Zinc‐Mediated Photodegradation of Nitrophenols in Aqueous Solution: Devising Environmental Mitigation Strategies With Ultrafast Spectroscopic Insights

ABSTRACT Nitrophenols are environmental pollutants found in waterways and the atmosphere, which can be a significant source of nitrous acid under UV light. In this work, we investigate nitrophenols in aqueous solution upon UV irradiation with a suite of steady‐state and ultrafast electronic and vibrational spectroscopies. We find that ortho ‐ and para ‐nitrophenol are highly sensitive to zinc as a heterogeneous catalyst, accelerating the transformation of nitrophenols with/without UV light. Steady‐state absorption, femtosecond stimulated Raman spectroscopy (FSRS), and quantum calculations reveal that UV light triggers excited‐state intramolecular proton transfer to form an aci ‐nitro intermediate that is stabilized by zinc. We photoexcite the metastable intermediate and obtain femtosecond transient absorption signatures. The redshifted ground‐state absorption and lengthened excited‐state lifetime of the aci ‐nitro intermediate are beneficial for photocatalytic removal with irradiation greater than 400 nm. Furthermore, the water O–H stretching band is tracked by FSRS to unveil a uniquely active role of water during the catalytic reaction of ortho ‐nitrophenol, rationalizing its reduced photosensitivity in organic solvents such as methanol. These neat treatment strategies are also applied to para ‐ and meta ‐nitrophenol and nitrophenolate. Our mechanistic insights into the photosensitivity of nitrophenols can inspire more effective cost‐efficient treatment strategies for atmospheric aerosols and water systems.