Deep blue phosphorescence of the iridium(III) complexes containing N-heterocyclic carbene ligands

Article abstract of DOI:10.1080/15421406.2012.703807

Previously, the iridium complexes containing N-Heterocyclic carbene (NHC) ligands have caught attention because of their phosphorescence for organic light-emitting diodes (OLEDs). It was reported that the use of strong-field ligands such as carbenes shoul

Full text of DOI:10.1080/15421406.2012.703807

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Deep Blue Phosphorescence of the
Iridium(III) Complexes Containing N-
Heterocyclic Carbene Ligands
Hyejoo Lee a , Galim Noh b , Boyoung Kim c & Yunkyoung Ha a
a Department of Chemical Engineering and Department of Science ,
Hongik University , Seoul , 121-791 , Korea
b Department of Chemical Engineering , Hongik University , Seoul ,
121-791 , Korea
c Department of Information Display Engineering , Hongik
University , Seoul , 121-791 , Korea
Published online: 17 Sep 2012.
To cite this article: Hyejoo Lee , Galim Noh , Boyoung Kim & Yunkyoung Ha (2012) Deep Blue
Phosphorescence of the Iridium(III) Complexes Containing N-Heterocyclic Carbene Ligands, Molecular
Crystals and Liquid Crystals, 567:1, 193-199, DOI: 10.1080/15421406.2012.703807
To link to this article: http://dx.doi.org/10.1080/15421406.2012.703807
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Mol. Cryst. Liq. Cryst., Vol. 567: pp. 193–199, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.703807
Deep Blue Phosphorescence of the Iridium(III)
Complexes Containing N-Heterocyclic Carbene
Ligands
HYEJOO LEE,1 GALIM NOH,2 BOYOUNG KIM,3
AND YUNKYOUNG HA1,∗
1Department of Chemical Engineering and Department of Science,
Hongik University, Seoul 121-791, Korea
2Department of Chemical Engineering, Hongik University, Seoul 121-791, Korea
3Department of Information Display Engineering, Hongik University,
Seoul 121-791, Korea
Previously, the iridium complexes containing N-Heterocyclic carbene (NHC) ligands
have caught attention because of their phosphorescence for organic light-emitting
diodes (OLEDs). It was reported that the use of strong-field ligands such as carbenes
should result in an increase of the blue phosphorescent efficiency of their metal com-
plexes. In this study, the new NHC ligands, 1-(4-fluorophenyl)-3-methyl benzimidazolate
(fpmb) and 1-naphthyl-3-methyl benzimidazolate (nmb), were introduced to the iridium
complexes to develop phosphorescent dyes for OLED.
We investigated photoabsorption and photoluminescence (PL) properties of the
iridium complexes and studied their bandgaps with cyclic voltammetry (CV). Deep blue
and green phosphorescence was observed with these complexes, and the bandgaps be-
tween their highest occupied molecular orbitals (HOMOs) and the lowest unoccupied
molecular orbitals (LUMOs) were correlated with CV data. We also studied the elec-
troluminescence (EL) properties of the new iridium complexes as possible phosphors
for the application to OLED.
Keywords Organic light-emitting diode; Blue phosphorescence; iridium complex;
carbene
Introduction
During the past decade, organic light-emitting diodes (OLEDs) have been extensively
studied because of their fundamental physical properties and possibility of application for
many kinds of displays [1]. Ir(III) tricyclometallated complexes have caught attention for
their phosphorescence performance for OLEDs [2–6]. For the fabrication of full color
display, the colors red, green, and blue are required for the complexes, and their emission
colors can be tuned by the structures of the ligands in the complexes.
In 2005, Sajoto et al. reported the iridium complexes containing phenylbenzimida-
zolinato and phenylimidazolinato carbene ligands that exhibit near-UV phosphorescence
∗Address correspondence to Yunkyoung Ha, Department of Science, Hongik University, Sangsu-
dong, Mapo-gu, Seoul 121-791, Korea (ROK). Tel.: (+82)2-320-1490; Fax: (+82)2-3142-0335.
E-mail: ykha@hongik.ac.kr
[395]/193

194/[396]
H. Lee et al.
with a relatively high quantum efficiency at room temperature. One of the homoleptic
cyclometallated carbene complexes, Ir(pmb)3 where pmb is 1-phenyl-3-methylimidazolin-
2-ylidiene, was reported with unique luminescence in the near-UV region (ca. 380 nm) at
room temperature [1]. Such high phosphorescence is due to the strong ligand-field effect by
∗
the carbene ligands. As a result, the nonradiative dπ –dσ state is kept largely away from the
emissive triplet state, and therefore the complexes were actually used as emissive materials
for OLEDs [7,8].
The emission color of the complexes depends on the energy gap between LUMO and
HOMO energy levels and the resulting luminescence is mainly determined by their ligands.
Previous studies in which the benzimidazole carbene ligands were used in the complexes
can help us to predict that benzimidazole ring moiety tunes the LUMO level and the other
part of ligand such as a phenyl group affects the HOMO level.
In this study, we synthesize two iridium complexes. One is Ir(fpmb)3 (fpmb = 1-(4-
fluorophenyl)-3-methyl benzimidazolate) and the other is Ir(nmb)3 (nmb = 1-naphthyl-3-
methyl benzimidazolate). Both fpmb and nmb ligands have a benzimidazole ring which
make the LUMO level of complexes almost same. The fpmb ligand has a fluorine-substituted
phenyl ring in which F lowers the HOMO levels. Thus, Ir(fpmb)3 can emit blue lumi-
nescence. Meanwhile, the nmb ligand that contains naphthalene moiety has extensive π-
conjugation length and its iridium complexes may undergo a bathochromic shift in emission
wavelength due to extension of π-conjugation on the ligands.
Experimental
All reagents were purchased from Aldrich Co. and Strem Co., and used without further
purification. All reactions were carried out under an argon atmosphere. Solvents were dried
by standard procedures. All column chromatography was performed with the use of silica
gel (230-mesh, Merck).
Synthesis of Ligands.
1-(4-fluorophenyl)-3-methyl benzimidazolium iodide (fpmb). The 1-(4-fluorophenyl)-
3-methyl benzimidazolium iodide was prepared according to the literature [9] with the
corresponding precursor, 4-fluoroiodobenzene and benzimidazole. Yield: 45%
Naphthyl-3-methyl benzimidazolium iodide (nmb). This compound was prepared
from the reaction of 1-iodonaphthalene and benzimidazole, according to the similar pro-
cedure described for 1-(4-fluorophenyl)-3-methyl benzimidazolium iodide (fpmb). Yield:
34%
Synthesis of Iridium Complexes.
Ir(fmpb)3. 1-(4-fluorophenyl)-3-methyl benzimidazolium iodide (0.93 g, 2.6 mmol),
IrCl3·3H2O (0.25 g, 0.84 mmol), silver carbonate (0.37 g, 1.3 mmol), sodium carbon-
ate (0.14 g, 1.3 mmol), and ethoxyethanol (40 mL) were heated at reflux for 20 h. After
the mixture was cooled to room temperature and diluted with water, and the resultant
precipitate was filtered off and washed with water and methanol. The crude product was
chromatographed on silica gel column with dichloromethane and purified by recrystalliza-
1
tion. Yield: 35%. FAB-MS: calculated 868; found 868. HNMR (DMSO, 400 MHz) : δ
8.34–6.05 (m, 21H, aromatic C-H); 3.33 (s, 9H, N-CH3)

Deep Blue Phosphorescence of Iridium Complexes
[397]/195
Ir(nmb)3. This compound was prepared from the reaction of with IrCl3nH2O and
1-naphthyl-3-methyl benzimidazolium iododide, according to the similar procedure de-
scribed for Ir(fpmb)3. Yield: 14%.
Measurements
UV-visible absorption spectra were obtained from Hewlett Packard 8425A spectrometer.
Photoluminescence (PL) spectra were measured on a Perkin Elmer LS 50B spectrometer.
UV-Vis and PL spectra of iridium complexes were measured in 10−5 M dilute CH2Cl2 solu-
tion. Cyclic voltammograms were obtained at scan rate of 100 mV/s with Electrochemical
1
Analyzer of CH Instruments. H NMR spectra were obtained from a 400 MHz NMR at
Sogang University, and mass spectra were determined on JEOL, JMS-AX505WA, HP 5890
Series II Hewlett-Packard 5890A (capillary column) at Seoul National University in Korea.
3. Results and Discussion
The synthesis of the ligands and their iridium complexes was straightforward, but the
overall yields were rather low presumably due to instability of the carbene ligands dur-
ing the reaction. The ligands, 1-(4-fluorophenyl)-3-methyl benzimidazolium iodide and
1-naphthyl-3-methyl benzimidazolium iodide, was prepared from the reaction of benzimi-
dazolium iodide precursor with 4-fluoroiodobenzene and 1-iodonaphthalene, according to
the reported procedure [4]. Their iridium complexes were synthesized via one step, accord-
ing to the procedure reported previously [7]. The overall synthetic schemes are illustrated
in Figure 1.
The UV-Vis absorption spectra of the complexes in CH2Cl2 are shown in Figure 2.
The absorption spectra of Ir(fpmb)3 and Ir(nmb)3 have the strong bands appearing in the
ultraviolet part of the spectrum between 230 and 280 nm. These bands are assigned to
the spin-allowed 1(π → π∗) transitions of the ligands in the complexes. The weak bands
from 280 nm extended to 320 nm can be assigned to the spin-allowed metal-ligand charge
transfer band (1MLCT). The weaker absorption bands at the longer wavelengths can be
attributed to the spin-forbidden 3MLCT and spin-orbit coupling enhanced 3π-π∗ transition
[10]. In the case of Ir(nmb)3, weak absorption beyond 330 nm was also observed due to
extended π-conjugation by naphthalene moiety while Ir(fmpb)3 showed the shoulder peak
only around 320 nm.
The photoluminescence (PL) spectra of the Ir complexes in 10−5 M CH2Cl2 solution
are shown in Figure 3. The PL patterns of the complexes are different: While Ir(fmpb)3
showed a sharp peak in emission, Ir(nmb)3 exhibited a rather wide emission band with a
shoulder peak in front. The emission maxima also occur at somewhat different wavelengths.
Ir(fmpb)3 and Ir(nmb)3 exhibited the emission maxima at 435 and 530 nm, respectively.
Ir(fmpb)3 containing the electron-withdrawing fluorine (F) substituents in the phenyl ring
experiences blue-shift in emission, compared to Ir(nmb)3 containing naphthyl substituents.
It indicates that the fluorinated phenyl ring contributes to energy gap increase by lowering
the HOMO levels with electron-withdrawing substituent. On the other hand, π-conjugation
effect by the naphthyl substituent in the ligand leads its iridium complex to a bathochromic
shift in photoemission.
We investigated the electrochemical properties of the Ir complexes by the cyclic voltam-
metry (CV), which reveal their positions of the HOMO/LUMO [11,12]. The detailed CV
data were summarized in Table 1. The oxidation potentials which indicate the HOMOs
of the complexes were irreversible in the range of 0.41–0.48 V relative to an internal

196/[398]
H. Lee et al.
Table 1. Physical parameters for the complexes.
Ir complex
λem/nma
Eox/Vb
HOMO/eVc LUMO/eVd
ꢀE/eVd
Ir(fmpb)3
Ir(nmb)3
435
530
0.41
0.48
−5.21
−5.28
−1.39
−1.83
3.82
3.45
b
aMeasured in CH2Cl2 solution. scan rate: 100 mV/s, Electrolyte: tetrabutylammonium hexaflu-
orophosphate. The potentials are quoted against the internal ferrocene standard. cDeduced from the
equation HOMO = − 4.8 − Eox. dCalculated from the optical edge ꢀE = LUMO − HOMO.
(a) The ligands, fpmb and nmb
-fpmb
F
N
NH
CuI, Cs2CO3
N
CH3I
N
I
F
F
N
I-
N+
-nmb
I-
I
N
N
NH
N
CH3I
CuI, Cs2CO3
N
N+
(b) The iridium complexes
L, Ag2CO3 ,Na2CO3
IrCl3·nH2O
Ir(L)3
F
L =
N
N
N
N
fpmb
nmb
Figure 1. The synthesis of ligands and their iridium complexes.

Deep Blue Phosphorescence of Iridium Complexes
[399]/197
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Ir(fpmb)3
Ir(nmb)3
250
300
350
400
450
500
550
600
Wavelength (nm)
Figure 2. UV-Vis absorption spectra of Ir(fmpb)3 and Ir(nmb)3 in a 10−5 M CH2Cl2 solution.
ferrocenium/ferrocene reference (Fc+/Fc). The reduction potentials of the complexes in
this study were calculated to be between −1.39 and −1.83 eV, according to optical-edge
estimation with their photoabsorption spectra. Using the equation HOMO = −4.8 − Eox,
the HOMO of Ir(fpmb)3 was calculated to be −5.21 eV, that gives the larger energy gap
(ꢀE) of 3.82 eV.
Ir(fpmb)3
Ir(nmb)3
1.0
0.8
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Figure 3. PL spectra of Ir(fmpb)3 and Ir(nmb)3 in a 10−5 M CH2Cl2 solution.

198/[400]
H. Lee et al.
Figure 4. (a) EL spectra and (b) CIE coordinates of Ir(fmpb)3.
The electroluminescent (EL) device containing Ir(fpmb)3 as a phosphorescent dopant
was fabricated and its performances were investigated. The EL spectra of Ir(fpmb)3 showed
the emission peak at 475 nm with the CIE coordinates of (0.18, 0.27), as shown in Figure 4.
The patterns of its EL spectra were mostly consistent with the PL results, indicating that
emission was originated from the iridium complex in the emitting layer. However, the EL
maxima of 475 nm was red-shifted, compared with the PL maxima of 435 nm and we
attribute this shift to involvement of solvent CH2Cl2 in the measurement of solution PL
spectra.
Conclusions
The iridium complexes chelated with N-Heterocyclic carbene (NHC) ligands were synthe-
sized and their luminescence properties were investigated. Ir(fmpb)3 and Ir(nmb)3 prepared
in this study exhibited the blue and green emission at 435 and 530 nm, respectively. We
expected that Ir(fpmb)3 might exhibit blue luminescence since the fpmb ligand has a
fluorine-substituted phenyl ring in which F lowers the HOMO levels, leading to the large
band-gap of its iridium complex. As a result, Ir(fpmb)3 showed deep blue emission at
435 nm in PL spectra and CIE coordinates of (0.18, 0.27) in EL. On the other hand,
Ir(nmb)3 showed a red shift on photoemission due to π-conjugation effect of the ligand in
the complex.
Acknowledgment
This work was supported by National Research Foundation of Korea Grant funded by the
Korean Government (NO. 2011-0003765).
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