Quantitative Evaluations Of Decay Mechanism Of Transition Metal Complexes And Ligands Decoration For Higher Quantum Efficiency

Organic light-emitting diodes?OLEDs?are a kind of display screen with advantages in wide viewing angle,strong contrast ratio and so forth.However,this display technique still suffered in the performance and endurance of blue,red,green,and white light emitting.The previous fluorescent OLEDs were gradually replaced by phosphorescent light-emitting ones,due to their theoretically higher efficiency.Among phosphorescent materials,transition metal complexes are remarkable,and they are the focus of this investigation.In a framework of OLED,both charge transfer materials and light emitting materials are indispensable,in this thesis,we focus on the mechanism of electronic decay from excited states.Through qualitative and quantitative theoretical investigation,both radiative and non-radiative decay processes were evaluated for certain typical complexes containing iridium ion?Ir????.For the radiative decay processes,the spin-orbit couplings between singlet and triplet states were calculated.For non-radiative decay processes,the Fermi's golden rule was employed in computation of the rate,and the analysis of group vibrations was conduct through resolving the main normal modes that contributing the most to the reorganization energies.Subsequently,new iridium compounds with high quantum efficiencies could be designed with the help of the analysis results mentioned above.This thesis is divided into six chapters.The first chapter is the preface,which introducing the functions and structures of OLED,and summarizing the developing history of OLED.The second chapter is the foundation of theory relating to researches in this thesis.It contains the basics of quantum mechanics,the computational method of quantum chemistry,and the principle of basis sets.Besides,the theory of spectroscopy was also concerned in this chapter for the assessment of electronic decay mechanism.For the third to the sixth chapter,a series Ir???complexes were evaluated with an integrated theoretical investigation method,to uncover the reason of large non-radiative decay rate.Meanwhile,some new Ir???complexes were designed to enhance the quantum efficiency.The main research contents are as follows:1.For purpose of calculating radiative and non-radiative rate constants,an integrated theoretical investigation method was employed on several common Ir???complexes,to verified its feasibility.This theoretical scheme was firstly applied to the famous fac-Ir?ppy?₃ complexes,the consistence between experimental and computational results impelled us to uncover the reason of non-emissive Ir?ppy?₂DBM at ambient temperature.The largest non-radiative decay rate of Ir?ppy?₂DBM accounted for its relative low quantum efficiency.Through decomposing typical normal modes that contributing to the large non-radiative decay process revealed that the vibrations of DBM ancillary ligand were responsible for its non-emissive character.Based on this theoretical analysis,a new complex was designed,and a higher quantum efficiency was achieved logically.2.On the basis of our integrated theoretical methods on investigating photophysical properties of Ir???complexes,the influences of substitution groups on the ancillary ligand were then evaluated.A series of Ir???complexeswiththesamecyclometalatingligand2-?2,4-difluorophenyl?pyridine?dfppy?were chosen.The ancillary ligands have a same skeleton structure of biimidazole.The differences between them is the substituent groups on ancillary ligands with electron-donating or–withdrawing properties.The calculational results showed that substituting electron-withdrawing groups leading to increase of twist in biimidazole-type ligand,thereby decreasing the quantum efficiency.3.Subsequently,the same theoretical approach set its foot in influences of substituent groups on cyclometalating ligands.There was a kind of Ir??? complexes with a same ancillary ligand,picolinate,and with a same main part of the cyclometallating ligand,2',6'-difluoro-2,3'-bipyridine.The only difference between them was different substituent groups on cyclometalating ligand.Experimental results showed that electron-withdrawing substituent groups could enhance the quantum efficiency,while electron-donating substituent groups could suppress the quantum efficiency.The calculated results through our integrated theoretical scheme were consisted with the experimental findings.However,when analyzing the origins of large non-radiative decay rates,it was revealed that the vibrations of bulky substituent groups contribute the most to the activation of non-radiative decay processes.4.In the process of non-radiative decay,vibrational relaxations constitute a large part,which is also embodied in theoretical evaluations.In the non-radiative formula,the reorganization and Huang-Ryhs factor evaluated the degree of molecular deformation in electronic transition from excited state to ground state.Based on this fact,we replaced the hydrogen atom with deuterium,in order to increase the nuclear mass and further decrease its amplitude.A series of well-known Ir???complexes were chosen for this theoretical test.Results show that deuteriation on the ligand that relating to electronic transition enable to increase the quantum efficiency.