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)

A series of dimethyl-substituted tris(pyridylphenyl)iridium(III) derivatives [(n-MePy-n'-MePh)(3)Ir] [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 degreesC) 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 (lambda(max) = 512 nm) with a higher PL quantum efficiency (phi(PL) = 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 Ir-III 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/cm(2) 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 (eta(ext) = 11 % at 1 mA/cm(2)) and brightness (4543 cd/m(2) at 20 mA/cm(2)) than Ir(ppy)(3) (eta(ext) = 6.0% at 1 mA/cm(2); 3156 cd/m(2) at 20 mA/cm(2) 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 It metal atom have a great influence on absorption, emission, redox potentials and electroluminescence. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.