Dynamical Friction Effects on the Photoisomerization of a Model Protonated Schiff Base in Solution

Photoisomerization involving a conical intersection (CI) for a model protonated Schiff base (PSB) in modeled water and acetonitrile solvents is examined with the inclusion of energy- and momentum-transfer effects described via a generalized Langevin equation (GLE) frictional approach and surface-hopping dynamics. Short-time GLE frictional effects on the model's three coordinates, the intramolecular bond length alternation and torsional PSB coordinates and a solvent coordinate, eliminate several unphysical features associated with a no-friction inertial description and have the general feature of accelerating nonadiabatic transitions to the ground electronic state. The inertial prediction of equal probability formation of ground-state trans and cis isomer products subsequent to the Franck-Condon excitation of the ground cis isomer is replaced by the GLE prediction of a preferential higher proportion of ground-state trans isomer, that is, a successful cis to trans photoreaction. This preference is solvent-dependent and is enhanced in water solvent with its higher friction intensity and short time scales. For the fast water solvent motion, the nonadiabatic transitions to the S(0) ground state are centered around the CI seam (which is due to the solvent coordinate's role as a tuning coordinate), facilitating direct transitions to the ground-state trans isomer. In contrast, for the slower acetonitrile solvent motion, the decay occurs, on average, away from the CI seam in regions with a finite free-energy gap between the excited and ground states, resulting in reduced trans isomer production. Some directions for the extension of the model description are also discussed.