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We report the synthesis of a new homologous series of symmetric bent-shaped liquid crystal dimers bearing oxybiphenylcarboxylate mesogens linked by a pentamethylene spacer. The dimers were characterised by optical, thermal, X-ray diffraction and computational techniques. Shorter dimers showed an I-N-SmA-SmC phase sequence with strong first-order phase transitions. For the longer dimers, the unconventional uniaxial SmA phase present in shorter members collapsed. A 9:1 binary mixture of a bent dimer with a non-mesogenic dimer exhibited upon the SmA-SmC phase transition a maximum layer contraction of 1%.

Ionic liquid Crystals constitute highly versatile materials that have drawn much interest these past few years in the fields of academic research and industrial development. In this respect, the present article is intended as an update of K. Binnemans review published in 2005, but focusing exclusively on the imidazolium cation - the most widely studied. Herein, imidazolium-containing thermotropic liquid crystalline materials will be sorted by molecular structure (mono-, bis-, poly-imidazolium compounds, with symmetrical and non-symmetrical structures) and discussed. Their physico-chemical properties will be exposed in order to adduce the relevancy and potential of the imidazolium platform in various fields of research.

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 ...

Two ferroelectric three‐ring chiral esters, one with a partially fluorinated alkyl chain and another with a cyano terminal group, were mixed with a structurally similar compound having an alkyl terminal chain. In their mixtures an antiferroelectric phase was induced. The phase behaviour, spontaneous polarisation, tilt angle, smectic layer spacing and helical pitch of both systems were determined. The mechanism of the induction of an antiferroelectric phase is different in both cases, with highly tilted phases in former system and less tilted phases in the latter.

Two series of polymeric mixed-valent diruthenium (II,III) aliphatic carboxylates of formula Ru2[O2C(CH2) n−2CH3]4X (where X=dodecylsulfate (DOS) anion and n=8, 9, 16 and 18, or X=octylsulfonate (OS) anion and n=8, 10, 12, 14 and 18) were synthesized and characterized. Their liquid crystalline properties and crystalline (Cr) structures were analysed; the Cr phases are lamellar in all cases. For long chain DOS derivatives (n=16 and 18) hexagonal and rectangular columnar mesophases (Colh and Colr) with melting temperatures close to 140°C were observed. For long chain OS derivatives (n≥10) a Colh mesophase was observed, with melting temperatures between 140 and 190°C.

A new metallomesogen, the dihydroxo[5,10,15,20-tetrakis(4-n-dodecylphenyl)porphinato]silicon(IV) complex, C12TPPSi(OH)2, was synthesized and its mesomorphism was investigated in terms of the axial hydrogen bond interaction in the stacked columnar structure. It was found that this compound exhibits a 3D plastic lamellar mesophase with a columnar structure, and the axial hydroxyl groups are connected by a very weak hydrogen bond interaction in the column. This causes a dramatic increase of the clearing point for the mesophase, even though the stacking periodicity is far larger (c. 9 A) than that typically found for a columnar meosphase (c. 3.5 A).

A series of linear oligo (p-phenylene)s containing three, five and seven phenylene groups, modified with short lateral and long terminal alkoxy chains, were synthesized via Palladium complex-catalysed cross-coupling reactions. The thermotropic liquid crystalline and UV absorption/emission properties of these compounds were studied. It was observed that tri (p -phenylene)s develop a rich mesomorphism including tilted smectic type mesophases (SmC and SmF/SmI) and the nematic phase, whereas penta- and hepta-(p-phenylene)s substituted with short lateral chains develop only the nematic phase. From these observations it is clear that the short lateral chains hinder the layered molecular packing typical of smectic phases and promote the formation of the less ordered liquid-like nematic phase. Mesophases appeared at lower temperatures when longer end chains were used. The optical properties studied by UV-Vis and emission spectroscopy indicate that these systems are promising candidates for blue-emitting layers in...

This paper deals with the synthesis of vanadyl phthalocyanines substituted with eight alkoxy chains in the peripheral (2, 3, 9, 10, 16, 17, 23, 24) positions. The alkoxy chain length was varied, and octa-octyloxy (C8H17O), octa-dodecyloxy (C12H25O) and octa-hexadecyloxy (C16H33O) substituted vanadyl phthalocyanine complexes were prepared. Studies by polarizing optical microscopy and small angle X-ray diffraction (XRD) revealed that all the complexes are liquid crystalline and that these metallomesogens exhibit a columnar phase. The symmetry of the 2D lattice is rectangular, with a c 2 mm space group, as determined by the indexation of the XRD reflections; hence a rectangular columnar phase (Colr) was assigned. A double periodicity, although weak, along the axis of the columns was found, which indicates some degree of pairing or dimerization. A tentative explanation based on an antiferroelectric stacking is given. Transition enthalpies were determined by differential scanning calorimetry. The compounds sta...

In this work we report the characterization of two organosiloxane liquid crystalline compounds by means of DSC, polarizing optical microscopy and X-ray diffraction. These compounds can be used for dye guest-host ferroelectric displays. We focus this investigation on the molecular organization of the SmC phases of the two pure compounds to help elucidate the physical behaviour of mixtures with different concentrations of the dye and the guest host. The existence of longitudinal and transverse dipoles in the molecules of the dye and the chiral guest-host respectively are responsible for the different molecular organizations in the SmC mesophases of each compound. Taking into account the experimental results and the complexity of the molecules, we present coherent models to explain the molecular arrangements in the mesophases of both compounds.

In the framework of a systematic investigation of the molecular organization in different mesophases exhibited by biforked molecules, we present a structural study of the smectic C phase of a biforked compound containing heptyloxy chains. With a combination of dilatometry measurements and X-ray diffraction on polydomain samples, together with a detailed X-ray investigation on oriented samples, it was possible to describe precisely the packing of the molecules within the smectic C layers. The result is a large tilt of the long aromatic cores of about 50°–60° with respect to the normal to the layer planes, whereas the terminal aliphatic chains are close to the normal to the planes with a small tilt angle of about 20°–30°. For the first time, these two angles have been directly observed on the X-ray patterns of oriented samples.

The aim of this work was to determine by X-ray diffraction the nature of the UV-induced changes in the smectic C* organization of a calamitic liquid crystalline material containing the UV-sensitive azobenzene group in the rigid core moiety. As a result of the reversible trans-cis-isomerization, a decrease of all transition temperatures (1.1°C for the SmA-SmC* transition) was observed. In the SmC* phase, this corresponds to an increase in layer spacing (up to 0.5 A); in the SmA phase, the detected increase in layer spacing was less than a tenth of this. The increase in layer spacing is interpreted in terms of molecules in the cis- configuration expelled from the smectic layers.

The mesomorphic properties of 27 homologues of the N-alkylpyridinium alkylsulphate series were studied by X-ray diffraction and dilatometry. All of these compounds exhibit single smectic A phases whose layer spacings are between 0.64 and 0.8 times the molecular length. Polarizing optical microscopy observation shows that these peculiar layer spacings are not due to the tilting of the molecules; a new packing model is proposed to explain the particular structure of these thermotropic ionic liquid crystal phases.

The liquid crystalline behaviour and the electro-optical properties of mixtures of two pure ferroelectric enantiomers are studied as a function of the enantiomeric excess and compared with those of the corresponding racemate. It appears clearly that, depending upon the value of the enantiomeric excess, distinct thermotropic sequences are observed. The transition temperatures change significantly, and an additional SmB phase is observed for the racemate and for the mixtures of neighbouring concentrations. Moreover, the variation of the spontaneous polarization as a function of the enantiomeric excess is found to be strongly nonlinear. All these experimental observations show unambiguously that the thermotropic behaviour, the polar order, and consequently all the related electro-optical properties of the ferroelectric liquid crystal materials studied in the present work depend significantly upon the optical purity of the materials.

The thermotropic mesophase behaviour of an asymmetrically shaped and highly substituted tetrabenzotriazaporphyrin (TBTAP) derivative has been studied by optical microscopy, DSC and X-ray diffraction. The TBTAP macrocycle differs from the more common phthalocyanine ring system by the substitution of a methine for a nitrogen at one meso-position. The TBTAP core was substituted with a single heptadecyl chain at this meso-position and four neopentyl groups were statistically distributed around the peripheral benzene rings. In contrast to an earlier study of this system which described the structure of the mesophase as discotic lamellar, detailed X-ray diffraction study indicates that the TBTAP derivative forms a disordered hexagonal columnar mesophase (Colhd), with a weak tendency towards antiparallel orientation of neighbouring molecules observed in the form of a weak pseudo-centred rectangular packing.

A structural characterization of two series of ferroelectric low molar mass organosiloxanes is described. X-ray and dilatometry studies have been used to determine the impact of variations in mono- or di-mesogen architecture, halogen substitution and siloxane content on the mesophase structure and thermal behaviour. All of the compounds exhibit smectic C* mesophases with high tilt angles. Bromo substituted compounds are consistently found to exhibit higher tilt angles than their fluoro analogues, and a strong odd-even effect in melting temperature is observed for the dimesogenic bromo series.

The liquid crystal properties of binary mixtures of copper- and palladium-containing, twinnedcalamitic, metal organic complexes with the electron acceptor 2,4,7-trinitro-9-fluorenone (TNF) are reported. The smectic A to isotropic transition temperature of the pure metal organic component is increased on mixing. Furthermore, only smectic A behaviour is exhibited by the mixtures compared with smectic A and C phases for the pure metal organic complexes. The stabilization of the smectic A phase in these mixtures is found to be due to a weak charge transfer type interaction.

A new series of ethylpyridinium salts N-substituted with a 4′-methoxybiphenyl-4-yloxyalkyl group has been synthesized and the mesomorphic properties examined. The effect of positioning the ethyl group at each of the three available positions (2; 3- or 4-) on the pyridinium ring was examined and compounds with alkyl chains containing odd numbers of carbon atoms from C5 to C11 were investigated. It was found that the longer chain (C9-C11) 2- and 4-ethylpyridinium compounds form smectic phases, whereas the corresponding 3-substituted compounds do not. X-ray diffraction studies indicated that the smectic phases were of types A, B and in one case E, with the molecules lying in a head to tail fashion in single layers with interdigitated alkyl chains. We explain the difference in mesogenic properties of the three different ring-substituted compounds in terms of packing considerations in the ionic regions of the structures. It is suggested that the 3-derivatives pack more efficiently than the 2- and 4-su...

Reaction of β-enaminoketones with lanthanide salts leads to mesomorphic complexes displaying an SA mesophase. The mesophase was characterized using X-ray scattering, EPR and magnetic susceptibility. A very high magnetic anisotropy was deduced for a complex containing dysprosium.

The phase behaviour and the structural characterization of a new series of linear dimeric and cyclic tetrameric molecules which contain three distinct parts, a cyanobiphenyl aromatic core (A), a paraffin chain (P) and a central siloxane group (B) with the A-P-B-P-A sequence, are described. Incompatible with one another, these parts tend to locate themselves in three separate sub-layers superposed in a partially bilayered smectic A structure. Each sub-layer adapts its internal structure in order to be appropriate for superposition, as in the case of organosiloxane molecules with the A-P-B sequence reported previously.

Reaction of pyridine with 1-bromoalkanes leads to N-alkylpyridinium bromides which can be further reacted with silver dodecylsulphate to give N-alkylpyridinium dodecylsulphates in good yield. These salts are mesomorphic and their mesophases have been characterized by optical microscopy, DSC and X-ray scattering techniques.

We demonstrate that the addition of small amounts of a novel azo-dye to a ferroelectric liquid crystal and illumination with low intensity (∼ 0.8 mW cm−2) UV light can result in reversible, isothermal phase transitions and dramatic changes in the properties of the system. In particular we examine light induced transitions between the S∗1, and S∗C phases and the consequent photomechanical regulation of spontaneous polarization.

One phasmid and fourteen biforked mesogens with thiobenzoate end groups have been synthesized and studied. The phasmid displays a hexagonal columnar phase with a lattice parameter a = 45.5 A at 70°C. This phase is not observed for the biforked mesogens which exhibit only a nematic phase, a smectic C phase for short chains and an unknown phase for long chains. The tilt angle of the molecules in the smectic C phase is about 50° (the layer thickness is 38.2 A for a molecular length l = 61 A in the case of IA (n = 12). The crystal structure of compound IB (n = 12) is monoclinic and the molecules adopt a zigzag form.

The synthesis and the properties of new ferroelectric liquid-crystalline compounds with a tolane rigid core and an optically active alkyl sulphinate group are reported. These compounds contain two chiral centres: a sulphinate group and a branched aliphatic chain. The two pure diastereomers having opposite configurations on the sulphur have been isolated. The mesomorphic behaviour of these two compounds is different, only one exhibiting an enantiotropic chiral smectic C phase; this clearly demonstrates the role of the sulphur configuration in determining the thermotropic properties. The mesomorphic transition temperatures, the structural behaviour as a function of temperature and preliminary measurements of the spontaneous polarization are presented.

A homologous series of binuclear copper (II) linear chain alkanoates together with two branched chain and one aromatic substituted copper (II) alkanoates have been synthesized and studied by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. All of these are crystalline at room temperature, they are mesomorphic in nature above c. 100°C, with the exception of copper propionate which remains crystalline up to its thermal decomposition above 200°C. A systematic study has shown that the linear chain alkanoates, starting from the pentanoic derivative, produce columnar mesophases with hexagonal symmetry. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two dimensional hexagonal lattice. The repeat unit in a column is a binuclear dicopper tetracarboxylate complex. Two transition regimes have been detected leading from the crystal to the columnar mesophase: one dominated by the interactions between the polar heads, t...

A mixture containing 70 per cent 4-cyanobenzylidene-4′-n-octylaniline (CBOOA) and 30 per cent 4-n-dodeciloxybenzylidene-4′-cyanoaniline (DOBCA) has been studied by optical microscopy, differential scanning calorimetry, X-ray diffraction and, primarily, by NMR spectroscopy. The smectic phase of the mixture is a partial bilayer smectic A phase (SAd), in which about one half of the molecules are associated in pairs through their polar cyano end groups. The molecular dynamics in the smectic Ad phase of the mixture was studied by proton spin-lattice relaxation. Self-diffusion and rotations/reorientations were found to be essential relaxation mechanisms while the contribution of the order director fluctuations seems to be very small in all frequency regions. The correlation times associated with the molecular rotational motion around the short molecular axis, τS, are of the order 10−10 s and the ratio τS/τL ≈6.2. The translational diffusion coefficients D∥ are of the order 10−11m2s−1. The possible cont...

The syntheses and structural properties of new binuclear ruthenium (II, III) complexes with long chain carboxylates, of general formula Ru2(RCO2)4X (R = C8H17, C11H23 and X = C1, RCOO), are reported. The thermodynamic behaviour of these compounds is strongly influenced by the nature of the counterion X. The chloro complexes (X = CI) are not mesomorphic whereas the carboxylato species (X = RCOO) are shown to exhibit a thermotropic columnar mesophase, thus providing the first example of a mixed-valent liquid-crystalline material.

A series of N-alkylpyridinium halides ω-substituted with a 4-methoxybiphen-ylyloxy mesogenic group were synthesized and characterized. The molecules of these compounds contain three distinct parts: a flexible aliphatic chain, a rigid polarizable aromatic core, and a positively charged pyridinium ring associated with a negatively charged counterion. Their thermotropic liquid-crystalline behaviour was studied by differential scanning calorimetry and optical microscopy. Three types of smectic mesophase, namely A, B, and E, were identified by X-ray diffraction. Their structure consists of single layers of upright molecules laterally arranged head to tail. Segregated from the non-ionic parts of the molecules, the ionic end groups are set in double layers with the oppositely charged species facing each other and equally distributed between the two sub-layers. For the ordered smectic E phases, the anions are arranged in rows along the rectangular two dimensional unit cell diagonals with the pyridinium r...

Three series of metal-free phthalocyanines with alkyl, alkoxy, and alkoxymethyl side groups have been studied with X-ray diffraction. The structure of the columnar mesophases was described as a two dimensional (pseudo) hexagonal packing of indefinitely long stacks of phthalocyanine molecules. From the slope of the squared intercolumnar distance as a function of the number of methylene groups in the peripheral chains, and using the known value of the volume of one methylene group in the liquid state, the stacking period of the molecules along the columnar axis was calculated. It was found to be equal to 3·5 A for the alkoxy and 47middot;9 A for the alkyl and alkoxymethyl derivatives. This is interpreted to mean that in the alkoxy series the molecules are oriented upright with respect to the columnar axis, while in the alkyl and alkoxymethyl series the molecules are tilted by an angle of 46°. Taken as circular for the alkoxy, the aromatic cores of the columns may be assumed elliptical in shape for ...

Four new chiral diarylethane α-chloroester series have been synthesized and studied. All of the derivatives are mesogenic and display a ferroelectric S∗c phase. In addition to this S∗c phase, some of them exhibit other ferroelectric S∗1 and/or S∗F phases. Transition temperatures and enthalpies are reported. Several electrooptical properties such as the spontaneous polarization and response times are given and discussed. Layer thickness and tilt angle in the smectic S∗c phases of series II compounds were determined by X-ray diffraction.

A homologous series of poly(dialkylsilanes), with alkyl side chains from butyl to tetradecyl, has been studied using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. It was shown that these polymers undergo a first order transition from a low temperature crystalline to a high temperature columnar mesomorphic phase; the transition temperatures range from 9 to 82°C. As the length of the side chains is increased, the transition temperature initially drops drastically and then rises again. These two regimes are connected to the relative part played in the phase transition by the aliphatic side chains and the polymer backbone. The enthalpies also revealed two regimes. Starting from the butyl polymer the enthalpy first grows more and more rapidly and, beyond a chain length of about ten methylene groups, it then grows linearly. From the Y intercept of the corresponding curve it was inferred that the transition into the mesomorphic phase is followed by a stabilizat...

Low angle X-ray scattering studies have been used to identify the mesophase of some calamitic lanthanide mesogens as smectic A, while magnetic birefringence studies have shown a huge magnetic anisotropy for these complexes.

Reaction of 3,4-dialkoxystilbazoles (n-3,4-OPhVPy) with silver dodecylsulphate (AgDOS) leads to complexes of the formula [Ag(n-3,4-OPhVPy)2][DOS]. These complexes are polycatenar liquid crystals and show mesophases which are characteristic of this structural type, namely, hexagonal, columnar and cubic. On cooling, some of the materials do not crystallize, but form stable glasses instead. All the mesophases have been fully characterized by small angle X-ray diffraction.

Acetylenic and diacetylenic mesomorphic monomers have been prepared in order to obtain long conjugated polymers able to give high non-linear optical hyperpolarizabilities. Here we report the synthesis of such monomers incorporating cholesteryl and methoxybiphenyl groups; their structural and thermal behaviour are described. The occurrence of mesophases in the acetylenic and diacetylenic derivatives is discussed as a function of the spacer length and of the size of the mesogen moiety. The diacetylenic derivatives containing the methoxybiphenyl groups do not exhibit any liquid-crystalline behaviour but are able to polymerize under U.V. radiation.

Structures of the thermotropic mesophases of lead(II) decanoate are reassigned following optical and X-ray diffraction studies. These results, and those of D.S.C., Raman and 207Pb N.M.R. spectroscopy, indicate formation of a lower temperature mesophase involving mainly increased lateral disorder, and a higher temperature Lα (smectic A) phase resulting from chain disordering and decreased lead-carboxylate interaction. Comparison of experimental thermodynamic data for the phase transitions with theoretical data in the literature indicates that the entropy change for the lower to higher mesophase transition is dominated by the increase in chain disorder.

Lead(II) carboxylates with even chain lengths from octanoate to octadecanoate have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction, and dilatometry, together with Raman, infrared, and 207Pb N.M.R. spectroscopy. The reassignment of the high temperature mesophase as smectic A (Lx) for the octanoate to dodecanoate is confirmed. This phase is most stable for the decanoate. The intermediate temperature mesophase is a highly ordered three dimensional lamellar structure based on ionic layers. The alkyl chains in this phase are partially disordered (melted) through the introduction of gauche conformations and show decreased lateral interactions; however, they remain largely in an extended conformation due to the constraints imposed by the ionic network. Melting of the alkyl chains occurs therefore in more than one step. Re-cooling to the crystalline phase appears to result in a different polymorphic crystalline form (albeit still lamellar) from the...

The synthesis of several 4-(4′-(4″-n-alkoxypiperidino)-trans-styryl)pyridine-N-oxides, a new liquid-crystalline series, is described. Its mesomorphic behaviour is studied, and it is shown that different members of the series exhibit partially bilayered smectic A phases; the degree of head-to-head association of the molecules, in the smectic A phase, is found to decrease when the temperature or the length of the aliphatic chain increases. This new type of smectogen should possess interesting non-linear optical properties.

Binuclear copper(11) complexes of fatty acids crystallize at room temperature in a lamellar lattice that has been characterized by X-ray diffraction. A transition to a thermotropic columnar mesophase is observed at about 110-120°C for each compound of the series n = 12 to n = 22, n being even and equal to the number of carbon atoms in the corresponding fatty acid. This columnar mesophase has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. Columns of polar copper carboxylate groups are surrounded by disordered aliphatic chains, and form a two-dimensional hexagonal lattice. The repeating unit in a column is a binuclear dicopper tetracarboxylate complex.

The thermotropic phase transitions and the mesophase structure of two long-chain alkyl pyridinium halides are described. From small angle X-ray diffraction, optical microscopy, differential scanning calorimetry and dilatometry, the single liquid-crystalline phase found in each material is identified as smectic A. It is argued that the molecules within a layer are oriented alternately and packed in an interdigitated fashion, with the anions sandwiched between the pyridinium rings.

A series of 4-cyanoalkoxybenzylidene-4'-alkylanilines (CN-nO. m; n = 3, 4, 6 and m = 4, 5, 8) have been synthesized. Their thermotropic liquid-crystalline polymorphism has been investigated using optical microscopy, differential scanning calorimetry and dilatometry. Nematic, smectic A and smectic B mesophases have been identified. The structure of the smectic phases has been studied with X-ray diffraction. Depending upon the compound and the temperature, the smectic layers have been found to be either single (A1) or double (A2) layers of molecules. Unexpected from a simple comparison with the well-known behaviour of the very similar nO. m smectogens, which belong to the class of the so-called symmetric and therefore exhibit single-layered smectics only, this structural behaviour has been interpreted to mean the importance of the dipole interactions of the terminal cyano groups. For CN-60.8, a transition has been detected at 64°C between the single and double layered structure.