Brown carbon chromophores at environmental air-water interfaces often act as photosensitizers that absorb sunlight and subsequently transfer energy to nearby molecules, initiating a wide variety of chemical reactions. Despite their importance to understanding daytime chemistry at these air-water interfaces, little is known about the role of the solvation environment on the photophysical properties of these photosensitizers. In this work, we present a joint experimental-theoretical study of the vibrational and photophysical properties of microhydrated protonated and deprotonated 4-benzoylbenzoic acid (4-BBA), a key model system for environmental photosensitizers. We find that for protonated 4-BBAH+∙(H2O)0-1, representing photosensitizers in very acidic conditions, a single bright state dominates the UV–Vis spectrum between 280 and 400 nm. Comparing the experimental UV–Vis spectra and quantum chemistry-predicted spectra of 4-BBAH+∙(H2O)0-2, we find that the degree of microhydration has little effect on the UV–Vis spectra or the orbitals of the dominant feature. For deprotonated 4-BBA‒, representing photosensitizers in basic conditions, quantum chemistry calculations predict that the UV–Vis spectra are ∼3x weaker in intensity than the brightest 4-BBAH+∙(H2O)0-1 features and were not observed experimentally. Quantum chemistry calculations predict one dominant UV–Vis feature is present in the spectra of 4-BBA‒∙(H2O)0-2, which exhibit minor shifts with degree of microhydration. While 4-BBA in bulk solution over a range of pH values has relatively weak absorption within the solar actinic region, we show that microhydrated 4-BBA has bright transitions within the actinic region. This indicates that the complex structure of environmental air-water interfaces can shift the absorption maximum of photosensitizers into the actinic region for enhanced absorption of sunlight and subsequent enhancement of photosensitizer-driven reactions.
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