Abstract
Eight new classes of positive dielectric anisotropic liquid crystals with polar NCS and CN end groups, comprising alkyl as well as alkenyl side-chains are presented. The isothiocyanates (NCS compounds) exhibit low bulk viscosities η(22°C) ∼ 11 cP as well as low rotational viscosities γ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</inf> . The latter are up to 35 percent lower than those of structurally comparable CN compounds. We show that nematics with elastic ratios as low as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k_{3}/k_{1} = 0.6</tex> result when combining suitable heterocyclic rings with four alkenyls and/or NCS groups. These unusually low <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k_{3}/k_{1}</tex> ratios as well as the low elastic expression <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K = k_{1} + (k_{3} - 2k_{2})/4</tex> , combined with their large dielectric anisotropies Δε ∼ 20, render the compounds applicable for highly multiplexable low-threshold liquid-crystal displays. From numerical calculations, 160:1 multiplex ratios of TN-LCD's with a contrast of 5:1 appear feasible. While the new compounds reduce the low end of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k_{3}/k_{1}</tex> range of positive dielectric nematics by ∼40 percent, thus improving the multiplexibility of TN-LCD's considerably, it is shown that supertwist birefringence mixtures comprising three alkenyls exhibit rather large <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k_{3}/ k_{1}</tex> ratios. An SBE mixture is presented that leads to multiplexing ratios exceeding 400:1 at a contrast of at least 10:1 between 0° and 50°C without external temperature compensation. The in situ temperature compensation is achieved by novel cholesteric additives.
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