Synergisms, structure-material relations and display performance of novel fluorinated alkenyl liquid crystals

Abstract

Several new classes of low polarity, low viscosity and optically weakly anisotropic two and three ring fluorinated liquid crystals with alkenyl side chains are presented. Synergisms which lead to a number of favourable material properties are shown to result from systematic changes of (a) para and lateral fluorination of the benzene ring in the rigid cores, (b) variations of double bond positions in the alkenyl side chains and (c) from replacing cyclohexane core rings by dioxane rings. A broad range of dielectric anisotropies −0.3 < Δ∊ < 13, broad nematic meso-phases, low rotational, λ and bulk viscosities, n, (22°C) < 10cP as well as visco elastic ratios as low as λ/k (22°C) = 3.8 × 10–12m-12s-1 follow; were K = K11 + (K33 - 2K22)/4. As a consequence a wide range of optical threshold voltages V10 and short response times result in field-effect liquid crystal displays. The low ratios κ/Δ∊ of the heterocyclic representatives among the new compounds lead in twisted nematic displays to threshold voltages which are comparably as low as those of (polar) phenylcyclohexane compounds. In agreement with our earlier results pronounced odd-even effects are shown to result from shifting the aklenyl double bond in the new compounds from even to odd side chain sites. This causes Δ∊ to increase by 40 per cent and γ₁ by 33 per cent. Both odd-even effects are shown to be due qualitatively to the increase of side chain order parameter which is expected from shifting the alkenyl double bond from even into odd sites.