Separation of photogenerated radical ion pairs in viscous solutions

Encounter theory is used for the comparative study of two geminate reactions of backward electron transfer (recombination), followed by photoinduced bimolecular forward transfer. The latter can be affected by the initial correlations between similarly charged reactants, but a proper account of the electrostatic interactions shows that they are not responsible for the contrasting viscosity dependence of the recombination efficiency in these reactions. Instead, the observed qualitative difference in the experimental results is attributed to a parallel change of refraction index and dielectric constant in the solvent mixtures used for the viscosity variation in one of the reacting systems. Also it is demonstrated that neither the contact nor exponential models of transfer rates allow us to explain the nonmonotonic viscosity dependence of recombination efficiency, only the true Marcus' expression for the rates provides the explanation.