Bent‐core mesogens based on semi‐flexible dicyclohexylmethane spacers

New, bent‐core mesogens are described in which the core of the molecule is a semiflexible, di(4‐aminocyclohexyl)methane spacer. The compounds show nematic, columnar nematic and columnar phases as shown by a combination of X‐ray diffraction and optical microscopy. The potential of these new mesogens as biaxial nematic candidates is considered.     

Invited Article

Several series of phasmids and biforked mesogens or more generally of polycatenar mesogens, i.e. rod-like core mesogens ending in six, five, four or three aliphatic chains (respectively hexa-, penta-, tetra- and tricatenars) are reviewed. According to the number and positions of the aliphatic chains, lamello-columnar, columnar, cubic, smectic and nematic phases appear.     

Functional organic materials based on polymerized liquid-crystal monomers: supramolecular hydrogen-bonded systems

Functional organic materials are of great interest for a variety of applications. To obtain precise functional properties, well-defined hierarchically ordered supramolecular materials are crucial. The self-assembly of liquid crystals has proven to be an extremely useful tool in the development of well-defined nanostructured materials. We have chosen the illustrative example of photopolymerizable hydrogen-bonding mesogens to show that a wide variety of functional materials can be made from a relatively simple set of building blocks. Upon mixing these compounds with other reactive mesogens, nematic, chiral nematic, and smectic or columnar liquid-crystalline phases can be formed that can be applied as actuators, sensors and responsive reflectors, and nanoporous membranes, respectively.     

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.     

Mesomorphic self-assembly of 1,3-Diacylaminobenzenes

Some new examples of hydrogen-bonded 1,3-diacylaminobenzene mesogens are presented. This odd type of polymeric self-hydrogen bonded supramolecular association allows a nematic and/or a columnar order to be obtained. Such a mesogenic hydrogen-bonded arrangement (MHB) should be able to stabilize diverse mesomorphic molecular edifices such as layers, columns and tubes.     

New biforked mesogen series

Four new series of biforked mesogens derived from 3,4-dialkoxybenzoic acids or 3,4-dialkoxybenzaldehydes have been prepared and studied. These derivatives exhibit an interesting polymorphism in which both columnar and lamellar mesophases exist in the same series and in some cases in the same compound. The short chain derivatives exhibit a nematic and/or a smectic C phase but those with long chains display the hexagonal, rectangular or oblique columnar phases. In some of them, a centred cubic phase is found.     

The influence of lateral apolar substituents on the mesomorphic behaviour of tetracatenar liquid crystals

Several series of new tetracatenar mesogens consisting of a five-ring aromatic core and bearing one or more apolar lateral substituents in the central benzene ring of the molecule have been synthesized and their mesomorphism is characterized by polarization microscopy, differential scanning calorimetry, and X-ray diffraction methods. Introduction of one, two, or four methyl groups into these tetracatenar mesogens destabilized columnar organization typical of the long-chain homologs, but did not suppress smectic C and nematic phases. Introduction of larger substituents did, however, suppress lamellar organization (C₁₅H₃₁) or mesomorphism (tert-butyl) totally.     

To be or not to be – nematic liquid crystals from shape-persistent V-shaped nematogens with the ‘magic angle’

The tetrahedral bending angle in V-shaped nematogens was claimed to be the optimum for finding a biaxial nematic liquid crystal phase. The benzo[1,2-b:4,3-b’]dithiophene core, recently successfully applied as a tetrahedral bending unit in mesogens with lateral flexible chains, is here embedded in a scaffold with only terminal chains, which conventionally promotes the formation of nematic phases at low temperature. A series of new mesogens has been successfully prepared, realising hockey-stick, hockey-stick dimer and V-shaped molecular topologies. Only the hockey-stick mesogens assemble in uniaxial nematic phases over a broad temperature range. Single crystal structure analysis of a hockey-stick and V-shaped compound reveal remarkable similarities with the benzodithiophene core wrapped by aliphatic chains. A model explaining the absence of nematic mesophases in the family of V-shaped, shape-persistent mesogens with terminal aliphatic chains is presented and results in the proposal of a new design for biaxial nematogens.     

Mesomorphic donor-acceptor twin molecules with covalently linked sheet-like pentaalkyne and nitrofluorenone sub-units

Charge-transfer (CT) twin mesogens are presented which consist of sheet-like pentayne donor moieties with different peripheral substituents chemically linked with nitro substituted fluorenone sub-units via flexible alkyl spacers. A novel plastic rectangular columnar (Colrp) phase is reported for the laterally unsubstituted member of the pentayne based CT twins. The phase is characterized by a three-dimensional crystal-like correlation of ordered columns in a rectangular lattice, while the molecular sub-units are still able to perform relaxation motions. The attachment of peripheral substituents to the pentayne moieties results in a distortion of the three-dimensional positional order and the five-fold alkyl substituted homologues exhibit a rectangular columnar ordered (Colro) mesophase. The rectangular lattice symmetries originate from the chemical linkage of the flat donor and acceptor parts of the molecules placed in an alternating manner within neighbouring columns, and the regular intracolumnar periodicity is enhanced by charge-transfer interactions within the columns. A further structural modification consists of the incorporation of an asymmetric carbon into the spacer sequence resulting in pentayne based CT-twin mesogens displaying a nematic columnar mesophase with a helical twisting of the columns (N*col). Dielectric investigations reveal an unusual dynamic behaviour of the donor-acceptor pentaynes. The glass relaxation process is characterized by the occurrence of two relaxation modes, both following a WLF behaviour, a feature which has not previously been reported to our knowledge for columnar phase-forming disc-like liquid crystal materials.     

Laterally linked mesogens in main chain polymers

The syntheses of liquid-crystalline main chain polyesters with laterally linked mesogens are presented. The terminal groups of the mesogens and the spacers fixation of mesogens, the polyesters with 1,4-di(p-alkoxybenzoyloxy)benzenes 1,4-di(p-alkoxybenzolyloxy)benzenes, 2,5-di(p-alkoxyphenyl)pyrimidines or a 4,4'-bis(p-ethoxyphenylazo)biphenyl derivative. Despite the unusual bilateral fixation of mesogens, the polyesters with 1,4-di(p-alkoxybenzoyloxy)benzenes show mainly monotropic phases which were found to be nematic in investigations with the polarizing microscope, in DSC measurements, temperature-dependent measurements of the Kerr constant and X-ray diffraction experiments. The monotropic nematic character remains when varying the terminal groups of the mesogen as well as the length of the n-alkanedioic acid spacer. However, when the spacer is branched, crystallization can be disturbed effectively and a stable nematic phase can be obtained. All of the polyesters with only 2,5-di(p-alkoxyphenyl)-pyrimidines as mesogens are amorphous. The reason probably lies in the unsymmetric 4,6-substitution of the pyrimidine ring. In contrast to this, oligomers with a 4,4'-bis(p-ethoxyphenylazo)biphenyl derivative show broad, predominantly enantiotropic nematic phases with relatively high clearing temperatures. With the knowledge gained about relations between structure and properties, especially of polyesters with 1,4-(p-alkoxybenzoyloxy)benzenes as mesogens, polyesters with this mesogenic group but with various architectures have been compared. Speculations about possible arrangements of the structural elements have been included to explain the phase behaviour of the polyesters with laterally fixed mesogens.     

刚柔相嵌液晶高分子的赝二级相变温度

讨论了刚柔相嵌液晶高分子的向列相一各向同性相转变与其分子结构的关系．给出了该一级相变的赝二级相变温度Ｔ＊与这类液晶高分子的液晶基元和间隔基的长度、柔顺性（相关长度）以及它们之间的相互作用的关系的解析表示式．分析了液晶基元与间隔基连接处的表观弯曲（接口效应）对Ｔ＊的影响．文中的结论与实验相符．     

Liquid‐Crystalline Mesogens Based on Cyclo[6]aramides: Distinctive Phase Transitions in Response to Macrocyclic Host–Guest Interactions

Producing macrocyclic mesogens that are responsive to guest encapsulation presents a significant challenge. Cyclo[6]aramides, a type of macrocycle with a hydrogen‐bond‐constrained backbone, exhibit thermotropic lamellar, discotic nematic, hexagonal, and rectangular columnar mesophases over a considerably wide temperature range, including at room temperature. Additionally, cyclo[6]aramides show unusual mesophase transitions from lamellar to hexagonal columnar phase mediated by macrocyclic host–guest (H–G) interactions between the macrocycles and alkylammonium salts. The phase transition, triggered by an organic guest engaging in H–G interactions with a macrocyclic cavity, provides a novel strategy for manipulating the properties of liquid‐crystalline materials. The crystal structure of a homologous cyclo[6]aramide reveals a disk‐shaped, near‐planar molecular backbone that facilitates intermolecular π–π stacking and leads to columnar assembly.     

刚柔相嵌液晶高分子的赝二级相变温度

讨论了刚柔相嵌液晶高分子的向列相－各向同性相转变与其分子结构的关系，给出了该一级相变的赝二级相变温度Ｔ与这类液晶高分子的液晶基元和间隔基的长度，柔顺性（相关长度）以及它们之间的相互作用的关系的解析表示式，分析了液晶基元与间隔基连接处的表观弯曲（接口效应）对Ｔ的影响，文中的结论与实验相符。     

侧基含纳米构筑单元的甲壳型液晶高分子的多层次自组装

甲壳型液晶高分子可以呈现超分子柱或片层的链构象,因此可以作为超分子液晶基元形成多种液晶相态,如六方柱状相、柱状向列相、六方柱状向列相、近晶相等.将纳米构筑单元,如一维的二联苯、二维的苯并菲、三维的多面体低聚倍半硅氧烷(POSS)等,引入到甲壳型液晶高分子中,所得聚合物可以自组装形成在亚十纳米和近纳米尺度的多级有序结构.这些结构具有尺寸可控及单分散的优点,可望在有机光电、纳米多孔膜以及纳米光刻等领域有着广阔的应用前景.本文主要介绍了将二联苯、偶氮苯、棒状多苯结构、苯并菲和POSS基元引入到甲壳型液晶高分子中制备多级组装结构的相关工作.     

Self-assembly of discotic mesogens in solution and in liquid crystalline phases: effects of substituent position and hydrogen bonding

The effects of functional group position on the phase behavior of discotic mesogens was examined for a series of dibenzophenazine derivatives bearing a carboxylic acid, methyl carboxylate, or nitro group. In all cases, changing the position of the group from the "top" to the "side" of the aromatic core led to dramatic differences in the phase behavior, both in terms of the stability of the liquid crystalline phases as well as the types of mesophases formed. For the non-hydrogen bonding ester and nitro derivatives, moving the substituent to the side of the core led to a lowering of the clearing temperatures or loss of liquid crystallinity. Carboxylic acid derivatives exhibit broad mesophases irrespective of the position of the acid group, but mesogens bearing this group on the side of the core exclusively form Col(h) phases, whereas those with an acid group on the top of the core exhibit more varied mesomorphism, with the formation of Col(h), Col(r), and nematic phases. Contrary to expectations, the presence of a carboxylic acid group on the side of the core does not appear to lead to the formation of dimeric structures in the liquid crystalline phase, although the columnar structures appear to be stabilized by intermolecular hydrogen bonding along the columns. These derivatives also form pi-stacked dimers in solution; the structure of these dimers are consistent with the proposed structure of the columnar phases.     

Star-shaped oligobenzoates: non-conventional mesogens forming columnar helical mesophases

Star-shaped mesogens with a phloroglucinol or a trimesic acid core and oligobenzoate arms with up to five repeating units have been synthesised. These non-conventional mesogens form various columnar mesophases over a broad temperature range. The liquid-crystal phases were characterised by optical microscopy, differential scanning calorimetry, X-ray diffraction, dilatometry and solid-state NMR spectroscopy. In addition to the high-temperature hexagonal columnar phases, the columnar self-assemblies undulate upon cooling and consequently form higher-ordered body-centred orthorhombic columnar 3D structures. A model of E-shaped folded conformers helically displaced along the columns is proposed. Helical preorganisation in the hexagonal phase precedes the transition to the low-temperature phases. Space filling and nano-segregation compete in the self-organisation process, thus aliphatic chains and the polar oligobenzoate scaffold are not perfectly separated in these star-shaped mesogens.     

Luminescence of nematic lanthanide-containing mixtures

The possibility of obtaining eutectic binary nematic liquid crystal systems containing lanthanide mesogens was studied. As lanthanide mesogens, we used lanthanide(III) tris-(β-diketonate) adducts exhibiting nematic mesomorphism. The luminescence of the compositions containing Eu(III) adducts was investigated.     

Full miscibility of disk- and rod-shaped mesogens in the nematic phase

Mesogens containing rod-shaped moieties as well as one disk-shaped group exhibit nematic phase behavior and are entirely miscible with rod- and disk-shaped mesogens in the nematic phase. In the phase diagram, an extra nematic phase appears at nearly equimolar disk-rod ratios, where the nematic biaxial phase has been predicted.     

Columnar versus smectic order in systems of charged colloidal rods

We study the stability of inhomogeneous liquid crystalline states in systems of monodisperse, stiff, charged rods. By means of a bifurcation analysis applied to the Onsager free energy for charged rods in strongly nematic states, we investigate nematic-smectic and nematic-columnar instabilities as a function of the Debye screening length kappa(-1). While the nematic-smectic transition clearly preempts the nematic-columnar one in the regime of strong screening (i.e., small kappa(-1)) a marked stability of hexagonal columnar order is observed at larger screening lengths. The theoretical results are substantiated by Brownian dynamics computer simulation results based on the Yukawa site model. Our findings connect to experiments on tobacco mosaic virus rods, in particular, but might be relevant for soft rodlike mesogens in strong external directional fields in general.     

Nematic tribenzosilatranes

The linkage of benzylsilatrane groups to laterally functionalized mesogens allows the investigation of molecular systems which contain propeller shaped mesogens and exhibit nematic phase behaviour.