Coordination Mode-Regulated Lanthanide Supramolecular Hydrogels with Tunable Luminescence and Stimuli-Responsive Properties

Hydrogels with tunable luminescence and stimuli responsiveness have attracted wide interest, owing to the desire to design soft machines and/or wearable electronic devices. Yet, it is challenging to obtain those that possess multicolorimetric change properties when exposed to complex mechanical stimuli and are strong, elastic, and capable of self-healing. Strong polymers with adjustable and switchable mechanical and fluorescence properties were designed using fluorescent lanthanide (Eu/Tb) as a cross-linker. The resulting compounds contained dynamic coordination and hydrogen bonds forming the physical cross-linking network; sodium 4-(allyloxy)pyridine-2,6-dicarboxylate (SPD) and H2O ligands coordinated with Eu/Tb(III) centers through different coordination modes and interactions. Three SPDs coordinated with Tb(III), and two SPDs and three weaker H2O coordinated with Eu(III). As a result, Eu/Tb–ligand bonds with different degrees of cleavages showed changes in the fluorescence color when exposed to complex mechanical stimuli. Also, for the reversible formation and dissociation of the Eu/Tb–SPD complexes, the designed hydrogel showed potential application in human fingerprint sensing.