Khoa V. Le

Stable amorphous blue phase of bent-core nematic liquid crystals doped with a chiral material

We report an induction of the blue phase III (BPIII) at a relatively low and wide (over 20 °C) temperature range in nematogenic achiral bent-core liquid crystals doped with a high twisting power chiral material. The pitch decreases with increasing chiral dopant ratio, and easily reaches the ultraviolet wavelength, so that completely dark texture is obtained under crossed polarizers. Electrooptical switching was achieved in a time range of a few to a few tens of milliseconds. We propose for the stabilization of BPIII that broad-temperature range smectic nano-clusters inhibit the long-range order of the double twisted helical structures, and also inhibit possible separation of chiral dopants from the mixture.

Liquid crystalline amorphous blue phase and its large electrooptical Kerr effect

An amorphous blue phase III with low and wide thermal range (∼20 °C) including room temperature is induced by doping a bent-core nematic with a strong chiral material. We confirm that the electrooptical response is due to the Kerr effect, with the Kerr constant being up to two orders of magnitude larger than conventional Kerr materials such as nitrobenzene.

Flexoelectric effect in a bent-core mesogen

The flexoelectricity in a bent-core liquid crystal was evaluated quantitatively using two independent electrooptic methods. The absolute value of the bend flexoelectric coefficient e 3 was measured to be about 15.8 pC m−1. Our results revealed that there is little difference between the flexoelectricity of bent-core mesogens and that of conventional calamitic mesogens, i.e. in the same pC m−1 order. The principles of the methods used are reviewed in detail.

Transition between widened BPs by light irradiation using photo-active bent-core liquid crystal with chiral dopant

Upon UV irradiation of a bent-core liquid crystal (LC) bearing an azo linkage doped with chiral molecules, a photo-induced transition takes place from BPI to BPIII. BPIII is stabilised over 20 °C, while the widening of the BPI range is not so remarkable. The mechanism of photo-induced BPI–BPIII transition is also discussed.

Unusual temperature dependence of smectic layer structure associated with the nematic–smectic C phase transition in a hockey-stick-shaped four-ring compound

In a newly designed four-ring asymmetrical bent-core compound, we observed smectic-C-type diffuse layer reflection over the entire nematic temperature range. At the nematic–smectic C phase transition, a sharp layer reflection emerges in addition to the diffuse reflection with different layer tilt angles.

Chiral Superstructure Mesophases of Achiral Bent-Shaped Molecules – Hierarchical Chirality Amplification and Physical Properties

Chiral mesophases in achiral bent-shaped molecules have attracted particular attention since their discovery in the middle 1990s, not only because of their homochirality and polarity, but also due to their unique physical/physicochemical properties. Here, the most intriguing results in the studies of such symmetry-broken states, mainly helical-nanofilament (HNF) and dark-conglomerate (DC) phases, are reviewed. Firstly, basic information on the typical appearance and optical activity in these phases is introduced. In the following section, the formation of mesoscopic chiral superstructures in the HNF and DC phases is discussed in terms of hierarchical chirality. Nanoscale phase segregation in mixture systems and gelation ability in the HNF phase are also described. In addition, some other related chiral phases of bent-shaped molecules are shown. Recent attempts to control such mesoscopic chiral structure and the alignment/confinement of HNFs are also discussed, along with several examples of their fascinating advanced physical properties, i.e. huge enhancement of circular dichroism, electro- and photo-tunable optical activities, chirality-induced nonlinear optics (second-harmonic-generation circular difference and electrogyration effect), enhanced hydrophobicity through the dual-scale surface morphological modulation, and photoconductivity in the HNF/fullerene binary system. Future prospects from basic science and application viewpoints are also indicated in the concluding section.

Heat-driven and electric-field-driven bistable devices using dye-doped nematic liquid crystals

We have demonstrated memory and rewritable bistable devices based on an anchoring transition of a nematic liquid crystal on a perfluoropolymer surface. Spontaneous orientation changes between planar and homeotropic occur on cooling and heating with a large temperature hysteresis. Photo (heat) addressing is possible from homeotropic to planar using dye-doped samples. For a coumarin dye, photoaddressed images are preserved even after heating up the sample to the isotropic temperature, whereas, for a 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye the images can be erased by decreasing the temperature out of the thermal hysteresis. Orientation switching also occurs by applying an electric field with a response time of several milliseconds depending on the field strength.

Nanosize-Induced Optically Isotropic Nematic Phase

We fabricated, in a polymer matrix, liquid crystal (LC) nanosized droplets with a correlation length ξ of about 140 nm, which appear as an optically isotropic film. Differential scanning calorimetry (DSC) and light scattering measurements gave unambiguous evidences of an existence of nematic LC (NLC) order and fluctuation over a wide temperature range. The correlation length obtained by light scattering was consistent to the droplet size determined by a scanning electron microscope (SEM). The dynamic electro-optic (EO) response in such an isotropic NLC (IsoN) phase was found to be very fast, tens of µs, in a confined geometry because of the local short-range nematic order in the IsoN phase. This type of EO effect is very attractive for next-generation LC displays and light waveguides because of (1) very dark view in the absence of a field, (2) very fast response being independent of temperature and applied electric field, (3) gray-scale display capability with a constant response time, and (4) unnecessity of any surface treatment.

Large-scale self-organization of reconfigurable topological defect networks in nematic liquid crystals

Topological defects in nematic liquid crystals are ubiquitous. The defects are important in understanding the fundamental properties of the systems, as well as in practical applications, such as colloidal self-assembly, optical vortex generation and templates for molecular self-assembly. Usually, spatially and temporally stable defects require geometrical frustration imposed by surfaces; otherwise, the system relaxes because of the high cost of the elastic energy. So far, multiple defects are kept in bulk nematic liquid crystals by top-down lithographic techniques. In this work, we stabilize a large number of umbilical defects by doping with an ionic impurity. This method does not require pre-patterned surfaces. We demonstrate that molecular reorientation controlled by an AC voltage induces periodic density modulation of ions accumulated at an electrically insulating polymer interface, resulting in self-organization of a two-dimensional square array of umbilical defects that is reconfigurable and tunable.

Characterization of Nematic Phase of Banana Liquid Crystal

We report the first detailed optical and electrical characterizations of the nematic (N) phase of a banana liquid crystal, i.e., pretilt angle, and optical and dielectric anisotropies, and point out the difference from those of rodlike molecules. The pretilt angle measurements using a conventional crystal rotation method show that the pretilt angle is very small (less than 1°) even in cells with a polyimide film yielding a high pretilt angle like 8° for calamitic molecules. We also measured the birefringence Δn in the N phase as a function of temperature and estimated the orientational order parameter S. The medium shows a small negative dielectric anisotropy in the N phase and the anisotropy becomes large in the B2 phase mainly because of the tilt of the molecules from the layer normal.