Excited State Nonadiabatic Dynamics Of o-Nitrophenol: Theoretical Insight into Pump-probe Experiments and Dissociation Mechanisms

Photoexcitation of o-nitrophenol is known to lead to many subsequent nonadiabatic events, such as internal conversion, intersystem crossing, excited state intramolecular proton transfer and dissociation to form HONO, an important atmospheric species. In this work, time-resolved photoionization and time-resolved photoelectron spectra were taken by pumping o-nitrophenol at 255 nm (4.9 eV) and probing at 153 nm (8.1 eV). The measurements revealed a time dependent ion fragment signal of mass 109 amu. Based on our computations this signal is attributed to the formation of the 2-hydroxyphenoxy ion and NO. Most previous experimental time resolved studies of o-nitrophenol excited the molecule to the first excited singlet state (S₁). In this work, however, at 4.9 eV, o-nitrophenol is pumped to the fourth excited singlet state (S₄) which allows the observation of the dissociation channel leading to 2-hydroxyphenoxy and NO as the dominant one. Electronic structure calculations were used to interpret the experiments and provide insight into the dynamics. Calculations based on multireference perturbation theory (CASPT2) of the potential energy surfaces created by linear interpolation of internal coordinates along the reaction coordinate of NO dissociation through a bicyclic, spiro like, intermediate showed that the photodissociation is most likely to occur on the first excited triplet state surface (T₁). Therefore, the findings provide strong evidence for internal conversion, intersystem crossing and NO formation following photoexcitation to S₄. The combination of photoelectron and photoion spectroscopy, together with computational results, provides robust evidence of intersystem crossing that is difficult to establish with only a single technique.