Primary photophysical processes in benzene. Part 3.—Effect of concentration on triplet yields

Abstract

The triplet yield in benzene solutions irradiated at 254 nm has been measured as a function of temperature and concentration. The increase in triplet yield in going from 0.112 M (0.23) to 11.2 M (0.56) at 25°C is attributed to a high probability of intersystem crossing from the singlet excimer. An apparently anomalous temperature dependence of the intrinsic intersystem crossing yield from dimer, ϕ^o_TD, is attributed to rapid decay of the triplet dimer, probably by diradical formation. The transfer of excitation energy from the triplet excimer to but-2-ene is endothermic and inefficient and triplet yield measurements at higher temperatures by the but-2-ene isomerization technique can be affected. The formation and rapid radiationless disapearance of the triplet excimer explains the very short lifetime of the benzene triplet in solution. A complex kinetic situation arises in which neither triplet lifetimes nor yields are measured by the addition of excited state scavengers under all conditions.