Synergetic Effect of Fluorine Substitution and Substitution Position on Two Competing ESIPT Pathways in BR through DFT/TD-DFT Investigation

In this work, we systematically investigated the synergistic effect of fluorine (F) substitution and substitution position on the excited-state intramolecular proton transfer (ESIPT) mechanisms of 2,2'-(1-(4-bromophenyl)-1H-1,2,4-triazole-3,5-diyl)diphenol (BR) with dual intramolecular hydrogen bonds (IHBs) using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The geometric optimization of the enol (E) form and its single (SPT1, SPT2) and double proton-transfer (DPT) keto forms in the ground (S₀) and excited (S₁) states revealed that there is no stable DPT configuration in the S₀ and S₁ states. Structural changes, infrared vibration analysis, topological parameters, and core-valence bifurcation (CVB) indices collectively confirm that both IHBs are enhanced upon photoexcitation. Furthermore, the IHB of O₄-H₅···N₆ (IHB2) in the S₁ state is consistently stronger than the corresponding IHB of O₁-H₂···N₃ (IHB1), which facilitates proton transfer along IHB2. The experimental absorption and fluorescence spectra of BR were reproduced at the TD-CAM-B3LYP/6-311+G(d,p) level. The potential energy curves revealed that the energy barrier of the ESIPT process occurring along with IHB2 is much lower than that along with IHB1, implying that the single ESIPT path from E to SPT2 (PT2 pathway) is preferred from the kinetic and thermodynamic viewpoints. The substituted position of the F atom has an effect on the ESIPT barrier of the PT2 pathway. It can be found that the F-substitution at the C₁/C₄/C1'/C3' position reduced the ESIPT barrier, while the F-substitution at other positions increased the ESIPT barrier.