Helical morphologies of thermotropic liquid-crystalline chiral Schiff-based rod-coil amphiphiles

Abstract

A series of Schiff-based rod-coil molecules possessing thermotropic liquid-crystalline (LC) character have been prepared whereas a sugar-based moiety was introduced to the chain end of the molecules so as to create chiral amphiphiles for self-assembly. The self-assembly of the chiral Schiff-based rod-coil amphiphiles gave rise to a variety of specific LC textures. A banded morphology under polarized light microscopy (PLM) can be observed; the appearance of the banded texture is strongly dependent upon the length of hydrophobic tail that determines the twisting power of self-assembled hierarchical superstructures with helical sense. As a result, the banded spherulites are identified as quaternary helical morphology with a collection of the tertiary chiral structures (i.e., helical twists) so as to give regular extinction in PLM attributed to zero-birefringence effect. Consistent with the observation of helical morphologies, the occurrence of chiral smectic C (SmC*) phase can only be found in samples with enough alkoxyl chain length, suggesting the existence of strong correlation for morphological evolution from molecular level to macroscopic object with the formation of SmC*. A hypothetic model about the bilayer structure within the SmC* structure is thus given to elucidate the morphological evolution. Consequently, the self-assembly of the chiral amphiphiles with thermotropic LC character represents the mechanism for the chirality transfer in different length scales.