Eu³⁺-Directed Supramolecular Metallogels with Reversible Quadruple-Stimuli Response Behaviors

Smart luminescent materials that have the ability to reversibly adapt to external environmental stimuli and possess a wide range of responses are continually emerging, which place higher demands on the means of regulation and response sites. Here, europium ions (Eu³⁺)-directed supramolecular metallogels are constructed by orthogonal self-assembly of Eu³⁺ based coordination interactions and hydrogen bonding. A new organic ligand (L) is synthesized, consisting of crown ethers and two flexible amide bonds-linked 1,10-phenanthroline moieties to coordinate with Eu³⁺. Synergistic intermolecular hydrogen bonding in L and Eu³⁺-L coordination bonding enable Eu³⁺ and L to self-assemble into shape-persistent 3D coordination metallogels in MeOH solution. The key to success is the utilization of crown ethers, playing dual roles of acting both as building blocks to build L with C₂-symmetrical structure, and as the ideal monomer for increasing the energy transfer from L to Eu³⁺'s excited state, thus maintaining the excellent luminescence of metallogels. Interestingly, such assemblies show K⁺, pH, F^-, and mechano-induced reversible gel-sol transitions and tunable luminescence properties. Above findings are useful in the studies of molecular switches, dynamic assemblies, and smart luminescent materials.