Unravelling the photochemical pathways for aqueous p-nitrophenol excited by 320 nm ultraviolet radiation Deborin Ghosh 1 , Nicholas Lau 2 , Min-Hsien Kao 1 , William Whitaker 1 , Ian P. Clark 3 Partha Malakar 3 , Sandra Gómez 2 , Graham A. Worth 2 , Thomas A.A. Oliver 1 , Helen H. Fielding 2 , A.J. Orr-Ewing 1 1 School of Chemistry, University of Bristol, UK, 2 Department of Chemistry, University College London, UK, 3 Central Laser Facility, Research Complex at Harwell, UK Early-time dynamics of nitroaromatics and their corresponding bases can give valuable insights into photo- induced reactions relevant to atmospheric and environmental processes. In this work, femtosecond transient broadband infra-red and electronic absorption spectroscopy have been applied to explore the ultrafast dynamics of a tropospherically important organic compound p-nitrophenol in aqueous media of different pHs. After excitation to the lowest electronically excited state, transient electronic and IR absorption spectra were recorded between 350 to 900 nm and 1400 cm -1 to 1800 cm -1 respectively for delay times from sub-picosecond up to several microseconds. Several excited state dynamics have been monitored in our experiment and their kinetics have been quantitatively measured. After rapid (~10 ps) intersystem crossing from the S 1 excited state to the triplet manifold, the p-nitrophenol (T 1 ) state decays by excited-state proton transfer (ESPT) to the surrounding water. The resulting p-nitrophenolate anion builds up in its ground electronic state (S 0 ) over a ~5 ns timescale controlled by the ESPT and subsequent relaxation of the anion from an excited triplet state. Under neutral or acidic conditions, the p-nitrophenolate (S0) is re-protonated to recover p-nitrophenol in its electronic ground state with close to 100% efficiency. The observed photochemical behaviour of the p-nitrophenolate anion in aqueous and non-aqueous media agrees with prior work by Michenfelder et al.. 1 The experimental observations help to explain why nitroaromatic compounds such as p-nitrophenol are resistant to photo-oxidative degradation in the environment. References 1. 1.N. C. Michenfelder, H. A. Ernst, C. Schweigert, M. Olzmann, A.-N. Unterreiner, Phys. Chem. Chem. Phys. 20 , 10713 (2018).
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