Hierarchical Superstructures with Helical Sense in Self-Assembled Achiral Banana-Shaped Liquid Crystalline Molecules

Abstract

The self-assembly of 1,3-phenylene bis[4-(4-n-heptyloxybenzoyloxy)-benzoates] (BC7) is studied to examine the formation of helical morphologies from achiral banana-shaped liquid crystal molecules at different self-assembling levels. Various hierarchical superstructures including flat-elongated lamellar crystal, left- and right-handed helical ribbons, and tubular texture are observed while the BC7 molecules self-assemble in THF/H(2)O solution. By contrast, only plate-like morphology is observed in the self-assembly of achiral linear shaped 1,4-phenylene bis[4-(4-n-heptyloxybenzoyloxy)-benzoates] (LC7) molecules, indicating that the chirality of the self-assembled texture is strongly dependent upon the molecular geometry of the achiral molecules. The formation of the helical superstructures, namely hierarchical chirality, is attributed to the conformational chirality from the achiral banana-shaped liquid crystalline molecules, as evidenced by significant optical activity in time-resolved circular dichroism experiments. Selective area electron diffraction is performed to examine the structural packing of the hierarchical superstructures. As observed, the molecular disposition of the lamellar crystal is identical to that of the helical superstructure. Also, the diffraction patterns of the helical superstructures appeared arc-like patterns consisting of a series of reflections, suggesting that the helical morphology resulted from the curving of the lamellar crystals through a twisting and bending mechanism. Consequently, the model of molecular disposition in the self-assembled helical superstructures from the achiral banana-shaped molecules is proposed. The morphological evolution in this study may provide further understanding with respect to the chiral information transfer mechanism from specific molecular geometry to hierarchical chirality in the achiral banana-shaped molecules.