Effect of steric hindrance on the dynamics of charge recombination within geminate ion pairs

The dynamics of charge recombination within geminate ion pairs formed by electron transfer (ET) quenching of excited aromatic hydrocarbons by aliphatic and aromatic amines was investigated using picosecond transient grating spectroscopy. With aliphatic donors, the rate constant of back ET, k(BET), shows a substantial decrease with increasing steric encumbrance around the N atom. No correlation between k(BET) and the exergonicity of the process was observed. This effect is ascribed to a decrease of the electronic coupling matrix element, V, which is affected by both the distance between the N atom of the donor and the aromatic plane of the acceptor and by the delocalization of the hole upon increasing the bulkiness of the alkyl substituents. With aromatic amines, k(BET) is substantially slower than with the unhindered amines. This is also explained in terms of a smaller value of V because of charge delocalization.