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

Abstract

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 ϵ_||(f) falling on the kilohertz frequencies with practically constant value of the perpendicular permittivity ϵ_⊥ 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 1–10 kHz at 20°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.

Keywords:

• nematic liquid crystal
• dual frequency mixture
• relaxation frequency
• cross-over frequency

Acknowledgements

The work was carried out under Polish Ministry of Sciences and Higher Education, Key Project POIG.01.03.01-14-016/08 ‘New Photonic Materials and their Advanced Applications’, Polish Ministry of Sciences and Higher Education OR-000-144-12, and PBS 847.