Nonradiative Decay and Stability of N-Heterocyclic Carbene Iridium(III) Complexes

Devices based on deep-blue emitting iridium(III) complexes with N-heterocyclic carbene (NHC) ligands have recently been shown to give excellent performance as phosphorescent organic light-emitting diodes (PHOLEDs). To facilitate the design of even better deep-blue phosphorescent emitters, we carried out density functional theory (DFT) calculations of the lowest triplet (T₁) potential-energy surfaces upon lengthening the iridium-ligand (Ir-C) bonds. Relativistic time dependent DFT calculations demonstrate that this changes the nature of T₁ from a highly emissive metal-to-ligand charge transfer (³MLCT) state to a metal centered (³MC) state where the radiative decay rate is orders of magnitude slower than that of the ³MLCT state. We identify the elongation of an Ir-C bond on the NHC group as the pathway with the lowest energy barrier between the ³MLCT and ³MC states for all complexes studied and show that the barrier height is correlated with the experimentally measured nonradiative decay rate. This suggests that the thermal population of ³MC states is the dominant nonradiative decay mechanism at room temperature. We show that the ³MLCT → ³MC transition is reversible, in marked contrast to deep-blue phosphors containing coordinating nitrogen atoms, where the population of ³MC states breaks Ir-N bonds. This suggests that, as well as improved efficiency, blue PHOLEDs containing phosphors where the metal is only coordinated by carbon atoms will have improved device lifetimes.