Star-shaped triphenylene discotic liquid crystalline oligomers and their hydrogen-bonded supramolecular complexes with simple acids

The synthesis and self-assembly properties of four novel star-shaped oligomers containing triphenylene discotic mesogens are reported. These materials have been synthesized in good yields by the Cu-catalyzed coupling cycloaddition reaction (CuAAC) between 1,3,5,-tris(2-trimethylsilylethynyl)benzene and three equivalents of the appropriate discogen bearing a pending terminated-azide alkyl substituent. The molecular structures of the discotic oligomers, thereof consisting of three lateral triphenylenes linked together by various soft methylene spacers to a central 1,3,5-tris(triazolyl)benzene, have been fully characterized by H-1 and C-13 NMR, and their purity confirmed by elemental analysis. Their thermal and liquid crystalline supramolecular organization behaviours have been studied by combining polarized optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. These investigations showed that only the two oligomers with short spacers display a hexagonal columnar mesophase, while those with long spacer are not mesomorphic. Scanning tunneling microscopy revealed that one of the mesomorphic oligomer self-assembles on a solid/liquid interface into a well ordered two-dimensional architecture that closely resembles to the proposed bulk mesomorphic supramolecular organization deduced from X-ray measurements. Furthermore, columnar mesophases have been induced in the corresponding hydrogen-bonded supramolecular complexes obtained by reaction of one of the nonmesogenic oligomeric derivative with various simple acids, e.g. hexanoic, benzoic, and 4-hexyloxybenzoic acids, respectively, in molar ratios of 1 : 2 and 1 : 3. The optical properties of these D-A dyads have been studied by UV-Vis absorption and fluorescent emission.