Near-Infrared Luminescent Ru(II) and Os(II) Complexes.

Two Ru(II) and Os(II) homoleptic complexes based on a phenanthroline-pyridylidene tridentate ligand have been designed to reach NIR emission by combining strong σ-donation, provided by the unconventional N-heterocyclic carbene (NHC) pyridylidene moiety, with the low-lying π-accepting phenanthroline scaffold. Detailed experimental and computational investigations of the photophysical properties of this novel family of cyclometallated complexes reveal emission squarely falling in the NIR domain peaking at λem = 790 nm for the Os(II) complex, while emission is hypsochromically shifted at λem = 747 nm for the Ru(II) homologue in deaerated CH3CN solution at room temperature. Despite the triplet metal-to-ligand charge transfer (3MLCT) nature that characterizes the excited state in both complexes, density functional theory (DFT) calculations allow to ascribe the remarkable optical shift observed in the latter to the larger spin-orbital coupling (SOC) effects exerted by the heavier metal.