Aggregation Induced Symmetry Breaking Of Photoactive Structures Enhances The Performance Of Photocatalytic Water Splitting For Hydrogen Production

As an essential secondary energy source,hydrogen energy is significant for limiting and reducing carbon emissions.Photocatalytic splitting of water for hydrogen production is a green and sustainable pathway for hydrogen production.Organic semiconductor photocatalysts have received broad concerns in the field of photocatalytic due to their adjustable electronic structure and physical and chemical properties.However,organic semiconductor photocatalysts are not conducive to the separation of light-generated charges due to their high exciton binding energy,which limits the photocatalytic activity.In this paper,the effect of aggregation on the charge separation and photocatalytic activity of pyrene active structures was researched.It is discovered that the aggregation of pyrene molecules can induce symmetry breaking,which built an electric field in situ to promote the separation and transport of photogenerated charges and significantly enhanced the performance of photocatalytic hydrogen production.Meanwhile,we found that the aggregation of 1-hydroxypyrene,1-nitropyrene and perylene molecules also significantly enhanced the separation efficiency of photogenerated charges and photocatalytic hydrogen production activity,confirming the universality of enhancing photogenerated charge separation by photoactive molecules aggregation.The main research contents and results are as follows:(1)The pyrene molecules with high symmetry structure were selected and the pyrene aggregates were obtained by increasing the concentration of pyrene in isopropanol.It was found that the aggregation of pyrene could broaden the light absorption from the UV region to the visible region.The aggregation of pyrene is confirmed by various characterizations and theoretical calculations to spontaneously break its structural symmetry,induce the generation of dipole electric fields,and significantly accelerate the separation and transfer of photogenerated charges.Hydrogen production evaluation by photocatalysis showed that the pyrene aggregates exhibited enhanced hydrogen production activity.Meanwhile,non-covalent interactions in pyrene aggregates provide conditions for optimizing the physicochemical and electronic properties of pyrene aggregates.For example,2-amino-5-cyanopyridine and 3,4-diaminopyridine small molecules could be inserted into pyrene aggregates through π-π interactions,which further enhanced the separation of photogenerated charges and the performance of photocatalytic hydrogen production,with the quantum yield of hydrogen production up to 20.77% at 400 nm.(2)The aggregation effects of the analogues of pyrene(1-hydroxypyrene,1-nitropyrene and perylene)were systematically investigated.The aggregates of pyrene analogs all exhibit red-shifted light absorption properties relative to the light absorption of pyrene analogs monomer.In particular,a large dipole moment is exhibited by all these aggregates,which accelerates the carrier separation and verifies the universality of the strategy to enhance photogenerated charge separation by aggregation-induced symmetry breaking.Meanwhile,the charge separation properties of these aggregates can be further modulated and optimized by in-situ embedding of heterogeneous small molecules(e.g.2-amino-5-cyanopyridine and 3,4-diaminopyridine),thus effectively enhancing the performance of photocatalytic hydrogen production.This work promotes carrier separation by aggregation-induced symmetry breaking of photoactive structures,creating a new pathway for the separation of photogenerated charges.