Germanium liquid crystals

Liquid-crystalline compounds containing germanium atoms were synthesised and assessed for liquid-crystalline properties. These new compounds generally possess smectic C phases, and many also possess nematic, smectic A and higher order smectic phases. The germanium-containing liquid crystals were incorporated into smectic C mixtures. These mixtures tend to exhibit little change in smectic C*?layer thickness over temperature. This characteristic is associated with de Vries smectic A materials, but measurements show that, although they have high smectic C stability, the materials' smectic cone angles are small. Measurement of smectic cone angle versus temperature of an exemplar material and its analogues containing carbon and silicon in place of the germanium, all show small cone angles which fall smoothly and extrapolate to zero as the smectic C*?to smectic A transition is approached. These measurements largely explain the observed small layer changes and establish that the materials are not first-order de Vries materials. They must be located elsewhere along the de Vries-orthogonal continuum of smectic A phases.     

Preliminary Communication : Induction of smectic A phases by electron donor-acceptor interaction between calamitic mesogens and 2,4,7-trinitrofluorenone

The mesomorphic properties of conventional rod-like liquid crystals (diphenyl1,3,4-thiadiazoles, diphenylpyrimidines, diphenyltriazines, diphenyltetrazines and p-terphenyl derivatives), of macrocyclic liquid crystals and of dimesogens can be influenced by addition of the electron acceptor 2,4,7-trinitrofluorenone (TNF). Thereby nematic and smectic C phases are suppressed and smectic A phases can be stabilized or induced. Long and branched terminal chains result in a strong stabilization of the SA phase, whereas no smectic phase is induced to accompany the nematic phases of mesogens with short terminal chains.     

The effect of phase transitions on the response times of ferroelectric liquid crystal devices

The response times of ferroelectric liquid crystal devices are of great importance, and are known to be related to various physical properties of the chiral smectic C* phase, such as the spontaneous polarization, the cone angle and the rotational viscosity. We report measurements of the effect of the smectic phase transitions on the response times and show that these are influenced by the smectic C* to smectic A transition temperature.     

Competition between micro-segregation and anti-parallel alignment of an amphiphilic rod-like liquid crystal

Recently, we reported that a rod-like molecule, 4-[4-(7-hydroxyheptyloxy)phenyl]-1-(4-hexylphenyl)-2,3-difluorobenzene, exhibited a nematic phase with a layered structure and smectic C phases consisting of three states. We prepared a homologous series of the rod-like molecules in which a 2,3-difluoro-1,4-diphenylbenzene unit and a hydroxyl unit are connected via a flexible methylene spacer. We investigated their physical properties using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Although they showed nematic, smectic A and smectic C phases, the phase structures were found to depend on the flexible spacer length. Those compounds possessing a shorter spacer length than a heptyl unit exhibited bilayered smectic A and smectic C phases, whereas those possessing a longer spacer length than a heptyl unit showed conventional monolayered smectic A and smectic C phases. A nematic phase with a smectic-like layer ordering was observed for the compound possessing an octyl unit. The structure–property relations of the amphiphilic compounds are discussed.     

Symmetric esters derived from 1,3,4-oxadiazole: synthesis, mesomorphic properties and structural study by semi-empirical calculations

Three homologous series of symmetric esters derived from 2-alkylthio-5-( p -hydroxy)phenyl-1,3,4-oxadiazole (series 3 , 4 and 5 ) were synthesized and their liquid crystalline properties investigated by optical microscopy and differential scanning calorimetry. Depending on the chain length, nematic and smectic C phases were observed in the series 3 ; smectic A and smectic C phases in series 4 . None of the homologues of series 5 shows mesomorphic properties. These three series are compared with other previously reported oxadiazoles. A structural study of AM1 semi-empirical calculations is also described.     

Symmetric esters derived from 1,3,4-oxadiazole: synthesis,mesomorphic properties and structural study by semi-empirical calculations

Three homologous series of symmetric esters derived from 2-alkylthio-5-(p -hydroxy)phenyl-1,3,4-oxadiazole (series 3, 4 and 5) were synthesized and their liquid crystalline properties investigated by optical microscopy and differential scanning calorimetry. Depending on the chain length, nematic and smectic C phases were observed in the series 3 ; smectic A and smectic C phases in series 4. None of the homologues of series 5 shows mesomorphic properties. These three series are compared with other previously reported oxadiazoles. A structural study of AM1 semi-empirical calculations is also described.     

Coupling of liquid crystalline and polymer network properties in LC-elastomers

Electromechanical and electro-optical properties which result from an interplay of ferroelectricity and elasticity in slightly crosslinked ferroelectric liquid crystalline elastomers are studied in the vicinity of the smectic C-smectic A phase transition. In the chiral smectic C phase, the formation of the network during the crosslinking procedure stabilizes the polar orientation which is present. On heating the smectic C network into the chiral smectic A phase, an induced tilt is observed (mechanoclinic effect). The internal mechanical stress induced by the network formation appears to act like an external electric bias-voltage. The results are discussed in terms of a simple Landau model.     

The stabilization of smectic A phases in mixtures of twinned-calamitic, metal organic complexes with 2,4,7-trinitro-9-fluorenone

The liquid crystal properties of binary mixtures of copper- and palladium-containing, twinnedcalamitic, metal organic complexes with the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF) are reported. The smectic A to isotropic transition temperature of the pure metal organic component is increased on mixing. Furthermore, only smectic A behaviour is exhibited by the mixtures compared with smectic A and C phases for the pure metal organic complexes. The stabilization of the smectic A phase in these mixtures is found to be due to a weak charge transfer type interaction.     

Liquid crystalline properties of 4-alkoxyphenyl 3-fluoro- and 3-methyl-4-thiocyanatophenyl terephthalates

The preparation and thermal properties of homologous series of 4-alkoxyphenyl 3-fluoroand 3-methyl-4-thiocyanatophenyl terephthalates are described. Both homologous series show nematic phases; smectic A and C phases are also formed in the higher homologous. The methyl group and fluorine atom adjacent to the thiocyanate group lower the clearing points, in comparison with the unsubstituted compounds. The layer structures for both smectic A and C phases were characterized by X-ray diffraction experiments. The effects of the substituents on liquid crystalline properties are discussed.     

Synthesis and study of 4-(4'-n-alkoxybenzoyloxybenzoyl)-4'-n-butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n-propyl derivatives exhibit nematic phases, n-butyl to n-decyl derivatives exhibit smectic and nematic mesophases, whereas n-dodecyl to n-octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd-even effect for nematic-isotropic transition temperatures. Nematic-isotropic and smectic-cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin-layer chromatography and spectral data.     

Layer distortions induced by a magnetic field in planar samples of smectic C liquid crystals

This article derives theoretical results for the onset of the Helfrich-Hurault transition in smectic C liquid crystals induced by a magnetic field applied parallel to the smectic layers. A suitable quadratic energy in terms of the smectic layer displacement u is derived from the nonlinear version of the smectic C energy. This energy is minimized via averaging to enable the calculation of a critical field strength H_c for the onset of layer distortions. Comparisons are made with known results for the corresponding geometry in the smectic A case. An estimate for the value of the smectic C elastic constant A₁₂ can also be made by considering characteristic length scales.     

Structural assignment and molecular order of three-ring mesogen by 13C NMR spectroscopy in mesophase

A homologous series of rod-like molecules with three phenyl rings in the core and terminal alkoxy chains are synthesised from mesogenic two-ring aldehyde by coupling with non-mesogenic 4-alkoxy anilines. The mesophase properties are evaluated with hot-stage optical polarising microscopy and differential scanning calorimetry, and accordingly, all the molecules exhibited enantiotropic nematic and smectic C phases along with monotropic low-temperature smectic phases. For a representative homologue, the existence of smectic C phase is further confirmed by noticing a sharp reflection at small angle region in powder X-ray diffraction which varies with change in temperature. The main focus of the investigation, however, is the clear demonstration of chemical shift assignment of static ¹³C NMR of a representative three-phenyl ring mesogen in smectic C phase. In this novel approach, the static ¹³C NMR spectral data of synthetic mesogenic intermediate namely two-ring aldehyde are utilised for the chemical shift assignment of three-ring mesogens. Further, the orientational order parameter of two-ring aldehyde in smectic A and three-ring mesogen in smectic C phase is carried out by measuring the ¹³C-¹H dipolar couplings by 2D separated local field spectroscopy.     

Ferroelectric smectic C liquid crystal mixtures: Relationship between molecular structure,ferroelectricity and electro-optic response

The relationship between the physical properties and the molecular structure of a number of smectic C compounds has been investigated. Special emphasis was placed on the study of a new class of laterally substituted smectic C compounds incorporating a trans-1,4-disubstituted cyclohexane ring. We have investigated the influence of new chical dopants on the ferrolectric and physical properties of smectic C mixtures. New chiral smectic C liquid crystal mixtures have been developed. Due to the low response times, low birefringence and excellent chemical and U.V. stability, these mixtures are particularly well suited for electro-optic device applications.     

The effect of fluorinated terminal chains on the mesomorphic properties of 4,4'-disubstituted phenyl benzoates

A variety of 4,4'-disubstituted phenyl benzoates having a terminal chain containing multifluorine atoms, attached directly to the benzene ring or through an ester group, have been synthesized and their mesomorphic properties determined by hot stage polarizing optical microscopy. These properties were compared to those of the corresponding hydrogenated esters and to other esters containing rigid terminal chains. Usually transition temperatures were higher and mesophase ranges wider than those observed for the parent compounds but no nematic phases were found. Any mesophase seen was usually a smectic A phase sometimes accompanied by a smectic C phase. Crystal E phases were found along with the smectic A phase in alkyl or alkoxy esters having a C₉F₁₉CO₂ chain on the acid side. A first order smectic A-smectic C transition was observed in the ester with CN on the acid side and O₂CC₇F₁₅ on the phenol side. A comparison of the effect of a terminal fluorinated chain and a lateral fluorine group on one set of esters is also included.     

Properties of new ferroelectric materials

A homologous series of chiral 4-(3-methylpentyl)benzenethio-4′-n-alkoxy-benzoates has been studied. These thioesters display a ferroelectric, chiral smectic C phase in addition to cholesteric and smectic A phases. A comparison is made between the thioester series and a phenylbenzoate, having the same molecular end group. The effect of the different central linkage on the transition temperature, and on the physical and ferroelectric liquid crystal (FLC) properties has been investigated. Several mixtures, containing these thioester components, were calculated and formulated to obtain room temperature chiral smectic C phases. Spontaneous polarization P_s values and electro-optical response times are correlated with chemical structures. Although these thioesters have very low P _s values, they are useful components for FLC mixtures because of their convenient chiral smectic C temperature ranges and their low viscosities.     

Liquid crystal properties of new ionic liquid crystal compounds having a 1,3-dioxane ring

New pyridinium type chiral ionic liquid crystal materials having a 1,3-dioxane ring in the central core, (+)- N -2-methylbutyl-4-(5-alkyl-1,3-dioxan-2-yl)pyridinium bromides ( 6 ), were synthesized. Mixture A was made by adding 5.9 wt % of compound 6-3 to the ferroelectric liquid crystal mixture Merck 4655-100; its phase transitions and some liquid crystal properties were measured. In spite of a large decrease of P s for mixture A, its response time A was the same as that of Merck 4655-100. The dielectric constants perpendicular to the molecular axis for the smectic C* and smectic A phases were measured for the mixture and Merck 4655-100: in the smectic C phase, these dielectric constants were almost the same (4.5~5.5), but in the smectic A phase, the perpendicular dielectric constant of the mixture was 47.0 and that of Merck 4655-100 was 4.5.     

Layer distortions induced by a magnetic field in planar samples of smectic C liquid crystals

This article derives theoretical results for the onset of the Helfrich-Hurault transition in smectic C liquid crystals induced by a magnetic field applied parallel to the smectic layers. A suitable quadratic energy in terms of the smectic layer displacement u is derived from the nonlinear version of the smectic C energy. This energy is minimized via averaging to enable the calculation of a critical field strength H _c for the onset of layer distortions. Comparisons are made with known results for the corresponding geometry in the smectic A case. An estimate for the value of the smectic C elastic constant A ₁₂ can also be made by considering characteristic length scales.     

Synthesis and study of 4‐(4′‐n‐alkoxybenzoyloxybenzoyl)‐4′′‐n‐butoxyanilides

Thirteen compounds with ester and amide linkages were synthesized and their mesogenic properties evaluated. Methyl to n‐propyl derivatives exhibit nematic phases, n‐butyl to n‐decyl derivatives exhibit smectic and nematic mesophases, whereas n‐dodecyl to n‐octadecyl derivatives exhibit only smectic phases. All the smectic homologues exhibit smectic C phases. Middle members of the homologous series exhibit polymorphism of smectic mesophase. A plot of transition temperatures versus number of carbon atoms in the alkoxy chain reveals an odd–even effect for nematic–isotropic transition temperatures. Nematic–isotropic and smectic–cholesteric thermal stabilities of the prepared compounds (series I) are higher compared to those of previously reported compounds, series A, B and C. The results indicate that a simple reversal of a central linkage has a dramatic effect on the appearance of smectic mesophase in a homologous series. The structures of the synthesized compounds were characterized using elemental analysis, thin‐layer chromatography and spectral data.     

Novel phase behaviour in two (smectic/cholesteric) liquid crystal mixtures

The phase behaviours of mixed liquid crystal systems having either Sm/N or Sm/Ch properties have been studied. The (smectic/nematic) binary system formed smectic phases over a wide and much enhanced range of temperature (42 C) and a broad concentration range (0-90 wt %). The ternary smectic/cholesteric system, in appropriate concentration ranges, exhibited the smectic A phase, a TGBA-like twist grain boundary A phase, the cholesteric phase and blue phases. The TGBA-like phase appeared in the cholesteric-smectic phase transition range. Three textures (chiral pitch, fan-shaped and scale-like) for the cholesteric phase of the ternary smectic/cholesteric mixtures were observed in the ranges 0-7, 7-43 and 43 wt % respectively, of cholesteric CB15, in a binary Sm/N mixture.     

Smectic ordering in athermal systems of rodlike triblock copolymers

The phase behavior of the system of parallel rigid triblock copolymers is examined using the second virial density functional theory. The triblock particle consists of two identical infinitely thin hard rods of finite lengths on the opposite ends of one central hard cylinder with nonzero length and diameter. Stability analyses and free energy calculations show that the system of parallel particles can form not only uniform nematic and smectic A phases but also a smectic C phase. The stability and structure of the tilted structure are controlled by only the diameter and the length of the central cylinder segment. Interestingly, the diameter affects only the layer tilting and the periodicity, but not the packing fraction of the nematic to smectic-C transition. For all values of cylinder length the usual smectic A and smectic C transitions compete with each other and no nematic-columnar transition is observed. At low and high cylinder lengths the smectic A phase is stabilized first, while the smectic C is the most stable for intermediate length values.