Tautomerism and Behavior of 3-Hydroxy-2-phenyl-4H-chromen-4-ones (Flavonols) and 3,7-Dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-diones (Diflavonols) in Basic Media: Spectroscopic and Computational Investigations

Abstract

Absorption and emission spectroscopic investigations and computational predictions have shown that neutral molecules of flavonols and diflavonols can exist in the ground and excited states in one or two tautomeric forms stabilized by intramolecular (in aprotic media) or intermolecular (with solvent molecule(s), in protic media) hydrogen bonds. Electronic excitation creates conditions for the transformation of tautomeric forms, accompanied by proton transfer, reflected in fluorescence spectra. Proton transfer is also probable in monoanions of diflavonols in protic media. The OH groups involved in hydrogen bonds exhibit a proton-donating ability characterized by the respective acidity constants. The electronically excited diflavonols are relatively strong acids if they lose one proton. With increasing basicity of the medium, anionic forms occur, which exhibit spectral characteristics and emission abilities different from those of neutral molecules. These features open up possibilities for the analytical use of these compounds as spectral probes sensitive to the properties of liquid phases--from neutral to strongly basic. The less intensively studied diflavonols seem to be more promising than flavonols for these purposes, since they are more lipophilic, polarizable, polar, and sensitive to basic features of the environment.