Photoacidity as a tool to rationalize excited state intramolecular proton transfer reactivity in flavonols

This work presents the determination of acidic strengths at the electronic ground and excited states (pK_a and ) of three flavonol derivatives using electronic absorption and fluorescence emission spectroscopy. The differences of the pK_a and values were successfully correlated with the molecular structures according to the substitution pattern at the flavonol structure (hydrogen, diethylamino or fluoro moieties). In order to obtain more information about the observed photoacidity of these superacids, geometry optimizations and excitation energy calculations were performed at the CAM-B3LYP/6-311++G(d,p) level for their neutral, protonated and deprotonated species.