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Charge-transfer Processes (charge-transfer + process)
Selected AbstractsEnergy- and Charge-Transfer Processes in a Perylene,BODIPY,Pyridine Tripartite ArrayEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2008Mohammed A. H. Alamiry Abstract A novel boron dipyrromethene (BODIPY) dye has been synthesized in which the F atoms, usually bound to the boron center, have been replaced with 1-ethynylperylene units and a 4-pyridine residue is attached at the meso -position. The perylene units function as photon collectors over the wavelength range from 350 to 480 nm. Despite an unfavorable spectral overlap integral, rapid energy transfer takes place from the singlet-excited state of the perylene unit to the adjacent BODIPY residue, which is itself strongly fluorescent. The mean energy-transfer time is 7,±,2 ps at room temperature. The dominant mechanism for the energy-transfer process is Dexter-type electron exchange, with Förster-type dipole,dipole interactions accounting for less than 10,% of the total transfer probability. There are no indications for light-induced electron transfer in this system, although there is evidence for a nonradiative decay channel not normally seen for F -type BODIPY dyes. This new escape route is further exposed by the application of high pressure. The meso -pyridine group is a passive bystander until protons are added to the system. Then, protonation of the pyridine N atom leads to complete extinction of fluorescence from the BODIPY dye and slight recovery of fluorescence from the perylene units. Quenching of BODIPY-based fluorescence is due to charge-transfer to the pyridinium unit whereas the re-appearance of perylene-based emission is caused by a reduction in the Förster overlap integral upon protonation. Other cations, most notably zinc(II) ions, bind to the pyridine N-atom and induce similar effects but the resultant conjugate is weakly fluorescent.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Computational studies of electron-transfer processes in old yellow enzymeINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001Ginger M. Chateauneuf Abstract Old Yellow Enzyme (OYE) is a flavoenzyme that was first isolated from brewer's bottom yeast. Homologues have been identified in other strains of yeast, bacteria, and plants. In plants, the OYE homologue functions enzymatically in the synthesis of plant hormones, but the biological function of OYE in yeast is still unknown. Flavin mononucleotide (FMN) is the cofactor that is noncovalently bound in the enzyme. OYE binds several phenolic ligands that serve as models for reactive biological substrates. These complexes have broad long-wavelength absorption bands, which have been ascribed to charge-transfer interactions, with the phenolate anion acting as the electron donor and the FMN as the acceptor [Abramovitz, A. S.; Massey, V. J Bio Chem 1976, 251, 5327,5336]. The computational characterization of these electronic transitions in the active site will help in understanding the biological processes in the enzyme. It was found that at several levels of computational methods, and through computationally mutating relevant amino acids, a charge-transfer process is occurring. This result agrees with previous experimental work and is consistent with all ultraviolet,visible spectrophotometric data. The preliminary results for the computational studies of these electron-transfer processes will be presented. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Photoinduced structural dynamics of polar solids studied by femtosecond X-ray diffractionACTA CRYSTALLOGRAPHICA SECTION A, Issue 2 2010Thomas Elsaesser Femtosecond X-ray diffraction allows for real-time mapping of structural changes in condensed matter on atomic length and timescales. Sequences of diffraction patterns provide both transient geometries and charge-density maps of crystalline materials. This article reviews recent progress in this field, the main emphasis being on experimental work done with laser-driven hard X-ray sources. Both Bragg diffraction techniques for bulk and nanostructured single crystals as well as the recently implemented powder diffraction from polycrystalline samples are discussed. In ferroelectric superlattice structures, coherent phonon motions and the driving stress mechanisms are observed in real time. In molecular crystals charge-transfer processes and the concomitant changes of the lattice geometry are analyzed. [source] Ferrocenyl-Ended Thieno,Vinylene Oligomers: Donor,Acceptor Polarization and Mixed-Valence Properties with Emphasis on the Raman Mapping of Localized-to-Delocalized TransitionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2009Juan Casado Dr. Abstract What's your role? New oligothiophene,vinylene compounds have been synthesized to study the role of the conjugated chain in two different cases (see scheme; MV=mixed valence). The electronic and molecular structures were analyzed by means of electronic, X-ray photoelectron, and Raman spectroscopy, together with thermo spectroscopy, electrochemistry, and DFT calculations. New oligothiophene,vinylene compounds have been synthesized in order to study the role of the conjugated chain in two different cases: 1),when push,pull action operates between an electron-donor and an electron-acceptor group at the ends of the thiophene,vinylene conjugated chain, and 2),when mixed-valence action is induced by single oxidation of the same chain functionalized at both terminal positions with ferrocene groups leading to competition between the donor groups. The electronic and molecular structures are analyzed by means of electronic, X-ray photoelectron and Raman spectroscopies, together with thermospectroscopy, electrochemistry and density functional theory calculations. The cyclic voltammetry processes have been followed by spectrochemistry. It is shown that the radical cation of the diferrocenyl derivative is a class,III mixed-valence system (i.e., fully delocalized) according to its Raman spectrum. Moreover, by Raman thermo-spectroscopy the thermal transition of this radical cation from a delocalized (class,III, room temperature) to a localized (class,II, ,160,°C) state is scanned. In all cases the Raman study is paralleled by an electronic absorption spectroscopic analysis. Structure,property relationships are proposed for molecules of two important fields of very active research as that of the non-linear optics (i.e., organic optoelectronic) and that of the mixed-valence systems (i.e., charge-transfer processes). [source] Organic Polyaromatic Hydrocarbons as Sensitizing Model Dyes for Semiconductor NanoparticlesCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 4 2010Yongyi Zhang Abstract The study of interfacial charge-transfer processes (sensitization) of a dye bound to large-bandgap nanostructured metal oxide semiconductors, including TiO2, ZnO, and SnO2, is continuing to attract interest in various areas of renewable energy, especially for the development of dye-sensitized solar cells (DSSCs). The scope of this Review is to describe how selected model sensitizers prepared from organic polyaromatic hydrocarbons have been used over the past 15 years to elucidate, through a variety of techniques, fundamental aspects of heterogeneous charge transfer at the surface of a semiconductor. This Review does not focus on the most recent or efficient dyes, but rather on how model dyes prepared from aromatic hydrocarbons have been used, over time, in key fundamental studies of heterogeneous charge transfer. In particular, we describe model chromophores prepared from anthracene, pyrene, perylene, and azulene. As the level of complexity of the model dye-bridge-anchor group compounds has increased, the understanding of some aspects of very complex charge transfer events has improved. The knowledge acquired from the study of the described model dyes is of importance not only for DSSC development but also to other fields of science for which electronic processes at the molecule/semiconductor interface are relevant. [source] |