Stable Liquid Crystalline Research Articles

Dynamic Motion of Twisted π System-Induced Temperature-Dependent Dielectric Response in the Neat Liquid State

Molecular assemblies of twisted π molecules of tetraphenylene with long alkoxy chains (1) and tetra[2,3]thienylene with long alkylamide chains (2) or long alkoxy chains (3) and their dielectric properties were investigated. Different degrees of intermolecular interaction of 1–3 afforded different molecular assemblies, including an ordered columnar structure, disordered columnar, and lamellar structures. The introduction of long alkyl chains enabled us to create thermally stable liquid crystalline or liquid states. Temperature-dependent dielectric measurement revealed that the dynamic flipping motion of the tetra[2,3]thienylene core of 3 induced a temperature- and frequency-dependent dielectric anomaly in the neat liquid phase. This flipping motion of the central tetra[2,3]thienylene π core occurred relatively easily in the liquid state with a high degree of freedom of molecular motion.

Liquid-crystalline fork-like dendrons

ABSTRACTDendritic molecules having several rigid-rod moieties can be applied to induce liquid crystallinity for a variety of non-mesomorphic functional molecules such as metal complexes, nanoparticles, fullerenes and π-conjugated molecules when these dendritic molecules are covalently bonded to those non-mesomorphic molecules. These complex molecules are called supermolecular liquid crystals. Due to the cooperation of several mesogenic moieties, these dendritic molecules exhibit very stable liquid-crystalline (LC) phases. We have used fork-shaped LC dendrons having two or three rigid-rod moieties to induce liquid crystallinity for functional molecules such as interlocked molecules and π-conjugated molecules. In these fork-like molecules, the rigid-rod cores are attached to the 3,4,5-position of the phenyl moieties through flexible spacer, and these molecules are bonded to functional molecules through the 1-position. They basically form smectic LC phases, which induce the layered arrangement of functional moieties. Here we report on a new family of fork-like mesogens containing a hydrogen bonding moiety or an ionic group. They are designed to build supramolecular materials.

Solvent-free Liquid Crystals and Liquids from DNA.

As DNA exhibits persistent structures with dimensions that exceed the range of their intermolecular forces, solid-state DNA undergoes thermal degradation at elevated temperatures. Therefore, the realization of solvent-free DNA fluids, including liquid crystals and liquids, still remains a significant challenge. To address this intriguing issue, we demonstrate that combining DNA with suitable cationic surfactants, followed by dehydration, can be a simple generic scheme for producing these solvent-free DNA fluid systems. In the anhydrous smectic liquid crystalline phase, DNA sublayers are intercalated between aliphatic hydrocarbon sublayers. The lengths of the DNA and surfactant are found to be extremely important in tuning the physical properties of the fluids. Stable liquid-crystalline and liquid phases are obtained in the -20 °C to 200 °C temperature range without thermal degradation of the DNA. Thus, a new type of DNA-based soft biomaterial has been achieved, which will promote the study and application of DNA in a much broader context.

An Effective Method for the Preparation of Stable LC Composites with High Concentration of Quantum Dots

Cholesteric liquid crystals doped with QDs present a special interest because the photonic bandgap structure of cholesteric media when combined with emission properties of QDs provides the opportunity to realize a variety of specifi c photooptical properties. Cholesteric mesophase has a 1D photonic bandgap structure, which stands for its unique optical properties such as Bragg selective light refl ection, huge optical rotation, and so on. [ 20 ] In several recent papers, the optical and photophysical properties of the cholesteric low-molar-mass and polymer liquid crystalline (LC) materials doped with QDs were demonstrated. [ 16‐18 ] For example, in recent publication, [ 16 ] the modulation of recombination lifetimes of CdSe/ZnS QDs (0.01‐0.02 wt%) dispersed in cholesteric low-molar-mass liquid crystal and time-resolved emission from QD ensembles in LC matrices with different alignment were investigated. The difference in decay time characteristics of QD’s emission depending on planar or homeotropic alignment was revealed in the case of overlapping of emission and photonic bandgap peaks. It was shown that coupling between the excitonic and the photonic cavity modes leads to the enhancement and modulation of spontaneous emission. In the papers [ 18,19 ] we have described the novel types of cholesteric QDs-doped materials with photo- and electro-tunable fl uorescence. The possibility of manipulation of fl intensity and degree of circular polarization (dissymmetry factor) was demonstrated for the fi rst time. Despite a great progress in the fi eld of the QD‐LC composites, the introduction of large concentration of QDs into liquid crystal media is still a challenging task. The most successful achievement in the area was published recently. [ 15 ] The authors of this work have offered an approach based on the surface

Remarkably Rich Variety of Nanostructures and Order–Order Transitions in a Rod–Coil Diblock Copolymer

A remarkably rich variety of nanophase-separated structures and various order–order transitions are observed in a series of low-molecular weight (MW) rod–coil block copolymers (BCPs) with the rod blocks of different lengths (LRod’s). The rod–coil diblock copolymer studied herein is poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS), in which PMPCS is a rod-like polymer and exhibits an MW-dependent liquid crystalline (LC) phase behavior. When the polymerization degree of the PMPCS rod block (NRod) is less than 32 (LRod < 8 nm), the PMPCS block is always amorphous in the entire temperature range. And the corresponding PDMS-b-PMPCS BCPs with NRod from 11 to 29 and the volume fraction of the PMPCS rod (fRod) from 43% to 67% self-assemble into various equilibrium nanostructures after annealed at temperatures above the glass transition temperatures of the PMPCS blocks. When NRod = 11 and fRod = 43%, the BCP forms a lamellar structure (LAM); when NRod = 15 and fRod = 51%, the BCP forms a double gyriod structure (GYR) ; when NRod = 20 and fRod = 57%, the BCP forms a GYR structure after annealed below 180 °C and transforms to the Fddd structure after annealed above 180 °C; when NRod = 29 and fRod = 67%, the nanostructure of the BCP after annealed below 180 °C is hexagonally packed cylinders (HEX) and changes to a body centered cubic structure (BCC) after annealed above 180 °C. When NRod > 32 (LRod > 8 nm), the PMPCS rod block is amorphous at low temperatures and transforms to a stable columnar LC phase after annealed at high temperatures. Correspondingly, the PDMS-b-PMPCS BCP with NRod = 44 and fRod = 75% forms a HEX structure after annealed at lower temperatures at which the PMPCS block is amorphous, and the nanostructure transforms to LAM after the sample is annealed at higher temperatures at which the PMPCS block enters into the LC phase. Therefore, only by a small change of the rod length in the low-MW PDMS-b-PMPCS rod–coil BCPs, various nanostructures including LAM, GYR, Fddd, HEX, and BCC are obtained. In addition, by increasing annealing temperatures, GYR-to-Fddd and HEX-to-BCC transitions are observed in the BCPs with the amorphous PMPCS, and a HEX-to-LAM transition occurs in the BCP when the LC PMPCS block undergoes an isotropic-to-LC phase transformation.

Insight into the liquid crystalline behavior of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene}: a typical thermotropic mesogen-jacketed liquid crystalline polymer

Based on the thermodynamic and kinetic analyses, the thermotropic hexagonal columnar liquid crystalline (LC) phase of poly-{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PBPCS) at high temperature is self-assembled by driving forces of the conformation entropy and macromolecular interaction, respectively. Because of the strong “coupling effect” between flexible backbone and rigid side chain, the molecular structure of PBPCS can be regarded as “dual chains” model that a flexible backbone and a rigid side chain are in parallel, the backbone and side chain are corresponded to different molecular thermal motion characteristics. When the orientation requirements of the side mesogenic chain are dominant, LC phase appears; when the random motions of flexible backbone are dominant, LC phase disappears. PBPCS demonstrates various types of metastable phases in the LC phase transition. Above LC phase transition temperature, molecular motion of rigid side chain is dominant; therefore, LC phase is formed. In the temperature range from glass transition to LC phase transition, molecular motion of flexible backbone is dominant; therefore, metastable LC phase may be relaxed partly and even disappear. The stability of LC phase can be determined by the kinetic factors, it is related to the residence time for LC phase formation and the interaction among the aggregated columnar macromolecular chains. Metastable LC phase disappeared in the subsequent cooling procedure, however, the stable LC phase was not.

SYNTHESIS AND CHARACERIZATION OF A SERIES OF MESOGEN-JACKETED LIQUID CRYATALLINE POLYMERS WITH LONG MESOGENIC SIDE CHAINS

A series of mesogen-jacketed liquid crystalline polymers,poly[2,5-(4′-alkoxybi-phenyloxycarbonyl) styrene](PnCbiPCS,n=4,6,8,10,14) with long mesogenic side chains,were prepared by atom transfer radical polymerization(ATRP).Molecular weights and molecular weight distributions of the polymers were measured by gel permeation chromatography(GPC).The liquid crystalline(LC) behavior was studied by a combination of differential scanning calorimetry(DSC),thermogravimetric analysis(TGA),polarized light microscopy(PLM),one-dimensional(1D) and two-dimensional(2D) wide-angle X-ray diffraction(WAXD),and small-angle X-ray scattering(SAXS).All the polymers had good thermal stablility and exhibited stable LC properties,and no isotropization temperatures were observed before the onset temperatures of thermal decomposition.1D SAXS and WAXD studies revealed that PnCbiPCS could form smectic structures with the long mesogenic side chains aligned parallel to each other.The d-spacings increased with increasing aliphatic tails.2D WAXD results showed that the sheared samples exhibited Smectic A structures with the layer normal perpendicular to the shear direction.Comparing with the results of poly{2,5-bis[(para-substituted phenyl)oxycarbonyl]styrene}s,one may indicate that the length of the mesogenic group is crucial in determining the LC structure.

Mesogen-Initiated Linear Polyglycerol Isomers: The Ordering Effect of a Single Cholesterol Unit on “Sticky” Isotropic Chains

Synthesis, thermal properties, and the liquid crystalline (LC) order of polymers consisting of a single mesogenic cholesterol unit and flexible, linear polyglycerol (PG) or poly(glyceryl glycerol) (PGG) chains have been investigated. Incorporation of the single mesogen has been achieved by using cholesterol directly as an initiator for the oxyanionic ring-opening polymerization (ROP) of ethoxyethyl glycidyl ether (EEGE) or isopropylidene glyceryl glycidyl ether (IGG). The controlled polymerization allowed the synthesis of a series of peculiar rod–coil type polyethers with molecular weights of 600–2300 g/mol, representing a degree of polymerization (DPn) of 4–30 for both PG and PGG with the polydispersity Mw/Mn in the range of 1.07–1.25. The resulting linear PGs exhibit extremely stable thermotropic LC order in a broad temperature range up to 260 °C, forming mainly layered smectic A (SmA) phases with varying layer thicknesses, depending on the degree of polymerization of the respective polymer structure. LC phases were observed up to a chain length of 26 glycerol units, while PGGs showed no LC order. This is explained both by the steric hindrance of the branched monomer units and the higher hydrophilicity of the polymer backbone. Permethylation of the cholesterol-PG samples resulted in strongly reduced LC order or in the entire loss of the self-assembly in LC phases, which is a consequence of the disappearance of hydrogen-bonding between the functional coil segments. Detailed characterization of the phase behavior of the polymers has been achieved by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small-angle X-ray scattering (SAXS), confirming the smectic layer structure of the materials.

Self-assembly of novel fluorescent silole derivatives into different supramolecular aggregates: fibre, liquid crystal and monolayer

Two novel organogelators based on 2,3,4,5-tetraphenylsilole functionalized with long-chain alkoxydiacylamido platforms (1a and 1b) were synthesized. The silole derivatives induced gelation of only hydrocarbon solvents and showed aggregation-induced emission (AIE) in the gel state, in contrast to very weak emission in solution. The polarized optical microscopic (POM) and field emission scanning electron microscopy (FE-SEM) studies exhibited that the xerogels formed fibrous structures. Hydrogen bonding and π-stacking interactions were the main driving forces for the formation of organogels, based on the FT-IR and absorption investigations. Differential scanning calorimetry (DSC) and POM studies indicated that both compounds 1a and 1b exhibited stable liquid crystalline (LC) phases over a wide temperature range. It is interesting that uniform and well-ordered monolayers were also obtained for both compounds on the HOPG surface. The new silole derivatives are quite unique because they can self-assemble into one-dimensional (fibres), three-dimensional (liquid crystal) and even two-dimensional (molecular monolayer) aggregates.

Design, synthesis, and self-assembly behavior of C3-symmetry discotic molecules via click chemistry

We prepared a series of C3-symmetry discogens with three 1,2,3-triazoles and a central benzene ring by a copper-catalyzed “click reaction” between 1,3,5-triethynylbenzene and 3,4,5-trialkoxybenzyl azides. According to the differential scanning calorimetry (DSC) data, compounds with octyl and dodecyl peripheries showed stable liquid crystalline (LC) phases at room temperature, and upon heating underwent an isotropization at 105.3 °C and 92.2 °C, respectively. Compound with octadecyl peripheries melted into a LC phase at 63.5 °C, and changed into a disordered liquid at 84.4 °C. As characterized by optical polarized microscopy (POM) and X-ray scattering techniques, all LC phases revealed hexagonally packed columns, the axes of which are aligned vertically on the glass substrate. Interestingly, the experimentally accessible crystalline phase of the compound with octadecyl chains kept the 2-D hexagonal columnar structure, although the interdisc distance within the columns changed at the transition from the LC to crystalline phase.

Truxene‐Based Columnar Liquid Crystals: Self‐Assembled Structures and Electro‐Active Properties

Columnar liquid-crystalline (LC) truxene derivatives containing branched flexible alkyl chains have been designed and synthesized. The dicyanomethylene and dithiafulvene substituents have been introduced into the pi-conjugated truxene framework to tune their electronic and redox properties as well as the molecular assembled structures. The pi-conjugated cores of dicyanomethylene- and dithiafulvene-appended truxenes adopt bowl-shaped conformations, giving rise to a large intrinsic dipole moment perpendicular to the aromatic framework. These molecules form stable columnar LC structures through intermolecular dipole-dipole interactions. The redox properties of LC truxene derivatives have been examined by cyclic voltammetry. The dicyanomethylene-appended truxene shows the reversible four-step electrochemical reductions, whereas the dithiafulvene-appended truxene undergoes three-step oxidations.

A Stimuli‐Responsive, Photoluminescent, Anthracene‐Based Liquid Crystal: Emission Color Determined by Thermal and Mechanical Processes

AbstractHere, a photoluminescent liquid crystal that exhibits a change of emission color on the metastable–stable phase transition induced by external stimuli is prepared. A 2,6‐diethynylanthracene derivative with amide groups and dendritic side chains exhibits a columnar phase on slow cooling from the isotropic phase and shows blue emission in this columnar phase. In contrast, a cubic phase is obtained by rapid cooling from the isotropic phase. In the cubic phase, the 2,6‐diethynylanthracene cores form excimers, resulting in yellow emission. While the columnar phase is a stable liquid‐crystalline (LC) phase, the cubic phase is a metastable LC phase. It is found that a change of the photoluminescent color from yellow to blue is observed on the cubic‐columnar phase transition induced by heating or mechanical shearing for this 2,6‐diethynylanthracene derivative in the cubic phase. This change of photoluminescent color is ascribed to the inhibition of excimer formation on the metastable–stable LC phase transition.

Facile Synthesis of Novel Monodisperse Liquid-crystalline Oligomers Based on Michael Adducts from Millipede Cyanoacetates

Abstract Facile synthesis of novel monodisperse liquid-crystalline (LC) oligomers was performed. The functionalization of the end group of 4 (and 6) millipede cyanoacetates with 8 (and 12) mesogenic units by Michael addition reaction produced a monodisperse LC octamer (and dodecamer) in high yields. Differential scanning calorimetry and optical polarizing microscopy revealed LC properties of the synthesized LC monodisperse oligomers. The monodisperse LC oligomers were morphologically stable glass-forming LC materials and exhibited a large variation in nematic phase temperatures in the range of approximately 80 to 300 °C.

Synthesis, Structure, and Computational Studies of Soluble Conjugated Multidentate Macrocycles.

Conjugated, shape-persistent macrocycles based on [3 + 3] Schiff-base condensation are of interest for supramolecular materials. In an effort to develop new discotic liquid crystals based on these compounds, a series of macrocycles with peripheral alkoxy groups of varying length have been prepared. The synthesis and mechanism of formation have been probed by isolation of oligomeric intermediates. A single-crystal X-ray diffraction study of one macrocycle revealed a nonplanar, strongly hydrogen-bonded structure. To our surprise, even with very long substituents, the macrocycles were not liquid crystalline. This has been rationalized by ab initio calculations that indicate the macrocycles are undergoing rotation of the dihydroxydiiminobenzene rings that may not allow a stable discotic liquid crystalline phase. These results provide new insight into the formation and properties of these large macrocycles and may provide guidance to developing stable liquid crystalline materials in the future.

Synthesis, Structure, and Computational Studies of Soluble Conjugated Multidentate Macrocycles

[Chemical reaction: See text] Conjugated, shape-persistent macrocycles based on [3 + 3] Schiff-base condensation are of interest for supramolecular materials. In an effort to develop new discotic liquid crystals based on these compounds, a series of macrocycles with peripheral alkoxy groups of varying length have been prepared. The synthesis and mechanism of formation have been probed by isolation of oligomeric intermediates. A single-crystal X-ray diffraction study of one macrocycle revealed a nonplanar, strongly hydrogen-bonded structure. To our surprise, even with very long substituents, the macrocycles were not liquid crystalline. This has been rationalized by ab initio calculations that indicate the macrocycles are undergoing rotation of the dihydroxydiiminobenzene rings that may not allow a stable discotic liquid crystalline phase. These results provide new insight into the formation and properties of these large macrocycles and may provide guidance to developing stable liquid crystalline materials in the future.

チアジアゾールのジスチリルベンゼン類似物とオリゴエチレンオキシド鎖から成る蛍光性液晶ポリエステル

Semi-rigid polyesters composed of distyrylbenzene analogues of 1, 3, 4-thiadiazole (TD) and oligoethylene oxide chains (m=3, 5, 6) were synthesized and their liquid crystalline (LC) and optical properties were evaluated. The resulting polyesters not only formed stable thermotropic LC smectic phase, but also fluoresced blue light in chloroform solutions and showed bluish-green fluorescent emission in solid states based on the TD analogue.

The relationship between the structures of semi-rigid homo- and copolyesters based on 5,5′-diphenyl-2,2′-bis(1,3,4-thiadiazole)s and their liquid crystalline and optical properties

New semi-rigid homo- and copolyesters composed of the quaterphenyl analogue of 2,2′-bis(1,3,4-thiadiazole) (BTD), 5,5′-diphenyl-2,2′-bis(1,3,4-thiadiazole) (DBTD), were prepared by high temprature solution polycondensation of monomers, 3,3′- and 4,4′-dioxydiundecanol derivatives of DBTD with four dimethyl esters, and their liquid crystalline (LC) and optical properties were investigated. DSC measurements, texture observations using a polarizing microscope equipped with a hot stage, and powder X-ray diffraction showed that the homopolymers containing the 4,4′-DBTD unit form more stable LC phases than those having the 3,3′-DBTD unit. The 4,4′-DBTD-containing polymers and the 3,3′-DBTD unit-poor copolymers, except for the 3,3′-DBTD unit-rich copolymers and the 3,3′-DBTD-containing homopolymers (which formed highly ordered smectic or crystal mesophases), displayed a thermotropic LC smectic C phase. Solution and solid state UV-vis and photoluminescent (PL) spectra showed that the polyesters display miximum absorbances and bluish-green or blue emission based on the DBTD unit, where the Stokes shifts were observed. The peak positions in the UV-vis and the PL spectra of homopolymers composed of the 4,4′-DBTD unit were at higher wavelengths than those in the corresponding 3,3′-DBTD-containing homopolymers, due to its more conjugated structure. In the copolymers the peak maxima were shifted to shorter wavelengths with the increase of 3,3′-DBTD content.

Stable Liquid Crystalline Phases of Colloidally Dispersed Exfoliated Layered Niobates.

AbstractFor Abstract see ChemInform Abstract in Full Text.

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.