Non‐symmetric liquid crystal trimers

Four homologous series of non‐symmetric linear liquid crystalline trimers, the 4‐[(4‐{n‐[4‐(4‐methoxy‐phenylazo)phenoxy]alkyloxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐methoxyphenylazo)phenoxy]hexyl esters, 4‐[(4‐{n‐[4‐(4‐cyanophenylazo)phenoxy]alkyloxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐methoxyphenylazo)phenoxy]hexyl esters, 4‐[(4‐{n‐[4‐(4‐methoxyphenylazo)phenoxy]alkyloxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐cyanophenylazo)phenoxy]hexyl esters and 4‐[(4‐{n‐[4‐(4‐cyanophenylazo)phenoxy]butoxy}benzylidene)amino]benzoic acid 6‐[4‐(4‐cyanophenylazo)phenoxy]hexyl esters, have been synthesized and characterized. In each series one spacer was held constant while the length of the second was varied from 3 to 12 methylene units. All these trimers were nematogenic and exhibited pronounced odd–even effects in both their clearing temperatures and associated entropies. Only two of the trimers exhibited a smectic phase, and specifically those containing two differing terminal mesogenic groups and an undecyl spacer. This strongly suggests the driving force for smectic phase formation is a combination of molecular shape and a specific interaction between unlike mesogenic units. The smectic phase has been assigned as a triply‐intercalated alternating smectic C phase, the first example of its kind.     

Oscillating Poiseuille flow in photo‐aligned liquid crystal cells

We present an experimental study of thin liquid crystal (LC) layers under the action of a harmonically varied pressure gradient. Optical measurements were performed to register the linear oscillations of a nematic director related to homeotropic and homeoplanar (hybrid) initial states. In the latter case one of the inner surfaces of the rectangular channels was treated by ultraviolet light to provide a relatively weak planar anchoring. The optical response of hybrid and homeotropic LC cells under an oscillating pressure gradient was investigated in relation to on the amplitude and frequency of the pressure gradient. A hydrodynamic model is developed taking into account the LC polar anchoring strength and the surface viscosity responsible for a fast LC surface dynamics. Our estimates show that the thickness of the boundary layer corresponding to the surface viscosity does not exceed 10^?6 m, and further experiments are needed with thinner LC cells and higher frequency oscillations to achieve a more precise value. An oscillating Poiseuille flow in the hybrid cell was found to be useful for characterizing elastic and viscous properties of a weakly anchoring LC surface layer in a fast surface dynamic process.     

Surface anchoring of nematic liquid crystal 8OCB on a DMOAP‐silanated glass surface

Dynamic light scattering spectroscopy has been used to determine the temperature dependence of the anchoring strength of the nematic liquid crystal 8OCB on DMOAP‐silanated glass surfaces inducing homeotropic alignment. Wedge‐type glass cells with known thickness profile starting from 150 nm to several microns have been used in the experiments. The relaxation rates of the nematic fluctuations with the wave vector perpendicular to the confining surfaces have been measured as a function of the cell thickness. Fitting of the thickness dependence of the relaxation rate allows for straightforward determination of the surface extrapolation length and therefore also the strength of the surface anchoring, which is 1×10^?4 J m^?2. The overall experimental accuracy of the experiments is discussed.     

Crystal structure of a liquid crystal non‐symmetric dimer: cholesteryl 4‐[4‐(4‐n‐butylphenylethynyl)phenoxy]butanoate

The crystal structure of cholesteryl 4‐[4‐(4‐n‐butylphenylethynyl)phenoxy]butanoate [phase sequence: Cr 155°C (46.1?J?g^?1) SmA 186.8°C (1.5?J?g^?1) TGB‐N* 204.7 (6?J?g^?1) I] has been solved from single crystal X‐ray diffraction data. The compound crystallizes in the monoclinic space group P2₁ with unit cell parameters: a?=?13.129(2), b?=?9.3904(10), c?=?17.4121(8)?Å, β?=?92.790(7)°, Z?=?2. The structure has been solved by direct methods and refined to R?=?0.0606 for 3?250 observed reflections. The bond distances and angles are in good agreement with the corresponding values for compounds containing phenyl and cholesterol moieties. The phenyl rings A and B are planar. The dihedral angle between the least‐squares planes of the two phenyl rings is 28°. The cholesterol moiety has the usual structure: the C and E rings have chair conformations, and the D and F rings adopt half‐chair conformations. The molecules in the unit cell are arranged in an antiparallel manner. The crystal structure is stabilized by an intermolecular C–H…O contact of 2.989(10)?Å.     

Rubbing angle effect on in‐plane switching liquid crystal displays

The rubbing angle effect on transmissive in‐plane switching liquid crystal displays is analysed by the Jones matrix method. Simulation results show that the optimum rubbing angle is around 30°–40°; the cell gap/birefringence product (dΔn) is about 0.33 µm. Increasing the rubbing angle can shorten the rise time and enlarge the grey scale voltage intervals. The optical characteristics are similar in two cells with different rubbing angles. These effects are particularly attractive for liquid crystal TV applications.     

Synthesis and liquid crystal properties of some 2,6‐disubstituted anthracenes

The synthesis and liquid crystalline properties of some new 2,6‐disubstituted anthracenes are described. Symmetrical alkylphenyl derivatives exhibit smectic mesophases, whereas simpler structures are non‐mesogenic. One unsymmetrically substituted anthracene exhibits a narrow nematic phase. The potential for these molecules to act as phototriggers has been investigated but efficient photodimerisation cannot be achieved.     

Improvements in morphological and electro‐optical properties of polymer‐dispersed liquid crystal grating using a highly fluorine‐substituted acrylate monomer

To improve the morphological and electro‐optical properties of a polymer‐dispersed liquid crystal (PDLC) grating, a type of highly fluorine‐substituted acrylate monomer was added to the prepolymer mixture. The morphologies of the PDLC gratings were investigated using atom force microscopy and scanning electron microscopy. The grating had a very clear polymer/LC interface after addition of 3.9 wt % of fluorine‐substituted monomer. The LC droplets in this case were much larger than the sample without fluorinated monomer. This phenomenon indicated that an almost complete phase separation had occurred. However, as the content of fluorine‐substituted monomer increased, the morphologies of gratings became less defined and the volumes of LC droplets were smaller. The diffraction efficiency (DE) decreased with increasing of fluoride content and the V ₉₀ increased simultaneously, which may be ascribed to the blurry interface and the small LC droplets. The highest DE (90%) and lowest V ₉₀ (70 V) were obtained simultaneously under the condition of 3.9 wt % fluoride added in the prepolymer. In addition, it was also found that the fluorine‐substituted monomer may disorder the alignment of LCs in the grating.     

Anomalous behaviour in the SmA*–SmCA* pre‐transitional regime of a chiral swallow‐tailed antiferroelectric liquid crystal

The temperature‐ and electric field‐dependent dielectric relaxation and polarisation of a new chiral swallow tailed antiferroelectric liquid crystal, i.e. 1‐ethylpropyl (S)‐2‐{6‐[4‐(4′‐decyloxyphenyl)benzoyloxy]‐2‐naphthyl}propionate (abbreviated as EP10PBNP), were investigated. The electric field‐induced dielectric loss spectra of EP10PBNP revealed electroclinic and anomalous dielectric behaviour in the chiral smectic A (SmA*)–chiral antiferroelectric smectic C (SmC_A*) pre‐transitional regime. From an analysis of thermal hysteresis of the dielectric constant, electric field‐induced polarisation and dielectric loss spectra, the appearance of a ferrielectric‐like mesophase is observed followed by an unstable SmC_A* phase in the SmA*–SmC_A* pre‐transitional regime.     

Ferroelectric liquid crystal dopants with a chiral (R,R)‐2,3‐difluorooctyloxy side‐chain: host dependence of the polarization power

The polarization powers δ_p of four chiral dopants with (R,R)‐2,3‐difluorooctyloxy side‐chains were measured in four liquid crystal hosts with isotropic (I)–nematic (N)–smectic A (SmA)–smectic C (SmC) phase sequences. The four chiral dopants differ in terms of their core structures: 2‐phenylpyridine (MDW950), biphenyl (5), 2‐phenylpyrimidine (6) and 2‐(3‐nitrophenyl)pyrimidine (7). In each case, δ_p varies with the structure of the liquid crystal host, which is consistent with the behaviour of so‐called Type II dopants that normally feature a chiral core structure. The δ_p(host) profile was found to depend on the degree of biaxiality of the dopant core structure, and on the degree of steric coupling between the chiral 2,3‐difluorooctyloxy side‐chain and the core. Conformational analyses at the B3LYP/6‐31G* level suggest that the 2,3‐difluorooctyloxy side‐chain is conformationally more rigid than conventional chiral side‐chains due to the added electrostatic repulsion of the two adjacent fluoro groups combined with the hyperconjugative ‘gauche effect’, and may therefore have a higher degree of biaxiality on the time average. This biaxial character should make the chiral side‐chain more sensitive to variations in quadrupolar ordering imposed by the SmC phase of the liquid crystal host, and may therefore explain the dependence of δ_p on the host structure reported herein.     

Improving the electro‐optical properties of bistable reflective cholesteric liquid crystal displays

We have improved the electro‐optical properties of a bistable cholesteric liquid crystal display (Ch‐LCD) that is driven by a 3+2 dynamic driving scheme (3+2 DDS). The best contrast ratio is achieved at the selection period of 1.2–1.5 ms/line and the temperature range 26–30°C. The suitable preparation period and evolution period for contrast ratio are 40 times and 20 times the selection time, respectively. In the 3+2 DDS, rubbing the homeotropic alignment layer increases reflectivity of the ON state and keeps the reflectivity of the OFF state at the same level, so reflectivity and contrast ratio alike increase as a result. However, in a delayed homeotropic reset driving method, when the homeotropic alignment layer is rubbed, the reflectivity of both the ON and OFF states increases, thus the contrast ratio decreases. The combination of driving method and aligned surface morphology influences the relaxation mechanism in the cholesteric texture. By optimizing panel condition, we have demonstrated an 8.4″ foldable VGA Ch‐LCD that exhibits high reflectivity and contrast ratio with an addressing speed of around 1.2 ms/line.     

Effect of the twisted alignment on the liquid crystal wave‐front corrector

The effect of twisted alignment on the phase modulation of a liquid crystal wave‐front corrector was investigated. First, the effect of twisted alignment is discussed in terms of the modulation principle of the liquid crystal molecule. Only partial incident light can be modulated because of the effect of the twisted alignment. Other unmodulated light will affect the correction accuracy and the resolution of the image. The blazed grating method is proposed to solve this problem. Adaptive correction was performed without the blazed grating method and the correction results are poor. A similar adaptive correction experiment was performed with the blazed grating method and a better correction result is obtained. The residual averaged wave‐front errors are PV = 0.101λ and RMS = 0.015λ and a resolvable image is obtained.     

Shear‐induced lamellar phase of an ionic liquid crystal at room temperature

The phase behaviour of a number of N‐alkylimidazolium salts was studied using polarizing optical microscopy, differential scanning calorimetry and X‐ray diffraction. Two of these compounds exhibit lamellar mesophases at temperatures above 50°C. In these systems, the liquid crystalline behaviour may be induced at room temperature by shear. Sheared films of these materials, observed between crossed polarisers, have a morphology that is typical of (wet) liquid foams: they partition into dark domains separated by brighter (birefringent) walls, which are approximately arcs of circle and meet at “Plateau borders” with three or more sides. Where walls meet three at a time, they do so at approximately 120° angles. These patterns coarsen with time and both T1 and T2 processes have been observed, as in foams. The time evolution of domains is also consistent with von Neumann's law. We conjecture that the bright walls are regions of high concentration of defects produced by shear, and that the system is dominated by the interfacial tension between these walls and the uniform domains. The control of self‐organised monodomains, as observed in these systems, is expected to play an important role in potential applications.     

Photoluminescence of a 5CB liquid crystal–organomontmorillonite nanoparticle heterocomposite

Photoluminescence (PL) of a heterocomposite, consisting of the nematic liquid crystal (LC) 4-pentyl-4´-cyanobiphenyl (5CB) and anisometric nanoparticles of montmorillonite (MMT) clay, modified by cetyltrimethylammonium bromide (CTAB) has been investigated at 4.2 and 300 K. The incorporation of this organoclay (B4) to 5CB decreases the emission intensity by 7–8 times due to efficient resonant quenching of the exciting energy by the organoclay. The spectrum shifts to a long-wave region, with this effect being considerably larger at low temperatures. Graphical separation of complex bands, corresponding to the bulk 5CB and 5СВ?+?В4 heterosystem at both temperatures revealed that the presence of the organoclay resulted in a significant growth of LC dimer quantity, shifting spectra towards longer wavelengths. Changes in the 5CB luminescence under organoclay influence can be explained by quite strong interphase interactions specified earlier by infrared spectroscopy between the MMT surface and LC, and by a realisation of more flat conformations of 5CB molecules. Confinement effects prevent full crystallisation of 5CB in the 5CB?+?B4 composite, and LC dimer structures located in the organoclay near-surface layers on the outer surface of the nanoparticles and inside its galleries remain in a larger amount, at low temperature, when compared to bulk 5CB. The remaining LC crystallises and photoluminescence from the 5CB monomers becomes intense.     

Synthesis and liquid crystal properties of a new class of calamitic mesogens based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives with wide mesomorphic temperature ranges

Liquid crystals based on substituted 2,5‐diaryl‐1,3,4‐thiadiazole derivatives (1a–1f, 3a and 3b) and 1,3,4‐oxadiazole analogues (2a–2f, 4a and 4b) were synthesised and characterised by ¹H, ¹³C nuclear magnetic resonance, Fourier transform infrared, mass spectrometry, high‐resolution mass spectrometry techniques and elemental analyses. The X‐ray crystal structure of 1e revealed that it contains tilted lamellar arrangement of molecules in the crystalline solid. The liquid crystal properties have been investigated by polarised‐light optical microscopy, differential scanning calorimetry and in‐situ variable‐temperature X‐ray diffraction. All compounds (except 2e and 2f) exhibited thermotropic liquid crystal behaviours with various mesophases (smectic A and C, nematic N or soft crystal E phases). Notably, the 1,3,4‐thiadiazole derivatives consistently have wider mesomorphic temperature ranges than those of the respective 1,3,4‐oxadiazole analogues. The solutions of all compounds in CH₂Cl₂ individually displayed one or two absorption bands with λ _max values at 297–355 nm and emitted with λ _max values at 363–545 nm and quantum yields of 0.12–0.73. Structure–property relationships of these compounds are discussed in the contexts of their molecular structures and weak intermolecular interactions.     

Dielectric properties of magnetic nanoparticles’ suspension in a ferroelectric liquid crystal

Colloids of elongated γ-Fe₂O₃ magnetic nanoparticles (NPs) in a ferroelectric liquid crystal (FLC) were studied. Decreasing the dielectric strength of the Goldstone mode and changing the value of Cole–Cole parameter were found in the suspensions. It was also shown that the effect of introducing magnetic particles into a FLC consists of increasing the electric field strength magnitude required for unwinding its helical structure. Effect of magnetic field on dielectric properties of the FLC colloid was also studied. Dielectric constant measured under static magnetic field is different for the FLC host and FLC doped with the NPs.     

Synthesis and liquid crystalline properties of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group

A series of novel compounds containing a 3‐fluoro‐4‐cyanophenoxy group were synthesized and fully characterized by IR and ¹H NMR, and their mesomorphic properties were studied. Seven compounds exhibited enantiotropic nematic phases and three compounds exhibited monotropic nematic phases, as confirmed by differential scanning calorimetry and polarizing optical microscopy. Selected properties of the liquid crystalline compounds synthesized were calculated by ab initio methods at a HF/6‐31G level. The bond lengths, bond angles and dihedral angles of the fragments with the same structure change little between the compounds. All the compounds with a terminal alkoxy chain approached a planar structure.     

In situ photochemical conversion from cinnamoyl‐functionalized liquid‐crystalline monomers to liquid‐crystalline dimers

Liquid‐crystalline (LC) monomers, which were functionalized with a cinnamoyl group on their extremity, were synthesized and irradiated with UV light in their LC phases. In the presence of a triplet sensitizer, most LC monomers were converted into the corresponding dimers, which were produced by the cycloaddition reaction of the cinnamoyl group. The photodimerization reaction could proceed while the LC phases were maintained, because the dimers showed LC phases whose temperature ranges were wider than those of the corresponding monomers. A ¹H NMR study of the LC dimers indicated that the cyclobutane unit dominantly had an anti‐head‐to‐head configuration, that is, δ‐truxinate. As the LC monomers, which had a phenyl biphenyl‐4‐carboxylate moiety as a mesogen, showed smectic A phases and the corresponding dimers also exhibited smectic A phases, we estimated the smectic layer distances by X‐ray diffraction analysis and found that the dimers adopted the structure in which the two mesogens aligned laterally and existed in the same smectic layer in the LC phases.