Orientation of a Helical Nanofilament (B4) Liquid‐Crystal Phase: Topographic Control of Confinement, Shear Flow, and Temperature Gradients

Abstract

The liquid-crystal (LC) phases of bent-core molecules have been of great interest because of their unusual polar and chiral properties, as well as their exotic collective behavior, including the formation of macroscopic chiral structures from achiral molecules. [ 1–9 ] The bent-core molecules strongly nanosegregate, leading to the formation of polar and chiral arrangements of the bent cores into planar or modulated layers. Subtle interactions between layers can be manipulated to produce a variety of ferroor anti-ferroelectric phases. Among the most mysterious and interesting of these new phases is the B4, a locally layered phase with hexatic or semicrystalline ordering of the layers, showing large optical rotation and light scattering which suggests a strongly chiral local structure. Recently, it has been shown that the B4 is a phase of spontaneously self-assembling helical nanofi laments (HNFs), the morphology of which is driven by a tendency for local saddle splay deformation of the semicrystalline layers. [ 7 ] These nanofi laments, shown in Figure 1 b for the bent-core material NOBOW (benzoic acid, 4-[(E)-([4-(nonyloxy)phenyl]imino) methyl]-,1,1’-(1,3-phenylene)ester) (Figure 1 a) grow from the higher temperature isotropic or fl uid bent-core phases with a well-defi ned pitch of the helical twist and lateral structure and dimensions. The bulk B4 is a close packing of HNFs in which the fi lament axes are parallel to one another and their layer twist becomes macroscopically coherent. In the fi lament growth process a nucleation event establishes the handedness of a fi lament and this handedness is transferred to subsequently growing fi laments to form large ( ≈ 100μ m dimension) homochiral domains with either right-handed and left-handed characteristics. Atomic force microscopy (AFM) imaging of the air/LC surface of the B4 reveals this bulk structure,