Effect of halogen substitutions on the ESDPT process and fluorescence properties of 1,2,4-triazole derivatives: A...

This work uses DFT and TD-DFT to elucidate how the dual-proton fluorescent probe Br-3N-2Et detects esterase activity and explains the origin of its experimentally observed fluorescence emission. Furthermore, the influence of halogen substitution on the excited-state intramolecular proton transfer (ESIPT) process in 3N-OH triazole derivatives is investigated in depth. By analyzing the geometric structures, infrared (IR) vibrational spectra, interaction region indicator (IRI) distributions, and topological parameters of the target molecules and their derivatives in both the ground (S₀) and excited (S₁) states, it is demonstrated that the intramolecular double hydrogen bonding interactions are significantly strengthened upon photoexcitation. Combined analysis of frontier molecular orbitals (FMOs) and electron-hole distribution characteristics reveals that excited-state charge redistribution plays a crucial role in facilitating ESIPT and excited-state double proton transfer (ESDPT). Moreover, potential energy surface (PES) scans confirm the feasibility of a stepwise ESDPT mechanism in the S₁ state. Computational results indicate that concerted dual-proton transfer is energetically inaccessible in the 3N-OH system. Instead, only sequential single-proton transfers occur-while the first proton transfer (ESIPT-I) is favorable, the second (ESIPT-II) is kinetically hindered following the initial transfer. Notably, the experimentally reported emission maximum at 498 nm does not originate from the previously proposed ESDPT tautomer but can be unambiguously assigned to the Br-3N-OH-PT1 tautomeric form. This work further demonstrates that halogen substitution effectively modulates the ESDPT behavior of 3N-OH derivatives: electron-withdrawing groups such asCF₃ andCl promote ESIPT, whereasF substitution inhibits it. These findings not only enhance the mechanistic understanding of novel dual-proton fluorescent probes but also provide valuable theoretical guidance for the rational design and development of halogen-substituted fluorescent probes with improved sensing performance.