An idealized bipolar host material and dopant are synthesized for deep‐red phosphorescent organic light‐emitting diodes (PhOLEDs). The host material exhibits a low‐lying lowest unoccupied molecular orbital and high thermal and morphological stability, while the dopant emits sharply at 616 nm with a full width at half‐maximum of only 39 nm. The new host/guest combination for the deep‐red device yields the highest device efficiencies to date.

Five iridium bis(carbene) complexes, [Ir(pmi)(2)(pypz)] (1), [Ir(mpmi)(2)(pypz)] (2), [Ir(fpmi)(2)(pypz)] (3), [Ir(fpmi)(2)(pyim)] (4), and [Ir(fpmi)(2)(tfpypz)] (5) (pmi=1-phenyl-3-methylimdazolin-2-ylidene-C,C(2'); fpmi=1-(4-fluorophenyl)-3-methylimdazolin-2-ylidene-C,C(2'); mpmi=1-(4-methyl-phenyl)-3-methylimdazolin-2-ylidene-C,C(2'); pypz=2-(1H-pyrazol-5-yl)pyridinato; pyim=2-(1H-imidazol-2-yl)pyridinato; and tfpypz=2-(3-(trifluoromethyl)-1H-pyrazol-5-yl)pyridinato), were synthesized and their structures were characterized by NMR spectroscopy, mass spectroscopy and X-ray diffraction. These complexes showed phosphorescent emission with the emission maxima between 453 and 490 nm. Various spectrophotometric measurements, cyclic voltammetric studies, and density functional theory (DFT) calculations show that, unlike most of the phosphorescent cyclometalated iridium complexes, the lowest unoccupied molecular orbital (LUMO) energy and the emissive state of these iridium complexes are mainly controlled by the N,N'-heteroaromatic (N^N) ligand. Despite the fact that the LUMO levels of these complexes are mainly on the N^N ligands, the efficiencies of the electroluminescent (EL) devices are very high. For example, the EL devices using [Ir(mpmi)(2)(pypz)], [Ir(fpmi)(2)(pypz)], and [Ir(fpmi)(2)(tfpypz)] as the dopant emitters exhibited light- to deep-blue electrophosphorescence with external quantum efficiencies of 15.2, 14.1, and 7.6% and Commission Internationale d'Énclairage (x,y) coordinates (CIE(x,y)) of (0.14, 0.27), (0.14, 0.18) and (0.14, 0.10), respectively.

The Raman and infrared spectra of fac-tris(2-phenylpyrazolato-N, C 2 0 )iridium (Ir(ppz) 3 ), bis[(4,6-difluorophenyl)-pyridinato-N,C 2 0 ](picolinato)iridium(FIrpic), fac-tris[1-(4-fluorophenyl)-3-methylimidazolin-2-ylidene-C,C 2 0 ]iridium [Ir(fpmi) 3 ], and fac-tris[1-(2,4-difluorophenyl)-3-methylimidazolin-2-ylidene-C,C 2 0]iridium [Ir(dfpmi) 3 ] were recorded, and complete vibrational analyses were performed. Quantum-chemical calculations using the density-functional theory methods B3LYP and B3PW91 were employed to obtain the vibrational wavenumbers and spectral intensities. The optimized geometries and vibrational wavenumbers agree with the experimental data, indicating that the calculations yielded accurate geometries for the iridium complexes. Based on comparison of the spectra of their analogues Ir(ppy) 3 and ligands and theoretical calculations, we assigned the vibrational wavenumbers for the metalÀligand modes: metalÀligand stretching bands ν(IrÀC) are at 270/321 cm À1 for Ir(ppz) 3 , 321 cm À1 for Firpic, 332 cm À1 (imidazole) and 286/303 cm À1 (phenyl) for Ir(fpmi) 3 , and 377 cm À1 (imidazole) and 283/299 cm À1 (phenyl) for Ir(dfpmi) 3 and metalÀligand stretching bands ν(IrÀN) are at 229/ 243 cm À1 for Ir(ppz) 3 and 267 cm À1 (picolinate) and 282 (phenyl) cm À1 for FIrpic. We observed greater vibrational wavenumbers for the metalÀligand bands in the fluoro complexes, indicating that the electron-withdrawing group F increases the strength of the bond from the ligand to iridium.

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