Liquid crystals cholesteric handedness

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... 

FIGURE 5.1 Helical superstructure of cholesteric liquid crystals (CLCs) with different handedness. 

LIGHT-INDUCED HANDEDNESS INVERSION IN CHOLESTERIC LIQUID CRYSTALS... 

Cell thickness-dependence of the reflection of a cholesteric liquid crystal in the planar state. The pitch of the liquid crystal is P = 350 nm. The refractive indices of the liquid crystal are tig = 1 -7 and = 1.5. The liquid crystal is sandwiched between two glass plates with the refractive index = 1.6. The incident light is circularly polarized with the same helical handedness as the liquid crystal. Neglect the reflection from the glass-air interface. Use two methods to calculate the reflection spectrum of the liquid crystal with the following cell thicknesses P, 2P, 5P and lOP. The first method is the Berreman 4x4 method and the second method is using Equation (2.186). Compare the results from the two methods. 

Cholesteric liquid crystals (CLCs) can also be used to make reflective polarizers. CLCs reflect circularly polarized hght with the same handedness as the helical stmcture of the liquid crystal. An unpolarized incident hght can be decomposed into a left-handed circular polarized hght and a right-handed circular polarized hght. One component is reflected and the other component is transmitted. The reflected hght is reflected toward the CLC polarizer by a back mirror and its handedness is converted to the opposite handedness and thus it passes the CLC polarizer. The transmitted circular polarized hght is converted into hnear polarization by a quarter waveplate. 

Usually, one prepares a sample of cholesteric liquid crystal by holding the material between two glass substrates. In order to know how each mode is being excited, we have to solve the problem with the proper boundary conditions. This has been done by de Vries. Using his result, we plot in Fig. 2, as a function of X, the polarizations of the incident fields which should feed exclusively into each of the two modes. For x al, both of them are close to circular polarization. The polarization of the incident field which feeds into the minus mode should have the same handedness as the helicity of the cholesteric structure. 

When a chiral substance is dissolved in a nematic liquid crystal, a cholesteric mesophase is obtained(8). The cholesteric structure is characterized by its handedness (P-or M-helix) and pitch. Equal amounts of enantiomeric solutes of equal optical purity induce helical structures with identical pitch and opposite handedness... 

A cholesteric liquid crystal contains both nematic and chiral moieties in a single molecular entity or as a binary mixture. Consisting of a helical stack of quasinematic layers, a well-aligned cholesteric film can be characterized by handedness and helical pitch length, p, as depicted in Figure 5 ... 

The pitch and handedness of the cholesteric helix can be affected by several factors temperature, chemical stmcture of the side chain and degree of polymerization. It was observed that in films casted from HPC cholesteric liquid crystal phase the pitch increases (Charlet and Gray 1987) with temperature. 

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