Stable Liquid Crystalline Phase Research Articles

Shape-persistent macrocycles with intraannular polar groups: synthesis, liquid crystallinity, and 2D organization.

The synthesis of macrocycles with intraannular polar ester groups and extraannular oligo-alkyl groups is described. The compounds exhibit stable liquid crystalline phases showing fan-shaped textures under the polarizing microscope, typical for a columnar order of the molecules. X-ray powder diffraction data of the LC phase indicate that the unit cell contains two symmetry-related units, a feature pointing most probably to a restricted rotation of the macrocycles within a stack. The X-ray data were further supported by solid-state NMR experiments, showing that the rigid core of the compounds does not rotate with kHz or higher frequencies within the column in the LC phase. Apart from the organization of the molecules in the LC phase, the 2D organization of the macrocycles at the solvent-highly oriented pyrolytic graphite (HOPG) interface was investigated and showed that these compounds are capable of nanofunctionalizing the HOPG surface in the multinanometer regime.

Stable liquid crystalline phases of colloidally dispersed exfoliated layered niobates.

Colloidally dispersed niobium oxide nanosheets obtained by exfoliation of layered niobates HNb(3)O(8) and HTiNbO(5) formed stable liquid crystalline phases; their liquid crystallinity was dependent on the niobate species exfoliated.

Synthesis and characterisation of a nematic homo-polyurethane

The presence of hydrogen bonds in the chemical structure of polymers promotes and stabilises the crystalline phase. For liquid crystalline (LC) polymers, the side insertion of aliphatic units to the mesogenic unit is a suitable artifice to decrease the crystalline stability, without significantly affecting the stability of the LC phases. Here, we report on the synthesis of a LC homo-polyurethane with high hydrogen bond concentration along the chain and bearing an n-pentyl side-chain. Rheological behaviour, thermal analysis, and X-ray diffraction show that the stable LC phase is the nematic.

Spectroscopic Investigation of Nitrate−Metal and Metal−Surfactant Interactions in the Solid AgNO3/C12EO10 and Liquid-Crystalline [M(H2O)n](NO3)2/C12EO10 Systems

Interactions of the nitrate ions in various metal nitrate salts with CnH2n+1(CH2CH2O)mOH (CnEOm)-type nonionic surfactants have been investigated both in the solid and in the liquid-crystalline (LC) systems. In the ternary system, the mixture of salt/water/CnEOm has a mesophase up to a certain concentration of salt, and the nitrate ions in this phase are usually in a free-ion form. However, upon the evaporation of the water phase, the nitrate ion interacts with the metal center and coordinates as either a bidentate or unidentate ligand. It is this interaction that makes the AgNO3 ternary system undergo a phase separation by releasing solid Ag(CnEOm)xNO3 complex crystals. In contrast, the salt/surfactant systems maintain their stable LC phases for months. Note also that the salt/surfactant systems consist of transition-metal aqua complexes in which the coordinated water molecules play a significant role in the self-assembly and organization of the nonionic surfactant molecules into an LC mesophase. Throughout this work, Fourier transform infrared spectroscopy has been extensively used to investigate the interactions of the nitrate ions with a metal center and the metal ions with the surfactant molecules. Polarized optical microscopy and X-ray diffraction techniques have been applied to investigate the nature of the crystalline and LC phases.

Synthesis and Properties of Liquid-Crystalline Semi-Rigid Polyesters Based on Isophthalates of Nitro- or Methoxyazobenzene and Dioxydiundecanols of Biphenyl or 2,5-Diphenyl-1,3,4-Thiadiazole

Semi-rigid polyesters having nitro- or methoxyazobenzene unit in the side chain and calamitic mesogen (biphenyl or 2,5-diphenyl-1,3,4-thiadiazole unit) in the main chain were prepared by melt polycondensation of diethyl isophthalates composed of the azobenzene units with three dioxydiundecanol derivatives of the mesogenic units and their thermotropic liquid crystalline (LC) and optical properties were evaluated. The assigned structures of polymers were characterized by FTIR and 13 C NLR spectroscopy and elemental analyses. Differential scanning calorimetry (DSC) measurements, optical texture observation, powder X-ray analyses and miscibility tests indicated that most of the polyesters form thermotropic LC (smectic B, C, and E) phases and the polymer having the nitroazobenzene in the side chain forms the most stable LC phase. Absorption spectra of the polyesters in solutions and in films showed absorption maxima based on the azobenzene or the 2,5-diphenyl-1,3,4-thiadiazole unit, whereas their emission spectra were not observable.

Short wavelength light reflecting films from side‐chain liquid crystal homopolymers with chiral spacers

AbstractA series of acrylate monomers with alkoxy tails of varying lengths are synthesised and polymerised. The butoxy analogue had a stable enantiotropic cholesteric liquid crystalline phase which formed a grandjean texture when prepared as a thin film between glass slides. The polymer was mixed with a low molar mass nematic liquid crystal in various proportions and the pitch of the chiral nematic phases were determined using a cano‐wedge cell technique. The polymer prepared from (S)‐2‐(4‐butoxyphenyl‐4′‐benzoyloxy)‐1‐methyl ethyl acrylate had a pitch length of 113 nm which indicates that the polymer film could be employed in optical devices requiring selective reflection of light with short wavelengths in the region of 170 nm. Copyright © 2001 John Wiley & Sons, Ltd.

Javelin-, Hockey Stick-, and Boomerang-Shaped Liquid Crystals. Structural Variations on p-Quinquephenyl

The ramifications of changing molecular geometry in a series of all-aromatic liquid crystals derived from p-quinquephenyl are reported. Substituting heterocyclic rings such as thiophene, oxadiazole, oxazole, or 1,3-phenylene into the p-quinquephenylene core affects molecular shape changes via the substituent's exocyclic bond angle. In general, we found that introducing nonlinearity into molecules depresses the melting transition temperature. The symmetric (boomerang-shaped) molecules, 2,5-bisbiphenyl-4-yl-1,3,4-oxadiazole, 2,5-bisbiphenyl-4-yl-oxazole, and 1,3-bisbiphenyl-4-yl-benzene, melt into isotropic phases showing small monotropic mesophases. By contrast, the asymmetric (hockey stick-shaped) mesogens, 2-terphenyl-4-yl-5-phenyl thiophene and 2-terphenyl-4-yl-5-phenyl-1,3,4-oxadiazole, exhibit more stable enantiotropic liquid crystalline phases. The hockey stick-shaped mesogens exhibit a smectic phase as well as a nematic phase. High-temperature X-ray determination of the smectic layer spacing gives an unambiguous picture of interdigitated, bilayerlike supramolecular architecture in the smectic phase. There are associated changes in the mesogen's electrostatic profile when a heterocycle is introduced into the quinquiphenylene framework (e.g., conjugation is perturbed). Our findings suggest that steric packing considerations dominate the phase preferences (nematic versus smectic phases). However, electronic considerations (conjugation) appear to control the range of mesomorphism in this new family of nonlinear liquid crystals.

Supramolecular hydrogen-bonded liquid crystals

The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted. Following the first and well-established examples of liquid crystal formation through the dimerization of aromatic carboxylic acids, several classes of compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular systems. In this review the main classes of liquid crystals prepared through hydrogen-bonding interactions are presented, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation. Rigid-rod anisotropic or amphiphilic-type molecules, appropriately functionalized with recognizable moieties, interact in the melt or in solution and lead to the formation of supramolecular complexes that may exhibit thermotropic liquid crystalline character. Depending on the nature, number and position of the groups able to form hydrogen bonds, a diversity of supramolecular structures, both dimeric and polymeric, have been obtained, affording in turn various liquid crystalline phases. The structure and stability of these hydrogen-bonded supramolecular complexes and their relation to the observed liquid crystalline phases are the main topics of this review.

Novel Class of Ionic Metallomesogens without Organic Ligands: Thermotropic Liquid-Crystalline Behavior of Tetrachlorometalate Salts and Distinct Polymorphic Transition in Smectic A Phases

The thermotropic liquid-crystalline behavior of the metal complex salts [(C18H37)(CH3)3N]2[MCl4] and [(C18H37)2(CH3)2N]2[MCl4] (M = Co, Ni, Cu, Zn, Cd) was evaluated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffractometry, and electronic absorption as well as by Raman spectroscopy. Introduction of divalent metal ions into the cationic amphiphiles was found to result in enhancement of their thermal properties. Although the parent tetraalkylammonium chlorides showed no liquid-crystalline phase (or indistinct transition behavior), the derived tetrachlorometalate salts exhibited a stable liquid-crystalline phase and simple phase transition behavior. For instance, the tetrachlorocobaltate salt, [(C18H37)2(CH3)2N]2[CoCl4], showed a smectic A (SA) phase in an extremely wide temperature range from 45 to 155 °C on heating. In the SA phase, the complexes were confirmed to form a layered self-assembly that is built up from alternating layers of tetrahedral [MCl4]2- ions and flexible alkyl segments. Furthermore, distinct polymorphism in the SA phase was observed for several tetrachlorometalate salts, which differs essentially from the SA−SA transition described in the usual rodlike liquid crystals.

Self-Assembly and Surface Structure of an Amphiphilic Graft Copolymer, Polystyrene-graft-ω-Stearyl–Poly(ethylene oxide)

Surface properties of the polystyrene-graft-ω-stearyl–poly(ethylene oxide) (PS-g-SPEO) have been characterized by X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), contact angle, and spin probe techniques. The XPS results indicate that the surface and bulk composition of the PS-g-SPEO copolymers differ remarkably from each other. The stearyl and EO components enrich at the copolymer/air interfaces due to the self-assembly of the stearyl groups. At the PS-g-SPEO-72.6 surface (the x in PS-g-SPEO-x indicates the bulk density of the SPEO in wt%), the self-assembly of the hydrophobic stearyl groups is strong enough to form a stable liquid crystalline phase as indicated by DSC. At polymer/water interfaces, PS-g-SPEO-72.6 presents a hydrophilic surface with low PEO mobility, whereas PS-g-SPEO-50.6 and PS-g-SPEO-31.0 present the hydrophobic surface with high PEO mobility. The two different types of the surfaces, with different characters in surface energy, surface mobility of PEO, and surface architecture of SPEO, will be quite valuable as models for detecting the synergistic action of the PEO chains and the stearyl groups (specific ligand for albumin binding) in protein solutions.

Synthesis of thermotropic polyurethanes containing imide units and their mesophase behavior

Thermotropic polyurethanes were synthesized from 1,6-hexane diisocyanate (HDI) as a diisocyanate, 1,6-hexane diol (HD), and rigid diols containing imide unit such as N,N′-bis(4-hydroxyphenyl)-3,4,3′,4′-biphenyl-dicarboxyimide (BPDI) or bis-N-(4-hydroxyphenyl)-4,4′-oxydiphthalimide (ODPI). The effects of structure difference between BPDI and ODPI and composition of HD/BPDI (ODPI) on the thermal and liquid crystalline behavior were studied. Thermotropic polyurethanes with an inherent viscosity of 0.59–0.70 were obtained. The melting temperature of BPDI-based polyurethanes were in the range of 150–290°C, however, those of ODPI-based polyurethanes were in the range of 150–190°C. All the polyurethanes based on ODPI (25–100 mole %) clearly exhibited a stable liquid crystalline phase, and BPDI-based polyurethane having 5–25% of BPDI showed a mesophase. The melting and isotropization temperatures (T m , T i ) andΔT(T i −T m ) increased with increasing BPDI and ODPI content. The polyurethanes based on BPDI has higher melting points and thermal stability compared to ODPI-based polyurethanes.

Liquid Crystalline Properties of Linear and Network Polymers Containing Allyl Groups as Lateral Substituents

The synthesis and the mesomorphic properties of liquid crystalline polymer networks and the linear polymers upon which networks are based are reported. The linear polymers, synthe-sised from trimellitic acid, 2-allyl ester with the appropriate 4,4′-l,n diphenoxyalkane, exhibit a nematic mesophase, as indicated by calorimetric analysis, X-ray diffraction data, and polarising microscopy. Polymer networks were prepared from the linear polymers in the aniso-tropic state by thermal crosslinking in the bulk producing a very stable liquid crystalline phase.

Two-Dimensional Superlattice Self-Formed by Novel Ionic Liquid Crystals and Its Photofunctional Property

ABSTRACTThermotropic liquid-crystalline behavior of various complex salts, which are structurally simple amphiphiles without rigid cores, was evaluated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffractometry. The phosphonium salts, consisting of positive phosphorus and negative chloride ions, were found to show a smectic A phase in which a homeotropic structure was formed spontaneously. It is significant that the phosphonium salts possess an advantageous feature as thermotropic liquid crystals and show a stable liquid-crystalline phase and a simple phase transition behavior in comparison with commonly available ammonium analogs. Furthermore, the introduction of divalent metal ions into the amphiphiles was revealed to result in enhancement of the thermal properties. Although the ammonium chlorides as parent compounds showed no liquid-crystalline phase (or indistinct transition behavior), the ammonium complexes possessing tetrachlorometalate ions exhibited clearly the smectic A phase in the expanded temperature range. Additionally, through the evaluation of dipolar alignment in the solid-state phosphonium assembly by second harmonic generation, it was assumed that the self-assembly can be regarded as layered polar thin films produced by two-dimensional ionic layers with an overall permanent electric polarization.

Liquid Crystalline Bispropargyl Thermosets

A series of rigid-rod bispropargyl thermoset monomers have been synthesized. These monomers were examined by differential scanning calorimetry (DSC) and hot stage polarized optical microscopy. Enantiotropic or monotropic nematic liquid crystalline phases were observed for all but two monomers. Partial curing of these reactive liquid crystalline monomers resulted in the formation of stable liquid crystalline phases with broad nematic phase ranges. In one example, a monomer that was neither enantiotropic nor monotropic exhibited a stable nematic phase after partial curing. DSC investigations indicated that the onset temperature of thermally induced cross-linking was approximately 260 °C and insensitive to the phase type. The rate of cure was insensitive to the phase in which the cure occurred due to the unusual reaction mechanism for the propargyl end group.

Synthesis and properties of two series of H-bonded and non-H-bonded thermotropic liquid crystal monomers

Two series of monomers, namely n-1-bromo-\\[4-(4-nitrophenylazo)phenyloxy]alkanes (Bn, n2, 3, 4, 5, 6, 8, 10) and N - n -\\[4-(4-nitrophenylazo)phenyloxy]alkyl diethanolamines (C n, n 3, 5, 6, 10), were synthesized and characterized. Their thermal behaviour was studied by differential scanning calorimetry (DSC), wide angle X-ray diffractometry (WAXD) and polarizing optical microscopy (POM) equipped with a hot stage. The results showed that the B n s (n6) exhibit monotropic nematic liquid crystalline behaviour; no liquid crystalline phase was found for the B ns (n6), while for the C ns, enantiotropic smectic liquid crystallinity for n 5, 6, 10 was seen, and for n 3 monotropic smectic phases were found. This different phase behaviour between B n and C n compounds is attributed to their different end groups. The FTIR analysis of C n indicated that there exists an intermolecular hydrogen bond between hydroxy groups, so that more stable liquid crystalline phase are formed. The effect of the length of the flexible chain on the thermal behaviour is also discussed.

Effect of Structural Variation on Photocurrent Efficiency in Alkyl-Substituted Porphyrin Solid-State Thin Layer Photocells

Three structurally variant alkyl-substituted porphyrins [zinc(II) 2,3,7,8,12,13,17,18-octa-n-decylporphyrin 2, metal(II) meso-5,10,15,20-tetra-n-undecylporphyrin 3 (metal = cobalt, copper, palladium, and zinc), and zinc(II) meso-5,10,15,20-tetrakis(phenoxy-n-nonyl)porphyrin 4] exist as photoconductive insulators and produce significant short circuit photocurrent and open circuit photopotential when irradiated as solid thin films in an indium−tin oxide sandwich cell. Liquid crystalline phase stability, the identity of the inserted metal, and the nature of its supramolecular packing all influence the observed photoelectrochemical response. Porphyrin 2, which has a stable liquid crystalline phase, produces higher steady-state photocurrents than do the corresponding meso-substituted porphyrins 3 and 4, although liquid crystallinity is not an absolute requirement for observation of a photoresponse. Among porphyrins 3, the zinc complex gave the highest steady-state photocurrent. When a phenyl ring orthogonal to the porphyrinic plane was present (as in 4), the observed photovoltaic response was significantly diminished, probably because of difficulties in achieving effective close packing.

Preliminary communication Hydrogen-bonded ionic liquid crystals

Ionic and non-ionic building units were connected via hydrogen bonds. Stable liquid crystalline phases were obtained by a suitable choice of the units. The ionic associates are not miscible with the respective non-ionic associates.

Synthesis and physical properties of novel side-chain type liquid crystalline poly(aryleneethynylene)s

Novel side-chain type liquid crystalline poly(aryleneethynylene)s with a biphenyl or azobenzene moiety in the side chain were obtained by coupling reaction between a dihalo-aromatic compound and a diethynyl-aromatic compound using a Pd complex as a catalyst. All polymers exhibit a stable enantiotropic liquid crystalline phase. The degree of orientation of the polymer backbone was confirmed by UV-vis measurements with polarized light. High quantum yield of fluorescence was obtained for these polymers.

Synthesis and physical properties of novel side-chain type liquid crystalline poly(aryleneethynylene)s

Novel side-chain type liquid crystalline poly(aryleneethynylene)s with a biphenyl or azobenzene moiety in the side chain were obtained by coupling reaction between a dihalo-aromatic compound and a diethynyl-aromatic compound using a Pd complex as a catalyst. All polymers exhibit a stable enantiotropic liquid crystalline phase. The degree of orientation of the polymer backbone was confirmed by UV-vis measurements with polarized light. High quantum yield of fluorescence was obtained for these polymers.

Photosensitization of Thin SnO2 Nanocrystalline Semiconductor Film Electrodes with Metalloporphyrins and Alkyl-substituted Metalloporphyrins

Modest fill factors (~0.2) and efficiencies for sensitized photocurrent generation are observed with porphyrins adsorbed to saturation on a nanocrystalline SnO 2 thin film employed as the working electrode in a photoelectrochemical cell. No dye aggregation is observed at the metal oxide/adsorbate interface, and no advantage in the photosensitization efficiency is seen with two porphyrins that exhibit a stable liquid crystalline phase over another porphyrin that does not.