How a toxic pollutant avoids photodegradation: Relaxation pathways of UV-excited 2,4-dinitrophenol in aqueous solution.

Nitroaromatic compounds are pollutants emitted from biomass burning and fossil fuel combustion. They are a major component of brown carbon aerosols, affecting radiative forcing in the lower atmosphere. Among these species, 2,4-dinitrophenol (DNP) is toxic to both plants and animals and is resistant to photodegradation when dissolved in water, such as in aqueous atmospheric aerosols. To understand this environmental photostability of DNP, the photochemistry of near-UV excited DNP in aqueous solution is investigated using transient absorption spectroscopy and time-resolved infrared spectroscopy, seamlessly spanning fs - μs timescales to reveal the pathways following photoexcitation. Building upon our understanding of simpler nitroaromatic species, and using linear-response time-dependent density functional theory (LR-TDDFT) to provide a framework for the interpretation of the results, the complex photochemistry of this species is unraveled. The majority of DNP relaxes within the singlet manifold, via intersection seams between the S₁ potential energy surface and the S₀ state, on timescales shorter than the few-picosecond limits of vibrational cooling. A second ground-state recovery pathway involves intersystem crossing from a region of the S₁ surface with nπ* electronic character into the triplet manifold, deprotonation to form the nitrophenolate anion and reprotonation in solution. Branching ratios between these pathways are influenced by the excitation wavelength. In aqueous solution, DNP will also exist as dinitrophenolate anions which, when photoexcited in the near-UV, undergo direct electronic relaxation on sub-ps timescales. Combinations of these pathways result in complete S₀ recovery, allowing this toxic species to resist solar photodegradation when dissolved in aqueous atmospheric aerosols.