Photochromic POM–Lanthanide Hybrids and Their Composite Hydrogels for Wearable Strain Sensing and Information Encryption

Abstract Organic–inorganic hybrid photochromic materials (OIHPMs) have attracted considerable attention owing to their rapid and reversible color changes upon light stimulation. In this paper, three Wells–Dawson-type P2W18-lanthanide–organic hybrid complexes (Ln = Eu, Ho, Nd) were hydrothermally synthesized: {[Eu(L)2(H2O)3][H3P2W18O62]} (1); {[Ho(L)2(H2O)3][H2P2W18O62]}·(L)·H2O (2); {[Nd2(L)4(H2O)6][P2W18O62]2}·(L)6·14H2O (3). They exhibit reversible photochromism upon UV irradiation, attributed to photoinduced electron transfer from POM oxygen donors to pyridine nitrogen acceptors. These complexes were incorporated into a sodium alginate/sulfobetaine methacrylate/polyacrylamide (SA/(AM-SBMA)) semi-interpenetrating hydrogel, yielding ASW composite hydrogels with enhanced mechanical properties and ionic conductivity. The hydrogels function as flexible strain sensors for real-time human motion monitoring. The Eu3+ complex shows characteristic red fluorescence dynamically quenched upon photochromism. The materials were further applied in inkless printing, QR code encryption, fingerprint anticounterfeiting, and photochromic PMMA films. This work provides a versatile strategy for multifunctional POM-based smart materials in wearable sensing and information security.