CONTEMPORARY-ISSUES IN ELECTRON-TRANSFER RESEARCH

This is an overview of some of the important, challenging, and problem atic issues in contemporary electron transfer research, After a qualit ative discussion of electron transfer, its lime and distance scales, e nergy curves, and basic parabolic energy models are introduced to defi ne the electron transfer process. Application of transition state theo ry leads To the standard Marcus formulation of electron transfer rare constants. Electron transfer in solution is coupled to solvent polariz ation effects, and relaxation processes can contribute to and even con trol electron transfer. The inverted region, in which electron transfe r rate constants decrease with increasing exoergicity, is one of the m ost striking phenomena in electron transfer chemistry. It is predicted by both semiclassical and quantum mechanical models, with the latter appropriate if there are coupled high- or medium-frequency vibrations, The intramolecular reorganizational energy has different contribution s from different vibrational modes, which, in favorable cases, can be measured on a mode-by-mode basis by resonance Raman spectroscopy. Alte rnatively, mode-averaging procedures are available for including multi mode contributions based on absorption or emission spectra. Rate const ants for intramolecular electron transfer depend on electronic couplin g and orbital overlap and, therefore, on distance. Mixed-valence syste ms have provided an important experimental platform for investigating solvent and structural effects and the transition between localized an d delocalized behavior. One of the important developments in electron transfer is the use of absorption and emission measurements to calcula te electron transfer rate constants. Ultrafast electron transfer measu rements have been used to uncover nonequilibrium relaxation effects, a n area that presents special challenges to the understanding of the dy namics and relaxation of the coupled processes. Electron transfer in t he sas phase offers substantial insights into the nature of the electr on transfer process. Similarly, electron transport in conductive polym ers and synthetic metals depends on the basic principles of electron t ransfer, with some special nuances of their own.