Fluorescence upconversion study of cis-stilbene isomerization

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Abstract
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.