Enhanced organic dye adsorption and photocatalytic activity through a metal–organic framework featuring heptanuclear/binuclear clusters and cage cavities†

A caged metal–organic framework based on heptanuclear and binuclear copper clusters, namely [Cu₉(L)₁₀(H₂O)₂][Me₂NH₂⁺]₂ (Cu-MOF) (L = rigid 6-(1H-tetrazol-5-yl)-2-naphthoic acid) was synthesized and structurally characterized by a solvothermal method. Single-crystal X-ray diffraction shows that Cu-MOF displays a three-dimensional porous anionic framework and exhibits a cage-cavity structure. The tri-functional ligand uses two coordination modes to bridge different SBUs to form a dodecahedral cage. Since Cu-MOF possesses both anionic and porous frameworks, it could be used for efficient and selective absorption of cationic organic dyes. Further studies truly showed that both the charge and size of the dye molecules affect the absorption of Cu-MOF. Besides, thanks to the electronic configuration and wide range of visible light absorption properties of the heptanuclear copper clusters, Cu-MOF could be used as a photocatalyst for energy transfer from oxygen molecules to singlet oxygen, and successfully oxidized benzyl alcohol to benzoic acid in air under additive-free mild conditions. Therefore, rationally prepared Cu-MOFs, guided by crystal engineering, demonstrate potential for application as organic dye adsorption separators and photocatalysts, and the feasibility of preparing functionally tailored frameworks using tri-functional ligands consisting of tetrazolium and carboxylate acid is presented.