Effect of solvent in the photoreduction and quenching of benzophenone by triethylamine, 1-azabicyclo[2.2.2]-octane and 1,4-diazabicyclo[2.2.2]octane

Photoreduction of benzophenone by triethylamine (TEA) is efficient in tert-butyl alcohol, acetone and acetonitrile (φ{symbol} = 0.8 - 1.1) and somewhat less efficient in benzene in which evidence of a light-absorbing quenching transient (LAT) is found. Photoreduction by the bridgehead bicyclic amine quinuclidine (1-azabicyclo[2.2.2]octane (ABCO)) is less efficient (φ{symbol} = 0.08 - 0.22) with lower quantum yield in the polar solvents and higher quantum yield in the less polar solvents despite formation of a LAT. Photoreduction by the corresponding bridgehead bicyclic diamine 1,4-diazabicyclo[2.2.2]octane (DABCO) is still less efficient (φ{symbol} = < 0.001 - 0.09) again with lower quantum yield in the polar solvent and the higher in the hydrocarbon. In 1:1 water-pyridine photoreduction of 4-carboxybenzophenone by TEA is efficient (φ{symbol}_ketyl = 1) by ABCO is very inefficient (φ{symbol}_ketyl = 0.0004) and by DABCO leads only to quenching. Low quantum yields for the reduction by ABCO and DABCO in polar solvents are accompanied by high values of the interaction rate constant k_ir. The results are interpreted in terms of charge transfer interaction between excited triplet carbonyl and non-bonding electrons of the amines. The effects of hyperconjugation and charge delocalization in the charge transfer complex (CTC) and the role of charge transfer complexation and of disproportionation and dissociation to free ions as light-wasting processes are discussed.