d → f Energy Transfer in a Series of IrIII/EuIII 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 IrIII 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 IrIII/LnIII dyads. Crystallographic analysis of several mononuclear iridium(III) complexes and one IrIII/EuIII 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 IrIII, 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 IrIII/EuIII dyads, Ir → Eu energy transfer occurs with varying degrees of efficiency, resulting in partial quenching of the IrIII-based blue emission and the appearance of a sensitized red emission from EuIII. 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 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.