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.