Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding pi-Conjugated Ligand

Abstract

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex,[Ir(dfppy)(2)(bpy-Im(2))](+), where dfppy=2-(2,4-difluorophenyl)pyridine andbpy-Im(2)=4,4'-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2',-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)(2)(bpy-Im(2))](+) with significantly expanding pi-conjugated ligand are compared to those of[Ir(dfppy)(2)(bpy)](+) (bpy=2,2'-bipyridine) and a free bpy-Im(2) ligand. The emission spectrum of [Ir(dfppy)(2)(bpy-Im(2))](+) shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im(2) showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of[Ir(dfppy)(2)(bpy-Im(2))](+) are due to the fast ILET process from (MLCTdfppy)-M-3 to (MLCT)-M-3/(LCbpy-Im2)-L-3, resulting in the phosphorescence emission originating from (MLCT)-M-3/(LCbpy-Im2)-L-3. On the other hand, the TA bands ofbpy-Im(2)are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of[Ir(dfppy)(2)(bpy-Im(2))](+)showes broad bands centered at 420 and 600 nm, attributed to the transitions from (MLCTdfppy)-M-3 and (MLCT)-M-3/(LCbpy-Im2)-L-3, respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from (MLCTdfppy)-M-3 to (MLCT)-M-3/(LCbpy-Im2)-L-3 in [Ir(dfppy)(2)(bpy-Im(2))](+). From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from (MLCTdfppy)-M-3 to (MLCT)-M-3/(LCbpy-Im2)-L-3 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding pi-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.