Pressure effects in a plastic columnar discotic triphenylene

The effect of hydrostatic pressure on the structure of a plastic columnar discotic triphenylene has been investigated. The goal was to determine whether pressure can be used to modify electronic properties via changes in structural properties of columnar discotics to any significant extent. The findings are that (i) the intra‐ and inter‐columnar distances are reduced in a nearly isotropic fashion, (ii) that the crystal sizes are reduced and (iii) that a transition takes place from a more highly ordered plastic columnar to a less ordered hexagonal columnar state with increasing pressure. The induced decrease of the molecular distances, amounting to 6% for pressures up to 17?kbar, are clearly too small to induce an appreciable modification of the electronic structure and thus opto‐electronic properties.     

The role of disorder on the electronic structure of conjugated polymers. The case of poly-2,5-bis(phenylethynyl)-1,3,4-thiadiazole

Insight into the electronic structure of disordered poly-2,5-bis(phenylethynyl)-1,3,4-thiadiazole in an amorphous region, in comparison to an ideal two-planar cofacial oligomer system, is pursued. The atomic structure of the amorphous polymer was obtained from classical molecular dynamics. It was subsequently used to calculate the electronic states and inter- and intrachain electronic coupling integrals using the density functional theory based charge patching method. The interchain electronic coupling integrals in the amorphous system were found to be an order of magnitude smaller than in the ordered system with similar distances between the chains. The results also suggest that the electronic structure of the whole system cannot be understood as a collection of the electronic structures of individual chains. The band gap of the whole system is significantly smaller than the band gaps of individual chains. This decrease originates from the disordered long range electrostatic potential created by the dipole moments of polymer repeat units, which should be minimized if one seeks good transport properties.     

Discotic liquid crystals: a new generation of organic semiconductors

Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given.     

Toward well-organised ionic discotic liquid crystals via versatile supramolecular approach

Various ordered structures of crystalline three-dimensional (3D) cubic, 2D columnar or 1D lamellar mesophases have been facilely achieved through host–guest interactions of electrically neutral host tris(18-crown-6)triphenylene and guest potassium sulfonates with alkyl tails of variant number and length. The convenient construction of functionalised ionic complexes and the flexibility of such a supramolecular approach offer a wide variety of possibilities to prepare various ordered functional soft materials, especially those in their 2D ordered columnar liquid crystalline mesophases may serve as promising electron and ion dual-channel transport organic electronic materials.     

Advances in Metal‐Free Heterocycle‐Based Columnar Liquid Crystals

Liquid crystals are ordered soft materials formed by self‐organized molecules and can potentially be used as new functional materials for electron‐, ion‐ or molecular‐transport; optical; and bio‐active materials. In particular, the columnar liquid crystals are promising candidates used in various optical and electronic devices. For this purpose, design and synthesis of unconventional materials are essential. In this review, we have summarized several approaches for the synthesis of columnar liquid crystals composed of various heterocyclic systems. We also outline their liquid crystalline and other relevant properties, and their suitability for applications in diverse fields.     

On the origin of the unique properties of supported Au nanoparticles

The unique catalytic activity of supported Au nanoparticles has been ascribed to various effects including thickness/shape, the metal oxidation state, and support effects. Previously, we reported the synthesis of ordered Au monolayers and bilayers on TiO(x), with the latter being significantly more active for CO oxidation than the former. In the present study, the electronic and chemical properties of ordered monolayer and bilayer Au films have been characterized by infrared reflection adsorption spectroscopy using CO as a probe and ultraviolet photoemission spectroscopy. The Au overlayers are found to be electron-rich and to have significantly different electronic properties compared with bulk Au. The common structural features of ordered Au bilayers and Au bilayer nanoparticles on TiO2(110) are described, and the exceptionally high catalytic activity of the Au bilayer structure related to its unique electronic properties.     

Small Tin Oxide Grains: Structural and Electronic Properties Evaluated using the Density Functional Theory

This study is motivated by the existence of many phases of tin oxide with a bulk crystalline structure and its purpose is the assessment if this property also applies to the clustered state of this material. Therefore small grains of a columnar and a spherical shape with a rutile lattice and a size up to 120 atoms have been considered and their structural and electronic properties are evaluated applying the Density Functional Theory. The central result of the calculations is that the rutile skeleton is retained starting from the size 20–40 atoms. The main factors contributing to this behaviour are the large interatomic distances in the rutile lattice and the irregular nature of the interatomic forces. Due to these effects, most of the structural and electronic parameters have a bulk-like behaviour with a nearly constant value. These values, however, have a clear dependence on the grain size and shape and significantly diverge from the bulk ones so that the grains have to be regarded as a new phase of the tin oxide materials.     

Controlled polymerization and self-assembly of halogen-bridged diruthenium complexes in organic media and their dielectrophoretic alignment

Lipophilic paddlewheel biruthenium complexes [Ru(2)(μ-O(2)CR)(3)X](n) (O(2)CR = 3,4,5-tridodecyloxybenzoate, X = Cl, I) self-assemble in organic media to form halogen-bridged coordination polymers. The polymerization is accompanied by spectral changes in π(RuO,Ru(2)) → π*(Ru(2)) and π(axial ligand) → π*(Ru(2)) absorption bands. These polymeric complexes form lyotropic liquid crystals in n-decane at concentrations above ~100 unit mM. The bridging halogen axial ligands (X = Cl or I) exert significant influences on their electronic structures and self-assembling characteristics: the chloride-bridged polymers give hexagonally aligned ordered columnar structure (columnar hexagonal phase, Col(h)), whereas the iodide-bridged polymers form less ordered columnar nematic (Col(n)) phase, as revealed by small-angle X-ray diffraction measurements. Chloro-bridged coordination polymers dispersed in n-decane are thermally intact even at the elevated temperature of 70 °C. In contrast, iodo-bridged polymers show reversible dissociation and reassembly phenomena depending on temperature. These halogen-bridged coordination polymers show unidirectional alignment upon applying alternating current (ac) electric field as investigated by crossed polarizing optical microscopy and scanning electron microscopy. The unidirectionally oriented columns of chloro-bridged polymers are accumulated upon repetitive application of the ac voltage, whereas iodo-bridged coordination polymers show faster and reversible alignment changes in response to turning on-and-off the electric field. The controlled self-assembly of electronically conjugated linear complexes provide a potential platform to design electric field-responsive nanomaterials.     

A triphenylene-containing side chain liquid crystalline ladder-like polysiloxane and its highly ordered superstructure

A liquid crystalline discotic triphenylene-containing side chain ladder-like polysiloxane (LPS) derivative was prepared by hydrosilylation. The superstructure was investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and isotherm surface pressure ( ∪)-area ( A ) diagrams based on LB-film experiments. The XRD results suggest that the discotic triphenylene units in the side chains stack to form a columnar structure and the columns further align with each other to form a board-like superstructure because of the semi-rigidity of the ladder-like backbone. This suggestion has been confirmed by parallel nanowire-like stripes observed in the AFM image. Mixing small amounts of the corresponding low molar mass molecules with the polymer leads to a more ordered and denser columnar stacking, as shown by sharpened XRD patterns and a ∪-A curve with a steeper slope, higher collapse pressure and lower collapse area than those obtained either for the low molar mass molecules or the polymer individually.     

On the ordering in new low gap semiconductors: PtSnS, PtSnSe, PtSnTe. Experimental and DFT studies

The crystallographic and electronic structures of PtSnS, PtSnSe and PtSnTe were investigated by X-ray structure analysis and density functional theory (DFT) calculations. Conductivity measurements and diffraction patterns show semiconducting ordered pyrite type related compounds containing SnX (X=S, Se, Te) entities. A scheme is presented to model ordered variants according to the relative orientation of the XY dumbbells. It represents the ullmannite, the cobaltite and a new rhombohedral structure type. The scheme allows for a systematic investigation of ordering preferences from first principles. According to the total electronic energy PtSnTe and PtSnSe prefer the cobaltite, PtSnS the rhombohedral structure type. The structural and electronic properties agree with experimental results. The three compounds are predicted to be narrow gap indirect semiconductors from conductivity measurements and band structure calculations.     

First-principles study of liquid gallium at ambient and high pressure

The static and dynamic properties of liquid Ga close to the melting line have been studied by first-principles molecular dynamics simulations at ambient and elevated pressure up to 5.8 GPa. Below 2.5 GPa, the nearest neighbor Ga-Ga separation shows little change, while the second and third coordination shells are compressed to shorter distances. This behavior is attributed to the gradual occupation of the interstitial sites. Detail analysis of the local geometry and dynamical behavior refutes the proposed existence of Ga(2) dimers in the liquid state. In fact, both the structure and electronic properties of the liquid are found to closely resemble that of the underlying Ga-II and Ga-III crystalline phases.     

Liquid crystalline order from ortho-phenylene ethynylene macrocycles

Triangular ortho-phenylene ethynylene (o-PE) cyclic trimers represent a novel member of shape-persistent macrocycles. Shape-persistent cyclic structures remain of great interest as molecular components in the fields of supramolecular materials, host-guest chemistry, and materials science. Novel discotic liquid crystalline properties are reported from triangular-shaped o-PE macrocycles containing branched alkoxy- and/or triethylene glycol (TEG) side chains using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The macrocycles self-assemble into thermotropic rectangular columnar (Colr) (for M1), hexagonal columnar (Colh) (for M2), and discotic nematic (for M3) mesophases at room temperature. This work shows clearly that electron-rich PE macrocycles can form LC materials. Alkyl side chains in M1 promote order, while hydrophilic side chains of M2 generate an amphiphilic structure that provides a different driving force for organization. The ability to create ordered self-assembling materials from these novel electron-rich macrocycles is important in nanotechnology.     

Liquid-crystalline physical gels

Liquid-crystalline (LC) physical gels are a new class of dynamically functional materials consisting of liquid crystals and fibrous aggregates of molecules that are called "gelators". Liquid-crystalline physical gels, which are macroscopically soft solids, exhibit induced or enhanced electro-optical, photochemical, electronic properties due to the combination of two components that form phase-separated structures. In this tutorial review, we describe the materials design and structure-property relationships of the LC physical gels. The introduction of self-assembled fibers into nematic liquid crystals leads to faster responses in twisted nematic (TN) mode and high contrast switching in light scattering mode. Furthermore, the LC physical gels can be exploited as a new type of materials for electro-optical memory. This function is achieved by the control of reversible aggregation processes of gelators under electric fields in nematic liquid crystals. Electronic properties such as hole mobilities are improved by the introduction of fibrous aggregates into triphenylene-based columnar liquid crystals. The incorporation of photochromic azobenzenes or electroactive tetrathiafulvalenes into the chemical structures of gelators leads to the preparation of ordered functional materials.     

Ordered packing of soft discoidal system

A novel mesoscopic simulation method is adopted to study the ordered packing of the anisotropic disklike particles with a soft repulsive interaction, which possesses a modified anisotropic conservative force type used in dissipative particle dynamics. We examine the influence of the shape of the particles, the angular width of the repulsion, and the strength of the repulsion on the packing structures. Specifically, an ordered hexagonal columnar structure is obtained in our simulations. Our study demonstrates that an anisotropic repulsive potential between soft discoidal particles is sufficient to produce a relatively ordered hexagonal columnar structure.     

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.     

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.     

Modular assembly of elliptical mesogens

Discotic mesogens featuring a pyridine ring were synthesized, and were found either to form ordered hexagonal columnar liquid crystalline phases or melt directly from a crystal to an isotropic liquid, depending on the position of the pyridyl nitrogen atom. Binary mixtures of the mesogenic pyridine derivatives with a similar discotic mesogen having a carboxylic acid group resulted in the formation of modular elliptical complexes through hydrogen bonding. The binary mixtures were found to exhibit ordered hexagonal columnar or ordered rectangular columnar and nematic mesophases, depending on the length of the alkyl chains, and displayed dramatically different properties from their constituent components. Binary mixtures of the non-mesogenic pyridine derivatives with carboxylic acid-functionalized discotic mesogens did not result in the formation of hydrogen-bonded complexes.     

Discotic liquid-crystalline materials based on porphycenes: a mesogenic metalloporphycene-tetracyanoquinodimethane (TCNQ) adduct

A number of substituted zinc(II) porphycenes and porphyrins have been synthesized as potentially mesogenic materials. One of the resulting porphycenes, bearing eight decyloxy chains, exhibits two mesophases, a transient lamellar phase (Lam) and a highly ordered lamello-columnar phase (L(Col)), with remarkably different structural characteristics. The same zinc(II) porphycene also forms an electron donor-acceptor (EDA) complex with tetracyanoquinodimethane (TCNQ), generating a hexagonal columnar mesophase (Col(h)) that is thermally stable up to ca. 200 degrees C. The EDA interaction between porphycene and TCNQ has been probed using electronic and vibrational spectroscopy. A mixture of zinc(II) porphyrins, isomeric with the above porphycene complex, forms a rectangular columnar mesophase (Col(r)).     

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.     

具有不同柱状结构的苯并菲盘状液晶化合物的合成及自组装

合成了3种含有不同长度烷基链的苯并菲盘状液晶化合物; 通过1H NMR 和 MALDI-TOF MS对其结构进行了表征; 利用差示扫描量热法(DSC)、热台偏光显微镜(POM)和小角X射线散射实验(SAXS)对3种液晶化合物的自组装行为进行了研究. 结果表明, 烷基链的长度对苯并菲盘状液晶化合物自组装结构的影响显著. 柔性链为辛基的苯并菲盘状液晶化合物自组装成六方柱状液晶相; 柔性链为十二烷基的化合物自组装成倾斜柱状液晶相; 而柔性链为十六烷基的化合物则未形成液晶相.