Theoretical Study On Luminescence Properties Of Pd(Ⅱ) Complexes With Carbazole And Pyridine Ligands

The Organic Light Emitting Diode is an important material in today’s display market.It has become a leader in modern display materials due to its advantages of ultra clear,ultra-thin,ultra wide display angle,ultra fast response speed and high contrast.Among them,in terms of device stability and luminescence efficiency,organometallic complex materials have remarkable advantages over inorganic and non-metallic materials.However,low stability of blue light emitting devices and triplet-triplet annihilation limit the application of metal complex materials in organic light-emitting diodes.Among various organometallic complexes,Pd（II）complexes with structurally rigid,moderate spin-orbit coupling effects,as well as rich electron transition types,are enable to utilize 100%excitons and reduce lifetimes of excited state through various light emitting pathways,making Pd（II）complexes as the potential materials for organic light emitting diode emission layers.In this thesis,using density functional theory and time-dependent density functional theory with Tamm-Dancoff approximation,the photophysical properties of a series of planar structures of carbazole and pyridine coordination groups with 5/6/6 and 6/6/6 membered rings have been studied.The luminescence mechanism of the metal assisted delayed fluorescence of the studied complexes was expounded from three aspects,and the important factors affecting their photophysical properties were discussed.The reason why the complex 3containing azacarbazole ligands exhibits a different luminescence mechanism in the solution and in the solid phase has also been explained.Geometrical structures.Complex 1 with imidazole ligands and complex 2 with methyl-substituted imidazole ligands undergo smaller structural deformation during excitation than complex 3.At the same time,by calculating the molecular planarity parameter and the span of deviation from plane,it is found that the planar stability of the 5/6/6 membered ring is greater than the 6/6/6 membered ring,which effectively inhibits the non-radiative transition process of complexes 1 and 2.These prove that the 5/6/6 structure is more stable and has higher internal quantum efficiency than the 6/6/6 structure.The relationship between geometric structures and luminescence properties was revealed.Electronic structures.The analysis of the natural transition orbital shows that the lowest excited singlet state of complexes 1 and 2 is a charge transfer transition,while the excited type of the lowest excited triplet state is mixed of the charge transfer and local excitation transition,so that the gap differenceΔE_STbetween the single and triplet states of the two complexes in solution significantly less than 3（S₁:CT transition,T₁:LE transition）.Therefore,complexes 1 and 2 are more prone to the reverse intersystem crossing,resulting in a metal assisted delayed fluorescence mechanism,whereas complex 3 is more likely to emit phosphorescence.Transition energies and transition rates.In this work,the important rates affecting the luminescence process of all complexes have been quantitatively calculated.According to the discussion of the factors that affect the mechanism of metal assisted delayed fluorescence(lifetime,ΔE_ST,spin-orbit coupling,reorganazation energy,etc.),it is found that when theΔE_STof complex 1 is between 0.134e V and 0.247 e V,the reverse intersystem crossing rate and the phosphorescent radiative decay rate can achieve a dynamic balance,and the complex emits light via the metal assisted delayed fluorescence mechanism;whenΔE_ST（27）0.134 e V,the reverse intersystem crossing rate is very fast,and only thermal activation delayed fluorescence occurs;whenΔE_ST（29）0.247 e V,the reverse intersystem crossing rate is very small and even disappears.Complex 2 is similar to 1,the range of the energy gapΔE_STunder metal assisted delayed fluorescence mechanism is 0.111-0.224 e V.At the same time,through solid phase environment simulation,it is found that the energy level of T₂state of complex 3 was lower than the S₁state,which makes its luminescence process cannot be simulated by the three state model（complexes 1 and 2;S₀,S₁,T₁）,instead of the four state model（S₀,S₁,T₁,T₂）.From a microscopic point of view,this work discusses the luminescence mechanism of metal-assisted delayed fluorescence and the important influencing factors.At the same time,the relationship between geometric structures and luminescence properties is clarified.These results provide theoretical reference for the future development of Pd（II）complexes with various luminescence mechanisms,and provide theoretical support for the market promotion and application of deep blue organic light emitting diode materials.