Steric effects in photoinduced electron-transfer reactions

Quantum yields for formation of separated radical ions are determined for the electron-transfer reactions of singlet excited cyanoanthracene acceptors with sterically hindered alkylbenzene donors, in acetonitrile. These yields are up to 4 times larger than those for unhindered alkylbenzene donors. The yields are controlled by the competition between separation and return electron transfer in the initially formed radical-ion pairs. From studies of the dependence of the steric effect on the driving force for return electron transfer, it is concluded that the main effect of the steric hindrance is to decrease the magnitude of the electronic coupling matrix element for electron transfer, thus decreasing the return electron transfer rate and increasing the separation yield. The sterically hindered donors can be divided into two groups depending upon whether the substituents on the benzene ring have hydrogens that are capable of hyperconjugative stabilization of the positive charge in the radical cation. The electron transfer reorganization parameters are measurably different for the donors that have these substituents compared to those that lack such hydrogen atoms.