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, ϕoTD, 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.