Oriented monolayers prepared from lyotropic chromonic liquid crystal

We use a layer-by layer electrostatic self-assembly technique to obtain in-plane oriented aggregates of mesogenic dye molecules cast from lyotropic chromonic liquid crystals (LCLCs) on mica substrates. The aqueous solutions of dye used for deposition are in the nematic phase. Atomic force microscopy and X-ray photoelectron spectroscopy of the dried film reveal that the LCLC molecules adsorb at the charged substrate preserving ordered aggregates of elongated shape characteristic of the nematic phase in the aqueous solution. These elongated aggregates of LCLC molecules form films with in-plane orientational order and are compositionally distinct from the substrate.     

Dispersion and orientation of single-walled carbon nanotubes in a chromonic liquid crystal

A post-synthesis alignment of individual single-walled carbon nanotubes (SWCNTs) is desirable for translating their unique anisotropic properties to a macroscopic scale. Here, we demonstrate excellent dispersion, orientation and concomitant-polarised photoluminescence of SWCNTs in a nematic chromonic liquid crystal. The methods to obtain stable suspension are described, and order parameters of the liquid crystal matrix and of the nanotubes are measured independently.     

Salt effects on the phase behavior, structure, and rheology of chromonic liquid crystals

Chromonic liquid crystals are formed by the addition of aromatic molecules such as disodium chromoglycate (cromolyn) to water. In this study, we investigate the addition of salts to the lyotropic nematic phase of cromolyn aqueous solutions. The addition of sodium and potassium salts shifts the isotropic-nematic phase boundary upward by more than 10 degrees C, so that samples that were isotropic at room temperature are transformed into nematic phases. Salt effects are predominantly dictated by the cation, not the anion, and appear to differ based on cation size. In contrast to small, hydrated cations like sodium, large, weakly hydrated cations such as tetraethylammonium and tetrabutylammonium shift the phase boundary downward, thus stabilizing the isotropic phase at the expense of the nematic one. The phase behavior results are highly correlated with viscosity measurements, with an upward shift in the phase boundary correlating with an increase in solution viscosity and vice versa. We also probe the microstructure in cromolyn-salt solutions, both indirectly by small-angle neutron scattering (SANS) and directly by cryo-transmission electron microscopy (cryo-TEM). The cryo-TEM images show the presence of rodlike aggregates that possibly undergo a higher order aggregation into bundles in the presence of salt.     

Synthesis and phase behavior of branched esters derived from cyclohexylbenzoic acid

A branched aldehyde on the basis of cyclohexylbenzoates and 3,5-dihydroxybenzoates was synthesized. For the characteristic of intermediates and the target substance TLC, elemental analysis, IR, NMR spectroscopy, and differential scanning calorimetry were used. It was found that at the increase in length and branching degree of aldehyde the final product acquires the tendency to transfer to the glassy state.     

A sharp J-band-like electronic absorption peak observed for the chromonic liquid crystal of bisazo dye Ponceau SS in water

Abstract  

The bisazo dye Ponceau SS forms an acute oblique (V-type) dimer in dilute aqueous solution. In concentrated aqueous solution, these V-type dimers are considered to stack to form V-type blocks of chromonic liquid-crystalline (LC) columns. The weak interactions among these LC columnar V-type blocks have been spectroscopically investigated. On increasing the concentration of Ponceau SS at 25 °C, the V-type LC columnar blocks form a rhombus-type LC columnar block at 2 wt% concentration. These rhombus-type LC columnar blocks are further aligned at an acute angle at 5 and 10 wt% concentrations. The rapidly cooled 10 wt% LC sample shows an unusually sharp J-band-like peak in the electronic absorption spectrum. The emergence of this J-band-like peak has been analyzed from the viewpoint of an exciton model, suggesting that many nearest neighbor unique molecules belonging to many different rhombus-type LC columnar blocks are aligned in a head-to-tail manner to give a giant quasi-linear head-to-tail-type exciton.     

Thermotropic liquid crystal polyesters derived from 4,4′-biphenyldicarboxylic acid and oxyalkylene spacers

Several polyesters derived from bibenzoic acid and oxyalkylene glycols have been prepared and the ability of formation of mesophases was studied. The phase behaviour was investigated by means of differential scanning calorimetry, wide-angle X-ray diffraction and real-time variable-temperature small-angle X-ray diffraction. A common feature of all the studied polyesters, with ether groups in the spacer, is the inhibited ability for developing three-dimensional structures, in contrast to the polymers with all-methylene spacers. Moreover, the latter polymers show a monotropic mesophase behaviour but the former exhibit an enantiotropic transformation. Smectic structures of different types have been detected: The polyester derived from 3,3'-oxybis(1-propanol) produces a smectic S_A structure, while the polyester from triethylene glycol leads to a smectic S_C mesophase. The influence of the spacers on the dynamic mechanical relaxations of these polybibenzoates has been studied and discussed.     

Proton NMR relaxation study of molecular dynamics of chromonic liquid crystal Edicol Sunset Yellow

Proton nuclear magnetic resonance (¹H NMR) relaxometry, over about five decades in Larmor frequency, and pulsed field gradient NMR were used to study the molecular dynamics in the chromonic nematic and isotropic phases of stacked molecules of the binary mixture composed by Edicol Sunset Yellow (ESY) and deuterated water. Our results evidence that in both phases collective motions are responsible for the spin-lattice relaxation dispersion in the Larmor frequency range below 1 MHz. In the nematic phase, the collective motion are attributed to columnar undulations within the stacked molecules, while, in the isotropic phase, the results are explained by local order fluctuations due to the formation of the stacks. The high frequency dispersion was explained by individual molecular motions like rotations around and perpendicular to the stack axis, and also self-diffusion.     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

Non-conventional multi-arm ester liquid crystal derived from cholic acid: synthesis,thermal and optical behaviour

ABSTRACT

With cholic acid as the core, five multi-arm ester liquid crystals were synthesised with different terminal substituents. B₁–B₅ were mesogenic arms which were linked to multifunctional chiral core cholic acid. The effect of electron-withdrawing and electron-donating groups on the mesogenic behaviour of the compounds was discussed. All products were structurally well characterised by elemental analysis, ¹HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. B₂, B₄, B₅ and C₂, C₄, C₅ and D₂, D₄, D₅ exhibited typical nematic texture and belonged to thermotropic enantiotropic nematic liquid crystals. B₃, C₃ and D₃ also displayed typical nematic texture of thermotropic monotropic nematic liquid crystal. B₁, C₁ and D₁ did not have mesogenic performance. Wider mesogenic range for 81.6°C of C₅ whose terminal substituent is nitro on heating cycle while 121.1°C on cooling cycle. The results indicated that terminal substituents have a pretty important effect on the mesogenic phase and range of multi-arm liquid crystal compounds. The mesomorphic behaviour of compounds with polar groups terminally substituted performance much better than those without polar groups. In this ester multi-arm liquid crystal system, electron-withdrawing groups terminally substituted behave better than those electron-donating groups substituted.     

Dielectric permittivity properties of a fulgide dye guest-host liquid crystal

The dielectric permittivity of the nematic liquid crystalline mixture E7, doped with a low concentration of the photochromic material thiophene fulgide, was studied and compared with the properties of the pure E7. Fulgides are a group of thermally stable photochromic materials. On irradiation with ultraviolet light, a ring-closure occurs, giving an isomer which is stable if the dye is kept in the dark. The isomerism induces changes to the steric, dipolar and electronic conjugation properties of the species. A capacitive technique was used to determine the dielectric constants of the fulgide-doped liquid crystalline mixtures. The parallel and perpendicular components of dielectric constant were measured using a single cell, in the latter case in the presence of a magnetic field. Measurements were made on the photochromic system before and after ultraviolet irradiation and a marked variation in the dielectric properties of the mixture was observed. This was shown to be entirely due to the differences in phase transition temperatures between the irradiated and non-irradiated guest-host mixtures.     

Planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic displays

We have explored the use of lyotropic chromonic liquid crystals (LCLCs) as promising materials for optical compensators for liquid crystal displays. We used aqueous solutions of disodium cromoglycate in the range of concentrations that yield a chromonic N phase. The N phase is aligned by rubbed polyimide layers giving planar orientation. Two different types of LCLC-based elements are described: (a) a uniformly aligned planar N phase that forms an optically uniaxial plate with a negative birefringence; (b) twisted cells of LCLC obtained by doping the N phase with chiral amino acids. Both planar and twisted N* phase cells were used for compensation of the positive birefringence of twisted nematic (TN) displays; the TN devices were appreciably improved. We achieved achromatic dark state and contrast ratio up to 50:1 at all directions within a 40° cone of viewing angle.     

Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal

We report on circularly polarized light emitted from a chiral nematic liquid crystal doped with a luminescent organolanthanide dye. The organolanthanide emission displays an extremely narrow spectral bandwidth of Δ λ_E≈ 8 nm. This is considerably narrower than the CNLC selective reflection bandwidth Δ λ_R≈60 nm. When conventional dyes with broader emission bandwidths are dissolved into CNLCs, the average degree of circular polarization g of emitted light is reduced from the maximum degree g _MAX ; this is due to the overlap of the emission band with the reflection band edges, and spectral regions outside the reflection band. Here, however, we can place the entire emission band inside the reflection band and achieve g ≈ g _MAX=1.27. Furthermore, a high degree of circular polarization is maintained under off-axis viewing up to a viewing angle of ≈ 30° to the normal.     

Nonacid nitration of benzenedicarboxylic and naphthalenecarboxylic acid esters

When treated with nitrogen dioxide in the presence of ozone and a catalytic amount of iron(III) chloride in inert organic solvent at -10 to +5 degrees C, benzenedicarboxylic acid diesters 1, 4, and 6 underwent smooth nitration to give the corresponding mononitro derivatives 2/3, 5, and 7, respectively, in good yield (kyodai nitration). Naphthalenecarboxylic acid esters 8 and 11 and naphthalene-1,8-dicarboxylic acid diester 16 were similarly nitrated in the absence of catalyst to give the expected nitro compounds 9/10, 12-15, and 17-22, respectively. Different from conventional nitration based on the combined use of concentrated nitric and sulfuric acids, no hydrolytic cleavage of the ester function was observed under these conditions. The isomer distribution has been determined for the nitration of naphthalenecarboxylic acid esters 8, 11, and 16, and spectral data were collected for less common nitro derivatives. A unique changeover of the orientation mode observed in the kyodai nitration of diester 16, from the initial exclusive meta to the final meta/para, has been discussed in terms of the competition between the electrophilic substitution process involving the nitronium ion (NO2+) and the addition-elimination sequence involving the nitrogen trioxide radical (*NO3).     

Restudy of the unusual phase behavior of the mesogen-jacketed liquid crystal polymers

1 Introduction Liquid crystals (LC) are a state of order between crystals and liquids. They have imperfect long range orders of orientation and position. Thus, they can be fluid like a liquid and they can have anisotropic prop-erties like crystals. For th…     

Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal

We report on circularly polarized light emitted from a chiral nematic liquid crystal doped with a luminescent organolanthanide dye. The organolanthanide emission displays an extremely narrow spectral bandwidth of ΔλE =8 nm. This is considerably narrower than the CNLC selective reflection bandwidth ΔλR =60 nm. When conventional dyes with broader emission bandwidths are dissolved into CNLCs, the average degree of circular polarization g of emitted light is reduced from the maximum degree g MAX ; this is due to the overlap of the emission band with the reflection band edges, and spectral regions outside the reflection band. Here, however, we can place the entire emission band inside the reflection band and achieve g ≈g MAX =1.27. Furthermore, a high degree of circular polarization is maintained under off-axis viewing up to a viewing angle of ≈30° to the normal.