Photoluminescent polymer hydrogels with stimuli-responsiveness constructed from Eu-containing polyoxometalate and imidazolium zwitterions

Inorganic-organic co-assembly of anionic polyoxometalates (POMs) with zwitterions provides a facile way to fabricate functional soft materials. In this paper, a translucent, photoluminescent polymer hydrogel was fabricated from Weakley-type POM Na9EuW10O36 (EuW10) and polymerizable imidazole-type zwitterion 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) via a one-step synthesis method. Detailed characterization indicated that the polymerization of double bonds in VIPS and electrostatic interactions between EuW10 and VIPS play important roles in the formation of the hydrogels. Additionally, the introduction of non-polymerizable zwitterions 3-(1-methyl-3-imidazolio)propanesulfonate (MIPS) or 3-(1-decyl-3-imidazolio)propanesulfonate (C10IPS) can improve the mechanical and luminous performances of the hydrogels. Especially, C10IPS with a long alkyl chain would more significantly alter the coordination environment of EuW10, and consequently resulted in a more efficient energy transfer process. Further investigations revealed that the chemical environment around the Eu3+ can be highly influenced by organic solvents with stronger coordination abilities than water molecules, such as acetone. The translucency and luminescence intensity of the hydrogels can be reversibly transformed after alternately immersing in acetone or H2O for several minutes. Our results provided a useful strategy for the fabrication of luminescent hydrogels by regulating the noncovalent interactions between POMs and zwitterions.