Synthesis, Stability and Sensitised Lanthanide Luminescence of Heterobimetallic d/f Terpyridine Complexes

The synthesis, solution behavior and photophysical properties of several heteronuclear bimetallic d/f complexes that utilise a Ru-II bis-terpyridine moiety as the sensitiser for Eu-III, Nd-III and Yb-III luminescence are reported and compared to a Gd-III analogue. The former compounds display sensitised emission in the visible and near-infrared (NIR) regions depending on the choice of the Ln(III) cation. We illustrate that sensitised lanthanide emission can operate by two distinctly different pathways that involve either the triplet ligand-centred ((LC)-L-3) excited state of the organic ligand or the triplet metal-to-ligand charge transfer ((MLCT)-M-3) excited state of the Ru-II complex. Owing to the electronic structure of this antenna and the higher density of metal-centred accepting states, a higher sensitization efficiency with Nd-III was observed than with Yb-III and Eu-III. Lastly, we have observed unusual sensitised emission from the upper (F-4(5/2), H-2(9/2)) excited state of Nd-III at approximately 800 nm, which we report here for the first time from a molecular species in solution.