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

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).