Influence of solvent on the electronic structure and the photochemistry of nitrophenols

Previous studies have suggested that the photochemistry of nitroaromatics in organic solvents can vary significantly from the photochemistry in aqueous solutions. This work compares the photodegradation of 2-nitrophenol (2NP), 4-nitrophenol (4NP), 2,4-dinitrophenol (24DNP), and 2,4,6-trinitrophenol (246TNP) in 2-propanol and water to better understand the photochemical loss of nitrophenols in atmospheric organic particles and aqueous droplets. Polychromatic quantum yields were determined by monitoring the loss of absorbance of each nitrophenol with UV/vis spectroscopy in the presence of an acid (undissociated nitrophenol) or base (nitrophenolate). There was no orderly variation between loss rates in the organic and aqueous phases: 2NP and 4NP had similar yields in the two solvents. 246TNP was an outlier in these results as it dissociated in both acidified 2-propanol and water due to its exceptionally strong acidity. A notable result is that only for 24DNP was a dramatically increased reactivity found in 2-propanol compared to that in water. Time-dependent density functional theory calculations were carried out to characterize the excited state energies and absorption spectra with a conductor-like polarizable continuum model or explicit solvation by a few solvent molecules. Explicit solvent calculations suggest the enhanced reactivity of 24DNP in 2-propanol is due to the strong interaction between a 2-propanol molecule and an -NO₂ group in the excited state. For the other nitrophenols, the solvent effects on electronic structure were minimal. Overall, the observations in this work suggest that solvent effects on the electronic structure and condensed-phase photochemistry of nitrophenols are minimal, with the exception of 24DNP.