SmAd Phase Research Articles

Photo-Responsive Behavior of Azobenzene Based Polar Hockey-Stick-Shaped Liquid Crystals.

A study on the photoswitching behavior of azobenzene-based polar hockey-stick-shaped liquid crystals (HSLCs) has been presented. Two new series of five phenyl rings based polar HSLCs have been designed and synthesized. Solution state photoisomerization of the synthesized materials was investigated thoroughly via UV-visible and 1 H NMR spectroscopic techniques, whereas solid-state photochromic behavior was elucidated via physical color change of the materials, solid-state UV-visible study, powder XRD, and FE-SEM techniques. The materials exhibited decent photochromic behavior for different potential applications. The thermal phase behavior of the superstructural assembly has been characterized via polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and temperature-dependent small and wide-angle X-ray scattering (SAXS/WAXS) studies. Depending upon the length of the terminal alkyl chain, nematic (N) and partially bilayer smectic A (SmAd ) phases were observed. DFT calculations revealed the favorable anti-parallel arrangement of the polar molecules that substantiate the formation of SmAd phase.

Simulation insights into the role of antiparallel molecular association in the formation of smectic A phases.

A simple dissipative particle dynamics (DPD) model is introduced, which can be used to represent a broad range of calamitic mesogens. The model allows for antiparallel association that occurs naturally in a number of mesogens with terminal dipoles, including the 4-n-alkyl-4'-cyanobiphenyl (nCB) series. Favourable antiparallel interactions lead to the formation of SmAd phases in which the layer spacing is intermediate between monolayer and bilayer. The model is easily tuned to vary the strength of antiparallel association and the SmA layer spacing, and to give either isotropic-smectic or isotropic-nematic-smectic phase sequences. The model allows for a range of other smectics: including SmA1 phases exhibiting microphase separation within layers, and smectics A structures with more complicated repeat units. For large system sizes (≥50 000 molecules) in the nematic phase, we are able to demonstrate the formation of three distinct types of cybotactic domains depending on the local interactions. Cybotactic domains are found to grow in the nematic-smectic pretransitional region as the system moves closer to TSN.

Electrical and electro-optical parameters of 4ʹ-octyl-4-cyanobiphenyl nematic liquid crystal dispersed with gold and silver nanoparticles

Thermodynamical, dielectric, optical and electro-optical characterisation of pure 8CB and its composites with gold and silver nanoparticles have been studied. Thermodynamical studies suggest a decrease in clearing temperature of the nanocomposite systems as compared to the pure system. Dielectric parameters of pure nematic liquid crystal and nanocomposites in the homeotropic and planar aligned samples have been measured in the frequency range of 1–35 MHz. Ionic conductivity increases significantly in nematic and smectic Ad (SmAd) phases, whereas dielectric anisotropy is almost unchanged for both the nanocomposites. Threshold voltage for Freederick transition, switching voltage and splay elastic constant have decreased in the case of nanocomposite systems. Relaxation frequency and activation energy of an observed relaxation mode corresponding to molecular rotation about the short axis increase in the SmAd phases of both the nanocomposites. The optical study suggests that due to dispersion of nanoparticles, the optical band gap has decreased.

New side chain liquid crystal aliphatic polycarbonate with pendant functionalized diosgenyl groups: I. Synthesis and mesomorphism

A new side chain liquid crystal aliphatic polycarbonate bearing side-functionalized diosgenyl mesogen, well-defined amphiphilic block copolymer mPEG 43 - b -P(MCC-DHO) 11 was synthesized through the ring-opening polymerization and coupling reaction. The chemical structures and average molecular weights of the monomers and copolymers obtained in this study were characterized using Fourier transform infrared (FT-IR), proton nuclear magnetic resonance (1H NMR) spectrum, and gel permeation chromatographic (GPC) measurement. Their mesomorphism was investigated with differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and variable-temperature x-ray diffraction (XRD) measurement. As a result, the monomer DHO and copolymer mPEG 43 - b -P(MCC-DHO) 11 all exhibited fan-shaped texture of a smectic A (SmA) phase on heating and cooling cycles. Furthermore, mPEG 43 - b -P(MCC-DHO) 11 revealed a LC state at body temperature. In addition, XRD suggested that mPEG 43 - b -P(MCC-DHO) 11 showed a SmAd phase and exhibited interdigitated packing of side chains.

From the discovery of the partially bilayer smectic A phase to blue phases in polar liquid crystals

The significance of Gray’s cyanobiphenyls in the discovery of the partially bilayer smectic A phase (SmAd) and in the formulation of mixtures exhibiting the blue phase is described. The properties of the SmAd phase and its relationships to molecular structures are reviewed. Induced phenomena such as the transformation of nematics into smectics, SmAd or SmA1, as well as the opposite transformation of smectics into nematics are described and discussed.

Liquid crystalline behaviour of hydrazide derivatives containing alkoxyazobenzene

Two series of dissymmetric hydrazide derivatives containing alkoxyazobenzene with nitro terminal group and octyloxy terminal group, N-4-alkoxyphenyl-N′-4-((4-nitrophenyl)azophenyl) benzohydrazide (Bn-NO2, n indicates the number of carbon atoms) and N-4-octyloxyphenyl-N′-4-((4-octyloxyphenyl)azophenyl) benzohydrazide (B8-B8), were designed and synthesised, and their liquid crystalline properties were investigated by means of differential scanning calorimetry, polarised optical microscopy and wide-angle X-ray diffraction. It was found that B8-B8 with octyloxy terminal chains displayed monolayer smectic C phase, whereas Bn-NO2 with nitro terminal group displayed SmAd phase, and intermolecular hydrogen bonding was confirmed as the driving force. In addition, the effect of hydrogen bonding, dipole–dipole interactions and steric hindrance effect on the liquid crystalline structures were also discussed.

Study of Structure Formation in Side-Chain Liquid Crystal Copolymers by Variable Temperature Fourier Transform Infrared Spectroscopy

The formation of smectic phases in side-chain liquid crystal copolymers, SCLCPs, containing sulfonic acid-based nonmesogenic units, has been investigated using variable temperature FT-IR microscopy. Two copolymers have been characterized, namely, the poly[10-(4-methoxy-4′-oxy-azobenzene) decyl methacrylate]–copoly[2-acrylamido-2-methyl-1-propanesulfonic acid]s, the X-MeOAzB/AMPS copolymers, containing X = 0.71 and 0.56 mol fraction of mesogenic side-chains, respectively. For comparative purposes the corresponding side chain liquid crystal homopolymer, poly[10-(4-methoxy-4′-oxy-azobenzene) decyl methacrylate], MeOAzB, has also been characterized. The 0.56-MeOAzB/AMPS copolymer exhibits a bilayer smectic A phase, in which the mesogenic side chains constitute one layer with a SmA1 packing arrangement and the sulfonic acid groups another; whereas in the smectic A phase shown by the 0.71-MeOAzB/AMPS copolymer, the acid groups are located within the smectic layers giving a partially interdigated SmAd phase and ...

Doping a Mixture of Two Smectogenic Liquid Crystals with Barium Titanate Nanoparticles

A mixture of two smectic liquid crystals was doped with harvested ferroelectric barium titanate nanoparticles and investigated with wide- and small-angle X-ray scattering during cooling from the isotropic phase. A decrease in the isotropic to nematic and in the nematic to partially bilayer smectic-A(d) (SmA(d)) phase transition temperatures was observed accompanied by an increase of the layer spacing in the SmA(d) phase.

Synthesis and Properties of Light-Emitting Polythiophene Derivatives Bearing Terphenyl Mesogenic Pendant

A series of polythiophenes containing terphenyl mesogenic side chain at the third position through ester bonding was synthesized by dehalogenative polycondensation. The thiophene monomers 2,5-bromo-3-{[(4-(4′-cyano)terphenyloxy)carbonyl]-methyl]}-thiophene [M(CN)] and 2,5-bromo-3-{[(4-(4′-methoxy)terphenyloxy)-carbonyl]methyl}-thiophene [M(OCH3)] were prepared via Suzuki reation and esterification reactions, in sequence. The structures and properties of the monomers and polymers were characterized and evaluated with nuclear magnetic resonance, infrared spectroscopy, thermogravimetry, differential scanning calorimetry, polarized optical microscopy, ultraviolet spectroscopy, and photoluminescence. All of the polymers were stable, losing little of their weights when heated to ≥300°C. The polymers showed good solubility and could be dissolved in common solvents such as CHCl3, THF, DMF, etc. The monomers exhibited enantiotropic SmAd phases with a bilayer arrangement in the heating and cooling processes. Due to short spacer methylene ester between the bulk terphenyl mesogenic side chain and rigid polythiophene main chain, the polymers could not exhibit liquid crystallinty at elevated temperature. The existence of the chromophoric terphenyl core endows the polymers with high photoluminescence. When their CH2Cl2 solutions were photoexcited, the polymers emitted a strong UV light about 400 nm.

A novel type of optically active helical liquid crystalline polymers: Synthesis and characterization of poly(p‐phenylene)s containing terphenyl mesogen with different terminal groups

AbstractSeries of poly(p‐phenylene)s (PPPs) containing terphenyl mesogenic pendants with cyano and methoxy terminal groups by flexible COO(CH2)6O bridge [P(CN) and P(OCH3)] are synthesized through Yamamoto polycondensation with Ni‐based complex catalysts. The effects of the structural variation on their properties, especially their mesomorphism, ultraviolet–visible (UV), and photoluminescence behaviors, are studied. All of the polymers are stable, losing little of their weights when heated to ≥340 °C. The polymers show good solubility and can be dissolved in common solvents. P(CN) with cyano terminal group shows enantiotropic SmAd phase with bilayer packing arrangement, while P(OCH3) with methoxy terminal group readily forms nematic and SmAd phase when heated and cooled. Photoexcitation of their solutions induces strong blue light emission. Compared with P(OCH3), the light‐emitting bands of polymer P(CN) is slightly redshifted to 428 nm and the emission intensity of P(CN) is much stronger, due to the existence of donor–acceptor pairs. More interestingly, both of the polymers exhibit obvious Cotton effect on the CD spectra, resulting from the predominant screw sense of the backbone. This indicates that the bulky mesogenic pendant orientating around the backbone will force the main chain with helical conformation in the long region due to steric crowdedness. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4723–4735, 2009

Synthesis and Helical Conformation of Novel Optically Active Liquid Crystalline Poly(p-phenylene)s Containing Cyanoterphenyl Mesogen as Pendant

Novel liquid crystalline poly(p-phenylene)s (PPP) bearing cyanoterphenyl mesogenic pendants with varying spacer lengths {−[C6H3−COO−terphenyl−CN]n−, P(0); −[C6H3−COO−(CH2)6−O−terphenyl−CN]n−, P(6)} were designed and synthesized, and the effects of structural variations on the liquid crystallinity behaviors and the optical properties of the polymers are studied. P(0) shows an SmA phase with faint birefringence, while P(6) readily forms the enantiotropic SmAd phase with a colorful fan texture, due to the longer flexible spacer favoring the better packing arrangements. Photoexcitation of their solutions induces strong blue light emission. The photoluminescence of P(6) in THF solution is much stronger and red-shifted to the visible spectral region, even extending to 600 nm, than that of P(0). The longer spacer may have better segregated the bulky chromophoric pendants, which effectively hampers the excitons from traveling to the quenching sites of the backbone and hence enhances the stronger emission in the photoluminescence. No significant shifts in the emission maxima are observed when the polymers are fabricated into thin films, suggesting that the segregation of the backbone effectively decreases the strong interchain interaction. Another interesting and outstanding property also could be found in this type of polymers. Due to the stric crowding, cyanoterphenyl mesogen pendants orientating around the main chain forces the main chain showing spiral conformation along the main chain in the long region; moreover, a short spacer is favorable.

Computational study through atomistic potentials of a partial bilayer liquid crystal: structure and dynamics

An ab initio derived force-field, previously (Cacelli et al., J. Phys. Chem. B, 2007, 111, 2130) tested and validated on the structural and dynamic properties of isotropic and nematic phases of the 4-alkyl-4′-cyanobiphenyl (nCB; n = 5–8) series, is employed here in molecular dynamics simulations to verify the model’s capability of reproducing the experimental partial double layer smectic phase (SmAd) shown by the 4-octyl-4′-cyanobiphenyl homologue. Lengthy (≃50 ns) NPT simulations, at 1 atm and 300 K, have been performed and the resulting mesophase was found to be in good agreement with most of the considered experimental data. Furthermore, by comparing the results obtained with systems of different size, the effect of both the number of molecules and smectic layers on mesophase stability is discussed. Particular attention has been also paid to the role of dipolar interactions within the double layer, by comparing the results obtained employing the force field with and without point charges. In the latter case, the SmAd phase was found to collapse into a single layer smectic phase. Finally dynamic properties have been computed from NVE trajectories and compared with their experimental counterparts.

Smectic ordering in side‐chain liquid crystalline polymers (LCPs) and in LCP–silica nanocomposites

The mesophase behaviour of a side‐chain liquid crystalline polyacrylate (LCP) grown by drying a solution has been investigated. This LCP, characterised by a short spacer (four carbon atoms) and a long tail (10 carbon atoms), displays, at increasing temperatures, SmC and SmAd phases. The effect of the mean molecular weight, i.e. the mean number of side chains per polyacrylate main chain (18 and 51) on the lamellar width, was studied. LCP–silica nanocomposites have been synthesised by a sol‐gel process in the presence of LCP in the solution, followed by subcritical drying. The mesophase behaviour of these nanocomposites was compared to that of the corresponding bulk LCP. The experimental methods were polarised optical microscopy, differential scanning calorimetry and synchrotron X‐ray scattering.

Synthesis and liquid crystalline properties of hydrazide derivatives: hydrogen bonding, molecular dipole, and smectic structures

A series of dissymmetric hydrazide derivatives with amino terminal group, N‐(4‐alkoxybenzoyl)‐N′‐(4′‐aminobenzoyl) hydrazine (Cn‐NH2, n indicates the number of carbon atoms), were designed and synthesised, and their liquid crystalline properties were investigated by means of differential scanning calorimetry, polarised optical microscopy, and wide‐angle X‐ray diffraction. It was found that these materials showed a SmAd phase, and intermolecular hydrogen bonding was confirmed as the driving force. Moreover, these results were further compared with those of their parent compounds, N‐(4‐alkoxybenzoyl)‐N′‐(4′‐nitrobenzoyl) hydrazine (Cn‐NO2) and the dimers, α, ω‐bis [N‐(4‐nitrobenzoyl)‐N′‐(benzoyl‐4′‐oxy) hydrazine] alkane (Nn), and the effect of hydrogen bonding, dipole‐dipole interactions on the smectic structures is discussed.

Structure and shear orientation of a side chain liquid crystal polymer studied by small angle X-ray scattering

We have studied the structure and shear flow behaviour of a side chain liquid crystal polymer, namely P12, in the smectic (SmAd) phase. Simultaneous oscillatory shear and small angle X-ray scattering experiments showed that, as expected, the shearing of P12 at high frequency and large strain amplitudes leads to the alignment of the smectic layer normals parallel to the neutral direction. It was also found that, on erasing the memory of the prior shear alignment through melting, P12 can unexpectedly orient with the smectic layers perpendicular to the shear flow direction (b-configuration). We speculate that the b-configuration is adopted due to a counter-balance between the rather long spacer in P12 which decouples mesogens from the backbone, and the oscillatory character of the shear applied to P12.

A polar biaxial smectic A phase in new unsymmetrical compounds composed of bent-core molecules

The synthesis and liquid crystalline properties of several new compounds unsymmetrically substituted about two different central phenyl rings are reported. All the compounds contain a strongly polar terminal cyano/nitro substituent. Some of the nitro-substituted compounds derived from 3-hydroxybenzoic acid show a transition from the partial bilayer uniaxial smectic A (SmAd) phase to a polar partial bilayer biaxial smectic A (SmAdPA) phase. The analogous cyano-substituted derivatives exhibit only a SmAd phase. However, compounds containing a 4-cyanobiphenyl moiety in the terminal position and derived from 1,3-dihydroxybenzene show the SmAd – SmAdPA transition. The mesophases were characterized using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction studies, conoscopic examination and miscibility studies.

Biaxial smectic A liquid crystal in a pure compound.

The synthesis and characterisation is reported of a low molecular weight organic compound to exhibit the biaxial smectic A (SmAb) phase, which shows a transition from the partial bilayer uniaxial SmAd phase to the SmAdb phase as the temperature is lowered.

Effect of chirality on phase transitions in re-entrant liquid crystals

The phase diagrams for enantiomers and their racemic counterparts have been compared in order to determine the influence of chirality on re-entrant phenomena. It has been found that chirality affects the location of the partially bilayer SmAd and monolayer smectic SmA1 phase areas, while the position of the smectic X phase remains nearly unchanged. This result shows that the N-SmA phase transition is influenced by a molecular chiral discrimination effect. Moreover, in enantiomeric systems, a N* re1-N* re2 phase transition has been detected by DSC within the re-entrant nematic gap which is limited by the smectic Ad and the SmX phases. Based on preliminary data, the SmX phase seems to be either a weakly tilted SmC antiphase or an incommensurate orthogonal mesophase.

Relaxation Phenomena in the Nematic, Nismatic Reentrant and Smectic A Phases Studied by Dielectrhic Spectroscopy

Dielectric properties and relaxation process of hinary mixture (MIX I) and quaternary mixture (MIX II), both composed of 4-alkylphenykarbonyloxy-4-eganophenyls and -4′-cyanobiphcnpls, have been studied by dielectric spectroscopy in the frequency rang from 5 fat to 10 GHz. Dielectric measurements have been done for two principal alignments of the N, SmAd and NR phases. In the caw of MIX II the parallel component of dielectric permittivity shows a pronounced decrease in the Sm Ad phase due to dipole-dipole correction. This anomaly is much weaker for MIX I. Lon frequency spectra for both mixtures consist of two well separated al, absorption peak. connected, respectively, with biphenyl and triphenyl ring entities. By fitting a sun of two Cole-COIF functions it was possible to compute dielectric parameters of the relaxation process observed. In both cases the relaxation times exhibit Arrhenius type behavior with activation energy higher in the NR than and N phases. At high frequencies (from 10MMz to 10GHz) a broad spectrum, originating from a number of relaxation processes, have been observed.

Mesogenic properties of liquid crystals having a chiral semiflexible joint

Abstract Two homologous series of mesogenic enantiomers with chiral semiflexible joint, -C*H(CH3)CH2COO-, are examined. High twist power and presence of blue phases with short lattice period are found for eleven examined compounds. Some effects of chirality in reentrant system are discussed. On phase diagram of enantiomers the SmAd phase area is shifted toward longer homologues as compared to racemates. In result, an island of the SmAd phase in the cholesteric sea exists on binary phase diagrams of R- and S- enantiomers. Triple divergence of the pitch in the cholesteric phase of a pure compound is observed.