Femtosecond Studies of Orientationai Anisotropy Decay of Benzopyranthione in the Excited 52 State in Hydrocarbons

Reorientational dynamics of 4H-l-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, «-heptane, «-octane, «-decane, «-dodecane and cyclohexane. The dependence of the rotational time constant, r„,-, 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 orientationai anisotropy is observed for the first time for a molecule in the S2 state. 90 Femtosecond Studies of Anisotropy Decay of BPT in the Excited S2 State in Hydrocarbons 7.