Spin-orbit coupling controlled spin chemistry of Ru(bpy)3 2+ photooxidation : detection of strong viscosity dependence of in-cage backward electron transfer rate

The magnetic field dependence of qce, the cage escape yield of radicals generated by photoelectron transfer between Ru(bpy)3zt and methylviologen (MV++) was investigated for a series of solvent mixtures of increasing viscosity, obtained by adding ethylene glycol (EGLY) to H20/ACN. From the experimental results the absolute values of the rate parameters characterizing the primary radical ion pair (kce for cage escape, zs for spin relaxation, and bet for spin-allowed backward electron transfer) have been obtained. Particularly noteworthy is the strong viscosity dependence of bet, which can be assigned to dielectric solvent relaxation as the rate limiting step of backward electron transfer in the EGLY containing solvents.