Synthesis Of Chiral Metal-Organic Frameworks By Post-Modification Strategy And Their Applications

Chiral metal-organic frameworks（CMOFs）are considered to be ideal platforms for enantioselective recognition and separation due to their diverse structure and rich chiral active sites.However,it is not easy to synthesize CMOFs by conventional solvothermal method under the constraint of many factors,and developing a simple and effective method to obtain CMOFs is still a long-term pursuit.In this thesis,post-synthetic modification（PSM）strategy was used to obtain CMOFs based on achiral MOFs through chiral covalent modification,and on this basis,selective sensing of chiral enantiomers were further explored.1.Two dimensional layered Eu-MOF was successfully prepared by solvothermal method,and the adjacent layers were interleaved byπ···πinteraction of 1,10-phenanthroline,thus forming a three-dimensional supramolecular structure.D-Cam was grafted onto the framework of Eu-MOF by post-synthetic modification strategy（PSM）to obtain chiral modified CMOF（Eu-MOF-Cam）.The IR spectrum showed that horseshoe-shaped double peaks at 3000-4000cm^-1due to the stretching vibration of N-H bond on amino group.Further,the double peaks merged into a single peak and the characteristic peak belonging to the C=O bond in the amide group appears at 1710cm^-1after PSM.At the same time,XPS spectra showed that the-NH₂ group on the ligand was transformed into a secondary amide group after modification.The CD spectrum shows obvious positive cotton absorption at 358 nm,indicating that chirality is transferred from D-Cam to the framework after PSM.It can be concluded that D-Cam was successfully modified by covalent binding.Then it was found that Eu-MOF-Cam exhibited selective fluorescence quenching effect on R/S-mandelic acid（R/S-MA）enantiomers through fluorescence sensing experiments.By fluorescence titration experiments,it is found that the former causes more obvious quenching when the same concentration of R-or S-MA was added.The K_sv values of R-and S-MA are 3.3×10³M^-1 and1.29×10³M^-1,respectively,by fitting the titration data with S-V equation.By comparing the K_sv values,the enantioselectivity factorαof Eu-MOF-Cam for MA is 2.56.2.UiO-66（Zr）-NH₂was successfully prepared by solvothermal method,and the amino groups on Ui O-66（Zr）-NH₂ frameworks were grafted with L/D-proline（L/D-Pro）respectively to obtain chiral modified CMOFs（Ui O-L/D-Pro）.The IR spectrum indicated that there were amide bonds formed in the MOF skeleton after PSM.In the CD spectrum,there is a mirror-symmetric circularly polarized absorption at 370nm,indicating that chirality was transferred from L/D-Pro to the MOF framework.Through the fitting analysis of XPS spectrum,new chemical environments were formed around N 1s after PSM,which was consistent with the results of infrared analysis.In addition,the formation of amide bond effectively improves the electron transport efficiency and the fluorescence intensity after modification is greatly enhanced.When L/D-phenylalanine（L/D-Phe）was added to Ui O-L/D-Pro suspension,the fluorescence response of various configuration substrates show differences.Ui O-L-Pro has better fluorescence enhancement effect on D-Phe,the association constants K_BH（D-Phe）and K_BH（L-Phe）are 311.5M^-1and 74.9M^-1,respectively,and theαvalue is 4.15.UiO-D-Pro showed better fluorescence enhancement on L-Phe,K_BH（D-Phe）and K_BH（L-Phe）were 107.1M^-1 and 226.2M^-1,withαvalue of 0.47.It is concluded that the chiral environment based on L/D-Pro plays a key role in the selective recognition process.3.MOF-1114（Tb）was successfully prepared by solvothermal method,and then L/D-tartaric acid（L/D-Tar）as a chiral source was introduced into the MOF framework by PSM to obtain chiral modified CMOFs（MOF-L/D-Tar）.It was proved by IR,XPS and CD spectra that L/D-Tar was grafted into the MOF framework in covalent modification after PSM,and the chirality was successfully transferred to the overall MOF framework.Fluorescence sensing experiments show that modified MOF-L/D-Tar can selectively quenching D/L-His fluorescence respectively.MOF-L-Tar can be better quenched by D-His,and the quenched constants K_sv（D-His）and K_sv（L-His）are 402.9M^-1 and 152.7 M^-1,respectively,and the enantioselectivity factorαis 2.64.MOF-D-Tar was better quenched by L-His,K_sv（D-His）and K_sv（L-His）are 77.1M^-1and 286.9M^-1,respectively,and the enantioselectivity factorαwas 0.27.The enantioselective recognition may be caused by the difference in binding force between CMOFs framework and chiral molecules,for the heterochiral guest,the relative complementary configuration can produce stronger hydrogen bonding,while the hydrogen bonding with the homochiral guest is weaker.