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 (T 1 ) potential-energy surfaces upon lengthening the iridium–ligand (Ir–C) bonds. Relativistic time dependent DFT calculations demonstrate that this changes the nature of T 1 from a highly emissive metal-to-ligand charge transfer ( 3 MLCT) state to a metal centered ( 3 MC) state where the radiative decay rate is orders of magnitude slower than that of the 3 MLCT state. We identify the elongation of an Ir–C bond on the NHC group as the pathway with the lowest energy barrier between the 3 MLCT and 3 MC 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 3 MC states is the dominant nonradiative decay mechanism at room temperature. We show that the 3 MLCT → 3 MC transition is reversible, in marked contrast to deep-blue phosphors containing coordinating nitrogen atoms, where the population of 3 MC 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.