Origin of Helix in Achiral Banana-Shaped Molecular Systems

Abstract

Helical structures were confirmed for both the SmA b and SmBlue phases of banana-shaped molecular systems from observations of the microscopic fringe pattern and the selective reflection of blue color, respectively. X-ray and optical microscopy indicate that the helical axes in the SmA b and SmBlue phases are normal and parallel to the smectic layer, respectively. In these two helical phases, 13C NMR spectra show two C=O peaks, suggesting two different configurations of ester group, whereas only one C=O peak appears in the isotropic and crystal phases. This indicates that the two C=O groups in the mesogenic core are not in the same plane but are twisted. The addition of chiral dopant makes the dichroic ratio of the right- and left-circularly polarized scattered light positive or negative in the SmBlue phase, although the wavelength (∼430 nm) of the scattering peak does not change significantly. The origin of the helix will be discussed in view of the twisted molecular conformation (conformational chirality) and the escape from macroscopic polarization.

Helical nanofilaments of bent-core liquid crystals with a second twist

“…1c) and they also become yellow, related to the absorption of the Schiff base in the molecules. The blue appearance of the lower homologues was noted in a previous work 8 (and attributed to a helix along the smectic layers), but the absence of the blue reflection in the higher homologues has never been discussed.…”

“…The studied phenylene bis(alkoxyphenyliminomethyl) benzoate series 8,19 with n ¼ 7, 8, 9 and 12 carbons in their alkyloxy tails were synthesized by D Lötzsch in G Heppke's group at the Iwan Stranski Institute of the Technical University, Berlin, Germany. Their molecular structures, phase sequences, colour images with the corresponding reflection spectra and the small angle X-ray scattering (SAXS) results are shown in Fig.…”

“…Freeze-fracture transmission electron microscopy (FFTEM) studies of the B4 phase revealed an assembly of twisted layers stacked to form chiral nanobundles with around wB40 nm widths and hB100 nm half pitch 6 . This helical nanofilament (HNF) phase is particularly interesting owing to its unique hierarchical nanostructure 7,8 in which the individual filaments further selforganize into oriented arrays of filaments that are phase coherent with respect to their twist. The bulk HNF phase expels most guest molecules, such as rod-shaped liquid crystals [9][10][11][12][13][14][15] , and their unique structure makes it attractive for the design of bulk and surface structures with controlled porosity and roughness at the nanometre and micrometre scales.…”

“…In this paper we describe cryo-TEM studies of the phenylene bis(alkoxyphenyliminomethyl) benzoate series 8,19 with n ¼ 7, 8, 9 and 12 that form room temperature HNF phases. The most important result of the study is that the nanofilament layers stack on top of each other in a twisted configuration; the carbon substrate has only minor effect on the bulk structure of the helical nanofilaments.…”

Impact of Intermediate and Terminal Groups on the Thermal Stability of Bent-Core Liquid Crystals

“…However they have not attracted much interest until the synthetic work of Matsunaga [8] in the early of 1990, the discovery of the mesogenic properties of bent-core molecules has opened up a major new and exciting dimension in the science of thermotropic liquid crystals (LCS). There are a broad implications for the general field of soft condensed matter including the observation of Ferro electricity and spontaneous breaking of chiral symmetry in smectic phases composed of molecules that are not intrinsically [9]. An increase in the temperature, liquid crystal phases in bent-core mesogens appear different from what is commonly known.…”