Preparation Of Conjugated Polymers Containing B←N Bond And Research On Their Application In Photocatalytic Hydrogen Production

With the increasing depletion of fossil energy sources,inexhaustible green solar energy has received widespread attention as an emerging energy source,among which sunlight-driven hydrogen production from water cracking in the presence of high-efficiency photocatalysts is considered as an effective way to solve the energy crisis.At present,photocatalysts mainly include inorganic semiconductors,organic semiconductors and organic-inorganic hybrid materials.Early inorganic semiconductors represented by titanium dioxide have disadvantages such as poor response in the visible region and weak stability of photocatalytic hydrogen production,which limit its development,while new organic semiconductors represented by conjugated polymers have gradually attracted the attention of researchers due to their easily adjustable electronic structure and strong response in the visible region.However,the rate of hydrogen production in organic semiconductors is severely limited by the large photogenerated Frenkel exciton binding energy,which leads to the ineffective diffusion of electrons and holes into the catalyst interface.Recently,our group found that organoboron units embedded as acceptor units in the conjugated polymer backbone can effectively improve the exciton dissociation and transport ability and can effectively promote the hydrogen production performance of photocatalytic water cracking,but due to the organoboron units are sensitive to water and oxygen,need to be wrapped by large resistance groups,resulting in the three-ligand organoboron materials have difficulties in the exposure of active sites and poor stability.Under the cognition that the organoboron unit facilitates carrier separation and transport,we use the nitrogen lone pair electrons to combine with the boron 2p vacant orbitals to form the B←N coordination bond,which can still facilitate carrier separation and transport when the B←N coordination bond is embedded in the conjugated skeleton due to the electronegativity difference of the boron and nitrogen elements that give the B←N coordination bond a large dipole moment（5.2 D）.In addition,the embedded B←N coordination bond can effectively reduce the energy band of the conjugated material and improve its hydrophilicity.In this work,we have developed organic materials with hydroxide stabilized tetra-ligand boron-nitrogen coordination bonds and applied them to photocatalytic cracking of aquatic hydrogen,aiming to solve the problems of difficult exposure and lack of stability of the active centers of organoboron unit conjugated polymers.Work 1:To solve the problems of insufficient hydrogen precipitation activity and poor stability of triaryl boron conjugated polymers reported by the group previously,two new water-oxygen stabilized D-A conjugated polymers,PBNN and PBNP,are reported in this chapter.The polymer PBNP can reach a photocatalytic hydrogen production rate（HER）of 9445μmol g^-1 h^-1 under AM 1.5G(100 m W cm^-2)simulated sunlight irradiation and an apparent quantum yield of AQY_{420 nm}=5.98%under 420nm single-beam irradiation,and can maintain an efficient hydrogen production rate for more than 32 hours.Compared with the all-carbon conjugated polymer PCCP constructed by C-C bonding,the PBNP with B←N bonds has superior exciton dissociation and transport ability due to the strong built-in electric field,large dipole moment and low E_b.In addition,the introduction of B←N bonds enhances the absorbance and hydrophilicity of the conjugated polymer containing B←N bonds.The work in this chapter provides a new way to construct efficient photocatalytic cracking of aquatic hydrogen photocatalysts.Work 2,we further explain the importance of the boron unit in promoting charge transport through electronic energy level,surface electrostatic potential,dipole moment,bond length and aromaticity.The HER of conjugated polymers containing B←N bonds was studied by in situ photodeposition of 0%,1%,3%,5%,and 10%Pt in chloroplatinic acid,and the HER of PBM loaded with 1%Pt co-catalyst under simulated solar irradiation at AM 1.5G(100 m W cm^-2)was an impressive 50388μmol g^-1 h^-1,which was 3.03 times higher than that without the co-catalyst.The other conjugated polymer containing B←N bonds PBMP,which is obtained by Suzuki-Miyaura coupling copolymerization with benzene unit,has less nitrogen content in its conjugated backbone,so its best performance is 40132μmol g^-1 h^-1,which is only1.39 times higher;while the performance of the unconjugated boron-conjugated polymer P29,which has high nitrogen content,is only improved to 4788μmol g^-1 h^-1after adding Pt co-catalyst,which is much lower than that of the conjugated polymer containing B←N bonds.The work in this chapter both explains the importance of boron and nitrogen units for photocatalytic performance and provides new ideas for improving the synergy between conjugated polymers containing B←N bonds and Pt co-catalysts.Work 3:To address the problems of palladium residue and poor absorption in the visible region in the Suzuki-Miyoura coupling polymerization of conjugated polymers containing B←N bonds.PBN-Ni,PBP-Ni and PBS-Ni were obtained by replacing the methyl group with a benzene ring and introducing narrow band gap thiophene on the basis of the previous conjugated polymer PBN and by nickel-catalyzed self-polymerization of Yamamoto coupling.Palladium-residual PBN,the property of PBN-Ni was 17,435μmol g^-1 h^-1,indicating that the palladium residual was not absolutely responsible for the high photocatalytic activity of the conjugated polymers containing B←N bonds;in addition,the optical band gap of the conjugated polymer PBS-Ni with the introduction of narrow band gap thiophene was as low as 1.93 e V and the absorption edge was up to 643 nm,and its light absorption capacity covered most of the visible light The present work provides new insights for improving the response of conjugated polymers containing B←N bonds in the visible region.