Green‐Light Activation of Push–Pull Ruthenium(II) Complexes

Synthesis, characterization, electrochemistry, and photophysics of homo‐ and heteroleptic ruthenium(II) complexes [Ru(cpmp)(2)](2+) (2(2+)) and [Ru(cpmp)(ddpd)](2+) (3(2+)) bearing the tridentate ligands 6,2’’‐carboxypyridyl‐2,2’‐methylamine‐pyridyl‐pyridine (cpmp) and N,N’‐dimethyl‐N,N’‐dipyridin‐2‐ylpyridine‐2,6‐diamine (ddpd) are reported. The complexes possess one (3(2+)) or two (2(2+)) electron‐deficient dipyridyl ketone fragments as electron‐accepting sites enabling intraligand charge transfer (ILCT), ligand‐to‐ligand charge transfer (LL'CT) and low‐energy metal‐to‐ligand charge transfer (MLCT) absorptions. The latter peak around 544 nm (green light). Complex 2(2+) shows (3)MLCT phosphorescence in the red to near‐infrared spectral region at room temperature in deaerated acetonitrile solution with an emission quantum yield of 1.3 % and a (3)MLCT lifetime of 477 ns, whereas 3(2+) is much less luminescent. This different behavior is ascribed to the energy gap law and the shape of the parasitic excited (3)MC state potential energy surface. This study highlights the importance of the excited‐state energies and geometries for the actual excited‐state dynamics. Aromatic and aliphatic amines reductively quench the excited state of 2(2+) paving the way to photocatalytic applications using low‐energy green light as exemplified with the green‐light‐sensitized thiol–ene click reaction.