Frequency-Driven Self-Organized Helical Superstructures Loaded with Mesogen-Grafted Silica Nanoparticles

Photonic Multilayer Structure Induced High Near-Infrared (NIR) Blockage as Energy-Saving Window

Energy-saving window that selectively blocks near-infrared (NIR) is a promising technology to save energy consumption. However, it is hard to achieve both high transmittance in visible light and high reflectance in NIR for the energy-saving windows. Here, a TiO₂ /Ag/TiO₂ /SiO₂ /TiO₂ multilayer is demonstrated on a glass substrate to selectively block NIR while maintaining high transmittance to visible light. The thickness of a TiO₂ /Ag/TiO₂ structure is first design and optimized; the metal layer reflects NIR and the dielectric layers increase transmittance of visible light with zero reflection condition. To further enhance NIR-blocking capability, a TiO₂ back reflector is implemented with a SiO₂ spacer to TiO₂ /Ag/TiO₂ structure. The back reflector can induce additional Fresnel reflection without sacrificing transmittance to visible light. The optimal TiO₂ (32 nm)/Ag (22 nm)/TiO₂ (30 nm)/SiO₂ (100 nm)/TiO₂ (110 nm)/glass shows solar energy rejection 89.2% (reflection 86.5%, absorption 2.7%) in NIR, visible transmittance 69.9% and high long-wave (3 ≤ λ ≤ 20 µm) reflectance > 95%. This proposed visible-transparent, near-infrared-reflecting multilayer film can be applied to the windows of buildings and automobiles to reduce the energy consumption.

Smart Window Based on Angular-Selective Absorption of Solar Radiation with Guest–Host Liquid Crystals

In this study, we analyzed angular-selective absorption in a guest–host liquid crystal (GHLC) cell for its application in smart windows. For reducing the energy consumption, angular-selective absorption is desired because the light transmitted through windows during the daytime is predominantly incident obliquely from direct sunlight. Owing to the absorption anisotropy of guest dichroic dyes, a GHLC cell can absorb the obliquely incident light, while allowing people to see through windows in a normal view. Therefore, the cell can provide a comfortable environment for occupants, and reduce the energy required for cooling by blocking the solar heat incident from the oblique direction. The GHLC cell can be switched between the transparent and opaque states for a normal view. The rising (falling) time was 6.1 (80.5) ms when the applied voltage was 10 V.

Electro- and Photo-Driven Orthogonal Switching of a Helical Superstructure Enabled by an Axially Chiral Molecular Switch

Self-organized functional soft materials, enabled by specific chemical architectures, are currently attracting tremendous attention because of their stimuli-responsive attributes and applications in advanced technological devices. A novel axially chiral molecular switch containing two azo linkages and six terminal alkyl chains on two elongated rod-shaped wings, that exhibits superior solubility, high helical twisting power, and reversible photoisomerization in an achiral liquid crystal host, is synthesized and utilized in the development of a photoresponsive, self-organized helical superstructure, that is, cholesteric liquid crystal (CLC). The planar CLC adopts a standing helix (SH) configuration because of surface alignment layers on the substrates. This SH can be transitioned to a lying helix configuration, enabling tunable diffraction gratings under the application of electric field. Adjustment of the initial pitch of the planar CLC by photoirradiation yields the diffraction gratings with stripes either parallel or perpendicular to the rubbing direction upon the application of an appropriate electric field. Tunable beam steering along orthogonal directions has been demonstrated. Such tunable stimuli-responsive soft materials fabricated with artificial chiral switches show great potential in optics, photonics, and beyond.

Nature-Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self-Assembly to DNA Mesophase and Nanocolloids

Liquid crystals (LCs) are omnipresent in living matter, whose chirality is an elegant and distinct feature in certain plant tissues, the cuticles of crabs, beetles, arthropods, and beyond. Taking inspiration from nature, researchers have recently devoted extensive efforts toward developing chiral liquid crystalline materials with self-organized nanostructures and exploring their potential applications in diverse fields ranging from dynamic photonics to energy and safety issues. In this review, an account on the state of the art of emerging chiral liquid crystalline nanostructured materials and their technological applications is provided. First, an overview on the significance of chiral liquid crystalline architectures in various living systems is given. Then, the recent significant progress in different chiral liquid crystalline systems including thermotropic LCs (cholesteric LCs, cubic blue phases, achiral bent-core LCs, etc.) and lyotropic LCs (DNA LCs, nanocellulose LCs, and graphene oxide LCs) is showcased. The review concludes with a perspective on the future scope, opportunities, and challenges in these truly advanced functional soft materials and their promising applications.

Visible-Light-Driven Halogen Bond Donor Based Molecular Switches: From Reversible Unwinding to Handedness Inversion in Self-Organized Soft Helical Superstructures

Visible-light-driven molecular switches endowing reversible modulation of the functionalities of self-organized soft materials are currently highly sought after for fundamental scientific studies and technological applications. Reported herein are the design and synthesis of two novel halogen bond donor based chiral molecular switches that exhibit reversible photoisomerization upon exposure to visible light of different wavelengths. These chiral molecular switches induce photoresponsive helical superstructures, that is, cholesteric liquid crystals, when doped into the commercially available room-temperature achiral liquid crystal host 5CB, which also acts as a halogen-bond acceptor. The induced helical superstructure containing the molecular switch with terminal iodo atoms exhibits visible-light-driven reversible unwinding, that is, a cholesteric-nematic phase transition. Interestingly, the molecular switch with terminal bromo atoms confers reversible handedness inversion to the helical superstructure upon irradiation with visible light of different wavelengths. This visible-light-driven, reversible handedness inversion, enabled by a halogen bond donor molecular switch, is unprecedented.

Parameter Space Design of a Guest-Host Liquid Crystal Device for Transmittance Control

A transmittance-control device requires a high transmittance difference between its transparent and opaque states. In this paper, we propose a systematic approach to find the condition for the maximum transmittance difference in a guest-host liquid crystal (GHLC) cell. To this end, we calculated the transmittance difference as we varied the cell gap and dye concentration. The transmittance of a GHLC cell is dependent on the alignment of dye molecules, cell gap, and dye concentration. We used a constant-transmittance contour map to find the condition for the desired transmittance of LC cells in each state and the transmittance difference of each LC mode. We experimentally confirmed that the design of a GHLC cell with the desired performance could be achieved through the proposed design process.

Alignment Control of Nematic Liquid Crystal using Gold Nanoparticles Grafted by the Liquid Crystalline Polymer with Azobenzene Mesogens as the Side Chains

The gold nanoparticles highly grafted by a liquid crystalline polymer (LCP) with azobenzene mesogens as the side chain (denoted as Au@TE-PAzo NPs) are successfully designed and synthesized by the two-phase Brust-Schiffrin method. The chemical structures of the monomer and polymer ligands have been confirmed by nuclear magnetic resonance, and the molecular weight of the polymer is determined by gel permeation chromatography. The combined analysis of transmission electron microscopy and thermogravimetric analysis shows that the size of the nanoparticles is 2.5(±0.4) nm and the content of the gold in the Au@TE-PAzo NPs is ca. 17.58%. The resultant Au@TE-PAzo NPs can well disperse in the nematic LC of 5CB. The well-dispersed mixture with appropriate doping concentrations can automatically form a perfect homeotropic alignment in the LC cell. The homeotropic alignment is attributed to the brush formed by Au@TE-PAzo NPs on the substrate, wherein the Au@TE-PAzo NPs gradually diffuse onto the substrate from the mixture. On the contrary, the pure side chain LCPs cannot yield vertical alignment of 5CB, which indicates that the alignment of 5CB is ascribed to the synergistic interaction of the nanoparticles and the grafted LCPs. Moreover, Au@TE-PAzo NPs show excellent film-forming property on account of their periphery of high densely grafted LCPs, which can form uniform thin film by spin-coating. The resultant thin film also can prompt the automatical vertical alignment of the nematic 5CB. Further, upon alternative irradiation of UV and visible light, the alignment of 5CB reversibly switches between vertical and random orientation because of the trans-cis photoisomerization of the azobenzene group on the periphery of Au@TE-PAzo NPs. These experimental results suggest that this kind of nanoparticles can be potentially applied in constructing the remote-controllable optical devices.

Instant Locking of Molecular Ordering in Liquid Crystal Elastomers by Oxygen-Mediated Thiol-Acrylate Click Reactions

Liquid crystal elastomers (LCEs) with intrinsic anisotropic strains are reversible shape-memory polymers of interest in sensor, actuator, and soft robotics applications. Rapid gelation of LCEs is required to fix molecular ordering within the elastomer network, which is essential for directed shape transformation. A highly efficient photo-cross-linking chemistry, based on two-step oxygen-mediated thiol-acrylate click reactions, allows for nearly instant gelation of the main-chain LCE network upon exposure to UV light. Molecular orientation from the pre-aligned liquid crystal oligomers can be faithfully transferred to the LCE films, allowing for preprogrammed shape morphing from two to three dimensions by origami- (folding-only) and kirigami-like (folding with cutting) mechanisms. The new LCE chemistry also enables widely tunable physical properties, including nematic-to- isotropic phase-transition temperatures (T_N-I ), glassy transition temperatures (T_g ), and mechanical strains, without disrupting the LC ordering.

Stimuli-Directed Dynamic Reconfiguration in Self-Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors

Dynamic controllability of self-organized helical superstructures in spatial dimensions is a key step to promote bottom-up artificial nanoarchitectures and functional devices for diverse applications in a variety of areas. Here, a light-driven chiral overcrowded alkene molecular motor with rod-like substituent is designed and synthesized, and its thermal isomerization reaction exhibits an increasing structural entropy effect on chemical kinetic analysis in anisotropic achiral liquid crystal host than that in isotropic organic liquid. Interestingly, the stimuli-directed angular orientation motion of helical axes in the self-organized helical superstructures doped with the chiral motors enables the dynamic reconfiguration between the planar (thermostationary) and focal conic (photostationary) states. The reversible micromorphology deformation processes are compatible with the free energy fluctuation of self-organized helical superstructures and the chemical kinetics of chiral motors under different conditions. Furthermore, stimuli-directed reversible nonmechanical beam steering is achieved in dynamic hidden periodic photopatterns with reconfigurable attributes prerecorded with a corresponding photomask and photoinduced polymerization.

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 KH₂ PO₄ 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.