Supramolecular Catalysis And Photochemical Properties Of Porphyrin-based Metal-Organic Cages

The confinement effect has played important role in the catalytic activity and selectivity displayed by enzymes.An in-depth study of the feature and mechanism of substrate,electron and energy transfer in host-guest systems will be significant for both developing synthetically useful catalysts inspired by enzymes and understanding fundamental questions with regard to mechanisms of enzyme action.In this regard,metal-organic cages（MOCs）are good candidates for emulating the properties of enzyme active sites.However,most MOCs were lack of functional diversity and metal sites to active and bind the guests,which limited their applications in the field of enzyme mimics.Porphyrin compounds are of particular importance in catalysis due to the great stability of the aromatic porphyrin core,strong light capacity and Lewis acid or redox center from its metalated form.In this thesis,a series of M₈L₆ type cube cages were constructed by metalloporphyrin and dynamic active zinc（Ⅱ）ion,and their chemical properties could be modulated by introducing different central metal into porphyrins.The host-guest supramolecular systems with metalloporphyrin cage host and series of target guests have been employed to be supramolecular mimic of cytochrome P450,supramolecular catalyst and supramolecular photochemistry systems.And these supramolecular systems were applied for the models to investigate the effect of confinement effect on substrates,electron and energy transfer.（1）Two Lewis acid cubic cage Zn-1 and Zn-2 with different caviy sizes were assembled from Zn（Ⅱ）-porphyrin ligands（Zn-TAPP）,which were used to catalyze acyl transfer reactions between N-acetylimidazole（NAI）and various pyridylcarbinol（PC）regioisomers（2-PC,3-PC,and 4-PC）,aiming to inveatigate the effects of substrates transfer and preorganization in supramolecular catalysis:the results that the reaction of 2-PC was found to be highly efficient with high rate enhancements(k_cat/k_uncat=283 for Zn-1 and 442 for Zn-2),and there is little change in activity after five turnovers.A systemic investigation of the supramolecular catalysis occurring within these two hosts was performed,in combination with a host-guest binding study and density functional theory calculations（DFT）.The results all suggested that,besides the confinement effect caused by the inner microenvironment,substrate transfer,including the encapsulation of the reactant and the release of products,was considered to be a quite important factor in supramolecular catalysis within a molecular container.（2）A redox-active cubic cage Mn-1 with large cavity sizes was obtained through combination of Mn（Ⅱ）-porphyrin（Mn-TAPP）,2-formylpyridine and zinc（Ⅱ）ions.Metal-organic cube Mn-1 with included FMN showed the potential for reductive catalysis in the presence of nicotinamide adenine dinucleotide（NADH）,and was further worked as artificial models of cytochrome P450 enzymes,aiming to investigate the electron transfer in the host-guest system:this supramolecular system was capable of efficiently activating dioxygen and catalyzing the oxidation of benzyl alcohol.Control experiments suggested that the close proximity between FMN and Mn（Ⅲ）porphyrins forced by the host-guest interaction might benefit the electron transfer process from the FMN cofactor to the metal centers.This artificial system efficiently collected electron pairs from NADH and rapidly delivered single electrons to Mn（Ⅱ）-porphyrin,with 20-fold rate enhancement compared with the monmer in solution.（3）By introducing photoactive Pd（Ⅱ）-porphyrin（Pd-TAPP）as ligand backbone,the cubic cage Pd-1 with hydrophobic cavity was achieved through subcomponent self-assembly.The host-sensitizer Pd-1 formed stable 1:2 complexes with the guest-emitter perylene,and further investigated the machinism of the photoinduced energy and electron transfer between the host-guest complexation:the efficiency of triplet-triplet energy transfer was significantly enhanced,and was higher than that of monomer porphyrin L_Pd and perylene.However,the fluorescence efficiency of perylene was sharply low because of the formation of perylene excimer within the cage Pd-1 confined cavity.A supramolecular host-guest complex peryleneìPd-1 was used for photocatalytic proton reduction because of the strong host-guest interaction and the long fluorescence lifetime of perylene excimer.Under the same conditions,the complex peryleneìPd-1 could produce 4mmol H₂ during an irradiation of 24 h,which was 50 times more efficient than that of monomer palladium（Ⅱ）-porphyrin L_Pd.