In Pursuit of Low Energy Phosphorescence: Late Metal Coordination Complexes of the Planar, π-extended Bipyridyl Ligand 6,6',7,7'-Biphenanthridine.

The coordination chemistry of the planar, doubly π-extended bipyridine analog, 6,6',7,7'-biphenanthridine (p-biphe), is presented. The phenanthridine units in p-biphe are fused together at the 6- and 7- positions, and the resulting rigid ligand is compared with the more flexible parent "biphe" fused only at the 6-positions. p-Biphe is intensely fluorescent in solution with a much higher quantum yield, but, unlike biphe, at 77 K the fluorescence is not accompanied by any significant phosphorescence. Two four-coordinate Cu(I) complexes and pseudo-octahedral Ru(II) and Ir(III) complexes are described: [Cu(p-biphe)₂]⁺, [(P^P)Cu(p-biphe)]⁺, [Ru(bpy)₂(p-biphe)]²⁺, and [Ir(ppy)₂(p-biphe)]⁺, isolated as PF₆ salts (P^P  =  4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; bpy = 2,2'-bipyridine; ppy = 2-phenylpyridine). The complexes are strongly coloured due to intense low-energy absorption to charge-transfer excited states based on TDDFT calculations. The acceptor nature of the p-biphe ligand is clear from multiple reduction events observed electrochemically. The Ir(III) complex displays remarkably low-energy phosphorescence (λ_max = 812 nm) in solution at room temperature with a vibrational progression evident extending into the near IR, with no contamination from visible light emission. While the heteroleptic Cu(I) complex is phosphorescent at 77 K, no emission is detectable from [Cu(p-biphe)₂]⁺ or [Ru(bpy)₂(p-biphe)]²⁺, likely due to competitive nonradiative decay.