Fluorescence of Benzene. The Effects of Solvent and Temperature on the Quantum Yield

To investigate the mechanism of the solvent and thermal quenching of fluorescence, the absolute quantum yield of benzene excited at 254 mμ has been determined in four deaerated solvents as a function of concentration ${\text{(10}}^{\text{−4}}{\text{–10}}^{\text{−2}}\mathrm{M)}$ and temperature (77°–330°K). In the limit of low temperature, the yield in the four solvents was found to be within experimental error (± 20%) of the yield 0.18 determined at 298°K for the vapor by Noyes, Mulac, and Harter. In the limit of high temperature, the yield approached zero in all the solvents at all concentrations. Similarities between the fluorescence properties of benzene in solution and vapor suggest that the thermal quenching may be related to an intramolecular process, not intersystem crossing, with an activation energy about 2000 cm⁻¹, and which is induced by solute–solvent interactions.