Blue Phases of Chiral Liquid Crystals

The history of blue phases can be traced back more than a century. In 1888, Reinitzer [1], an Austrian botanist, examined cholesteryl benzoate (a chiral organic compound) under an optical microscope. He observed that this material exhibited a blue color just below the isotropic phase as it cooled, and the color disappeared almost immediately. It is known now that cholesteryl benzoate exhibits blue phases - and they got their name because this first observed such phase had a blue color. This blue phase is also the first liquid crystal phase reported in hterature. 

Non-chiral liquid crystal transforms directly from the isotropic phase into the nematic phase. If there is a spatial fluctuation in these phases (i.e. spatial non-uniformity) a physical property / (F) can be expressed in terms of a set of Fourier components / ( ) exp (- / r), where q is 

Fundamentals of Liquid Crystal Devices, Second Edition. Deng-Ke Yang and Shin-Tson Wu. 2015 John Wiley Sons, Ltd. Published 2015 by John Wiley Sons, Ltd. 

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D.-K. Yang, Optical studies of blue phase III of chiral liquid crystals, dissertation, University of Hawaii, 1989. 

This interaction term, which is proportional to the first pseudoscalar term in a multipole expansion of the intermolecular interaction energy, as obtained by Goossens [79], [77], [80], was sufficient to obtain a rich polymorphism of chiral liquid crystal phases given by cholesteric and several blue phases in dependence on temperature and the chirality parameter c measuring the strength of the energy of chiral interaction, both for calamitic and discotic chiral Gay-Beme molecules [77], [81]. 

Taushanoff S, Le KV, Williams J, Twieg RJ, Sadashiva BK, Takezoe H, Jakli A (2010) Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material. J Mater Chem 20 5893-5898... 

In the case of thermotropic liquid crystals, a surface description is mathematically useful, but physically misleading, since these surfaces are fictional they serve only to describe the three-dimensional variation of molecular orientation. An alternative description of blue phases in tem s of close-packing of chiral rods can be found in die next Chapter (section 5.1.8). 

Chirality is also an important aspect of liquid crystals. The introduction of chiral moieties into the chiral smectic phases induces functions such as ferroelectricity and antiferroelectricity. A few of the unconventional chiral liquid crystals are described in Chapter 1. The blue phase is one of the exotic chiral liquid crystalline phases. In Chapter 3, Kikuchi introduces the basic aspects and recent progress in research of the blue phase. Recently, the materials exhibiting the blue phases have attracted attention because significant photonic and electro-optic functions are expected from the materials. 

Blue phases exist in a narrow temperamre region between the isotropic and cholesteric phases. As temperamre is decreased, the order of appearance of the blue phases is BPIII, BPII, and BPI [15-17]. Whether a chiral liquid crystal has a blue phase depends on its molecular stmcmre and chirality. The blue phases can be identified by an optical microscope under reflection mode. BPI and BPII have bright and colorful multi-domain crystal plate textures, while BPIII has a dim uniform foggy texmre [5,18]. Therefore, BPIII is also called the fog phase. As will be discussed later, BPI and BPII have cubic crystal structures while BPIII has an amorphous stmcmre. 

It can be seen from liquation (13.41) that the Fourier components with m = 2 (for right-handed liquid crystals) have lower free energy than the components with other m values, and therefore the blue phase only has the components with m = 2, as shown in Equation (13.70). The reflected light is circularly polarized with the same handedness as the chirality of the liquid crystal. 

In recent years a wide variety of chiral liquid crystalline phases has been discovered in certain chiral materials and these are mentioned above in the section on optical polarising microscopy. However, DSC has often not revealed the presence of such phases. In the case of the blue phases and the TGBA phase, the mesophase ranges are often too short to provide a distinct enthalpy peak. Of course, in some materials this situation is trae of the more usual liquid crystal phases. The transitions between the S( ferri phase and... 

This chapter reviews the present understanding of blue phases. Blue phases are distinct thermodynamic phases that appear over a narrow temperature range at the helical-isotropic boundary of highly chiral liquid crystals. In the absence of electric fields, there can be three blue phases BPI and BPII, both of which have cubic symmetry and BPIII, which possesses the same symmetry as the isotropic phase. Figure 7.1 shows schematically the phases in both nonchiral and chiral nematics. For nonchiral nematics, including racemic mixtures (with equal numbers of left- and right-handed versions of the same molecule) and even weakly chiral nematics, the nematic (or weakly chiral) phase heats directly to the isotropic phase. When the chirality is high, however, as many as three blue phases may appear. 

In conclusion, electric field effects in liquid crystals is a well-developed branch of condensed matter physics. The field behavior of nematic liquid crystals in the bulk is well understood. To a certain extent the same is true for the cholesteric mesophase, although the discovery of bistability phenomena and field effects in blue phases opened up new fundamental problems to be solved. Ferroelectric and antiferroelectric mesophases in chiral compounds are a subject of current study. The other ferroelectric substances, such as discotic and lyotropic chiral systems and some achiral (like polyphilic) meso-genes, should attract more attention in the near future. The same is true for a variety of polymer ferroelectric substances, including elastomers. 

OPTICAL ACTIVITY IN THE ISOTROPIC AND BLUE PHASES OF A CHIRAL LIQUID CRYSTAL... 

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