Enantiotropic SmC Research Articles

Preparation and properties of novel liquid crystal ionomers bearing benzoxazole-based mesogenic side-chains and their membranes

A new heterocyclic mesogenic compound, 6-[4-(5-chloro-2-benzoxazolyl)phenoxy]-hexanethiol (CBPHT), is designed and synthesized, which shows an enantiotropic SmC mesophase with the liquid crystal phase ranges of 5.67 °C and 22.99 °C on heating and cooling, respectively. The CBPHT-based side-chain liquid crystal polymers (LCP-x) are obtained via a nucleophilic substitution between CBPHT and polyepichlorohydrin, and also shows an enantiotropic SmC mesophase with narrow temperature range. Furthermore, novel side-chain liquid crystal ionomers (LCI-x) with enantiotropic SmC mesophase are prepared via quaternization of N-methylimidazole with LCP-x, where polyepichlorohydrin, CBPHT and methylimidazolium chloride act as polymer main-chain, mesogenic side-chain, and ionic group, respectively. In addition, LCI-x membranes can be fabricated via a solution casting method, and exhibit high thermal stability, good mechanical properties and high ion conductivity. For example, LCI-0.20 with a low ion exchange capacity of 0.12 mequiv. g−1 has a large elongation at break (385 %) and a high chloride ion conductivity (20.68 mS cm−1), indicating a high potential for application as polymer electrolyte membrane.

Design of unsymmetric coumarin chalcone derivatives with tunable self-assembling behavior

Unsymmetric coumarin chalcone derivatives were designed with unsymmetric alkoxy chain on both terminal of the molecules to find their effect on the overall mesomorphic properties. In the first homologous series, compounds with fixed octyloxy chain on chalcone end and varied alkoxy chain length at coumarin end of the molecule (n = 10,12,14,16,18) have been analyzed for their mesomorphic properties. In this series, all the Compounds showed stable enantiotropic SmC mesophase. In the second homologous series, compounds with fixed octyloxy chain on coumarin end and varied alkoxy chain length at chalcone end (n = 10,12,14,16,18) have been studied for their mesomorphic properties. In the second homologues series, compounds (n = 10,12) exhibited monotropic nematic mesophase. For higher analogues (n = 16,18), enantiotropic SmA mesophase was observed. Further, the powder X-ray diffraction analysis of the three compounds from both the series at 112 °C showed diffraction pattern quite close to identified mesophase type. To understand effect of variation of terminal chain, DFT theoretical study was carried out for both different series and tried to understand correlation between mesomorphic properties and structural parameters.

Colorful patterned polyacrylate films prepared using cholesteric liquid-crystalline mixtures with a smectic order

ABSTRACT Polymer-stabilised cholesteric liquid crystal (PSCLC) films with selective circularly polarised light reflection have attracted much attention for their applications as polarisers, energy-saving windows and for displays. Herein, CLC mixtures were prepared using a nematic LC LC242 and a chiral compound S-6 with enantiotropic SmA and SmC* phases, which exhibited a cholesteric phase with a smectic order. After photopolymerisation, the PSCLC films with fingerprint structure at the surfaces and supramolecular helical structure inside were obtained. Due to the existence of short-range smectic order in the cholesteric structure, the Bragg reflection bands were broadened. For a CLC mixture, with increasing temperature, the short-range smectic order was suppressed, and the selective Bragg reflection band shifted to the short wavelength. Based on this thermochromic behaviour, colourful PSCLC patterns and gratings were prepared, which were able to be applied in decoration and anti-counterfeiting. Since the PSCLC films can be obtained within several seconds under air, large-area films can be prepared on coating lines at low-cost.

Anti-counterfeiting Behaviour of Organic-Inorganic Hybrid Silica Film with a Chiral Smectic C Structure.

Artificial photonic materials based on chiral liquid-crystalline nanostructures have attracted increasing interests for their wide applications as sensors, anti-counterfeit measures, displays and colour filters. Although the structurally coloured films with chiral nematic structures or blue phases have been prepared, the ones fabricated by fixing chiral smectic C (SmC*) phases have been rarely reported. In this work, organic-inorganic hybrid silica (OIHS) films with a SmC* structure were reported for the first time. An organosilane (CSC) with an enantiotropic SmC* phase was synthesized. The OIHS films with a SmC* structure were fabricated by the polycondensation of CSC under an acidic condition. The colour patterns can be observed in the oblique view, but not be observed in the vertical view, which is mainly due to the light scattering of the nanoparticles on film surface and the selective Bragg reflection of film inside. Therefore, such properties make the OIHS films promising candidates for anti-counterfeiting applications.

Mesomorphic behaviors of some acrylate liquid crystals and their application for polarizer preparation

ABSTRACT Two series of acrylate liquid crystals with enantiotropic SmA and SmC phases were synthesised. With increasing the alkoxy chain length, SmC phase was favourable to be formed. The mesophases were confirmed by taking small angle X-ray scattering patterns. Moreover, a diacrylate liquid crystal with an enantiotropic SmA phase was synthesised. Smectic liquid crystal mixtures were prepared using the diacrylate liquid crystal (or a mixture of an acrylate liquid crystal and the diacrylate one) and a dichroic dye. After photopolymerisation at the SmA phase, polymeric polarisers were obtained. The degree of polarisation was increased with increasing the dye concentration and film thickness.

A series of organosilane liquid crystals and their application for the preparation of structurally colored organic–inorganic hybrid silica films

ABSTRACT A series of organosilane liquid crystals (OSn) with different alkoxy chain lengths were synthesised. Both enantiotropic nematic and SmC phases were identified. With increasing the alkoxy chain length, SmC phase is feasible to be formed. Both the clearing points and SmC-nematic transition temperatures show odd-even effects. Small angle X-ray scattering characterisation indicated that the SmC phase of OS6 showed a mono-layered structure. Cholesteric liquid crystal mixtures were prepared using OS1, a bola-type organosilane (Bola-1) and a chiral dopant. After polycondensation under an acidic condition, structurally coloured organic–inorganic hybrid silica films were obtained. With increasing the weight ratio of Bola-1/OS1, the reflection band of the film shifted to short wavelength.

Lateral fluoro-substitution driven molecular packing difference in a series of pyridinium-based chiral ionic liquid crystals

ABSTRACT A series of pyridinium-based chiral ionic liquid crystals (ILCs) and the intermediated, non-ionic ones (NILCs), were designed and synthesised. All ILCs exhibit enantiotropic SmA phases, while all NILCs exhibit enantiotropic cholesteric and SmA phases. In addition, few of NILCs and ILCs exhibit enantiotropic SmC* phases. Due to the Coulombic force, the clearing points of the ILCs are higher than those of the corresponding NILCs. For ILCs, the lateral fluoro-substitution not only decreases the clearing points, but also drives the formation of bilayer molecular packing structures at the smectic phases.

Molecular packing handedness dominated by the chirality of the lactic acid residue near the liquid crystalline core

ABSTRACT Eight dichiral compounds containing two lactic acid residues and one perfluorocarbon chain are synthesised. Six of them are liquid crystals with an enantiotropic SmA phase. Two of them are liquid crystals with enantiotropic SmA and SmC* phases. Although (R,R)-3 and (R,R)-4 molecules pack in monolayer structures at the crystalline states, (S,R)-3 and (R,S)-4 molecules pack in bilayer structures at the crystalline states. The optical activity of these compounds is mainly controlled by the chirality of the lactic acid residue near the liquid crystalline core. The dichiral compounds can self-assemble into nanotubes or twisted nanoribbons in methanol. Circular dichroism characterisation indicated that the stacking handedness of the liquid crystalline cores in the organic self-assemblies was also dominated by the chirality of the lactic acid residue near the liquid crystalline core.

A series of fluorinated liquid crystals with an alanine residue

ABSTRACTA series of fluorinated compounds with an L-alanine residue are synthesised. Both enantiotropic SmA and SmC* phases were identified. The clearing points decreased with increasing the alkoxy chain length. The intramolecular hydrogen bond was proposed to decrease the melting points of them and broaden the temperature ranges of the mesophases. The optical purity also played an important role in the thermostability of the mesophases. The compounds derived from L-butyrine and L-norvaline also show SmA and SmC* phases. The clearing point decreases with increasing the alkyl chain length of the amino acid residue.

Liquid crystalline behaviours of some 3-perfluoroalkylbenzoate esters

ABSTRACTA series of 3-perfluoroalkylbenzoate esters are synthesised. They tend to exhibit enantiotropic SmA and SmC phases. The clearing points are decreased with increasing the alkoxy chain length and show odd-even effect. With increasing the fluorocarbon chain length, the melting and clearing points increase; however, the SmC phase is suppressed.

Terminal Effect of the Semi-fluorocarbon Chain on the Liquid Crystalline Behaviors of Some Compounds

Two series of chiral compounds with semi-perfluorocarbon chains and alkoxy ones, nBP4F and nBP6F, are synthesized. Enantiotropic SmA phase is identified in all of these compounds. When the alkoxy chain length is short, enantiotropic SmB is also identified. When the alkoxy chain length is longer than 6 carbons, enantiotropic SmC* phase is formed. Their clearing points increase with increasing the perfluorocarbon chain length. Comparison of compounds with -CF3, -CHF2 and -CF2Cl groups at the terminals of the fluorocarbon chains, the clearing point sequence is -CHF2 < -CF2Cl < -CF3; and the melting point sequence is -CF3 < -CHF2 < -CF2Cl. Therefore, the compounds with semi-perfluorocarbon chains show the widest liquid crystalline temperature range. Moreover, the SmC* phase of the compounds with -CF3 at the terminals shows the highest thermostability.

Luminescent mesogenic borondifluoride complexes with the Schiff bases containing salicylideneamines and β-enaminoketones core systems

Three new families of borondifluoride complexes 1a–c derived from salicylideneamines 2a and β-enaminoketonates 2b–c were reported, and their mesomorphic and optical properties were also investigated. One single crystal and molecular structure of nonmesogenic BF2 complex 1c (n=10) was resolved and the geometry of the central boron atom was tetrahedron. A larger dihedral angle of 81.3° between the two phenyl rings observed in crystal lattice was attributed to the lack of liquid crystallinity. Boron complexes 1a formed monotropic SmA phases, while boron complexes 1b exhibited enantiotropic SmC mesophases. The optical property of the boron complexes was dependent on their molecular structures, and they emitted a blue–to–green emission at λmax=476–541nm in the solution and 488–550nm in the solid state. This is the first group of mesogenic BF2 complexes with the Schiff bases derived from respective salicylideneamines and β-enaminoketones.

Synthesis, characterization and phase transition studies in 4-hexyloxy benzylidene 4’-alkoxyanilines

ABSTRACTSynthesis and the characterization through polarizing optical microscope (POM) textures in number of 4-hexyloxy benzylidene 4’-alkoxyanilines, 6O.Om with m = 4, 6 to 10 liquid crystalline materials are carried out. Furthermore, the density and thermal expansion coefficient results reveal that the phase transitions present, viz., isotropic–nematic, nematic–SmC and SmC–SmI show first-order nature as expected. The transition temperatures obtained thorough the differential scanning calorimetry is found to be in agreement with the literature data. The first two compounds exhibit only the nematic phase while the next four compounds show enantiotropic SmC phase as per the literature data. The compound with m = 10 exhibits monotropic SmI phase. However, we, the authors, are able to observe this phase in addition to SmC and nematic through POM only as per the literature. The parameters calculated across the phase transitions and in the phases are in agreement with the body of the data available.

Induction of ionic smectic C phases: a systematic study of alkyl-linked guanidinium-based liquid crystals

ABSTRACTA series of ionic liquid crystals with an alkoxy biphenyl unit tethered via an alkyl spacer to a guanidinium head group were synthesised and the mesomorphic properties were studied by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction (XRD; WAXS and SAXS). Whereas all symmetrical guanidinium chlorides with the same chain lengths in alkyl tail and spacer displayed enantiotropic SmA2 phases, monotropic SmC2 phases with 1–2 K temperature range were only formed for chain lengths ≥ C10. Shifting the calamitic core more closely to the ionic head group by decreasing the tether length and simultaneously increasing the terminal alkyl chain improved the stability of both SmA and SmC phases considerably and led to enantiotropic SmC phases for the guanidinium chloride with C14 alkyl tail and C6 spacer. An even more pronounced effect was detected during anion exchange. Bromide, iodide, hexafluorophosphate, thiocyanate and triflate suppressed any SmC phase, whereas tetrafluoroborate behaved similar to chloride maintaining the SmC phase. However, acetate stabilised the SmC phase at the expense of the SmA phase. Based on temperature-dependant XRD measurements, a bilayer structure was proposed.

Optically active, three-ring calamitic liquid crystals: the occurrence of frustrated, helical and polar fluid mesophases

Five enantiomeric (R&amp;S) pairs of liquid crystalline Schiff-bases prepared show a ferroelectric SmC* phase, besides N* and BP phases.

Synthesis, Characterization And Mesomorphic Properties of Side Chain Liquid Crystalline Oligomer Having Schiff Base Type Mesogenic Group

A new liquid crystalline oligomer OLC was synthesized from its monomer having Schiff-base type mesogenic group MLC via free radical polymerization. The chemical structures of all compounds were confirmed using UV, FTIR, 1 H NMR, 13 C NMR and MS spectroscopy. Schiff-base type thermotropic system based on side chain oligomer containing long aliphatic branching was studied to determine the change in mesophase behaviour of the Schiff-base type mesogenic groups. A combination of differantial scanning calorimetry (DSC) and polarize optical microscopy (POM) demonstrated that the oligomer behaves similar to its monomer and both of them exhibit enantiotropic SmC and monotropic SmX mesophases. The oligomerization of the liquid crystal monomer gives rise to decreased transition temperatures whereas it has no influence on the type and stability of the mesophase formed. Namely, simply through oligomerization, we can greatly vary transition temperatures of the mesogenic groups.

Tuning the mesogenic properties of 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals: the effect of a phenoxy end-group in two sterically equivalent series

Two sterically equivalent series of phenoxy-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals were synthesised, and their mesogenic properties were characterised by polarised optical microscopy and differential scanning calorimetry (DSC). The phenoxy end-group causes a significant increase in melting point and inhibits – at least partially – the mesomorphism of these materials relative to the parent isomers; in most cases, the broad enantiotropic SmC phase formed by the parent isomers is suppressed by the addition of the phenoxy end-group. However, detailed analyses by small-angle X-ray scattering and monodomain 2D X-ray scattering suggest that these compounds form a SmA phase with a partially intercalated bilayer structure in which the phenoxy end-groups are nanosegregated. Such an intercalated bilayer structure might enable the tuning of smectogenic properties by appropriate substitution of the phenoxy end-groups.

Synthesis, characterization and mesomorphic investigations of ester-substituted aroylhydrazones possessing a lateral hydroxyl group

The synthesis, characterization and investigation of a new liquid crystalline series of ester containing aroylhydrazones with a lateral hydroxyl group, N-[4-(4′-alkoxy)benzoyloxy-2-hydroxy-benzylidene]-N′-[4″-alkoxybenzoyl]hydrazine (Cm,nLH), with the same or different peripheral alkoxy chains, and some of their nickel(II) and copper(II) complexes are described. All the ligands, except those with no terminal chains on either end of the molecule, exhibit an enantiotropic SmC mesophase, as evidenced by polarizing optical microscopy and differential scanning calorimetric studies. It has been found that addition of a lateral hydroxyl group along with ester-substitution to the aroylhydrazone core increases the mesomorphic as well as thermal stability of the ligands. Also, the coordination of Ni2+ and Cu2+ with the aroylhydraozone core inhibits the mesomorphic potential of the ligands. A temperature dependent Raman study of one of the members, C12,12LH (m=n=12) has been made to identify phase transitions and to understand the molecular rearrangement as a result of changes in intermolecular interactions at the phase transition. DFT calculations have been performed to obtain the stable electronic structure of the ligand (C6,6LH) and its nickel(II) complex.

Ferroelectric liquid crystals with axially chiral 2,2′-spirobiindan-1,1′-dione cores

The axially chiral mesogens 5-alkoxy-5′-[(4-alkoxybenzoyl)oxy]-2,2′-spirobiindan-1,1′-dione (QL7-n) were synthesised as racemic mixtures and resolved as single enantiomers by preparative chiral phase HPLC. The shorter homologues (R)-QL7-8 and QL7-9 form enantiotropic N* and SmC* phases, whereas the longer homologues (R)-QL7-10 and QL7-12 form an enantiotropic SmC* phase only. Texture analysis showed that the helical pitch of the N* phase is below the wavelength range of visible light; a helical pitch of 187 nm was measured at T – TNI = –5 K by selective reflection using enantiomerically enriched mixtures of (R)- and (S)-QL7-8 (0.35 ≤ ee ≤ 0.80). Spontaneous polarisations were measured as a function of temperature in the SmC* phase by the triangular wave method. PS values at saturation range from 102 nC cm–2 for (R)-QL7-8 to 120 nC cm−2 for (R)-QL7-9, which are up to three times greater than the highest polarisation previously reported for an axially chiral SmC* mesogen. Optical tilt angles θ were measured as a function of temperature and showed a sharp rise consistent with first-order N*–SmC* or I–SmC* transitions, with values at saturation ranging from 42 to 44°.

Synthesis and smectogenic properties of novel phloroglucinol-based star-shaped liquid crystals containing three peripheral alkyloxylated Schiff base arms

A series of symmetrical trimeric liquid crystalline compounds of which the molecular structure with a central core of 1,3,5-benzene attached by three rod-like mesogenic Schiff base moieties via the propylene spacers and ether linkages has successfully been synthesised and characterised by infra red and nuclear magnetic resonance spectroscopic techniques. All the star-shaped compounds in this series exhibit predominantly SmC phase except the analogue possessing terminal C8H17 group. It is apparent that the members with even parity from C10H21 to C16H33 show enantiotropic SmC phase while the member with longest terminal chain of C18H37 is inclined to monotropic smectogen. The X-ray diffraction measurements reveal that the tilted smectic layer structures of the SmC phase are confirmed to have an obvious sharp peak at small angles of 2θ ∼ 1.03°–1.48° with d-spacing values of 4.01–4.58 nm, which are corresponding to tilt angles of ∼48° in the SmC phase.