Is photoisomerization of stilbene in solution directly promoted by solvent collisions?

Abstract

Gas phase thermal isomerization rates of trans-stilbene at 300 K obey the Arrhenius law. In solution the rates increase ten-fold and depend only very weakly on excitation wavelength or intramolecular temperature. Both observations are difficult to reconcile within RRKM theory. Previously discussed mechanisms, like restricted IVR, nonadiabaticity, solvent-dependent barriers, or excitation-induced cooling, may explain the increase of the rates in solution but not their dependence on the excitation wavelength. The latter suggests that solvent collisions can directly promote the isomerization.