Solution, structural and photophysical aspects of substituent effects in the N^N ligand in [Ir(C^N)2(N^N)]+ complexes

The syntheses and properties of a series of eleven new [Ir(ppy)2(N^N)][PF6] complexes (Hppy = 2-phenylpyridine) are reported. The N^N ligands are based on 2,2-bipyridine (bpy), substituted in the 6- or 5-positions with groups that are structurally and electronically diverse. All but two of the N^N ligands incorporate an aromatic ring, designed to facilitate intra-cation face-to-face π-interactions between the N^N and one [ppy](-) ligand. Within the set of ligands, 6-(3-tolyl)-2,2'-bipyridine (5), 4,6-bis(4-nitrophenyl)-2,2'-bipyridine (9), and 4,6-bis(3,4,5-trimethoxyphenyl)-2,2'-bipyridine (10) are new and their characterization includes single crystal structures of 9, and two polymorphs of 10. A representative [Ir(ppy)2(N^O)](+) complex is also described. We report solution NMR spectroscopic, photophysical and electrochemical properties of the complexes, as well as representative solid-state structural data. The solution (1)H NMR spectroscopic data illustrate different dynamic processes involving the substituents attached to the bpy domain in the ligands. In degassed MeCN and at room temperature, the [Ir(ppy)2(N^N)][PF6] complexes are orange emitters with λ(em)max in the range 575 to 608 nm; however, quantum yields are very low. The most promising complexes were evaluated in light-emitting electrochemical cells leading to bright and stable devices with rather good external quantum efficiencies.