Unveiling the Mechanism of o-fluorophenol Ultrafast Photodecay

Excited states play a central role in essential natural processes. For example, tyrosine, a common natural compound, is shielded from UV damage through ultrafast nonradiative photodecay, driven by phenol’s butterfly mode vibration. Intramolecular H-bonding in phenol derivatives like o-fluorophenol disrupts this decay process. The relaxation mechanism of o-fluorophenol remains incompletely understood, as it exhibits both ultrafast and nanosecond decay rates that may follow distinct pathways. Nonadiabatic dynamics simulations were performed using complete active space self-consistent field theory, and transition probabilities were computed using the Zhu-Nakamura theory. Relaxation from S₂ is notably inhibited, contrary to prior phenol photodynamic simulations. Relaxation through H-tunneling from S₁ is rare. In contrast to the hypothesis, decay via butterfly vibration does not lead to H-dissociation and follows a distinct conical intersection for a longer time. A fraction of the trajectories undergoing H-dissociation persisted on S₁, supporting the nanosecond lifetime. These findings highlight the pivotal role of intramolecular H-bonding in impeding the ultrafast relaxation of phenols. Also, theoretical elucidation clarified ambiguous experimental decay routes, revealing two distinct mechanisms. These insights advance the fundamental understanding of excited-state processes in aromatic compounds.