Abstract
Abstract
Excited-state intramolecular proton transfer (ESIPT) is a key photophysical process closely related to the optical properties and antioxidant activity of functional molecules, yet its substituent-dependent regulation in nontraditional ESIPT systems remains unclear. In this work, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed to investigate the ESIPT mechanisms, hydrogen-bond dynamics, electronic structure evolution, and antioxidant properties of 2-(2-hydroxyphenyl)pyrimidine (HPP) and its –NH
2
and –NO
2
-substituted derivatives. Geometric optimizations, potential energy surface scans, and noncovalent interaction analyses reveal that the electron-donating –NH
2
group strengthens the intramolecular O–H⋯N hydrogen bond and induces a barrierless ESIPT process with direct formation of the keto excited state (K*). In contrast, the electron-withdrawing –NO
2
group stabilizes an excited enol-like intermediate (E*), leading to a stepwise proton transfer pathway with a small activation barrier. Frontier molecular orbital and natural bond orbital analyses demonstrate that photoexcitation triggers substituent-dependent charge redistribution, which governs proton transfer feasibility. Moreover, ionization potential and global reactivity index calculations indicate enhanced antioxidant activity in the excited state, particularly for HPP and HPP–NH
2
. These results establish a clear relationship between substituent effects, ESIPT pathways, and antioxidant performance, providing theoretical insight for the rational design of ESIPT-based photoactive antioxidants.