Kinetics of nonequilibrium electron transfer in photoexcited ruthenium(II)-cobalt(III) complexes

Kinetics of photoinduced electron transfer reactions in [Ru(II)(L-L)Co(III)](5+) complexes have been investigated in the framework of the stochastic point-transition model. The model involves the medium and intramolecular nuclear reorganization as well as fast relaxation of intramolecular high frequency vibrations and description of the medium relaxation in terms of two time scales. The model has allowed reproducing the experimental data (Torieda, H.; Nozaki, K.; Yoshimura, A.; Ohno, T. J. Phys. Chem. A2004, 108, 4819) of forward and backward electron transfer kinetics, including the low yield of electron transfer products and its variation with solvent. These results have lent support to the important role of hot backward electron transfer in the formation of low yield of electron transfer products. The experimentally observed significant decrease of the product yield in more viscous solvents has been shown to be a direct consequence of the hot transition efficiency increase. A weak opposite dependence, also revealed in experiments, has been elucidated in terms of two time scales of the solvent relaxation. Solvent relaxation is well-known to involve at least two stages: the inertial one (fast) and the diffusive one (slower) with the time scales weakly dependent on and proportional to solvent viscosity, respectively. When hot transitions are terminated at the stage of the inertial relaxation, the yield of the electron transfer products is nearly independent of the diffusive time scale; otherwise a strong increase of the yield in more viscous solvents should be observed.