Nematic ordering and crystal structure of liquid crystals containing a 4-chlorophenyldifluoroethylene unit

Partially fluorinated liquid crystals have many desirable properties such a s high impedance and low viscosity. The compounds trans-1-(trans-4-alkylcyc lohexyl)-2-(4-chlorophenyl)-difluoroethylene have very low viscosities and reasonable nematic ranges, and are very suitable for electro-optic displays . The crystal structure of a compound in this homologous series has been de termined by X-ray diffraction. The orientational ordering for this compound and for another homologue in their nematic phases has been studied by the use of one and two dimensional C-13 NMR. The proton-decoupled C-13 spectra are complex due to splittings from the two F-19 nuclei, but they have been analysed by combining chemical shift anisotropy and variable angle spinning studies. Carbon-proton and carbon-fluorine dipolar coupling constants were obtained, and the order parameters for the two conjugated molecular segmen ts (the chlorophenyl ring and the difluoroethylenic link) were calculated f rom the dipolar coupling constants. The temperature dependences of the loca l order parameters of each fragment were studied.