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

Abstract

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.