Salicylimine-based Covalent Organic Frameworks With Proton/Metal Ions Locking For Photocatalytic Properties

With the rapid development of industrialization,the problems of energy shortage and environmental pollution caused by pesticide synthesis are becoming more and more serious,so it is urgent to find a green and sustainable development method.As an emerging,high-efficiency and energy-saving technology,semiconductor photocatalysis technology has been widely used in the fields of energy development and organic transformation.As the key to photochemical energy conversion,photocatalysts can make electrons flow to the reduction center continuously to drive the catalytic reaction.As an emerging semiconductor photocatalyst,covalent organic frameworks（COFs）are organic porous materials with a certain crystalline state based on covalent bonding,due to the fullyπ-conjugated framework structure and highly ordered stackedπElectron channels have received extensive attention in the field of photocatalysis.Imine-linked（C=N）COFs have shown promising properties in the field of photocatalysis as a kind of easily synthesized and highly stable COFs.In the current study,based on the characteristic of N atoms with lone pair electrons,imine-bonded COFs can enhance their photocatalytic performance and construct active sites by combining protons or metals to form weak D and strong A structures.However,this binding is unstable,so how to construct a highly stable imine-bonded COFs that can bind protons and metals is a difficult problem.Based on the theory of coordination chemistry,this paper innovatively introduces salicyl groups into the skeleton near the imine bond,chelates protons and metals through the synergistic effect of（N,O）,enhances the photocatalytic activity of COFs materials and constructs stable materials.metal active sites.This paper provides a new path for the fine-tuning of COFs,which has important theoretical value and will greatly promote the wide application of such materials in the field of photocatalysis.The specific research contents are as follows:1.Using Tfppy（R）（R=H,F,OMe,OH）monomer and PDAN,Benzidine,4,4’’-DIAMINO-P-TERPHENYL synthesized 12 COFs materials through Schiff base reaction,and used ascorbic acid as a sacrificial reagent to compare the performance of photocatalytic water splitting to verify the effect of different groups on the degree of protonation of the materials.Specifically,in the photocatalytic hydrogen production system,the COF-Tfppy（R）material will be in-situ protonated during the photocatalytic hydrogen production process,and the protonation degree of COFs materials with different groups is different.It was found that when the COFs with salicyl groups（-OMe,-OH）were used for photocatalytic hydrogen production,the hydrogen production performance reached 804μmol h^-1 and 729μmol h^-1,which were comparable to those without The COFs of the salicylimine structure（-H,-F）have a very large improvement.Then we used TEOA as a sacrificial agent to prove that the protonated COFs＿AA material is the real photocatalyst in the hydrogen production process.Finally,through a series of test characterizations,it is proved that the introduction of salicylimine structure is beneficial to the protonation of the material and promotes the electron transport of the material,thereby improving the photocatalytic hydrogen production performance of the material;2.Based on the research content of the first part,the coordination of Ni²⁺is used to construct active sites by using the structure of salicylimine（N,O coordination）,and the research of photocatalytic organic transformation is carried out.Specifically,COF-Tfppy（OH）＿Ni²⁺material was used for photocatalytic C-O coupling reaction of phenyl bromide with alcohol,and C-N coupling reaction of phenyl bromide with arylamine and alkylamine.The C-O coupling reaction was carried out under the irradiation of a 6 W blue LED,and the COF-Tfppy（OH）Bp＿Ni²⁺material successfully converted p-bromoxynil（0.5mmol）to p-cyanoanisole within 12 h;C-N coupling reaction was carried out under the irradiation of 18 W blue LED,COF-Tfppy（OH）＿Ni²⁺material successfully synthesized p-bromoxynil（0.5 mmol）and aniline into 4-（benzene）Amino)benzonitrile within 48 h,and when the reaction system is heated to 85℃,the reaction substrate can be extended to alkylamines,and a good yield can still be obtained,and the cycle experiments also prove that the material has a certain stability.Finally,we also proved that the catalytic effect of（N,O coordination）is better than that of（N,N coordination）through the comparison of reaction efficiency and simple characterization.