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)₂acac (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 ¹H NMR spectra. The ligand based absorption is red shifted by 35 nm in asymmetric Ir(mpp)₂acac compared to the symmetric Ir(mpp)₂acac. The MLCT absorption peak positions are similar in both cases, but the extinction coefficients of the asymmetric Ir(mpp)₂acac are higher than those of symmetric Ir(mpp)₂acac. The two isomers exhibit similar PL emission spectra but PL lifetimes of two isomers are different. The lifetime of the symmetric Ir(mpp)₂acac is 1.74 μs, whereas that of the asymmetric Ir(mpp)₂acac is 0.9 μs. The asymmetric Ir(mpp)₂acac shows higher electrochemical oxidation potential than that of the symmetric Ir(mpp)₂acac. The polymer light emitting devices are fabricated by blending 8 wt% of Ir(mpp)₂acac in the PVK. The polymer light emitting device with symmetric Ir(mpp)₂acac shows a maximum external quantum efficiency (η_ext) of 0.48% at 73 mA/cm² and a luminance of 12,300 cd/m². The polymer light emitting device with asymmetric Ir(mpp)₂acac shows a maximum external quantum efficiency of 0.32% at 100 mA/cm² and a luminance of 7,500 cd/m². This results show that the symmetry of metal complex can affect the photophysical properties as well as the device performance.

Keywords:

• symmetry
• Ir(III) complex
• isomer
• polymer light emitting devices

Acknowledgment

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100024785) and APRI-Research Program through a grant provided by Gwangju Institute of Science and Technology.

Notes

^†These authors contributed equally to this work.