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.

本研究制备了一系列5-N-芳基氨基噻唑（5-N-arylaminothiazoles）：将仲硫代酰胺衍生得到的硫代酰胺二阴离子与硫代甲酰胺进行反应，随后依次通过碘单质完成氧化。X射线衍射（X-ray）分析显示，该类化合物的结构相较于平面构象呈现高度扭曲状态。其分子取向可通过2、4、5位取代基的空间位阻及/或电子相互作用进行调控。所制备的5-氨基噻唑（5-aminothiazoles）可发射覆盖蓝色至橙色区间的荧光。尽管其吸收光谱不受溶剂极性影响，但荧光发射行为却与溶剂极性密切相关：在极性更强的溶剂中，荧光发射峰发生红移。本研究通过Lippert–Mataga图以及基态与激发态间的偶极矩变化，对上述现象开展了分析探讨。该类化合物在固态下同样可发射蓝色至橙色区间的荧光。对5-氨基噻唑的循环伏安法（cyclic voltammetry）测试表明，其呈现出单电子氧化的可逆氧化还原峰。当在5位氮原子所连苯基上引入给电子基团时，其氧化半峰电位会出现降低。本研究通过密度泛函理论（DFT）计算，确定了最高占据分子轨道（HOMO）与最低未占据分子轨道（LUMO）的能级。最后，热重-差热分析（TG-DTA）结果显示，该类化合物具备良好的热稳定性。