Galangin: Effect of solution concentration on absorption and emission spectra

Galangin is a bioactive flavonol found in a variety of plants. Upon photoexcitation, its molecular structure enables excited-state intramolecular proton transfer (ESIPT). This study combines experimental and computational approaches to analyze how solution concentration and solvent type influence galangin's absorption and fluorescence properties. UV-Vis and fluorescence spectra were measured for galangin in methanol and dimethyl sulfoxide across a wide concentration range and compared with its solid-state emission. Time-dependent density functional theory calculations were performed for three tautomeric forms (Enol, Keto O3, Keto O5) for two conformational isomers of the galangin molecule to interpret the observed spectral features. The results demonstrate that ESIPT processes are significantly influenced by the environment: the fluorescence emission from the Keto O3 form dominates in low-concentration solutions, while in galangin solutions with higher concentrations and in its solid state, the Keto O5 form predominates in the fluorescence spectra.