Band Gap Regulation And Photochemical Applications Of Covalent Organic Frameworks

Solar energy is a kind of sustainable energy with strategic significance.Reasonable and effective development and utilization of solar energy is an important measure to solve the crisis of energy shortage and environmental pollution.The utilization of solar energy by researchers is mainly reflected in the conversion from light energy to electric energy,from light energy to heat energy and from light energy to chemical energy.Among them,photo-chemical conversion is concerned by chemical researchers.In recent years,a large number of inorganic semiconductors,organic semiconductors and hybrid materials have been designed,synthesized and applied to photocatalytic reactions or photodynamic therapy,and important research progress has been made.At present,photo-chemical conversion involves hydrogen evolution,carbon dioxide reduction in the context of carbon neutralization,photocatalytic organic conversion,degradation of organic pollutants and photodynamic therapy in cancer treatment.These applications are also encouraging scientists to design and develop novel materials that are highly photoactive,recyclable and costeffective.Covalent organic frameworks(COFs)are one of the hot topics in current research.COFs usually have the advantages of high crystallinity,high specific surface area and excellent photothermal stability.Moreover,monomers can be designed and selected in advance to prepare functional COFs.COFs with photoactivity generally have a wide light absorption range to capture photons and a narrow band gap to promote the electron transition.Meanwhile,the long-range ordered structure is also conducive to the migration of photogenerated electrons and holes to reduce the compound loss.The design strategy of D-A type COFs can effectively promote the electron transfer process in the frame,narrow the band gap and obtain a wide light absorption range.Therefore,the band gap and energy level of the material can be effectively controlled through the reasonable design of donor and receptor units in the framework,and the preparation of photoactive COFs can be realized.In this thesis,two strategies of direct design of D-A type COFs or post-modified strong electron-absorbing unit were used to prepare electron donor-acceptor COFs based on imine bond and apply them to photochemical studies:(1)D-A type TPPy-PBT-COF was constructed by solvothermal method by selecting electrongiving pyrene unit and strong electron-absorbing benzothiadiazole unit.The COFs have high specific surface area,high crystallization property,wide light absorption range and small band gap energy level.Based on its excellent photochemical and photophysical properties,we applied it to the aerobic cross dehydrogenation coupling reaction under sunlight,and achieved 86% catalytic yield,and realized efficient conversion of gram-grade raw materials.(2)By controlling the proportion of raw materials,we reserved the aldehyde group defect for pure electron-donor COFs(TPB-DMTP-COF),and then reacted the aldehyde group with the strong electron-absorbing dicyanoinhydrin unit to obtain D-A type COFs.Compared with the unmodified COFs,the post-modified D-A type COFs has a significant red shift(about 200 nm)in absorption,and its band gap is also significantly reduced,which is beneficial for photo driven catalysis or biological applications.Both photodynamic experiment and confocal microscope observation with reactive oxygen probe showed that the modified D-A type COFs produced reactive oxygen species under light.Later,when the modified D-A type COFs was applied to cell death experiment,it was found that the modified D-A type COFs had negligible dark toxicity and good killing ability of cancer cells.