4-(5-Alkyl-2-pyridinyl)phenyl alkanoates: Some novel smectic materials for display devices

Abstract

Three homologous series of 4-(5-alkyl-2-pyridinyl)phenyl alkanoates have been prepared. The synthesis and the liquid crystal transition temperatures of these esters are reported. Many members of these three ester series exhibit enantiotropic, wide range smectic F mesophases and some narrow range smectic C mesophases. However, in admixture with a chiral smectic C base mixture, the esters often induce a substantial increase in the chiral smectic C–smectic A transition temperature and, at the same time, decrease the temperature of crystallization. Thus, the temperature range of the chiral smectic C mesophase is substantially extended at both high and low temperatures. Ordered smectic mesophases are only observed at very low temperatures, if at all. These chiral smectic C mixtures are characterized by short switching times in surface-stabilized ferroelectric liquid crystal displays (SSFLCD). These novel esters are of especial interest for short-pitch chiral smectic C mixtures for short pitch bistable ferroelectric liquid crystal displays (SBFLCD) with short response times.     

Current topics in smectic liquid crystal research.

Interest in the smectic liquid-crystalline state of matter received a substantial boost with the discovery by Meyer in the mid-1970s that a chiral smectic C (SmC*) phase exhibits a spontaneous electric polarization, and with the subsequent demonstration by Clark and Lagerwall of the surface-stabilized SmC* ferroelectric liquid crystal at the beginning of the 1980s. Since then, chiral smectic phases and their plethora of polar effects have dominated the research in this field, which today has reached a mature state where the first commercial microdisplay applications are now shipping in millions-per-year quantities. In this Review we discuss some of the topics of highest interest in current smectic liquid crystal research, and address application-relevant research (de Vries-type tilting transitions without defect generation and high-tilt antiferroelectric liquid crystals with perfect dark state) as well as more curiosity-driven research (the nature and origin of the chiral smectic C subphases and their intermediate frustrated states between ferro- and antiferroelectricity).     

Naphthyl benzoates versus phenyl benzoates A molecular structure-liquid crystal and ferroelectric behaviour relationship study

A comparative study of the liquid crystal and ferroelectric properties of several achiral, racemic and non-racemic 2,6-naphthyl benzoates and their 1,4-phenyl benzoate analogues is reported. In terms of pure compounds, both central core structures give rise to similar mesophase sequences; however, the naphthalene derivatives exhibit broader and more thermally stable liquid crystalline states. On the other hand, both central core units give rise to equivalent electric and optoelectronic properties when the compounds are used as chiral dopants in ferroelectric liquid crystal mixtures.     

New chiral dopants for ferroelectric liquid crystals

New chiral dopants for application in ferroelectric liquid crystal mixtures are presented. They derive from chirally modified biphenylcyclohexylcarbonitriles, lactates and optically active 2-cyano-2-methylalkanoates. Synthetic routes to the new compounds are shown and the physical properties of the new compounds in a non-chiral smectic C host are reported.     

The characteristics of SnPc-doped ferroelectric liquid crystal cells

Tin phthalocyanine (SnPc)-doped polymer films were used to align ferroelectric liquid crystals (FLC). A stripe-shaped domain structure was obtained spontaneously in the initial alignment of the surface-stabilized cells. Atomic force microscopy (AFM) was introduced to investigate the aligning films. It was discovered that the modified aligning films show a polymer aggregate chain structure oriented along the rubbing direction that may play an important role in the FLC alignment. The stability, memory capability and response time of the cell were improved and enhanced through the doping process.     

Fast-switching effect using viscous chiral-nematic materials

We report on chiral-nematic mixtures containing viscous chiral dopants, composed of rigid rod-like chiral molecules, in order to solve the problem of backflow in the middle layers of the liquid crystal cells. During the study, the viscoelastic properties of the liquid crystal compositions have been optimised, as well as the helical twisting power of the chiral compounds, the anchoring energy and the pre-tilt angle of alignment materials, the ratio between the cell gap and the helical pitch (d/P₀). After optimisation, we prepared test cells with low operating voltage – from 1.5 till 5.0 V, fast response time (τ_on + τ_off) – less than 1 ms, wide operating temperature range from ?40°C till +100°C, and good thermal stability. It should be noted that the response time of the new electro-optical mode does not strongly depend on the cell gap, which is in contrast to other known operating modes. We have achieved rather linear than quadratic dependence of the response time when changing the thickness of the cell.     

Phase diagrams and physical properties of binary ferroelectric mixtures based on a series of chiral α-cyanocinnamate derivatives

We report a detailed investigation of binary mixtures composed of members of the homologous series of (S)-4′-(2-n-alkoxypropanoyloxy)biphenyl-4-yl 4-n-alkoxy- f-cyanocinnamates. The phase transition curves for these mixtures have no minima. On the contrary, eutectic behaviour was obtained if the members of this homologous series were mixed with structurally different chiral cinnamic acid derivatives and other ferroelectric liquid crystal materials. Spontaneous polarization, tilt angle and dielectric constant results were obtained for three of the single compounds and nine ferroelectric mixtures.     

Ferroelectric compounds with chiral (S)-1-methylheptyloxycarbonyl terminal chain – their miscibility and a helical pitch

Compounds with the same chiral terminal chain but different structure of rigid core and non-chiral terminal chain were added to basic highly tilted ferroelectric compound in the aim of establishing whether the miscibility of phases, especially ferroelectric phase, exists in these compounds and what are the helical pitch and tilt angle temperature dependence in bicomponent systems. In certain systems, we obtained a mixture with very short helical pitch at broad temperature range of ferroelectric phase, which open opportunities for the formulation of multicomponent mixtures for fast switching ferroelectric liquid crystals modes.     

Achiral swallow-tailed materials with 'antiferroelectric-like' structure and their potential use in antiferroelectric mixtures

Achiral 'swallow-tailed' liquid crystalline materials are known to give alternating-tilt smectic C phases (S_Calt) which have structural similarities to the chiral antiferroelectric phases denoted as S*_CA. This paper describes the synthesis and characterization of three achiral branchedalkyl 4-(4'-dodecyloxybiphenyl-4-carbonyloxy)-3-fluorobenzoates. Optical microscopy and differential scanning calorimetry confirm that these materials show S_Calt and overlying S_A phases. The compounds were investigated as potential hosts which could be doped with a chiral ferroelectric liquid crystal so as to provide a viable- antiferroelectric mixture. These studies (microscopy and differential scanning calorimetry), to characterize the properties of the mixtures, show that antiferroelectric phases are induced. However, switching studies show that the antiferroelectric phases are extremely stable, a property which is almost certainly a consequence of the length of the lateral branching groups (ethyl, propyl and butyl).     

Molecular rotational potentials in the SC* phase of binary ferroelectric liquid crystal mixtures

The spontaneous polarization and tilt angle have been measured as a function of temperature and concentration in the S_C* phase of four binary ferroelectric liquid crystal mixtures. The substances have been chosen in a way that different situations could be studied, for example mixtures of a chiral substance (P_s non-zero) with a non-chiral one (P_s = 0) and mixtures of chiral compounds with the same sign of P_s as well as with opposite signs. The results have been interpreted by means of a molecular rotational model and potentials associated with the tilt as well as with the chirality have been determined as a function of concentration.     

Influence of chirality on phase transitions in ferroelectric liquid crystals

The transition temperatures between various smectic liquid crystal phases are determined as a function of the enantiomeric excess for three different chiralracemic systems (i.e. binary mixtures consisting of a chiral enantiomer and its racemate). It is shown that transitions involving a ferroelectric phase occur in the chiral compounds at higher temperatures compared to their racemates, the temperature shift being proportional to the square of the enantiometric excess. In contrast, for transitions between two non-ferroelectric phases no difference between the chiral and the racemic compounds is found. Various reasons for the experimental behaviour are discussed. A chirality dependence of the transition temperature is also observed for the smectic A-isotropic transition.     

Effect of fluorination on the phase sequence,dielectric and electro-optical properties of ferroelectric and antiferroelectric mixtures

Four multicomponent room temperature mixtures were formulated using a pyrimidine-based achiral matrix as host and four terphenyl-based ester chiral compounds as dopants. Among the four mixtures, two exhibit antiferroelectric and two other exhibit ferroelectric phases at room temperature. Dopant molecules differ from each other by position and number of fluorine atom substitution at the benzoate group of the molecular rigid core. Number and location of fluorine atoms in the dopant structure show significant effect on the phase sequences as well as on different physical properties. All the mixtures found to have moderate spontaneous polarisation, high tilt angle, very fast switching time and low viscosity which are important for liquid crystal-based display applications.     

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.     

Shear aligned polymer dispersed ferroelectric liquid crystal devices

Fast switching liquid crystal devices can be produced by forming a dispersion of ferroelectric liquid crystal droplets in a polymer film. Such PDFLCs have been fabricated using a polymerization-induced phase separation technique involving ultraviolet photopolymerization, during which the film was sheared to obtain a uniform orientation of the liquid crystal medium. These birefringence devices show fast response times (sub-millisecond), optimum tilt angle (22.5°), and good contrast (∼ 30:1) at room temperature, using ferroelectric switching. We studied the tilt angles, response times and contrast ratio as a function of voltage and temperature to determine the effects of the preparation parameters on the electro-optic behaviour of these devices. Using a ferroelectric liquid crystal with long helical pitch, such devices appear to be bistable.     

Material characteristics of an active matrix LCD based upon chiral smectics

We describe the material properties of a fast switching active matrix ferroelectric liquid crystal mode. The mode relies on a monostable monodomain having a high transmission and full grey scale capability. Experimental data are compared with the results of switching model calculations and the switching mechanism is discussed. The mode is suitable for video and multimedia application and compatible with conventional amorphous silicon thin film transistors due to the small value of the spontaneous polarization.     

Material characteristics of an active matrix LCD based upon chiral smectics

We describe the material properties of a fast switching active matrix ferroelectric liquid crystal mode. The mode relies on a monostable monodomain having a high transmission and full grey scale capability. Experimental data are compared with the results of switching model calculations and the switching mechanism is discussed. The mode is suitable for video and multimedia application and compatible with conventional amorphous silicon thin film transistors due to the small value of the spontaneous polarization.     

Synthesis and characterization of chiral compounds containing cationic groups as chiral dopants in liquid crystals

Novel chiral molecules containing cationic groups, (N-[4-triethylammoniomethyl]-benzoyl ester)-ethyl lactate chloride and bi-(N-[4-triethylammoniomethyl]-benzoyl ester)-isosorbide chloride, were designed and synthesized. Chemical structures of the molecules were characterized by elemental analysis, FT-IR, and (1)H NMR. The photochemical properties of the chiral compounds and their textures in nematic liquid crystals (LCs) were investigated by optical rotation, circular dichroism (CD), and polarizing optical microscopy (POM). The novel chiral molecules exhibited good optical activity. The chiral compound based on a L-ethyl lactate chiral center had a left-handed configuration. The chiral compound based on an isosorbide chiral center had a right-handed configuration. The cationic polar groups did not affect the direction of optical rotation, but could effluence the molar rotation of chiral compounds. The mixtures with dopants showed oily streak textures. Doping of a nematic phase liquid crystal with the chiral molecules converted it to the cholesteric phase.     

Thermal analysis of binary liquid crystalline mixtures

Two series of binary mixtures composed of bent shaped and rod like molecules are reported. The first star shaped bent core molecules were synthesized and used as a component of binary mixtures. The chiral rod like compounds having commensurable length with the arms of the bent core compounds have been chosen for these mixtures. The resulted compositions show various thermotropic liquid crystalline phases that are characteristic to both types of liquid crystalline materials. In case of mixing the rod like molecules to the bent core compound the B2, B7 and induced B1 phases have been observed. While using the star-shaped bent core and chiral rod like compounds in mixture, the paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic B phases were preferred. The appearing mesophases were investigated by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction methods.     

Switchable columnar metallomesogens

Chiral columnar liquid crystals have recently appeared as a promising new type of ferroelectric materials. To date, all the columnar liquid crystals that have been reported to show ferroelectric switching consist of organic compounds. However, metal-containing liquid crystals open this field to a significant number of new structures and offer the possibility of adding to the ferroelectric behavior other properties inherent to the presence of metals in the structure, such as magnetism, as well as the use of new methods of characterization (EPR, synchrotron radiation, etc.). The potential of columnar metal-containing liquid crystals as ferroelectric materials has been demonstrated even though only a few organic columnar ferroelectric liquid crystals have been described. As a first approach to this type of material, this concepts article describes the results obtained with chiral metal beta-diketonates that show ferroelectric switching in the columnar mesophase. It has been shown that these materials have a helical columnar arrangement in the mesomorphic state, and a chiral superstructure has been proposed from circular dichroism studies. This type of supramolecular structure plays a fundamental role in the ferroelectric properties of these compounds. The discussion is mainly focused on the strategy employed for the molecular design, and on the interpretation of the mesophase structure and the electrooptic effect. The use of a diverse range of techniques, both those commonly used in the field of liquid crystals and those that are more specific will be highlighted, and the principles of these specific techniques are summarized together with a justification of their applicability to this study.