Bis-Tridentate Iridium(III) Phosphors Bearing Functional 2-Phenyl-6-(imidazol-2-ylidene)pyridine and 2-(Pyrazol-3-yl)-6-phenylpyridine Chelates for Efficient OLEDs

Proligands to the monoanionic tridentate chelate 4-(tert-butyl)-2-(2,4-difluorophenyl)-6-(3-isopropyl-imidazol-2-ylidene)pyridine ((phpyim-H₂)PF₆) and dianionic tridentate chelates derived from functional 2-pyrazol-3-yl-6-phenylpyridine chelates, i.e. L1-H₂-L5-H₂, have been synthesized and characterized. Treatment of (phpyim-H₂)PF₆ with [Ir(COD)(μ-Cl)]₂ in the presence of sodium acetate, followed by heating at 200 °C with 1 equiv of the dianionic chelate, afforded the respective charge-neutral, bis-tridentate Ir(III) complexes [Ir(phpyim)(Ln)] (1-5; n = 1-5). The hydride complex [Ir(phpyim)(L5-H)(H)] (6) was made when the "one-pot" reaction of (phpyim-H₂)PF₆, [Ir(COD)(μ-Cl)]₂, and L5-H₂ was carried out at 140 °C. Complex 6 is likely an intermediate in the formation of 5, as it is converted to 5 on heating to 200 °C. Compounds 1-6 have been characterized by NMR spectroscopy and, in the cases of 1, 5, and 6, by X-ray structural analysis. TD-DFT computations confirmed that the emission bands are derived from ³MLCT transitions involving the chelates L1-L5, resulting in a wide range of emission wavelengths from 473 (cyan) to 608 nm (orange-red) observed for 1 - 5. A series of green- and red-emitting organic light-emitting diodes (OLEDs) with a simplified trilayer architecture were fabricated using the as-prepared Ir(III) complexes 2 and 5, respectively. A maximum external quantum efficiency of 18.8%, a luminance efficiency of 58.5 cd/A, and a power efficiency of 57.4 lm/W were obtained for the green-emitting OLEDs (2), which compares with 15.4%, 10.4 cd/A, and 9.0 lm/W obtained for the red-emitting OLEDs (5). The high efficiencies of these OLED devices suggest great potential for these bis-tridentate Ir(III) metal phosphors in the fabrication of multicolored OLED devices.