Competition between helical and heliconical twist in the development of complex soft matter structures

Abstract

Chirality strongly influences the self-assembly of soft matter systems, yielding new superstructures with numerous potential applications. Here, two series of 4-cyanoresorcinol-based bent-core liquid crystals (BCLCs) in which molecular chirality is provided by the introduction of a branched (S)-2-alkyloxypropyl end chain at both or only one end of the aromatic core have been synthesized and characterized to study the effects of chain branching and molecular chirality on liquid crystalline (LC) self-assembly. Two new LC phases with tilted and twisted arrangements of the molecules, restricted rotation around the long axis, preferred face-to-face stacking of the polyaromatic cores, and only a short correlation length of lamellar ordering were discovered; one is achiral and isotropic (M_Iso) while the other is chiral and low birefringent (M_L*). With decreasing density of the chain branching (i.e., stereogenic centers), the achiral M_Iso phase is replaced by the birefringent heliconical M_L* phase, an amorphous type III blue phase (BPIII), a heliconical tilted smectic phase (SmC*), and a chiral cybotactic nematic phase composed of SmC* clusters (N_CybC*). In this N_CybC* phase, the twist is heliconical, in contrast to the conventional non-cybotactic chiral nematics where it is helical. The heliconical twist is favored and the helical twist is suppressed by the emergence and growth of SmC-type cybotactic clusters. At the transition to a denser, face-to-face-packed core, saddle-splay emerges, and helical twist can become stronger, supporting layer deformation and causing a transition from the heliconical SmC* via M_L* to the achiral M_Iso phase as the density of chain-branched stereogenic centers increases and temperature decreases.