Photoinduced Bending Research Articles

Surface and Bulk Effects in Photochemical Reactions and Photomechanical Effects in Dynamic Molecular Crystals.

The increasing number of reports on photomechanical effects in molecular crystals necessitates systematic studies to understand the intrinsic and external effectors that determine and have predictive power of their type and magnitude. Differential light absorption and product gradient between the surface and the bulk of the crystal are often invoked to qualitatively explain the mechanical response of crystals to light; however, the details on how this difference in photochemical response accounts for macroscopic effects such as surface modification, deformation, or disintegration of crystals are yet to be established. Using both bulk- and surface-sensitive analytical techniques, a rare instance of benzylidenefuranone crystals is studied here, and it is capable of several distinct types of photomechanical response including surface striation and delamination, photosalient effect (ballistic disintegration and motion), and photoinduced bending by dimerization. The results provide a holistic view on these effects and set the stage for the development of overarching theoretical models to describe the photomechanics in the ordered solid state.

Precise Actuation of Bilayer Photomechanical Films Coated with Molecular Azobenzene Chromophores.

Bilayer photomechanical films are fabricated by depositing one layer of molecular azobenzene chromophores onto flexible low-density polyethylene substrates. The photoinduced bending and unbending behavior of five azobenzene derivatives including azobenzene, 4-hydroxy-azobenzene, 4-((4-hydroxyphenyl)diazenyl)bezoitrile, 4-((4-methoxyph-enyl)diazenyl)phenol, and 4-(phenyldiazenyl)phenol is systematically studied by considering the incident light intensity and the thickness of the coated chromophore layers. Precise control of photoinduced curling of the bilayer film is successfully achieved upon irradiation with two beams of UV light, and the curled films can be recovered by thermal relaxation in the dark. The easily fabricated bilayer films show fast photomechanical response, strong photoinduced stress, and stability similar to crosslinked polymeric films.

Synthesis of well-defined easily crosslinkable azobenzene side-chain liquid crystalline polymers via reversible addition–fragmentation chain transfer polymerization and photomechanical properties of their post-crosslinked fibers

The synthesis of a series of well-defined easily crosslinkable azobenzene (azo) side-chain liquid crystalline polyacrylates (including both homopolymers with an N-hydroxysuccinimide carboxylate-substituted azo mesogen and copolymers with both the above crosslinkable azo mesogen and a non-crosslinkable azo mesogen) via reversible addition–fragmentation chain transfer (RAFT) polymerization and photomechanical properties of their post-crosslinked fibers are described. The RAFT polymerizations of the azo monomers proved to be living and well-controlled, as revealed by their linear kinetic plots as well as the linear increase of the molecular weights of the azo polymers with monomer conversions and their low molar-mass dispersities (Đ⩽1.17). All the obtained azo polymers exhibited high thermal stability and relatively low glass transition temperatures, but only the azo homopolymers with longer flexible spacers and azo copolymers showed liquid crystallinity. In addition, the crosslinked azo polymer fibers with easily controlled diameters and high alignment order were obtained by the first fabrication of the uncrosslinked azo polymer fibers by the simple melt spinning method and their subsequent crosslinking with a difunctional primary amine under mild conditions. The contents of the crosslinkable azo mesogen in the polymers had significant influence on the photomechanical properties of their post-crosslinked fibers, and those with appropriate crosslinking densities could exhibit fast and reversible photoinduced bending and unbending behaviors and excellent photodeformation fatigue resistance even at close to room temperature.

Photo-induced bending in a light-activated polymer laminated composite.

Light activated polymers (LAPs) have attracted increasing attention since these materials change their shape and/or behavior in response to light exposure, which serves as an instant, remote and precisely controllable stimulus that enables non-contact control of the material shape and behavior through simple variation in light intensity, wavelength and spatially controlled exposure. These features distinguish LAPs from other active polymers triggered by other stimuli such as heat, electrical field or humidity. Previous examples have resulted in demonstrations in applications such as surface patterning, photo-induced shape memory behavior, and photo-origami. However, in many of these applications, an undesirable limitation has been the requirement to apply and maintain an external load during light irradiation. In this paper, a laminated structure is introduced to provide a pre-programmed stress field, which is then used for photo-induced deformation. This laminated structure is fabricated by bonding a stretched elastomer (NOA65) sheet between two LAP layers. Releasing the elastomer causes contraction and introduces a compressive stress in the LAPs, which are relaxed optically to trigger the desired deformation. A theoretical model is developed to quantitatively examine the laminated composite system, allowing exploration of the design space and optimum design of the laminate.

Dependence of photoinduced bending behavior of diarylethene crystals on irradiation wavelength of ultraviolet light.

The dependence of the photoinduced bending behavior of diarylethene crystals on the ultraviolet light irradiation wavelength was investigated. When irradiated with 365 nm light, a crystal of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (1a) bends toward the incident light. On the other hand, when irradiated with 380 nm light, the crystal of 1a first bends away from the light source and then bends toward the incident light. To explain this bending behavior, we propose a comprehensive mechanism based on the depth of the photochromic reaction from the crystal surface. This mechanism is successfully supported by the change of cell parameters associated with the photochromic reaction upon irradiation with 380 nm light, which was determined by in situ X-ray crystallographic analysis.

Photoreversible current ON/OFF switching by the photoinduced bending of gold-coated diarylethene crystals.

Gold-coated diarylethene crystals exhibited photoreversible bending upon alternating irradiation with ultraviolet (UV) and visible light. The bending behavior can be well explained by the extended bimetal model, which we propose here. Moreover, we have demonstrated that it can be used as an actual electrical circuit photoswitch.

Mechanism of Photoinduced Bending and Twisting in Crystalline Microneedles and Microribbons Composed of 9-Methylanthracene

The solid-state photodimerization of 9-methylanthracene is used as a model system to investigate how crystal morphology and reaction dynamics affect photomechanical deformations of single microcrystals. By varying the crystallization conditions, two different crystal shapes, microneedles and microribbons, are grown on a clean water surface. The microribbons twist under irradiation, while the microneedles bend. In both shapes, the maximum deformation occurs at roughly the midpoint of the reaction, while further dimerization causes the crystals return to their original shapes. Powder X-ray diffraction patterns establish that the needles and ribbons have the same crystal orientation and that the photoreaction proceeds in a crystal-to-crystal manner. NMR spin-lattice relaxation measurements are consistent with the rapid formation of large (>100 nm) dimer crystal domains. Simultaneous measurement of the needle bending and monomer fluorescence signal allows us to correlate the bending with the reaction progress. The behavior is qualitatively reproduced by a model in which the motion is driven by strain between spatially distinct reactant and product domains, also called heterometry. We consider several different mechanisms that could give rise to these spatially distinct domains. The ability to control the photoinduced crystal deformation by manipulating crystal shape and solid-state reaction kinetics suggests that photoreactive molecular crystals may be useful for generating well-defined motions on small length scales.

Photoinduced bending behavior of cross-linked azobenzene liquid-crystalline polymer films with a poly(oxyethylene) backbone.

Cross-linked azobenzene liquid-crystalline polymer films with a poly(oxyethylene) backbone are synthesized by photoinitiated cationic copolymerization. Azobenzene moieties in the film surface toward the light source are simultaneously photoaligned during photopolymerization with unpolarized 436 nm light and thus form a splayed alignment in the whole film. The prepared films show reversible photoinduced bending behavior with opposite bending directions when different surfaces of one film face to ultraviolet light irradiation.

Model for Photoinduced Bending of Slender Molecular Crystals

The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity).

Photoinduced Bending Behaviour of Cross-linked Liquid Crystalline Polymers with High Cross-linking Densities

Highly cross-linked liquid crystalline polymer (CLCP) films with a long spacer and azobenzene chromophores (of varying concentrations and positioned differently) were prepared. The effects of various factors on the photoinduced bending behaviour of the highly CLCPs were explored. Long spacer groups and high cross-linking density resulted in considerably faster bending of the CLCP films relative to those reported in the literature that feature shorter spacer groups and smaller cross-linking densities. The azobenzene chromophores at the cross-linking points significantly influenced the photoinduced bending behaviour of the CLCP films relative to those at side chains. Furthermore, reduced concentrations of the azobenzene chromophores produced faster bending and larger photoinduced stress of the CLCP films that contained chromophores at the side chains as opposed to those incorporating chromophores at the cross-linking points. Moreover, the CLCP film with relatively low azobenzene concentration and small thickness exhibited bending in the reverse direction following maximum bending under continuous UV irradiation, thereby showing that photomobile behaviour of the material system.

Crystal Thickness Dependence of Photoinduced Crystal Bending of 1,2-Bis(2-methyl-5-(4-(1-naphthoyloxymethyl)phenyl)-3-thienyl)perfluorocyclopentene

The photoinduced crystal bending of a photochromic diarylethene derivative, 1,2-bis(2-methyl-5-(4-(1-naphthoyloxymethyl)phenyl)-3-thienyl)perfluorocyclopentene (1a), has been particularly investigated. The rodlike crystal of 1a shows reversible photoinduced bending upon alternating irradiation with ultraviolet (UV) and visible light. The photoinduced crystal bending can be repeated over 80 cycles. The rodlike crystal of 1a shows different bending behavior depending on the faces irradiated with UV light. This is ascribed to the molecular orientation viewed from the faces. Furthermore, we found that the bending speed depends on the crystal thickness, and the curvature change against the crystal thickness is well-fitted to Timoshenko’s bimetal model. These findings provide a new useful strategy to design for the photomechanical actuators.

Synthesis of Reactive Azobenzene Main-Chain Liquid Crystalline Polymers via Michael Addition Polymerization and Photomechanical Effects of Their Supramolecular Hydrogen-Bonded Fibers

A new and efficient strategy for obtaining a series of reactive azobenzene (azo)-containing main-chain liquid crystalline polymers (LCPs) is described, which involves the first design and synthesis of acrylate-type azo monomers with different length of flexible spacers and an amino end-group (in its trifluoroacetate salt form) and their subsequent Michael addition polymerization under mild reaction conditions. The resulting polymers showed rather high thermal stability, relatively low glass transition temperatures, a broad temperature range of smectic C liquid crystalline phase, and reversible photoresponsive behavior. The presence of secondary amino groups in the backbones of these azo main-chain LCPs not only made them highly reactive precursors for various new functional linear and cross-linked azo LCPs but also led to the formation of hydrogen-bonding interactions among their polymer chains. Supramolecular hydrogen-bonding cross-linked LCP fibers were directly fabricated by using the simple melt spinning method, which proved to have a high order of mesogen along the fiber axis and exhibit good mechanical properties, fast and reversible photoinduced bending and unbending behaviors, and large photoinduced stress (240 kPa) at close to ambient temperature as well as excellent photodeformation fatigue resistance.

Photoinduced Bending of a Large Single Crystal of a 1,2‐Bis(4‐pyridyl)ethylene‐Based Pyridinium Salt Powered by a [2+2] Cycloaddition

Rolled-up crystals: Photoinduced bending of large single crystals has been realized based on a simple organic small molecule (see picture). The bending process is accompanied with high visual fluorescence contrast, which is essential for remote detection of photomechanical work. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

PHOTOINDUCED BENDING UPON PULSED IRRADIATION IN AZOBENZENE-CONTAINING CROSSLINKED LIQUID-CRYSTALLINE POLYMERS

We have investigated the photoresponsive behavior of homogeneously aligned low-azobenzene-concentration crosslinked liquid-crystalline polymers. We compared the thermal contraction and photocontraction of the films, and show that upon UV irradiation, they bend efficiently towards the actinic light source. The photoinduced bending is studied both under continuous-wave irradiation and irradiation with nanosecond pulses. Pulsed irradiation revealed that the photomechanical response can take place within sub-millisecond time scale, and that even a single laser pulse can cause the sample to bend.

Location of the Azobenzene Moieties within the Cross-Linked Liquid-Crystalline Polymers Can Dictate the Direction of Photoinduced Bending.

We present a simple way to control the photoinduced bending direction of azobenzene-containing cross-linked liquid-crystalline polymers. By changing the location of the photoactive azobenzene moieties from cross-links to side-chains, the bending direction of the sample is reversed under identical irradiation conditions. In addition to providing a versatile route toward directionality control of the photoinduced macroscopic motions, this observation highlights the complicated nature of the photomechanical response of azobenzene-containing cross-linked liquid-crystalline polymers, showing that the photomobile behavior can be determined by seemingly small details on the materials design.

An Improved Photoinduced Model for Silicon Based Microcantilever

The paper presents an improved analytical model of the photoinduced bending of the silicon based microcantilever beam. The objective of the present work is to explain the bending of the microcantilever beam in terms of photoinduced as well as thermally induced effects. The thermal effect has been included in the model by incorporating phonon assisted recombination mechanism, which play essential role in the deposition of the thermal energy, by justifying the whole process within the beam. The proposed model successfully explains the strain generating processes in the beam, which was not explained in earlier model given by Guo et al. 1 The results obtain on the basis of proposed improved model shows that the thermally induced bending is a late process as compared to photoinduced generation effect. The result has been compared and contrasted with experimental results reported by Datskos et al. 2 Good agreement has been found between theoretical and experimental result.

Photodeformable CLCP material: study on photo-activated microvalve applications

A kind of photodeformable polymer material, crosslinked liquid-crystalline polymer (CLCP) incorporated with azobenzene moieties, is studied as microvalve membrane actuator for microfluidic system applications. In the photomechanical analysis, the photo-induced bending effect of such polymer film is equivalent to a moment Tm and an axial force Nm applied on it, which are related to irradiation time and intensity of the driving UV light. A linear elastic beam model is established to calculate the maximal force that the valve actuator can resist and to analyze the deformation at any irradiation time in FEA. An experimental setup is built to record the flow rate in one operate cycle under various irradiation or preload conditions. The open time can reach 8 s and the close time 6 s. The results show that CLCP films have great potential in microvalve applications in biological engineering, drug delivering, etc. It has advantages of wireless, remote control and green energy.

Preparation of Crosslinked Liquid-Crystalline Polymers by Electron-Beam Irradiation and Their Photoinduced Bending Behavior

We proposed a new approach to fabricate bilayer films with a crosslinked azobenzene liquid-crystalline (LC) polymer. The polyethylene (PE) substrates were coated with linear azobenzene LC polymers, and then the azobenzene layers were crosslinked by irradiation with electron beams (EBs). In the EB-irradiated films with ordered azobenzene moieties, exposure to UV light caused trans-cis isomerization and the change in alignment of azobenzene moieties. Bending of the bilayer films could be induced toward an actinic light source when they were irradiated from the side of the azobenzene layer, while they bent away from the actinic light source upon UV irradiation from the side of the PE layer. In both cases, the bent films reverted to the initial flat state by turning off the light, which could be repeated at room temperature.

Visible light induced bending and unbending behavior of crosslinked liquid-crystalline polymer films containing azotolane moieties

Monodomain crosslinked liquid-crystalline polymer (CLCP) films with three crosslinking densities were prepared by thermal polymerization of the mixtures of mono and diacrylates, both of which contain azotolane chromophores. Thermodynamic properties of the monomer, the crosslinker, their mixtures and the CLCP films were determined by differential scanning calorimetry. Mesomorphic properties were studied using a polarizing optical microscope and a polarized UV-vis absorption spectrometer. Due to a longer conjugated structure of the azotolane moieties in side chains, free-standing CLCP films underwent photoinduced bending upon exposure to short-wavelength visible light at 436 nm and the bent films returned to their initial states by alternating the wavelength of the actinic visible light to 577 nm. The bending could also be induced by 530 nm visible light though the bending speed decreased with the increasing of the wavelength of the actinic light. It was also observed that the bending speed of the films was related to the order parameter, the light intensity and the temperature. The mechanical force of the films generated upon exposure to visible light at 436 nm increased with the increasing of the crosslinking density and the light intensity but decreased with the increment of the temperature.

Can sunlight drive the photoinduced bending of polymer films?

Photoinduced bending and unbending behavior of crosslinked liquid-crystalline polymers containing azotolane moieties in side chains occurred upon irradiation with sunlight, according to the trans–cisphotoisomerization of the azotolane moieties.