Template-directed assembly of two cadmium diphosphonates with or without the luminescent sensing ability for Hg2+

By using 2, 2'-biimidazole (2, 2′-biim) and 2, 2′-bipyridine (2, 2′-bipy) as structure-directing agents (SDAs), two cadmium diphosphonates, [Cd₂(H₂L)₂(H₂O)] (1) and [Cd₅(HL)₂(H₂L)₂]·4H₂O (2) (H₄L = 4–F–C₆H₄CH₂N(CH₂PO₃H₂)₂), have been hydrothermally synthesized. The introduction of different N-heterocyclic aromatic SDAs results in the diverse structures, thermal stabilities, and luminescent properties. Structural analyses reveal that both 1 and 2 show three-dimensional (3D) supramolecular structures wherein the adjacent layers are further connected by moderately π–π stacking interactions or hydrogen-bond interactions. Among them, the thermal stability of 1 is higher than 2. Surprisingly, 1 displays excellent luminescent performance, whereas 2 shows no luminescent signal. Based on the intense luminescence of 1, it can serve as a high selectively and sensitively luminescent sensor for Hg²⁺ with a low detection limit of 3.1 × 10^–7 M in aqueous solution. Additionally, the probable sensing mechanism for 1 has also been investigated in detail, which can be assigned to the weak interaction between the uncoordinated O sites from the H₂L^2– ligands and Hg²⁺.

Graphical abstract

Two cadmium diphosphonates with different structures and luminescent properties have been hydrothermally obtained by using N–heterocyclic aromatic (2, 2′–biim and 2, 2′‒bipy) compounds as SDAs. 1 displays excellent luminescent performance, whereas 2 shows no luminescent signal. Furthermore, 1 can serve as a luminescent sensor for Hg²⁺ in water