Photomechanical crystals are an promising class of smart materials with the ability to convert absorbed light energy into mechanical work. While the majority of photomechanical crystals are categorized as either T-type (thermally reversible) or P-type (photochemically reversible), a novel subset has recently emerged, exhibiting the unique capacity to sustain continuous motion through a feedback loop when exposed to light. These peculiar photomechanical crystals, although relatively scarce, hold significant promise for applications in nanomedicine and microrobotics. By strategically selecting the photochemical engine driving molecular motion, crystal engineering, and controlling crystal morphology, we have successfully expanded the repertoire of continuously actuating light-powered photomechanical crystals, thereby opening up new avenues for their potential utilization. One particularly promising application for these crystals involves their evolution as active matter components. For instance, for derivatives of compound A, long crystalline microwires (ca 1 mm thick) subjected to UV (365 nm) or visible light continuously undergo flexing, bending, and translational motion while suspended in an aqueous medium. When these microwires are combined in suspension, they exhibit autonomous collective motion under UV light irradiation, forming larger superstructures that move collectively. This cooperative behavior among the microwires bears resemblance to the dynamics observed in active matter systems. Furthermore, derivatives of compound B, forms flat crystalline microribbons upon precipitation from an aqueous surfactant solution. When illuminated with visible light, these microribbons display intricate and continuous three-dimensional motion, reminiscent of “jellyfish” motion which can be finely modulated by adjusting light intensity (figure-1). Notably, this motion exhibits remarkable endurance, showing no signs of fatigue. In addition to these examples, we will provide a brief overview of their synthesis and discuss other noteworthy derivatives that illustrate light-powered continuous actuation.

Christopher Bardeen

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