Applications twist effects

This effect has been observed experimentally in comparatively thick cells (d 50 /xm) [113]. In cells with d 20 /xm, the final twisted state (in the field) proves to be insufficiently stable and the nematic liquid crystal layer is gradually transformed into a planar structure. The addition of small quantities of cholesteric liquid crystals to the initial nematic mixture enables a stable twisted structure to be achieved with the application of a field and improves the electrooptical characteristics of the device. The electrooptical response of electrically induced twist nematic cells includes intensity oscillations observed both in the switching on and switching off regimes [114]. These oscillations take place due to the variation of birefringence, which are not important in the usual twist effect. 

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... 

Simple application of either the Brown or the Y-T equation to the substituent effects on p/tR+ for a,a-diphenylethylenes [31(X,Y)] (in Table 9) is not necessarily successful (Goethals et ai, 1978). The difficulties arise mainly from stereochemical factors the RHF/6-31G optimization of the 1,1-diphenylethyl carbocation [31C ] provides a propeller geometry shown in Fig. 17. The phenyl rings are twisted from the plane of the sp carbocation centre by 23.4° and 34.0° (Fujio et ai, unpublished). This difference is probably due to the steric non-equivalence of the methyl group. 

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... 

The principles of Twisted Intramolecular Charge Transfer (TICT) formation are outlined and many examples given. The most recent developments in the understanding of these states both from the theoretical and the phenomenological viewpoint are reviewed. Emphasis is given to the effect of broadened angular distributions and to the possibility of multiple TICT channels. Application possibilities with respect to dye developments and fluorescent probes are outlined as well as various ways of intermolecular reactivity of TICT molecules. 

The combination of properties exhibited by these fluids, and their effectiveness at very low lubricant add-on levels, make them particularly effective as finishes or finish ingredients for texturing and draw-twisting. They are also effective for knitting operations. Specific applications include ... 

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