Turning on the Fluorescence of Flavonoid Anions: Synergistic Roles of Spatial Constraints and Microbasicity in Poly(vinyl alcohol)/DMSO Films

Deprotonated flavonoids typically undergo efficient nonradiative relaxation in solution, rendering them nonemissive. In this study, we successfully activate the fluorescence of such anions by embedding five representative flavonoids─galangin, apigenin, luteolin, kaempferol, and quercetin─into DMSO-doped poly(vinyl alcohol) (PVA/DMSO) films. The unique microenvironment of the composite film provides dual functionalities: intrinsic microbasicity facilitates flavonoid deprotonation, while spatial constraints effectively suppress the nonradiative decay pathways. Consequently, strong visible-light emission is observed for all flavonoids except galangin upon excitation at 440 nm. Time-dependent density functional theory calculations were employed to model the emission energies of various anionic species. By correlating the calculated trends with experimental spectra, we identify the emissive species as anions deprotonated specifically at the C_4'-OH position, rationalizing the absence of emission from galangin, which lacks this moiety. This work not only demonstrates a general strategy to unlock the fluorescence of nonemissive anionic chromophores through synergistic matrix control but also establishes PVA/DMSO films as a versatile platform for modulating the photophysics of charged organic molecules.