posted on 2019-10-18, 12:34authored byBingqing Liu, Mohammed A. Jabed, Jiali Guo, Wan Xu, Samuel L. Brown, Angel Ugrinov, Erik K. Hobbie, Svetlana Kilina, Anjun Qin, Wenfang Sun
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[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 1π,π*/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 1π,π*/1CT/1d,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 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 Ir2 was used as a dopant for organic light-emitting diode (OLED) fabrication.
The device displayed a yellow emission with a maximum current efficiency
(ηc) of 71.29 cd A–1, a maximum
luminance (Lmax) 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.