Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan (R.O.C.).
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan (R.O.C.).
J Am Chem Soc. 2021 Aug 18;143(32):12715-12724. doi: 10.1021/jacs.1c05602. Epub 2021 Aug 6.
We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4-chromen-4-one (), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S absorption at 383 nm. Upon photoexcitation, exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm. Upon methylation on the thiol group, , lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for , followed by a tautomer emission lifetime of 120 ps. In sharp contrast to , both and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4-chromen-4-one () are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible π* contribution to the S' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For , the extension of π-electron delocalization by the diethylamino electron-donating group endows the S' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.
我们首次在室温溶液中观察到硫醇质子的激发态分子内质子转移（ESIPT）反应。这一现象由 3-硫代黄酮的衍生物（）证实，即 2-（4-（二乙氨基）苯基）-3-巯基-4-色满-4-酮（），它具有-S-H···O═ 分子内氢键（虚线表示），并在 383nm 处有 S 吸收。光激发后，在环己烷中以 710nm 处显示出明显的红色发射，斯托克斯位移异常大，为 12230cm。巯基甲基化后，失去硫醇质子的，在 472nm 处显示出正常的斯托克斯位移发射。这些结果，结合计算方法，明确得出了硫醇型 ESIPT 的结论。进一步的时间分辨研究表明，对于，ESIPT 速率是不可分辨的（<180fs），随后是互变异构体发射寿命为 120ps。与相比，和 3-巯基-2-（4-（三氟甲基）苯基）-4-色满-4-酮（）均不发光。详细的计算方法表明，所有研究的硫醇都经历了热有利的 ESIPT。然而，一旦形成质子转移互变异构体，硫原子上的孤对电子会对 S'态（prime 表示质子转移互变异构体）产生不可忽视的 π贡献，其中弛豫主要由非辐射失活控制。对于，二乙氨基供电子基团扩展了 π 电子离域，使 S'态主要处于 ππ构型，表现出明显的互变异构体发射。该结果在 ESIPT 领域开辟了一个新的篇章，涵盖了非典型的硫分子内氢键及其相关的 ESIPT 在环境温度下。
