Competing Nonadiabatic\nRelaxation Pathways for Near-UV\nExcited <i>ortho</i>-Nitrophenol in Aqueous Solution

Nitrophenols are atmospheric pollutants found in brown\ncarbon aerosols\nproduced by biomass burning. Absorption of solar radiation by these\nnitrophenols contributes to atmospheric radiative forcing, but quantifying\nthis climate impact requires better understanding of their photochemical\npathways. Here, the photochemistry of near-UV (λ = 350 nm) excited <i>ortho</i>-nitrophenol in aqueous solution is investigated using\ntransient absorption spectroscopy and time-resolved infrared spectroscopy\nover the fs to μs time scale to characterize the excited states,\nintermediates, and photoproducts. Interpretation of the transient\nspectroscopy data is supported by quantum chemical calculations using\nlinear-response time-dependent density functional theory (LR-TDDFT).\nOur results indicate efficient nonradiative decay via an S<sub>1</sub>(ππ*)/S<sub>0</sub> conical intersection leading to hot\nground state <i>ortho</i>-nitrophenol which vibrationally\ncools in solution. A previously unreported minor pathway involves\nintersystem crossing near an S<sub>1</sub>(nπ*) minimum, with\ndecay of the resulting triplet <i>ortho</i>-nitrophenol\nfacilitated by deprotonation. These efficient relaxation pathways\naccount for the low quantum yields of photodegradation.