Effects of Template and Molecular Nanostructure on the Performance of Organic–Inorganic Photomechanical Actuator Membranes

Abstract

Porous anodic aluminum oxide (AAO) membranes template the growth of photochromic crystalline nanowires. The resulting organic–inorganic composite can function as a photomechanical bending actuator. In order to investigate how the nanostructural properties of both the organic and inorganic components affect the photomechanical response, the composite mechanical properties are characterized using a variety of methods. There is a significant variation in both morphology and elastic modulus for two commercially available AAO templates with nominally identical pore diameters of 200 nm. After these templates are filled with diarylethene molecules that undergo a ring‐open to ring‐closed photoisomerization, the light‐generated curvature and mechanical work are evaluated using two different methods. The templates with a lower average elastic modulus (16 GPa vs 68 GPa) generate almost an order of magnitude more photomechanical work. The dependence of the photomechanical response on the chemical structure of the photochrome is assessed by comparing the performance of a diarylethene that undergoes a crystal expansion to that of one that undergoes a contraction, which leads to a decrease in curvature. Both the inorganic template and the organic active component play important roles in the overall photomechanical response, with substantial room to improve the performance.

Performance of Composite Glass–Diarylethene Crystal Photomechanical Actuator Membranes

“…Similar to the AAO templates, the glass templates exhibit strong light scattering, but some fraction of the incident light is transmitted to the other side, suggesting that light penetration will not limit the photochemistry. In a previous study of AAO templates, we found considerable variations in the surface pore morphology, even when comparing the top and bottom of a single template . This top–bottom asymmetry also led to an asymmetric elastic modulus for these templates, complicating any analysis of how the host mechanical properties influence the performance of the composite.…”

“…Although the hybrid molecular crystal approach to photoactuation is promising, it also faces some difficulties. Porous AAO templates are fabricated by electrochemical etching, and we found considerable structural variability among different templates, as well as a nonuniform pore diameter across the template . They also have a limited range of pore diameters and thicknesses .…”

Future‐Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

“…Subsequently, Bardeen et al further explored the effects that the nano-structural properties of the inorganic AAO template and organic DAE component had on the photomechanical response behavior. [257] Among the two commercially available AAO templates selected, the template with the lower average elastic modulus generated photomechanical work that was about an order of magnitude higher than the other. Adjusting the elastic modulus of the inorganic template was thus an effective way to improve the performance of the material.…”

Patterning submicron photomechanical features into single diarylethene crystals using electron beam lithography

“…35 To create macroscopic actuator structures based on this photochromism, we recently developed a bottom-up approach that assembled crystalline diarylethene nanowires inside a porous ceramic host material. 36,37 This approach was successful in generating useful mechanical motion, e.g. tilting mirrors, but variations in the template quality and filling made the production of reproducible samples a challenge.…”