Influence of C3′- and C4′-substitutions on fluorescence, crystal packing, and physicochemical properties of flavonol

Natural flavonols exhibit a wide range of pharmacological activities and possess unique dual ESIPT (Excited-State Intramolecular Proton Transfer) fluorescence, making them sensitive to microenvironments. This sensitivity allows for the detection of metal ions, anions, small ligands, and biomacromolecules. However, the diversity in their structure, including the number and position of hydroxyl groups and potential chemical modifications, complicates the relationship between structure and fluorescence, posing challenges for their practical use as fluorescent probes. In this study, we focus on fine-tuning the ESIPT fluorescence, crystal packing, physicochemical properties, and ADMET (Absorption, Distribution, Metabolism, Excretion, and Toxicity) characteristics of a series of flavonols. We achieve this by introducing hydroxy, methoxy and benzyl groups at the C3' and C4' positions of the 2-phenyl side ring. The photophysical properties of the synthesized flavonols were systematically examined by UV-vis and fluorescence measurements in terms of their structure-property relationship. Our findings indicate that the nature and position of the substituent groups in flavonols can significantly influence their crystal packing in the solid state, tuning contributions of intra- and intermolecular hydrogen bonding and the ESIPT behavior. Lastly, through fluorescence titration and molecular docking calculations, we explored how the introduction of a bulky benzyl moiety and its alteration between C3' and C4' positions can influence the binding interactions of flavonols with β-glucosidases. We believe our findings shed light on the structure-fluorescence relationship in flavonols and open up new opportunities for the design of innovative flavonol-based probes.