Diphenyl(1-naphthyl)phosphine Ancillary for Assembling of Red and Orange-Emitting Ir(III) Based Phosphors; Strategic Synthesis, Photophysics, and Organic Light-Emitting Diode Fabrication

Treatment of a series of dinuclear Ir(III) complexes [(fnazo)₂Ir(μ-Cl]₂, [(fpiq)₂Ir(μ-Cl]₂, and [(fppy)₂Ir(μ-Cl]₂ with diphenyl(1-naphthyl)phosphine (dpnH) in decalin at 100 °C afforded the simple adducts, trans-N,N′-[(fnazo)₂Ir(dpnH)Cl] (1a), trans-N,N′-[(fpiq)₂Ir(dpnH)Cl] (1b), and trans-N,N′-[(fppy)₂Ir(dpnH)Cl] (1c), for which the C^∧N cyclometalating reagents, that is, fnazoH, fpiqH and fppyH, stands for 4-(4-fluorophenyl)quinazoline, 1-(4-fluorophenyl)isoquinoline and 4-fluorophenylpyridine, respectively. Single crystal X-ray diffraction study on 1a revealed existence of two trans-N,N′ cyclometalates, with both chloride and dpnH donors located at the positions opposite to the phenyl substituents. Subsequent heating of 1a−1c at higher temperature afforded the second isomer (2a−2c), showing formation of cis-N,N′ orientation for the aforementioned cyclometalates. Further thermolysis of either trans or cis-Ir(III) complexes 1 or 2 in presence of sodium acetate, which serves as both activator and chloride scavenger, gave successful isolation of a mixture of two fully cyclometalated Ir(III) complexes trans-N,N′-[(C^∧N)₂Ir(dpn)] (3a−3c) and cis-N,N′-[(C^∧N)₂Ir(dpn)] (4a−4c). Structural and photophysical properties of complexes 3a−3c and 4a−4c were measured and compared. Time-dependent density functional theory (DFT) studies suggested that, upon changing the C^∧N cyclometalates from quinazolinyl, isoquinolinyl, and, finally, to pyridyl fragment, the lowest unoccupied molecular orbitals (LUMOs) are gradually shifted from the cyclometalating nitrogen heterocycles to the 1-naphthyl group of the phosphine chelate and, concomitantly altered the photophysical properties. An organic light-emitting diode (OLED) using orange-red phosphors 4a and 4b has been successfully fabricated. At the practical brightness of 500 cd·m⁻², decent external quantum efficiency of 10.6% and 12.5% could be reached for 4a and 4b, respectively, revealing the usefulness of relevant molecular architecture in designing triplet OLED emitters.