Synthesis, structure, photo- and electro-luminescence of an iridium(iii) complex with a novel carbazole functionalized β-diketone ligand

Article abstract of DOI:10.1039/c3ra44432e

A new iridium complex containing coumarin derivative as a cyclometalated ligand (¹L) and a carbazole-functionalized β-diketonate ( ²L) as the ancillary ligand, namely, Ir(iii)bis(3-(pyridin-2-yl) coumarinato-N,C⁴)(1-(9-butyl-9H-carbazol-3-yl)-4,4,4-trifluoro- butane-1,3-dionato-O,O) (Ir(¹L)₂(²L)), was synthesized. The crystal structure of Ir(¹L)₂( ²L) was determined via combined wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM), which showed π-π the interactions of Ir(¹L)₂(²L) molecules stacking along the crystal axes. The doped light-emitting diodes using this novel Ir(¹L)₂(²L) complex as the phosphorescent dopant were fabricated. At a Ir(¹L)₂(²L) concentration of 6.0 wt%, a green-yellow emitting OLED was achieved with a maximum external quantum efficiency (EQE) of 6.11% and a maximum luminous efficiency of 22.55 cd A^-1 at the current density of 6.06 mA cm ^-2, and a maximum luminance of 6653 cd m^-2 at 10.7 V. Furthermore, two reference complexes Ir(¹L)₂(acac) and Ir(¹L)₂(TTA) were also used as emitters to fabricate OLED devices with the same device configuration. The maximum luminous efficiency of Ir(¹L)₂(acac) doped device was measured to be 20.04 cd A^-1 at 2.15 mA cm^-2 (10.0 wt%), while the doped device of Ir(¹L)₂(TTA) had a maximum luminous efficiency of 16.59 cd A^-1 at 1.36 mA cm^-2. The better performances of Ir( ¹L)₂(²L) doped devices could be largely attributed to an improved hole-transporting property due to the introduction of the carbazole moiety. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3ra44432e

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Synthesis, structure, photo- and electro-
luminescence of an iridium(III) complex with a novel
Cite this: RSC Adv., 2014, 4, 554
carbazole functionalized b-diketone ligand†
a
b
c
a
a
Tianzhi Yu,‡* Yan Cao,‡ Wenming Su, Chengcheng Zhang, Yuling Zhao,
a
b
b
b
Duowang Fan, Mingjun Huang, Kan Yue and Stephen Z. D. Cheng*
1
A new iridium complex containing coumarin derivative as a cyclometalated ligand ( L) and a carbazole-
2
functionalized b-diketonate ( L) as the ancillary ligand, namely, Ir(III)bis(3-(pyridin-2-yl)coumarinato-
4
1
2
N,C )(1-(9-butyl-9H-carbazol-3-yl)-4,4,4-trifluoro-butane-1,3-dionato-O,O) (Ir( L)
2
( L)), was synthesized.
1
2
The crystal structure of Ir( L)
2
( L) was determined via combined wide angle X-ray diffraction (WAXD) and
1
2
2
transmission electron microscopy (TEM), which showed p–p the interactions of Ir( L) ( L) molecules
1
2
stacking along the crystal axes. The doped light-emitting diodes using this novel Ir( L)
2
( L) complex as
1
2
the phosphorescent dopant were fabricated. At a Ir( L)
2
( L) concentration of 6.0 wt%, a green-yellow
emitting OLED was achieved with a maximum external quantum efficiency (EQE) of 6.11% and a
ꢀ1
ꢀ2
maximum luminous efficiency of 22.55 cd A at the current density of 6.06 mA cm , and a maximum
ꢀ2
1
1
luminance of 6653 cd m at 10.7 V. Furthermore, two reference complexes Ir( L)
2 2
(acac) and Ir( L) (TTA)
were also used as emitters to fabricate OLED devices with the same device configuration. The maximum
Received 16th August 2013
Accepted 5th November 2013
1
ꢀ1
ꢀ2
luminous efficiency of Ir( L)
2
(acac) doped device was measured to be 20.04 cd A at 2.15 mA cm
1
ꢀ1
(
10.0 wt%), while the doped device of Ir( L) (TTA) had a maximum luminous efficiency of 16.59 cd A at
2
DOI: 10.1039/c3ra44432e
ꢀ2
1
2
1
.36 mA cm . The better performances of Ir( L) ( L) doped devices could be largely attributed to an
2
www.rsc.org/advances
improved hole-transporting property due to the introduction of the carbazole moiety.
complex to possess wide range of phosphorescence color, high
emission quantum yield and good stability, Ir(III) complexes
1
. Introduction
1
5,16
Phosphorescent organic light-emitting diodes (PhOLEDs) have with mixed ligands have been synthesized.
The photo-
been intensively studied due to their high external quantum physical properties and device performance parameters, such
1
–7
efficiencies. Owing to the strong spin–orbital coupling of as the lowest unoccupied molecular orbital (LUMO) energy, the
heavy-metal complexes, both singlet and triplet excitons can be luminous efficiency and emission color of these Ir(III)
harvested for light, and thus potentially 100% internal complexes can be tuned by modifying the chemical structure of
2
,15,17–24
quantum efficiency can be obtained in PhOLEDs. The most the cyclometalating ligands.
For example, Rehmann
24
well studied organometallic compounds are mainly complexes and co-workers
reported PhOLEDs fabricated with an
iridium(III) complex of coumarin-6(iridium(III)bis(3-(2-benzo-
Among organometallic compounds, cyclometalated iridium thiazolyl)-7-(diethylamino)-coumarinato-N,C4 )(6-((3-ethyl-oxe-
II 1,8,9 III 2–7
II 10,11
I
12–14
of noble metal ions such as Pt ,
Ir , Os ,
and Re .
0
complexes are the most promising emitting materials due to tan-3-yl)methoxy)hexylacetoacetate)) as the emitter,
a
ꢀ1
their high quantum efficiency, brightness, color diversity and maximum luminous efficiency of 18.4 cd A and a power
ꢀ
1
short excited-state lifetime. In order to develop the Ir(III) efficiency of 11.7 lm W
were achieved at 5 V and at a
ꢀ
2
brightness of 100 cd m
.
Neutral homoleptic tris-cyclometalated Ir(C^N) complexes
3
a
Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of
based on 2-phenylpyridine (ppy) are one of the most popular
Education), Lanzhou Jiaotong University, Lanzhou 730070, P. R. China. E-mail:
25–27
and thoroughly studied green emissive iridium complexes.
yutianzhi@hotmail.com; Fax: +86-931-4938756; Tel: +86-931-4956935
b
College of Polymer Science and Polymer Engineering, The University of Akron, Ohio, Recently, we have reported three coumarin-based iridium(III)
28,29
4
c
4325, USA. E-mail: scheng@uakron.edu; Fax: +330 972-8626; Tel: +330 972-6931
complexes,
the cyclometalated ligand and acetylacetonate, thenoyltri-
uoroacetonate and picolinate were used as the ancillary
monoanionic ligand, respectively. These iridium complexes
exhibit intensive green emissions, indicating that they are
useful for the fabrication of organic light-emitting diodes.
in which 3-(pyridin-2-yl)coumarin was used as
Printable electronics research center, Suzhou Institute of Nano-Tech and Nano-
Bionics, Chinese Academy of Sciences, Suzhou 215123, China

†
Electronic supplementary information (ESI) available: The structure
1
2
determination of Ir( L)
2
( L) crystal is described detailedly. Table S1 and
Fig. S1–S4 are also given. See DOI: 10.1039/c3ra44432e
‡
These authors are contributed equally.
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mica sheet, which was placed in a tetrahydrofuran atmosphere.
1
2
The single crystals of Ir( L) ( L) were gradually grown within
2
several hours under the tetrahydrofuran vapor. The mica sheet
was immersed into water and the carbon lm was oated onto
the water, which was picked up onto copper grids for TEM
characterization.
One-dimensional WAXD experiments were carried out on a
Rigaku MultiFlex 2 kW tube-anode X-ray with a scanning rate of
ꢁ
ꢀ1
1
min at room temperature. The silicon powder was used to
ꢁ
calibrate the peak postions in the high-angle region (>15 ) and
silver behenate in the low-angle region (<15 ).
ꢁ
2
1
2
2
Scheme 1 Synthetic routes of L and Ir( L) ( L).
2
.2 Synthesis and characterization of the Ir(III) complex
1
2
(Ir( L)
2
( L))
In this work, a new coumarin-based iridium(III) complex,
2
.2.1 1-(9-Butyl-9H-carbazol-3-yl)-4,4,4-triuorobutane-1,3-
1
2
1
2
Ir( L)
2
( L), where L is 3-(pyridin-2-yl)coumarinato, L is 1-(9-
2
dione ( L)
butyl-9H-carbazol-3-yl)-4,4,4-triuoro-butane-1,3-dionato, was
designed and synthesized to study the effect of the improved
hole-transporting property of the complex by modifying the
9
-Butylcarbazole. A mixture of carbazole (4.00 g, 23.9 mmol),
n-butyl bromide (3.27 g, 23.9 mmol) and sodium hydroxide
27.50 g, 687.5 mmol) in dimethyl sulfoxide (60 mL) was stirred
in a 250 mL three-necked ask for 4 h at room temperature. The
mixture was then poured into water, and extracted with ether
(
1
2
b-diketonate ligand. The PhOLEDs fabricated with Ir( L) ( L) as
2
an emitter exhibited a maximum external quantum efficiency
(
A
EQE) of 6.11% and a maximum luminous efficiency of 22.55 cd
(
3 ꢂ 100 mL). The combined organic phase was washed with
ꢀ
1
ꢀ2
at the current density of 6.06 mA cm . The synthetic routes
water (2 ꢂ 100 mL) and dried over anhydrous magnesium
sulfate. Aer ltration and evaporation of the solvent, the crude
product was puried by silica column chromatography with
petroleum ether as the eluent to give 9-butylcarbazole (4.86 g,
2
1
2
of the ancillary ligand ( L) and Ir( L)
Scheme 1.
2
( L) were shown in
ꢁ
1
9
7
7
1%). m.p. 58–59 C. H NMR (CDCl
3
, d, ppm): 8.10–8.08 (d, J ¼
2. Experimental section
.6 Hz, 2H), 7.47–7.43 (m, 2H), 7.40–7.38 (d, J ¼ 8.0 Hz, 2H),
.24–7.20 (m, 2H), 4.31–4.27 (t, J ¼ 7.2 Hz, 2H), 1.88–1.81 (m,
2.1 Materials and methods
Carbazole was purchased from Tianjin Guangfu Fine Chemical 2H), 1.44–1.36 (m, 2H), 0.95–0.92 (t, J ¼ 7.2 Hz, 3H).
Research Institute (China) and recrystallized from methanol
before use. n-Butyl bromide was obtained from Tanjing Kaixin (4.78 g, 21.4 mmol) in dichloromethane (30 mL), anhydrous
Chemical Co. Ltd. (China). Ethyl triuoroacetate and acetyl ZnCl (4.38 g, 32.14 mmol) was added quickly. The mixture was
3-Acetyl-9-butylcarbazole. To a solution of 9-butylcarbazole
2
chloride were purchased from Aladdin Reagent Inc. and Alfa stirred, and then a solution of acetyl chloride (2.52 mL, 32.1
0
Aesar China Co. Ltd, respectively. 4,4 -Bis(9-carbazolyl)biphenyl mmol) in dichloromethane (15 mL) was added dropwise. The
0
00
(
CBP), 4,4 ,4 -tris(N-3-methylphenyl-N-phenylamino)triphenyl- reaction mixture was stirred at room temperature for about
amine (m-MTDATA), 1,3,5-tris(N-phenylbenzimidazol-2-yl)ben- 24 h, and then cooled water (100 mL) was added to the mixture.
0
0
0
zene (TPBi) and N,N -bis-(naphthyl)-N,N -diphenyl-1,1 -biphenyl- The product was extracted with dichloromethane (3 ꢂ 100 mL).
0
4
,4 -diamine (NPB) were purchased from Electro-Light Tech- The combined organic phase was washed with water (2 ꢂ
nology Corp., Beijing. Other solvents were analytical reagents.
100 mL) and dried over anhydrous magnesium sulfate. Aer
The cyclometalated coumarin ligand (3-(pyridine-2-yl) ltration and evaporation of the solvent, the crude product was
1
coumarin, L) and cyclometalated Ir(III)m-chlorobridged dimmer puried by silica column chromatography with a mixture of
1
1
25,26
(
( L)
2
Ir(m-Cl)
2
Ir( L)
2
) were prepared as previously described.
ethyl acetate–petroleum ether (1 : 10) as eluent to afford 3-
ꢀ
1
1
IR spectra (400–4000 cm ) were carried out using a Shi- acetyl-9-butylcarbazole as a primrose yellow oil (4.71 g, 83%). H
1
madzu IRPrestige-21 FT-IR spectrophotometer. H NMR spectra NMR (CDCl , d, ppm): 8.74 (d, J ¼ 1.6 Hz, 1H), 8.16–8.10 (m, 2H),
3
were obtained on Unity Varian-500 MHz. Elemental analyses 7.53–7.49 (m, 1H), 7.44–7.38 (q, J ¼ 8.0 Hz, 2H), 7.31–7.25 (q, J ¼
were obtained using an Elemental Vario-EL automatic 7.6 Hz, 1H), 4.33–4.29 (t, J ¼ 7.2 Hz, 2H), 2.72 (s, 3H), 1.87–1.82
elemental analysis instrument. Melting points were measured (q, J ¼ 7.6 Hz, 2H), 1.42–1.36 (q, J ¼ 7.2 Hz, 2H), 0.96–0.93 (t, J ¼
by using an X-4 microscopic melting point apparatus (Beijing 7.2 Hz, 3H).
2
Taike Instrument Limited Company). UV-vis absorption and
1-(9-Butyl-9H-carbazol-3-yl)-4,4,4-triuorobutane-1,3-dione ( L).
photoluminescent spectra were recorded on a Shimadzu UV- A suspension of fresh sodium (0.356 g, 15.5 mmol) in anhydrous
2
550 spectrometer and on a Perkin-Elmer LS-55 spectrometer, toluene (30 mL) was placed in a three-necked ask. The mixture
ꢁ
respectively. The electroluminescent spectra were measured on was heated up to 120 C and stirred vigorously till the sodium
a PR-650 SpectraScan Colorimeter.
was molten and scattered, then ethyl triuoroacetate (2.20 g,
The WAXD/TEM samples were prepared by casting a drop of 15.5 mmol) was added into the mixture. Meanwhile, a solution
1
2
2
Ir( L) ( L) solution in chloroform (0.01 wt%) onto carbon coated of 3-acetyl-9-butylcarbazole (4.107 g, 15.5 mmol) in toluene
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(
1
15 mL) was added dropwise. The reaction mixture was stirred at 3645 DC power supply combined with a 1980A spot photometer
20 C for about 24 h. The mixture was poured into cooled water and were recorded simultaneously. All measurements were
ꢁ
(100 mL), and then acidied with dilute hydrochloric acid to done in the air at room temperature without any encapsulation.
pH ¼ 3. The mixture was extracted with CHCl (3 ꢂ 100 mL), and
3
the combined organic phase was dried over anhydrous MgSO
The solvent was removed under reduced pressure and the
residue was puried by column chromatography on silica gel 3.1 Molecular design and synthesis of Ir( L) ( L)
4
.
3
. Results and discussion
1
2
2
with a mixture of ethyl acetate–petroleum ether (1 : 15) as eluent
1
2
The chemical structure of Ir( L)
2
( L) is shown in Fig. 1a and the
to obtain 1-(9-butyl-9H-carbazol-3-yl)-4,4,4-triuorobutane-1,3-
1
2
molecular model of Ir( L)
2
( L) is shown in Fig. 1b with carbon,
oxygen, nitrogen, iridium and uorine atoms shown as purple,
red, blue, orange and yellow balls, respectively. Based on the
model established, the length of Ir( L)
1
dione as a yellow oil (3.21 g, 57.4%). H NMR (CDCl , d, ppm):
3
8
8
.74–8.73 (d, J ¼ 1.6 Hz, 1H), 8.18–8.16 (d, J ¼ 8.0 Hz, 1H), 8.08–
.06 (q, J ¼ 7.2 Hz, 1H), 7.56–7.52 (t, J ¼ 7.2 Hz, 1H), 7.47–7.43 (t,
1
2
2
( L) along the molecular
J ¼ 6.4 Hz, 2H), 7.35–7.32 (t, J ¼ 7.2 Hz, 1H), 6.71 (s, 2H), 4.36–
1
2
long a axis is about 2.0 nm and the width of Ir( L)
2
( L) along the
4
0
.32 (t, J ¼ 7.2 Hz, 2H), 1.92–1.88 (m, 2H), 1.44–1.36 (m, 2H),
molecular short b axis that is perpendicular to the long axis is
ꢃ1.0 nm.
.99–0.93 (m, 3H).
4
2.2.2 Ir(III)bis(3-(pyridin-2-yl)coumarinato-N,C )(1-(9-butyl-
1
1
2
The cyclometalating ligand L and the complexes Ir( L) (a-
1
9
(
H-carbazol-3-yl)-4,4,4-triuoro-butane-1,3-dionato-O,O) (Ir( L)
2
-
1
cac) and Ir( L)
2
(TTA) were synthesized according to the proce-
dure previously reported. The synthetic route of the ancillary
ligand L and the complex Ir( L) ( L) is shown in Scheme 1.
2
1
1
L)). The chloro-bridged dimer complex (( L) Ir(m-Cl) Ir( L) )
29
2
2
2
(0.200 g, 0.149 mmol), 1-(9-butyl-9H-carbazol-3-yl)-4,4,4-tri-
2
1
2
2
uorobutane-1,3-dione (0.269 g, 0.744 mmol) and sodium
carbonate (0.710 g, 6.70 mmol) were reuxed in dichloroethane
under nitrogen atmosphere for 12 h. Aer cooling, a small
quantity of water was added. The mixture was extracted with
dichloromethane (3 ꢂ 50 mL). The organic phase was washed Fig. 2 shows a 1D WAXD power pattern of Ir( L) ( L). The d-
1
2
3
.2 Ir( L)
2
( L) single crystal morphology and structure
determination
1
2
2
with water (2 ꢂ 50 mL) and dried over anhydrous MgSO
4
. Aer spacings of the rst ve strong diffraction peaks are 2.01 nm
ltering, the ltrate was evaporated to dryness under reduced (2q ¼ 4.44 ), 1.43 nm (2q ¼ 6.19 ), 1.00 nm (2q ¼ 8.79 ), 0.84 nm
ꢁ
ꢁ
ꢁ

ꢁ
ꢁ
pressure. The crude was puried by chromatography on silica gel (2q ¼ 10.57 ), and 0.74 nm (2q ¼ 11.90 ), respectively. The
using ethyl acetate–petroleum ether (1 : 6, v/v) as the eluent to d-spacing values of the rst and third diffraction peaks are
1
2
1
1
2
give Ir( L)
d, ppm): 9.19 (t, 2H, J ¼ 8.8, aryl–H), 8.45 (d, 1H, J ¼ 1.6, aryl–H), respectively, which are estimated based on the molecular
.15–8.10 (m, 2H, aryl–H), 8.04 (d, 1H, J ¼ 5.2, aryl–H), 7.95–7.82
m, 3H, aryl–H), 7.51 (t, 1H, aryl–H), 7.42 (d, 1H, aryl–H), 7.35–
2
( L) as a yellow powder (0.11 g, 71%). H NMR(CDCl
3
,
2
consistent with the length and width of Ir( L) ( L) molecule,
8
(
7
1
6
7
.23 (m, 6H, aryl–H), 7.10–7.06 (m, 1H, aryl–H), 7.01–6.97 (m,
H, aryl–H), 6.71–6.66 (m, 2H, aryl–H), 6.39 (s, 1H, aryl–H), 6.25–
.17 (m, 2H, aryl–H), 4.29 (t, 2H, J ¼ 6.8, N–CH –), 1.83 (t, 2H, J ¼
2
.6, –CH –), 1.43–1.35 (m, 2H, –CH –), 0.93 (t, 3H, J ¼ 7.2, –CH ).
2
2
3
1
3
3
C NMR (CDCl , d, ppm): 184.34, 181.44, 165.75, 156.80, 150.88,
1
1
1
47.89, 143.14, 141.07, 139.04, 132.02, 130.87, 128.88, 127.99,
26.64, 125.64, 123.77, 122.74, 121.13, 120.91, 120.73, 120.20,
16.30, 109.23, 108.83, 91.96, 76.99, 43.09, 30.96, 20.46, 13.80.
Anal. calc. for C48
33 3 3 6
H F IrN O (%): C, 57.82; H, 3.34; N, 4.21.
1
2
2
Fig. 1 Chemical structure (a) and molecular model (b) of Ir( L) ( L).
Found: C, 58.03; H, 3.29; N, 4.32. MS (MALDI-TOF): m/z 1020.2
+
[
M + Na] .
2.3 PhOLEDs fabrication and characterization
The multilayer PhOLEDs with a device architecture of ITO/m-
MTDATA (10 nm)/NPB (20 nm)/CBP:complex (x wt%, 30 nm)/
TPBi (30 nm)/LiF (1 nm)/Al (100 nm) were fabricated by vacuum-
deposition method. All organic layers were sequentially
ꢀ
5
deposited without breaking vacuum (2 ꢂ 10 Pa). Thermal
ꢀ
1
˚
deposition rates for organic materials, LiF and Al were ꢃ2 A s
,
ꢀ
1
ꢀ1
˚
˚
ꢃ1 A s and 10 A s , respectively. The active area of the
2
devices was 12 mm . The EL spectra and Commission Inter-
nationale de L’Eclairage (CIE) coordinates were measured on a
Hitachi MPF-4 uorescence spectrometer. The characterization
of brightness–current–voltage (B–I–V) were measured with a Fig. 2 One dimension wide X-ray diffraction pattern of Ir( L) ( L).
2
1
2
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model shown in Fig. 1b. Fig. 3a is a bright eld TEM image of 3.3 UV-vis absorption and photoluminescence spectra
1
2
one elongated ribbon-like Ir( L) ( L) single crystal. Fig. 3b is
2
The UV-vis absorption and photoluminescence spectra of
a SAED pattern with the [001] zone taken from the crystal
in Fig. 3a.
Combining the d-spacing calculations of the WAXD pattern
and the crystal tilting experiments of TEM, the unit cell
1
2
1
1
2 2 2
Ir( L) ( L), Ir( L) (acac) and Ir( L) (TTA) were measured in
diluted dichloromethane solutions at room temperature, as
1
2
presented in Fig. 5. In UV-vis absorption spectrum of Ir( L)
there are eight major absorptions at 213, 240, 276, 288, 348, 383,
05 and 436 nm, in which the absorption bands below 383 nm
2
( L),
1
2
parameter of Ir( L)
2
2
( L) crystal can be determined to be a ¼
4
ꢁ
.03 nm, b ¼ 0.99 nm, c ¼ 1.49 nm, a ¼ b ¼ g ¼ 90 . The space
could be attributed to spin-allowed p / p* transition of the
group is assigned to be P222 by the international crystallo-
graphic table due to no extinction ED spots found in Fig. S1a–
ligands and the band around 383 nm can be assigned to the
1
spin-allowed metal-to-ligand charge transfer (MLCT) (metal–
1
2
S1f (see ESI†). There are two Ir( L) ( L) molecules in one unit
cell. The unit cell parameters are calculated in the Cerius
2
ligand-charge-transfer), and the bands at the longer wave-
lengths (405 and 436 nm) can be assigned to both spin–orbit
2
molecular package, and the preliminary packing model of
3
3
coupling enhanced (p / p*) and spin-forbidden MLCT
1
2
Ir( L)
shown in Fig. 4a–c. The calculated density (dcal. ¼ 1.108 g cm
based on the unit cell parameters is found in accordance with
2
( L) single crystal is established along three crystal axes as
30–32
transitions.
ꢀ
3
)
1
The absorption spectra of the complexes Ir( L)
Ir( L) (TTA) strongly resemble each other, indicating that the
absorption spectra are mainly decided by the L ligand, and that
2
(acac) and
1
29
2
ꢀ3
the experimental data (dmea. ¼ 1.102 g cm ).
1
the ancillary ligands, acetylacetone and thenoyltriuoroaceton,
scarcely play a role in the absorptions in the complexes
1
28
compared with the absorption of L ligand. The stronger
absorption bands at 200–350 nm can be assigned to the spin-
1
allowed (p / p*) transitions of the cyclometalated coumarin
1
ligand L in the complexes. The weak absorption bands (at 377,
1
4
09 and 433 nm for Ir( L) (TTA) and at 385, 419 and 439 nm for
2
1
1
Ir( L) (acac)) can be ascribed to the admixture of MLCT,
MLCT and (p / p*) states.
2
3
3
Based on Fig. 5, the photoluminescence spectra of the
1
2
1
1
2 2 2
complexes Ir( L) ( L), Ir( L) (acac) and Ir( L) (TTA) in
dichloromethane solutions greatly resemble each other, all of
which exhibit green emissions with a maximum main peak and
a shoulder peak. But the photoluminescence spectrum of
1
2
Fig. 3 (a) TEM bright filed morphology of the Ir( L)
2
( L) single crystal
grown at room temperature by chloroform solution evaporation under
tetrahydrofuran atmosphere. (b) A [001] zone-SAED pattern corre-
sponding to the crystal within the circle area in (a).
1
2
complex Ir( L) ( L) (l
¼ 531 nm, l
¼ 568 nm) shows a blue
2
max
min
1
shi of 2 nm in comparison with that of complexes Ir( L) (acac)
2
1
and Ir( L) (TTA) (l
¼ 533 nm, l
¼ 570 nm) due to lower
2
max
min
1
ligand-eld strength of ligand L anion than acac and TTA
15,33
anions.
The quantum yields of the complexes Ir( L)
1
2
1
2
( L), Ir( L)
2
(acac)
1
and Ir( L)
2
(TTA) at room temperature were measured to be
1
2
Fig. 4 Molecular packing model of Ir( L)
2
( L) single crystal established Fig. 5 Normalized UV-vis absorption and photoluminescence spectra
2
1
2
1
1
by Cerius modeling package (a) ac-plane projection; (b) bc-plane of Ir( L)
2
( L), Ir( L)
2
(acac) and Ir( L)
2
(TTA) in diluted dichloromethane
ꢀ6
ꢀ1
projection; (c) ab-plane projection.
solutions (C ¼ 1 ꢂ 10 mol L ).
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ꢀ
6
1
2
1
L
(
0.36%, 9.15% and 9.08% from the THF solutions (ca. 10 mol level of Ir( L)
2
( L) was determined to be ꢀ5.21 eV regarding the
ꢀ
1
) by an absolute method using the Edinburgh Instruments energy level of ferrocene/ferrocenium as ꢀ4.80 eV. The optical
1
2
FLS920) integrating sphere excited at 380 nm with the Xe lamp. band gap of Ir( L) ( L) was estimated to be 461 nm, which
2
1
2
1
1
2
The lifetime decays of the complexes Ir( L) ( L), Ir( L) (acac) corresponds to 2.69 eV. Then the LUMO of Ir( L) ( L) is calcu-
2
2
2
1
and Ir( L)
2
(TTA) were measured to be (s
¼ 9.22 ms) and (s
1
¼ 0.72 ms, s
1
¼ 9.05 ms), lated to be ꢀ2.52 eV.
(s
1
¼ 0.79 ms, s
1
1
¼ 0.81 ms, s
1
¼ 9.27 ms) by a
On the basis of the oxidation potentials together with the
1
time-correlated single photon counting spectrometer using extrapolated absorption edge data of Ir( L)
2
(acac) and
1
Edinburgh Instruments (FLS920) with a microsecond ashlamp Ir( L)
as the excitation source (repetition rate 90 Hz) at room be estimated. For Ir( L)
2
(TTA), their HOMO and LUMO energy levels can similarly
1
2
(acac), the determined HOMO and
temperature.
LUMO energy levels are ꢀ5.28 eV and 2.61 eV, while for
1
Ir( L) (TTA), the results are ꢀ5.30 eV and ꢀ2.59 eV.
2
1
2
3
.4 LUMO and HOMO energy levels of Ir( L) ( L) by cyclic
2
3.5 PhOLEDs performance
voltammetry measurement
1
2
The devices with a conguration of ITO/m-MTDATA (10 nm)/
NPB (20 nm)/CBP:complex (x wt%, 30 nm)/TPBi (30 nm)/LiF
(1 nm)/Al (100 nm) were fabricated. The diagram of HOMO and
LUMO energy levels of the materials are depicted in Fig. 6. The
emitting layers are consisted of host materials CBP and dopants
The LUMO and HOMO energy levels of Ir( L) ( L) were calcu-
2
lated from cyclic voltammetry (CV) measurement and absorp-
tion spectrum. Ir( L) ( L) was dissolved in dichloromethane
2
with tetra-n-butylammonium tetrauoroborate (0.1 mol L ) as
the electrolyte. A platinum working electrode and a saturated
Ag/AgCl reference electrode were used. Ferrocene was used for
potential calibration. Fig. S3† shows the cyclic voltammogram
of Ir( L) ( L), from which the oxidation onset at 0.81 V was
clearly observed. At the same condition, the oxidation peak and
the reductive peak were observed at 0.55 and 0.25 V, respec-
1
2
ꢀ1
1
2
1
1
2 2 2
of the complexes Ir( L) ( L) or Ir( L) (acac) or Ir( L) (TTA) at
different concentrations (x wt%). m-MTDATA, NPB and TPBi
were used as hole injection, hole transport and electron trans-
port materials, respectively. LiF was used as the electron-injec-
tion layer.
From the energy diagram, strong hole trapping can be
expected due to the lower HOMO levels of the complexes than
that of CBP. On the other hand, little variation in LUMO level
between the complex and CBP allows the electron to be injected
into the complex easily. Moreover, it is clear that NPB and TPBi
can act as substantial electron-blocking and hole-blocking
layers, respectively. As a result, effective electron–hole pair
connement in the emission layer can be expected.
1
2
2
+
tively, then the E1/2 (Fc/Fc ) is 0.40 V. Thus the HOMO energy
The electroluminescence (EL) spectra of PhOLEDs are shown
in Fig. S4.† From Fig. S4a,† when the doping concentration of
1
2
Ir( L)
2
( L) is 6 wt%, the shape of EL spectra did not change at
1
2
different applied voltages. The EL spectra of Ir( L)
2
( L) resemble
closely its PL spectrum in thin lm with a maximum main peak
at 531 nm and a shoulder peak at 570 nm. It can be seen that the
EL spectra of devices with different doping concentrations of
Fig. 6 Energy-level diagram of the materials in the device. Device
configuration: ITO/m-MTDATA/NPB/CBP:complex (x wt%)/TPBi/
LiF/Al.
1
1
Ir( L) (acac) and Ir( L) (TTA) also did not change and resemble
2
2
closely their respective PL spectra in thin lms (Fig. S4b and
1
2
Fig. 7 (a) Current efficiency vs. current density characteristics of the devices at various Ir( L)
2
( L) doping concentrations; (b) current density–
1
2
luminance–voltage characteristics of the device at 6.0 wt% Ir( L)
CBP:complex (x wt%)/TPBi/LiF/Al.
2
( L) doping concentration. Device configuration: ITO/m-MTDATA/NPB/
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1
2
Table 1 EL performances of the Ir( L)
2
( L) doped devices
ꢀ2
ꢀ1
ꢀ2
Entry on (V) max (cd m
V
L
)
LEmax (cd A
)
LE (20 mA cm
)
EQEmax (%)
CIEx,y
ꢀ2
ꢀ2
ꢀ2
4.0 wt%
6.0 wt%
8.0 wt%
6.3
6.4
6.6
6066
6653
8972
11.40 @ 20.0 mA cm
22.55 @ 6.06 mA cm
18.00 @ 9.32 mA cm
11.40
19.22
16.46
3.09%
6.11%
4.88%
(0.473, 0.507)
(0.479, 0.510)
(0.481, 0.504)
Fig. 8 (a) Current efficiency vs. current density characteristics and (b) current density–luminance–voltage characteristics of the device at 10.0 wt
1
%
2
Ir( L) (acac) doping concentration. Device configuration: ITO/m-MTDATA/NPB/CBP:complex (x wt%)/TPBi/LiF/Al.
ꢀ
2
ꢀ2
S4c†). It was also found that the EL spectra of PhOLEDs and the brightness is between 6066 cd m and 8972 cd m . By
1
2
1
incorporated with the complexes Ir( L)
2
( L) and Ir( L)
2
(acac) comparing the performance of different doping concentrations,
and Ir( L) (TTA) were independent of the applied voltages and the 6.0 wt% Ir( L) ( L) doped device has a maximum efficiency
doping concentrations of Ir( L) ( L) and Ir( L) (acac) and of 22.55 cd A at 6.06 mA cm , which has a green-yellow color
with a CIExy of (0.479, 0.510). Even at 20 mA cm , the doped
Fig. 7a shows the relationship between luminous efficiency device has a maximum efficiency of 19.22 cd A . A maximum
and the current density in PhOLEDs fabricated with different external quantum efficiency (EQE) of 6.11% was achieved. From
1
1
2
2
2
1
2
1
ꢀ1
ꢀ2
2
2
1
ꢀ2
Ir( L) (TTA).
2
ꢀ
1
1
2
Ir( L)
2
( L) doping concentrations. It was found that all PhO- the results, it is indicated that 6.0 wt% is the optimal doping
1
2
LEDs have higher luminous efficiencies at low current densities, concentration of Ir( L)
2
( L). The luminance and current density
and then the luminous efficiencies fall off at higher current vs. voltage and current density vs. voltage characteristics of the
densities. Table 1 summarized the performances of PhOLEDs devices at 6.0 wt% doping concentration are shown in Fig. 7b.
1
2
with various Ir( L) ( L) doping concentrations in CBP host. The The device shows a turn-on voltage of about 6.4 V, the maximum
2
ꢀ
2
turn-on voltages (Von) of the devices are between 6.3 V and 6.6 V, brightness is about 6653 cd m at 10.7 V.
Fig. 9 Luminance and current density (inset) vs. voltage characteris-
1
tics of the devices at various Ir( L)
2
(TTA) doping concentrations. Device Fig. 10 Current efficiency vs. current density characteristics of the
1
2
configuration: ITO/m-MTDATA/NPB/CBP:complex (x wt%)/TPBi/ devices at various Ir( L) (TTA) doping concentrations. Device config-
LiF/Al.
uration: ITO/m-MTDATA/NPB/CBP:complex (x wt%)/TPBi/LiF/Al.
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1
2
Table 2 EL performances of the Ir( L) (TTA) doped devices
ꢀ2
ꢀ1
ꢀ2
Entry
V
on (V)
L
max (cd m
8470
)
LEmax (cd A
)
LE (20 mA cm
)
EQEmax (%)
CIEx,y
ꢀ
ꢀ
ꢀ
ꢀ
2
2
2
2
6
1
1
2
.0 wt%
8.8
7.0
6.8
6.6
16.59 @ 1.36 mA cm
12.79 @ 2.44 mA cm
13.35 @ 3.82 mA cm
10.16 @ 3.01 mA cm
12.52
10.47
10.76
8.39
4.37%
3.48%
3.61%
2.78%
(0.384, 0.592)
(0.400, 0.578)
(0.395, 0.581)
(0.402, 0.575)
2.0 wt%
8.0 wt%
3.0 wt%
10 540
10 840
10 970
1
2
In order to compare the electrophosphorescence perfor- enlarged by p–p stacking of Ir( L)
2
( L) molecules shown in the
1
2
1
1
2
mances of the Ir( L)
2
( L), the devices using Ir( L)
(TTA), respectively, as doped emitters were fabricated strated that the complex Ir( L)
with the same device conguration of ITO/m-MTDATA/NPB/ porting property and a larger exciton recombination area than
2 2
(acac) and Ir( L) ( L) crystal structure (Fig. 4). All these results demon-
1
1
2
Ir( L)
2
2
( L) has a better hole-trans-
1
1
1
CBP:complex (x wt%)/TPBi/LiF/Al. For the complex Ir( L)
the doped devices with different concentrations (6.0, 8.0, 10.0 reduction of the triplet–triplet annihilation. The results show
and 12.0 wt%) were investigated, in which the 10.0 wt% doped that the complex Ir( L) ( L) is a better electroluminescent
2 2 2
(acac), the complexes Ir( L) (acac) and Ir( L) (TTA) leading to the
34
1
2
2
device has a maximum efficiency. Thus, we chose 10.0 wt% as phosphor for OLEDs.
1
the optimal doping concentration of Ir( L)
2
(acac). Fig. 8 displays
the luminous efficiency versus the current density characteris-
4
. Conclusion
tics and the current density–luminance–voltage characteristics
1
of PhOLEDs with 10.0 wt% Ir( L)
2
(acac) doping concentrations, A novel iridium complex containing coumarin derivative as a
1
respectively. The turn-on voltage of these PhOLEDs is the same. cyclometalated ligand ( L) and a carbazole-functionalized
ꢀ2
2
1
2
It is at ꢃ6.4 V, the maximum brightness is ꢃ20 399 cd m at b-diketonate ( L) as the ancillary ligand, Ir( L) ( L), were
2
1
4 V, and the PhOLEDs exhibit a maximum luminous efficiency successfully synthesized and characterized. The crystal struc-
ꢀ
1
ꢀ2
of 20.04 cd A at 2.15 mA cm , and a maximum external ture has been determined via combined WAXD and SAED
quantum efficiency (EQE) of 5.22%. The EL color of the PhO- experiments, and it is orthorhombic with a space group of P222.
1
2
LEDs is green-yellow with a CIExy of (0.375, 0.599). The lumi- The molecular packing model of Ir( L) ( L) single crystal shows
2
1
2
2
nous efficiency declines relatively faster with the increase of that the p–p interaction favors the stacking of Ir( L) ( L)
ꢀ
2
current density at low current density (<100 mA cm ), this is molecule in three dimensional space. This could form a
probably attributed to the faster increase of triplet–triplet (T–T) channel for hole transportation which enhances the hole
3
3
1
2
annihilation and eld-induced quenching effects. At current mobility. The PhOLEDs fabricated by the complex Ir( L) ( L)
2
ꢀ
2
density of 100 mA cm , the luminous efficiency of the PhO- possess better performance as compared with those fabricated
ꢀ
1
1
1
LEDs reaches 14.88 cd A
.
by similar complexes Ir( L) (acac) and Ir( L) (TTA). The better
2 2
Fig. 9 presents the current density and luminance versus bias performance of Ir( L) ( L) is mainly attributed to the high PL
1
2
2
1
voltage curves of the PhOLEDs fabricated with Ir( L)
2
(TTA) at quantum efficiency and low triplet–triplet (T–T) annihilation of
1
2
2
different doping concentrations 6.0, 12.0, 18.0 and 23 wt%. Ir( L) ( L).
Fig. 10 shows the luminous efficiency versus current density
1
characteristics of the PhOLEDs at various Ir( L)
2
(TTA) doping
Acknowledgements
concentrations. The CIExy color coordinates of the devices are
near x ¼ 0.395 and y ¼ 0.582, which correspond to the green- This work was supported by the National Natural Science
yellow region of the CIE chromaticity diagram. The perfor- Foundations of China (61166003 and 60776006), and the
ꢀ2
mance of 6.0 wt% doped device had a luminance of 8470 cd m
Program for Changjiang Scholars and Innovative Research
at 15 V and a maximum current efficiency of 16.59 at 1.36 mA Team in University (IRT0629). Also, it was supported by US NSF
ꢀ
2
cm and a maximum external quantum efficiency (EQE) of (DMR-0906898) and the Joint-Hope Education Foundation.
.37%. As the doping concentration increased, the luminance
4
ꢀ
2
of the devices increased from 10 540 to 10 970 cd m and the
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