Photophysics and Photochemistry of Diphenyl Sulfone. 2. Investigation of the S1 Decay Pathways

An extensive analysis of the SI deactivation pathways of diphenyl sulfone (DPS) in alcohols and some other solvents is presented.The combined results of steady-state photolysis, time-resolved fluorescence, and steady-state fluorescence measurements atvariable temperature and of SI -- TI intersystem crossing quantum yield determinations enable a complete characterizationof the SI photophysics and photochemistry of DPS to be obtained. The main S1 decay channel appears to be an activatedSI -- T, intersystem crossing (Ea= 2 kcal/mol), other than SI -- TI. Although the fluorescence emission and SI -- TI quantum yields show some solvent dependence, they play minor roles in the solvents studied. The state eventually reachedin the SI -- T, activated intersystem crossing is proposed to be a dissociative 3ss* state localized on a C-S bond, on thebasis of experimental evidence and of some preliminary theoretical data. A simple rationalization of the observations is provided by a model potential energy diagram obtained as an extension of the one previously proposed to account for the results concerning the TI decay of the same molecule. Two hypotheses on the mechanism of the abovementioned SI -- T, activated intersystem crossing are put forward and briefly discussed