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Two novel heteroleptic iridium complexes containing 4-phenylquinazoline (pqz) as a cyclometalating ligand, namely Ir1 and Ir2, are designed and synthesized. Methoxy groups are incorporated into the 6- and 7-sites of pqz rings to tune the physical properties and the emission performance of the complexes. The introduction of methoxy groups in Ir1 and Ir2 causes a slight blue shift by 3–5 nm relative to the reference complex Ir(pqz)2(acac) (630 nm), and moderate photoluminescence quantum yields of 0.35 and 0.38 and short lifetimes of 0.79 and 0.64 μs, respectively, are obtained. Ir1 and Ir2 are used as doped emitters to fabricate phosphorescence organic light-emitting diodes and pure red electroluminescence (EL) is achieved. In comparison with the reference complex Ir(pqz)2(acac), dramatically improved EL performance with color purity and efficiencies are realized for both Ir1 and Ir2. In particular, the Ir2 device exhibits a maximum external quantum efficiency of 26.22% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.65, 0.34), which is close to the best efficiencies reported for other pqz based red iridium phosphors but advantageous in terms of a simple device structure with a single emitting layer. [ABSTRACT FROM AUTHOR]

Blue iridium phosphors are always precious and desirable for both fundamental research and industrial applications in organic light-emitting diodes (OLEDs). Three heteroleptic cyclometalated iridium(III) complexes, namely, Ir1, Ir2 and Ir3, incorporating 2-(3,5-bis(trifluoromethyl)phenyl)-pyrimidine (tfmphpm) or 2-(2,4-difluorophenyl)-pyrimidine (dfppm) as the main ligand, and 2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridine (fppz) or 2-(5-(4-(trifluoromethyl)phenyl)-2H-1,2,4-triazol-3-yl)pyridine (Htaz) as the ancillary ligand, were developed for application in OLEDs. Owing to the introduction of the strong-field CF3-containing ancillary ligand and the incorporation of electron-withdrawing F or CF3 groups on the HOMO-lying C-related aromatic rings of the C^N cyclometalating ligands, the phosphorescence of these Ir(III) complexes peaked at 457–459 nm. In comparison with the well-known commercial sky-blue iridium complex FIrpic (bis(4′,6′-difluorophenylpyridinato) iridium(III) picolinate, 475 nm), the phosphorescence of these complexes is hypsochromically shifted by 16–18 nm, more closely approaching standard blue light. They have high phosphorescence quantum yields in solution. The phosphorescent OLEDs employing these complexes as doped emitters display impressive performance. In particular, the PhOLED based on Ir3 achieves an outstanding maximum external quantum efficiency of 21.23% with Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.26), which is about 27% higher than that of the control device using FIrpic as a dopant (16.63%). [ABSTRACT FROM AUTHOR]

The weak electron donating ability of carbazole is most suitable for constructing the donor-acceptor type blue emitters, but its low spatial requirements caused by the five-membered ring bridged structure is not favorable for spatial separation of frontier molecular orbitals. So many carbazole based compounds do not exhibit thermally activated delayed fluorescence (TADF). Herein, a more sterically demanding group was introduced at the 1-site of carbazole to form 1-methylcarbazole (1-MeCz), in order to construct a versatile donor for blue TADF materials. 1-MeCz was used as donor in combination with triazine and pyrimidine as acceptors to design novel compounds 1-MeCz-TRZ and 1-MeCz-Pm. It was observed that the presence of the methyl group at the 1-site of carbazole enhanced the twisted angles and reduced frontier molecular orbital overlapping, successfully switching the blue emission from the normal fluorescence of the methyl-free reference compounds (Cz-TRZ and Cz-Pm) to TADF of 1-MeCz-TRZ and 1-MeCz-Pm. The organic light-emitting diodes of 1-MeCz-TRZ and 1-MeCz-Pm exhibited blue emission at 450 and 458 nm with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.11) and (0.17, 0.18), and external quantum efficiencies of 13.07% and 7.53%. This study provides a versatile, simple and practical design strategy with 1-MeCz to construct pure blue TADF emitters in combination with various acceptors. Increasing the torsion angle of molecule by introducing methyl at 1-site of carbazole is proved to be an effective donor design strategy for blue emitters to change mechanism from normal fluorescence to thermally activated delayed fluorescence. [Display omitted] • 1-Methylcarbazole (1-MeCz) was designed and prepared as universal donor to construct pure blue TADF emitters. • The steric hindrance effect of the methyl group is capable of facilitating donor twisting and HOMO/LUMO separation and lead to TADF. • Two novel blue TADF-OLEDs of these emitters realized pure blue emission with CIE (0.15, 0.11) and external quantum efficiencies of 13.07%. [ABSTRACT FROM AUTHOR]