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.

本研究通过将仲硫酰胺（secondary thioamides）衍生得到的硫酰胺二阴离子（thioamide dianions）与硫代甲酰胺（thioformamides）反应，再经碘依次氧化，制备了一系列5-N-芳氨基噻唑（5-N-arylaminothiazoles）类化合物。X射线衍射分析（X-ray analyses）结果显示，该类化合物的结构相较于平面构象呈现高度扭转。其分子取向可通过2、4、5位取代基的位阻效应及/或电子相互作用（steric and/or electronic interactions）进行调控。
该类5-N-芳氨基噻唑可发射出从蓝色至橙色的宽谱荧光。尽管其吸收光谱（absorption spectra）不受溶剂极性（solvent polarity）影响，但荧光发射却受溶剂极性调控：在极性更强的溶剂中，发射波长发生红移。我们通过Lippert–Mataga图以及基态与激发态间的偶极矩变化，对上述现象开展了考察。该类化合物在固态下同样可发射出从蓝色至橙色的荧光。
对该类5-N-芳氨基噻唑的循环伏安法测试结果表明，其存在单电子氧化（one-electron oxidation）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（electron-donating groups）时，该氧化过程的半波电位会发生降低。通过密度泛函理论（DFT）计算，我们获得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）的能级。最后，热重-差热分析（TG-DTA）结果证实，该类化合物具备良好的热稳定性。