Low Polar Exciplexes As Intermediates in Excited-State Electron Transfer Reactions

M.G. Kuzmin and N.A. Sadovskii

XVIth IUPAC Symposium on Photochemistry, Helsinki, Finland, July 21-26, 1996, Book of Abstracts, p.64.

ABSTRACT. Conventional mechanism of excited-state electron transfer reactions supposes that contact radical ion pairs are formed directly in a primary step of interaction between excited molecules and electron donor or acceptor (in polar solvents they can dissociate to give free radical ions). Recently, some evidences for the formation, even in polar media, of low polar exciplexes, with a medium contribution from electron transfer state, were obtained. The nature of such exciplexes and their role in the formation of radical ions and triplet states are of primary importance for the discussion of mechanisms of excited-state electron transfer reactions.
The effects of the solvent polarity and of the driving force of electron transfer on the properties of these exciplexes (polarity, enthalpy of formation, rate constants of intersystem crossing and dissociation leading to free radical ions) are discussed.
Experimental data on temperature dependence of radical ions and triplet states formation in polar solvents, as well as the kinetics of their formation and decay and dependence of their quantum yield on the quencher concentration give evidence that both radical ions and triplet states are formed from the exciplex as a transient rather than by direct interaction of excited molecules with a quencher. Apparent activation energies of their formation were found to be rather low and sometimes negative for the excited donor-acceptor systems with Gibbs energy of electron transfer close to zero, which can be explained only by the transient exciplex formation. Intersystem crossing, dissociation leading to radical ions and emission occur to be main pathways of the exciplex decay.
The effects of media polarity and electron transfer driving force on apparent excited state quenching rate constants and yields of radical ions and triplet states are discussed in terms of the rate constants and activation energies of intersystem crossing and exciplex dissociation leading to radical ions.

Laboratory of Photochemistry