Selective Reflection Wavelength Research Articles

TGBs below SmA*: Mesophase behaviours and optical properties

Re-entrant mesophase behaviours are studied including the relaxation effect of TGB phases, and an abnormal phase transition of TGBC*-SmA*. For the re-entrant TGBA mesophase between SmA* and TGBC*, its temperature range can be expanded by doping method and meanwhile its selective reflection wavelength moves to the visible light range. Furthermore, this series of dimers are compared with other series to study the structure–property relationship. This research provides an important reference for preparing materials exhibiting multiple mesophases that reflect visible light within multiple temperature ranges.

Geometrical Optics Analysis of Diffraction in Patterned Cholesteric Liquid Crystals

A geometrical optics method is developed to analyze the diffraction in patterned cholesteric liquid crystals. We extend the equation for the center wavelength of selective reflection in a conventional cholesteric liquid crystal to the wavelength edges of the diffraction in patterned cholesteric liquid crystals. This method can be used to determine the reflection bandwidth and its wavelength edges of patterned cholesteric liquid crystals which depends on a slant angle and an incident angle. Furthermore, we also investigate a difference in the diffraction between planar and slanted patterned cholesteric liquid crystals using the geometrical optics method.

Temperature-Dependent Circularly Polarized Luminescence of a Cholesteric Copolymer Doped with a Europium Complex

The design of new materials for non-contact temperature sensors is an important task for scientists working in the fields of chemistry, physics, and materials science. In the present paper, a novel cholesteric mixture based on a copolymer doped with a highly luminescent europium complex was prepared and studied. It was found that the spectral position of the selective reflection peak strongly depends on temperature and a shift towards shorter wavelengths is observed upon heating with an amplitude of more than 70 nm, from the red to green spectral range. This shift is associated with the existence and melting of clusters of smectic order, as confirmed by X-ray diffraction investigations. The extreme temperature dependence of the wavelength of selective light reflection provides a high thermosensitivity of the degree of circular polarization of the europium complex emission. The highest values of the dissymmetry factor are observed when the peak of selective light reflection fully overlaps with the emission peak. As a result, the highest sensitivity of 65%/K for luminescent thermometry materials was obtained. In addition, the ability of the prepared mixture to form stable coatings was demonstrated. The obtained experimental results, i.e., the high thermosensitivity of the degree of circular polarization, and the ability to form stable coatings allow us to consider the prepared mixture as a promising material for luminescent thermometry.

Preparation and characterisation of liquid crystal elastomer films with selective reflectivity

ABSTRACT In this paper, the liquid crystal elastomer films (A’BG series) have been prepared by grafting polymerisation, using the 4-allyloxybenzoic acid cholesteryl ester (MB) as liquid crystal monomer, dodecane-1,12-diyl bis(undec-10-enoate) (MG) as flexible cross-linker and polymethyl hydrosiloxane (containing 35 Si-H) as the main chain. The A’BG series films are cholesteric liquid crystal elastomer films (LCEs) with a liquid crystal region above 100°C. With the crosslinker content decreasing, A’BG-2 and A’BG-3 display selective reflection in UV region and visible region, respectively. The selective reflection wavelength of A’BG-3 occurs blue shift with the temperature increasing. The glass transition temperatures (Tg) of the A’BG series films are 16.4–40.8°C and A’BG series films display shape memory properties near room temperature. The A’BG series elastomer films have a high thermal stability and the temperatures of the 5% mass loss are above 300°C. The backbone siloxane structures in the A’BG series elastomer films are highly stable, and the -COOChol group is relatively easy to decompose.

Transparent and iridescent photonic films with intelligent responsive ability based on electrospun core-shell nanofibrous membranes.

The fabrication of transparent and iridescent photonic films that possess intelligent responsiveness by membrane electrospinning is challenging due to the lack of periodic changes in the refractive index (RI) of electrospun membranes. Herein, transparent and iridescent photonic films are prepared through electrospinning core-shell polyacrylonitrile/glucose-containing polyvinyl alcohol (PAN/PVA@GLU) membranes, infiltrated with a cellulose nanocrystal/polyvinyl alcohol/glucose (CNC/PVA/GLU) suspension, followed by evaporation-induced co-assembling. The as-prepared transparent and iridescent photonic films exhibited reversible changes in selective reflection wavelengths ranging from the visible light to the near-infrared region in response to alternate changes in the relative humidity (RH). Thus, the films could be used as an alcohol dipstick by choosing solvents with different polarities such as alcohol-water mixtures of different ratios. Moreover, the films were highly deformable with a strain at failure up to 14.91% without a compromise of strength. In sum, the current work demonstrates a strategy for the design and fabrication of transparent and iridescent photonic films with intelligent responsiveness using electrospinning, and a soft material platform for developing scalable colorimetric sensors and optically active components.

When Chirophotonic Film Meets Wrinkles: Viewing Angle Independent Corrugated Photonic Crystal Paper.

Light manipulation strategies of nature have fascinated humans for centuries. In particular, structural colors are of considerable interest due to their ability to control the interaction between light and matter. Here, wrinkled photonic crystal papers (PCPs) are fabricated to demonstrate the consistent reflection of colors regardless of viewing angles. The nanoscale molecular self-assembly of a cholesteric liquid crystal (CLC) with a microscale corrugated surface is combined. Fully polymerizable CLC paints are uniaxially coated onto a wrinkled interpenetrating polymer network (IPN) substrate. Photopolymerization of the helicoidal nanostructures results in a flexible and free-standing PCP. The facile method of fabricating the wrinkled PCPs provides a scalable route for the development of novel chirophotonic materials with precisely controlled helical pitch and curvature dimensions. The reflection notch position of the flat PCP shifts to a lower wavelength when the viewing angle increased, while the selective reflection wavelength of wrinkled PCP is remained consistent regardless of viewing angles. The optical reflection of the 1D stripe-wrinkled PCP is dependent on the wrinkle direction. PCPs with different corrugated directions can be patterned to reduce the angular-dependent optical reflection of wrinkles. Furthermore, 2D wavy-wrinkled PCP is successfully developed that exhibit directionally independent reflection of color.

力を可視化できるキラル液晶エラストマーの動的挙動制御

Strain sensors are a key for the development of next-generation soft robots. For such applications, the electronic sensing using flexible electronic materials are continuing to be at the forefront of the sensing technology. Recently, optical-sensing technology have attracted much attention because of high spatiotemporal resolution, high sensitivity and noninvasively. Especially, chiral-nematic liquid-crystalline elastomers have much attention to be applied to optical strain sensors because of the change of selective reflection wavelength in response to the strain. Here, we introduce our recent research on optical strain sensors using chiral-nematic liquid-crystalline elastomers. There is remarkable maturity in the development of numerous stimuli-responsive liquid-crystalline elastomers. However, the control of the recovery process after the removal of the stimuli has remained difficult. Interestingly, our simple materials design concept of “layering the elastomers with other materials” allows us to arbitrary control the recovery process in both macroscopic deformation and microscopic molecular orientation. Based on the concept, we have succeeded to fabricate a mechano-optical sensor with high sensitivity and high spatiotemporal resolution. This concept would open a pathway to control the recovery process in various stimuli-responsive liquid-crystalline elastomers.

Multiple Anti-Counterfeiting Composite Film Based on Cholesteric Liquid Crystal and QD Materials.

A composite film with multiple anti-counterfeiting features was demonstrated by superposing quantum dots (QDs) polymer matrix (film A) and cholesteric liquid crystal film (film B) together. The first-line and second-line anti-counterfeiting characteristics were successfully implemented by employing thermochromic, angular photochromic, and circularly polarized discoloration of film B, respectively. By initiatively utilizing the different relative positions between the fluorescence emission peak (λem) of film A and the central selective reflection wavelength (λm) of film B at different temperatures, which resulted in changes in the fluorescence spectra or the different presence of latent patterns, the most important third-line anti-counterfeiting feature was successfully achieved.

Chiral, Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal.

The front cover artwork is provided by the group of Prof. Emeritus Yasushi Yokoyama and Prof. Takashi Ubukata. The image shows the generation of a colored image, whose origin is the UV-controlled selective reflection of visible light from a cholesteric liquid crystal containing chiral photochromic molecules. Read the full text of the Article at 10.1002/cphc.202000309.

Chiral, Thermally Irreversible and Quasi-Stealth Photochromic Dopant to Control Selective Reflection Wavelength of Cholesteric Liquid Crystal.

A chiral and thermally irreversible photochromic fulgide derivative incorporating an (R)-binaphthol unit in its acid anhydride moiety was used for the photoswitching of the pitch length of cholesteric liquid crystals. Since the absorption maximum wavelengths of both thermally stable photoisomers are nearly in the UV region (quasi-stealth photochromism), it can be exposed to visible light without inducing photochromic reactions. Therefore, when the photoswitching molecule is added to a permanent cholesteric liquid crystal whose reflection light wavelength is in the visible region, the UV light-induced photochromic reaction of the photoswitching molecule changes the wavelength of the reflection light in the visible light region. We have succeeded in regulating the color of cholesteric liquid crystalline cells between red and blue upon UV light irradiation. Attempts to introduce this system in polymer dispersed cholesteric liquid crystals are also described.

Optical properties of selective diffraction from Bragg-Berry cholesteric liquid crystal deflectors

The operational bandwidth of a cholesteric liquid crystal deflector based on the Bragg-Berry effect is analyzed using the two-dimensional finite-difference time-domain method. Despite its similarity in structure with a conventional cholesteric mirror under oblique incidence, the bandwidths of selective reflection and selective diffraction are different. The selective reflection wavelength from the cholesteric mirror has a cosine dependence on the Bragg angle, while that of selective diffraction from the cholesteric deflector has a cosine-squared dependence on the slant angle. We also propose equations that approximate the selective diffraction bandwidth of the deflector. The equations can be used to find the helical pitch required to achieve a deflector with a specified deflection angle and operational wavelength, thereby facilitating the development of cholesteric liquid crustal based diffractive optical elements.

1,2-Dithienyldicyanoethene-Based, Visible-Light-Driven, Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices.

Reported here is the first example of a 1,2-dithienyldicyanoethene-based visible-light-driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10-fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.

1,2‐Dithienyldicyanoethene‐Based, Visible‐Light‐Driven, Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

AbstractReported here is the first example of a 1,2‐dithienyldicyanoethene‐based visible‐light‐driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10‐fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.

Tuning helical twisting power and photoisomerisation kinetics of axially chiral cyclic azobenzene dopants in cholesteric liquid crystals

ABSTRACTSignificant attention has been paid to improve the helical twisting power (β) and Δβ between the two different isomers of axially chiral azobenzene dopants in cholesteric liquid crystals (CLCs); however, the correlations between the vales (β and Δβ) with the molecular structures as well as photoisomerisation kinetics are far from clear. In this study, a series of binaphthyl-azobenzene cyclic dopants R1–R3 with different lengths of alkoxy chain was synthesised, which exhibited photochemically reversible trans–cis isomerisation in both organic solvents and liquid crystal hosts. When doping into a nematic liquid crystal, dopant R2 with one linking alkoxy group showed the highest values of β and Δβ. The results revealed that the β value was related to the dihedral angle between two naphthyl planes and the miscibility between the dopants and the host molecule. Moreover, Δβ was also depended on the photoisomerisation quantum yields. With increasing length of alkoxyl chain, the photoisomerisation rate constant of dopants increased upon ultraviolet irradiation and decreased for the reverse process upon visible light irradiation either in isotropic acetonitrile or in CLCs. These results enable the precise tuning of the pitch and selective reflection wavelength of CLCs.

Influence of shorter backbone and cholesteric monomer percentage on the phase structures and thermal-optical properties of linear siloxane tetramers containing cholesterol and benzene methyl ether groups

A series of thermochromism liquid crystalline materials with an unusual characterization of the centre selective light reflection wavelength (λm) vs. Xchol was prepared.

Nanocomposite based on a liquid crystal doped with aluminum nitride nanotubes for optical sensor of sulfur dioxide

AbstractIt has been demonstrated the possibilities of designing the material for optical sensor primary transducer. The nanocomposites based on a cholesteric liquid crystal with aluminum nitride for material of primary transducer of sulfur dioxide optical sensor were studied. The spectral characteristics of studied nanocomposite under the influence of sulfur dioxide were studied. The shift of central wavelength of selective reflection under the sulfur dioxide concentrations was observed.

New azobenzene-based chiral-photochromic substances with thermally stable Z-isomers and their use for the induction of a cholesteric mesophase with a phototunable helix pitch

Novel photosensitive azobenzene-containing substances with sorbitol- and lactate-based chiral fragments were synthesized to serve as chiral-photochromic dopants in cholesteric mixtures. Owing to the presence of chlorine or methyl lateral substituents in the azobenzene fragment of the dopants, the photoinduced Z-form of their molecules is highly stable. The photooptical properties of the prepared cholesteric mixtures were studied and it was shown that UV-irradiation of the planarly-oriented films results in a shift of the selective light reflection wavelength to a long-wavelength spectral region due to E–Z isomerization of the azobenzene moieties. This process is thermally and photooptically reversible and the rate of blue-light-induced helix twisting is comparable with the rate of the untwisting process, whereas the thermal relaxation completes only after ca. 40 days at room temperature (for a chlorine-substituted chiral-photochromic dopant). It was found that this value is almost two orders of magnitude less in comparison with the non-substituted chiral-photochromic dopant also studied in this work. The obtained results demonstrate the promising properties of the synthesized compounds for creation of novel photoswitchable cholesteric materials.

Chiroptical properties of cholesteric liquid crystals of chitosan phenylcarbamate in ionic liquids

Chitosan phenylcarbamate (CtsPC) samples were synthesized with different degrees of substitution (DS) ranging from 2.3 to 3.5, and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the concentrated lyotropic system formed a cholesteric liquid-crystalline phase and mostly exhibited vivid colorations owing to selective reflection of visible light. Ultraviolet–visible–near-infrared spectroscopy and wide-angle X-ray diffractometry were used to evaluate the maximum wavelength of selective light reflection and the distance and rotational difference between adjacent nematic thin layers in the cholesteric mesophase, respectively. The cholesteric helical pitch increased with decreasing polymer concentration, increasing temperature or increasing DS. The helical sense remained right-handed under the adopted measurement conditions, although CtsPCs are known to form a left-handed cholesteric structure in polar organic solvents, such as N,N-dimethylformamide (DMF). Accordingly, an inversion of the cholesteric sense was achieved by combination of [C2Mim][N(CN)2] and DMF as solvent; circular dichroism and optical rotatory dispersion spectroscopy verified that the sense of CtsPC cholesterics changed from left-handed to right-handed with increasing [C2Mim][N(CN)2] content in the mixed solvent. Chitosan phenylcarbamate (CtsPC) samples were synthesized and their lyotropic liquid crystallinity in ionic liquids was investigated. 1-Ethyl-3-methylimidazolium dicyanamide ([C2Mim][N(CN)2]) was found to be a novel solvent for CtsPC, and the lyotropic system formed a cholesteric liquid-crystalline phase and exhibited vivid colorations owing to selective light reflection. The cholesteric helical pitch was controllable by polymer concentration, temperature or the degree of phenylcarbamoyl substitution. Although the helical sense of CtsPC/[C2Mim][N(CN)2] cholesterics remained right-handed under the adopted measurement conditions, an inversion of cholesteric sense was achieved by using a mixed solvent of [C2Mim][N(CN)2]/N,N-dimethylformamide.

Red, Green and Blue Reflections Enabled in an Optically Tunable Self‐Organized 3D Cubic Nanostructured Thin Film

A new light-driven chiral molecular switch doped in a stable blue phase (BP) liquid crystal allows wide optical tunability of three-dimensional cubic nanostructures with a selective reflection wavelength that is reversibly tuned through the visible region. Moreover, unprecedented reversible light-directed red, green, and blue reflections of the self-organized three-dimensional cubic nanostructure in a single film are demonstrated for the first time. Additionally, unusual isothermal photo-stimulated less ordered BP II to more ordered BP I phase transition was observed in the system.

偶氮分子掺杂对蓝相液晶选择性反射波长的影响

偶氮分子掺杂对蓝相液晶选择性反射波长的影响