Thermotropic MIDA Boronates as a Case Study for the Role of Dipolar Interactions in Liquid Crystalline Self-Assembly

Liquid crystalline self-assembly of mixtures of rod- and wedge-shaped MIDA boronates

Rod-like MIDA boronates form smectic mesophases, while wedge-shaped MIDA boronates self-assemble into columnar mesophases. However, the phase behavior of mixtures is less understood. In order to obtain further insight on the molecular self-assembly of MIDA boronate mixtures two series of binary mixtures of rod-like and wedge-shaped mesogens were prepared. The phase behavior was studied using differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). The study revealed a strong dependency of the mesophase structure on the mesogen composition. Usage of a less bulky columnar mesogen suppressed the formation of columnar mesophases in the mixture and led to a decrease in melting and clearing temperatures. The phase behavior is discussed in terms of the packing parameter model typically applied for lyotropic liquid crystals.

Fluorenone imidazolium salts as novel de Vries materials

In ionic liquid crystals (ILCs) tilted mesophases such as SmC required for electro-optic devices are quite rare. We report a design concept that induced the SmC phase and enabled de Vries-like behaviour in ILCs. For this purpose, we synthesized and characterized a library of ILC derivatives ImR(On,Ym)X which consist of a rigid central fluorenone core containing an alkoxy or thioether side chain and connected via a flexible spacer to an imidazolium head group. The mesomorphic properties were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). Temperature-dependent measurements of smectic layer spacing d by small-angle X-ray scattering (SAXS) and of optical tilt angles by POM demonstrate that ILCs ImR(On,Ym)X undergo SmA–SmC phase transitions with maximum layer contraction values between 0.4% and 2.1%. The lowest reduction factor R of 0.2 at the reduced temperature T − T_AC = −10 K was calculated for Im(O12,S14)Br. Electron density calculations indicated a bilayer structure. Furthermore, temperature dependent emission studies show that self-assembling has a strong influence on the emission intensity of these ILCs.

ILCs consisting of cationic head group–spacer–fluorenone central core–side chain show de Vries-like behaviour.

Large Electro-Optic Kerr Effect in Ionic Liquid Crystals: Connecting Features of Liquid Crystals and Polyelectrolytes

The electro-optic Kerr effect in simple dipolar fluids such as nitrobenzene has been widely applied in electro-optical phase modulators and light shutters. In 2005, the discovery of the large Kerr effect in liquid-crystalline blue phases (Y. Hisakado et al., Adv. Mater. 2005, 17, 96-98.) gave new directions to the search for advanced Kerr effect materials. Even though the Kerr effect is present in all transparent and optically isotropic media, it is well known that the effect can be anomalously large in complex fluids, namely in the isotropic phase of liquid crystals or in polyelectrolyte solutions. Herein, it is shown that the Kerr effect in the isotropic phase of ionic liquid crystals combines the effective counterion polarization mechanism found in polyelectrolytes and the unique pretransitional growth of the Kerr constant found in the isotropic phase of nematic liquid crystals. Maximum Kerr constants in the order of several 10^-11  m V^-2 (ten times higher than the Kerr constant of the toxic nitrobenzene and less temperature sensitive than Kerr constants of nematic liquid crystals) make ionic liquid crystals attractive as new class of functional materials in low-speed Kerr effect applications.