ИСТИНА

New approach to kinetics of excited-state electron transfer reactions has been developed recently, which tooks into account substantial decrease of the barrier in contact pairs of reactants, due to strong electronic coupling (concatenated mechanism), and implies transient formation of the exciplex [1-3]. In contrast to conventional approaches to these reactions [4] which consider them either in terms of the transition state theory, as a preliminary thermally activated reorganization of the medium and reactants necessary for degeneration of electronic levels in molecules of reactants and products (Marcus mechanism) or in terms of radiationless quantum transitions, which requires no preliminary activation and occurs in exergonic region, this transient exciplex formation mechanism provides natural explanation for many well known important features of ET reactions, being contrary to another two theories, and new possibilities to control their rates and quantum yields. There are two important features that distinguish this mechanism: (1) a very small energy barrier, caused by the strong electronic coupling, which decreases the activation energy and (2) a competition of the formation of final products (radical ions) with efficient radiationless decay in a transient exciplex, caused by its relatively long lifetime (~ 10-8 – 10-11 s), in contrast to the transition state (~ 10-13 – 10-15 s). Radiationless decay in transients can decrease the products quantum yield substantially. Several features of the excited-state ET reactions inherent to the concatenated mechanism and contradicting common Marcus and radiationless transition mechanisms are discussed using numerous experimental data on their kinetics. 1) Nonexponential fluorescence decay of parent M*. 2) An absence of an exciplex emission cannot testify against the transient exciplex formation since many exciplexes have much smaller emission rate constants and lifetimes than parent excited molecules and exciplex emission spectra can overlap significantly with parent reactant fluorescence. 3) Sublinear dependence of 0/ on the quencher concentration in contrast to the linear dependence of fluorescence quantum yields. 4) Non-Arrhenius (bell shape) dependence of quenching constant on inverse temperature. 5) Negative values of apparent activation enthalpy of quenching. 6) Weak solvent polarity effect. 7) Quenching of singlet excited states is not frequently followed by the decrease of the triplet state yield. 8) The dependence of apparent activation free energy G‡Q on GET* frequently does not follow either Marcus equation (3) or Weller equation (5) and has rather small slope at GET* > 0.3 eV. These features arise due to to equilibrium formation of transient exciplex and can be used for clarification/elucidation of actual mechanism of electron transfer in particular cases.