Experimental data supporting "Environmental photosensitizers can exhibit enhanced actinic absorption in microhydrated clusters compared to solution," DOI: 10.1016/j.ijms.2025.117431.

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 studyof 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.

摘要：环境气水界面（air-water interfaces）处的布朗碳发色团（Brown carbon chromophores）通常作为光敏剂（photosensitizers），吸收太阳光后将能量传递至邻近分子，引发各类化学反应。尽管此类发色团对理解气水界面的日间化学过程至关重要，但学界对溶剂化环境如何影响这类光敏剂的光物理性质仍知之甚少。本研究针对微水合的质子化与去质子化4-苯甲酰苯甲酸（4-benzoylbenzoic acid，4-BBA）——一类环境光敏剂的关键模型体系——的振动与光物理性质，开展了实验与理论相结合的研究。研究发现，对于代表强酸性环境中光敏剂的质子化4-BBAH+∙(H2O)0-1，其280至400 nm区间的紫外-可见光谱（UV–Vis spectrum）由单一亮态主导。通过对比4-BBAH+∙(H2O)0-2的实验紫外-可见光谱与量子化学（quantum chemistry）预测光谱，我们发现微水合程度对该光谱及主导特征的轨道结构影响极小。对于代表碱性环境中光敏剂的去质子化4-BBA‒，量子化学计算预测其紫外-可见光谱强度约为最亮的4-BBAH+∙(H2O)0-1特征峰的1/3，且未在实验中被观测到。量子化学计算显示，4-BBA‒∙(H2O)0-2的光谱中存在单一主导紫外-可见特征峰，其峰位随微水合程度仅发生微小偏移。尽管在宽pH范围的本体溶液中，4-BBA在太阳光化学活性区域（solar actinic region）内的吸收相对较弱，但我们的研究表明，微水合状态下的4-BBA在该活性区域内存在明亮的电子跃迁。这意味着环境气水界面的复杂结构可将光敏剂的吸收峰位移至太阳光化学活性区域，从而增强其对太阳光的吸收效率，并进一步强化光敏剂驱动的化学反应。