Liquid crystals chiral nematic

In particular most of the early studies on CPL were based on the incorporation of a luminescent achiral chromophore in a chiral nematic or cholesteric liquid crystal. Chiral nematic liquid crystals (CNLC) are intrinsically birefringent and exhibit a helical supramo-lecular architecture, which is characterized by the pitch length p (Figure 5.11). 

FIGURE 5.11 Supramolecular, helical architecture and definition of pitch length p of chiral nematic liquid crystals. 

M Voigt, M Chambers, and M Grell, Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal, Liq. Cryst., 29 653-656, 2002. 

Fig. 13. Tilted 2D spectrum of natural abundance 2H Q-COSY experiment of ( )-l-pentyn-ol dissolved in PBLG/chloroform chiral nematic liquid crystal. The spectrum was symmetrized prior to the tilting procedure. Note that all isotopomers of the mixture are clearly visualized. (Reproduced by permission of American Chemical Society.)...

By far the most important commercial applications of nematic liquid crystals are in the a multi-billion dollar display industry. Cholesteric, or chiral nematic liquid crystals have been used in coloured guest-host displays and in thermography/ther-mochromic applications. 

The history of liquid crystals started with the pioneer works of Reinitzer and Lehmann (the latter constructed a heating stage for his microscope) at the end of the nineteenth century. Reinitzer was studying cholesteryl benzoate and found that this compound has two different melting points and undergoes some unexpected color changes when it passes from one phase to another [1]. In fact, he was observing a chiral nematic liquid crystal. 

The first application described was as temperature sensors by using a chiral nematic liquid crystal, which displays different colors at different temperatures. It is also worth noting that many common fluids are in fact liquid crystals. Soap, for instance, is a liquid crystal, and forms a variety of liquid crystal phases depending on its concentration in water. 

If the molecules of a liquid crystal are optically active Ichiral), then the nematic phase is not formed. Instead of the director being locally constant as is the case for nematics, the director rotates in helical fashion throughout the sample, Within any plane perpendicular to the helical axis the order is nematic-like. In other words, as in a nematic there is only orienlalional order in chiral nematic liquid crystals, and no positional order. 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

Nakata M, Takanishi Y, Watanabe J, Takezoe H (2003) Blue phases induced by doping chiral nematic liquid crystals with non-chiral molecules. Phys Rev E 68 041710-1-6... 

Several exciting phenomena described for non-chiral nematic systems were also reported for nanoparticle-doped chiral nematic liquid crystals. We mentioned the work of Kobayashi et al., who, most notably, demonstrated a frequency modulation twisted nematic (FM-TN) mode and fast switching characteristics using metal nanoparticles as dopants [301-307, 313, 314],... 

Our group pursued another approach of combining the properties of nanoparticles with chiral nematic liquid crystal phases. The idea was to decorate gold nanoparticles with chiral molecules known to be strong inducers of chiral nematic phases. To realize the idea, we prepared a series of alkylthiol-capped gold nanoparticles, either pure monolayer or mixed monolayer, with all or about every second of the alkylthiols end-functionalized with (5)-naproxen (e.g., 6 in Fig. 11) [349]. 

Finally, dispersions of MWCNT in chiral nematic liquid crystals were studied as well. These experiments suggested no change in the helical twisting characteristics of the chiral nematic phase. However, the MWCNTs were thought to disrupt the translational order in the SmA phase (decrease of the SmA-N phase transition) yet follow the twist of the nematic director in the chiral nematic phase [498]. 

Recently, true circular polarizers have been developed and are available from commercial sources. These are fashioned by sandwiching a chiral nematic liquid crystal between parallel glass windows. These devices are characterized by a high transmission and extinction ratios greater than 1000. They operate over a fairly narrow range of wavelengths about a central wavelength. 

Liquid crystalline (LC) solutions of cellulose derivatives form chiral nematic (cholesteric) phases. Chiral nematic phases are formed when optically active molecules are incorporated into the nematic state. A fingerprint texture is generally observed under crossed polarizers for chiral nematic liquid crystals when the axis of the helicoidal structure is perpendicular to the incident light (Fig. 2). 

All the above theories are derived for rigid rod nematic liquid crystal systems. The rheological behavior of chiral nematic liquid crystals is more complex and less understood than that of nematic systems. Rey introduced a model based on rigid rod chiral nematic liquid crystals to describe permeation shear flow and small amplitude oscillatory shear flow. The model can predict some common phenomena of chiral nematic liquid crystals, e.g., the three-region... 

Chiral disubstituted PEDOTs have recently been prepared for the first time via transetherification of 3,4-dimethoxythiophene monomers with chiral glycols followed by potentiodynamic oxidation.174 An alternative approach to optically active PEDOTs has also been recently described, which involves the electrochemical polymerization of the EDOT monomer in aqueous hydroxypropyl cellulose (HPC) as a polymer lyotropic liquid crystal to give a chiral PEDOT/HPC hybrid.175 The PEDOT prepared in this chiral nematic liquid crystal exhibited optically active electrochromism in that it could be electrochemically switched between a dark blue reduced state and a sky blue oxidized form that exhibited a different CD spectrum. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

In general, the term liquid crystal is used to describe an intermediate phase between liquid and solid occurring in some organic compounds. The phase of liquid crystal can be divided into two mesophases smectic and nematic. Nematic liquid crystals can be further divided as chiral nematic or archiral nematic. In chiral nematic liquid crystals, sterol-related compounds are called cholesteric, and non-sterol-based compounds are termed chiral nematic. For heat transfer applications, encapsulated forms of chiral nematic [71] or the composite liquid crystal sheets of the cholesteric type [72] are commonly used. Recently, the application of micro-encapsulated liquid crystals has become more popular in heat transfer measurements because of the fast response and easy paintbrush or spray application to the test surface. 

Lee H K, Doi K, Harada H, Tsutsumi O, Kanazawa A, Shiono T, Ikeda T. 2000. Photochemical modulation of color and transmittance in chiral nematic liquid crystal containing an azobenzene as a photosensitive chromophore. J Phys Chem B 104(30) 7023 7028. 

Ruslim C, Ichimura K. 2001. Photocontrolled alignment of chiral nematic liquid crystals. Adv Mater 13(9) 641 644. 

Ruslim C, Ichimura K. 2002. Photoswitching in chiral nematic liquid crystals inter action selective helical twist inversion by a single chiral dopant. J Mater Chem... 

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