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

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 (max = 512 nm) with a higher PL quantum efficiency (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 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 (ext = 11% at 1 mA/cm2) and brightness (4543 cd/m2 at 20 mA/cm2) than Ir(ppy)3 (ext = 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.