PHOTOISOMERIZATION IN DENSE GASES AND LIQUIDS

The pressure and temperature dependence of the rate coefficient for the singlet state photoisomerization of trans-stilbene and all-trans-diphenylbutadiene in supercritical and liquid alkane solvents reveal the importance of specific solvent effects modifying the reaction path on the potential energy surface. In the gas-liquid transition range the barrier height for the reaction seems to be lowered due to increasing solvation of the reactants in solute-solvent clusters. At higher pressures, the transition to the Smoluchowski-limit within each solvent is well described by Kramers' model. Differences between solvents can be attributed to a solvent-dependent barrier shape. Deviations from this description only appear for stilbene at higher viscosities. In part, they seem also be due to a specific influence of the compressed solvent on the reaction path and to a frequency dependence of the friction. Solvent-size dependent microfriction effects do not seem to be responsible for the observed solvent dependence of the reaction rates. Manifestations of multidimensional barrier crossing show up in the strong temperature dependence of the rate coefficient at constant solvent self-diffusion coefficient. Apparently, the reaction path on the energy surface changes with temperature leading to an effectively temperature dependent height and shape of the barrier for the reaction with increasing excitation of "perpendicular" modes. Possible reasons for the striking difference of the friction dependence of the rate coefficient between stilbene and diphenylbutadiene at intermediate to high friction are also discussed.