Red-emitting [Ru(bpy)2(N-N)]2+ photosensitizers: emission from a ruthenium(II) to 2,2'-bipyridine (3)MLCT state in the presence of neutral ancillary "super donor" ligands.

The synthesis and characterization of a novel family of [Ru(II)(bpy)2(N-N)](PF6)2 (bpy = 2,2'-bypyridine) 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 Ru(III/II) couple, which is ∼500 mV less positive than the Ru(III/II) couple in Ru(bpy)3(2+). The (1)MLCT and (3)MLCT states of all of the complexes (530-560 nm/732-745 nm) are shifted bathochromically in comparison to those of Ru(bpy)3(2+) (450 nm/620 nm). These values are in good agreement with DFT and TD-DFT calculations.