Hexakis(4-iodophenyl)-peri-hexabenzocoronene -: A versatile building block for highly ordered discotic liquid crystalline materials

Hexakis (4-iodophenyl)-peri-hexabenzocoronene (5), a novel functionalizable mesogenic building block, was prepared by rational multistep synthesis. Although sparingly soluble in common solvents, it can be obtained in pure form and then functionalized via Hagihara-Sonogashira coupling to give a series of highly ordered columnar liquid crystalline molecules 14a-c. The total synthesis involves five 6-fold transformations, all in excellent to near quantitative isolated yields. Their thermotropic liquid crystalline behavior was studied by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXID). Compared to the normal alkyl-subsituted hexabenzocoronenes (HBCs), 14a-c exhibit more highly ordered columnar mesophases, including three-dimensionally ordered superstructures (helical columnar mesophase). These could arise from additional intracolumnar,7-,7 interactions between, and space-filling requirements introduced by, the rigid-rod side groups. Atomic force microscopy (AFM) revealed self-assembled bundles of columnar aggregates in spin-coated films and isolated several micron-long nanoribbons composed of a defined number of columns in drop cast films.