The Study Of Singlet And Triplet Energy Splitting Based On Density Functional Theory

Phosphorescence materials have received much attention from chemists in recent years due to their promising applications in bioimaging,chemical detection,dyesensitized solar cells and organic light-emitting diodes.The phosphorescence process is mainly the transition of energy from the triplet to the singlet state,so how to accurately describe the singlet-triplet energy splitting is the most critical step to be solved in theoretical studies,which is also the main focus of this paper.Density functional theory(DFT)is the most important research method in current theoretical studies because of its efficiency and accuracy.Therefore,in this paper,the author focuses on the singlet-triplet energy splitting using DFT,the time-dependent density functional theory(TDDFT),the unrestricted density functional theory(UDFT)and the multistate density functional theory(MSDFT)methods are chosen to investigate the singlet-triplet energy splitting of small molecular systems(divalent radicals)and medium-sized molecules(anthraquinone and derivatives).The main areas of research are as follows.(1)In this paper,the singlet-triplet energy splitting of small molecule systems,namely divalent radicals,is studied by MSDFT.and the influencing factors of MSDFT in the active space construction and the choice of functionals are discussed.The results show that,in terms of active space construction,the orbitals obtained by the complete active space self-consistent field(CASSCF)method are more accurate than those obtained by the restricted open-shell density functional theory(ROKS-DFT).In terms of functional selection,the mean unsigned error(MUE)of the M06 series is much smaller than that of other functionals,making the M06 series more suitable for MSDFT to study the singlet-triplet energy splitting of divalent radicals.(2)In this paper the singlet-triplet energy splitting of medium-sized molecules,namely anthraquinone and derivatives,is further investigated.The performance of TDDFT and UDFT are compared.The results show that the MUE of UDFT is much smaller than that of TDDFT,and that UDFT is more suitable for the calculation of the singlet-triplet energy splitting of medium-sized molecules.In addition,the M06 series of functionals has the smallest MUE,which suggests that the M06 series of functionals is more suitable for the study of singlet-triplet energy splitting of medium-sized molecules.More importantly,the singlet-triplet energy splitting of the medium-sized molecules is studied by MSDFT on the minimal basis.The results of singlet-triplet energy splitting of anthraquinone and derivatives by MSDFT are compared with those of CC2.It is found that the minimum MUE(1.65 e V)is obtained by PBE,which is much larger than that of TDDFT and UDFT.This shows that the dynamic electron correlation energy is more important in the study of singlet-triplet energy splitting,so it is necessary to increase the active space size when studying the singlet-triplet energy splitting of larger systems with MSDFT.The systematic study of singlet-triplet energy splitting in this paper is expected to provide a theoretical foundation for the subsequent calculation and simulation of phosphorescence materials and the improvement of excited state methods.