Temperature effects on radical ion and triplet quantum yields in excited-state electron transfer reactions of 1,12-benzperylene in acetonitrile

Activation energies of 1,12-benzperylene fluorescence quenching (EQ) in acetonitrile by a series of electron donors and acceptors and of radical ion (ER) and triplet state (ET) formation were determined from Arrhenius plots of the quantum yields. The quenchers were chosen in the range of Gibbs energies of electron transfer ΔGet=−100 to 6 kJ mol−1. Surprisingly low EQ, ER and ET values were obtained when ΔGet>−5 kJ mol−1. The effects of temperature are discussed from the point of view of two alternative mechanisms: the conventional mechanism of direct electron transfer and that of transient exciplex formation. Evidence for the latter is given.