Liquid Crystalline Assembly Research Articles

ChemInform Abstract: Porous Metals from Colloidal Templates

This article reviews the synthesis, characterization, and potential applications of macroporous metals created from colloidal crystal templates. Porous metals formed in this way are over 70 % air—resulting in a net surface area of several square meters for a thin film only one centimeter across—and are sturdy enough to withstand routine handling. The voids of these samples are highly ordered over large length scales, and are interconnected by small windows that give solvents and gases ready access to the entire internal surface of the metal. Moreover, since the porous structure is ordered and the surface is relatively flat, the materials possess striking optical features reminiscent of highly blazed gratings. [1] Finally, the metal of the solid matrix is actually nanocrystalline. The unique combination of both micro- and nanostructure, as well as the material’s thin-film format, suggests that these samples may be useful in a wide variety of applications ranging from sensors to catalysis. [2,3] The application of templating methods to the synthesis of macroporous metals is in its early stages, though the general area of template chemistry has been very active over the last decade. The term templating refers to a technique that involves first the formation of a temporary medium, the template, whose interstices are then filled with another material. The template is subsequently removed chemically or thermally, leaving behind a porous material that is an inverse replica of the template’s microstructure. The architecture and form of the resulting porous sample thus depend directly on the characteristics of the starting template. Templates for the production of mesoporous materials, i.e., materials containing pores with diameters less than 50 nm, include liquid-crystalline surfactant assemblies [4‐6] and anodically etched alumina membranes. [7] Such materials can be used to form high-surface-area metals with pore sizes of 10‐50 nm, typically with cylindrical voids. [8‐11] The subject of this review, however, is the development of template routes for the production of macroporous metals, specifically materials with pore diameters between 200 and 1000 nm, in a free-standing format with highly ordered void structures.

Smectic liquid crystals based on hexaazatriphenylene: potential organic n-type semiconductor

We report the synthesis of dibenzohexaazatriphenylene derivative DBHAT. The addition of electron-withdrawing groups on the dibenzohexaazatriphenylene core leads to a high electron-accepting capability. DBHAT exhibits reversible four-step reduction waves at half-wave potentials (E1/2) of −0.42, −0.94, −1.35 and −1.80 V vs. Ag/AgCl. Furthermore, DBHAT is able to form a smectic liquid crystalline assembly over a wide temperature range. With strong dipole–dipole interactions, the smectic liquid crystals show extremely highly stabilized mesophases.

Columnar liquid crystalline assembly of doubly discotic supermolecules based on tetra-triphenylene-substituted phthalocyanine

A series of doubly discotic supermolecules with four triphenylene (Tp) mesogens attached to a phthalocyanine (Pc) core via flexible alkyl spacers were designed and synthesized. The samples were denoted as Pc(Tp)4. Two alkyl chain lengths (C5 and C12) in the Tp arms and two spacer lengths (C6 and C10) were used to study columnar liquid crystalline (LC) self-assemblies in Pc(Tp)4 samples. The mesophase morphology was studied by polarized light microscopy and X-ray diffraction techniques. When the spacer length was relatively long (C10), a normal hexagonal columnar structure with the unit cell dimension close to those for the parent Tp or Pc liquid crystals was observed. The Pc columns packed either randomly or in a hexagonal superlattice in the Tp matrix. When the spacer length was short (C6), the Tp arms and the Pc core were tightly coupled together, and thus the whole supermolecules stacked together to form a super-LC column. A column-in-column rectangular mesophase with unit cell dimensions close to the size of the entire supermolecule was observed.

Recent progress in morphology control of supramolecular fullerene assemblies and its applications

Self-organisation is an elegant tool for the creation of assemblies controlled in all dimensions with tunable properties in natural as well as artificial supramolecular systems. Especially, the supramolecular organisation of fullerene (C(60)) using π-stacking interaction to form various functional assemblies is of particular importance as it can provide excellent optoelectronic properties. Interestingly, the insufficient solubility of C(60) has been overcome through the noncovalent interaction with other hosts and covalent functionalisation with organic moieties. This has resulted in supramolecular assemblies at the nano/micro/macro scales under different preparation conditions. The developments in the area of fullerene self-assembly during the last few decades have significantly contributed to the sensible design and fabrication of organic electronic devices. A summary of the very recent reports regarding the organisation of pristine C(60), its coassembly with other hosts, unique polymorphs of fullerene derivatives, functional liquid crystalline assemblies, donor/acceptor heterojunctions and its applications will be presented in this tutorial review. Future research directions in which the supramolecular fullerene assembly may achieve more precision and improve the efficiency of the photovoltaic devices are also discussed.

Self‐Assembly in Helical Columnar Mesophases and Luminescence of Chiral 1H‐Pyrazoles

Chiral polycatenar 1H-pyrazoles self-assemble to form columnar mesophases that are stable at room temperature. X-ray diffraction and CD studies in the mesophase indicate a supramolecular helical organization consisting of stacked H-bonded dimers. The liquid-crystalline compounds reported are 3,5-bis(dialkoxyphenyl)-1H-pyrazoles that incorporate two or four dihydrocitronellyl chiral tails. It can be observed that the grafting of these branched chiral substituents onto the 3,5-diphenyl-1H-pyrazole core has a beneficial role in inducing mesomorphism, because isomeric linear-chain compounds are not liquid crystalline; this is not the usual scheme of behavior. Furthermore, the molecular chirality is transferred to the columnar mesophase, because preferential helical arrangements are observed. Films of the compounds are luminescent at room temperature and constitute an example of the self-organization of nondiscoid units into columnar liquid-crystalline assemblies in which the functional molecular unit transfers its properties to a hierarchically built superstructure.

“Bicontinuous Cubic” Liquid Crystalline Materials from Discotic Molecules: A Special Effect of Paraffinic Side Chains with Ionic Liquid Pendants

Triphenylene (TP) derivatives bearing appropriate paraffinic side chains with imidazolium ion-based ionic liquid (IL) pendants were unveiled to display a phase diagram with liquid crystalline (LC) mesophases of bicontinuous cubic (Cub(bi)) and hexagonal columnar (Col(h)) geometries. While their phase transition behaviors are highly dependent on the length of the side chains and the size of the ionic liquid pendants, TPs with hexadecyl side chains exclusively form a Cub(bi) LC assembly over an extremely wide temperature range of approximately 200 degrees C from room temperature when the anions of the IL pendants are relatively small. Wide-angle X-ray diffraction analysis suggested that the Cub(bi) LC mesophase contains pi-stacked columnar TP arrays with a plane-to-plane separation of approximately 3.5 A. Consistently, upon laser flash photolysis, it showed a transient microwave conductivity comparable to that of a Col(h) LC reference.

π‐Conjugated Oligothiophene‐Based Polycatenar Liquid Crystals: Self‐Organization and Photoconductive, Luminescent, and Redox Properties

AbstractA series of liquid‐crystalline (LC) π‐‐conjugated oligothiophenes bearing three or two alkoxy chains at their extremities has been designed and synthesized. These polycatenar oligothiophenes form various LC nanostructures including smectic, columnar, and micellar cubic phases. These properties depend on the number and length of the terminal alkoxy chains. The hole mobilities for the oligothiophenes have been measured. The layered smectic and columnar structures are capable of transporting holes, leading to mobilities of up to 0.01 cm2 V−1 s−1. The columnar LC assemblies have also been explored to produce linearly polarized light‐emission. Fine red polarized fluorescence is observed from a uniaxially aligned film of the oligothiophenes. The redox properties of the oligothiophenes both in solutions and in films have been examined. The oligothiophenes exhibit electrochromism upon applying an oxidative potential. The present design strategy is useful for fabricating a variety of functional electro‐active molecular assemblies.

Self-assembly of functional columnar liquid crystals

Columnar liquid crystals have recently been developed as dynamically functional anisotropic materials. In this feature article, some recent examples of the materials design and functionalisation of columnar liquid crystals are described. Not only disklike molecules but also molecules with unconventional shapes can form columnar liquid-crystalline assemblies with nanosegregated structures. Moreover use of non-covalent interactions such as hydrogen bonding, ionic and electron donor-acceptor interactions leads to the formation of supramolecular functional columnar materials that are stimuli-responsive. Nanosegregated columnar molecular order can also be used for one-dimensional transportation of electrons, ion and molecules.

Liquid Crystalline Corannulene Responsive to Electric Field

An amide-appended corannulene derivative (3) with tribranched paraffinic side chains self-assembles into a hexagonal columnar liquid crystalline (LC) mesophase over a wide temperature range from 154 to -10 degrees C. In contrast with columnar LC assemblies of planar polycyclic aromatic hydrocarbons, the LC assembly of nonplanar 3 responds to an applied electric field and can align homeotropically to the electrode surface. Even after the electric field is switched off, this alignment is memorized for a long period of time unless the material is heated above the clearing temperature.

PH-Induced Nanosegregation of Ritonavir to Lyotropic Liquid Crystal of Higher Solubility than Crystalline Polymorphs

Birefringent spherical vesicles of ritonavir (RTV) are formed by increasing the pH of aqueous solutions from 1 to 3 or to 7 and by addition of water to ethanol solutions at room temperature. Increasing the pH creates supersaturation levels of 30-400. Upon this change in pH, the solutions become translucent, implying that some kind of RTV assembly was formed. Small spherical vesicles of narrow size distribution are detectable only after a few hours by optical microscopy. The vesicles show similar X-ray diffraction patterns and differential scanning calorimetry (DSC) behavior to amorphous RTV prepared by melt-quenching crystalline RTV. Examination by polarized optical microscopy suggests that these are lyotropic liquid crystalline (LLC) assemblies. Small-angle X-ray scattering and synchrotron X-ray diffraction further support the presence of orientational order that is associated with a nematic structure. RTV self-organizes into various phases as a result of the supersaturation created in aqueous solutions. The LLC vesicles do not fuse but slowly transform to the polymorphs of RTV (in days), Form I and finally Form II. Amorphous RTV in aqueous suspension also undergoes a transformation to a mesophase of similar morphology. Transformation pathways are consistent with measured dissolution rates and solubilities: amorphous > LLC >> Form I > Form II. The dissolution and solubility of LLC is slightly lower than that of the amorphous phase and about 20 times higher than that of Form II. RTV also self-assembles at the air/water interface as indicated by the decrease in surface tension of aqueous solutions. This behavior is similar to that of amphiphilic molecules that induce LLC formation.

Nanostructured columnar and cubic liquid-crystalline assemblies consisting of unconventional rigid mesogens based on triphenylmethanes

Liquid-crystalline (LC) molecules of unconventional shapes that form columnar and micellar cubic structures have been synthesized using triarylmethyl moieties as building blocks. The molecules have bowl- and dumbbell-shape. Despite the rigidity and bulkiness of the triarylmethyl moieties, the molecules form columnar and micellar cubic LC phases. The bowl-shaped molecules containing one triarylmethyl moiety show LC phases. The LC temperature ranges of the dumbbell-shaped molecules containing two triarylmethyl moieties connected by rigid rods are wider than those of bowl-shaped molecules containing one triarylmethyl moiety. The UV–vis spectroscopy of the dumbbell-shaped molecules having a terphenyl moiety reveals that the terphenyl moieties aggregate in the mesophase.

Organization of Rigid Wedge−Flexible Coil Block Copolymers into Liquid Crystalline Assembly

The synthesis and characterization of wedge−coil block molecules with a poly(ethylene oxide) coil of 77 (1a), 91 (1b), 114 (1c), and 182 (1d) ethylene oxide units are described. The self-assembling behavior of these block polymers in the melt state was investigated by optical polarized microscopy, differential scanning calorimetry (DSC), X-ray scattering measurements, and transmission electron microscopy (TEM). Block polymers self-organize from 1-D lamellar to 2-D hexagonal columnar liquid crystalline phases as the length of the PEO chain increases. This result demonstrates that supramolecular nanostructures can be controlled in a rigid wedge−flexible coil block copolymer system through introduction of a long flexible coil.

Polymerization in Surfactant Liquid Crystalline Phases

We report the formation of cross-linked polysiloxane particles having rodlike and sheetlike morphologies by condensation of monomers in organized liquid crystalline mesophases of nonionic surfactants. The characteristic diffraction pattern obtained from the liquid crystalline surfactant assembly is preserved during polymer synthesis. The polymer colloids are rodlike when synthesized in a hexagonal mesophase and are sheetlike when synthesized in a lamellar mesophase. Interestingly, the size of the polymer particles is on the order of micrometers, viz. 3 orders of magnitude larger than the characteristic size of the surfactant mesophase. Thus, the polymers phase separate from the liquid crystalline phase during polymerization, and organize to resemble the mesophase geometry. The polymer particles are organized so as to minimize the elastic distortion of the surfactant mesophase after they phase separate. We speculate that the observed particle morphologies are formed due to the slow kinetics of polycondensa...

Discotic liquid crystals stabilized by interionic interactions: imidazolium ion-anchored paraffinic triphenylene

Incorporation of imidazolium ion functionalities into the paraffinic side-chain termini of a triphenylene derivative resulted in stabilization of a columnar mesophase of its liquid crystalline assembly, which was retained over a wider temperature range down to 4 degrees C by an externally added imidazolium-based ionic liquid.

Osmotic Stress Measurements of Intermolecular Forces in Ordered Assemblies Formed by Solvated Block Copolymers

Intermolecular interactions in ordered (lyotropic liquid crystalline) assemblies formed by hydrated poly(ethylene oxide)−poly(propylene oxide) (PEO−PPO) block copolymers (Pluronics or Poloxamers) have been probed using the osmotic stress method. This method involves the equilibration, following the removal or the addition of an appropriate amount of water, of hydrated block copolymer samples with a system (aqueous polymer solution or water vapor) of known osmotic pressure (in the range 0.05−3000 atm). The primary result from such an experiment is a relationship between osmotic pressure (and consequently the corresponding chemical potential and activity of the solvent water) and block copolymer volume fraction. The osmotic pressure of the two PEO−PPO block copolymer−water systems examined here increased exponentially from 5 × 103 to 3 × 108 Pa over the 6−99.9 wt % block copolymer concentration range. The osmotic pressure of the PEO−PPO block copolymer−water systems in the block copolymer concentration rang...

Liquid crystalline assemblies of collagen in bone and in vitro systems

Precise descriptions of the three-dimensional arrangements of collagen in bone are essential to understand the mechanical properties of this complex tissue. Transmission electron microscopy (TEM) analysis of decalcified human compact bone in section reveals characteristic patterns forming regular series of nested arcs. Such patterns are a direct consequence of an organization described as a twisted plywood and relate the distribution of collagen fibrils in osteons with that of molecules in cholesteric liquid crystals. The hypothesis that liquid crystalline properties are involved in the morphogenesis of dense collagen matrices was supported by data obtained in vitro. At a molecular level, acid-soluble collagen molecules spontaneously assemble, at concentrations of 50 mg/ml or more, in precholesteric-banded patterns and cholesteric phases, identified by polarized light microscopy. In a more physiological context, these results were conforted, with the precursor molecule of collagen, procollagen, soluble at neutral pH. This protein spontaneously forms liquid crystalline precholesteric phases corresponding to banded patterns and birefringent cords. Stabilization of the liquid crystalline collagen, induced by pH modification and fibril formation, shows characteristic morphologies in TEM, which directly mimic arrays described in vivo. Undulating fibrils are indeed similar to crimp morphologies described in tendons and continuously twisting fibrils, and give rise to arced patterns similar to supra-molecular architectures identified in compact bone.

1B11 イオン性液体の低次元組織化 : 液晶性分子集合体とそのイオン伝導特性(化学・材料)

1B11 イオン性液体の低次元組織化 : 液晶性分子集合体とそのイオン伝導特性(化学・材料)

Supramolecular organization of block oligomers based on rod-shaped mesogen into liquid crystalline assembly

The development of novel supramolecular materials with nanometer-scale architectures and the effect of these architectures on the materials' properties are presently of great interest in molecular design. Liquid crystalline assemblies of rod like mesogenic molecules containing flexible coils (rod-coil molecules) provide a facile entry into this area. Rod-coil molecules are demonstrated to self-assemble into a rich variety of different liquid crystalline structures of nanoscale dimensions through the combination of shape complementarity and repulsive interaction of rigid and flexible parts as an organizing force. The mesophases include smectic, hexagonal columnar, bicontinuous cubic, honeycomb like and discrete micellar phases. The unconventional mesophases are induced by changing the rod to coil volume fraction, by controlling the number of rod-coil repeating units and by increasing the rod-coil molecular length. Additionally, the liquid crystalline structures of rod-coil molecules are also discussed with respect to their manipulation by complexation of polyether chains with lithium cations or addition of polar solvent, from lamellar, bicontinuous cubic to hexagonal columnar mesophases.

Self-assembled aerogel-like low dielectric constant films

A series of new spin-on nanoporous silica films with controlled porosity and dielectric constants were prepared by evaporation induced self-assembly (EISA) during spin-coating. Starting with a homogeneous solution of soluble silica and surfactant/swelling agent prepared in an alcohol/water solvent, preferential evaporation of alcohol during spin-coating results in the hierarchical self-assembly of surfactants plus swelling agent and soluble silica into a templated microemulsion system. Silicate polymerization followed by removal of surfactant and swelling agents via immediate calcination results in a highly porous silica film of the liquid crystalline assembly with controlled porosity ranging from 50% to 90% and dielectric constant ranging from 1.3 to 2.6. TEM and surface acoustic wave (SAW) techniques were used to characterize the microstructure of the films. The dielectric constant, Young's modulus, and hardness of the final films are also discussed.

Spontaneous Organization of Supramolecular Rod-Bundles into a Body-Centered Tetragonal Assembly in Coil−Rod−Coil Molecules

The synthesis and characterization of coil−rod−coil molecules of 4,4‘-bis[4-methyloxypoly(propyleneoxy)propyloxy-4‘-biphenyloxymethyl]biphenyl with a poly(propylene oxide) coil of 3 (7), 6 (8), 9 (9), 13 (10), 17 (11), and 22 (12) propylene oxide units are described. These molecules self-assemble into ordered structures that differ significantly on variation of the length of poly(propylene oxide) coil. Coil−rod−coil molecule 7 self-organizes into lamellar crystalline and bicontinuous cubic liquid crystalline assemblies, while 8 shows a hexagonal columnar liquid crystalline assembly. Remarkably, increasing the length of coil induces discrete supramolecular aggregates that self-assemble into a birefringent 3-D superlattice. The molecules 9 and 10 assemble into discrete supramolecular aggregates that spontaneously organize into a novel 3-D tetragonal lattice with a body-centered symmetry in the crystalline and melt states. Further increasing the length of coil as in the case of 11 and 12 induces only a body-centered tetragonal crystalline phase, while the liquid crystalline phase in these molecules is suppressed. X-ray diffraction experiments and density measurements showed that the aggregation of these molecules into a discrete supramolecular structure gives rise to aromatic rod bundles with hockey puck-like cylindrical shape encapsulated by phase-separated coil segments which results in the formation of oblate aggregates. This nonspherical oblate shape is believed to be responsible for the formation of a body-centered tetragonal phase. These results demonstrate that supramolecular structures, from 1-D lamellar to 3-D tetragonal superlattices, formed by the self-assembling process of molecular rods can be controlled in a systematic and predictive way by simple variation of the length of grafted coils.