Photochemistry of Charge Transfer Excited States

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This award, from the Inorganic, Bioinorganic and Organo- metallic Chemistry Program, is for the support of studies of the photochemistry of excited states of two types of rhenium complexes in which the rhenium atom has six d-electrons. In the first case the complex will contain a bidentate diimine ligand that serves as an electron acceptor in the excited state and a monodentate ligand that serves as an electron donor in the excited state, and which is reactive in its electron deficient form. Photochemical excitation of these molecules will generate a ligand-ligand charge transfer (LLCT) excited state. Objectives of this phase of the investigation include: 1) delineation of the photochemical reactions that occur andidentification of reactive intermediates produced via LLCT excitation, 2) determination of the kinetics of rearrangements and bond fragmentations of the reactive groups, and 3) utilization of the rate constants obtained in 2) to determine the effects of structural and energetic factors on the lifetime of the LLCT excited state. In the second case, sigma-to-pi antibonding excitation processes in rhenium(I) complexes that contain bidentate diimine and alkyl ligands will be examined. Here the results of bond scission in the excited state will produce metal- and carbon-based radicals. A primary objective of this work is to relate rhenium-carbon bond strength to the rate of excited state bond fragmentation. %%% Charge transfer in photochemically generated excited states is a fundamental aspect of biological light energy harvesting systems. How charge transfer states are sufficiently stabilized in biological systems so that productive reactions occur faster than charge recombination is a mystery that continues to stimulate exciting research. Successful emulation of biological systems could mean more efficient utilization of solar energy for technological purposes; determination of the reactivity of the excited state in systems such as those to be examined here may be useful in this regard. The chemical systems under study may also be useful as photochemically activated catalysts for polymerization reactions.