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.     

Blue phase liquid crystals affected by graphene oxide modified with aminoazobenzol group

Liquid-crystalline blue phases (BPs) are stable only for very narrow temperature range between the isotropic and the chiral nematic phase that severely hinders their applicability. Herein, the aminoazobenzol group was chemically grafted onto epoxy group of graphene oxide (GO) via addition reaction. Successful grafting of aminoazobenzol group was confirmed using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), UV–vis absorption spectra and thermogravimetric analysis (TGA). The resultant aminoazobenzol group–modified GO sheets, which is reduced (RGO-Az), were easily redispersable in common organic solvents or liquid crystals (LCs). By doping different contents of RGO-Az, nanosheets could stabilise BP and increase the BP range. When doped with 0.5 wt% RGO-Az, the mixtures show the wider range with 5.9°C than the range with 3.6°C of BPLCs without RGO-Az. Meanwhile, the phase sequence and the range of the aforementioned phases are reproducible upon heating and cooling, which shows that the BPs doped with RGO-Az nanosheets are thermodynamically stable.     

Optical activity in CE6 A new metastable blue phase?

Abstract

The optical rotation of a mixture of 60 per cent of chiral and 40 per cent of racemic CE6 as an 18 μm thick sample placed between glass plates treated with PI has been measured. This mixture exhibits one blue phase (BP1) on heating over a temperature range of about 0·1°C. On cooling the sample on the other hand, the BP region is expanded to 0·6°C and is divided into two regions. One region (BP1) (of range about 0·38°C below the isotropic–blue phase transition) shows two Bragg wavelengths which increase with decreasing temperature. For the second region (BPS) (of range about 0·22°C above the cholesteric–blue phase transition), one Bragg wavelength decreases with decreasing temperature, and a third Bragg wavelength appears. At constant temperature both phases remained stable for a period of several days.     

Inducing monodomain blue phase liquid crystals by long-lasting voltage application during temperature variation

We demonstrate a new successful technique for inducing a monodomain in blue phase (BP) liquid crystals (BPLCs). By application of an AC voltage during the cooling or heating of the material into the BP temperature range, a fairly homogeneous BP area was obtained, compared to the case without voltage. We determined that applying a voltage of 3.3 V/µm was very effective in terms of obtaining a monodomain: for four of the seven different applied cooling/heating rates an area ratio of more than 99% was obtained for one particular BP orientation. This study describes a successful and replicable technique to make a BPLC monodomain, which is important for the development of BPLCs and other applications.     

Comparison of electro-optical switching between polymer-stabilised cubic and amorphous blue phases

We prepared polymer-stabilised cubic and amorphous blue phase liquid crystals consisting of the same materials at different UV-curing temperatures using photopolymerisation. Then we investigated the effects of polymer stabilisation on their physical properties. The UV-curing temperature does not affect phase stabilisation of the polymer-stabilised amorphous BPIII (PS-BPIII). Temperatures of the PS-BPIII were found to be extended to more than 50 K. However, a critical temperature exists in the polymer stabilisation process for the PS-cubic BP. Although no substantial difference in response speed was found between the polymer-stabilised cubic and amorphous structures, larger hysteresis and higher residual birefringence were observed in the PS-cubic BP than in the PS-BPIII. Amorphous BPIII with an amorphous network of disclination lines has no critical temperature for the polymer stabilisation and realises a high-performance display.     

Selective stabilisation of blue phase liquid crystal induced by distinctive geometric structure of additives

We successfully stabilised liquid crystalline blue phases (BPs) by introducing two suitable additives with different geometric molecular structures, linear-shaped cobalt oleate complex (Co-OL) or tetrahedral-shaped tetraoctadecylammonium bromide (TODAB), into a liquid crystal (mixture of 4?-pentyl-4-biphenylcarbonitrile, JC-1041XX and chiral dopant). The BPs temperature range and phase sequence depending on the addition amount and shape of additives were systematically investigated to determine the optimal concentration and shape dependency required to achieve a stabilising effect for BPs. From the polarising optical microscope results, the BPs temperature range for all of the samples with additives was not only broadened but also shifted to room temperature compared to that of BPs without additives. The widest BPs temperature range was increased to 15.3°C by the addition of 3 wt% Co-OL. According to the UV-vis reflection spectroscopy results, the Co-OL has a more significant stabilisation effect on BP I than on BP II, and the widest BP I range increases to 11.0°C. On the other hand, TODAB is effective for BP II stabilisation with the broadest BP II range reaching 1.8°C. These selective stabilisation effects are attributed to the specific shape of additives that closely match the structures of the disclination lines of the BPs.     

Influence of monomer structure on the properties of blue phase liquid crystal

In this paper, four kinds of polymer-stabilized blue phase liquid crystals (PS-BPLCs) with different monomer were designed and prepared. The morphology, temperature ranges and electro-optical properties of the blue phase were studied and discussed. The temperature ranges of four BPLCs are more than 90°C, and all samples can be stabilized well at room temperature. Referring to electro-optical performance, on-state voltage for mono-functional monomers systems is reduced from 75 V to 65 V at 497 nm. On-state voltage for tri-functional monomers systems is reduced from 107 V to 85 V at 497 nm. These results are helpful to the application of display and photonic devices.     

Enlarging the Kerr constant of polymer-stabilised blue phases with a novel chiral monomer

Polymer-stabilised blue phase liquid crystals (PS-BPLC) have attracted many attentions in the area of display and photonic applications because of the superior electro-optical properties. Before application, the problem of high driving voltage should be resolved first. In this report, a new chiral monomer (C5011) has been synthesised and its influence on PS-BPLC was investigated. By adopting this chiral monomer, the Kerr constant of the PS-BPLC could be increased dramatically, which means improved driving capacity can be achieved.     

Photoinduced liquid crystal blue phase by bent-shaped cis isomer of the azobenzene doped in chiral nematic liquid crystal

A photoresponsive azobenzene molecule DCAZO2 with two cholesteryl groups linked to both sides of the azobenzene group is doped in a mixture of nematic liquid crystal E7 and chiral dopant S811 (61.9 wt% E7, 36.1 wt% S811 and 2.0 wt% DCAZO2). Cooled from isotropic phase to 33.0°C, chiral nematic liquid crystal (N*LC) was formed in the sample and then the temperature was kept unchanged at 33.0°C. UV light irradiation induces the trans–cis photoisomerisation and thus an obvious phase transition. When the azobenzene groups isomerise to a cis-saturated state, the UV light was turned off and the white light was turned on at the same time. The bent-shaped cis isomer then turns back to the planar trans isomer gradually. A blue–green platelet texture representing cubic blue phase (BP) was observed and the size of the platelets was increased along with the cis–trans isomerisation. UV–vis absorption spectra indicate that the photoinduced BP exists when the isomerisation degree is between 79% and 18%, and further cis–trans isomerisation change BP back into N*LC. The large geometric structure of the cholesteryl groups and the large bent angle θ of the cis isomer are supposed to be responsible for the interesting result.     

Combustion synthesis and thermal expansion of RE6UO12 (RE = Dy and Tb)

Citrate–nitrate combustion method was adopted for the synthesis of RE₆UO₁₂ (RE = Dy and Tb). These compounds were characterized by X-ray diffraction. Thermal expansion coefficient of these compounds were measured in the temperature range of 298–1,273 K by high temperature X-ray powder diffractometry (HT-XRD) and compared with other rare earth compounds reported in the literature. There was no observed phase transition in Dy₆UO₁₂, but Tb₆UO₁₂ showed a second-order phase transition at 670 K which was confirmed using differential scanning calorimeter. The average volume thermal expansion coefficient of Dy₆UO₁₂ in the temperature range of 298–1,273 K is (29.82 ± 4.02) × 10^?6 and that of Tb₆UO₁₂ in the temperature range of 298–673 K is (13.76 ± 2.64) × 10^?6 K^?1.     

Blue phase stability of n-OCB homologue chiral nematic liquid crystal mixtures

Blue phase (BP) stability of a chiral nematic liquid crystal (LC) mixture is dependent upon chemical structure as well as physical properties. In this study, the blue phase temperature range dependent on alkyl chain length was investigated in order to evaluate the relationship between blue phase stability and the molecular structures of four kinds of 4-n-alkyloxy-4'-cyanobiphenyl (n-OCB) homologue chiral nematic LC mixtures composed of rod-like nematic LCs. It was confirmed that the blue phase temperature range was strongly dependent upon the molecular parity, K ₃₃/K ₁₁ and the helical twist power of the n-OCB homologues chiral nematic LC mixtures.     

Transflective BPIII mode with no internal reflector

A transflective device without reflector using room temperature blue phase III (BPIII) material is demonstrated in this study. In this device, the coupling of an induced birefringence and field-induced BP, relating the ordered orientation of the double-helix cylinders, causes the reflection and transmission. Compared with other reported transflective liquid crystal devices, the BPIII device shown here does not need any type of internal reflector. Well-matched voltage-dependent transmittance and reflectance curves can be obtained easily without considering the cell gap and incident wavelength. The total response time is less than 2 ms, which is also independent of the cell gap. The experimental results exhibit a simple way to get a transflective device with good ability based on the electro-optical properties of BPIII.     

Magnetocaloric properties of dendrimer complexes of Fe(III) with substituted Schiff base

In dendrimer complexes of iron (III) with Schiff base (three complexes of iron (III) based on azomethine 4,4′-dodecyloxybenzoyloxybenzoyl-4-salicylidene-2-aminopyridine, a significant magnetocaloric effect (MCE) and heat capacity was found the first time. It was found that the magnitude of MCE depends on the nature of the counter-ion of the complex. MCE were measured with a microcalorimeter over the temperature range of 278–320 K and in a magnetic induction of 0–1.0 T. The temperature dependences of the MCE dendrimer complexes of iron (III) with Schiff base were obtained for the first time. For all the samples studied, the existence of extreme temperature dependence of MCE in the range of temperatures 300–350 K, which is possibly the result of the magnetic phase transition, is shown. The correlation between the thermotropic mesomorphism with the magnetic phase transition in complexes has been established.     

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.     

Temperature-dependent Raman study of pure and silver nanoparticles dispersed N-(4-n-heptyloxybenzylidene)-4’-n-butylaniline (7O.4)

Temperature-dependent micro-Raman study of C-H in-plane bending mode of aromatic rings, C-N and C=N stretching of linking group (-C(H)=N) and C=C stretching of rings of pure and silver nanoparticles dispersed (0.5% and 1% by weight) Schiff’s base liquid crystal (LC) compound, N-(4-n-heptyloxybenzylidene)-4’-n-butylaniline (7O.4) in 500–2250 cm^?1 region has been done. The change in Raman spectral parameters (peak position and linewidth) at crystal–smecticG (K–smG) and smecticG–smecticC (smG–smC) gives the evidence of charge shift at phase transition which is associated with changes in orientation and vibrational freedom of the molecules. The peak position of the Raman bands shows blue shift for 0.5 wt% dispersed sample, whereas it shows red shift for 1 wt% dispersed sample. The blue and red shifts of the Raman bands indicate an increase and decrease in the charge density, respectively. The optimised structure and theoretical room temperature Raman spectra of 7O.4 were obtained using density functional theory. The vibrational assignment using potential energy distribution is reported using vibrational energy distribution analysis (VEDA).     

Understanding the Solid Solution–Aqueous Solution Equilibria in the KCl + RbCl + H<Subscript>2</Subscript>O System from Experiments,Atomistic Simulation and Thermodynamic Modeling

The solid solution–aqueous solution (SSAS) equilibria in the KCl + RbCl + H₂O system were redetermined at 298.15 K. The experimental data for (K,Rb)Cl were consistent with the formation of a continuous solid solution without miscibility gaps. The Schreinemakers’ wet residues method and an XRD quantitative analysis technique based on the Vegard approach were applied to determine the chemical composition of the solid solution phase of (K,Rb)Cl. The compositions of (K,Rb)Cl derived from the Vegard approach are in accordance with those from the wet residues method. The thermodynamic properties of mixing of the (K,Rb)Cl solid solution were theoretically predicted using atomistic simulations. From these simulations, a regular solution behavior is recognized that is consistent with the knowledge of the thermodynamic properties of mixing of (K,Rb)Cl obtained from SSAS equilibrium studies, but the predicted regular solution model parameter A ₀ is significantly larger than that regressed from the SSAS equilibrium data. Finally, a thermodynamic model was developed for representing the SSAS equilibria and element partitioning in the KCl + RbCl + H₂O system as a function of temperature that can be used for predicting the SSAS equilibria in the studied system over the temperature range 273.15–373.15 K.     

Optical activity in CE6 A new metastable blue phase?

The optical rotation of a mixture of 60 per cent of chiral and 40 per cent of racemic CE6 as an 18 μm thick sample placed between glass plates treated with PI has been measured. This mixture exhibits one blue phase (BP1) on heating over a temperature range of about 0·1°C. On cooling the sample on the other hand, the BP region is expanded to 0·6°C and is divided into two regions. One region (BP1) (of range about 0·38°C below the isotropic-blue phase transition) shows two Bragg wavelengths which increase with decreasing temperature. For the second region (BPS) (of range about 0·22°C above the cholesteric-blue phase transition), one Bragg wavelength decreases with decreasing temperature, and a third Bragg wavelength appears. At constant temperature both phases remained stable for a period of several days.     

High pressure optical measurements on BP III and the isotropic phase of a highly chiral liquid crystal

The optical activity and light transmission of the blue phases and isotropic phase of 4'-(2-methylbutylphenyl)-4'-(2-methylbutyl)-4-biphenylcarboxylate (CE2) are investigated under high pressure. The temperature ranges of the first (BP I) and third (BP III) blue phases show only the slightest change, if any, with increasing pressure. This is the first time BP III, a phase which is not understood at all, has been investigated under pressure. In addition, an alternative analysis to the one reported previously is reported which confirms that the pretransitional optical activity in the isotropic phase changes very slightly due to pressure, and even this small change seems to be due to small changes in pitch and index of refraction. Taken together, these observations indicate that high pressure has little effect on both the BP III phase and the transition to the isotropic phase in highly chiral systems, even though the BP III phase differs significantly from the other blue phases and strong short range fluctuations make the transition appear continuous optically.     

Spatially and electrically tunable random lasing based on a polymer-stabilised blue phase liquid crystal-wedged cell

ABSTRACT

We demonstrate a spatially and electrically tunable random lasing based on polymer-stabilized blue phase liquid crystal (PS-BPLC)-wedged cell. The spatially tunable random lasers can be obtained from the laser dye-doped PS-BPLC-wedged cell through changing the pump positions, where the emission wavelength of the random laser can be tuned due to the thickness gradient of the wedged cell, which affects the scattering mean free path. Additionally, applying different electric fields can also tune the laser emission wavelength. The changing of refractive index due to the Kerr effect leads to a change in the scattering mean free path, resulting in shift of lasing wavelength. This PS-BPLC-wedged cell device has a great potential in applications of speckle-free imaging, document coding, biomedicine and other photonic devices.     

Low-temperature dynamics of (S)-4-(1-methylheptyloxy)-4ʹ-cyanobiphenyl (8*OCB) and (S)-4-(2-methylbutyl)-4ʹ-cyanobiphenyl (5*CB) in disordered crystalline and glassy phases

The inelastic neutron scattering (INS) spectra were measured for two materials of chiral molecules: (S)-4-(1-methylheptyloxy)-4?-cyanobiphenyl (8*OCB) and (S)-4-(2-methylbutyl)-4?-cyanobiphenyl (5*CB), revealing solid state polymorphism with two partially disordered crystalline phases I and II and glassy state of liquid and of crystalline phase in each substance. The experiments were performed in the energy range up to 30 µeV in the temperature range from 4 to 35 K. For 8*OCB the elastic scans were measured as well up to 300 K illustrating well the phase diagram. For all solid phases of both substances in the µeV range of INS spectra, the existence of the excess density of vibrational states over that typical for fully ordered crystalline phases was evidenced. Contribution of this so-called boson peak occurred to be much larger in glass of isotropic phase than in the phase II and glass of phase I of 8*OCB, while for 5*CB it was larger in the phase I and glass of phase II than in glass of cholesteric phase. The quasi-elastic broadening of elastic peak corresponding to stochastic reorientations in the ns time scale was detected for both substances. Comparison of the results obtained for glassy and crystalline phases of 8*OCB and 5*CB compounds have been given and confronted with those obtained previously in meV energy range.