Three Wells–Dawson/LanthanideHybrids: FromPhotochromic Crystals to Multifunctional Ionic Hydrogels for WearableSensors and Inkless Printing

Stimuli-responsive materials have attracted widespread interest due to their broad application prospects in fields such as optical anticounterfeiting, sensor technology, and information storage. In this paper, three Wells–Dawson-type polyoxometalate/lanthanide-based hybrids were hydrothermally synthesized: {[Sm2(L)4(H2O)6][H4P2W18O62]2}·(L)2 (1),{[Dy(L)2(H2O)3] [H2P2W18O62]}·(L)·2H2O (2), and {[La2(L)2(H2O)12][P2W18O62]2}·(L)6·2H2O (3). The three complexes all displayed reversible photochromism under xenon light irradiation. Mechanistic studies (EPR, XPS) reveal that the color change is due to electron transfer from the oxygen atoms of the POM framework to the pyridine nitrogen atoms of the viologen-based ligand. The complexes were incorporated into polyacrylamide/ionic liquid (PAM/PIL) hydrogels, displaying excellent mechanical performance with a distinct lanthanide-dependent trend (La > Sm > Dy), high stretchability (874% strain), conductivity, and reversible photochromism. The hydrogel demonstrates accurate sensing of human motion, such as finger bending, wrist movement, and facial expressions, pointing to potential use in electronic skin. In contrast, the La-based coating on filter paper has been used for inkless printing of patterns, QR codes, and fingerprint-based anticounterfeiting. This combined strategy holds promise for responsive materials in optical and flexible electronics.