Photo-induced Charge Transfer Of MOFs And Their Application In Organic Transformations

Metal-organic frameworks(MOFs)are a kind of crystalline porous materials formed by inorganic metal nodes(called secondary building units,SBUs)and organic ligands.Due to the variety of metal clusters and bridging ligands and the diversity of connecting ways,MOFs have rich pore structures,high specific surface areas,and designability.These materials have wide applications in a large variety of fields such as gas storage and separation,sensing,drug delivery,catalysis and etc.Among them,heterogeneous catalysis has a huge development prospect.In terms of catalytic applications,MOFs have the following advantages:i)the permanent pores of MOFs can promote the transport of substrates and products;ii)the evenly dispersed and high-density catalytic sites can increase the catalytic efficiency;iii)the tunability of the porous structure provides a versatile and tunable microenvironment for the catalytic sites;iv)metastable homogeneous catalysts can be stabilized in the porous frameworks by means of solid loading or confinement to avoid self-polymerization and inactivation;v)The solid catalysts are easy to recycle,maintaining catalytic efficiency;vi)The well-defined crystal structure and pore surface/internal isolated catalytic sites in MOFs facilitate the understanding of the catalytic mechanism and the interaction of the substrate molecules.The composition of metals nodes and organic ligands gives the MOF materials semiconductor-like behavior that can be used for photocatalysis.Many different processes may occur in MOFs under light irradiation,such as ligand-to-metal charge transfer(LMCT),metal-to-ligand charge transfer(MLCT),and delocalized ligand?-?*transitions.However,in previous reports,the detection of the charge separation process in the photocatalysis of MOFs was still lacking,and only one half of the reaction intermediate was detected,usually an electron-reducing metal ion such as Ti3+,whereas a cation radical(for example,the excited ligands)is still undetectable due to the short life of the excited state.Therefore,the collection of the intermediates from both electrons and holes,which still remains a challenge in the photocatalysis of MOFs,benefits the understanding the charge separation process in MOFs.Based on the recent works in our laboratory,this paper develops ligand-functionalized MOFs,sulfonate-functionalized MIL-101-SO3H and porphyrin-based MOF PCN-222 to study photocatalytic organic tranformations,elucidating the charge-separation process of porphyrinic MOF under light irradiation and the effect of LMCT process on the enhancement of sulfonic acid-functionalized MOF.The main research results obtained in this paper are as follows:1.Porphyrinic MOF(PCN-222)can efficiently catalyze the oxidative coupling of different amines under visible light in an open atmosphere.In addition to the 1O2 generated by porphyrin ligands based on energy transfer process,which can directly perform the reaction,the synergetic effect between electron and hole in charge transfer process greatly promotes the reaction.Therefore,PCN-222 exhibits excellent photocatalytic activity,selectivity and cyclability,far superior to corresponding TPPCOOMe ligand where only the energy transfer process exists.It was confirmed by ESR tests for the first time that photo-excited PCN-222 generates oxygen-centered active sites on Zr-oxo clusters at the reduction end and porphyrinic cation radicals at the oxidation ends,respectively.For the first time direct evidences of charge separation process can be provided to support semiconductor-like behavior of MOFs.Moreover,the intermediate products in which electrons and holes participate have been confirmed by the ESR tests,and it is clear to understand the reaction mechanism based on the charge transfer process.This work not only provides a profound understanding of the charge transfer/separation in MOFs,but it will also further stimulate the research on photocatalytic organic transformations of MOFs.2.The sulfonate functionalized MOF,MIL-101-SO3H,as a solid Br(?)nsted acid catalyst,catalyzes a variety of acid catalyzed organic reactions.In particular,we have demonstrated that its activity is significantly enhanced under illumination and even exhibits higher activity than the conventional strong acid H2SO4.To the best of our knowledge,this is not only the first work on the promotion of acid-related activities of MOFs under light irradiation,but also the first work where the activity of Br(?)nsted acid is higher than that of inorganic strong acids.The superior catalytic activity is due to the Lewis acidity,the enrichment of the substrates by the MOF pores,the enhanced proton release by light-induced electron transfer and the proton aggregation inside the MOF cage,promoting the acid catalytic activity simultaneously.It is exciting that this result will pave the way for the development of light-enhanced catalysis by other materials in many industrial processes.