Smectic Liquid Crystal Phases Research Articles

On the change in helix handedness at transitions between the SmC* and phases in chiral smectic liquid crystals

Using a discrete model for the synclinic SmC* and the anticlinic phases we give a theoretical explanation for the fact that the helix twisting sense reverses at a transition between these phases (direct transition or via the so‐called chiral smectic C ‘subphases’) and we derive an explicit expression for the helical pitch in the phase. As the theory shows and as we also demonstrate experimentally, the reversal is of a different nature from helix inversions within a single phase, where the inversion is always coupled to a pitch divergence. At a clinicity change the common behaviour is instead pitch‐shortening on approaching the phase transition and the associated helix twisting sense reversal. The phenomenon may be put to use in smart mixing in order to control the helix pitch, either for achieving long pitch for surface‐stabilized ferroelectric and antiferroelectric liquid crystal displays; or a very short pitch, in the case of devices utilizing the deformed helix mode.

Liquid Crystal and Crystal Structure of Octahomotetraoxacalix[4]arenes

Octahomotetraoxacalix[4]arenes bearing long alkyl chains on their lower rim were prepared. Ester 4a existed in a 1,2-alternate conformation in its crystal structure, which was examined by single-crystal X-ray diffraction analysis. To prepare liquid crystalline materials possessing calixarene moieties by self-assembling, carboxylic acid derivatives 5 were synthesized. Among them, 5c, the octadecyloxy derivative, showed smectic liquid crystal phase. Homooxacalixarenes 5 also formed liquid crystal phases with longer layer distances when two equivalent moles of 1,2-ethylenediamine were added as a linker. These phases were investigated with X-ray diffraction, differential scanning calorimetry, and polarized optical microscopy.

Derivation by Electronic Structure Calculations of the Aspect Ratios (Length/Diameter) for Homologous Series of Calamitic Liquid Crystals

ABSTRACT 4′-Methyl-biphenyl-4-carbonitrile and the 4′-Methoxy-biphenyl-4-carbonitrile homologous series are known to exhibit calamitic (nematic and/or smectic) liquid crystal phases. In this work, aspect ratios x (length/diameter) for these molecules are computed from electronic structure calculations using ab initio HF/3-21G//PM3 level of theory/numerical precision. Aspect ratios were calculated from ovality Ov, defined as Ov=(A/4π)/(3V/4π)2/3, where V is the molecular volume and A the surface area. Derivations of thermotropic liquid crystal aspect ratios as a function of Ov, by this model are in agreement with observed values and permit an adequate description of the liquid crystal molecular geometry.

Gelation of smectic liquid crystal phases with photosensitive gel forming agents.

This paper studies the potential of 3 newly synthesized low molecular weight gelling agents (2 of them with azo groups) with the semicarbazide group as a H-bonding motif to gel the smectic phase of a commercial chiral smectic C material. To detect gelation two methods were used: the stabilization of the director pattern, which measures the interaction of the physical gel network with the LC-director, and the suppression of the onset of electrohydrodynamic instabilities, which measures the increase of viscosity. Special emphasis was put on the influence of the photochemical - isomerization of the gelling agents on gelation.From the stabilization of the director pattern an elastic component can be seen by the memory effect (two different switching times). Photochemical - isomerization destroys this memory effect. The increase of the threshold field for the onset of electrohydrodynamic instabilities in free-standing films measures the increase of viscosity and the supression of flow. Both methods detect the same melting temperature of the gel and the same reduction of the gelling ability upon - isomerization. This proves that the gelling abilities of the new azo-containing gelling agents can be reversibly switched on and off by photoisomerization.

Monodisperse Oligofluorenes with Keto Defect as Models to Investigate the Origin of Green Emission From Polyfluorenes: Synthesis, Self‐Assembly, and Photophysical Properties

Oligofluorenes (a trimer, pentamer, and heptamer) with one fluorenone unit in the center (OFnK: n=3, 5, or 7) were synthesized and used as models to understand the origin of the low-energy emission band in the photoluminescence and electroluminescence spectra of some polyfluorenes. All compounds form glasses with T(g) at 30 degrees C (OF3 K), 50 degrees C (OF5 K) and 57 degrees C (OF7 K). Oligomers OF5 K and OF7 K exhibit smectic liquid crystal phases that undergo transition to isotropic melts at 107 and 205 degrees C, respectively. Oligomer OF5 K could be obtained in form of single crystals. The X-ray structure analysis revealed the helical nature of the molecule and a helix reversal defect located at the central fluorenone unit. The packing pattern precludes formation of excimers. Electrochemical properties were investigated by cyclic voltammetry. The ionization potential (I(p)) and electron affinity (E(a)) were calculated from these data. Studies of the photophysical properties of OFnK in solution and thin film by steady-state and time-resolved fluorescence spectroscopic measurements suggest efficient funneling of excitation energy from the photoexcited fluorene segments to the low-energy fluorenone sites by both intra- and intermolecular hopping events whereby they give rise to green emission. Intermolecular energy transfer was investigated by using a model system composed of a highly defect free polyfluorene PF2/6 doped by OFnK. Förster-type energy transfer takes place from PF2/6 to OFnK. The energy-transfer efficiency increases predictably with increasing concentration of OFnK.

1,24-Dibromotetracosane

In the title compound, C24H48Br2, the mol­ecule is centrosymmetric and the mol­ecular skeleton including both terminal Br atoms has an all-trans conformation. The mol­ecules form layers with a thickness of c/2. These features are similar to the smectic CA phase of liquid crystals.

Induced and spontaneous deracemization in bent-core liquid crystal phases and in other phases doped with bent-core molecules

Recently discovered chiral properties of several bent-core smectic liquid crystal phases are summarized and discussed in detail under the assumption that typical bent-core molecules may exist in chiral conformational states and are achiral only on average. Results of atomistic computer simulations are presented which indicate the existence of strongly chiral conformational states for typical bent-core mesogens. A theory is developed which describes a possible shift of equilibrium between left- and right-handed conformations in a macroscopically chiral phase. The theory describes a chirality induction in the B2 bent-core phase and a reduction of the helical pitch in cholesteric and chiral SmC* phases doped with bent-core molecules. Finally, the possibility of spontaneous deracemization in bent-core smectic phases is discussed in detail.

Highly Efficient Photogeneration of Surface Relief Structure and Its Immobilization in Cross-Linkable Liquid Crystalline Azobenzene Polymers

A new family of azobenzene containing polymer applicable for photoinduced surface relief formation is reported. An azobenzene containing acrylate was copolymerized with methacrylate monomers bearing an oligo(ethylene oxide) chain via free radical copolymerization, yielding soft liquid crystalline polymers. These copolymers displayed a smectic liquid crystal phase at room temperature. After preexposure to ultraviolet light, thin films of the liquid crystalline polymers showed highly sensitive photoinduced material transfer to generate the surface relief structures. The typical exposure dose required for full polymer migration was as low as 50 mJ cm-2. The inscribed surface relief structures could be rapidly and fully erased either upon incoherent nonpolarized ultraviolet light irradiation or by heating close to the clear point of the soft liquid crystalline polymers. On the other hand, the chemical cross-linking, using a mixed vapor of hydrogen chloride and formaldehyde after surface relief inscription, re...

Origin of spontaneous polarization, tilt, and chiral structure of smectic liquid-crystal phases composed of bent-core molecules: a molecular model.

A simple molecular model is proposed for novel bent-core smectic phases that enables one to explain the origin of the experimentally observed chiral structure of the B2 phase composed of nonchiral banana-shaped molecules. It is shown that in the perfectly ordered smectic phase the distributed dispersion interaction between banana-shaped molecules stabilizes the spontaneous polarization and may be responsible for the tilt of the director. The orientation of the spontaneous polarization with respect to the tilt plane is determined by the balance between the dispersion and electrostatic dipole-dipole intermolecular interactions. In particular, sufficiently strong dipole-dipole interaction promotes the B2 phase where the polarization is normal to the tilt plane. The actual chiral structure of each smectic layer in the B2 phase appears as a result of the symmetry breaking. In the case of small molecular dipoles the nonchiral polar smectic phase is formed where the spontaneous polarization is parallel to the tilt plane. The role of the opening angle and of the axial ratio of banana-shaped molecules is also considered and a phase diagram is presented.

1,20-Dibromoicosane

The mol­ecule of the title compound, C20H40Br2, is centrosymmetric and the skeleton has an all-trans conformation including both terminal Br atoms. In the crystal structure, the mol­ecules form a layer in which the long axes of the mol­ecules are inclined to the basal plane of the Br atoms. The layers are arranged in a zigzag manner between the neighboring layers, forming a herringbone motif, just like the smectic CA phase of liquid crystals.

Structure and Rheology of Hydrogen Bond Reinforced Liquid Crystals

Phase behavior and structure in both the crystalline and liquid-crystalline states is dependent on molecular size and shape and competition between various noncovalent intermolecular interactions. Numerous guanidinium organomonosulfonate (GS) crystals have been shown to adopt layered architectures because of their propensity to form a two-dimensional hydrogen-bonded network consisting of the sulfonate moieties (S) and complementary guanidinium (G) ions. This characteristic has now been demonstrated for nearly all of a series of guanidinium alkylbenzenesulfonates (GCnBS), in which the alkyl chain length (n) ranges from 0 to 16 carbons, and some members of a series of guanidinium alkylbiphenylsulfonates (GCnBPS). Single-crystal X-ray diffraction reveals that these layered architectures include previously observed simple bilayer and continuously interdigitated layer architectures as well as a new “box bilayer” architecture. Whereas the related sodium organosulfonates, phenylalkanes, and biphenylalkanes alone do not form liquid-crystal phases upon heating, the reinforcement provided by the hydrogen-bonded GS network leads to the transformation of these crystalline phases to persistent smectic liquid-crystal phases at elevated temperatures for n ≥ q (as reported previously for some of the GCnBS compounds). This chain length corresponds to single-crystal packing motifs that signal the onset of a structure-directing role for alkane−alkane interactions. X-ray diffraction reveals that the d-spacings of the smectic phases increase monotonically with n, consistent with a common structure that is independent of the preexisting crystal architecture. This behavior can be ascribed to facile rearrangements of the hydrogen-bonding connectivity in the GS sheet during the crystal-to-smectic transition. Several GCnBS compounds also exhibit polymorphic transitions involving rearrangement of the hydrogen-bond connectivity at temperatures below the smectic transition. The general rheological behavior of the GCnBS smectic phases was consistent with that reported for other small-molecule smectics; however, the viscosity of the GCnBS smectic phases was at least 2 orders of magnitude larger. The large viscosities observed in the GCnBS smectic phases are attributed to the persistence of intermolecular hydrogen bonding in the liquid-crystal state.

Generation of Stable Calamitic Liquid-Crystal Phases with Lateral Intermolecular Hydrogen Bonding

Rodlike molecules consisting of the anchoring and the spinning parts generated stable smectic liquid-crystal phases despite the lateral intermolecular hydrogen bonding.

Theoretical model for the discrete flexoelectric effect and a description for the sequence of intermediate smectic phases with increasing periodicity.

A general phenomenological description and a simple molecular model is proposed for the "discrete" flexoelectric effect in tilted smectic liquid crystal phases. This effect defines a polarization in a smectic layer induced by a difference of director orientations in the two smectic layers adjacent to it. It is shown that the "discrete" flexoelectric effect is determined by electrostatic dipole-quadrupole interaction between positionally correlated molecules located in adjacent smectic layers, while the corresponding dipole-dipole interaction is responsible for a coupling between polarization vectors in neighboring layers. It is shown that a simple phenomenological model of a ferrielectric smectic liquid crystal, which has recently been proposed in the literature, can be used to describe the whole sequence of intermediate chiral smectic C* phases with increasing periods, and to determine the nonplanar structure of each phase without additional assumptions. In this sequence the phases with three- and four-layer periodicities have the same structure, as observed in the experiment. The theory predicts also the structure of intermediate phases with longer periods that have not been studied experimentally so far. The structures of intermediate phases with periodicities of up to nine layers are presented together with the phase diagrams, and a relationship between molecular chirality and the three-dimensional structure of intermediate phases is discussed. It is considered also how the coupling between the spontaneous polarization determined by molecular chirality and the induced polarization determined by the discrete flexoelectric effect stabilizes the nonplanar structure of intermediate phases.

Parallel and Perpendicular Lamellae on Corrugated Surfaces

We consider the relative stability of parallel and perpendicular lamellar layers on corrugated surfaces. The model can be applied to smectic phases of liquid crystals, to lamellar phases of short-chain amphiphiles and to lamellar phases of long-chain block copolymers. The corrugated surface is modelled by having a single $q$-mode lateral corrugation of a certain height. The lamellae deform close to the surface as a result of chemical interaction with it. The competition between the energetic cost of elastic deformations and the gain in surface energy determines whether parallel or perpendicular lamellar orientation (with respect to the surface) is preferred. Our main results are summarized in two phase diagrams, each exhibiting a transition line from the parallel to perpendicular orientations. The phase diagrams depend on the three system parameters: the lamellar natural periodicity, and the periodicity and amplitude of surface corrugations. For a fixed lamellar periodicity (or polymer chain length), the parallel orientation is preferred as the amplitude of surface corrugation decreases and/or its periodicity increases. Namely, for surfaces having small corrugations centered at long wavelengths. For a fixed corrugation periodicity, the parallel orientation is preferred for small corrugation amplitude and/or large lamellae periodicity. Our results are in agreement with recent experimental results carried out on thin block copolymer films of PS-PMMA (polystyrene-polymethylmethacrylate) in the lamellar phase, and in contact with several corrugated surfaces.

Liquid-Crystalline Collapse of Pulmonary Surfactant Monolayers

During exhalation, the surfactant film of lipids and proteins that coats the alveoli in the lung is compressed to high surface pressures, and can remain metastable for prolonged periods at pressures approaching 70 mN/m. Monolayers of calf lung surfactant extract (CLSE), however, collapse in vitro, during an initial compression at ∼45 mN/m. To gain information on the source of this discrepancy, we investigated how monolayers of CLSE collapse from the interface. Observations with fluorescence, Brewster angle, and light scattering microscopies show that monolayers containing CLSE, CLSE-cholesterol (20%), or binary mixtures of dipalmitoyl phosphatidylcholine(DPPC)-dihydrocholesterol all form bilayer disks that reside above the monolayer. Upon compression and expansion, lipids flow continuously from the monolayer into the disks, and vice versa. In several respects, the mode of collapse resembles the behavior of other amphiphiles that form smectic liquid-crystal phases. These findings suggest that components of surfactent films must collapse collectively rather than being squeezed out individually.

Molecular dipoles and tilted smectic formation: a Monte Carlo study.

We investigate the possibility of forming a tilted smectic liquid crystal phase by suitably positioning two permanent dipoles in a rodlike molecule. We show, using Monte Carlo simulations, that a tilted smectic is formed from ellipsoidal Gay-Berne particles with two off-center outboard dipoles when these are directed along or at 60 degrees from the rod axis where they are located, but not when they are perpendicular to it. The properties of the phases obtained are studied in some detail.

Phase behavior of bent-core molecules.

Recently, a new class of smectic liquid crystal phases characterized by the spontaneous formation of macroscopic chiral domains from achiral bent-core molecules has been discovered. We have carried out Monte Carlo simulations of a minimal hard spherocylinder dimer model to investigate the role of excluded volume interactions in determining the phase behavior of bent-core materials and to probe the molecular origins of polar and chiral symmetry breaking. We present the phase diagram of hard spherocylinder dimers of length-diameter ratio of 5 as a function of pressure or density and dimer opening angle psi. With decreasing psi, a transition from a nonpolar to a polar smectic A phase is observed near psi=167 degrees, and the nematic phase becomes thermodynamically unstable for psi<135 degrees. Free energy calculations indicate that the antipolar smectic A (SmAP(A)) phase is more stable than the polar smectic A phase (SmAP(F)). No chiral smectic or biaxial nematic phases were found.

An experimental and theoretical investigation into the reflection spectra of SmC* and SmCA* phases

Analysis of the reflection spectra of chiral smectic liquid crystal phases is used to provide detailed information on physical parameters, including refractive indices, helicoidal pitch and tilt angle. Numerical models of the antiferroelectric and ferroelectric phases are constructed, based on a 4 × 4 matrix technique, and a downhill simplex algorithm is employed to fit the model to reflection spectra measured from free-standing films of the materials of interest. The temperature dependence of the refractive indices, dispersion, tilt angle and pitch are reported for three different liquid crystalline materials. The accuracy of the fitting method is around 1%, better than the experimental error, and this is confirmed by comparison of the parameters determined by the fitting program with those measured by independent methods.

Ferroelectric liquid crystalline polymers based on mesogens with halogen-containing terminal groups

A new series of halogen-containing side chain ferroelectric liquid crystal polymers was synthesized. Mesophases were characterized by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and molecular simulation. The behaviour of the liquid crystalline phase was investigated with variation of chiral centres, spacer units and grafted ratios. It was found that the thermal stability and temperature range of the chiral smectic C phase decreased with increasing length of the oligo-oxyethylene spacer, and decreasing mesogenic group content. The bulky substituent attached to the chiral centre reduces molecular packing in smectic liquid crystal phases, which disturbs the orientation of the side chain liquid crystal polymer. Furthermore, the influence of molecular structure on electrooptical properties of FLCPs has been studied by broad band dielectric spectroscopy (from 0.1 to 1 × 106 Hz).

Molecular orientation arrangements in the smectic-C* variant liquid-crystal phases.

Recent experiments have identified three-layer and four-layer distorted helical structures in the smectic liquid-crystal phases Sm C*(FI1) and Sm C*(FI2), respectively. However, no theories have explained the existence of all these phases. A discrete phenomenological model of the free-energy is analyzed and found to predict the stability of distorted three-layer and four-layer structures, as well as simple helical solutions in smectic liquid crystals. A simple physical picture is provided to explain the stability of the phases exhibiting distorted helical structures.