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.     

The effects of azo-oxadiazole-based bent-shaped molecules on the temperature range and the light-responsive performance of blue phase liquid crystal

A series of light-responsive azo-oxadiazole-based bent-shaped molecules was synthesised and characterised. Their effects as dopants on the blue phases (BPs) range of the chiral nematic liquid crystal (N*LC) matrix and light-responsive properties including the photo-inducing change of UV absorption properties, photo-adjusting the BP structure as well as the temperature range were investigated. It was found that the azo-oxadiazole-based bent-shaped molecules with branched terminal had better miscibility in LC host than the molecules with straight terminal, and that increasing the length of rigid core of bent-shaped molecules will greatly improve the effect of widening the BP temperature range.     

Effect of specific rotation of chiral dopant and polymerization temperature on reflectance properties of polymer stabilized cholesteric liquid crystal cells

A series of polymer stabilized cholesteric liquid crystal (PSCLC) cells were prepared by photo‐polymerization of a cholesteric liquid crystal (Ch‐LC) mixture containing a nonreactive LC, a nematic diacrylate and a novel cholesteryl monomer. The influence of the specific rotation and concentration of the chiral dopants, and the polymerization temperature on reflection properties was investigated. The results demonstrate that the reflection band was broadened after polymerization for all the systems both left‐handed S811 and right‐handed R1011 as the chiral dopant, which is speculated to be a result of an inhomogeneous consumption of the chiral monomer within the system. Additionally, the polymer temperature plays an integral role in the observed reflection spectra, and at optimum polymerization temperature the broadband reflection effect becomes much more pronounced. Scanning electron microscopy (SEM) was used to examine the role of microscopic changes of the polymer network induced by polymerization temperature. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1562–1570, 2008     

Thermal tuning band gap in cholesteric liquid crystals

The phase of a liquid crystal (LC) changing from a nematic phase to a cholesteric (Ch) mesophase is achieved by adding different ratios of chiral dopants S811. By studying the transmission spectrum, we are able to measure the helical pitch in cholesteric phase. The pitch in the mixtures of nematic E7 and chiral dopants S811 as a function of the concentration of the dopant and temperature is investigated. The sensitivity of the selective reflection notch of the cholesteric phase to the thermal tuning depends strongly on the ratios of the chiral dopants. It reveals that the influence of temperature is more profound for those cholesteric liquid crystals (CLCs) which exhibit smectic A (SmA) at lower temperatures. When fitted using Keating's formula, the helical pitch calculated from our experimental results lies on the predicted curve. Optimised ratios of the mixture CLCs for the optimised reflection band with the specified wavelength ranging from 467 nm to 2123 nm are suggested.     

Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACT

A series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, ¹H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase.     

Mesomorphic Characteristics of Induced Chiral Nematic Phase of [Smectic LCP,PS(4BC/DM)/Nematic LC,E7/Chiral Dopant,CB-15]-Ternary Composite System

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The role of crosslinking and polarity in the dark relaxation of azobenzene-based,polymer-stabilised cholesteric liquid crystals

Prior examinations have reported that polymer stabilisation of azobenzene-based cholesteric liquid crystal (CLC) mixtures can reduce the time necessary for complete colour restoration in the dark from three days to as few as five minutes. This work extends upon these prior examinations by exploring and elucidating the role of crosslinker concentration and monomer polarity on the colour restoration of a representative CLC mixture composed of a high HTP bis(azo) binapthanyl chiral dopant (QL76) mixed into the cyanobiphenyl nematic liquid crystal host MDA-00-1444. The impact of these variables was unexpectedly convoluted. In all the formulations examined here, polymer stabilisation dramatically reduces the time for complete colour restoration of the starting reflection notch. In mixtures based on nonpolar liquid crystal monomers, increasing the crosslinker concentration reduces the time necessary for complete colour restoration. However, the dependence on crosslinker concentration reverses in mixtures composed from polar liquid crystal monomers in which increasing the crosslinker concentration serves to increase the time necessary for complete colour restoration.     

Memory effects in chiral nematic liquid crystals doped with functionalised single-walled carbon nanotubes

Octadecylamine-functionalised single-walled carbon nanotubes (SWCNTs) were dispersed into nematic liquid crystals (LCs) doped with chiral molecules. The collective orientation of nematic LC molecules in helical layers was manipulated by varying dopant concentration. Highly anisotropic nature of SWCNTs enhanced the anisotropy of the LC as confirmed by polarised fluorescence spectroscopy. The π–π interaction of SWCNTs present in the planar alignment layers and twisted nematic LC molecules affects the molecular relaxation process. An irreversible electro-optic memory in the material has been observed.     

Why chiral tartaric imide derivatives give large helical twisting powers in nematic liquid crystal phases: substituent-effect approach to investigate intermolecular interactions between dopant and liquid crystalline molecules

Novel C₂-symmetric chiral dopant derivatives, namely, N-substituted (2R, 3R)-2,3-bis(4-(4-octyloxyphenyl)benzoyloxy)succinimides1a-h, were synthesised, and the effects of the N-substituent and imide-carbonyl groups on the helical twisting powers (HTPs) were investigated in two nematic liquid crystalline compounds, 4-n-pentyl-4?-cyanobiphenyl (5CB) and N-(4-ethoxybenzylidene)-4-n-butylaniline (EBBA). As a result, it was clarified that the bulkiness of the N-substituents has a significant correlation with the HTPs, and the imide-carbonyl group interacts strongly with the cyano group of 5CB to give high HTPs in the nematic phases. However, it is assumed that the imide-carbonyl groups of the dopants do not have strong electrostatic attractive interactions with EBBA molecules in the nematic phase to afford the moderate HTPs.     

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.     

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.     

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.     

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.     

Dye induced great enhancement of broadband reflection from polymer stabilized cholesteric liquid crystals

A series of polymer stabilized cholesteric liquid crystals (PSCLCs) films were prepared from cholesteric liquid crystal (Ch‐LC) mixtures containing different components such as non‐reactive LC monomer, polymerizable monomer, chiral dopant, dye, and photoinitiator upon polymerization. The influence of the polymerizable monomer and dye of Ch‐LC mixtures on the reflection properties was investigated. The reflection bandwidth for all the samples can be increased by photo‐polymerization, and the network upon polymerization derived from two different polymerizable monomers with both one and two functional groups is more effective than that from one polymerizable monomer for broadening the reflection band. Especially, a dye‐doped Ch‐LC film can reflect incident light with the bandwidth over the wavelength range of 550–2350 nm, which is due to a greater pitch gradient formed inside of Ch‐LC film. The gradient pitch network structure was firstly demonstrated by scanning electron microscopy (SEM) with the film prepared from high diacrylate monomer concentration and subsequently proved by using a wash‐out/refill method. The nematic liquid crystals monomers was infiltrated into the polymer network that was prefabricated by removing the low molar weight LCs from the original PSCLCs film, and SEM exhibited the existence of a pitch gradient across the film thickness. The refilled nematic liquid crystals film showed broadband reflection after polymerzition, too. Copyright © 2011 John Wiley & Sons, Ltd.     

Adsorption processes during the filling of a display with a smectic A LC mixture

The separation of a smectic A liquid crystal (LC) mixture during the filling of a large area display was studied. This process is governed by the selective adsorption of the mixture's ionic dopant on the surface of the display substrates, leading to a non-uniform distribution of the dopant in the LC over the display area and resulting in defective regions. Conductivity measurements were performed to indicate the distribution of the dopant concentration in the LC over the display area. The consequences of mixture separation on the structure of the LC and on the display's electro-optical properties are discussed. Factors to reduce the extent of separation of the mixture are revealed using a general theory of chromatography. Increasing the cell gap, decreasing the area of the glass surface by reducting the gap between the ITO electrodes, and increasing the back pressure during filling, effectively reduce the significance of the separation process. The results obtained can be used when designing and filling displays based on the electrically reversible memory effect in a smectic A LC or displays based on dynamic light scattering in a nematic LC.     

Cholesteric mesophase of the hydrogen-bonded blends of liquid crystalline ionogenic copolymers with a low molecular weight chiral dopant

A family of a new hydrogen-bonded complexes based on comb-shaped LC copolymers containing the monomer units of cyanobiphenyl derivative and n-alkyloxy-4-oxybenzoic acid with a chiral dopant on the base of 4-pyridinecarboxylic acid and L -menthol, was prepared. At concentrations of chiral groups 1–25 mol %, the induction of cholesteric phase was observed. Temperature dependences of selective light reflection wavelengths were studied, and helix twisting power was calculated. Depending on the type of copolymer nematic matrix, this value is changed in the range from 12.1 to 19.6 µm⁻¹. It was shown that an increase of a distance between the chiral dopant and the main polymer chain results in a lower values of helix twisting power. With respect to optical properties, the chiral nematic phase in the hydrogen-bonded complexes is comparable to classical cholesteric copolymers, in which the chiral group is covalently bound to polymer chain. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3215–3225, 1999     

The influence of flexible spacer and the chirality of the core on the formation of chiral nematic phase of symmetric dimers containing trifluoromethyl terminal

Seven symmetric liquid crystal (LC) dimers containing different chiral cores and LC arms have been synthesised, termed EPDA-(R,S)PD, TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD, TFBA-(R)PD, TFBDA-IB and TFBA-IB, respectively. TFBDA-(R,S)PD, TFBA-(R,S)PD, TFBDA-(R)PD and TFBDA-IB displayed chiral smectic A (SmA*) phase, while EPDA-(R,S)PD, TFBA-(R)PD and TFBA-IB exhibited chiral nematic (N*) phase. The effects of flexible spacer, structural type of LC arms and the chirality of the cores on the thermal properties of the dimers and the formation of N* phase have been studied. The results indicated that the chiral core was prone to induce the N* phase in LC dimer that contained nematic arms although the chirality of the core is very weak, while for the smectic LC arms containing CF3 terminal, the removal of the flexible spacer between chiral core and rigid LC units was conducive to the formation of N* phase. For example, TFBDA-(R)PD and TFBDA-IB displayed SmA* phase, while TFBA-(R)PD and TFBA-IB exhibited N* phase. However TFBA-(R,S)PD did not display N* phase, which reminded us that the chirality of the core and the conformation of the dimer also played an influence in the formation of the LC phase.     

Study on selective reflection properties of chiral nematic liquid crystalline composites with a non‐uniform pitch distribution

The preparation is reported of particles of photopolymerisable monomer/chiral dopant composites with a crystalline (Cr)‐chiral nematic (N*) phase transition. By mixing particles with different pitches of the N* phase in the Cr phase and crosslinking the liquid crystal (LC) monomer molecules by photopolymerisation in the planarly oriented N* phase, an N*‐LC composite film with a non‐uniform pitch distribution was obtained. Experimental results show that the bandwidth of the reflection spectrum and the location of reflection band of the composite films can be controlled accurately by controlling the pitch lengths of the N* phase of the particles. Effects of polymerisation temperature and UV intensity on the non‐uniform pitch distribution of N*‐LC composite films were investigated.     

Lattice structure in liquid-crystal blue phase with various chiral concentrations

Lattice structures, including reflection lattice planes and lattice constant, of liquid-crystal blue phase I (BPI) are studied via the measurements on reflection spectrum and Kossel diagram as concentration of a chiral dopant is changed. Peaks of the reflection wavelength in BPI are mainly dominated by the lattice plane and the lattice constant, which are affected by the chiral concentration. In the chiral nematic state, as decreasing the chiral concentration the reflection peak will shift to a longer wavelength because the helical pitch linearly depends on the chiral concentration and becomes longer. However, this dependence of the chiral concentration and reflection wavelength is broken in the BPI. The reflection peak of BPI moves to a short wavelength when the chiral concentration is less due to the contraction of the lattice constant as well as helical pitch. Moreover, when the concentration of the chiral dopant increases over a certain value, a discontinuous shift in reflection peak occurs due to the production of the different lattice planes. It means that the relationship between the chiral concentration and the helical pitch in BPI is not the same as it in the chiral nematic phase and should be reconsidered.     

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.