Solvent-mediated electronic coupling: The role of solvent placement

The role of solvent location in mediating electronic coupling between electron donor and acceptor groups is investigated. The temperature-dependent electron-transfer rate constant in a C-clamp shaped donor-bridge-acceptor (DBA) molecule with a 7-Angstrom donor-to-acceptor separation is used to evaluate the solvent reorganization energy and the electronic interaction between the donor and acceptor sites. By studying the reaction in an homologous series of alkylbenzene solvents, it is demonstrated that the donor-acceptor electronic interaction is greatly reduced in solvents that are too bulky for their aromatic ring to position itself between the donor and acceptor groups. The temperature dependence of the reaction free energy for charge separation, Delta(r)G, is directly determined from the experimental data. This allows parametrization of a molecular-based solvation model and provides a means to estimate the outer-sphere reorganization energy and its temperature dependence in aromatic solvents.