Experimental and Theoretical Study of Fluorescent Properties of Morin.

Morin (M) is one of the most widely distributed flavonoids with several beneficial effects on human health, and has the potential of being used as a possible treatment for COVID-19. To achieve a better understanding of the process of M dissolution, the fluorescent (FL) emission from M solutions prepared with different polar and nonpolar solvents (methanol, DMSO, and chloroform) was measured and compared with the FL emission from M powder and M crystals. In the FL spectra of the solutions with high M concentration, as well as in the spectra of M in solid state, two features, at 615 nm and 670 nm, were observed. As the solution concentration decreases, the maxima of FL spectra of the M solutions in all considered solvents shift to the blue side of the spectrum until reaching the value of 520 nm. To explain the experimental results, the TDDFT-M06-2X/6-31++G(d,p) method was used to determine the possible electronic transitions in the M molecule. The computations show that the FL emission in the spectral range of detection of our setup (405-800 nm) is related to the excited state intramolecular proton transfer (ESIPT). Comparison of the experimental data with the computations strongly suggests that in low-concentrated solutions, the FL emission is mostly due to electronic transitions in the keto OH3 form, whereas in aggregated states, the dominate contribution to the FL emission spectra is due to the transitions in keto OH5 form. Moreover, the time evolution of the M solutions FL spectra was observed, measured and explained for the first time.

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.