Concentration-Dependent Fluorescence Emission of Quercetin

Quercetin (Q) is an important antioxidant with high bioactivity and the potential of being used as SARS-CoV-2 inhibitor. The fluorescence (FL) emission from Q solutions made with different polar and non-polar solvents (methanol, acetone, and chloroform) was measured and compared with the FL emission from Q powder and from Q crystals. In the FL spectra of the solutions with high Q concentration, as well as in the spectra of Q in solid state, two features, at 615 nm and 670 nm, were observed. As the solution concentration decreases, the intensity of those peaks decreases and a peak at 505 nm arises. The FL emission of low concentration solutions displayed only that peak. Calculations for the Q molecule in each solvent, performed using time-dependent density functional theory (TDDFT), show that the emission at 505 nm is associated with the excited state intramolecular proton transfer (ESIPT) of the -OH3 group proton. Our calculations also show that the feature at 615 nm, which is observed in solid state Q and also in the emission of the high concentrated solutions, is related to the -OH5 proton transfer.

Automated summary

The study investigated the fluorescence properties of quercetin (Q) solutions prepared in different solvents, revealing characteristic peaks for high concentration Q solutions and solid Q, with only one peak observed at low concentrations, all associated with proton transfer.