Return electron transfer within geminate radical ion pairs. Observation of the Marcus inverted region

Abstract

In recent years the physical and chemical properties of photoinduced electron-transfer reactions have been extensively studied with particular emphasis on maximizing the efficiency of charge separation. However, energy-wasting return electron transfer, especially within the primary geminate radical ion pair for singlet-state reactions, often results in low quantum yields for free-ion formation. Detailed studies of the mechanisms and kinetics of product formation for several photosensitized electron-transfer reactions suggest that a relationship exists between the thermodynamics and the kinetics of the return electron transfer process. In this work the authors summarize the results of laser flash photolysis studies which were specifically designed to study this relationship. The results provide a clear example of the Marcus inverted region in these processes. Experiments were performed in degassed acetonitrile at room temperature using 9,10-dicyanoanthracene (DCA) and 2,6,9,10-tetracyanoanthracene (TCA) as the excited-state sensitizers and electron acceptors and naphthalene derivatives, diphenylacetylene, and biphenyl as the electron donors. Absolute quantum yields for formation of free radical ion (Theta/sub sep/) were determined by using conventional laser flash photolysis.