Theoretical Insights into Ultrafast-Decaying and Long-Lived States of <i>ortho</i>-Nitrophenol upon Photoexcitation in the Gas Phase

Nitrophenols in the atmosphere are chromophore pollutants that absorb sunlight. Among them, <i>ortho</i>-nitrophenol (<i>o</i>-NP) stands out due to its strong intramolecular hydrogen bond which facilitates excited-state intramolecular proton transfer (ESIPT) and influences its photoisomerization and dissociation. Time-resolved experiments have captured both ultrafast-decaying and long-lived signals. Here, we employ non-adiabatic molecular dynamics simulations and the reaction space projector method to elucidate the photoinduced reactions of <i>o</i>-NP. Our simulations show that photoexcited <i>o</i>-NP in the S₁(ππ*) state undergoes ultrafast ESIPT followed by rapid decay through one of two non-adiabatic pathways: ultrafast internal conversion to the S₀ state and intersystem crossing (ISC) to the triplet states. Furthermore, trajectories undergoing ISC remain trapped in triplet states, with reverse ISC to the singlet states rarely observed. These trajectories are clearly linked to the long-lived excited-state signal, providing more conclusive computational evidence of the origin.