Superexchange Electron Tunneling Mediated by Solvent Molecules:  Pulsed Electron Paramagnetic Resonance Study on Electronic Coupling in Solvent-Separated Radical Ion Pairs

Nanosecond pulsed electron paramagnetic resonance spectroscopy is applied to characterize exponential decay constants (β) of the squared electronic coupling matrix element (V DA 2) in transient, solvent-separated radical ion pairs (RIP) composed of quinone anions and several cation radicals in aprotic liquid solutions of N,N-dimethylformamide, DMSO, and benzonitrile. The distance dependence of singlet−triplet energy splitting (2J) is shown to be described by β in V DA for charge-recombination processes. We show that the radical pair mechanism (RPM) electron spin polarization (P RPM) is quite sensitive to β. The β value is characterized by using the stochastic Liouville equation to fit the experimental P RPM values. The β values (from 0.8 to 1.0 Å-1) manifest that V DA is governed by the superexchange mechanism mediated by the intervening solvent molecules from a result that the β increases with increasing the tunneling energy gap (ΔG eff) for solvent oxidation or reduction in several intermolecular electron-transfer systems. We propose a simple three-dimensional model of V DA, in which the through-solvent tunneling pathways are exponentially increased with the increase in the intermolecular distance in bulk, condensed media. This model explains the ΔG eff dependence of β, including the data previously reported on the charge-transfer reactions both in liquid and frozen (77 K) solutions. Effective solvent−solvent coupling is estimated to be v B ≈ 850 cm-1 at a mean nearest-neighbor distance of 5.7 Å. This relatively large magnitude of v B may agree with dynamical amplifications of the effective coupling by low-frequency motions of the mediators as reported in charge-transfer reactions in biological systems. (Balabin, I. A.; Onuchic, J. Science 2000, 290, 114 and Troisi, A.; Orlandi, G. J. Phys. Chem. B 2002, 106, 2093.)