Twist-Effect

If the directions x and y of the planar orientation of nematic liquid crystal molecules on opposite electrodes are perpendicular to each other and the material has a positive dielectric (or diamagnetic) anisotropy, then. 

When the applied field exceeds a certain threshold value [280] 

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Fig. 24. Three principal types of orientational effects induced by electric (E) and magnetic (H) fields in nematic low molecular liquid crystals. At the top of the figure the initial geometries of molecules are shown. Below the different variants of the Frederiks transition — splay-, bend- and twist-effects are represented...

Fig. 2.22. Non-planarity of trityl cations compromise between maximum benzylic resonance (planarizing effect) and minimum aryl/aryl repulsion (twisting effect).

In order to avoid conjugation and twist effects, for this comparison two compounds having aliphatic substituents only have been selected, namely 2,5 -dimethyl-1,6,6aA-trithiapentalene and compound (8b) of Table 4. For the first of these compounds, the order of S—S bonds is 0.5 and the bond length (Table 1) is 2.358 A. For (8b), the order of bonds a-b and d-e is 0.67 and the bond lengths are 2.183 and 2.172 A, respectively the order of the bonds b-c and c-d is 0.33 and the bond lengths are 2.580 and 2.583 A, respectively. It appears that there is a rather good correlation between this assumed bond order and the observed bond length ... 

Figure 54. Correlation diagram depicting the dithiolene ligand twist effects on the energy of the metal orbitals of M(dithiolene)3 complexes. [Adapted from (413).]...

Fig. 9.7 Geometry of the twist effect that may be used for observation of director relaxation and determination of viscosity coefficient yi. Dash line shows the distortion profile In the magnetic field H,...

The electrooptical (or magnetooptical) effect corresponding to the geometry in Fig. 4.4(c) is called the T-effect. It is caused by a pure twist deformation and is most amenable to theoretical analysis. Unfortunately, it is less suitable for experimental investigation, since the field induces optical biaxiality in the nematic liquid crystal. The effect, for which the relaxation kinetics were considered in Section 4.1.6, must not be confused with the twist effect which includes combined T, S, and B deformations (to be discussed in Section 4.2). 

The typical transmission-voltage curve (TVC) of the twist effect for normal light incidence is shown in Fig. 4.12 for the twist cell placed between parallel polaroids. As mentioned above, the deformation of the director, measured by birefringence [66] or capacity [73], begins for smaller voltages U = C/tw then visible variations of the twist cell transmission. In view of the Mauguin requirement (4.53), the optical threshold of the twist effect decreases for the smaller values of the cell thickness [74] and the optical anisotropy of the liquid crystal as well as for director pretilt on the boundaries [76]... 

Matrix Addressed Displays and Multiplexing Capability of Twist-Effect Materials... 

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