Design,Synthesis And Application Of Covalent Organic Frameworks Based On High-connected Building Units

Covalent organic frameworks（COF）are created by connecting organic units with covalent bonds,resulting in crystalline porous materials that exhibit long-range ordered structures.The highly customizable design of building units and the crystalline nature of the materials give COF a distinct pore structure with accurate functional sites.These characteristics make COF ideal for gas adsorption and separation,photoelectric catalysis,energy storage,and drug sustained release.COF are divided into two categories:Two-dimensional（2D）COF and Two-dimensional（3D）COF based on dimensions.Due to the diversity and high functionality of construction units,most reports focus on 2D COF.2D COF with excellent interlayer regulation have significant advantages in stable structures and electron transport properties but are restricted by mass diffusion due to their 2D stacking.3D COF,on the other hand,have a periodic three-dimensional skeleton that is built using three-dimensional units,which gives them high specific surface area,diverse pore structure,low density,and dense active sites,but the structural instability and poor conductivity limit their practical applications.Currently,the majority of 3D COF are constructed using three-dimensional building units with 4-or 6-connected,and it wasn’t until 2020 that the formation of 3D COF using three-dimensional building units with 8-connected was reported,the lack of rich three-dimensional construction units remains a significant bottleneck in their synthesis.The basis of its application study is the functionalization of 3D COF structures,the majority of the functionalized 3D COF structures characterized up to this point are be limited in dia topology,which have a tendency to produce severe respiratory effects and highly interpenetrated.This severely restricts the development of 3D COF applications.In light of this,the paper aims to focus on the synthesis and functional design of COF with quasi-three-dimensional and 3D new net structures using high-connected construction units,in order to explore their potential applications.The research will primarily consist of the following three parts.（1）Based on highly connected heteroatom flexible building units,COF with unique quasi-three-dimensional structure could be synthesized.In the second chapter,we successfully synthesized two cyclotriphosphazene covalent organic frameworks（Q3CTPCOF:JUC-611 and JUC-612）and their nanosheets（JUC-610-CON）by[6+3]imine bond and applied them to ORR electrocatalysts.The abundant electrophilic structure in Q3CTP-COF induces the generation of high-density carbon-active sites,and the unique double-stacking of carbon-active sites in the structure formed by[6+3]imine bond makes carbon-active sites easier to expose,accelerating diffusion of substances during the ORR reaction.In particular,due to the weakπ-πinteraction between layers,Q3CTP-COF is easy to peel off into nanosheets（NS）.Q3CTP-COF NS exhibits efficient ORR catalytic activity（half-wave potential of 0.72 V in basic electrolytes）,better than most COF-based ORR electrocatalytic materials reported so far.In addition,Q3CTP-COF NS can be used as a cathode material for zinc-air batteries(power density of 156 m W cm^-2at 300 m A cm^-2).COF and its nanosheets with high density and exposed active site could be achieved by rationally selecting highly connected institutionally building units,definitely promoting the development of metal-free carbon-based electrocatalysts.（2）Starting from the basic concepts of reticular chemistry,we are able to display topological information such as vertices and edges on the RCSR website for a specific topology.Based on covalent bond connection and network extension,COF is able to be obtained by selecting appropriate institutional building units,so reasonable design of the geometry of institutional building elements is an effective method for synthesizing 3D COF with various topologies.In Chapter 3,we synthesized two 3D COF（termed as JUC-643 and JUC-644）by polycondensation between a linear linker and two newly designed 8-connected（8-c）monomers in D2d symmetry,and explored its performance in terms of gas adsorption.The crystal structures of both COF were determined or simulated to be an unreported[4+3（+2）]-connected topology jcb.In these structures,the structure resembled a 6-coordinated pcu topology because adjacent monomers were connected in the same axial direction by a unique intermediate double helix structure.This was a subversive example that did not appear in traditional topological analysis,indicating the entire building block may not always be considered as a topological vertex.Two more novel topologies named as jca and jcc were then defined via symmetry improvement and vertex division.This paper provides a new perspective and promising strategy on obtain of 3D COF with novel topologies via geometrical design of their organic building blocks.（3）The advancement and progress of 3D COF heavily relies on the creation of innovative topologies and the addition of functionality.Therefore,in Chapter 4,utilizing an 8-connected（8-c）construction unit and a linear linker（hydrazone derivatives）,we synthesized a 3D hydrazone COF（JUC-645）with stimulating response.Notably,JUC-645 demonstrated reversible E/Z isomerization at various p H levels while maintaining high crystallinity porosity and chemical stability.JUC-645loaded with anticancer drugs（Ara-C）in phosphate buffer exhibited a drug release rate nearly three times higher at p H=4.8 compared to p H=7.4,indicating effectively improved drug targeting.Our findings in this paper not only broadened the topological structure of 3D COF functionalization,but also promoted potential applications in the biomedical field as a drug carrier.