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

Abstract

Spin-orbit coupling (SOC) and magnetic field effects (MFEs) on the efficiency {eta}{sub net} of net electron transfer in the reductive and oxidative quenching of excited Ru{sup II} 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{prime}-bis(ethoxycarbonyl)-2,2{prime}-bipyridine)ruthenium(2+) quenched by anilines X-An (X = H, 4-Cl, 4-Br, 2-I, 4-I). Increasing SOC of the substituent X leads to a decrease of relative {eta}{sub net} values (1.0, 0.79, 0.45, 0.22, 0.12). With X = 4-I {eta}{sub net} is decreased by 12.5 {plus minus} 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 (Ru{sup II}(2,2{prime}-bipyridine){sub 3}){sup 2+} and methylviologen (MV{sup 2+}) as a quencher. Here a MFE of {minus}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{sup {center dot}+} radicals was measured by continuous photolysis using EDTA as a sacrificial electron donor to reduce Ru{sup III}. The MFE shows a gradual onsetmore » 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 {eta}{sub net} and not the rate of quenching k{sub q}, as recently asserted by other authors. The MFE is attributed to SOC effects in the Ru{sup III} complex constituent of the primary radical pair.« less