On the excitation wavelength dependence and Arrhenius behavior of stilbene isomerization rates in solution

Photoisomerization of trans-stilbene in solution occurs much faster than in jets or gases at collisionless conditions. Besides, solution reaction rates are nearly independent on λexc (excitation wavelength), while in jets the dependence is very pronounced. These facts are difficult to reconcile within transition state or RRKM theories. Mechanisms proposed so far may explain the difference in the rates but not their λexc dependence in solution. The discrepancy can be resolved by assuming that solute–solvent collisions promote isomerization directly, in addition to intramolecular activation. The two activation mechanisms can be distinguished by the weak λexc dependence.