Structural characterization of organosiloxane liquid crystals with a transverse or longitudinal dipole

In this work we report the characterization of two organosiloxane liquid crystalline compounds by means of DSC, polarizing optical microscopy and X-ray diffraction. These compounds can be used for dye guest-host ferroelectric displays. We focus this investigation on the molecular organization of the SmC phases of the two pure compounds to help elucidate the physical behaviour of mixtures with different concentrations of the dye and the guest host. The existence of longitudinal and transverse dipoles in the molecules of the dye and the chiral guest-host respectively are responsible for the different molecular organizations in the SmC mesophases of each compound. Taking into account the experimental results and the complexity of the molecules, we present coherent models to explain the molecular arrangements in the mesophases of both compounds.     

Ferroelectric and antiferroelectric low molar mass organosiloxane liquid crystals

The present paper is a study of the ferroelectric and antiferroelectric behaviour in low molar mass organosiloxane liquid crystal materials classed as mono-mesogens (AB type) and bi-mesogens (ABA type). A systematic study of series of materials all based on the same chiral mesogenic moiety is presented. The mesogen is a biphenylyl benzoate with a halogen X attached laterally to the benzoate ring which is closest to the chiral centre. Three homologous series, where the halogen is either fluorine, chlorine or bromine, are investigated. The number of silicon atoms in the linkage or end group is varied between two and nine. It is found that the materials have a broad (40 C) ferroelectric or antiferroelectric phase with a high P s (90nC cm2). The siloxane moiety forces the tilt angle to a temperature independent value close to 45. The antiferroelectric order is observed only in bi-mesogens with an odd number of silicon atoms in the siloxane link. The antiferroelectric order is attributed to the conformation of the molecule rather than to antiferroelectric interactions between layers.     

Design of liquid crystals with "de Vries-like" properties: organosiloxane mesogen with a 5-phenylpyrimidine core

According to a new design strategy for "de Vries-like" liquid crystal materials, we report the synthesis and characterization of an organosiloxane mesogen with a 5-phenylpyrimidine core that forms SmA and SmC liquid crystal phases. This new material is characterized by a maximum layer contraction of 1.2% upon SmA-SmC phase transition and is comparable to the best "de Vries-like" materials reported heretofore.     

Optical and electro-optical properties of bimesogenic organosiloxane antiferroelectric liquid crystals with molecular tilt approaching 45 degrees

We have studied the optical and electro-optical properties of three different bimesogenic siloxane materials. The dimers are symmetrical and the two mesogenic groups are connected by a siloxane unit containing three Si atoms and methylene spacers. The mesogens contain F, Cl and Br substituents, respectively, laterally attached to the phenyl ring lying closest to the chiral centre of the mesogen. These materials each exhibited a broad temperature range antiferroelectric phase and large molecular tilt, which is almost temperature-independent. We have shown that, at low temperatures, the molecular tilt in the antiferroelectric phase is the same as the apparent molecular tilt in the field-induced ferroelectric state and is close to 45°. It was found that the antiferroelectric phase of these compounds, aligned in such way that the dimeric molecules were lying in a plane parallel to the confining substrates, exhibited very low in-plane birefringence. For this reason, samples of the compounds inserted between crossed polarizers in the antiferroelectric state exhibited an extremely dark state, with contrast ratios as high as 1000:1, which did not change noticeably on rotating the sample between the crossed polarizers. It was shown that the optically isotropic siloxane moiety did not contribute to the optical properties, viz. birefringence or tilt angle, but served to enhance the promotion of the antiferroelectric phase and the temperature independent properties. We also showed the existence of a linear electro-optic response in the pretransitional region of the antiferroelectric phase of these compounds over a moderate range of fields.     

Pretransitional behavior in the isotropic phase of a nematic liquid crystal with the transverse permanent dipole moment

The results presented give the evidence for the quasicritical, pretransitional behavior of dielectric properties in the isotropic phase of a rodlike nematic liquid crystal with the transverse permanent dipole moment. Studies were conducted in 2-cyano-4-pentylbiphenyl 4-(trans-4-pentylcyclohexyl) benzoate, focusing on the static-and ionic-dominated low-frequency (LF) regions. For the static dielectric permittivity [epsilon(')(T)] the application of the derivative analysis revealed the pretransitional anomaly associated with the specific heat exponent alpha approximately 0.5. For the LF domain the contribution to epsilon(')(T) from residual ionic impurities follows a linear temperature dependence on approaching the isotropic-nematic (I-N) transition. This dependence and pretransitional anomalies of electric conductivity and dielectric modulus can be associated with the influence of prenematic fluctuations. "Linear" dielectric studies were supported by the static nonlinear dielectric effect measurements, which delivered reliable estimations of the temperature of the hypothetical continuous phase transition T(*) and the discontinuity of the I-N transition DeltaT approximately 1.7 K.     

Structural characteristics of polysaccharide-based thermotropic liquid crystals

Main mesomorphic properties of cellulose and cello- and chito-oligosaccharide derivatives are overviewed, and their structuring principles are briefly discussed with some new analyses incorporated.     

Transverse dipole association and negative dielectric anisotropy of nematic liquid crystals

Measurements are reported of the dielectric properties of three laterally substituted dicyano compounds with large molecular dipole moments perpendicular to the long molecular axis. Results are given for the materials in solution, in the isotropic liquid state, and for one compound in the nematic state. There is evidence of a high degree of local ferroelectric order in all phases, and dipole correlation factors in excess of 3 were obtained. The significance of these observations for macroscopic ferroelectrically ordered and biaxial liquid-crystalline phases is considered.     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Synthesis and characterization of thiophene-containing liquid crystals

Six series of liquid crystal materials containing a 2,5-disubstituted thiophene unit were synthesized. The liquid crystal compounds obtained were characterized by NMR, differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction techniques. The properties of liquid crystalline phases were investigated as a function of spacer units, number of aromatic core rings and different terminal moieties. Cyano, methoxy and iodo groups were used as terminal groups. It is found that: (i) compounds having one thiophene ring and one phenyl ring connected by an ester group, with a length/breadth value of 2.1, exhibit no mesophase, while other compounds containing two biphenyl rings, with a length/breadth ratio of 2.7, show mesophases; (ii) the polarity of terminal groups and the flexible spacer length significantly affect the thermal behaviour of these compounds; (iii) the nematic transition range of cyano-containing compounds decreases with increasing length of the flexible spacer, and long alkenyloxy chains tend to facilitate the formation of the smectic phase and suppress the nematic phase in all the mesogenic compounds synthesized.     

Synthesis and characterization of novel imidazolium-based ionic discotic liquid crystals with a triphenylene moiety

Two novel triphenylene-tethered imidazolium salts were synthesized either by the quaternization of 1-methylimidazole with an ω-bromo-substituted triphenylene or by the quaternization of a triphenylene-substituted imidazole with methyl iodide. Their chemical structures were determined by ¹H NMR, ¹³C NMR, IR, UV spectroscopy and elemental analysis. The thermotropic liquid crystalline properties of these salts were investigated by polarizing optical microscopy and differential scanning calorimetry. These triphenylene-based imidazolium salts with bromide or iodide as counterion show columnar mesophase properties over a wide temperature range.     

Structural factors controlling the self-assembly of columnar liquid crystals

A series of disc-shaped molecules were prepared by the condensation of 1,2-diamines with 2,3,6,7-tetrakis(hexyloxy)phenanthrene-9,10-dione to investigate the relationship between changes in molecular structure and the self-assembly of columnar liquid crystalline phases. A comparison of compounds with different core sizes indicated that molecules with larger aromatic cores had a greater propensity to form columnar phases, as did compounds substituted with electron-withdrawing groups. In contrast, molecules with electron-donating substituents were nonmesogenic. The clearing temperature of columnar phases increased linearly with the electron-withdrawing ability of the substituents, as quantified by Hammett sigma-values. The observed trends can be rationalized in terms of the strength of pi-pi interactions between aromatic cores in the liquid crystalline phases and suggest that both electrostatic interactions and dispersion forces play important roles in the self-assembly of these materials.     

Study of the molecular configuration and the dipole moment in fluorinated liquid crystals

We have investigated the relationship between the molecular configuration and dipole moment of some fluorinated liquid crystals (LCs). The geometries of the molecules were preliminarily optimized at empirical AM1 and then were further optimized at B3LYP/6‐31G(d) level. The dipole moment has been calculated. It is strongly influenced by the position and number of fluorine substituents in the benzene ring of the molecule. The polarizability, mean polarizabilities, and anisotropic polarizability of the phenylbicyclohexane (PBC) fluorine substituents are also given and discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Synthesis and characterization of non-symmetrical liquid crystals containing a cholesteryl ester moiety

In this paper,a series of chiral non-symmetrical liquid crystals(nBA-chol)consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core with different terminal alkyl chain has been synthesized and investigated for their liquid crystalline properties.Effects of numbers of methylene units in the terminal alkyl chain on the phase transition temperatures and on the temperature-dependent pitch lengths of the chiral liquid crystals have been studied.The long terminal alkyl chain tends ...     

Synthesis and characterization of novel thermotropic liquid crystals containing a dimethylamino group

Novel thermotropic liquid crystals containing a terminal dimethylamino group were synthesized via a multi-step route. Three phenyl ring units linked with ester and azomethine groups constituted the mesogenic core. All the compounds were characterized by spectroscopic techniques, polarizing microscopy and differential scanning calorimetry. An enantiotropic nematic phase was observed for all the systems studied.     

Reminiscences from a life with liquid crystals

The author looks back over a scientific career of almost 50 years spent mainly in research on liquid crystals (LCs) and carried out largely in University service and latterly for about five years in Industry. In this review, the development through its rapid escalation starting in the early 1970s and stemming from the development of applications for LCs in electro-optical displays, itself set on course by the author's own discovery of the first materials enabling the production twisted nematic displays. The author's other contributions widened into a general review of current trends and activities in the field, with some emphasis being placed on threats to fundamental research posed by diminished funding and the current pressures on researchers to engage heavily in short term, 'wealth creating' projects. of UK LC research is traced mainly of commercially viable, long life in the field are discussed and     

Preparation and characterization of polyfluoroaniline/organosiloxane hybrid films

Polyfluoroaniline (PFANI)/organosiloxane hybrid films were directly prepared from PFANI emulsions and γ-glycidoxypropyltrimethoxysilane (GPTMS) through a sol–gel method. The influence of GPTMS/FANI molar ratios on the morphology and properties of PFANI/GPTMS hybrid films was characterized by fourier-transform infrared, scanning electron microscopy, thermo gravimetric, contact angle measurement and potentiodynamic polarization analysis. It was found that the hybrid films are highly water repellent. In addition, increasing the GPTMS content could significantly improve the thermal behavior and corrosion resistant ability of the hybrid films. However, the water repellency decreases slowly as the GPTMS/FANI molar ratio increases.     

Synthesis and characterization of polymerizable photochromic liquid crystals containing a spiro-oxazine group

Novel polymerizable photochromic liquid crystal materials containing either the 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]naphth[2,1-b][1,4]oxazine] group or the 1,3-dihydro-1,3,3-trimethylspiro[2H-indole-2,3'-[3H]pyrido[3,2-f][1,4]benzoxazine] group as the photochromic moiety, and the biphenylene unit as the mesogenic moiety, were synthesized. Thermal and physical properties of the compounds were examined using differential scanning calorimetry, optical polarizing microscopy, wide-angle X-ray diffractometry and UV-visible spectrophotometry. The photochromic compounds containing both photochromic and mesogenic moieties showed metastable mesophases; These tended to crystallize with time and on heating. The crystallization behaviours were strongly dependent on the structure of the terminal group of alkylene spacer and the position of the spiro-oxazine bound to the mesogen.