Spin-orbit coupling induced electron spin polarization in photoinduced electron transfer reactions

Abstract

The observation of net-absorptive chemically induced dynamic electron polarization (CIDEP) spectra induced by spin-orbit coupling (SOC) interactions due to heavy atoms gives proof of the generation of a contact radical pair (RP) or triplet exciplex as the intermediate. Recent time-resolved EPR and pulsed EPR studies on the spin dynamics are reviewed. The SOC interaction causes simultaneous changes in the orbital and spin operators, leading to sublevel-selective back electron transfer from the short-lived intermediate to the singlet ground state. The intrinsic enhancement factor of the SOC-induced polarization increases with increasing atomic number of the heavy atom substituent. On the other hand, the radical yield decreases remarkably with increasingly heavy atoms. The build-up rate of the SOC-induced CIDEP signal gives the formation rate of the RP-type triplet exciplex. Quantitative analyses of the spin dynamics provide kinetic parameters such as the back electron transfer rate, escape rate, relaxation times of the precursor of the excited triplet state and free radicals. It has been shown that the experiments on the magnetic field effect on the radical yield complement the spin polarization dynamics studies.