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Inter‐Ligand Energy Transfer Process in an Ir‐Complex with Expanding π‐Conjugated Ligand
Author(s) -
Hwang Jeongwook,
Ahn Mina,
Choi Jungkweon,
Wee KyungRyang
Publication year - 2020
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.202000565
Subject(s) - bathochromic shift , photochemistry , chemistry , bipyridine , ligand (biochemistry) , iridium , phosphorescence , ultrafast laser spectroscopy , conjugated system , intramolecular force , fluorescence , quantum yield , crystallography , spectroscopy , stereochemistry , crystal structure , biochemistry , physics , receptor , quantum mechanics , polymer , organic chemistry , catalysis
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 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 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 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 3 MLCT dfppy and 3 MLCT/ 3 LC bpy‐Im2 , respectively. Time‐resolved spectroscopic results confirm the efficient ILET dynamics from 3 MLCT dfppy to 3 MLCT/ 3 LC bpy‐Im2 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 3 MLCT dfppy to 3 MLCT/ 3 LC bpy‐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.