Quantum Chemistry Study On The Structure,luminescent Mechanism And Efficiency Of Nitrogen-containing Heterocyclic Ligand Multidentate Metal (Cu(?),Au(?)) Complexes

Organic Light Emitting Diode?OLED?technology,which is generally considered as a great breakthrough in display filed,has developed rapidly in recent years.Most of Liquid Crystal Display?LCD?and inorganic materials have been replaced by the OLED materials in the display field rapidly with the advantages of large area and flexible display.The high-performance OLED devices require not only accurate fabrication technology and well coordination among layers,but also the higher efficiency of the emissive layer materials.As an important kind of emissive layer materials for OLED devices,metal complexes have irreplaceable advantages of inorganic and non-metallic organic materials in terms of flexibility,luminous efficiency and stability.There are two main luminescent mechanisms,phosphorescence and thermally activated delayed fluorescence?TADF?.The metal complexes can effectively mix the singlet and triplet excitons to obtain fast?reverse?intersystem crossing progress,which is attributed to the strong spin-obit coupling effect caused by the metal ion,then completed the emission of phosphorescence or TADF.Theoretically,the quantum yield of metal complexes can reach 100%whether they are phosphorescent emission or TADF emission.Based on the related literatures,it is found that the effects of structure of chelated ligand on the luminescent properties were obvious.Because the luminescent mechanism and efficiency caused by the variety ligands were volatile and complicated,there is still no universal conclusion to predict the luminescent efficiency of metal complexes.In this paper,the series of metal complexes with N-containing heterocyclic ligand have been studied by DFT/TDDFT methods.The electronic properties,luminescent properties,luminescent mechanism and quantum yield were studied detailedly in the aspects of the radiative decay transition and non-radiative decay transition process,which can provide valuable theoretical reference for the synthesis and design of metal complexes.This paper mainly includes the following sections:?1?New insight into the effect of N^N ligand isomerization and methyl modification on the phosphorescence properties of Cu???complexes with?1-?2-pyridyl?pyrazole/imidazole?ligands.In this part,a series of[Cu?N^N??P^P?]⁺complexes were investigated,in which their ground and excited states structures,absorption and emission spectra,radiative decay rate?k_r?and non-radiative decay rate(k_nr)were predicted by DFT/TD-DFT methods.The results indicate the additional methyl could increase the ratio of the d orbital in frontier molecular orbitals of Cu???complexes,and then enlarge the ³MLCT?%?.Meanwhile,the substitution of the parazole with an imidazole group could reduce the ratio of³MLCT involved in emission transition.Furthermore,vibration analysis reveals that a methyl group substitution in the ortho-position of N₂ increases the steric hindrance,and then suppresses the vibration of benzene and the non-radiative dissipation caused by methyl,which reduces the non-radiative decay rate(k_nr)finally.Our studies provide the intrinsic reason for the high PLQY of[Cu?N^N??P^P?]complexes with ortho-methyl modification.?2?A Study on the Modification of Azole Rings to Regulate the Transition Dipole Moment,MLCT and T₁ Structural Distortion of 2-Pyridyl-azole Cu???Complexes for High Phosphorescence Performance.It is generally believed that the participation of metal ion on the transition is proportional to the radiative transition,which means the higher the participations of Cu???,the faster the radiative transition rate.But,based on the study of a series of Cu???complexes with2-Pyridyl-azole ligands,it is found that based on the effective spin-orbit coupling,the large transition diploe moments?the S_n state has both effective SOC and large transition dipole moments of S_n?S₀ transition?of the singlet excited state is the decisive factor to obtain high phosphorescence.The results illustrate that a higher participation of metal ion on transition is not the only determinant factor of the phosphorescence efficiency.Consequently,in the development of the 2-pyridyl-azole Cu???luminescent materials,the high quantum yield materials can be designed by adjusting the substituent at C₃ position and the strength of electron donor-acceptor groups at C₅ position to balance the charge transfer transition modes.?3?A theoretical study on the influence of N-containing heterocyclic ligands on the luminescence mechanisms?phosphorescence or TADF?of Au?III?complexes.In this section,DFT and TDDFT methods were adopted to discuss a series of Au?III?complexes,and the effect on the electronic properties and luminescence mechanism caused by the modification of ligands were studied.It was found that the influence on energy level of HOMO was weakened by the addition of carbazole group on complex 5.Meanwhile,the local-excitation character was shown in the T₁ state due to the elongation of the bond Au-C₃.These lead to a significant increase of?E_ST and SOC.At the same time,the reorganization energy between the S₁ and T₁ states,enlarged obviously because of the rotation angle of the benzene linked between the Au?III?and N₂ atom.As a result,the reverse intersystem crossing rate(k_RISC)was too small that the exciton could not reach to the S₁ state,and finally phosphoresce through the T₁ state.The results illustrate that although the SOC between the S₁ and T₁ states is one of the necessary factors for the TADF,the probability of TADF can not be estimated completely according to the value of SOC.More attention should be paid to whether the energy between S₁ and T₁ is conducive to the thermal activation and mixing of the S₁ and T₁ states.If the the thermal activation is not obtained,TADF phenomenon may not occur.Our studies provide the relationship between the structure and luminescent properties,as well as the reasonable theoretical support for the research of TADF materials.