Volker Reiffenrath

New liquid-crystalline compounds with negative dielectric anisotropy

Abstract Until now, liquid-crystalline compounds with high negative dielectric anisotropy were usually realized by a lateral cyano group. A drawback to these cyano substituted liquid crystals, namely the considerable increase in the viscosity and the reduction of the thermodynamic stability of the mesophase, has been circumvented by preparing 2,3-difluorobenzene derivatives. A universal method to prepare a variety of classes of liquid-crystalline compounds containing the 2,3-difluoro-phenylene moiety has been developed. The new materials are characterized by high negative Δϵ values of up to -6, and viscosities comparable with the non-fluorinated compounds. The introduction of the two fluorine atoms also leads to an increase in K 33/K 11. They also suppress higher ordered smectic phases and transform SA into Sc phases. The new compound classes are promising materials for liquid crystal mixtures for various applications as electrically controlled bire-fringent, supertwisted nematic and ferroelectric liquid ...

Physical properties of diphenyldiacetylenic liquid crystals

Optical and electro‐optic properties of two highly conjugated diphenyldiacetylenic liquid crystals (LCs) [1.4‐bis‐(4‐propylphenyl)‐butadiyne, abbreviated as PTTP‐mm] and their eutectic mixture were characterized in the nematic range. These LCs are found to exhibit extraordinarily high birefringence and very low viscoelastic coefficient. Their figure of merit is significantly higher than that of the widely used commercial LC mixture BDH‐E7. Potential application of PTTP‐mm in modulating infrared radiation is addressed.

Improved liquid crystals for active-matrix displays using high-tilt-orientation layers

Liquid crystal mixtures containing new Super Fluorinated Materials as highly dielectrically positive compounds have been developed. The new experimental LC mixtures have threshold values as low as 1.42 V in Viewing-angle Independent Panels, utilizing TN cells with an optical retardation of 0.5 micrometers . Broad nematic phase ranges from -40 to +100 and even +110 degree(s)C have been realized with threshold voltages of 1.8 V and below. Only a small increase of the flow viscosity had to be accepted. Also experimental LC mixtures with higher birefringence values suitable for displays with an optical retardation of 1.0 to 1.1 micrometers , having threshold values of about 2.3 V, have been developed. The surface tilt angles of the new liquid crystal mixtures have been investigated on several commercially available, as well as on new experimental polyimide orientation layers. All polyimides investigated were pre-imidized materials and showed suitable voltage holding ratios for AMDs. On all different alignment layers investigated here, the new liquid crystals lead to increased surface tilt angles, compared to the SFM mixtures available so far. The temperature dependent decrease of the surface tilt angle of the LCs on these new polyimides has also been investigated. Especially for the new LCs optimized for high surface tilt angles the temperature dependence of the surface tilt angle is stronger than for the same LC on high tilt orientation layers for STN application. Nevertheless, even at a temperature of 60 degree(s)C tilt angles of about 2 degree(s) have been obtained. This helps to reduce the occurrence of reverse tilt domains in AM displays. The strong temperature dependence of the tilt angle, found for some combinations of liquid crystals and orientation layers, can compensate the temperature dependence of the threshold voltage in the range from -20 to +40 degree(s)C.