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

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.     

Synthesis and X-ray study of new columnar heteroaromatic salts

The synthesis of 2,4,6-triarylpyrylium tetrafluoroborates substituted by six long alkyloxy chains (n = 8 and 12) is described. These salts exhibit a discotic mesophase from room temperature to 200°C. X-ray diffraction experiments on powder and oriented samples show that this phase is an ordered hexagonal columnar phase, D_ho.     

Structural study of crystalline and columnar copper (II) soaps

A homologous series of binuclear copper (II) linear chain alkanoates together with two branched chain and one aromatic substituted copper (II) alkanoates have been synthesized and studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. All of these are crystalline at room temperature, they are mesomorphic in nature above c. 100°C, with the exception of copper propionate which remains crystalline up to its thermal decomposition above 200°C. A systematic study has shown that the linear chain alkanoates, starting from the pentanoic derivative, produce columnar mesophases with hexagonal symmetry. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two dimensional hexagonal lattice. The repeat unit in a column is a binuclear dicopper tetracarboxylate complex. Two transition regimes have been detected leading from the crystal to the columnar mesophase: one dominated by the interactions between the polar heads, the other by the interactions between aliphatic chains. In the special case of the butyric derivative, the columnar mesophase obtained is rectangular in symmetry. Instead of being oriented perpendicular to the columnar axis and superposed in a four fold helicoidal fashion, the repeat units in the columns are tilted and all shifted in the same direction with respect to one another.     

Discotic Liquid Crystals of Transition Metal Complexes VI. Monotropic Lamella Mesomorphism of Tetrakis(N-Alkyldithiolato)Dinickel(II) and Bis(N-Alkylxanthato)Nickel(II) Complexes

It was found that each of the tetrakis(n-alkyldithiolato)dinickel(II), (C_n-DTA)₄-Ni₂, complexes where n-alkyl is n-pentyl through n-dodecyl, exhibits a broken-fan texture on cooling from an isotropic liquid, and that the phase gave a characteristic lamella structure X-ray diffraction powder pattern. Furthermore, the infrared spectrum of this phase is more similar to that of the isotropic liquid than that of the crystal. Therefore, the phase can be described as a monotropic lamella mesophase. Interestingly, each of the complexes of bis(n-alkylxanthato)nickel(II), (C_n-Xan)₂Ni, (n = 5, 7, 9, 11) exhibits double (triple) melting behavior via the isotropic liquid, whereas each of the complexes of (C_n-Xan)₂Ni (n = 4, 6, 8, 10, 12) shows ordinary single melting behavior. Such unique double melting accompanied by an even-odd effect appears to be the first example of this in the long chain substituted compounds. Each of the complexes of (C_n-Xan)₂ Ni (n = 9, 11, 12) has a monotropic lamella mesophase exhibiting a large broken fan texture.     

Structure of the liquid crystalline state in hexakis (4-(4'-alkyloxy)biphenoxy)cyclotriphosphazenes

X-ray measurements on hexakis(4-(4'-alkyloxy)biphenoxy)cyclotriphosphazenes [PN-(OC₆H₄C₆H₄OC_nH_{2n + 1})₂]₃ (HACP, n = 7-9), confirm the previous mesophase identification. The apparent molecular length measured in the mesophase compares to twice the length of an alkyloxybiphenyl side group. Specific features are added to the usual features of the nematic and smectic diffraction patterns which show that the molecular arrays in directions parallel and perpendicular to the director both reflect the peculiar shape of the cyclotriphosphazenes.     

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

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.     

Thermotropic mesomorphism in a branched chain copper(II) carboxylate and its pyrazine complex

The thermotropic mesomorphism of tetrakis(μ-(2,2-(dioctyl(acetato))-O,O')bis-[copper(II)] has been investigated by optical microscopy and X-ray diffraction. This compound was found to form a hexagonal discotic phase analogous to those reported for the copper(II) salts of n-alkanoic acids, but in contrast to these materials the mesophase of the branched chain compound was found to extend to below room temperature. Complexation of the copper(II) branched chain carboxylate with the difunctional ligands pyrazine and 4,4'-dipyridyl resulted in liquid-crystalline materials whose mesophase structure could not be established. The possibility of these complexes existing as polymeric/oligomeric entities is discussed.     

Structure of gyroid mesophase formed by monodendrons with fluorinated alkyl tails

The structure of a three-dimensional mesophase has been studied by the methods of small-angle X-ray diffraction and reconstruction of electron-density maps of the cubic lattice. In the oriented samples based on monodendrons with partially fluorinated alkyl tails, this mesophase has been shown to coexist with a two-dimensional columnar phase through a wide temperature interval. Epitaxial relationships between (10) planes of the hexagonal lattice and (211) planes of the cubic lattices lead to the ten-point pattern of azimuthal intensity distribution for the first X-ray 211 reflection and to the six-point intensity distribution for the second 220 reflection. The observed 12-point pattern of the 220 reflection is due to the presence of twin “crystallites” of the three-dimensional phase, and their [110] axis is parallel to the axis of cylinders in the columnar phase. The reconstructed electron-density maps show that the regions with increased electron density, which are composed of fluorinated aliphatic tails, form a bicontinuous gyroid structure.     

Liquid crystalline semifluorinated ionic dendrimers

Poly(propyleneimine) [PPI-(NH₂) _n ] and poly(amidoamine) [PAMAM-(NH₂) _n ] dendrimers with n = 4, 8, 16, 32 and 64, have been functionalized with a semifluorinated carboxylic acid [CF₃(CF₂)₇CH₂CH₂COOH] on the surface. The thermal and liquid crystal properties of the resulting ionic dendrimers have been studied by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dendrimers show a smectic A (SmA) mesophase, except that derived from the fifth generation of PPI, which displays a columnar mesophase. The mesomorphic behaviour of the dendrimers is attributed to the formation of ionic bonds and to the microsegregation of the dendritic branches and the fluorinated peripheral chains.     

Discotic liquid crystals of transition metal complexes X. Phthalocyanine derivatives substituted with n-alkyloxyphenyl side chains

Octakis(alkyloxyphenyl)-phthalocyanine derivatives ((C_nOph)₈PcH₂ n = 8, 10 12 and 18), their corresponding copper (II) complexes ((C_nOph)₈PcCu, n = 10, 12 and 18) and octakis(2-ethylhexyl)-PcH₂ have been synthesized and their mesomorphic properties characterized. (C_nOph)₈PcH₂, (n = 12 and 18) exhibit hexagona disordered columnar mesophases whereas (C_nOph)₈PcCu (n = 12 and 18) yields a rectangular disordered columnar liquid crystal. An alkylphenyl-oxymethyl derivative, (C₁₂phOCH₂)₈PcH₂, was synthesized to determine the influence of the connecting link between the side chains and the phthalocyanine macrocycle on the mesomorphic properties.     

Coexistence of two different side chain packing structures in a smectic phase of liquid-crystalline side chain polymers

Ten varieties of liquid-crystalline side chain polymers, poly(cholesteryl-ω-(methacryloyloxy)alkanoates) (pChMO-n, n = 1, 2, 3, 4, 5, 7, 9, 10, 11 and 15; the carbon number of the alkyl chain), were studied by differential scanning calorimetry and small angle X-ray scattering. On and after the first cooling run from the isotropic state, these polymethacrylates gave the same smectic phase. X-ray investigations showed that pChMO-n with short spacers (n = 1-7) has a two layer (bilayer) S_A packing structure, and pChMO-n with a longer spacer (n= 15) has a single layer (monolayer) S_A packing structure. However, these two types of packing structure appear simultaneously in pChMO-n (n = 9s-11) below their phase transition temperature. To clarify the manner of the coexistence of the two different structures the smectic layer spacing and X-ray diffraction patterns were examined by small angle X-ray scattering at various temperatures.     

Thermotropic liquid crystals based on chito-oligosaccharides. II. Discotic columnar liquid crystals in chitobiose octaalkanoates and chitotriose hendecaalkanoates

Chitobiose octaalkanoates and chitotriose hendecaalkanoates with varying acyl pendant lengths were synthesized and their mesophase properties studied. Both series of derivatives showed an enantiotropic mesophase in a wide temperature region below 200°C. An X-ray diffraction analysis revealed the mesophase to be of a hexagonal columnar type, in which the columns built up by a periodic stacking of chitobiose or chitotriose cores are packed into a two dimensional hexagonal lattice. The mesophase is thus similar to the hexagonal ordered columnar (D_ho) phase in discotics. Compared with cello-oligosaccharide counterparts, the diameter of the column is fairly large and the stacking period somewhat short; these can be interpreted as resulting from the intermolecular hydrogen bonding which is formed between the secondary amide group in the C2 position and the ester group.     

Mesophase behaviour and thermal stability of octa-alkoxy substituted phthalocyaninatocobalt (II) complexes

Cobalt (II) phthalocyanines substituted with eight alkoxy chains in the peripheral (2, 3, 9, 10, 16, 17, 23, 24) positions were prepared. The alkoxy chain length was varied between n-butyloxy (C₄H₉O) and n-octadecyloxy (C₁₈H₃₇O). Studies by polarizing optical microscopy and high temperature X-ray diffraction revealed that all the complexes are liquid crystalline and that they exhibit a hexagonal columnar mesophase (Col_h). Transition enthalpies were determined by differential scanning calorimetry. The clearing point could only be observed for compounds with a chain length longer than C₁₃H₂₇O. Both the melting and clearing points decrease with increasing chain length. The transition temperatures of these discotic metallomesogens are higher than those of the corresponding metal-free phthalocyanines, but are comparable with those of the corresponding copper (II) compounds. The thermal decomposition of the compounds was studied by thermogravimetry.     

Mixed-valent diruthenium (II,III) aliphatic carboxylates: columnar mesophases in dodecylsulfate and octylsulfonate derivatives

Two series of polymeric mixed-valent diruthenium (II,III) aliphatic carboxylates of formula Ru₂[O₂C(CH₂)_n-2CH₃]₄X (where X=dodecylsulfate (DOS) anion and n=8, 9, 16 and 18, or X=octylsulfonate (OS) anion and n=8, 10, 12, 14 and 18) were synthesized and characterized. Their liquid crystalline properties and crystalline (Cr) structures were analysed; the Cr phases are lamellar in all cases. For long chain DOS derivatives (n=16 and 18) hexagonal and rectangular columnar mesophases (Col_h and Col_r) with melting temperatures close to 140°C were observed. For long chain OS derivatives (n≥10) a Col_h mesophase was observed, with melting temperatures between 140 and 190°C.     

Photoluminescent nickel(II)-metallomesogens derived from salphen ligands: influence of halogens at the spacer on mesomorphism and emission properties

Three new series of photoluminescent nickel(II) metallomesogens, [NiL]; H₂L = N,N′-Bis(4-n-alkoxysalicylidene)-4-fluoro/bromo/chloro-1,2-diaminobenzene (n = 12, 14, 16) based on ‘salphen’ ligands have been synthesised and their mesomorphic and photophysical properties explored. The complexes, isolated as orange microcrystalline solids were characterised by elemental analyses, FT-IR, ¹H NMR and UV-visible spectroscopy. Thermal studies show all the compounds to be enantiotropic liquid crystals displaying columnar mesophase over a wide temperature range. Electronegativity and steric requirement of the halogen substituent at the ligand’s spacer remarkably influence the 2-D packing of the columns in the lattice in these complexes controlling the supramolecular mesomorphic order and photoluminescence. The mesophase behaviour of the fluoro-substituted complex is characterised by a transition from a columnar oblique (p1) to columnar rectangular (p2mm) phase, former stable till ambient temperature. The chloro and bromo analogues, on the other hand, displayed exclusively columnar rectangular (p2mm) mesophase with the former transforming into a glassy state and latter into a crystalline phase during cooling to ambient temperature. Molecular model based on interdigitated anti-parallel and back to back arrangements in the different columnar mesophase are proposed on the basis of X-ray diffraction (XRD) studies. The complexes emit in the blue region when excited with near UV wavelength.     

Mesomorphic phase transitions of a series of D-phase compounds

Mesomorphic phase transitions of 4'-n-alkoxy-3'-nitrobiphenyl-4-carboxylic acids (ANBC) with numbers of carbons (n) in the alkoxy group ranging from 11 to 22 have been studied by differential scanning calorimetry (DSC) and polarizing optical microscopy. The D phase, a mesophase of particular interest through its being optically isotropic, was observed for the n = 17, 19, 20, 21, and 22 members of the ANBCs, as well as for the n = 16 and 18 members, as reported previously. The S_c-D phase transition temperature decreased with increasing n, so that the temperature range of the D phase extended over 64° at n = 22. In the n = 15 member, the D phase was certainly observed on first heating, but was not seen on subsequent cooling and second heating processes.     

Smectic, columnar and cubic mesophases in binary systems of hemiphasmidic and calamitic amphiphilic diols

A first example of an amphiphilic hemiphasmid consisting of a biphenyl rigid core connected to a hydrophilic 5,6-dihydroxy-3-oxahexyloxy group at one end and carrying two lipophilic dodecyloxy chains at the other end has been synthesized by a Pd-catalysed cross coupling reaction. The liquid crystalline properties of this compound have been investigated by polarized light microscopy, by differential scanning calorimetry and by X-ray diffraction. It exhibits a thermotropic hexagonal columnar mesophase which is stabilized on addition of formamide. On addition of ethylene glycol, a bicontinuous cubic mesophase is induced. Furthermore, binary mixtures of this compound with structurally related single chain amphiphiles have been investigated. Besides a smectic A phase, an induced columnar mesophase and a cubic phase were found in these mixtures.     

Studies of the thermotropic mesophase behaviour exhibited by a highly asymmetric tetrabenzotriazaporphyrin derivative

The thermotropic mesophase behaviour of an asymmetrically shaped and highly substituted tetrabenzotriazaporphyrin (TBTAP) derivative has been studied by optical microscopy, DSC and X-ray diffraction. The TBTAP macrocycle differs from the more common phthalocyanine ring system by the substitution of a methine for a nitrogen at one meso-position. The TBTAP core was substituted with a single heptadecyl chain at this meso-position and four neopentyl groups were statistically distributed around the peripheral benzene rings. In contrast to an earlier study of this system which described the structure of the mesophase as discotic lamellar, detailed X-ray diffraction study indicates that the TBTAP derivative forms a disordered hexagonal columnar mesophase (Col hd ), with a weak tendency towards antiparallel orientation of neighbouring molecules observed in the form of a weak pseudo-centred rectangular packing.     

Homologous series of liquid-crystalline metal free and copper octa-n-alkoxyphthalocyanines

Two series of discotic liquid crystals, one based on metal-free and the other on copper containing phthalocyanine (Pc) as the central core and eight peripherally n-alkoxy groups (C₆-C₁₂) as substituents have been synthesized and their mesomorphic properties and structural parameters have been determined. These compounds represent a series totally devoid of polymorphism, the mesophase in all cases being D_ho. In the solid state an orthorhombic two dimensional lattice with four molecules per unit cell and a presumably tilted stacking is present, while in the mesophase the planes of the molecules are on average perpendicular to the direction of the stacks. The aggregation behaviour of the C₈, C₁₀ and C₁₂ homologues was studied as a function of solvent polarity and of concentration. The electrical properties of the copper complexes of the n-dodecoxy and n-octoxy derivatives have been determined by means of complex impedance spectroscopy and compared with those of the non-mesogenic CuPc.     

Spectroscopic and structural studies on metal halide complexes of 4-vinylpyridine

Fourier transform infrared spectra (4000–400 cm⁻¹) are reported for metal(II) halide 4-vinlypridine complexes of the following stoichiometries: [MX₂(4-vipy]_n (n=4, M=Ni, X=Cl or Br; n=2, M=Cd, X=Cl, Br or I) and assignment are given for all the observed bands. These spectra were compared with X-ray powder diffraction patterns of complexes. It is shown that the proposed structures for these complexes derived from FTIR spectra are consistent with the X-ray powder diffraction measurements and the elemental analysis results. Coordination effect on 4-vinylpyridine has also been investigated.