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

The photoinduced cis-trans isomerization of all-trans retinal protonated Schiff base is studied using the full multiple spawning method. Our model allows for two classes of electronic transitions: exciton migration and quenching by coupling of the electronic and nuclear degrees of freedom. We find that quenching and exciton transfer are two temporally disjoint events and that exciton transport is highly directed. The observed selectivity in photoproducts 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 migration even when thermodynamic equilibrium among the excited electronic states is not reached.