Two tetranuclear Cd-based metal–organic frameworks for sensitive sensing of TNP/Fe3+ in aqueous media and gas adsorption

Abstract

Two novel 3D Cd-MOFs, namely {[Cd₄(HDDCP)₂(4,4′-bibp)₂(H₂O)₂]·2.5(DOA)·1.5(H₂O)}_n (1) and {[Cd₂(HDDCP)(1,4-bib)(H₂O)]·H₂O}_n (2), where H₅DDCP = 2,4-di(2′,5′-dicarboxylphenyl)benzoic acid, 4,4′-bibp = 4,4′-bis(imidazolyl)biphenyl and 1,4-bib = 1,4-bis(1-imidazoly)benzene, have been solvothermally synthesized and structurally characterized by single crystal X-ray diffraction, elemental analysis (EA), thermogravimetric analysis (TGA) and infrared (IR) analysis. The structural analysis showed that 1 should be regarded as a complex 3D structure based on different 1D Cd–(COO) or Cd–(COO)–(COOH) rods as SBUs, and 2 was viewed as a complex 3D framework based on 1D Cd–(COO) rods as SBUs. MOFs 1 and 2 have good stability in boiling water, and alkaline/acidic aqueous solutions (pH = 4–10). The adsorption measurements indicated that the adsorption capacity of 1 for CO₂ and CH₄ is greater than that of 2, while the adsorption selectivity of 1 for CO₂ is superior to that for CH₄, which is attributed to the fact that 1 has a larger porosity and CO₂ has characteristics such as a linear structure, larger polarizability and significant quadrupole moment. Furthermore, luminescence studies revealed that both 1 and 2 can be used as a highly selective and sensitive sensor for sensing TNP or Fe³⁺ ions in aqueous solution or soil samples, and the fluorescence quenching mechanism was explored in this work.