Exciplex mechanism of excited state electron transfer reactions in polar media

Abstract

Experimental data provide strong evidence for the transient formation of exciplexes in excited state electron transfer reactions, even in polar media. The exciplex mechanism of excited state electron transfer involves the reversible formation of low polarity exciplexes from excited molecules at positive or slightly negative free energies of electron transfer (i.e. in the kinetic region). This means that the mechanism of electron transfer involves a gradual electron shift from electron donor to acceptor and conjugated simultaneous reorganization of the reactants and media, rather than an electron jump after preliminary reorganization. The apparent quenching rate depends on the equilibrium constant of formation and lifetime of the exciplex rather than on the reorganization energy of the reactants and media. Transient exciplex formation provides new pathways for excitation decay (internal conversion and intersystem crossing) which compete with the formation of the products of electron transfer and lead to a decrease in their yield and, at the same time, produce an increase in the total rate of excited state quenching.