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).
