Phases cholesteric

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

FIGURE 5.51 The cholesteric phase of a liquid crystal. In this phase, sheets of parallel molecules are rotated relative to their neighbors and form a helical structure. 

The three classes of liquid crystals differ in the arrangement of their molecules. In the nematic phase, the molecules lie together, all in the same direction but staggered, like cars on a busy multilane highway (Fig. 5.49). In the smectic phase, the molecules line up like soldiers on parade and form layers (Fig. 5.50). Cell membranes are composed mainly of smectic liquid crystals. In the cholesteric phase, the molecules form ordered layers, but neighboring layers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules (Fig. 5.51). 

Ito et al. [152] described the crystal structure of 4-[(S)-2-methylbutyl]phe-nyl 4 -hexylbiphenyl-4-carboxylate which shows a smectic A phase and a cholesteric phase. The molecules are arranged in a tilted smectic-like layer structure. Within the layers, the long molecular axes are tilted (30°). However, the compound exhibits no smectic C phase. 

P-chiral dibenzophosphole oxide (52a) (Scheme 14) shows liquid crystalline behaviour [52], a property that is of interest in the area of electro-optical displays [53]. Chiral resolution of (52a) was achieved by column chromatographic separation of the diastereoisomers obtained following coordination of the o -benzophosphole (52b) to chiral cyclometallated palladium(II) complexes [52]. Notably, the presence of a stereogenic P-centre is sufficient to generate a chiral cholesteric phase. 

Similar to the sitnation with DNA structures formed under osmotic stress, DNA strands in cation-condensed bundles were found to be hexagonally packed and to possess liqnid crystalline order. For example, spermine and spermidine-condensed samples were fonnd to contain a cholesteric phase [70]. Surprisingly, DNA condensed with (Co(NH3)6) failed to exhibit a liquid crystalline ordering [47]. 

The introduction of a second chiral atom in the system leads to a reduction in the mesogenic properties and only a monotropic chiral nematic transition is observed for compound 23. However, when this compound is cooled down from the isotropic liquid state at a cooling rate of 0.5 °Cmin , very unusual blue phases BP-III, BL-II and BP-I are observed in the range 103-88 °C. Blue phases usually require pitch values below 500 nm. Hence the pitch value of the cholesteric phase for 23 must be very short, suggesting that the packing of two chiral carbons forces a faster helical shift for successive molecules packed along the perpendicular to the director. 

Several 4-(3-alkyl-2-isoxazolin-5-yl)phenol derivatives that possess liquid crystal properties have also been obtained (533-535). In particular, target compounds such as 463 (R = pentyl, nonyl) have been prepared by the reaction of 4-acetoxystyrene with the nitrile oxide derived from hexanal oxime, followed by alkaline hydrolysis of the acetate and esterification (535). A homologous series of 3-[4-alkyloxyphenyl]-5-[3,4-methylenedioxybenzyl]-2-isoxazolines, having chiral properties has been synthesized by the reaction of nitrile oxides, from the dehydrogenation of 4-alkyloxybenzaldoximes. These compounds exhibit cholesteric phase or chiral nematic phase (N ), smectic A (S4), and chiral smectic phases (Sc ), some at or just above room temperature (536). 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

A very different model of tubules with tilt variations was developed by Selinger et al.132,186 Instead of thermal fluctuations, these authors consider the possibility of systematic modulations in the molecular tilt direction. The concept of systematic modulations in tubules is motivated by modulated structures in chiral liquid crystals. Bulk chiral liquid crystals form cholesteric phases, with a helical twist in the molecular director, and thin films of chiral smectic-C liquid crystals form striped phases, with periodic arrays of defect lines.176 To determine whether tubules can form analogous structures, these authors generalize the free-energy of Eq. (5) to consider the expression... 

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

Figure 7.2 Helical arrangement of director in right-handed cholesteric phase.

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Several natural10 and synthetic (e.g., polyisocyanates11) polymers form lyotropic cholesterics with the appropriate solvent also micellar systems formed by amphiphilic molecules and water, if chirality is introduced by either using a chiral amphiphile or adding a chiral dopant, can give cholesteric phases.12... 

Recently, a promising theoretical treatment was introduced by Ferrarini et al.22 which, in selected cases, leads to the effective calculation of the helical sense and pitch of the induced cholesteric phases.23 Attempts to relate the cholesteric handedness of lyotropic cholesterics to the helical sense of the polymers were first reported by Sato and co-workers.11... 

Before attempting to develop any theory correlating molecular to cholesteric handedness, one must be completely sure of the experimental data. A cholesteric phase is fully described by its handedness and pitch, and often also knowledge of the pitch variations with temperature is fundamental. In particular, the determination of the handedness is quite a delicate matter. Before discussing the methods currently used to determine handedness and pitch, the principal textures of the cholesteric phase must be briefly reviewed The planar or Grandjean textures are obtained in thin cells by rubbing the cell walls (with... 

Some of these cholesteric systems are well-characterized The structure and handedness of the macromolecule is unequivocally known and so is the pitch and handedness of the cholesteric phase. A few attempts were made to correlate the polymer structure to the cholesteric handedness. 

Sq and negative Hq (Figure 7.12), so the handedness of the polymers are correlated to those of the cholesteric phases and the apparent discrepancy depends only on the comparison of the two results at a single temperature. Also in these cases the most common model of Figure 1.9a seems to be followed.46 It should be remarked that in all these cases, Hq and Sq have opposite signs.41... 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

For an electrostatic interpretation of the formation of the cholesteric phase of DNA, which does not give, however, a stereochemical correlation between the macromolecular and cholesteric handedness, see Komyshev, A. A. Leikin, S. Phys. Rev. Lett. 2000, 84. 

The CD reflection spectra are quite sharp at all temperatures, and the reflection wavelength, corresponding to the optical pitch of the TChLC phase, increased progressively with temperature from 500 nm at 70 °C to 1,000 nm at 140 °C. It was considered that the positive sign of the CD reflection band indicated M screw sense helicity of the cholesteric phase. Very recently, a smectic A-cholesteric phase transition was also observed for PDMBS.348... 

Figure 54 (a) CD reflection bands of PDMBS cholesteric phases at various temperatures (cooling run) and (b) UV and CD absorption spectra at 80°C.346 Reprinted with permission from Watanabe, J. Kamee, H. Fujiki, M. Polymer J. 2001,33, 495-497, 2001 Society of Polymer Science Japan. 

Navard and Haudin studied the thermal behavior of HPC mesophases (87.88) as did Werbowyj and Gray (2), Seurin et al. (Sp and, as noted above, Conio et al. (43). In summary, HjPC in H2O exhibits a unique phase behavior characterized by reversible transitions at constant temperatures above 40 C and at constant compositions when the HPC concentration is above ca. 40%. A definitive paper has been recently published by Fortin and Charlet ( who studied the phase-separation temperatures for aqueous solutions of HPC using carefully fractionated HPC samples. They showed the polymer-solvent interaction differs in tiie cholesteric phase (ordered molecular arrangement) from that in the isotropic phase (random molecular arrangement). 

The influence of molar mass on the cholesteric phases properties for the system CTC/dietliylene glycol monoetliyl ether was investigated by Siekmeyer and Zugenmaier (106). The pitch of the cholesteric... 

Quasi-nematic or compensated cholesteric phases were formed by CTC dissolved in mixtures of methylpropyl ketone (MPK) and DEME. CTC/MPK has a right-handed twist but CTC/DEME a left-handed one (109). Siekmeyer et al. (IIQ) studied the phase behavior of the ternary lyotropic system CTC/3-chlorophenylurethane/triethyleneglycol monoethyl ether. 

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