LC Host Research Articles

Tuning of optical properties and phase behavior of Nanomaterial-stabilized blue phase liquid crystals

HypothesisNanoparticles of various shapes and sizes can affect the optical properties and blue phase (BP) stabilization of BP liquid crystals (BPLCs). This is because nanoparticles, which are more compatible with the LC host, can be dispersed in both the double twist cylinder (DTC) and disclination defects in BPLCs. ExperimentsThis study presents the first systematic study of the use of CdSe nanoparticles having various sizes and shapes (spheres, tetrapods and nanoplatelets) to stabilize BPLCs. Unlike previous studies using commercial nanoparticles (NPs), we custom-synthesized NPs with the same core and nearly identical long chain hydrocarbon ligand materials. Two LC hosts were used to investigate the NP effect on BPLCs. FindingsThe size and shape of nanomaterials greatly influence the interaction with LCs, and the dispersion of NPs in the LC medium affects the position of the BP reflection band and the stabilization of BPs. Spherical NPs were found to be more compatible with the LC medium than tetrapod shape and platelet shape NPs, resulting in a wider temperature range of BP and a redshift of the reflection band of BP. In addition, the inclusion of spherical NPs tuned the optical properties of BPLCs to a significant extent, whereas BPLCs with nanoplatelets displayed a limited influence on the optical properties and temperature window of BPs due to poor compatibility with LC hosts. The tunable optical behavior of BPLC as a function of the type and concentration of NPs has not been reported.

Strain-induced recognition of molecular and chirality in cholesteric liquid crystal droplets for distance and curvature sensing.

Chirality is universal in nature and in biological systems, and the chirality of cholesteric liquid crystals (Ch-LC) is both controllable and quantifiable. Herein, a strategy for precise chirality recognition in a nematic LC host within soft microscale confined droplets is reported. This approach facilitates applications in distance and curvature sensing as well as on-site characterization of the overall uniformity and bending movements of a flexible device. Due to interfacial parallel anchoring, monodisperse Ch-LC spherical microdroplets show radial spherical structure (RSS) rings with a central radical point-defect hedgehog core. Strain-induced droplet deformation destabilizes the RSS configuration and induces the recognition of chirality, creating "core-shell" structures with distinguishable sizes and colors. In practice, an optical sensor is achieved due to the rich palette of optically active structures that can be utilized for gap distance measuring and the monitoring of curvature bending. The properties reported here and the constructed device have great potential for applications in soft robotics, wearable sensors, and advanced optoelectronic devices.

Color Tuning in the Luminescence of Two Oligomers Derived from N,N′-(Naphthalenediyl)bis-phenylimine Containing Oligomers

Two oligomers with different molecular weight were produced. Fluorescence properties were examined both in solutions and in the solid state. In the solid state, molecular weight turned out to be an important factor influencing the aggregation structures and therefore the emission of the compounds, in colour and intensity. The neat compounds emit two different colours, blue and yellow, with increasing the molecular weight, whereas both compounds are blue emitters in solution and dispersed in host polymers. Dissolved in a LC host polymer and in a PVK matrix, they displayed noteworthy photoluminescence quantum yields.

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

ABSTRACTFour 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.

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

ABSTRACTA 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.

The effects of silica nanoparticles on blue-phase liquid crystals

ABSTRACTWe investigate the effects of hydrophobic silica nanoparticles (SNs) on blue-phase liquid crystals (BPLCs). The optical microscope and reflection spectra observation reveal that a tiny hydrophobic SN dopant stabilises the BPLC phase, and widens the temperature range of the BP I phase. Furthermore, the doped dilute SNs can fine-tune their positions to relax the formation stress of the BPLC lattices, and slightly increase the platelet sizes of the BPLCs. The doped SNs also decrease the driving voltage and response time of the BPLC cell, because the added SNs decrease the elastic constant of the LC host and the relaxation time constant of the BPLC mixture.

Amplified Photon Upconversion by Photonic Shell of Cholesteric Liquid Crystals

As an effective platform to exploit triplet-triplet-annihilation-based photon upconversion (TTA-UC), microcapsules composed of a fluidic UC core and photonic shell are microfluidically prepared using a triple emulsion as the template. The photonic shell consists of cholesteric liquid crystals (CLCs) with a periodic helical structure, exhibiting a photonic band gap. Combined with planar anchoring at the boundaries, the shell serves as a resonance cavity for TTA-UC emission and enables spectral tuning of the UC under low-power-density excitation. The CLC shell can be stabilized by introducing a polymerizable mesogen in the LC host. Because of the microcapsule spherical symmetry, spontaneous emission of the delayed fluorescence is omnidirectionally amplified at the edge of the stop band. These results demonstrate the range of opportunities provided by TTA-UC systems for the future design of low-threshold photonic devices.

PS-VA模式中RM残留量对信赖性的影响

Well known PS-VA technology has been extensively used for TV application, but the image sticking issue is still a concern. In order to improve the image sticking performance of PS-VA TV panel, the condition of both UV1 and UV2 process are investigated to find out the relationship between the residual amount of RM and the image sticking performance. It shows that the image sticking performance is related to the RM residual amount. As we know, ion density is a factor of image sticking issue. Based on analysis on the relationship between ion density and RM residual amount, we develop a new technology to reduce the image sticking issue in PS-VA mode. Furthermore, another method is provided in this work to improve the image sticking performance by designing a new LC host mixture which has lower residual amount of RM. The technical data is obtained including ion density data measured by Toyo 6254 and surface profile investigation by SEM and AFM.

Polarized photoluminescence of the polymer networks obtained by in situ photopolymerization of fluorescent monomer in a nematic liquid crystal

Reported herein are the in situ creation and characterization of crossed-linked copolymer networks with a polarized photoluminescence. The π-extended fluorene-based reactive mesogen with photopolymerizable acrylate groups has been copolymerized with conventional reactive mesogen RM-257 in a uniformly aligned nematic LC host. Networks are localized at surfaces as thin films. The copolymer networks exhibit a strong optical anisotropy with a uniaxial orientation of the mesogens, templated by nematic order of the host LC. As a result, the network shows dichroic absorption for excitation and polarized photoluminescence. The degree of the orientational order of the fluorophore has been decreased during the polymerization process. The order parameter of the fluorophore has been curtailed from S = 0.57 for the reaction mixture prior to polymerization to S = 0.32 for the bare polymer networks after polymerization and removal of unreacted LC molecules.

Light Propagation and Photoactuation in Densely Cross-Linked Azobenzene-Functionalized Liquid-Crystalline Polymers: Contribution of Host and Concerted Isomerism

The photomechanical behavior of azobenzene-functionalized liquid-crystalline polymers (azo-LCPs) is closely related to UV light propagation. Here, we report the effect of light absorption by the LC host and the concerted isomerism of azobenzene on this property. In situ measurements of light absorptivity and computer simulations revealed that the light propagation of a liquid-crystalline polymer is affected by the UV absorption of the LC host under the same concerted isomerism of azobenzene. The lower UV light absorption of the LC host results in deeper UV light propagation and a sharper isomerization profile, which eventually induces faster bending when compared to the higher UV light absorption of the LC host. On the other hand, increasing the azobenzene concentration in the polymer greatly decreases the speed of light propagation, which is regarded as one of the factors inhibiting further increases in the bending speed of azo-LCP films.

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

ABSTRACTA 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.

55.3: A Low‐Voltage and Fast‐Response Infrared Spatial Light Modulator

We report a low‐voltage and fast‐response polymer network liquid crystal (PNLC) infrared phase modulator. By optimizing the UV curing temperature and LC host, we achieved V2ϰ= 22.8V at λ=1.55μm and response time about 1ms. Such a PNLC‐based spatial light modulator is useful for adaptive optics, adaptive lens, and laser beam steering.

Aggregate‐induced emission in light‐emitting liquid crystal display technology

AbstractNowadays, liquid crystal displays (LCDs) with light‐emission are considered as energy‐efficient devices and are promising alternatives to conventional LCDs. To realize such possibility, strong fluorescent materials with a dichroic properties are required. Aggregate‐induced emission (AIE) is an unusual photophysical phenomenon shown by some luminogenic materials that will be highly emissive in their aggregate state. In this work, we studied the AIE effect of a luminescent liquid crystalline molecule TPE‐PPE in our LC system as a luminophore dopant. The result showed the excellent AIE effect that higher concentration of luminogen in the nematic LC host induced stronger luminescent intensity. Through exposure of a photoisomeriable alignment material sulfonic‐dye‐1, the photopatterning of a light‐emitting LC device was achieved with the use of the TPE‐PPE/nematic LC mixture.

LC mixture with promoted electro-optical properties for LCD applications by using a highly UV-resistant dopant

A highly ultraviolet (UV)-resistant dopant 3HBBTfV consisting of an α-trifluomethyl vinylbenzene (TfV) moiety was synthesized and mixed with a commercial LC host (A) to promote its dielectric anisotropy in LC mixture C (A:3HBBTfV = 9:1 w/w). Their electric-optical properties were studied and compared with those of an analogous commercial LC mixture B composed of LC mixture A (as a commercial host) mixed with the same weight ratio (10% wt.) of a commercial component 3HBEBTf containing a 3,4,5-trifluorobenzene (BTf) moiety. The electro-optical properties of voltage holding ratio (VHR) values and voltage-transmission (V-T) curves of LC mixture C were sustained after the UV irradiation. These results showed that dopant 3HBBTfV (acting as an UV stabilizer) demonstrated a higher stability under UV exposure in contrast to commercial component 3HBEBTf. Therefore, LC mixture C (containing UV-resistant dopant 3HBBTfV) exhibited good UV resistance, high dielectric anisotropy, and excellent electro-optical properties.

Low voltage polymer network liquid crystal for infrared spatial light modulators.

We report a low-voltage and fast-response polymer network liquid crystal (PNLC) infrared phase modulator. To optimize device performance, we propose a physical model to understand the curing temperature effect on average domain size. Good agreement between model and experiment is obtained. By optimizing the UV curing temperature and employing a large dielectric anisotropy LC host, we have lowered the 2π phase change voltage to 22.8V at 1.55μm wavelength while keeping response time at about 1 ms. Widespread application of such a PNLC integrated into a high resolution liquid-crystal-on-silicon (LCoS) for infrared spatial light modulator is foreseeable.

Recent advances on polymer‐stabilized blue phase liquid crystal materials and devices

ABSTRACTPolymer‐stabilized blue phase liquid crystals (PS‐BPLC) have become an increasingly important technology trend for information display and photonics applications. BPLCs exhibit several attractive features, such as reasonably wide temperature range, submillisecond gray‐to‐gray response time, no need for alignment layer, optically isotropic voltage‐off state, and large cell gap tolerance. However, some bottlenecks such as high operation voltage, hysteresis, residual birefringence, charging issues due to the large capacitance, and relatively low transmittance remain to be overcome before their widespread applications can be realized. Recent progress on BPLC materials and devices has shown great promise. To realize the electro‐optic effect of PS‐BPLC, two driving modes: in‐plane switching and vertical field switching mode, have been developed. The material system of PS‐BPLC, including nematic LC host, chiral dopant, and polymer network, are discussed. Each component plays an essential role affecting the stability and electro‐optic properties of PS‐BPLC. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40556.

A low voltage submillisecond-response polymer network liquid crystal spatial light modulator

We report a low voltage and highly transparent polymer network liquid crystal (PNLC) with submillisecond response time. By employing a large dielectric anisotropy LC host JC-BP07N, we have lowered the V2π voltage to 23 V at λ = 514 nm. This will enable PNLC to be integrated with a high resolution liquid-crystal-on-silicon spatial light modulator, in which the maximum voltage is 24 V. A simple model correlating PNLC performance with its host LC is proposed and validated experimentally. By optimizing the domain size, we can achieve V2π < 15 V with some compromises in scattering and response time.

Figure of Merit of Polymer-Stabilized Blue Phase Liquid Crystals

A figure of merit (FoM), defined as the ratio of Kerr constant to response time, is used to characterize and compare the electro-optic performance of polymer-stabilized blue phase liquid crystals (BPLCs). Increasing the dielectric anisotropy of LC host enhances Kerr constant and lowers the operation voltage, but its associated high viscosity dramatically increases the response time. As a result, the FoM may not be necessarily higher. For a given BPLC device, an optimal temperature exists where FoM reaches maximum. The proposed FoM provides useful guidelines for optimizing BPLCs for display and photonic applications.

Polyfluorinated cycloalkoxyphenyl ether systems as dopants for liquid crystal display applications

The effects on response times of representative commercially available dielectrically negative (−LC) and positive (+LC) type liquid crystal mixtures doped with small quantities of various tetrafluorinated cycloalkoxyphenyl ether derivatives were investigated. The synthesis of various polyfluoroaryl systems by nucleophilic aromatic substitution processes is described and electro-optical measurements concerning the rise and decay times and voltage holding ratios of each doped LC formulation are reported. The response time of the dielectrically negative LC host was improved upon doping with each of the tetrafluorinated cycloalkoxyphenyl ether derivatives, although no improvement in the performance of the dielectrically positive LC host was achieved.

Ultranarrow PbS Nanorod-Nematic Liquid Crystal Blend for Enhanced Electro-optic Properties

Dispersion of ultranarrow PbS nanorods of 1.8 nm diameter, encapsulated by a fluidlike soft trioctylamine layer, in the nematic liquid crystal (LC ZLI-4792) results in a novel soft matter type blend with enhanced electro-optic properties. Despite having a cubic rock-salt structure, the ultranarrow rods possess a finite inherent dipole moment and show excellent miscibility in the LC host. The local ordering significantly affects the global ordering of the blend allowing a more rapid response of the electro-optic properties. These unique blends are obtainable only by doping with one-dimensional nanorods of ultrasmall dimensions and illustrates how a nonmesogenic element can be introduced within a liquid crystal for improvement of the switching properties of the blend. The unique blend could be a model for fundamental conceptual advances in general understanding of interaction behavior, leading consequently to a significant technological advancement for superior device fabrications.