Synthesis And Electroluminescence Properties Of Novel Orange Iridium Complexes

The advent of the information age promotes the rapid development of flat-panel displays. The organic light-emitting diodes (OLEDs) has been established as most promising next-generation display technology due to many merits. Among the various electroluminescent materials, phosphorescent material is much more promising than the fluorescent material owing to its propability to realize a theoretical internal quantum efficiency of100%. With the increasingly intensive investigation in this field, a large amount of phosphorescent materials have been developed, which can emit light with various colors ranging from deep blue to near infrared.Cyclometallated iridium complexes are a type of important electroluminescent materials, which have short excited-state lifetime and high phosphorescence efficiency. Now, iridium complexes have been regarded as the most promising phosphors in OLEDs field and can be used for commercial purpose. Bis-cyclometallated iridium complexes based on the2-phenylbenzothiozole ligand have been widely studied as electroluminescent materials previously, which show good light-emitting performance and stability in the device. The structure of the ligand is easy to modified, and lots of iridium complexes were derived from it. The concurrent iridium complex derivatives based on2-phenylbenzothiazole ligand can be divided into two groups. One type is those with simple substituents on either phenyl ring of the ligand, and the other group is those with2-aryl-benzothiazole ligand, in which the aryl is aromatic structure rather than phenyl. By structure modification in these ways, the performance has been improved and the emission color could be tuned from yellow to red. However, as far as we know, there has been no report that the benzene ring of benzothiazole part in the2-phenylbenzothiazole ligand is replaced by other aromatic structure.In this desertation, based on the2-phenylbenzothiazole as ligand model, we designed and synthesized five novel cyclometalating ligands by replacing the benzene ring of benzothiazole part by carbazole (L-Cz and L-CzPO), dibenzofuran (L-DBF), dibenzothiaphene (L-DBT), or naphthane(L-Np). Four bis-cyclometalating heteroleptic iridium complexes were prepared using these ligands and acetylacetone(acac) as ancillary ligand. The photoluminescent and electroluminescent properties of these iridium complexes were investigated systematically. In addition, the influence of these novel ligand structures to the emissive properties of the resultant iridium complexes was studies. To the best of our knowledge, these cyclometalating ligands and the corresponding iridium complexes are reported for the first time.The chemical structures of the four iridium complexes were characterized by the Nuclear Magnetic Resonance and the Mass Spectrometry, and their photophysical and electrochemical properties were measured through UV-vis absorption spectra, emission spectra, cyclic voltammetry, etc. Compared to the bis-cyclometallated iridium complex(Ir-Bt) containing2-phenylbenzothiazole ligand, these four iridium complexes show red-shifted emission. In dichloromethane solutions, the phosphorescent emission wavelengths of these iridium complexes are ranging from563nm to600nm at room temperature, which belong to orange region. Light-emitting devices were fabricated by high-vacuum vapor deposition techniche, using CBP as the host material and these iridium complexes as the doping emitters. All these iridium complexes exhibited orange electrophosphorescence with good performance. In particular, the maximum luminance efficiency of the device based on Ir-Np has reached55cd/A, being among the best results for the orange phosphorescent organic light emitting devices reported so far.