Theoretical Study On The Photophysical Properties Of Blue Phosphorescent Iridium (?) Complexes

Organic light-emitting diodes?OLEDs? have attracted much attention due to their great potential applications in the flat-panel display device and solid-state lighting source. Phosphorescent materials based on metal complexes, especially for Ir?III? complexes, are one of the most important OLED materials. These luminescent materials could potentially emit light from both the singlet and triplet excitons through the spin-orbit coupling?SOC? provided by the central transition metal atom leading to the high phosphorescence quantum yield and good electroluminescence performance. Unfortunately, the stable and efficient blue-emitting metal phosphor, particularly for the deep-blue emitting one, is still rare as compared to the red- and green-emitting phosphors. To uncover the relationship between structure and property of the complex, quantum chemistry methods are performed to deeply study the geometries, electronic structures, quantum yields, and electroluminescent performance. Additionally, a series of new Ir?III? complexes are designed through the judicious modification of the ligand?s?. Moreover, their photophysical properties are also evaluated theoretically. It is expected that an in-depth understanding of the structure-property relationship might open a high-efficiency way to explore the new complexes. The following three systems are studied systematically by density functional theory?DFT? and time-dependent DFT?TD-DFT? calculations in the thesis:1. The photophysical properties of a series of Ir?III? complexes with the skeleton of dfppy as primary ligand and pic as ancillary ligand are investigated. Except for the six experimentally synthesized Ir?III? complexes, seven new Ir?III? complexes are theoretically designed to explore the influence of different substituted groups and positions on the phosphorescent properties. The ground-state and the lowest-lying triplet excited-state geometries for complexes are optimized at the B3LYP/6-31G?d?-LANL2 DZ level. Simultaneously, the electron density distributions of frontier molecular orbitals?FMOs? and the variation of energy levels are analyzed in detail. On the basis of the optimized geometries, the absorption and emission spectral features are simulated by means of the TD-LC-BLYP and TD-M06-2x functionals, respectively. Additionally, the phosphorescent quantum yields(?_em) of these Ir?III? complexes are analyzed through a qualitative discussion on the variation of the radiative?kr? and nonradiative?knr? rate constants, respectively. Moreover, the electroluminescence?EL? performance is also theoretically evaluated. After introducing the phenyl substituted groups on the pyridine?4a? or difluorophenyl rings?4b? of the primary ligand, the energy gap between the highest occupied and lowest unoccupied molecular orbitals?HOMO and LUMO? is decreased. Correspondingly, their absorption spectra undergo a red-shifting and have a stronger absorption strength. Furthermore, the combinations of larger ³MLCT-³MC energy gap, smaller ?ES1-T₁, and higher contribution of ³MLCT in the emission process result in the higher quantum yields for complexes 4a and 4b. According to the results, the newly-designed complexes 4a and 4b are considered to have potential application as blue-emitting materials with better equilibrium between the hole transport??hole? and electron transport??electron?.2. The phosphorescent properties of seven Ir?III? complexes with dfppy as primary ligand and pytz as ancillary ligand are systematically studied by DFT and TD-DFT calculations. On the basis of the calculated results, three newly-designed Ir?III? phosphors were theoretically proposed by incorporation of the stronger electron-withdrawing group into the 2,4-difluorophenyl and modulation of the nature of the groups substituted in the pyridine of the ancillary ligand. Their possibilities to be blue-emitting phosphors are theoretically evaluated by the EL performance and phosphorescence quantum yield. The results show that introducing the electron-withdrawing substituted group?-CF₃/-NO₂? into the difluorophenyl ring might improve the electron injection capability and lower the hole injection ability. Moreover, the addition of the suitable electron-withdrawing group into the difluorophenyl ring is beneficial to improve the performance of Ir?III? complexes. At the same time, the electron-donating group should be added at the pyridine ring of pytz ligand, which is the guarantee to obtain a higher quantum yield.3. The optoelectronic properties of homoleptic fac-Ir?C^N?3 complexes employing different phenylimidazole-based ligands are systematically investigated. Compared with the heterocyclic bis-cyclometalated Ir?III? complexes, the homoleptic derivatives usually have the higher photochemical stability and longer lifetime. To deeply understand the phosphorescent properties for tris-cyclometalated Ir?III? complexes, seven new complexes are theoretically proposed by introduction of the different substituents into the phenyl ring or imidazole ring in phenylimidazole ligand. The geometrical and electronic structures as well as the optoelectronic properties of three synthesized and seven designed Ir?III? complexes are investigated systematically by DFT and TD-DFT calculations. Furthermore, their ionization potential?IPv?, electron affinity?EAv?, the reorganization energies??hole and ?electron?, and the radiative and non-radiative rate constants are compared. The following results can be obtained:?1? The HOMO-LUMO energy gap is greatly decreased by introduction of the isophthalaldehyde group into the phenyl ring or imidazole ring in phenylimidazole ligand. As a result, their absorption and emission spectra present red-shifting to be the potential red-emitting phosphors.?2? Other complexes are all blue-emitting materials, indicating that the effect of the substituted position on the emitting color is negligible.?3? The addition of the substituent on the para-position of the phenyl ring in phenylimidazole ligand would increase the quantum yield and EL performance as compared with that on the imidazole ring.