Eutectic mixtures with plastic columnar discotics: molecular structure,phase morphology and kinetics of phase separation

We report the properties of eutectic mixtures of triphenylenes displaying a highly ordered columnar phase with a low molar mass non‐discotic compound. Such highly ordered triphenylenes display large charge carrier mobilities which are strongly controlled by the state of order in the discotic phase. The motivation was to establish how the state of order—molecular order, phase morphology, temperature ranges of phase stabilities and macroscopic orientational order—can be influenced by mixing. The studies reveal that the molecular order, in particular the mutual arrangement of the columns and the intracolumnar order, are unaffected by dilution of the discotic compound, whereas the phase morphology and the kinetics of phase separation change significantly with dilution. Rod‐shaped discotic domains with a hexagonal cross‐sectional area are formed via a nucleation process and the rods grow linearly as a function of time. Both the pure discotic phase as well as the discotic domains forming during phase separation can be macroscopically ordered by orientation layers.     

Eutectic mixtures with plastic columnar discotics: molecular structure, phase morphology and kinetics of phase separation

We report the properties of eutectic mixtures of triphenylenes displaying a highly ordered columnar phase with a low molar mass non-discotic compound. Such highly ordered triphenylenes display large charge carrier mobilities which are strongly controlled by the state of order in the discotic phase. The motivation was to establish how the state of order—molecular order, phase morphology, temperature ranges of phase stabilities and macroscopic orientational order—can be influenced by mixing. The studies reveal that the molecular order, in particular the mutual arrangement of the columns and the intracolumnar order, are unaffected by dilution of the discotic compound, whereas the phase morphology and the kinetics of phase separation change significantly with dilution. Rod-shaped discotic domains with a hexagonal cross-sectional area are formed via a nucleation process and the rods grow linearly as a function of time. Both the pure discotic phase as well as the discotic domains forming during phase separation can be macroscopically ordered by orientation layers.     

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.     

Fractal analysis of a discotic texture

The dendritic-type texture displayed by low molar mass discotic charge transfer systems possessing a columnar hexagonal ordered phase has been subjected to a fractal analysis. The diffusion limited aggregation approach was used to simulate the growth of the texture. The results are that the texture displays a fractal geometry and that the prominent features of the discotic texture are correctly reflected by the simulated clusters, including the fractal dimension. The conclusion is that the texture is conditioned by the lattice structure of the discotic phase and the kinetic processes, the sticking rules in the terms of diffusion limited aggregation model, responsible for the growth of the texture.     

Synthesis and analytical properties of a novel columnar metallomesogenic polymer

A metallomesogenic side-chain polymer with copper carboxylato discotic units in stacks prepared by covalent bonding of 14-pentadecenoic acid, stearic acid and poly(methylhydrosiloxane) is described. The physico-chemical and thermal properties of both monomeric and polymeric metallomesogens were determined by elemental analysis, IR, polarizing optical microscopy, thermal gravimetric analysis and differential scanning calorimetry. The polymeric states showed a discotic lamellar phase at 50-95 degrees C and an ordered discotic hexagonal phase at 95-200 degrees C. By dynamic coating, the metallomesogenic polymer was crosslinked to the capillary wall via benzoyl peroxide. The wall-coated capillary columns (15 m x 0.25 mm I.D.) were used for the separation of phenols. Factors affecting the retention and the sample selectivity were examined. Van 't Hoff plots as a function of temperature indicated that phase transitions were occurring. Thermodynamic properties of the analytes in this system were also studied. For the determination of a mixture of 3-aminophenol, 2-chlorophenol, 2-nitrophenol, 4-nitrophenol, o-methylphenol, m-methylphenol, p-methylphenol, 2,4-dichlorophenol, 2,4-dimethylphenol, 2,4-dinitrophenol, 2,4,6-trichlorophenol, 2,4,6-trimethylphenol, 4-bromophenol, 3-methyl-4-chlorophenol, pentachlorophenol, and unsubstituted phenol, the calibration graphs for most phenols were linear over the range of 10-1000 microg ml(-1) and the mass detection limits were in the ng range based on three times standard deviation of seven measurements of the lowest peak that could be detected.     

Light-induced formation of curved needle texture by circularly polarized light irradiation on a discotic liquid crystal containing a racemic chromium complex

We have found that the discotic nematic liquid crystal, hexakis(4-nonylphenylethynyl)benzene (HNEB), doped with the racemic chromium complex Cr(Ocacac)₃, shows a novel straight-needle texture with hexagonal columnar alignments, changing to a curved-needle texture under irradiation of circularly polarized light (CPL). This novel phenomenon is specific to the mixture of HNEB and Cr(Ocacac)₃. The formation of curved needles means that chiroselective photoinversion of racemic Cr(Ocacac)₃ by CPL irradiation induces a needle direction change in a discotic liquid crystal. The change in chirality of Cr(Ocacac)₃ in HNEB induced by CPL irradiation, and the resulting nano-segregation of its enantiomers during cooling from the isotropic to mesophase of HNEB, are considered to influence changes in the alignment of columns and/or small domains of column aggregates in the discotic liquid crystal.     

The first lanthanide(III) monoporphyrin complex liquid crystal

Hydroxy [5,10,15,20-tetra[ p -decyloxy- m -methyloxy)phenyl]porphyrin Yb(III) exhibits a discotic hexagonal columnar phase, it is the first example of a monoporphyrin rare earth complex liquid crystal.     

A re-entrant Col ho phase

The formation of a re-entrant columnar discotic hexagonal ordered phase (re-entrant Col ho phase) is reported for the asymmetrically substituted triphenylene 3,6,7,10,11-penta- pentyloxytriphenylene-2-yl pivaloate (pivaloate) and its structural and dynamic properties described. The re-entrant phase behaviour is strongly modified by doping the pivaloate with the electron acceptor 2,4,7-trinitrofluorenone (TNF).     

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.     

Discotic liquid crystalline poly(propylene imine) dendrimers based on triphenylene

The design, synthesis, and mesomorphic properties of a new series of homodendrimers consisting of the commercially available poly(propylene imine) (PPI) dendrimers (G = 1-5), PPI-(NH(2))(n)() (n = 4, 8, 16, 32, 64), functionalized with a discotic triphenylene moiety are reported. The liquid crystalline behavior was investigated by means of differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), and X-ray diffractometry (XRD). All of the homodendrimers showed mesomorphic properties, with the second to fifth generations giving a hexagonal columnar mesophase (Col(h)) and the first generation a rectangular columnar mesophase (Col(r)). The X-ray study reveals that these mesophases show a highly ordered structure with segregation of triphenylenes and dendrimers into separate columns and a regular stacking distance inside the triphenylene columns. GPC analysis showed that the dendrimers had good monodispersity and MALDI-TOF studies of the first three generations gave good evidence that all of the terminal amino groups of the dendrimers were functionalized with a discotic unit.     

Room-temperature fluorescent liquid crystalline dimers based on discotic 1,3,4-oxadizole

A series of liquid crystalline dimers DOXD-n (n = 6–10) based on discotic 1,3,4-oxadiazole have been synthesised and its thermal properties have been investigated by means of polarised optical microscopy, differential scanning calorimetry, variable-temperature X-ray diffraction and thermogravimetric analyses. Most of the dimers display a discotic nematic phase or a hexagonal columnar phase at room temperature and exhibit photoluminscence in solution, in solid state and in liquid crystalline phase. The relationship between the properties and states of the matter is discussed briefly. Notably, the fluorescent intensity in liquid crystalline state decreased apparently compared to that in solid state due to the self-quenching aggregates.     

The first lanthanide(III) monoporphyrin complex liquid crystal

Hydroxy [5,10,15,20-tetra[P-decyloxy- ^m -methyloxy)phenyl]porphyrin Yb(III) exhibits a discotic hexagonal columnar phase, it is the first example of a monoporphyrin rare earth complex liquid crystal.     

Self-assembled discotic nematic liquid crystals formed by simple hydrogen bonding between phenol and pyridine moieties

New discotic nematic liquid crystals have been prepared through intermolecular hydrogen bonding between the core of 1,3,5-trihydroxybenzene (phloroglucinol, PG) or 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) and the peripheral molecules of stilbazole derivatives. The various nematic phases formed by new hydrogen bonding building blocks were investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The first discotic complexes of PG and trans-4-alkoxy-4′-stilbazoles exhibited nematic columnar (N_C) and hexagonal columnar phases depending on the length of alkyl chains, which were considered as the basic discotic structure. Several structural variations on the building blocks were attempted to examine their effects on the liquid crystalline properties of discotic complexes. The nematic lateral phase (N_L) with enhanced intercolumnar order was observed for the complexes of PG and trans-4-cyanoalkoxy-4′ stilbazoles due probably to the strong dipole interactions between cyano groups at the end of alkoxy chains. By introducing the nonlinear structure in three arms of supramolecular discotic mesogen, a discotic nematic phase (N_D) was observed for the complex of THPB and trans-4-octyloxy-4 -stilbazole. The single hydrogen bonding between phenol and pyridine moieties in this study provides a simple and effective method for preparing the rarely found discotic nematic liquid crystals.     

Alpha-helical-within-discotic columnar structures of a complex between poly(ethylene oxide)-block-poly(l-lysine) and a hexa-peri-hexabenzocoronene

Poly(ethylene oxide)-block-poly(l-lysine) (PEO-PLL) was complexed with an amphiphilic hexa-peri-hexabenzocoronene (HBC). This produced a thermotropic liquid crystalline material (PEO-PLL-HBC), which was investigated by FTIR spectroscopy and differential scanning calorimetry as well as by wide- and small-angle X-ray scattering. It was found that the poly(l-lysine) blocks form an alpha-helical secondary structure. Each helix is surrounded symmetrically by six discotic columns of HBC, which gives an alpha-helical-within-discotic column structural entity. The dense packing of these entities produces hexagonal sublattices (formed by the columns) in the frame of a two-dimensional hexagonal lattice (formed by the helices). An order-order transition from a columnar structure Col1 to Col2 was found at 54 degrees C. The unit cell constants are 5.75 nm (Col1) and 6.60 nm (Col2). The larger unit cell size of Col2 was explained by a higher intracolumnar order of the latter in which the packing distance of the disklike HBC cores is well-defined (0.353 nm). PEO-PLL-HBC combines essential features of liquid crystals with a basic structural element of proteins into a single material.     

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.     

Anomalous columnar order of charged colloidal platelets

Monte Carlo computer simulations are carried out for a model system of like-charged colloidal platelets in the isothermal-isobaric ensemble (NpT). The aim is to elucidate the role of electrostatic interactions on the structure of synthetic clay systems at high particle densities. Short-range repulsions between particles are described by a suitable hard-core model representing a discotic particle. This potential is supplemented with an electrostatic potential based on a Yukawa model for the screened Coulombic potential between infinitely thin disklike macro-ions. The particle aspect-ratio and electrostatic parameters were chosen to mimic an aqueous dispersion of thin, like-charged, rigid colloidal platelets at finite salt concentration. An examination of the fluid phase diagram reveals a marked shift in the isotropic-nematic transition compared to the hard cut-sphere reference system. Several statistical functions, such as the pair correlation function for the center-of-mass coordinates and structure factor, are obtained to characterize the structural organization of the platelets phases. At low salinity and high osmotic pressure we observe anomalous hexagonal columnar structures characterized by interpenetrating columns with a typical intercolumnar distance corresponding to about half of that of a regular columnar phase. Increasing the ionic strength leads to the formation of glassy, disordered structures consisting of compact clusters of platelets stacked into finite-sized columns. These so-called "nematic columnar" structures have been recently observed in systems of charge-stabilized gibbsite platelets. Our findings are corroborated by an analysis of the static structure factor from a simple density functional theory.     

A computer simulation of model discotic dimers

We set up a model for discotic liquid crystal dimers and study, by means of Monte Carlo simulations, their phase behaviour and self–assembling properties, in comparison with the simpler monomeric case. Each discotic dimer is described by two oblate Gay–Berne ellipsoids connected by a flexible spacer, modelled by a harmonic “spring” of three different lengths. We find that dimerization in general yields produces a significant change on the phase behaviour, with an increase of the columnar–nematic transition temperature, a widening of the nematic region and the apparent suppression of the crystalline phase in favour of the columnar phase up to very low temperatures. Longer spacers prove to ease the formation of columns and to increase the orientational order. Contribution to the Fernando Bernardi memorial issue.     

Composite structure of liquid crystal/polymer nanotubes revealed by high-angle annular dark-field scanning transmission electron microscopy.

We have applied high-angle annular dark-field microscopy to the characterization of the structure of template-grown nanotubes composed of a polymer and a discotic liquid crystalline material. Selective staining of the liquid crystal phase with ruthenium tetroxide was used to develop adequate Z-contrast that allows us to distinguish between the two phases. At appropriate staining conditions, we could clearly visualize, in the annular dark-field mode, a 5-15-nm thin liquid crystalline layer precipitated on the inner surface of the polymer tubes. Cryo-electron diffraction has shown high alignment of the discotic columns within the layer parallel to the tube axis. However, although the polymer/liquid crystal phase separation is almost complete, the wetting behavior of the polymer in relation to the template appears to be sensitively influenced by kinetic factors.     

Spherical Supramolecular Structures Constructed via Chemically Symmetric Perylene Bisimides: Beyond Columnar Assembly

Like other discotic molecules, self-assembled supramolecular structures of perylene bisimides (PBIs) are commonly limited to columnar or lamellar structures due to their distinct π-conjugated scaffolds and unique rectangular shape of perylene cores. The discovery of PBIs with supramolecular structures beyond layers and columns may expand the scope of PBI-based materials. A series of unconventional spherical packing phases in PBIs, including A15 phase, σ phase, dodecagonal quasicrystalline (DQC) phase, and body-centered cubic (BCC) phase, is reported. A strategy involving functionalization of perylene core with several polyhedral oligomeric silsesquioxane (POSS) cages achieved spherical assemblies of PBIs, instead of columnar assemblies, due to the significantly increased steric hindrance at the periphery. This strategy may also be employed for the discovery of unconventional spherical assemblies in other related discotic molecules by the introduction of similar bulky functional groups at their periphery. An unusual inverse phase transition sequence from a BCC phase to a σ phase was observed by increasing annealing temperature.     

Discotic charge transfer twins Structure and mesophase behaviour of covalently linked triphenylenes and trinitrofluorenones

Discotic charge transfer twins, a novel class of discotic liquid-crystalline compounds were studied. These compounds consist of triphenylene units (as donors) which are chemically linked via flexible spacers of various lengths to trinitrofluorenone units (acting as acceptor). They display a liquid-crystalline phase over a wide temperature range extending up to 240-260°C. Based on X-ray analysis a structural model is proposed for the liquid-crystalline phase: the molecules are arranged in columns in such a way that mixed stacks occur, the intercolumnar packing possesses an orthorhombic symmetry. The neighbouring columns are connected along specific directions via flexible spacers which give rise to highly anisotropic structural properties of this columnar liquid-crystalline phase.