Coordinative Guest Recognition Triggers Macroscale Deformation of Covalently Linked Metal-Organic Polyhedra Polymer Gels.

Molecular recognition plays a central role in supramolecular chemistry. The concept of molecular recognition initially emerged from discrete molecular systems and has now been expanded to solid-state soft materials; however, there remains a lack of knowledge to elucidate the relationship between molecular recognition events and the modulation of macroscale properties. Here, we present a new type of polymer gel that discriminates structurally similar guests by utilizing coordination interactions. Our approach uses Rh(II)-based metal-organic polyhedra (MOPs) as both coordinative recognition sites and cross-linking nodes. By connecting MOPs with flexible, deformable poly(ethylene glycol)s, covalently linked MOP polymer gels are formed while preserving accessible coordination sites. The resulting MOP polymer gels exhibit macroscale shrinkage and swelling behavior in response to solvent exchange and guest molecule recognition. Specifically, ditopic coordinative guests induce significant gel shrinkage, whereas a structurally similar noncoordinative guest results in negligible volumetric changes. This difference highlights the capability of the MOP polymer gel to distinguish subtle structural differences of the guests at the coordinative recognition sites and to propagate the recognition events into macroscale shrinkage. Correspondingly, the storage modulus of the gel increases upon guest-induced shrinkage, which indicates that coordinative guest recognition reinforces the polymer network within the gel. These results establish a correlation between coordinative guest recognition and macroscale changes in the volumetric deformation and mechanical properties of the gels. This covalently linked MOP polymer gel system paves the way for designing stimuli-responsive, functional soft materials.