The mesomorphic behaviour of cyanopropylalkyldimethylammonium bromides

Abstract

A homologous series of N-cyanopropyl-N-alkyl-N,N-dimethylammonium bromides was synthesized and characterized. Its thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy, dilatometry, and X-ray diffraction. A smectic A mesophase was thus identified. The smectic layers were found to be formed of two ionic planes alternately separated by sub-layers of disordered alkyl chains and cyanopropyl groups. The smectic structure was compared with that of the crystal at room temperature and also that of the smectic T structure of the N,N-dialkyl-N,N-dimethylammonium bromides already described in the literature.     

The mesomorphic behaviour of cyanopropylalkyldimethylammonium bromides

A homologous series of N-cyanopropyl-N-alkyl-N,N-dimethylammonium bromides was synthesized and characterized. Its thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy, dilatometry, and X-ray diffraction. A smectic A mesophase was thus identified. The smectic layers were found to be formed of two ionic planes alternately separated by sub-layers of disordered alkyl chains and cyanopropyl groups. The smectic structure was compared with that of the crystal at room temperature and also that of the smectic T structure of the N,N-dialkyl-N,N-dimethylammonium bromides already described in the literature.     

Langmuir-Blodgett films as aligning layers for homeotropic alignment of liquid crystal molecules

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

The crystalline and liquid crystalline structures of benzyl-tri-octadecylammonium bromide complete the puzzle: how do Group VA halide salts with one-four long n-alkyl chains pack?

The first single crystal structure of a Group VA halide salt with three equivalent long n-alkyl chains, benzyltrioctadecylammonium bromide (BzN18Br), is reported. It consists of alternating interdigitated and non-interdigitated regions of alkyl chains separated by ionic planes. Two chains per molecule are paired and extend to one side in a non-interdigitated region. The third chain is on the opposite side of the ionic plane and pairs intermolecularly to form an adjacent, interdigitated region. The thickness of two nearly extended molecules defines the bilayer unit-two ionic planes flanked by a region with intramolecularly paired chains and separated by an interdigitated chain region. Powder X-ray diffraction and optical microscopy data of liquid crystalline BzN18Br are consistent with an enantiotropic smectic A₂ (SmA₂) phase: the three n-alkyl chains of each molecule are projected from one side of an ionic plane, and head groups of neighbouring molecules are oriented head-to-head, in a non-interdigitated bilayer assembly. The structure of BzN18Br fills an important gap in our knowledge about the crystal packing of ammonium and phosphonium salts with one-four equivalent long n-alkyl chains. A comparison of their packing arrangements is made and the transitional nature of the BzN18Br structures is demonstrated. Although salts with one, two, or three long n-alkyl chains form SmA₂ phases, each is distinctive in its molecular packing. A large molecular reorganization accompanies the crystal-to-liquid crystal transition of BzN18Br.     

Thermotropic liquid crystalline behaviour of piperazinium and homopiperazinium alkylsulphates

A series of 1,4-piperazinium di-n-alkyl sulphates was synthesized and compared with an analogous series of 1,5-homopiperazinium di-n-alkyl sulphates. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. For the piperazinium salts, two ordered smectic phases were established, in which the lateral packing of the molecules within the layers is rectangular or tetragonal. Both phases are characterized by an alternate periodic packing of the positive and negative ionic groups in the polar sublayer, while the lipophilic sublayers of the alkyl chains are in a disordered conformation. Substitution of the piperazinium by the less symmetrical homopiperazinium group disfavours organization within the smectic layer and as a result smectic A phases were obtained.     

Thermotropic liquid crystalline behaviour of piperazinium and homopiperazinium alkylsulphates

A series of 1,4-piperazinium di- n -alkyl sulphates was synthesized and compared with an analogous series of 1,5-homopiperazinium di- n -alkyl sulphates. Their thermotropic liquid crystalline behaviour was studied by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction. For the piperazinium salts, two ordered smectic phases were established, in which the lateral packing of the molecules within the layers is rectangular or tetragonal. Both phases are characterized by an alternate periodic packing of the positive and negative ionic groups in the polar sublayer, while the lipophilic sublayers of the alkyl chains are in a disordered conformation. Substitution of the piperazinium by the less symmetrical homopiperazinium group disfavours organization within the smectic layer and as a result smectic A phases were obtained.     

Pyrrolidinium Ionic Liquid Crystals

N‐Alkyl‐N‐methylpyrrolidinium cations have been used for the design of ionic liquid crystals, including a new type of uranium‐containing metallomesogen. Pyrrolidinium salts with bromide, bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate, thiocyanate, tetrakis(2‐ thenoyltrifluoroacetonato)europate(III) and tetrabromouranyl counteranions were prepared. For the bromide salts and tetrabromouranyl compounds, the chain length of the alkyl group C_nH_2n+1 was varied from eight to twenty carbon atoms (n=8, 10–20). The compounds show rich mesomorphic behaviour: highly ordered smectic phases (the crystal smectic E phase and the uncommon crystal smectic T phase), smectic A phases, and hexagonal columnar phases were observed, depending on chain length and anion. This work gives better insight into the nature and formation of the crystal smectic T phase, and the molecular requirements for the appearance of this highly ordered phase. This uncommon tetragonal mesophase is thoroughly discussed on the basis of detailed powder X‐ray diffraction experiments and in relation to the existing literature. Structural models are proposed for self‐assembly of the molecules within the smectic layers. In addition, the photophysical properties of the compounds containing a metal complex anion were investigated. For the uranium‐containing mesogens, luminescence can be induced by dissolving them in an ionic liquid matrix. The europium‐containing compound shows intense red photoluminescence with high colour purity.     

Microwave-promoted solvent-free synthesis of N-(diphenylmethylene)glycine alkyl esters

The efficient synthesis of N-(diphenylmethylene) glycine alkyl esters was achieved for the first microwave irradiation under solvent-free condition, using PEG or quaternary ammonium salts as phase transfer catalysts (PTCs). Under the optimum conditions, N-(diphenylmethylene) glycine alkyl esters were obtained in excellent yields in most cases.     

Synthesis and thermotropic liquid-crystalline properties of N-alkylpyridinium bromides substituted with a terphenylene moiety

Molecules containing a terphenylene core, two alkyl chains and a pyridinium ring associated with its bromine counterion were synthesised and their liquid crystalline properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results were compared with those of chemical intermediates, which also develop a liquid crystalline behaviour. Both intermediates and pyridinium salts showed a rich polymorphism at temperatures ranging from around 100 to 200°C and 115 to 220°C, respectively. X-ray results indicate that both intermediates and pyridinium salts develop tilted smectic mesophases with molecules stacked in single and double layers, respectively. The tilt angle of some of these compounds decreases so markedly upon cooling that molecules attain almost an orthogonal position. The stacking of molecules in the smectic layers was explained in terms of the mutual repulsion interactions between the terphenylene core, the alkyl chains and the ionic species (the pyridinium ring associated with its counterion) and it was proposed that the π–π interactions between the long aromatic cores counterbalance the strong forces between the ionic species, leading to a full segregation of these molecular parts in periodic sublayers. A molecular arrangement model is proposed for these salts.     

Effects of the alkyl side-chain length on the structures of poly[oxy(N-alkylsulfonylmethyl)ethylene]s

The structures of poly[oxy(N-alkylsulfonylmethyl)ethylene]s (ASE-Ns) were examined with X-ray, IR, DSC, and polarized optical microscopy. The structures of ASE-Ns were strongly dependent on the alkyl side-chain length. ASE-2 and ASE-3, the shortest ones, were amorphous materials. ASE-4 and ASE-5 showed nematic characteristics. ASE-6–ASE-12 had smectic A structures. ASE-14 and ASE-16 could be labeled as more ordered structures higher than smectic A (probably smectic B or smectic E). The d-spacings of the first small-angle reflections were double the most extended side-chain length and linearly increased with a slope of 2.50 Å per methylene unit, regardless of the structural phases. The maintenance of the double-layered structure in all ASE-Ns may be due to the strong dipole–dipole interactions at both sides of the layers against the main chain for all amorphous, nematic, and smectic phases of ASE-Ns. The double-layered structure was maintained above the isotropic temperature, indicating that dipole–dipole interactions were not destroyed although the alkyl side chains melted during the isotropic transition. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1868–1874, 2004     

Synthesis and characterization of liquid crystalline polyacrylates and non-liquid crystalline polypyrroles from bifunctional monomers

Two kinds of polymerizations of liquid-crystalline N-{{W-{4-[4-(11-acryloyloxy)undecanoxybenzoyl]biphenyleneoxy}alkyl}}pyrrole gave a side-chain liquid-crystalline polyacrylate containing pyrrole group and a non-liquid crystalline polypyrrole containing acrylate group. Liquid crystallinity was determined by DSC and optical polarizing microscope measurements. The monomers having hexyl or decyl group as the alkyl group exhibited an undefined smectic, smectic B, and smectic A phases on the heating stage. The radically polymerized polyacrylate derivatives containing the pyrrole group showed smectic A, smectic C, and undefined smectic phases on the cooling stage. On the other hand, soluble N-substituted polypyrrole derivatives containing the terminal acrylate group which were prepared by chemical oxidative polymerization by ferric chloride catalyst did not show liquid crystallinity. Structure analysis of the polypyrroles performed by ¹H and ¹³C-NMR and FT-IR spectroscopies demonstrated that the polymerization occurred at the pyrrole ring. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3877–3887, 1999     

Three ring dioxanes as dopants enhancing the stability of the smectic C phase

Abstract

Searching for compounds which could be useful as modifiers of smectic C mixtures, we have synthesized four homologous series of three ring dioxanes, 2BBD, 5BBD, 2CBD, and 5CBD. Their phase transition temperatures and enthalpies were measured and their liquid crystal phases identified. Compounds belonging to series n-BBD form smectic B_cr phases for shorter alkyl chains, and smectic B_cr and A phases, for longer chains. Compounds belonging to the n-CBD series exhibit the smectic A phase, but those with longer alkyl chains have exclusively smectic B phases and those with short tails have other low temperature, highly ordered smectic phases. The compounds were added to smectic C mixture and it was found that some can be useful as dopants. Compounds with longer alkyl tails in the molecule are more suitable for this purpose; the type of ring in the core is less important.     

Effects of alkyl chain length on properties of 1-alkyl-3-methylimidazolium fluorohydrogenate ionic liquid crystals

A series of 1‐alkyl‐3‐methylimidazolium fluorohydrogenate salts (C_xMIm(FH)₂F, x=8, 10, 12, 14, 16, and 18) have been characterized by thermal analysis, polarized optical microscopy, IR spectroscopy, X‐ray diffraction, and anisotropic ionic conductivity measurements. Liquid crystalline mesophases with a smectic A interdigitated bilayer structure are observed from C₁₀ to C₁₈, showing a fan‐like or focal conic texture. The temperature range of the mesophase increases with the increase in the alkyl chain length (from 10.1 °C for C₁₀MIm(FH)₂F to 123.1 °C for C₁₈MIm(FH)₂F). The distance between the two layers in the smectic structure gradually increases with increasing alkyl chain length and decreases with increasing temperature. Conductivity parallel to the smectic layers is around 10 mS cm^?1 regardless of the alkyl chain length, whereas that perpendicular to the smectic layers decreases with increasing alkyl chain length because of the thicker insulating sheet with the longer alkyl chain.     

Anisotropic Ionic Mobility of Lithium Salts in Lamellar Liquid Crystalline Polymer Networks

New mesogens presenting smectic A (SmA) phases and capable of hosting lithium salts are designed. The mesogens comprise a vinyl‐functionalized spacer to allow further reaction to the polymer backbone, an aromatic core and ethylene oxide chains, able to coordinate lithium ions. Copolymerizing these monomers with a suitable crosslinker yields the first lithium containing liquid crystalline elastomers (LCEs). The SmA structure where the ethylene oxide chains are microphase separated in layers is fixed by the crosslinking and permanent macroscopic orientation is obtained. Diffusion and conductivity measurements of the monomer sample show a large anisotropy of the ion mobility (100 for the cation and 400 for the anion). In the elastomer the anisotropy of the lithium mobility is comparable to that in the monomers.     

Metal‐Ion‐Induced Switch of Liquid‐Crystalline Orientation of Metallomacrocycles

A new series of shape‐persistent imine‐bridged macrocycles were synthesized based on dynamic covalent chemistry. The macrocycles had an alternating sequence of dibenzothiophene and N,N′‐bis(salicylidene)‐ethylenediamine (salen) tethering branched alkyl chains. The macrocycles and tetranuclear metallomacrocycles bearing long and branched alkyl chains exhibited thermotropic columnar liquid‐crystalline phases over a wide temperature range and the metallomacrocycles greatly depended on the characteristics of the coordinated metal ions. The metal‐free macrocycle showed a liquid‐crystalline phase with a lamellar structure and poor birefringence. In sharp contrast, the macrocyclic Ni complex showed a columnar oblique liquid‐crystalline phase, whereas the Pd and Cu complexes showed columnar liquid‐crystalline phases with a lamellar structure. The macroscopic organization and thermal properties of the corresponding liquid‐crystalline metallomacrocycles were significantly dependent on the subtle structural differences among the planar macrocycles, which were revealed by single‐crystal X‐ray crystallographic analysis of the macrocycles with shorter alkyl chains.     

Enantiotropic nematic phases of quaternary ammonium halide salts based on trioctadecylamine

The neat phases of a series of quaternary ammonium halides based on trioctadecylamine have been investigated by optical microscopy, differential scanning calorimetry, and H NMR spectroscopy. Phase behaviour depends primarily on the structure of the fourth substituent on nitrogen and the thermal history of the samples. For instance, the iodide salts with methyl, ethyl, or propyl as the fourth substituent form enantiotropic nematic phases, while those with a proton or a longer alkyl substituent exhibit complex phase behaviour that does not include a liquid crystalline phase. Possible reasons are advanced for the unprecedented formation of nematic phases.     

Novel diphenylacetylene-based room-temperature liquid crystalline molecules with alkylthio groups,and investigation of the role for terminal alkyl chains in mesogenic incidence and tendency

In this study, we offer a new family of alkylthio-containing diphenylacetylene-based liquid crystalline molecules (the so-called tolanes) showing nematic phases stable down to room temperature and high birefringence, with insights into the role for terminal alkyl chains in mesogenic incidence and tendency. A number of asymmetric tolane homologues having various alkyl chain lengths in the alkylthio and alkyl groups at each p-, p’-position were synthesised, and their phase transition behaviour was investigated by polarising optical microscope observation, differential scanning calorimetry and wide-angle X-ray diffraction measurement. Consequently, several homologues exhibited monotropic nematic or highly ordered smectic (soft crystal) phases stable down to room temperature. It is found that a long alkyl group on the side opposite to an alkylthio group is prerequisite for mesogenic incidence. In addition, a nematogenic homologue exhibited a higher birefringence value of 0.20 compared to that of 0.19 for an alkoxy counterpart.     

Self‐Assembly of Imidazolium‐Based Rodlike Ionic Liquid Crystals: Transition from Lamellar to Micellar Organization

By using aryl‐amination chemistry, a series of rodlike 1‐phenyl‐1H‐imidazole‐based liquid crystals (LCs) and related imidazolium‐based ionic liquid crystals (ILCs) has been prepared. The number and length of the C‐terminal chains (at the noncharged end of the rodlike core) and the length of the N‐terminal chain (on the imidazolium unit in the ILCs) were modified and the influence of these structural parameters on the mode of self‐assembly in LC phases was investigated by polarizing microscopy, differential scanning calorimetry, and X‐ray diffraction. For the single‐chain imidazole derivatives nematic phases (N) and bilayer SmA₂ phases were found, but upon increasing the number of alkyl chains the LC phases were lost. For the related imidazolium salts LC phases were preserved upon increasing the number and length of the C‐terminal chains and in this series it leads to the phase sequence SmA–columnar (Col)–micellar cubic (Cub_I/Pm3n). Elongation of the N‐terminal chain gives the reversed sequence. Short N‐terminal chains prefer an end‐to‐end packing of the mesogens in which these chains are separated from the C‐terminal chains. Elongation of the N‐terminal chain leads to a mixing of N‐ and C‐terminal chains, which is accompanied by complete intercalation of the aromatic cores. In the smectic phases this gives rise to a transition from bilayer (SmA₂) to monolayer smectic (SmA) phases. For the columnar and cubic phases the segregated end‐to‐end packing leads to core–shell aggregates. In this case, elongation of the N‐terminal chains distorts core–shell formation and removes Cub_I and Col phases in favor of single‐layer SmA phases. Hence, by tailoring the length of the N‐terminal chain, a crossover from taper‐shaped to polycatenar LC tectons was achieved, which provides a powerful tool for control of self‐assembly in ILCs.     

Synthesis and thermal behaviour of liquid crystalline pyridinium bromides containing a biphenyl core

A range of new pyridinium bromides was synthesized by the quaternization of different substituted pyridines with a group containing a biphenyl core and alkyl chains of differing lengths. The phase behaviour of the pyridinium bromides was studied by differential scanning calorimetry, polarizing optical microscopy and powder X-ray diffraction. It is shown that pyridinium moieties, linked to a rod-like biphenyl core via an alkyl spacer, can form ionic liquid crystals. Unsubstituted pyridinium groups promote mesomorphism. Liquid crystalline phases are formed only from 2- and 4-ethyl substituted pyridinium groups with sufficiently long alkyl terminal chains and spacers; i.e. decyl chains on both sides of the biphenyl core. Both the substitution pattern at the pyridinium group and the alkyl chain length have an influence on the polymorphism of the smectic phases. 3,5-Dimethyl substitution hinders mesophase formation.     

New possibility for an organic semiconductor: a smectic liquid crystalline semiconductor having a long conjugated core and two long alkyl chains

We report a new possibility for liquid crystalline organic semiconductors. These materials exhibit smectic liquid crystalline phases, in which the molecules assume a smectic molecular order by self‐assembly. Because of the strong dispersion force among long alkyl chains, on cooling, smectic molecular order was retained at room temperature. A charge transport ability was also retained. The conductivity of a device having smectic liquid crystalline order is about 5×10⁷ that of a device with no smectic order. The current?–?voltage characteristic of the device has a very sharp increase at low threshold voltage (5?V). A high carrier mobility of 1.8×10^‐2 was observed in the smectic phase of one of the compounds studied (e).