Jia Xueli, Meng Ju, Liu Yufang
Henan Key Laboratory of Infrared Materials & Spectrum Measures and Applications, School of Physics, Henan Normal University Xinxiang, Henan 453007, PR China.
J Phys Chem A. 2024 Sep 26;128(38):8096-8104. doi: 10.1021/acs.jpca.4c04647. Epub 2024 Sep 12.
A detailed theoretical investigation on the excited state intramolecular proton transfer (ESIPT) directionality and dynamics behavior of 3-(benzo[]thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with two unsymmetric proton acceptors (N and O) has been performed. The hydrogen bond O-H···N in BTHMB-a formed by the O-H group with the N atom or O-H···O in BTHMB-b formed by the O-H group with the O atom is enhanced upon photoexcitation, and the strength of the O-H···N bond is stronger, which will drive the O-H proton to the N atom. Potential energy curves further confirm that ESIPT occurs in the N atom because of the smaller energy barrier (0.39 kcal/mol). Results of dynamics simulations manifest that no surface hopping exists between the S and S states within 300 fs, and ESIPT time constants of BTHMB-a and BTHMB-b are 48 and 151 fs, respectively. While the reverse ESIPT is observed in BTHMB-b at 294 fs, implying that the O-H proton is transferred to the N atom instead of the O atom. The consistency of the calculated absorption (390 nm) and fluorescence spectra (443 and 602 nm) of BTHMB-a with the experimental values (390, 410, and 605 nm) confirms this conclusion again. The charge distribution analysis shows that the charge on the proton acceptors increases, and the O atom has higher electronegativity because it has more negative charges. The minimum surface electrostatic potential on the N atom in BTHMB-b correlating with the pKb value is -47.38 kcal/mol, indicating that the N atom has strong basicity. Therefore, the basicity of the N atom dominates the ESIPT process rather than the electronegativity of the O atom.
对具有两个不对称质子受体（N和O）的3-（苯并[]噻唑-2-基）-2-羟基-5-甲氧基苯甲醛（BTHMB）的激发态分子内质子转移（ESIPT）方向性和动力学行为进行了详细的理论研究。光激发后，BTHMB-a中由O-H基团与N原子形成的氢键O-H···N或BTHMB-b中由O-H基团与O原子形成的氢键O-H···O增强，且O-H···N键的强度更强，这将促使O-H质子转移至N原子。势能曲线进一步证实，由于能垒较小（0.39 kcal/mol），ESIPT发生在N原子上。动力学模拟结果表明，在300 fs内S态和S态之间不存在表面跳跃，BTHMB-a和BTHMB-b的ESIPT时间常数分别为48和151 fs。虽然在294 fs时在BTHMB-b中观察到反向ESIPT，这意味着O-H质子转移至N原子而非O原子。BTHMB-a的计算吸收光谱（390 nm）和荧光光谱（443和602 nm）与实验值（390、410和605 nm）的一致性再次证实了这一结论。电荷分布分析表明，质子受体上的电荷增加，且O原子具有更高的电负性，因为它带有更多负电荷。BTHMB-b中与pKb值相关的N原子上的最小表面静电势为-47.38 kcal/mol，表明N原子具有较强的碱性。因此，N原子的碱性而非O原子的电负性主导了ESIPT过程。
