Theoretical Research On Luminescence Mechanism And Molecular Design Of Thermally Activated Delayed Fluorescence Of Cuprous Halide Complexes

Organic Light-Emitting Diode（OLED）technology is gaining popularity in the field of optical display due to its flexibility,lightness,and self-illumination capabilities.The luminous efficiency of the device is determined by the material used for the light-emitting layer,which serves as the core component of an OLED device,making it essential for achieving self-illumination.Designing and developing luminescent materials with excellent performance is therefore critical for the advancement of the OLED industry.While conventional fluorescent materials have low exciton utilization,and phosphorescent materials are costly,cuprous（Cu（Ⅰ））halide complexes with thermally activated delayed fluorescence（TADF）properties are both cheap and environmentally friendly.Their triplet excitons can be converted into singlet excitons through the reverse intersystem crossing（RISC）processes,resulting in a theoretical internal quantum efficiency of up to 100%,making Cu（Ⅰ）halide complexes ideal candidates for high-performance OLED luminescent materials.This paper investigates the luminescence mechanism of a series of Cu（Ⅰ）halide complexes from the perspective of theoretical calculations,using quantum chemical calculations.The geometric configuration,electronic structure,front molecular orbital distribution,and the leap rate of each luminescence process of the complexes were analyzed to reveal the relationship between the structure of Cu（Ⅰ）halide complexes and their TADF mechanism,as well as the luminescence efficiency.The paper’s main research contents are as follows:1.Investigating the effect of the nitrogen ring ligand structure on the luminescence mechanism of Cu（Ⅰ）halide complexes through theoretical calculations on three Cu（Ⅰ）halide complexes with slightly different structures of the nitrogen ring ligands.It was found that the introduction of the pyridine ring to extend theπ-conjugation of the nitrogen ring ligands can effectively reduce the overlap between HOMO and LUMO,make the complex have a smaller energy difference(ΔE_ST)between the lowest singlet state（S₁）and the lowest triplet state（T₁）,and restrain the structural relaxation caused by the plane distorted vibration of nitrogen ring ligands,obtain smaller recombination energy（λ）.This allows for the fast RISC process,resulting in efficient TADF properties.However,extending theπ-conjugation of the nitrogen ring ligand using the benzene ring changes the leaping property of the T₁,which has obvious local excitation characteristics,makeingΔE_STincrease.While changing the vibrational modes that contribute most to the recombination energy in the inter system crossing（ISC）and RISC processes,λincreases and the structural rigidity decreases,hindering the（R）ISC process smooth realization,leading to phosphorescence emission instead.2.Designing three new Cu（Ⅰ）halide complexes based on experimentally reported Cu（Ⅰ）halide complexes with high TADF performance by changing the species of substituents on the bidentate phosphorus（P^P）ligand to investigate the effects of the substituent steric hindrance and electronic properties on the TADF performance of the Cu（Ⅰ）halide complexes.It was found that increasing the spatial site resistance of the complexes by introducing substituents into P^P ligands can suppress structural distortion during excitation and enhance the structural rigidity of the complexes.When the electron-accepting ability of the substituents was stronger,theΔE_STof the complexes could be reduced,and the spin orbit coupling（SOC）effects could be enhanced,allowing the complexes to possess more rapid inter system crossing（ISC）and RISC processes.When the substituents have strong electron-donating ability,it is more favorable to reduce the nonradiative energy dissipation during the fluorescence process of the complex.In addition,the newly designed complex B3 shows a faster transition rate than the experimental molecule B2 in both fluorescence radiation as well as ISC and RISC processes,which has the potential to be an efficient TADF material and is worthy of synthesis.3.The effects of halogen atom species on the TADF properties of Cu（Ⅰ）halide complexes were investigated,and the results showed that decreasing of the electronegativity of halogen atoms effectively extended the distribution range of the complex’s HOMO.This reduced the overlap between the HOMO and LUMO of the Cu（Ⅰ）halide complex,resulting in a decreasedΔE_STand enabling fast ISC and RISC processes.Furthermore,greater electronegativity of the halogen atoms in the Cu（Ⅰ）halide complexes led to more intense oscillations and an increased fluorescence rate.In addition,reducing the electronegativity of halogen atoms can reduce the proportion of MLCT in the electron transition of the complex,which can effectively restrain the geometric distortion of Jahn-Teller caused by MLCT and increase the quantum yield.