Photochemical Properties Of Coumarin-based Metal-organic Cage Architecture

Metal-organic cage are a class of supramolecular compounds formed by the coordinate self-assembly of metal ions and organic ligands.At present,metal-organic cage compounds are widely used in many fields,such as molecular recognition,active species stabilization and catalysis.Metal-organic cage can specifically recognize encapsulated guest molecules due to their controllable composition and unique internal cavity.Coumarin with excellent photophysical properties was introduced into organic ligands to synthesize a new metal-organic octahedral Zn-BTC.A total of 12 coumarin groups were uniformly distributed on the edge of the metal-organic cage,which was developed as a light funnel for photocatalytic E-Z isomerization of olefin and selective recognition of carbohydrate molecules.（1）Three organic ligands containing ONO tridentate ligands were designed and synthesized through Schiff-base reaction,and the topology of M₆L₄ metal-organic octahedron was obtained through coordinate self-assembly.The metal-organic octahedron as energy transfer catalyst was investigated for E-Z isomerization.The metal-organic octahedral Zn-BTC has sufficient cavity which allows the encapsultation of guest molecules.The results of UV-Vis spectrometric titration,fluorescence titration and isothermal calorimetric titration showed that Zn-BTC and guest trans-stilbene formed 1:1 host-guest complex.Under the optimum conditions,cis-stilbene was obtained by photocatalytic E-Z isomerization in CHCl₃solution with the concentration of Zn-BTC（5.0μM）and the concentration of trans-stilbene（0.1 M）,and the TON value could reach 6400.The efficiency of the supramolecular system was almost 16 times higher than that of the mononuclear complex Zn-BSC,indicating the advantage of the supramolecular system in photocatalytic energy transfermation.Free radical trapping agent（TEMPO）was added under the catalytic condition of Zn-BTC to probe the reaction mechanism.It was confirmed that the isomerization reaction was carried out in the intermediate state of free radical.The above experiment shows that coumarin as a photosensitizer can be applied to catalytic E-Z isomerization reaction through energy transfer process and can be extended to a variety of olefin derivatives.（2）Metal-organic octahedral Zn-BTC can selectively recognize glucosamine among ten kinds of saccharides via the output signal of fluorescence quenching.Fluorescence titration showed that the octahedral cage structure of Zn-BTC and glucosamine molecule could form a 1:2 inclusion complex with a binding constant of 1.6×10⁸ M^-2.Furthermore,NMR titration shows that Zn-BTC and glucosamine were indeed formed host-guest inclusion compounds through hydrogen bonds,and the amide group in the octahedral Zn-BTC was used as the hydrogen bonding acceptor.High-resolution mass spectrometry showed that octahedral Zn-BTC could form a 1:2 host-guest inclusion complex with glucosamine and was stable in solution.Under the same conditions,the ligand H₃BTC failed to recognize glucosamine,indicating that the formation of host-guest complex played an important role in the recognition process.