Substituent and Heteroatom Electronegativity Effects on ESIPT, Fluorescence Properties, and Antioxidant Activity of...

Excited-state intramolecular proton transfer (ESIPT) plays an important role in regulating the fluorescence behavior and photophysical properties of flavonoids. In this work, 5,3'-dihydroxy-7,4'-dimethoxyflavone and its derivatives were systematically investigated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations to clarify the effects of substituents (-CN/-NH₂) and heteroatom electronegativity variation (O→S substitution) on ESIPT behavior, fluorescence properties, and antioxidant activity. The calculated excited-state potential energy curves reveal that the ESIPT process is barrierless and spontaneous in all investigated systems, resulting in fluorescence emission exclusively from the keto excited-state (K*) configuration. Simulated fluorescence spectra exhibit large Stokes shifts characteristic of ESIPT systems. The introduction of the electron withdrawing -CN group and O→S substitution strengthens the intramolecular hydrogen bond, lowers the proton transfer barrier, and further promotes the ESIPT process. Moreover, the -CN substituted derivatives display enhanced oscillator strengths and relatively large S₁-S₀ energy gaps, indicating improved fluorescence efficiency. In contrast, the electron donating -NH₂ group induces pronounced red shifted emission but severely quenches fluorescence due to extremely low oscillator strengths and narrowed energy gaps. Antioxidant analysis based on the HAT, SPLET, and SET-PT mechanisms demonstrates that antioxidant activity strongly depends on substituent effects, hydroxyl site selectivity, and solvent polarity. The -NH₂ group significantly enhances radical scavenging ability by facilitating the HAT and SET-PT pathways, whereas the -CN group mainly promotes the SPLET mechanism in methanol solution through stabilization of deprotonated anionic species. Furthermore, the 3'-OH site was identified as the dominant radical scavenging active site due to its lower thermodynamic energy requirements. These findings provide theoretical insight into the regulation of ESIPT behavior, fluorescence emission, and antioxidant activity in flavonoid systems, and offer guidance for the rational design of multifunctional fluorescent and bioactive materials.