pH Dependence of the speciation and optical properties of 4-benzoylbenzoic acid

Organic chromophores initiate much of daytime aqueous phase chemistry in the environment. Thus, studying the absorption spectra of commonly used organic photosensitizers is paramount to fully understand their relevance in environmental processes. In this work, we combined UV-Vis spectroscopy, ¹H-NMR spectroscopy, quantum chemical calculations, and molecular dynamics simulations to investigate the absorption spectra of 4-benzoyl benzoic acid (4BBA), a widely used photosensitizer and a common proxy of environmentally relevant chromophores. Solutions of 4BBA at different pH values show that protonated and deprotonated species have an effect on its absorbance spectra. Theoretical calculations of these species in water clusters provide physical and chemical insights into the spectra. Quantum chemical calculations were conducted to analyze the UV-Vis absorbance spectra of 4BBA species using various cluster sizes, such as C₆H₅COC₆H₄COOH·(H₂O)_n, where n = 8 for relatively small clusters and n = 30 for larger clusters. While relatively small clusters have been successfully used for smaller chromophores, our results indicate that simulations of protonated species of 4BBA require relatively larger clusters of n = 30. A comparison between the experimental and theoretical results shows good agreement in the pH-dependent spectral shift between the hydrated cluster model and the experimental data. Overall, the theoretical and empirical results indicate that the experimental optical spectra of aqueous phase 4BBA can be represented by the acid-base equilibrium of the keto-forms, with a spectroscopically measured pK_a of 3.41 ± 0.04. The results summarized here contribute to a molecular-level understanding of solvated organic molecules through calculations restricted to cluster models, and thereby, broader insight into environmentally relevant chromophores.