N-Heterocyclic Carbenes: Versatile Second Cyclometalated Ligands for Neutral Iridium(III) Heteroleptic Complexes

Abstract

With 2-(2,4-difluorophenyl)pyridine (dfppy) as the first cyclometalated ligand and different monoanionic N-heterocyclic carbenes (NHCs) as the second cyclometalated ligands, 16 blue or greenish-blue neutral iridium(III) phosphorescent complexes, (dfppy)2Ir(NHC), were synthesized efficiently. The obtained Ir(III) complexes display typical phosphorescence of 455-485 nm with quantum yields up to 0.73. By modifying the phenyl moiety in the NHCs with electron-withdrawing substituents (e.g., -F or -CF3) or replacing it with N-heteroaromatic rings (pyridine or pyrimidine), the HOMO-LUMO gaps are broadened, and the emissions shift to the more blue region accordingly. Furthermore, to extend the application scope of NHCs as the second cyclometalated ligands, five other Ir(III) complexes from blue to red were synthesized with different first cyclometalated ligands. Finally, the organic light-emitting diodes using one blue emitter exhibit a maximum current efficiency of 37.83 cd A(-1), an external quantum efficiency of 10.3%, and a maximum luminance of 8709 cd m(-2). Our results demonstrate that NHCs as the second cyclometalated ligands are good candidates for the achievement of efficient phosphorescent Ir(III) complexes and corresponding devices.

Molecular Engineering of Iridium Blue Emitters Using Aryl N‐Heterocyclic Carbene Ligands

“…Both complexes exhibit a distorted octahedral geometry around the Ir center with the pyridine ligands trans to each other. All bonds involving Ir are >2 Å in length with the Ir–C4(NHC) bond length in the range 2.056–2.099 Å for both complexes, similar to those described for other Ir III –NHC cyclometalated, , , , , and non‐cyclometalated complexes . The Ir–NHC bond in each case is shorter than the Ir–Pyr bond, which indicates a stronger bonding interaction.…”

“…The oxidation and reduction potentials measured in a 0.1 m Bu 4 NPF 6 solution in MeCN show that the value of Δ E red for each of the complexes is nearly constant throughout the series and similar (+30 to –140 mV difference) to that observed for [FIrPic]. Instead of selectively modifying just one of the energy levels, as was observed with electron‐withdrawing arylNHC ligands, both the HOMO and LUMO were modified (Figure b). The complex with the highest ground‐state energy (most destabilized HOMO, 1.14 V vs. NHE) E (S + /S) value is Ir1 due to the dimethoxybenzene cyclometalating ligand and imidazole chelating group.…”

“…Such tuning of energy levels widens the HOMO–LUMO gap. As was described in detail by Zuo and co‐workers very recently for the [(dfppy) 2 Ir(arylNHC)] scaffold (and by others for other ligand classes), the ancillary ligand also greatly impacts the photophysical properties . As a practical example, starting from [(ppy) 2 Ir(acac)] (ppy = 2‐phenylpyridine; acac = acetylacetonate) and changing the ancillary ligand can result in either a redshift (as in 2,2′‐bipyridine or phenanthrene) or a blueshift [as in (CN) 2 ,, bis(1‐methylimidazol‐3‐yl‐2‐idene)methane, Im 2 CH 2 , or 2‐picolinic acid] of the observed emission (Figure ).…”

“…A recent study by Zuo and co‐workers showed that by using progressively more electron‐withdrawing arylNHC ancillary ligands, the emission energy could be increased to that of the sky‐blue [FIrPic] emitter by selective stabilization of the HOMO energy level, as is also the case for [FIrPic] . Because of the success of Zuo and co‐workers, we sought to better understand the effect of NHC ancillary ligands bearing cyclometalating moieties with electron‐donating functionalities.…”

“…To ascertain the influence of NHC ligand π‐donor strength, the NHC ligands chosen contained either imidazole ( Ir1 – Ir3 ) or 1,2,4‐triazole ( Ir4 ). To better understand how the electron‐donating ability of both the Ir metal center and the NHC ligand affects emission energy, aryl wingtip groups of different donating and accepting strengths were used as cyclometalating ligands (Figure ) , , , …”