Tetrabenzocyclododecatetraene A new core for mesogens exhibiting columnar mesophases

Abstract

Octasubstituted derivatives of tetrabenzocyclododecatetraene are found to exhibit thermotropic mesophases. The molecules of these mesogens are highly flexible with an average four-fold symmetry. Optical and differential scanning calorimetry measurements are presented on five ester and ether derivatives. All mesophases appear to be columnar with a negative optical anisotropy. From the five compounds studied the four which are derived from aliphatic ester and ether are optically uniaxial, while the mesophase of the decylbenzoyloxy derivative could be biaxial. Miscibility measurements performed with some discotic and pyramidic mesogens of similar molecular dimensions suggest that the mesophases of the tetrabenzocyclododecatetraene core form new families of columnar mesophases.     

Elliptical diffusion of dye in hexagonal columnar polycatenar mesophases

We have studied polycatenar compounds which exhibit hexagonal columnar mesophases. In the planar orientation of these mesophases, the elliptical diffusion of the dissolved dyes is visualized by taking several pictures of the sample. The diffusion ratio D// /D is deduced for these columnar mesophases. Furthermore, using a classical optical absorption technique, we present measurements of dye diffusion in the same mesophases. The diffusion constants are measured in two geometries, along and perpendicular to the columns. The diffusion anisotropy ratios are in agreement with those deduced from the ellipse axes. The structure of these new columnar mesophases exhibited by rod-like mesogens is compared with that of disk-like mesogens.     

Elliptical diffusion of dye in hexagonal columnar polycatenar mesophases

We have studied polycatenar compounds which exhibit hexagonal columnar mesophases. In the planar orientation of these mesophases, the elliptical diffusion of the dissolved dyes is visualized by taking several pictures of the sample. The diffusion ratio D// /D is deduced for these columnar mesophases. Furthermore, using a classical optical absorption technique, we present measurements of dye diffusion in the same mesophases. The diffusion constants are measured in two geometries, along and perpendicular to the columns. The diffusion anisotropy ratios are in agreement with those deduced from the ellipse axes. The structure of these new columnar mesophases exhibited by rod-like mesogens is compared with that of disk-like mesogens.     

Kojic acid—a new fragment for the preparation of bent-core mesogens

Kojic acid was used to synthesize bent-core mesogens. Antiferroelectric polar smectic mesophases, an oblique and a rectangular columnar mesophase were found. The mesophases were characterized by X-ray scattering, electro-optical measurements, polarizing microscopy and DSC. On the base of these measurements, models of the mesophases are proposed.     

Synthesis and characterization of a discotic uranium-containing liquid crystal

The synthesis of several UO(2)-alaskaphyrin derivatives functionalized with aliphatic chains is described. These compounds are found to be liquid-crystalline and form columnar mesophases, as judged from polarizing optical microscopy studies, and represent the first uranium-containing discotic mesogens.     

Kojic acid—a new fragment for the preparation of bent‐core mesogens

Kojic acid was used to synthesize bent‐core mesogens. Antiferroelectric polar smectic mesophases, an oblique and a rectangular columnar mesophase were found. The mesophases were characterized by X‐ray scattering, electro‐optical measurements, polarizing microscopy and DSC. On the base of these measurements, models of the mesophases are proposed.     

Self-assembly of discotic mesogens in solution and in liquid crystalline phases: effects of substituent position and hydrogen bonding

The effects of functional group position on the phase behavior of discotic mesogens was examined for a series of dibenzophenazine derivatives bearing a carboxylic acid, methyl carboxylate, or nitro group. In all cases, changing the position of the group from the "top" to the "side" of the aromatic core led to dramatic differences in the phase behavior, both in terms of the stability of the liquid crystalline phases as well as the types of mesophases formed. For the non-hydrogen bonding ester and nitro derivatives, moving the substituent to the side of the core led to a lowering of the clearing temperatures or loss of liquid crystallinity. Carboxylic acid derivatives exhibit broad mesophases irrespective of the position of the acid group, but mesogens bearing this group on the side of the core exclusively form Col(h) phases, whereas those with an acid group on the top of the core exhibit more varied mesomorphism, with the formation of Col(h), Col(r), and nematic phases. Contrary to expectations, the presence of a carboxylic acid group on the side of the core does not appear to lead to the formation of dimeric structures in the liquid crystalline phase, although the columnar structures appear to be stabilized by intermolecular hydrogen bonding along the columns. These derivatives also form pi-stacked dimers in solution; the structure of these dimers are consistent with the proposed structure of the columnar phases.     

Modular assembly of elliptical mesogens

Discotic mesogens featuring a pyridine ring were synthesized, and were found either to form ordered hexagonal columnar liquid crystalline phases or melt directly from a crystal to an isotropic liquid, depending on the position of the pyridyl nitrogen atom. Binary mixtures of the mesogenic pyridine derivatives with a similar discotic mesogen having a carboxylic acid group resulted in the formation of modular elliptical complexes through hydrogen bonding. The binary mixtures were found to exhibit ordered hexagonal columnar or ordered rectangular columnar and nematic mesophases, depending on the length of the alkyl chains, and displayed dramatically different properties from their constituent components. Binary mixtures of the non-mesogenic pyridine derivatives with carboxylic acid-functionalized discotic mesogens did not result in the formation of hydrogen-bonded complexes.     

Star-shaped oligobenzoates: non-conventional mesogens forming columnar helical mesophases

Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non-conventional mesogens form various columnar mesophases over a broad temperature range. The liquid-crystal phases were characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and solid-state NMR spectroscopy. In addition to the high-temperature hexagonal columnar phases, the columnar self-assemblies undulate upon cooling and consequently form higher-ordered body-centred orthorhombic columnar 3D structures. A model of E-shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low-temperature phases. Space filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and the polar oligobenzoate scaffold are not perfectly separated in these star-shaped mesogens.     

New biforked mesogen series

Four new series of biforked mesogens derived from 3,4-dialkoxybenzoic acids or 3,4-dialkoxybenzaldehydes have been prepared and studied. These derivatives exhibit an interesting polymorphism in which both columnar and lamellar mesophases exist in the same series and in some cases in the same compound. The short chain derivatives exhibit a nematic and/or a smectic C phase but those with long chains display the hexagonal, rectangular or oblique columnar phases. In some of them, a centred cubic phase is found.     

Columnar and smectic liquid crystals based on crown ethers

Unsymmetrical benzo[15]crown-5 ethers 5 with one lateral ortho-terphenyl unit bearing alkoxy side chains of varying chain lengths (C5-C14) were prepared from 3,4-dialkoxyphenylbromides 2. Complexation with metal salts MX (M = Na, Cs) afforded the corresponding derivatives MX5. The uncomplexed crown ethers 5 h and 5 i, with dodecyloxy and tetradecyloxy side chains, respectively, exhibit liquid crystalline properties. In the series of complexed crown ethers, liquid crystal properties appeared as early as NaI5 f with C9H19 side chains. Whereas the uncomplexed 5 h,i form smectic mesophases, the complexed NaI5 g and NaI5 h exhibit textures typical of columnar mesophases. These results were supported by X-ray diffraction measurements (WAXS, SAXS), which revealed smectic (5 h,i), rectangular columnar (NaI5 g), and hexagonal columnar (NaI5 h) mesophases. As the liquid crystalline phase might retain packing features of the solid-state structure, single-crystal X-ray analyses were also performed for some of the uncomplexed and complexed crown ether derivatives. The complex NaI(3)5 a displays a sandwich-type structure, with the crown ether cores mutually antiperiplanar and maintaining an almost perfect crown conformation. In contrast, non-mesogenic uncomplexed crown ether 5 b displays a layer-type ordering in the solid phase.     

A generalized model for the molecular arrangement in the columnar mesophases of polycatenar mesogens. Crystal and molecular structure of two hexacatenar mesogens

The columnar mesophases of two series of hexacatenar palladium(II) mesogens have been studied in detail by a combination of X-ray diffraction on aligned and unaligned samples and dilatometry. The results of these studies, combined with the results of two single crystal structure determinations, have allowed a model of the molecular arrangement in the columnar phases to be proposed. This model differs in detail from that generally accepted for the arrangement of polycatenar mesogens in columnar phases, and a new model is proposed which accounts for both new and existing data.     

Molecular dynamics and conformational behaviour of mesogenic resorcinarenes

The dynamic and conformational behaviour of four mesogenic resorcinarenes exhibiting columnar mesophases have been studied by a combination of broadband dielectric spectroscopy and deuterium solid state NMR. Broadband dielectric spectroscopy provided evidence for two relaxation processes present both in the mesophase and in the isotropic liquid. The high frequency process II, common to all mesogens, has been assigned to the libration of the carbonyl groups of the ester junctions between the core and the side chains. The low frequency process I, present in conformationally mobile mesogens 1 and 2, has been attributed to the ring inversion process of the macrocyclic core associated with dipole inversion along the columnar axis. Deuterium solid state NMR performed on 4, the deuteriated analogue of 1, confirmed the molecular dynamics attribution for process I, assigning the ring inversion to the interconversion of the two equivalent crown conformations.     

Molecular dynamics and conformational behaviour of mesogenic resorcinarenes

The dynamic and conformational behaviour of four mesogenic resorcinarenes exhibiting columnar mesophases have been studied by a combination of broadband dielectric spectroscopy and deuterium solid state NMR. Broadband dielectric spectroscopy provided evidence for two relaxation processes present both in the mesophase and in the isotropic liquid. The high frequency process II, common to all mesogens, has been assigned to the libration of the carbonyl groups of the ester junctions between the core and the side chains. The low frequency process I, present in conformationally mobile mesogens 1 and 2 , has been attributed to the ring inversion process of the macrocyclic core associated with dipole inversion along the columnar axis. Deuterium solid state NMR performed on 4 , the deuteriated analogue of 1 , confirmed the molecular dynamics attribution for process I, assigning the ring inversion to the interconversion of the two equivalent crown conformations.     

1,3,5-Triazine(trithiophenylcarboxylate) esters form metastable monotropic nematic discotic liquid crystal phases

Conformationally flexible cores that generate large free volume when stacked and have an insufficient number of side-chains for filling the lateral space around each core have been proposed as design criteria for discotic liquid crystals that display nematic rather than columnar mesophases. Presented here are 1,3,5-triazines substituted with three thiophene carboxylate ester groups that fulfil these design criteria. Nine derivatives with linear, branched, and chiral ester chains were synthesised and their mesomorphism was studied by polarised optical microscopy, differential scanning calorimetry, and powder x-ray diffraction. Triazines with linear propyl to octyl chains display monotropic nematic discotic mesophases that crystallise within hours or days or when mechanically agitated. Packing structures for the nematic phases are proposed based on the single crystal structure of the ethyl ester. Their unusually slow nucleation and crystal growth can be attributed to the large number of possible low energy conformations that arise from the conformationally flexible core. Incorporation of racemic and chiral branched chains expectedly lowers the melting points of these derivatives but, unexpectedly, enhances nucleation and crystal growth well above the temperature ranges of their hypothetical nematic phases. Chiral nematic discotic mesophases were obtained for binary mixtures of derivatives with chiral branched and linear chains.     

Non-symmetrical bent-shaped liquid crystals based on a laterally substituted naphthalene central core with four ester groups

In the continuation of our study of the role of direction of the linking ester group and lateral substitution, we present three series of bent-core liquid crystals based on 7-hydroxynaphthalene-2-carboxylic acid. The mesomorphic properties were investigated by polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical methods. For non-substituted and laterally methyl-substituted compounds, columnar phases of broken-layer type and a smectic A phase were found. With the increasing length of the terminal alkyl chain, an increasing stability of the lamellar smectic A phase was observed. Substitution with a methyl group led to substantial narrowing of the mesophase range and the chloro-substituted materials exhibited no mesophases. The results are also discussed in context with the earlier studied naphthalene based mesogens.     

Poly(vinyl ether)s and poly(propenyl ether)s containing mesogenic groups: A new class of side-chain liquid-crystalline polymers

A new series of thermotropic liquid-crystalline polymers, poly(vinyl ether)s and poly(propenyl ether)s, containing the mesogen 4-methoxy-4′-hydroxybiphenyl in the side chain and different spacers between the mesogens and the backbone were synthesized and characterized by DSC and optical microscopy. Their mesomorphic behavior was compared to that of the corresponding polymethacrylates. In general the poly(propenyl ether)s showed lower transition temperatures and larger thermal stability ranges of their mesophases than the respective polymethacrylates, reflecting the effect of substituting the rigid ester bond connecting the spacer to the backbone in polymethacrylates by a more flexible ether bond. Also, poly(propenyl ether)s showed more ordered mesophases than the polymethacrylates, suggesting that a better decoupling of the mesogen and polymer backbone is obtained through the ether linkage.     

Dimeric liquid‐crystalline epoxyimine monomers: Influence of dipolar moments on mesomorphic behavior and the formation of liquid‐crystalline thermosets

We examine some of the structural aspects that influence the mesomorphic behavior of liquid‐crystalline dimeric epoxy resins with imine groups in the mesogens. We synthesized two new series of monomers and compared them with previously synthesized monomers. Compared with previously studied series, the imine group in the new monomers is oriented differently with respect to the ether and ester groups linked to the end of the mesogenic unit. Our results confirmed the importance of polarization of the mesogenic groups and the presence of an ester group in the inner position in the formation of smectic mesophases. By curing with primary and tertiary amines, we demonstrate that these two requirements are necessary if liquid‐crystalline thermosets are to be obtained with different degrees of order. These studies were carried out with differential scanning calorimetry, polarized optical microscopy, and X‐ray diffraction. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1465–1477, 2003     

Connector type‐controlled mesophase structures in poly(propyl ether imine) dendritic liquid crystals of identical dendrimer generations

Poly(propyl ether imine) (PETIM) dendrimers of one to three generations are used as dendritic cores to identify the influence of varying connector types that connect the dendritic core with peripheral mesogens on the emerging liquid crystalline (LC) properties. The LC properties vary in these dendritic liquid crystals, even when the dendrimer generations and thus the number of peripheral mesogenic moieties remain identical. PETIM dendrimer generations one to three, ester and amide connectors varying with succinates, phthalates, and succinamides, are studied herein. Cholesteryl moieties are installed at the peripheries through the above connectors to induce mesogenic properties. These modified dendritic liquid crystals reveal a layered mesophase structure in most ester and amide connector‐derivatives, whereas a third‐generation phthalate ester dendrimer favors a rectangular columnar mesophase structure. A transition from layered to a rectangular columnar structure results by a mere change in the connector varying between a succinate or succinamide or phthalate, within one particular dendrimer generation and without altering the underlying dendrimer core or the number of mesogenic moieties. The study demonstrates that in dendritic liquid crystals with essentially identical chemical constitutions, a change in the connector type connecting the mesogen with the dendrimer core is sufficient to change the mesophase structures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3665–3678     

Mesomorphic properties of some 1,4,8,11,15,18,22,25-octa-alkoxymethylphthalocyanines

Abstract

The mesophase behaviour of a number of non-peripherally octa-substituted phthalocyanine derivatives has been studied by optical microscopy, differential scanning calorimetry (DSC) and X-ray diffraction. A homologous series of straight chain alkoxymethyl derivatives has exhibited both rectangular and hexagonal columnar mesophases, with the rectangular phases being favoured by the shorter chains. Two branched chain derivatives were found to give rectangular columnar phases at room temperature. A detailed analysis of the X-ray data has shown some differences from the analogous n-alkyl compounds. It has been deduced that the increased polarizability and flexibility of the ether linkage allows the disc-like molecules to approach more closely face to face with correspondingly thicker columns.