Singlet and triplet energy splitting in the radical ion pairs generated by photoinduced electron-transfer reactions
Chemically induced dynamic electron polarization (CIDEP) spectra generated by photoinduced electron-transfer reactions have been studied using continuous-wave time-resolved EPR and FTEPR spectroscopy. It has been suggested that the energy difference (DEST) between the singlet and triplet states of radical ion pairs (RIPs) is dependent on the charge recombination free energy (DGCR) and reorganization energy (l). The result is interpreted in terms of the spin-selective stabilization or destabilization of RIP states by electronic coupling with the S0 and locally excited triplet states of the donoracceptor pair at equilibrium distance in the reaction coordinate. It has been verified that a charge transfer-type exchange interaction governs the DEST. Magnitudes as well as the sign of DEST can be predicted at the distance of the solvent-separated RIP based on DGCR and l values.