Liquid cholesteric

Partial photochemical decomposition of racemic alkyl aryl sulphoxides in the presence of chiral amines as sensitizers gave non-decomposed sulphoxides in optically active form with optical purity of about 3%339. The report340 on the use of cholesteric liquid crystalline reaction media to change the enantiomeric composition of racemic sulphoxides at high temperatures could not be reproduced341. 

See Chapters on TGB phases and cholesteric liquid crystals in this volume... 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

The Helfrich-Prost model was extended in a pair of papers by Ou-Yang and Liu.181182 These authors draw an explicit analogy between tilted chiral lipid bilayers and cholesteric liquid crystals. The main significance of this analogy is that the two-dimensional membrane elastic constants of Eq. (5) can be interpreted in terms of the three-dimensional Frank constants of a liquid crystal. In particular, the kHp term that favors membrane twist in Eq. (5) corresponds to the term in the Frank free energy that favors a helical pitch in a cholesteric liquid crystal. Consistent with this analogy, the authors point out that the typical radius of lipid tubules and helical ribbons is similar to the typical pitch of cholesteric liquid crystals. In addition, they use the three-dimensional liquid crystal approach to derive the structure of helical ribbons in mathematical detail. Their results are consistent with the three conclusions from the Helfrich-Prost model outlined above. 

Thermotropic cholesterics were officially discovered2 in 1888 by the Austrian botanist Friederich Reinitzer, while studying the melting of cholesterol esters cholesteryl benzoate first melted to give a cloudy liquid that, at higher temperature, turned into an ordinary clear liquid. The cloudy liquid (the mesophase) was a thermotropic cholesteric liquid crystal. These phases... 

In Chapter 7, Gottarelli and Spada extend further the theme of liquid crystalline materials. They note that although much is understood, important mysteries remain about the relationships between the handedness of cholesteric liquid crystalline phases and the chirality of the constituting building blocks. This may seen surprising given the intense interest in these materials, including their... 

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). 

VI Kopp, B Fan, HKM Vithana, and AZ Genack, Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals, Opt. Lett., 23 1707-1709, 1998. 

Under certain conditions, stiff rod-like helical polymers can spontaneously form lyotropic or thermotropic cholesteric liquid crystal (TChLC) phases. 

The use of photoreactive liquid crystal systems in imaging devices is not unprecedented. As early as 1971, Sackmann showed for the first time the photochemical change of pitch in cholesteric liquid crystals (H). Since then several techniques for using liquid... 

Another mechanism of chiral amplification that extends over an even larger scale has been reported by Huck et al. [119] The molecule 12-(9 H-thioxantbene-9 -yli-dene-12H-benzo[a]xanthene (Fig. 11.6), which has no chiral center, nevertheless exists, like the helicenes, in two chiral forms defined by their enantiomeric configurations. Consistent with the discussion in Section 11.2.3, a small net handedness (ca. 0.7 %) could be induced in racemic solutions of this molecule by use of ultraviolet CPL. However, introducing 20 wt% of this molecule, which contained a 1.5% chiral excess of one roto-enantiomer, into a nematic phase of liquid crystals produced macroscopic (100 pm) regions of a chiral cholesteric liquid crystal phase. The... 

Similar to cholesteric liquid crystals, the nematics have an orientational long-range order with the deviation that the direetion of the preferred orientation does not rotate (Fig. 2a). If, however, a chiral mesogen is dissolved in a nematic liquid crystal, the latter will be transformed into a eholesterie liquid crystal. 

Liquid crystals are mainly used for decorative purposes in cosmetics. Cholesteric liquid crystals are particularly suitable because of their iridescent color effects, and find applications in nail varnish, eye shadow, and lipsticks. The structure of these thermotropic liquid crystals changes as a result of body temperature, resulting in the desired color effect. In recent times, such thermotropic cholesteric liquid crystals have been included in body care cosmetics, where they are dispersed in a hydrogel. Depending whether this dispersion requires stirring or a special spraying process, the iridescent liquid crystalline particles are distributed statistically in the gel (Estee Lau-... 

Supramolecular systems, e.g. cholesteric liquid crystals (see Chapter 5, section 5.2)... 

Macroscopical helical structures formed by cholesteric liquid crystals have been used as chiral helical media for the asymmetric synthesis of helicenes. 

Since Robinson [1] discovered cholesteric liquid-crystal phases in concentrated a-helical polypeptide solutions, lyotropic liquid crystallinity has been reported for such polymers as aromatic polyamides, heterocyclic polymers, DNA, cellulose and its derivatives, and some helical polysaccharides. These polymers have a structural feature in common, which is elongated (or asymmetric) shape or chain stiffness characterized by a relatively large persistence length. The minimum persistence length required for lyotropic liquid crystallinity is several nanometers1. 

Optically active polymerizable liquid-crystal monomers, (I), providing polymeric cholesteric liquid-crystal films were prepared by Seki et al. (2) and used in optical applications. 

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