Solvent-dependent nonadiabatic dynamics and excited-state branching of cis -stilbene from QM/MM-MD simulations

Cis-stilbene exhibits ultrafast excited-state dynamics in solution, where photoisomerization and photocyclization compete on the picosecond timescale and are strongly influenced by solvent polarity. Despite extensive experimental studies, the molecular-level mechanism by which solvent environments control nonadiabatic relaxation and product branching remains unclear. Here, we perform QM/MM surface-hopping ab initio molecular dynamics simulations based on MRSF-TDDFT to investigate the excited-state dynamics of cis-stilbene in explicit nonpolar (n-hexane) and polar (methanol) solvents. The simulations qualitatively reproduce the experimental solvent trend, including the shorter excited-state lifetime and increased trans-isomer yield in methanol. Trajectory analyses revealed that twisted structures formed along the torsional coordinate exhibit enhanced dipole moments due to charge localization character in the S1 state. While this dipole enhancement occurs in both solvents, only in polar methanol does electrostatic stabilization significantly reduce the S1-S0 energy gap, thereby promoting nonadiabatic transitions over a broader configurational space. These results demonstrate that solvent polarity reshapes the excited-state branching landscape of cis-stilbene by modulating nonadiabatic transitions through electrostatic stabilization of charge-separated twisted geometries. The present study provides direct computational insight into solvent-controlled photoreaction dynamics and highlights the importance of explicit solvent effects in ultrafast processes in solution.