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Title: Aesculetin Exhibits Strong Fluorescent Photoacid Character
Coumarins are bioactive molecules that often serve as defenses in plant and animal systems, and understanding their fundamental behavior is essential for understanding their bioactivity. Aesculetin (6,7-dihydroxycoumarin) has recently attracted attention due to its ability to act as an antioxidant, but little is known about its photophysical properties. The fluorescence lifetimes of its neutral and anion form in water are 19 ± 2 ps and 2.3 ± 0.1 ns, respectively. Assuming the short lifetime of the neutral is determined by ESPT, we estimate k(PT )~ 5 × 10^10 s^–1. Using steady-state and time-resolved fluorescence spectroscopy, we determine its ground and excited-state p𝐾𝑎1 to be 7.3 and –1, respectively, making it one of the strongest photoacids of the natural coumarins. Aesculetin exhibits a strong pH dependence of the relative fluorescence quantum yield becoming much more fluorescent above p𝐾𝑎1. The aesculetin anion has slightly photobasic character. We also report that aesculetin forms a fluorescent catechol-like complex with boric acid, and this complex has a p𝐾𝑐 of 5.6.
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Journal of fluorescence
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National Science Foundation
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