Correlating the transition dipole moment orientation of phosphorescent emitter molecules in OLEDs with basic material properties

The orientation of the emissive dipole moment of seven iridium-based phosphorescent emitter molecules commonly used in organic light-emitting diodes (OLEDs) is investigated. The orientation of Ir(ppy)(3), Ir(ppy)(2)(acac), Ir(chpy)(3), Ir(dhfpy)(2)(acac), Ir(BT)(2)(acac), Ir(MDQ)(2)(acac), and Ir(piq)(3) is determined by measuring the angle dependent spectral radiant intensity of the transverse magnetic polarized emission from p-i-n OLEDs comprising these emitters. The experimental data are compared to the intensity calculated by a multilayer simulation method that includes the anisotropy factor describing the average dipole orientation. Surprisingly, among these molecules, Ir(ppy)(3) is the only emitter showing an isotropically distributed transition dipole moment. In order to correlate our results with basic molecular properties, the permanent dipole moment and the size of the molecules are calculated by density functional theory (DFT). The dipole-dipole potential obtained for Ir(ppy)(3) is more than 2.5 times larger than those for all other emitter molecules investigated here, indicating that this parameter is correlated with the transition dipole moment orientation.