Various columnar phases formed by bent-core mesogens

Two new compounds containing bent-core mesogens are presented in which a flexible central methyleneoxycarbonyl unit acts as the bent fragment. The compounds form different two-dimensional columnar ('B 1 ') phases both with a rectangular lattice but different in the angle of the director of the molecules with respect to the a -axis of the lattice. Based on X-ray diffraction measurements structural models are proposed which cannot be described with the common model of p 2 mg symmetry for B 1 phases of bent-shaped molecules. The new models are of layer group symmetry ( p 11 n and pm 2 1 n ) and are discussed as a new common model for two-dimensional columnar phases of bent-shaped mesogens.     

Polycatenar liquid crystals based on bent-shaped chalcone and cyanopyridine molecules

In this study, the synthesis, structural characterisation and mesomorphic and optical properties of seven new bent-shaped and polycatenar bent-shaped compounds derived from chalcone and cyanopyridine are reported. The mesomorphic behaviour was investigated by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and X-ray diffraction (XRD) and correlated with the molecular structure. Two bent-core hexacatenars molecules (Ic and IIc) presented liquid crystalline properties, showing a hexagonal columnar (Col_h) phases at room temperature, being each disc constituted by two mesogens. Optical studies were also performed for the final molecules, being conducted by ultraviolet-visible and fluorescence spectrometry. The cyanopyridine derivatives show moderate luminescence quantum yields, ranging between 18% and 27%, with emission maxima around 371 nm. It is also shown that while the chalcone central unit favours a calamitic liquid crystalline behaviour in molecules with lower number of aliphatic chains, a polycatenar structure with cyanopyridine as the central unit favours a Col_h arrangement, also providing luminescence properties to the molecule.     

Pyrrolidinium Ionic Liquid Crystals

N‐Alkyl‐N‐methylpyrrolidinium cations have been used for the design of ionic liquid crystals, including a new type of uranium‐containing metallomesogen. Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate, thiocyanate, tetrakis(2‐ thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group C_nH_2n+1 was varied from eight to twenty carbon atoms (n=8, 10–20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncommon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly ordered phase. This uncommon tetragonal mesophase is thoroughly discussed on the basis of detailed powder X‐ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self‐assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds containing a metal complex anion were investigated. For the uranium‐containing mesogens, luminescence can be induced by dissolving them in an ionic liquid matrix. The europium‐containing compound shows intense red photoluminescence with high colour purity.     

High speed parallel synthesis of banana-shaped molecules and phase transition behaviour of 4-bromo-substituted derivatives

Novel derivative series of the well known bent-shaped P-n-O-PIMB liquid crystal mesogens, referred to as ‘4Br-P-n-O-PIMB’, ‘4Cl-P-n-O-PIMB’ and ‘5Cl-P-n-O-PIMB’, having halogen atoms substituted on the phenyl ring in the central core, were synthesized by solution phase parallel synthesis based on a combinatorial approach. The mesomorphic behaviour and physical properties of all the new compounds were studied by means of optical microscopy, differential scanning calorimetry, X-ray and circular dichroism spectroscopy. We found interesting transitional behaviour for the 4Br-P-n-O-PIMB homologous series. The homologues with alkyl tails having carbon numbers of n=3–10, 12, 14 exhibit rather complicated mesomorphic behaviour, which is strongly sensitive to n. The chiral fluid smectic B2 phase with SmC_AP_A structure and unidentified smectic Bx phase were observed in the homologues with n=9, 10, 12, 14 and n=3–5, respectively. Interestingly, every member exhibits frustrated and/or helical ordered phases at low temperatures, designated as X1, X2, and X3 phases, which result from a spontaneous escape from a macroscopic polarization. The mesomorphic behaviour and mesophase structures differ remarkably from those of the parent P-n-O-PIMB homologues, suggesting that substitution of the halogen atoms at the central core essentially creates a particular interaction between molecules.     

Influence of halogen substituent on the mesomorphic properties of five-ring banana-shaped molecules with azobenzene wings

Three new series of bent-shaped molecules with 4-chlororesorcinol, 4-bromoresorcinol or 4-fluororesorcinol as the central unit, and azobenzene with different alkoxy chain length as side arms were synthesised. The mesophase behaviour was investigated by polarising optical microscopy, and differential scanning calorimetry. A representative example has also been characterised by X-ray diffraction (XRD) studies. It is found that almost all of the materials prepared are monotropic liquid crystalline. Depending on the substituent at the central unit and on the chain length nematic phases, B₆ phases, a B₄-like dark conglomerate phase and a modulated/undulated anticlinic SmC phase were found. As a unique feature, upon reducing the chain length a transition from nematic to B₆-type smectic phases was observed, which is reverse to usually observed phase sequences. The UV–vis absorption spectroscopy was also performed to study the effect of light-induced trans–cis-isomerisation on the prepared compounds.     

The mesophases of octa-alkanoyloxy-9,10-anthraquinone

Abstract

Nine members of the octa-n-alkanoyloxy-9,10-anthraquinone series, ranging from octanoyloxy (n = 8) to hexadecanoyloxy (n = 16) (where n is the number of carbon atoms per chain), were prepared and their mesomorphic properties studied by differential scanning calorimetry and optical microscopy. All the compounds studied are mesomorphic with the higher homologues exhibiting two (for n ≧ 12) or even three (for n ≧ 14) columnar mesophases. For n = 8 to 11, the phase sequence is C-D_B-I, where D_B is an optically biaxial mesophase with a two-dimensional pgg symmetry. For n = 12, 13, the phase sequence is C-M₃-D_B-I, where M₃ is a highly ordered phase whose structure has not been identified. Finally for n = 14 to 16, the phase sequence is C-M₃-D_B-D_A-I, where D_A is an hexagonal phase. In all cases, the phases change from high to low order with increasing temperature and all transitions are first order. Compared with the corresponding hexa-substituted naphthoquinones the present series generates larger mesomorphic ranges and the higher homologues exhibit a uniaxial phase which is lacking in the hexa-substituted series. The increase in volume occupied by the aliphatic chains in the octa-substituted series is evidently sufficient to reduce the interaction between the aromatic cores of the different columns to permit the stability of a uniaxial phase.     

Electro-optical investigations and effect of asymmetry in bent-core liquid crystals

We report the synthesis and mesomorphic properties of a homologous series (10a–10g) of bent-core molecules constructed through covalent linkage of structurally non-symmetrical rod-like mesogens connected with a 1, 3-phenylene unit. The study of homologous series underlines the importance of length and nature of terminal chains. The homologues of shorter chains show a typical non-switchable rectangular columnar B₁ phase, while the switchable lamellar (B₂) phase is induced on moving to higher homologues. X-ray diffraction patterns indicate the presence of B₁ and B₂ mesophases. Polarised optical microscopy investigations under electric field in the B₂ phases revealed the existence of anticlinic antiferroelectric texture. The measured spontaneous polarisation value in one of the compounds is 936 nC cm^?2, a high polarisation value in bent-core liquid crystals.     

New Type of Columnar Liquid Crystal Superlattice in Double-Taper Ionic Minidendrons

Wedge-shaped molecules, such as dendrons, are among the most important building blocks for directed supramolecular self-assembly. Here we present a new approach aimed at widening the range and complexity of potential mesophases by introducing double-tapered mesogens. Two series of compounds are presented, both alkali metal salts (Li, Na, Cs) of 3,4,5-tris-alkoxybenzoic acid with a second tapered tris-alkoxyaryl group attached at the end of an alkoxy chain. The double-tapered compounds all display an unusual hexagonal columnar phase consisting of one ionic and three non-ionic columns per unit cell. The cation size has an unexpectedly drastic effect on unit cell size. Unlike most columnar phases, the current phases show unusually high dimensional stability on heating, and high stiffness in spite of being 80–85 % aliphatic, attributed to their molecular topology. The described approach may lead to co-assemblies of multifunctional materials, for example, parallel p- and n-semiconducting nanowires or parallel ionic and electronic conductors.     

Non-symmetrical bent-shaped compounds containing a chiral moiety

The synthesis and mesomorphic properties of new non-symmetrical bent-shaped compounds containing a chiral moiety are reported. These materials were built up from 3-hydroxybenzoic acid as a central unit with a chiral terminal group derived from lactic acid. A columnar phase of the B_1REV type with tilted molecules was observed in a wide temperature range down to room temperature. The ferroelectric-like character of switching with one peak in a half-period of the triangular electric field was observed and supported by texture observations and electro-optical changes. A longer terminal group ending with a double bond makes the compounds ready for polymerisation.     

Synthesis and characterization of achiral banana‐shaped liquid crystalline molecules containing bisnaphthyl moieties

Three analogous series of symmetric banana‐shaped liquid crystalline molecules containing bisnaphthyl units were synthesized and characterized. The effects of linking groups (on the side wings) and lateral meta‐fluoro substitutions (on the middle outer rings) on the mesogenic properties were examined. The type of mesophase depends on the lengths of terminal alkoxy chains. Thus, achiral molecules with shorter flexible chains (n = 8) exhibit a rectangular columnar (B₁) phase, while analogous derivatives with longer flexible chains (n = 12) display the B₂ phase. All lateral meta‐fluoro substituted analogues (series II) possess the lowest isotropization temperatures and the narrowest mesophasic ranges of the B₁ and B₂ phases. The B₁ and B₂ phases were confirmed by X‐ray diffraction, polarizing optical microscopy (POM) and electro‐optical (EO) switching experiments. An electric field‐induced transition from an antiferroelectric (tristable) state to a ferroelectric (bistable) state was observed in the EO measurements. Spontaneous polarization (by switching current response), tilt angle of chiral domains (by POM), and transmittance–voltage measurements of the B₂ phase in related compounds have been surveyed in this study.     

Syntheses and Properties of meso‐Substituted Porphyrin Mesogens with Triazole Linkages and Peripheral Alkyl Chains

Discotic mesogens P/n‐M (n=12, 16, 18, M=2 H, Zn and Cu) bearing a porphyrin core, triazole linkages and peripheral 3,4,5‐trialkoxybenzyl units have been synthesized by a click‐chemistry approach. The thermal behavior, photophysical properties and morphologies of these compounds were investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), XRD, UV and PL, SEM and TEM. These compounds can self‐assemble into hexagonal columnar phases in their pure states and form organogels in 1,4‐dioxane with unusually flower‐like sphere morphology. The supramolecular complexes of P/18‐Zn with C₇₀ or 4,7‐di‐4‐pydriyl‐2,1,3‐benzothadiazole can display hexagonal columnar phases too. Additionally, zinc porphyrin compounds P/n‐Zn show binding selectivity to Cu²⁺ among a series of cations in THF/H₂O.     

A new structural type of liquid crystal Crystal structures and thermotropic mesophases of dihydroxytetraalkyldisiloxanes: Columnar phases with hydrogen bonded assemblies

Abstract

Previous structural investigations of mesogenic organosilicon compounds (1, 3-dihydroxytetraalkyldisiloxanes, [R₂(OH)Si]₂O, R = C _n H_2n+1) which form thermotropic phases are outlined. The crystal and molecular structure determination of a non-mesogenic member of this series [Me₂(OH)Si]₂O is described. It is inferred that the mesophases formed by this family of compounds belong to a new structural type with columnar stacks of the molecules laced together with hydrogen bonds.     

Formation of a homochiral antiferroelectric ground state in asymmetric bent-shaped molecules

New asymmetric bent-shaped molecules were synthesised and their mesomorphic properties and phase structures investigated by means of electro-optical measurements. All of the compounds formed fluid smectic phases and showed the homochiral antiferroelectric (SmC_AP_A) ground structure of the B2 phase. Comparing the new molecules with the previously reported classical symmetric bent-shaped molecules, we concluded that the modification of the chemical structure, especially the asymmetrical design of the bent-shaped molecules, is very significant for the interlayer interaction, influencing the formation of a specific polar order in banana phases.     

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.     

Synthesis and structure of lateral halogenated V-shaped liquid crystal molecules based on a 1,7-naphthalene central core and alkylthio tails

A series of small angle bent-core (V-shaped) mesogens carrying 1,7-naphthalene as a central core linked with lateral halogenated (chlorinated or fluorinated) Schiff-base side wings and alkylthio terminal tails of variable carbon number (n = 12, 16) was synthesised in order to reduce the transition temperature and improve the phase stability of bent-core liquid crystal molecules. Differential scanning calorimetry (DSC), polarising optical microscopy (POM), small-angle X-ray scattering system (SAXS) and two-dimensional X-ray diffractometer were applied to ascertain the mesomorphic structure and phase transition temperatures of the compounds. The results confirm that all the molecules show thermotropic liquid crystalline behaviour and exhibit hexagonal columnar phase (Col_h) in a certain temperature range. Compared with the homologous compounds without lateral halogen, the cleaning point temperature of lateral halogenated V-shaped compounds generally decrease and the Col_h phase ranges are more extensive. The influence of lateral chlorine on the cleaning point temperature is more obvious, as well as the effect of lateral fluorine on the range of Col_h phase.     

Novel mesophases in fluorine substituted banana-shaped mesogens

A new homologous series of achiral banana-shaped mesogens ('Dn') has been synthesized and studied by the classical techniques (optical microscopy, differential scanning calorimetry, X-ray diffraction, miscibility studies and electro-optic investigations). The short homologues (D6-D8) exhibit a two-dimensional phase 'B1_x' different from a B1 phase with a rectangular lattice. The longer homologues (D9-D14) present a mesophase which displays the defects of the B7 phase of the PIMB-NO₂ compounds. Nevertheless the D9-D14 mesophase is not miscible with the B7 phase, and contrary to B7, exhibits a bistable behaviour ('ferroelectric' type) suggesting at least a B7 variant.     

Creation of nematic and reentrant nematic phases in bi- and multi-component smectic mixtures

Abstract

Experimental results referring to the transformation of smectic phases, mainly smectic A, into nematic and reentrant nematic phases are reviewed. A new explanation of some experimental results is proposed. Factors which are responsible for the depression of smectic phases in mixtures of polar mesogens are discussed and the possibility of forming mixtures with a broad temperature range of nematic phase from smectic compounds, which can be useful for liquid crystal displays (LCDs), is shown. A nematic gap observed in some cases between monolayer (S_A1) or monolayer and partially bilayer (S_Ad) smectics results from the differences in the organization of the molecules in the smectic layers. It is concluded that polar phase from smectic A₁ phases can be divided into two groups: (a) the first one is characteristic for compounds with the -NCS, -F, -CI, -I or ?COC _m H_{2m + 1} terminal group. The spacing of the smectic layer slowly expands with the increase in alkyl chain length and the structure of the smectic A₁ phase slowly changes to be more like the smectic A_d phase (d/1 > 1). It is proposed that such a smectic is called an enhanced monolayer smectic (S_A1e (b) the second one is typical for compounds with the -CN terminal group. This kind of smectic A₁ phase is rapidly transformed into the smectic A_d phase with increasing alkyl chain length. These latter monolayer mesogens easily form the reentrant nematic phase when they are mixed with other polar smectic mesogens.     

Bent-core mesogens with biphenyl moieties: observation of a B7 to B4 phase transition

Four new bent-core mesogens containing biphenyl moieties are reported. All these, except the first homologue, exhibit B₇ and B₄ banana phases, both of which are known to have a helical structure. The B₇ to B₄ phase transition is very rare, this being perhaps only the second report of the observation of such a phase sequence. The first homologue exhibits only one mesophase X₁ with textural features reminiscent of the B₁ phase. All the compounds synthesized are characterized by spectral data. The mesophases exhibited by these compounds are characterized by polarizing optical microscopy, differential scanning calorimetry and X-ray studies.     

Banana-shaped mesogens: a new homologous series of compounds exhibiting the B7 mesophase

The synthesis and characterization of a new homologous series of compounds, the 2-cyano-1,3-phenylene bis[4-(4-n-alkoxyphenyliminomethyl)benzoates] derived from 2-cyanoresorcinol is reported. All the compounds are enantiotropic mesogens and exhibit the fascinating B₇ mesophase. The characterization of the mesophase was performed using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical studies.     

Columnar liquid crystals in oligosaccharide derivatives. II. Two types of discotic columnar liquid-crystalline phase of cellobiose alkanoates

Abstract

A series of cellobiose octaalkanoates, Cel-II-n (n is the carbon number of the alkyl chain), with n = 7–14 were prepared and their mesogenic properties examined by differential scanning calorimetry, optical polarizing microscopy and X-ray diffraction. All of these compounds form enantiotropic discotic columnar phases, in which the columns are built up by a regular stacking of the cellobiose moieties and are packed in a two dimensional lattice. Homologues with n = 9–14 form the D_ho phase only whilst the compound with n = 7 forms the D_ro phase. The n = 8 compound forms the D_ho phase at higher temperatures and the D_ro phase at lower temperatures. Structural parameters obtained from X-ray diffraction studies are presented for both phases.