title: 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.
authors.affiliation: State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, Center for Molecular Organic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University , Nanjing 210093, P. R. China.
authors.forename: Tian-Yi
authors.initials: TY
authors.lastname: Li
authors.forename: Xiao
authors.initials: X
authors.lastname: Liang
authors.forename: Liang
authors.initials: L
authors.lastname: Zhou
authors.forename: Chen
authors.initials: C
authors.lastname: Wu
authors.forename: Song
authors.initials: S
authors.lastname: Zhang
authors.forename: Xuan
authors.initials: X
authors.lastname: Liu
authors.forename: Guang-Zhao
authors.initials: GZ
authors.lastname: Lu
authors.forename: Li-Sha
authors.initials: LS
authors.lastname: Xue
authors.forename: You-Xuan
authors.initials: YX
authors.lastname: Zheng
authors.forename: Jing-Lin
authors.initials: JL
authors.lastname: Zuo