Preparation And Properties Of Functional Covalent Organic Frameworks

Covalent organic frameworks（COFs）have the advantages of controlled structure,functionalization,porosity,crystallinity and high stability,and have shown excellent applications in the fields of adsorption,energy storage,sensing and detection,which have attracted considerable attention by researchers.The functionalization of the COFs is strongly related to the organic building blocks and the nature of the COFs.Based on the topology,target COFs can be synthesized by selection of functionalized organic building blocks with suitable preparation methods.Although there are numerous methods for the preparation of COFs,such as the traditional"one-step synthesis"method for the preparation of functionalized COFs,there are some limitations in application.The emerging"transformation method"is currently more focused on the preparation and transformation mechanism of COFs,but ignores the preparation and application of functionalized COFs.It is important to investigate the preparation methods and application properties of functionalized COFs.In recent years,our group has synthesized a large number of functionalized COFs and fluorescent covalent organic polymers by changing the organic building blocks,and achieved better applicability.Based on this,we select suitable functionalized organic building blocks and fluorescent covalent organic polymers as COF precursors to realize the preparation of functionalized COFs.The aim of this thesis is to synthesize different functionalized COFs by using different methods,and to regulate the reaction conditions to prepare the target molecules through selecting suitable organic building blocks or covalent organic polymers,so as to achieve the modulation of material properties and application and to expand the preparation methods of functionalized COFs and deepen the study of material functionality.The main research contents are as follows:1.Functionalized COF material with dual-mode response to acids was prepared by selecting"one-step method".TPBD-TA COFs with weak acceptor interactions were prepared by Schiff-base condensation using N,N,N’,N’-tetra（4-aminophenyl）-1,4-phenylenediamine（TPBD）with nitrogen-rich structure and terephthalaldehyde（TA）as organic building blocks.The COF has good crystalline,porous and chemical/physical stability,so it can be used to detect organic and inorganic acids in different kinds of solutions.The protonation of the nitrogen atoms on the imine bond affects the distribution of the electron clouds on the COF,so that the COF exhibits acidichromism for acids in different solutions（aqueous,organic solvents and aqueous-organic phase mixtures）,i.e.,the color of the COF changes from red to dark red,and its UV-vis absorption spectrum exhibits a significant shift of absorption to a larger wavelength band with the detection limit as low as 0.4μmol.Due to the weak donor-receptor interaction of TPBD-TA COF and the weak ability on solvent to ionize H⁺,this allows COF to be used as a chemical sensor to detect acids in different solutions,breaking the limitation of detection environment,and realizing the ability of the COF to detect acids in different solutions.The COF can detect acids in different solution systems.In addition,based on the acidichromism property of the COF,we realize the application of information encryption and broaden the application scope of the functionalized COF.2.Functionalized COFs were prepared by dynamic transformation method.The imine nanospheres TFPB-ED HS were prepared by Schiff-base reaction using 1,3,5-tris（p-formylphenyl）benzene（TFPB）and ethylenediamine（ED）as the building blocks.Due to the weak fluorescence emission intensity and amorphous structure of TFPB-ED HS,which lead to the unsatisfactory metal ion response properties of the nanospheres,we improved the polymer function by changing the molecular structure of the nanospheres.2,5-diethoxybenzene-1,4-di（formylhydrazine）（DETH）,greater degree of conjugation and more hydrogen bonding structure,was introduced into the nanospheres to induce the exchange reaction of monomers by using dynamic covalent chemistry.The functionalized hydrazide-COF（Transfromed-TFPB-DETH COF）was prepared.Since both imine-nanospheres and hydrazide-COF are reversibly covalent bonds,the transformations from the nanosphere to the COFs can be achieved by changing the reaction conditions.The of the conversion process from nanospheres to COF was monitored through using characterization methods such as XRD,IR and SEM.To demonstrate the universality of this conversion strategy,we synthesized additional hydrazide-COFs with different structures by using the nanosphere conversion method.All these COFs exhibited good crystallinity,porosity,stability and fluorescence properties.In addition,due to the abundant heteroatoms of the hydrazide building blocks,these heteroatoms and Cu²⁺can form coordination interactions,which affects the electronic distribution of the COFs and leads to the fluorescence quenching of the COFs,making them fluorescence sensors for the detection of Cu²⁺with detection limits as low as 0.082mmol L^-1,0.26mmol L^-1 and 1.15mmol L^-1,respectively.Based on the dynamic covalent chemistry principle,we have prepared functionalized COFs with greater degree of conjugation and crystallinity by using nanosphere with lower degree of conjugation,which achieves the detection of specific metal ions.The transformation method provides more references for the preparation of the functionalized COFs.3.Based on the principle of dynamic covalent chemistry,the functionalized COFs were transformed from nanosphere.On the basis of the research on the preparation of COFs molecules by the"transformation method"of nanospheres in the second part of the work and in order to achieve the response of large content of water to organic reagents,imine-nanosphere（TAPB-TA-OH NS）was prepared by the condensation of 1,3,5-tris（4-aminophenyl）benzene（TAPB）and 2,5-dihydroxy-p-phenylene terephthalaldehyde（TA-OH）.Due to the reversibility of imine bond,the transformation from nanosphere to crystalline COF（TAPB-TA-OH COF）was achieved by adjusting the reaction solvents and the reaction temperature.The TAPB-TA-OH COF was characterized by XRD,IR as well as electron microscopy,which proved that the COF has good crystallinity,porosity and stability.Meanwhile,TAPB-TA-OH COF exhibited different photophysical properties to different solvents,among which the COF responded specifically to water molecules,i.e.Meanwhile,the COF changed from orange to black when it met water.Finally,the COF was prepared into COF test paper,which was used for water-responsive erasable test paper.