Columnar Liquid Crystalline Phase Research Articles

Semi-flexible star-shaped molecules: conformational analysis of nano-segregated mesogens forming columnar liquid-crystal phases

Abstract The structural prerequasites are investigated, which make star-shaped molecules suitable precursors for the formation of columnar liquid-crystalline phases. Electronic structure calculatio...

Polarization switching in a columnar liquid crystalline urea as studied by optical second-harmonic generation interferometry

The polar order and its switching characteristics have been investigated by means of optical second-harmonic generation interferometry in a compound N, N'-bis(3,4,5-trialkoxylphenyl)urea (R=n- C16 H33) being connected by intermolecular hydrogen bondings to form a columnar liquid crystalline phase. The polar structure is formed along the column by applying an electric field and is cooperatively switched by reversing the field. The polar order is relaxed to a nonpolar state within a few milliseconds by terminating the field. No macroscopic polar order exists at least in a range of a visible wavelength scale in the absence of a field.

High Charge‐Carrier Mobility in π‐Deficient Discotic Mesogens: Design and Structure–Property Relationship

Hexaazatrinaphthylene (HATNA) derivatives with six alkylsulfanyl chains of different length (hexyl, octyl, decyl and dodecyl) have been designed to obtain new potential electron-carrier materials. The electron-deficient nature of these compounds has been demonstrated by cyclic voltammetry. Their thermotropic behaviour has been studied by means of differential scanning calorimetry and polarised optical microscopy. The supramolecular organisation of these discotic molecules has been explored by temperature-dependent X-ray diffraction on powders and oriented samples. In addition to various liquid crystalline columnar phases (Col(hd), Col(rd)), an anisotropic plastic crystal phase is demonstrated to exist. The charge-carrier mobilities have been measured with the pulse-radiolysis time-resolved microwave-conductivity technique. They are found to be higher in the crystalline than in the liquid crystalline phases, with maximum values of approximately 0.9 and 0.3 cm(2) V(-1) s(-1), respectively, for the decylsulfanyl derivative. Mobilities strongly depend on the nature of the side chains.

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.

Tuning organogels and mesophases with phenanthroline ligands and their copper complexes by inter- to intramolecular hydrogen bonds.

A novel family of highly functionalized molecules consisting of a central 4-methyl-3,5-diacylaminobenzene platform linked in close proximity to the methyl group by two lateral aromatic rings each equipped with two long alkoxy chains has been rationally designed. The presence of amide tethers and a chelating phenanthroline fragment connected via an ester dipole formed a new class of gelating reagents and mesomorphic materials. A few of these compounds have the tendency to form macromolecule-like aggregates through noncovalent interactions in hydrocarbon solvents and were found to exhibit thermotropic cubic mesophases. In light of the X-ray molecular structure of the methoxy ligand, an infinite network maintained by intermolecular hydrogen bonds as well as by pi-pi stacking of the phenyl subunits was evidenced. FT-IR studies confirm that the common driving force for aggregation in the organogels and microsegregation in the mesophase is the occurrence of a tight intermolecular H-bonded network that does not persist in diluted solution. This situation is switched when the ligands are interlocked by a copper(I) cation. A strong intramolecular H-bond confirmed by X-ray diffraction of a single crystal for the methoxy case provides very stable complexes but inhibits the gelation of the solvents. Heating the complexes bearing long paraffin chains (n = 12 and 16) in the dried state leads to a self-organization into a columnar liquid-crystalline phase in which the columns are arranged along a 2D oblique symmetry as deduced from powder XRD experiments. In this case, the complexes with the appended counteranions self-assemble in a specific way to form columns. A striking observation is that the intramolecular hydrogen bond persists in the mesophase as it does in solution without any evidence of an extended network. As far as we are aware, these ligands and complexes are rare examples in which organogelation and thermotropic mesomorphic behavior could be observed in parallel with molecules bearing a chelating platform. Due to the synthetic availability of the 4-methyl-3,5-diacylaminobenzene core and the simplicity by which the chelating platforms can be graphed, this methodology represents a practical alternative to the production of functionalized organogelators and mesomorphic materials.

An ABC Stacking Supramolecular Discotic Columnar Structure Constructed via Hydrogen-Bonded Hexamers

An ABC intracolumn stacking scheme was discovered in a discotic columnar liquid-crystalline phase, which formed by self-assembly of hexameric supramolecular disks. Strong DDA-AAD hydrogen bonding dictated the formation of hexameric disks. Molecular structural simulation suggested that interplay among secondary interactions (π−π, electrostatic, and steric) between the hexameric disks determined the formation of this columnar phase.

A new type of square columnar liquid crystalline phases formed by facial amphiphilic triblock molecules.

A series of three novel liquid crystalline amphiphilic molecules is reported which are composed of three incompatible molecular parts, a rigid terphenyl core, two lipophilic decyloxy chains in the terminal 4- and 4' '-positions, and a polar group in the lateral 2'-position. The polar group comprises a polyether chain, an amide group, and a polyhydroxyalkyl end group (1-acylamino-1-deoxy-d-sorbitol derivatives). The self-organization of these compounds was studied by polarized light microscopy, differential scanning calorimetry, and different X-ray diffraction techniques. These investigations confirm a novel liquid crystalline phase with a square 2D-lattice (square columnar mesophase, plane group p4mm). This structure is built up by a set of three distinct columns, namely columns containing the polar lateral groups, columns incorporating the alkyl chains, and ribbons of the rodlike terphenyl units. The calamitic cores form walls bounding square-shaped channels occupied by the microsegregated polar lateral chains. The lipophilic columns containing alkyl chains are at the corners interconnecting the aromatic rods end-to-end.

Self-assembly of C(3)(h) and C(s) symmetric keto-enamine forms of tris(N-salicyl-ideneanilines) into columnar phases: a new family of discotic liquid crystals.

A new class of discotic liquid crystals derived from tris(N-salicylideneanilines) existing exclusively in their C3h and Cs keto-enamine tautomeric forms is reported. These compounds self-assemble to form columnar liquid crystalline phases with a small core-core separation as evidenced by X-ray diffraction studies.

Synthesis and study of novel azopyridine-containing hexacatenar silver mesogens

Azopyridine-containing hexacatenar silver complexes have been synthesized and their liquid crystalline and photoisomerization behaviours investigated. These silver complexes (1a–d) exhibit hexagonal columnar liquid crystalline phases, whose structures were confirmed using polarizing optical microscopy and small angle X-ray diffraction.

Supramolecular chirality of thermotropic liquid-crystalline folic acid derivatives.

Helical self-assembled columns form supramolecular chiral cubic and columnar liquid-crystalline phases (see picture). The complexation of hydrogen-bonded disks of folic acid derivatives that have oligo(glutamic acid) moieties and lipophilic alkyl chains with nonchiral ions leads to the self-assembly of chiral Pm3n cubic and hexagonal columnar structures.

Nanohybrids from Liquid Crystalline Extended Amphiphilic Dendrimers

A novel extended amphiphilic dendrimer with linear poly(ethylene oxide) (PEO) attached to a PEO-like dendritic core as hydrophilic fraction and eight docosyl chain branches as hydrophobic fraction has been prepared for the use as structure-directing agent for silica-type materials. The extended dendrimer exhibits a hexagonal columnar liquid crystalline phase in the melt. Organically modified inorganic precursors and the extended dendrimer co-assemble into nanostructured hybrids. Hybrids with 0.44 weight fraction (fw) of aluminosilicate show a lamellar morphology, while hybrids with 0.21 fw exhibit a cylindrical structure. Nanostructures were characterized by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results suggest that dendrimer-based amphiphiles may provide an exciting platform for the formation of multifunctional organic-inorganic nanostructured hybrid materials with unique structural characteristics.

One-dimensional ion transport in self-organized columnar ionic liquids.

New fan-shaped ionic liquids forming columnar liquid crystalline phases have been prepared to obtain one-dimensional ion-transporting materials. The ionic liquids consist of two incompatible parts: an imidazolium-based ionic part as an ion-conducting part and tris(alkyloxy)phenyl parts as insulating parts. Two compounds having octyl and dodecyl chains have been synthesized. Self-assembly of these materials leads to the formation of thermotropic hexagonal columnar liquid crystalline states at room temperature. Anisotropic one-dimensional ionic conductivities have been successfully measured by the cells having comb-shaped gold electrodes. The self-organized columns have been aligned macroscopically in two directions by shearing perpendicular and parallel to the electrodes. The ionic conductivities parallel to the column axis are higher than those perpendicular to the axis. The incorporation of lithium salts in these columnar materials leads to the enhancement of the ionic conductivities and their anisotropy. These materials would be useful for anisotropic transportation of ions at the nanometer level.

Smectic and Columnar Liquid Crystalline Phases through Charge-Transfer Interactions

New heterocyclic electron donors based on a 1,3,5-triazine nucleus are presented. Three phenyl rings are grafted to the triazine core either via secondary amino groups or by a direct C,C-linkage and a specific number of decyloxy chains is attached to the molecular periphery. The compounds are non-liquid crystalline in their pure states. Lamellar or columnar mesophases are induced by attractive interactions with electron acceptors.

Columnar liquid crystalline phases through hydrogen bonding and nanoscale segregation

Two columnar phases forming 2,4,6-triarylamino-1,3,5-triazines have been investigated in binary mixtures with calamitic and non-liquid crystalline benzoic acids carrying one or two alkoxy chains at the aromatic core. The triazines form hydrogen bonded aggregates with the complementary acids. Each investigated equimolar mixture exhibits a columnar mesophase due to segregation of the H-bonded polar core region from the lipophilic aliphatic molecular segments. The cross sectional shape of cylindrical aggregates and, therefore, the two-dimensional lattice symmetries, hexagonal or rectangular, are defined by the number of alkoxy chains of the benzoic acid component.

Inelastic X-ray scattering studies of phonons in liquid crystalline DNA

A high resolution X-ray scattering method is used to measure spectra of the collective density oscillations propagating along the axis of a rod-like supra-molecular system made of a shear-aligned columnar hexagonal liquid crystalline phase of DNA dispersed in water. The dynamic structure factor is extracted from the spectrum using a generalized three effective eigenmode (GTEE) theory. The dynamic structure factor consists of three Lorentzian lines, one central Rayleigh and two symmetrically shifted Brillouin peaks, from which the phonon frequency and damping can be obtained. We investigated two systems: a 40 wt.% calf-thymus Na–DNA of molecular weight 8.4 × 106 Da (13 000 base pairs) in water, and 40 wt.% Na–DNA in 0.085 M MgCl2. The phonon dispersion relations obtained show an oscillatory behavior as a function of Q (magnitude of the scattering vector) in the range of 2 nm−1 to 30 nm−1, similar to that obtained from lipid bilayers before. The dispersion relation of 40 wt.% DNA rods in pure water, for example, starts with a linear dispersion at low Q with a high-frequency sound speed of about 3100 m s−1, similar to that observed in bulk water, reaching a maximum of ω = 12 meV at Q = 9 nm−1, going down to a valley at Q = 18.7 nm−1, coming up to a secondary (lower) maximum at Q = 25 nm−1 and eventually going down at Q = 30 nm−1. One major distinctive feature of the DNA systems from liquids is that the dispersion relation in the shear aligned DNA extends to a considerably higher Q than that observed in liquids. The extended range of Q includes values corresponding to those in the second Brillouin zone in a crystalline system. This latter feature implies that a DNA rod can be regarded approximately as a one-dimensional crystal, as far as the density oscillations are concerned. In the system with 40 wt.% Na–DNA in 0.085 M MgCl2, we observe a significantly greater phonon damping compared to the one in water.

13C-detected 1H– 2H separated local field NMR spectroscopy

We present a new NMR method for measuring 1H– 2H dipolar couplings in macroscopically oriented media. To overcome the lack of dipolar resolution in 1D 1H and 2H spectra of deuterated molecules, we use a 2D heteronuclear correlation experiment where 1H chemical shifts and 1H– 2H dipolar interactions in the first dimension are correlated with 13C chemical shifts and 2H– 13C dipolar interactions in the second dimension. The technique is demonstrated on a columnar liquid-crystalline phase.

Template polymerization of columnar architectures based on the salts of a carboxylic acid and 2-amino alcohols: application to the molecular recognition of 2-amino alcohols.

The salts of trialkoxybenzoic acids and 2-amino alcohols showed a columnar liquid crystalline phase; in the case of the salt of a polymerizable acid and norephedrine, the photopolymerization proceeded efficiently in the liquid crystalline state, and the resultant solid adsorbed 2-amino alcohols size, regio-, and enantio-selectively.

Photoconductivity Measurements in the Discotic Columnar Phase of a few Anthraquinone Derivatives

It has recently been shown that the structure of the columnar phase of discotic liquid crystals is suitable for fast transport of photogenerated charge carriers. Although the magnitude of the charge carrier mobility exhibited by these systems is still small compared to those in organic single crystals, the fluidity and the resulting self-healing nature of the liquid crystalline columnar phases hold promises for potential applications. In this paper we report on steady-state photoconductivity measurements carried out on three anthraquinone derivatives exhibiting the columnar phase. Upon illumination of the sample with UV light the current through the sample increases abruptly and reaches a photo-stationary value within a short span; the fastest observed response time is 17 ms. It was found that the anisotropy of the photoconductivity, defined as the ratio of the photocurrent along and perpendicular to the column axis, reaches a value of about 450. We also discuss the relation between the chemical structure of the molecule and the maximum value of the photocurrent observed.

Structure and Dynamics in Columnar Discotic Materials: A Combined X-ray and Solid-State NMR Study of Hexabenzocoronene Derivatives

The solid columnar discotic and liquid crystalline (LC) phases formed by alkyl-substituted hexabenzocoronene (HBC) mesogens have been investigated by a combination of X-ray scattering and a variety of advanced solid-state NMR methods. Correlations between chemical structure, molecular packing and dynamics are established. Maximum charge carrier mobility is observed for a crystal-like stacking of the disks in the column with optimized π−π interactions. For systems with lower intracolumnar order the charge carrier mobility is slightly lower and slow axial motion of the disks around the column axes occurs.

Electro-Optic Switching by Helicene Liquid Crystals

Binary mixtures of the aggregating helicene 1 with dodecane have, at dodecane concentrations >30 vol %, a nematic liquid crystalline phase and, at dodecane concentrations ≲5 vol %, a hexagonal columnar liquid crystalline phase. The helical structure with donor and acceptor groups at opposite ends gives rise to a molecular dipole moment that is parallel to the helix axis and a positive dielectric anisotropy. Accordingly, the helix axes orient parallel to an applied electric field, providing the basis for electro-optic switching.