Two AIE-Ligand-Based 2-D Luminescent Metal-Organic Frameworks as Fe³⁺ Sensors

The design and synthesis of high-performance sensors are very important but remain great challenges. In this work, a new aggregation-induced-emission (AIE) molecule 4,4'-(((9H-fluoren-9-ylidene)methylene)bis(4,1-phenylene))dipyridine (L) was successfully synthesized and first developed as a functional ligand to construct two isomorphic metal-organic frameworks (MOFs) [M(L)(OBBA)] [M²⁺ = Cd²⁺ (1), Co²⁺ (2); H₂OBBA = 4,4'-oxybisbenzoic acid]. They adopt [M₂(COO)₄] flywheel clusters, OBBA^2- bridges, and terminal L ligands as building units to form isomorphic 2-D networks with Lewis base active cites (uncoordinated pyridyl N). Both 1 and 2 exhibit excellent water, pH, and thermal stabilities and extremely efficient Fe³⁺ sensing abilities in the water environment. The quenching constants and detection limits reach the best levels reported so far. The sensing mechanism of 1 and 2 toward Fe³⁺ is studied in depth, and the difference in their sensing performance is also explained.