Deuterium, methyl, and solvent effects on the phosphorescence lifetime of benzene

The phosphorescence lifetimes of benzene and a number of its methyl and deutero derivatives have been investigated in a wide variety of solvents at temperatures below 65 °K. Deuterium effects indicate a dominant role for the aryl hydrogens in the radiationless process. Methylation increases the triplet decay rate by approximately 25% per methyl group in a nearly linear fashion. Intensity data indicate that this is not due to ``allowed'' character introduced by any reduction in molecular symmetry as is theoretically predicted to be the case. Perfluorocarbons act as inert solvents for benzene derivatives whereas typical hydrocarbons produce 50% or greater changes in the lifetimes even at very low temperatures. The data suggest a common mechanistic pathway (or perturbed state) for both the effects of solvent and methyl groups