Time-Resolved EPR Study on Reorganization Energies for Charge Recombination Processes in Nanometer-Separated Radical Ion Pairs

Abstract

The reorganization energies for the intermolecular charge recombination processes have been determined for the electron donor-acceptor systems involving the quinones and methoxy aromatic molecules by time-resolved EPR andcyclic voltammetry measurements in polar solvents. We have carefully examined what distance between the donor and acceptor molecules is effective for the radical pair mechanism (RPM) chemically induced dynamic electron polarization (CIDEP) detection using the stochastic Liouville equation on the basis of the charge transfer (CT) interaction mechanism in the singlet-triplet energy splitting (2J) in radical ion pairs. The RPM CIDEP shows that the signs of the J are inverted from positive to negative with decreasing temperature. The results are interpreted by the CT interaction mechanism. With the observations of the J ∼ 0 condition, the individual values of the total and solvent reorganization energies (As) are determined for several 1.2 nm separated donor-acceptor systems. The individual As values ranging from 1.27 to 1.39 eV in the present donor-acceptor systems agree with those calculated from the Marcus continuum dielectric model at 1.2 nm donor-acceptor separation within the deviation of ∼0.1 eV. In the dimethoxynaphthalene (DMN)-duroquinone (DQ) system, the solvent reorganization around the methoxy groups is dominated due to the localization of the electron spin density in the DMN cation radical.