UC Riverside

Photomechanical actuators are smart materials that transform photon energy into mechanical energy without any wired connections. Photonmechanical molecular crystals have shown superior performance on nanoscopic and microscopic scales due to their high elastic moduli, high-energy density and fast response time compared with polymer actuators. However, the development of macroscopic crystal actuators that can be used in the real world has yet to be demonstrated.

The work in this dissertation solves the problem by organizing photomechanical nanocrystals in porous inorganic templates Al2O3. It combines the best of both worlds where it is processable like polymers but it also attains high performance of organic crystals. 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, a diarylethene (DAE) derivative that undergoes a unimolecular photoisomerization between its ring-open and ring-closed isomers was used as the active organic element. The isomers are thermally stable but can be switched back and forth using UV and visible light for hundreds of cycles. Very small (sub-mg) amounts of material can move masses that are 105 times larger without being physically connected to an external power source. A light-to-work conversion efficiency for a molecular crystal photomechanical actuator has been for the first time quantitatively estimated, providing a starting point for future optimization.

Additional studies investigated how both the organic and inorganic components could affect the photomechancial response. Two alumina templates with different morphology and elastic modulus were compared. The template with a lower average elastic modulus generate about one order of magnitude more photomechanical work. Two diarylethene derivatives with different crystal deformations were studied to determine the dependence of photomechanical response on chemical and crystal structure.

The hybrid approach was then extended to porous SiO2 membranes with variable thicknesses in order to improve optical penetration and reproducibility. The photon-induced bending could be repeated for 150 cycles without loss of performance which suggests good fatigue resistance. The thickness dependence of the performance was studied, revealing thinner templates gave larger displacements. This work demonstrates that the hybrid method for making photomechanical actuators on macroscopic scales can be extended to other host materials and template geometries without sacrificing performance.