Design,Synthesis And Properties Of Schiff-based Covalent Organic Frameworks

Under the background of the deteriorating global environment and the urgent search for new energy alternatives,low-cost and environmentally friendly photochemical technology has provided new hope for overcoming the current energy and environmental crisis and has been widely studied by many researchers.Since the mid-20th century,new porous materials have experienced decades of development and research.Among them,covalent organic frameworks（COFs）are completely designable crystal materials that aggregate organic building blocks through strong covalent bonds.The open and infiniteπ-conjugated structure and various topologies of COFs endow COFs with permanent porosity,long-range order and rigid skeleton,which make them potentially valuable in a wide range of applications,including gas storage and separation,energy storage and conversion devices,drug delivery,chemical sensing and heterogeneous catalysis.In the field of photochemistry,COFs stand out because of their excellent physical and chemical properties and become an ideal candidate for photosensitizers in photochemical systems.The research content of this paper mainly includes the following three parts:1.In the second chapter of this paper,we used the’bottom-up’synthesis strategy to synthesize a hydrazone-bonded covalent organic framework material（SH-COF）with excellent hydrophilicity and rich chelating sites under typical solvothermal conditions.The SH-COF with specific binding sites was applied to detect and remove Cu²⁺and Hg²⁺in water.In the experiment,SH-COF can achieve visible color changes to the naked eye,and exhibits high selectivity,high sensitivity,low detection limit(0.244μM for Cu²⁺,0.239μM for Hg²⁺)and real-time response.In addition,the high selectivity and interaction of Cu²⁺and Hg²⁺with SH-COF were confirmed by XPS,EDX and FT-IR,and the sensing mechanism of SH-COF for Cu²⁺and Hg²⁺was explored.2.In the third chapter,an imine-based highly fluorinated TFA-TTA-COF was synthesized by polycondensation of 2,3,5,6-tetrafluoroterephthalaldehyde（TFA）and4,4’,4’’-（1,3,5-triazin-2,4,6-triyl）triphenylamine（TTA）.The highlyπ-conjugated structure increases the stability of the material and improves the effective separation and charge transfer of photogenerated carriers.The introduction of fluorine atoms activates the aldehyde group on the benzene ring,enhances its electrophilicity,effectively improves the crystallinity of COF,and increases the hydrophilicity of the material.Benefiting from the synergistic effect of photoelectric properties,hydrophilicity and inherent porosity of COF materials,TFA-TTA-COF not only exhibits satisfactory photodegradation efficiency for Chrome Black-T（EBT）and Chrome Black-A（EBA）azo dyes in water,but also exhibits high photocatalytic performance for the oxidative coupling of benzylamine in aqueous phase（the conversion rate of most substrates exceeds 90%）,and can be recycled more than five times without losing activity.As expected,the excellent visible light absorption capacity and enhanced photocatalytic performance of TFA-TTA-COF can be attributed to the pre-designed framework structure.3.In the fourth chapter of this paper,three kinds of ketoenamine group COFs（HB-COF,DHB-COF and THB-COF）were designed and synthesized by the aldehyde amine condensation reaction of oxalic dihydrazide and benzene-1,3,5-tricarbaldehyde derivatives with different numbers of hydroxyl.In order to verify the continuous attention to the relationship between the structures and properties of COFs,these three materials were used for the autoxidation of benzylamine and the cross-coupling of benzylamine and aniline.The results show that the less the ketoenamine group,the better the photophysical properties and photoelectric properties.