Structural characterization of organosiloxane liquid crystals with a transverse or longitudinal dipole

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.     

Structural and conformational investigations in SmA and different SmC phases of new hockey stick-shaped compounds

New meta -substituted homologous three-ring mesogens, the 4-(3- n -decyloxyphenyliminomethyl) phenyl 4- n -alkyloxybenzoates, have been synthesized, which are non-linear due only to the attachment of one of the alkyloxy groups in a meta -position. The mesophases were studied by optical microscopy, differential scanning calorimetry, NMR spectroscopy, X-ray diffraction, and electro-optical and dielectric measurements. Unusual phase behaviour was observed on varying the length of the terminal chain. The most interesting finding is the occurrence of two polymorphic tilted smectic phases designated as SmC 1 and SmC 2 . The existence of these phases was revealed by calorimetric studies and also from the pronounced difference in optical textures. It was shown from NMR measurements that the molecular orientation changes from a synclinic to an anticlinic arrangement in the SmC 1 to SmC 2 phase transition. It has also been shown, using NMR, that the SmC 1 →SmC 2 phase transition in these compounds is accompanied by a conformational change in the molecular fragment containing the aromatic ring with the meta -substituted terminal alkyloxy chain. This conformational change is linked to a change in the shape of the molecules and leads to a different packing of the molecules within the layers of the SmC 2 phase. From dielectric measurements an increase by a factor of two was detected in the molecular mobility at the transition into the low temperature SmC 2 phase. This finding supports a change in the packing as result of conformational changes.     

Unusual molecular packing within the low symmetry mesophases of alkyl- and alkoxy-acylhydrazinatonickel(II) liquid crystals

Three series of novel thermotropic metallomesogens have been synthesized and characterized using microscopy and DSC. Unexpected X-ray scattering is observed within all of the observed mesophases. Diffuse in-layer reflections are observed both at angles corresponding to approximately twice the molecular width and to the side-by-side separation of molecules within the nematic, SmA and SmC mesophases. Furthermore, the tilt angles within the SmC phases are observed to decrease as the terminal chain lengths decrease. Conoscopic observations show the SmC phases to be strongly biaxial as expected, but surprisingly, weak biaxiality is also observed within both the SmA and nematic mesophases. A model to explain the results is proposed, in which the cores remain orthogonal to the layers, whilst the overall molecular tilt necessary for SmC phase formation is provided by the contribution of the terminal alkyl chains to the overall molecular shape. IR spectroscopy is used to confirm the model.     

Synthesis and mesomorphic properties of 7-acyloxy-3-(4-acyloxyphenyl)-4H-1-benzopyran-4-one

A new homologous series of 7-acyloxy-3-(4-acyloxyphenyl)-4H-1-benzopyran-4-ones was synthesized and characterized by elemental analysis and spectroscopic techniques along with thermal behaviour study using differential scanning calorimetry. Texture observation was performed under the polarizing optical microscope from which the derivatives were found to exhibit different molecular organizations as exemplified by various mesophases. Whilst the compounds containing butanoyloxy and hexanoyloxy groups behave as nematogens, the other compounds with heptanoyloxy and longer side chains show a SmC phase. The octadecanoyloxy derivative was the only member showing a monotropic SmC phase. The difference in phase behaviour is discussed based on the molecular structure and the presence of a lateral dipole which can be associated with the intermolecular interaction within the mesophase.     

Antiferroelectric and ferrielectric liquid crystalline low molar mass materials and polymers

Novel liquid crystalline Low Molecular Mass (LMM) materials bearing two chiral lactate groups, as well as compounds of like structure to MHPOBC have been synthesized. All the LMM compounds exhibit the SmC*A (antiferroelectric) and/or SmC*gamma (ferrielectric) phases. The mesogens have been incorporated as pendant groups on polymer backbones of three different natures to prepare side chain liquid crystalline copolysiloxanes, homopolysiloxanes and homopolyacrylates. The investigation of the polymers by means of miscibility studies showed that the antiferro- and ferri-electric phases are strongly destabilized in the coposiloxanes and homopolysiloxanes, while in the homopolyacrylates a large temperature range mesophase is found to be miscible with the SmC*A phase. The physical properties of the mesophases and their stability, both for the LMM materials and the polymers, are presented and discussed.     

Synthesis and mesomorphic properties of some esters of trans-4-n-alkoxycinnamic and trans-4-n-alkoxy-alpha-methylcinnamic acids exhibiting ferroelectric and antiferroelectric phases

The synthesis and mesomorphic properties of several homologues of (S)-(+)-1-methylheptyl 4-[4(trans-4-n-alkoxycinnamoyloxy)benzoyloxy]benzoates (series I) and (S)-(+)-1-methylheptyl 4-[4-(trans-4-n-alkoxy-alpha-methylcinnamoyloxy)benzoyloxy]benzoates (series II) are described. The influence of the alpha -methyl group on the different mesophases and the clearing temperatures has been evaluated. Many homologues of both the series show a ferroelectric chiral smectic C (SmC*) phase while the antiferroelectric smectic C phase (SmC*A) is exhibited by compounds of series I. The mesophases have been identified by optical polarizing microscopy, differential scanning calorimetry and miscibility studies. The spontaneous polarization, the helical pitch and the tilt angle of two of the compounds have also been measured.     

Detecting the adsorption of dye molecules in homogeneous poly(propylene imine) dendrimer monolayers by surface plasmon resonance sensor

In this work, we studied guest-host interactions between various dye molecules and the fifth-generation poly(propylene imine) (PPI-5) dendrimers in aqueous solutions using a surface plasmon resonance (SPR) sensor. The effect of the properties of guest and host molecules (e.g., charge and shape) and media (e.g., pH and ion strength) on affinity between guest and host molecules was investigated. Based on an immobilized homogeneous monolayer of PPI-5 dendrimer tethered to carboxyl-terminal self-assembled monolayers, the adsorption behavior of a group of dye molecules in PPI-5 was obtained. Results show that the strong affinity of PPI-5 to Rose Bengal and erythrosine B is attributed to the good match in charge and shape between the cavities of the dendrimer and the dye molecules. Maximum adsorption around a pH value of 7 was observed. The kinetic behaviors of different dye molecules in dendrimers were also studied. A fundamental understanding of guest-host interactions in dendrimers will guide the design of new-generation sensors and drug delivery carriers.     

Novel smectic liquid crystals based on benzo[c]cinnoline: their synthesis,mesomorphism, opto- and electro-chemical properties

A series of novel calamatic liquid crystals based on a polar benzo[c]cinnoline moiety were efficiently prepared through a facile route. Rich smectic mesophases were induced by the monoalkylated and dialkylated molecular design, including highly ordered smectic mesophases in the rectangular and hexagonal orders. Dialkylated phenylbenzocinnoline derivatives showed a very wide temperature range over 150°C for smectic C (SmC) phase, while the monoalkylated ones only presented the low-ordered mesophases, which exhibited a bilayer structure in crystalline phase. From pronounced reversible redox waves in a cyclic voltammogram and low-lying lowest unoccupied molecular orbital level of about ?3.2 eV indicated possible electron-transporting behaviour. In addition, a switching behaviour originating from ferroelectricity in SmC* induced by chiral dopant was observed.     

Thermal analysis and X-ray studies of chiral ferroelectric liquid crystalline materials and their binary mixtures

Summary Thermal properties of a homologous series of ferroelectric liquid crystals S-(-)-[4-(2-n-alkoxy-propionyloxy)]biphenyl-4'-[n-alkoxy-(3,5-dimethyl)]benzoate have been investigated by polarizing optical microscopy and differential scanning calorimetry. The mesophases were identified and confirmed by X-ray too. Three binary mixtures were prepared from the individual homologues. In one of the mixtures (Mix1), the ferroelectric SmC* phase has broadened and became enantiotropic. This mesophase remained monotropic in the other two mixtures (Mix2, Mix3). The chiral nematic N* phase did not appear in Mix1, but remained monotropic for the other two mixtures. Two molecular parameters, the layer spacing and the average intermolecular distance have been calculated from the X-ray results for the homologues and their mixtures. An intercalated tail-to-tail packing of molecules was found both in the single compounds and their mixtures resulting in the layer spacing about half of the molecular length of the single compounds.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^-2 on reversal of an applied electric field.     

Symmetric 3,5-pyrazole and isoxazole heterocycles comprising a bent core unit: synthesis and mesomorphic characterisation

The synthesis and characterisation of some new liquid crystalline (LC) heteroaromatic compounds containing the five-membered pyrazole/isoxazole rings is reported. Some of the compounds exhibited enantiotropic LC properties. The transition temperatures and LC textures of the mesophases were determined using polarised optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray studies. The five-membered heterocyclic compounds with the pyrazole core exhibit smectic C (SmC) phase, whereas isoxazoles show SmC and nematic (N) phases.     

Ferroelectric liquid crystals from banana-shaped achiral molecules with vinyl end groups

Banana-shaped achiral compounds, the 1,3-phenylene bis[4-(alkenyloxyphenyliminomethyl)benzoate]s, were synthesized with varying length of the alkenyl group; their ferroelectric properties are described. The smectic mesophases, including a switchable chiral smectic C (SmC*) phase, were characterized by differential scanning calorimetry, polarizing optical microscopy and the triangular wave method. The presence of vinyl groups at the ends of the linear side-wings in the banana-shaped achiral molecules, containing a Schiff's base mesogen, induced a decrease in melting temperature and formation of the switchable SmC* phase in the melt. The compound having the octenyloxy group exhibited a spontaneous polarization of 120 nC cm^?2 on reversal of an applied electric field.     

Synthesis and mesomorphic properties of fluoro-substituted chiral liquid crystals derived from (S)-lactic acid with alkoxyethanols

A homologous series of fluoro-substituted chiral liquid crystals derived from (S)-lactic acid and alkoxyethanols were prepared for investigation. Mesophases and their corresponding transition temperatures were identified by polarized optical microscopy and differential scanning calorimetry. The compounds exhibit a broad temperature range in the ferroelectric chiral smectic C (SmC*) phase; in particular, compounds with shorter alkyl chain lengths have an SmC* phase at ambient temperature. The physical properties of the ferroelectric SmC* phase, such as switching current, spontaneous polarization, optical tilt angle and electro-optical response, were also measured. The effects of fluoro substituent on the mesophases and physical properties of the chiral liquid crystals are discussed.     

Mesophase behaviour of laterally di-fluoro-substituted four-ring compounds

Four new groups of the di-fluoro-substituted 4-(2′-(or 3′)-fluoro phenylazo)-2-(or 3-) fluoro phenyl-4″-alkoxyphenylazo benzoates (In–IVn) were prepared and investigated for their mesophase behaviour. An alkoxy group of variable chain length (n = 6, 10 and 14 carbons) is attached to the terminal phenylazo benzoate moiety, and two lateral fluoro substituents are attached individually with different orientations to the other two adjacent rings. The molecular structures of the prepared compounds were confirmed by Fourier transform infrared spectroscopy and ¹H NMR spectroscopy. The study aims to investigate the steric effect of the spatial orientation and relative positions of the two lateral fluorine atoms on the mesomorphic properties in their pure states. The mesophase behaviour was investigated via differential scanning calorimetry and mesophases were identified by polarised light microscopy. The investigation shows that these compounds exhibit high enantiotropic mesophases (SmC and N) and broad mesophase temperature range. The type and stability of the mesophase depends on the length of the terminal alkoxy chain and the position the two fluoro substituents. A comparison between these investigated compounds with their corresponding three-ring analogues was discussed.     

Elliptical diffusion of dye in hexagonal columnar polycatenar mesophases

We have studied polycatenar compounds which exhibit hexagonal columnar mesophases. In the planar orientation of these mesophases, the elliptical diffusion of the dissolved dyes is visualized by taking several pictures of the sample. The diffusion ratio D// /D is deduced for these columnar mesophases. Furthermore, using a classical optical absorption technique, we present measurements of dye diffusion in the same mesophases. The diffusion constants are measured in two geometries, along and perpendicular to the columns. The diffusion anisotropy ratios are in agreement with those deduced from the ellipse axes. The structure of these new columnar mesophases exhibited by rod-like mesogens is compared with that of disk-like mesogens.     

Self-association of rhodamine dyes in different host materials

The aggregation of rhodamine 6G in liquid crystalline solution (anisotropic host) was studied using polarised spectroscopy and in a guest-host system. The self-association of rhodamine B was investigated in molecular sieves of type AlPO(4)-5 (microporous host) using diffuse reflectance spectroscopy. Also, the molecular interaction of rhodamines in normal solvents (isotropic hosts) was studied using visible spectroscopy for comparison. Therefore, the role of the host nature in the different phases on the self-association of the guest molecules has been investigated and compared. The absorption spectrum of the rhodamine dye in liquid crystalline host is affected by a specific interaction related to the alignment by the liquid crystal property as well as solvent polarity. Due to the existence of a large amount of water molecules absorbed into channels and cavities of aluminophosphate molecular sieve, the maximum absorption wavelengths of the dye loaded AlPO(4)-5 is affected by aqueous environment of the aluminophosphate pores.     

Elliptical diffusion of dye in hexagonal columnar polycatenar mesophases

We have studied polycatenar compounds which exhibit hexagonal columnar mesophases. In the planar orientation of these mesophases, the elliptical diffusion of the dissolved dyes is visualized by taking several pictures of the sample. The diffusion ratio D// /D is deduced for these columnar mesophases. Furthermore, using a classical optical absorption technique, we present measurements of dye diffusion in the same mesophases. The diffusion constants are measured in two geometries, along and perpendicular to the columns. The diffusion anisotropy ratios are in agreement with those deduced from the ellipse axes. The structure of these new columnar mesophases exhibited by rod-like mesogens is compared with that of disk-like mesogens.     

Chromonic liquid crystals formed by CI Acid Red 266 and related structures

Chromonic liquid crystals are currently receiving increased attention because they have applications in a wide range of products. In this study, we have compared the chromonic mesophase behaviour of four azo dyes with similar chemical structures. Our objective is to determine if there is an obvious link between mesophase formation and dye chemical structure. Orange G does not form mesophases over the concentration range examined (saturated solution > ~20–30 wt%). The other three compounds all form nematic (N) and hexagonal (H) mesophases, but over very different concentration ranges. X-ray diffraction shows that the ordered Edicol Sunset Yellow (ESY) aggregates present in the mesophases have a single molecule cross section, while those of CI Acid Red have a cross section equivalent to six to eight molecules, probably organised in a ‘water-filled pipes’ structure. NMR quadrupole splittings of ²H₂O demonstrate that water binding to the aggregates is similar to that found for surfactant lyotropic mesophases. The sodium (²³Na) quadrupole splittings for Orange II and CI Acid Red are similar to the values found for surfactant hexagonal phases, suggesting that most sodium ions are ‘bound’ to the aggregates. This is unlike the behaviour of ESY where only one of the two sodium ions is bound.     

Novel antiferroelectric liquid crystals with a phenylpiperazine moiety in the mesogenic core structure

A novel series of antiferroelectric liquid crystalline compounds having a phenylpiperazine unit and a biphenyl or a phenyl moiety in the core structure was synthesized using the optically active alcohols 2-methyl octanol and 1,1,1-trifluoromethyl octanol. Most of the compounds exhibited enantiotropic SmC*A, SmC* and SmA mesophases. The phase transition temperatures, response times, sponta A neous polarizations and the field-transmittance hysteresis loops were monitored. These data were compared with those of the MHPOBC compounds, which represent the most conventional core structure of the biphenyl/phenyl combination. The replacement of the biphenyl moiety, with the phenylpiperazine unit, reduced the isotropization temperature as well as the SmC*A-SmC transition point. It was found that some of the new compounds, which consisted ofAan octyl or a nonyl terminal chain with a 2-methyloctyl 2-fluorophenylcaboxylate group in the core structure, could have a potential application in display devices. Some work on blends of these compounds is also described.     

Lyotropic liquid-crystalline mesophases and a novel, solid physical form of some water-soluble, reactive dyes

A novel, solid physical form has been observed when some water-soluble, reactive dyes are isolated from aqueous solution, as sodium salts, by the addition of sodium chloride. This quasi-crystalline form has a fibrous morphology, is birefringent but is not crystalline. Dyes of this type are known to form lyotropic liquid-crystalline mesophases in water. Preliminary X-ray diffraction investigations, reported here, for the mesophases formed by two such dyes indicate that they have columnar structures of the type first proposed for the lyotropic mesophases of the disodium chromglycate/water system and subsequently for other drug and dye molecules. X-ray and electron diffraction studies of the quasi-crystalline form show that it has a closely related columnar structure. The quasi-crystalline form is postulated to result from the formation and subsequent precipitation of columnar dye aggregates, as sodium chloride is added to the aqueous dye solution.