Symmetry and Liquid Crystals

Liquid crystals are aggregates of individual molecules due to moderate intermolecular interactions, which are roughly divided into lyotropic liquid crystals and thermotropic liquid crystals. The latter includes smectic, nematic, cholesteric, and discotic phases depending on the molecular arrangement and symmetry, as well as changes in the external field, such as temperature and pressure.

Currently, the scale of the global liquid crystal industry is approximately USD 75 billion, most of which is for display applications using nematic liquid crystals (including chiral nematic liquid crystals). Research and development of cholesteric and smectic liquid crystals are also actively carried out for sensors, organic semiconductors, solar cells, etc.

Generally, a liquid crystal device confines liquid crystal molecules in a cell and achieves a desired liquid crystal alignment by interfacial force. The same is true for thin-film devices. Liquid crystals are inherently anisotropic media, causing symmetry/asymmetry configuration, which can be controlled by an external field and used as an optical or electrical device.

In this Special Issue entitled “Symmetry and Liquid Crystals”, we focus on the symmetry of the chemical structure of liquid crystal molecules, the asymmetry of the alignment process at the substrate interface, the asymmetry of the electric field applied to the liquid crystal layer, and the effect of an inclined external field stress on the liquid crystal elastomer.

Chiral liquid crystals based on extended π-conjugated units form the chiral nematic phase (N*) and chiral smectic C phase (SmC*).

In his review paper, Funahashi [1] introduces research on circular polarized (CP) photo-luminescence using these materials, as well as CP electro-luminescence using chiral conjugated polymers, and introduces examples of their applications in CP light emitters. In addition, he shows that ferroelectric liquid crystals (FLCs) consisting of a phenylterthiophene skeleton and chiral alkyl side display a bulk photovoltaic effect, suggesting the possibility of FLC for solar cell applications.

Blue phase (BP), which exhibits a quasi-isotropic phase, is attracting attention for use in wearable devices, such as volume holograms and smart glasses. Ozaki et al. [2] reviews various techniques and mechanisms to achieve a uniform orientation of BP crystal planes and achieve an azimuthal orientation of BPs in arbitrary directions. These technologies, including thermal gradients, electric fields, alignment films, and the three-dimensional control of interfaces by nanostructures, will bring considerable progress to 3D photonic crystals.

Next, Kikuchi et al. [3] optimize the monomer material and process to selectively aggregate and polymerize only at the disclination part in the BP phase structure, and succeed in lowering the monomer concentration while maintaining the BP stability. This technology can considerably reduce the high drive voltage, which is a problem with conventional BP devices.

On the other hand, Yamaguchi et al. [4,5] create prototypes of asymmetric TN and HAN-LCD with different azimuthal anchoring energies of the upper and lower alignment-film substrates, examining the electro-optical characteristics of the cells in detail. As a result, they show that the voltage enabling 100% modulation can be considerably reduced when the weak anchoring energy is less than the critical value.

Needless to say, the mesomorphic behavior of liquid crystals greatly depends on their molecular structure, including symmetry and polarity. Mohammady et al. [6] synthesize N-arylidene-4-alkyloxbenzenamines with different alkyl chain lengths and experimentally and theoretically investigate the relationship between alkyl chain length, thermal stability, and SmA phase range expansion. They also show that the increased π–π stacking arising from the pyridine ring’s extra N-atom contribution and the lone pairs of electrons on the oxygen atom of the alkoxy chains were closely related to these physical properties.

A diluter is a material that can be mixed with nematic liquid crystals to considerably reduce their rotational viscosity. However, conventional diluters contain a vinyl group in their structure, so they are prone to deterioration due to UV irradiation, making them difficult to use in polymer-sustained–vertical-aligned (PS-VA) and polyimide-free (PI-less) cells. Mizusaki et al. [7] develop a highly reliable diluter with a symmetrical structure and realized PS-VA and PI-free IPS with high-speed response and a high-voltage holding ratio by using it in combination with a reactive monomer containing an azobenzene moiety.

Regarding the high-speed response of IPS and FFS-LCD, Kobayashi et al. [8] adopt a dynamic retarder to adjust the phase advancement magnitude, that is, the axial direction can be controlled by the electric field to achieve asymmetric optically compensated IPS and FFS-LCD, demonstrating that the response time in the switching-off process is approximately twice as fast as the conventional process.

Residual DC voltage (VrDC) is generated by various factors and causes image sticking. Several methods for evaluating VrDC have been developed, such as the C-V hysteresis method, flicker minimization method, and dielectric absorbance method. However, precise VrDC measurements are difficult, especially in IPS or FFS-LCDs. Mizusaki and Ishihara [9] devise an I-V curve shift technique by applying triangular voltage, succeeding in measuring VrDC with high accuracy.

Liquid crystal elastomers with spontaneous polarization are suitable for use as electrically controllable soft actuators and have been actively researched recently. Hiraoka et al. [10] deform a liquid crystalline elastomer with wedge-shaped mesogens derived from cholesterol into a horseshoe shape, observing macroscopic polarization due to the flexoelectric effect and clarifying its mechanism of polarization formation.