Diffusion-model analysis of effective CIDEP distance in solvent-separated radical-ion pair

Abstract

The radical pair mechanism (RPM) of chemically induced dynamic electron polarization (CIDEP) is theoretically analyzed to determine what intermolecular separations (r eff) effectively contribute to the CIDEP generated from diffusive, separated radical-ion pairs (RIP) in terms of the chargetransfer interaction in the singlet-triplet energy splitting (J) by taking into account the distance-dependent electronic coupling and reorganization energy. The diffusion-model analysis reveals that the hyperfine-dependent RPM polarization (P RPM) phase is varied with the driving force (−ΔG CR) for the charge-recombination (CR) process and that the boundary −ΔG CR between the opposite phases coincides well with the total reorganization energy around the diffusible separation distance,r eff=1.2 nm, between the ion radicals. For the first time, ther eff is well described by the exponent parameter (β) in the distance-dependent electronic coupling, suggesting that the RPM CIDEP detection can be applied to characterize the electronic coupling in individual solvent-separated RIP systems. It has been concluded that, in contrast to the spin exchange interaction of the neutral radical pairs, the characteristic long-range charge-transfer interaction enables us to utilize the simple diffusion-model analysis to successfully evaluate ther eff and theP RPM in homogeneous liquid polar solvents.