Femtosecond studies of orientational anisotropy decay of benzopyranthione in the excited S2 state in hydrocarbons

Reorientational dynamics of 4H-1-benzopyrane-4-thione (BPT) in the S2 state, in hydrocarbon solution, is studied from the anisotropy decay of fluorescence upconversion and pump–probe absorption transients with femtosecond time resolution. The anisotropy decay is found to be monoexponential in n-hexane, n-heptane, n-octane, n-decane, n-dodecane and cyclohexane. The dependence of the rotational time constant, τor, on the solvent viscosity is approximated by a linear function with a slope of 2.6 ± 1 ps/cP indicating slip conditions. The decay of orientational anisotropy is observed for the first time for a molecule in the S2 state.