Generation by Electron Transfer of an Emitting State Not Observed by Photoexcitation in a Linked Ru(bpy)₃²⁺−Methyl Viologen

The molecule in which a methyl viologen (MV 2+ ) moiety is coupled through a three-carbon chain to one bipyridine in Ru(bpy) 3 2+ (bpy = 2,2‘bipyrine), [Ru(bpy) 2 (4-(2-(1‘-methyl-4−4‘-bipyridinediium-1-yl)propyl)-4‘-methyl-2,2‘-bipyridine), abbreviated Ru II (bpy) 2 (MV 2+ -bpy) 4+ ], does not emit upon photoexcitation because of rapid quenching of the MLCT state by the MV 2+ . The integrated steady-state photoemission intensity is <1/1000 that of Ru(bpy) 3 2+ under the same conditions. However, quite efficient emission [∼1/5 that of Ru(bpy) 3 2+ ] is found when Ru III (bpy) 2 (MV 2+ -bpy) 5+ is electrogenerated at a Pt electrode in aqueous solution and reacts with a suitable coreactant (tri- n -propylamine or oxalate), where the excited state is produced by an electron-transfer reaction. Electrogenerated chemiluminescence is also observed for this species in acetonitrile solutions upon potential cycling via annihilation of the reduced (+1) and oxidized (+5) species.