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

Abstract

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 ¹H 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.