Effect of Molecular Diffusion on the Spin Dynamics of a Micellized Radical Pair in Low Magnetic Fields Studied by Monte Carlo Simulation.

Magnetic field effect is a powerful tool to study dynamics and kinetics of radical pairs (RPs), which are one of the most important intermediates for organic photon-energy conversion reactions. However, quantitative discussion regarding the relationship between the modulation of interelectron interactions and spin dynamics at low magnetic fields (<10 mT) is still an open question. We have studied the spin dynamics of a long-lived RP in a micelle by newly developed Monte Carlo simulation, in which fluctuations of the exchange and magnetic dipolar interactions by in-cage diffusion are directly introduced to the time-domain spin dynamics calculation. State-dependent relaxation/dephasing times of a few to a few tens of nanoseconds are obtained by simulations without hyperfine interactions (HFIs) as a function of the mutual diffusion constant (similar to 10(-6) cm(2)/s). Simulations with the HFIs exhibit incoherent singlet-triplet (S-T) mixings resulting from interplay between the HFIs and the fluctuating spin-spin interactions. The experimentally observed incoherent S-T mixing of similar to 20 ns at 3 mT for a singlet-born RP in a sodium dodecyl sulfate micelle is reproduced by the simulation with reasonable diffusion coefficients. The computational method developed here contributes to quantitative detection of molecular motion that governs the recombination efficiency of RPs.