Discotic liquid crystalline hexabenzocoronenes carrying chiral and racemic branched alkyl chains: supramolecular engineering and improved synthetic methods

In this paper we present the synthesis and characterization of three different thermotropic liquid crystalline derivatives of hexabenzocoronene (HBC), substituted at the periphery by six chiral or racemic branched (3,7-dimethyloctanyl) chains, namely the chiral 5a, racemic 5b, and mono-bromo functionalized 18 carrying five alkyl chains. Improved methods for synthesis of HBC precursors, largely relying on transition-metal catalyzed coupling reactions, are also described. Solubility and processability are increased while the thermal K→DH transitions are shifted to lower temperatures, relative to analogous HBCs carrying n-alkyl chains. In case of substitution with a chiral branched alkyl chain, 5a, strong signals have been recorded by means of circular dichroism spectroscopy. All new compounds were characterized by 1H, 13C NMR, and UV–Vis spectroscopy, and FD-MS. Preliminary mesophase characterization is carried out by differential scanning calorimetry (DSC) and polarized light microscopy. In the course of the DSC experiments, optically pure and racemic HBC derivatives were compared.