Ionic interaction induced novel ordered columnar mesophases in asymmetric discotic triphenylene salts

Abstract

Asymmetric triphenylene imidazolium salts with different spacer lengths were successfully synthesized through quarternization of ω‐bromo‐substituted triphenylenes with 1‐methyl imidazole. The asymmetry in ω‐bromo‐substituted triphenylenes tended to destroy liquid crystallinity in the sample. However, highly ordered columnar mesophases with a lamellar microphase segregation were induced by ionic interactions among the imidazolium salts, and the lamellar morphology was visualized by transmission electron microscopy. On the basis of an X‐ray diffraction study on shear oriented samples, a novel rectangular columnar phase with a plane group of pm was observed for a triphenylene imidazolium salt with a spacer length of C11, while an oblique columnar phase was determined for a triphenylene imidazolium salt with a C8 spacer. Due to the asymmetric molecular shape and ionic interactions in the triphenylene imidazolium salts, the columnar liquid crystalline phase was extended to below room temperature (c. −20°C) for samples with spacer lengths of C8 and C11.