Unsymmetrical trimesogens exhibiting the undulated twist grain boundary (UTGBC*) mesophase

A new set of trimesogens consisting of a cholesteryl ester, a biphenyl and an azobenzene mesogenic segment joined together through either odd-even, even-even or odd-odd central alkylene spacers has been synthesized and the liquid crystalline properties investigated. The trimesogens having even-even and odd-odd central alkylene spacers exhibit the undulated twist grain boundary phase (UTGBC*) mesophase over a temperature range of ~40-50°C.     

Organometallic chiral liquid crystals: bis4-[ω-(cholest-5-en-3-yloxycarbonyl)alkoxy]phenylethynylmercury complexes

We report the synthesis and thermal behaviour of the first chiral organometallic trimesogens having bis[(4-phenyl)ethynyl]mercury as the central core connected on either side to cholesteryl ester moieties through either even-even (C₄) or odd-odd (C₃, C₅ and C₇) central alkylene spacers. All the organomercury trimesogens exhibit a chiral nematic phase. These mercury complexes are extremely sensitive to heat, in a manner reminiscent of other reports.     

Organometallic chiral liquid crystals: bis{4-[ω-(cholest-5-en-3-yloxycarbonyl)alkoxy]phenylethynyl}mercury complexes

We report the synthesis and thermal behaviour of the first chiral organometallic trimesogens having bis[(4-phenyl)ethynyl]mercury as the central core connected on either side to cholesteryl ester moieties through either even-even (C₄) or odd-odd (C₃, C₅ and C₇) central alkylene spacers. All the organomercury trimesogens exhibit a chiral nematic phase. These mercury complexes are extremely sensitive to heat, in a manner reminiscent of other reports.     

New side chain liquid crystalline polymers: synthesis and thermal properties of side chain polyacrylates having segmented spacers

In order to acquire high mobility of a mesogenic group, we propose a new type of side chain liquid crystal polymer having segmented spacers, consisting of binary moieties of an oligo(ethylene oxide) segment and analkylene segment. Six kinds of polyacrylates having segmented spacers consisting of different lengths of an oligo(ethylene oxide) and an alkylene, and having 4'-cyanobiphenyl as a mesogenic group have been synthesized, and their thermal properties investigated. Some of these polyacrylates exhibit a mesophase and show an extremely low glass transition temperature compared with LC polyacrylates having ordinary alkylene spacers.     

New side chain liquid crystalline polymers: synthesis and thermal properties of side chain polyacrylates having segmented spacers

In order to acquire high mobility of a mesogenic group, we propose a new type of side chain liquid crystal polymer having segmented spacers, consisting of binary moieties of an oligo(ethylene oxide) segment and analkylene segment. Six kinds of polyacrylates having segmented spacers consisting of different lengths of an oligo(ethylene oxide) and an alkylene, and having 4'-cyanobiphenyl as a mesogenic group have been synthesized, and their thermal properties investigated. Some of these polyacrylates exhibit a mesophase and show an extremely low glass transition temperature compared with LC polyacrylates having ordinary alkylene spacers.     

Thermotropic liquid‐crystalline polymers having five‐membered heterocycles as mesogens, 3. New semi‐rigid thermotropic liquid‐crystalline polyesters based on a twin biphenyl analogue of 1,3,4‐thiadiazole

New semi‐rigid thermotropic liquid crystalline (LC) polyesters containing a twin biphenyl analogue of 1,3,4‐thiadiazole in the main chain were prepared by melt polycondensation of the bismethyl ester derivative of twin 2‐phenyl‐1,3,4‐thiadiazole having a decamethylene segment in the central part with three aliphatic diols. The polymer with an octamethylene segment forms a monotropic nematic phase and those with decamethylene and dodecamethylene segments form enantiotropic smectic phases, although their LC states are unstable. The melting and isotropization temperatures decrease with increasing length of alkylene spacers.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

The influence of alkylene spacers on conformational and thermal properties of poly (aryl methacrylates)

The effect of introducing alkylene spacers on the thermal and conformational properties of poly(aryl methacrylates) has been studied. Poly(2-phenylethyl-1-methacrylate) (PPEMA) and poly(4-phenylbutyl-1-methacrylate) (PPBMA) samples of narrow poly-dispersities and covering a very broad range of molecular weights were produced by free radical solution polymerization, followed by fractionation. These materials were studied by a combination of differential scanning calorimetry, light scattering, and viscometry measurements to allow evaluation of their glass transition temperatures (T_g) and characteristic ratios (C_∞). These results are compared with literature data available for poly(phenyl methacrylate) (PPMA), which has no alkylene spacer, and poly(benzylmethacrylate) (PBMA), which has a methylene spacer. A progressive decrease in T_g is observed as the length of the alkylene spacer is increased, reflecting the enhancement of side group (local) flexibility. C_∞ decreases substantially on going from PPMA to PBMA but then increases on going to PPEMA and PPBMA. This behavior is attributed to the fact that while side group flexibility increases for the materials with longer alkylene spacers, and this would be expected to decrease C_∞, the side group size is progressively increasing. Larger side groups generally increase C_∞ of polymethacrylates. Thus the observed C_∞ behavior for these polymers reflects both the size and flexibility of the substituents. © 1994 John Wiley & Sons, Inc.     

Cholesterol-based dimeric liquid crystals: synthesis and mesomorphic behaviour

Chiral non-symmetric dimeric liquid crystals consisting of a cholesteryl ester moiety as chiral entity and a biphenyl aromatic core, interconnected through n-butyl (C₄) or n-pentyl (C₅) parity alkylene spacers, have been synthesized and investigated for their liquid crystalline properties. All the dimers exhibit enantiotropic mesophases. The first member of the dimers having the C₄ central spacer exhibit only the chiral nematic (N*) mesophase, while the higher homologues also show smectic A (SmA) and twist grain boundary (TGB) mesophases. The dimers of the other series containing the C₅ central spacer also have stable SmA, TGB and N* mesophases, except for the first which does not show the TGB phase. Both series of compounds show a weak odd-even effect with terminal alkyl chain substitution, while the spacer length has a marked influence on the phase transition temperatures.     

Hydrogel-Stiffening and Non-Cell Adhesive Properties of Amphiphilic Peptides with Central Alkylene Chains

Amphiphilic peptides bearing terminal alkyl tails form supramolecular nanofibers that are increasingly used as biomaterials with multiple functionalities. Insertion of alkylene chains in peptides can be designed as another type of amphiphilic peptide, yet the influence of the internal alkylene chains on self-assembly and biological properties remains poorly defined. Unlike the terminal alkyl tails, the internal alkylene chains can affect not only the hydrophobicity but also the flexibility and packing of the peptides. Herein, we demonstrate the supramolecular and biological effects of the central alkylene chain length inserted in a peptide. Insertion of the alkylene chain at the center of the peptide allowed for strengthened β-sheet hydrogen bonds and modulation of the packing order, and consequently the amphiphilic peptide bearing C2 alkylene chain formed a hydrogel with the highest stiffness. Interestingly, the amphiphilic peptides bearing internal alkylene chains longer than C2 showed a diminished cell-adhesive property. This study offers a novel molecular design to tune mechanical and biological properties of peptide materials.     

Solid-state CP/MAS 13C-NMR studies of naphthalene-based thermotropic polyesters and model compounds

Thermotropic aromatic polyesters based on 2,6-naphthalenedicarboxylic acid and 4,4′-dihydroxy-1,6-diphenoxyhexane 1a and -decane 1b have been synthesized by solution polymerization. The solid-state structures of these polyesters have been examined by high-resolution solid-state CP/MAS (cross polarization/magic angle spinning) and solution ¹³C-NMR. For precipitated original samples, alkylene spacers were generally in the all-trans form in the solid state. For once-melted samples, torsional gauche conformations were introduced to the spacers. The mesophase of the polyesters was identified as nematic. The temperature ranges of the nematic state of 1a and 1b were much wider than those of analogous polymers 2a and 2b based on terephthalic acid. For these polyesters, the substitution of the 2,6-naphthalene ring for the benzene ring induced no appreciable change in the conformation of the diphenoxy alkylene units in the solid state and on the melting points. Thermotropic ester model compounds, i.e., bis(4-butoxyphenyl) 2,6-naphthalate 3a and bis(4-butylphenyl) 2,6-naphthalate 3b have been prepared and characterized by both solid-state and solution NMR, which helped the interpretation of the solid-state structures of the polyesters. These spectra were compared with those of terephthalate-based related compounds 4a and 4b . The solid-state spectra suggest that the butoxyphenyl group of 3a and the butylphenyl group of 3b formed almost the same conformations as those of 4a and 4b , respectively.     

Liquid-crystalline polyesters based on branched propylene spacers

A new promising concept of spacer modification in main chain liquid crystalline polymers is the use of branched alkylene spacers. In this article we give as a first example the polyesters of p, p′-terphenyl-4,4′-dicarboxylic acid as mesogenic unit and disubstituted propylene spacers. The steric restrictions of the spacers lower the melting points compared with the polyesters with linear spacer but they do not flexibilize the main chains as linear alkylspacers do. This leads to a residual stability range of the liquid crystalline state in the homologues of polyesters with disubstituted spacer. The melting and clearing points are lowered to the same extent. Because of the solubility of the higher homologues in common solvents it is possible for the first time to determine molecular masses of this polymer class based on p, p′-terphenyl-4,4′-dicarboxylic acid and aliphatic diols more exactly. X-ray scattering indicates that long side chains are arranged parallel to the main chain and have no longer influence on transition temperatures and intermolecular distances. © 1996 John Wiley & Sons, Inc.     

A new synthetic approach to side-chain liquid crystalline polyacrylates containing siloxane spacers and their thermal behaviours

The syntheses of novel side-chain liquid crystalline polyacrylates (LCPAs) containing siloxane spacers were carried out according to a convenient route by preparing alcohol compounds having a mesogenic group linked with a siloxane bond as key intermediates of the monomers. A relatively large mesogenic group was necessary to exhibit a mesophase in this system. The introduction of a siloxane spacer lowered the temperature range of mesophase of LCPAs resulting from the decrease of T , as compared with the corresponding LCPAs containing an alkylene spacer. It was also revealed that the chemical structure of spacer component played an important role for the assembly structure in the mesophase. g     

Liquid crystalline dimers with bent-core mesogenic units

Two series of dimers, in which two identical aromatic bent core mesogenic units are connected via an organosiloxane or an alkylene spacer, have been investigated. The dimers with a spacer consisting of a trisiloxane central group and relatively long alkylene groups show an intercalated tilted smectic structure. The layer spacings appear to be very weakly dependent on the terminal chain lengths. In these dimers the smectic phase is stabilized for the compounds with a short terminal chain. Dimers with an aliphatic alkylene spacer are liquid crystalline only when the spacer is relatively short and the terminal chains are long. For these dimers a monolayer tilted smectic phase is observed.     

Influence of chemical structure on the optical properties of new side-chain polymers containing cholesterol

Poly[1-(cholesteryloxyhexyloxy)ethylene] (PHET) and poly[1-(cholesteryloxycarbonyl-hexyloxy)ethylene] (PHES) were prepared by reacting poly(vinyl alcohol) with cholesteryloxyhexyloxy bromides (CHB) or cholesteryloxycarbonylhexyloxy bromides (CEHB), and their thermal and optical properties were investigated. PHET and PHES exhibited monotropic cholesteric phases; however, their thermal behaviours depended on the cholesteryl groups and alkylene spacers with different chemical structures. PHET did not display reflection colours over its entire cholesteric range, whereas PHES did display reflection colours. These results suggested that the thermal stability and helical twisting power (HTP) of these polymers strongly depend on the difference in the chemical structures of the flexible spacer via cholesterol. The mesophase properties of PHET and PHES differed substantially from those of poly(cholesteryl-ω-acryloyloxyalkanoates). The results indicate that the mode of chemical linkage between the side-chain group and the main chain as well as that between the alkylene spacer and side chain play important roles in determining the thermal stability, mesophorphic structure and HTP of the cholesteric mesophases.     

A phase polymorphism N?SAd?I in liquid-crystalline polymethacrylates with 4′-trifluoromethoxyazobenzene mesogenic side groups

The mesophase behaviour of liquid-crystalline polymethacrylates with 4′-trifluoromethoxyazobenzene mesogens and alkylene spacers $ \left( {\rlap{--} ({\rm CH}_{\rm 2} \rlap{--} )_n ,n = 2 - 6} \right) $ in the side chains was investigated and compared with that of the corresponding non-fluorinated polymers. The fluorinated polymers with spacer lengths n = 5 and 6 are the first side-group liquid-crystalline polymethacrylates showing a nematic phase below a smectic A phase.     

Liquid crystalline properties and dependence of transition temperatures on the length of the flexible alkylene spacer of symmetric dimers composed of bent-core units

Three new series of symmetric dimers composed of bent-core molecules connected by a flexible alkylene spacer have been synthesized and their mesomorphic properties studied. The effect of varying the length of the terminal chains for fixed odd and even spacers, as well as varying the spacer length for a fixed terminal chain length, on the mesomorphic properties has been investigated. Remarkably, all the dimers investigated show the same type of mesophase, which has been characterized as a Col_obP_F phase, on the basis of X-ray diffraction and electro-optical measurements. It is found that for a fixed spacer length, the thermal range of the mesophase increases with increase in the number of methylene units in the terminal chain. On the other hand, interestingly an odd-even effect is observed when the length of the flexible spacer is varied; dimers having an odd number of methylene units possess higher clearing points.     

A deuteron NMR study of axial motion and side chain conformation in the mesophase of discotic liquid crystal main-chain polymers

Pulsed deuteron NMR spectroscopy has been used to examine the axial motion and the side-chain conformation in both oriented and unoriented mesophases of discotic liquid crystal main-chain polymers based on pentyloxy or heptyloxy substituted triphenylenes. Lineshape simulations show that the rotational motion of the triphenylene rings about the column axes can be described by an inhomogeneous distribution of reorientation angles around 45°. However, only about 60% of the discs are involved in such large amplitude motions; the remaining 40% have reorientation angles below 10°. This illustrates the severe restrictions imposed on the rotation of the discs by the interlinkage of the columns via the alkylene spacers. Furthermore, the simulations demonstrate that, at the -carbon, the side chains show very little fast internal motion, but have a relatively complex conformation involving a disorder which does not change on the microsecond timescale. Since such a disorder is not present in the corresponding monomeric samples it is ascribed to the presence of the spacers. These results are also consistent with the presence of large sterical hindrances between the first side chains segments of adjacent discs, and they indicate a correlated reorientation of the discotic units within a column.     

Salicylaldimine-based symmetric dimers: synthesis and thermal behaviour

Several symmetrical dimers comprised of salicylaldimine-moieties connected through ester linkages to the termini of odd-parity alkanediols have been synthesized and investigated for their thermal behaviour. In order to understand the structure-property relations, the lengths of the central alkylene spacers (C₃ and C₅) as well as those of the terminal alkoxy chains (C₆ to C₂₂) have been varied. The dimers with a C₃-alkylene spacer are non-liquid crystalline, while some of the compounds having a C₅-alkylene spacer exhibit liquid crystalline properties. The dimer, with a C₅-alkylene spacer and C₆-alkoxy tails, shows an intercalated smectic C (SmC_c) phase, whereas the C₈, C₁₀, C₁₁, and C₁₂ homologues are non-mesomorphic. The higher homologues of this series with C₁₆, C₁₈, C₂₀ and C₂₂ alkoxy tails show a mesophase that has the signatures of a two-dimensional banana (B₁) phase. This mesophase is enantiotropic in the C₁₆ and C₁₈ homologues while it is monotropic in the other homologues. In these dimers, the spacer length has a remarkable influence on the thermal behaviour.     

Selective extraction of higher fullerenes using cyclic dimers of zinc porphyrins

Higher fullerenes (>/=C76) were selectively extracted from a fullerene mixture obtained from a combustion-based industrial production source by cyclic dimers of beta-unsubstituted porphyrin zinc complexes 2C5-2C7 with C5-C7 alkylene spacers as host molecules. Results of single extraction of the fullerene mixture with 2C5-2C7 together with a beta-substituted analogue of 2C6 (1C6) and spectroscopic titration of 2C6 and 1C6 with C60, C70, and C96 indicated that the host selectivity toward higher fullerenes is much dependent on the structure of the porphyrin units and the size of the host cavity. Sequential three-stage extraction of the fullerene mixture with the best-behaved 2C6 resulted in considerable enrichment in very rare fullerenes C102-C110 (<0.1 abs %) up to 82 abs % (C76-C114, 99 abs %) (356 nm) of total fullerenes.