Cholesteric phase structure

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. 

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. 

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]. 

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... 

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. 

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... 

According to the helical structure, the cholesteric phase (n ) is optically uniaxial negative, where the ordinary refractive index n0 nt is larger than the extraordinary... 

The following table lists the liquid crystalline materials that are useful as gas chromatographic stationary phases in both packed and open tubular column applications. In each case, the name, structure, and transition temperatures are provided (where available), along with a description of the separations that have been done using these materials. The table has been divided into two sections. The first section contains information on phases that have either smectic or nematic phases or both, while the second section contains mesogens that have a cholesteric phase. It should be noted that each material may be used for separations other than those listed, but the listing contains the applications reported in the literature. 

The terminus of chirality induction is used for processes, in which the structural information of a chiral molecule is transferred to an initially achiral collective which then will form a superstructural chiral phase. One of the most prominent examples can be found in the field of liquid crystals The doping of a nematic LC phase with chiral mesogenes (dopants) can lead to a twisted, helical cholesteric phase. Noteworthy is the fact that the length scales of the chiral information that characterizes the involved species can differ by several orders of magnitude a few Angstrpms in a single chiral molecule, but the pitch of a helical cholesteric phase amounts typically a few microns. 

Supramolecular architectures are highly sensitive to chiral perturbations in general, and in systems that form liquid crystals in particular. Small amounts of enantiopure guest molecule added to a nematic host can induce a transition to a cholesteric phase, and the helical organization in the mesoscopic system is very sensitive to the structure of the guest molecule. Chiral amplification was successfully achieved in such liquid crystals, using CPL as the chiral trigger for the phase transition [183]. 

The liquid crystal melt, which comes into being at the glass-rubber transition or at the crystal-melt transition, may have several phase states (Mesophases) one or more smectic melt phases, a nematic phase and sometimes a chiral or cholesteric phase the final phase will be the isotropic liquid phase, if no previous decomposition takes place. All mesophase transitions are thermodynamically real first order effects, in contradistinction to the glass-rubber transition. A schematic representation of some characteristic liquid crystal phase structures is shown in Fig. 6.13, where also so-called columnar phases formed from disclike molecules is given. 

FIG. 6.13 Schematic representation of some characteristic liquid crystal phase structures. Nematic, smectic and cholesteric phases formed from rod-like molecules columnar phases formed from disc-like molecules (from Jansen, 1996). 

Figure 2.3 Schematic representation of the periodical helical structures of the chiral nematic (cholesteric) phase. The pitch of the helix corresponds to the rotation of the director through 360°. There is no layered structure in a chiral nematic. N. phase.

Cholestogenic compounds (i.e. such which produce cholesteric phases) are optically active, i.e. their molecules are chiral under equivalent conditions the enantiomers form countercurrent, but otherwise identical helical structures. The close relationship of cholesteric and nematic phases is emphasized by the fact that a racemic mixture of cholestogenic compounds does not form a mesophase but a nematic one (Leclercq et al., 1969). A nematic phase can also be formed by mixtures of non-enantiomeric cholesto-gens which tend to form oppositely coiled structures on their own. However, the ratio... 

It is possible to conceive of situations where the chemical linking of molecular components around a template is not as crucial as the formation of defined, non-covalent interactions during templating. This may be exemplified by the polymerisation of a nematic liquid crystalline crosslinker in the presence of a template, a non-polymerisable cholesteric mesogen [23]. The chiral dopant forces the crosslinker to form a cholesteric phase. After polymerisation of the crosslinker, the polymer still exhibits a helicoidal structure which is stable over a wider temperature range than the initial cholesteric phase. It is not reported in this work whether extraction of the chiral mesogen has been attempted or not. 

Unfortunately, there is no report on the detailed physical characterization of these polymers. Such information as unidirectional twist angle and form optical rotation, as well as their dependence on chemical structures and temperature, can be very useful in further understanding the molecular orientations of the polymers in the cholesteric phase. In contrast, a number of studies have been made on the physical-chemical properties of cholesteric lyotropic polymer systems, especially polypeptides. 

Hgure 4.33 Representation of the local structure of some chiral mesophases on heating. The matches describe the relative orientations of chiral molecules in space. As the temperature is raised, the system transforms from a crystalline phase (left) to a cholesteric phase (centre) characterised by a single twist, to a double-twist blue" phase (right). 

A molecule with three cholesterol units and one biphenyl unit according to Scheme 1 connected to a tetrasiloxane shows an elongated structure (Fig. 3). Molecules of this shape can be arranged in nematic, smectic, or cholesteric phases. 

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