Electronic Effects on the ESIPT and Antioxidant Activity of the Dihydrochalcone Phloretin

Context A dynamic balance between oxidation and antioxidation is essential for maintaining normal physiological functions in the human body. Disruption of this balance, resulting from insufficient antioxidant capacity, leads to oxidative stress and subsequent cellular damage. Phloretin (PHL), a naturally occurring dihydrochalcone, exhibits distinctive antioxidant activity that is closely linked to its excited-state intramolecular proton transfer (ESIPT) characteristics. Despite its potential, the systematic influence of substituent modifications on the ESIPT process and the resulting antioxidant activity remains largely unexplored. In this study, we designed six novel PHL derivatives by introducing electron-donating groups (-OH, -NH₂, -CH₃) and electron-withdrawing groups (-CN, -NO₂, -CHO) to elucidate the regulatory effects of these functional groups on the ESIPT mechanism and antioxidant performance. Using density functional theory (DFT) and time-dependent DFT (TD-DFT), we systematically investigated the geometric structures, infrared spectra, reduced density gradient, molecular orbitals, potential energy curves, and antioxidant activities of PHL and its derivatives. Our results reveal that while pristine phloretin lacks an E* state during its ESIPT process, the incorporation of strong electron-withdrawing groups (-NO₂, -CHO) restores the typical four-state ESIPT pathway. Notably, electron-donating and electron-withdrawing groups exert opposing regulatory effects: the former facilitate the ESIPT process and substantially enhance antioxidant activity, whereas the latter suppress ESIPT and concurrently diminish antioxidant capacity. This work establishes a crucial theoretical foundation for the rational design of high-performance antioxidants through targeted functional group engineering to precisely modulate ESIPT and antioxidant activity. Methods All computational studies in this paper were performed using the Gaussian 16 software package (Revision B.01), while GaussView 6.0.16 was employed for the analysis and visualization of compounds. To identify the most suitable computational method for the phloretin system, four functionals—B3LYP, B3PW91, Cam-B3LYP, and PBE were evaluated by comparing calculated absorption maxima with experimental data. Based on this benchmark, ground-state (S₀) geometries were optimized using the B3LYP functional in conjunction with the TZVP basis set, while excited-state (S₁) optimizations were carried out using the long-range corrected Cam-B3LYP functional with the same basis set. Frequency analyses confirmed the absence of imaginary frequencies for all optimized structures, verifying that they correspond to genuine local minima on the potential energy surface. Solvent effects were modeled using the integral equation formalism of the polarizable continuum model (IEFPCM) to simulate a methanol environment. Antioxidant activity was assessed by calculating the ionization potential (IP). Post-processing analyses, including reduced density gradient (RDG) analysis and frontier molecular orbital (FMO) visualizations, were conducted using the Multiwfn 3.8 (dev) and VMD 1.9.3 software.