Effect of polar and non-polar media on photoinduced ring-opening of 1,3-cyclohexadiene: A QM/MM-based surface hopping approach

Solute-solvent interactions can affect the electronic state energies and crossing points, which in turn shape the photochemical pathways of a system. In this study, we investigated the effect of solvent polarity on the photochemical ring-opening reaction of the 1,3-cyclohexadiene molecule to yield the hexatriene photoproduct using water and n-hexane as solvent media. We used classical molecular dynamics to collect the initial geometries for further QM/MM-based surface hopping simulations. Using multireference state-averaged complete active space self-consistent field theory for QM and the OPLS-AA force field and TIP3P water model for the MM region, the excited-state dynamics in both solvent media were observed. Ensemble-averaged bond length evolution, electronic state energies, and adiabatic populations were analyzed. This exploration reveals that the polar medium can facilitate faster and more efficient ring-opening, with a relatively higher quantum yield (61.7% ± 2.8%) compared to the non-polar medium (53.7% ± 2.9%), owing to the stabilization of polarized electronic states and the enhanced accessibility of the conical intersection regions. Consistently, the excited-state decay was observed to proceed more rapidly in the polar solvent environment. Furthermore, solvation shell analysis confirms the dynamic role of water in influencing the reaction pathway. These findings underscore the critical role of solvent environments in modulating photochemical processes and can help guide the design of better systems in areas like molecular switches, light-based medicine, and solar energy.