Functional star-shaped tris(triazolyl)triazines: columnar liquid crystal, fluorescent, solvatofluorochromic and electrochemical properties

Abstract

Columnar liquid crystals with a C₃ star-shaped heteroaromatic nitrogen-rich core, i.e. 2,4,6-tris(1′,2′,3′-triazol-4′-yl)-1,3,5-triazine (TTT), that show luminescence and solvatofluorochromism, and are also electron-acceptors are described. The core has been extended with polyalkoxybenzoyloxyphenyl groups situated at the 1 position of the 1,2,3-triazole rings to obtain molecules with three, six or nine n-decyloxy chains (series TB) or three, six or nine (S)-3,7-dimethyloctyloxy chains (series TB*). The preparation of the target compounds involved a copper-catalysed alkyne–azide cycloaddition (CuAAC) “click chemistry” procedure with a 1,3,5-triazine precursor and aromatic azides and this proved to be a versatile way to functionalise the periphery of the heteroaromatic core. Comparison of these compounds and those bearing polyalkoxyphenyl substituents (series T) has led to a deeper understanding of the self-assembly of the compounds with the TTT core. It was found that they self-organise into columnar mesophases, in which two molecules with a low-symmetry conformation occupy on average a columnar stratum. These molecules tend to adopt a polar conformation instead of the apolar C₃ conformation and arrange in an antiparallel assembly. For some compounds the molecules in this arrangement have enough flexibility to tilt upon lowering temperature or by applying an electric field. Interestingly, the hexagonal columnar arrangement is preserved in a glassy state at room temperature. Furthermore, the mesophases show luminescence in the blue region of the visible spectrum depending on the peripheral substitution and molecular structure. We also report a novel property of TTT compounds (series T, TB and TB*) as they are able to act as solvatofluorochromic probes, a property that allowed us to estimate the overall polarity of the liquid crystalline medium. An aromatic azide precursor was also found to be mesomorphic and exhibited a monolayer SmA mesophase.