The cooperative effects of heterocycles, NCS-polar groups, and double bonds on the material properties of new nematic liquid crystals

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
1
. 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
k_{3}/k_{1} = 0.6
result when combining suitable heterocyclic rings with four alkenyls and/or NCS groups. These unusually low
k_{3}/k_{1}
ratios as well as the low elastic expression
K = k_{1} + (k_{3} - 2k_{2})/4
, 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
k_{3}/k_{1}
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
k_{3}/ k_{1}
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.