In Search of Through-Solvent Electronic Coupling in Flexible Biradicals

Recently there have been several reports of computations of electronic coupling matrix elements in donor−acceptor (D/A) complexes.1 These calculations, which were performed at several different levels of theory, were carried out in order to assess the importance of distance, orientation, and solvent on the electron transfer matrix element, V. The role of the solvent in electronic coupling is of interest as it addresses the issue of mechanism2 and medium effects in D/A chemistry, which is of particular importance in biological D/A systems.3 Direct measurement of V is generally not possible in D/A systems because the rate information obtained also depends heavily on nuclear reorganization energies in both the D/A complex and the solvent molecules.4 For this reason, there is very little experimental data to directly compare with the new calculations.5