Supramolecular self-assembly of discotic liquid crystalline supermolecules

A series of discotic liquid crystalline supermolecules based on triphenylene (Tp) were designed, synthesized, and characterized. Phase behaviors and self-assemblies of those discotic liquid crystalline supermolecules were studied by differential scanning calorimetry (DSC), polarized light microscopy (PLM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). ^ Hydrogen bond-induced supramolecular self-assembly of a series of doubly discotic supermolecules based on Tp and porphyrin (Py) linked via amide and ester bonds have been studied. As shown by temperature-dependent and polarized FTIR, all amide bonds were associated together via hydrogen bonding below the clearing point and hydrogen-bonding was aligned along porphine columns. In addition, molecular shape played an important role in the self-assembly. When the spacer length between Tp and Py was no shorter than or similar to the Tp alkyl arm length, a rectangular board-like overall molecular shape was adopted. Therefore, lamellar structures were obtained. When the spacer length was much shorter than the Tp alkyl arms, a regular columnar phase was obtained. ^ The importance of molecular shape in supermolecular self-assembly was further studied in a series of hybrid star liquid crystalline supermolecules based on eight Tps as peripheral arms and a polyhedral oligomeric silsesquioxane (POSS) molecule as the central scaffold. When the spacer length was very short (C2), a column-within-column super hexagonal columnar phase was observed. With increasing the spacer length to CO, alternating POSS-Tp lamellar morphology with a rectangular columnar symmetry was observed. For the sample with a spacer of C10, an oblique columnar phase was determined by XRD, and inverted columnar morphology with four Tp columns forming a super column within the POSS/alkyl chain matrix was observed by TEM. ^ Similar phenomena were also observed in the tetra-Tp-substituted phthalocyanine (Pc) system. When the spacer length was relatively long, a normal hexagonal columnar structure with the unit cell dimension close to those for the parent Tp or Pc liquid crystals were observed. When the spacer length was short, the whole supermolecules thus stacked together to form a super liquid crystalline column, and a column-in-column rectangular mesophase with unit cell dimensions close to the size of the entire supermolecule was observed.