Photoswitchable sol-gel transitions and catalysis mediated by polymer networks with coumarin-decorated Cu24L24 metal-organic cages as junctions.

Photoresponsive materials that change in response to light have been studied for a range of applications. These materials are often metastable during irradiation, returning to their pre-irradiated state after removal of the light source. Herein, we report a polymer gel comprising poly(ethylene glycol) star polymers linked by Cu24L24 metal-organic cages/polyhedra (MOCs) with coumarin ligands. In the presence of UV light, a photosensitizer, and a hydrogen donor, this "polyMOC" material can be reversibly switched between Cu-II, Cu-I, and Cu-0. The instability of the MOC junctions in the Cu-I and Cu-0 states leads to network disassembly, forming Cu-I/Cu-0 solutions, respectively, that are stable until re-oxidation to Cu-II and supramolecular gelation. This reversible disassembly of the polyMOC network can occur in the presence of a fixed covalent second network generated in situ by copper-catalyzed azide-alkyne cycloaddition (CuAAC), providing interpenetrating supramolecular and covalent networks.