Relationship between ESIPT properties and antioxidant activities of 5-hydroxyflavone derivates*

It is of great significance to study the relationship between the excited state intramolecular proton transfer (ESIPT) properties and antioxidant activities of compounds in the field of life sciences. In this work, two novel compounds 5HF–OMe and 5HF–NH2 are designed through introducing a methoxy- and amino-group into the structure of 5-hydroxyflavone (5HF) respectively. The relationship between the ESIPT reaction and antioxidant activities of the three compounds is studied via the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated potential energy curves suggest that the rate of ESIPT reaction will gradually slow down from 5HF to 5HF–OMe and 5HF–NH2. In addition, the antioxidant activities of the three compounds gradually enhance from 5HF to 5HF–OMe and 5HF–NH2, which can be seen from the calculated energy gaps and ionization potential values. Interestingly, the above results imply that the rate of ESIPT reaction has a negative relationship with the antioxidant activities of the compounds, i.e., the slower rate of ESIPT reaction will reflect the higher antioxidant activity of the compound, which will provide valuable reference for detecting the antioxidant activity of compound via the photophysical method.