5‑N-Arylaminothiazoles as Highly Twisted Fluorescent Monocyclic Heterocycles: Synthesis and Characterization

A series
of 5-N-arylaminothiazoles was prepared by reacting thioamide dianions
derived from secondary thioamides with thioformamides, followed by
sequential oxidation with iodine. X-ray analyses demonstrated that
they adopt structures that are highly twisted from planar conformations.
Their orientations were tuned by the steric and/or electronic interactions
of the substituents at their 2-, 4-, and 5-positions. The 5-aminothiazoles
exhibited a range of fluorescent emissions, from blue to orange. Although
the absorption spectra were independent of the polarity of the solvent,
fluorescent emissions were influenced by the polarity of the solvent:
in more polar solvents, the emissions were red-shifted. These phenomena
were examined in terms of Lippert–Mataga plots and the change
in the dipole moment between the ground and excited states. They also
exhibited emissions in the solid state, again from blue to orange.
Cyclic voltammetry of the 5-aminothiazoles showed reversible waves
of one-electron oxidation. The half-potential of the oxidation was
reduced by the introduction of electron-donating groups to the phenyl
groups on the nitrogen atom at the 5-position. DFT calculations were
carried out to determine the energy levels of the HOMO and LUMO. Finally,
the results of TG-DTA showed that they are thermally stable.

本研究通过二级硫代酰胺衍生的硫代酰胺二阴离子与硫代甲酰胺发生反应，随后经碘依次氧化，制备得到一系列5-N-芳基氨基噻唑类化合物。X射线衍射（X-ray）分析结果表明，此类化合物的结构与平面构象存在显著扭曲；其分子取向可通过2、4、5位取代基的空间位阻与/或电子相互作用进行调控。该类5-氨基噻唑类化合物展现出从蓝色到橙色的广谱荧光发射行为。尽管其紫外-可见吸收光谱不受溶剂极性影响，但荧光发射特性却与溶剂极性密切相关：在极性更强的溶剂中，发射波长发生红移。本研究借助李珀特-马塔加（Lippert–Mataga）绘图法，并结合基态与激发态间的偶极矩变化，对上述光物理现象进行了系统分析。此类化合物在固态下同样展现出从蓝色至橙色的荧光发射。对5-氨基噻唑类化合物的循环伏安法（Cyclic Voltammetry, CV）测试显示，其存在可逆的单电子氧化还原峰；当在5位氮原子所连苯基上引入给电子基团时，该氧化半波电位会发生负移。通过密度泛函理论（Density Functional Theory, DFT）计算，我们获取了该类化合物的最高占据分子轨道（Highest Occupied Molecular Orbital, HOMO）与最低未占据分子轨道（Lowest Unoccupied Molecular Orbital, LUMO）能级。最后，热重-差热联用分析（TG-DTA）结果表明，此类化合物具有优异的热稳定性。