Ingeniously regulating the antioxidant activities of hydroxyanthraquinone-based compounds via ESIPT reaction: Combining experiment and theory methods

The relationships between the excited state intramolecular proton transfer (ESIPT) behaviors and antioxidant activities of three hydroxyanthraquinone (HAQ) compounds containing 2,6-DHAQ, 1,5-DHAQ and 1,5-DAAQ were systematically explored via the experimental and theoretical methods. The measured femtosecond transient absorption indicates that 1,5-DAAQ exhibits the slower ESIPT reaction than 1,5-DHAQ due to its larger S 1 -state energy barrier in the scanned potential energy surfaces , which means that the transformation of 1,5-DAAQ from enol form to keto form is more difficult. Based on this, the antioxidant activities of the compounds in enol- and keto-form were studied. Concretely, the antioxidant activities of 1,5-DHAQ and 1,5-DAAQ in the keto form are stronger than that in the enol form, which is estimated from the calculated ionization potential (IP) values. In addition, compared with that at the S 0 state, the involved three compounds in the enol and keto forms have the lower IPs at the S 1 state, revealing that their antioxidant activities will be enhanced upon the photo-excitation. Derived from the difficulty of ESIPT reactions and antioxidant activities of the three compounds in the experiments and theory, it can be concluded that antioxidant activity of the compound can be regulated through the transformation of enol- and keto-form in the ESIPT reaction. This new discovery can help to design and synthesize more efficient ESIPT-based antioxidants in experiments.