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

The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2 (bpy-Im2 )]+ , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2 =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-Im2 )]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2 (bpy)]+ (bpy=2,2'-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2 (bpy-Im2 )]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 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-Im2 )]+ are due to the fast ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/3 LCbpy-Im2 . On the other hand, the TA bands of bpy-Im2 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-Im2 )]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCTdfppy and 3 MLCT/3 LCbpy-Im2 , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 in [Ir(dfppy)2 (bpy-Im2 )]+ . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3 MLCTdfppy to 3 MLCT/3 LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance.