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

In this paper we present the synthesis and characterization of three differ ent thermotropic liquid crystalline derivatives of hexabenzocoronene (HBC), substituted at the periphery by six chiral or racemic branched (3,7-dimeth yloctanyl) chains, namely the chiral 5a, racemic 5b, and mono-bromo functio nalized 18 carrying five alkyl chains. Improved methods for synthesis of HB C precursors, largely relying on transition-metal catalyzed coupling reacti ons, are also described. Solubility and processability are increased while the thermal K -->D-H transitions are shifted to lower temperatures, relativ e to analogous HBCs carrying n-alkyl chains. In case of substitution with a chiral branched alkyl chain, 5a, strong signals have been recorded by mean s of circular dichroism spectroscopy. All new compounds were characterized by H-1, C-13 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, opti cally pure and racemic HBC derivatives were compared. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.