Photochemical isomerization in solution. Photophysics of diphenyl butadiene

We have examined the radiative and nonradiative decay rates of diphenyl butadiene in hydrocarbon and polar solvents as a function of temperature and pressure. We demonstrate that the radiative rate constant has a small dependence on solvent polarizability, in contrast to what is observed for diphenylhexatriene. We make an explicit separation of the internal molecular barrier to isomerization and the barrier due to frictional forces in the solvent. The energy of the internal barrier to isomerization is less than the activation energy observed in solution, and depends on the polarizability and polarity of the solvent. In contrast to diphenylhexatriene, it decreases, in more polar solvents. We compare the viscosity dependence of the barrier crossing process with Kramers’ theory, the recent kinetic theory result of Skinner and Wolynes [J. Chem. Phys. 69, 2143 (1978)], and the free volume theory of Gegiou, Muzkat, and Fischer, [J. Amer. Chem. Soc. 90, 12 (1968)]. Our data clearly show that the isomerization process is in the intermediate friction limit and we obtain experimental values for the parameters of the three theories. Over the viscosity region studied, Kramers’ and Skinner and Wolynes’ equations coincide exactly and do not reproduce the functional dependence of the radiationless rate pre‐exponential factor as well as the free volume theory. We speculate that it may be necessary to consider the frequency dependence of medium viscosity.

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