Carbohydrate rod conjugates: ternary rod-coil molecules forming complex liquid crystal structures

T-shaped polyphilic triblock molecules, consisting of a rodlike p-terphenyl unit, a hydrophilic and flexible laterally attached oligo(oxyethylene) chain terminated by an 1-acylamino-1-deoxy-D-sorbitol unit, and two end-attached lipophilic alkyl chains, have been synthesized by palladium-catalyzed cross-coupling reactions as the key steps. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light microscopy, differential scanning calorimetry (DSC), and X-ray scattering. We investigated the mode of self-organization as a function of the length and position of the lateral polar chain and the length of the terminal alkyl chains. Depending on the size of the polar and lipophilic segments, a series of unusual liquid crystalline phases was detected. In three of these phases, the space is divided into three distinct periodic subspaces. In addition to a hexagonal channeled layer phase (ChL(hex)) consisting of layers that are penetrated by polar columns, there are also two honeycomb-like network structures formed by square (Col(squ)/p4mm) or pentagonal cylinders (Col(squ)/p4gm). The cylinder walls consist of the terphenyl units fused by columns of alkyl chains, and the interior contains the polar side chains. In addition, a hexagonal columnar phase was observed in which the polar columns are organized in a continuum of terphenyls and alkyl chains with an organization of the terphenyl cores tangentially around the columns with the long axis perpendicular to the columns. For one compound, a reversal of birefringence was observed, which is explained by a reorientation of the terphenyl cores. The addition of protic solvents induces lamellar phases.     

Calamitic bolaamphiphiles with (semi)perfluorinated lateral chains: polyphilic block molecules with new liquid crystalline phase structures

Novel bolaamphiphiles consisting of a rigid biphenyl unit, two terminal polar 1,2-diol units and laterally attached (semi)perfluorinated chains have been synthesized via palladium-catalyzed cross coupling reactions as the key step. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light optical microscopy, DSC, and X-ray scattering, and the influences of the length, number, structure, and position of the lateral chain on the mesomorphic properties were studied. A wide variety of unique liquid crystalline phases were found upon elongation of the lateral semiperfluorinated chains. For short- and medium-chain length a series of columnar phases were observed, and upon further elongation of the lateral chain a series of novel mesophases with layer structures were found. In the columnar phases, the nonpolar lateral chains segregate into columns, which are embedded in honeycomb-like networks of cylinders consisting of the biphenyl units. Strings of hydrogen-bonding networks of the diol groups provide cohesive forces, which maintain the overall structure. Changing the length of the lateral chains influences the diameter of the columns and thus determines the number of biphenyl units which are required to surround these columns. The number of these units [four (c2mm, p4mm), five (p2gg), six (p6mm), eight (c2mm) or 10 (p2gg)] defines the shape of the cylinders as well as the lattice type of the columnar phase. It is proposed that the columnar phases with a p2gg lattice result from the regular organization of pairs of cylinders which have a pentagonal cross sectional shape. In the mesophases with layer structure the aromatic rodlike cores are arranged parallel to the layer planes, and the onset of orientational and positional ordering of the biphenyl segments leads to a sequence of subtypes for these lamellar phases (Lam(Iso)-Lam(N)-Lam(X)).     

Distinct columnar and lamellar liquid crystalline phases formed by new bolaamphiphiles with linear and branched lateral hydrocarbon chains

A universal building block for the convergent synthesis of a wide variety of different T-shaped ternary amphiphiles was developed and used for the synthesis of a series of new liquid-crystalline materials composed of a rigid biphenyl core with polar glycerol groups at both ends and linear or branched alkyl chains in a lateral position. In addition, compounds with bulky achiral (2,4,6-trimethylphenoxy, adamantane-1-carboxylate, benzoate) or chiral (menthyl or cholesteryl) substituents attached to the end of the lateral alkyl chain were also investigated. In all cases the lateral chains were connected to the aromatic core by an ether linkage. The effect of the ether linking unit on mesophase stability and mesophase type is discussed with respect to conformational effects. The liquid-crystalline phases were investigated by polarizing microscopy, calorimetry, and X-ray diffraction of surface aligned samples. Upon enlarging the lateral chains a series of different polygonal cylinder phases was observed, which were replaced by lamellar phases and a non-cylinder hexagonal columnar phase by further increasing the size of these substituents. Remarkably, only pentagonal, hexagonal, and giant hexagonal cylinder phases could be observed, whereas mesophases composed of cylinders with a smaller number of sides are missing. No distinct chirality effects were observed for the menthyl- and cholesteryl-substituted compounds. However, the rodlike shape of the polycyclic cholesteryl core leads to a unique phase structure combining an organization of the alicyclic cholesteryl cores perpendicular to the layer planes and the aromatic biphenyl cores parallel to the layer planes.     

The trapezoidal cylinder phase: a new mode of self-assembly in liquid-crystalline soft matter

A new rectangular columnar liquid crystalline phase with p2gg lattice is reported, which represents a polygonal cylinder array composed of cylinders with trapezoidal cross section. In these polygonal cylinders, one of the sides has a different length and is composed of a different material than the others. This tiling pattern was obtained in two series of T-shaped facial amphiphilic triblock molecules in which a rigid rod-like p-terphenyl core is substituted laterally by a polar and flexible oligoethylene glycol chain, terminated either by a hydrogen-bonding COOH group or by a Li carboxylate group, and having identical or different alkyl groups in the terminal positions. The trapezoidal cylinder phase provides an improved packing for relatively long and rigid alkyl chains at lower temperature and more space inside the polygonal cylinders than triangular cylinders. This combination of conformational and space-filling effects leads to different phase sequences. The trapezoidal cylinder phases pave the way to a new level of complexity in LC engineering and show the huge potential of the general concept of polyphilic tectons for the design of new complex soft matter structures.     

Liquid crystalline bolaamphiphiles with semiperfluorinated lateral chains: competition between layerlike and honeycomb-like organization

Novel bolaamphiphilic triblockmolecules consisting of a rigid biphenyl unit, with a polar 2,3-dihydroxypropyloxy group and a phenolic OH group at opposite ends, as well as a semiperfluorinated chain in a lateral position have been synthesized via palladium catalyzed cross coupling reactions as the key steps. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light microscopy, DSC and X-ray scattering, and the influence of the length of the lateral chain on the mesomorphic properties was studied. The compound with the shortest chain as well as the long chain derivatives form lamellar mesophases composed of segregated layers of the bolaamphiphilic moieties and sublayers comprising the fluid lateral chains. The layers within the lamellar phases of the short chain compound adopt a positional correlation, leading to a 2D lattice (Col(r)/p2mm), whereas the layers of the lamellar phases of the long chain derivatives are noncorrelated (Lam). Compounds with a medium chain length organize into columnar phases, where the nonpolar lateral chains segregate into columns, which are embedded in networks of regular (Col(h)) or stretched (Col(r)/c2mm) hexagonal cylinder shells consisting of the bolaamphiphilic units. In total, an unusual phase sequence was found, where, with respect to the chain length, columnar mesophases occur between two mesophases with layer organization.     

Formation of columnar mesophases by calamitic molecules: a modulated SmA phase in mixtures of amphiphilic and bolaamphiphilic biphenyl derivatives

Binary mixtures of bolaamphiphilic biphenyl derivatives with each other and with amphiphilic biphenyl derivatives were investigated by means of optical microscopy. The miscibility of the bolaamphiphiles is very sensitive to the molecular length of the components. The SmA phases of bolaamphiphiles with the same length are completely miscible. If the length difference between the two components of the binary system increases, a miscibility gap occurs. Due to their different phase structures (bilayer versus monolayer) no miscibility in the SmA phases was found for amphiphilic and bolaamphiphilic compounds with comparable molecular lengths. However, in some cases a novel mesophase was induced in the contact region. This mesophase was investigated by X-ray diffraction. It represents a two-dimensionally modulated (columnar) phase with a rectangular lattice (Col r), but the local order is similar to that of disordered smectics. Its formation is explained in terms of ribbon structures resulting from the collapse of smectic bilayers, in strong analogy to the antiphases (SmA) of terminally polar calamitic mesogens.     

Interfacial assembly of a series of cinnamoyl-containing bolaamphiphiles: spacer-controlled packing, photochemistry, and odd-even effect

A series of bolaamphiphiles with 4-hydroxycinnamoyl head groups and different length of the alkyl spacers (n = 6-12) were designed to investigate their photochemistry in the organized films obtained from the air/water interface. It has been found that both the length and odd-even number of the spacers can finely tune the molecular packing as well as the photochemistry. When the spacer length was changed from 6 to 12 methylene units, the assemblies changed from J aggregate to H aggregate. The molecules with even-numbered polymethylene spacer tend to form three-dimensional nanorod structure at the air/water interface. For the assembly of derivatives with odd-numbered spacers, diverse morphologies such as nanospirals and nanofibers were observed depending on the chain length and the surface pressures. The different packing of bolaamphiphiles could subsequently affect the photochemistry of the cinnamoyl groups in the organized films. The spacer effect in the assembly can be understood from the cooperation between H-bond of the phenolic hydroxyl and the amide groups, π-π stacking as well as the hydrophobic interactions of the alkyl spacer. A packing model was proposed to explain the phenomenon.     

Evaluation of smectic biphenylcarboxylate ester liquid-crystalline polysiloxane stationary phases for capillary column gas chromatography

Mesomorphic biphenylcarboxylate esters were coupled via flexible aliphatic hydrocarbon spacers to a polysiloxane backbone. The influence of spacer length, percent mesomorphic substitution, and crosslinking of the stationary phase on liquid-crystalline transition temperatures and on chromatographic performance was investigated. Unique selectivity and good efficiency over a wide temperature range for gum and cross-linked liquid-crystalline phases were demonstrated by the separation of various isomeric polycyclic aromatic compounds.     

Amphiphilic taper-shaped oligomer exhibiting a monolayer smectic A to columnar phase transition

We prepared two series of liquid-crystalline oligomers composed of phenylpyrimidine based mesogenic cores, alkyl spacers and semiperfluorinated alkyl chains in one series and only alkyl chains in the second series. Their physical properties were investigated using optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. The oligomers possessing a semiperfluorinated alkyl chain show a phase sequence of SmC–SmA–Col when the number of phenylpyrimidine moieties was increased. The compound possessing three phenylpyrimidine cores was found to exhibit monolayer SmA and Col phases. On the other hand, the oligomer composed of an alkyl chain and three phenylpyrimidine cores showed only a nematic phase. We discuss effects of the shape amphiphilicity and the hydrocarbon–fluorocarbon amphiphilicity on phase transition behaviour of the amphiphilic oligomers.     

λ-Shaped liquid crystal trimers with dual terminal cholesteryl moieties: synthesis and concomitant of N*, SmA and cholesteric glassy phases

Two homologous series of λ-shaped chiral liquid crystal trimers composed of a laterally substituted benzylidene-aniline as the central core armed by two cholestryl ester moieties via odd-even alkyl spacer are synthesised. All the compounds are mesogenic exhibiting both chiral nematic (N*) and SmA phases except for trimers bearing long spacers (n = 9–10). A pronounced odd-even effect is observed on the phase transition temperatures and clearing enthalpies when the spacer length is varied in which the even-parity members show higher value. The widening of N* phase upon elongation of the alkyl spacer can be interpreted as a result of the destabilisation of SmA phase. Full recrystallisation from the cholesteric phase upon cooling is not observed for all the compounds bearing long spacer, especially oxydecanoyl spacer. Instead, the anisotropic fluid vitrified to form the cholesteric glassy state characteristic of polymer at low temperature. This finding is also evident from the polarising optical microscope whereby non-crystalline texture which resembles the Grandjean texture with various reflection colours is observed upon cooling to low temperature. The effect of the lateral substituents on the liquid-crystalline behaviour is also discussed.     

Odd-even effects in the thermotropic and optical properties of chiral triplet liquid crystals

A series of chiral triplet liquid crystalline compounds with a central biphenyl group to which two dihydrocholesteryl groups are attached via two flexible alkanoate spacers has been prepared and investigated. As a function of spacer length, these compounds show strong odd-even effects for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phases. Asymmetrical compounds with one odd and one even number of methylene groups in their spacers have properties intermediate between those of symmetrical compounds with two odd or two even spacers.     

Synthesis and characterization of new ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers

Two series of ferroelectric liquid crystals containing a phenyl biphenyl carboxylate mesogenic group and oligooxyethylene spacers were synthesized. The mesomorphic behaviours of the compounds were characterized using differential scanning calorimetry (DSC), optical polarizing microscopy (POM) and powder X-ray diffraction measurements. These series all contain a smectic A (S_A), a chiral smectic C (S_C) and a crystal E (C_r E) phase sequence. The ordered smectic Cr E phase was observed for the short spacer chain (n = 0, 1, 2) homologues of the two series. Moreover, the mesomorphic properties are discussed as a function of the oligooxyethylene spacer length, and the position of the aromatic core rings. These results indicate that the structures with a rigid part (biphenyl group) closer to the flexible spacer chain (oxyethylene unit) tend to form the symmetrical smectic S_A and Cr E phases.     

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.     

Synthesis and self-assembly behavior of 2,5-diphenylethynyl thiophene based bolaamphiphiles

Synthesized 2,5-diphenylethynyl thiophene based bent bolaamphiphiles can self-organize into square honeycomb LC phases with p4mm symmetries in the presence of water. UV and PL measurements indicate fl uorescent properties making them potential candidates for application in fl uorescence sensor devices     

The Effect of Molecular Weight on the Thermal Properties of Polystyrene-Based Sidechain Liquid-Crystalline Polymers

Abstract

Sidechain liquid-crystalline polymers were prepared by the derivatization of three poly(4-hydroxystyrene) fractions of different molecular weights (M_w = 1.0 × 10⁴, 2.2 × 10⁴ and 3.0 × 10⁴). 4-Cyanoazobenzene and 4-cyanobiphenyl were incorporated as mesogenic groups with ether-linked methylene spacers of varying length. The polymers all exhibited a smectic A phase, with the exception of the propyl member of the cyanobiphenyl series for which no liquid-crystalline behavior was observed. For short spacers the thermal properties were insensitive to molecular weight changes in the backbone, whereas small but consistent differences in the transition temperatures and entropies were observed as the number of methylene groups in the spacer increased.     

Investigations of comblike polysiloxanes. I. The influence of spacer length on dielectric relaxation studies of aligned samples

Dielectric relaxation studies on aligned liquid-crystalline comblike polysiloxanes are presented. The polymers differ only in the length of the flexible spacer separating the mesogenic side groups from the polysiloxane backbone. The characteristic features of the observed relaxation process as a function of frequency and temperature, in both the liquid-crystalline and isotropic phases, are described, and the effect of the spacer length is discussed. Within the liquid-crystalline phase a narrow single loss process is observed, due to the relaxation of the mesogenic side groups around the polymer backbone, whereas in the isotropic phase a much broader loss curve is seen. The transition from an aligned polymer mesophase to the randomly aligned isotropic phase is also studied in detial, and comparison with earlier work on comblike polyacrylate and methacrylate liquid crystals is presented.     

d-xylose-based bolaamphiphiles: Synthesis and influence of the spacer nature on their interfacial and membrane properties

A two-step synthesis, with good yields, of d-xylose-based bolaamphiphiles is described. The monolayer properties, the adsorption behavior and membrane destabilization properties of two bolaamphiphiles differing by their spacers (presence or absence of one double bond) were studied. The presence of one unsaturation has no influence on the interfacial organization at low compression but impairs the stability of the monolayer at high compression. Saturated and unsaturated molecules are suggested to adopt a loop structure at the interface at low compression. The higher degree of freedom of the saturated hydrophobic spacer does not affect the initial diffusion step of the bolaform from the subphase to the interface but greatly slows the arrangement step at the interface. However, once at the interface, their surface-active properties are similar. The higher flexibility of the saturated analogue spacer also greatly increases its lipid vesicle destabilizing property. Its rearrangement within the lipid bilayer is in favour of the formation of inverted phases, facilitating membrane fusion.     

Synthesis and ring-opening polymerization of optically pure mesogenic malolactonates

Optically pure malolactonate monomers containing biphenyl mesogenic groups with either an ethylene or a hexamethylene spacer were prepared from optically pure malic acid and polymerized with alkylaluminoxane catalysts to form a series of new chiral side chain liquid-crystalline polymers, which contained the chiral centres in the backbone. The mesogenic malolactonate monomers were determined to be optically pure by ¹H NMR spectroscopy of the β-lactone complexed with a chiral europium shift reagent. Both the methylaluminoxane and isobutylaluminoxane catalysts gave polymers having bimodal molecular weight distributions, the latter catalyst yielded a larger amount of the higher molecular weight fraction than the former. The polymers showed high optical rotations, high degrees of isotactic stereoregularity, and enantiotropic liquid-crystalline properties, all of which were influenced by the molecular weight distribution. Copolymers of malolactonate monomers with different spacers were also prepared and characterized.     

The first examples of monodispersive liquid crystalline tetramers possessing four non-identical anisometric segments

The molecular design, synthesis and thermal behaviour of the first liquid crystalline tetramers composed of four non-identical mesogenic entities connected covalently through paraffinic spacers are presented. These new molecular structures consisting of a tolan (half disc-shaped), azobenzene (photoactive), biphenyl and cholesteryl ester (thermochromic) cores, connected through either an even-even-odd or an odd-even-odd paraffinic spacer, exhibit features of a columnar mesophase.     

LC-POLYIMIDES. 37. POLY(ESTER-IMIDE)S DERIVED FROM N,N′-BIS(4-CARBOXY-PHENYL)BIPHENYL-3,3′-4,4′-TETRA-CARBOXYLIC IMIDE AND VARIOUS α,ω-DIHYDROXYALKANES

A new imide monomer was prepared from ethyl-4-amino-benzoate and biphenyl-3,3′-4,4′-tetracarboxylic anhydride (BTCA). This new momoner was polycondensed with various α,ω-dihydroxyalkanes. The resulting poly(ester-imide)s, PEIs, were characterized by elemental analyses, inherent viscosities DSC-measurements optical microscopy, and X-ray measurements using synchrotron radiation. All PEIs formed a crystalline smectic H-phase in the solid state. Depending on the length of the spacer individual members of this series formed either an enantiotropic smectic-A phase, a monotropic smectic-A phase or no liquid crystalline phase at all. Long spacers and odd numbers of CH₂-groups favored the formation of liquid-crystalline (LC-) phases.