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Devices based on deep-blue emitting iridium (III) complexes withN-heterocyclic carbene (NHC) ligands have recently been shown to give excellentperformance as phosphorescent organic light-emitting diodes (PHOLEDs). Tofacilitate the design of even better deep-blue phosphorescent emitters wecarried 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 demonstratethat this changes the nature of $T_1$ from a highly-emissive metal-to-ligandcharge transfer ($^3$MLCT) state to a metal centered ($^3$MC) state where theradiative decay rate is orders of magnitude slower than that of the $^3$MLCTstate. We identify the elongation of an Ir-C bond on the NHC group as thepathway with lowest energy barrier between the $^3$MLCT and $^3$MC states forall complexes studied and show that the barrier height is correlated with theexperimentally measured non-radiative decay rate. This suggests that thethermal population of $^3$MC states is the dominant non-radiative decaymechanism at room temperature. We show that the $^3$MLCT $\rightarrow$ $^3$MCtransition is reversible, in marked contrast to other deep blue phosphorscontaining coordinating nitrogen atoms, where the population of $^3$MC statesbreaks Ir-N bonds. This suggests that, as well as improved efficiency, bluePHOLEDs containing phosphors where the metal is only coordinated by carbonatoms will have improved device lifetimes.

Non-radiative decay and stability of $N$-heterocyclic carbene iridium(III) complexes