Smectic liquid crystals from supramolecular guanidinium alkanesulfonates

The thermotropic polymorphism of a series of guanidinium alkanesulfonates (6 < or = n < or = 18) was investigated by optical microscopy and differential scanning calorimetry. Hydrogen bonding was analyzed by infrared spectroscopy. Molecular volumes were measured by dilatometry. The structure of the crystal, smectic A, and ordered smectic phases observed were studied by X-ray diffraction and utilized to prove that the supramolecular arrangement of the molecules in the crystal survives in the smectic phases at high temperature.     

Smectic A liquid crystals from dihydrazide derivatives with lateral intermolecular hydrogen bonding

Six dissymmetrical dihydrazide derivatives, N-(4-alkoxybenzoyl)-N′-(4′-nitrobenzoyl) hydrazine (Cn-NO₂) and N-(4-alkoxybenzoyl)-N′-(4′-biphenyl carbonyl) hydrazine (Cn-Ph), were synthesized and investigated by means of differential scanning calorimetry, polarized optical microscopy and wide angle X-ray diffraction. The compounds exhibit smectic A₁ phase. Based on the results of ¹H NMR and variable temperature FT-IR spectroscopy, lateral intermolecular hydrogen bonding between -CO and -N-H groups was proposed and the effect of hydrogen bonding on the phase transitions was discussed. It was concluded that the combination of lateral intermolecular hydrogen bonding and microphase segregation stabilized the smectic A phase.     

Designer ionic liquid crystals based on congruently shaped guanidinium sulfonates

Ionic liquid crystals are mesogenic compounds that consist of cations and anions, usually rod-like cations and spherical anions. Herein we report a new method for the synthesis of ionic liquid crystals by using cations and anions of the same molecular shape with oppositely charged head groups. Thus, 4-alkoxyphenylpentamethylguanidinium 4-alkoxyphenylsulfonate ion pairs have been synthesised. 4-Alkoxyphenylpentamethylguanidinium iodides were also prepared to determine the influence of congruently shaped anions, in comparison with their spherical counterparts, on mesophase behaviour, which was investigated by differential scanning calorimetry (DSC), polarising optical microscopy (POM) and X-ray diffraction (XRD). All the liquid crystalline salts exhibit smectic A mesophases with strongly interdigitated bilayer structures. The guanidinium sulfonate ion pairs show mesomorphic properties from shorter alkyl chain lengths (≥C(9)) and lower melting points (≈10 K), whereas the corresponding guanidinium iodides are liquid crystalline for longer alkyl chain lengths (≥C(14)). For chains with ≥C(18), however, the mesophase range decreases for the sulfonate ion pairs, but not for the iodide salts.     

Novel supramolecular columnar liquid crystals based on thiosemicarbazides

Novel liquid crystal materials based on 3,4-di-n-alkoxybenzoylthiosemicarbazides (3a–h, n = 5–10, 12, 14) were synthesised. The mesomorphic properties of these compounds were characterised and studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. Compound 3a did not show mesomorphic properties; 3b shows a monotropic hexagonal columnar (Col_h) phase. Compounds 3c–h display an enantiotropic Col_h phase. The mesomorphic properties were found to be dependent on the length of alkoxy side chains. In N,N-dimethylformamide solution, all the compounds displayed a room temperature emission with λ_max at 361–332 nm. A thermogravimetric analysis also was performed.     

From tectons to luminescent supramolecular ionic liquid crystals

New phosphorescent and room-temperature liquid-crystalline materials were obtained by combining dicyanometallate anions with dicationic bisamidinium based tectons bearing four peripheral lipophilic pyrogallate moieties.     

Mesophase behaviour of azobenzene-based angular supramolecular hydrogen-bonded liquid crystals

New types of angular 1:1 hydrogen-bonded supramolecular complexes via hydrogen-bond formation between 4-alkoxyphenylazo benzoic acids (In) and 4-(3?-pyridylazo)-4??-alkoxybenzoates (IIm) with various alkoxy chains (from 6 to 16 carbons) were prepared and investigated for their mesophase behaviour by differential scanning calorimetry (DSC) and polarised-light microscopy (PLM). All prepared homologues were found to be dimorphic, possessing smectic C and nematic mesophases. The formation of 1:1 hydrogen-bonded supramolecular liquid crystals (LCs) complexes was confirmed by FTIR and UV?visible (UV?vis) absorption spectroscopy. The study revealed that nematic transition enhancement (ΔT) decreases with the increase of the alkoxy chain length on the base complement, while it increases with the increase of the chain attached to the acid complement of the complex, that is the stability of the nematic phase is more dependent on the length of the acid component.     

Star-shaped supramolecular ionic liquid crystals based on pyridinium salts

Series of novel star-shaped liquid crystals containing [1,1?-biphenyl]-4,4?-diyl diisonicotinate mesogens with various counterions (Br^?, B-SO₃^?,C-SO₃^?,H₂PO₄^?, BF₄^?) have been synthesized and characterized, which display a nematic phase. The molecular structures of the liquid crystals, thereof consisting of rod-like mesogens linked together by different long alkyl spacers to a small disc-like core of central benzene-1,3,5-triyl triisonicotinate, have been fully characterized by ¹H NMR. Their thermal and supramolecular organization behaviours have been studied by combining polarized optical microscopy, differential scanning calorimetry and small-angle X-ray scattering. These investigations showed that the mesophase temperature range increased with increasing alkyl chain length and the clearing point decreased with increase of anion size. These star-shaped liquid crystals have a long and ordered molecular structure, the electron delocalization of the π-π stacking conjugation effect and the electrostatic attraction of ionic make them have good ferroelectric properties and ionic conductivity properties. The interest in preparing ionic liquid crystal with a nematic phase lies in the technological applications as it is well known that the nematic phase has the highest fluidity of all liquid crystalline phases and hence the possibility to align it by applying an external electric/magnetic field, commonly used in electro-optical devices.     

Smectic liquid crystals. VIII. Some new laterally substituted smectic C compounds

Abstract

The synthesis and liquid crystal transition temperatures of a new class of compounds incorporating a trans-1,4-disubstituted cyclohexane ring have recently been reported. Many of these novel materials exhibit an enantiotropic smectic C mesophase over a wide temperature range and are suitable for commercial display device applications. The influence of several lateral substituents in various positions on the liquid crystal transition temperatures of these compounds has now been investigated systematically. A linear relationship between the size of the lateral substituent and the magnitude of the transition temperatures of various liquid crystal mesophases has been established for several of the series synthesized. A number of series incorporating a lateral substituent and an optically active centre have also been prepared and similar effects have been found.     

Mimicking the behaviour of rigid rod molecules. Smectic H liquid crystals from amphiphilic quaternary ammonium salts

The effect of secondary hydrogen-bonding interactions on the crystalline and liquid crystalline phases of quaternary ammonium salts functionalised with a carboxylic group attached at the polar head through a decyl spacer of a homologous series of N-alkyl-N-carboxydecyl-N,N-dimethylammonium bromides was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. The low-temperature crystal phases were found to have a lamellar structure in which the ammonium bromide groups are arranged within the layers in two distinct planes, alternately separated by single layers of alkyl chains and double layers of carboxydecyl chains coupled through the carboxyl end groups. At higher temperatures, although these molecules were made from soft flexible chains, smectic H mesophases were identified. The smectic layers were found to be formed by the same two ionic planes alternately separated by the alkyl and carboxydecyl sub-layers. The smectic structure was compared with the three-dimensional positional order observed in the smectic T phase of dihydroxyl functionalised quaternary ammonium salts already described in the literature.     

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.     

Preparation of conjugated polymeric columns with a hexagonal symmetry from star-shaped supramolecular liquid crystals

New conjugated polymeric columns with a hexagonal symmetry were prepared via topochemical polymerisation of star-shaped supramolecular liquid crystals formed by hydrogen bonding between a phloroglucinol core and pyridine derivatives containing a diacetylenic group in the alkyl chain. The mesomorphic properties of the supramolecular monomer and its photopolymerisation behaviour were investigated. The supramolecular liquid crystal exhibited a rectangular columnar mesophase. Photopolymerisation of supramolecular monomer along the column axis in the liquid crystalline state provided well-ordered conjugated polydiacetylenic columns with a two-dimensional hexagonal symmetry. Fourier transform infrared and ultraviolet–visible spectroscopy affirmed that conjugated polydiacetylenes were produced by 1,4-polymerisation of the supramolecular monomer along the column axis. X-ray diffraction analysis showed that a two-dimensional columnar order in the supramolecular monomer was maintained after photopolymerisation, and that the resulting polydiacetylene had a hexagonal array of conjugated columns. Our controlled methodology provides a new route to conjugated polymeric columns with highly ordered structures by self-assembly and polymerisation of star-shaped supramolecular liquid crystals.     

Preliminary communication Properties of nonlinear supramolecular liquid crystals containing thiophenedicarboxylic acids

Novel mesogenic supramolecules have been constructed from the 2 :1 molar ratio of trans 4-alkoxy-4-stilbazoles CnPS (1) complexed with 2,5-thiophenedicarboxylic acid THDA (4) to form the kinked hydrogen-bonded (H-bonded) complexes (CnPS)-THDA. The analogous H-bonded complexes (CnPP)2-THDA consisting of the 2:1 molar ratio of 4-alkoxypyridines CnPP (2) and THDA (4) are also compared. In contrast to linear complexes (CnPS)2-TA prepared from the 2 :1 molar ratio of CnPS (1) and terephthalic acid TA (5), supramolecular liquid crystals with kinked molecular structures (CnPS)2-THDA are generated by introducing the thiophene unit into the H-bonded complexes. In addition, the chiral complex (C5*PS)2-THDA composed of an optically active proton acceptor (S)-(-)-4-(2-methylbutoxy)-4-stilbazole C5*PS ( 3 ) and THDA ( 4 ) (2 :1 molar ratio) is reported. Significantly, the first thiophene based supramolecular liquid crystals have been constructed in this study, and the mesogenic properties of the supramolecules can be easily adjusted not only by the nonlinear shape of the thiophene unit but also by the dipole moment derived from the lone-pair electrons of the sulphur hetero-atom.     

Fabrication of conjugated polymeric nanochannels from star-shaped supramolecular liquid crystals containing two different photoreactive groups

New conjugated polymeric nanochannels were prepared via photopolymerisation and core extraction of star-shaped supramolecular liquid crystals formed by simple hydrogen bonding between a phloroglucinol core and pyridine derivatives containing two different photoreactive groups. The polymerisable supramolecular liquid crystal exhibited a discotic columnar mesophase. Photopolymerisation of supramolecular liquid crystalline monomer provided cross-linked conjugated polymeric columns in which the diacetylene and acrylate moieties were selectively polymerised along the vertical and horizontal axes, respectively. Chemical treatment of the cross-linked conjugated columns with sodium hydroxide resulted in the removal of the phloroglucinol core to produce the cross-linked conjugated polymeric nanochannels with an estimated effective pore diameter of about 1 nm. The controlled methodology in the present study can be used to design and fabricate conjugated nanoporous organic materials with intriguing ordered structures.     

Smectic liquid single crystal elastomers

In the liquid crystalline (I.c.) state the backbone of l.c-polymers has to adapt an anisotropic conformation that is consistent with the I.c.-phase structure. For macroscopically uniformly aligned samples this anisotropic backbone conformation can be obtained by applying a suitable mechanical field on chemically crosslinked I.c-polymers. Introducing the network anisotropy a priori by synthesis, stable macroscopically aligned I.c.-networks can be realized. We have called these networks Liquid Single Crystal Elastomers (LSCE) because their physical properties resemble that of organic or inorganic single crystals.     

Synthesis and mesophase behaviour of branched azobenzene-based supramolecular hydrogen-bonded liquid crystals

Five types of new pyridine-based azo compounds, namely, 4-(4′-pyridylazo)-4′‘-alkoxybenzoates or 4-(4′-pyridylazo)-4′’-substituted benzoates with straight or branch terminal were prepared as hydrogen bond (H-bond) acceptors to build mesogens. After self-assembled with different carboxylic acids, the resulted H-bond complexes were confirmed for the stability of H bond by fourier transform infrared spectroscopy (FTIR) spectroscopy, and investigated for the mesophase behaviour by differential scanning calorimetry and polarised optical microscopy. The influence of terminal chains in azo accepter or acid donor and the structure of acid donors on the mesogenic behaviour were discussed in detail.     

Tilted supramolecular structures of amphiphilic glycolipids in straight-core smectic C liquid crystals

Optical microscopic and the electric polarization studies show that amphiphilic glycolipids form tilted supramolecular structures in straight-core tilted smectic liquid crystals. Glycolipid and chiral smectic C (SmC) structures show spontaneous polarization while glycolipid and non-chiral SmC structures show no spontaneous polarization. We deduce from these observations that glycolipid molecules in chiral SmC structures nano-segregate in tilted double layers sandwiched between layers of straight-core liquid crystal molecules. In non-chiral SmC structures, tilted glycolipid molecules are nano-segregated in the layers of straight-core liquid crystal molecules. This indicates that, in contrast to the currently accepted interdigitated model, amphiphilic glycolipids may already have a tilted structure.     

Design and characterization of novel supramolecular liquid crystals involving multiple hydrogen bonds

A novel type of liquid crystalline system, namely 5-[4-(4-methoxyphenylazo)phenyloxyalkyloxy]isophthalic acidhydrazides (Dn, n = 3, 6, 10) was designed and synthesized. The FTIR spectra of the Dn compounds were examined in the range 4000-1000 cm^-1 at different temperatures. The results showed that the stretching vibration frequencies of the NH and C=O groups, and the NH₂ deformation shifted to lower frequencies with decrease of temperature, while out-of-plane NH bending shifted slightly to higher frequency. Intermolecular hydrogen bonding between NH, NH₂ and C=O groups is proposed and the schematic supramolecular Dn structure is given. This may explain the higher Dn transitional temperatures and their smectic phase behaviour compared with that of 5-[4-(4-methoxyphenylazo)phenyloxyalkyloxy]isophthalic acid diethyl esters.