Reactive Mesogen Research Articles

Polarization‐Dependent and Color‐Tunable Electrochromic Smart Windows: Uniaxially Oriented and Polymer‐Stabilized Viologen‐Based Liquid Crystal Networks

AbstractViologen‐based reactive mesogen (VRM) is newly designed and synthesized for the development of polarization‐dependent, color‐tunable, and transparent electrochromic (EC) smart windows. By controlling electric field, the color of VRM can be intentionally tuned due to the formation of dication (D state), cation‐radical (C state), and neutral state (N state). Based on the deep understanding of phase transition of VRM, a uniaxially oriented VRM hierarchical nanostructure is successfully constructed on the macroscopic area by simple coating and self‐assembly processes. The subsequent photopolymerization significantly improves thermal, mechanical, and chemical stabilities of the VRM thin film. In addition to the control of color, the transmittance of uniaxially oriented VRM‐based EC smart window can be precisely tuned by rotating a linear polarizer.

Mesomorphic and optical properties of azo-ester liquid crystalline reactive mesogens with enhanced thermal stability

A series of azo-ester-linked polymerizable liquid crystalline compounds M1-M3, containing lateral methyl substitution and different terminal alkoxy substituents, e.g., −OCH3, −OCH2CH3, and −OCH2CH2CH3 have successfully been synthesized and characterized. The chemical structures of the prepared compounds were confirmed by different spectroscopic techniques. Thermogravimetric analysis revealed that all the compounds exhibited excellent thermal stability and showed two-staged decomposition patterns. The optical microscopic study revealed that all compounds displayed only the nematic phase and lateral substituent played a crucial role in the formation of single mesophase. The broad X-ray scattering peaks for M1, M2, and M3 in the 2.5–5.5 nm−1 region was observed at temperatures of 154 °C, 85 °C, and 129 °C, respectively, upon cooling from the isotropic liquid, confirming the existence of nematic mesophase. The electronic spectra of M1-M3 showed two main absorption bands with λmax ranges of 263–266 and 369–377 nm. The E-Z photoisomerization study of M1-M3 showcased that the processes followed the first-order kinetic, and the isomerization rate decreased as the alkoxy chain length increased. On the contrary, the Z-E thermal isomerization rates for M1-M3 showed insignificant changes as the alkoxy chain length increased.

Patterned Liquid Crystal Polymer Thin Films Improved Energy Conversion Efficiency at High Incident Angles for Photovoltaic Cells.

In this report, micro-patterned silicon semiconductor photovoltaic cells have been proposed to improve the efficiency in various incident sunlight angles, using homeotropic liquid crystal polymers. The anisotropic liquid crystal precursor solution based on a reactive mesogen has good flowing characteristics. It can be evenly coated on the silicon solar cells' surface by a conventional spreading technique, such as spin coating. Once cured, the polymers exhibit asymmetric transmittance properties. The optical retardation characteristics of the coated polymer films can be eventually determined by the applicable coating and curing parameters during the processes. The birefringence of light then influences the optical path and the divergence of any encountered sunlight. This allows more photons to enter the active semiconductor layers for optical absorption, resulting in an increase in the photon-to-electron conversion, and thus improving the photovoltaic cell efficiency. This new design is straightforward and could allow various patterns to be created for scientific development. The experimental results have evidenced that the energy conversion efficiency could be improved by 2-3% for the silicon photovoltaic cells, under direct sunlight or at no inclination, when the liquid crystal polymer precursor solution is prepared at 5%. In addition, the efficiency could be much more significantly improved to 14-16% when the angle is inclined to 45°. The unique patterned liquid crystal polymer thin films provide enhanced energy conversion efficiency for silicon photovoltaic cells. The design could be further evaluated for other solar cell applications.

Stepwise Progression of Dye-Induced In Situ Photoalignment and Subsequent Stabilization for Noncontact Alignment of Liquid Crystals.

Noncontact alignment of liquid crystals (LCs) is crucial for large-area and ultrahigh definition (UHD) display manufacturing. This research presents an innovative approach to the photoalignment of LCs, aiming to overcome challenges associated with uniformity and assembly in large-sized and UHD displays. Using homogeneously dissolved, nonionic azobenzene chromophores sensitive to both visible and UV light, we demonstrate an in situ stepwise progression of dye-induced LC alignment and subsequent stabilization using reactive mesogen (RM). Both dual-wavelength and single-wavelength approaches enable stepwise interfacial modifications for LC alignment and stabilization. The dye-induced LC alignment is rewritable, allowing for the creation of various patterns and gray-level alignments. The stability of the alignment is ensured through cross-linked RM layers, providing a robust and permanent solution for LC alignment without the need for delicate mechanical treatments. Importantly, this method addresses the challenges associated with conventional photoalignments, including various dye-induced approaches and high-energy photoalignment. The proposed method exhibits high-quality electro-optical switching, azimuthal anchoring strength, and stability against thermal, radiation, and ac-field stresses, making it a promising candidate for commercial mass production, especially in the fabrication of large-sized and UHD LC displays.

Thermal colour map of 4D smart microtags made by Two-Photon Lithography in cholesteric reactive mesogens

ABSTRACT The helicoidal architecture of cholesteric reactive mesogens allows for smart 4D responsive microdevices that reflect selectively the wavelength and polarisation of the incident light. Two-photon lithography is an effective 4D microprinting technology which also enables fine tuning of the Bragg reflection, as a function of the energy delivered during the manufacturing and the polymerisation degree. When CRMs objects undergo temperature variation, thermal expansion causes the elongation of the effective helix pitch and the consequent red-shift of the photonic band gap. A thermal characterisation of the optical response of CRMs microcylinders is presented, showing a tunable red-shift from 2 to 37 nm in the 30–150°C range. The red-shift is enhanced for tall structures, with high polymerisation degree. This characterisation is exploited to add a security label to a 4D cholesteric QR code.

The good, the bad and the ugly faces of cyanobiphenyl mesogens in selected tracks of fundamental and applied liquid crystal research

ABSTRACT Liquid crystal-forming cyanobiphenyls are truly extraordinary molecules that have had an enormous impact on liquid crystal research and applications since they were first synthesised. This impact is, on the one hand, due to the exceptionally convenient physical properties of the main characters, 5CB and 8CB, allowing easy experiments at room temperature, as well as their commercial availability at reasonable cost. On the other hand, the cyanobiphenyl chemical structure leads to some quite peculiar characteristics in terms of organisation at the molecular scale, which are sometimes well recognised and even utilised, but often the awareness of these peculiarities is not strong. This perspective article reviews the use of cyanobiphenyls in making liquid crystal shells and liquid crystal core fibres, in sensing, as a medium for simultaneously aligning and dispersing carbon nanotubes, and as highly useful solvents for reactive mesogens that can be polymerised into anisotropic networks. This choice is very much motivated by how cyanobiphenyls have impacted our group’s research throughout the years, which is the basis for the examples I provide. Nevertheless, I believe they serve well to illustrate the immense usefulness of cyanobiphenyls in innovating research and applications related to liquid crystals.

Progress in Fabrication and Applications of Cholesteric Liquid Crystal Microcapsules.

Liquid crystals (LCs) are well known for inherent responsiveness to external stimuli, such as light, thermal, magnetic, and electric fields. Cholesteric LCs are among the most fascinating, since they possess distinctive optical properties due to the helical molecular orientation. However, the good flow, easy contamination, and poor stability of small-molecule LCs limit their further applications, and microencapsulation as one of the most effective tools can evade these disadvantages. Microencapsulation can offer shell-core structure with LCs in the core can strengthen their stability, avoiding interference with the environment while maintaining the stimuli-responsiveness and optical properties. Here, we report recent progress in the fabrication and applications of cholesteric LC microcapsules (CLCMCs). We summarize general properties and basic principles, fabrication methods including interfacial polymerization, in-situ polymerization, complex coacervation, solvent evaporation, microfluidic and polymerization of reactive mesogens, and then give a comprehensive overview of their applications in various popular domains, including smart fabrics, smart sensor, smart displays, anti-counterfeiting, information encryption, biomedicine and actuators. Finally, we discuss the currently facing challenges and the potential development directions in this field.

Ionic Conductivity Switchable and Shape Changeable Smart Skins with Azobenzene‐Based Ionic Reactive Mesogens

AbstractTo develop smart ionic skins capable of multi‐responsiveness and switchable ionic conductivity, a stimuli‐responsive and ionic conductive azobenzene‐based monomer is newly synthesized, uniaxially oriented, and polymerized for anisotropic liquid crystal elastomers (LCEs). Since the uniaxially oriented monodomain LCE with ionic asymmetric azobenzene monomers (i‐AAM) is prepared by thermal oligomerization, uniaxial stretching, and subsequent photopolymerization, the stimuli‐responsive i‐AAM LCE is thermally contracted by increasing the temperature above TNI as well as is bent along the aligned direction by irradiating it with UV light. In addition to the change of shape, the ionic conductivity of the LCE is reversible in response to heat and light stimuli. Polydomain i‐AAM LCE polymerized without the stretching process exhibits higher ionic conductivity than stretched monodomain LCE due to the initially formed stable ionic pathways. Ionic conductivity can also be switched simultaneously by polarity changes in response to the photoisomerization of i‐AAM, increased mobility from heat, and an elastic mechanoresponse from external stresses. These i‐AAM LCE‐based soft grippers exhibit stimuli‐responsive grasping and releasing actuation and detect ionic conductivity switches finely, suggesting potential as smart skins for advanced soft robots.

Gradient polarization volume grating with wide angular bandwidth for augmented reality.

Angular bandwidth, which is critical to field-of-view, plays important role in diffractive optical waveguide augmented reality display. However, design and fabrication of large angular bandwidth is still a challenge. Herein, we demonstrate a liquid crystal reflective gradient polarization volume grating with three-dimensional gradient periodic structure for waveguide near-eye display. Two-beam polarization interference with special designed periodic gradient photomask are applied to chiral-dopant reactive mesogens doped with ultraviolet dye for generating gradient three-dimensional configuration of liquid crystals, resulting in gradient polarization volume grating with extended angle bandwidth of 61° while keeping 80% diffraction efficiency, with peak efficiency near 100%. The proposed gradient polarization volume grating provides an effective method to broaden the angular bandwidth in waveguide for wide field-of-view augmented reality display.

Coatable Negative Dispersion Retarder: Kinetically Controlled Self-Assembly Pathway of Butterfly-Shaped Molecular Building Blocks for the Construction of Nanocolumns.

Disc-shaped building blocks with columnar phases have attracted attention for their potential in optical applications, including a retarder. However, to achieve coatable high-performance optical films, it is essential to understand a subtle interaction balance between building blocks and relevant self-assembled behaviors during material processing. Herein, we studied a self-assembled nanocolumn evaluation of linear butterfly-shaped dendrons (T-A3D) consisting of thiophene-based conjugated core and flexible alkyl dendron. X-ray diffraction provided insight into the unique hexagonal columnar liquid crystal phase of T-A3D, driven by intermolecular hydrogen bonding and coplanarity of the thiophene-based conjugated core. The formation of a self-assembled nanocolumn with high mobility enabled the uniaxial orientation of butterfly-shaped T-A3D on the aligned rod-shaped nematic reactive mesogens, resulting in a transparent and colorless two-layered negative retarder. The self-assembled nanocolumn consisting of butterfly-shaped molecule would break a new ground for developing advanced optical thin films.

Controlled Mesoscopic Growth of Polymeric Fibers Using Liquid Crystal Template.

Orientation-controlled polymeric fiber is one of the most exciting research topics to rationalize the multi-functionality for various applications. In order to realize this goal, the growth of polymeric fibers should be controlled using various techniques like extrusion, molding, drawing, and self-assembly. Among the various candidates to fabricate the orientation-controlled polymeric fibers, the template-assisted assembly guided by a liquid crystal (LC) matrix is the most promising because the template can be manipulated easily with various methods like surface anchoring, rubbing, geometric confinement, and electric field. This review introduces the recent progress toward the directed growth of polymeric fibers using the LC template. Three representative LC-templated polymerization techniques to fabricate fibers include chemical or physical polymerization from the monomers mixed in LC matrix, patterned fibers formed from LC-templated reactive mesogens, and orientation-controlled nanofibers by infiltrating vaporized monomers between LC molecules. The orientation-controlled polymeric fibers will be used in electro-optical switching tools, tunable hydrophilic or hydrophobic surfaces, and control of phosphorescence, which can open a way to design, fabricate, and modulate nano- to micron-scale fibers with various functions on demand. This paper focuses on oriented polymeric fibers via the liquid crystal templating method. The stimuli-responsive characteristics and directional spacing of liquid crystal present a synthesis method of highly oriented fiber bundles. The paper has three sections according to the types of monomers: monomers without liquid crystallinity, reactive mesogens, and infiltrating vaporized monomers. This article is protected by copyright. All rights reserved.

Encryptable Electrochromic Smart Windows: Uniaxially Oriented and Polymerized Hierarchical Nanostructures Constructed by Self-Assembly of Tetrathiafulvalene-Based Reactive Mesogens.

Tetrathiafulvalene (TTF)-based reactive mesogens (TTF-E and TTF-T) are synthesized, self-assembled, uniaxially oriented, and polymerized for the development of encryptable electrochromic smart windows. Electrochemical and spectroscopic experiments prove that the self-assembled TTF mixture (TTFM, TTF-E:TTF-T = 1:1) can reversibly switch the absorption wavelength of the TTF chromophore according to the redox reactions. Based on the identification of the phase transition and crystallographic structure, uniaxially oriented hierarchical nanostructures are easily constructed on the macroscopic area by simple coating and a self-assembly process. Subsequent polymerization of hierarchical nanostructures of TTFM significantly enhances thermal and mechanical stabilities and makes it possible for them to be fabricated as an electrochromic device. The angularly dependent correlation between the anisotropy of mesogens and the linearly polarized light allow us to demonstrate TTFM as smart windows capable of various optical security applications, including privacy protection and information encryption.

Negative biaxial retarder made from reactive molecules with X-shaped core structure

Although biaxial retarder films have merits of optical applications, limited numbers of the biaxial retarder have been developed using reactive mesogen (RM) materials. New X-shaped RM (XRM) molecules were synthesized and optical retarder films were fabricated using mixtures of a rodlike RM and the XRM. The in-plane retardation, the out-of-plane retardation and the NZ coefficient of the retarder films were investigated. The retarder films represented negative biaxial property or z-plate property depending on the chemical structure and the concentration of XRM.

Synthesis and Study of Reactive Mesogen in a Spin Valve Structure

Synthesis and Study of Reactive Mesogen in a Spin Valve Structure

Synthesis and Surface Anchoring Property of Partially Fluorinated Hockey Stick-Shaped Reactive Mesogens

Synthesis and Surface Anchoring Property of Partially Fluorinated Hockey Stick-Shaped Reactive Mesogens

Liquid Crystal-Templated Porous Microparticles via Photopolymerization of Temperature-Induced Droplets in a Binary Liquid Mixture.

Porous polymeric microspheres are an emerging class of materials, offering stimuli-responsive cargo uptake and release. Herein, we describe a new approach to fabricate porous microspheres based on temperature-induced droplet formation and light-induced polymerization. Microparticles were prepared by exploiting the partial miscibility of a thermotropic liquid crystal (LC) mixture composed of 4-cyano-4'-pentylbiphenyl (5CB, unreactive mesogens) with 2-methyl-1,4-phenylene bis4-[3-(acryloyloxy)propoxy] benzoate (RM257, reactive mesogens) in methanol (MeOH). Isotropic 5CB/RM257-rich droplets were generated by cooling below the binodal curve (20 °C), and the isotropic-to-nematic transition occurred after cooling below 0 °C. The resulting 5CB/RM257-rich droplets with radial configuration were subsequently polymerized under UV light, resulting in nematic microparticles. Upon heating the mixture, the 5CB mesogens underwent a nematic-isotropic transition and eventually became homogeneous with MeOH, while the polymerized RM257 preserved its radial configuration. Repeated cycles of cooling and heating resulted in swelling and shrinking of the porous microparticles. The use of a reversible materials templating approach to obtain porous microparticles provides new insights into binary liquid manipulation and potential for microparticle production.

Preview the Products on Display at Display Week 2023

Preview the Products on Display at Display Week 2023

P‐12.13: Novel Phase Separation Reactive Mesogen for Speeding Polymer‐stabilized Vertical Alignment Process

Well known the polymer‐stabilized vertically aligned (PS‐VA) liquid crystal display (LCD) has been extensively using for TV application. However, for the purpose of energy saving during the UV exposure process, there is still no available reactive mesogen solution at PS‐VA mode. In this paper, through the Gaussian program calculation, we have investigated the novel reactive mesogen with high reactivity at the wavelength of 313 nm compared to the current commercial reactive mesogen. By using this novel reactive mesogen at PS‐VA process, there is a positive performance to shorten 74% both UV1 and UV2 curing time. The polymer bump size of novel reactive mesogen is quite smaller after UV curing process. After UV curing process by using both novel reactive mesogen and current commercial reactive mesogen under the same host mixture separately, the optical and reliability performance are also comparable in between.

Electrospun Core-Sheath Fibers with a Uniformly Aligned Polymer Network Liquid Crystal (PNLC).

Electrospun polymer-liquid crystal (PLC) fibers have potential applications such as wearable sensors and adaptive textiles because of their rapid response and high flexibility. However, existing PLC fibers only have a narrow responsive range and poor resistance to heat and chemicals. Herein, a new type of PLC fiber is prepared by using a coaxial electrospinning process. The core solution is 4'-pentyl-4-biphenylcarbonitrile (5CB), and the sheath solution is a mixture containing 13 wt % PVP and 10 wt % reactive mesogen (RM). After UV exposure of the fibers, 5CB in the core and RM diffusing from the core are cross-linked into an LC polymer. The fibers have a highly uniform morphology with an average diameter of 3.2 ± 0.5 μm, and mesogens inside the fibers align unidirectionally with the long axis of the fibers. The fibers show a broad phase-transition temperature range between 13.5 and 155.5 °C and have a response time of less than 10 s. The temperature range can also be controlled by adjusting components in the electrospun fibers and UV exposure time. The core-sheath fibers prepared in such a manner exhibit excellent heat and chemical resistance with reversible optical responses. Moreover, when the fibers are exposed to volatile organic compounds (VOCs) such as toluene, the fibers show a rapid optical response to toluene vapor within 25 s. This study demonstrates that the fibers are potentially useful for preparing flexible temperature and chemical sensors.

Molecular Engineering for Chiroptical Smart Paints: Photo‐Tunable and Polymerizable Helical Nanostructures by Cyanostilbene‐Based Reactive Mesogens (Advanced Optical Materials 6/2023)

In article number 2202707, Kwang-Un Jeong and co-workers present chiroptical smart paints using cyanostilbene-based reactive mesogens. The helical nanostructures in the developed paint are precisely controlled and fixed by UV light and subsequent blue light irradiation. Corresponding chiroptical properties are investigated in detail. Furthermore, advanced optical information storages and encryptions are successfully demonstrated using the proposed chiroptical smart paints.