Several Types of Bilayer Smectic Liquid Crystals with Ferroelectric and Antiferroelectric Properties in Binary Mixture of Dimeric Compounds

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.

Electric Switching Behaviors and Dielectric Relaxation Properties in Ferroelectric, Antiferroelectric, and Paraelectric Smectic Phases of Bent-Shaped Dimeric Molecules

This study reports the electric switching behaviors and dielectric properties of the ferroelectric smectic-A (SmAP_F), anti-ferroelectric smectic-A (SmAP_A), anti-ferroelectric SmC_AP_A, and smectic-A (SmA) phases formed by mixing the bent-shaped dimeric molecules, α,ω-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentanes. These four phases each show characteristic features. The SmAP_F shows a low threshold electric field for ferroelectric switching and a large dielectric strength due to the collective fluctuation mode of dipoles at around 500 Hz. Both the threshold electric field and dielectric strength are strongly dependent on the cell thickness. The threshold field decreases to 0.1 V μm^-1, and the dielectric strength increases up to a huge value of 10,000 as the cell thickness increases up to 80 μm. The SmAP_A also shows a similar collective mode at around 2 kHz with a relatively small dielectric strength (around 200), which may be induced by the anti-phase rotation of dipoles in adjacent layers. In these collective modes, the dielectric strength is found to be inversely proportional to the switching threshold field. On the other hand, another anti-ferroelectric SmC_AP_A as well as the paraelectric SmA show only the non-collective mode (i.e., rotational relaxation of individual molecules around their short axes) at a high frequency of around 100 kHz.

H-Shape mesogenic dimers – the spacer parity effect

It was found that for H-shape dimers, the parity of the number of atoms in the spacer linking the mesogenic cores has important influence on the mesogenic properties only for the molecules with weak mesogenic core anisotropy.

Optical properties of unsymmetrical azomethines with one imine bonds

We have explored the photoluminescence (PL) and electronic properties, that is, orbital energies and resulting energy gap calculated theoretically by density functional theory (DFT) of four unsymmetrical (UAz1-UAz4) azomethines. All of the investigated compounds exhibited mesomorphic behavior. The photoluminescence studies have shown that molecular structure of the imines influenced both the PL properties and HOMO-LUMO levels of azomethines. Azomethines emitted violet, blue or green light. The effect of excitation wavelength and concentration on the PL properties has been detected as well. Unsymmetrical imine UAz3 posses carbazole unit exhibited lower both HOMO and LUMO energies compare to others investigated azomethines. Additionally, azomethines were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Two unsymmetrical imines (UAz2 and UAz3) exhibited irreversible oxidation behavior. The HOMO level of the compound with ethylcarbazole (UAz2) moiety was observed at -5.22 eV, while for the UAz3 with phenoxybiphenyl unit was found at -5.64 eV. The data obtained by theoretical calculation using DFT method was very similar to the results obtained by electrochemical measurements.

Biaxial and antiferroelectric structure of the orthogonal smectic phase of a bent-shaped molecule and helical structure in a chiral mixture system

We examined the biaxial and antiferroelectric properties in the Smectic-AP(A) (Sm-AP(A)) phase of bent-shaped DC-S-8. The biaxiality, which results from the existence of a secondary director, was well established from birefringence observations in the homeotropically aligned Sm-AP(A). By entering into Sm-AP(A) phase, the birefringence (Δn, difference between two refractive indices of short axes) continuously increased from 0 to 0.02 with decreasing temperature. The antiferroelectric switching and second harmonic generation (SHG) activity on the field-on state were also observed in the Sm-AP(A) phase, and the evaluated spontaneous polarization (P(S)) value strongly depended on temperature. The temperature dependence of Δn and P(S) resembles each other and follows Haller's approximation, showing that the biaxiality is due to polar packing in which the molecules are preferentially packed with their bent direction arranged in the same direction, and that the phase transition of Sm-AP(A) to Sm-A is second order. The biaxiality was further examined in chiral Sm-AP(A)(*). Doping with chiral components induced the helical twisting of the secondary director in the Sm-AP(A)(*) phase, which was confirmed by observing the reflection of the circular dichroism (CD) bands in the homeotropically aligned cell. The helical pitch of Sm-AP(A)(*) is tunable in the range of 300-700 nm wavelength with a variation in the chiral content of 5 to 10 weight (wt)%.

Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit

Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic phases (N(cybA) and N(cybC)), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and cybotactic nematic phases (SmC, N(cybC)), which are replaced by the nontilted CybA phases and nematic phases composed of SmA-type clusters (N(cybA)). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving biaxial nematic phases of the orthorhombic type. The orthogonal (N(cybA)) and skewed (N(cybC)) cybotactic nematic phases were distinguished by XRD and optical observations.

Structural characteristics of the B6 phase for a bent-core molecular system observed through the B1-B6 transition

A bent-shaped molecule exhibiting the mesophase sequence of B6-B1 was studied to understand the structural characteristics of the B6 phase compared with those of the B1 phase. A well-oriented sample was prepared in a magnetic field and examined by wide-angle x-ray diffraction measurements through the B6-B1 phase transition. The B6 phase has been considered to show only the (002) reflection, but this B6 phase showed broad scattering just inside the (002) reflection. The broad scattering has an intensity maximum at a very similar position to that of the (101) reflection in B1 and evolves into the well-defined (101) reflection on cooling into B1. Thus, B6 has a similar frustrated structure to B1, but the size of the antidomain in B6 may be dynamically distributed while B1 possesses an antidomain of definite size.

Liquid crystal dimers and higher oligomers: between monomers and polymers

The underlying theme of this Critical Review is the relationship between molecular structure and liquid crystalline behaviour in a class of materials referred to as liquid crystal oligomers. For the purposes of this review, a liquid crystal oligomer will be defined as consisting of molecules composed of semi-rigid mesogenic units connected via flexible spacers. Much of the review will be devoted to structure-property relationships in the simplest oligomers, namely dimers, in which just two mesogenic units are connected by a single spacer. Along the way we will see how this molecular architecture has been exploited to address issues in a range of quite different areas and has given rise to potential applications for these materials. On the whole, only compounds in which the mesogenic units are linked essentially in a linear fashion will be considered while structures such as liquid crystal dendrimers and tetrapodes fall outside the scope of this review. The review will be of interest not only to scientists working directly in this area but in particular to those interested in understanding the relationships between structure and properties in polymers, and those designing materials for new applications.

Structure of a B6-like phase formed from bent-core liquid crystals determined by microbeam x-ray diffraction

We studied the structure of the Bx phase formed from the short terminal homolog, 1,3-(4-bromobenzene) bis[4-(4-n-butoxyphenylliminomethyl)benzoate] (4Br-P-4-O-PIMB), by focusing a microbeam of x ray on the well-developed fan-shaped texture. From the highly oriented x-ray patterns detected at the two states of DC-ON and DC-OFF, the Bx structure was definitely illustrated. It is a kind of frustrated one similar to the B1 phase: the molecules lie perpendicularly to the layer, and the frustration takes place perpendicularly to the bent direction. Unlike in the B1 phase, however, the size of the resulting antidomain is not definite, but fluctuates from position to position as observed in the B6 phase.