Aza-Fused Conjugated Ladder Polymers:Synthesis And Photocatalytic Applications

In order to cope with the increasing environmental pollution and energy shortage,people pay their attention to solar energy,an inexhaustible source of clean energy.Among them,the use of semiconductor catalysts for photocatalytic water splitting is a effective way to directly convert solar energy into chemistry energy.The photocatalytic water splitting reaction can be divided into two half reactions of oxygen evolution reaction and hydrogen evolution reaction.The oxygen evolution reaction is more hard than the hydrogen evolution reaction.The formation of oxygen is a kinetically slow process which involves complex multielectron transfers and requires a large overpotential.Therefore,it has become a major obstacle to restrict the use of photocatalytic water splitting for large-scale production of clean energy.In recent years,organic polymeric photocatalysts such as carbon nitride have been actively used in the field of photocatalytic water splitting due to their structural flexibility,adjustable energy band structure,rich source of elements and high chemical resistance.However,organic polymeric materials generally have the disadvantages of low carrier separation efficiency,weak water oxidation driving force and limited light absorption,which limit their application to catalyze water to produce oxygen under visible light.As a result,the development of photocatalysts that can efficiently produce oxygen under visible light is of great significance to the development of photocatalysis.Conjugated ladder polymers(cLPs),in which all the backbone units on the polymer main-chain are π-conjugated and fused,have attracted great interest owing to their intriguing properties,remarkable chemical and thermal stability,and potential suitability as functional organic materials.Meanwile,cLPs are analogous to graphene nanoribbons,which combine the excellent charge transport property of graphene with opened band gaps as high-performance semiconducting materials.Therefore,they have extremely strong photocatalytic application potential,but few people study their application in the field of photocatalysis.In this work,we report an efficient approach to synthesize aza-fused conjugated ladder polymers via condensation of 1,2,4,5-benzenetetramine and 2,5-dihydroxy-1,4-benzoquinone,piperazine-2,3,5,6-tetraone,pyrene-4,5,9,10-Tetraone.The test results show that they have a narrow band gap and a low valence band position that can ensure possess effectively visible light absorption and have enough over-potential to drive water oxidation reaction.Further experiments also show that the prepared polymers can undergo oxygen production reaction under visible light,especially cLP-1 polymer.The oxygen production performance of cLP-1 is better than most reported organic polymeric catalysts,and these photocatalysts still maintain high catalytic activity and structural stability after multiple cycles of testing.In addition,we also analyze the reasons for the difference in photocatalytic activity of the three materials through photoelectric test,and further improved the photocatalytic activity of the material by depositting cobalt hydroxide cocatalyst.It is the first time to use conjugated ladder polymers to split water to produce oxygen in visible light,which greatly broadens its application in the field of photocatalysis and provides new research ideas for the design and synthesis of polymer semiconductor catalysts with high photocatalytic performance in the future.