Direct and Indirect Photodegradation of Daidzein in Different Dissociated Forms in Water: Theoretical Analysis Based on DFT/TDDFT

Daidzein (DAID), as a common phytoestrogen, is often detected in the environment. In this paper, the photodegradation mechanism and UV absorption spectra of three different dissociation forms of DAID in water were studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The photodegradation mechanism includes direct photolysis and indirect photolysis by reacting with ·OH in water. The reaction types of indirect photolysis of DAID⁰ with ·OH are OH-addition and H-abstraction, and H-abstraction is more favorable than OH-addition. The indirect photolysis reaction type of DAID⁻ and DAID²⁻ with ·OH is OH-addition. The degree of difficulty in the reaction of the three dissociation forms of DAID in water with ·OH is: DAID⁰ < DAID⁻ < DAID²⁻. Results have shown that DAID can undergo degradation and can be transformed by reacting with ·OH in water. In addition, the direct photolysis reaction is also considered in the excited state to reveal the mechanism of the photochemical reaction. The direct photolysis pathway of DAID in water is the cleavage of C–C bond and C–O bond, but the E_a values required for these reactions are higher than the E_a values for the indirect photolysis reactions, indicating that direct photolysis is not the main pathway of photodegradation. The method of quantum chemical calculation in this study is helpful to understand the photochemical transformation of DAID in water.