Spectroscopic Properties of Flavonoids in Various Aqueous-Organic Solvent Mixtures

The characteristic fluorescence properties of quercetin (QCT) and apigenin (API) were studied in various CH3OH-H2O and CH3CN-H2O mixed solvents. The structure of QCT is completely planar. API is not planar at the ground state but becomes nearly planar at the excited state. If the molecules are excited to the S-1 state in organic solvents, QCT exhibits no fluorescence due to excited state intramolecular proton transfer (ESIPT) between the -OH and the carbonyl oxygen, but API shows significant fluorescence because ESIPT occurs slowly. If the molecules are excited to the S-2 state, both QCT and API exhibit strong S-2 -> S-o emission without any dual fluorescence. As the H2O composition of both solvents increases, the fluorescence intensity decreases rapidly due to the intermolecular hydrogen bonding interaction. The theoretical calculation further supports these results. The change in fluorescence properties as a function of the solvatochromic parameters was also studied.