Electronic Energy Levels of Dysprosium(III) ions in Solution. Assigning the Emitting State and the Intraconfigurational 4f-4f Transitions in the Vis-NIR Region and Photophysical Characterization of Dy(III) in Water, Methanol, and Dimethyl Sulfoxide.

Dysprosium(III) ions are the third most luminescent lanthanide(III) ions. Dy(III) is used as dopant in optical fibers and as shift reagent in NMR imaging and is the element at the forefront of research in single-molecule magnets. Nonetheless, the excited state manifold of the dysprosium(III) ion is not fully mapped and the nature of the emitting state has not been unequivocally assigned. In the work reported here, the photophysical properties of dysprosium(III) triflate dissolved in H2O, MeOH, and DMSO have been studied in great detail. The solvates are symmetric, all oxygen donor atom complexes where the coordination number is 8 or 9. By comparing protonated and deuterated solvents, performing variable temperature spectroscopy, and determining the excited state lifetimes and luminescence quantum yields, the solution structure can be inferred. For the three complexes, the observed electronic energy levels were determined using absorption and emission spectroscopy. The Dy(III) excited state manifolds of the three solvates differ from that reported by Carnall, in particular for the low lying 6F-states. It is shown that dysprosium(III) complexes primarily luminesce from the 4F9/2 state, although thermal population of, and subsequent luminescence from the 4I15/2 state is observed. The intrinsic luminescence quantum yield is moderate (∼10%) in DMSO- d6 and is significantly reduced in protonated solvent as both C-H and O-H oscillators act as efficient quenchers of the 4F9/2 state. We are able to conclude that the emitting state in dysprosium(III) is 4F9/2, that the m J levels must be considered when determining electronic energy levels of dysprosium(III), and that scrutiny of the transition probabilities may reveal the structure of dysprosium(III) ions in solution.