Solvent-dependent triplet-state behaviour of nitrophenol and nitroanisole isomers

Triplet states of nitroaromatic compounds are populated rapidly by intersystem crossing (ISC) after ultraviolet or visible photoexcitation to electronically excited singlet states. These triplet state populations play a major role in the photochemistry of the nitroaromatic species, with implications for their use in synthetic chemistry and for their longevity as environmental pollutants. Nitrophenols have also been shown to display photoacidic behaviour from an excited triplet state, undergoing excited state proton transfer to solvent. In this work, the solution-phase behaviours of meta and para isomers of photoexcited nitrophenol and nitroanisole are investigated using transient absorption spectroscopy. By comparing nitrophenols to their respective nitroanisoles in a range of solvents, the photoacidic pathways can be isolated from the photophysics of excited-state relaxation. The electronic characters of the lowest-energy triplet states of the anisoles are solvent-dependent, with the T₁ state of para-nitroanisole having predominantly ³nπ* character in acetonitrile solution, and ³ππ* character in water. The corresponding ³ππ* states for nitrophenols lead to photoacidic behaviour in proton-accepting solvents. In methanol, meta-nitrophenol undergoes faster proton transfer to solvent (τ = 10-20 ps) than its para isomer, whereas in water the photoacidic behaviour of nitrophenols is rate-limited by the ∼10 ps ISC into the triplet manifold, and the neutral T₁ state populations cannot be detected. The triplet-state nitrophenolate anions produced by deprotonation have distinct spectra and relax by ISC to the anionic ground (S₀) state. This new understanding of the rich photophysics and photochemistry of these systems can inform synthetic procedures and models of the environmental fate of nitroaromatic compounds.