Spin-orbit coupling and magnetic field effects in photoredox reactions of ruthenium(II) complexes

Spin-orbit coupling (SOC) and magnetic field effects (MFEs) on the efficiency ηnet of net electron transfer in the reductive and oxidative quenching of excited RuII trisbipyridyl complexes have been investigated by nanosecond laser flash spectroscopy and by a special continuous photolysis technique. The reductive process was studied with tris[4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine]ruthenium(2+) quenched by anilines X-An (X = H, 4-C1,4-Br, 2-1, 4-1). Increasing SOC of the substituent X leads to a decrease of relative ηnet values (1.0, 0.79, 0.45, 0.22, 0.12). With X = 4-1 ηnet is decreased by 12.5 ± 2.5% 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 [RuII(2,2'-bipyridine)3]2+ 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 RuIII. 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 ηnet and not the rate of quenching kq, as recently asserted by other authors. The MFE is attributed to SOC effects in the RuIII 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.