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 el ectron transfer (ET) quenching of excited aromatic hydrocarbons by aliphati c and aromatic amines was investigated using picosecond transient grating s pectroscopy. With aliphatic donors, the rate constant of back ET, k(BET), s hows a substantial decrease with increasing steric encumbrance around the N atom. No correlation between k(BET) and the exergonicity of the process wa s observed. This effect is ascribed to a decrease of the electronic couplin g matrix element, V, which is affected by both the distance between the N a tom of the donor and the aromatic plane of the acceptor and by the delocali zation of the hole upon increasing the bulkiness of the alkyl substituents. With aromatic amines, k(BET) is substantially slower than with the unhinde red amines. This is also explained in terms of a smaller value of V because of charge delocalization.