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 3MLCTdfppy to 3MLCT/3LCbpy‐Im2, resulting in the phosphorescence emission originating from 3MLCT/3LCbpy‐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 3MLCTdfppy and 3MLCT/3LCbpy‐Im2, respectively. Time‐resolved spectroscopic results confirm the efficient ILET dynamics from 3MLCTdfppy to 3MLCT/3LCbpy‐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 3MLCTdfppy to 3MLCT/3LCbpy‐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. Energy transfer: As π‐conjugation expands from bpy ligand to bpy‐Im2 ligand, energy from main ligand to ancillary ligand is efficiently transferred. Species‐associated spectra (SAS) reveal that in [Ir(dfppy)2(bpy‐Im2)]+, ILET occurs without overlap with other species from the main dfppy ligand to the ancillary bpy‐Im2 ligand.