Structurally Diverse Zn/Cd Coordination Polymers for Recyclable Fluorescence Sensing of Fe3+ in Water

Abstract

Structurally diverse luminescent zinc/cadmium coordination polymers (LCPs) based on the π-conjugated bis-imidazole ligand bipmo were successfully constructed, namely, [Zn(bipmo)(NO3)2]n (1), [Cd(bipmo)(NO3)2]n (2), [Cd2(bipmo)(1,3-BDC)2(H2O)]n·2nH2O (3), and [Cd3(bipmo)2(1,4-BDC)3]n·2nCH3OH (4) (bipmo = bis(4-(1H-imidazol-1-yl)phenyl)methanone, 1,3-H2BDC = isophthalic acid, and 1,4-H2BDC = terephthalic acid). Owing to the tunable coordination behavior of the ligand and the synergistic effects of auxiliary ligands, these LCPs exhibit remarkable structural diversity, ranging from a 1D chain and two distinct 2D networks to an unprecedented 3D framework with a point symbol of {441·64}. LCPs 1 and 3 were identified as selective and recyclable fluorescent probes for Fe3+ in water, with detection limits of 7.13 and 7.19 μM, respectively. Comparative investigations reveal that structural diversity and the resulting accessibility of uncoordinated donor sites play key roles in determining the fluorescence sensing behavior, providing insight into the structure–sensing relationship in Zn/Cd coordination polymers. Furthermore, the fluorescence quenching originates from the synergistic effects of the inner filter effect (IFE) and static quenching through ground-state interactions between Fe3+ and the uncoordinated ketonic oxygen in 1 or the uncoordinated imidazole nitrogen in 3.