Hydrogen Detachment in Diprotic Aromatics: O-H versus N-H Photodissociation in m-Aminophenol.

While the photochemistry of aromatics with a labile hydrogen atom has been broadly characterized through molecular beam experiments and electronic structure simulations, much less is known regarding the photodissociation of species with more than one exchangeable hydrogen. Fundamental questions like the relative probabilities for the scission of the different X-H bonds, the involved excited states and reaction mechanism, or even the possible photogeneration of diradicals and H2 as a byproduct, remain largely unexplored. On the basis of nonadiabatic molecular dynamics simulations at the TDDFT level, confronted and validated against CASSCF/NEVPT2 calculations, this study addresses these questions for m-aminophenol, a diprotic heteroaromatic compound featuring hydroxyl and amino functions. We find unequal propensities for O-H and N-H dissociation, and the stabilization of the radical products. Data science tools allow us to identify the reaction coordinates leading to dissociation in each case, identifying two very different pathways that result in the observed trends.