Metal-Ligand Electron-Transfer Complexes: Experimental Investigations of the Limits of Semiclassical Electron- Transfer Theory

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In this project in the Inorganic, Bioinorganic, and Organometallic Chemistry Program, Dr. Harry B. Gray and Dr. Jay R. Winkler of the California Institute of Technology will explore metal-ligand electron-transfer (ET) processes under conditions where semiclassical ET theory should break down. To determine the role of tunneling, a series of Ir(I) complexes in which the ET rates at high driving forces are substantially slower than theoretically predicted will be studied. To aid in the theoretical interpretation of the ET rates, nuclear reoganization parameters, temperature, and isotope effects on ET rates will be determined. A systematic investigations of bimolecular ET reactions of Ir(I) dimers will show whether electronic excited products can reduce the effective ET reaction driving force. Photoinduced ET reactions in the region where ET is slower than predicted tend to be faster than recombination processes at comparable driving forces. Based on this fact, coordinatively unsaturated metal complexes with weakly coupled electron acceptors will be investigated to determine whether long-lived charge-transfer states can be prepared by direct light absorption. Some of the most important processes which occur in chemical and biological systems involve the transfer of electrons and the rate of such transfers is often critical to the operation of the process. A semi-empicical theoretical model has been developed to account for these rates. In this study specially designed iridium complexes will be used to probe the factors which this theory uses to describe the rate of electron transfer. Such studies are important both to the basic understanding of electron transfer and to the development of new methods of, for example, solar energy conversion.