Design And Synthesis Organic Conjugated Polymers For Hydrogen Evolution

Owing to the massive use of fossil energy,human being is facing with energy shortage crisis and environmental pollution,and thus highly desires to develop clean and renewable new energy sources.Hydrogen energy is abundant,sustainable,green,and can be stored and transported in a variety of forms,and thus has been regarded as a clean energy with great potential for development.Different from conventional electrolysis and photoelectrolysis of water that consume a lot of energy,the use of solar energy directly photo-catalyzes water splitting into hydrogen is an ideal method.Compared with inorganic semiconductors,organic semiconductor photocatalysts have advantages in terms of structural diversity,designability,composability and easy regulation of energy levels and band gaps.Although a variety of different forms of organic semiconductors,including linear conjugated polymers,conjugated microporous polymers,and covalent organic frameworks,have been successively developed as catalysts for photocatalytic hydrogen evolution from water,it is not yet understood which structure is most advantageous.Moreover,most of the reported photocatalysts are active only under 500 nm light exposure,and cannot efficiently utilize visible and near infrared light with long wavelengths,which account for a large amount of energy in the solar spectrum.In view of the fact that linear conjugated polymer is more easily synthesized with high yield and clear structure than other types of polymer semiconductor materials,such as conjugated microporous polymers and covalent organic frameworks,the thesis work focused on such structure,designed,synthesized and studied six linear conjugated polymer photocatalysts by selecting particular moieties and combination styles as well as via a side chain conjugation strategy.In the second chapter,we selected three characteristic groups,namely,the hydrophobic electron donor triphenylamine（TPA）,the pyrropyrrolidone（DPP）based hydrophobic electron acceptor and the hydrophilic electron acceptor dibenzothiophene sulfone（DBTS）,and the linear conjugated polymers,P（TPA-DBTS）,P（DPP-DBTS）and P（TPA-DPP）,were obtained by pairing these three moieties.The results show that the polymer P（TPA-DBTS）has a characteristic of effective photoinduced intramolecular charge transfer,and the polymer is relatively hydrophilic,so it obtains the best photocatalytic hydrogen production performance with the hydrogen evolution rate reaching 14040μmol g^-1 h^-1.The DPP-containing polymers have broad absorption in visible light region,but the polymer backbones fail to produce effective photoinduced charge transfer,resulting in poor photocatalytic hydrogen production performance.The polymer constructed by DPP and hydrophilic group DBTS gives a medium hydrogen evolution rate of 672μmol g^-1 h^-1,while that combined with hydrophobic TPA unit reports the smallest hydrogen evolution rate,54μmol g^-1 h^-1.This work proves that the performance of organic semiconductor photocatalysts is greatly determined by the moiety selection and combination.Proper electron donor and electron acceptor collocation is conducive to the separation and migration of photoinduced electron-hole pairs,and proper hydrophilicity is conducive to the contact between polymer and water,all of which are beneficial to improve photocatalytic hydrogen evolution performance.In chapter 3,we propose a side chain conjugate extension strategy to improve photocatalytic hydrogen production performance of linear polymers.To this end,we synthesized three polymers:P0 is benzodithiophene（BDT）/DBTS alternating conjugated copolymer;P1 inserts two thiophene units into P0 backbone,and thus is a main-chain conjugation-extended polymer;P2 places these two thiophene units in the side chain of BDT in P0 backbone with a side chain conjugation-extension fashion.As compared with P0,P1 with a main-chain-extended conjugation displays a red-shifted absorption spectrum and a longer excited state lifetime.But thermostability,crystalline property,hydrogen-evolution overpotential and carrier mobility have not been improved.As a result,P1 exhibits a hydrogen evolution rate 4 times as that of P0.In the case of side-chain-extended conjugation,P2achieves the largest red-shifted absorption spectrum,hydrogen-evolution overpotential,thermal stability,crystalline property,excited state life and carrier mobility,so it shows a 152-fold increase in hydrogen evolution rate,reaching20314μmol g^-1 h^-1.Moreover,P2 can be used in combination with a variety of sacrificial reagents.And it is highly stable and reusable.Most importantly,the apparent quantum yield（AQY）of P2 is very high,reaching 7.04%at 500 nm,5.06%at 550 nm,and also active at 600 nm,making it one of the few photocatalysts that can utilize the highest energy density of the green light of sunlight.This work demonstrates that the side chain conjugation strategy can improve the performance of a linear conjugated polymer photocatalyst by enhancing light absorption capacity,adjusting optical material bandgap and energy level,prolonging the excited electron lifetime,and facilitating exciton separation and transfer.