Design,Synthesis And Fluorescence Properties Of Moisture-resistant Zn₄O-based Metal-organic Frameworks Based On Substituted Thiophene Dicarboxylic Acid Ligands

Zn₄O-based metal-organic frameworks(MOFs)are characterized by high specific surface area,adjustable porous structure and easy functionalization.They have promising application in many fields such as molecular selective absorption and separation,heterogeneous catalysis,small molecule sensing,drug delivery and so on.However,most of the Zn₄O-based MOFs are sensitive to water molecules and unstable in the environment.The previous studies in our group showed that covalent modification of hydrophobic aromatic groups onto carboxylic acid ligands to cover water-sensitive Zn₄O structural units can significantly improve the stability of such structures.Encouraged of those results,we tried to introduce other fluorescent conjugated aromatic groups into the structure to assemble Zn₄O-based MOFs with good moisture resistance and fluorescence.The following is the experimental results and conclusions:1.Design and synthesis of 3-methyl-4-(naphthalen-2-yl)thieno[2,3-b]thiophene-2,5-dicarboxylic acid(L₁).The target compound Zn₄O(L₁)₃(MOF-1)was successfully synthesized by the reaction of zinc nitrate under solvothermal conditions.MOF-1 has a good porous structure with a specific surface area of 1102.5m²g^-1.PXRD data showed that the basic framework of MOF-1 remains intact and showed good moisture resistance when it was placed in an atmosphere with an average relative humidity of about 60%for 7 days.The fluorescence properties of MOF-1 showed that MOF-1 has blue-violet emission,and its fluorescence properties was applied to detect DCN molecules in solution.2.Design and synthesis of 3-(9-ethyl-9H-carbazol-3-yl)-4-methylthieno[2,3-b]thiophene-2,5-dicarboxylic acid(L₂).The target compound Zn₄O(L₂)₃(MOF-2)was successfully synthesized by the reaction of zinc nitrate with zinc nitrate under solvothermal conditions.MOF-2 has a good porous structure with a specific surface area of 1287.7 m²g^-1.PXRD analysis showed that the introduction of carbazole group makes MOF-2 have good moisture resistance,even water stability,and kept it in water for at least one week.Fluorescence measurements showed that it has pure blue emission,and its fluorescence properties are expected to be applied to detect the presence of nitro compounds in solution.3.Design and synthesis of 3-(4-(diphenylamino)phenyl)-4-methyl-thieno[2,3-b]thiophene-2,5-dicarboxylic acid(L₂).The target compound Zn₄O(L₂)₃(MOF-3)was successfully prepared by the reaction of zinc nitrate with zinc nitrate under solvothermal conditions.MOF-3 has a good porous structure with a specific surface area of 1102.5m²g^-1.PXRD analysis showed that the introduction of triphenylamine group makes MOF-3 moist,but its stability is slightly worse than that of MOF-1-2,which is due to the tension and structural defects caused by steric resistance.Fluorescence measurements showed that MOF-3has green emission,and its fluorescence properties and the ability to detect nitro compounds had been preliminarily studied.