Photoinduced Deformations Research Articles

Ultraviolet‐visible‐near‐infrared light‐responsive soft materials: Fabrication, photomechanical deformation and applications

AbstractIn recent years, the advances in light‐responsive soft materials with fascinating properties and functions have attracted tremendous attention, which are also enlightening when attempting to achieve the goals of complex deformations, motions, or attractive applications by precise regulation. Attractively, light is not only a clean and inexhaustible energy but also can be controlled remotely, quickly and accurately in a non‐contact way. Moreover, light‐responsive soft materials are capable of amplifying photo‐triggered molecular changes at the microscopic scale into macroscopic deformations, that is, directly converting the input light energy into the output mechanical work, therefore enabling potential applications in the field of actuators and functional devices. To date, some wonderful reviews have reported the progress in photo‐driven soft materials. However, the research progress in ultraviolet, visible (Vis) and near‐infrared (NIR) light‐driven soft materials containing azobenzene or other non‐azobenzene moieties has not been reported yet. In this review, we summarize recent progress in light‐responsive soft materials in terms of preparation methods, response wavelengths and potential applications. Firstly, the preparation methods of photoresponsive soft materials are introduced. Subsequently, photoinduced macroscopic deformations or motions are summarized, in which Vis and NIR light‐responsive behaviors are especially highlighted. Finally, the potential applications of photoresponsive soft materials are classified. To guide the future work for researchers, the existing problems and future development prospects of light‐responsive soft materials are proposed.

Photosalience and Thermal Phase Transitions of Azobenzene- and Crown Ether-Based Complexes in Polymorphic Crystals.

Stimuli-responsive molecular crystals have attracted considerable attention as promising smart materials with applications in various fields such as sensing, actuation, and optoelectronics. Understanding the structure-mechanical property relationships, however, remains largely unexplored when it comes to functionalizing these organic crystals. Here, we report three polymorphic crystals (Forms A, B, and C) formed by the non-threaded complexation of a dibenzo[18]crown-6 (DB18C6) ether ring and an azobenzene-based ammonium cation, each exhibiting distinct thermal phase transitions, photoinduced deformations, and mechanical behavior. Structural changes on going from Form A to Form B and from Form C to Form B during heating and cooling, respectively, are observed by single-crystal X-ray crystallography. Form A shows photoinduced reversible bending, whereas Form B exhibits isotropic expansion. Form C displays uniaxial negative expansion with a remarkable increase of 44% in thickness under photoirradiation. Force measurements and nanoindentation reveal that the soft crystals of Form A with a low elastic modulus demonstrate a significant photoresponse, attributed to the non-threaded molecular structure, which permits flexibility of the azobenzene unit. This work represents a significant advance in the understanding of the correlation between structure-thermomechanical and structure-photomechanical properties necessary for the development of multi-stimulus-responsive materials with tailored properties.

Separation of the Mechanisms of Photoinduced Deformations in Crystals Using Time-Resolved X-ray Diffractometry

Separation of the Mechanisms of Photoinduced Deformations in Crystals Using Time-Resolved X-ray Diffractometry

Is it possible to maintain photomechanical properties of crystalline diarylethenes after thermal amorphization?

The photomechanical properties of the photochromic 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DTE) crystals, largely documented since the discovery by Irie et al. of its reversible photoswitching of crystallographic plane morphology, is renewed with a novel approach consisting on suppressing gradually dynamical effects through thermal amorphization. In the crystalline case, the reversibility of UV/Visible induced plastic deformations have been observed as expected. In addition, a number of large jumps for both cycles are reported for the first time for this molecule in contrast to being observed only during the first UV irradiation cycle for other DTEs. In the amorphous case, several striking results have been obtained: (i) the still present reversible photoinduced plastic deformations mimicking the crystalline case are interpreted in terms of apparent polyamorphism; (ii) the total vanishing of the jumps brings the evidence of the crucial role of the crystallinity for the propelling surface in contact with the substrate; (iii) the discovery of a never reported material jump during the first visible cycle. This last result is rationalized combining two features –a thermal one, a glass transition temperature very close to the room temperature (Tg ≈ 34°C) and –a photochemical one, an inefficient photoreversion allowing the other vibrational relaxation pathway to proceed inducing the required temperature rise to overpass the phase transition (Solid → Liquid) responsible for the jump.

Multidimensional‐Encryption in Emissive Liquid Crystal Elastomers through Synergistic Usage of Photorewritable Fluorescent Patterning and Reconfigurable 3D Shaping

AbstractAdvanced encryption/decryption strategies are of great significance for information protection and data security. It is highly desirable yet quite challenging to develop functional materials for encrypting/decrypting information more effectively. Herein, a novel emissive liquid crystal elastomer (LCE) for multidimensional and multistage encryption is proposed for the first time, through synergistic utilization of phototunable fluorescence and the photoprogrammable shape. The fluorescent LCE is fabricated by incorporating an aggregation‐induced‐emission α‐cyanodiarylethene‐based hydrogen‐bonded complex and azobenzene derivative into the LCE networks through covalent bonding. Because of the photoisomerization of both these two photosensitive derivatives under the same exciting light, the consequent LCE films exhibit photoinduced reversible fluorescence changes and shape deformations that are suitable for data storage and encryption. On this basis, the collaborative usage of photolithography‐based 2D fluorescent images and photoprogramming 3D shape configurations can lead to multidimensional and multistage encryption. Moreover, this encryption strategy is reprogrammable, allowing for repeatable encoding and decrypting. The results demonstrated here reveal that the reversible phototunable fluorescent LCE materials exhibit promising applications in data storage and encryption.

Modeling of the photo-induced stress in azobenzene polymers by combining theory and computer simulations.

It has been shown recently that the photo-induced deformations in azobenzene-containing polymers of a side-chain architecture can be explained by means of the so-called orientational approach. The explanation is based on the following sequence of steps: (i) reorientation of azobenzenes under illumination, (ii) reorientation of the polymer backbones coupled mechanically to azobenzenes, and (iii) development of large stress in a material. Step (i) is based on the angle selective absorption of the azobenzene chromophore, which is a well established fact. Step (iii) has been validated in a series of recent theoretic studies in an infinite coupling limit. Concerning step (ii), in a real material, the backbone-azobenzene coupling will be always finite, resulting in a decrease of the effective torque sensed by the backbones and in a time delay in their reorientation. To study the relevance of these effects in detail, we perform coarse-grained molecular dynamics simulations of side-chain azobenzene-containing oligomers in bulk at conditions close to the glassy state. The focus is on the dynamical properties of such a system and on its response to the illumination, with the latter modeled either as an orientation potential applied to the azobenzenes or via their stochastic photo-isomerization. By matching the amount of light-induced stress evaluated in both cases, we obtained the equivalent orientation potential as a function of the illumination intensity and the system density.

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.

Photo-Induced Bending Behavior of Post-Crosslinked Liquid Crystalline Polymer/Polyurethane Blend Films.

Photoresponsive blend films with post-crosslinked liquid crystalline polymer (CLCP) as a photosensitive component and flexible polyurethane (PU) as the matrix are successfully fabricated. After being uniaxially stretched, even at low concentration, the azobenzene-containing CLCP effectively transfers its photoresponsiveness to the photoinert PU matrix, resulting in the fast photo-induced bending behavior of whole blend film thanks to the effective dispersion of CLCP. Specifically, the blend film shows photo-induced deformations upon exposure to unpolarized UV light at ambient temperature. The film unbends after thermal treatment, and the randomly orientated mesogens in the film can be realigned by the mechanical stretching, which endows the film with a reversible deformation behavior. The photosensitive blend film possesses favorable mechanical property and good processability at low cost, and it is a promising candidate for a new generation of actuators.

Strong focusing higher-order laser modes: transverse and longitudinal optical fields

The distribution of transverse and longitudinal optical fields in tightly focused higher-order laser beams is investigated. Polarization-dependent fingerprints of transverse and longitudinal optical fields are experimentally captured by means of photoinduced surface deformations in azobenzene polymer thin films.

Visualization of Longitudinal and Transverse Components of Strongly Focused Optical Field by means of Photo-Reactive Azopolymers

Most important problems in modern photonics are fabrication, visualization and characterization of nanomaterials at optical frequencies. A number of optical techniques uses tightly focused laser beams to access longitudinal electromagnetic fields, which are directed towards the wave vector. In this Letter, the distribution of transverse and longitudinal optical fields in tightly focused laser beams, polarized in a new fashion, is investigated. Polarization dependent fingerprints of transverse and longitudinal optical fields are experimentally captured by means of photoinduced surface deformations in azobenzene polymer thin films.

Stress-free layers in photoinduced deformations of photoelastomer beams

Nematic liquid crystals combined with long molecular chains to form liquid crystal elastomers are capable of large extension. When such liquid crystal elastomers contain azo dyes to constitute photoelastomers, illumination can trigger large contraction. Beams made from such photoelastomers possess a non-uniform illumination and hence photostrain across their cross-section, resulting in bending and highly non-linear stress distribution. Due to the non-linear stress distribution, there can be more than one stress-free layers within the beam. In this paper, we present a dimensionless parametric study of nematic photoelastomer beams under the combined effects of light and mechanical loads. We show how the number of stress-free layers depends on three dimensionless parameters. The paths traced out by the system in the space of dimensionless parameters by varying the different real parameters are investigated, showing how the number of stress-free layers changes when e.g.the thickness or the mechanical load of the elastomer beam is varied. These results are important if the strain induced director rotation is not negligible.

Photoinduced macroscopic vector deformations in chalcogenide glasses

Photoinduced vector deformations in amorphous Se films on viscous and rigid substrates have been studied for comparison with those in As2S3. In both the films, illumination of focused and unfocused linearly-polarized bandgap light produces, respectively, anisotropic M-shape deformations and wavy wrinkles. The M-deformations occur in parallel to the electric field, but the wavy directions are contrastive; parallel and orthogonal to the electric field in Se and As2S3 films on grease. The unfocused illumination also produces corrugations in As2S3 films on glass substrates. We assume that atomic and optical forces are responsible for these deformations.

Opposite photo-induced deformations in azobenzene-containing polymers with different molecular architecture: Molecular dynamics study

Photo-induced deformations in azobenzene-containing polymers (azo-polymers) are central to a number of applications, such as optical storage and fabrication of diffractive elements. The microscopic nature of the underlying opto-mechanical coupling is yet not clear. In this study, we address the experimental finding that the scenario of the effects depends on molecular architecture of the used azo-polymer. Typically, opposite deformations in respect to the direction of light polarization are observed for liquid crystalline and amorphous azo-polymers. In this study, we undertake molecular dynamics simulations of two different models that mimic these two types of azo-polymers. We employ hybrid force field modeling and consider only trans-isomers of azobenzene, represented as Gay-Berne sites. The effect of illumination on the orientation of the chromophores is considered on the level of orientational hole burning and emphasis is given to the resulting deformation of the polymer matrix. We reproduce deformations of opposite sign for the two models being considered here and discuss the relevant microscopic mechanisms in both cases.

Linear and Nonlinear Photoinduced Deformations of Cantilevers

Glassy and elastomeric nematic networks with dye molecules present can be very responsive to illumination, huge reversible strains being possible. If absorption is appreciable, strain decreases with depth into a cantilever, leading to bend that is the basis of micro-opto-mechanical systems (MOMS). Bend actually occurs even when Beer's law suggests a tiny penetration of light into a heavily dye-doped system. We model the nonlinear opto-elastic processes behind this effect. In the regime of cantilever thickness giving optimal bending for a given incident light intensity, there are three neutral surfaces. In practice such nonlinear absorptive effects are very important since heavily doped systems are commonly used.

Polarization-dependent photoinduced mechanical deformations in covalent chalcogenide glasses

The optomechanical effect and photoinduced anisotropic deformation induced by illumination of linearly polarized light have been comparatively studied for glassy As2S3 and Se films. The two materials show qualitatively the same behaviors in the mechanical effect and the deformation, which suggests that these photoinduced phenomena are inherent to the covalent chalcogenide glass. But, the two phenomena show different dependences upon intensity, spectrum and exposure time of excitation light, which imply different underlying mechanisms.

Longitudinal anisotropy of the photoinduced molecular migration in azobenzene polymer films

The effects of tightly focused, higher-order laser beams on the photoinduced molecular migration and surface deformations in azobenzene polymer films are investigated. We demonstrate that the surface relief is principally triggered by longitudinal fields, i.e., electric fields polarized along the optical axis of the focused beam. Our findings can be explained by the translational diffusion of isomerized chromophores when the constraining effect of the polymer-air interface is considered.

Photomechanical Effects in Azobenzene‐Containing Polymer Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Photomechanical Effects in Azobenzene-Containing Polymer Systems

Several kinds of photodeformable polymer systems containing azobenzenes, capable of generating photomechanical effects, are reviewed. In polymer amorphous systems including monolayers, gels and solid films, trans-cis photoisomerization of the azobenzene moieties accompanied by geometrical and dipolar changes caused contraction and expansion of the systems. Recently, polymer liquid-crystalline elastomers containing azobenzenes, a novel system able to produce the photomechanical effects, have been developed to achieve fast and large photoinduced deformations. It is interesting that not only photoinduced contraction but also photoinduced bending have been achieved. These deformations are ascribed to the subtle variation in microscopic ordering during the photochemical phase transition of the polymer liquid-crystalline elastomers. This new photomechanical system is potential for application in driving micromachines and nanomachines without the aid of batteries, motors and gears.

Excited-State Forces within a First-Principles Green’s Function Formalism

We present a new first-principles formalism for calculating forces for optically excited electronic states using the interacting Green's function approach with the GW Bethe-Salpeter-equation method. This advance allows for efficient computation of gradients of the excited-state Born-Oppenheimer energy, allowing for the study of relaxation, molecular dynamics, and photoluminescence of excited states. The approach is tested on photoexcited carbon dioxide and ammonia molecules, and the calculations accurately describe the excitation energies and photoinduced structural deformations.

Photoinduced surface deformations in ion-conducting Ag–As–S glasses. I. Isotropic deformations produced by small light spots

Prominent surface deformations appear in Ag–As–S chalcogenide glasses when exposed to focused laser beams. Deformation behaviors change with the beam diameter, the photon energy, the temperature at which illumination is provided, and the sample composition. In thin AgAsS2 films exposed at room temperature to a 2.0 eV light spot with a diameter of 2 μm, an isotropic convex structure with a height of ∼0.1 μm appears. The expanded region accompanies an Ag-content enhancement by ∼3 at. %, which suggests that the deformation is caused by photoinduced accumulation of Ag+ ions.