Dilatometric investigation of thermotropic linear chain copper (II) alkanoates

The molar volumes of three binuclear copper (II) alkanoates (cupric octadecanoate, docosanoate, and tetracosanoate) were measured over the temperature range 30°–200 °C, which encompasses their phase transition to a columnar mesophase. A sharp volume increase in a narrow temperature interval about the transition was observed, consistent with a first-order phase transition. Values obtained for the partial molar volume of a methylene group in the columnar mesophase (16.9±0.3 cm³/mol) are between those for crystalline paraffins and for other smectic or discotic mesogens, respectively, indicating a disordered state, but not a complete fusion, of the aliphatic chains of the copper soaps in their columnar mesophase. The stacking period of the binuclear copper complexes in a column was derived from a combination of dilatometric and X-ray data. Its value, 4.64±0.08 Å, is independent of the chain length of the complex and of the temperature.     

Investigation of the Structures of the Crystalline and Columnar Phases of Linear Chain Copper(II) Alkanoates

Binuclear copper(11) complexes of fatty acids crystallize at room temperature in a lamellar lattice that has been characterized by X-ray diffraction. A transition to a thermotropic columnar mesophase is observed at about 110-120°C for each compound of the series n = 12 to n = 22, n being even and equal to the number of carbon atoms in the corresponding fatty acid. This columnar mesophase has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two-dimensional hexagonal lattice. The repeating unit in a column is a binuclear dicopper tetracarboxylate complex.     

Synthesis and Mesophase Behavior of Phenylthiophene Based Amphiphilic Molecules

Novel amphiphilic molecules consisting of a rigid 2‐phenylthiophene core, with a polar flexible tri(oxylethylene) moiety attached to the phenyl ring and one or two alkyl chains attached to the thiophene ring at the other side have been synthesized by using Ni(II) and Pd(0) catalyzed coupling reaction as key steps. The tri(oxylethylene) moieties were terminated with hydroxyl group, sodium carboxylate group and lithium carboxylate group respectively. The thermotropic and solvent induced liquid crystalline behavior of these substances was investigated by polarized optical microscopy, differential scanning calorimetry and X‐ray diffraction. Thereby the influence of the terminal groups attached to the tri(oxylethylene) moities as well as the influence of the length and the number of the alkyl chains on the mesophase behavior were investigated. The single alkyl chain Na‐carboxylate termianted derivatives show smectic A phases, double alkyl chain Na‐carboxylate terminated derivatives show a thermo tropic hexagonal columnar mesophase, while columnar mesophases are found in both single and double alkyl chain Li‐carbonate terminated derivatives. The model for molecular organization in the hexagonal columnar mesophase is established.     

Amphiphilic N-benzoyl-1-amino-1-deoxy-D-glucitol derivatives forming thermotropic lamellar, columnar and different types of cubic mesophases

Novel amphiphilic glucamine derivatives have been synthesized. These are N-benzoyl-1-deoxy1-methylamino-D-glucitols and N-benzoyl-1-amino-1-deoxy-D-glucitols carrying one, two or three aliphatic chains (CnH2n 1O- with n 3, 6 and 12) grafted to the benzamido group. The thermotropic mesophases of these compounds were studied by thermal polarizing optical microscopy and differential scanning calorimetry, and some also by X-ray scattering. Depending on the number and the length of the alkyl chains lamellar, bicontinuous cubic, hexagonal columnar or inverted micellar cubic mesophases were detected by analogy with lyotropic systems. In the contact region between lamellar phases of the single chain amphiphiles and micellar cubic phases of the mesomorphic triple chain compounds, hexagonal columnar phases can be induced. A hexagonal columnar phase was also induced in the contact region between a bicontinuous and a micellar cubic mesophase. The lyotropic liquid crystalline behaviour of the dodecyloxy substituted N-benzoyl-1-deoxy-1-methylamino-D-glucitols was investigated by the solvent penetration method using ethylene glycol as protic solvent. On increasing the solvent content, the double chain compound forms a cubic and a lamellar mesophase and the triple chain compound forms a hexagonal columnar lyomesophase. The dodecyloxy substituted compounds were also investigated with respect to their behaviour as thin films at the air-water interface using a Langmuir film-balance. Different types of pi/Aisotherms were observed whereby the molecular areas at collapse were determined either by the size of the carbohydrate head group (single chain compounds) or by the number of alkyl chains (double and triple chain compound).     

Mesomorphic properties of symmetrical and asymmetrical triphenylene homologues possessing fluoroalkylated side chains

Two types of homologues (symmetrical and asymmetrical in rotational symmetry) of novel triphenylene compounds possessing fluoroalkyl and alkyl side chains were synthesized via an alternative method. X-ray diffraction and DSC measurements showed that these homologues are thermotropic liquid crystals with a hexagonal columnar (Col_h) mesophase. The phase transition temperatures (Col_h-Iso) for both symmetrical and asymmetrical fluoroalkyloxytriphenylenes increase to about 180°C, and are independent of fluoromethylene chain and the rotational symmetry of chemical structure. The Col_h phase of symmetrical and asymmetrical fluoroalkyloxytriphenylenes possessing three fluoroalkyl side chains are more stable than fluoroalkyloxytriphenylenes possessing six fluoroalkyl side chains and alkyloxytriphenylenes. The X-ray diffraction patterns for symmetrical and asymmetrical fluoroalkyloxytriphenylenes, fluoroalkyloxytriphenylenes and alkyloxytriphenylenes in the wide-angle region are compared.     

Dendromesogens: liquid crystal organizations of poly(amidoamine) dendrimers versus starburst structures

A new series of liquid crystalline poly(amidoamine) (PAMAM) dendrimers is described. These dendrimers are made by attaching to the 0-, 1-, 2-, 3-, and 4-generation of PAMAM-terminal promesogenic units that carry two decyloxy chains in the 3- and 4-positions of their peripheral aromatic ring. X-ray diffraction studies show that all the compounds display a hexagonal columnar mesophase. A high density of aliphatic chains imposes a curved interface with the promesogenic units that forces the molecules to adopt a radial conformation, and therefore, the columnar structure. A model for the supramolecular organization of the different generations within the columnar mesophase is proposed based on the variation of some of the structural parameters.     

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.     

Mesomorphic complexes through hydrogen bonding between alkoxy-substituted triarylmelamines and semiperfluorinated benzoic acids

The thermal behaviour of two 2,4,6-triarylamino-1,3,5-triazines carrying either three or six peripheral alkoxy chains have been investigated in binary mixtures with two-chain and three- chain partially fluorinated benzoic acids by means of polarizing microscopy, differential scanning calorimetry and X-ray scattering. The melamines form hydrogen-bonded aggregates with the complementary carboxylic acids. Each investigated equimolar mixture exhibits a hexagonal columnar mesophase. In the case of the six-fold alkoxy-modified triazine the individual columns are built up by descrete hetero-dimers with a circular cross-sectional shape. The segregation of fluorinated from lipophilic side chain regions leads to a superstructure within the hexagonal lattice. The hexagonal columnar phases of 1:1 compositions of the triazine, incorporating just three aliphatic chains with the semiperfluorinated benzoic acids, are formed by pairs of H-bonded dimeric supermolecules.     

Self-assembling behavior of amphiphilic dendron coils in the bulk crystalline and liquid crystalline states

We prepared a series of amphiphilic dendron coils (1-3) containing aliphatic polyether dendrons with octadecyl peripheries and a poly(ethylene oxide) (PEO) coil (DP = 44). The molecular design in this study is focused on the variation of dendron generation (from first to third) with a fixed linear coil, upon which the thermal and self-assembling behavior of the dendron coils was investigated in the bulk. All the dendron coils exhibit two crystalline phases designated as k1 (both crystalline octadecyl chains and PEO) and k2 states (crystalline octadecyl chains and molten PEO). Crystallinities for both octadecyl peripheries and the PEO decrease as generation increases. In particular, the dendron coil (3) containing third generation shows a drastic reduction of the PEO crystallinity, which is attributed to the considerable chain folding and plasticization effects by the largest hydrophilic dendritic core segment. All the crystalline phases are bilayered lamellar morphologies. On going from k1 to k2, the periodic lamellar thickness decreases in the dendron coil (1) with first generation, but interestingly increases in 3. After melting of octadecyl peripheries, 1 shows no mesophase (i.e., liquid crystalline phase). Additionally, dendron coil 2 (3) displays a network cubic mesophase with Ia3d symmetry (micellar cubic with Pm3n) which is transformed into a lamellar (hexagonal columnar) mesophase upon heating. Remarkably, the temperature-dependent mesomorphic behavior in 2 and 3 is a completely reverse pattern in comparison with conventional linear-linear block copolymers. The unusual bulk morphological phenomena in the crystalline and liquid crystalline phases can be elucidated by the dendron coil architecture and the associated coil conformational energy.     

Mesomorphic properties of copolyesters of 3,6-linked triphenylene-based units and polymethylene spacers

Main chain discotic liquid crystalline polymers consisting of triphenylene-based units and alkyl spacers (C8, C10 and C12), connected by ester linkages in the 3- and 6-positions of triphenylene, have been synthesized and their mesomorphic properties were studied by DSC, polarizing optical microscopy and X-ray diffraction. It was found that these polymers exhibit a hexagonal columnar (Col h ) mesophase with intracolumnar order over a wide temperature range. The clearing temperature decreases on increasing the spacer length. It was found that the clearing temperatures are rather higher than that of the corresponding triphenylene monomer having six hexyloxy chains. These polymers form an ordered columnar mesophase, while the corresponding monomeric mesogen shows a disordered columnar phase. In the polymeric system, the fluctuations of the disc-like units in the mesophase are restricted by the connection of the mesogenic units, which stabilizes the columnar mesophase.     

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.     

Influence of acetylene-containing peripheral chains on the mesomorphic properties of triphenylene-based liquid crystals

Three series of novel acetylene-containing triphenylene-based liquid crystals have been prepared. One class of material is symmetrical with the acetylenic unit located at the terminus of a peripheral alkynyloxy chain in a peripheral position at the terminus of an alkoxy chain. These symmetrical hexa-substituted materials possess high melting points which renders mesomorphism monotropic. A second class comprises materials where only one of the six peripheral units contains a terminal alkyne chain. The asymmetry confers relatively low melting points and an enantiotropic hexagonal columnar mesophase is exhibited for all materials, albeit to a lower temperature than the known parent systems. The third series of materials also comprises unsymmetrical structures, here containing one acetylenic unit conjugated with the triphenylene core and five alkoxy chains. This third series of materials show enhanced π–π interactions and enhanced space-filling effects of the acetylenic unit close to the core which enhance mesophase stability markedly over the hexaalkoxy analogues. This result shows that rigidity of peripheral chains, as opposed to molecular symmetry, is the significant factor for the reduction of mesophase stabilities.     

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.     

Mesomorphic properties of copolyesters of 3,6-linked triphenylene-based units and polymethylene spacers

Main chain discotic liquid crystalline polymers consisting of triphenylene-based units and alkyl spacers (C8, C10 and C12), connected by ester linkages in the 3- and 6-positions of triphenylene, have been synthesized and their mesomorphic properties were studied by DSC, polarizing optical microscopy and X-ray diffraction. It was found that these polymers exhibit a hexagonal columnar (Col_h) mesophase with intracolumnar order over a wide temperature range. The clearing temperature decreases on increasing the spacer length. It was found that the clearing temperatures are rather higher than that of the corresponding triphenylene monomer having six hexyloxy chains. These polymers form an ordered columnar mesophase, while the corresponding monomeric mesogen shows a disordered columnar phase. In the polymeric system, the fluctuations of the disc-like units in the mesophase are restricted by the connection of the mesogenic units, which stabilizes the columnar mesophase.     

Induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system

The induction of a nematic columnar phase in a discotic hexagonal ordered phase forming system is achieved by mixing hexakispentyloxytriphenylene 1 with a long chain derivative of trinitrofluorenone 3. The difference in chain length has a strong influence on the packing behaviour due to steric effects. The long hydrocarbon chains of the acceptor introduce a strong asymmetry into the electron donor acceptor complex. It could be shown by differential scanning calorimetry, optical microscopy and X-ray measurements that a nematic columnar phase is formed. In this mesophase the triphenylenes form columns but no hexagonal or orthorhombic lattice is built up. Each column behaves like a rod-like nematic mesogen. To prove that the long hexadecane alkyl chains of the acceptor are responsible for this induction, the acceptor 3 was mixed with the non-liquid-crystalline triphenylene derivative 2 containing six hexadecyloxy side groups. The long alkyl chains of the acceptor dissolve perfectly in the side chain region of the discs. No asymmetry is induced and the columns formed can be arranged on a hexagonal lattice resulting in a D_ho phase.     

Structural anisotropy study by resonance Raman scattering in nematics with different dyes

Ionic liquid crystals based on congruent ion pairs composed of mesogenic cations and anions of similar shape provide an attractive tool for the tuning of mesophase properties. Here, the effect of the number and symmetry of lipophilic side chains and the type of head group on the phase type and thermal mesophase properties was probed by the synthesis and investigation of two series of novel guanidinium and imidazolium sulphonates and compared with the corresponding iodides. Their mesomorphic properties were examined via differential scanning calorimetry, optical polarising microscopy and X-ray diffraction. While derivatives bearing only one alkoxy chain in either cation or anion with up to three alkoxy chains in total within the ion pairs display smectic A mesophases, hexagonal columnar mesophases were observed for all other compounds with four or five alkoxy chains totally irrespective of the head group. However, with increasing steric bulk, i.e. with a total of six alkoxy chains, the symmetry of the aryl moiety in the anion as well as the type of head group becomes relevant, resulting in hexagonal columnar or plastic phases for guanidinium sulphonates with symmetrical anions, while those with unsymmetrical anions were non-mesomorphic. In contrast, the corresponding imidazolium sulphonates displayed cubic phases for the combinations of symmetrical cation/symmetrical anion and symmetrical cation/unsymmetrical anion. If both ions are unsymmetrically substituted, the imidazolium sulphonate displayed a hexagonal columnar phase. The results further demonstrate the utility of the congruent ion pairs for tailor-made ionic liquid crystals.     

Hierarchical organization of photoresponsive hydrogen-bonded rosettes

Hydrogen-bonded disk-shaped aggregates (rosettes) composed of azobenzene-appended melamine and barbiturate or cyanurate are investigated in view of their hierarchical organization and photoresponsive behavior by (1)H NMR and UV/vis spectroscopies, dynamic light scattering, and gelation behavior in aliphatic solvents and liquid crystalline behavior in bulk state. In the bulk state the rosette possessing a sterically bulky tridodecyloxyphenyl substituent in the barbiturate component stacks in an offset arrangement to form a rectangular columnar mesophase, whereas in aliphatic solvents it does not hierarchically organize into higher-order columnar aggregates. This drawback is improved by exchanging the barbiturate component into a more sterically nondemanding N-dodecylcyanurate component. The resulting new rosette stacks in a face-to-face arrangement to form a hexagonal columnar mesophase in the bulk state and hierarchically organizes into elongated fibrous aggregates in cyclohexane, which eventually leads to the formation of organogel. Dynamic light scattering and UV-vis experiments upon UV-irradiation of the columnar aggregates in cyclohexane revealed that the dissociation and the reformation of columnar aggregates can be controlled by the trans-cis isomerization of the azobenzene moiety. Molecular modeling indicates that the rosette possessing cis-azobenzene side chains loses its planarity. Using this photoinduced morphological change of the rosette, photoresponsive organogel is created by the use of a disk-shaped supramolecule the first time.     

Self-Assembly and Fluorescence of Tetracationic Liquid Crystalline Tetraphenylethene

A series of tetraguanidinium tetraphenylethene (TPE) arylsulfonates with different chain lengths was prepared via ionic self-assembly of tetraguanidinium TPE chloride and the respective methyl arylsulfonates. Liquid crystalline properties were studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. Tetraguanidinium TPE arylsulfonates with chain lengths of C₈–C₁₂ displayed hexagonal columnar mesophases over a broad temperature range, while derivatives with longer chains showed oblique columnar phases. In solution all compounds displayed aggregation-induced emission behaviour. Temperature-dependent luminescence spectra of the bulk phase of the tetraguanidinium TPE arylsulfonate with C₁₄ side chains revealed a strong luminescence both in the solid state and the oblique columnar mesophase. The emission behaviour was rationalized by a unique combination of restriction of intramolecular rotation of the TPE core, Coulomb interaction between the guanidinium cations and π–π interactions of the anionic arylsulfonate moieties.     

Liquid crystalline metal-free phthalocyanines designed for charge and exciton transport

A joint theoretical and experimental study of the electronic and structural properties of liquid crystalline metal-free phthalocyanines bearing a strong potential for charge and exciton transport has been performed. The synthesis of such compounds has been triggered by quantum chemical calculations showing that: (i) hole transport is favored in metal-free phthalocyanines by their extremely low reorganization energy (0.045 eV) and large electronic splittings; and (ii) the efficiency of energy transfer along the one-dimensional discotic stacks is weakly affected by rotational disorder due to the two-dimensional character of the molecules. We have synthesized two metal-free phthalocyanines with different branched aliphatic chains on the gram scale to allow for a full characterization of their solid-state properties. The two compounds self-organize in liquid crystalline mesophases, as evidenced by optical microscopy, differential scanning calorimetry, X-ray powder diffraction, and molecular dynamics simulations. They exhibit a columnar rectangular mesophase at room temperature and a columnar hexagonal mesophase at elevated temperature.     

Mesomorphic phase transition behavior of novel triphenylene compounds possessing fluoroalkylated side chains

This report discusses the effect of fluoroalkyl chain on the mesomorphism. Several homologues of novel triphenylene compounds possessing fluoroalkylated side chains were synthesized. Studies of X-ray diffraction, DSC and texture observations by polarized microscope revealed that these homologues show hexagonal columnar (Col_h) mesophase. These homologues made columnar mesophase stabilize and the melting point increase, as compared with corresponding alkyloxytriphenylenes. In the case of fluoroalkyloxytriphenylenes possessing fluoromethylene side chains, the increase of the fluoromethylene chain length stabilized columnar mesophase, made the phase transition enthalpy (Col_h-Iso) and entropy (Col_h-Iso) increase. It is considered that these results are due to the fluorophilic interaction, and that the fluorophilic interaction is important for stabilizing columnar mesophase.