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

Abstract

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 A P 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.

Three Distinct Polar Phases, Isotropic, Nematic, and Smectic-A Phases, Formed from a Fluoro-Substituted Dimeric Molecule with Large Dipole Moment

“…On the other hand, f r decreases with the decreasing temperature in the N F and SmAP F phases, both of which can be considered to be a thermally activated phenomenon like the standard Arrhenius one. , The Arrhenius plots reveal good linear relationships (see Figure S4), giving activation energies of 13 and 45 kJ mol –1 for the N F and SmAP F phases, respectively. These values are comparable with those obtained for other bent-shaped molecules. − ,− …”

“…Furthermore, Nishikawa et al also reported that DIO with fluorine possesses a large dipole moment of 9.4 D, resulting in a high dielectric constant of 10,000 and a high P s of 4.4 μC cm –2 . Similar results have been reported in fluorine-substituted bent-shaped molecules; an increase in the number of aromatic rings and fluorine substitution was found to result in an increase in the dipole moment. − Based on this strategy, a di-5­(3FM-C4) molecule with 3 benzene rings having one fluorine substitution in each is prepared. The dipole moment of its one-side mesogenic core is estimated as 11.2 D (see Figure ), which is relatively larger than those previously reported …”

“…It is known that the value of Δε for the collective mode increases with the cell thickness. 43,44,46,48 When it is compared with cells having the same cell thickness, Δε of 8000 in 3 μm cell is larger than those reported for bent-shaped [42][43][44]47,48 and rod-like molecules. 25,28,30−32 Figure 11 shows the temperature dependence of Δε and f r , respectively.…”

“…On cooling from the N F phase, the SmAP F phase appears as a homogeneous fan-shaped texture (see Figure c,f), which is well developed regardless of the homeotropic and homogeneous orientation in the preceding N F phase. This preferential homogeneous alignment suggests that the molecules in this phase assume the bent-shaped conformation. − …”

“…On field-ON, neither of the fan shapes of the texture nor the extinction positions change, but only the birefringence colors change from yellow to red. This characteristic texture change is attributable to the biaxial nature of the SmAP F phase composed of the bent-shaped molecules: initially, both the molecular axis and bent (polar) direction of the bent molecules lie parallel to the cell’s surface, while on field-On, the bent direction becomes perpendicular to the cell. − In this SmAP F phase, the E c is increased from 1 to 3.5 Vμm –1 with a decrease of the temperature from 160 to 100 °C, which can be considered to be a thermally activated phenomenon . If it is treated as having an Arrhenius-like thermal activation, the pseudo-activation energy can be obtained as 28 kJ mol –1 .…”