Changing the Role of 2,2′-Bipyridine from Secondary Ligand to Protagonist in [Ru(bpy)2(N-N)]2+ Complexes: Low-Energy, Red Emission from a Ruthenium(II)-to-2,2′-Bipyridine 3MLCT State

Two new bidentate ligands (1 and 2) with bicyclic guanidine moieties were synthesized and attached to a Ru-II(bpy)(2) core (bpy = 2, 2'-bipyridine) to afford complexes 3 and 4, which were characterized by spectroscopic and electrochemical methods. Complex 4 was further characterized by X-ray crystallography. In cyclic voltammetric studies, both complexes show a Ru-II/III couple, which is 500 mV less positive than the Re-II/III couple of Ru(bpy)(3)(2+). The (MLCT)-M-1 and (MLCT)-M-3 states of 3 (560 nm/745 nm) and 4 (550 nm/740 nm) are significantly red-shifted with respect to Ru(bpy)32+ (440 nm/620 nm). Compounds 3 and 4 exhibit emission from a Ru-II-to-bpy (MLCT)-M-3 state, which is rarely the emitting state at lambda > 700 nm in [Ru(bpy)(2)(N-N)](2+) complexes.