Emissive Ruthenium-Bisdiimine Complexes with Chelated Thioether Donors

The photophysical properties of ruthenium(II) complexes of bidentate 2-[(alkylthio)methyl]pyridine (N,S) ligands have been systematically investigated. The co-ligands in the heteroleptic complexes were diimines (N,N ligands, e.g., bpy, phen) or tripodal tetradentate tris(2-pyridylmethyl)amine (tpa). Their X-ray structures revealed little variation in the Ru-S (2.31-2.33 Å) and Ru-N (2.06-2.11 Å) bond lengths. Despite this, considerable variation can be observed in the electrochemistry and spectroscopy measurements. The presence of the thioether sulfur donor results in a shift to a more positive potential of the Ru ²⁺/Ru ³⁺ process, and the absorption maxima are consequently blueshifted in [Ru(N,N) ₂(N,S)] ²⁺ complexes compared with the [Ru(N,N) ₃] ²⁺ analogues. The [Ru(N,N) ₂(N,S)] ²⁺ complexes display strong room-temperature emission from ³MLCT states, governed by the N,N donors. This is corroborated by low-temperature steady-state emission studies, which revealed typical ³MLCT emission profiles and excited-state lifetimes of around 5-10 μs.