Spin-orbit coupling controlled spin chemistry of Ru(bpy)32+ 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 b e t for spin-allowed backward electron transfer) have been obtained. Particularly noteworthy is the strong viscosity dependence of b e t , which can be assigned to dielectric solvent relaxation as the rate limiting step of backward electron transfer in the EGLY containing solvents. Introduction One of the main interests in photoelectron transfer reactions is their eminent potential for converting light energy into chemical energy. This potential is based on the general property of electron transfer processes to exhibit very fast rates while at the same time conserving most of the primary electronic excitation energy in the formation of energy rich radical ion pairs (RIPS). However, the thermodynamic potential of these primary RIPS can be utilized only if the primary charge separation is followed by further separation of the oxidized and reduced species (orin a solid matrixby a separation of the produced electron/hole pair through subsequent electron transfer steps), a process that has to compete with backward electron transfer (BET) regenerating the reactants in their ground states whereby all of the absorbed photon energy would be finally wasted. Kinetically, the processes in the primary RIP are usually characterized by two rate constants: kce for cage escape (CE) and bet for the energy dissipating BET, from which the efficiency qce of CE is obtained as qce = kce /(kce t be t ) . For freely diffusing reactants only qce is usually observable from which kbet may be estimated by employing a theoretical estimation of kcbased e.g. on the Eigen-Debye equation (1). For chemically linked donor and acceptor systems (dyads) b e t can be measured by time-resolved spectroscopy since the CE is blocked and the rate of the forward electron transfer is not limited by diffusion. In particular, such methods have been also applied to the reaction of photoexcited Rutrisbipyridine ( R u ( b p ~ > 3 ~ + ) with methylviologen (MVtt), a most popular photoelectron transfer system because of its potential to function as an efficient sensitizing system in the photochemical splitting of water (2) . What has been disregarded, however, in most of the pertinent investigations, is the role of electron spin as a kinetically controlling factor, which can be unequivocally demonstrated through a magnetic field effect on the observed CE efficiency qce. In a series of investigations (3-8) we have shown how one can take advantage of this effect to determine the details of the BET/CE kinetics. In this paper we will report the first results of a spin chemical investigation studying the solvent viscosity dependence of the processes occuring in the photogenerated RIP (Ru(bpy)$+*..MV+*). Spin Chemistrv of BET Since the photoreactive MLCT state of Ru(bpy)32+ is of predominant triplet character and electron spin is conserved as far as possible in a fast electron transfer reactions, the primary RIP (Ru@py)$+*.. MV+*). originates with predominant triplet spin. This is true in spite of the strong spin-orbit coupling