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.