Probing the Photochemical Funnel of a Retinal Chromophore Model via Zero Point Energy Sampling Semiclassical Dynamics

In this work, both static and dynamic aspects of the Z --> E photoisomerization of the (Z)-pentadieniminium cation, a minimal model of retinal (the chromophore of rhodopsin proteins), are investigated on the basis of ab initio CASSCF potential energy surfaces taking into account the full set of vibrational degrees of freedom of the molecule. In particular, the structure of the lowest lying one-dimensional cross-section of the intersection space formed by the S-1 and S-0 energy surfaces has been mapped along the Z --> E isomerization of the central double bond from 0degrees to 180degrees. The evolution of a semiclassical wave packet toward the S-1/S-0 intersection seam on the basis of 70 independently calculated zero-point energy sampled trajectories has been examined. The results indicate that the photodynamics of the Z --> E isomerization is controlled by a small segment of the intersection space which intercepts the excited-state reaction path. In addition to the effects of surface topography, the influence of the kinetic energy on the rate of the Z --> E isomerization is also investigated.