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-difluorophenylene moiety has been developed. The new materials are characterized by high negative A& values of up to -6 , and viscosities comparable with the nonfluorinated compounds. The introduction of the two fluorine atoms also leads to an increase in K,3/K,i. 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 birefringent, supertwisted nematic and ferroelectric liquid crystal displays.

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It is demonstrated by model calculations and experimental results, that the range of viewing angles of a twisted nematic (TN) display can be considerably improved by using a liquid-crystal material with a low refractive index anisotropy Dn<0.1.

Active matrix liquid crystal displays belong to one of the fastest growing fields in display research and development. Several pocket TVs using this technology are already available commercially. High quality displays for TV and instrumentation require an understanding and optimization of liquid crystal material parameters. We have, therefore, investigated the influence of the elastic, dielectric and optic properties on the electrical resistance of the liquid crystal as well as on the response times and the viewing angle dependence of active matrix displays. Based on correlations derived from our investigations, we have developed new liquid crystals which have suitable properties and a very high electrical resistivity.