Characterisation of azo dye and carbon nanoparticle-doped guest–host liquid crystalline matrix

During the recent years, liquid crystals (LCs) have much attention owing to their wide range of possible application and structural properties in electronics and optics. In this study, two different azo structured dyes (Disperse Yellow 3 and 7) and their mixture were separately doped to each of two different nematic LCs (E8 and E63). Their solubilities (except dye mixture), order parameters, textures and phase transition temperatures were determined. Single-walled nanotubes and fullerene C60 in a small amount were separately added to each of these solutions, and the experiments were repeated at the final stage analogously except solubilities. The solubilities of dyes in the LC E63 were lower than those of E8. The highest order parameter value was attained with Yellow 7 dye in E63 nematic host.     

Orientation in Nematic Liquid Crystals Doped with Orange Dyes and Effect of Carbon Nanoparticles

Some properties of nematic liquid crystal E7 doped with two disperse orange dyes used together and effect of addition of carbon nanoparticles (single walled carbon nanotube or fullerene C₆₀) on them were studied. Two dyes (disperse orange 11 and 13) having high solubility and order parameter were used as co-dopants. A notable increase in order pa-rameter was obtained comparing to that of liquid crystal doped with single dye. When carbon nanoparticles were used as dopant, a decrease in order parameter was observed at low temperatures while it increased at high temperatures. When applied voltage changed, the order parameter abruptly increased in its threshold value and saturated in higher voltages as expected. An appreciable change in textures was not observed with addition of dopants. This addition gave rise to an increase in nematic-isotropic phase transition temperatures compared with that of pure liquid crystal.     

Mesomorphic phase transitions and electro-optical properties of benzyl fluoroalkyl thioether liquid crystalline compounds

Four series of liquid crystalline (LC) compounds containing benzyl perfluoroalkyl thioether groups (BFT), 4-n-alkoxyphenylbenzyl perfluoroalkyl thioethers (nO-BBFT-FmF), 4-n-alkoxyphenylbenzyl perfluoroisoalkyl thioethers (nO-BBFT-FmIF), 4-n-propylcyclohexylbenzyl perfluoroalkyl thioethers (3-C1BFT-FmF) and 4-ethylcyclohexyl-3'-fluorobenzyl perfluoroalkyl thioethers (2-C1FBFT-FmF), were synthesized and their mesomorphic phase transitions and electro-optical properties investigated. nO-BBFT-FmF and nO-BBFT-FmIF have SmA, SmB and CrE phases, while 3-C1BFT-FmF and 2-C1FBFT-FmF showed a SmB phase. It was found that the appearance of a LC phase and the thermal stability were closely connected with the chemical structures of the rigid core part and the terminal groups. The electrooptical properties were investigated for BFT using ZLI-1132 as nematic solvent. Some of the BFT compounds have a good solubility of 15 wt % in ZLI-1132 and can be used as a component for lowering the refractive anisotropy (Delta n) of the host ZLI-1132 solvent.     

Mesomorphic phase transitions and electro-optical properties of benzyl fluoroalkyl thioether liquid crystalline compounds

Four series of liquid crystalline (LC) compounds containing benzyl perfluoroalkyl thioether groups (BFT), 4-n-alkoxyphenylbenzyl perfluoroalkyl thioethers (nO-BBFT-FmF), 4-n-alkoxyphenylbenzyl perfluoroisoalkyl thioethers (nO-BBFT-FmIF), 4-n-propylcyclohexylbenzyl perfluoroalkyl thioethers (3-C1BFT-FmF) and 4-ethylcyclohexyl-3'-fluorobenzyl perfluoroalkyl thioethers (2-C1FBFT-FmF), were synthesized and their mesomorphic phase transitions and electro-optical properties investigated. nO-BBFT-FmF and nO-BBFT-FmIF have SmA, SmB and CrE phases, while 3-C1BFT-FmF and 2-C1FBFT-FmF showed a SmB phase. It was found that the appearance of a LC phase and the thermal stability were closely connected with the chemical structures of the rigid core part and the terminal groups. The electrooptical properties were investigated for BFT using ZLI-1132 as nematic solvent. Some of the BFT compounds have a good solubility of 15 wt % in ZLI-1132 and can be used as a component for lowering the refractive anisotropy (Delta n) of the host ZLI-1132 solvent.     

Mesomorphic phase transitions and electro-optical properties of benzyl fluoroalkyl ether derivatives

Three series of liquid crystalline (LC) compounds containing benzyl fluoroalkyl ether groups (BFE), 4-n-propylbicyclohexylbenzyl fluoroalkyl ethers (3-C2BFE-X,X: fluoroalkyl group), 4-n-alkylcyclohexylbenzyl fluoroalkyl ethers (m-C1BFE-X, m: carbon no. ofn-alkyl group)and 4-n-alkoxyphenylbenzyl fluoroalkyl ethers (mO-BBFE-X), were newly synthesized for several fluoroalkyl groups (X) such asn-fluoroalkyl, branched fluoroalkyl, and 2-perfluoropropoxy-2,3,3,3-tetrafluoropropyl. The mesomorphic phase transitions and electro-optical properties were investigated for BFE. 3-C2BFE-X and m O-BBFE-X have SmB phases, and SmA and/or E phases, respectively, while m-C1BFE-X showed no mesophases. The occurrence and thermal stability of the LC phases are related to the chemical structures of the rigid core part and the terminal groups. The electro-optical properties were investigated for 15 wt% of BFE in host ZLI-1132. It was found that the BFE compounds have a good solubility of more than 15 wt% in ZLI-1132 and can be used as a component for lowering the refractive anisotropy of the host ZLI-1132 solvent.     

Preliminary communication Electro-optical properties of benzyl fluoroalkyl ether derivatives

Several new fluorinated liquid crystal compounds, 4- n -propylbicyclohexylbenzyl fluoroalkyl ethers (C2BFE) and a 4- n -propylcyclohexylbenzyl 2-perfluoropropoxy-2,3,3,3-tetrafluoropropyl ether (C1BFE), were synthesized and their phase transition behaviours and electro-optical properties investigated by DSC, polarizing optical microscopy, Abbe refractometry and other physical methods. All the C2BFE compounds showed a layered mesophase like SmB, while C2BFE had no mesophase. All the fluorinated compounds have good solubility in ZLI-1132 (more than 15 wt %) and can be used as a component for lowering the refractive anisotropy (Delta n ) of the nematic solvent ZLI-1132.     

Irreversible visual sensing of humidity using a cholesteric liquid crystal

Irreversible optical sensing of humidity by a doped cholesteric liquid crystal is achieved by using a thin film of nematic host E7 with a binaphthylorthosilicate ester as dopant (guest). The film changes its color from blue (to green to orange to red) to colorless when exposed to humidity as the dopant is hydrolyzed.     

2H-N.M.R. studies of flexibility and orientational order in nematic liquid crystals

Deuteron magnetic resonance spectroscopy (²H-N.M.R.) has been used to investigate the effect of the nematic environment on the flexibility and orientational order of two perdeuteriated cyanobiphenyl homologues: 4-methyl-4'-cyanobiphenyl (1CB-d₁₁) and 4-n-pentyl-4'-cyanobiphenyl (5CB-d₁₉). The systems studied were low concentrations of 1CB-d₁₁ and 5CB-d₁₉ dissolved in the nematic phases of 5CB, N-(-4-ethoxybenzylidene)-4'-n-butylaniline (EBBA), Merck ZLI-1132 (1132) and a 55wt% mixture of 1132: EBBA. The spectra are dramatically different in these environments. Previous studies on small solutes have suggested that in the 55wt% 1132: EBBA mixture (at 301.4 K) the dominant orienting mechanism depends on the size and shape of the molecule which suggests that it is a short range repulsive interaction. This interaction has been modelled by treating the liquid crystal as an elastic continuum and the solute as a collection of van der Waals spheres which stretch the liquid crystal in the two dimensions perpendicular to the director. The distortion of the liquid crystal depends on the dimensions of the solute, and the elastic energy is described in terms of a Hooke's law force constant, k. The model is extended to include flexible liquid crystal molecules and quadrupolar couplings are calculated for each conformation of the 5CB chain. Statistical averaging over all conformations gives an excellent fit to the experimental spectrum. The results for 1CB and 5CB show that in the other nematic phases contributions from additional mechanisms must be included. Previous studies of ²H₂ and other solutes indicate that the additional mechanism is the interaction between the solute molecular quadrupole moment and the mean electric field gradient of the liquid crystal.     

Induction and structural control of chiral nematic phases by the use of photoresponsive tris(beta-diketonato) Co(III) and Ru(III) complexes

Two kinds of tris(beta-diketonato) metal(III) complexes denoted as [M(acac)2(LC12)] and [M(acac)2(Lazo)] (M=Ru(III) and Co(III); acac=acetylacetonato; LC12=1,3-didodecyloxyphenyl-1,3-propanedionato; Lazo=3-[4'-(4'-(butoxy)phenylazo)phenyl]-pentane-2,4-dionato) were synthesized and optically resolved into Delta, Lambda isomers. Here, LC12 and Lazo were designed to be elongated perpendicular to and in parallel with the molecular C 2 axis, respectively. The metal complexes were doped into three kinds of nematic liquid crystals (MBBA, EBBA, and ZLI-1132). Both dopants induced chiral nematic phases with relatively high helical twisting powers. With a purpose to clarify the mechanisms of helical induction, order parameters (S) were determined by means of polarized UV-vis measurements on nematic samples doped with racemic Co(III) complexes. As a result, the long axes of the elongated ligands in both [Co(acac)2(LC 12)] and [Co(acac)2(trans-Lazo)] were found to align in the direction of the director vector with S=0.50+/-0.05 and 0.60+/-0.05, respectively. Based on the results, the photoresponsive behavior of Delta- or Lambda-[Ru(acac)2(Lazo)] was interpreted in terms of the variation of S accompanied with the cis-trans isomerization of the azobenzene moiety in Lazo.     

Helical twisting power and compatibility in twisted nematic phase of new chiral liquid crystalline dopants with various liquid crystalline matrices

ABSTRACT

Four chiral dopants exhibiting smectic LC phases themselves were prepared and their helical twisting power (HTP) and thermal phase behaviour in mixtures with four various LC hosts were studied. The influence of host liquid crystal on HTP was evaluated and generally higher values were found for hosts with high birefringence. Unexpectedly, high enhancement was found for an LC-chiral dopant pair, both having a similar aromatic core – biphenyl ring substituted with polar group. All studied chiral dopants exhibited limited compatibility with the LC hosts in twisted nematic phase at room temperature. For one of the studied mixtures, it was able to obtain single twisted nematic phase with selective light reflection band with maximum at wavelength about 1.0 µm. Carboxylic acid-type dopants exhibited total compatibility with the studied host in single twisted nematic phase at elevated temperatures, allowing preparation of mixtures with reflection band in the visible range. In case of the carboxylic acid dopants, blue phases for optimised compositions were observed. Intermolecular hydrogen bonding between carboxylic acid proton and pyridine nitrogen of chiral dopants was found. Doping the LC host with these dopants led to slight enhancement of HTP value and higher solubility in the LC host.     

Linkage effects of chromium(III) acetylacetonato units on chiral induction of liquid crystal phases

The linkage effects of polynuclear metal complexes on chiral induction have been studied by application of the chiral oligomers of acetylacetonato chromium(III) units as a dopant, inducing chiral nematic phases. The compounds were prepared by reacting [Cr(acac)(3)] (acac = acetylacetonato) and 1,1,2,2-tetraacetylethane (taetH(2)) in solid phase at 160 degrees C. Binuclear diastereomers were separated on a silica gel column, followed by chromatographic resolution on a chiral column packed with an ion-exchange adduct of Delta-[Ru(phen)(3)](2+) (phen = 1,10-phenanthroline) and synthetic hectorite. An enantiomeric pair (DeltaDelta- and LambdaLambda-[Cr(acac)(2)(taet)Cr-(acac)(2)]) and a meso species (DeltaLambda-[Cr(acac)(2)(taet)Cr(acac)(2)]) were identified. The binuclear enantiomers were doped into a room-temperature nematic liquid crystal, N-methoxybenzylidene-4-n-butylaniline. Helical twisting power (beta(M)) was found to be +97.9 and -88.9 microm(-1) for LambdaLambda- and DeltaDelta-[Cr(acac)(2)(taet)Cr(acac)(2)], respectively. The values were compared with beta(M) for the monomeric enantiomers (+99.5 and -91.0 microm(-1) for Lambda- and Delta-[Cr(acac)(3)], respectively). The results are interpreted on the basis of the surface chirality model. DeltaDelta-[Cr(acac)(2)(taet)Cr(acac)(2)] was found to photoisomerize both in a hexane solution and in a liquid crystal phase of ZLI-1132. The quantum yield of photoisomerization in a liquid crystal phase was lowered to ca. 30% of that in a hexane solution.     

The phase behaviour and optical properties of a nematic/chiral dopant liquid crystalline mixture system

The phase behaviour and aggregation states of a binary mixture of a nematic liquid crystal and a chiral dopant have been investigated. The nematic liquid crystal E7 was miscible with the chiral dopant S811 over their entire concentration range. Binary E7/S811 mixtures formed the N* phase for S811 contents under 20%, and the SmA* phase for S811 contents between 40% and 90%. BP and TGBA* frustrated phases were found during cooling, for S811 contents between 25% and 35%. The helical pitches of the binary mixtures decreased with increasing chiral dopant content. From XRD profiles, the orientational ordering of the binary composites was found to increase with increasing chiral dopant content.     

2H-N.M.R. studies of flexibility and orientational order in nematic liquid crystals

Abstract

Deuteron magnetic resonance spectroscopy (²H-N.M.R.) has been used to investigate the effect of the nematic environment on the flexibility and orientational order of two perdeuteriated cyanobiphenyl homologues: 4-methyl-4′-cyanobiphenyl (1CB-d ₁₁) and 4-n-pentyl-4′-cyanobiphenyl (5CB-d ₁₉). The systems studied were low concentrations of 1CB-d ₁₁ and 5CB-d ₁₉ dissolved in the nematic phases of 5CB, N-(-4-ethoxybenzylidene)-4′-n-butylaniline (EBBA), Merck ZLI-1132 (1132) and a 55wt% mixture of 1132: EBBA. The spectra are dramatically different in these environments. Previous studies on small solutes have suggested that in the 55wt% 1132: EBBA mixture (at 301.4 K) the dominant orienting mechanism depends on the size and shape of the molecule which suggests that it is a short range repulsive interaction. This interaction has been modelled by treating the liquid crystal as an elastic continuum and the solute as a collection of van der Waals spheres which stretch the liquid crystal in the two dimensions perpendicular to the director. The distortion of the liquid crystal depends on the dimensions of the solute, and the elastic energy is described in terms of a Hooke's law force constant, k. The model is extended to include flexible liquid crystal molecules and quadrupolar couplings are calculated for each conformation of the 5CB chain. Statistical averaging over all conformations gives an excellent fit to the experimental spectrum. The results for 1CB and 5CB show that in the other nematic phases contributions from additional mechanisms must be included. Previous studies of ²H₂ and other solutes indicate that the additional mechanism is the interaction between the solute molecular quadrupole moment and the mean electric field gradient of the liquid crystal.     

Bis-chelated imine-alkoxytitanium complexes: novel chiral dopants with high helical twisting power in liquid crystals

Enantiomerically and diastereomerically pure bis-chelated imine-alkoxytitanium complexes 6 and 7 have been synthesized and used as chiral dopants for converting nematic into cholesteric phases. The dopants were tested in mainly commercially available nematic liquid crystalline compounds or mixtures: LC1 (BASF), ZLI-1695 and ZLI-1840 (Merck), as well as N-(4-methoxybenzylidene)-4'-butylaniline (MBBA). The values of the helical twisting power (HTP) were determined by the Grandjean-Cano method. Exceptionally high helical twisting powers were obtained. Thus, the titanium complex 6 h displayed a HTP value of 740 microm(-1) in MBBA, the highest HTP value reported. The helical twisting power has been found to depend strongly on the structure of the nematic phase and the substitution pattern of the chiral ligand in the titanium complexes 6 and 7. Crystal structure analysis of 6 f confirmed the A,R,R configuration of the metal complex. The chiral imine ligands 4 and 5 were derived from the regioisomeric amino alcohols 1 and 2.     

Enlargement of blue-phase stability for rod-like low-molecular-weight chiral nematic liquid crystal mixtures

In this study, we investigated the enlargement of liquid crystal (LC) blue-phase (BP) temperature range using the rod-like low-molecular-weight cyano phenyl-type chiral nematic LC with various core group and chiral dopant concentrations. Also, the electro-optic response time was investigated for them. We found that the BP temperature range was strongly dependent upon the core structure and the chiral dopant concentration for the chiral nematic LC mixtures having the same terminal group. Also, we found a stable BP with a wide temperature range (more than 6 K), including a BP-isotropic coexistence state over 13.5 K upon heating and cooling processes and very fast response time (less than 1 ms), by using the cyano phenyl-type chiral nematic LC mixture with a high molecular aspect ratio and a high chiral dopant concentration.     

Nematic phase transition and texture dynamics

ABSTRACT

Research advances over the past decade in the areas of nematic phase transition and texture dynamics are reviewed. Research studies applying theoretical techniques able to resolve the length and time scales inherent to liquid crystal (LC) dynamics are focused on: coarse-grained molecular dynamics and continuum mechanics. The focus on LC dynamics is due to their importance in both fundamental and technological processes involving complex LC textures and texture transitions. Meta-stable textures frequently occur in soft matter systems, thus knowledge of LC textures that result in free energy minima for a specific system is not sufficient to characterise its behaviour. Resolution of dynamics enables researchers to predict with more accuracy observable LC textures and texture transitions. As is reflected in the reviewed research, LC dynamics simulations have enabled both validation of simulations with existing experimental observations and predictive results, which augment direct experimentation. While the outlook is positive as a result of this, several key challenges stymie further progress: (i) the availability of validated open-source software implementing nematic dynamics simulation methods, (ii) development of suitable visualisation and characterisation methods for transient three-dimensional LC textures, and (iii) inclusion of thermal fluctuations in nematic dynamics models.     

Effects of thiophene-based mesogen terminated with branched alkoxy group on the temperature range and electro-optical performances of liquid crystalline blue phases

A series of symmetrically thiophene-based bent-shaped molecules with branched terminal was synthesised and characterised. Then, their effects as dopants on the blue phase (BP) range of the chiral nematic liquid crystal (N*LC) host were investigated. It was found that the bent-shaped dopants with branched terminal had better miscibility in LC host than the bent-shaped dopants with straight terminal, and contributed to induce BP and enhance the BP temperature range, with the maximum BP temperature range about 20.4°C. Besides, the electro-optical (E-O) performances of the blue phase liquid crystal doped with Th-BC6 (a bent-shaped dopant with the widest induced BP range in N*LC) were also explored. It was found that the drive voltage reduced first and then increased with the doping amount of Th-BC6 increasing. When the doping amount of Th-BC6 was about 15 wt%, the hysteresis could be strikingly reduced.     

Effects of non-additive conductivity variation for a nematic liquid crystal caused by magnetite and carbon nanotubes at various scales

Effect of magnetite and single-walled carbon nanotubes (CNTs) on the electric conductivity of 6CHBT nematic liquid crystal (LC) dispersed in polyvinyl alcohol is studied. The chosen technology of mixture homogeneising is shown to result in a LC dispersed in a polymer matrix (PDLC) with average LC droplet size of 500 nm (nano-PDLC). The conductivity of the nano-PDLC films with simultaneously introduced magnetite and nanotubes is shown to be smaller than the sum of the conductivities of films with nanoparticles of each type introduced separately. The main reason for this effect is assumed to be the formation of deep electron trap levels from complexes of nanoparticles of different type and thereby a decrease of the electron conductivity across the polymer which is the main mechanism of charge transfer in nano-PDLC. The effect of magnetite and the CNTs on the conductivity of the homogeneous LC and the micro-PDLC is analysed in comparison with the nano-PDLC.     

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

ABSTRACT

Three 1,4-disubstituted anthraquinone dyes with bis(4-n-butylphenyl) substituents connected via amine or amide linking groups have been studied as guest molecules dissolved in the nematic host E7. UV-visible absorption spectroscopy has shown each of the dyes to exhibit multiple absorption bands in the visible region, and dichroic order parameters obtained from polarised spectra of aligned guest–host samples were shown to differ significantly between the bands for each dye, and between the dyes. Time-dependent density functional theory calculations indicated that each dye exhibits several transitions, giving transition dipole moment vectors with a range of orientations, and fully atomistic molecular dynamics simulations of the guest–host mixtures showed differences in the calculated molecular alignments of the dyes. Combining the results from these two sets of calculations enabled a comparison of molecular alignment models based on the moments of inertia and the surface tensors of the dyes. The match between calculated and experimental values was improved significantly when using the surface tensor rather than the moment of inertia model, indicating that the shapes of the molecular surfaces of these dyes are crucial to their alignment. A novel method of calculating polarised UV-visible absorption spectra of dyes in liquid crystal hosts is also presented.     

A simple method to measure the twist elastic constant of a nematic liquid crystal

We demonstrate a simple method for measuring the twist elastic constant (K₂₂) of a nematic liquid crystal (LC). By adding some chiral dopant to an LC host, the LC directors rotate 180° in a homogeneous cell, which is known as 180° super-twisted nematic (STN) cell. By preparing two such STN cells with different chiral concentrations and measuring their Fréedericksz threshold voltages, we can obtain the K₂₂ and helical twisting power simultaneously. In the whole process, there is no need to measure the pitch length. Our obtained K₂₂ values agree well with those reported by using other methods.