A novel series of anthraquinone-based discotic liquid crystals with bulky substituents: synthesis and characterization

A novel series of anthraquinone-based discotic liquid crystals with bulky substituents, namely 1,5-dibenzyloxy-2,3,6,7-tetraalkyloxy-9,10-anthraquinones, has been synthesized starting from gallic acid. This is, to our knowledge, the first example of bulky substitution in a discotic C₂-symmetric molecule forming columnar phases. Except for the lowest homologue, all members of this series are found to exhibit columnar mesophases; the low temperature mesophase appears to be three-dimensionally ordered whereas the high temperature mesophase is hexagonal columnar (Col_h). We find that the introduction of benzyl substituents for alkyl chains (in the 1,5-positions) on the anthraquinone hexaalkoxylates stabilizes the three-dimensionally ordered phase, whereas it destabilizes the Col_h phase, compared with the anthraquinone hexaalkoxylates. Interestingly, the three-dimensionally ordered phase extends down to -50 °C, making these new derivatives suitable for device applications.     

Prediction, synthesis and investigation of mesomorphism of new octa(benzo-15-crown-5)-substituted derivatives of phthalocyanine and its metallo complexes [Zn(II), Ni(II)]

First calculation of molecular parameters (MP) of new octa-(benzo-15-crown-5)-substituted phthalocyanine derivatives and of its metallo complexes Zn(II), Ni(II) (Ib, Ic) is carried out. A prediction was made of the possible formation therefrom of metaphases, and for that purpose the synthesis of these compounds and investigation of their mesomorphic properties was performed. For compounds Ia-Ic the formation of high-temperature enantiotropic mesophases was established typical for discotic mesogenes, in good agreement with the prediction. Proceeding from the data on miscibility of 2,3,9,10,16,17,23,24-octa-[(4′-benzo-15-crown-5)-oxy]-phthalocyanine Ia with discotic mesogene 2,3,6,7,10,11-hexa(cyclohexanebenzoyloxy)triphenylene (II) displaying nematic mesomorphism the compound Ia was assigned to the group of nematic mesogenes and by the texture of the mixture to discotic mesogenes forming columnar type nematic phase (N _Col). Convergence of the results of prediction and experimental data is about 83%.     

Columnar mesophases and phase behaviors of novel polycatenar mesogens containing bi-1,3,4-oxadiazole

A new series of liquid-crystalline bi-1,3,4-oxadiazole derivatives (2,2′-bis(3,4,5-trialkoxyphenyl)-bi-1,3,4-oxadiazole, BOXD-Tn, n=3, 4, 5, 6, 7, 8, 10, 14) were designed and synthesized. They have been confirmed to give rise to columnar mesophases. The columnar mesophases for BOXD-Tn (n=5, 6, 7, 8, 10) could be supercooled to −20 °C on the cooling runs. A room temperature Col_ho phase was obtained for BOXD-T14. All BOXD-Tn exhibit good fluorescence properties either in cyclohexane or in solid state.     

Mesomorphic phase transition behaviour of photopolymerisable liquid crystalline triphenylene ether compounds

This report discusses the preparation and the unusual mesomorphism of three homologues of photopolymerisable triphenylene ether compounds. These homologues showed ‘a cold crystallisation’ on heating differential scanning calorimetry curves, and this phenomenon is attenuated with increase methylene units, spacers between the triphenylene core and the terminal photopolymerisable acrylate group. Classical textures of hexagonal columnar (Col_h) mesophase were observed by polarising optical microscopy for all three photopolymerisable mesophase compounds described here. Also described is the mesomorphism of their intermediates, hydroxylalkoxytriphenylenes. In some cases, the discotic columnar hexagonal mesophases were confirmed by wide angle X-ray scattering techniques.     

The stability of columns comprising alternating triphenylene and hexaphenyltriphenylene molecules: variations in the structure of the hexaphenyltriphenylene component

Previous investigations of complementary polytopic interaction (CPI) columnar mesophases, in which the columns are built up of alternating hexaalkoxytriphenylene (HAT) and hexaphenyltriphenylene (PTP) molecules, concentrated mainly on the effect of variations in the structure of the HAT component. This investigation is concerned with the effect of variations in the structure of the PTP component and, in particular, variations in the position of an alkoxy side chain in the phenyl ring. Stable columnar mesophases are obtained when a hexyloxy substituent is placed in the meta‐ or para‐position but not in the ortho‐position. In the case of the meta‐ and para‐substituted systems, the two‐component CPI columnar phases are stable over a considerably larger temperature range than the one‐component HAT systems. The evidence suggests that unfavourable PTP/PTP stacking is as much a driving force for the formation of these mixed stacks as is favourable PTP/HAT stacking, but both need to be explained in terms of the sum of atomically dispersed van der Waals and coulombic interactions. On cooling from the isotropic into the Col_h phase, the columnar phase based on a 1:1 mixture of hexakis(hexyloxy)tripenylene and the meta‐hexyloxy‐substituted PTP gives an unusual texture consisting of ‘viking‐axe’‐shaped structures.     

Self-assembly of a coil-rod-coil block molecule complexed with LiCF3SO3

Enantiotropic liquid crystalline phases were induced by the complexation of docosyl 4-(4-oxy-4'-biphenylcarbonyloxy)-4'-biphenyl carboxylate, containing poly(ethylene oxide) of twelve ethylene oxide subunits, with 0.25 mol of LiCF₃SO₃ per ethylene oxide unit. The existence of oblique columnar, rectangular columnar (Col_rec), and smectic A (SmA) mesophases of this complex was demonstrated by a combination of polarized optical microscopy and X-ray diffraction. This complex was observed to transform from the rectangular columnar phase to a lamellar phase on heating. The unusual phase transition behaviour in this complex is believed to originate from the mixing entropy of the added LiCF₃SO₃. Also, the Col_rec-SmA phase transition was accompanied by a change of the packing structure from bilayer to monolayer.     

Flavylium Salts: A Blooming Core for Bioinspired Ionic Liquid Crystals

Thermotropic ionic liquid crystals based on the flavylium scaffold have been synthesized and studied for their structure-properties relationship for the first time. The mesogens were probed by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). Low numbers of alkoxy side chains resulted in smectic (SmA) and lamello-columnar (Lam_Col) phases, whereas higher substituted flavylium salts showed Col_ro as well as ordered and disordered columnar (Col_ho, Col_hd) mesophases. Mesophase width ranged from 13 K to 220 K, giving access to room temperature liquid crystals. The optical properties of the synthesized compounds were probed towards absorption and emission properties. Strong absorption with maxima between 444 and 507 nm was observed, and some chromophores were highly emissive with quantum yields up to 99 %. Ultimately, mesogenic and dye properties were examined by temperature-dependent emissive experiments in the solid state.     

Molecular structure and mesomorphism in the series of discotic esters derived from benzene,triazine, biphenyl,and triphenylene

New mesogenic discotic triazine-based esters were synthesized and shown to form a low-temperature monotropic columnar mesophase. Using as examples the newly synthesized compounds and previously prepared discotic esters on the basis of benzene, biphenyl, and triphenylene, the effects of the nature and size of the central fragment and the number, position, length, and rigidity of peripheral substituents therein on the mesomorphic properties (in particular, the ability to form columnar and nematic mesophases) were examined. The main structural factors stabilizing columnar and nematic mesophases were revealed. Nematic mesomorphism was shown to be favored by increased size of the central fragment of the discogen molecule and by the presence of cyclohexane fragments in peripheral substituents.     

Ferroelectrically Switching Helical Columnar Assembly Comprising Cisoid Conformers of a 1,2,3‐Triazole‐based Liquid Crystal

The 1,2,3‐triazole molecule, which is a product of click chemistry, possesses a high dipole moment and can be a useful polar motif for ferroelectric columnar liquid crystal (LC) materials—though it has not been used to date. Herein, we report the helical assembly and ferroelectric switching properties of a columnar liquid crystal comprising a naphthalene core and 1,2,3‐triazolyl linkages. The molecule assembles into a double‐stranded helical columnar LC structure (Col_hel). The X‐ray simulations of cisoid and transoid columnar models suggest that the helical assembly comprises cisoid conformers with a non‐zero dipole moment. The helical columns in the Col_hel phase are aligned homeotropically under an electric field. The ferroelectric switching of the axial polarization can be observed in the temperature range of 105–115 °C in the Col_hel phase, wherein the triazolyl hydrogen bonding along the column axis is weakened. The ferroelectric switching event is attributed to the rotation of the polar triazolyl units in response to the electric field.     

Room‐Temperature Columnar Mesophases in Triazine–Gold Thiolate Metal–Organic Supramolecular Aggregates

Supramolecular mono‐ and dinuclear liquid‐crystalline gold(I) aggregates have been synthesized by means of hydrogen bond interactions of 2,4,6‐triarylamino‐1,3,5‐triazine with thiolate moities of gold metalloacids [Au(PR₃)(SC₆H₄COOH)] or [μ‐(binap){Au(SC₆H₄COOH)}₂], in 1:1 and 2:1 molar ratio, respectively. All of the supramolecular aggregates display a stable columnar hexagonal mesophase (Col_h) at room‐temperature. The supramolecular arrangement within the columns consists of the one‐dimensional stacking of triazine units, with the core of the attached metallic thioacid fragments acting as the fourth branch. The phosphine‐containing moieties of the metallic thioacid protrude out in the aliphatic continuum. These complexes do not show metallophilic interactions, but this strategy appears very promising for the future design of room‐temperature LC mesophases containing interacting metallic fragments.     

Enhanced conductivity and dielectric absorption in discotic liquid crystalline columnar phases of a vanadyl complex

A liquid crystalline vanadyl complex has been studied by DSC, polarizing optical microscopy, the reversal current technique, X-ray diffraction and frequency domain dielectric spectroscopy. The compound exhibits three columnar phases: rectangular ordered (Col_ro), rectangular disordered (Col_rd), and hexagonal disordered (Col_hd), all of which show a dielectric relaxation process at low frequencies. In the Col_ro low temperature phase this process seems to be connected with a slow relaxation of polarized polymeric chains inside the columns (mHz frequency range). However, in the Col_hd high temperature disordered phase this relaxation is faster (Hz range). It is interesting that the liquid crystalline phases studied show enhanced conductivity which changes by four orders of magnitude from 10^?9 S m^?1 in the orientationally disordered crystal (an ODIC phase) to 10^?5 S m^?1 in the Col_hd high temperature phase. Such a value of the conductivity is typical for semiconducting materials.     

Synthesis and characterization of liquid crystalline tetra- and octa-substituted novel phthalocyanines

Methylene-bridged tetra- and octa-(13,17-dioxa nonacosane-15 sulfanyl)-substituted metal free- and Ni(II) phthalocyanines were synthesized from the corresponding phthalonitrile (3, 4) derivatives in the presence of the anhydrous metal salt (NiCl₂) or a strong organic base. The new compounds were characterized by elemental analyses, UV/Vis, IR, NMR and mass spectra. The mesogenic properties of these materials were studied by differential scanning calorimetry (DSC), optical microscopy and X-ray diffraction investigations. X-ray diffraction patterns of the mesophase confirm that tetra- and octa-substituted compounds (3a–b, 4a–b) form hexagonal columnar mesophases (Col_h). We indicated that addition of the methylene-bridged phthalocyanine (Pc) core can either decrease the liquid crystal phase transition temperatures or extend the liquid crystal temperature range to include room temperature. Also, the Pc compounds (3a, 3b, 4a and 4b) are liquid crystals at room temperature. These properties of the Pc complexes provide some advantages such as easily obtaining an ordered film for sensor applications. Computational modelling work was combined with X-ray diffraction investigation to validate the diameter of the phthalocyanine molecule (3b).     

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.     

Ester-functionalized phthalonitriles and zinc phthalocyanines via palladium-catalyzed cyanation of 4,5-dichlorophthalates

Dicyanophthalates 3 were synthesized via Pd-catalyzed two-fold cyanation of the corresponding 4,5-dichlorophthalates with Zn(CN)₂. Appropriate modification of reaction conditions allowed one-pot synthesis of the corresponding zinc phthalocyanines 7 bearing eight peripheral alkyl ester groups. Powder X-ray diffraction study of a mesogenic zinc phthalocyanine bearing branched alkyl substituents revealed a rare case of a transition between two columnar rectangular liquid crystalline mesophases with different symmetry.     

A novel dimeric discotic liquid crystal based on anthraquinone

A novel dimer based on the disc-like anthraquinone core-bis[1-(4-nitrobenzyloxy)-2,3,6,7-tetrapentyloxy-9,10-anthraquinon-5-oxy]do and its corresponding monomer 1-hexyloxy5-(4-nitrobenzyloxy)-2,3,6,7-tetrapentyloxy-9,10-anthraquinone, have been synthesized and characterized. X-ray diffraction studies show that this novel dimer exhibits a hexagonal columnar phase (Col_h) with correlation among the molecular cores along the column. The Col_h phase of the dimer exists over a very wide temperature range, extending from 176°C down to at least 60°C (the lowest temperature reached in DSC), whereas the monomer exhibits a Col_h phase at high temperature and a three-dimensionally ordered columnar phase (Col_x) at low temperature.     

Long- and short-range order in the mesophases of laterally substituted calamitic mesogens and their radial octapodes

The mesomorphic behavior of a calamitic mesogen (4'-undecyloxybiphenyl-4-yl-4-octyloxy-2-(pent-4-en-1-yloxy)benzoate) and of a supermesogenic octapode formed by the side-on attachment of the mesogen to a octasilsesquioxane central core is studied by X-ray diffraction and polarizing optical microscopy. The calamitic compound is found to have a nematic phase that has biaxial domains (cybotactic clusters) of tilted layers throughout its entire temperature range. Domains of analogous structure are also found in both the nematic and the hexagonal columnar mesophases exhibited by the obctapode compound. The spacing of the layers forming the domains is found to have the same, essentially temperature independent value for the calamitic monomer and for the octapode, in both its mesophases. Comparison with compounds of analogous structure shows that this value is determined by the length of the rigid part of the mesogenic unit. Variation of the latter length is shown to have no effect on the size of the hexagonal lattice of the octapode columnar phase or on the stacking distance within the columns. The presence of the biaxial domains in the nematic phase is discussed in connection with the phase biaxiality that has been observed in structurally related tetrapode compounds and the possibility of field induced macroscopic biaxial nematic order.     

Thermally stable azo-substituted bent-core nematogens: observation of chiral domains in the nematic mesophases composed of smectic nano clusters

In an effort to synthesise thermally stable bent-core nematogens with fairly low transition temperatures and wide nematic mesophase range, we have synthesised five new series of azo-substituted bent-core compounds without Schiff’s base unit. Here, we studied the effect of different lateral substituents (–CH₃ and –Cl), at two different positions in one of the arms, on the mesogenic properties. We found that such variation in the molecular architecture has a clear-cut impact on the transition temperatures as well as the liquid crystalline properties. The mesogenic properties of these compounds were studied using polarising optical microscopy and differential scanning calorimetry. The exact nature of different mesophases is investigated using X-ray diffraction studies. The results obtained indicated the presence of smectic nano clusters in the nematic mesophases of these compounds. We observed domains of opposite handedness in these nematic phases. The unusual properties of these mesophases were investigated by electro-optical studies. The electro-convection pattern study showed that these nematic mesophases are of negative dielectric anisotropic in nature. The compounds synthesised here exhibit photo-switching both in solution and in their nematic mesophases, a property that can be exploited for practical applications.     

Columnar metallomesogens based on vanadyl(VI) and cobalt(II) complexes of N,N'-bis[3-(3,4-dialkoxyphenyl)3-oxopropenyl]ethylenediamines

The preparation, characterization and mesomorphic properties of vanadyl(VI) and cobalt(II) complexes derived from N, N -bis[3-(3,4-dialkoxyphenyl)-3-oxopropenyl]ethylenediamines are described. These half-disk-shaped molecules exhibited columnar mesophases over a wide range of temperature, as characterized by DSC analysis and polarizing optical microscopy. The structure of the mesophases was also confirmed as columnar hexagonal (Col_ho) by powder XRD. The vanadyl complexes were found to have substantially higher clearing temperatures and wider mesophase temperatures than the analogous cobalt complexes. The influence of the metal centres on the mesomorphic temperatures may be attributed to better intermolecular dative association in the vanadyl complexes than in the cobalt complexes.     

An Unusual Stacking Transformation in Liquid‐Crystalline Columnar Assemblies of Clicked Molecular Propellers with Tunable Light Emissions

The columnar liquid‐crystalline (LC) and fluorescence properties of three‐dimensional molecular propellers based on tetraphenylethylene is reported. X‐ray scattering studies reveal an unusual transition from a rectangular (Col_rec) to a hexagonal columnar (Col_hex) phase. In contrast to second‐order intercolumnar transitions based on a common tilt mechanism, the transition is first order and involves an unprecedented zigzag stacking of aromatic propellers in the Col_rec phase. A sudden change in emission color from sky blue to green occurs rapidly and reversibly at this transition, which is due to the planarization of the propeller mesogen.     

The influence of sodium chloride and urea on chromonic liquid crystals formed by CI Acid Red 266

Chromonic liquid crystals are currently receiving renewed interest with particular attention on the Edicol Sunset Yellow (ESY)/water system, which forms columnar nematic and hexagonal phases. CI Acid red 266 is structurally fairly similar to ESY and also forms columnar nematic and hexagonal phases but at much lower concentrations (>1%). In this study, we have examined the influence of sodium chloride and urea on chromonic liquid crystals formed by CI acid red 266. The techniques employed were polarising microscopy, X-ray diffraction and ²H NMR. Sodium chloride moves the concentration at which mesophases form to higher values. Once formed, the mesophases are stable to slightly higher temperatures. Screening of the interstack electrostatic repulsions by added electrolyte appears to be responsible for the changes. Urea can be added in fairly large concentrations (up to 25 wt%) without significant changes in mesophase stability. X-ray diffraction measurements show that there is little change in the aggregate structure with added urea. NMR measurements on urea and water ordering show that urea has much larger order parameters than water. Both order parameters are much smaller than values reported for ESY, but this is simply because of the lower dye concentrations. The larger order parameters for urea appear to arise from some intercalation of urea into the acid red 266 stacks. There is no evidence for changes in ‘water structure’ by the addition of urea.