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

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)benozoic acid and 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 complex 3D structures based on different 1D Cd-(COO) or Cd-(COO)-(COOH) rod as SBUs, and 2 was viewed as complex 3D frameworks based on 1D Cd-(COO) rod as SBUs. MOFs 1 and 2 have good stability in boiling water, alkaline/acidic aqueous solutions (pH = 4 ~ 10).The adsorption measurement indicated that the adsorption capacity of 1 for CO2 and CH4 is greater than that of 2, meanwhile, the adsorption selectivity of 1 for CO2 is superior to that of CH4, which is attributed to the fact that 1 has a larger porosity and the characteristics of CO2, such as 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.