Optical switching and alignment of antiferroelectric liquid crystals containing an azo group

The optical properties of two kinds of photochromic antiferroelectric liquid crystal (AFLC) containing an azo group have been examined. Depending on the substituting group at the chiral centre, these AFLCs showed different photoinduced phase transitions by Ar⁺ laser light irradiation. This phase transition has led to a new form of optical switching controlled by a bias voltage. In addition, it was found that these AFLCs caused an azimuthal photoalignment effect regulated by linearly polarized Ar⁺ laser light irradiation. These phenomena were applied to image storage.     

Synthesis and characterisation of reactive liquid crystals containing an azo group

A new series of liquid crystal (LC) monomers – not only contain a double bond but also contain an azo group – were designed and synthesised. The length of side groups in the LC monomers containing azobenzene ester varied from 1 to 2 methylene units, and the length of the substituted groups in the main chain varied from 1 to 3 methylene units. The molecular structures of the intermediates and target compounds were confirmed by Fourier transform infrared, ultraviolet and visible spectrum and nuclear magnetic resonance (NMR) spectroscopy. The thermal phase behaviour of the LC monomers was investigated by polar optical microscopy coupled with hot stage and differential scanning calorimetry (DSC). In this paper, the effect on the LC zone with the substituents were investigated, and with the increased methylene of main chain, the melting point and the phase transition temperature of the substance will be lowered and LC regions will be narrower.     

Thresholdless switching induced by polar anchoring in antiferroelectric liquid crystals

We present a novel thresholdless switching mode in an antiferroelectric liquid crystal cell which is stabilized by the presence of polar anchoring at the cell surfaces and the antiferroelectric nature of the material. We also suggest other possible configurations which are induced by strong polar anchoring and possess quite different director structures and optical characteristics.     

Thresholdless switching induced by polar anchoring in antiferroelectric liquid crystals

We present a novel thresholdless switching mode in an antiferroelectric liquid crystal cell which is stabilized by the presence of polar anchoring at the cell surfaces and the antiferroelectric nature of the material. We also suggest other possible configurations which are induced by strong polar anchoring and possess quite different director structures and optical characteristics.     

Studies of the alignment properties of antiferroelectric liquid crystals by X-ray diffraction

It is observed optically that in a parallel rubbed antiferroelectric liquid crystal device, the texture consists of domains with two distinct optic axes, which make equal and opposite angles with the rubbing direction. It is proposed that this is caused by a large electroclinic effect at the surfaces during layer formation in the SmA* phase. This hypothesis is verified by finding the layer structure in single, parallel and skew rubbed devices by using X-ray diffraction.     

Orthoconic antiferroelectric liquid crystals

The ubiquitous liquid crystal display (LCD) is based on nematic liquid crystals (LCs) and has during 40 years developed from simple few digit displays into high-resolution flat-panel displays. A last and very important step towards the present TVs was the combination with thin film transistors that also led to the introduction of several new switching modes. Despite the enormous success of this technology, there is presently a renewed interest in LCs with faster electrooptic response, especially for future 3D vision display systems and possibly for field-sequential-colour (FSC) generation displays. Here, I focus on the so-called orthoconic antiferroelectric LCs (OAFLCs), which can provide the fast switching of conventional antiferroelectric LCs, but combined with a potentially much higher optical contrast. The reason is that the dark state of orthconic materials has a homeotropic optic axis, which makes the extinction ideally complete between crossed polarisers and independent of in-plane alignment fluctuations characteristic of all antiferroelectric LCs. The basic features and device physics issues of orthoconic LCs are reviewed and the most important remaining challenges to be met in terms of materials development are discussed in this article. Furthermore, a few examples of new applications made possible with the use of present and future orthoconic materials are given.     

Optical and dielectric properties of antiferroelectric liquid crystals and their surface effects

Abstract

A detailed study of the dielectric and optical properties of the ferroelectric liquid crystal material (R)-4′-(3-methoxycarbonyl-2-propoxycarbonyl)phenyl 4-(4-(n-octyloxy)phenyl)benzoate (3MC2PCPOPB) has been carried out. It has been found that an anomalous temperature dependence of the dielectric constant in 3MC2PCPOPB is due to the antiferroelectric and ferrielectric properties. A T(temperature)-E(electric field) phase diagram has been obtained on the basis of the apparent tilt angle measurements. In a thin cell (< 3 μm), both ferroelectric and antiferroelectric domains are simultaneously observed over a wide temperature range, and the complete antiferroelectric phase does not appear even at low temperature. A characteristic texture in which boundary focal conics are aligned parallel to a smectic layer has been observed. The movement of the zig-zag defect line caused by the application of the voltage is also observed.     

Optical and electro-optical switching processes in polymeric liquid crystals

Low-molar-mass and polymeric liquid crystals exhibit large nonlinear optical responses, due to a combination of orientational order and fluidity. Polymeric liquid crystals have the disadvantage of longer response times but offer the advantage that the liquid-crystalline state and any modulation imposed on it can be frozen-in in the anisotropic glassy state. This holds in particular for light-induced reorientations of the director and thus of the optical axes based on a photoselection process of photostationary mesogens.     

Morphology and switching of holographic gratings containing an azo dye

The effects of an azo dye on the diffraction efficiency, morphology and electro-optic properties of the transmission mode of a holographic polymer dispersed liquid crystal (LC) have been studied. The azo dye induced an induction period which otherwise does not exist, followed by a gradual increase of the diffraction efficiency to a saturation value which increased with increasing azo dye content, as a result of the azo dye reorientating LC molecules within the droplet. The increased diffraction efficiency was caused by the decreased droplet coalescence which was due to the hindered migration of the LC by the dye molecules, and to LC orientation induced by azo dye molecules giving a high refractive index contrast. The droplet size decreased with increasing dye content. The dye also lowered the threshold voltage due to the high dielectric anisotropy caused by the presence of a strong on-axis dipole moment and decreased the response time.     

A novel class of banana-shaped azo compounds exhibiting antiferroelectric switching behaviour

The first banana-shaped liquid crystal materials with photo-active azo linkages are reported. The higher temperature mesophase X₁ (believed to be B₆) above the B₁ phase exhibited by these azo compounds shows antiferroelectric switching characteristics. The observation of banana phases in azo compounds assumes significance from the point of view of photochromic studies.     

New antiferroelectric liquid crystals with a trifluoromethyl group at the chiral centre

New antiferroelectric liquid crystals (AFLCs) with a trifluoromethyl group at the chiral centre were synthesized using optically active 3-(4-methoxyphenyl)-4,4,4-trifluorobutanoic acid. The new AFLCs showed the phase sequence on cooling: isotropic liquid (I)-smectic A (S_a) phase-antiferroelectric (AF) phase-crystal (Cr). In the antiferroelectric phase, a d.c.-threshold, a double hysteresis loop, a small dielectric constant value and two sharp switching-current peaks were observed. These are the first examples that are AFLCs which do not have a carbonyl moiety in the molecule.     

New liquid crystals containing the benzothiazol unit: amides and azo compounds

Two series of calamitic liquid crystals containing a benzothiazole ring within the central core and two different linkage groups (amide and azo) have been prepared and their liquid crystalline properties studied and compared with those of the analogous series of imines. The influence of the linkage group within the central core has been proven to determine the variety of mesomorphism displayed by the compounds. The compounds with imine and azo linkages behave in a similar way and exhibit typical nematic and smectic C mesophases. Compounds incorporating an amide linkage show a poorer mesomorphism and mainly present a smectic C mesophase.     

A novel class of banana-shaped azo compounds exhibiting antiferroelectric switching behaviour

The first banana-shaped liquid crystal materials with photo-active azo linkages are reported. The higher temperature mesophase X₁ (believed to be B₆) above the B₁ phase exhibited by these azo compounds shows antiferroelectric switching characteristics. The observation of banana phases in azo compounds assumes significance from the point of view of photochromic studies.     

A phenomenological model of antiferroelectric liquid crystals

Abstract

We present the analysis of static properties of a simple phenomenological model of antiferroelectric liquid crystals, capable of explaining some different sequences of phase transitions found experimentally. Stability analysis shows the existence of three simply modulated phases. Results are summed up in phase diagrams with dependence on the parameters of the model. The existence of doubly modulated incommensurate phases is predicted in parts of the phase diagrams.     

Antiferroelectric phase in liquid crystalline compounds containing an azo group in their molecular core

Novel lactic acid derivatives containing an azo group in their molecular core have been synthesised and their mesomorphic properties established. Compounds with a short aliphatic non-chiral chain exhibited the SmA* phase and ferroelectric (FE) SmC* phase. The monotropic antiferroelectric (AF) phase was found in the homologues with a longer terminal chain. Spontaneous polarisation and tilt angle were measured for the FE and AF phases. Dielectric spectroscopy in the frequency range 1 Hz–10 MHz was carried out and dielectric properties established within the temperature range of the smectic phases. Two high frequency modes were detected in the AF phase. Small-angle X-ray measurements were conducted, and the temperature dependence of layer spacing values is established and discussed.     

New antiferroelectric liquid crystalline materials containing a keto group and two lactate groups

New chiral liquid crystalline materials containing a keto group and two lactate units attached to the mesogenic core have been synthesized and studied. All the new compounds show the paraelectric SmA, ferroelectric SmC* and antiferroelectric SmC*_A phases each over rather broad temperature ranges. The properties of the phases were characterized by DSC, electro-optical and dielectric studies. The spontaneous polarization, tilt angle, helical pitch, and switching times were determined for the polar phases.     

Investigations on the field-induced switching behaviour in an antiferroelectric liquid crystal

The switching currents and field-induced apparent tilt angles in an antiferroelectric liquid crystal, (R)-MHPOBC, were measured. The structural differences among different smectic C* subphases may sensitively reflect the field or temperature dependence of the apparent tilt angle. In a thin cell, the apparent tilt angle was found to change in two steps as a function of field strength in the SmC*alpha and SmC* phases: a steep increase at lower fields and a small linear increase at higher fields. The steep increase in apparent tilt angle is divided into two parts via a plateau in the ferrielectric SmC*gamma phase. Stepwise change with a plateau is also seen in the relation of apparent tilt angle versus temperature under various electric fields. Moreover, the apparent tilt angle in the vicinity of the plateau is almost temperature- and field-independent, implying a preferred orientation of the molecules in the ferrielectric state. The influence of the cell thickness on the structural changes was also investigated.     

New antiferroelectric liquid crystalline materials containing a keto group and two lactate groups

New chiral liquid crystalline materials containing a keto group and two lactate units attached to the mesogenic core have been synthesized and studied. All the new compounds show the paraelectric SmA, ferroelectric SmC* and antiferroelectric SmC*_A phases each over rather broad temperature ranges. The properties of the phases were characterized by DSC, electro-optical and dielectric studies. The spontaneous polarization, tilt angle, helical pitch, and switching times were determined for the polar phases.     

Structure of surface-stabilized antiferroelectric liquid crystals cells

A comprehensive expression describing the director profile and energy parameters of surface-stabilized antiferroelectric liquid crystals has been derived. The expression is based on a previously developed ferroelectric liquid crystal model in which two contiguous smectic layers have been simultaneously computed. The antiferroelectric behaviour is mathematically described as an additional energy term relating these layers to each other through a coupling constant. The new energy term gives significant differences in the material behaviour, as compared with the ferroelectric phase. Electrically induced antiferroelectric-ferroelectric phase transitions are predicted as well. The effect of every term on the general expression has been analyzed. Comparisons with ferroelectric results are also included.     

Circular dichroism in ferroelectric and antiferroelectric liquid crystals

Liquid crystal induced circular dichroism (LCICD) measurements were made to investigate the pretransitional phenomena in the S_A phase just above the S_A-S*_C and S_A-S*_CA phase transitions of both the first and the second order. The pretransitional LCICD in S_A was observed in the second order phase transition to S*_c and S*_cA, suggesting the existence of a dynamic helical structure in S_A. Such behaviour disappears when the transition is of the first order. It seems that the handedness of the dynamic helix in S_A is the same as that in S*_c even when the lower temperature phase is S*_cA. This is explained as a result of a dominant contribution of ferroelectric soft mode.