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 frill 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 pi 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 Forster (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 pi stacking between components. In some cases, an initial photoinduced electron-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.