Photogeneration of High Pretilt Angles of Nematic Liquid Crystals by Non‐Polarized Light Irradiation of Azobenzene‐Containing Polymer Films

A vertical‐alignment (VA) cell of nematic liquid crystals (LCs) was prepared using photoirradiated thin films of a poly(methacrylate) with mesogenic moieties of 4‐trifluoromethoxyazobenzene as the side chains. Optical anisotropy was generated by oblique irradiation of the azobenzene‐containing polymer films with non‐polarized UV light, followed by annealing treatment to enhance the photodichroism, which displayed thermal stability. The combination of oblique exposure to non‐polarized UV light and subsequent annealing treatment brought about high pretilt angles of nematic LCs so that a photoaligned VA LC cell was fabricated. The photopatterned LC cell exhibited electro‐optical properties with excellent optical quality when a voltage was applied even after heating at 100 °C for several hours.     

Alignment of liquid crystals on a topographically nano-patterned polymer surface prepared by a soft-imprint technique

In this study, we have fabricated polyacrylate substrates having a nano-patterned surface topography using a soft-imprint technique. The planar alignment of liquid crystals (LCs) along the direction of nanogrooves has been generated. Twisting behavior of nematic LCs has been also observed with a perpendicularly assembled LC cell and the cell parameters can be estimated by using the Soutar and Lu method. By comparing the anchoring energies obtained, accordingly, it has been demonstrated that the polymer nanogroove pattern has a comparable influence on LC alignment to the conventional rubbing process. It has been also shown that the artificial topography of the line grooves on the conventionally rubbed surface has a significant influence on the anchoring stability of the LC molecules.     

An Orientation‐Controlled Pentacene Film Aligned by Photoaligned Polyimide for Organic Thin‐Film Transistor Applications

We have demonstrated that the photoalignment method could be used to control the structural anisotropy of pentacene films, an active semiconducting layer, in thin‐film transistors (TFTs) with conspicuously anisotropic electrical characteristics. The photoaligned pentacene films were characterized with respect to structure and morphology using X‐ray diffraction and atomic force microscopy. Compared to the pentacene films that are not controlled, a maximal 25‐times increase in field‐effect mobility (up to 0.75 cm V^–1 s^–1) has been achieved in the photoaligned pentacene‐based TFTs by aligning pentacene orientation parallel to the direction of current flow with the help of a photoaligned polyimide layer. Mobility anisotropic ratio in the range of 2.7–8.3 for the current flow parallel and perpendicular to the alignment of the photoaligned pentacene films have been observed for photoaligned pentacene‐based TFTs.     

Super‐Fast Switching of Twisted Nematic Liquid Crystals on 2D Single Wall Carbon Nanotube Networks

We have developed a high performance liquid crystal (LC) alignment layer of ultra‐thin single wall carbon nanotubes (SWNTs) and a conjugated block copolymer nanocomposite that is solution‐processible for conventional twisted nematic (TN) LC cells. The alignment layer is based on the non‐destructive solution dispersion of nanotubes with a poly(styrene‐b‐ paraphenylene) (PS‐b‐PPP) copolymer and subsequent spin coating, followed by conventional rubbing without a post‐annealing process. Topographically grooved nanocomposite films with two dimensionally (2D) networked SWNTs embedded in a block copolymer matrix were created using a rubbing process in which bundles of SWNTs on the composite surface were effectively removed. The LCs were well aligned with a stable pre‐tilt angle of approximately 2° on our extremely transparent nanocomposite, which gave rise to superfast switching of the TN LC molecules that was approximately 3.8 ms, or four times faster than that on a commercial polyimide layer. Furthermore, the TN LCD cells containing our SWNT nanocomposite alignment layers exhibited low power operation at an effective switching voltage amplitude of approximately 1.3 V without capacitance hysteresis.     

Magnetic and Transparent Composites by Linking Liquid Crystals to Ferrite Nanoparticles through Covalent Networks

The fabrication of transparent, flexible, and optically homogeneous magnetic composites containing ferrite (Fe₃O₄) nanoparticles, liquid crystals (LCs), and siloxane backbones is reported. The transparent magnets are achieved by covalently bonding LCs to the siloxane backbones and then linking them to dopamine‐functionalized ferrite nanocrystals. They exhibit simultaneous high transparency and strong magnetic properties. A remarkable feature of these films is that the surface morphology of the LC‐attached ferrite films can be tuned by an external magnetic field, demonstrating a striped surface in the direction of the field. We show that the LC‐attached film can act as an alignment layer to orient LCs, enabling the development of LC alignment surfaces on the basis of these nanomagnet–LC polymer composites.     

Fabrication and Characterization of Polymer/Liquid Crystal Composite Diffractive Optics by Multiphoton Methods

Direct‐write multiphoton photolithography is used to prepare electrically switchable diffraction gratings having spacings as small as 4 μm. Surface‐relief gratings are written into poly(methyl methacrylate) films using a sample‐scanning confocal microscope and are characterized by using contact‐mode atomic force microscopy. The resulting polymeric channels are filled with nematic liquid crystals (LCs) and sandwiched between indium tin oxide‐coated coverslips to obtain functional devices. These devices exhibit diffraction efficiencies approaching 30 %. Microscopic LC organization and field‐induced reorientation dynamics within these devices are characterized by static and dynamic polarization‐dependent multiphoton excited fluorescence microscopy. LCs are found to align predominantly along the channel axis, but exhibit some disorder near the channel walls, resulting from nanometer‐scale polymer surface roughness. LC reorientation in response to an electric field is rapid (<1 ms) and uniform, whereas field‐free LC relaxation is relatively slow (>20 ms). Both reorientation and relaxation are influenced by orientationally anchored LCs near the channel walls.     

Nematic Anisotropic Liquid‐Crystal Gels—Self‐Assembled Nanocomposites with High Electromechanical Response

The uniqueness of liquid crystals (LCs) lies in the large anisotropies of their properties, which can be utilized to generate high electromechanical responses. In a properly oriented LC polymer system, an external electric field can induce reorientation of the mesogenic units possessing a dielectric anisotropy, which, when coupled with the shape anisotropy of the mesogenic units, can in turn produce large mechanical strain. Anisotropic LC gels, which can be obtained by in‐situ photopolymerization of the reactive LC molecules in the presence of non‐reactive LC molecules in an oriented state, are an example of such liquid‐crystal polymer systems. It is shown here that a homeotropically aligned LC gel in its nematic phase exhibits high electrically induced strain (> 2 %) with an elastic modulus of 100 MPa and a high electromechanical conversion efficiency (75 %) under an electric field of 25 MV/m. These anisotropic LC polymeric materials could provide a technologically compatible system for such applications as artificial muscles and as microelectromechanical devices.     

Highly Oriented Nanofibrils of Regioregular Poly(3‐hexylthiophene) Formed via Block Copolymer Self‐Assembly in Liquid Crystals

A novel method making use of block copolymer self‐assembly in nematic liquid crystals (LCs) is described for preparing macroscopically oriented nanofibrils of π‐conjugated semiconducting polymers. Upon cooling, a diblock copolymer composed of regioregular poly(3‐hexylthiophene) (P3HT) and a liquid crystalline polymer (LCP) in a block‐selective LC solvent can self‐assemble into oriented nanofibrils exhibiting highly anisotropic absorption and polarized photoluminescence emission. An unusual feature of the nanofibrils is that P3HT chains are oriented along the fibrils' long axis. This general method makes it possible to use LCs as an anisotropic medium to grow oriented nanofibrils of many semiconducting polymers insoluble in LCs.     

Chiroptical Resolution and Thermal Switching of Chirality in Conjugated Polymer Luminescence via Selective Reflection using a Double‐Layered Cell of Chiral Nematic Liquid Crystal

An optically resolvable and thermally chiral‐switchable device for circularly polarized luminescence (CPL) is first constructed using a light‐emitting conjugated polymer film and a double‐layered chiral nematic liquid crystal (N*‐LC) cell. The double‐layered N*‐LC cell with opposite handedness at each layer is fabricated by adding each of two types of N*‐LCs into each of the cells, and the N*‐LCs consist of nematic LCs and chiral dopants with opposite chirality and different mole concentrations. The selective reflection band due to the N*‐LC is thermally shifted so that the band wavelength is close to the luminescence band of the racemic conjugated polymer, such as disubstituted polyacetylene (diPA), yielding CPL with opposite handedness and high dissymmetry factor values (|g_lum|) of 1.1–1.6 at low and high temperatures. The double‐layered N*‐LC cell bearing the temperature‐controlled selective reflection is useful for generating CPLs from racemic fluorescent materials and for allowing thermal chirality‐switching in CPLs, which present new possibilities for optoelectronic and photochemical applications.     

Alignment of a Perylene‐Based Ionic Self‐Assembly Complex in Thermotropic and Lyotropic Liquid‐Crystalline Phases

The thermotropic and lyotropic liquid‐crystalline (LC) phases of the ionic self‐assembled complex N,N′,‐bis(2‐(trimethylammonium)ethylene)‐perylene‐3,4,9,10‐tetracarboxyldiimide‐bis(2‐ethylhexyl)sulfosuccinate have been studied using polarizing microscopy, differential scanning calorimetry (DSC), and X‐ray scattering techniques. A two‐dimensional (2D) columnar thermotropic LC phase with π–π stacking of the perylene tectonic units and a lyotropic LC phase in dimethyl sulfoxide (DMSO) have been found. Different techniques have been applied to align both systems and included: surface interactions, electric and magnetic fields, shear force, and controlled domain formation at the LC–isotropic phase‐transition front (PTF). Characterization of the alignment in films has been performed using polarized UV‐vis spectroscopy and transmission null‐ellipsometry. The best results have been obtained for alignment of the material in a lyotropic phase by controlled domain formation at the PTF of the LC–isotropic phase transition. In this case, a dichroic ratio of 18 is achieved with packing of columns of perylenediimide tectons perpendicular to the PTF.     

An Air‐Supported Liquid Crystal System for Real‐Time and Label‐Free Characterization of Phospholipases and Their Inhibitors

A new air‐supported liquid crystal (LC) system for analyzing interfacial phenomena that occur based on the molecular interaction between LCs and adsorbed molecules of interest at the aqueous/LC interface is reported. Compared with existing LC‐based detection systems, the miniature system reported here requires less sample and involves simpler preparation. Using this system, the enzymatic hydrolysis of various phospholipases such as phospholipase A₂ (PLA₂), phospholipase C (PLC) and phospholipase D (PLD) are characterized. The hydrolysis of phospholipid monolayers self‐assembled at aqueous/LC interface induces an orientational response from the LCs. As a result, an optical signal that reflects the spatial and temporal distribution of phospholipids during the enzymatic reaction can be generated in a real‐time manner. When well‐known phospholipase inhibitors are introduced together with respective phospholipases, no orientational response of LCs is observed. In the case of inhibitors MJ33 and compound 48/80, cross‐inhibitions among phospholipases are also observed. This work demonstrates that the air‐supported LC system provides a facile label‐free assay for characterizing phospholipase activities and for screening enzyme inhibitors. It could potentially be useful for different high throughput and cost‐effective enzyme screening assays.     

Computational Chemistry‐Guided Design of Selective Chemoresponsive Liquid Crystals Using Pyridine and Pyrimidine Functional Groups

Computational chemistry‐guided designs of chemoresponsive liquid crystals (LCs) with pyridine or pyrimidine groups that bind to metal‐cation‐functionalized surfaces to provide improved selective responses to targeted vapor species (dimethylmethylphosphonate (DMMP)) over nontargeted species (water) are reported. The LC designs against experiments are tested by synthesizing 4‐(4‐pentyl‐phenyl)‐pyridine and 5‐(4‐pentyl‐phenyl)‐pyrimidine and quantifying LC responses to DMMP and water. Consistent with the computations, pyridine‐containing LCs bind to metal‐cation‐functionalized surfaces too strongly to permit a response to either DMMP or water whereas pyrimidine‐containing LCs undergo a surface‐driven orientational transition in response to DMMP without interference from water. The computation predictions are not strongly dependent on assumptions regarding the degree of coordination of the metal ions but are limited in their ability to predict LC responses when using cations with mostly empty d orbitals. Overall, this work identifies a promising new class of chemoresponsive LCs based on pyrimidine that exhibits enhanced tolerance to water, a result that is important because water is a ubiquitous and particularly challenging chemical interferent in chemical sensing strategies based on LCs. The work also provides further evidence of the transformative utility of computational chemistry methods to design LC materials that exhibit selective orientational responses in specific chemical environments.     

Electric‐Field‐Controlled Alignment of Rod‐Shaped Fluorescent Nanocrystals in Smectic Liquid Crystal Defect Arrays

Periodic micro‐arrays of straight linear defects containing nanoparticles can be created over large surface areas at the transition from the nematic to smectic‐A phase in a nanoparticle–liquid crystal (LC) composite material confined under the effect of conflicting anchoring conditions (unidirectional planar vs normal) and electric fields. Anisomeric dichroic dye molecules and rod‐shaped fluorescent semiconductor nanocrystals (dot‐in‐rods) with large permanent electric dipole and high linearly polarized photoluminescence quantum yield align parallel to the local LC molecular director and follow its reorientation under application of the electric field. In the nano‐sized core regions of linear defects, where the director is undefined, anisotropic particles align parallel to the defect whereas spherical quantum dots do not show any particular interaction with the defect. Under application of an electric field, ferroelectric semiconductor nanoparticles in the core region align along the field, perpendicular to the defect direction, whereas dichroic dyes remain parallel to the defect. This study provides useful insights into the complex interaction of anisotropic nanoparticles and anisotropic soft materials such as LCs in the presence of external fields, which may help the development of field‐responsive nanoparticle‐based functional materials.     

2D Versus 3D Crystalline Order in Thin Films of Regioregular Poly(3‐hexylthiophene) Oriented by Mechanical Rubbing and Epitaxy

Highly oriented films of regioregular poly(3‐hexylthiophene) (P3HT) are prepared by two methods: mechanical rubbing and directional epitaxial crystallization. The structure, nanomorphology, and optical and charge‐transport properties of the oriented films are investigated by electron diffraction, high resolution transmission electron microscopy (HR‐TEM), absorption spectroscopy, and transistor field‐effect measurements. In rubbed films, P3HT chains align parallel to the rubbing direction and the crystalline domains orientation changes from preferential edge‐on to flat‐on orientation. The maximum in‐plane orientation probed by absorption spectroscopy is a function of the polymer molecular weight M_w; the lower the M_w, the higher the in‐plane orientation induced by rubbing. The anisotropy of field‐effect mobility measured parallel and perpendicular to the rubbing shows the same trend as the absorption. The M_w‐dependence of the orientation is explained in terms of chain folding and entanglement that prevent the reorientation and reorganization of the π‐stacked chains, especially when M_w ≥ 50 kDa. For comparison, P3HT films are oriented by directional epitaxial crystallization using a zone‐melting technique. Electron diffraction and HR‐TEM show that epitaxial and rubbed films differ in terms of intralamellar order within layers of π‐stacked chains. Comparison of UV‐vis absorption spectra for the different samples suggests that the vibronic structure is sensitive to intralamellar disorder.     

How far has the molecular alignment of liquid crystals been elucidated?

Liquid crystal displays (LCDs) - born in late 1960s - have become a 45 billion-dollar industry in 2004. Today's LCD panels cover a wide range of sizes from 0.2 to 82 in diagonal. The electro-optical characteristics of any field effect LCD are determined by the orientation of its liquid crystal (LC) molecules at the display boundaries. Until recently most LCD alignment processes were based on rubbing polymer coated LCD substrates with a cloth. Despite its key importance, the complex mechanism governing the alignment of LC molecules on display substrates is still not well understood. Do the microgrooves generated by rubbing induce LC-alignment or does the stretching of polymer chains as a result of brushing cause it? To overcome the drawbacks of conventional aligning techniques, new alignment processes have recently been proposed and developed. Examples are micro embossing, ion bombardment, fringe field effects and photo-alignment. In this paper, some mechanisms of LC alignment are discussed and various alignment techniques and methods to investigate the interaction of LC molecules with substrates are reviewed.     

Sub‐5 nm Dendrimer Directed Self‐Assembly with Large‐Area Uniform Alignment by Graphoepitaxy

Directed self‐assembly (DSA) using soft materials is an important method for producing periodic nanostructures because it is a simple, cost‐effective process for fabricating high‐resolution patterns. Most of the previously reported DSA methods exploit the self‐assembly of block copolymers, which generates a wide range of nanostructures. In this study, cylinders obtained from supramolecular dendrimer films with a high resolution (<5 nm) exhibit planar ordering over a macroscopic area via guiding topographical templates with a high aspect ratio (>10) and high spatial resolution (≈20 nm) of guiding line patterns. Theoretical and experimental studies reveal that this property is related to geometrical anchoring on the meniscus region and physical surface anchoring on the sidewall. Furthermore, this DSA of dendrimer cylinders is demonstrated by the non‐regular geometry of the patterned template. The macroscopic planar alignment of the dendrimer nanostructure reveals an extremely small feature size (≈4.7 nm) on the wafer scale (>16 cm²). This study is expected to open avenues for the production of a large family of supramolecular dendrimers with different phases and feature dimensions oriented by the DSA approach.     

Simultaneous Edge‐on to Face‐on Reorientation and 1D Alignment of Small π‐Conjugated Molecules Using Room‐Temperature Mechanical Rubbing

In this study, room‐temperature mechanical rubbing is used to control the 3D orientation of small π‐conjugated molecular systems in solution‐processed polycrystalline thin films without using any alignment substrate. High absorption dichroic ratio and significant anisotropy in charge carrier mobilities (up to 130) measured in transistor configuration are obtained in rubbed organic films based on the ambipolar quinoidal quaterthiophene (QQT(CN)4). Moreover, a solvent vapor annealing treatment of the rubbed film is found to improve the optical and charge transport anisotropy due to an increased crystallinity. X‐ray diffraction and atomic force microscopy measurements demonstrate that rubbing does not only lead to an excellent 1D orientation of the QQT(CN)4 molecules over large areas but also modifies the orientation of the crystals, moving molecules from an edge‐on to a face‐on configuration. The reasons why a mechanical alignment technique can be used at room temperature for such a polycrystalline film are rationalized, by the plastic characteristics of the QQT(CN)4 layer and the role of the flexible alkyl side chains in the molecular packing. This nearly complete conversion from edge‐on to face‐on orientation by mechanical treatment in polycrystalline small‐molecule‐based thin films opens perspectives in terms of fundamental research and practical applications in organic optoelectronics.     

Electrotunable Non‐reciprocal Laser Emission from a Liquid‐Crystal Photonic Device

A liquid crystal (LC) photonic device with an anisotropic optical heterojunction structure has been fabricated. The device has a phase‐retarding nematic LC (NLC) layer sandwiched between two polymer cholesteric LC films with right‐handed helices of different pitches. Electrotunable non‐reciprocal light transmittance and unidirectional circularly polarized (CP) lasing emission have been successfully demonstrated for this device structure. Two left CP (LCP) lasing emission peaks are observed at the edges of the overlapping region between the two photonic bands in the structure and are shifted upon the application of a voltage. In contrast, a non‐reciprocal right CP (RCP) lasing emission peak emerges at one of the band edges and diminishes upon the application of a voltage. These phenomena are interpreted based on the selective reflection of RCP light and the reorientation of the NLC molecules by the application of a voltage.     

A Versatile Method to Fabricate Highly In‐Plane Aligned Conducting Polymer Films with Anisotropic Charge Transport and Thermoelectric Properties: The Key Role of Alkyl Side Chain Layers on the Doping Mechanism

A general method is proposed to produce oriented and highly crystalline conducting polymer layers. It combines the controlled orientation/crystallization of polymer films by high‐temperature rubbing with a soft‐doping method based on spin‐coating a solution of dopants in an orthogonal solvent. Doping rubbed films of regioregular poly(3‐alkylthiophene)s and poly(2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[3,2‐b ]thiophene) with 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄TCNQ) yields highly oriented conducting polymer films that display polarized UV–visible–near‐infrared (NIR) absorption, anisotropy in charge transport, and thermoelectric properties. Transmission electron microscopy and polarized UV–vis–NIR spectroscopy help understand and clarify the structure of the films and the doping mechanism. F₄TCNQ^? anions are incorporated into the layers of side chains and orient with their long molecular axis perpendicular to the polymer chains. The ordering of dopant molecules depends closely on the length and packing of the alkyl side chains. Increasing the dopant concentration results in a continuous variation of unit cell parameters of the doped phase. The high orientation results in anisotropic charge conductivity (σ) and thermoelectric properties that are both enhanced in the direction of the polymer chains (σ = 22 ± 5 S cm^?1 and S = 60 ± 2 µV K^?1). The method of fabrication of such highly oriented conducting polymer films is versatile and is applicable to a large palette of semiconducting polymers.     

液晶共面取向的等离子处理方法

在受等离子辐照过的一些有机和无机的基板上，获得了高质量的液晶共面取向。与已知的用来改善顶部锚定及预倾角各向同性的等离子处理方法不同，新方法是将等离子束调整到倾斜射向待取向的基板。在所用的辐射参数范围内，所有基板上的LC取向的易取向轴(easy axis)都平行于等离子体传播方向。研究了LC的预倾角和锚定能与等离子束的入射角、辐照时间、能量以及辐照电流密度等的依赖关系。经等离子处理过的基板上，方位角、锚定能与用光取向方法得到的相近，而预倾角与摩擦产生的类似。透过率-电压曲线与等离子处理的和摩擦工艺处理的非常接近。与摩擦取向相同，等离子诱导的取向具有很高的温度和光照的稳定性。也考虑了采用等离子／偏振紫外光和等离子／摩擦处理等组合方法来制作LC图形。