Supramolecular Mechanophore Additives Enable Mechanochromic Hydrogels

Mechanochromic hydrogels that report deformation through optical signals require mechanophores that function in aqueous environments, are activated at low force, and allow integration into the hydrogel matrix. Here, we report a new strategy to address these challenges that involves deploying a supramolecular mechanophore as a water-soluble macromolecular additive. The mechanophore developed for this purpose features a loop-forming architecture, in which a naphthalene diimide (NDI) quencher and a 9,10-bis(phenylethynyl)anthracene (BPEA) fluorophore are connected by a short spacer. Intramolecular charge-transfer interactions between these moieties induce loop formation and quench the BPEA emission. Mechanical deformation unfolds the loop, disrupts the charge-transfer interaction, and increases the BPEA emission. The functionalization of this loop-forming motif with poly(oligo(ethylene glycol) methyl ether methacrylate) (pOEGMA) affords a macromolecular water-soluble additive that can simply be blended into different hydrogel matrices, including cross-linked poly(acrylamide) and pOEGMA. These hydrogels exhibit a pronounced increase in emission intensity upon swelling, stretching, and compression. Spectroscopic investigations reveal correlations between the applied deformation and the optical response and show that the mechanophore activation is fully reversible. The results establish macromolecular supramolecular mechanophores as a general strategy for imparting reversible mechanochromism to hydrogels.