Synthesis And Study Of Electroluminescent Property Of Yellow-emitting Iridium (Ⅲ) Complexes

Currently, organic light-emitting diode (OLED) is a very popular research direction owing to potential applications in flat panel display and solid-state lighting. Phosphorescent materials play an important role in OLED, because they can utilize both singlet and triplet excitons, and reach a theoretical internal quantum efficiency of100%. Iridium (Ⅲ) complexes are the most promising phosphorescent material because of their remarkable strong spin-orbit coupling and rather short phosphorescent lifetime. For iridium (Ⅲ) complex, the organic ligand chromophore can greatly affect the electroluminescence efficiency and the spectroscopic property of the complex. Therefore, developing new ligands and modifying existing ligands can obtain different light-emitting complexes and then achieve full color display.Yellow is an intermediate color, usually with the typical CIE coordinates of (0.50,0.50). The iridium (Ⅲ)complex containing the2-phenylbenzothiazole (bt) as cyclometalating ligand and the acetylacetone (acac) as ancillary ligand, i.e.(bt)2Ir(acac), is one of the earliest reported yellow phosphors. It has a short phosphorescent lifetime of1.8μs, maximum emission wavelength of565nm, CIE coordinates of (0.51,0.49). For performance optimization and wavelength tuning, a lots of (bt)2Ir(acac) derivatives have been developed so far. For example, our group obtained several iridium (Ⅲ) complexes by introducing various substituents on the6-position of2-phenylbenzothiazol ligand, and the performance of (CF3-bt)2Ir(acac) was the best with emission peak at564nm, which is very similar to that of (bt)2Ir(acac). The luminance efficiency and power efficiency of (CF3-bt)2Ir(acac) have reached as high as76.0cd A-1and39.8lm W-1, being one of the best data for yellow phosphorescent OLED reported so far.In this desertation, we designed and synthesized eight2-phenylbenzothiazole derivatives by introducing the electron-withdrawing-F and-CF3or electron-donating diphenylamino substituent into the4,5, or7-site of benzothiazole ring. Using these derivatives as cyclometalating ligands and acetylacetone (acac) as ancillary ligand, six heteroleptic iridium (Ⅲ) complexes have been prepared. The new ligands and iridium (Ⅲ) complexes were characterized by MS and1H NMR. The photophysical, electrochemical, and electroluminescent properties of these iridium (III) complexes have been systematically studied. The room-temperature phosphorescence peaks for all these iridium (Ⅲ) complexes are the range of563～568nm, belonging to yellow light. In electrochemical measurements, they exhibited one-electron oxidation waves with the onset potentials at0.83～0.94V. In order to study the electroluminescent property of Ir-(bt-5-CF3), Ir-(bt-5-F), Ir-(bt-5-Ph2N), Ir-(bt-7-CF3) and Ir-(bt-7-F), we prepared OLEDs using these iridium (III) complexes as doped emitters. The OLEDs structure is as follows:ITO/PEDOT:PSS (45nm)/TCTA (6nm)/(Ir (III) complex:CBP (5wt%,30nm)/TPBI (45nm)/LiF (1nm)/Al (150nm). All these iridium (III) complexes exhibited yellow electrophosphorescence with good performance. In particular, the performance of Ir-(bt-7-CF3) is the best with the maximum brightness of46060cd m-2, the luminance efficiency and power efficiency of64.0cd A-1and28.7lm W-1, and CIE coordinates of (0.49,0.50), which are among the best data for yellow phosphorescent OLEDs reported so far.