Luminescent bis-tridentate iridium(III) complexes: Overcoming the undesirable reactivity of trans-disposed metallated rings using –N^N^N–-coordinating bis(1,2,4-triazolyl)pyridine ligands

Abstract

Nine new iridium(III) complexes featuring two tridentate ligands have been synthesised of the form Ir(N^C^N)(–N^N^N–), where N^C^N represents a cyclometallating ligand based on 1,3-di(2-pyridyl)benzene and –N^N^N– is a doubly deprotonated bis(1,2,4-triazolyl)pyridine. Three examples of each ligand have been used, with different substituents in the central aryl ring of the former and para-substituted aryl groups in the 5-positions of the triazole rings of the latter. Two of the complexes have been structurally characterised in the solid-state by X-ray diffraction, confirming the mutually orthogonal arrangement of the two ligands. Unlike related tris-cyclometallated complexes of the type Ir(N^C^N)(C^N^C), which are unstable with respect to photoactivated cleavage of the trans-disposed Ir–C bonds, the new complexes show no evidence of instability. They are phosphorescent in the green region of the spectrum with lifetimes around 200 ns and quantum yields up to 3%, apparently limited by non-radiative decay processes in particular. Although there is some variation in performance with substitution pattern, the only discernible trend is that complexes of the 4-methoxy-substituted N^C^N ligand are the better emitters. Three examples of related complexes of the form Ir(N^C^N)(N^N–)Cl – incorporating a bidentate 1,2,4-triazolylpyridine – have also been prepared. They show no room-temperature emission but the properties at 77 K are similar to those of the bis-tridentate systems.