Investigations On The Structure-Property Relationship Of Perylene Diimides Supramolecular Photocatalyts

A new class of organic photocatalysts,perylene diimides（PDI）supramolecular materials,with full visible spectrum response have been successfully developed.In this paper,a variety of characterization methods have been adopted to investigate the molecular structure,supramolecular self-assembly mechanism,organic crystal structure,photoelectric physicochemical properties,organic electron energy band structure,photocatalytic oxidation and anticancer properties of PDI supramolecular photocatalysts.The relationship of structure-property between supramolecular stacking structure and its photocatalytic oxidation and anticancer properties have been studied systematically and in-depth.We present that one-dimensional supramolecular organic nanofibers,self-assembled by a carboxy-substituent PDI molecule through H-typeπ-πstacking and hydrogen bonding,can act as a robust and effective photocatalyst for both organic pollutants degradation and water oxidation under full visible light without the apparent need for an added metal co-catalysts.Compared to molecular and unassembled PDI,such PDI supramolecular nanofibers exhibit superior photocatalytic activity under visible light.The high activity of PDI supramolecular nanofibers mainly stems from the establishment of the organic semiconductor band structure and the nanocrystallization.As a discrete energy level,molecular PDI has no photocatalytic activity due to the inefficiency of photo-generated carrier separation.Whereas,organic semiconductor band structure of PDI has been established afterπ-πstacking,in which effective photo-generated separation occurs.At the same time,due to the short mean free path of organic semiconductors,the photo-carrier separation efficiency of bulk-phase PDI materials is low,and the photocatalytic performance is weak.Nano-crystallization can greatly increase the photo-generated carrier separation efficiency of PDI supramolecular materials,thus enhance its photocatalytic activity.Furthermore,combining with the theoretical and experimental methods,the effect ofπ-πsupramolecular packing on its electronic energy level structure have been fully elucidated.Higherπ-πsupramolecular packing means a largerπ-electron conjugated system,which leads to a smaller bandgap,a deeper valence band position,enhanced light absorption and photo-oxidation capability.Meanwhile,a largerπ-electron conjugated structure can effectively promote the migration and separation of photo-generated carriers.As the two nitrogen positions at the imides of PDI are nodes in theπ-orbital wave function,the introduction of side-chain impose little effect on the electronic properties of PDI molecule,offering wide options to modify theπ-πstacking arrangement,which in turn allows for investigating the structure-property relationship of PDI supramolecular nanostructures.Here,two PDI derivatives modified with different length of linear carboxy-substituent side-chains are employed to construct H/J-type aggregated PDI supramolecular nanostructures.The effects of H/J-type stacking on the photocatalytic mechanism and activity of PDI have been investigated.As a face-to-face arrangement,H-aggregates have higherπ-electron conjugation and show more semiconductor characteristics,which results in higher carrier separation and migration efficiency under irradiation.Whereas,J-aggregates exhibit more molecular properties due to its lowπ-electron conjugation caused by head-to-tail stacking mode.As a result,H-aggregated PDI mainly forms superoxide radicals（·O₂^-）and holes（h⁺）through electron-transfer（ET）.In contrast,J-aggregated PDI mainly generates singlet oxygen species（¹O₂）via energy-transfer（EnT）.Benefit from the stronger oxidizability of·O₂^-and h⁺,H-aggregated PDI shows higher photocatalytic activity for small molecule degradation and oxygen evolution under visible light.Whereas,J-aggregated PDI exhibits potential application in photocatalytic anti-cancer treatment owing to its high ¹O₂ quantum yields under red light radiation.