January 26, 2022 feature
Superelasticity of a photoactuating chiral crystal
Superelasticity is an elastic response to an applied external force that occurs via phase transformation. The resulting actuation of the materials is an elastic response to external stimuli, including light and heat. While superelasticity and actuation are deformations resulting from stimulus-induced stress—a phenomenological difference exists between the two, depending on the force. In a new report now published in Communications Chemistry, Takuya Taniguchi and a research team in advanced science and engineering, data science, life sciences and materials science in Japan, described a molecular crystal that manifested superelasticity during photo-actuation under light irradiation. The crystal showed step-wise twisted actuation based on two effects, including photoisomerization and photo-triggered phase transition. They simulated the actuation behavior on a dynamic multi-layer model to reveal progressive photoisomerization and phase-transition in the crystal, while indicating superelasticity induced by modest stress as a result of photoproduct formation. The work provides successful simulations of step-wise twisted actuation to indicate superelasticity-induced by light.
Stress and strain are fundamental in material mechanics to characterize deformation behaviors of materials and structured objects. When an external force is applied to an object, depending on the mechanical properties, it deforms due to elastic or plastic responses, followed by fracture. In some instances, superelasticity leads to pseudo-deformation due to a phase transition resulting from the applied force. While the manifestation of superelasticity is well-known with shape memory alloys, molecular crystals have also shown superelasticity despite their rigid and fragile appearance. As a result, researchers are looking into the response of molecular crystals relative to the applied force, known as crystal adaptronics, with substantial curiosity. Such materials have shown the capacity to form an elastic response such as actuation when stress is induced to materials, by external stimuli, including light, heat, and electromagnetic fields. While conventional actuators contain hard materials, organic smart actuators are soft and flexible, with promising applications as soft robots, and wearable electronics. In this work, Taniguchi et al. showed that the molecular crystal of the enol-(s)-1 compound showed superelasticity during photoactuation and characterized the elastic properties and actuation performance of the material to introduce a dynamic multi-layer model and simulate stepwise twisted actuation via finite element analysis. The results revealed the progression of photoisomerization and photo-triggered phase transition (PtPT) in the crystal, to indicate superelasticity induced by internal stress of the photoproducts. The outcome successfully simulated the process of stepwise twisted actuation to provide the first indication of superelasticity induced by light.
Elastic properties and actuation performance
The scientists obtained thin plate-like crystals of enol-(s)-1 and determined the elastic modulus of the material from load-displacement curves by beam bending or compression depending on the loaded crystal face. They then calculated the elastic response and Young's modulus to determine the maximum strain and stress based on the elastic response. Young's moduli were much smaller on the side- and cross-section faces of the material, compared to the elastic property loaded on the top face. The strain and stress also reflected the anisotropy of the crystal structure to suggest the existence of weaker intermolecular interactions. The previously determined crystal structure of enol-(s)-1 formed two relatively stronger intermolecular interactions due to the layer-by-layer stacking architecture, where the anisotropy of the crystal structure was consistent with the observed elastic moduli. During the experiments, the team noted crystal deformation when irradiated by UV light, which could be divided into three steps. In the first step, bending occurred toward the light source due to photoisomerization. The second step was twisted due to the progression of photo-triggered phase transition and the third step simply led to bending towards the light source due to photoisomerization after its completion. Taniguchi et al. quantified step-wise actuation, which occurred within a few seconds of photoillumination and ceased after, to facilitate the stepwise relaxation of simple bending, twisted unbending, and simple unbending via back isomerization reactions to reverse propagate the phase transition. The team assessed the actuation performance of the material under UV light and determined the value to be similar to salicylideneamine crystals and some azobenzene crystals.
Manifestation conditions of photo-triggered phase transition (PtPT) and simulations.
Since the origin of PtPT manifestation was photoisomerization, characterization of the conversion ratio of photoisomerization was therefore important. To accomplish this, Taniguchi et al. used Fourier-transform infra-red measurements and identified the difference between the enol and trans-keto forms that occurred during intramolecular proton transfer. The estimates indicated how the PtPT started when the ratio of photoproducts were 1 percent in the crystal based on time-series behaviors. They then assessed the dependence of light intensity on the PtPT by irradiating the crystal at several intensities for the results to indicate phase transitions resulting from the effect of photoproducts. Taniguchi et al. next simulated the actuation behavior and indicated the superelasticity of the materials. The twist-bent deformation occurred by PtPT, at the onset of the photo-process in smaller crystals. They reproduced actuation behavior by simulating the material mechanics to regulate actuation and understand the underlying mechanisms. During simulations, the enol-(s)-1 crystal showed simple stepwise processes including simple bending, twisted bending and simple bending again. The team performed further investigations to understand the dependence of thickness of the material to clarify how photoisomerization and PtPT progressed. During all observations the team did not observe any local melt.
Outlook: Structure dynamics and the proposed mechanisms
The scientists further studied manifestation mechanisms of photo-triggered phase transition (PtPT) and dynamics of the crystal structure using diffracted X-ray blinking. The method allowed them to measure time-resolved X-ray diffraction images and analyze the time-series intensity at each pixel of specific diffraction via an autocorrection function to clarify dynamic changes in the crystal structure before and after exposure to UV light. The number of photoproducts increased under UV light up to a steady-state. The team credited the modest stress from the photoproducts to be crucial for super-elastic deformation at PtPT (photo-triggered phase transition). In this way, Takuya Taniguchi and colleagues developed a first-in-study investigation of superelasticity during actuation by light, to accomplish step-wise twisted actuation by simulating a dynamic multi-layer model to identify super-elastic behavior of phase transition in the enol-(s)-1 crystal. The work can contribute to the development of new mechanical materials.
Ray H. Baughman et al, Carbon Nanotube Actuators, Science (2002). DOI: 10.1126/science.284.5418.1340
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