Hexagonal Columnar Liquid Crystal Research Articles

Novel liquid crystals with high fluorescence: Synthesis, mesomorphic and photophysical properties of cholesterol-triazine-BODIPY trimers

By step-by-step etherification on cyanuric chloride with BODIPY unit and cholesterol units, two novel cholesterol-triazine-BODIPY trimers (compounds 6 and 7 with one and two cholesterol unit, respectively) were designed and synthesized in high yields. The investigation on mesomorphic properties suggested that compound 6 with one cholesterol unit was a nematic liquid crystal but compound 7 with two cholesterol units was a hexagonal columnar liquid crystal. Both compounds 6 and 7 exhibited the strong fluorescence with high quantum yields and larger Stokes shifts than their precursors. The cholesterol-triazine moiety played crucial role for producing liquid crystalline property and enhancing fluorescence.

Novel fluorescent liquid crystals: synthesis, mesomorphism and fluorescence of triphenylene-Bodipy derivatives based on 1,3,5-triazine core

ABSTRACTThe novel triphenylene-Bodipy derivatives 6 and 7 based on 1,3,5-triazine core were designed and synthesised by introducing BODIPY unit and triphenylene units sequentially onto cyanuric chloride. The Bodipy derivative 6 with one triphenylene unit was a nematic liquid crystal but the derivative 7 with two triphenylene units was a hexagonal columnar liquid crystal. The investigation on photophysical properties suggested that both of them exhibited excellent fluorescence with high fluorescence quantum yields and the Stokes shifts were larger than their Bodipy precursors. This research presented a good example of design and synthesis of columnar Bodipy liquid crystal with high fluorescence and large Stokes shift.

Vapor-Deposited Glasses with Long-Range Columnar Liquid Crystalline Order

Anisotropic molecular packing, particularly in highly ordered liquid crystalline arrangements, has the potential for optimizing performance in organic electronic and optoelectronic applications. Here we show that physical vapor deposition can be used to prepare highly organized glassy solids of discotic liquid crystalline systems. Using grazing incidence X-ray scattering, atomic force microscopy, and UV–vis spectroscopy, we compare three systems: a rectangular columnar liquid crystal, a hexagonal columnar liquid crystal, and a nonmesogen. The packing motifs accessible by vapor deposition are highly organized for the liquid crystalline systems with columns propagating either in-plane or out-of-plane depending upon the substrate temperature during deposition. The structures formed at a given substrate temperature can be understood as resulting from partial equilibration toward the structure of the equilibrium liquid crystal surface during the deposition process.

Influence of chirality on the thermal and electric properties of the columnar mesophase exhibited by homomeric dipeptides.

We present the first investigation of the influence of chirality on the thermal and electric properties in a biologically important homomeric dipeptide that exhibits a hexagonal columnar liquid crystal mesophase. The peptide employed has two chiral centres, and thus the two possible enantiopures are the (R,R) and (S,S) forms having opposite chirality. The measurements reported the span of the binary phase space between these two enantiopures. Any point in the binary diagram is identified by the enantiomeric excess Xee (the excess content of the R,R enantiopure over its S,S counterpart). We observe that the magnitude of Xee plays a pivotal role in governing the properties as evidenced by X-ray diffraction (XRD), electric polarization (Ps), dielectric relaxation spectroscopy (DRS) measurements, and the isotropic-columnar transition temperature. For example, XRD shows that while other features pointing to a hexagonal columnar phase remain the same, additional short-range ordering, indicating correlated discs within the column, is present for the enantiopures (Xee = ±1) but not for the racemate (Xee = 0). Similarly, an electric-field driven switching whose profile suggests the phase structure to be antiferroelectric is seen over the entire binary space, but the magnitude is dependent on Xee; interestingly the polarization direction is axial, i.e., along the column axis. DRS studies display two dielectric modes over a limited temperature range and one mode (mode 2) connected with the antiferroelectric nature of the columnar structure covering the entire mesophase. The relaxation frequency and the thermal behaviour of mode 2 are strongly influenced by Xee. The most attractive effect of chirality is its influence on the polar order, a measure of which is the magnitude of the axial polarization. This result can be taken to be a direct evidence of the manifestation of molecular recognition and the delicate interplay between chiral perturbations and the magnitude of the polar order, a feature attractive from the viewpoint of devices based on, e.g., remnant polarization-a currently hot topic. To add further dimension to the work, the DRS measurements are also extended to elevated pressures.

Crowded Star Mesogens: Guest-Controlled Stability of Mesophases from Unconventional Liquid-Crystal Molecules.

The molecular design of crowded hexasubstituted star mesogens based on a benzene core and alternating substitution with oligo(phenylenevinylene) arms and aryl units generates free space between the conjugated arm scaffolds. Various arylcarboxy building blocks, decorated with alkoxy chains, have been incorporated in the void by mixing, hydrogen bonding or covalent bonds to the aryl groups. The mesogens assemble in columnar stacks ranging from soft crystals to rectangular and hexagonal columnar liquid crystals, revealed by polarized optical microscopy, differential scanning calorimetry, X-ray scattering and modelling. The stability of the mesophases is crucially influenced by the binding mode of the arylcarboxy guest building blocks. The origin of the variation in clearing temperature is unravelled by modelling, cohesive energy density considerations and solid-state NMR spectroscopy. The control over the transition temperature is important for the formation of aligned thin films and thus for potential applications.

Self-Assembled Hierarchical Superstructures from the Benzene-1,3,5-Tricarboxamide Supramolecules for the Fabrication of Remote-Controllable Actuating and Rewritable Films.

The well-defined hierarchical superstructures constructed by the self-assembly of programmed supramolecules can be organized for the fabrication of remote-controllable actuating and rewritable films. To realize this concept, we newly designed and synthesized a benzene-1,3,5-tricarboxamide (BTA) derivative (abbreviated as BTA-3AZO) containing photoresponsive azobenzene (AZO) mesogens on the periphery of the BTA core. BTA-3AZO was first self-assembled to nanocolumns mainly driven by the intermolecular hydrogen-bonds between BTA cores, and these self-assembled nanocolumns were further self-organized laterally to form the low-ordered hexagonal columnar liquid crystal (LC) phase below the isotropization temperature. Upon cooling, a lamello-columnar crystal phase emerged at room temperature via a highly ordered lamello-columnar LC phase. The three-dimensional (3D) organogel networks consisted of fibrous and lamellar superstructures were fabricated in the BTA-3AZO cyclohexane-methanol solutions. By tuning the wavelength of light, the shape and color of the 3D networked thin films were remote-controlled by the conformational changes of azobenzene moieties in the BTA-3AZO. The demonstrations of remote-controllable 3D actuating and rewritable films with the self-assembled hierarchical BTA-3AZO thin films can be stepping stones for the advanced flexible optoelectronic devices.

Room temperature liquid crystal of symmetric gallic trimer containing cyanuric core: synthesis and mesomorphism

Three novel gallic monomer 7, dimer 10 and trimer 12 with conjugated cyanuric core were designed and synthesised by Schiff-base condensation mode in yields of 80–90%. Their structures were characterised by fourier transform infrared spectroscopy, 1H NMR (nuclear magnetic resonance), electrospray ionization mass spectrometry and elemental analyses. Their mesomorphic behaviours were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The gallic monomer 7 has no mesomorphic property, but the dimer 10 and trimer 12 possess good mesomorphic properties. The trimer 12 with high symmetry exhibited the typical hexagonal columnar liquid crystal at room temperature. The temperature range of mesophase is as wide as 149°C (14–163°C). The results suggested that the more gallic units and symmetric structures are favourable for excellent mesomorphic properties.

New Advances in the One-Dimensional Coordination Polymer Copper(II) Alkanoates Series: Monotropic Polymorphism and Mesomorphism

The polymorphism in the copper(II) alkanoates, recently discovered for one member, has been thoroughly studied for the whole series, from 3 to 16 C atoms. Three polymorphic phases have been found, all of them sharing the same molecular unit, the paddle-wheel, which grows forming a 1D coordination polymer or catena. The three polymorphs are defined by a different packing of these catenae and a specific arrangement of the alkyl chains. Ten new crystal structures of those compounds have been solved by high resolution powder diffraction and presented in this paper. The polymorphism in this series has been found to be monotropic and is responsible for the complex thermal behavior observed. The most characteristic feature, the endothermic–exothermic effect, has been explained for the first time in these compounds by a combination of data from differential scanning calorimetry (in normal and modulated modes), powder X-ray diffraction and Fourier transform infrared spectroscopy. These techniques, together with small-angle X-ray scattering and optical microscopy, were used to analyze the hexagonal columnar discotic liquid crystal phase of copper(II) alkanoates. Thus, new information has been found in the packing and stacking of the discs formed by the paddle-wheel units, also maintained in the mesophase.

Hexagonal columnar liquid crystals as a processing additive to a P3HT:PCBM photoactive layer

Trihydrazone-functionalized cyanopyridine liquid crystals are introduced into the photoactive layer as processing additives to acquire an improved interpenetrating network and charge carrier mobility.

High Temperature Phase Transitions of Graphene Oxide Paper from Graphite Oxide Solution

Graphene oxide paper (GOP) can be prepared through simplified filtration of a graphite oxide solution. It possesses similar properties to graphene. In this study, the graphite oxide solution was synthesized from commercial graphite by means of Hummer's method. It corresponds to the dried GOP that was prepared by deposition on a cellulose filter. It is found that the mesophase of the dried graphene oxide papers obtained from the graphite was thermotropic hexagonal columnar liquid crystal. Its higher temperature transitions were found at 80 °C, 150 °C and 170 °C–180 °C. Therefore, it could be used for thermal storage and conductive materials in the future.

Smectic assemblies in C₃-symmetric hexa-alkylated liquid crystals: transformation from smectogen to discogen via hydrogen bonding.

In this paper, we report a C3-symmetric liquid crystal (LC) with sixfold alkyl peripheries exhibiting an unusual smectic E-like organization in the LC state. Based on conformational considerations, the smectic assembly is attributed to the formation of an endo-type Y conformer of asymmetric triazolyl and benzylic groups that cannot be accessed in other C3-symmetric molecules exclusively showing columnar assemblies. The Y conformers form a two-dimensional oblique lattice in the aromatic layers of the ordered smectic phase. In addition, the Y-shaped molecule in the smectic phases can change into a circular shape by the 1 : 1 hydrogen-bonding interaction with a gallic acid derivative, which leads to a hexagonal columnar LC phase. The triazole-based LC design concept proves the smectic LC assembly in the C3-symmetric system, and provides the supramolecular manipulation of LC morphologies.

Helical self-assembly of optically active phthalocyanine derivatives in mesophase. Effect of the position of chiral substituents on the handedness and mesophase textures

Two phthalocyanine derivatives with peripheral chiral (S)-2-methylbutanol moieties on the α- or β-position of the phthalocyanine ring, namely {(1,8,15,22-tetra[(S)-2-methylbutoxy]} phthalocyaninato copper complex (S)-CuPc(α-OC5H11)4 (1) and {(2,3,9,10,16,17,23,24-octa[(S)-2-methylbutoxy]} phthalocyaninato copper complex (S)-CuPc(β-OC5H11)8 (2) were synthesized. Their helical self-assembly behavior in mesophase were comparatively investigated by electronic absorption and circular dichroism spectroscopy, differential scanning calorimeter, polarized optical microscope and X-ray diffraction technique. In mesophase, (S)-CuPc(α-OC5H11)4 (1) self-assembled into rectangular columnar liquid crystals with left-handed fan texture. However, hexagonal columnar liquid crystals with well-ordered, rare, right-handed spherulites were formed from (S)-CuPc(β-OC5H11)8 (2). These results clearly indicated the effect of the position of chiral substituents on the handedness and mesophase textures of self-assembled liquid crystals and will be helpful towards rational design and preparation of chiral liquid crystal materials.

Propeller-Shaped Fused Oligothiophenes: A Remarkable Effect of the Topology of Sulfur Atoms on Columnar Stacking

Propeller-shaped regioisomers of fused oligothiophenes F9T(endo), F9T(anti), and F9T(exo) were successfully synthesized. DFT calculations indicated that their core parts are distorted from planarity due to intramolecular steric repulsions involving large sulfur atoms. In contrast with soft crystalline F9T(anti) and F9T(exo), F9T(endo) self-assembles into a hexagonal columnar liquid crystal (Col(h) LC), displaying a clear X-ray diffraction (XRD) due to its stacked π-conjugated core. In each LC column, well-organized intermolecular S-S contacts are developed triple-helically along the columnar axis with a helical pitch of 4.04 nm. Among LC semiconductors reported to date, Col(h) LC F9T(endo) displays a top-class charge-carrier mobility (0.18 cm(2) V(-1) s(-1)) with a distinct ambipolar character featuring well-balanced hole and electron mobilities. A thin film, prepared by mixing F9T(endo) with soluble fullerene PCBM, shows a photovoltaic response, when the fullerene content is large enough to compensate a small absorptivity of F9T(endo) for visible light.

Reversible Nucleation, Growth, and Dissolution of Poly(γ-benzyll-glutamate) Hexagonal Columnar Liquid Crystals by Addition and Removal of a Nonsolvent

We have investigated the process of nucleation and growth and its reversal (i.e., dissolution) of ordered poly(γ-benzyl l-glutamate) (PBLG) objects in thin film solutions containing a few percent of α-helical PBLG dissolved in chloroform. Nucleation, growth, and dissolution rate were controlled by adding and removing, respectively, defined amounts of a nonsolvent (methanol), introduced through the vapor phase by regulating its flow rate and vapor pressure. Adding methanol to the isotropic polymer solution allowed for the induction of nucleation and growth, even with polymer solutions of very low concentrations, which were significantly below the solubility limit (equilibrium volume fraction). The variation of the number density of nuclei with the supersaturation ratio was found to fit well the predictions of the classical nucleation theory, for all equilibrium concentrations. For a given supersaturation ratio, fewer objects were nucleated for lower equilibrium concentrations.

Filaments formed in the hexagonal columnar liquid crystal phase of star-shaped oligobenzoates

We investigate the stability and structure of freely suspended filaments of columnar liquid crystal phases. In the hexagonal columnar phase, filaments with aspect ratios of the order of 20:1 and more can be formed. They can be cooled into a low-temperature body-centred orthorhombic columnar phase without breaking. The slow optic axis is perpendicular to the filament axis and the materials are negatively birefringent in the high-temperature phase. At the transition into the low-temperature phase, the sign of the birefringence is inverted. Within the lifetime of the filaments, three regimes can be distinguished. First, the filaments thin exponentially over a period of about 2 hours. Thereby, the axial alignment improves and the filaments adopt a cylindrical geometry. These cylindrical filaments can persist quasi-stationary for several hours. Fluctuations finally lead to slight inhomogeneities in the filament diameter. This causes Laplace pressure gradients and induces an axial redistribution of material, and the filaments unavoidably rupture within about half an hour.

Mesogen jacketed liquid crystalline polyacetylene containing triphenylene discogen: synthesis and phase structure

Triphenylene-containing acetylenes with one or three methylene units as spacers and the corresponding mesogen-jacketed liquid crystalline polyacetylenes (MJLCPAs) were designed and synthesized, the mesomorphic properties and phase behaviors of the monomers and novel side-chain liquid crystalline polymers were investigated. The monomers [HCC(CH2)mC18H6(OC6H13)5; m = 1, 3] are prepared by consecutive etherization, coupling, and etherization reactions, and the chemical structures were confirmed by mass spectroscopy and 1H/13C-NMR. The phase behaviors of the monomers were investigated by differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide-angle X-ray diffraction (WAXD). The results show that both of the monomers form a hexagonal columnar liquid crystal phase at room temperature. The monomers are polymerized using [Rh(nbd)Cl]2 as catalyst and producing soluble polymers in the yields of 55% and 52%, respectively. The chemical structures and phase behaviors of the two polymers are characterized and evaluated by IR, NMR, TGA, DSC, and WAXD analyses. Both of the two polymers show enhanced thermal and chemical stability with thermal decomposition temperatures up to higher than 340 °C due to the protection of the “jacketed effect” of the side-chain triphenylenes wrapped around the rigid polyacetylene main-chain. The polymers adopt a columnar shaped structure and self-organized into hexagonally packed columnar phase. The relative electron density maps of the columnar structure are also reconstructed.

Highly crystalline iron/iron oxide nanosheets via lyotropic liquid crystal templating

A simple, room temperature reduction-hydrolysis of FeCl3 in H2O/Triton X-100 serving as a lyotropic hexagonal columnar liquid crystal template is an effective and mild approach for preparing highly crystalline iron/iron oxide nanosheets.

Liquid Crystals: Mesomorphic Organization and Thermochromic Luminescence of Dicyanodistyrylbenzene‐Based Phasmidic Molecular Disks: Uniaxially Aligned Hexagonal Columnar Liquid Crystals at Room Temperature with Enhanced Fluorescence Emission and Semiconductivity (Adv. Funct. Mater. 1/2012)

(2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(3,4,5-tris(dodecyloxy)phenyl)acrylonitrile) (GDCS) molecules self-assemble into supramolecular disks consisting of a pair of molecules in a side-by-side dis-position assisted by secondary bonding interactions of the lateral polar cyano group, which, in turn, constitute the hexagonal columnar LC structure. As demonstrated on page 61 by Dongho Kim, Soo Young Park, and co-workers, uniaxially aligned liquid crystal (LC) and crystalline GDCS microwires with enhanced fluorescence and semiconductivity are successfully fabricated by using a micromolding in capillaries (MIMIC) process.

Mesomorphic Organization and Thermochromic Luminescence of Dicyanodistyrylbenzene‐Based Phasmidic Molecular Disks: Uniaxially Aligned Hexagonal Columnar Liquid Crystals at Room Temperature with Enhanced Fluorescence Emission and Semiconductivity

AbstractA new dicyanodistyrylbenzene‐based phasmidic molecule, (2Z,2′Z)‐2,2′‐(1,4‐phenylene)bis(3‐(3,4,5‐tris(dodecyloxy)phenyl)acrylonitrile), GDCS, is reported, which forms a hexagonal columnar liquid crystal (LC) phase at room temperature (RT). GDCS molecules self‐assemble into supramolecular disks consisting of a pair of molecules in a side‐by‐side disposition assisted by secondary bonding interactions of the lateral polar cyano group, which, in turn, constitute the hexagonal columnar LC structure. GDCS shows very intense green/yellow fluorescence in liquid/solid crystalline states, respectively, in contrast to the total absence of fluorescence emission in the isotropic melt state according to the characteristic aggregation‐induced enhanced emission (AIEE) behavior. The AIEE and two‐color luminescence thermochromism of GDCS are attributed to the peculiar intra‐ and intermolecular interactions of dipolar cyanostilbene units. It was found that the intramolecular planarization and restricted molecular motion associated with a specific stacking situation in the liquid/solid crystalline phases are responsible for the AIEE phenomenon. The origin of the two‐color luminescence was elucidated to be due to the interdisk stacking alteration in a given column driven by the specific local dipole coupling between molecular disks. These stacking changes, in turn, resulted in the different degree of excited‐state dimeric coupling to give different emission colors. To understand the complicated photophysical properties of GDCS, temperature‐dependent steady‐state and time‐resolved PL measurements have been comprehensively carried out. Uniaxially aligned and highly fluorescent LC and crystalline microwires of GDCS are fabricated by using the micromolding in capillaries (MIMIC) method. Significantly enhanced electrical conductivity (0.8 × 10−5 S•cm−1/3.9 × 10−5 S•cm−1) of the aligned LC/crystal microwires were obtained over that of multi‐domain LC sample, because of the almost perfect shear alignment of the LC material achieved in the MIMIC mold.

Amphiphilic dendritic peptides: Synthesis and behavior as an organogelator and liquid crystal

New amphiphilic dendritic peptides on dendritic polyaspartic acid were designed and synthesized. The organogel and liquid crystal properties of these amphiphilic dendritic peptides were fully studied by field-emission SEM, temperature dependent FT-IR, differential scanning calorimetry, polarization optical microscopy and X-ray diffraction experiments. Amphiphilic dendritic peptides G3 show good organogel properties with a minimum gelation concentration as low as 1 wt %. Furthermore, amphiphilic dendritic peptides G3 can form a hexagonal columnar liquid crystal assembly over a wide temperature range.