Lau, Nicholas Albert; (2025) Photochemical dynamics of nitroaromatic molecules in the gas-phase and in solution. Doctoral thesis (Ph.D), UCL (University College London).

Text Thesis_Corrected_NL.pdf - Accepted Version Access restricted to UCL open access staff until 1 March 2026. Download (48MB)

Abstract

Nitroaromatic compounds are major components of brown carbon aerosols formed from biomass burning which absorb in the visible and near ultraviolet (UV) and contribute to atmospheric radiative forcing. Aqueous aerosols are exposed to visible and near-UV radiation where absorptions to the singlet excited state manifold of various nitroaromatics are located. Femtosecond transient absorption (TA) spectroscopy, time-resolved infrared (TRIR) spectroscopy, time-resolved photoelectron imaging (TRPEI), and quantum chemistry calculations have been used to investigate the relaxation processes following photoexcitation at wavelengths in the UVA and UVC bands (400-315 and 280-100 nm respectively). An overview of dynamics following photoexcitation is provided. Technical details of the experiments, the data analysis procedures used, and descriptions of the computational chemistry methods are also provided. TA and TRIR measurements on aqueous nitrobenzene following photoexcitation at 355 nm are presented and compared with analogous measurements in methanol, acetonitrile, and cyclohexane. Coupling to the solvent bath through hydrogen bonds slows the internal conversion process to the ground state and allows for efficient disposal of energy without forming photoproducts. TA and TRIR measurements on 3-nitrophenol under acidic, neutral, and basic conditions following photoexcitation at 320 (pH 1-7) and 390 nm (pH 14) are presented. 3-Nitrophenol undergoes a rapid intersystem crossing to the triplet manifold on which an excited state proton transfer occurs to form 3-nitrophenolate. Protonation of the anion occurs on an extended pH-dependent nanosecond timescale. TA measurements on 2- and 4-nitrotoluene in methanol following photoexcitation at 355 nm reveal a rapid population of the triplet manifold and, for 2-nitrotoluene, the formation of an aci-nitro product existing to timescales outside the experimental window. TRPEI measurements on gas-phase nitrobenzene following photoexcitation at 240.5 nm reveal two ultrafast processes which are attributed to a rapid cascaded decay to the lowest singlet excited state and the subsequent lifetime of this state.

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