Design,Synthesis And Photocatalytic Performance Of Two Dimensional Covalent Organic Frameworks

As a new type of crystalline porous organic polymers,covalent organic frameworks(COFs)are constructed from organic building blocks according to"Reticular Chemistry",which have unique aesthetic structure and permanent porosity.Besides the advantages of low density,large specific surface area,good stability and so on,they have great potential for their diversity in synthesis strategy,building unit,topological structure and linked type.In fact,COFs have been widely used as a multifunctional platform on gas adsorption and separation,sensing,proton conduction,energy storage and heterogeneous catalysis.Two dimensional covalent organic frameworks(2D-COFs)possess periodic columnar?-arrays and one-dimensional nanopores.The long-range ordered pores provide an ideal environment for the adsorption,diffusion and reaction of small molecules.The columnar conducting channels formed by?-?stacking can effectively promote the separation,diffusion and migration of photogenerated electrons and holes.These characteristics make 2D-COFs into a potential candidate for heterogeneous photocatalysts.Herein,a series of covalent organic frameworks with photocatalytic activity were designed and synthesized at the molecular level by introducing different functional groups,adjusting the energy structure and changing the linked type.The materials were characterized by infrared,powder X-ray diffraction,solid-state nuclear magnetic resonance and elemental analysis.The microstructure of materials was observed by electron microscopy.The potential of these materials as heterogeneous photocatalysts was evaluated from nitrogen adsorption and desorption measurement,chemical stability,UV-Vis absorption and electrochemical measurement.According to the energy level distribution,the applications of these materials in photocatalytic organic reactions,photocatalytic hydrogen evolution from water were explored.The research content will be introduced from the following four aspects:1.The electron donor-acceptor(D-A)structure not only increase the absorption of visible light,but also facilitate the separation and migration of photogenerated charges,thus improving the photocatalytic performance of the materials.Benzothiadiazole group is an excellent photo-active group with electron-deficient properties.Pyrene group is an excellent photoelectric active group with electron-donating properties.The introduction of pyrene group can improve the crystallinity,specific surface area and stability of materials.Therefore,a novel D-A covalent organic framework,COF-JLU22,was designed and synthesized by introducing these two excellent functional groups.The material not only possessed large specific surface area,strong crystallinity and good stability,but also had wide visible light absorption range,sensitive photoelectric response and narrow band gap.Therefore,COF-JLU22 could be used as a metal-free and stable photocatalyst for the reductive dehalogenation of phenacyl bromide derivatives and the?-alkylation of aldehydes,which had good catalytic efficiency and wide substrate applicability under visible light irradiation,and the material still maintained catalytic activity and structural stability after five times of reuse.It is the first time to show the application of crystalline 2D-COF in organic photocatalytic reduction system.2.In chapter 3,we reported an effective strategy to guide us to obtain excellent COF based photocatalysts,which is to screen them from a series of isomorphic COFs.Using1,3,6,8-tetrakis(4-aminophenyl)pyrene as the knot,three kinds of isomorphic covalent organic frameworks(COF-JLU23?25)with different linkers were designed and synthesized.On the one hand,they have similar crystallinity and topological structure due to the similarity of link structure.On the other hand,because of the different electronic structure of dialdehydes,the optical absorption and band structure of these materials can be easily adjusted.And the effects of energy level adjustment in isomorphic COF on photocatalytic organic reactions were investigated.The introduction of 4,4'-([1,2,5]thiadiazolo[3,4-c]pyridine-4,7-diyl)dibenzaldehyde as an electron-deficient unit into the framework can induce wide visible light absorption and rapid transfer of photogenerated carriers,thus promoting the electron and energy transfer process to produce reactive oxygen species.In the C-3 formylation and thiocyanation of indoles,COF-JLU24 showed the best photocatalytic activity.In addition,as a metal-free photocatalyst,the material exhibited a wide range of substrate adaptability and good recyclability.3.Trifluoromethyl compounds are widely used in medicine,functional materials and pesticides.Among them,trifluoromethylated carbonyl compounds are multifunctional building blocks for the synthesis of a variety of fluorine-containing compounds.However,up to now,it has not been reported that the synthesis of the important compounds using porous organic polymers as photocatalysts.Therefore,a novel D-A covalent organic framework,COF-JLU32,was synthesized by using triazine group as electron-withdrawing group and benzothiophene group as electron-donating group.The material not only had large specific surface area,strong crystallinity and good stability,but also possessed high charge separation efficiency.The synthesis of trifluoromethylated carbonyl compounds from aromatic olefins by photo-induction with CF₃SO₂Na as fluorine source under the catalysis of COF-JLU32 showed good catalytic efficiency.However,COF-JLU32 exhibited poor reusability in the harsh reaction conditions due to its reversible skeleton.Therefore,the amide-linked covalent organic framework(COF-JLU33)was formed by oxidation of COF-JLU32 through post-modification.Because of the strong irreversibility of amide bond,COF-JLU33possessed stronger stability.Besides,the material not only maintained the characteristics of strong crystallinity and high charge separation efficiency,but also achieved good results in yield,substrate adaptability and recycling.4.As a new type of photocatalytic platform,2D-COFs have broad application prospects in solar hydrogen production,but their low efficiency in solar energy capture and rapid charge recombination of material hinder the improvement of performance.The sp²c-COFs are expected to make a significant breakthrough in photocatalytic hydrogen evolution because of the stronger stability and higher?-conjugation.In this chapter,the electron-deficient triazine group and the electron-rich benzothiophene group were introduced into 2D-COF by different links to construct two kinds of three component donor-acceptor(TCDA)materials.The two novel TCDA-COFs not only possessed large specific surface area,good crystallinity and excellent stability,but also had wide light absorption range,high efficiency of charge generation and separation.Compared with the imine-linked COF-JLU36,the sp²c-linked COF-JLU35 can capture sunlight more effectively and inhibit charge recombination.Under 420-780 nm light of irradiation,COF-JLU35 exhibited a high hydrogen evolution rate of 70.8 mmol g^-1 h^-1and excellent reusability,which is superior to the most advanced COF-based photocatalysts reported so far.