Twisted nematics/cholesterics

Fig. 5.17. The helicoidal director field of the twisted nematic (cholesteric) organization is illustrated. The very small unidirectional twist from chiral mesogen to chiral mesogen causes the spiral supramolecular organization of the director that turns through 180° after traversing a distance P/2 (half of the cholesteric pitch).

Figure 6.2 Discotic molecules in a (a) nematic state ND, (b) twisted nematic discotic state Np (P/2 is half of cholesteric pitch), (c) columnar state, ordered D0 and disordered Dd, (d) hexagonal ordered columnar state Dho two-dimensional packing arrays for columnar structures in (e) hexagonal, Colh rectangular, Colr oblique, Col0b.

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... 

Research Focus Method for preparing polymerizable chiral intermediates containing optically active isosorbide to prepare twisted nematic or cholesteric phases. 

As the cholesteric phase is a twisted nematic phase, the orientational long range order of the mesogenic molecules, characterized by Eq. (3) ... 

That both phenomena arise as a consequence of macroscopic solvent order and not Intimate solvent-solute Interactions Is clear Saeva and 01In (75) have shown that solute LCICD spectra can be observed In twisted nematic phases only Nakazaki et al. (76) find an excess of one enantiomer of hexahelicene Is produced photochemlcally from achiral precursors In twisted nematic phases no LCICD spectra or optical Induction occurs In untwisted nematic phases and the handedness of the twist can be correlated with the sign of the LCICD and the preferred product enantiomer. Furthermore, Isotropic phases of cholesteric mixtures display no discernible LCICD spectra (12, 67) and the enantiomeric excesses In products of photolablle reactants In Isotropic phases are near zero (51). 

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. 

While the vast majority of studies on chiral induction were mainly concerned with the induction of the chiral (twisted) nematic or cholesteric phase, more recently induction of the smectic C phase in the smectic C has come to the fore, with a special emphasis on the way chirality is transferred between molecules [ 115]. It should also be noted that comparison of the chiral induction phenomena in the two types of LC phase and in other media can provide useful information concerning mechanisms of transfer and amplification of stereochemical information [116]. 

Fig. I. Schematic diagrams of the contrast versus the applied field for (a) a twisted nematic, (b) a cholesteric guest-host bistable display (Wysocki ei al., 1972 Ohtsuka and Sukamoto, 1973), and (c) a bistable LCD (Boyd et al., 1982).

Twisting a nematic structure around an axis perpendicular to the average orientation of the preferred molecular axes, one arrives at the molecular arrangement commonly called cholesteric (Kelker and Hatz, 1980). The twisted nematic phase is optically uniaxial, however with the axis perpendicular to the (rotating) director. Such a mesophase combines the basic properties of nematics with the implications of chirality The structure itself is chiral and as a consequence, a non-identical mirror image exists as it is shown schematically in Fig. 4.6-7. Besides the order parameters mentioned before, the essential characteristics of a cholesteric mesophase are the pitch, i.e., the period of the helical structure as measured along the twist axis, and its handedness, i.e., whether the phase is twisted clockwise or anticlockwise. 

Such twisted nematic phases are called induced cholesteric solutions and - as schematically outlined in Fig. 4.6-9 - enantiomers cause countercurrently twisted structures. As discussed by Korte and Schrader (1981) this effect offers the potential of sensitively characterizing the chirality of small amounts of optically active compounds. There are no restrictions as to the type of chirality, and the experiments can advantageously be based on infrared spectroscopy. The application of induced cholesteric solutions was later reviewed again by Solladie and Zimmermann (1984). The host phase is the more twisted the more of the optically active guest compound is dissolved. Quantifying the twist by the inverse pitch z and the concentration by the molar fraction x, the ability of a chiral. solute to twist a given nematic host phase is characterized by the helical twisting power (HTP Baessler and Labes, 1970). For small values of a this quantity P is defined by the relation... 

A lyotropic, nematic solution of cellulose was formed in a NH3/NH4SCN solvent in what are presumably good solvent compositions. Evidence strongly suggests that the twisted nematic or cholesteric structure that results when solutions of chiral cellulose chains interact may be repressed or compensated so that interactions among chiral centers are minimized. Our reasoning is based a body of experimental evidence which includes ... 

In general, cholesteric liquid crystals are found in optically active (chiral) mesogenic materials. Nematic liquid crystals containing optically active compounds show cholesteric liquid crystalline behavior. Mixtures of right-handed and left-handed cholesteric liquid crystals at an adequate proportion give nematic liquid crystals. From these results cholesteric liquid crystals are sometimes classified into nematic liquid crystals as twisted nematics . On the other hand, cholesteric liquid crystals form batonnet and terrace-like droplets on cooling from isotropic liquids. These behaviors are characteristic of smectic liquid crystals. Furthermore, cholesteric liquid crystals correspond to optically negative mono-axial crystals, different from nematic... 

The cholesteric liquid, which is a spontaneously twisted nematic, behaves like a negative uniaxial crystal, so that light vibrating perpendicular to the molecular layers shows maximum velocity. Linearly polarized light transmitted perpendicular to the molecular layers shows rotation of its electrical vector along a helical path. 

Fig. 3.5.24. Possible helical configurations of disclination pairs in twisted nematics or cholesterics of large pitch (see figs. 4.2.2 and 4.2.3).

We now consider defect structures in the cholesteric liquid crystal. Treating the cholesteric as a spontaneously twisted nematic,... 

Fig. 4.5.5. Theoretical variation of the apparent viscosity with pitch P = 2n/q for flow normal to the helical axis of a cholesteric (or twist nematic) at low shear rates. Plot of versus P for twisted PAA. The separation between the...

The nematic phase (N, ) is exhibited by relatively few compounds examples are hexakis((4-octylphenyl)ethynyl)benzene (fig. 6.1.1(A)) and the hexa-n-alkyl and alkoxybenzoates of triphenylene (fig. 6.1.1(e)). The Nd phase has an orientationally ordered arrangement of the discs with no long-range translational order (fig. 6.1.2(f)). Unlike the usual nematic of rod-like molecules, is optically negative, the director n now representing the preferred axis of orientation of the disc normal. The properties of this phase will be discussed in greater detail in 6.5. A twisted nematic (or cholesteric) phase, with the helical axis normal to the director, has also been identified. ... 

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