  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 (PES) upon lengthening the iridium-ligand
(Ir-C) bonds. Relativistic time dependent-DFT (TDDFT) 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 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 non-radiative decay rate. This suggests that the
thermal population of $^3$MC states is the dominant non-radiative decay
mechanism at room temperature. We show that the $^3$MLCT $\rightarrow$ $^3$MC
transition is reversible, in marked contrast to other 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.
