A butterfly-like yellow luminescent Ir(iii) complex and its application in highly efficient polymer light-emitting devices

Article abstract of DOI:10.1039/c2jm34992b

A novel butterfly-like Ir(iii) complex is designed, synthesized, and characterized for highly efficient yellow phosphorescent polymer-based light-emitting diodes (PLEDs). The device shows a maximum external quantum efficiency of 19.2%, luminance efficiency of 40 cd A^-1 and Commission International de L'Eclairage (CIE) color coordinates of (0.49, 0.50) at J = 1.2 mA cm^-2.

Full text of DOI:10.1039/c2jm34992b

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PAPER
A butterfly-like yellow luminescent Ir(III) complex and its application in highly
efficient polymer light-emitting devices†
a
a
Lei Fu, Mei Pan, Yan-Hu Li,b Hong-Bin Wu,b Hai-Ping Wang,a Cheng Yan,a Kang Li,a Shi-Chao Wei,a
*
a
Zi Wang and Cheng-Yong Su
a
*
Received 27th July 2012, Accepted 3rd September 2012
DOI: 10.1039/c2jm34992b
A novel butterfly-like Ir(III) complex is designed, synthesized, and characterized for highly efficient
yellow phosphorescent polymer-based light-emitting diodes (PLEDs). The device shows a maximum
external quantum efficiency of 19.2%, luminance efficiency of 40 cd Aꢀ1 and Commission International
de L’Eclairage (CIE) color coordinates of (0.49, 0.50) at J ¼ 1.2 mA cmꢀ2
.
methods. However, until now, PLEDs still faced the problem of
the relatively low device efficiency of fluorescent polymer-emit-
ting layers. One solution is that the mixture of phosphorescent
Ir(III) complexes in polymer-emitting layers may provide a
significant breakthrough in obtaining both large display area and
high device efficiency.8 In this regard, improvement of the
luminance efficiency (Fp) is a prerequisite for the successful
application of Ir(III) complex based PLEDs. However, other
factors such as the electron or hole transport and injection
abilities, solubilities, and film-forming properties of Ir(III)
complex will also affect the electroluminesence (EL) performance
of the PLED devices significantly. In this article, we report a
butterfly-like bright yellow luminescent Ir(III) complex based on
a carbazole-substituted tridentate N^C^N ligand. Its application
in PLEDs gives a maximum external quantum efficiency of
19.2%, which is among the highest quantum efficiency for yellow
light PLED ever reported.9
Introduction
Ir(III) complexes are one kind of promising phosphorescent
emitters which have attracted intensive study in recent years.1
Due to their efficient intersystem crossing owing to the large
spin–orbital coupling mediated by the heavy metal core, Ir(III)
complexes feature unmatched photoluminescence efficiencies,
relatively short (ms range) phosphorescence lifetimes and
versatile color tuning abilities via ligand control. Typically, Ir(III)
complexes can emit blue to red phosphorescence upon opto- or
electro-stimulating.2–4 However, reports of yellow luminescent
Ir(III) complexes remain relatively rare. In recent years, due to the
emerging demand for white light illumination, mixing yellow
with blue luminescence or phosphorescence has become an
important choice in the fabrication of white light emitting diodes
(LEDs).5–7 Furthermore, yellow constitutes one of the most
sensitive colors to human eyes and is widely used in traffic
signals, wild field rescues or art decoration. Therefore, the
fabrication of yellow light emitting Ir(III) complexes is quite
essential in both the scientific and the practical view. Due to this,
the application of efficiently emitting Ir(III) complexes in poly-
mer-based light emitting diodes (PLEDs) has made significant
progress in recent years. Compared with the standard vacuum
evaporation-based organic light emitting diodes (OLEDs),
PLEDs feature improved large-area display or illumination
ability, as well as facile and inexpensive solution processing
Experimental
General details
The 1H NMR spectra were recorded on a Varian/Mercury-Plus 300
NMR spectrometer using tetramethylsilane (TMS) as an internal
reference. Elemental analyses (C, H, N) were tested on a Vario EL
instrument. Mass spectra were determined by AccuTOF CS JMS-
T100CS spectrometer. UV-Vis absorption spectra were measured
on a Shimadzu UV-2450 spectrometer. The photoluminescence
(PL) spectra were recorded on an Edinburgh Analytical Instru-
ments FLS920 spectrometer. The emission quantum yield of the
complex was measured in degassed CH2Cl2 solution, using
Rhodamine 6G in degassed ethanol as the standard. Cyclic vol-
tammetry (CV) was performed on a CHI760D electrochemical
workstation at room temperature using 0.1 mol Lꢀ1 tetra(n-butyl)
ammonium hexafluorophosphate (TBAP) as the supporting elec-
trolyte and degassed CH2Cl2 as the solvent. The ferrocenium/
ferrocene couple was used as the internal standard.
aMOE Laboratory of Bioinorganic and Synthetic Chemistry/KLGHEI of
Environment and Energy Chemistry, State Key Laboratory of
Optoelectronic Materials and Technologies, School of Chemistry and
Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275,
China. E-mail: panm@mail.sysu.edu.cn; cesscy@mail.sysu.edu.cn; Fax:
+86 20-8411-5178
bInstitute of Polymer Optoelectronic Materials and Devices, State Key
Laboratory of Optoelectronic Functional Materials and Devices, South
China University of Technology, Guangzhou 510640, P. R. China
† Electronic
supplementary
information
(ESI)
available:
Crystallographic information, TG curve, PL spectra. See DOI:
10.1039/c2jm34992b
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The new ligand L1 (1,3-bis(1-(6-(9H-carbazol-9-yl)hexyl)-1H-
benzo[d]imidazol-2-yl)benzene) was synthesized by introducing
carbazole groups with long alkyl chains on a tridentate N^C^N
ligand, and then the butterfly-like Ir(III) complex 1 (Scheme 1)
was synthesized from IrCl3 with L1 and 2-phenylpyridine
(ppy).10 Both the ligand and complex were characterized by
Synthesis of 1,3-bis(1-(6-(9H-carbazol-9-yl)hexyl)-1H-benzo[d]
imidazol-2-yl)benzene (L1). The benzimidazole derivatives
MBImB (2.5 mmol), the bromide Br-Cz (5 mmol), aqueous
NaOH (50%, 0.5 g) and TBAB (80 mg, 0.25 mmol) were dis-
solved in 20 mL butanone. The mixture was heated to reflux for 8
h, and the resulting precipitate complex was collected. White
power with yield of 86% was obtained. 1H NMR (300 MHz,
DMSO, TMS) d 8.07 (dd, J ¼ 14.4, 9.4 Hz, 5H), 7.86 (d, J ¼ 7.7
Hz, 2H), 7.65 (dt, J ¼ 5.3, 3.3 Hz, 3H), 7.52 (d, J ¼ 7.3 Hz, 2H),
7.41–7.16 (m, 12H), 7.09 (t, J ¼ 7.2 Hz, 4H), 4.16 (dt, J ¼ 13.5,
6.9 Hz, 8H), 1.50 (s, 8H), 1.02 (s, 8H). ESI-MS m/z: 809.6.
1
means of H NMR, MS, and elemental analyses. The synthetic
details are as follows:
Synthesis
of
1,3-bis(1H-benzo[d]imidazol-2-yl)benzene
(MBImB). Isophthalic acid (1.66 g, 10 mmol) and benzene-1,2-
diamine (2.16 g, 20 mmol) were dissolved in 15 mL poly-
phosphoric acid and 15 mL phosphoric acid. The mixture was
heated to 190 ꢁC for 12 h, then cooled to 100 ꢁC and poured into
water. The precipitate was filtered and then recrystallized with
50% ethanol as a white product with yield of 72%. 1H NMR (300
MHz, dimethyl sulfoxide (DMSO), TMS) d 9.03 (s, 1H), 8.24
(dd, J ¼ 7.8, 1.7 Hz, 2H), 7.72 (t, J ¼ 7.8 Hz, 1H), 7.66–7.55 (m,
4H), 7.25–7.16 (m, 4H), 1.11–1.00 (m, 1H). electrospray ioniza-
tion mass spectrometry (ESI-MS) m/z: 310.1.
Preparation of Ir(III) complex (1). Under N2 atmosphere,
IrCl3$4H2O (370 mg, 1 mmol) was added to a solution of the
ligand L1 (1.1 mmol) in methanol (20 mL). The mixture was
heated to 75 ꢁC for 8 h and then cooled to room temperature. The
green precipitates were filtered, washed with chloroform 3 times
and then with ether 3 times. Crude dichloro-bridged iridium
dimers ([Ir(L1)Cl2]2) were obtained. The complex was insoluble
in common organic solvents and was therefore used without
purification for further preparation. After that, the mixture of
the dimers and 2-phenylpyridine (ppy, 1 g) in 15 mL glycerol was
heated by microwave irradiation (600 W) for 10 min. After the
mixture was cooled to room temperature, the addition of water
resulted in the formation of an orange precipitate, which was
collected by filtration and washed with water and ether. The
purification was performed by flash column chromatography
using CH2Cl2 and CHCl3 as eluent to afford complex 1 with a
Synthesis of 9-(6-bromohexyl)-9H-carbazole (Br-Cz). Carba-
zole (1.67 g, 10 mmol), 1,6-dibromohexane (5.5 mL, 36 mmol),
tetrabutylammonium bromide (TBAB) (160 mg, 0.5 mmol), and
aqueous NaOH (50%, 1 g) were added to benzene (5 mL). The
mixture was stirred at room temperature for 48 h. The organic
phase was separated and washed by water (2 ꢂ 30 mL) and then
dried with MgSO4. The solvent was removed in vacuo, and the
residue was purified by flash column chromatography using
petroleum ether (boiling range 60–90 ꢁC) : ethyl acetate ¼ 10 : 1
as eluent to afford white needle crystals (2.38 g, 60%). 1H NMR
(300 MHz, CDCl3, TMS) d 8.10 (d, J ¼ 7.7 Hz, 2H), 7.50–7.37
(m, 4H), 7.25–7.19 (m, 2H), 4.32 (t, J ¼ 7.1 Hz, 2H), 3.37 (t, J ¼
6.7 Hz, 2H), 1.92 (t, J ¼ 12.3, 6.1 Hz, 2H), 1.82 (dd, J ¼ 14.6, 6.7
Hz, 2H), 1.53–1.35 (m, 4H). ESI-MS m/z: 330.1.
1
yield of 26%. H NMR (300 MHz, CD2Cl2, TMS) d 10.51 (dd,
J ¼ 5.5, 1.2 Hz, 1H), 8.15–8.04 (m, 5H), 7.86 (d, J ¼ 7.8 Hz, 2H),
7.68 (dd, J ¼ 9.0, 4.0 Hz, 1H), 7.52–7.32 (m, 10H), 7.21 (dd, J ¼
11.5, 8.7, 7.7 Hz, 9H), 6.94 (dd, J ¼ 8.2, 7.1 Hz, 2H), 6.55–6.46
(m, 1H), 6.27 (t, J ¼ 7.4 Hz, 1H), 6.21 (d, J ¼ 8.2 Hz, 2H), 5.73 (d,
J ¼ 7.6 Hz, 1H), 4.71–4.54 (m, 4H), 4.27 (t, J ¼ 7.0 Hz, 4H),
2.08–1.98 (m, 4H), 1.91–1.81 (m, 4H), 1.29 (s, 4H). ESI-MS m/z:
Scheme 1 Synthesis of the ligand L1 and complex 1.
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1154.8. Anal. calcd (%) for C67H59IrN7Cl: C, 67.63; H, 8.24; N,
5.00; found: C, 67.90; H, 7.84; N, 5.27.
contributed by the ancillary ligand ppy. The planes of the
primary and ancillary ligands are almost perpendicular (dihedral
angle is 100ꢁ). The Ir–C bond lengths between Ir3+ and L1 and
between Ir3+ and ppy are 2.005 and 2.036 A, respectively, which
ꢀ
PLED fabrication and measurement
ꢀ
ꢀ
are much shorter than Ir–Nppy (2.160 A) and Ir–Cl (2.481 A)
bonds.
Devices based on the iridium(III) complex 1 with a structure of
indium tin oxide (ITO)/Poly(3,4-ethylenedioxythiophene) poly-
(styrenesulfonate) (PEDOT:PSS) (4083)/EML (Emitting Mate-
rial Layer) (80 nm)/CsF (1.5 nm)/Al (100 nm) were fabricated.
UV-Vis absorption and photoluminescent properties
To examine the photophysical properties of the complex, UV-Vis
and PL spectra were measured. The UV-Vis spectrum of complex
1 in CH2Cl2 shows the 1MLCT (metal-to-ligand charge transfer)
band at 404, 427 and 477 nm, in addition to the p–p* transition
at 266 nm for ppy and 294, 347 nm for the L1 ligand (Fig. 2a).
Besides these, the band around 510–550 nm can be assigned as a
3MLCT band characteristic of Ir(III) complex. Fig. 2b shows the
emission spectra of the complexes in solid state and in CH2Cl2
solution (1 ꢂ 10ꢀ5 mol Lꢀ1) at 298 K. As shown in Fig. 2b, the
solid and solvent (CH2Cl2, 1 ꢂ 10ꢀ5 mol Lꢀ1) state complex 1
exhibit intense luminescence with a maximum at 570 and 558 nm,
respectively. The CIE coordinates for the two emissions are
calculated to be (0.50, 0.50) and (0.49, 0.50), falling into the
yellow light region.
PVK
poly(N-vinylcarbazole) : PBD
(2-tert-butylphenyl-5-
biphenyl-1,3,4-oxadiazole) (100 : 40) doped with 1, 2, 4, 8% of
iridium complex 1 were used as EML. All raw materials were
purchased commercially. ITO glass substrates were washed in
turn with acetone, substrate cleaning detergent, deionized water
and isopropyl alcohol, then oven dried. The cleaned substrates
were treated with oxygen plasma to improve the work function of
ITO. After that, a 40 nm thick PEDOT:PSS film was spin-coated
onto the pre-cleaned and dried ITO glass and baked in a vacuum
oven at 80 ꢁC for 8 h. Then the PVK, PBD and complex 1 with
different ratios were dissolved separately in chlorobenzene and
spin-coated on top of the dried PEDOT:PSS layer to obtain an
emitting layer (80 nm) inside a glove box under vacuum. The
assembly was then annealed for 20 min at 100 ꢁC. Subsequently,
a layer of CsF (1.5 nm) and a layer of Al (100 nm) were vacuum
evaporated at 3 ꢂ 10ꢀ4 Pa on top of the EML polymer layer.
Except for the PEDOT:PSS, which was spin-coated in air, other
processes were performed in the glove box. EL spectra and
current density–voltage–luminance (J–V–L) characteristics were
collected by a PR705 photometer and a Keithley 236 source
meter, respectively. The external quantum efficiency was cali-
brated in an integrating sphere (Labsphere, IS080).
A noteworthy observation is that the photoluminescent
quantum yield (F) for complex 1 is quite outstanding, reaching
77% in CH2Cl2 when using Rhodamine 6G as the standard. This
indicates efficient intra- and intermolecular energy transfer in the
complex. The luminescent lifetimes (s) of the complex were
measured in air at 298 K and 77 K, as 131 and 1241 ns, respec-
tively. We can see that low temperature significantly reduces non-
radiative pathways for triplets and increases the lifetime of the
luminescence in the Ir(III) complex.
Results and discussion
Optimized geometrical structure of complex 1
Fig. 1 shows the coordination unit structure of complex 1,
optimized by theoretical calculations with HOMO or LUMO
orbitals highlighted. The calculations were performed using the
B3LYP hybrid functional implemented in the Gaussian-03 suite
of programs.11 The basis sets used were 6-31G(d) for C, H, N and
LanL2DZ for Ir. We can see that the central ion, Ir3+, is six
coordinated by three nitrogen atoms and two carbon atoms from
L1 and ppy, and one chlorine atom helps to satisfy the octahedral
coordination sphere. Two of the nitrogen atoms and one of the
carbon atoms are offered by the ligand L1, forming N^C^N type
coordination, and the other nitrogen and carbon atoms are
Fig. 2 (a) UV-Vis (CH2Cl2, 1 ꢂ 10ꢀ5 mol Lꢀ1) and (b) photo-
luminescent emission spectra (solid and solvent in CH2Cl2, 1 ꢂ 10ꢀ5 mol
Fig. 1 Schematic representation of the optimized geometrical structure
and HOMO (left), LUMO (right) orbitals of complex 1.
L
ꢀ1) of complex 1.
22498 | J. Mater. Chem., 2012, 22, 22496–22500
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Electrochemical properties
The CV responses for the complex dissolved in CH2Cl2 are
presented in Fig. 3. We can see that the complex has single
reversible oxidative wave between 1.73 and 1.96 V vs. normal
hydrogen electrode (NHE) and between 0.80 and 1.33 vs. ferro-
cene, which may be formally attributed to the oxidation of the
iridium metal centre [Ir3+/4+]. In addition, the complex has two
reduction peaks which belong to the reduction of the cyclo-
metallated ligand and ancillary ligand ppy. According to the
equation EHOMO ¼ ꢀ(Eox(vs.
+ 4.8) eV and ELUMO
Fc/Fc+)
ꢀ(EHOMO + Eg),12 the HOMO and LUMO of complex 1 in
¼
CH2Cl2 were calculated to be ꢀ6.13 and ꢀ3.71 eV, respectively.
Theoretical calculations
To understand the nature of the electrochemical and optical
properties of the complex, we used density functional theory
(DFT) and time-dependent density functional theory (TD-DFT)
calculations to examine the frontier orbitals and electronic
transitions in complex 1. From Fig. 1, we can see that the HOMO
orbitals of the complex comprise contributions mainly from the
iridium atom, the chlorine atom and the phenyl ring of the ppy
ligands, while the LUMO orbitals are mainly contributed by the
benzene and pyridine rings of the cyclometalated ligands (Table
S1†). The delocalization of the HOMO and LUMO orbitals of
the complex is consistent with the experimental evidence from
electrochemistry and photoluminescence.
The calculated TD-DFT singlet state transitions support the
assignment of the absorption bands from the experimental data.
The absorption bands above 400 nm are mainly assigned to the
energy transitions between the frontier orbitals, i.e. between the d
orbitals of Ir(III) center and p* orbitals of the ligand. Therefore,
these peaks are generally endowed with MLCT characters.
Electrophosphorescent properties
The iridium complex 1 which exhibited high photoluminescent
quantum yield at room temperature was selected for PLED
device fabrication. The device structure used is ITO/
PEDOT:PSS(4083)/EML(80 nm)/CsF(1.5 nm)/Al(100 nm), in
which ITO was used as the anode, PVK : PBD (100 : 40) doped
with 1, 2, 4, 8% of iridium complex as the emitter and CsF/Al as
the cathode.
Fig. 4 Electroluminescent performance of the PLED devices based on
complex 1.
Devices with the iridium complex emit strong orange-yellow
light with an emission maximum at 570 nm (Fig. 4a). We can see
that the maximum emission wavelength and the shape of the
electroluminescent spectra are nearly the same as those in the
photoluminescent spectra, which illustrates that both EL and PL
originate from the same emitting core. The CIE coordinates of
the devices and other details are listed in Table 1. We can see that
all the electroluminescence falls into the orange-yellow region.
Fig. 4b and c show the luminance–voltage–current (L–C–I)
and the external quantum efficiency–current density–power
efficiency characteristics of the devices, respectively. All devices
show low turn-on voltages between 4.7 and 7.0 V. The device
with a doping ratio of 4% appears to show the highest perfor-
mance, with brightness of 7455 cd mꢀ2 at 9.3 V, external
quantum efficiency (EQE) of 19.2% and luminance efficiency
Fig. 3 CV outline of complex 1 in CH2Cl2.
(LE) of 40.1 cd Aꢀ1
.
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Table 1 Detailed electroluminescent performance of the PLED devices
based on complex 1
3 (a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and
S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4; (b) A. Fukase,
K. L. T. Dao and J. Kido, Polym. Adv. Technol., 2002, 13, 601; (c)
M. H. Ho, B. Balaganesan, T. Y. Chu, T. M. Chen and
C. H. Chen, Thin Solid Films, 2008, 517, 943; (d) J. W. Kang,
D. S. Lee, H. D. Park, J. W. Kim, W. I. Jeong, Y. S. Park,
S. H. Lee, K. Go, J. S. Lee and J. J. Kim, Org. Electron., 2008, 9,
452; (e) M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki and Y. Taga,
Appl. Phys. Lett., 2001, 79, 156.
4 (a) T. Akira, T. Takao, O. Shinjiro, K. Atsushi, M. Takashi,
M. Kiyoshi, F. Manabu and I. Satoshi, Jpn. Kokai Tokkyo Koho,
2003, 26; (b) C. Adachi, M. A. Baldo, S. R. Forrest, S. Lamansky,
M. E. Thompson and R. C. Kwong, Appl. Phys. Lett., 2001, 78,
1622; (c) S. Lamansky, P. Djurovich, D. Murphy, F. A. Razzaq,
H. E. Lee, C. Adachi, P. E. Burrows, S. R. Forrest and
M. E. Thompson, J. Am. Chem. Soc., 2001, 123, 4304; (d)
A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide,
J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi,
S. Okada, M. Hoshino and K. Ueno, J. Am. Chem. Soc., 2003, 125,
12971; (e) L.-L. Wu, C.-H. Yang, I.-W. Sun, S.-Y. Chu, P.-C. Kao
and H.-H. Huang, Organometallics, 2007, 26, 2017; (f) S. O. Jung,
Q. Zhao, J. W. Park, S. O. Kim, Y. H. Kim, H. Y. Oh, J. Kim,
S. K. Kwon and Y. Kang, Org. Electron., 2009, 10, 1066.
Compared with Ir(III) complexes with similar structures, the
electroluminescent performance of the PLED devices based on
complex 1 is substantially higher, which can be mainly explained
by the efficient hole transport and injection ability, due to the
introduction of carbazole groups on the Ir(III) complex,13 as well
as the good solubility and film-forming property due to the
introduction of long alkyl chains.
5 (a) C. Y. Mei, J. Q. Ding, B. Yao, Y. X. Cheng, Z. Y. Xie, Y. H. Geng
and L. X. Wang, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1746;
(b) S. Tokito, T. Iijima, T. Tsuzuki and F. Sato, Appl. Phys. Lett.,
2003, 83, 2459.
6 (a) Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing and
F. Wang, Adv. Funct. Mater., 2009, 19, 84; (b) R. Wang, D. Liu,
H. Ren, T. Zhang, H. Yin, G. Liu and J. Li, Adv. Mater., 2011, 23,
2823; (c) Q. Wang and D. Ma, Chem. Soc. Rev., 2010, 39,
2387.
7 (a) Y. Sun and S. R. Forrest, Appl. Phys. Lett., 2007, 91, 263503; (b)
Y. Sun and S. R. Forrest, Org. Electron., 2008, 9, 994.
8 H. Wu, L. Ying, W. Yang and Y. Cao, Chem. Soc. Rev., 2009, 38,
3391 and references there in.
9 (a) N. R. Park, G. Y. Rye, S. J. Moon, J. H. Seo, Y. K. Kim and
D. M. Shin, Mol. Cryst. Liq. Cryst., 2011, 538, 75; (b) S. Wang,
H. Lou, Y. Liu, G. Yu, P. Lu and D. Zhu, J. Mater. Chem., 2001,
11, 2971; (c) C.-C. Kwok, H. M. Y. Ngai, S.-C. Chan,
I. H. T. Sham, C.-M. Che and N. Zhu, Inorg. Chem., 2005, 44, 4442.
10 (a) A. J. Wilkinson, H. Puschmann, J. A. K. Howard, C. E. Foster
and J. A. G. Williams, Inorg. Chem., 2006, 45, 8685; (b) S. Obara,
M. Itabashi, F. Okuda, S. Tamaki, Y. Tanabe, Y. Ishii, K. Nozaki
and M. Haga, Inorg. Chem., 2006, 45, 8907.
11 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven,
K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi,
V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega,
G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota,
R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli,
J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador,
J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels,
M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision D.01), Gaussian, Inc., Wallingford, CT, 2004.
Conclusions
In conclusion, a highly efficient yellow-emitting iridium(III)
complex is assembled from a carbazole-substituted benzimid-
azole-based N^C^N type ligand. The fabricated PLEDs based on
the complex showed maximum external quantum efficiency and
luminance efficiency of 19.1% and 40.2 cd Aꢀ1, respectively.
These performances of the Ir(III) complex are some of the highest
ever reported for yellow emitting PLEDs. The combination of
good efficiency and color specificity elevates the material to a
promising candidate for phosphorescent dopants of PLEDs,
especially in the fabrication of white-light display or illumination
devices.
Acknowledgements
This work was supported by the National Basic Research
Program of China (973 Program, 2012CB821701), NSF of China
(Grants U0934003, 20903120, 21121061, 21173272), the RFDP
of Higher Education of China, the Fundamental Research Funds
for the Central Universities, and FDYT.
References
1 C. Ulbricht, B. Beyer, C. Friebe, A. Winter and U. S. Schubert, Adv.
Mater., 2009, 21, 4418 and references there in.
2 (a) S. J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008,
20, 4189; (b) R. J. Holmes, S. T. Forrest, Y. J. Tung, R. C. Kwong,
J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett.,
2003, 82, 2422; (c) Y. Zhen, S. H. Eorn, N. Chopra, J. Lee, F. So
and J. Xue, Appl. Phys. Lett., 2008, 92, 223301; (d) Y. Kawamura,
K. Goushi, J. Brooks, J. J. Brown, H. Sasabe and C. Adachi, Appl.
Phys. Lett., 2005, 86, 71104; (e) R. Ragni, E. A. Plummer,
K. Brunner, J. W. Hofstraat, F. Babudri, G. M. Farinola, F. Naso
and L. De Cola, J. Mater. Chem., 2006, 16, 1161; (f) L. L. Wu,
C. H. Yang, L. W. Sun, S. Y. Chu, P. C. Kao and H. H. Huang,
Organometallics, 2007, 26, 2017; (g) Y. C. Chui, J. Y. Hung, Y. Chi,
C. C. Chen, C. H. Chang, C. C. Wu, Y. M. Cheng, Y. C. Yu,
G. H. Lee and P. T. Chou, Adv. Mater., 2009, 21, 2221.
12 K. R. J. Thomas, M. Velusamy, J. T. Lin, C. H. Chien, Y. T. Tao,
Y. S. Wen, Y. H. Hu and P. T. Chou, Inorg. Chem., 2005, 44, 5677.
13 Y. Wong, C. L. Ho, Z. Q. Gao, B. X. Mi, C. H. Chen, K. W. Cheah
and Z. Lin, Angew. Chem., Int. Ed., 2006, 45, 7800.
22500 | J. Mater. Chem., 2012, 22, 22496–22500
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