Highly Stretchable and Luminescent Polymer Hydrogels\nCross-Linked by a Minute Amount of Water-Soluble Lanthanide Organic\nPolyhedra

Polymer\nnetworks cross-linked by the metal–organic polyhedra\nhave attracted dramatic attention in recent years due to the high\ndegrees of freedom in tuning their structures and functions. Particularly,\nthe 4f electrons from lanthanide ions have offered the lanthanide\norganic complex with unique magnetic and optical properties. Herein,\nfor the first time, we proposed a facile scheme to construct polymer\nhydrogels with superior stretchability, strong luminescence, and magnetic\nresonance relaxation properties via the incorporation of a minute\namount of lanthanide organic polyhedra as the cross-links. A bipyridyl\nbis-tetrazole ligand, bearing an alkene group, was synthesized for\ncoordination with Ln<sup>3+</sup> (Eu<sup>3+</sup> and/or Gd<sup>3+</sup>) ions in water to form lanthanide organic polyhedra complexes (K<sub>24</sub>Ln<sub>8</sub>L<sub>12</sub>), which were subsequently used\nas cross-linkers (0.0083–0.033 mol % of the monomer) for the\nfree radical polymerization with acrylamide monomers to obtain Ln-polyhedra\nhydrogels. Thanks to the large functionalities and the dynamic feature\nin lanthanide metal coordination complexes as the network junctions,\na tiny amount of cross-linker has rendered these hydrogels superior\nstretchability exceeding 4200% and tensile stress at break beyond\n400 kPa at a stretching rate of 100 mm/min. In addition, Eu<sup>3+</sup> and Gd<sup>3+</sup> polyhedra can offer hydrogels with a high luminescence\nquantum yield and efficient magnetic resonance relaxation properties,\nrespectively. In particular, Eu<sup>3+</sup> and Gd<sup>3+</sup> ions\ncan coassemble with the ligand to form K<sub>24</sub>Eu<sub><i>x</i></sub>Gd<sub>8–<i>x</i></sub>L<sub>12</sub> polyhedra, simultaneously rendering the cross-linked hydrogels superior\nstretchability, strong luminescence, and fast spin–lattice\nrelaxation rate, and thus a great potential for applications in both\nluminescent and magnetic resonance imaging.