d → f Energy Transfer in a Series of Ir^III/Eu^III Dyads: Energy-Transfer Mechanisms and White-Light Emission

An extensive series of blue-luminescent iridium(III) complexes has been prepared containing two phenylpyridine-type ligands and one ligand containing two pyrazolylpyridine units, of which one is bound to Ir^III and the second is pendant. Attachment of {Ln(hfac)₃} (Ln = Eu, Gd; hfac = anion of 1,1,1,5,5,5,-hexafluoropentanedione) to the second coordination site affords Ir^III/Ln^III dyads. Crystallographic analysis of several mononuclear iridium(III) complexes and one Ir^III/Eu^III dyad reveals that in most cases the complexes can adopt a folded conformation involving aromatic π stacking between a phenylpyridine ligand and the bis(pyrazolylpyridine) ligand, but in one series, based on CF₃-substituted phenylpyridine ligands coordinated to Ir^III, the steric bulk of the CF₃ group prevents this and a quite different and more open conformation arises. Quantum mechanical calculations well reproduce these two types of “folded” and “open” conformations. In the Ir^III/Eu^III dyads, Ir → Eu energy transfer occurs with varying degrees of efficiency, resulting in partial quenching of the Ir^III-based blue emission and the appearance of a sensitized red emission from Eu^III. Calculations based on consideration of spectroscopic overlap integrals rule out any significant contribution from Förster (dipole–dipole) energy transfer over the distances involved but indicate that Dexter-type (exchange) energy transfer is possible if there is a small electronic coupling that would arise, in part, through π stacking between components. In some cases, an initial photoinduced <i>electron</i>-transfer step could also contribute to Ir → Eu energy transfer, as shown by studies on isostructural iridium/gadolinium model complexes. A balance between the blue (Ir-based) and red (Eu-based) emission components can generate white light