Theoretical insights into ESIPT mechanism of the two protons system BH-BA in dichloromethane solution

Abstract In a previous experiment, a novel triazole derivative excited intramolecular proton transfer fluorophore BH-BA has been successfully synthesized and showed obvious dual fluorescence phenomenon in dichloromethane (DCM) [Zhang, Y. J. et al. Angew. Chem. Int. Ed. 2019, 10, 1002]. But a detailed and reasonable explanation is lacking in theory. In order to explain the mechanism, our work mainly focuses on the dual fluorescence phenomenon of BH-BA in DCM. Through detailed molecular structure theory calculations, we find that the two-proton system has two possible reaction approaches for proton transfer isomerization. Which proton transfer reaction approach is associated with the dual fluorescence observed experimentally? A series of theoretical studies have confirmed the effect of the excited intramolecular proton transfer (ESIPT) process of BH-BA in DCM on its photophysical properties. We fully optimized the geometric configurations, at the same time, the infrared spectra (IR) were also calculated and analyzed theoretically, indicating that two excited state intramolecular hydrogen bonds were enhanced. After that, the analysis of frontier molecular orbitals (FMOs) and Mulliken charge illustrated the redistribution of charge during photoexcitation, essentially explaining that the enhanced hydrogen bonding provided the driving force for the ESIPT processes. The reduced density gradient (RDG) isosurfaces were used to visually contrast between two intramolecular hydrogen bonding strengths. What's more, after analyzing the molecular potential energy curves and the transition-state structures, we got the most likely reaction approach of ESIPT. The absorption and emission spectra were simulated by using density functional theory (DFT) and time-dependent density functional theory (TDDFT), the calculated spectra values were in line with the absorption and emission energies in experiment. Therefore, we concluded that the dual fluorescence observed in the experiment attributed to the ESIPT reaction along the intramolecular hydrogen bond O1-H2···N3 instead of O4-H5···N6.