Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes: Preparation, Structures, Electrochemistry, Photophysical Properties and Their Applications in Organic Light‐Emitting Diodes (OLEDs)

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Abstract
A series of dimethyl‐substituted tris(pyridylphenyl)iridium(III) derivatives [(n‐MePy−n′‐MePh)3Ir] [n = 3, n′ = 4 (1); n = 4, n′ = 4 (2); n = 4, n′ = 5 (3); n = 5, n′ = 4 (4); n = 5, n′ = 5 (5)] have been synthesized and characterized to investigate the effect of the substitution of methyl groups on the solid‐state structure and photo‐ and electroluminescence. The absorption, emission, cyclic voltammetry and electroluminescent performance of 1−5 have also been systematically evaluated. The structures of 2 and 4 have been determined by a single‐crystal X‐ray diffraction analysis. Under reflux (> 200 °C) in glycerol solution, fac‐type complexes with a distorted octahedral geometry are predominantly formed as the major components in all cases. Electrochemical studies showed much smaller oxidation potentials relative to Ir(ppy)3 (Hppy = 2‐phenylpyridine). All complexes exhibit intense green photoluminescence (PL), which has been attributed to metal‐to‐ligand charge transfer (MLCT) triplet emission. The maximum emission wavelengths of thin films of 1, 3, 4 and 5 at room temperature are in the range 529−536 nm, while 2 displays a blue‐shifted emission band (λmax = 512 nm) with a higher PL quantum efficiency (ΦPL = 0.52) than those of complexes 1 and 3−5; this is attributed to a decrease of the intermolecular interactions. Multilayered organic light‐emitting diodes (OLEDs) were fabricated by using three (2, 3 and 4) of these IrIII derivatives as dopant materials. The electroluminescence (EL) spectra of the devices, which have the maximum peaks at 509−522 nm, with shoulder peaks near 552 nm, are consistent with the PL spectra in solution at 298 K. The devices show operating voltages at 1 mA/cm2 of 4.9, 5.6, 5.1, and 4.6 V for Ir(ppy)3, 2, 3, and 4, respectively. In particular, the device with 2 shows a higher external quantum efficiency (ηext = 11% at 1 mA/cm2) and brightness (4543 cd/m2 at 20 mA/cm2) than Ir(ppy)3 (ηext = 6.0% at 1 mA/cm2; 3156 cd/m2 at 20 mA/cm2) and other Ir(dmppy)3 derivatives, (dmppy = dimethyl‐substituted ppy), under the same conditions. The methyl groups at the meta (Ph) and para (Py) positions to the Ir metal atom have a great influence on absorption, emission, redox potentials and electroluminescence. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)