Construction Of Luminescent Covalent Organic Framework And Its Application In Arsenic/Uranium Detection

Covalent organic frameworks（COFs）are a class of periodic porous crystalline materials,consisting of organic building units spliced by covalent bonds.Due to their pre-designable molecular types,highly flexible topology,high porosity and large specific surface area,COFs become attractive candidates for a vast variety of applications.Imine-linked COFs have extendedπ-conjugated properties and uniformly distributed active sites,which bind with toxic ions such as arsenic and uranium,demonstrating great potential in environmental detection.However,most of the COFs that linked by imine bonds are weakly luminescence or nonluminescence,which apparently limits the application of imine-linked COFs in promising optical fields.Therefore,improving the luminescence effects of imine-linked COFs is of great importance for further expansion of its application in environmental analysis.This paper mainly focuses on the preparation of novel fluorescent luminescent systems and electrochemiluminescent systems and their applications in detecting arsenic ion and uranyl,The main research contents are as follows:1.Construction of uranyl fluorescence probes based on Eu（III）-grafted covalent organic frameworks.The covalent organic framework（TFPB-Bpy）with C=N linkage was synthesized by Schiff-condensation reactions carried out between monomer1,3,5-tris（4-formylphenyl）benzene（TFPB）and monomer 5,5’-diamino-2,2’-bipyridine（Bpy）.Based on a large number of nitrogenous sites in the framework,Eu³⁺was further grafted onto the COF skeleton,and a two-dimensional COF hybrid material(Eu³⁺@TFPB-Bpy)was successful preparation,in which not only maintained the stability of organic ligand,but also endowed excellent optical properties due to the introduction of Eu³⁺.The fluorescence resonance energy transfer（FRET）between Eu³⁺@TFPB-Bpy and uranyl ion(UO₂²⁺)led to the fluorescence enhancement of Eu³⁺@TFPB-Bpy.With the increased of UO₂²⁺concentration,the fluorescence of Eu³⁺@TFPB-Bpy increased proportionally,a novel Eu³⁺@TFPB-Bpy-based fluorescence“turn-on”sensor was constructed for UO₂²⁺determination,and the detection limit was 1.7 n M,and the linear relationship is in a wide range（0.005～5μM）.The high sensitivity and selectivity of Eu³⁺@TFPB-Bpy to UO₂²⁺in aqueous phase provides a new thought for the design and synthesis of rare earth COFs hybrid materials and its application in environmental analysis.2.Structural isomerism of covalent organic frameworks with different electrochemiluminescence effects and its application for detecting arsenic.COFs isomers were synthesized by Schiff-condensation reactions and Povarov post-modification reactions,which are different in the quinoline linkages positions（TFPB-BD（OMe）₂-H and TAPB-BD（OMe）₂-H）,TFPB-BD（OMe）₂-H possessed significantly superior photoelectrochemical properties and electrochemiluminescence performance than TAPB-BD（OMe）₂-H.Photoelectric tests and DFT calculation results demonstrated that the isomeric configurations of isomeric COFs in the framework led to significant structural polarity differences,affecting carrier transfer and electronic communication behavior.TFPB-BD（OMe）₂-H with a stronger polar interaction drove the separation of excited carriers,which led to a smaller energy difference between the excited and ground states,thereby more favoring for the formation of electrochemiluminescence（ECL）excited states.Also,the ordered conjugate skeleton of COFs provided high-speed charge transport channel for the transport of electrons.In addition,an ECL probe based on TFPB-BD（OMe）₂-H was prepared to achieve the detection of toxic As（V）,the detection limit was as low as 0.33 n M,with a good linear relationship between 0.001 and 5μM.The ECL probe has good practicability for the detection of As（V）in water samples,and the recovery rate is 93.2%～98.5%.These findings provide guidance on regulating COFs performance from the perspective of linkage orientation and broadening the application of ECL technology in the determination of environmental pollutants.3.Cyclization imine-based covalent organic framework for enhanced electrochemiluminescence performance.Imine-linked covalent organic framework（TFPB-BD）wassynthesizedbycondensationreactionof1,3,5-tris（4-formylphenyl）benzene（TFPB）with 4,4’-diaminobiphenyl（BD）.Due to the rotational instability of the imine bond,the energy may be excited through a non-emission dissipation pathway,making TFPB-BD emit poorly.Phenylethynyl and4-ethynylbenzonitrile were introduced into TFPB-BD framework via Povarov reactions,namely TFPB-BD-H and TFPB-BD-CN,respectively.Compared to TFPB-BD,both TFPB-BD-H and TFPB-BD-CN significantly improved the ECL performance after transforming imine to quinoline.Moreover,the ECL signal of TFPB-BD-CN,which had strongly electronegative side group,was three times higher than that of TFPB-BD-H.In this paper,the covalently locked imine bond improves the degree of planar conjugation along the skeleton and eliminates the nonradiative energy loss,achieving the enhancement of ECL.In addition,the introduction of different molecules enables further access to the effect of the push-pull electron interaction of the side chain on the optical properties of materials and charge transport and thus modulates the intensity of ECL,developing molecular structure design strategy for obtaining tunable ECL signals from nonluminescent imine-linked COFs.