Highly Stable and Efficient Light-Emitting Electrochemical Cells Based on Cationic Iridium Complexes Bearing...

A series of bis-cyclometalated iridium(III) complexes of general formula [Ir(ppy)₂(N^∧N)][PF₆] (ppy^- = 2-phenylpyridinate; N^∧N = 2-(1H-imidazol-2-yl)pyridine (1), 2-(2-pyridyl)benzimidazole (2), 1-methyl-2-pyridin-2-yl-1H-benzimidazole (3), 2-(4'-thiazolyl)benzimidazole (4), 1-methyl-2-(4'-thiazolyl)benzimidazole (5)) is reported, and their use as electroluminescent materials in light-emitting electrochemical cell (LEC) devices is investigated. [2][PF₆] and [3][PF₆] are orange emitters with intense unstructured emission around 590 nm in acetonitrile solution. [1][PF₆], [4][PF₆], and [5][PF₆] are green weak emitters with structured emission bands peaking around 500 nm. The different photophysical properties are due to the effect that the chemical structure of the ancillary ligand has on the nature of the emitting triplet state. Whereas the benzimidazole unit stabilizes the LUMO and gives rise to a ³MLCT/³LLCT emitting triplet in [2][PF₆] and [3][PF₆], the presence of the thiazolyl ring produces the opposite effect in [4][PF₆] and [5][PF₆] and the emitting state has a predominant ³LC character. Complexes with ³MLCT/³LLCT emitting triplets give rise to LEC devices with luminance values 1 order higher than those of complexes with ³LC emitting states. Protecting the imidazole N-H bond with a methyl group, as in complexes [3][PF₆] and [5][PF₆], shows that the emissive properties become more stable. [3][PF₆] leads to outstanding LECs with simultaneously high luminance (904 cd m^-2), efficiency (9.15 cd A^-1), and stability (lifetime over 2500 h).