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 PF 6 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.