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Abstract
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 BBA+∙(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.
Supplementary materials
Title
Supporting Information for: Environmental Photosensitizers Can Exhibit Enhanced Actinic Absorption in Microhydrated Clusters Compared to Solution
Description
Content:
Fig. S1. Calculated vibrational spectrum of 4-BBAH+ tagged with a N2 molecule.
Fig. S2. Comparison of the experimental 4-BBAH+ fingerprint spectrum with calculated spectra for protonation at the ketone group and at the carboxylic acid group.
Figs. S3-S5. Calculated vibrational spectra, electronic spectra, and NTOs for higher-energy conformers of 4-BBAH+.
Figs. S6-S8. Calculated vibrational spectra, electronic spectra, and NTOs for higher-energy conformers of 4-BBAH+∙H2O.
Figs. S9-S11. Calculated vibrational spectra, electronic spectra, and NTOs for higher-energy conformers of 4-BBAH+∙(H2O)2.
Fig. S12. Comparison of UV photodissociation mass spectra of 4-BBAH+ and 4-BBAH+∙H2O.
Fig. S13. UV photodissociation spectrum of 4-BBAH+∙N2.
Fig. S14. NTOs of the weaker higher-energy transitions predicted for 4-BBAH+∙(H2O)0-2.
Fig. S15. Ketone and carboxylate CO stretch region of 4-BBA‒.
Fig. S16. Calculated UV spectra of neutral 4-BBA∙(H2O)0-2.
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