M.G. Kuzmin
XX ICP International Conference on Photochemistry, Moscow, Russia, July 30 - August 4, 2001, Book of Abstracts, p. 335.
ABSTRACT. Excited-state electron transfer reaction are commonly considered as controlled by activation energies necessary for preliminary reorganization of medium and reactants to achieve a degeneration of electronic levels in reactants and products. Different approach proposed recently assumes gradual electron shift coupled with medium and reactants reorganization what provides much lower p.Ч..&.energy barrier due to substantial electronic coupling of initial and final states and possible formation of transients - exciplexes with partial charge transfer, when electronic coupling matrix element is above 0.1 eV. Numerous data exist which show that electron transfer rate constants decrease dramatically when the driving force of of electron transfer DGet > 0. The question arises whether the distinction between experimental and diffusional rate constants gives an evidence for the existence of true potential barrier or it can be just a consequence of complicated reaction mechanism? Available experimental data on the temperature dependence of excited states quenching rate constants demonstrate that apparent activation enthalpies DHq. become negative at DGet < -0.1 eV. This induced us to reconsider the conventional interpretation of activation barrier in electron transfer reactions. The analysis of temperature dependence of apparent rate constants for transient exciplex formation reactions show that apparent quenching rate constants are close to the diffusion limit at low temperatures but at higher temperatures reaches a maximum and starting to decrease (>250 K). Formal "activation energy" appears to be an illusion of potential barrier (which is concerned in fact with enthalpy rather than with Gibbs energy). Actually the reaction rate is controlled by the enthalpy of the transient exciplex formation and activation energy of its decay by diss.И..*'into radical ions (final products of electron transfer), intersystem crossing and internal conversion. Experimental data indicate smaller activation enthalpy of exciplex transformations as compared with its dissociation into parent reactants. Formally, low activation enthalpy and high activation entropy are typical for such reactions, that is flat but narrow valley corresponds to the reaction pathway. Therefore to ensure the real existence or lack of potential barrier in particular reaction and to use the kinetic data for verification of one or other theoretical model one should study experimentally the temperature dependence of experimental rate constants. The use of formal activation energy DG. obtained from measurements of rate constants at fixed temperature is misleading and creates an illusion of confirmation instead of revealing the real reaction mechanism. Quenching rate constants cannot be used with a confidence also for evaluations of redox potentials of quenchers or excited molecules. References: 1. Kuzmin M.G. Pure Appl.Chem., 1993, vol 65, p.1653. 2. Kuzmin M.G. J. Photochem.Photobiol., A:Chem., 1996, vol 102, p.51. This work is supported by grant of Russian Foundation for Basic Research (99-03-32337).