Theoretical Study On Phosphorescent Properties Of Iridium And Platinum Complexes

As one of the potential candidates for next generation flat-panel displays and solid-state lighting,organic light-emitting diodes?OLEDs?have attracted numerous attentions owing to a lot of advantages.Phosphorescent OLED,composed by transition metal complexes,is considered to be one of the most promising candidates,since the spin-forbidden nature of the T₁?S₀ radiative relaxation is broken due to the strong spin-orbit coupling?SOC?effect induced by the transition metal.As a result,the quantum efficiency is up to unity by harvesting both both the singlet and triplet excitons.Among them,iridium and platinum complexes have attracted more attention than other metal complexes due to their high quantum efficiency and good photophysical properties.The scarity of full-color display and the low external quantum efficiency have greatly blocked the large scale application of OLED.To elucidate the relationship between structure and property of transition metal complex and evaluate the quantum efficiency would be helpful to develop highly efficient phosphorescent complexes.However,it is difficult to quantitatively calculate the quantum efficiency due to the complexity of the phosphorescence process.In this thesis,the geometrical configuration,spectral properties and phosphorescent properties of the complexes are studied by the density functional theory?DFT?and time-dependent density functional theory?TD-DFT?to clarify the relationship between the structure and properties of the complexes and their phosphorescence properties.More importantly,the quantum efficiency of transition metal complex has been calculated by different models and methods including qualitative,semi-quantitative and quantitative methods.The research work of this thesis mainly includes the following four parts:1.The complexity of emissive process for five heteroleptic Ir?III?complexes with4-methyl-2',6'-difluoro-2,3'-bipyridine as primary ligand is unveiled by the PBE0,TD-PBE0 and quadratic response TD-B3LYP methods to explore the effects of different ancillary ligands on the phosphorescence properties.The emission wavelengths,radiative decay rate constants?k_r?and nonradiative decay processes of thermal activation are investigated.In addition,their emission rules are comfirmed.Complex with N,N'-di-tert-butylbenzamidinate as ancillary ligand follows the Kasha rule and other complexes obey dual emission scenarios.The newly designed molecules are potential blue-green phosphorescent materials.In this system,the quantum efficiency is discussed by semi-quantitative method.The k_r is quantitatively determined by the quadratic response TD-B3LYP method.The triplet potential energy surfaces are constructed to elucidate the factors that affect the temperature-dependent nonradiative rate constants(k_nr?T?).Complex with N,N'-di-tert-butylbenzamidinate as ancillary ligand has the higher quantum efficiency in all the investigated complexes because of the larger k_r and smaller k_nr.2.The phosphorescent properties of two experimental synthesized Ir?III?complexes?DPQ?₂Ir?pic?and?DPQ?₂Ir?dpm?and two designed Ir?III?complexes?DPQ?₂Ir?ozl?and?DPQ?₂Ir?iml?are theoretically investigated by the PBE0 and TD-PBE0 methods with the aim to explore the effect of ancillary ligand.The emission rules and the quantum efficiency are explored.In addition,their solubility in common organic solvents are also studied.The k_r is calculated,and the transition dipole moment,spin-orbit coupling matrix element,and singlet-triplet splitting energy related with the k_r are also analyzed to further uncover the crucial factors to affect the k_r.While the nonradiative rate constant(k_nr)for phosphorescence is qualitatively estimated from both temperature-independent nonradiative rate constant(k?_nr)and temperature-dependent nonradiative rate constant(k_nr?T?)viewpoints.The results show that the emissive wavelengths of?DPQ?₂Ir?ozl?and?DPQ?₂Ir?iml?are in the red region.Complex?DPQ?₂Ir?iml?has the larger quantum efficiency because of both larger k_r and smaller k_nr.Moreover,it has the better solubility in three common organic solvents than other three Ir?III?complexes,which is beneficial for reducing production cost.Therefore,the variation of ancillary ligand is also an advisable choice to develop red-emitting Ir?III?complex with ideal quantum efficiency.3.The properties of three iridium complexes with 3-methyl-6-?2?,4?-difluoro-pyridinato?pyridazine as primary ligand are theoretically investigated by the B3LYP and TD-B3LYP methods.On the basis of experimental reported complex?dfpypya?₂Ir?pic?,other two complexes are theoretically designed by introduction of the hydroxyl group into the ancillary ligand.The difference between?dfpypya?₂Ir?pic-OH?and?dfpypya?₂Ir?pic-OH??is whether the intramolecular hydrogen bond is formed.The main goal is to explore the influence of intramolecular hydrogen bond on quantum efficiency.All three complexes are potential blue emitting phosphorescent materials.Their emission spectra and emissive rules are studied with the previous similar methods.The quantum efficiency is determined by three different methods including the semi-quantitative and quantitative methods.The quantum efficiency of complex?dfpypya?₂Ir?pic-OH?is the highest,which is mainly attributed to the fact that it has the smallest k_nr?T?.Construction a hydrogen bond is a method to improve the quantum efficiency,which paves a new avenue to build blue emissive phosphor.Moreover,the intramolecular hydrogen bond is also helpful to enhance the stability.4.The phosphoresscent properties of a series of platinum complexes with isoquinolinyl pyrazole ligands,including four experimental reported and three new designed complexes,are studied by the B3LYP,TD-B3LYP and M06 methods.The effect of different aromatic ligands or substituents on the?-?stacking interaction and photophysical properties are predominantly investigated.Besides monomer,the possibility to form dimer,trimer,and tetramer by?-?stacking interaction is investigated.In addition,the effect of different coordination environments on excitation energy and quantum efficiency is also explored.The results show that the intermolecular?-?stacking interaction energy is weakened when the isoquinolinyl ring is replaced by the pyridyl ring step by step.The incorporation of conjugated or electron-donating substituents on the pyridyl azolate would improve the?-?stacking interaction.Compared with pyridyl pyrazole,the coordination of platinum with isoquinoline pyrazole is beneficial to the improvement of quantum efficiency.However,the different substituents on the pyridyl ring have a negligible influence on the quantum efficiency.