Four-ring Liquid Crystals Research Articles

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 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 fco values in the range of 1–10 kHz at 20°C, have been prepared and investigated. The cross-over frequency fco is close to the relaxation frequency fr. The discussion refers to relations among the relaxation frequency, compound structure and viscosity of a liquid crystal medium.

Synthesis and mesomorphic properties of four-ring liquid crystals containing 2,3-difluorophenyl and 1,3-dioxane units

Two series of novel liquid crystalline materials were synthesized: 2-(2,3-difluoro-4-alkoxyphenyl)-5-[4-(4-alkylcyclohexyl)phenyl]-1,3-dioxanes (A m-n and 2-(2,3-difluoro-4-alkylphenyl)-5-[4-(4-alkylcyclohexyl)phenyl]-1,3-dioxanes (B m-n ). Their mesmorphic properties were characterized by polarizing optical microscopy and differential scanning calorimetry. All these compounds exhibit broad mesomorphic phases, and most of them show wide smectic C phases and the phase sequence Cr-SmC-SmA-N-I. The relationship between properties and chemical structures is discussed in detail.

Synthesis and mesomorphic properties of four-ring liquid crystals containing cyclohexyl, phenyl and pyridyl units

Two series of chiral and achiral 4-[(3-n-alkoxycarbonylpyridyl)-6-ethynyl]phenyl 4-(trans-4-n-alkylcyclohexyl)benzoate liquid crystals have been synthesized. Their mesomorphic properties were observed and measured by polarizing optical microscopy and DSC. A broad chiral smectic C phase was observed when terminal chains contained a chiral centre.

Synthesis and mesomorphic properties of three- and four-ring liquid crystals containing cyclohexyl, phenyl and pyridyl units

Two series of 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and 4-(3-alkoxycarbonylpyridyl-6-ethynyl)phenyl 4-(trans-4-n-alkylcyclohexyl)benzoates have been prepared. Their mesmorphic properties were observed and measured by polarizing optical microscopy and differential scanning calorimetry. The relationship between structures and properties is discussed, showing that the ferroelectric phase can be observed when chiral terminal chains exist.