Short-Time Dynamics of Radical-Ion Pairs Produced by Photoinduced Electron Transfer in Solution: The Magnetic Field Effect

Kinetics of radical-ion pairs (RIPs) formed by photoinduced electron transfer in solution, as well as triplet and singlet products of their recombination, are studied within a general theory of spin-selective charge transfer assisted by diffusion of reactants in solution. The RIPs are assumed to be created in the singlet state, and their coherent singlet–triplet evolution is described in terms of isotropic hyperfine interaction (HFI) and Δg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta g$$\end{document} mechanisms. A set of quantum-classical model equations is solved numerically using the time propagator splitting technique. Numerical simulations are carried out on a prototype photochemical reaction involving bimolecular electron transfer between 9,10-dimethylanthracene (DMeA) and phthalonitrile (PN) in acetonitrile (ACN) solution. Time-dependent populations of all electronic and spin states, as well as spatial distributions of reactants in the course of forward and backward charge transfer are calculated and analysed. Particularly, spatial profiles of charge recombination (CR) in singlet and triplet RIPs are shown to differ significantly, with a significant part of the singlet RIPs undergoing distant (non-contact) recombination. The effect of a strong (saturating) magnetic field on the triplet CR product yield in these reactions is studied. For the HFI-induced coherent spin transitions, the time-dependent magnetic field effect is shown to decrease with time. A phenomenon of suppressing the triplet CR product yield in RIPs with the HFI- and Δg\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta g$$\end{document}-induced coherent spin transitions in moderate magnetic fields is investigated and its physical origins are discussed.