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) [Lui’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.

中文翻译：

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N,N,-双(水杨烯)-(2-(3''4'-二氨基苯基)苯并噻唑)激发态分子内质子转移的理论研究

摘要 在最近的实验中，合成了一种具有双氢键结构的新型发色团（N, N,-bis (salicylidene)-(2-(3″4′-diaminophenyl) benzothiazole) (BTS))，其典型的激发态为在二氯甲烷 (DCM) 中观察到分子内质子转移 (ESIPT) 荧光现象 [Lui 的 Gustavo Teixeira Alves Duarte 等人。物理。化学 化学 物理，2019, 21, 1172–1182]。然而，由于分子内双分子氢键结构，ESIPT反应路径和反应机理尚未阐明。这使得研究人员进一步给BTS分子的研究和应用带来了很大的困难和局限性。在我们的研究中，分子的ESIPT反应机理主要采用密度泛函理论（DFT）和瞬态密度泛函理论（TD-DFT）方法，基于CAM-B3LYP/6-31G(d,p)/SDM计算层次详细研究。我们充分优化了 S0 和 S1 态的几何形状以获得键长、键角、红外光谱 (IR) 并对其进行了理论分析，发现分子内氢键在激发态下会增强。随后，前沿分子轨道（FMO）说明了光激发过程中电子密度分布的重新分布，基本解释了增强的氢键为 ESIPT 过程提供了驱动力。通过还原密度梯度 (RDG) 直观地比较两个分子内氢键强度。我们扫描静电势 (ESP)，计算 ESIPT 反应能垒和过渡态结构，以定量研究可能性、顺序、ESIPT 反应的难度。这种方法可以更好地了解 BTS 的激发态分子内质子转移是如何在 DCM 中发生的。结论是BTS的两个ESIPT过程不是同时发生的，而是逐步反应（BTS→BTS-A→BTS-D）。请注意，BTS-A→BTS-D 比 BTS→BTS-A 质子转移过程更可能发生在 S1 状态。