Hao Jiaojiao, Yang Yang
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
J Phys Chem A. 2021 Dec 9;125(48):10280-10290. doi: 10.1021/acs.jpca.1c05530. Epub 2021 Nov 30.
An asymmetric two-way proton transfer molecule 3-(benzo[]-thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with the function of white-light emission was synthesized in a recent experiment (Bhattacharyya, A.; Mandal, S. K.; Guchhait, N. 2019, 123, 10246). The particularity of this molecule is that there are two possible forms, one of which contained a six-membered H-bonded network toward a N atom (BTHMB-NH) present in the molecule as a proton acceptor and the other was toward an O atom (BTHMB-OH). Unfortunately, the experimental work lacked the theoretical explanation about the determination of the BTHMB-NH form and its excited-state intramolecular proton transfer (ESIPT) process under different solvents. Therefore, this study has explored these two points by means of the time-dependent density functional theory (TDDFT) method. The calculated relative energy and potential energy profile (PEP) of the transformation between BTHMB-NH and BTHMB-OH forms illustrated that BTHMB-NH was more stable, and the transfer from BTHMB-NH to BTHMB-OH was almost impossible at both S and S states under all solvents due to high potential energy barriers (PEBs) (11.67-21.59 kcal/mol). These calculated results provided the theoretical explanation and verification for the conclusion that the BTHMB molecule exists in the BTHMB-NH form in the experiment. Subsequently, the constructed PEPs of the ESIPT process for BTHMB-NH have proved that it was prone to the ESIPT process due to low PEBs (0.11-0.28 kcal/mol) at the S state. In particular, as the solvent polarity increased, the intensity of the intramolecular hydrogen bond (IHB) (O-H···N) increased and the ESIPT process was more likely to occur. In addition, the twisted intramolecular charge-transfer (TICT) process was studied to explore the possible fluorescence quenching pathway of BTHMB-NH. Based on the PEPs of BTHMB-NH-T as a function of the N-C-C-C dihedral angle at the S and S states, it is seen that the S state TICT process was inhibited due to the large PEBs (16.45-23.93 kcal/mol). Although the S state PEBs have been greatly reduced, they were still maintained at about 3.60 kcal/mol (3.60-3.84 kcal/mol), and hence, this process was still relatively difficult to occur. Due to the fact that BTHMB can be regarded as a standard in future designs involving red light and solvent-specific white-light emitters, a certain amount of investigative work on the ESIPT process was done in detail, and it paved the way for future research on the directionality of ESIPT in double ESIPT probes.
近期的一项实验合成了一种具有白光发射功能的不对称双向质子转移分子3-(苯并[]噻唑-2-基)-2-羟基-5-甲氧基苯甲醛（BTHMB）（Bhattacharyya, A.; Mandal, S. K.; Guchhait, N. 2019, 123, 10246）。该分子的特殊之处在于存在两种可能的形式，其中一种含有朝向分子中作为质子受体的N原子的六元氢键网络（BTHMB-NH），另一种则朝向O原子（BTHMB-OH）。遗憾的是，实验工作缺乏关于BTHMB-NH形式的确定及其在不同溶剂下的激发态分子内质子转移（ESIPT）过程的理论解释。因此，本研究通过含时密度泛函理论（TDDFT）方法对这两点进行了探索。计算得到的BTHMB-NH和BTHMB-OH形式之间转化的相对能量和势能剖面图（PEP）表明，BTHMB-NH更稳定，并且在所有溶剂下，由于高势能垒（PEB）（11.67 - 21.59千卡/摩尔），从BTHMB-NH到BTHMB-OH的转移在S和S态几乎都不可能发生。这些计算结果为实验中BTHMB分子以BTHMB-NH形式存在的结论提供了理论解释和验证。随后，构建的BTHMB-NH的ESIPT过程的PEP证明，由于在S态下低PEB（0.11 - 0.28千卡/摩尔），它易于发生ESIPT过程。特别地，随着溶剂极性增加，分子内氢键（IHB）（O-H···N）的强度增加，ESIPT过程更有可能发生。此外，还研究了扭曲分子内电荷转移（TICT）过程，以探索BTHMB-NH可能的荧光猝灭途径。基于BTHMB-NH-T在S和S态下作为N-C-C-C二面角函数的PEP，可以看出S态TICT过程由于大的PEB（16.45 - 23.93千卡/摩尔）而受到抑制。尽管S态的PEB已大幅降低，但仍保持在约3.60千卡/摩尔（3.60 - 3.84千卡/摩尔），因此，该过程仍然相对难以发生。由于BTHMB可被视为未来涉及红光和溶剂特异性白光发射体设计的标准，对ESIPT过程进行了一定量的详细研究工作，为未来双ESIPT探针中ESIPT方向性的研究铺平了道路。
