Construction of a three-state responsive framework from a bistable photoswitch

The integration of photoswitchable elements into porous solids provides fascinating opportunities to modulate material properties. However, the fabrication strategies of these materials are dedicated only to preserving the function of the photoswitches in the solid while introducing new properties beyond those offered by the individual components remains challenging. Here, we present a three-state porous aromatic framework constructed from a bistable overcrowded alkene-based photoswitch. The framework was fabricated using a Yamamoto coupling polymerization of an engineered hexadentate monomer to yield swellable and hierarchical microporous/mesoporous architectures with densely integrated photoswitches. The interplay of hierarchical porosity, flexible backbone, and reversible photoisomerization between two isomers generates three unique and distinct porosity states that can be accessed in sequence upon application of external stimuli to induce sponge-like behavior. This material represents a major step toward light-responsive materials capable of harnessing limited molecular-scale motion and converting it into an on-demand response over hierarchical length scales toward a practical output.