The theoretical study of excited-state intramolecular proton transfer of N, N,-bis (salicylidene)-(2-(3″4′-diaminophenyl) benzothiazole)

Abstract

In recent experiments, a novel chromophore (N, N,-bis (salicylidene)-(2-(3″4′-diaminophenyl) benzothiazole) (BTS)) with double hydrogen bond structure was synthesized, and the typical excited-state intramolecular proton transfer (ESIPT) fluorescence phenomenon was observed in dichloromethane (DCM) [Luı’s Gustavo Teixeira Alves Duarte et al. Phys. Chem. Chem. Phys., 2019, 21, 1172–1182]. However, due to the molecule dual-intramolecular hydrogen bonding structure, the ESIPT reaction path and reaction mechanism have not been elucidated. This makes the researchers further bring great difficulties and limitations to the research and application of BTS molecules. In our research, the ESIPT reaction mechanism of molecules mainly uses density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods, which are studied in detail based on CAM-B3LYP/6-31G (d, p)/SDM calculation level. We fully optimize the geometry of S0 and S1 states to obtain bond length, bond angle, infrared spectrum (IR) and analyze them theoretically, it is find that the intramolecular hydrogen bonds would be enhanced under the excited state. Subsequently, the frontier molecular orbitals (FMOs) illustrated the redistribution of electron density distribution during photo-excitation, basically explaining that the enhanced hydrogen bonding provides a driving force for the ESIPT process. Two intramolecular hydrogen bond intensities are visually compared by the reduction density gradient (RDG). We scan the electrostatic potential (ESP), calculate ESIPT reaction energy barriers and the transition-state structures to quantitatively investigate the possibility, sequence, difficulty of ESIPT reactions. This method provides a better understanding of how the excited state intramolecular proton transfer of BTS occurs in the DCM. The conclusion is that the two ESIPT processes of BTS are not simultaneous, but stepwise reaction (BTS→BTS-A→BTS-D). Note that BTS-A→BTS-D are more likely to occur in S1 states than BTS→BTS-A proton transfer processes.