Spin-orbit coupling induced magnetic field effects in electron-transfer reactions with excited triplets: the role of triplet exciplexes and radical pairs in geminate recombination

The magnetic field dependence of free-radical yield in the electron-transfer quenching of methylene blue triplet by picdoaniline has been determined between 0.00 and 3.30 T in methanol/ethylene glycol mixtures of various viscosities by using laser flash spectroscopy and a photostationary flow technique. The observed decrease of the free-radical yield with the magnetic field is interpreted by heavy-atom-induced spin-orbit coupling causing magnetic field sensitivity according to the triplet mechanism (TM) in intermediate triplet exciplexes and to the Ag type radical pair mechanism (RPM) in geminate triplet radical pairs originating from dissociation of the triplet exciplexes. Analytical expressions are provided for a treatment of a combination of both mechanisms including the case of reversible formation of the triplet exciplex from the geminate radical pair. The formalism of Pedersen developed for the high field radical pair mechanism and modified by Vollenweider and Fischer to account for effects of exchange interaction is generalized to include various boundary conditions for the electron spin density matrix suggested in the literature to describe the effects of encounters and chemical reaction. With a physically consistent choice of TM and RPM parameters model calculations provide a very good quantitative fit of the observed magnetic field and viscosity dependence of the yield of free radicals.