Liquid‐Crystalline Ordering as a Concept in Materials Science: From Semiconductors to Stimuli‐Responsive Devices

While the unique optical properties of liquid crystals (LCs) are already well exploited for flat‐panel displays, their intrinsic ability to self‐organize into ordered mesophases, which are intermediate states between crystal and liquid, gives rise to a broad variety of additional applications. The high degree of molecular order, the possibility for large scale orientation, and the structural motif of the aromatic subunits recommend liquid‐crystalline materials as organic semiconductors, which are solvent‐processable and can easily be deposited on a substrate. The anisotropy of liquid crystals can further cause a stimuli‐responsive macroscopic shape change of cross‐linked polymer networks, which act as reversibly contracting artificial muscles. After illustrating the concept of liquid‐crystalline order in this Review, emphasis will be placed on synthetic strategies for novel classes of LC materials, and the design and fabrication of active devices.     

Synthesis of cyclopentyloxy terphenyl liquid crystals with negative dielectric anisotropy

Liquid crystalline materials possessing negative dielectric anisotropy (Δε) have attracted considerable attention because they can be used to formulate eutectic mixture for several display applications. The negative Δε can be achieved by introducing a lateral polar substituent onto the mesogen core of a liquid crystalline material, and fluorine atoms are usually used because of the small size and high electronegativity. 2,3-ortho-Difluoro substituent liquid crystals exhibit stable and profound physical properties such as the low viscosity, strong negative Δε, and high solubility. To avoid the decrease of the mesophase range, 2,3-ortho-difluorophenyl is often placed next to the terminal chain. In this paper, we have synthesised a new series of 2,3-ortho-difluoroterphenyl liquid crystals with a negative dielectric anisotropy. Ether oxygen is introduced to the structure because it can further increase the lateral dipole moment. 3-Alkylcyclopentane end group decreases both the melting point and the viscosity, which makes it an appropriate substituent for negative Δε 2,3-ortho-difluoroterphenyl liquid crystals. The mesomorphic properties as well as the optical anisotropies of the synthesised liquid crystals are discussed in this paper. All compounds in the series are found to possess highly negative dielectric anisotropy, wide mesophase ranges; therefore, they have a great potential to formulate eutectic mixture for liquid crystal displays.     

Advances in Metal‐Free Heterocycle‐Based Columnar Liquid Crystals

Liquid crystals are ordered soft materials formed by self‐organized molecules and can potentially be used as new functional materials for electron‐, ion‐ or molecular‐transport; optical; and bio‐active materials. In particular, the columnar liquid crystals are promising candidates used in various optical and electronic devices. For this purpose, design and synthesis of unconventional materials are essential. In this review, we have summarized several approaches for the synthesis of columnar liquid crystals composed of various heterocyclic systems. We also outline their liquid crystalline and other relevant properties, and their suitability for applications in diverse fields.     

Triphenylene-based discotic liquid crystals: recent advances

Since the early work of Chandrasekhar and his co-workers on hexaesters of benzene published in 1977, discotic liquid crystals (DLCs), in particular, triphenylene-based DLC materials have been investigated intensively, especially over the last decade. The first successful commercialisation of triphenylene-based DLCs has been accomplished in Fuji ‘Wide-View’ optical compensation films. DLCs represent a broad well understood class of soft matter which possess the ability to self-organise into highly anisotropic and ordered structures such as columns that function not only as organic anisotropic semiconductors, but also contribute to the development of new smart materials in the field of organic electronics for many device applications such as photovoltaic devices, light-emitting diodes, field-effect transistors, memory elements, and sensors. Over the last 35 years, more than 1000 triphenylene derivatives have been synthesised and investigated starting from structure-properties to structure-device performance relationships. The very first review by Cammidge and Bushby followed by Kumar summarised the chemistry and physical properties of triphenylene-based discotics up to 2003. In this review, progress in the research of triphenylene DLC materials since 2004 is comprehensively outlined.     

Discotic liquid crystals: from tailor-made synthesis to plastic electronics

Most associate liquid crystals with their everyday use in laptop computers, mobile phones, digital cameras, and other electronic devices. However, in contrast to their rodlike (calamitic) counterparts, first described in 1907 by Vorl?nder, disklike (discotic, columnar) liquid crystals, which were discovered in 1977 by Chandrasekhar et al., offer further applications as a result of their orientation in the columnar mesophase, making them ideal candidates for molecular wires in various optical and electronic devices such as photocopiers, laser printers, photovoltaic cells, light-emitting diodes, field-effect transistors, and holographic data storage. Beginning with an overview of the various mesophases and characterization methods, this Review will focus on the major classes of columnar mesogens rather than presenting a library of columnar liquid crystals. Emphasis will be given to efficient synthetic procedures, and relevant mesomorphic and physical properties. Finally, some applications and perspectives in materials science and molecular electronics will be discussed.     

碲化镓纳米片中单斜晶相和六方晶相共存的光谱特征（英文）

快速发展的二维层状材料为构建新型高效的微纳发光器件和光电探测器件提供了新的平台.其中碲化镓纳米片备受关注,这是因为碲化镓从少层到块体都具有极强的发光性能,因而可以降低二维微纳器件的制备条件.然而碲化镓纳米片具有单斜和六方等多种晶相,使其发光特性和光谱结构异常复杂,目前还缺少对不同厚度碲化镓纳米片荧光光谱的系统研究.本文研究了不同厚度碲化镓纳米片的荧光光谱和拉曼光谱,以及荧光光谱从低温(4 K)到室温的演化行为.通过光谱分析,发现单斜晶相和六方晶相可能同时共存于多层碲化镓纳米片中.为了阐述相关实验结果,构建了如下原子结构模型:其中六方晶相位于纳米片的顶层与底层,而单斜晶相位于纳米片的中间位置.基于温度依赖的荧光光谱,可以获得六方晶相的光学带隙为1.849 eV,且该激子峰只能在较高温度(>200 K)下和声子辅助下才能表现出显著的荧光发射行为.进一步的实验结果确定了六方晶相的激子-声子相互作用强度约为1.24 meV/K.本工作证实了碲化镓纳米片中单斜晶相与六方晶相共存的实验现象,为研究碲化镓纳米片的晶相变化和激子动力学,以及探究碲化镓的奇异物理化学特性,提供了新的见解.     

Application of electric fields in NMR

Anisotropic orientation states in materials such as liquid crystals, liquid crystalline polymers or polymeric electrets can be induced by the application of electric fields. This behaviour is exploited extensively in displays or sensors. Anisotropic optical properties induced by relatively high electric field strengths are used in the well known displays made from liquid crystalline materials. The anisotropy of electric properties in polymeric materials gives rise to properties such as the high piezoelectricity of fluorine containing electrets. Attempts have been made to investigate the orientational state induced by electric fields after poling using NMR techniques. However, there is only access to the irreversible part of the electrically induced orientation. In-situ application of (high) voltages during NMR experiments enables the investigation of the influence of the electric field on molecular orientational states or electrically-induced morphological changes. The NMR methods have been modified to investigate the dynamics of reorientation processes in electric fields and to extend the possible time range towards below1ms. The application of high voltages in NMR experiments requires experimental specifications. Some of our experiences are outlined in this paper.     

蓝色有机电致发光材料及器件的研究进展

有机电致发光器件因在全彩平板显示和固态照明领域中具有广阔的应用前景, 而受到人们的广泛关注。 时至今日, 与现有的红色和绿色有机电致发光材料和器件相比, 具有优越综合性能的蓝色有机电致发光材料和器件却始终匮乏。 相对而言, 蓝光材料具有较宽的能隙, 因而很难获得低电压、高效率和良好稳定性的深蓝光器件。 通常, 白色有机电致发光器件可以通过混合三基色或者两种颜色的方法获得。 但是无论哪种方法, 蓝光材料均是必不可少的。 另外, 还可以通过能量传递将蓝光转化为红光和绿光。 因此, 研发出具有优越综合性能的蓝光材料对有机电致发光器件的推广及应用十分关键。 本文综述了近年来蓝色荧光材料、蓝色磷光材料的研究进展以及蓝光材料在蓝色和白色有机电致发光器件中的应用, 并结合现有工作, 对蓝色有机电致发光材料的研究和应用前景进行展望。     

A novel generation of photoactive comb‐shaped polyamides for the photoalignment of liquid crystals

A new approach to the synthesis of photoactive comb‐shaped homo‐ and copolyamides containing azobenzene, cinnamate, and coumarin side groups for photoalignment of liquid crystals was elaborated. Photooptical properties and photoorientational ability of these polymers with respect to liquid crystals were studied. It was shown that polarized UV irradiation of all spin‐coated polyamides leads to orientation of liquid crystalline molecules deposited on the polyamide thin films. The synthesized polymers containing cinnamate and coumarin side groups as well as azobenzene‐containing cyano‐ and nitro‐substituted polymers demonstrated good orientation ability in relation to liquid crystals displaying photoinduced planar orientation with high dichroism values within the range of 0.68–0.72. Contrary to the above‐mentioned polyamides, azobenzene‐containing fluorosubstituted polymers induced a homeotropic orientation of liquid crystals. It was shown that the synthesized photoactive polyamides can be considered as promising photoalignment materials for application in display technology, photonics, and other “smart” optical devices. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4031–4041     

Research progress on hybrid organic-inorganic perovskites for photo-applications

Hybrid organic-inorganic perovskite materials have attracted significant attention of most researchers in recently years, which is ascribed to the superior photoelectric properties, such as the suitable band gaps for harvesting sunlight, and exhibit high optical adsorption, high charge-carrier lifetimes and long diffusion lengths. The photodetectors, light-emitting diodes, solar cells and photocatalysts represent the remarkable applications for the hybrid organic-inorganic perovskite materials. Herein, we review the recent progress of hybrid organic-inorganic perovskite-based photodetectors, light-emitting diodes, solar cells and photocatalysts. The challenges and outlook for the hybrid organic-inorganic perovskite-based photodetectors, light-emitting diodes, solar cells and photocatalysts are considered.     

Diverse Synthetic Transformations Using 4-Bromo-3,3,4,4-tetrafluorobut-1-ene and Its Applications in the Preparation of CF2CF2-Containing Sugars,Liquid Crystals,and Light-Emitting Materials

Organic molecules with fluoroalkylene scaffolds, especially a tetrafluoroethylene (CF₂CF₂) moiety, in their molecular structures exhibit unique biological activities, or can be applied to functional materials such as liquid crystals and light-emitting materials. Although several methods for the syntheses of CF₂CF₂-containing organic molecules have been reported to date, they have been limited to methods using explosives and fluorinating agents. Therefore, there is an urgent need to develop simple and efficient approaches to synthesize CF₂CF₂-containing organic molecules from readily available fluorinated substrates using carbon-carbon bond formation reactions. This personal account summarizes the simple and efficient transformation of functional groups at both ends of 4-bromo-3,3,4,4-tetrafluorobut-1-ene and discusses its synthetic applications to biologically active fluorinated sugars and functional materials, such as liquid crystals and light-emitting molecules.     

Chiral Side Groups Trigger Second Harmonic Generation Activity in 3D Octupolar Bipyrimidine‐Based Organic Liquid Crystals

The design of efficient noncentrosymmetric materials remains the ultimate goal in the field of organic second‐order nonlinear optics. Unlike inorganic crystals currently used in second‐order nonlinear optical applications, organic materials are an attractive alternative owing to their fast electro‐optical response and processability, but their alignment into noncentrosymmetric film remains challenging. Here, symmetry breaking by judicious functionalization of 3D organic octupoles allows the emergence of multifunctional liquid crystalline chromophores which can easily be processed into large, flexible, thin, and self‐oriented films with second harmonic generation responses competitive to the prototypical inorganic KH₂PO₄ crystals. The liquid‐crystalline nature of these chiral organic films also permits the modulation of the nonlinear optical properties owing to the sensitivity of the supramolecular organization to temperature, leading to the development of tunable macroscopic materials.     

Analytical devices based on light-emitting diodes – a review of the state-of-the-art

A general overview of the development of the uses of light-emitting diodes in analytical instrumentation is given. Fundamental aspects of light-emitting diodes, as far as relevant for this usage, are covered in the first part. The measurement of light intensity is also discussed, as this is an essential part of any device based on light-emitting diodes as well. In the second part, applications are discussed, which cover liquid and gas-phase absorbance measurements, flow-through detectors for chromatography and capillary electrophoresis, sensors, as well as some less often reported methods such as photoacoustic spectroscopy.     

Hydrogen‐Bonded Liquid Crystalline Materials: Supramolecular Polymeric Assembly and the Induction of Dynamic Function

Liquid crystals are molecular materials that combine anisotropy with dynamic nature. Recently, the use of hydrogen bonding for the design of functional liquid crystalline materials has been shown to be a versatile approach toward the control of simple molecularly assembled structures and the induction of dynamic function. A variety of hydrogen‐bonded liquid crystals has been prepared by molecular self‐assembly processes via hydrogen bond formation. Rod‐like and disk‐like low‐molecular weight complexes and polymers with side‐chain, main‐chain, network, and guest‐host structures have been built by the complexation of complimentary and identical hydrogen‐bonded molecules. These materials consist of closed‐type hydrogen bondings. Another type of hydrogen‐bonded liquid crystals consists of open‐type hydrogen bonding. In this case, the introduction of hydrogen bonding moieties, such as hydroxyl groups, induces microphase segregation leading to liquid crystalline molecular order. Moreover, liquid crystalline physical gels have been prepared by the molecular aggregation of hydrogen‐bonded molecules in non‐hydrogen‐bonded liquid crystals. They show significant electrooptical properties. An anisotropic gel is a new type of anisotropic materials forming heterogeneous structures.     

Research progress on hybrid organic–inorganic perovskites for photo-applications

Hybrid organic–inorganic perovskite materials have attracted significant attention of most researchers in recently years, which is ascribed to the superior photoelectric properties, such as the suitable band gaps for harvesting sunlight, and exhibit high optical adsorption, high charge-carrier lifetimes and long diffusion lengths. The photodetectors, light-emitting diodes, solar cells and photocatalysts represent the remarkable applications for the hybrid organic–inorganic perovskite materials. Herein, we review the recent progress of hybrid organic–inorganic perovskite-based photodetectors, light-emitting diodes, solar cells and photocatalysts. The challenges and outlook for the hybrid organic–inorganic perovskite-based photodetectors, light-emitting diodes, solar cells and photocatalysts are considered.     

环戊二噻吩中的硫原子被CH2、SiH2取代对化合物光电性质影响的理论研究

有机电致发光材料是国际上的研究热点之一。本文采用量子化学方法,研究了环戊二噻吩CH2和SiH2取代对其与2,1,3-苯并噻二唑和三苯胺的化合物光电性质的影响。研究结果表明,CH2取代对母体分子的电子和光谱性质的影响比SiH2取代明显。SiH2取代使吸收和发射光谱的振子强度增大的程度大于CH2取代,更有利于发光强度的提高。CH2和SiH2取代衍生物的空穴和电子重组能的差值极小,可以作为有机电致发光二极管中双极性电荷传输材料。静电势能结果表明,SiH2取代衍生物的稳定性高于母体分子的稳定性。通过探索分子结构与性质间关系,为实验设计合成新的有机电致发光材料提供了理论支持。     

Cross dipole stacking in the crystal of distyrylbenzene derivative: the approach toward high solid-state luminescence efficiency

Through adding two substituent phenyl groups on distyrylbenzene, we have obtained the cross stacking of 2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds (trans-DPDSB) in crystalline state. In such a cross-stacking mode, the solid-state emission exhibits high-intensity, having characteristics similar to its single molecule. The organic light-emiiting diodes (OLEDs) with attractive performance have been achieved using trans-DPDSB as a light-emitting layer, and the amplified spontaneous emission of the needlelike crystals has been observed.     

Synthesis,mesomorphic behaviour and optoelectronic properties of phosphorus-based thermotropic liquid crystalline dendrimers

A new series of thermotropic phosphorus-based liquid crystalline (LC) dendrimers based on a thiophosphoryl-phenoxymethyl(methylhydrazono) core (thiophosphoryl-PMMH) up to the fifth generation has been synthesised by solution condensation of aldehyde groups, surface-functionalised thiophosphoryl-PMMH dendritic substrates of generation numbers G_0.5 to G_5.5, with the appropriate molar equivalents of the pro-mesogenic n-hexadecylaniline mono-functional building block. Their chemical composition has been confirmed by ¹H/¹³C/³¹P nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, elemental analysis. Optical properties have been studied by ultraviolet-visible absorption, photoluminescence spectroscopy and polarised optical microscopy, and thermal characteristics by differential scanning calorimetry. Electrical studies have been made using the current-voltage characteristics of organic light-emitting diodes consisting of multi-layered indium tin oxide/dendrimer/aluminium tris(8-hydroxyquinoline)Al architecture. It has been demonstrated that the molecular engineering approach adopted can successfully lead to phosphorus-containing dendritic organic semiconductors (OSCs) which show tunable mesomorphic behaviour (extension of the observed smectic mesophase) and (opto) electronic properties, owing to their peripheral decoration with a tunable number of azomethine-based optically active chromophoric units. This rare combination of ‘tunable by design’ properties makes this series of thermostable thiophosphoryl-PMMH-based LC dendrimers a particularly appealing class of OSCs for use in optically and/or electronically active layers of (opto)electronic devices such as light-emitting diodes, field-effect transistors, solar cells and lasers.     

High-refractive index acrylate polymers for applications in nanoimprint lithography

The development of polymeric optical materials with a higher refractive index,transparency in the visible spectrum region and easier processability is increasingly desirable for advanced optical applications such as microlenses,image sensors,and organic light-emitting diodes.Most acrylates have a low refractive index(around 1.50)which does not meet the high perfo rmance requirements of advanced optical materials.In this research,three novel acrylates were synthesized via a facile one-step approach and used to fabricate optical transparent polymers.All of the polymers reveal good optical properties including high transparency(≥90%)in the visible spectrum region and high refractive index values(1.6363)at 550 nm.Moreover,nanostructures of these acrylate polymers with various feature sizes including nanogratings and photonic crystals were successfully fabricated using nanoimprint lithography.These results indicate that these acrylates can be used in a wide range of optical and optoelectronic devices where nanopatterned films with high refractive index and transparency are required.     

Effect of cholesteric liquid crystalline elastomer with binaphthalene crosslinkings on thermal and optical properties of a liquid crystal that show smectic A‐cholesteric phase transition

A side‐chain polysiloxane cholesteric liquid crystalline elastomer (ChLCE) with binaphthalene derivate as crosslinkings and cholesterol derivate as liquid crystalline monomers was designed and synthesized. A binaphthyl chiral dopant (CD) was synthesized as well. The chemical structures and liquid crystalline properties of the ChLCE and the CD were characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance spectroscopy, element analyses, differential scanning calorimetry and polarizing optical microscopy measurements. The helical twisting power of the ChLCE exhibited a turning point with changing temperature and was smaller than that of the CD. In addition, the effect of the ChLCE on phase transition temperatures and thermal‐optical properties of a liquid crystal that show smectic A (SmA)‐cholesteric (Ch) phase transition was studied. Worthily, the testing of the reflection wavelength with changing temperature suggested that the adding of the ChLCE in liquid crystals that show SmA‐Ch phase transition can expedite their SmA‐Ch transition. In addition, the network structure of the ChLCE may play a significant role in the accelerating of the transition. These properties provided theoretical and experimental foundations for applying ChLCE in thermally sensitive liquid crystal devices requiring fast response, such as thermally controllable windows, materials and displays. Copyright © 2012 John Wiley & Sons, Ltd.