Theoretical Investigation On The Generation Of Singlet Oxygen From Triplet Oxygen Catalyzed By The Complex Of Anthraquinone And Potassium Tert-butoxide

Singlet oxygen is a vital reactive oxygen species(ROS)due to its unique electronic structure and strong oxidation,and plays a key role in medicine,biology and atmospheric chemistry.The research on the singlet oxygen has attracted the attention of many theoretical and experimental chemists in the past few decades.Since oxygen molecules exist in nature in the form of triplet state,singlet oxygen generally needs to be obtained by some special experimental methods,such as the reduction of endoperoxides,photosensitization methods and some chemical synthesis.Among these methods,the generation of singlet oxygen by triplet state quenching is an efficient and convenient means of generating the singlet oxygen.However,some generating stable triplet state photosensitive compounds are required.Anthraquinones are photosensitive organic molecules that generate stable triplet state.And they have a wide variety of species,unique electronic structures and easy modification.They are important compounds that can be used to generate the singlet oxygen.Recently,Li Mingde’s group generated a complex(AQ@[K(O-t Bu)]4)by mixing anthraquinone and potassium tert-butoxide,and found that the complex can directly catalyze the generation of singlet oxygen from triplet oxygen only at room temperature without light.More interestingly,when AQ@[K(O-t Bu)]4 was irradiated with light,another stable complex(AQ@[K(O-t Bu)]4*)was found to catalyze the generation of singlet oxygen from triplet oxygen.However,the electronic structures,photochemical properties,and reaction mechanisms for the generation of singlet oxygen of these two complexes are still unknown.In this paper,the density functional theory(DFT)and high-level ab-initio method(NEVPT2)are used to systematically study the electronic structures and photochemical properties of AQ@[K(O-t Bu)]4 and AQ@[K(O-t Bu)]4*.And the reaction mechanism of the two complexes catalyzing the generation of singlet oxygen from triplet oxygen is systematically described.The results suggested that these two complexes are stable triplet state complexes.The electrons in potassium atoms of two complexes showed the strong delocalization behavior,resulting in strong charge separation of the complex,which leads to the anthraquinone part having a negative charge and existing in the form of anthraquinone anion radical,[AQ ? ]-.Further the research found that the difference in electronic structure between AQ@[K(O-t Bu)]4and AQ@[K(O-t Bu)]4* is only the electronic structure of the tert-butoxide group.Therein,the electronic structure of tert-butoxide group of AQ@[K(O-t Bu)]4 and AQ@[K(O-t Bu)]4* is π-type and σ-type,respectively.This finding is a good explanation for the essence that the two complexes have the same UV-Vis(the excitation modes of both are the excitation of the electron in [AQ ? ]-).In addition,we also used the multistate density functional theory(MSDFT)to study the reaction process of the complex to catalyze the generation of singlet oxygen.The results showed that the singlet oxygen catalyzed by the complex has the open-shell electronic structure.The works,in this paper,revealed the microcosmic mechanism of generating singlet oxygen catalyzed by AQ@[K(O-t Bu)]4,provided theoretical support for the experiment of generating singlet oxygen from anthraquinones at room temperature without light,and broaden the way for anthraquinone and its derivatives to generate singlet oxygen.