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.