Compounds with low relaxation frequency and dual frequency mixtures useful for active matrix addressing

Dozens of three- and four-ring liquid crystal derivatives of alkylcyclohexyltolanes, alkylbicyclohexyltolanes and alkylbicyclohexylbiphenyls with a terminal polar group F or trifluoromethoxy (OCF3), cyano (CN), isothiocyanato (NCS) have been synthesised. Their dielectric properties upon temperature and electric field frequency f have been investigated. Compounds dissolved in a negative dielectric medium exhibit strong dispersion of the parallel permittivity E-||(f) falling on the kilohertz frequencies with practically constant value of the perpendicular permittivity E-perpendicular to in this frequency range. This causes a change of sign of the dielectric anisotropy from positive at low frequencies to negative at high frequencies. Many so-called dual frequency mixtures, convenient for active matrix addressing, with the f(co) values in the range of 1-10kHz at 20 degrees C, have been prepared and investigated. The cross-over frequency f(co) is close to the relaxation frequency f(r). The discussion refers to relations among the relaxation frequency, compound structure and viscosity of a liquid crystal medium.