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 RuII(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 RuII/III couple, which is 500 mV less positive than the RuII/III couple of Ru(bpy)32+. The 1MLCT and 3MLCT 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 RuII-to-bpy 3MLCT state, which is rarely the emitting state at λ > 700 nm in [Ru(bpy)2(N−N)]2+ complexes.