Azobenzene-containing Materials Research Articles

HYDROPHOBIC PHOTORESPONSIVE AZOBENZENE-CONTAINING MATERIALS PREPARED BY CLICK REACTION

With the rapid development of intelligent materials in recent years, intelligent hydrophobic materials have become a new research direction, and hydrophobic surfaces with adjustable wettability have enormous application potential in many fields. Generally speaking, surface wettability is determined by the combined effect of surface chemical composition and surface morphology. Therefore, the photoresponsive wettability of azobenzene-modified surfaces can be achieved by constructing rough surface of micro-nano scale and reducing the surface free energy. In this paper, we synthesized two photo responsive hydrophobic azobenzene derivatives (AAAB-PFDT and UAAB-PFDT), characterized their structure, thermal degradation behavior, and microscopic morphology. The light conversion performance of the two materials in dispersion was also studied using a UV–Vis spectrometer. By depositing the materials on the glass slide, the measurements of the water contact angle (WCA) under different lighting conditions were performed, demonstrating that both materials exhibit excellent photoswitchable hydrophobicity and reversibility.

Nanocomposite Photoanisotropic Materials for Applications in Polarization Holography and Photonics.

Photoanisotropic materials, in particular azodyes and azopolymers, have attracted significant research interest in the last decades. This is due to their applications in polarization holography and 4G optics, enabling polarization-selective diffractive optical elements with unique properties, including circular polarization beam-splitters, polarization-selective bifocal lenses, and many others. Numerous methods have been applied to increase the photoinduced birefringence of these materials, and as a result, to obtain polarization holographic elements with a high diffraction efficiency. Recently, a new approach has emerged that has been extensively studied by many research groups, namely doping azobenzene-containing materials with nanoparticles with various compositions, sizes, and morphologies. The resulting nanocomposites have shown significant enhancement in their photoanisotropic response, including increased photoinduced birefringence, leading to a higher diffraction efficiency and a larger surface relief modulation in the case of polarization holographic recordings. This review aims to cover the most important achievements in this new but fast-growing field of research and to present an extensive comparative analysis of the result, reported by many research groups during the last two decades. Different hypotheses to explain the mechanism of photoanisotropy enhancement in these nanocomposites are also discussed. Finally, we present our vision for the future development of this scientific field and outline its potential applications in advanced photonics technologies.

Controlling the Structure and Morphology of Organic Nanofilaments Using External Stimuli.

In our continuing pursuit to generate, understand, and control the morphology of organic nanofilaments formed by molecules with a bent molecular shape, we here report on two bent-core molecules specifically designed to permit a phase or morphology change upon exposure to an applied electric field or irradiation with UV light. To trigger a response to an applied electric field, conformationally rigid chiral (S,S)-2,3-difluorooctyloxy side chains were introduced, and to cause a response to UV light, an azobenzene core was incorporated into one of the arms of the rigid bent core. The phase behavior as well as structure and morphology of the formed phases and nanofilaments were analyzed using differential scanning calorimetry, cross-polarized optical microscopy, circular dichroism spectropolarimetry, scanning and transmission electron microscopy, UV-vis spectrophotometry, as well as X-ray diffraction experiments. Both bent-core molecules were characterized by the coexistence of two nanoscale morphologies, specifically helical nanofilaments (HNFs) and layered nanocylinders, prior to exposure to an external stimulus and independent of the cooling rate from the isotropic liquid. The application of an electric field triggers the disappearance of crystalline nanofilaments and instead leads to the formation of a tilted smectic liquid crystal phase for the material featuring chiral difluorinated side chains, whereas irradiation with UV light results in the disappearance of the nanocylinders and the sole formation of HNFs for the azobenzene-containing material. Combined results of this experimental study reveal that in addition to controlling the rate of cooling, applied electric fields and UV irradiation can be used to expand the toolkit for structural and morphological control of suitably designed bent-core molecule-based structures at the nanoscale.

Generation of sub-micrometer surface relief gratings in an azobenzene-containing material using a 355 nm laser

Controlled effective fabrication of surface relief gratings (SRG) of different periods and dimensionality in an azobenzene-containing polyelectrolyte material using an UV-laser operating at 355 nm was demonstrated for the first time. The application of an UV-laser in combination with Lloyd's interferometer allows the accurate control of grating period and the generation of gratings with small period down to 180 nm. The SRG formation is related to the kinetics of the steady state of the trans-cis photoisomerization of a PAZO film at UV exposure. The replication of the manufactured gratings using soft lithography is demonstrated as well.

Accurate Morphology-Related Diffraction Behavior of Light-Induced Surface Relief Gratings on Azopolymers

Holographic relief gratings can be fabricated directly on the surface of azobenzene-containing materials (or simply azomaterials) without additional development steps. Despite often being described to have ideal sinusoidal profiles, the developing surface morphology in large amplitude gratings can affect the light distribution of the writing interferogram, causing profile deformations and deviations in the expected diffraction behavior. In this work, we characterize the temporal evolution of the surface relief grating (SRG) morphology fabricated by means of interference lithography on azopolymer films, quantitatively relating the results of a Fourier analysis of their surface profiles to the accurate measurement of efficiencies in the transmitted diffraction orders. A reliable surface structuration dynamics is empirically extracted from the analysis and used to formulate a simple but detailed diffraction model that, within the scalar diffraction theory, exhaustively describes the diffraction behavior of real SRGs without the need of complex rigorous electromagnetic theories. Our results add a deeper insight in the quantitative description of the morphology and the diffraction behavior of SRGs, which can contribute to their adoption for operating high-performance, reconfigurable, compact, and lightweight diffractive optical devices.

Coherent beam amplification with dynamic holograms in glass-forming molecular azobenzene-based materials

Self-diffraction of two coherent light waves that record a holographic grating in thin films of azobenzene-containing molecular glass-forming materials is used for intensity enhancement of a weak beam in expense of the strong beam intensity. The intensity of a weak probe beam is doubled at interaction length of only 8 µm thus pointing to a very high specific coupling constant. The characteristic time of the intensity redistribution is relatively long, on the order of 200 s. Consequently, the underlying process of pronounced Kerr-type optical nonlinearity is related, most probably, to the photoinduced mass transfer.

Optical and chemical control of the wettability of nanoporous photoswitchable films.

Wettability is a central surface property of functional thin films. Here, we present a nanoporous film made of an azobenzene-containing metal-organic framework material where the wettability is controlled by photoswitching of the fluorinated azobenzene moieties and by reversible incorporation of guest molecules with different polarities in the pores. Using both, the optical and the chemical stimuli, the water contact angle was modified over a wide range, from 23° to 97°.

Isomerization and reorientation of Disperse Red 1 in poly(ethyl methacrylate).

Irradiation of azobenzene-containing polymer materials with light causes cis-trans isomerization and reorientation of azobenzene moieties and thereby changes in the optical properties of the materials. In this study, the film of poly(ethyl methacrylate) doped with the azobenzene derivative Disperse Red 1 (DR1) has been irradiated with the linearly polarized light of 546nm. The time profiles of optical anisotropy and absorbance measured during irradiation have been analyzed using a kinetic model. Based on the analysis of the time profiles, we conclude that the light-induced reorientation of DR1 molecules occurs in confined environments where trans → cis isomerization is hindered, while in roomy environments, there is no reorientation. In the confined environment, reorientation occurs due to the environmental changes caused by the isomerization attempts of the DR1molecule. The polymer environment affects thermal cis → trans and light-induced trans → cis isomerizations of the DR1molecule differently, suggesting that the spatial requirements for these processes to proceed are different. The thermal isomerization does not result in the reorientation of DR1 molecules in roomy environments.

Photo-induced changes in azobenzene-containing soft materials

Photocontrol of molecular alignment is an exceptionally-intelligent and useful strategy. It enables us to control optical coefficients, peripheral molecular alignments, surface relief structure, and actuation of substances by means of photoirradiation. Epoxy linear polymers with azobenzene and alkyl side groups were synthesized and studied. They showed interesting and variable photo-response upon visible-light irradiation due to trans–cis–trans photoisomerization of the azobenzene units. The influence of physical crosslinks between alkyl chains on optical and mechanical properties was also investigated. Mechanical characterization of the polymers was done by nanoindentation. Light induced reversible or irreversible changes in the polymer after being irradiated with visible light, depend on the polarization states of light used. When using circularly polarized light, reversible mechanical response is observed. Permanent structural changes in films are related to anisotropic alignment of azobenzene molecules (after being illuminated with linear polarized light) and/or physical crosslinks of alkyl chains. This study demonstrated the importance of the polymer architecture and the presence of domains of alkyl chains on the potential photo responsiveness.

Light-Associated Surface Wrinkling-Based Metrology for the Photosoftening Characterization in Azobenzene-Polymer Supramolecular Complexes.

As an intriguing characteristic of azobenzene-containing materials (azo-materials), photoinduced changes in mechanical properties (e.g., photosoftening) have stimulated many efforts both theoretically and experimentally. Here a simple yet powerful tool (i.e., a light-associated surface wrinkling-based method) to study the photosoftening effect in azobenzene-polymer (azo-polymer) supramolecular complexes is reported. The photo-induced modulus decrease of supramolecular complex films is deduced by analyzing the change of critical wrinkle wavelength of strain-induced surface wrinkling, in the case of varying experiment parameters. In particular, thanks to the facile modular tunability of the supramolecular system, the photosoftening effect has been systematically investigated as a function of azo-moiety content and the molecular weight of the host polymer. Notably, a photosoftening coefficient that is related to the chemical composition/structure of azo-polymers is introduced, and a simple formula that can quantify the connection of the photosoftening with external irradiation conditions and internal chemical factors of azo-polymers is derived for the first time. The obtained results are of great importance not only to enhance understanding of the photosoftening mechanism, but also to thoroughly apply it in diverse smart fields.

Azobenzene-based sinusoidal surface topography drives focal adhesion confinement and guides collective migration of epithelial cells

Surface topography is a key parameter in regulating the morphology and behavior of single cells. At multicellular level, coordinated cell displacements drive many biological events such as embryonic morphogenesis. However, the effect of surface topography on collective migration of epithelium has not been studied in detail. Mastering the connection between surface features and collective cellular behaviour is highly important for novel approaches in tissue engineering and repair. Herein, we used photopatterned microtopographies on azobenzene-containing materials and showed that smooth topographical cues with proper period and orientation can efficiently orchestrate cell alignment in growing epithelium. Furthermore, the experimental system allowed us to investigate how the orientation of the topographical features can alter the speed of wound closure in vitro. Our findings indicate that the extracellular microenvironment topography coordinates their focal adhesion distribution and alignment. These topographic cues are able to guide the collective migration of multicellular systems, even when cell–cell junctions are disrupted.

Modeling of Stripe Patterns in Photosensitive Azopolymers.

Placed at interfaces, azobenzene-containing materials show extraordinary phenomena when subjected to external light sources. Here we model the surface changes induced by one-dimensional Gaussian light fields in thin azopolymer films. Such fields can be produced in a quickly moving film irradiated with a strongly focused laser beam or illuminating the sample through a cylindrical lens. To explain the appearance of stripe patterns, we first calculate the unbalanced mechanical stresses induced by one-dimensional Gaussian fields in the interior of the film. In accordance with our orientation approach, the light-induced stress originates from the reorientation of azobenzenes that causes orientation of rigid backbone segments along the light polarization. The resulting volume forces have different signs and amplitude for light polarization directed perpendicular and parallel to the moving direction. Accordingly, the grooves are produced by the stretching forces and elongated protrusions by the compressive forces. Implementation into a viscoplastic model in a finite element software predicts a considerably weaker effect for the light polarized along the moving direction, in accordance with the experimental observations. The maximum value in the distribution of light-induced stresses becomes in this case very close to the yield stress which results in smaller surface deformations of the glassy azopolymer.

Photoswitchable Adhesives Using Azobenzene-Containing Materials.

Photoinduced reversible solid-to-liquid transitions of azobenzene-containing materials can control adhesion. Photoswitchable adhesives based on azobenzene-containing small molecules and polymers are under intense investigation. The melting points or glass transition temperatures of such azobenzene-containing materials in trans and cis forms are above and below room temperature, respectively. Photoswitching of these materials results in reversible trans-cis isomerization and solid-to-liquid transitions. The solid trans azobenzene-containing materials have strong adhesion and the liquid cis azobenzene-containing materials have weaker adhesion. In this Minireview, we introduce adhesives based on azobenzene-containing small molecules and polymers. The remaining challenges and perspectives in the field of photoswitchable adhesives using azobenzene-containing materials are also discussed.

Deep surface relief grating in azobenzene-containing materials using a low-intensity 532 nm laser

The inscription of deep surface relief grating (SRG) in glassy monomeric azobenzene materials using a low intensity diode pumped solid state (DPSS) laser operating at 532 nm is demonstrated. Previously developed epoxy-based star-like azobenzene-containing material was applied. Both deep SRG and multi-dimensional structures can be fabricated using this cheap, effective material in combination with a low cost laser source. The dependencies of the SRG performance (amplitude, inscription rate, stability) on exposure conditions (time, intensity) and layer thickness are presented. The replication of the inscribed gratings using polydimethylsiloxane is demonstrated. The results show that effective all-optical fabrication of surface relief master-gratings can be realized using low cost chemistry and widely used low-intensity coherent light source operating at 532 nm.

Photoinduced deformations in chalcogenide glasses: What are the driving forces?

Chalcogenide glasses including As2S3 and Se are known to exhibit a variety of photoinduced deformations such as volume expansions, anisotropic shape changes, and surface ripples. These deformations are produced by photoinduced viscous material flows which are caused by some driving forces, while the origins are controversial. We propose a guiding idea that the driving force arises from atomic and optical mechanisms; the former from structural disordering and intermolecular alignment and the latter from radiation force and torque. Previously proposed models such as Coulombic and electric-gradient forces are criticized. We also compare these deformations with those in azobenzene-containing organic materials.

From nanoscopic to macroscopic photo-driven motion in azobenzene-containing materials

Abstract The illumination of azobenzene molecules with UV/visible light efficiently converts the molecules between trans and cis isomerization states. Isomerization is accompanied by a large photo-induced molecular motion, which is able to significantly affect the physical and chemical properties of the materials in which they are incorporated. In some material systems, the nanoscopic structural movement of the isomerizing azobenzene molecules can be even propagated at macroscopic spatial scales. Reversible large-scale superficial photo-patterning and mechanical photo-actuation are efficiently achieved in azobenzene-containing glassy materials and liquid crystalline elastomers, respectively. This review covers several aspects related to the phenomenology and the applications of the light-driven macroscopic effects observed in these two classes of azomaterials, highlighting many of the possibilities they offer in different fields of science, like photonics, biology, surface engineering and robotics.

Direct Microscopic Observation of Domain Rearrangement Mechanism of Photo-Orientation Process in Azobenzene-Containing Materials.

The evolution of the domain structure of two azobenzene-containing materials during irradiation with polarized light was monitored using polarized optical microscopy with additional λ-waveplate for discerning the domains oriented parallel and perpendicular to irradiating light polarization. It is shown that the process of photo-orientation consists in growth of perpendicularly oriented domains and diminishing of parallel domains via the movement of domain boundaries. These data confirm the mechanism of photo-orientation via domain structure rearrangement.

Monte-Carlo simulation of the birefringence kinetics in azobenzene-containing materials

The photoinduced optical anisotropy (birefringence and dichroism) in azobenzene-containing materials gives rise to a variety of applications in photonics by way of controlling the anisotropic properties at a nanoscale level. These applications require modeling the photoisomerization process kinetics. This is a complex and stochastic task that depends on many parameters. The paper presents a computationally effective Monte-Carlo simulation of recording, relaxation and erasure of the photoinduced anisotropy for a large number of 3D-oriented molecules at small angular increments between the neighboring orientations. The modeling is in good agreement with the experiment for measuring the birefringence in the PAZO polymer.

Molecular-Level Study of Photoorientation in Hydrogen-Bonded Azopolymer Complexes

To optimize azobenzene-containing materials for applications such as rewritable waveguides and holographic data storage, it is imperative to understand the effect of the azobenzene structure on the photoresponse of the material. Supramolecular materials, in which a complexed photoactive azobenzene controls the motion or other properties of a passive polymer, are uniquely convenient for studying the impact of specific chemical modifications. Here, we use polarization modulation infrared structural absorbance spectroscopy (PM-IRSAS) to study hydrogen-bonded supramolecular azobenzene complexes using poly(4-vinylpyridine) (P4VP) as a model polymer. We show that changing the tail group from hydrogen (AH) to cyano (ACN) induces greater angular redistribution of the chromophores and, remarkably, provokes P4VP pyridine ring orientation. Increasing the degree of complexation decreases the saturated orientation of both AH and ACN, whereas for P4VP/ACN it increases the pyridine orientation as well as the orientation...

Azobenzene-based polymers: emerging applications as cell culture platforms.

The fabrication of biomaterials whose properties are activated or inhibited on demand via light is appealing for fundamental biological studies as well as for the development of new applications in tissue engineering and regenerative medicine. One of the most widely used molecules in light-controlled systems is azobenzene for its ability to isomerise in response to light. In this minireview, the fundamental landmarks towards the application of azobenzene-containing materials as cell culture substrates will be highlighted, foreseeing their massive use as next-generation cell-instructive materials.