Is photoisomerization of stilbene in solution directly promoted by   solvent collisions?
  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. 
S. A. Kovalenko, A. L. Dobryakov, N. P. Ernsting
physics.chem-ph