Spontaneous Polarization Characteristics in Polar Smectic Phases of Fluoro-Substituted Bent-Shaped Dimeric Molecules

Three kinds of bent-shaped dimeric molecules are synthesized by fluorine substitution of C16 molecules, and influences of the substitution on the polar smectic phases are examined. The fluorine-substituted C16 molecules form the SmAP_F and SmC_AP_A phases. The transition temperatures decrease by 20-30 °C without significantly changing the temperature span of the smectic phase, and the switching rates to the ferroelectric state become 5-10 μs, which are fairly shorter than 250 μs of C16. These behaviors are considered to be caused by the decrease in the intermolecular force and the decrease in the viscosity. The anchoring behavior also appears to be different. On the indium tin oxide (ITO)-coated cell, the fluorine-substituted molecules are homogeneously aligned with the bent (polar) axes perpendicular to the surface, while the bent axes of ordinary bent-shaped molecules lie parallel to the surface. This may be attributable to the repulsion between the fluorine and ITO electrodes. Further, the fluorine substitution can increase the dipole moment of the molecule. The largest dipole moment obtained is 7.94 D, and this leads to a huge reversal polarization of 2.42 μC cm^-2, which is much higher compared to those reported in the bent-shaped molecules.