Cyanobiphenyls displays

Correlations between molecular structure and the LC state have been fruitfully employed to design 4-alkyl and 4-alkoxy-4 -cyanobiphenyls, which form the basis for electrooptic display devices. The reasoning behind the design of these nematogens is as follows. In a family of nematogens of the general formula... 

The compounds which first found applications in display devices were the 4-alkyl-4 -cyanobiphenyls 17 and the 4-alkoxy-4 -cyanobiphenyls 18 (Gray and colleagues, 1973) . It has subsequently proved possible to incorporate other structural features and thereby modify the liquid crystal characteristics in interesting and sometimes useful ways. For... 

Figure 24. Biomesogenic structures a) (Bio)meso-gens displaying order-disorder distributions in CPK-presentation (left to right and top to bottom) hexa-n-alkanoyl-oxybenzene discoid - Chandrasekar s first non-rodlike liquid crystal [28 a, 51c] enantiomeric cholesteric estradiol- and estrone-derivatives [ 17 a, c, d, 26 f, 51 a, s, u] Reinitzer s cholesterolbenzoate [21, 22] - together with the acetate the foundation stones of liquid crystal history [21, 22] Kelker s MBBA -first liquid crystal fluid at ambient temperature [ 13 f, g] Gray s cyanobiphenyl nematics for electrooptic displays [25 a, 51 e] lyotropic lecithin membrane component [7 a, 14, 27 d, 52 a] and valinomycin-K -membrane carrier [7 a, 35] thermotropic cholesteryl-side-chain-modifiedpolysiloxanes with the combination of flexible main-chain and side-chain spacers [51 a, h] thermotropic azoxybenzene polymers with flexible main-chain spacers [51a] thermotropic cya-...

At the present time, as shown by the pocket calculator, materials are available with adequate electro-optic performance where the duty cycle is 1 in 2, 3 or 4. Where temperature compensation is used, 1 in 7 duty cycle is possible O with mixtures of cyanobiphenyls and benzoate esters. For a duty cycle of 1 in 10, temperature compensation is a must and the electro-optic performance is determined principally by the threshold sharpness. By way of example. Fig. 36 shows a viewing cone plot for ZLl 1253 (dotted line curves) compared with a sharper threshold material (1.65) (solid line curves) in an 8y, low tilt cell. By single frequency addressing, a 1 in 10 duty cycle is approaching the maximum for an acceptable performance in a twisted nematic display. 

Since the cyanobiphenyl derivatives had proved to be excellent materials for electro-optic displays, equivalent homopolymer poly-siloxane liquid crystals, based on these low molar mass mesogens, were synthesized. Their properties are summarized in Table 1. Since the homopolymers only exhibited smectic phases, copolymers were prepared using destabilizing 2-methyl-substituted ester side-groups. As shown in Table 2, at sufficiently high ester-group concentrations, a... 

Cyanobiphenyl liquid crystals were reported in the early 1970s by Gray and coworkers and were responsible for the conunerciaUzation of liquid crystal displays. 

Mixtures containing cyanobiphenyl liquid crystals are widely used in twisted nematic (TN) liquid crystal displays. They commonly contain 5CB (Fig. 5.1), for which the elastic constants have been measured. At 25°C,... 

Going back to the earlier 1970s, with the advent of the cyanobiphenyls and later the cyclohexane and pyrimidine analogs, not only was the display device industry provided with a wealth of novel, useful materials, but also those in fundamental research were given a range of stable, room temperature liquid crystal materials for study by a growing panoply of experimental and theoretical... 

This theoretical prediction has been confirmed by detailed X-ray diffraction studies which also enable accurate measurements of 7 [11]. Figure 2 displays the X-ray diffraction intensity liq ) as a function of the scattering wave vector q obtained at two temperatures in the SmA phase of 80CB [4 - -octyloxy-4-cyanobiphenyl]. The quasi-long-range smectic layer order gives the... 

A few examples of nematogens. Cr means crystal, 1 stands for isotropic, and N denotes the nematic phase. PAA, MBBA, and EBBA were the most studied materials until the appearance of the cyanobiphenyls, which are mostly used in displays due to their increased stability (an example is shown in row 2). The bottom row shows an example of the rigid molecules without flexible end chains. This and similar materials have nematic phase only at very high temperatures. 

Cyanobiphenyl compounds are well-known for their widespread practical utility in displays. The data in TABLE 2 are based on pVT measurements which provide estimates of the volume discontinuity at the nematic-isotropic transition as well as the pressure and temperature points of phase equilibrium. The general range of AV is from 0.2 to 1.0 cm /mol. The fitting constants presented in TABLE 3 were evaluated by a least squares fit to the second order polynomial defined above. The constant term is approximately the room pressure transition temperature. The data fit the polynomial well showing a mild curvature which increases at higher pressures. No obvious trends are apparent from the magnitudes and signs of the parameters. 

Before going on to discuss briefly other basic liquid crystal phases, it is worth recording some points about the development of nematic liquid crystals, since a substantial part of this book concerns these substances. The first nematic liquid crystal, p-azoxyanisole (PAA), was synthesised by Gattermann and Ritschke [102], who reported their results in 1890 it was the first liquid crystal not based on a naturally occurring substance and went on to be extensively investigated by Lehmann and others. The first relatively stable room temperature nematic liquid crystal, 4-methoxybenzylidene-4 -butylaniline (MBBA), was synthesised by Kelker and Scheurle [142] in 1969, but, for various reasons, this substance was not considered suitable for some applications. The nematic liquid crystal 4-pentyl-4 -cyanobiphenyl (5CB) is an example of a stable room temperature material designed for use in twisted nematic displays (discussed in Section 3.7) and was synthesised by Gray and co-workers in the early 1970 some details can be found in articles [117, 118). 

Figure 11.1 Proposed supramolecular organization of 3 within the smectic A phase. The interdigitation is illustrated by the red and green cyanobiphenyl units the red units belong to the dendrimer that is displayed on the drawing, and the green units belong to dendrimers of adjacent layers. Compounds 2 and 4 show a similar supramolecular organization.

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