Enhanced quantum efficiency of cationic iridium(III) complexes with carbazole moiety as a steric hindrance unit

Two cationic iridium(III) complexes with different degrees of steric hindrance, namely, [Ir(L1)2(bpy)]PF6 (1) and [Ir(L2)2(bpy)]PF6 (2) (where HL1, HL2 and bpy ligands are 1,2-diphenylbenzo[d]imidazole, 9-(4-(2-phenylbenzo[d]imidazol-1-yl)phenyl)-9H-carbazole and 2,2′-bipyridine, respectively), have been designed and synthesized. Photoluminescence measurements show that the photoluminescence quantum yield of complex 2 with the introduction of carbazole moieties as steric hindrance units is up to 36% in the neat film, which is obviously higher than that (27%) of the model complex 1. On basis of the quantum calculations and photophysical data, we conclude that the carbazole moieties effectively reduce the intermolecular interaction by enhancing the steric hindrance of the complex, which inhibits the quenching process without altering the nature of the excited-state. In addition, the results of thermogravimetric analysis (TGA) and cyclic voltammetry (CV) measurements reveal that complexes 1 and 2 have potential applications in electroluminescent devices.