Temperature‐ and Light‐Regulated Gas Transport in a Liquid Crystal Polymer Network

Azobenzene‐containing liquid crystal polymer networks (LCNs) are developed for temperature‐ and light‐regulated gas permeation. The order in a chiral‐nematic LCN (LCN*) is found to be essential to couple the unique structure of the membrane and its gas permeation responses to external stimuli such as temperature and varying irradiation conditions. An LCN membrane polymerized in the isotropic phase exhibits enhanced N₂ permeation with increasing temperature, like most traditional polymers, but barely responds to exposure with 455 and 365 nm light. In sharp contrast, a reversible decrease of N₂ transport is observed for the LCN* membrane of exactly the same chemical composition, but molecularly ordered, when submitted to an elevated temperature. More importantly, alternating in situ illumination with 455 and 365 nm light modulates reversibly N₂ permeation performance of the LCN* membrane, through the trans–cis isomerization of azo moieties. The authors postulate that, besides the anisotropic deformation of LCN*, the decreased order in LCN* membrane caused by external stimuli (i.e., increasing temperature or UV light illumination) is responsible for an inhibition of gas permeation. These results show potential applications of liquid crystal polymers in the gas transport and separation, and also contribute to the development of “smart” membranes.     

物理技术与方法在溶致液晶结构研究中的应用

现代物理技术与方法的发展为液晶科学的研究提供了丰富的手段。文章介绍了在溶致液晶研究中的一些常用物理技术与方法如偏光显微镜、量热法、X射线，同时重点介绍了一些新兴的非常规研究手段诸如中子散射技术、光谱技术、电子显微镜、扫描探针显微镜(扫描隧道显微镜、原子力显微镜)。并对诸多技术与方法近年来在国内外的应用进展做了简单介绍，且对各种技术与方法的优缺点进行了类比。     

一种聚合物/液晶体系的结构与模型分析

为提高液晶器件的响应速度,介绍了一种基于聚合物/液晶(预应力液晶)的结构与制作工艺,其响应速度可达毫秒级.对制作过程中预应力液晶结构(液晶微滴的尺寸和形状)与制作工艺条件(聚合物含量、紫外曝光光强、聚合温度)的关系进行了分析,并建立了预应力液晶的简易模型,给出了其响应时间与液晶微滴的尺寸和剪切比之间的关系.结果证明这种模型与实验结果基本相符.     

Smectic Liquid Crystal Defects for Self‐Assembling of Building Blocks and Their Lithographic Applications

Recently, it has been reported that liquid crystal (LC) defects can be used to create highly periodic templates by controlling the surface anchoring and the elastic properties of LC molecules. The self‐assembled defect ordering of the LC materials takes advantage of the ability to achieve fast stabilization of molecular ordering and structure due to the reversible and non‐covalent interactions of the LC molecules. In this Featre Article, the defect structures of liquid crystalline materials will be demonstrated by the surface anchoring and elastic properties. A particular focus are the focal conic domains (FCDs) that are commonly observed in SmA liquid crystals and their lamellar lyotropic counterparts, which form periodic defect ordered structures. In addition, methodologies for creating lithographic templates from the defect order will be described. Finally, the review closes with a discussion of toric focal conic domain arrays that have been fabricated in this manner and used for various applications.     

Reversible Formation of Gold Halides in Single‐Crystal Hybrid‐Perovskite/Au Interface upon Biasing and Effect on Electronic Carrier Injection

Solar cells, light emitting diodes, and X‐ray detectors based on perovskite materials often incorporate gold electrodes, either in direct or indirect contact with the perovskite compound. Chemical interactions between active layers and contacts deteriorate the operation and induce degradation, being the identification of the chemical nature of such interfacial structures an open question. Chemical reactivity of gold in contact with the perovskite semiconductor leads to reversible formation of oxidized gold halide species and explains the generation of halide vacancies in the vicinity of the interface. Electrical biasing induces contact reaction and produces modifications of the current level by favoring the ability of perovskite/Au interfaces to inject electronic carriers. The current injection increment does not depend on the halogen source used, either extrinsically by iodine vapor sublimation of Au electrodes, or intrinsically by bias‐driven migration of bromide ions. In addition, the formation of a dipole‐like structure at the reacted electrode that lowers the potential barrier for electronic carriers is confirmed. These findings highlight adequate selection of the external contacts and suggest the need for a deeper understanding of contact reactivity as it dominates the operation characteristics, rather than being governed by the bulk transport properties of the charge carriers, either electronic or ionic.     

Ultralarge Contraction Directed by Light‐Driven Unlocking of Prestored Strain Energy in Linear Liquid Crystal Polymer Fibers

Anisotropic 1D contraction motion of polymeric actuating materials has drawn growing interests in fields ranging from soft robotics to biomimetic muscles. Although light‐driven liquid crystal polymers (LCPs) represent promising candidates to realize contraction (<20%) triggered remotely and spatially, there remain multitudes of challenges to develop an LCP system possessing ultralarge contraction rate. Here, a novel strategy combining shape memory effect and photochemical phase transition is presented to realize light‐driven contraction as large as 81% in a newly designed linear liquid crystal copolymer, where the eutectic mesogens of azobenzene and phenyl benzoate self‐organize into the smectic B phase. Importantly, this highly ordered structure as the switching segment firmly locks the stress‐induced strain energy, which is rapidly released by reversible trans–cis photoisomerization that destroys the lamellar liquid crystal phase, therefore leading to such ultralarge contraction. Fibers serve as light‐driven building blocks to achieve precise origami, to mimic the recovery of a “broken” spider web and to screen objects in different sizes, laying new ground for advanced applications of light‐driven LCPs from biomimetic robots to human assists.     

A Stimuli‐Responsive,Photoluminescent, Anthracene‐Based Liquid Crystal: Emission Color Determined by Thermal and Mechanical Processes

Here, a photoluminescent liquid crystal that exhibits a change of emission color on the metastable–stable phase transition induced by external stimuli is prepared. A 2,6‐diethynylanthracene derivative with amide groups and dendritic side chains exhibits a columnar phase on slow cooling from the isotropic phase and shows blue emission in this columnar phase. In contrast, a cubic phase is obtained by rapid cooling from the isotropic phase. In the cubic phase, the 2,6‐diethynylanthracene cores form excimers, resulting in yellow emission. While the columnar phase is a stable liquid‐crystalline (LC) phase, the cubic phase is a metastable LC phase. It is found that a change of the photoluminescent color from yellow to blue is observed on the cubic‐columnar phase transition induced by heating or mechanical shearing for this 2,6‐diethynylanthracene derivative in the cubic phase. This change of photoluminescent color is ascribed to the inhibition of excimer formation on the metastable–stable LC phase transition.     

Self‐Assembly Behavior of an Oligothiophene‐Based Conjugated Liquid Crystal and Its Implication for Ionic Conductivity Characteristics

In this work, a joint experimental and computational study on the synthesis, self‐assembly, and ionic conduction characteristics of a new conjugated liquid crystal quaterthiophene/poly(ethylene oxide) (PEO4) consisting of terminal tetraethyleneglycol monomethyl ether groups on both ends of a quaterthiophene core is performed. In agreement with molecular dynamic simulations, temperature‐dependent grazing‐incidence wide angle X‐ray scattering and X‐ray diffraction indicate that the molecule spontaneously forms a smectic phase at ambient temperature as characterized both in bulk and thin film configurations. Significantly, this smectic phase is maintained upon blending with bis(trifluoro‐methanesulfonyl)imide as ion source at a concentration ratio up to r = [Li⁺]/[EO] = 0.05. Nanosegregation between oligothiophene and PEO moieties and π–π stacking of thiophene rings lead to the formation of efficient 2D pathways for ion transport, resulting in thin‐film in‐plane ionic conductivity as high as 5.2 × 10^?4 S cm^?1 at 70 °C and r = 0.05 as measured by electrochemical impedance spectroscopy. Upon heating the samples above a transition temperature around 95 °C, an isotropic phase forms associated with a pronounced drop in ionic conductivity. Upon cooling, partial and local reordering of the conducting smectic domains leads to an ionic conductivity decrease compared to the as‐cast state.     

Biosensor Array of Interpenetrating Polymer Network with Photonic Film Templated from Reactive Cholesteric Liquid Crystal and Enzyme‐Immobilized Hydrogel Polymer

A biosensor array is fabricated using an interpenetrating polymer network consisting of photonic film templated from reactive cholesteric liquid crystal (CLC) and enzyme‐immobilized polyacrylic acid (PAA). The solid‐state photonic film on the glass substrate is successfully templated by ultraviolet (UV) curing of the reactive CLC mixture of a reactive mesogen mixture of RMM 727 (from Merck) and a nonreactive chiral dopant of (S)‐4‐cyano‐4′‐(2‐methylbutyl)biphenyl following the extraction of the chiral dopant. The acrylic acid monomer mixed with a cross‐linker of tri(propylene glycol) diacrylate is infiltrated into the extracted space of the photonic film, and UV‐cured with a photomask to obtain a patterned array‐dot film. The interpenetrated cholesteric liquid crystal/hydrogel polymer network (CLC‐hydrogel‐IPN) array is further functionalized in the individual dots with urease, for a model study of biosensor array applications. The dots of the CLC‐hydrogel‐IPN array respond independently to the urea by a color change with high sensitivity and stability. Thus, the patterned CLC‐hydrogel‐IPN can be used as a new biosensor array for cost‐effective and easy visual detection without any sophisticated instruments.     

A Facile Method for the Growth of Organic Semiconductor Single Crystal Arrays on Polymer Dielectric toward Flexible Field‐Effect Transistors

Polymer dielectrics with intrinsic mechanical flexibility are considered as a key component for flexible organic field‐effect transistors (OFETs). However, it remains a challenge to fabricate highly aligned organic semiconductor single crystal (OSSC) arrays on the polymer dielectrics. Herein, for the first time, a facile and universal strategy, polar surface‐confined crystallization (PSCC), is proposed to grow highly aligned OSSC arrays on poly(4‐vinylphenol) (PVP) dielectric layer. The surface polarity of PVP is altered periodically with oxygen‐plasma treatment, enabling the preferential nucleation of organic crystals on the strong‐polarity regions. Moreover, a geometrical confinement effect of the patterned regions can also prevent multiple nucleation and misaligned molecular packing, enabling the highly aligned growth of OSSC arrays with uniform morphology and unitary crystallographic orientation. Using 2,7‐dioctyl[1]benzothieno[3,2‐b]benzothiophene (C8‐BTBT) as an example, highly aligned C8‐BTBT single crystal arrays with uniform molecular packing and crystal orientation are successfully fabricated on the PVP layer, which can guarantee their uniform electrical properties. OFETs made from the C8‐BTBT single crystal arrays on flexible substrates exhibit a mobility as high as 2.25 cm² V^?1 s^?1, which has surpassed the C8‐BTBT polycrystalline film‐based flexible devices. This work paves the way toward the fabrication of highly aligned OSSCs on polymer dielectrics for high‐performance, flexible organic devices.     

Effects of Heteroatom Substitutions on the Crystal Structure,Film Formation,and Optoelectronic Properties of Diketopyrrolopyrrole‐Based Materials

Substitution of the heteroatoms in the aromatic end‐groups of three diketopyrrolopyrrole containing small molecules is investigated to evaluate how such substitutions affect various physical properties, charge transport, and the performance in bulk heterojunction solar cells. While the optical absorption and frontier orbital energy levels are insensitive to heteroatom substitution, the materials' solubility, thermal properties, film morphology, charge carrier mobility, and photovoltaic performance are altered significantly. Differences in material properties are found to arise from changes in intra‐ and intermolecular interactions in the solid state caused by heteroatom substitution, as revealed by the single crystal structures of three compounds. This study demonstrates a systematic investigation of structure–property relationships in conjugated small molecules.     

Liquid Phase Heteroepitaxial Growth of Moisture‐Tolerant MOF‐5 Isotype Thin Films and Assessment of the Sorption Properties by Quartz Crystal Microbalance

The moisture‐tolerant metal‐organic frameworks (MOFs) of formula [Zn₄O(L)₃]_n (L = di‐substituted carboxypyrazolate derivatives) are fabricated as thin films on self‐assembled monolayer (SAM) functionalized gold substrates by employing the step‐by‐step liquid phase epitaxial (LPE) deposition method in a continuous operation mode. The in situ monitoring of the deposition by quartz crystal microbalance (QCM) and grazing incidence X‐ray diffraction reveal different growth regimes and crystallinities of the obtained thin films in dependence of the chosen alkyl side chain functionality at the carboxypyrazolate linkers, L. To overcome the relatively poor crystallinity and low porosity of a particular homostructured metal‐organic framework type B film, the step‐by‐step heteroepitaxial growth of this MOF B on top of the crystallite surfaces of a well‐grown and lattice‐matched MOF type A is applied. This approach enables the fabrication of oriented, core‐shell‐like MOF B @ A surface mounted heterocrystals as an intergrown homogeneous coating for the selective adsorption of volatile organic compounds. The accessible pore volumes of the individual components and the heterostructured films are characterized by performing adsorption measurements of different organic probe molecules using an environmentally controlled QCM instrument. The results show good adsorption capacity, excellent size exclusion selectivity for alcohols, and a high degree of moisture‐tolerance of the heteroepitaxial MOF films.