Electronic energy funnels in cis-trans photoisomerization of retinal protonated Schiff base

The photoinduced cis-trans isomerization of all-trans retinal protonated Sc hiff base is studied using the full multiple spawning method. Our model all ows for two classes of electronic transitions: exciton migration and quench ing by coupling of the electronic and nuclear degrees of freedom. We find t hat quenching and exciton transfer are two temporally disjoint events and t hat exciton transport is highly directed. The observed selectivity in photo products is interpreted in terms of an electronic energy "funnel", which is attributed to the Schiff base. The limits of the concept of an electronic energy "funnel", when electron-phonon coupling is strong, are discussed and it is argued that such funnels can induce selectivity in excitation migrat ion even when thermodynamic equilibrium among the excited electronic states is not reached.