Effects of Symmetry of Ir (III) Complex on the Photophysical Properties and Device Performances

Abstract

The optical, electrochemical properties of two isomers of Ir(mpp)(2)acac (mpp = anion of 3-methyl-2-phenyl-pyridine and acac = acetylacetonate) and the device performance of polymer light emitting devices using two isomers as a triplet emitter have been studied. The two isomers show the symmetric and asymmetric coordination of two mpp ligands to Ir(III), which is confirmed from the H-1 NMR spectra. The ligand based absorption is red shifted by 35 nm in asymmetric Ir(mpp)(2)acac compared to the symmetric Ir(mpp)(2)acac. The MLCT absorption peak positions are similar in both cases, but the extinction coefficients of the asymmetric Ir(mpp)(2)acac are higher than those of symmetric Ir(mpp)(2)acac. The two isomers exhibit similar PL emission spectra but PL lifetimes of two isomers are different. The lifetime of the symmetric Ir(mpp)(2)acac is 1.74 mu s, whereas that of the asymmetric Ir(mpp)(2)acac is 0.9 mu s. The asymmetric Ir(mpp)(2)acac shows higher electrochemical oxidation potential than that of the symmetric Ir(mpp)(2)acac. The polymer light emitting devices are fabricated by blending 8 wt% of Ir(mpp)(2)acac in the PVK. The polymer light emitting device with symmetric Ir(mpp)(2)acac shows a maximum external quantum efficiency (eta(ext)) of 0.48% at 73 mA/cm(2) and a luminance of 12,300 cd/m(2). The polymer lig