Optically active mesogenic triphenylenedicarboxyimides: Effects of axially connected aromatic ring at the stereogenic center on liquid crystal and self-assembly properties

We describe the synthesis and self-assembly behaviour of a series of ten optically active triphenylenedicarboxyimide (TDI) derivatives with a stereogenic center attached to the imide nitrogen carrying an end aromatic ring. All TDIs show an enantiotropic hexagonal columnar mesophase with high clearing temperatures up to 260 °C, which means that branching of the introduced stereocenter does not disrupt the columnar π−stacking of the TDI cores. Although all chiral TDIs in THF solution showed CD signal indicative of optical activity, chiral supramolecular structures were only observed in the columnar mesophase of three pairs of chiral TDIs (2, 4 and 5) having one end phenyl or naphthyl ring linked directly or close to the chiral center. DFT calculations suggests that the axially connected aromatic ring at the stereocenter provides steric repulsion upon stacking, thereby leading to the formation of chiral supramolecular structures. Besides, a small amount exclusively single-handed helical ribbons along with bundles of straight ribbons were simultaneously observed in SEM image of self-assembled R- or S-5 in mixed solution of ethyl acetate and ethanol, suggesting that the configuration of the stereogenic center is not the only factor responsible for inducing helical structures in solution.