Fluorescence upconversion study of cis-stilbene isomerization

The isomerization dynamics of cis-stilbene in the first excited singlet state were studied by the technique of fluorescence upconversion. Lifetime measurements were made with subpicosecond resolution in 2-propanol, decanol, n-hexane, and tetradecane. The cis-stilbene fluorescence decay curves are single exponential in all solvents except for decanol, where they are adequately described by a double exponential. A weak viscosity dependence of the decay times is observed in both alcohols and alkanes. These results are discussed in terms of the Bagchi, Fleming, Oxtoby theory [J. Chem. Phys. 78, 7375 (1983)] for activationless electronic relaxation in solution, and the limitations of hydrodynamic models of microscopic friction. The fluorescence decay times of α,α′ dideutero cis-stilbene (cis-stilbene-D2) in tetradecane are found to be approximately 20% greater than those of cis-stilbene. A picosecond component in the fluorescence anisotropy decay measurements made in 2-propanol suggest that we are directly measuring motion along the reaction coordinate.