously determined and the relationship of their63Cu resonance frequency to the average Cu---N bond length is compared to that of the analogous lutidine or collidine complexes. It is concluded that there are probably significant differences between the electronic structures of the pyridine complexes and those of the pyrazole or imidazole series. | ||||||||
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| Intersystem crossing is the crucial first step determining the quantum efficiency of very many photochemical and photophysical processes. Spin-crossover compounds of first-row transition metal ions, in particular of Fe(II), provide model systems for studying it in detail. Because in these compounds there are no competing relaxation processes, intersystem crossing rate constants can be determined over a large temperature interval. The characteristic features are tunnelling at temperatures below ∼80 K and a thermally activated process above ∼ 100 K. This, as well as the twelve order of magnitude increase of the low-temperature tunnelling rate constant on going from a spin-crossover compound with a small zero-point energy difference to a low-spin compound with a substantially larger one, can be understood on the basis of a nonadiabatic multiphonon process in the strong vibronic coupling limit. | ||||||||
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| Quantum chemical calculations based on density functional theory have been performed on Cr(CO)6, (eegr6-C6H6)Cr(CO)3 and (eegr6-C6H6)Cr(CO)2(CS) at the local and nonlocal level of theory using different functionals. Good agreement is obtained with experiment for both optimized geometries and metal-ligand binding energies. In particular, a comparison of metal-arene bond energies calculated for the (eegr6-C6H6)Cr(CO)3 and (eegr6-C6H6)Cr(CO)2(CS) complexes correlates well with kinetic data demonstrating that substitution of one CO group by CS leads to an important labilizing effect of this bond, which may be primarily attributed to a larger pgr-backbonding charge transfer to the CS ligand as compared with CO. | ||||||||
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| The parametrization of the EHMO-ASED method we have recently suggested for organometallics is shown to be also applicable, in principle without any modification, to derive the major structural parameters of second-row transition metal systems such as carbonyls or metallocenes. Furthermore, this model leads to satisfactory results when used to calculate the structure of compounds as large as (N-methylindole)tricarbonylchromium(0) or (phenylo.xazoline) tricarbonylchromlum(0) with full geometry optimization of the ligands. | ||||||||
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| The tetragonal Er3+ ion associated with the interstitial F− ion along the [100] axis in CaF2 is studied using ENDOR. The parameters of the transferred hyperfine interaction and of the nuclear Zeeman interaction of the surrounding fluorine ions are determined. Anomalously large values of the pseudo-nuclear Zeeman effect on the F− nuclei are found. The theoretical analysis of these parameters is carried out in a frame of operator techniques in the theory of transferred hyperfine interactions. A number of useful relations for practical calculations of the values of the local field at ligand nuclei are reported. | ||||||||
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| The absorption spectra of the ferrate (VI) ion (FeO2- 4) in K2MO4 (M = S, Se, Cr, Mo) host lattices consist of a series of relatively weak bands at low energy, which can be assigned to transitions within the partially filled 3d shell and some intense bands at higher energy, which are assigned to ligand-to-metal charge transfer transitions (LMCT). In the near-infrared (NIR) region sharp lines are observed belonging to the spin-forbidden spin-flip transitions3A2→ 1E and 3 A2 → 1 A1. The lowest excited state is the1E state, serving as initial state for1E →3A2 sharp-line luminescence at around 6200 cm-1. Another luminescence is observed centered at 9000 cm-1, which is assigned to the3T2 →3A2 transition. It is rather broad and three orders of magnitude weaker than the1E luminescence at 30K as a result of efficient non-radiative relaxation processes to the1E state. The temperature dependence of the total intensity and the lifetime of the1E →3A2 luminescence is understood within a complex scheme of radiative and non-radiative processes. | ||||||||
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| Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged. | ||||||||
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| A study of the effect of an external electric field on spectral holes burnt at different frequencies in the inhomogeneous absorption band of a centrosymmetric squaraine dye, bis [4-(diethylamino)-2-hydroxyphenyl] squaraine (DEAH), in polymers of different polarity is presented. Average matrix induced dipole moment differences of about 1 D and 0.37 D were measured in the directions parallel and perpendicular to the long axis of DEAH. In all polymers investigated, the induced dipole moment difference decreased from the higher to the lower frequencies. Solvatochromic shift measurements were performed in order to elucidate the origin of this effect. The matrix field inducing the dipole moment is also partially responsible for the frequency shift of the absorption of DEAH. With increasing matrix field, the absorptiion frequency is shifted to the blue due to electrostatic interaction with the local dipoles of DEAH. The contribution of the electrostatic interactions to the frequency shift is smaller than the dispersion interactions by two orders of magnitude in polystyrene, but increases slightly in more polar polymers. | ||||||||
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| The well-resolved absorption, excitation, and luminescence spectra of [Ir(ppy)2bpy]+ (ppyH = 2-phenylpyridine, bpy = 2,2'-bipyridine) in different media at cryogenic temperatures are presented. In solutions and glasses at ambient temperature the lowest energy excited state corresponds to an Ir - bpy charge-transfer excitation whereas in the crystalline host lattice [Rh(ppy)2bpy]PF6 the lowest excited state at 21 450 cm-1 is assigned to a 37r-r* excitation localized on the cyclometalating ppy- ligands. The next higher excited Ir - bpy charge-transfer state has shifted to 21 820 cm-', only 300 cm-I above the 3LC excited state. The close proximity of the 3LC and 3MLCT excited states and the large spin-orbit coupling constant of Ir3+ induce a strong mixing of charge-transfer character into the 3LC lowest excited states, resulting in increased oscillator strengths, reduced lifetimes, short axis polarized transitions, and a large zero-field splitting of 10-15 cm-1. | ||||||||
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| Making use of the phenomenon of light-induced spin-crossover in the [Zn1−xFex(ptz)6](BF4)2 spin-crossover system, very high diffraction efficiencies η can be achieved in non-degenerate four-wave-mixing. In the mixed crystal with x=0.1 and at 76 K, i.e. at a temperature where the system is predominantly in the low-spin state, a value for η of 43% was obtained. This is attributed to a phase grating due to the large difference in metal---ligand bondlength between the low-spin ground state and the light-induced high-spin state. The rate constant for the decay of the laser-induced grating as a function of temperature is found to be exactly twice the one of the high-spin→low-spin relaxation, as expected for a dilute system in the absence of cooperative effects. | ||||||||
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| The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr9 3- (M=Cr3+, Ho3+) dimers as discrete units, and the magnetic excitations of mixed Yb3+-Cr3+ and Yb3+-Ho3+ dimers could thus be observed. The Yb3+-Cr3+ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr3+, has to be included in the exchange Hamiltonian. For the Yb3+-Ho3+ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho3+ ion. Taking into account the full (2J+ 1) ground-state multiplet of Ho3+, as well as anisotropic exchange, gives a satisfactory description of this dimer. | ||||||||
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| Spectral hole-burning studies of nile red and cresyl violet in polyvinylbutyral and polyvinylformal films have been performed. From the shape of spectral holes under the influence of an electric field, the dipole moment difference between the ground and excited state of both dyes has been determined. The Stark effect was investigated at different positions in the inhomogeneously broadened absorption band of the guest molecules. The observed dipole moment difference decreases with increasing wavelength. This variation is caused by the matrix induced dipole moment. For nile red, which is a neutral and polar molecule, the distribution of induced dipole moments is strongly correlated with the orientation of its ground state dipole moment. In the case of cresyl violet perchlorate, which is a salt, this distribution is anisotropic for guests absorbing in the blue part of the inhomogeneous band but becomes more isotropic as the absorption wavelength increases. The wavelength dependence of the observed dipole moment is much stronger and is ascribed to the existence of the cresyl violet perchlorate salt in different states of solvation. | ||||||||
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| A study of the hole-burning mechanisms of bis[4-(diethylamino)-2-hydroxyphenyl]squaraine (DEAH) and bis[4-(disethylamino)-phenyl]squaraine (DEA) in hydrogen-bonding and non-hydrogen-bonding polymers is presented. Intramolecular H-bonding is only possible for DEAH. In all systems, the spectral holes are not persistent and decay with a distribution of rates ranging from 10-5s-1 to about 1 s-1, the time resolution of the experiment. In H-donating matrices, this distribution varies with the burning wavelength. From the hole-burning efficiencies and the kineticsof the hole refilling, four different types of nonphotochemical hole-burning mechanisms are postulated. The efficiency of these mechanisms depends mainly on the occurrence of processes slowing down the relaxation to the initial product state. | ||||||||
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| The conformational analysis of TMABN by three different methods X-ray analysis, photoelectron spectroscopy, and UV molar absorption coefficient yields a twist angle of the dimethylamino group of 60-70° in the ground state, whereas DMABN is not far from planar in qualitative agreement with the predictions from force field calculations (QCFF/PI and MM3). Dipole moment determinations by the thermochromic method agree with those from other methods (solvatochromism, electrochromism and time resolved microwave absorption) in that the excited state dipole moment of TMABN is very large, as well as that of the TICT state of DMABN. Its value increases somewhat with solvent polarity. This is explained by a nuclear polarizability model. The force field calculations are used to predict twist angle values for various sterically hindered DMABN derivatives | ||||||||
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| The constructive interference between two Stark-effect-broadened holograms produced by spectral hole burning is discussed. The holograms are burned at the same frequency but at different external electric-field values. The phase difference is selected to be zero so that constructive interference between the waves diffracted by each grating occurs. Experimentally it is found that a dip in the hologram efficiency that is not predicted by previous theory occurs for all reconstruction external electric-field values in the region between the original burn values. This dip is interpreted as being due to the time nonlinearity of the hologram burn process. The dip corresponds to those molecules, oriented in a specific direction with respect to the electric field, for which no Stark shift occurs and that are therefore resonant with the laser during the production of both holograms. The width of the anomalous feature is close to that of the hologram when the hologram is reconstructed at the original burn external electric-field strength. Other molecular orientations may be selected by burning pairs of holographic gratings at other combinations of the frequency and the electric field. | ||||||||
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| A series of [Cr(benzene)(CO)2L] complexes with L = PPh3, P(OMe)3, PPh2 ((−)-menthyl), P(OPh)2(O-(−)-menthyl), P(O-(−)-menthyl)3 were subjected to a nueleophile addition/acylation sequence to give trans-5,6-disubstituted cyelohexadienes. Low-to-moderate asymmetric induction was observed with the chira] ligands. Experimental and theoretical evidence for an alkylation at the metal center trans to the P ligand is presented, and a crystal structure determination of a [Cr(η5-cyclohexadienyl)(P(OMe)3)(CO)2SnPh3] complex is included. | ||||||||
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| The 5T2(HS)1A1(LS) intersystem crossing rates have been determined for a number of Fe(II) coordination compounds between 10 and 270 K using time-dependent optical spectroscopy. Strong deviations from Arrhenius kinetics with nearly temperature independent tunneling at low temperatures and a thermally activated behavior at elevated temperatures with apparent activation energies smaller than the classical energy barrier were found. The tunneling rates range from ~10−6 s−1 for the doped spin crossover system [Zn1−xFex(ptz)6](BF4)2 to ~106 s−1 for the doped low-spin (LS) system [Zn1−xFex(bipy)3](PF6)2. The large range of 12 orders of magnitude in the low temperature tunneling rates as well as the activated region can be understood in terms of nonadiabatic multiphonon relaxation. Values for the Huang–Rhys parameter S of 40–50 and for the reduced energy gap p of 1–12 are estimated for the present series of compounds. The validity of an inverse energy gap law in the strong vibronic coupling limit with Sp is borne out by experiment. | ||||||||
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| Recently, we have discovered a fascinating photophysical effect in spin crossover complexes of iron(II) : Light-Induced Excited Spin State Trapping (LIESST). At sufficiently low temperatures, the low spin state (1A1) can be converted quantitatively to the high spin state (5T2) by irradiating the sample into the 1A1 → 1T1 d-d absorption band (—540 nm). The resulting metastable HS state has a very long lifetime at low temperatures, in some cases it does not decay noticeably over a period of several days at 10 K. Only at temperature above some critical temperature does thermal relaxation back to the LS state set in. The sample can also be reconverted to the LS state by irradiating into the 5T2 → 5E absorption band (∞50 nm). The system thus behaves like an optical switch. The relative positioning - horizontally and vertically - of the potential wells of the two spin states is crucial for the lifetime of the metastable HS state. | ||||||||
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| Temperature-dependent Raman measurements of n-propylammonium chloride solutions in water, concentrated hydrochloric acid and methanol yield experimentalgauche-trans enthalpy differences of 0.21 ± 0.13, 0.51 ± 0.26 and 0.37 ± 0.11 kcal mol−1, respectively. Raman spectra in different aqueous solutions show that thegauche-trans equilibrium in solution is affected by the total chloride concentration. Thegauche-trans enthalpy difference for CH3CH2CH2NH3 + is found by 4-31G ab initio calculations to be 0.26 kcal mol−1. | ||||||||
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| The ESR of small concentration of Gd 0.03<y<0.06 substituting for Y in single crystals of Gd y Y1−y Ba2Cu3O6−x has been measured. In the insulating compound, withx ~0.1, and the superdconducting materials with 30 K <T c < 80 K, the measurements were performed at X-band, 9.3 GHz, and Kα-band, 36 GHz, over a large temperature range aboveT c. Angular dependence measurements exhibit a spectrum which is fully resolved in certain directions, but only partially resolved, because of exchange narrowing, in other directions. Comparisons between the spectra in the insulating and superconducting compounds shows similar angular dependent behavior. This seems to indicate that the origin of the exchange narrowing is the same in both compounds. Since this narrowing in the insulating compound arises from interaction with, or via, the Cu magnetic system, it is implied that there is a similar, perhaps fluctuating, system in the superconducting state. Preliminary measurements of the temperature dependence of the line widths may indicate the presence of spin pairing at about 110 K, above the actualT c of 70 K. The crystal field parameters areD = 3B 0 2 = 1307MHz,B 0 4 = 3.014MHz andB 4 4 = -11.43MHz, for the semiconducting sample. Theg-value is 1.989 ± 0.005. These values change only slightly in the superconducting crystals. | ||||||||
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| It is known that [Fe (2-mephen)3] (ClO4)2 in the solid state is an iron(II) spin-crossover system which shows light-induced excited spin state trapping (LIESST). The thermal spin-crossover behaviour of the complex [Fe(2-mephen)3]2+ embedded in various polymer matrices is similar to the solid-state behaviour, and a light-induced long-lived excited state is observed at temperatures below 50 K. Relaxation curves show that polymer matrices are not very homogeneous media. | ||||||||
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| [Fe(ptz)6l(BF4)2 (ptz= 1-propyltetrazole) and the mixed crystals [Znl,Fe,(ptz)6] (BF4)2 are Fe(I1) spin-crossover compoundsthat exhibit light-induced excited-spin-state trapping. It is shown that (a) for x I 0.1 a single-ion treatment of both the spinequilibrium ( M H L = 510 (12) cm-', ASHL = 5.1 (2) cm-'/K at T = 100 K) and the relaxation from the excited high-spin state (.Eao = 810 (30) cm-I, A - 105/s) is appropriate and (b) for 0.2 I x I 1 cooperative effects observed in the relaxation from the high-spin state are of long-range nature and therefore of elastic rather than of electronic origin. | ||||||||
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| Low-temperature luminescence and absorption spectra were recorded of VClz doped into MgCl, and CdCl, as well as of the pure compound. There is evidence for excitation energy transfer in Velz down to 5 K. In the diluted materials the luminescence remains unquenched up to 200 K (Cd,,V,Cl2) and 250 K (MgI-,V,CI2). The broad-band 4T2, - 4Azs luminescence transition is highly structured in the diluted samples. There is multiple evidence for a Jahn-Teller effect in the 4Tz, state with an estimated Jahn-Teller energy of the order of 200 cm-I. Polarized absorption and Zeeman meausurements were used to assign the 4Az, -,E,, *TI, transitions. | ||||||||
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| We have obtained the room temperature Raman spectra of the three salts n-PrNH3X (X = Cl, Br and I). The concentration ofgaucheisomers increases when passing from the chloride to the iodide. We have evaluated experimentally the lattice contribution to the energy difference betweengauche andanti conformers. A force field calculation yielded an energy difference of 0.85 kcal mol−1 between the two conformations of the propyl-ammonium moiety. | ||||||||
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