Tilted Smectic Phases Research Articles

Synthesis and mesomorphic properties of laterally fluorinated alkyl 4′′-alkylterphenyl-4-yl carbonate liquid crystals

Fifteen series of homologues of a variety of mono-, di- and trifluorosubstituted alkyl 4′′-alkylterphenyl-4-yl carbonates have been synthesized and their mesomorphic properties have been determined. From among 95 prepared compounds, 40 pure nematogens have been found, as well as 55 mesogens with orthogonal and tilted smectic phases in broad temperature ranges. The type and combination of the LC phase strongly depend on the position and number of the fluorine atoms. Physical properties and correlations between the molecular core fluorosubstitution, the length of the terminal chains and the type and sequence of the liquid crystalline phases, have been determined. The compounds are useful for the formulation of nematic mixtures as well as ferroelectric ones.

New mesomorphic benzothiazol derivatives: Synthesis and characterization

In this paper, the synthesis of new mesomorphic benzothiazolyl derivatives, 6-ethoxy-2-[4-(4-alkyloxybenzoyloxy)benzylidenamino]benzothiazoles, is presented. The structures of the title compounds were elucidated using spectroscopic techniques, such as FT-IR, NMR (1H and 13C), elemental analysis and EI-MS. The mesomorphic behaviours of these compounds were determined by differential scanning calorimetric and polarizing optical microscopic techniques. Compounds exhibited nematic and tilted smectic phases upon heating from crystal phase. An obvious odd-even effect was observed in this homologous series.

High permittivity of chiral tilted smectic phases in a binary mixture

Characterization of structure and properties of the bicomponent mixture of partially fluorinated chiral three ring ester with an chiral alkyl compound by applied different methods such as the differential scanning calorimetry, the electro-optic and the dielectric spectroscopy, and the helical pitch measurement is presented. In particular, we investigate the influence of the texture changes in order to determine the phase transitions in the mixture under investigation.

X-ray Diffraction, Calorimetric, and Dielectric Relaxation Study of the Amorphous and Smectic States of a Main Chain Liquid Crystalline Polymer

Liquid crystalline polymers (LCPs) are complex systems that include features of both orientationally ordered mesophases and amorphous polymers. Frequently, the isotropic amorphous state cannot be studied due to the rapid mesophase formation. Here, a new main chain LCP, poly(triethyleneglycol methyl p,p'-bibenzoate), PTEMeB, has been synthesized. It shows a rather slow mesophase formation making possible to study independently both the amorphous and the liquid crystalline states. The structure and phase transitions of PTEMeB have been investigated by calorimetry, variable-temperature MAXS/WAXS employing synchrotron radiation, and X-ray diffraction in oriented fibers. These experiments have pointed out the presence of two glass transitions, related to the amorphous or to the liquid crystal phases. Additionally, the mesophase seems to be a coexistence of orthogonal and tilted smectic phases. A dielectric relaxation study of PTEMeB over broad ranges of temperature and pressure has been performed. The dynamic glass transition turns out to be slower for the amorphous state than for the liquid crystal. Monitoring of the α relaxation has allowed us to follow the isothermal mesophase formation at atmospheric pressure. Additionally, the dynamical behavior at high pressures has pointed out the fast formation of the mesophase induced by sudden pressure changes.

New hockey stick compounds with a lateral methyl group showing nematic, synclinic and anticlinic smectic C phases

New hockey stick-shaped mesogens with a lateral methyl group inserted between the m-alkyloxy-chain and the azomethine connecting group, 4-(3-n-alkyloxy-2-methyl-phenyliminomethyl) phenyl-4-n-alkyloxycinnamates have been synthesised. The interesting feature of these hockey stick-shaped mesogens is the appearance of the nematic phase in addition to two polymorphic tilted smectic phases – the synclinic smectic C (SmCs) and the anticlinic SmCa phase. Physical characterisation of these mesogens has been carried out using polarising optical microscopy, differential scanning calorimetry, optical birefringence, X-ray diffraction (XRD), electro–optical and dielectric spectroscopy measurements. The SmCs–SmCa phase transition is accompanied by only a small calorimetric signal but causes a pronounced change in the optical textures. Orientational order parameters determined from the birefringence measurements have been compared with those estimated from XRD patterns. Dielectric measurements point to the formation of small ‘soft’ ferroelectric clusters responsible for a low frequency absorption range.

強・反強誘電性フラストレーションの縮退解消による3層・4層以外の超構造をもつ2軸性副次相

強・反強誘電性フラストレーションの縮退解消による3層・4層以外の超構造をもつ2軸性副次相

Novel Supramolecular Side-Chain Banana-Shaped Liquid Crystalline Polymers Containing Covalent- and Hydrogen-Bonded Bent Cores

Novel supramolecular side-chain banana-shaped liquid crystalline (LC) polymer complexes AmBn-N bearing various (m/n) molar ratios of hydrogen- and covalent-bonded bent-core components were acquired by the free radical polymerization, where the hydrogen-bonded (H-bonded) structures were executed via mixing equimolar portions of proton donor (H-donor) polymers AmBn (homopolymers/copolymers) and pyridyl proton acceptor (H-acceptor) bent cores (small molecules). The influences of the molar ratios of bent-core H-bonded components in side-chain banana-shaped LC polymers and their corresponding polymer complexes on mesomorphic and electro-optical properties were first investigated. The voltage-dependent antiferroelectric properties of spontaneous polarization (Ps) values in the polar smectic phase of the supramolecular side-chain banana-shaped copolymers were also reported. The nematic and tilted smectic phases were verified by XRD measurements, and the SmCP phase was further confirmed by the triangular wave met...

Nematic phases of bent-core mesogens

Bent-core mesogens derived from 4-cyanoresorcinol with terminal alkyl chains have been synthesized and investigated by polarizing microscopy, XRD and electro-optical methods. Short chain compounds have exclusively nematic phases which can be cooled to ambient temperature. These nematic phases are similar to ordinary nematic phases with only nearest neighbour correlation (N) whereas long chain compounds form SmC-type cybotactic clusters and these cybotactic nematic phases (NcybC) can be regarded as strongly fragmented SmC phases. The chain length dependent as well as temperature dependent structural transition from N to NcybC is continuous and associated with a change of the position and intensity of the small angle scattering in the XRD patterns. Moreover, a temperature dependent stepwise transition from cybotactic nematic phases to different types of non-polar and tilted smectic phases (SmC(I) and SmC(II)) is observed with a mesophase composed of elongated, but not yet fused cybotactic clusters (CybC) as an intermediate state of this transition. This improves the understanding of the nature and special properties of the nematic phases formed by bent-core molecules as well as their transition to smectic phases and it paves the way to new materials with spontaneous or field-induced biaxial nematic phases at ambient temperatures.

Degeneracy lifting near the frustration points due to long-range interlayer interaction forces and the resulting varieties of polar chiral tilted smectic phases

To gain a clear understanding of ferroelectricity and antiferroelectricity together with their frustration in chiral tilted smectic liquid crystals, we have constructed E–T phase diagrams by drawing field-induced birefringence contours in the prototype binary mixture system of MHPOCBC and MHPOOCBC. The results obtained are discussed in terms of the theoretical model proposed by Emelyanenko and Osipov; we have insisted on the appropriateness of specifying the biaxial subphases, which emerge sequentially in the temperature-induced transition, by the relative ratio of ferroelectric and antiferroelectric orderings in the superstructure unit cell, such as , where qT = [F]/([A] + [F]). Additional subphases other than the ordinary subphases with three- and four-layer superstructures, and , have been established to exist, firmly for and less adequately for and . Likewise, we have observed several stable superstructures during the field-induced transition from the biaxial subphases to unwound SmC*, and have tried to specify them using qE = |[R] − [L]|/([R] + [L]), where [R] and [L] refer to the numbers of smectic layers with directors tilted to the right and to the left, respectively, in a unit cell of the superstructure. We have also found the characteristic field-induced deformation of the uniaxial subphase and attempted to understand it in terms of the devil's staircase due to soliton condensation reported recently by Torikai and Yamashita (41).

Chirality-Induced Liquid Crystalline Properties of Seven-Ring Trimeric Mesogens Incorporating Dual Chiral Centers

Two series of novel trimers with benzylidene-1,4-phenylenediamine core units attached by two (S)-2-methylbutyl 4-(4′-phenyl)benzoate moieties via methylene groups at terminal ends have been synthesized and characterized. In each series, the title compounds differ from each other in term of spacers -OCnH2nO- wherein n ranges from 5 to 12. The difference of the second series from their analogues in the first series is due to the presence of two OCH3 groups residing at the meta position in the benzylidene-1,4-phenylenediamine mesogenic core. The trimers without lateral OCH3 groups exhibit the SmC* and SmA phases in addition to the N* phase suggesting the possession of high molecular shape anisotropy. The higher-order tilted smectic phases and the blue phase are also observed for the members with odd n numbers in the first series. In the second series, however, the smectogenic characteristics are absent. Whilst the even-parity trimers are nematogenic, the trimers with odd spacers are not mesogenic. The trimers without lateral OCH3 groups in the first series also exhibit higher mesophase thermal stability in comparison with their analogues in the second series.

Molecular-statistical approach to a behavior of ferroelectric, antiferroelectric and ferrielectric smectic phases in the electric field

The fundamental theoretical approach derived in A.V. Emelyanenko et al., Phys. Rev. E 74, 011705 (2006) is complemented by a consideration of the influence of the homogeneous electric field on Sm-CA*, biaxial intermediate phases, and Sm-C*. The crucial role of the induced polarization is investigated for the first time. The evolution of any tilted smectic phase in the electric field is found to meet the two thresholds. The first threshold corresponds to the unwinding process, and the second one corresponds to the phase transition into the bi-domain structure of Sm-C*, where the tilt plane has some contribution either along or against the electric field, while the average direction may still be perpendicular to the electric field. The tilt plane in the monodomain (conventional) structure preceding the second threshold is the same in every unwound phase, and is perpendicular to the electric field. No 3D distortion in Sm-CA* is predicted on application of the electric field. The entire electric-field-temperature phase diagrams including the possibility of existence of the maximal number of tilted smectic phases are plotted and compared with the experimental ones. The numerical calculations in the framework of this fundamental study are done with help of AFLC Phase Diagram Plotter software developed by the author and available at his web page.

Computer simulations of linear rigid particles that form chiral superstructures and tilted smectic phases

We study the superstructures and phases formed by linear rigid coarse-grained particles using molecular dynamics simulations in the isothermal–isobaric ensemble (NPT). Chiral barrel-shaped and chiral column superstructures self-assemble from these building blocks at low pressures and Monte Carlo parallel tempering and energy minimization simulations demonstrate that chiral superstructures are minima of the potential energy surface for these rigid linear building blocks. At higher pressures the nematic, smectic A, smectic C, and smectic I phases are formed. By varying the spacing between interaction centres in our particles, we show that the discrete nature of the interactions can be tuned to favour both tilted smectic phases and chiral superstructures and our findings point to design rules that can be applied to nanoparticle and colloidal building blocks for the targeted synthesis of the structures we present. Interestingly, superstructural chirality appears as a common theme in systems composed of achiral linear particles with Lennard-Jones interaction sites that are appropriately spaced.

On the balance between syn- and anticlinicity in smectic phases formed by achiral hockey-stick mesogens with and without chiral dopants

A series of achiral hockey-stick-shaped mesogens forming tilted smectic liquid crystal phases of synclinic SmC- as well as anticlinic SmCa-type was prepared and characterized. While all homologues exhibit both phases, the balance shifts from anticlinic to synclinic order upon elongation of the terminal chain at the meta- position, defining the hockey-stick shape. The elongation also leads to an increased kinetic hindrance of the transition between syn- and anticlinic phases and a decreased transition enthalpy. These observations indicate that a well-defined kink (short meta-substituted chain) promotes the anticlinic structure while a higher flexibility between kinked and rod-shape (long meta-substituted chain) promotes synclinic order. An intermediate chain-length homologue was selected as host material for doping with syn- and anticlinic rod-shaped chiral dopants, respectively, at varying concentrations. Opposite of what might be expected the balance between syn- and anticlinic order was not simply dictated by the choice of dopant. Instead, both types of tilting order prevailed with roughly the same strength as in the achiral host regardless of which chiral material was added, up to concentrations well beyond normal doping conditions. Thus, at least with hockey-stick-shaped achiral hosts, syn- as well as anticlinic chiral compounds can be used effectively as chiral dopants without necessarily having an important impact on the clinicity of the resulting mixture. The hockey-stick design concept should be useful in producing achiral anticlinic-forming mesogens for low-polarization, long-pitch antiferroelectric liquid crystal mixtures. Finally, we point out that a mixture study like the one carried out here yields a conclusive means of establishing the clinicity of achiral tilted smectics, an endeavour that can sometimes be far from trivial.

Successive Phase Transitions in Antiferroelectric Liquid Crystal Systems—Axial Next-Nearest-Neighbor XY Model with Biquadratic Interaction

An axial next-nearest-neighbor XY model is studied as a model of chiral liquid crystals which exhibit many ferro-, ferri- and antiferroelectric tilted smectic phases. Depending on the values of interaction parameters, this model exhibits Ising symmetric (i.e., the tilt directions of directors are parallel or anti parallel) phases or XY symmetric phases. Phases with each type-of-symmetry show the character of devil's staircase, which has been observed in experiments.

Theory of the intermediate tilted smectic phases and their helical rotation

A molecular-statistical theory for the entire sequence of the chiral tilted smectic phases is derived. Uniaxial and biaxial subphases were found to be stable in different temperature ranges depending on the molecular parameters. The model of a chiral molecule possessing a strong transverse terminal dipole moment and a quadrupole moment located in the molecular core was used. Direct dispersion and electrostatic interactions (modulated by shape) between molecules located in the same or in the neighboring smectic layers are taken into account. An effective long-range interaction arises after the minimization of the free energy with respect to polarization vectors. If the molecular quadrupole moment is small, only uniaxial phases with different periodicities arise. Their periodicity may be tens and hundreds of layers (Sm-C*), or approximately two layers (Sm-C*)(Sm-C*A), or several layers (Sm-C*alpha). In the presence of the nonpolar biaxial ordering (in addition to polarization) there is a cap-shaped border in the phase diagram that separates Sm-C*A, Sm-C*, and Sm-C*alpha. If the molecules are nonchiral, Sm-CA, Sm-C, and the de Vries's phases arise instead of the three phases mentioned above. If the molecular quadrupole moment is large, the left "arm" of the border breaks into two lines, and a sequence of biaxial subphases arises in the area between these two lines. Among these biaxial subphases, the one with periodicity of three smectic layers appears to be the broadest in the temperature range. In addition, the subphases with different periodicities were found to be stable in narrow temperature ranges. The long helical rotation in every biaxial subphase is calculated. It is found to change sign between the three-layer subphase and Sm-C*, and may diverge in the four-layer subphase if it arises. All calculations are done with help of (A)FLC Phase Diagram Plotter software developed by the first author and available at his web-page.

Synthesis and characterization of main chain thermotropic liquid crystalline polyesters based on methyl 4-[4′-(ω-hydroxyalkoxy)biphenyl-4-yl]-4-oxobutyrate: Effects of keto group and connectivity on mesophasic characteristics of biphenyl based AB-type polyesters

A series of biphenyl based main chain thermotropic liquid crystalline polyesters, poly(hydroxyalkoxybiphenyl oxobutyrate) [ PHBOB-2, n ( n = 2–8)], containing various methylene spacers was synthesized by using the novel AB-type self-polycondensation route from the monomers, methyl 4-[4′-(ω-hydroxyalkoxy)biphenyl-4-yl]-4-oxobutyrate ( HBOB-2, n). The mesophase characteristics of these polyesters was studied using DSC, polarized light optical microscopy and XRD techniques. The transition temperatures were found to decrease upon increasing the spacer length with an even polymer exhibiting higher transition temperature than the odd counterpart. The mesophase type also indicated an odd–even oscillation with the even members showing normal smectic phase and odd members exhibiting tilted smectic phase. The comparison of mesophasic characteristics of these polyesters with that of structurally similar AB-type polyesters indicated that the total number of atoms connecting the mesogen in a polymeric repeat unit governs the nature of the mesophase while the rigidity of the system dictates the magnitude of transition temperatures.

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.

Odd–even effect on SmC* formation in chiral liquid crystal dimers

A homologues series of chiral symmetric dimer was prepared from ( S)-2-(6-methoxy-2-naphthyl)propionic acid. Structural effects on the mesomorphic and physiochemical properties were investigated in terms of variation in spacer length between two mesogenic units. The mesophase textures and their corresponding transition temperatures were identified by polarised optical microscopy and DSC measurements. The prepared dimeric compounds exhibit BP, N*, SmA* and SmC*/SmB* mesophase sequence. An odd–even effect was observed in forming tilted smectic phases with various length of spacers. Compounds containing even numbers of methylene spacer form SmC* mesophase, whereas odd spacers exhibited SmB* textures. Spontaneous polarisation was measured and maximum of 33.5 nC/cm −1 was observed for BDPNP-6.

Smectic Layer Structures in Complex Geometries—Modelling Complex Layer Structures in Smectic Liquid Crystals

In order to understand the operation of liquid crystal devices based on tilted smectic materials it is highly important to also understand the formation of the smectic liquid crystal layer structures. However, in many situations this is not easy to do. It is well established that in devices filled with materials that have an upper lying SmA phase a “chevron” structure forms when cooled from the SmA to the tilted smectic phase due to the layer thinning combined with layer packing constraints. But it is less well understood how the structure is influenced by the application of an electric field. Experimental work has been done in both ferroelectric and anti-ferroelectric phases which shows a transition from chevron towards bookshelf structure. In this paper we present some of our recent theoretical work in the area of smectic layer structures where “melting” of the smectic layer ordering is allowed.

SF5-Terminated Fluorinated Schiff Base Liquid Crystals

Novel liquid crystals based on fluorinated carbon chains and terminated with SF5 groups are presented for the first time. Attachment of an SF5 group separated from a benzylidine group by a fluorinated spacer leads to monotropic nematic and smectic phases, while an enantiotropic nematic phase is observed in mixtures. The phase sequence of these new compounds is comparable to hydrocarbon analogues of nO.m (n-alkoxy-m-alkylbenzylidine) series. Despite a long alkoxy chain, the nematic → smectic A transition is closer to second order, with ≈ 0.995 values that are even higher than for the nCB (n-alkylcyanobiphenyls) series. Single crystal and quantum computational studies suggest that electrostatic interactions play a major role in governing the value of MTCP. Molecular packing in the crystal structure exhibits monoclinic P21/a symmetry. A flexible synthetic strategy that allows the synthesis of another homologous series with a bend in the molecular structure is also presented. The bent core structure of 10O.2 SF5 investigated shows a monotropic, tilted smectic phase for the higher C10 alkoxy chain. The compounds in this series reveal a high degree of undercooling (well above 50 °C), which is rationalized using finite size effects on nucleation.