Tuning the emission and morphology of cyclometalated iridium complexes and their applications to organic light-emitting diodes

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Abstract
We have synthesized two highly efficient phosphorescent iridium metal complexes, Ir(DPQ)2(acac) and Ir(FPQ)2(acac), which are based on cyclometalated quinoline ligands, and discuss details of their electrochemical behavior and photophysical properties (viz. absorption and photoluminescence). Single-crystal X-ray diffraction studies of Ir(DPQ)2(acac) reveal a distorted octahedral geometry, in which the quinoline N atoms and the C atoms of the orthometalated phenyl groups are located at mutual trans and cis positions, respectively. In contrast, Ir(FPQ)2(acac) is an amorphous solid and undergoes a glass transition at 92 °C, which we attribute to the presence of the long di-n-octyl chains in the fluorenyl groups. The phosphorescence of these Ir complexes originates from the dominant 3MLCT excited state shifts to red that occur upon introducing a phenyl substituent and/or a large conjugating aromatic ring into the ligand. A polymer light-emitting diode (PLED) device that uses Ir(FPQ)2(acac) as a phosphorescent dopant and a PVK/PBD blend as the host material produces very high efficiency (an external quantum efficiency of 8.16% at 100 mA cm−2) and a pure-red emission with 1931 CIE (Commission Internationale de L'Eclairage) chromaticity coordinates of (x = 0.68, y = 0.32).