Solvent-mediated electronic coupling: The role of solvent placement

The role of solvent location in mediating electronic coupling between elect ron donor and acceptor groups is investigated. The temperature-dependent el ectron-transfer rate constant in a C-clamp shaped donor-bridge-acceptor (DB A) molecule with a 7-Angstrom donor-to-acceptor separation is used to evalu ate the solvent reorganization energy and the electronic interaction betwee n the donor and acceptor sites. By studying the reaction in an homologous s eries of alkylbenzene solvents, it is demonstrated that the donor-acceptor electronic interaction is greatly reduced in solvents that are too bulky fo r their aromatic ring to position itself between the donor and acceptor gro ups. The temperature dependence of the reaction free energy for charge sepa ration, 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 temperatur e dependence in aromatic solvents.