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+ (22+ ) and [Ru(cpmp)(ddpd)]2+ (32+ ) 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 (32+ ) or two (22+ ) 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 22+ 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 32+ 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 22+ paving the way to photocatalytic applications using low-energy green light as exemplified with the green-light-sensitized thiol-ene click reaction.