Visible Light-Triggered Microactuators and Dynamic Surfaces from Hydrogels Containing Photoswitchable Host–Guest Complexes

Light-responsive hydrogels garner considerable interest due to their ability to undergo a wide variety of shape transformations that can be triggered remotely and with fine spatiotemporal control. To this end, photoswitchable supramolecular complexes offer a convenient route to tune hydrophilicity of hydrogels; however, thus far, this approach has required the use of potentially damaging ultraviolet light. To address this limitation, we introduce a light-responsive supramolecular hydrogel that can be addressed entirely with visible light using a hydrogel with pendant thiomethyl-substituted arylazopyrazole (SMe-AAP) in a solution of beta-cyclodextrin (β-CD). Upon irradiation with purple light, trans–cis isomerization triggers a swelling response in the hydrogels that is rapid, stable, and reversible under green light. Remarkably, this result suggests stronger binding of β-CD to the cis isomer, which is opposite to the behavior for most reported azo photoswitches. To shed further insights into this behavior, we characterize the binding between SMe-AAP and β-CD using calorimetry and 2D-NOESY NMR spectroscopy. Finally, we demonstrate that the SMe-AAP hydrogels can be used to fabricate functional materials, including micron-scale bending actuators and surface wrinkles with precisely patterned morphologies.