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 boolean AND N)][PF6] complexes (Hppy = 2-phenyl-pyridine) are reported. The N boolean AND 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 boolean AND N ligands incorporate an aromatic ring, designed to facilitate intra-cation face-to-face pi-interactions between the N boolean AND N and one [ppy](-) ligand. Within the set of ligands, 6-(3-tolyl)-2,2'-bipyridine (5), 4,6-bis(4-nitro-phenyl)-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 boolean AND 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 H-1 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 boolean AND N)][PF6] complexes are orange emitters with lambda(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.