Exposing Solvent's Roles in Electron Transfer Reactions: Tunneling Pathway and Solvation

Abstract

We describe studies of electron transfer in donor−spacer−acceptor molecules for which the highly curved spacer topology imparts a vacant cleft along the “line-of-sight” between the electron donor and electron acceptor moieties. The electron transfer kinetics in nondipolar and weakly polar solvents allow experimental determination of the reaction free energy as a function of solvent structure and temperature. These data were used to parametrize a molecular solvation model developed by Matyushov. The model provides reasonable estimates of reaction free energy in solvents that are too polar for its direct measurement and provides reasonable values of the solvent reorganization energy in all solvents. Successful modeling of the solvation enables quantitative study of the factors that control electron tunneling through molecules located in the solute's cleft, i.e., the electronic coupling. Electron tunneling in these systems is mediated by the unoccupied orbitals of the solvent (“electron-mediated superexchange”). The solvent molecule's presence within the cleft is critical for effective electronic coupling, and its motion modulates the electronic coupling magnitude. These studies demonstrate and quantify the importance of electron tunneling “pathways” through noncovalent contacts for this model system and indicate that such pathways can contribute significantly to electron-transfer processes in biological and chemical systems.