Design, Synthesis And Optoelectronic Properties Of Blue Phosphorescent Iridium Complexes

PHOLEDs have drawn extensive attention in Display and Lighting industry, due to itsaccess to obtaining high efficiency by fully utilizing energy of excited states.Phosphorescent iridium complexes are deemed to be the most promisingphosphorescent materials because of their several advantages, such as high efficiency,high stability and tunability of color. Compared to green-and red-emitting iridiumcomplexes, the blue ones show lower performances, not only in aspects of colorpurity and efficiency, but also in respect of stability. Iridium complex named FIrpic isstill the best blue-emitting iridium complex material so far, on account of its highquantum efficiency, high thermal stability and its easily preparation. However, its lowcolor purity has limited its development in a large extent. Great efforts have beenmade to optimize the color purity, luminous efficiency and stability of iridiumcomplexes by using different ligand structures and to develop a variety ofblue-emitting iridium complexes. However, the performances of most materials wereworse. Several materials have saturated blue emission but no favorable efficiency andstability, on the contrary, some efficient ones have low color purity. In total,high-efficient deep-blue-emitting iridium complexes are still in great demand.There are three common structures of ligands for blue-emitting iridiumcomplexes and they are phenylpyridine-based ones, aryl-five-membered-heterocycle-based ones and NHC-based ones. It is effective to realize high-energyemission by introducing electron-withdrawing groups to benzene rings, or introducing electron-donating groups to pyridine rings, or selecting ancillary ligands owningstronger electron-withdrawing property for phenylpyridine-based complexes.Complexes based on aryl-five-membered-heterocycle ligands or NHC ligands usuallyexhibit true-blue and deep-blue emission due to the high triplet energies of ligands.According to the principle of realizing blue emission, we have designed andsynthesized and then characterized several series of complexes, and studied therelationships between the ligands and optoelectronic properties of the iridiumcomplexes, attempting to obtain high-efficient deep-blue-emitting materials.In chapter2, we selected two fluorinated2-phenyl-2H-[1,2,3]triazole-basedcyclometalating ligands and prepared two novel iridium complexes. Two complexesboth exhibit no emission at the room temperature, but intense true-blue emission at77K, indicating that it is feasible to realize high-energy emission by utilizing this kind ofligands. We put forward the possible reason for the extremely low efficiency of thesecomplexes at the room temperature by means of theoretical simulations. We thoughtthat the conjugacy and rigidity of the ligands are quite poor and the structures of theligands were distorted in triplet excited states. The severe wagging is likely todissipate the excitation energy, thus resulting in the extremely low PL efficiency.According to the previous research, we knew that NHC ligands owned hightriplet energy and strong coodination ability. Firstly, we designed and prepared a serieof complexes with NHC ancillary ligands. We carefully studied the influence of NHCligands on the performances of the materials compared with complexes with picolinicacid ligand. We demonstrated that the employing of NHC ligands was beneficial toachieving high efficiency, and the NHC ligands had some certain impacts on thelifetimes and color purities of complexes. In addition, we prepared a novel complexbased on phenyltriazole-type NHC ligand, which exhibited a efficient deep-blueemission with a CIE coordinates of (0.15,0.06), and a phosphorescence quantumefficiency of19.21%. We studied the EL property of this complex preliminarily andproved it to be a potential efficient deep-blue-emitting material. According to the principle of designing a blue-emitting iridium complex, weintroduced a new electron-withdrawing group called cyanato group to the liganddfppy, hoping to lower the HOMO energy levels and then achieve high-energyemission. We selected three ancillary ligands to synthesize three true-blue-emittingiridium complexes and deeply researched into the optical and electronic properties.The main peaks of their PL spectra are450nm~460nm and show a blue shiftcompared to that of complex FIrpic. The quantum efficiencies in film are75%~90%for the three complexes. Furthermore, we studied the influences of ancillary ligandson the optoelectronic properties of this kind of complexes.