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) (bpy-Im )] , where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im =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) (bpy-Im )] with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy) (bpy)] (bpy=2,2'-bipyridine) and a free bpy-Im ligand. The emission spectrum of [Ir(dfppy) (bpy-Im )] shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy) (bpy-Im )] are due to the fast ILET process from MLCT to MLCT/ LC , resulting in the phosphorescence emission originating from MLCT/ LC . On the other hand, the TA bands of bpy-Im are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy) (bpy-Im )] showes broad bands centered at 420 and 600 nm, attributed to the transitions from MLCT and MLCT/ LC , respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from MLCT to MLCT/ LC in [Ir(dfppy) (bpy-Im )] . From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from MLCT to MLCT/ LC 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.