The proton coupled electron transfer and electron transfer reactions of photochemically prepared charge separated states of biomimetic assemblies will be addressed experimentally ad theoretically with the goal of suggesting efficient charge separating reaction networks. A homologous series of biomimetic compounds has been designed that permits a comparative study of electron transfer and proton coupled electron transfer reactions. Well characterized charge separated states of a series of these donor/acceptor complexes will be prepared optically by picosecond laser excitation. The competition between recombination and subsequent propagation of charge separated states will be monitored on the picosecond time scale by the use of time absorption spectroscopy. These experiments will permit the role of solvent dynamics, as well as energetics, on charge recombination steps to be explored. In accord with theoretical predictions that charge recombination occurring in the inverted electron transfer regime will be strongly affected by solvent dynamical properties, electron transfer rates of the biomimetic donor/acceptor complexes will be measured in solvents providing a wide range of dielectric relaxation times, as may be evocative of a protein environment. Analytic developments will focus on the consequences of solvent dynamical effects on rates in dielectrically complex solvents, and molecular dynamics simulations will be carried out to provide a detailed framework within which to interpret the experiments. As local solvation effects are important to biological electron transfer, laser induced fluorescence and time-of-flight mass spectrometry will be used to measure gas phase electron transfer rates of selectively solvated donor/acceptor complexes in supersonic jets. Electron transfer theories incorporating molecular dynamics simulations to characterize the role of specific solvation will be carried out to aid the experimental program. With the electron transfer chemistry elaborated, the role of proton motion on electron transfer will be investigated. A study of proton coupled electron transfer reactions has been initiated on a series of donor/acceptor pairs that are intramolecularly juxtaposed by a cyclic hydrogen bonded network of a dicarboxylic acid dimer or quinhydrone. Theoretical developments point to a significant proton/deuteron isotope effect on and an atypical temperature dependence of the electron transfer rate induced by proton motion. The effect of proton transfer mediation on electron transfer rates will be assessed in well defined experimental systems with the goal of elucidating these effects in the biological context.
Showing the most recent 10 out of 78 publications
