What Optical Electron-Transfer Reactions Can Teach Us about Electrode Kinetics and Electrocatalysis

A number of ingredients are known or thought to be important in determining rates of electrochemical reactions and, therefore, mechanisms of electrocatalysis. These ingredients may include bond reorganization, solvent reorganization, electronic coupling, ion pairing, proton demand, “work terms”, and free-energy driving-force effects. Although evidence exists, in general, for a detectable role for each one of these, for specific cases there is inevitably uncertainty about the relative importance of each effect. Fortunately, optical electron-transfer measurements provide an unambiguous way of partitioning rates into their component effects. The theme of this article is the use of optical processes to learn about molecular level kinetic events and, by inference, molecular level electrocatalysis. We begin by exploring the relationships between electrode processes and analogous optical reactions. Following that, some applications to problems of electrochemical interest—largely drawn from our own recent work—are illustrated. Although the focus is mainly on uncomplicated, single-electron-transfer reactions, preliminary studies of more complicated processes like proton-coupled electron transfer are also mentioned.