Constructing an AIE Diphenylquinoxaline-Based Luminescent Europium-Organic Framework for Multiple Substance Sensing

Luminescent lanthanide metal–organic frameworks (Ln-MOFs) draw great interest due to their promising luminescent properties and wide applications in sensor design. In this work, a europium-organic framework (DPQ-Eu) has been synthesized under solvothermal conditions based on the aggregation-induced emission (AIE) ligand 4, 4′-(quinoxaline-2, 3-diyl)­dibenzoic acid (H₂DPQ). Structural analyses show that DPQ-Eu adopts the monoclinic C2/c space group and forms a 3, 6-connected 2D porous network with a topological symbol of {4²·6}₂{4⁴·6⁹·8²}. The formation of DPQ-Eu contributes an absolute quantum efficiency of 21.49% compared with that of the AIE ligand H₂DPQ (0.02%) mainly due to the coordination-induced emission, providing DPQ-Eu with a great potential of being a luminescent sensor. Fluorescent investigations show that DPQ-Eu has a selective and sensitive response to Fe³⁺ and Hg²⁺ among various metal ions by ratiometric emission changes of the decreased emission at 415 nm and increased emission at 676 nm, with the detection limits of 25.99 and 39.70 nM and good reusability (5 times). The sensing behaviors probably can be ascribed to the promoted antenna effect from the ligand to the Eu­(III) center by the coordination of Fe³⁺/Hg²⁺ to the quinoxaline moiety. For anion sensing, DPQ-Eu can respond to Cr₂O₇^2– by emission quenching at both 415 and 676 nm with a detection limit of 9.88 nM and 5 times reusability, probably associated with the energy competing absorption and reduced energy transfer from the ligand to the Eu­(III) center. In addition, DPQ-Eu shows ratiometric emission changes to CN^– by the emission enhancement and decrease at 415 and 676 nm, respectively, with a detection limit of 8.19 nM and five-run recycling tests, probably corresponding to the inhibition of energy transfer from the ligand to the Eu­(III) center caused by the binding interactions between the CN^– and Eu­(III) nodes.