Synthesis Of Organic Conjugated Polymers By C-H Activation And It’s Photocatalytic Hydrogen Production Performance

Light driven hydrogen production can transform solar energy into clean energy hydrogen energy,which is expected to solve the crisis of energy and environmental.Organic conjugated polymers,as a new type of photocatalyst,have the characteristics of flexible design,diversified synthesis methods,and adjustable electronic structure.In this study,the C-H bond activated cross-coupling method is used to realize one-step reaction synthesis of organic conjugated polymers.It possess the advantages of high atom utilization,few steps,and low cost,which is in line with the concept of green chemistry.At the same time,an electron donor-acceptor（DA）conjugated system was introduced into the conjugated framework.In the polymers,the benzothiadiazole unit was used as the electron acceptor unit（A）and the bithiophene unit was used as the electron donor unit（D）.The unique structure can increase the dipole moment and promote carrier separation.In addition,different bridging groups were introduced to study the effect of organic conjugated polymers with different spatial structures on their hydrogen production performance.The specific research contents are as follows:1.With fluorene as the bridging unit,benzothiadiazole as the electron acceptor unit（A）,and bithiophene as the electron donor unit（D）,three types of D-A type linear conjugated polymerizations were constructed based on the C-H bond activated cross coupling method.Various characterization results show that the band gap of the organic conjugated polymer（2.07eV-2.28eV）can be effectively optimized by adjusting the ratio of benzothiadiazole（BT）to bithiophene.With the content of BT in the conjugated polymer increases,the absorption edge is gradually red-shifted,and the absorption wavelength is shifted from 693.3 nm to 716 nm.Under visible light irradiation,the linear conjugated polymer which the D:A ratio is 1:0.02 has the best photocatalytic activity,and the hydrogen generation rate（HER）of photolysis water reaches 52.3μmol g^-11 h^-1.In addition,the experimental conditions for light driven hydrogen production such as hydrogen production solutions were studied and optimized.When the system was DMF/ascorbic acid/water,the prepared organic conjugated polymer had the highest hydrogen production performance.The above results show that with the increase of the content of the BT acceptor unit in the linear conjugated polymer,the enhancement of the DA effect is beneficial to the charge transfer.But the increase of the polymer conjugate length will also lead to the increase of the charge recombination rate.Therefore,suitable D:A ratio is beneficial to improve the photocatalytic performance of polymers.2.With spirobifluorene was used as the bridging unit,BT unit is used as the electron acceptor unit（A）,and bithiophene unit is used as the electron donor unit（D）,a series of DA types are constructed based on the C-H bond activation cross-coupling conjugated polymer.The spirobifluorene exhibits an orthogonal three-dimensional structure due to the steric hindrance effect,so the molecular structure of the cross-linked conjugated polymer is approximately the dimer of the linear conjugated polymer in the previous chapter.The experimental results show that when D:A is 1:0.02,it has an excellent specific surface area of 279.6 m² g^-1,and its HER reaches1379.90μmol g^-11 h^-11 under visible light irradiation,which is 26 times the linear conjugate with the same D:A ration in the above chapter.The porous structure possesses greatly increases the contact area between the polymer and water,and provides more active sites in the photocatalytic reaction.3.With fluorene as the bridging unit,benzothiadiazole unit as the electron acceptor unit（D）,and bithiophene unit as the electron donor unit（A）,four DA type crosslinks were constructed based on the C-H bond activation cross-coupling method.The rigid planar structure of the pyrene unit improves the planarity of the cross-linked conjugated polymer backbone.Among them,the planarity of the cross-linked structure enhances the performance of the photo-generated charge transport and separation of the cross-linked conjugated polymer.The photocatalytic activity comparison reveals that under visible light irradiation,the HER of the cross-linked conjugated polymer without benzothiadiazole reached 15.98 mmol g^-11 h^-1.Compared with spirofluorene as the linking unit in the cross-linked conjugated polymer,the conjugated polymer containing the pyrene unit has stronger photogenerated charge transport and separation performance,which improves the hydrogen production rate of photocatalyst under visible light irradiation.The main reason is that the pyrene unit has a higher degree of conjugation,which is conducive to charge transport.On the other hand,the planarity of the pyrene unit also greatly improves the charge separation efficiency of the polymer,which improves the utilization rate of photo-generated electrons.