Ultrafast Experiments on the Role of Vibrational Modes in Electron Transfer

Abstract

The role of vibrational modes in ultrafast photoinduced intramolecular electron transfer reactions is explored. Femtosecond resolved experiments on two chemical classes will be described, namely, metalmetal intervalence electron transfer in mixed valence compounds and intramolecular charge recombinatiodseparation in organic donor/acceptor compound class, the betaines. The results have been analyzed to reveal the complex interactions of intramolecular and intermolecular modes in electron transfer reactions. I. INTRODUCTION The study of charge transfer processes, and in particular homogeneous electron transfer (ET) in solution, is at the forefront of the study of the molecular details of chemical reactions in liquids. The last decade has brought progress to many central problems in ET research, including the role of the solvent in these reactions, the involvement of vibrational degrees of freedom, the role of nuclear tunneling of the solvent and solute (and related quantum mechanical effects), the pathway and mechanism of electronic interactions, including long range ET, and other aspects of biological and heterogeneous ET. Recent reviews of the foundations of ET theories have been given by Newton and Sutinl and Marcus and Sutin.* Theories of ET reactions in solution are formulated in terms of a model in which the transferring electron is localized at a hmr molecular site in the reactant arid at a different acceptor molecular site in theproduct. Figure 1A portrays the usual curves for an ET reaction with no nuclear degrees of freedom other than the solvent coordinate. The coupling of the solvent coordinate to the ET is quantified by h, the solvent reorganization energy. AGO is the reaction free energy (driving force) for the ET. A central expression in ET theozv due to Marcus is as follows (h+AG)2 AG*= 4h where AG* is the activation energy for the ET reaction.