In the 21st century, soft materials will become more important as functional materials because of their dynamic nature. Although soft materials are not as highly durable as hard materials, such as metals, ceramics, and engineering plastics, they can respond well to stimuli and the environment. The introduction of order into soft materials induces new dynamic functions. Liquid crystals are ordered soft materials consisting of self-organized molecules and can potentially be used as new functional materials for electron, ion, or molecular transporting, sensory, catalytic, optical, and bio-active materials. For this functionalization, unconventional materials design is required. Herein, we describe new approaches to the functionalization of liquid crystals and show how the design of liquid crystals formed by supramolecular assembly and nano-segregation leads to the formation of a variety of new self-organized functional materials.

Functional liquid-crystalline assemblies: self-organized soft materials.

A smectic liquid-crystal phase made from achiral molecules with bent cores was found to have fluid layers that exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt. These instabilities combine to form a chiral layer structure with a handedness that depends on the sign of the tilt. The bulk states are either antiferroelectric-racemic, with the layer polar direction and handedness alternating in sign from layer to layer, or antiferroelectric-chiral, which is of uniform layer handedness. Both states exhibit an electric field-induced transition from antiferroelectric to ferroelectric.

https://science.sciencemag.org/content/sci/278/5345/1924.full.pdf

Spontaneous formation of macroscopic chiral domains in a fluid smectic phase of achiral molecules

An overview on the recent developments in the field of liquid crystalline bent-core molecules (so-called banana liquid crystals) is given. After some basic issues, dealing with general aspects of the systematisation of the mesophases, development of polar order and chirality in this class of LC systems and explaining some general structure–property relationships, we focus on fascinating new developments in this field, such as modulated, undulated and columnar phases, so-called B7 phases, phase biaxiality, ferroelectric and antiferroelectric polar order in smectic and columnar phases, amplification and switching of chirality and the spontaneous formation of superstructural and supramolecular chirality.

/pdf/bent-core-liquid-crystals-polar-order-superstructural-17m3bst2sc.pdf

Bent-core liquid crystals: polar order, superstructural chirality and spontaneous desymmetrisation in soft matter systems

Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

https://iopscience.iop.org/article/10.1143/JJAP.45.597/pdf

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

This critical review begins with a brief, but essential, introduction to the special nature of liquid crystal materials, their peculiar properties, and their commercial applications, followed by an introductory insight into the remarkable nature of the fluoro substituent, and its fascinating influence on the properties of organic compounds. However, the main focus of the review is to discuss the enormous amount of exciting research on fluorinated liquid crystals that has been reported. The small size of the fluoro substituent enables its incorporation into all types of liquid crystal, including calamitic, discotic, banana, lyotropic, and polymers, without ruining the liquid crystalline nature of the material. However the fluoro substituent is larger than hydrogen, and hence causes a significant steric effect, which combined with the high polarity, confers many fascinating, and often remarkable, modifications to melting point, mesophase morphology and transition temperatures, and the many other very important physical properties, such as dielectric anisotropy, optical anisotropy, and visco-elastic properties. There are many different positions within a liquid crystal structure where a fluoro substituent can be located, including (i) a terminal position, (ii) within a terminal chain, as a semi-fluorinated or as a perfluorinated chain, or as one fluoro substituent at a chiral centre, (iii) as part of a linking group, and (iv) a lateral position in the core section. Such variety enables the interesting and advantageous tailoring of properties, both for the fundamental purposes of establishing structure–property relationships, and for materials targeted towards commercially-successful liquid crystal display applications.

Fluorinated liquid crystals – properties and applications

The synthesis of a banana-shaped molecule is reported and it is found that the smectic phase which it forms is biaxial with the molecules packed in the best direction into a layer. Because of this characteristic packing, spontaneous polarization appears parallel to the layer and switches on reversal of an applied electric field. This is the first obvious example of ferroelectricity in an achiral smectic phase and is ascribed to the C2v symmetry of the molecular packing.

Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules

Helical structures were confirmed for both the SmAb 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 SmAb 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.

https://iopscience.iop.org/article/10.1143/JJAP.36.6455/pdf

Origin of Helix in Achiral Banana-Shaped Molecular Systems

Spontaneous formation of two types of helical structure is observed in
two smectic phases consisting of banana-shaped achiral molecules for the
first time. Local optical resolution occurs to form spatially resolved
helices of both handednesses.

Spontaneous helix formation in smectic liquid crystals comprisingachiral molecules

We found that the frustration, i.e., unusual density modulation, is induced on ferroelectric SmAb and HexBb phases composed of achiral banana-shaped molecules. The frustration takes place along the bent direction of molecules, with four or five molecules included in each inversion unit. Such an unusual frustrated structure may be explained as a result of a two-dimensional escape from spontaneous polarization in the ferroelectric phases.

Frustrated Structure Induced on Ferroelectric Smectic Phases in Banana-Shaped Molecular System

We investigated the ferroelectric behavior in novel ferroelectric liquid crystals consisting of banana-shaped achiral molecules. Under the application of a triangular-wave electric field, we observed a single peak corresponding to the switching current in the higher-order smectic ( SmX2) phase as well as the smectic Ab ( SmAb) phase, where ferroelectricity had been already reported. This result proves that the spontaneous polarization is switchable with a single hysteresis loop in SmX2 as well as in SmAb. We also discussed the dielectric behavior in both the phases.

https://iopscience.iop.org/article/10.1143/JJAP.36.L1201/pdf

Teruki Niori

Papers

Distinct ferroelectric smectic liquid crystals consisting of banana shaped achiral molecules

Origin of Helix in Achiral Banana-Shaped Molecular Systems

Spontaneous helix formation in smectic liquid crystals comprisingachiral molecules

Frustrated Structure Induced on Ferroelectric Smectic Phases in Banana-Shaped Molecular System

Ferroelectric Properties in Banana-Shaped Achiral Liquid Crystalline Molecular Systems