Red-Emitting [Ru(bpy)₂(N-N)]²⁺ Photosensitizers: Emission from a Ruthenium(II) to 2,2′-Bipyridine ³MLCT State in the Presence of Neutral Ancillary “Super Donor” Ligands

The synthesis and characterization of a novel family of [RuII(bpy)2(N-N)]­(PF6)2 (bpy = 2,2′-bipyridine) complexes are reported, where N-N = pyridine/pyrimidine/pyrazine functionalized in different positions with the electron-donating bicyclic hexahydropyrimidopyrimidine (hpp) unit. A series of bidentate ligands 1a–5a were synthesized in good to high yields (55–96%). The corresponding complexes 1b, 2b, and 5b were prepared in n-butanol, while complexes 3b and 4b were prepared in a mixture of n-butanol and water (1/1, v/v) in modest to good yields (23–76%). Both ligand and complex structures were fully characterized by a variety of techniques, including X-ray crystallography. In cyclic voltammetric studies, all the complexes exhibit a RuIII/II couple, which is ∼500 mV less positive than the RuIII/II couple in Ru­(bpy)32+. The 1MLCT and 3MLCT states of all of the complexes (530–560 nm/732–745 nm) are shifted bathochromically in comparison to those of Ru­(bpy)32+ (450 nm/620 nm). These values are in good agreement with DFT and TD-DFT calculations.