Photomodulated Electro-optical Response in Self-Supporting Liquid Crystalline Physical Gels

Photoresponsive liquid crystal (LC) physical gels have attracted more and more attention because of the nature of strong response via light stimulus. Although many efforts on the breaking and recovering of physical gels through photoisomerization have been focused, fast electro-optical response and high mechanical properties even upon light irradiations are difficult to achieve at the same time. In this work, two kinds of azobenzene-containing gelators (<b>AG1</b> and <b>AG2</b>) with different terminal groups were designed and synthesized. Both gelators could induce the nematic LC P0616A self-assemble into anisotropic phase-separated LC physical gels at low contents. Their phase-transition behavior, thermal stability, microstructure, and mechanical strength were systematically studied. Compared with <b>AG2</b> in P0616A, the P0616A/<b>AG1</b> gels showed better mechanical property. When the gelator content was above 3 wt %, the P0616A/<b>AG1</b> gels possessed good self-supporting ability with a storage modulus more than 10⁴ Pa. Thus, the photoresponsive electro-optical properties and structures of P0616A/<b>AG1</b> gels were focused in detail. It was surprising that the electro-optical response speed of the P0616A/<b>AG1</b> gels could be promoted upon UV irradiation. In particular, the decay time (τ_off) was only about half when compared with the initial state, whereas the gels still exhibited good self-supporting ability; also the network of the LC physical gels had no change at macro- and microstructural levels. These exciting results would open a door for the application of this material in electro-optical devices

3D Image Storage in Photopolymer/ZnS Nanocomposites Tailored by “Photoinitibitor”

We synthesize zinc sulfide (ZnS) nanoparticles with a diameter of ∼5 nm and formulate novel photopolymer/ZnS nanocomposites for holographic recording. By taking advantage of the photoinitibitor, composed of 3,3′-carbonylbis­(7-diethylaminocoumarin) (KCD) and <i>N</i>-phenylglycine (NPG), with a capability of spatiotemporally tailoring the grating formation process, we successfully achieve high performance holographic photopolymer/ZnS nanocomposites with as high as 93.6% of diffraction efficiency (η), 26.6 × 10^–3 of refractive index modulation (<i>n</i>₁), 8.4 per 200 μm of dynamic range, and 9.8 cm/mJ of photosensitivity. In addition, for an aim of roughly describing the grating formation process, we establish a novel exponential correlation between the ZnS nanoparticles segregation degree (SD) and the ratio of photopolymerization gelation time (<i>t</i>_gel) to holographic mixture viscosity (<i>v</i>). Finally, we reconstruct and display 3D images that are clearly identifiable to the naked eye through a master technique, opening a versatile class of potential applications in high capacity data storage, stereoadvertisements, and anticounterfeiting

Spatial and Temporal Control of Thiol-Michael Addition via Photocaged Superbase in Photopatterning and Two-Stage Polymer Networks Formation

Photochemical processes enable spatial and temporal control of reactions, which can be implemented as an accurate external control approach in both polymer synthesis and materials applications. “Click” reactions have also been employed as efficient tools in the same field. Herein, we combined photochemical processes and thiol-Michael “click” reactions to achieve a “photo-click” reaction that can be used in surface patterning and controlled polymer network formation, owing to the ease of spatial and temporal control through use of photolabile amines as appropriate catalysts

Holographic Plastics with Liquid Crystals

Holography via laser interference is a powerful technique for precise processing of plastics by creating ultrafine structures down to the nanometer level. As all of the information from the laser can be reconstructed in these ultrafine structures by periodic refractive index modulation, the produced holographic plastics have been recognized to be indispensable for flexible and lightweight three-dimensional displays, augmented/virtual reality, high-density data storage, advanced anticounterfeiting, etc. Particularly, the marriage of liquid crystals (LCs) with holographic plastics not only is profitable for facile holographic processing but also can impart versatile stimuli-response functions. However, despite extensive research on this interdisciplinary field, several fundamental questions are still unclear. (1) Is there any simple form to illustrate how the refractive index modulation manipulates light propagation? (2) Does the refractive index modulation work in the same way for different types of holograms? (3) What are the big challenges for future practical applications? With these questions in mind, this perspective presents several important equations for both transmission and reflection holograms, summarizes updated advances in the field, and finally calls for endeavors to meet the urgent needs in rapidly growing information technology

Deformation Drives Alignment of Nanofibers in Framework for Inducing Anisotropic Cellulose Hydrogels with High Toughness

Deformation-driven alignment of macromolecules or nanofibers leading to anisotropy is a challenge in functional soft materials. Here, tough cellulose hydrogels that exhibited deformation-induced anisotropy are fabricated by reacting cellulose with a small amount of epichlorohydrin (EPI) in LiOH/urea solution and subsequent treating with dilute acid. The loosely cross-linked network that was obtained via chemical cross-linking of cellulose with EPI as a large framework maintained the elasticity of hydrogels, whereas nanofibers produced by the acid treatment formed physical cross-linked networks through hydrogen bonds which could efficiently dissipated mechanical energy. Meanwhile, the nanofibers could further aggregate to form submicrobundles and participate in the formation of frameworks during the acid treatment. Under deformation, the nanofibers and submicrobundles in the physical networks synchronize easily to align with the large framework, generating the rapidly responsive birefringence behaviors with highly stable colors. Thus, the cellulose hydrogels possessing sensitively mechano-responsive behavior could be utilized as a dynamic light switch and a soft sensor to accurately detect small external force, respectively. This work opens a novel pathway to construct tough and mechanoresponsive hydrogels via a green conversion of natural polysaccharide

Photoinitiation and Inhibition under Monochromatic Green Light for Storage of Colored 3D Images in Holographic Polymer-Dispersed Liquid Crystals

Holographic photopolymer composites have garnered a great deal of interest in recent decades, not only because of their advantageous light sensitivity but also due to their attractive capabilities of realizing high capacity three-dimensional (3D) data storage that is long-term stable within two-dimensional (2D) thin films. For achieving high performance holographic photopolymer composites, it is of critical importance to implement precisely spatiotemporal control over the photopolymerization kinetics and gelation during holographic recording. Though a monochromatic blue light photoinitibitor has been demonstrated to be useful for improving the holographic performance, it is impractical to be employed for constructing holograms under green light due to the severe restriction of the First Law of Photochemistry, while holography under green light is highly desirable considering the relatively low cost of laser source and high tolerance to ambient vibration for image reconstruction. Herein, we disclose the concurrent photoinitiation and inhibition functions of the rose bengal (RB)/<i>N</i>-phenylglycine (NPG) system upon green light illumination, which result in significant enhancement of the diffraction efficiency of holographic polymer-dispersed liquid crystal (HPDLC) gratings from zero up to 87.6 ± 1.3%, with an augmentation of the RB concentration from 0.06 × 10^–3 to 9.41 × 10^–3 mol L^–1. Interestingly, no detectable variation of the ϕ^1/2<i>k</i>_p/<i>k</i>_t^1/2, which reflects the initiation efficiency and kinetic constants, is given when increasing the RB concentration. The radical inhibition by RBH^• is believed to account for the greatly improved phase separation and enhanced diffraction efficiency, through shortening the weight-average polymer chain length and subsequently delaying the photopolymerization gelation. The reconstructed colored 3D images that are easily identifiable to the naked eye under white light demonstrate great potential to be applied for advanced anticounterfeiting

Precisely Tuning Helical Twisting Power via Photoisomerization Kinetics of Dopants in Chiral Nematic Liquid Crystals

It has been paid much attention to improve the helical twisting power (β) of dopants in chiral nematic liquid crystals (CLCs); however, the correlations between the β value and the molecular structures as well as the interaction with nematic LCs are far from clear. In this work, a series of reversibly photo-switchable axially chiral dopants with different lengths of alkyl or alkoxyl substituent groups have been successfully synthesized through nucleophilic substitution and the thiol–ene click reaction. Then, the effect of miscibility between these dopants and nematic LCs on the β values, as well as the time-dependent decay/growth of the β values upon irradiations, has been investigated. The theoretical Teas solubility parameter shows that the miscibility between dopants and nematic LCs decreases with increasing of the length of substituent groups from dopant <b>1</b> to dopant <b>4</b>. The β value of chiral dopants in nematic LCs decreases from dopant <b>1</b> to dopant <b>4</b> both at the visible light photostationary state (PSS) and at the UV PSS after UV irradiation. With increasing of the length of substituent groups, the photoisomerization rate constant of dopants increases for trans–cis transformation upon UV irradiation and decreases for the reverse process upon visible light irradiation either in isotropic ethyl acetate or in anisotropic LCs, although the constant in ethyl acetate is several times larger than the corresponding value in LCs. Also, the color of the CLCs could be tuned upon light irradiations. These results enable the precise tuning of the pitch and selective reflection wavelength/color of CLCs, which paves the way to the applications in electro-optic devices, information storage, high-tech anticounterfeit, and so forth

Surface Decoration on Polymeric Gate Dielectrics for Flexible Organic Field-Effect Transistors via Hydroxylation and Subsequent Monolayer Self-Assembly

A simple photochemical reaction based on confined photocatalytic oxidation (CPO) treatment and hydrolysis was employed to efficiently convert C–H bonds into C–OH groups on polymeric material surfaces, followed by investigation of monolayer self-assembly decoration on polymeric dielectrics via chemical bonding for the organic field-effect transistors (OFETs) applications. This method is a low temperature process and has negligible etching effect on polymeric dielectric layers. Various types of self-assembled monolayers have been tested and successfully attached onto the hydroxylated polymeric dielectric surfaces through chemical bonding, ensuring the stability of decorated functional films during the subsequent device fabrication consisting of solution processing of the polymer active layer. With the surface decoration of functional groups, both n-type and p-type polymers exhibit enhanced carrier mobilities in the unipolar OFETs. In addition, enhanced and balanced mobilities are obtained in the ambipolar OFETs with the blend of polymer semiconductors. The anchored self-assembled monolayers on the dielectric surfaces dramatically preclude the solvent effect, thus enabling an improvement of carrier mobility up to 2 orders of magnitude. Our study opens a way of targeted modifications of polymeric surfaces and related applications in organic electronics

Facile Image Patterning via Sequential Thiol–Michael/Thiol–Yne Click Reactions

Freestanding substrates with high refractive index modulation, good oxygen resistance, and low volume shrinkage are critical in photolithography for the purpose of high density data storage, image patterning and anticounterfeiting. Herein, we demonstrate a novel paradigm of direct holographic image patterning via the radical-mediated thiol–yne click reaction subsequent to the base-catalyzed thiol-Michael addition reaction. With the benefit of a newly synthesized alkyne monomer, 9-(2-((2-(prop-2-yn-1-yloxy)­ethyl)­thio)­ethyl)-9<i>H</i>-carbazole (POETEC), holograms with as high as 96% diffraction efficiency, refractive index modulation of 0.0036, dynamic range of 5.6 per 200 μm and volume shrinkage of 1.1%, are successfully patterned in an aerobic environment. Uniquely and distinctly, an inhibitor is unnecessary to prevent the initiation of the sequential reaction in this framework

Monochromatic Visible Light “Photoinitibitor”: Janus-Faced Initiation and Inhibition for Storage of Colored 3D Images

Controlling the kinetics and gelation of photopolymerization is a significant challenge in the fabrication of complex three-dimensional (3D) objects as is critical in numerous imaging, lithography, and additive manufacturing techniques. We propose a novel, visible light sensitive “photoinitibitor” which simultaneously generates two distinct radicals, each with their own unique purpose–one radical each for initiation and inhibition. The Janus-faced functions of this photoinitibitor delay gelation and dramatically amplify the gelation time difference between the constructive and destructive interference regions of the exposed holographic pattern. This approach enhances the photopolymerization induced phase separation of liquid crystal/acrylate resins and the formation of fine holographic polymer dispersed liquid crystal (HPDLC) gratings. Moreover, we construct colored 3D holographic images that are visually recognizable to the naked eye under white light