Cholesteric liquid crystal phases

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... 

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. 

The role of supramolecular chemistry in materials is perhaps expressed most impressively in liquid crystals, in which slight variations of chiral content can lead to dramatic influences in the properties of the mesophases. The helical sense of these mesophases is determined not only by intrinsically chiral mesogens but also by the use of dopants which more often than not interact with achiral host LCs to generate chiral phases (Fig. 7). These phenomena are important both scientifically and technologically, most notably for the chiral smectic and cholesteric liquid crystal phases [68-71]. These materials—as small molecules and as polymers [72,73]—are useful because their order... 

Another saturated tetrahydrofuryl core has found application as a component of liquid crystals. Cholesteric liquid crystal polymers are useful as photostable UV filters in cosmetic and pharmaceutical preparations for the protection of human epidermis and hair against UV radiation, especially in the range 280-450nm <2000DEP19848130>. Fused bifuran 81 is a suitable monomer for the preparation of these desired polymers as it contains the requisite characteristics of having more than one chiral, bifunctional subunit type which is capable of forming a cholesteric liquid crystal phase with a pitch of <450 nm. It also contains an achiral aromatic or cycloaliphatic hydroxyl or amino carboxylic acid subunit, achiral aromatic or cycloaliphatic dicarboxylic acids, and/or achiral aromatic or cycloaliphatic diols or diamines. Polymers prepared from suitable monomers, such as diol 81, can also be used as UV reflectors, UV stabilizers, and multilayer pigments. 

The blue phases occur in cholesteric systems of sufficiently low pitch, less than about 5000 A. They exist over a narrow temperature range, usually 1 C, between the cholesteric liquid crystal phase and the isotropic liquid phase (see (1.3.5)). The first observation of a blue phase was described by Reinitzer himself in his historic letter to Lehmann as follows On cooling (the liquid phase of cholesteryl benzoate) a violet and blue phenomenon appears, which then quickly disappears leaving the substance cloudy but still liquid. Although Lehmann recognized it as a stable phase, not until the 1970s was it generally accepted that the blue phases are thermodynamically distinct phases. The nature of these phases has now become a subject of considerable interest to condensed matter physicists. 

Describe how a cholesteric liquid crystal phase differs from a nematic phase. 

A sufficient amount of oriented chiral molecules can be obtained in an induced cholesteric liquid crystal phase if the induced helical structure has been untwisted by an electric field. In the following description tensors are needed for the sake of simplicity (At least there are three tensors required the transition moment tensor (absorption tensor ,y), the rotational strength tensor (circular dichroism tensor A ,y), and the order tensor g,y33 (i,j= 1,2,3). If the molecules do not possess any symmetry, the principal axes of all of these tensors are differently oriented with respect to the molecular frame (the coordinate system in which only the three diagonal elements of a tensor are different from zero).) The only tensorial property, needed here explicitly, is the existence of three coordinates (components) of a tensor with respect to three specially chosen mutually perpendicular axes. This means that three information instead of one information about a molecule are needed instead of one CD value, namely Ae, three CD values, namely As, (i=l, 2, 3), have to be introduced. Ac is then one-third of a sum of the three so-called tensor coordinates of the CD tensor ... 

Fingerprint texture of the chiral nematic (cholesteric) liquid crystal phase. 

We have shown theoretically and experimentally that third-harmonic generation can be collinearly phase matched in cholesteric liquid crystals. Phase matching is achieved since the momentum mismatch between the fundamental and the third harmonic is compensated by the lattice momentum which is present due to the periodicity of the helical structure of the cholesteric medium. Many different phase-matching conditions exist. Analogous to electron-electron interaction in a periodic lattice,... 

Cholesteric liquid crystal phase is obtained when a nematic phase is doped with chiral molecules. Chiral molecules are optically active and are known to show optical rotary dispersion in the order of l7cm. However in the cholesteric phase they induce rotation of the long axes of the liquid crystal molecules (the director n) about a helix as shown in Figure 1. 

For the investigated photopolymerization-induced banded texture of ethyl-cya-noethyl cellulose/acrylic acid/copper acrylate cholesteric liquid crystalline solutions, the results indicate that the cholesteric liquid crystal phase can be fixed through photopolymerization. 

The -dependence of the decay rate F of tracer polystyrene in polymethylmethacrylate benzene was measured by Numasawa, et a/.(60). The value of V/q increases at large q, as also seen for dilute polymers in simple solvents as discussed in Chapter 11. The effect of a phase transition on Ds was observed by Russo, et al, who examined poly(y-benzyl-a,L-glutamate) pyridine(61). An isotropic-cholesteric liquid crystal phase transition occurs for this rodlike polymer at elevated c. The value of Ds c) increases dramatically at the transition, but on both sides of the transition Ds c) decreases as c is increased. 

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. 

A given material may possess either the nematic liquid crystal phase or the cholesteric liquid crystal phase, but none axe known to possess both. Cholesterics will not feature greatly in this book nevertheless, a brief discussion involving them is incorporated into the mathematical description given in Section 2.2.2. 

---