The enhanced excited-state intramolecular proton transfer energy barrier of flavonols induced by deprotonation

The barrierless excited-state intramolecular proton transfer (ESIPT) is believed to account for the non-radiative decays of flavonols composed of 5-hydroxyl group. However, the ESIPT mechanisms of flavonol anions have never been elucidated. In this work, by using the time-dependent density functional theory (TDDFT) calculations, we have determined the barrierless ESIPT in kaempferol and galangin, in agreement with their non-emissive properties. In contrast, deprotonation at the position 7 of them is demonstrated to decrease the basicity of proton acceptor and acidity of proton donor in the excited state, largely increasing the ESIPT barrier and leading to the fluorescence emission from the normal state. A further deprotonation of mono-deprotonated kaempferol is inferred to induce blue shifted emission. These results elucidate the nature of emissive flavonol anions and give a deep insight into the optical properties of flavonols in different matrices.