Tunable Surface Defects In Conjugated Polymer For Photocatalytic C-N Couplings

Conjugated polymers are a new generation of photocatalyst with visible-light response,have structural tunability,made from plentiful sources,and have low production costs.In recent years,conjugated polymers have shown excellent visible-light activity in photocatalytic organic synthesis.However,the inefficient charge transfer and lack of active sites for conjugated polymers severely impede their catalytic applications.Defects engineering can form localized electronic states,which can not only generate active sites,but also offer the bridge for electron and energy transfer from catalyst to the adsorbed molecules.In this dissertation,point defects were constructed and modulated on the p-type polyanilines（PANIs）and n-type polyimides（PIs）to improve their photocatalytic C-N coupling performances,which facilitates the adsorption and activation of benzylamine and oxygen,respectively.Sepecific research results are as follows:（1）HCl-doped polyaniline（PANI-ES）and PANIs of different oxidation states（PANI-LB,PNAI-EB and PANI-PB）were prepared by the typical oxidative polymerization and subsequent redox reaction,respectively.The photocatalytic performances of PANIs towards selective benzylamine oxidation reactions under visible light were investigated and a probable mechanism using PANI-ES as the photocatalyst was proposed.The results show that the band gaps of PANI-LB,PNAI-EB and PANI-PB are 3.04,2.94and 1.50 e V,respectively.As the oxidation state increases,the band gap becomes narrower.In comparison with PANI-EB,a new polaron band above the top of valence band is formed in PANI-ES.Therefore,the UV-Vis absorption of PANI-ES at 420 nm is significantly enhanced and the band gap of PANI-ES is reduced to 2.65 e V.PANI-PB containing more bipolaron structures with the highest degree of oxidation exhibits the highest conversion of dark reaction（82.6%）.However,PANI-ES containing more polaron structures exhibits the highest light-induced activity（54.9%）.The UV-Vis analysis and theoretical calculation results show that the formation of the surface complex between PANI-ES and adsorbed amine should be responsible for the enhanced visible light activity.Proposed benzylamine oxidation mechanism of PANI-ES:a visible light absorbing surface complex is formed by the hydrogen bonding interaction between benzylamine and the polaron structure in PANI-ES.Then,one electron of amino nitrogen atom is excited under the irradiation of visible light and transferred to oxygen adsorbed on the catalyst to generate superoxide radical.Finally,cationic radical intermediate of benzylamine reacts with superoxide radical to release benzaldehyde,which then reacts with unreacted benzylamine to yield imine.（2）Polyanilines with different content of polarons and bipolarons（ES_x）were prepared by the doping process of emeraldine base form of polyaniline（EB）with different concentration of HCl.The contribution of polarons and bipolarons to the conductivity of ES_x was further confirmed respectively,and their effects on selective benzylamine oxidation reaction under visible light were investigated.The results show that ES_0.5 has the highest content of polarons,while ES_0.001 and ES₁ possess relatively high content of bipolarons.Both polarons and bipolarons can form surface complexes with benzylamine,which extends the range of visible-light absorption.The increase of the concentration of polarons is beneficial to improve the conductivity of ES_xwith the same degree of oxdiation.The conductivity of polyaniline affects the separation efficiency of photo-generated carriers,and good conductivity can reduce the probability of photo-generated electron-hole recombination,which thereby promotes the photocatalytic activity.Due to the highest concentration of polarons,ES_0.5 exhibits the highest light-induced activity（55.9%）and its conductivity is 0.06 S cm^-1.（3）Oxygen vacancies-rich polyimide（R-PI）and oxygen vacancies-deficient polyimide（D-PI）were synthesized by thermal polymerization of the salt monomer and ordinary monomers,respectively.The effect of oxygen vacancies（OVs）on the energy band structures,separation efficiency of photogenerated carriers and the activation of oxygen were investigated.The performances and stability of photocatalytic oxidation of benzylamine to imine under visible light were investigated and a possible photocatalytic reaction mechanism was proposed.The results show that the salt monomer reaches a molten state at the thermal polymerization temperature（275°C）and then polymerizes to oligomeric polyimides,resulting in high crystallinity.During the thermal polymerization process,a large amount of acetaldehyde was released,which induces a large number of OVs in R-PI.The concentration of OVs in R-PI is 26.1×10¹² spins mg^-1.The introduction of OVs in R-PI forms a new intermediate energy level near the bottom of the conduction band,which reduces the width of the bandgap from2.91 to 2.34 e V,and significantly enhances the absorption strength of visible region（450-700 nm）.The OVs-mediated defect state changes the transfer process of photo-generated electrons,which inhibits the recombination of photogenerated electron-hole pairs and thus significantly reduces the intensity of fluorescence.Oxygen can interact with the localized electron on OVs via Lewis acid-base interactions to form chemisorbed state.Chemisorbed oxygen spatially builds up a channel for the delivery of photoexcited electron to oxygen,which reduces the energy barrier of oxygen activation and enhances the transmission efficiency of photogenerated electrons.Due to the efficient ability of activating oxygen to superoxide,R-PI exhibits the highest photocatalytic performance（90.5%）over benzylamine oxidation under visible-light irradiation(300 m W cm^-2).Moreover,the conversion of R-PI is4.2 times that of D-PI under the irradiation of green light(50 m W cm^-2).Proposed benzylamine oxidation mechanism of R-PI:under the irradiation of visible light,benzylamine loses an electron to the photogenerated hole and forms a cationic radical intermediate,which reacts with superoxide radical to produce imine intermediate and H₂O₂.The imine intermediate can react with the free amine to furnish the coupling product.