Energy-Transfer Mechanisms in Ir^III -Eu^III Bimetallic Complexes

The bridging ligands in d-f bimetallic complexes play an important role in the excitation energy transfer (EET) process. To elaborate on the effect of the ligand on the EET process, a series of bridging ligands (μ-L), 1-(1',10'-phenanthrolin-2'-yl)-4,4,4-trifluorobutane-1,3-dione (phen3f), 1-(1',10'-phenanthrolin-2'-yl)-4,4,5,5,5-pentafluoropentane-1,3-dione (phen5f), 1-(2,2'-bipyridine-6-yl)-4,4,4-trifluorobutane-1,3-dione (bpy3f), and 1-(2,2'-bipyridine-6-yl)-4,4,5,5,5-pentafluoropentane-1,3-dione (bpy5f), and their corresponding iridium complexes, [(dfppy)₂ Ir(μ-L)] (dfppy=2-(4',6'-difluorophenyl)pyridinato-N,C2'), as well as their corresponding heteroleptic Ir^III -Eu^III complexes [{(dfppy)₂ Ir(μ-L)}₃ EuCl]Cl₂ were synthesized and characterized. Photophysical and kinetic results revealed that the alternation of the bridge ligand resulted in a systemic difference in the lowest triplet-state energy (T₁ ) of the iridium complexes, the EET efficiency from iridium complexes to the Eu^III ion, and a significant difference in the total luminescence quantum yields. Based on the nanosecond time-resolved phosphorescence spectra, a model for the energy-transfer mechanism was proposed for d-f bimetallic complexes, which indicated that the nonradiative relaxation of the excited energy of Eu^III , especially energy dissipation by means of the T₁ state, was the main reason for the discrepancy in the quantum yields of the four Ir^III -Eu^III complexes.