X-ray absorption spectroscopy of ground and excited rhenium-carbonyl-diimine complexes: evidence for a two-center electron transfer.

Steady-state and picosecond time-resolved X-ray absorption spectroscopy is used to study the ground and lowest triplet states of [ReX(CO)(3)(bpy)](n+), X = Etpy (n = 1), Cl, or Br (n = 0). We demonstrate that the transient spectra at both the Re L(3)- and Br K-edges show the emergence of a pre-edge feature, absent in the ground-state spectrum, which is associated with the electron hole created in the highest occupied molecular orbital following photoexcitation. Importantly, these features have the same dynamics, confirming previous predictions that the low-lying excited states of these complexes involve a two-center charge transfer from both the Re and the ligand, X. We also demonstrate that the DFT optimized ground and excited structures allow us to reproduce the experimental XANES and EXAFS spectra. The ground-state structural refinement shows that the Br atom contributes very little to the latter, whereas the Re-C-O scattering paths are dominant due to the so-called focusing effect. For the excited-state spectrum, the Re-X bond undergoes one of the largest changes but still remains a weak contribution to the photoinduced changes of the EXAFS spectrum.

New organic FET-like photoactive device, experiments and DFT modeling.

We present the possible construction of an organic FET-like photoactive device in which source-drain current through a phthalocyanine ( H(2)Pc film is affected by a photo-induced dipolar field in a photoactive "gate" electrode. The influence of the dipolar electric field on charge transfer between H(2)Pc molecules is modeled by DFT quantum-chemical calculations on H(2)Pc dimers and tetramers.

Combined experimental and theoretical DFT study of molecular nanowires negative differential resistance and interaction with gold clusters.

Electric-field-assisted assembly has been used to place rod-shaped metal nanowires containing 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecules onto lithographically defined metal pads. These junctions exhibited negative differential resistance. The quantum chemical approach was used to compare the properties of Au-bonded 4-[[2-nitro-4-(phenylethynyl) phenyl] ethynyl] benzenthiol molecule and a molecule that does not exhibit the negative differential resistance, Au-bonded 4-[[4-(phenylethynyl) phenyl] ethynyl] benzenthiol. The influence of the static electric field and charge variation were modelled for both systems.

Influence of the halogen ligand on the near-UV-visible spectrum of [Ru(X)(Me)(CO)2(alpha-diimine)] (X = Cl, I; alpha-diimine = Me-DAB, iPr-DAB; DAB = 1,4-diaza-1,3-butadiene): an ab initio and TD-DFT analysis.

The near-UV-vis electronic spectroscopy of [Ru(X)(Me)(CO)(2)(iPr-DAB)] (X = Cl or I; iPr-DAB = N,N'-di-isopropyl-1,4-diaza-1,3-butadiene) is investigated through CASSCF/CASPT2 and TD-DFT calculations on the model complexes [Ru(X)(Me)(CO)(2)(Me-DAB)] (X = Cl or I). Convergence of the calculated transition energies for the low-lying metal-to-ligand charge-transfer (MLCT), X-to-ligand charge-transfer (XLCT, X halide ligand), or sigma-bond-to-ligand charge-transfer (SBLCT) to experimental values is analyzed for both methods. On the basis of these accurate calculations, it is shown that whereas the lowest singlet state can be assigned to a nearly pure XLCT state in [Ru(I)(Me)(CO)(2)(Me-DAB)], its character is mainly MLCT in [Ru(Cl)(Me)(CO)(2)(Me-DAB)]. These results are in agreement with time-resolved emission/IR and resonance Raman experimental data. The experimental UV-vis bands are well reproduced by the CASSCF/CASPT2 calculations. The TD-DFT transition energies to the long-range charge transfer states are dramatically affected by the nature of the functional, with lowering leading to meaningless values in the case of nonhybrid functionals. Both methods reproduce well the red shift of the absorption bands on going from the chloride to the iodide complex as well as the shift of the strongly absorbing higher MLCT transition from the visible to the UV domain of energy.

UV-visible absorption spectra of [Ru(E)(E')(CO)(2)(iPr-DAB)] (E = E' = SnPh(3) or Cl; E = SnPh(3) or Cl, E' = CH(3); iPr-DAB = N,N'-Di-isopropyl-1,4-diaza-1,3-butadiene): combination of CASSCF/CASPT2 and TD-DFT calculations.

The UV-visible absorption spectra of [Ru(E)(E')(CO)(2)(iPr-DAB)] (E = E' = SnPh(3) or Cl; E = SnPh(3) or Cl, E' = CH(3); iPr-DAB = N,N'-di-isopropyl-1,4-diaza-1,3-butadiene) are investigated using CASSCF/CASPT2 and TD-DFT calculations on model complexes [Ru(E)(E')(CO)(2)(Me-DAB)] (E = E' = SnH(3) or Cl; E = SnH(3) or Cl, E' = CH(3); Me-DAB = N,N'-dimethyl-1,4-diaza-1,3-butadiene). The calculated transition energies and oscillator strengths allow an unambiguous assignment of the spectra of the nonhalide complexes [Ru(SnPh(3))(2)(CO)(2)(iPr-DAB)] and [Ru(SnPh(3))(Me)(CO)(2)(iPr-DAB)]. The agreement between the CASSCF/CASPT2 and TD-DFT approaches is remarkably good in the case of these nonhalide complexes. The lowest-energy part of the spectrum (visible absorption) originates in electronic transitions that correspond to excitations from the axial E-Ru-E' sigma(2) orbital into the low-lying pi(DAB) orbital (sigma-bond-to-ligand charge transfer, SBLCT, transitions), while the absorption between 25 000 and 35 000 cm(-1) is due to metal-to-ligand charge transfer (MLCT) excitations from the 4d(Ru) orbitals to pi(DAB) (MLCT). Above 35 000 cm(-1), the transitions mostly correspond to MLCT and SBLCT excitations into pi(CO) orbitals. Analysis of the occupied sigma orbitals involved in electronic transitions of the nonhalide complexes shows that the Kohn-Sham orbitals are generally more delocalized than their CASSCF/CASPT2 counterparts. The CASSCF/CASPT2 and TD-DFT approaches lead to different descriptions of electronic transitions of the halide complexes [Ru(Cl)(2)(CO)(2)(Me-DAB)] and [Ru(Cl)(Me)(CO)(2)(Me-DAB)]. CASSCF/CASPT2 reproduces well the observed blue-shift of the lowest absorption band on going from the nonhalide to halide complexes. TD-DFT systematically underestimates the transition energies of these complexes, although it reproduces the general spectral features. The CASSCF/CASPT2 and TD-DFT techniques differ significantly in their assessment of the chloride contribution. Thus, CASSCF/CASPT2 assigns the lowest-energy absorption to predominantly Ru --> DAB MLCT transitions, while TD-DFT predicts a mixed XLCT/MLCT character, with the XLCT component being predominant. (XLCT stands for halide (X)-to-ligand-charge transfer.) Analysis of Kohn-Sham orbitals shows a very important 3p(Cl) admixture into the high-lying occupied orbitals, in contrast to the CASSCF/CASSPT2 molecular orbitals which are nearly pure 4d(Ru) with the usual contribution of the back-donation to pi(CO) orbitals. Further dramatic differences were found between characters of the occupied sigma orbitals, as calculated by CASSCF/CASPT2 and DFT. They differ even in their bonding character with respect to the axial E-Ru and Cl-Ru bonds. These differences are attributed to a drawback of the DFT technique with respect to the dynamical correlation effects which become very important in complexes with a polar Ru-Cl bond. Similar differences in the CASSCF/CASPT2 and TD-DFT descriptions of the lowest allowed transition of [Ru(Cl)(2)(CO)(2)(Me-DAB)] and [Ru(Cl)(Me)(CO)(2)(Me-DAB)] were found by comparing the changes of Mulliken population upon excitation. This comparison also reveals that CASSCF/CASPT2 generally predicts smaller electron density redistribution upon excitation than TD-DFT, despite the more localized character of CASSCF/CASPT2 molecular orbitals.

The stable diiron(2.5) complex ion [(NC)(5)Fe(mu-tz)Fe(CN)5](5-), tz = 1,2,4,5-tetrazine, and its neighboring oxidation states.

The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and Mössbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.

Crucial role of Cu-S bonding for structural changes accompanying the reversible CuI/CuII transition in an unrestrained Cu(N[symbol: see text]S)2 coordination arrangement. An experimental and DFT study.

The structure of reversibly oxidizable [Cu(mmb)2](BF4) with 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) as bidentate N,S-donor ligand has been determined and compared with that of the copper(II) species [Cu(mmb)2(eta 1-ClO4)](ClO4). In the complex ions of the equilibrium [CuI(mmb)2](+) + ClO4- reversible e- + [CuII(mmb)2-(eta 1-ClO4)]+ the almost linear N-Cu-N backbone is invariant whereas the bonds to the thioether sulfur centers and especially the changing S-Cu-S angle (145.18(5) degrees for the CuII species, 109.33(3) degrees for the CuI form) reflect the metal oxidation state. In contrast to the perchlorate coordinating copper(II) species, [CuI(mmb)2](BF4) contains a cation with a very large vacant site at the metal center, resulting in elliptical channels within the crystal. DFT calculations on [CuI(mb)2]+, [CuII(mb)2]2+, and [CuII(mb)2(OClO3)]+ with mb = 2-methylthiomethyl-1H-benzimidazole confirm the essential role of the metal-sulfur bonds in responding to the reversible CuI/II electron transfer process, even in the absence of electronically stronger interacting thiolate sulfur centers or sophisticated oligodentate ligands.

Metal vs ligand reduction in complexes of 1,3-dimethylalloxazine (DMA) with copper(I), ruthenium(II), and tungsten(VI). Crystal structures of (DMA)WO2Cl2 and (bis(1-methylimidazol-2-yl)ketone)WO2Cl2.

The complexes [(DMA)Cu(PPh3)2](BF4) (1) (DMA = 1,3-dimethylalloxazine), [(DMA)Ru(bpy)2](PF6)2 (2), and (DMA)WO2Cl2 (3) were obtained as O4-N5-chelated species, as evident from an X-ray crystal structure analysis for 3 and from spectroscopy (NMR, IR, and UV-vis spectroelectrochemistry) for 1 and 2. The tungsten(VI) center in 3 has its oxide ligands in a cis/equatorial position and the chloride ligands in a trans/axial position; it also exhibits a relatively short bond to O4 (2.232(3) A) and a very long bond to N5 (2.462(3) A). Comparison with the new structurally characterized compound (BIK)WO2Cl2 (4) (BIK = bis(1-methylimidazol-2-yl)ketone), which has W-N bonds of about 2.30 A, confirms the unusual length of the W-N bond in 3, probably caused by repulsion between one of the oxo ligands and the peri-hydrogen atom (H6) of DMA. One-electron reduction of the complexes occurs reversibly at room temperature in THF (1, 2) or at 198 K in CH2Cl2 (3). EPR spectroscopy reveals that this process is ligand-centered for 1 and 2 but metal-centered for 3. Density functional methods and ab initio methodology are used to illustrate the correspondence in spin distribution between the radical anion pi systems of alloxazine and isoalloxazine ("flavosemiquinone").

Decomposition Products of s-Triazine Herbicides by Electron-Transfer in Acidic Aqueous Media

The reduction of atrazine and terbutylazine was preceded by protonation equilibrium. Three protonation sites of the s-triazine molecule determined the structure of the final reduction product. Protonation was investigated by the change of UV-Vis spectra. Two slightly different pKs corresponding to protonation on N5 and N1 heteroatoms were evaluated. The principal reduction pathway involved the cleavage of a chlorine atom. A small quantity of desethylatrazine was detected in the most acidic media.

Electron Transfer in Donor-Acceptor Molecules of Substituted Naphtoquinones: Spectral and Redox Properties of Internal Charge Transfer Complexes

The investigated compounds were substituted naphtoquinones containing aniline moiety separated from the quinoid system by a methylene bridge. The nature of substituents on both donor and acceptor parts influenced the degree of formation of an internal charge transfer complex. Properties of the internal charge transfer interactions were investigated by voltammetry, spectroscopy, and in situ spectroelectrochemistry. Quantum chemical interpretation of experiments is given.