Nonadiabatic Excited-State Dynamics of Ortho-Fluorophenol

The coupling between nuclear motion and electronic structure strongly influences nonradiative relaxation following photoexcitation. Substituted phenols serve as benchmark systems for probing how intramolecular hydrogen bonding and geometrical distortions affect ultrafast excited-state dynamics. We present a computational study of ortho-fluorophenol using on-the-fly nonadiabatic molecular dynamics with Zhu-Nakamura hopping probabilities, performed at the CASSCF(12,9)/6-311++G** level. Trajectory ensembles were initialized in either the ππ* (S₁) or πσ* (S₂) excited states. Trajectories initiated from S₁ showed no relaxation within 100 fs, consistent with experimentally inferred long-lived tunneling behavior. In contrast, 44% of trajectories initiated on S₂, 44% underwent internal conversion to S₁ within sub-100 fs. Competing relaxation channels were identified: 27% of trajectories exhibited prompt O–H bond cleavage within 20 fs, while 17% relaxed through a leaving-group-preserving pathway driven by low-frequency out-of-plane distortions involving the C–F, C–OH, and C–H coordinates. Relaxation to the ground state remained limited (8%) over the simulated interval, and no C–F bond dissociation was observed. These results illustrate how selective vibrational mode coupling controls branching between dissociative and non-dissociative decay pathways in hydrogen-bonded aromatic chromophores.