Inter-ligand energy transfer process in an Ir-complex with expanding π-conjugated ligand.

2020 
The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy) 2 (bpy-Im 2 )] + , where dfppy = 2-(2,4-difluorophenyl)pyridine and bpy-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 π-conjugated ligand were 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 )] + showed no shift upon changing the solvent polarity, whereas the free ligand bpy-Im 2 showed the bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to the intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy) 2 (bpy-Im 2 )] + are due to the fast ILET process from 3 MLCT dfppy to 3 MLCT/ 3 LC bpy-Im2 , resulting in the phosphorescence emission originating from 3 MLCT/ 3 LC bpy-Im2 . On the other hand, the TA bands of bpy-Im 2 were 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 )] + showed broad bands centered at 420 and 600 nm, attributed to the transitions from 3 MLCT dfppy and 3 MLCT/ 3 LC bpy-Im2 , respectively.
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