Phenanthrothiophene‐Triazine Star‐Shaped Discotic Liquid Crystals: Synthesis, Self‐Assembly, and Stimuli‐Responsive Fluorescence Properties

Triazine-based mesogens with three pending discoid arms have been shown to self-organize into rectangular columnar mesophases and form gels in various solvents. The solvatochromic-, acidochromic- and metal recognition-response photoluminescence of these mesogens offer applications in stimuli-responsive fluorescence anti-counterfeiting and as selective metal-ion recognition.

Abstract

Lipophilic biphenylthiophene- and phenanthrothiophene-triazine compounds, BPTT n and CPTT n, respectively, were prepared by a tandem procedure involving successive Suzuki-Miyaura coupling and Scholl cyclodehydrogenation reactions. These compounds display photoluminescence in solution and in thin film state, solvatochromism with increasing solvent's polarity, as well as acidochromism and metal ion recognition stimuli-responsive fluorescence. Protonation of BPTT10 and CPTT10 by trifluoroacetic acid results in fluorescence quenching, which is reversibly restored once treated with triethylamine (ON-OFF switch). DFT computational studies show that intramolecular charge transfer (ICT) phenomena occurs for both molecules, and reveal that protonation enhances the electron-withdrawing ability of the triazine core and reduces the band gap. This acidochromic behavior was applied to a prototype fluorescent anti-counterfeiting device. They also specifically recognize Fe³⁺ through coordination, and the recognition mechanism is closely related to the photoinduced electron transfer between Fe³⁺ and BPTT10/CPTT10. CPTT n self-assemble into columnar rectangular (Col_rec) mesophase, which can be modulated by oleic acid via the formation of a hydrogen-bonded supramolecular liquid crystal hexagonal Col_hex mesophase. Finally, CPTT n also form organic gels in alkanes at low critical gel concentration (3.0 mg/mL). Therefore, these star-shaped triazine molecules possess many interesting features and thus hold great promises for information processing, liquid crystal semiconductors and organogelators.

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