Controllable Construction And Photocatalytic Oxidation Performance Of Functional Covalent Organic Frameworks

Covalent organic frameworks（COFs）,a class of materials are linked by covalent bonds of light elements,are a burgeoning type of porous polymeric materials with permanent porosity and certain crystallinity.Due to their low density,ordered channel and regulable structure,COFs have been widely applied into various fields,such as material storage,adsorption,transportation,catalysis,energy storage and conversion,sensors,sewage treatment and so on.Particularly,COF materials have shown remarkable photocatalytic properties in photocatalysis because of its largeπ-conjugated structure and ordered pore channels;also,its structure can efficiently promote charge separation and carrier migration in the skeleton under light irradiation.Although the COFs have achieved some achievements in photocatalysis,some drawbacks,such as weak visible light absorption,rapid recombination of electron-hole pairs,or low chemical stability,still restrict its application potentials.Based on these,in this paper,a series of visible-light-responsive COFs with excellent photocatalytic performance and stability were successfully designed and fabricated,and their photocatalytic properties and applications were explored in photocatalytic organic oxidation.First,an imine-linked anthraquinone functionalized COF（AQ-COF）that features nanofiber structure with staggered AB stacking mode was fabricated.Compared with the spherical particles AQ-COF_DMFsynthesized in previous literature,the AQ-COF presented the superior photocatalytic activity towards the selective oxidation of thioethers under visible light irradiation.A series of photoelectric tests revealed that the unique morphology and structure of AQ-COF might greatly enhance the separation efficiency of photogenerated electron-hole pairs,and thus improve its photocatalytic activity.By virtue of trapping experiments and spectroscopy analyses,the synergistic catalytic mechanism of superoxide radicals and singlet oxygen species for the selective oxidation of sulfides was proposed.Second,donor-acceptor conjugated organic polymer TTCOP was fabricated using cyanide trichloride and thiophene as monomers through the Friedel-Crafts reaction.To improve its photocatalytic performance,boron nitride（BN）was innovatively introduced into TTCOP network to construct heterojunction BN@TTCOP by a simple one-pot in-situ growth.A series of photoelectric tests proved that the addition of BN could significantly improve the light harvesting ability and bandgap structure of TTCOP.Also,the negatively charged BN might attract photogenerated holes from the surface of TTCOP through electrostatic interaction,thereby greatly promoting the photogenerated carrier separation and transfer.The catalytic property of BN@TTCOP photocatalyst doped with different amounts of BN was carefully investigated in the selective oxidation of sulfide under visible light,and BN@TTCOP-2 exhibited the highest catalytic activity,which was nearly twice higher than that of TTCOP.Finally,a visible-light-responsive covalent organic framework bearing benzothiadiazole and triazine moiety was successfully constructed.The morphology and crystallinity can be easily tuned by a simple variation in synthesis conditions.To unveil the structure-morphology-properties relationship,its analogues were also synthesized by modulating building block units.Systematic investigations indicated that the electronic structure and morphology of COFs might greatly affect the photoelectric and photocatalytic performance.Thereinto,COF containing benzothiadiazole and triazine units with ordered alignment and perfect crystalline nature was more beneficial for promoting charge transfer and separation.This COF also presented superior photocatalytic activity and good stability towards metal-free photocatalytic oxidation coupling of amines under visible light.This study reveals an inherent synergistic effect of the structure and morphology of COFs.A plausible reaction mechanism was proposed according to a series of characterization.