Relationship between antioxidant activity and ESIPT process based on flavonoid derivatives: A comprehensive analysis.

Antioxidant activity, as a topic of current interest, is discussed together with the excited state intramolecular proton transfer (ESIPT) process for three flavonoid derivatives, based on density functional theory (DFT)and time-dependent DFT (TD-DFT) methods, as well as DPPH free radical scavenging assay. The potential energy curves and transition states demonstrate that the three molecules can undergo only single proton transfer in the excited state, and all of them are ultrafast ESIPT processes. The absorption spectra of all the molecules show effective protection against UV radiation with low fluorescence intensity, especially Baicalein (Bai), which demonstrates their great potential for sunscreen applications. The density of states, HOMO energy values, global and local indices reveal that the antioxidant activity of the molecules after ESIPT process is enhanced, with Bai having the highest antioxidant activity, which is significantly attributed to the number and position of phenolic hydroxyl groups. Moreover, by comparing the DPPH free radical scavenging activity under the dark and UV radiation conditions, the radical scavenging activity (RSA) value in the UV radiation is remarkably higher than that in the dark condition, in which Bai achieves RSA value of 93.4%. Overall, the antioxidant activity of all three ESIPT-based flavonoid derivatives, especially Bai, is significantly elevated in the keto* form, which reinforces the significant relationship between antioxidant activity and ESIPT process, and provides new application prospects for molecules with ESIPT properties.