Influence of the smectic A-nematic transitional order on the formation of the smectic phases of 4-n-hexyloxybenzylidene-4′-n-butyIaniline and 4-n-butyloxybenzylidene-4′-n-octyIaniline from an electrically deformed nematic phase

Abstract

The crucial role of the smectic A-nematic transitional order for the formation of the smectic A, B and G phases from an electrically deformed nematic phase of the liquid crystal 4-n-hexyloxy-benzylidene-4′-n-butylaniline (6O.4) with a typical smectic A-nematic first order transition and the formation of the smectic A and B phases from an electrically deformed nematic phase of the liquid crystal (4-n-butyloxy-benzylidene-4′-n-octylaniline (40.8) with a smectic A-nematic second order transition has been demonstrated. The nematic phase was deformed by an AC voltage of 2U,_th 5U _th and 10U _th, where U _th is the threshold voltage which causes the appearance of the Fréedericksz transition in the homeotropic nematic layer. The smectic textures have been observed on free cooling of the nematic phase or after the use of an oven. The smectic A phase of the liquid crystal 60.4 was observed with the formation of a clear smectic A-nematic phase boundary while the smectic A phase of the liquid crystal 40.8 has been formed from intermediate pretransitional stripes, observed by Cladis and Torza [1]. The homeotropic anchoring of the direction was crucial for the formation of the smectic phases of the liquid crystal 40.8 but not significant for the liquid crystal 60.4.     

Pyrrolidinium Ionic Liquid Crystals

N‐Alkyl‐N‐methylpyrrolidinium cations have been used for the design of ionic liquid crystals, including a new type of uranium‐containing metallomesogen. Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate, thiocyanate, tetrakis(2‐ thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group C_nH_2n+1 was varied from eight to twenty carbon atoms (n=8, 10–20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncommon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly ordered phase. This uncommon tetragonal mesophase is thoroughly discussed on the basis of detailed powder X‐ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self‐assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds containing a metal complex anion were investigated. For the uranium‐containing mesogens, luminescence can be induced by dissolving them in an ionic liquid matrix. The europium‐containing compound shows intense red photoluminescence with high colour purity.     

Three ring dioxanes as dopants enhancing the stability of the smectic C phase

Abstract

Searching for compounds which could be useful as modifiers of smectic C mixtures, we have synthesized four homologous series of three ring dioxanes, 2BBD, 5BBD, 2CBD, and 5CBD. Their phase transition temperatures and enthalpies were measured and their liquid crystal phases identified. Compounds belonging to series n-BBD form smectic B_cr phases for shorter alkyl chains, and smectic B_cr and A phases, for longer chains. Compounds belonging to the n-CBD series exhibit the smectic A phase, but those with longer alkyl chains have exclusively smectic B phases and those with short tails have other low temperature, highly ordered smectic phases. The compounds were added to smectic C mixture and it was found that some can be useful as dopants. Compounds with longer alkyl tails in the molecule are more suitable for this purpose; the type of ring in the core is less important.     

Induced smectic phases in phase diagrams of binary nematic liquid crystal mixtures

To elucidate induced smectic A and smectic B phases in binary nematic liquid crystal mixtures, a generalized thermodynamic model has been developed in the framework of a combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for orientational ordering, McMillan free energy for smectic ordering, Chandrasekhar-Clark free energy for hexagonal ordering, and phase field free energy for crystal solidification. Although nematic constituents have no smectic phase, the complexation between these constituent liquid crystal molecules in their mixture resulted in a more stable ordered phase such as smectic A or B phases. Various phase transitions of crystal-smectic, smectic-nematic, and nematic-isotropic phases have been determined by minimizing the above combined free energies with respect to each order parameter of these mesophases. By changing the strengths of anisotropic interaction and hexagonal interaction parameters, the present model captures the induced smectic A or smectic B phases of the binary nematic mixtures. Of particular importance is the fact that the calculated phase diagrams show remarkable agreement with the experimental phase diagrams of binary nematic liquid crystal mixtures involving induced smectic A or induced smectic B phase.     

Twisted smectic A phases in side chain liquid crystal polymers

Abstract

The syntheses of two side chain liquid crystal polymers, a polyacrylate and a polymethacrylate, are reported. In each of the polymers the liquid-crystalline side group carries an asymmetric carbon atom, thereby making some of the liquid crystal phases formed by the polymers optically active and chiral. For the chiral polyacrylate smectic A and chiral ferroelectric smectic C phases are observed, however for the chiral polymethacrylate a cholesteric phase is detected above the smectic A phase. It is found that the smectic A to cholesteric phase transition is mediated by the formation of an intermediary twisted smectic A phase. This intermediary phase is a liquid-crystalline analogue of the Abrikosov flux phase found in Type II superconductors.     

1-((12-Bromododecyl)oxy)-4-((4-(4-pentylcyclohexyl)phenyl)ethynyl) benzene: Liquid crystal with aggregation-induced emission characteristics

The luminescent liquid crystals (LLCs) are expected to solve the conflicts between the aggregation caused quenching and the requirement of aggregation or self-organization for LCs. Herein, we developed a new strategy of applying aggregation-induced emission (AIE) phenomenon to the molecular design of LCs towards LLCs. In this report, a calamitic liquid crystal based on tolane with AIE characteristics was successfully synthesized and the chemical structure was characterized by 1H, 13C NMR, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) high-resolution mass spectra. The fluorescence behavior was studied by fluorescence spectroscopy and the liquid crystal phase behaviors were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM). The crystal structure was obtained by X-ray diffraction crystallography with P1 space group. Results demonstrated that the sample was AIE active and the LC phases sequence during cooling was nematic, smectic C and smectic B phase.     

A new synthetic approach based on (−)-menthone for chiral liquid crystals

Starting from (?)-menthone, a new chiral building block useful for liquid crystal preparation was synthesized. This chiral moiety was attached to selected phenols under mild conditions by esterification. Rigid cores of tolanebenzoates and phenylbenzoates were prepared using the palladium cross-coupling reaction or by traditional liquid crystal synthesis methods. This convergent approach ended with a second esterification or palladium cross-coupling reaction to furnish new liquid crystal materials with smectic A, smectic C* and N* phases, as well as blue phases (BP). Thermal behavior, and the effect of chiral moiety branches and molecular packing in the smectic phases, have been investigated using differential scanning calorimetry, optical microscopy and X-ray diffraction.     

Smectic polymorphism in a series of three-ring enaminoketone compounds

Abstract

Liquid crystalline properties of 1 - (4′ - alkoxyphenylamino) - 3 - (4′ - hexyloxyphenyl)-prop-1-en-3-ones, from methoxy to heptadecyloxy, have been examined by optical, DSC, and X-ray methods. The phase diagram for the series exhibits a rich polymorphism of tilted smectic phases, for example, five mesophases were found for the hexyloxy derivative. A characteristic feature of the phase diagram is a gap in the crystal G phase area. For the heptyloxy homologue, a direct crystal H-smectic F phase transition was found; in the case of shorter as well as longer terminal substituents, the phase sequence crystal H–crystal G–smectic F is observed. Calorimetric and X-ray studies revealed the existence of a tricritical point on the crystal G–smectic F transition line.     

Mimicking the behaviour of rigid rod molecules. Smectic H liquid crystals from amphiphilic quaternary ammonium salts

The effect of secondary hydrogen-bonding interactions on the crystalline and liquid crystalline phases of quaternary ammonium salts functionalised with a carboxylic group attached at the polar head through a decyl spacer of a homologous series of N-alkyl-N-carboxydecyl-N,N-dimethylammonium bromides was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. The low-temperature crystal phases were found to have a lamellar structure in which the ammonium bromide groups are arranged within the layers in two distinct planes, alternately separated by single layers of alkyl chains and double layers of carboxydecyl chains coupled through the carboxyl end groups. At higher temperatures, although these molecules were made from soft flexible chains, smectic H mesophases were identified. The smectic layers were found to be formed by the same two ionic planes alternately separated by the alkyl and carboxydecyl sub-layers. The smectic structure was compared with the three-dimensional positional order observed in the smectic T phase of dihydroxyl functionalised quaternary ammonium salts already described in the literature.     

Experimental characterization of hexatic smectic phases through electro-optic studies and dielectric relaxation spectroscopy

The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4′-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmC_A* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmC_A* and hexatic smectic phases were determined. The dielectric strength at the SmC_A* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmC_A* and hexatic smectic I* phases.     

长径比对椭球粒子液晶行为的影响

利用Gay-Berne模型, 结合分子动力学模拟方法, 研究了粒子长径比对椭球粒子液晶行为的影响, 考察了粒子长径比对向列相和近晶相的影响. 结果表明, 长径比相对较小的粒子有利于向列相的形成, 而长径比相对较大的粒子更有利于近晶相的形成. 分析了近晶相和向列相形成的动力学过程.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Monte Carlo simulations on mesophase formation using dipolar Gay-Berne model

We report the results of a Monte Carlo simulation of polar particles interacting via the Gay-Berne potential combining dipole-dipole interactions. Simulations were carried out on a system of 256 particles with either a zero dipole moment or longitudinal dipole moment located at the centre of the molecule. The system was found to spontaneously form nematic, smectic and crystal phases from an isotropic phase with a random configuration as temperature was decreased, irrespective of values of the dipole moment. The results do not give any indication of a net polarization even in the system with a strong dipole moment (μ* = 2.00). The transition temperature from the isotropic to nematic phase is not sensitive to the value of the dipole moment within the limits of statistical error, while the transition from the nematic to smectic phase depends on the strength of dipole moment. At lower temperatures forming the smectic or the crystal phase, the translational order along the director increases with increasing dipole moment. The dipolar interactions contribute to the long range ordering.     

具有氟烷基边链和手性中心液晶化合物的合成及液晶性研究

本文设计合成了十个含氟烷基边链和手性中心的液晶化合物，并通过DSC和偏光显微镜对它们的液晶性进行了研究。其中二环系液晶化合物不显示液晶相或仅显示单边近晶A相。三环系液晶化合物中较长的氟烷基边链有利于近晶相的形成，且当液晶核另一端的烷氧基链的长度适中时，在氟烷基边链和液晶核之间具有手性中心的液晶分子显示了手性近晶C相和其它液晶相。     

Three ring dioxanes as dopants enhancing the stability of the smectic C phase

Searching for compounds which could be useful as modifiers of smectic C mixtures, we have synthesized four homologous series of three ring dioxanes, 2BBD, 5BBD, 2CBD, and 5CBD. Their phase transition temperatures and enthalpies were measured and their liquid crystal phases identified. Compounds belonging to series n-BBD form smectic B_cr phases for shorter alkyl chains, and smectic B_cr and A phases, for longer chains. Compounds belonging to the n-CBD series exhibit the smectic A phase, but those with longer alkyl chains have exclusively smectic B phases and those with short tails have other low temperature, highly ordered smectic phases. The compounds were added to smectic C mixture and it was found that some can be useful as dopants. Compounds with longer alkyl tails in the molecule are more suitable for this purpose; the type of ring in the core is less important.     

Thermal Expansion of Liquid Crystalline Polyesters Having Phenanthrene Moieties. Simultaneous DSC-XRD method

Phase transition process of polyester from phenanthrene, poly(oxyheptamethyleneoxy-2,7-phenantrenedicarbonyl), a main chain type liquid crystalline polymer, was investigated by simultaneous DSC-XRD measurements using the synchrotron radiation facility (PF). Three exothermic DSC peaks were observed during cooling from the isotropic liquid state. These DSC peaks were assigned to the transition from the isotropic liquid to the smectic A, that from the smectic A to C, and that from the smectic C to the crystalline state, respectively, as determined by XRD profiles. The rate of transition from the smectic A to C was very slow compared with the liquid crystalline transition and the crystallization. From the DSC-XRD results, the thermal expansion along c-axis in the crystal and smectic phases are 4.1×10⁻⁴ and 1.4×10⁻³ nm K⁻¹ , respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Polarized infrared spectroscopic study on changes in molecular orientation and interaction during phase transitions of a ferroelectric liquid crystal with a naphthalene ring

Temperature-dependent polarized infrared spectra were measured over the temperature range 105-30°C for a ferroelectric liquid crystal with a naphthalene ring (FLC-1) in the isotropic, smectic A (SmA), and chiral smectic C (SmC*) phases to investigate its molecular conformation, interactions, and alignment in each phase. It has been found, from the temperaturedependent spectral changes in the 1610-1600 cm^-1 region, that the degree of twist between the naphthalene and benzene rings of FLC-1 changes with temperature. The peak intensity of the band at 1606 cm^-1 containing contributions from both the benzene and naphthalene ring stretching modes begins to decrease, not suddenly but gradually, upon going from the SmA phase to the SmC* phase, suggesting that the molecular orientation of the two rings changes gradually between the two phases. The frequencies of two CH₂ stretching bands suggest that the disorder of the alkyl chain of FLC-1 is similar for the liquid crystal phase and the isotropic liquid phase. The splitting of the core C=O stretching band indicates that the resonance system consisting of the benzene ring and the C=O group in the core part of FLC-1 is involved in two kinds of intermolecular interaction between adjacent molecules in the liquid crystal phase.     

Liquid crystalline behaviour of hydrogen-bonded complexes of alkoxycinnamic acids with octyloxystilbazole

Hydrogen-bonded liquid crystalline complexes have been obtained through 1:1 (molar ratio) complexation of 4-n-alkoxycinnamic acids (nCNA: n = 4, 8, 10, 12, where n is the number of carbons in the alkyloxy chain) and trans-4-octyloxystilbazole (8Sz). These hydrogen-bonded complexes (nCNA8Sz) form stable mesophases. The mesomorphic range was extended by the mixing of complexes. Hexatic modification of smectic B (SmB_h), smectic C (SmC), smectic A (SmA), and nematic mesophases of these complexes were determined by a combination of X-ray diffraction and polarizing optical microscopy. Transitions between the various smectic phases were deduced from the temperature-dependent layer spacing of nCNA8Sz. The layer spacing of these complexes in the SmB_h and SmA phases gradually increased with increasing alkoxy chain length. The favouring of smectic phases in these complexes is believed to originate from the increment of polarity of the mesogen by intermolecular H-bonding.     

Strain‐induced compression of smectic layers in free‐standing liquid crystalline elastomer films

The deformation of oriented smectic liquid crystal elastomer films with smectic layers parallel to the film surface was studied using optical reflectometry and small angle X‐ray diffraction. Reflectometry data show that in the chosen material, in‐plane strain causes a change in the optical thickness of the free‐standing films. Small angle X‐ray scattering was used to explore the molecular origin of this effect. The X‐ray scattering data confirm that the change in optical thickness originates from the compression of the individual smectic layers. The measured Poisson ratio in the smectic A and C* phases is close to ½, in contrast to the smectic elastomers investigated earlier by Nishikawa et. al. [Macromol. Chem. Phys. 200, 312 (1999)]. In this unique material, the molecular lattice dimensions can be reversibly controlled by macroscopic stretching of the oriented samples.     

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.