Inorganic Chemistry, Vol.58, No.21, 14377-14388, 2019
Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted N-Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow OLED Application
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)(2)Ir(R-NHC), where C<^>N and NHC refer to the cyclo-metalating ligand benzo[h]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 = NO2 (Ir1), CN (Ir2), H (Ir3), OCH3 (Ir4), N(CH3)(2) (Ir5)) or C5 of the benzimidazole ring (R = NO2 (Ir6), N(CH3)(2) (Ir7)). The configuration of Ir1 was confirmed by a single-crystal X-ray diffraction analysis. The ground- and excited-state properties of Ir1-Ir7 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)pi,pi*/(CT)-C-1(charge transfer)/(1)d,d transitions and very weak spin-forbidden (MLCT)-M-3 (metal-to-ligand charge transfer)/(LLCT)-L-3 (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the (1)pi,pi*/(CT)-C-1/(1)d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted complex Ir3. Except for the weakly emissive nitro-substituted complexes Ir1 and Ir6 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 (3)pi,pi*/(MLCT)-M-3 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 Ir2 was used as a dopant for organic light-emitting diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (eta(c)) of 71.29 cd A(-1), a maximum luminance (L-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.