Efficient Ground-State Recovery of UV-Photoexcited p-Nitrophenol in Aqueous Solution by Direct and Multistep Pathways

Nitroaromatic compounds are found in brown carbon aerosols emitted to the Earth's atmosphere by biomass burning, and are important organic chromophores for the absorption of solar radiation. Here, transient absorption spectroscopy spanning 100 fs-8 μs is used to explore the pH-dependent photochemical pathways for aqueous solutions of p-nitrophenol, chosen as a representative nitroaromatic compound. Broadband ultrafast UV-visible and infrared probes are used to characterize the excited states and intermediate species involved in the multistep photochemistry, and to determine their lifetimes under different pH conditions. The assignment of absorption bands, and the dynamical interpretation of our experimental measurements are supported by computational calculations. After 320 nm photoexcitation to the first bright state, which has ¹ππ* character in the Franck-Condon region, and ultrafast (∼200 fs) structural relaxation in the adiabatic S₁ state to a region with ¹nπ* electronic character, the S₁ p-nitrophenol population decays on a time scale of ∼12 ps. This decay involves competition between direct internal conversion to the S₀ state (∼40%) and rapid intersystem crossing to the triplet manifold (∼60%). Population in the T₁-state decays by excited-state proton transfer (ESPT) to the surrounding water and relaxation of the resulting triplet-state p-nitrophenolate anion to its S₀ electronic ground state in ∼5 ns. Reprotonation of the S₀-state p-nitrophenolate anion recovers p-nitrophenol in its electronic ground state. Overall recovery of the S₀ state of aqueous p-nitrophenol via these competing pathways is close to 100% efficient. The experimental observations help to explain why nitroaromatic compounds such as p-nitrophenol resist photo-oxidative degradation in the environment.