Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted N-Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow OLED Application

Abstract

The synthesis, crystal structure, and photophysics of a series of neutral cyclometalated iridium(III) complexes bearing substituted N-heterocyclic carbene (NHC) ancillary ligands ((C^∧N)₂Ir(R-NHC), where C^∧N and NHC refer to the cyclometalating ligand benzo[<i>h</i>]quinoline and 1-phenylbenzimidazole, respectively) are reported. The NHC ligands were substituted with electron-withdrawing or -donating groups on C4' of the phenyl ring (R <i>=</i> NO₂ (<b>Ir1</b>), CN (<b>Ir2</b>), H (<b>Ir3</b>), OCH₃ (<b>Ir4</b>), N(CH₃)₂ (<b>Ir5</b>)) or C5 of the benzimidazole ring (R = NO₂ (<b>Ir6</b>), N(CH₃)₂ (<b>Ir7</b>)). The configuration of <b>Ir1</b> was confirmed by a single-crystal X-ray diffraction analysis. The ground- and excited-state properties of <b>Ir1</b>-<b>Ir7</b> were investigated by both spectroscopic methods and time-dependent density functional theory (TDDFT) calculations. All complexes possessed moderately strong structureless absorption bands at ca. 440 nm that originated from the C^∧N ligand based ¹<i>π,π</i>*/¹CT (charge transfer)/¹d,d transitions and very weak spin-forbidden ³MLCT (metal-to-ligand charge transfer)/³LLCT (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the ¹<i>π,π</i>*/¹CT/¹d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted complex <b>Ir3</b>. Except for the weakly emissive nitro-substituted complexes <b>Ir1</b> and <b>Ir6</b> that had much shorter lifetimes (≤160 ns), the other complexes are highly emissive in organic solutions with microsecond lifetimes at ca. 540-550 nm at room temperature, with the emitting states being predominantly assigned to ³π,π*/³MLCT states. Although the effect of the substituents on the emission energy was insignificant, the effects on the emission quantum yields and lifetimes were drastic. All complexes also exhibited broad triplet excited-state absorption at 460-700 nm with similar spectral features, indicating the similar parentage of the lowest triplet excited states. The highly emissive <b>Ir2</b> was used as a dopant for organic light-emitting diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (η_c) of 71.29 cd A^-1, a maximum luminance (<i>L</i>_max) of 32747 cd m^-2, and a maximum external quantum efficiency (EQE) of 20.6%. These results suggest the potential of utilizing this type of neutral Ir(III) complex as an efficient yellow phosphorescent emitter.