Fabrication of Photoresponsive Crystalline Artificial Muscles Based on PEGylated Covalent Organic Framework Membranes

Seeking new photoresponsive materials with high energy conversion efficiency, good mechanical properties, as well as well-defined photoactuation mechanisms is of paramount significance. To address these challenges, we first introduced crystalline covalent organic frameworks (COFs) into the photoactuator field and created a facile fabrication strategy to directly install photoresponsive functional groups (i.e., acylhydrazone) on the skeletons of COFs. Herein, an approach to use polyethylene glycol (PEG) cross-linked dimers as the building blocks of the COF-42 platform was developed and afforded a series of uniform and freestanding membranes (PEG-COF-42) with outstanding mechanical properties (e.g., high flexibility and mechanical strength). Notably, these membranes possessed a fast mechanical response (e.g., bending) to UV light and good reversibility upon blue light or heating. After an in-depth investigation of the photoactuation mechanism via various techniques, we proposed a mechanism for the photoresponsive performance of PEG-COF-42: configurational change of acylhydrazone (i.e., E ↔ Z isomerization) accompanied by an excited-state intramolecular proton transfer (ESIPT) process intramolecularly transferring hydrogens from hydrogen donors (N-H) to hydrogen acceptors (oxygen in PEG). Moreover, attributed to the PEG moieties, PEG-COF-42 also demonstrated a vapor-responsive performance. This study not only broadens the application scopes of COFs but also provides new opportunities for the construction of multi-stimuli-responsive materials.