Preparation Of Organic Semiconductor Materials Containing Coupled Diacetylene For Hydrogen Production

Using sunlight to catalyze the reduction of water for hydrogen production is a method to convert solar energy into chemical energy and store it in hydrogen,an energy carrier that can be stored,transported and used without causing pollution.Compared with inorganic semiconductor catalysts,organic semiconductor catalysts have many advantages,such as diverse,designable,and synthesizable structure,tunable molecure frontier orbital energy levels,gap,and light-absorption properties.So far,a large number of organic semiconductor photocatalysts have been developed,including linear conjugated polymers,conjugate microporous polymers,covalent organic frameworks,etc..These materials are generally prepared by Pd-catalyzed cross-coupling reactions,including Suzuki,Stille and Sonogashira coupling reaction.The reactions involve two functional groups and usually use two or more than two monomers,and thus monomer feeding ratio becoming an important factor that influence the final polymerization results.However,it is rarely use oxidative coupling reaction of diacetylene,another kind of reaction for the construction of conjugated polymers,to prepare organic semiconductor photocatalysts.The polymerization requires only one monomer and does not use noble metal catalyst.Moreover,the prepared polymers keep the acetylene group at both two ends,which can be used for further polymerization or functionalization.To the best of our knowledge,only one work has been reported in the literature and the research is extremely inadequate.In this thesis,three characteristic diacetylene compounds were selected to synthesize their respective oxidation coupled homopolymers and two groups of pairwise random copolymers and block copolymers.Their basic properties and photocatalytic performance of hydrogen production from water were studied.This paper is divided into four chapters.In the second chapter of the thesis,we selected benzothiophene sulfone(DTP)a hydrophilic electron–accepting moiety,an triphenylamine(DPA)an electron-donating moiety,and benzodithiophene(BDT)an electron-donating unit with alkoxy side chain,and modified them with terminal acetylene group.By means of oxidation coupling polymerization,three homopolymers,PDTP,PDPA and PBDT,bearing diacetylene units were synthesized.It was found that the homopolymer PDTP has the highest photoelectric response,the accelerated separation of photogenerated electrons and holes,the best hydrophilicity,and hydrogen-evolution driving force,and thus diaplayed the highest hydrogen evolution rate of 1270?mol g^-1 h^-1.The properties of homopolymer PDPA and homopolymer PBDT were poor,and the hydrogen production rates were only 57.18?molg^-1h^-1 and 42.86?molg^-1h^-1,respectively.PDPA and PBDT have a larger fluorescence lifetime and slightly better light-absorpion properties than PDTP.However,due to their weak ability to separate electrons and holes,they cannot produce enough photoelectrons to reduce hydrogen protons,thus resulting in poor hydrogen evolution performance.Therefore,we conclude that the photoelectric response capability is very important for realize photocatalytic hydrogen production from water.In the third chapter of the article,since the two ends of the above synthesized polymers still retains the acetylene group,we can carry out the second polymerization reaction to synthesize block copolymers.We selected DTP and DPA monomers and block copolymer PDTPDPA-PD was synthesized.For purpose of comparation,we also synthesized random copolymer PDTPDPA-HJ.Compared with the homopolymers,the photocatalytic capacity of the synthesized block copolymer PDTPDPA-PD for hydrogen production was greatly improved,and the hydrogen evolution rate reached 2164?mol g^-1h^-1,the hydrogen evolution rate of PDTP reached 1270?mol g^-1h^-1.It was found that the block structure not only resulted in charge separation between the segments,which significantly enhanced the photoelectric response of the polymer,but also reduced the impedance of charge transfer.Therefore,block copolymers formed from donor and acceptor monomers was beneficial to polymer photocatalytic performance for hydrogen production from water.In the fourth chapter,DTP and BDT monomers were selected to synthesize similar block copolymer PDTPBDT-PD and random copolymer PDTPBDT-HJ along the same line in the third chapter.The studies found that the hydrogen evolution rate reached 289?mol g^-1h^-1 of random copolymer PDTPBDT-HJ,block copolymer PDTPBDT-PD has better performance of 381?mol g^-1h^-1.While,the PBDT homopolymer reached 42.86?mol g^-1h^-1,PDTP homopolymer reached 1270?mol g^-1h^-1.Studies found that,the block copolymer PDTPBDT-PD has better performance than PBDT homopolymer,but is inferior to PDTP homopolymer.Studies found that enhanced fluorescence intensity,no charge separation between the segments,and lower light-responsive capability.And its electrochemical impedance also increased obviously,which is unfavorable to electron transport,All these,result in the reducing of the active sites for proton reduction,and thus reduce hydrogen evolution performance.In summary,the oxidative coupling of diacetylene groups is a kind of excellent reaction for the construction of organic semiconductor photocatalysts.Not only can the polymerization reaction be carried out from a single monomer,but also the oligomers can be synthesized first and then the block copolymers can be further prepared.Therfore,more polymer catalyst systems with different structures can be achieved by diacetylene oxidative coupling polymerization.Whether the block copolymer is beneficial to the photocatalytic hydrogen production performance,it depends on the charge separation between the segments and its photoelectric response ability.