Emitting dipole orientation and molecular orientation of homoleptic Ir(III) complexes

Abstract

Origin and significance of emitting dipole orientation (EDO) of emitters in organic light-emitting diodes (OLEDs) have been studied extensively in recent years. However, EDO of homoleptic Ir(III) complexes (Homo-IrCs) have received less attention due to their three-fold molecular symmetry and resulting expectation to have random EDO. Herein, we report the EDOs of Homo-IrCs doped in various host materials. Interestingly the EDO of Homo-IrCs varies from preferred horizontal direction to rather vertical direction depending on dopant and host molecules. The variation of EDO of Homo-IrCs is correlated with the direction of the transition dipole moment (TDM) against the molecular C3 axis and the orientation of C3 axis of the dopants in films. The average orientations of the Homo-IrCs are extracted from measured the horizontal dipole ratios (Θs) and calculated TDM directions. Angle between the TDM direction and C3 axis in the Homo-IrCs under investigation are larger than 54.7° corresponding to the mutually orthogonal axes among the three transition dipole vectors so that the EDO of Homo IrCs can deviate from random orientation depending on the orientation of the C3 axis in films. Average molecular orientation of a HOMO-IrC in films varies by 10°–13° depending on host materials. In contrast, smaller variation (<5°) is observed among different Homo IrCs in a same host. The results indicate that host material is one of the dominating factors determining orientation of Homo-IrC molecules. Tris[(3,5-difluoro-4-cyanophenyl)-pyridine]iridium(III) (FCNIr) shows exceptionally high horizontal orientation, which likely originates from the dipole-dipole interaction between the dopant and hosts. FCNIr doped in diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide (TSPO1) shows strong preferred horizontal EDO with the Θ of 0.78, demonstrating the possibility of large Θ value of Homo-IrCs.