Construction Of Metal-imidazolate Cages And Their Photocatalytic And Photoluminescent Properties

Metal-organic cages(MOCs)are discrete supramolecular coordination compounds with internal cavities.They are widely used in gas storage and separation,molecular sensors,catalysis and drug carriers.In this paper,the application of cobalt-based metal-organic cages in photocatalytic water oxidation reaction and the photoluminescence properties of zinc-imidazolate cubic cages were studied.Water oxidizes to molecular oxygen is an essential challenge in the decomposition of water.In this work,inspired by the Mn₄Ca O₅ cluster in the natural photosynthesis system,a series of Co-based metal-organic cages for photocatalytic water oxidation were prepared.(MOC-1,[Co₂₀L1₁₂(OH)₁₂(H₂O)₄](Cl O₄)₈;MOC-2,[Co₈L2₆(H₂O)₆](BF₄)₆;MOC-3,[Co₈L3₆(NO₃)₆];MOC-4,[(Co₄^IIICo₄^IIL4₁₂Br₄)](Br)₃(NO₃).MOC-1 with both bis(?-oxo)bridged dicobalt and Co-O(O from H₂O),displays catalytic activity with an initial oxygen evolution rate of 80.4 mmol/g/h and a TOF of 7.49×10^-3 s^-1 in 10 min.In contrast,MOC-2 containing only Co-O(O from H₂O)in the structure results in a lower oxygen evolution rate(40.8 mmol/g/h,4.78×10^-3 s^-1),while the amount of oxygen evolved from the solution of MOC-4 without both active sites is undetectable.At the same time,isotope experiments with or without H₂¹⁸O as the reactant successfully demonstrated that the molecular oxygen was produced from water oxidation.Photophysical and electrochemical studies also further verified the catalytic activity of Co-based metal-organic cages.This proof-of-concept study indicates that metal-organic cages can work as potential efficient photocatalytic water oxidation catalysts(WOCs).In addition,a series of isostructural zinc-imidazolate cubic cages were designed and synthesized by subcomponent self-assembly.We used zinc chloride,5-Methyl-1H-imidazole-4-carbaldehyde and 4-R benzylamine(R=F,MOC-5;Cl,MOC-6;Br,MOC-7;I,MOC-8;CF₃,MOC-9)as raw materials,and the characterizations were carried out.It was found that they have good thermal stabilities and photoluminescence properties.The spectrometric results revealed these complexes have high quantum yields,MOC-5(42.7%)(27)MOC-6(45.8%)(27)MOC-8(64.9%)(27)MOC-7(82.7%)(27)MOC-9(92.2%).This results showed that the substituents of benzylamine had a significant effect on the photoluminescence properties of zinc-imidazolate cubic cages,providing a strategy for the development of high-performance luminescent materials.