Liquid crystals are used primarily its dielectrics in indicating devices, since the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to a person skilled in the art and can be based on various effects. Such devices are, for example, cells having dynamic scattering, DAP cells (deformation of aligned phases), guest/host cells, TN cells having a twisted nematic structure, STN cells ("super twisted nematic"), SBE cells ("super birefringence effect") and OMI cells ("optical mode interference"). For displays having a high content of information multiplexed, especially the actively controlled, e.g. TFT cells ("thin film transistor"), have recently become important besides the passively controlled. The most common indicating devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid crystal materials must have a good chemical, photochemical and thermal stability and must have a good stability towards electric fields. Further, they should have a suitable mesophase over a range which is as broad as possible (for example a nematic or a cholesteric phase for the cells mentioned above), but nevertheless should have a sufficiently low viscosity and in the cells should permit short response times, low threshold potentials and a high contrast. Further properties such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy must fulfil different requirements depending on the field of use and type of cell. For example, materials for cells having a twisted nematic structure should have a positive dielectric anisotropy which is as high as possible and at the same time should have a conductivity which is as low as possible. This latter property is above all of particular importance for TFT cells. Unfortunately, however, components having high dielectric anisotropy lead mainly to an increased conductivity in mixtures because of their improved solubility capacity for ionic impurities. Therefore, components which have a dielectric anisotropy which is as high as possible with at the same time a conductivity which is as low as possible are sought after.