Investigating the effects of supra molecularly caging ligands in [Ru(bpy)2L]2+ complexes

Five ruthenium(II) complexes [Ru(bpy)2(L)][PF6]2 in which L is a 6-substituted 2,2′-bipyridine ligand (ligands 1–3), 5-(2-picoloyl) (4), or an N,O-donor (5) have been prepared and characterized to investigate the role of the intramolecular caging effect of pendant aryl-substituents on the electronic and photophysical properties. The single crystal structures of three of the complexes are reported; in [Ru(bpy)2(L)][PF6]2 where L = 6-(selenophen-2-yl)-2,2′-bipyridine, intra-cation face-to-face π-stacking of the selenophenyl unit and one pyridine ring of an adjacent ligand is observed. In comparison to [Ru(bpy)3]2+, the energy of the MLCT band in [Ru(bpy)2(L)]2+ for L = 1, 2, 3 or 4 is little affected, but a red-shift of 35 nm is observed for [Ru(bpy)2(5)]2+. The new complexes are emissive at room temperature in degassed MeCN solution; values of λmaxem range from 613 to 689 nm for [Ru(bpy)2(L)]2+ (L = 1 to 5). The electrochemical behaviour of [Ru(bpy)2(L)][PF6]2 for L = 1, 2 and 3 is similar; in contrast, [Ru(bpy)2(4)][PF6]2 exhibits more complicated ligand-based redox behaviour, and for [Ru(bpy)2(5)][PF6]2, the presence of the N,O-donor shifts the metal-based oxidation to lower potential.