Wang Jian-Kai, Wang Chih-Hsing, Wu Chi-Chi, Chang Kai-Hsin, Wang Chun-Hsiang, Liu Yi-Hung, Chen Chao-Tsen, Chou Pi-Tai
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, Republic of China.
Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 10617, Taiwan, Republic of China.
J Am Chem Soc. 2024 Feb 7;146(5):3125-3135. doi: 10.1021/jacs.3c10405. Epub 2024 Jan 30.
The chapter on the thiol-related hydrogen bond (H-bond) and its excited-state intramolecular proton-transfer (ESIPT) reaction was recently opened where compound 4'-diethylamino-3-mercaptoflavone () undergoes ESIPT in both cyclohexane solution and solid, giving a 710 nm tautomer emission with an anomalously large Stokes shift of 12,230 cm. Considering the thiol H-bond to be unconventional compared to the conventional Pauling-type -OH or -NH H-bond, it is thus essential and timely to probe its fundamental difference between their ESIPT. However, thiol-associated ESIPT tends to be nonemissive due to the dominant π* character of the tautomeric lowest excited state. Herein, based on the 3-mercaptoflavone scaffold and π-elongation concept, a new series of 4'-substituted-7-diethylamino-3-mercaptoflavones, , was designed and synthesized with varied H-bond strength and 690-720 nm tautomeric emission upon ultraviolet (UV) excitation in cyclohexane. The order of their H-bonding strength was experimentally determined to be < < < , while the rate of -SH ESIPT measured by fluorescence upconversion was (398 fs) < (232 fs) < (123 fs) < (101 fs) in toluene. Unexpectedly, the strongest H-bonded gives the slowest ESIPT, which does not conform to the traditional ESIPT model. The results are rationalized by the trend of carbonyl oxygen basicity rather than -SH acidity. Namely, the thiol acidity relevant to the H-bond strength plays a minor role in the driving force of ESIPT. Instead, the proton-accepting strength governs ESIPT. That is to say, the noncanonical thiol H-bonding system undergoes an unconventional type of ESIPT.
最近开启了关于硫醇相关氢键（H键）及其激发态分子内质子转移（ESIPT）反应的章节，其中化合物4'-二乙氨基-3-巯基黄酮（）在环己烷溶液和固体中均发生ESIPT，产生710nm的互变异构体发射，斯托克斯位移异常大，为12230cm。考虑到硫醇H键与传统的鲍林型-OH或-NH H键相比是非传统的，因此探究它们在ESIPT之间的根本差异是必要且及时的。然而，由于互变异构最低激发态的主导π*特性，硫醇相关的ESIPT往往不发光。在此，基于3-巯基黄酮支架和π-延伸概念，设计并合成了一系列新的4'-取代-7-二乙氨基-3-巯基黄酮（），其具有不同的H键强度，在环己烷中紫外（UV）激发时产生690 - 720nm的互变异构体发射。通过实验确定它们的H键强度顺序为< < < ，而在甲苯中通过荧光上转换测量的-SH ESIPT速率为（398fs）< （232fs）< （123fs）< （101fs）。出乎意料的是，H键最强的 给出最慢的ESIPT，这不符合传统的ESIPT模型。结果通过羰基氧碱性而非-SH酸性的趋势得到合理解释。也就是说，与H键强度相关的硫醇酸度在ESIPT的驱动力中起次要作用。相反，质子接受强度控制ESIPT。也就是说，非经典硫醇H键系统经历了一种非传统类型的ESIPT。
