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.

reactions involving CH3 and CHO radicals erroneously increases the values of k, by a factor of 2-3. The inclusion of these reactions led to much lower values as shown in Figure 4.(26) Timonen, R. S.; Ratajczak, E.; Gutman, D. J . in a magnetic field of 1 T. These effects are interpreted as SOC effects of the X center on the spin-forbidden back electron transfer in triplet radical pairs. The oxidative process was studied with [R~"(2,2'-bipyridine)~]~+ and methylviologen (MV2+) as a quencher. Here a MFE of -10% at 1 T was observed on the yield of radicals detected on the nanosecond time scale. A detailed MF dependence between 0 and 1 T of the yield of MV" radicals was measured by continuous photolysis using EDTA as a sacrificial electron donor to reduce Ru"'. The MFE shows a gradual onset followed by a linear increase above 50-100 mT. The onset field decreases and the (negative) slope of the MFE increases as the solvent viscosity is increased in water/ethylene glycol mixtures. On the basis of measurements of lifetime and radical yield at various quencher concentrations, it is shown that the magnetic field mainly affects qnet and not the rate of quenching k,, as recently asserted by other authors. The MFE is attributed to SOC effects in the Ru"' complex constituent of the primary radical pair. Novel theoretical aspects arising in connection with the magnetokinetic behavior of radical pairs involving a strongly spin-orbit mixed Kramers doublet are outlined.