Effects of terminal chain length in hydrogen-bonded chiral switches on phototunable behavior of chiral nematic liquid crystals: helicity inversion and phase transition

Direct Digital Photonic Patterning of Hydrogen-Bonded Cholesteric Liquid-Crystalline Films

A modular tool box for photoresponsive cholesteric liquid crystals based on hydrogen-bonded assemblies is reported. By employing 3-azopyridines as photoswitch in cholesteric liquid-crystalline thin films, a fast and reversible blue shift is observed upon irradiation, allowing tuning of the structural color over the whole visible electromagnetic spectrum. Investigations of the materials via X-ray diffraction studies indicate that the blue shift is attributed to the photoinduced destruction of smectic clusters in the cholesteric phase, resulting in a contraction of the helical structure. Unprecedently, the use of a stereolithography 3D printer (SLA) allowed direct transfer of digital information into a multicolor photonic pattern, an important step toward photonic imaging and data storage.

Irradiation-Wavelength Directing Circularly Polarized Luminescence in Self-Organized Helical Superstructures Enabled by Hydrogen-Bonded Chiral Fluorescent Molecular Switches

Two light-driven chiral fluorescent molecular switches, (R,S,R)-switch 1 and (R,S,R)-switch 2, are prepared by means of hydrogen-bonded (H-bonded) assembly of a photoresponsive (S) chiral fluorescent molecule, respectively with a cyano substitution at different positions as an H-bond acceptor and an opposite (R) chiral molecule as an H-bond donor. The resulting two switches exhibit tunable and reversible Z/E photoisomerization irradiated with 450 nm blue and 365 nm UV light. When doped into an achiral liquid crystal, both switches are found to be able to form a CPL tunable luminescent helical superstructure. In contrast to the tunable CPL characteristics of the system incorporating switch 2, exposure of the system incorporating switch 1 to 365 nm and 450 nm radiation can lead to controllable different photostationary CPL behavior, including switching-off and polarization inversion. In addition, optical information coding is demonstrated using the system containing switch 1.

Near-infrared light-induced photoisomerization and photodissociation of a chiral fluorescent photoswitch in cholesteric liquid crystals assisted by upconversion nanoparticles

Upconversion-luminescence-induced reflective color switching and fluorescence tuning of a cholesteric liquid crystal (CLC) cells were investigated. The CLC system was constructed by co-doping a chiral fluorescence photoswitch, switch 5, and upconversion nanoparticles (UCNPs) into nematic LC media. Under irradiation with 980 nm NIR light, the UCNPs emit both 450 nm blue light and 365 nm UV light to induce the simultaneous Z-to-E and E-to-Z photoisomerization of switch 5. This continuous rotation-inversion movement further leads to an irreversible photoisomerization and photodissociation of dicyanodistyrylthiophene moieties in switch 5. As a result, the reflective color of the CLC cell changed from blue to red and the fluorescence intensity decreased as well when exposed to 980 nm NIR light. Finally, optically written reflective-photoluminescent dual mode CLC cells were further demonstrated.

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.

Origin and structure of liquid crystalline Blue Phase III

We report here an off-lattice NVT molecular dynamics simulation study of a system of polar chiral ellipsoidal molecules, which spontaneously exhibits Blue Phase III (BPIII), considering coarse-grained attractive-repulsive pair interaction appropriate for anisotropic liquid crystal mesogens. We have observed that suitable selection of chiral and dipolar strengths not only gives rise to thermodynamically stable BPIII but novel Smectic and Bilayered BPIII as well. Further, we have demonstrated that the occurrence of BPIII and its layered counterparts depend crucially on molecular elongation.

1,2-Dithienyldicyanoethene-Based, Visible-Light-Driven, Chiral Fluorescent Molecular Switch: Rewritable Multimodal Photonic Devices

Reported here is the first example of a 1,2-dithienyldicyanoethene-based visible-light-driven chiral fluorescent molecular switch that exhibits reversible trans to cis photoisomerization. The trans form in solution almost completely transforms into the cis form, accompanied by a 10-fold decrease in its fluorescence intensity within 60 seconds when exposed to green light (520 nm). The reverse isomerization proceeds upon irradiation with blue light (405 nm). When doped into commercially available achiral liquid crystal hosts, this molecular switch efficiently induces luminescent helical superstructures, that is, a cholesteric phase. The intensity of the circularly polarized fluorescence as well as the selective reflection wavelength of the induced cholesteric phases can be reversibly tuned using visible light of two different wavelengths. Optically rewritable photonic devices using cholesteric films containing this molecular switch are described.

Optically Rewritable Transparent Liquid Crystal Displays Enabled by Light-Driven Chiral Fluorescent Molecular Switches

Functional soft materials exhibiting distinct functionalities in response to a specific stimulus are highly desirable towards the fabrication of advanced devices with superior dynamic performances. Herein, two novel light-driven chiral fluorescent molecular switches have been designed and synthesized that are able to exhibit unprecedented reversible Z/E photoisomerization behavior along with tunable fluorescence intensity in both isotropic and anisotropic media. Cholesteric liquid crystals fabricated using these new fluorescent molecular switches as chiral dopants exhibit reversible reflection color tuning spanning the visible and infrared region of the spectrum. Transparent display devices have been fabricated using both low chirality and high chirality cholesteric films that operate either exclusively in fluorescent mode or in both fluorescent and reflection mode, respectively. The dual mode display device employing short pitch cholesteric film is able to function on demand under all ambient light conditions including daylight and darkness with fast response and high resolution. Moreover, the proof-of-concept for a "remote-writing board" using cholesteric films containing one of the light-driven chiral fluorescent molecular switches with ease of fabrication and operation is disclosed herein. Such optically rewritable transparent display devices enabled by light-driven chiral fluorescent molecular switches pave a new way for developing novel display technology under different lighting conditions.

Reversible chirality inversion of circularly polarized luminescence in a photo-invertible helical cholesteric superstructure

The first example of photo-driven reversible chirality inversion of circularly polarized luminescence in a helical cholesteric superstructure is reported.

Optically Reconfigurable Chiral Microspheres of Self-Organized Helical Superstructures with Handedness Inversion

Optically reconfigurable monodisperse chiral microspheres of self-organized helical superstructures with dynamic chirality were fabricated via a capillary-based microfluidic technique. Light-driven handedness-invertible transformations between different configurations of microspheres were vividly observed and optically tunable RGB photonic cross-communications among the microspheres were demonstrated.

Enhancement of the helical twisting power with increasing the terminal chain length of nonchiral bent-core molecules doped in a chiral nematic liquid crystal

We measured the helical twisting power (HTP) of nonchiral bent-core molecules doped in a chiral nematic liquid crystal, and found that HTP increases with the terminal chain length.

Matched elastic constants for a perfect helical planar state and a fast switching time in chiral nematic liquid crystals

Chiral nematic liquid crystals possess a self-assembled helical structure and exhibit unique selective reflection in visible and infrared light regions. Their optical properties can be electrically tuned. The tuning involves the unwinding and restoring of the helical structure. We carried out an experimental study on the mechanism of the restoration of the helical structure. We constructed chiral nematic liquid crystals with variable elastic constants by doping bent-dimers and studied their impact on the restoration. With matched twist and bend elastic constants, the helical structure can be restored dramatically fast from the field-induced homeotropic state. Furthermore, defects can be eliminated to produce a perfect planar state which exhibits high selective reflection.