Theoretical Study On Excited State And Photophysical Properties Of Transition Metal Complexes

Organic light-emitting diodes（OLED）are widely used in flat-panel displays and solid-state lighting due to their advantages of self-emitting light,low electron-driving and large-area luminescence.Therefore,the design and synthesis of luminous materials with high efficiency and stable performance have attracted much attention.For transition metal complexes,there are two main luminescence mechanisms that can achieve 100%exciton utilization:One is phosphorescent materials,with the aid of the spin-orbit coupling（SOC）of metal atoms,the singlet excitons through rapid intersystem crossing（ISC）to the triplet excited state to radiate phosphorescence,represented by platinum（Pt）and iridium（Ir）complexes.The other is thermally activated delayed fluorescence（TADF）materials,the nonluminous triplet excitons would reach the single excited state through reverse intersystem crossing（RISC）to radiate delayed fluorescence,at room temperature represented by silver（Ag）and gold（Au）complexes.Recent studies have shown that complexes centered on palladium（Pd）and copper（Cu）would have different luminescence properties when combined with different ligands,radiating phosphorescence alone,or radiating TADF alone,or radiating phosphorescence and TADF at the same time.The dual-emission mechanism is useful for achieving 100%internal quantum effciency（IQE）.Nowadays,there are few theoretical studies on this kind of complex,especially the properties of excited states.In this paper,we use quantum chemistry calculation methods to study the excited state properties of Pd and Cu complexes,explain their luminescence,and clarify the relationship between structure and property.The main research contents of this paper are presented as follows:（1）In the first part,in order to understand the interesting luminescence mechanism of molecules,phosphorescent molecule Pd（TZp-1）（in which the Tzp is 1,2,3-triazole）is selected as the parent.On the premise of not changing the central atom,the influence of the ligand on the luminescence mechanism of the complex was emphatically analyzed,aiming to realize different luminescence mechanisms by changing different ligands,finding out the reasons by means of theoretical calculations.In this work,in order to achieve the conversion from phosphorescence to TADF,the ligands were designed by introducing benzene rings in the triazole（Tz）portion while keeping the Ph-O-Cz portion fixed,with the expectation of enhancing the electron-withdrawing ability of the acceptor by extending the conjugation,and then replacing the benzene ring with pyrazine（Pz）to further enhance the electron-withdrawing ability of the ligand unit.The enhancement of the electron-withdrawing ability of the acceptor is conducive to formating obvious charge transfer within the molecule.This is beneficial to decrease the energy gap of singlet-triplet states.The reduction of the singlet-triplet energy gap promotes the generation of TADF in phosphorescent molecules,so as to change the luminescence mechanism of the molecule.（2）On the basis of understanding the mechanism of molecular luminescence,design and synthesis of efficient luminous complexes are still the most important thing.To understand the luminous efficiency of molecules in depth,we analyzed two tetrahedral Cu（N^N）（P^P）complexes with similar molecular structures and great efficiency differences and comprehensively studied the properties of excited states in detail,aim to revral the reason for the significant difference in quantum efficiency.The transition properties and excited state properties of the molecule were analyzed in detail by quantitative calculation,it’s found that benzene in the two P（Ph）₃ ligands of the complex molecule would produce two single excited states with similar energy leading to two paths of RISC after changing from“-C（CH₃）₂-”to“-NH-”.The single excited state,which is reached at a faster rate of RISC,is transition-forbidden and non-radiative,resulting the molecule almost not emitting.（3）The environment of the molecule also has an important influence on the luminescence behavior of the molecule.We found that Pd（Tzp）（in which the Tzp unit is 1,2,4-triazole）emits phosphorescence in solution phase alone,but emits phosphorescence and TADF simultaneously at the same time in the solid film phase.In order to explore the influence of the environment on molecular luminescence behavior,we used PCM solvent model to simulate dichloromethane（DCM）solution phase that the molecule is in.Simulate the PMMA solid film in the Mat Studio program according to the manual.The results show that the molecular configuration in the solid phase environment would be squeezed by the surrounding molecules more than that in the solvent phase.The change of the structure makes the energy gap between singlet and triplet excited states of the molecules decrease obviously,resulting in the existence of TADF phenomenon in the solid phase but not in the solvent phase.