Efficient Ground-State\nRecovery of UV-Photoexcited <i>p</i>‑Nitrophenol\nin Aqueous Solution by Direct and Multistep\nPathways

Nitroaromatic compounds are found in brown carbon aerosols\nemitted\nto the Earth’s atmosphere by biomass burning, and are important\norganic chromophores for the absorption of solar radiation. Here,\ntransient absorption spectroscopy spanning 100 fs–8 μs\nis used to explore the pH-dependent photochemical pathways for aqueous\nsolutions of <i>p</i>-nitrophenol, chosen as a representative\nnitroaromatic compound. Broadband ultrafast UV–visible and\ninfrared probes are used to characterize the excited states and intermediate\nspecies involved in the multistep photochemistry, and to determine\ntheir lifetimes under different pH conditions. The assignment of absorption\nbands, and the dynamical interpretation of our experimental measurements\nare supported by computational calculations. After 320 nm photoexcitation\nto the first bright state, which has <sup>1</sup>ππ* character\nin the Franck–Condon region, and ultrafast (∼200 fs)\nstructural relaxation in the adiabatic S<sub>1</sub> state to a region\nwith <sup>1</sup>nπ* electronic character, the S<sub>1</sub> <i>p</i>-nitrophenol population decays on a time scale\nof ∼12 ps. This decay involves competition between direct internal\nconversion to the S<sub>0</sub> state (∼40%) and rapid intersystem\ncrossing to the triplet manifold (∼60%). Population in the\nT<sub>1</sub>-state decays by excited-state proton transfer (ESPT)\nto the surrounding water and relaxation of the resulting triplet-state <i>p</i>-nitrophenolate anion to its S<sub>0</sub> electronic ground\nstate in ∼5 ns. Reprotonation of the S<sub>0</sub>-state <i>p</i>-nitrophenolate anion recovers <i>p</i>-nitrophenol\nin its electronic ground state. Overall recovery of the S<sub>0</sub> state of aqueous <i>p</i>-nitrophenol via these competing\npathways is close to 100% efficient. The experimental observations\nhelp to explain why nitroaromatic compounds such as <i>p</i>-nitrophenol resist photo-oxidative degradation in the environment.