PHOTOISOMERIZATION DYNAMICS OF DIPHENYLBUTADIENE IN COMPRESSED LIQUID ALKANES AND IN SOLID ENVIRONMENT

The investigation of the pressure dependence of the S1 photoisomerization of diphenylbutadiene in n-alkanes from ethane to n-dodecane allows us to differentiate between various models proposed to explain the observed viscosity dependence of the rate coefficient k(iso) for twisting about one of the double bonds. For each solvent we observe a linear dependence of k(iso) on the inverse of the solvent viscosity with a slope that increases with solvent size. Comparing this result with models for microscopic friction we find that the observed solvent size effect is significantly stronger than predicted. We conclude that a hydrodynamic description of frictional forces in terms of the zero frequency shear viscosity of the solvent is in agreement with the observed pressure and solvent dependence. We suggest, therefore, that the solvent size effect reflects variations in the shape of the reaction path with solvent that are possibly associated with the multidimensionality of the barrier crossing process. We also report nonradiative rate coefficients for DPB in solid solution.