Structural and electro-optical investigations of the smectic phase of chlorine-substituted banana-shaped compounds

Six members of a new homologous series of achiral banana-shaped molecules have been synthesized and studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy and X-ray diffraction. According to X-ray diffraction measurements on oriented samples, four homologues form an X-B2 phase which exhibits antiferroelectric switching behaviour. From electro-optical studies the spontaneous polarization and the tilt angle could be measured. An orientational order parameter of 0.8 was determined by C-13 NMR and this is nearly independent of the temperature. NMR investigations also give information about the real conformation of the molecules in the X-B2 phase. Dielectric measurements indicate that the rotation around the molecular long axis is clearly hindered because of the packing of the bent molecules within the smectic layers.