Azobenzene Mesogens Research Articles

Visualization of azobenzene mesogens trans-cis isomerization through structural color of photonic crystal

Azobenzene, a representative photoresponsive liquid crystal, has extensive applications in fields such as bionic robotics, actuators, artificial muscles, motors, and various functional devices. Photoisomerization (trans-cis isomerization) is the most important mechanism for its photoresponse. Currently, the isomerization of azobenzene mesogens can be measured through UV–vis absorption spectra, NMR techniques or transient grating methods. Visualization of the trans-cis isomerization of azobenzene mesogens still remains a challenge. In this study, a linear relationship is observed between the absorption intensity of azobenzene mesogens and the stopband shift of azobenzene inverse opals (AZOIOs) films during the isomerization process. This phenomenon can be used to determine the isomerization degree of azobenzene mesogens through the switching of the stopband of the AZOIOs film, enabling the visualization of the azobenzene mesogens’s configuration transition. The results of this study are highly significant for the design and fabrication of novel azobenzene devices.

Phase Behaviors and Photoresponsive Thin Films of Syndiotactic Side-Chain Liquid Crystalline Polymers with High Densely Substituted Azobenzene Mesogens.

Azobenzene-containing polymers (azopolymers) are a kind of fascinating stimuli-responsive materials with broad and versatile applications. In this work, a series of syndiotactic C1 type azopolymers of Pm-Azo-Cn with side-chain azobenzene mesogens of varied length alkoxy tails (n=1, 4, 8, 10) and different length alkyl spacers (m=6, 10) have been prepared via Rh-catalyzed carbene polymerization. The thermal properties and ordered assembly structures of thus synthesized side chain liquid crystalline polymers (SCLCPs) have been systematically investigated with differential scanning calorimetry (DSC), polarized optical microscopy (POM) and variable-temperature small/wide-angle X-ray scattering (SAXS/WAXS) analyses. P10-Azo-C1 and P10-Azo-C4 with shorter alkoxy tails exhibited hierarchical structures SmB/Colob and transformed into SmA/Colob at a higher temperature, while P10-Azo-C8 and P10-Azo-C10 with longer alkoxy tails only displayed side group dominated layered SmB phase and transformed into SmA phase at higher temperatures. For P6-Azo-C4 with a shorter spacer only showed a less ordered SmA phase owing to interference by partly coupling between the side chain azobenzene mesogens and the helical backbone. More importantly, the series high densely substituted syndiotactic C1 azopolymer thin films, exhibited evidently and smoothly reversible photoresponsive properties, which demonstrated promising photoresponsive device applications.

Photo- and Humidity-Responsive Liquid Crystal Copolymer Actuators Fabricated via Vapor-Assisted Alignment.

Deformable liquid crystal polymers (LCPs) driven by more than one external stimulus have received extensive attention in fields ranging from multifunctional soft robots to bionic actuators. Combining responsive liquid crystal with nonmesogenic responsive groups within polymer offers a versatile way to obtain multiresponsive LCPs. However, the incorporation of nonmesogenic responsive groups causes interruption in the assembly of mesogens and brings a challenge to the alignment of LCPs. Herein, a new method is put forward to facilitate uniform mesogen alignment by exerting water vapor in the film preparation process. Using this method, vapor-assisted alignment, the homeotropic alignment of azobenzene mesogens is achieved in a copolymer containing nonmesogenic poly(ethylene glycol) (PEG). The obtained copolymer films present photodeformation brought by azobenzene isomerization and humidity-responsive deformation resulting from the asymmetric swelling of film surfaces. The dual-responsive smart "blinds" and bionic flower actuators are fabricated to demonstrate the integration of the two different stimuli. This work is anticipated to provide a feasible alignment method for multiresponsive LCPs, showing the potential applications in soft robots, sensors, and biomimetic devices.

Photochromic Azobenzene Inverse Opal Film toward Dynamic Anti-Fake Pattern.

Azobenzene mesogens have garnered considerable research attention in the realm of photo-responsive materials due to their reversible trans-cis isomerization. In this paper, we demonstrate an azobenzene inverse opal film synthesized via photo-polymerization from a SiO2 opal template. The proposed design exhibits intriguing optical properties, including dynamic fluorescent features, distinct fluorescent enhancement, and an anti-fake micropattern with a switchable structure color. This work holds significant importance for advancing the development of novel optical devices.

Photothermal‐Responsive Crosslinked Liquid Crystal Polymers

AbstractCrosslinked liquid crystal polymers (CLCPs) are a promising type of smart polymer material with excellent two‐way shape memory behavior under the stimulation of various external conditions. Among the various external stimuli, light with the advantages of instantaneity, high accuracy, remote controllability, pollution‐free, etc., has been widely used in the control of CLCPs. Traditional photoresponsive CLCP systems based on photoisomerization are limited by the penetration depth of ultraviolet light, the number of useful photochemical reactions, and long‐term stability. Recently, a strategy for designing photoresponsive CLCPs based on the photothermal effect has attracted scientific attention. Photothermal materials in the CLCP matrix can convert light into heat through plasma resonance or energy transition under the light irradiation of a certain wavelength, and locally heat the CLCP matrix to beyond the liquid crystal‐to‐isotropic phase transition temperature, thus achieving reversible macroscopic deformation. Combining photothermal materials and CLCPs can decrease the dependence on specific photosensitive groups such as azobenzene mesogens and endow photoresponsive CLCPs with design flexibility. Herein, the photothermal‐responsive CLCPs under the stimulations of different light wavelengths and their application fields are reviewed. In addition, the current challenges in the field of photothermal‐responsive CLCPs are summarized, and a brief outlook on future development is proposed.

Synthesis and Self-Assembly of Silicon-Containing Azobenzene Liquid Crystalline Block Copolymers

The synthesis and self-assembly of a high interaction parameter silicon-containing liquid crystalline block copolymer (Si-LCBCP) are reported, containing thermally and photo-responsive azobenzene mesogens in the side chains which undergo molecular structural transitions, influencing phase transitions and orientation in the block copolymer. A series of the azobenzene-containing Si-LCBCPs, poly{dimethylsiloxane-b-11-[4-(4-cyanophenylazo)phenoxy]undecane methacrylate}, were synthesized through atom transfer radical polymerization, and the bulk self-assembly as well as the influence of the liquid-crystalline (LC) phase transition of the LC block on the microphase-separated nanostructure of the BCPs is described. As the volume fraction of the PMAAzo block (fPMAAzo) changes from 41 to 71%, lamellar, double gyroid (GYR), orthorhombic Fddd network, hexagonally packed cylindrical (HEX), and body centered cubic nanostructures were obtained. When fPMAAzo = 55–57%, a thermally reversible HEX-GYR-Fddd phase transition induced by the temperature change and LC phase transition was observed. Bulk morphologies are compared with thin-film self-assembly, and pattern transfer into silica nanostructures is demonstrated.

Aided- and self-assembly of liquid crystalline nanoparticles in bulk and in solution: computer simulation studies

ABSTRACT We review computer simulation studies of aided and self-assembly of nanoparticles that are decorated with liquid crystalline (mesogenic) ligands, termed hereafter as liquid crystalline nanoparticles (LCNPs). In bulk, LCNPs self-assemble into ordered morphologies, typically displaying a polydomain structure. We demonstrate that a range of monodomain morphologies can be grown by changing the density of ligands and employing external fields of specific symmetry, which act on mesogens. It is also demonstrated that the speed of self-assembly of LCNPs with chromophoric (e.g. azobenzene) mesogens can be increased by applying illumination at a certain wavelength and polarisation. Another case study covers the formation of an interconnected macromolecular network in a solution of the LCNPs in a polar solvent. Here, the network structure depends strongly on the decoration pattern of the LCNPs. Finally, the adsorption of LCNPs on a compatible liquid crystalline brush is discussed with a focus on the prerequisites and optimal conditions for this phenomenon. The review demonstrates the ability of classical particle-based models to produce a molecular-based understanding of the structure and interactions of LCNPs, and also to reproduce a wide set of physicochemical phenomena related to LCNP aided- and self-assembly processes.

Azobenzene functionalized poly(diphenylacetylene): Polymer synthesis and tunable fluorescent emission

AbstractWe prepared an azobenzene‐functionalized poly(diphenyl‐substituted acetylene) (named as PDPA‐azo) through the post‐polymerization modification route by using the activated ester strategy. The structures of the derived PDPA‐azo were fully characterized with comprehensive spectroscopic methods including NMR, FTIR, UV–vis, TGA, and so on. PDPA‐azo showed photo‐isomerization behavior due to the azobenzene moiety in the side chain. Since the short spacer between polymer main‐chain and side‐chain, PDPA‐azo could not form liquid‐crystal phase despite the presence of azobenzene mesogen in the polymer structure. It was noted that the modification of the precursor polymer PDPA‐PFP with primary amine‐functionalized azobenzene converted the fluorescence behavior from aggregation‐induced emission enhancement to aggregation‐caused quenching. The control experimental results suggested that the changes in fluorescent behaviors after irradiating under 365 and 254 nm UV lamps could be correlated to the photo‐induced isomerization of the azobenzene group. As a result, the fluorescence properties of PDPA‐azo can be adjusted by different external stimuli.

Photoinduced Contraction Fibers and Photoswitchable Adhesives Generated by Stretchable Supramolecular Gel

AbstractDynamically crosslinked supramolecular networks have emerged in the last decades as extremely stretchable and self‐healing materials thanks to the energy dissipation in weak and reversible interactions. Stretchable materials with tunable properties represent an effort to develop a charming class of smart devices. Once stretched, however, these materials’ properties are usually fixed due to strain‐induced crystallization (SIC), hindering their practical applications. Here, photoinduced contraction smart fibers and photoswitchable adhesives are generated from a dynamic supramolecular gel network, the SIC in which can be disrupted by UV irradiation to release the prestored strain energy. The dynamic sliding of azo‐bridged bipedal host molecules along the polymer chains renders the extreme stretchability of supramolecular gel under stretching forces. The oriented crystalline microdomains or crystallites through SIC upon stretching serve as physical crosslinks to store the contractile energy of fibers and improve the adhesion force of adhesives, and then are disoriented induced by trans‐to‐cis isomerization of azobenzene mesogens to release the prestored strain energy, resulting in such contraction of fibers and reduction of adhesion strength of adhesives. This work provides inspiration for exploring the structure–property relationship of supramolecular networks and transforming their clear potential to fabricate more remarkable synthetic smart materials.

Front Cover: Self‐assembly of Gold Nanoparticles into an Adjustable Plasmonic 3D Lattice using Janus‐type Forked Mesogenic Ligands (Chem. Asian J. 8/2022)

Gold nanoparticles (NPs) grafted with mixed-mesogen forked ligands self-assemble in a photoresponsive 3D superlattice with adjustable anisotropic plasmonic absorption. The gold NPs are arranged on a 3D superlattice intercalated by eggbox-shaped mesogen layers. Reducing graft density reduces the in-layer NP distance, enabling near-field plasmonic enhancement in a controlled fashion. The attached azobenzene mesogen allows repeated photoinduced order-disorder switching. More information can be found in the Research Article by Goran Ungar, Georg Mehl, Feng Liu et al.

Main-chain side-chain combined liquid crystalline polymers bearing azobenzene and biphenyl-based mesogens – segregation of the mesogens within two distinct layers

ABSTRACT Two different mesogenic units, namely biphenyl and azobenzene, were installed centrally within the polymer backbone and pendant alkylene segments; the purpose was to examine if chain folding of the backbone occurs so as to segregate the azobenzene and biphenyl units in adjacent layers of a smectic-type ordering. Two sets of isomeric main-chain side-chain liquid crystalline polymers (MCSC-LCPs), namely SC-AzoCx-MC-BPCx and SC-BPCx-MC-AzoCx (x=6 or 10), were synthesised; the former series has biphenyl in the backbone and azobenzene in the side-chain, while their locations are switched in the latter. It was observed that all four polymers were semicrystalline at room temperature and exhibited a smectic mesophase upon melting. DSC and X-ray scattering studies revealed that switching the locations of the azobenzene and biphenyls had little effect on; the phase transition temperatures and smectic layer spacings. A second series, namely SC-AzoCx-MC-BPCy and SC-BPCx-MC-AzoCy, where the length of alkylene spacers in the side-chain (Cx) and backbone (Cy) is different, was also studied; this was done to examine the influence of unequal alkylene segment lengths on the mesophase structure and interlayer spacing, and importantly, whether the zigzag folding-induced formation of a layered structure is retained, despite their unsymmetrical nature.

Self-assembly of Gold Nanoparticles into an Adjustable Plasmonic 3D Lattice using Janus-type Forked Mesogenic Ligands.

We report the formation of a 3D body-centred self-assembled superlattice of gold nanoparticles whose interparticle gap, and hence its plasmonic properties, are adjustable exclusively in the xy-plane. Thus, even though the particles are spherical, their anisotropic packing generates tailorable plasmonic dichroism. The gold nanoparticles are coated with forked ligands containing two mesogens: either two cholesterols ("twin"), one cholesterol and one azobenzene ("Janus"), or a mixture of the two. Beside the body-centered arrangement of gold nanoparticles, the structure also contains unusual two-dimensionally x-y dual undulated (eggbox-like) smectic-like layers of mesogens. Moreover, the presence of azobenzene mesogens allows the superlattice to be melted through UV-induced photo-isomerization; the process is reversible displaying low fatigue on repeated cycling.

Effect of Smectic Layers on Thermal Diffusivity of Side-Chain Polymer Liquid Crystals

The thermal conductivity (TC) of polymers is known to be effectively enhanced by liquid crystallinity. Liquid crystals (LCs) exhibit anisotropic TC with a greater magnitude in the orientation direction. However, the effects of other LC structures than the orientational order, such as smectic layer structures, on TC have not been well studied. This study investigated the correlation between LC structures and thermal diffusivity using a series of LC PMnMA polymethacrylates having azobenzene mesogens in the side chains. PMnMA formed nematic (N), smectic A (SmA), and smectic C (SmC) phases. These LCs were aligned by a magnetic field or large-amplitude oscillatory shear, yielding glassy LC films with mesogen long axes in the in-plane or out-of-plane direction. These glassy LCs increased thermal diffusivities in the LC orientation direction with the degree of LC orientation, whereas N and SmC LCs had smaller values than SmA LCs. These results demonstrated that smectic layers can enhance thermal diffusivity in their normal direction.

Fast Photoswitchable Order-Disorder Transitions in Liquid-Crystalline Block Co-oligomers.

Optically driven ordering transitions are rarely observed in macromolecular systems, often because of kinetic limitations. Here, we report a series of block co-oligomers (BCOs) that rapidly order and disorder at room temperature in response to optical illumination, and the absence thereof. The system is a triblock where rigid azobenzene (Azo) mesogens are attached to each end of a flexible siloxane chain. UV-induced trans-to-cis Azo isomerization, and vice versa in the absence of UV light, drive disordering and ordering of lamellar superstructures and smectic mesophases, as manifested by liquefaction and solidification of the material, respectively. The impacts of chemical structure on BCO self-assembly and photoswitching kinetics are explored by in situ microscopy and X-ray measurements for different mesogen end groups (NO2 or CN), and different carbon chain lengths (0C or 12C) between the siloxane and the mesogen. The presence of the 12C spacer leads to hierarchical ordering with smectic layers of mesogens existing alongside larger length-scale lamellae, versus only smectic ordering without the spacer. These hierarchically ordered BCOs display highly persistent lamellar sheets that contrast with the tortuous, low-persistence "fingerprint"-type structures seen in conventional block copolymers. The reordering kinetics upon removal of UV illumination are extremely rapid (<5 s). This fast response is due to the electron-withdrawing NO2 and CN, which facilitate cis-to-trans isomerization via thermal relaxation at room temperature without additional stimuli. This work elucidates structure-property relationships in photoswitching BCOs and advances the possibility of developing systems in which ordered nanostructures can be easily optically written and erased.

Novel chiral multi-arm liquid crystal polymers containing azobenzene mesogens and cholic acid

ABSTRACT Design and synthesis a series of azo multi-arm liquid crystal monomers (M1-M3) with cholic acid as the chiral core, followed by side chain graft copolymerisation with polymethylhydrosiloxane (PMHS) to synthesise the liquid crystal polymer PM series. All liquid crystal monomers and polymers were conformed to the molecular design in structure by FT-IR and NMR. The liquid crystal performance was tested by DSC and POM, and the results revealed that M1 and M2 were thermotropic enantiotropic cholesteric phase liquid crystals. Liquid crystal monomer M3 has no liquid crystal behaviour in both the up and down temperature processes. Polymers of the PM series exhibited different liquid crystal phases depending on the monomer content, with Grand-Jean texture at lower percentage and oil filament texture at higher percentage. The glass transition temperature and melting temperature decrease progressively with the increase in the liquid crystal monomer content, as opposed to the isotropic temperature.

Controlled Synthesis and Photophysical Properties of Liquid Crystalline Diblock Copolymers with Side⁃Chain Discotic Triphylene and Calamitic Azobenzene Mesogens

Controlled Synthesis and Photophysical Properties of Liquid Crystalline Diblock Copolymers with Side⁃Chain Discotic Triphylene and Calamitic Azobenzene Mesogens

Thickness-Dependent Photo-Aligned Thin-Film Morphologies of a Block Copolymer Containing an Azobenzene-Based Liquid Crystalline Polymer and a Poly(ionic liquid).

Photo-induced alignment of the thin-film morphologies of azobenzene-containing block copolymers (BCPs) is an effective method to obtain a uniaxial pattern of nanocylinders. Although film thickness is an important factor affecting the self-assembly of BCP thin films, the influence of film thickness on the photo-induced alignment of BCP thin-film morphology has never been systematically studied. Herein, we report the thickness-dependent photo-aligned film morphologies of the BCP containing an azobenzene-based liquid crystalline polymer and a poly(ionic liquid) (PIL), with a perfect uniaxial pattern of PIL nanocylinders. For films aligned with the unpolarized light (UPL), the out-of-plane PIL nanocylinders can be obtained in the film with a thickness of only 1L0 (∼30 nm, where L0 is the layer spacing of the hexagonally packed cylinder array), which is far lower than the thickness (more than 4L0) of the thermally annealed film needed to obtain the same morphology. This change is attributed to the orientation effect of UPL on azobenzene mesogens that suppresses the excluded volume effect. For the films aligned with linearly polarized light (LPL), to take advantage of the excluded volume effect to obtain the planar orientation of azobenzene mesogens, the thickness should be controlled to be no more than 3L0 to achieve an in-plane uniaxial alignment of PIL nanocylinders. The above relationship between the morphology and thickness of photo-aligned film eliminates the obstacles encountered in preparing films with well-ordered photo-aligned morphologies.

Thermal- and photo-responsive liquid crystalline elastomers fabricated using tung oil-based azobenzene

Azobenzene-based liquid crystalline elastomers with dynamic ester bonds exhibit high thermal- and photo-responsive properties. However, there are a few studies that focus on photo-induced and thermal-induced behaviors separately. Herein, novel thermal- and photo-responsive LCEs containing dynamic ester bonds were prepared from 4,4’-diglycidyloxy-azobenzene, sebacic acid and tung oil-based azobenzene, which acted as a side chain. The dynamic transesterification reactions on the network structure were confirmed through stress relaxation. The photo-induced and thermal-induced behaviors were also investigated. The monodomain LCEs film was obtained by uniaxial stretching at a temperature above the smectic–isotropic phase transition temperature (Ti ∼82 °C). Due to the breakage and formation of dynamic ester bonds and the order-disorder smectic phase transition caused by heat and the trans-cis isomerization of azobenzene mesogens, the thermal-induced stress of the monodomain LCEs film reached ∼3 MPa. The LCEs film also exhibited thermal-induced shape-memory properties. Investigation of its photo-responsive behavior further demonstrated that the monodomain LCEs film generated stress of ∼1.1 MPa while exhibited photo-induced deformation due to the trans-cis isomerization of azobenzene mesogens upon irradiation with 365 nm UV light. Finally, we observed that the monodomain LCEs film could undergo quickly photo-induced deformation and could be processed into a photo-driven gripper.

Inkless Rewritable Photonic Crystals Paper Enabled by a Light-Driven Azobenzene Mesogen Switch.

Rewritable paper, as an environment-friendly approach of information transmission, has potential possibility to conserve energy and promote a sustainable development of our society. Recently, photonic crystals (PCs) have become a research hotspot in the development of rewritable paper. However, there are still many shortcomings that limit the further application of PC paper, such as slow response sensitivity, short-cycle lifetime, poor storage stability, and so on. Herein, we constructed an optically rewritable azobenzene inverse opals (AZOIOs) with a thin film (ca. 1 μm) plated on an inverse opal structure based on the UV/vis switchable structure color of the sample. The top thin film acts as a protective layer to avoid the large deformation of the pore structure and the bottom inverse opal structure with refractive index/pore structure change that provides reversible structure color. Large, reversible, and rapid bandgap shift (ca. 60 nm, 2 s) of AZOIOs can be repeated more than 100 times under alternating UV/vis irradiation based on isomerization of high content of the azobenzene group. On-demand long-time preservation pattern can be obtained by the appearance of azobenzene's intrinsic color. The proof of concept for rewritable PC paper is demonstrated herein. Such inkless rewritable colorful paper paves a way for developing novel display technology.

Norbornenyl-based amphiphilic ABA-triblock azobenzene copolymers: Synthesis, photoresponsive and self-assembly properties

A series of well-defined ABA-triblock copolymers P(NB-PtBA)x-b-P(NB-PFP)y-b-P(NB-PtBA)x were synthesized by living ring-opening metathesis polymerization (ROMP) of norbornenyl-poly(tert-butyl acrylate) (NB-PtBA), norbornenyl-pentafluorophenyl (NB-PFP) and NB-PtBA in a sequential manner by the use of Grubbs’ third generation catalyst (G3). The corresponding amphiphilic ABA-triblock azobenzene copolymers P(NB-PAA)x-b-P(NB-Azo)y-b-P(NB-PAA)x were obtained in high yields after subsequent post-polymerization modifications, including the reaction of active esters with amine-functionalized azobenzene to introduce the azobenzene unit, and the hydrolysis reaction to remove the tert-butyl groups. Under alternating UV and visible light illumination, the light response behaviors of the polymers were demonstrated by UV–Vis spectroscopy, and the reversible changes were observed as a result of the reversible trans-cis photoisomerization of azobenzene mesogens. These amphiphilic copolymers can also spontaneously self-assemble into spherical nanoparticles in an aqueous medium, and the hydrodynamic diameter increases as the hydrophobic NB-Azo content increases.