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 1H and 13C 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 non-mesogenic 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.