A novel 3D luminescent Pb-CP based on a trifunctional group ligand for sensing heavy metal ions and nitrophenols

A novel trifunctional ligand H₄L (3,3′,3′',3′''-(11-(4-(1H-imidazol-1-yl)phenyl)-10,12-dihydrodiindeno[1,2-b:2′,1′e]pyridine 10,10,12,12-tetrayl)tetrapropionic acid) was synthesized through rational design using a multicomponent reaction. In addition, a new luminescent lead-based coordination polymer (CP-Pb) based on the ligand H₄L was synthesized and its structure was characterized by single-crystal X-ray diffraction. Two Pb atoms in CP-Pb are bridged to two COO^- groups to form a binuclear cluster. In addition, CP-Pb displays a 3D structure connected by 5-connected node, which can be simplified into a (4,6)-connected network with sqc topology. Furthermore, CP-Pb exhibit satisfactory sensitivity for the detection of Hg²⁺ and Cu²⁺ with low detection limits of 0.54 μM for Hg²⁺ and 0.63 μM for Cu²⁺, respectively. The possible fluorescence quenching mechanisms were systematically investigated through experiments and density functional theory. The strong interactions between Hg²⁺/Cu²⁺ ions and the uncoordinated pyridyl groups largely promote the intermolecular electron transfer process, thereby causing fluorescence quenching. While the sensing mechanism for nitrophenols was attributed to the co-existance of fluorescence resonance energy transfer (FRET), competitive absorption (CA), photo-induced electron transfer (PET) and the strong N‧‧‧H-O hydrogen bonding interactions between the CP-Pb and nitrophenols.