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Photophysical properties of structural isomers of homoleptic Ir-complexes derived from xylenyl-substituted N-heterocyclic carbene ligands

Authors
Yun, Bo-SunKim, Jin-HyoungKim, So-YoenSon, Ho-JinCho, Dae WonKang, Sang Ook
Issue Date
7-4월-2019
Publisher
ROYAL SOC CHEMISTRY
Citation
PHYSICAL CHEMISTRY CHEMICAL PHYSICS, v.21, no.13, pp.7155 - 7164
Indexed
SCIE
SCOPUS
Journal Title
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume
21
Number
13
Start Page
7155
End Page
7164
URI
https://scholar.korea.ac.kr/handle/2021.sw.korea/66043
DOI
10.1039/c9cp00553f
ISSN
1463-9076
Abstract
The phosphorescence properties of fac-Ir(pmp)(3), mer-Ir(pmp)(3), fac-Ir(dmpmp)(3) and mer-Ir(dmpmp)(3) (where pmp = 3-methyl-1-phenyl-2,3-dihydro-1H-imidazo[4,5-b]pyridine and dmpmp = 1-(2',6'-dimethylbiphenyl-2-yl)-3-methyl-2,3-dihydro-1H-imidazo[4,5-b]pyridine) in CH2Cl2 were investigated. At 77 K, the fac-isomers showed blue emission with a vibronic structure, while the mer-isomers showed less structured emissions. At 300 K, all complexes showed broad and markedly red-shifted emission spectra compared to those at 77 K. The quantum yields of the Ir(dmpmp)(3) isomers were very low, and their emission lifetimes were very short compared to those of Ir(pmp)(3). In order to understand the large differences between the photodynamic properties of Ir(pmp)(3) and Ir(dmpmp)(3), we performed femtosecond time-resolved transient absorption (TA) spectroscopic measurements. The TA spectra of Ir(dmpmp)(3) were almost the same as those of Ir(pmp)(3) at a short delay time. However, Ir(dmpmp)(3) showed a new broad TA band at around 720 nm with increasing delay time. The rise time of this band was ca. 10 ps for both isomers, and this may be attributed to the geometrical change in the excited state, which is associated with the steric hindrance of the bulky dimethylphenyl substituent. Actually, Ir(dmpmp)(3) showed a strong rigidochromic shift in the emission spectra with varying temperature. To understand the molecular orbitals and the energy levels, theoretical calculations were performed using density functional theory. As a result, structural displacement takes place accompanied by the fast migration of localization of excited states via intraligand charge transfer.
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