Light-Triggered Switching of Metallosupramolecular Polymer Systems

Metallosupramolecular polymers (MSPs) are formed through the formation of coordination complexes between monomers that contain multiple ligands and suitable metal salts. The assembly of MSPs is generally dynamic and reversible, which leads to stimuli-responsive materials and enables functions such as healing or recycling. Heat is arguably the most widely employed stimulus to manipulate MSPs, but the level of control that can be achieved is limited. Here, we report light-responsive MSP systems, whose response is based on an opto-chemical transduction principle. We combined the photoacid generator 2-(4-methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine (MBTT) with poly(acrylates) that comprise a few mol % of the 2,6-bis(1'-methyl-benzimidazolyl)pyridine (Mebip) ligand. The latter forms supramolecular cross-links upon the addition of metal salts, such as Zn²⁺, Eu³⁺, and Cu²⁺. We utilized titration experiments, optical spectroscopy, and rheology on model compounds and polymer systems to demonstrate that the MSP network can be rapidly disassembled upon optical activation of the photoacid generator, on account of protonation of the ligand and dissociation of the ML complex. Optorheological experiments reveal that the rheological properties of gels based on the MSP network, MBTT, and chlorobenzene can be drastically altered in an on-demand fashion by exposure to UV light.