Organic–Inorganic Hierarchical Self-Assembly into Robust Luminescent Supramolecular Hydrogel

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com through the excitation of OH vibrations.[5] On the other hand, the mechanical properties of supramolecular gels typically suffer from the relatively weak noncovalent interactions among the constituent components.[6] This drawback also limits construction of free-standing objects from the luminescent gels.[7] Herein for the first time, we report on robust, self-healable luminescent hydrogels based on the selfassembly of metal–organic complexes and inorganic components into hierarchical nanostructures driven by metal coordination and electrostatic interactions. To address the problem of luminescence quenching in hydrogels, we designed and synthesized one functional organic ligand consisting of a terpyridine moiety and two flexibly linked 3-methylimidazolium bromide units (tpy-mim2). In our study, exfoliated Laponite nanosheets coated with sodium polyacrylate (ASAP) was employed as the inorganic components to improve the mechanical property of hydrogels,[8] while europium(III) (Eu3+) tri-thenoyltrifluoroacetone (Eu(TTA)3) was utilized as the emission precursor whose luminescence is usually quenched in the presence of ASAP.[9] Coordinating the precursor with the terpyridine moiety of ligand tpy-mim2 led to formation of an stable amphiphilic monomer Eu(TTA)3tpy-mim2 by overcoming the competitive coordination of ASAP and water. This amphiphilic monomer self-assembled into supramolecular micelles in water where the Eu3+ complexes were buried in hydrophobic microenvironment, thus maintaining the luminescence of Eu3+ ions and rendering the hydrogels radiative with a long lifetime. Association of Laponite nanosheets coated with oxyanions with the imidazolium salts on the surface of the supramolecular assemblies via electrostatic interactions resulted in formation of robust luminescent hydrogels (Scheme 1).[8a,10] It is worth noting that the surrounding rich noncovalent interactions among different components within the hybrid organic–inorganic hierarchical hydrogels give rise to their rapidly and quantitatively self-healing property.