Novel yellow phosphorescent iridium complexes containing a carbazole-oxadiazole unit used in polymeric light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2011.04.005

Yellow iridium complexes Ir(PPOHC)₃ and (PPOHC) ₂Ir(acac) (PPOHC: 3-(5-(4-(pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl) -9-hexyl-9H-carbazole) were synthesized and characterized. The Ir(PPOHC) ₃ complex has good thermal stability with 5% weight-reduction occurring at 370 °C and a glass-transition temperature of 201 °C. A polymeric light-emitting diode using the Ir(PPOHC)₃ complex as a phosphorescent dopant showed a luminance efficiency of 16.4 cd/A and the maximum external quantum efficiency of 6.6% with CIE coordinates of (0.50, 0.49). A white polymeric light-emitting diode was fabricated using Ir(PPOHC)₃ which showed a luminance efficiency of 15.3 cd/A, with CIE coordinates of (0.39, 0.44). These results indicate that the iridium complexes containing a linked carbazole-oxadiazole unit are promising candidates in high-efficiency electroluminescent devices.

Full text of DOI:10.1016/j.dyepig.2011.04.005

Dyes and Pigments 91 (2011) 413e421
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel yellow phosphorescent iridium complexes containing
a carbazoleeoxadiazole unit used in polymeric light-emitting diodes
*
Huaijun Tang, Yanhu Li, Caihong Wei, Bing Chen, Wei Yang , Hongbin Wu, Yong Cao
Institute of Polymer Optoelectronic Materials and Devices, Key Lab of Specially Functional Materials of the Ministry of Education, South China University of Technology,
Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Yellow iridium complexes Ir(PPOHC)3 and (PPOHC)2Ir(acac) (PPOHC: 3-(5-(4-(pyridin-2-yl)phenyl)-
1,3,4-oxadiazol-2-yl)-9-hexyl-9H-carbazole) were synthesized and characterized. The Ir(PPOHC)3 complex
has good thermal stability with 5% weight-reduction occurring at 370 ꢀC and a glass-transition temperature
of 201 ꢀC. A polymeric light-emitting diode using the Ir(PPOHC)3 complex as a phosphorescent dopant
showed a luminance efficiency of 16.4 cd/A and the maximum external quantum efficiency of 6.6% with CIE
coordinates of (0.50, 0.49). A white polymeric light-emitting diode was fabricated using Ir(PPOHC)3 which
showed a luminance efficiency of 15.3 cd/A, with CIE coordinates of (0.39, 0.44). These results indicate that
the iridium complexes containing a linked carbazoleeoxadiazole unit are promising candidates in high-
efficiency electroluminescent devices.
Received 2 November 2010
Received in revised form
30 March 2011
Accepted 3 April 2011
Available online 21 April 2011
Keywords:
Iridium complex
Carbazole
Ó 2011 Elsevier Ltd. All rights reserved.
Oxadiazole
Polymeric light-emitting diode
Electroluminescence
White-light
1. Introduction
diodes (PLEDs) have been developed rapidly, because such devices
have brought together the advantages of solution-processed tech-
Organic light-emitting diodes (OLEDs) are under intense inves-
tigation because of their potential applications in full-colour flat-
panel displays, back-lit liquid crystal displays and solid-state
lighting sources [1e6]. Among electroluminescent (EL) materials,
the heavy transition metals, e.g. Pt(II) and Ir(III), complex phosphors
have attracted more attention, because both singlet and triplet
excitons can be harvested to generate light, in principle, their
theoretical internal quantum efficiency (QE) can be as high as 100%,
rather than the inherent 25% upper limit only from the radiative
decay of singlet excitons of the fluorescent precursors [7e10]. EL
materials also can be classified into small-molecule and polymer
materials according to the molecular weight, thus there are two
device fabrication technologies, vacuum-deposition (mostly based
on small-molecule materials) and solution processing (mostly
based on polymer materials). Compared with vacuum-deposition,
solution processing technology can offer some unique advantages,
such as low-cost, easy-processibility over a large surface area by
spin-coating, ink-jet printing, or screen-printing, and better control
of the level of the dopants [11e13]. In the past few years, small-
molecule complex phosphors doped polymeric light-emitting
nology and the high-efficiency of small-molecule materials [13e19].
For use in such EL devices, the small-molecule phosphors are
required to possess high solubility, high morphological and thermal
stabilities as well as high EL efficiency. Delightfully, the properties
such as emitting color, solubility, morphological and thermal
stabilities, even the efficiency of the metal complex phosphors can
be intentionally obtained via reasonable molecular design by
employing appropriate functional groups on the ligands [7,20].
Groups such as carbazole, triphenylamine and oxadiazole with
hole or electron-transporting functions are beneficial to improving
the performance of EL materials [7,20e29]. Recently, the synergic
action of hole and electron-transporting functional groups has
attracted great attention, because it can provide balance in electron
and hole fluxes and thus simplifies the device structure [24]. For
example, a series of bipolar triphenylamine/oxadiazole or carbazole/
oxadiazole derivatives have been synthesized and used as highly
efficient host materials [24e26]. Novel Pt(II) complexes containing
hole-transporting triarylamine and electron-transporting oxadia-
zole have been synthesized and used as phosphor guests [27]. Tri-
arylamine and oxadiazole substituted ferrocenes have been used as
hole-injection materials in polymeric light-emitting diodes [28].
A Eu(III) complex containing carbazole and oxadiazole has been
synthesized and used as a red light emitter [29].
* Corresponding author. Fax: þ86 20 87110606.
E-mail address: pswyang@scut.edu.cn (W. Yang).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.04.005

414
H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
Besides three primary colors of red, green and blue (RGB), yellow
slowly poured into water (about 500 mL) with stirring. The mixture
was extracted with CH2Cl2 (3 ꢁ 80 mL), washed with water and
dried (anhydrous Na2SO4). The solvent was distilled off and the
residue was chromatographed over silica gel, eluting with petro-
leum ether (60e90 ꢀC)/CH3COOC2H5 (volume ratio, 10:1). Yield 86%
(8.53 g), colorless needles, mp 65 ꢀC (DSC, lit. [36] mp
not only is another important monochromatic light, but also is a key
component in generating efficient white-light. For example, a yellow
iridium complex bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-
benzoimidazol-N,C3) iridium(acetylacetonate) ((fbi)2Ir(acac)) was
synthesized and used for fabricating highly efficient white OLEDs with
blue iridium complex bis(2-(4,6-difluorophenyl)-pyridinato-N,C2)
picolinate iridium(III) (FIrpic) [30e32]. Several yellow or orangee
yellow iridium complexes using 2-(9,9-diethylfluoren-2-yl)pyridine
(Flpy) or its derivatives as ligands were synthesized and also used for
fabricating highly efficient white OLEDs with FIrpic [13,14,33,34]. A
yellow iridium complex iridium(III) bis[3-phenylisoquinoline](2-(2H-
1,2,4-triazol-3-yl)pyridine) (Ir(3-piq)2(pt)) was used for fabricating
white PLEDs with the blue emission of host materials polyfluorene
(PFO) [35]. In this work, the ligand 3-(5-(4-(pyridin-2-yl)phenyl)-
1,3,4-oxadiazol-2-yl)-9-hexyl-9H-carbazole (PPOHC) containing
a carbazoleeoxadiazole unit and the derived yellow homoleptic
complex Ir(PPOHC)3 and heteroleptic complex (PPOHC)2Ir(acac)
(acac:acetylacetone) were synthesized and used in yellow and white
polymeric light-emitting diodes.
65.4e65.7 ꢀC). 1H NMR (300 MHz, CDCl3, 25 ꢀC, ppm):
d
¼ 8.13 (d,
2H, 3J ¼ 7.8 Hz, ArH), 7.41e7.51 (m, 4H, ArH), 7.22e7.27 (m, 2H, ArH),
4.31 (t, 2H, 3J ¼ 7.2 Hz, eNeCH2e), 1.84e1.93 (m, 2H, eNeCeCH2e),
1.29e1.56 (m, 6H, e(CH2)3e), 0.90 (t, 3H, eCH3). IR (KBr, cmꢂ1),
3050, 2954, 2923, 2855, 1897, 1777, 1593, 1484, 1452, 1325, 1238,
1216, 1151, 997, 749, 724. Element Anal. Calc. for C18H21N (%): C,
86.01; H, 8.42; N, 5.57. Found (%): C, 85.86; H, 8.63; N, 5.51.
2.1.3. 3-Formyl-9-hexyl-9H-carbazole (2)
POCl3 (2 mL, 21.5 mmol) was added dropwise into dried N,N-
dimethylformamide (DMF, 25 mL) with stirring in an ice bath.
Then 9-hexyl-9H-carbazole (5.02 g, 20.0 mmol) was added in, the
mixture was solwly heated to and maintained at 80 ꢀC for 6 h. The
cooled reaction mixture was slowly poured into ice-water (about
200 mL), and neutralized with Na2CO3. The mixture was extracted
with CH2Cl2 (3 ꢁ 50 mL), washed with water and dried (anhydrous
Na2SO4). The solvent was distilled off and the residue was chro-
matographed over silica gel, eluting with petroleum ether
(60e90 ꢀC) and CH2Cl2 (volume ratio, 2:1). Yield 78% (4.35 g),
brownish-yellow oil. 1H NMR (300 MHz, CDCl3, 25 ꢀC, ppm):
2. Experimental
2.1. Synthesis and characterization of the iridium(III) complexes
2.1.1. General information
Chemicals and reagents were obtained from commercial sour-
ces, and used without further purification unless otherwise noted.
Column chromatography was carried out on silica gel
(200e300mesh). Yields refer to the isolated pure compound.
1H NMR spectra were recorded on a Bruker AV300 spectrometer
operating at 300 MHz, tetramethylsilane (TMS) was used as internal
standard. IR spectra were recorded with a Bruker Vector 33 spec-
trometer. Elemental analyses were performed on Vario EL
Elemental Analysis Instrument (Elementar Co.). Ultravioletevisible
(UVeVis) absorption spectra were recorded on a HP8453E spec-
trophotometer (HewlettePackard Co.). Photoluminescence (PL)
spectra were recorded on an Fluorologe3 spectrophotometer (Jobin
Yvon Inc.). Differential scanning calorimetry (DSC) curves were
measured on a Netzsch DSC200 analyzer at a heating rate 10 ꢀC/min
under N2. In DSC measurements of the iridium complexes, both of
them underwent two heating cycles, the second heating followed
the first heatingecooling cycle (the first heating was from room
temperature to 250 ꢀC). Thermogravimetry (TG) curves were
measured on a Netzsch TG209 thermal analyzer at a heating rate
10 ꢀC/min under N2. Mass spectra were obtained from a Bruker
Esquire HCT PLUS liquid chromatography/mass spectrometer(LC/
MS, Bruker Corp.) with an electrospray ionization (ESI) interface
using acetonitrile as the matrix solvent. Cyclic voltammetry (CV)
was measured on a computer-controlled CHI800C electrochemical
analyzer (Shanghai Chenhua Instrument Co.). Anhydrous and
Aresaturated dichloromethane solutions of the iridium(III)
complexes containing 0.1 mol/L tetra-n-butylammonium hexa-
fluorophosphate (TBAPF6) as the supporting electrolyte were
scanned at 50 mV/s. A glass carbon electrode was used as a working
electrode, a Ag/AgCl electrode was used as a reference electrode,
a platinum wire was used as a counter electrode. Ferrocene (4.8 eV
under vacuum) was used as the internal standard material.
d
¼ 10.07 (s, 1H, eCHO), 8.55 (d, 1H, eArH), 8.12 (d, 1H,
3J ¼ 7.8 Hz, eArH), 7.98 (dd,1H, 3J ¼ 8.4 Hz, eArH), 7.50e7.55 (m,1H,
ArH), 7.34e7.44 (m, 2H, ArH), 7.29e7.31 (t, 1H, ArH), 4.25 (t, 2H,
3J ¼ 7.2 Hz, eNeCH2e), 1.80e1.90 (m, 2H, eNeCeCH2e), 1.25e1.39
(m, 6H, e(CH2)3e), 0.87 (t, 3H, eCH3). IR (KBr, cmꢂ1), 3052, 2955,
2929, 2857, 2727, 1893, 1686, 1593, 1469, 1383, 1352, 1339, 1329,
1240,1177,1135, 807, 765, 748, 730. Element Anal. Calc. for C19H21NO
(%):C, 81.68; H, 7.58; N, 5.01. Found (%): C, 81.83; H, 7.69; N, 5.14.
2.1.4. 3-Cyano-9-hexyl-9H-carbazole (3)
Ammonia (25e28%, 50 mL) was added in the mixture of
3-formyl-9-hexyl-9H-carbazole (5.58 g, 20.0 mmol) in tetrahydro-
furan (THF, 25 mL), then I2 (5.58 g, 22.0 mmol) was added in and
was stirred for 1.5 h. Na2S2O3 was added in until the black-
ishebrown vanished. The reaction mixture was extracted with
CH2Cl2 (3 ꢁ 30 mL), washed with water and dried (anhydrous
Na2SO4). The solvent was distilled off and the residue was chro-
matographed over silica gel, eluting with petroleum ether
(60e90 ꢀC) and CH2Cl2 (volume ratio, 3:1). Yield 92% (5.14 g), white
solid, mp 80 ꢀC (DSC). 1H NMR (300 MHz, CDCl3, 25 ꢀC, ppm):
d
¼ 8.41 (t, 1H, ArH), 8.13 (dt, 1H, 3J ¼ 7.8 Hz, ArH), 7.72 (dd, 1H,
3J ¼ 8.4 Hz, ArH),
d
¼ 7.55e7.57 (m, 1H, ArH), 7.44e7.49 (m, 2H,
ArH), 7.28e7.37 (m, 1H, ArH), 4.34 (t, 2H, 3J ¼ 7.2 Hz, eNeCH2e),
1.80e1.90 (m, 2H, eNeCeCH2e), 1.29e1.58 (m, 6H, e(CH2)3e), 0.88
(t, 3H, eCH3). IR (KBr, cmꢂ1), 3061, 2953, 2927, 2855, 2212, 1904,
1858, 1808, 1594, 1473, 1356, 1323, 1248, 1159, 1138, 1023, 908, 848,
802, 753, 727, 610. Element Anal. Calc. for C19H20N2 (%): C, 82.57; H,
7.29; N, 10.14. Found: C, 82.41; H, 7.38; N, 10.21.
2.1.5. 3-(1H-tetrazol-5-yl)-9-hexyl-9H-carbazole (4)
3-Cyano-9-hexyl-9H-carbazole (3) (4.14 g, 15.0 mmol), sodium
azide (14.63 g, 225.0 mmol) and ammonium chloride (12.04 g,
225.0 mmol) were added to DMF (150 mL) with mechanical
agitation and the mixture was heated to 120 ꢀC for 72 h. The cooled
reaction mixture were poured into ice-water (about 1 L) and
acidified with HCl to pH z 1.0. The precipitants were then filtered
and washed with plenty of water, dried in vacuum. Yield 84%
(4.00 g), white solid, mp 225 ꢀC (DSC). 1H NMR (300 MHz, DMSO-
2.1.2. 9-Hexyl-9H-carbazole (1)
After a mixture of KOH (14.00 g, 250.0 mmol) and carbazole
(6.60 g, 40.0 mmol) in acetone (80 mL) was stirred for 30 min,
another mixture of 1-bromohexane (9.9 g, 60.0 mmol) in acetone
(20 mL) was added dropwise in with stirring, and the stirring was
kept for 10 h after the addition was over. The reaction mixture was
D6, 25 ꢀC, ppm):
d
¼ 8.86 (s, 1H, ArH), 8.24 (d, 1H, 3J ¼ 7.8 Hz, ArH),

H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
415
CHO
KOH, n-C6H13Br
acetone, R.T.
POCl3
DMF, 80°C
N
N
H
N
1
2
N
N
N
CN
I2, NH3•H2O
NH
NaN3, NH4Cl
N
THF, R.T.
DMF, 120°C
N
3
4
KMnO4, KOH
SOCl2
4
N
N
N
COOH
COCl
CH3
pyridine, reflux
reflux
H2O, pyridine, 110°C
5
6
N
O
O
Ir
2
N
N
O
O
O
N
Cl
N
N
N
N
N
N
IrCl3•3H2O, Ar
Ir
Ir
2
Cl
C2H5OC2H4OH, reflux
(PPOHC)2Ir(acac)
2
O
O
O
N
N
N
N
Ir
N
N
N
3
N
N
O
PPOHC
(PPOHC)2Ir(¦Ì-Cl)2Ir(PPOHC)2
N
Ir(PPOHC)3
Scheme 1. Synthetic routes of iridium complexes.
8.12 (d, 1H, 3J ¼ 8.7 Hz, ArH), 7.82 (d, 1H, 3J ¼ 8.7 Hz, ArH), 7.66
(d, 1H, 3J ¼ 8.1 Hz, ArH), 7.52 (t, 1H, ArH), 7.28 (t, 1H, ArH), 4.44
(t, 2H, 3J ¼ 6.0 Hz, eNeCH2e), 1.71e1.86 (m, 2H, eNeCeCH2e),
1.17e1.35 (m, 6H, e(CH2)3e), 0.79 (t, 3H, eCH3). IR (KBr, cmꢂ1),
3421, 3062, 2955, 2926, 2855, 2771, 2722, 2631, 1883, 1812, 1632,
1602, 1570, 1491, 1468, 1447, 1404, 1378, 1355, 1246, 1196, 1158,
1056, 996, 909, 808, 749, 723. Element Anal. Calc. For C19H21N5 (%):
C, 71.45; H, 6.63; N, 21.93. Found (%): C, 71.28; H, 6.75; N, 21.97.
heated to 110 ꢀC. A saturated aqueous solution of KMnO4 (7.11 g,
45.0 mmol) was added dropwise into the aforementioned mixture
with stirring. The stirring was kept until the purple of the perman-
ganate vanished (checked by dropping the solution on the filter
paper), then the hot mixture was filtered on a vacuum filter, the filter
residue washed with hot water (3 ꢁ 30 mL). The filtrate was carefully
adjusted to pH ¼ 7.0 with concentrated hydrochloric acid after it was
cooled to room temperature, a large quantity of white precipitate was
produced and collected on a vacuum filter, washed with water and
dried in vacuum. Yield 65% (3.88 g), white solid, mp 237 ꢀC (DSC).
2.1.6. 4-(Pyridin-2-yl)benzoic acid (5)
KOH (2.80 g, 50.0 mmol) and 2-p-tolylpyridine (5.08 g, 30.0 mmol)
were added to a mixture of water (30 mL) and pyridine (50 mL), and
1H NMR (300 MHz, DMSO-D6, 25 ꢀC, ppm):
8.70e8.73 (m, 1H, ArH), 8.70e8.73 (m, 2H, ArH), 8.04e8.07 (m, 3H,
d
¼ 13.04 (s, 1H, eCOOH),

416
H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
ArH), 7.87e7.91 (q, 1H, ArH), 7.40e7.44 (m, 1H, ArH). IR (KBr, cmꢂ1),
3551, 3415, 3084, 3028, 2922, 2791, 2595, 2468, 1874, 1691, 1597, 1473,
1435, 1406, 1288, 1264, 1183, 1159, 1123, 1003, 811, 760. Element Anal.
Calc. for C12H9NO2 (%): C, 72.35; H, 4.55; N, 7.03. Found (%): C, 72.48; H,
4.72; N, 6.85.
in 2-ethoxyethanol (15 mL) was heated under reflux and argon for
24 h. The reaction mixture was poured in water (about 150 mL) after
it was cooled to room temperature. The orangeeyellow precipitate
was collected on a filter and washed by water and methanol alter-
nately. The crude product was purified by silica gel column chro-
matography with petroleum ether (60e90 ꢀC)/CH2Cl2/CH3COOC2H5
(volume ratio, 2:6:1) as eluent to afford pure product. Yield 21%
(101 mg), orangeeyellow solid. 1H NMR (300 MHz, CDCl3, 25 ꢀC,
2.1.7. 3-(5-(4-(Pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)-9-hexyl-
9H-carbazole (PPOHC)
4-(Pyridin-2-yl)benzoic acid (2.99 g, 15.0 mmol) was added into
SOCl2 (50 mL) and refluxed for 12 h to give 4-(pyridin-2-yl)benzoyl
chloride (6). The excess SOCl2 was removed by vacuum distillation.
After 4-(pyridin-2-yl)benzoyl chloride was cooled to room temper-
ature, the solution of 3-(1H-tetrazol-5-yl)-9-hexyl-9H-carbazole (4)
(4.79 g, 15.0 mmol) in anhydrous pyridine (80 mL) was added
dropwise in and then stirred under reflux in the presence of N2 for
6 h. After the reaction mixture was cooled to room temperature, it
was poured into the water (about 500 mL). The precipitate was
collected on a vacuum filter and washed with water. The pure
product was obtained by silica gel column chromatography, eluting
with petroleum ether (60e90 ꢀC) and CH2Cl2 (volume ratio, 1:1).
Yield 73% (5.18 g), white solid, mp 165 ꢀC (DSC). 1H NMR (300 MHz,
ppm):
d
¼ 8.83e9.28 (m, 3H, ArH), 8.63e8.74 (m, 3H, ArH),
8.24e8.55 (m, 3H, ArH), 8.07e8.20 (m, 6H, ArH), 7.88e8.00 (m, 4H,
ArH), 7.63e7.85 (m, 6H, ArH), 7.28e7.55 (m, 12H, ArH), 7.15e7.22 (m,
2H, ArH), 6.50e7.05 (m, 3H, ArH), 4.33 (t, 6H, 3J ¼ 6.9 Hz, eNeCH2e),
1.87e1.91 (m, 6H, eNeCeCH2e), 1.10e1.42 (m, 18H, e(CH2)3e), 0.88
(t, 9H, eCH3). IR (KBr, cmꢂ1), 3053, 2952, 2926, 2855, 2037, 2016,
1600,1562,1541,1487,1466,1430,1385,1352,1325,1249,1153,1059,
1024, 963, 893, 809, 781, 748, 728. MS (ESI, m/z): 1607.8 [M þ H]þ.
Element Anal. Calc. for C93H81IrN12O3 (%): C, 69.51; H, 5.08; N, 10.46.
Found (%): C, 69.38; H, 5.17; N, 10.62.
2.2. Fabrication and measurements of EL devices
CDCl3, 25 ꢀC, ppm):
d
¼ 8.90 (s, 1H, ArH), 8.77e8.79 (q, 1H, ArH),
Indium-tin oxide (ITO) coated glass with a sheet resistance of
15e20 U/, was used as substrate anode and was cleaned with
detergents, then with deionized water, acetone and isopropanol in
an ultrasonic bath. After oxygen plasma cleaning for 4 min, an
anode buffer layer of poly(3,4-ethylenedioxythiophene) doped
with poly(styrene sulfonate) (PEDOT:PSS) was spin-coated on the
ITO substrate and then dried by baking in a vacuum oven at 80 ꢀC
for 8 h. Then a light-emitting layer consisted of host materials
poly(N-vinylcarbazole) (PVK), electron-transporting materials 2-
(4-biphenylyl)-5-4-tert-butylphenyl-1,3,4-oxadiazole (PBD, used in
yellow-light devices) or 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadia-
zolyl]phenylene (OXD-7, used in white-light devices), and dopants
of iridium(III) complexes (PPOHC)2Ir(acac) or Ir(PPOHC)3 at
different concentrations was spin-coated on the PEDOT: PSS layer
in a glove box (Vacuum Atmosphere Co.) containing less than
10 ppm oxygen and moisture, then was baked on a hot plate at
100 ꢀC in inert atmosphere for 20 min to remove solvent (chloro-
benzene) residue. Electron-transporting layer, electron injection
layer (CsF) and subsequent cathode layer (Al) were thermally
evaporated with a base pressure of 3 ꢁ 10ꢂ4 Pa. The spin-coated
film thickness was measured by an Alfa Step 500 surface profil-
ometer (Tencor), layer thickness during thermal deposition was
monitored by a quartz crystal thickness/ratio monitor (Model:
STM-100/MF, Sycon). The current densityeluminanceevoltage
(IeLeV) characteristics were measured using a Keithley 236 source
measurement unit and a calibrated silicon photodiode. EL spectra
8.21e8.33 (m, 6H, ArH), 7.83 (t, 2H, ArH), 7.46e7.58 (m, 3H, ArH),
7.28e7.36 (m, 2H, ArH), 4.37 (t, 2H, 3J ¼ 6.6 Hz, eNeCH2e),1.85e1.95
(m, 2H, eNeCeCH2e), 1.28e1.44 (m, 6H, e(CH2)3e), 0.90
(t, 3H, eCH3). IR (KBr, cmꢂ1), 3049, 2956, 2925, 2853, 1950, 1880,
1780,1600,1560, 1494,1460, 1433,1326, 1253,1216,1190,1143,1097,
1062, 1013, 989, 964, 892, 856, 812, 785, 752, 731. MS (ESI, m/z):
473.3 [M þ H]þ. Element Anal. Calc. for C31H28N4O (%): C, 78.79; H,
5.97; N, 11.86. Found (%): C, 78.67; H, 6.08; N, 11.93.
2.1.8. (PPOHC)2Ir(m-Cl)2Ir(PPOHC)2
IrCl3$3H2O (0.42 g, 1.2 mmol), PPOHC (1.28 g, 2.7 mmol) and H2O
(8 mL) were added in 2-methoxyethanol (24 mL). The mixture was
heated under reflux and argon for 24 h and then cooled to room
temperature. The yellow precipitate was collected on a filter and
washed by water and methanol alternately. Yield 76% (1.21 g), yellow
solid. The dimer product was directly used for the next step after being
dried in vacuum without further purification and characterization.
2.1.9. (PPOHC)2Ir(acac)
Chloro-bridged dimer (PPOHC)2Ir(m-Cl)2Ir(PPOHC)2 (0.70 g,
0.3 mmol), acetylacetone (0.30 g, 3.0 mmol), and Na2CO3 (0.80 g,
7.5 mmol) in 2-ethoxyethanol (15 ml) was heated under reflux and
argon for 24 h. The reaction mixture was poured in water (about
150 mL) after it was cooled to room temperature. The orangeeyellow
precipitate was collected on a filter and washed by water and
methanol alternately. The crude product was purified by silica gel
column chromatography with petroleum ether (60e90 ꢀC)/CH2Cl2/
CH3COOC2H5 (volume ratio, 2:6:1) as eluent to afford pure product.
Yield 23% (160 mg), orangeeyellow solid. 1H NMR (300 MHz, CDCl3,
25 ꢀC, ppm):
d
¼ 8.68 (d, 4H, 3J ¼ 5.1 Hz, ArH), 8.16 (d, 2H, 3J ¼ 7.8 Hz,
ArH), 8.03e8.11 (q, 4H, ArH), 7.94 (t, 2H, 3J ¼ 7.5 Hz, ArH), 7.79 (d, 2H,
3J ¼ 6.3 Hz, ArH), 7.72 (d, 2H, 3J ¼ 8.1 Hz, ArH), 7.64 (d, 2H, 3J ¼ 7.8 Hz,
ArH), 7.27e7.56 (m, 10H, ArH), 5.30 (s, 1H, acaceCH), 4.32 (t, 4H,
3J ¼ 7.2 Hz, eNeCH2e), 1.82e1.89 (m, 4H, eNeCeCH2e), 1.57 (s, 6H,
acaceCH3), 1.15e1.37 (m, 12H, e(CH2)3e), 0.88 (t, 6H, eCH3). IR (KBr,
cmꢂ1), 3055, 2953, 2927, 2855, 2039, 1600, 1579, 1543, 1516, 1468,
1431,1395,1353, 1250,1155,1060, 1022, 963, 892, 810, 782, 749, 729.
MS (ESI, m/z): 1234.6 [M]þ. Element Anal. Calc. for C67H61IrN8O4 (%):
C, 65.19; H, 4.98; N, 9.08. Found (%): C, 65.28; H, 5.06; N, 9.18.
2.1.10. Ir(PPOHC)3
Chloro-bridged dimer (PPOHC)2Ir(m-Cl)2Ir(PPOHC)2 (0.35 g,
0.15 mmol), PPOHC (0.16 g, 0.34 mmol), and K2CO3 (0.47 g, 3.4 mmol)
Fig. 1. TG and DSC (inset) curves of the complexes.

H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
417
1.0
0.8
0.6
0.4
0.2
0.0
200 250 300 350 400 450 500 550 600
Wavelength (nm)
Fig. 3. UVeVis absorption spectra of the complexes and ligand (inset) in CH2Cl2
solution of 2.0 ꢁ 10ꢂ5 M.
Fig. 2. CV curves of the complexes in CH2Cl2 solution of 1 ꢁ 10ꢂ3 mol/L. Potentials
were recorded versus Fcþ/Fc (Fc is ferrocene).
3.3. Electrochemical properties
and the Commission Internationale de L’Eclairage (CIE) coordinate
were recorded using a spectrophotometer (SpectraScan PR-705,
Photo Research).
The cyclic voltammetry (CV) curves of the complexes are shown
in Fig. 2. All the complexes exhibit a one-electron reversible
oxidation couple at relatively low positive potentials and an irre-
versible oxidation couple at relatively high positive potentials. The
former are attributed to a metal-centered IrIII/IrIV oxidation couple
[14,40], the latter may originate from the electron-donating carba-
zole unit [40,41]. The oxidation potential of homoleptic complex
Ir(PPOHC)3 is lower than that of heteroleptic complex (PPOH-
C)2Ir(acac) and these values are similar to those of other homoleptic
and heteroleptic iridium complex homologs in the literature [14].
The value of the energy gap (Eg) of every iridium(III) complex was
3. Results and discussion
3.1. Synthesis
The synthesis procedure employed to obtained the iridium
complexes is shown in Scheme 1. The ligand PPOHC was synthe-
sized via a similar procedure to the reported by Lee [37], but
3-formyl-9-hexyl-9H-carbazole (2) was reduced to 3-cyano-9-
hexyl-9H-carbazole (3) by using I2 instead of NH2OH as reductant
at room temperature in 1.5 h, which constitutes a simpler and
milder reduction reaction. The iridium complexes were synthe-
sized from PPOHC and IrCl3$3H2O by a conventional way via
iridium dimer [14e17].
obtained from the spectroscopy absorption edge ( Eg ¼ 1240/la, edge
)
[14,42], together with the oxidation potentials, the HOMO and
LUMO energy levels relative to the energy level of ferrocene (4.8 eV
under vacuum) can be estimated [14], the results are listed in
Table 1.
3.4. UVeVis absorption and PL properties
3.2. Thermal properties
The UVeVis absorption spectra of the complexes in solution are
shown in Fig. 3. The absorption spectra of the complexes (PPOH-
C)2Ir(acac) and Ir(PPOHC)3 strongly resemble each other, that
indicates the absorption spectra are mainly decided by the PPOHC
ligand and the ancillary ligand, acetylacetone, scarcely plays a role
in the absorption in (PPOHC)2Ir(acac) [38]. The strong absorption
Thermogravimetry (TG) and differential scanning calorimetry
(DSC) curves of the iridium complexes are shown in Fig. 1.
Complexes (PPOHC)2Ir(acac) and Ir(PPOHC)3 have high thermal
stability with 5% weight-reduction temperatures (D
T5%) of 343 ꢀC
and 370 ꢀC, respectively. DSC curves show there is neither crys-
tallization nor melting peaks. The glass-transition temperatures
(Tg) of (PPOHC)2Ir(acac) and Ir(PPOHC)3 are 188 ꢀC and 201 ꢀC,
respectively. The amorphous character and high Tg of the iridium
complexes are resistant to crystallization and phase disengage-
ment, and are desirable for EL devices with high stability and effi-
ciency [38,39]. The results suggest that the n-hexyl substituent on
the carbazole unit plays a significant role in improving the amor-
phous nature and Tg values as well as the solubility [39,40].
bands at 200e420 nm can be assigned to the spin-allowed 1p p*
e
(PPOHC)2Ir(acac)
Ir(PPOHC)3
1.0
0.8
0.6
0.4
0.2
0.0
Table 1
Electrochemical properties.
Complex
E1/2,ox
Eonset,ox
HOMO
(eV)
LUMO
(eV)b
Eg
(eV)
(V)a
(V)a
(PPOHC)2Ir(acac)
Ir(PPOHC)3
0.61
0.54
0.48
0.36
ꢂ5.28
ꢂ5.16
ꢂ2.92
ꢂ2.73
2.36
2.43
450
500
550
600
650
700
750
Measured in Aresaturated CH2Cl2, 0.1 mol/L TBAPF6, scan rate 50 mV/s, vs. Fcþ
/
a
Wavelength (nm)
Fc couple.
b
LUMO ¼ HOMO þ Eg.
Fig. 4. PL spectra of the complexes in CH2Cl2 solution of 2.0 ꢁ 10ꢂ5 M.

418
H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
2%
4%
6%
2%
4%
6%
1.0
0.8
0.6
0.4
0.2
0.0
a
b
1.0
0.8
0.6
0.4
0.2
0.0
400 500 600 700 800
400 500 600 700 800
Wavelength (nm)
Wavelength (nm)
Fig. 5. EL spectra of the complexes (PPOHC)2Ir(acac) (a) and Ir(PPOHC)3 (b). Device configuration: ITO/PEDOT: PSS/PVK: PBD: complex (70: 30: x)/TPBI/CsF/Al.
transition of the ligand PPOHC in the complexes. In contrast to the
absorption spectrum of PPOHC in CH2Cl2 solution, such strong
absorption bands have a small red shift and slight differentiation
due to the conjugated coordination. The weak absorption bands (at
420e525 nm for (PPOHC)2Ir(acac) and 420e510 nm for Ir(PPOHC)3)
can be assigned to an admixture of 1MLCT (metaleligand charge
EL spectra of devices are shown in Fig. 5. The weak emission in
the region of 400e480 nm originated from the PVKePBD exciplex
[43] can be observed when the doping concentrations of the
complexes are 2 wt.-%, however, such emission peaks have not
emerged at 4 wt.-% and 6 wt.-%, because the low doping concen-
trations are not enough to consume the excitons of the PVK. Rela-
tive to the PL spectra of the complexes in solution, the EL spectra
show red shifts of 9e15 nm due to closer intermolecular contacts in
the solid-state [44]. The EL spectrum of (PPOHC)2Ir(acac) resembles
closely its PL spectrum, however, the EL spectrum of Ir(PPOHC)3
resembles its PL spectrum less so, the low shoulder on the peak on
the right side of the PL spectrum has become more intense in the EL
spectrum, and this disparity was possibly caused by slight optical
microcavity effect [45,46].
The device performances are listed in the Table 2. The turn-on
voltages (Von) of the devices are 5.6e7.0 V, the relatively high
driving voltages may be due to the large hole barrier at the
PEDOT:PSS/PVK interface (HOMOs of PVK w 5.80 eV versus
PEDOT w 5.20 eV) and the thicker emissive layer (70 nm) [14]. In
contrast to the devices fabricated by (PPOHC)2Ir(acac), the devices
fabricated by Ir(PPOHC)3 have lower turn-on voltages at the same
doping concentration which may be due to a closer match of the
transfer), 3MLCT and 3
pe
p* states [33,38]. The admixture of 3MLCT
and 3 p* with higher-lying 1MLCT is due to the strong spineorbit
pe
coupling induced by the iridium heavy atom [9,33,38].
The PL spectra of (PPOHC)2Ir(acac) and Ir(PPOHC)3 in solution
(
lex ¼ 350 nm) are shown in Fig. 4. The PL spectra of complexes
(PPOHC)2Ir(acac) and Ir(PPOHC)3 also resemble each other, both of
them have a maximum main peak and a shoulder peak, but the PL
spectra of complex (PPOHC)2Ir(acac) (lmax ¼ 555 nm) gives
a red shift of 9 nm in comparison with that of Ir(PPOHC)3
(
lmax ¼ 546 nm) due to higher ligand-field strength of acetylacet-
onate anion than PPOHC anion [14].
3.5. Device performances
Polymeric light-emitting diodes were fabricated with the config-
urations of ITO/PEDOT:PSS (40 nm)/emitting layer (70 nm)/TPBI
(30 nm)/CsF (1.5 nm)/Al (120 nm), emitting layer: PVK (70 wt.-%) : PBD
(30 wt.-%) : complex (x wt.-%). The emitting layers consisted of host
materials PVK, electron-transporting materials PBD, and dopants
of the complexes (PPOHC)2Ir(acac) or Ir(PPOHC)3 at different
concentrations (2, 4, 6 wt.-%); 1,3,5-tris(N-phenylbenzimidazol-2-yl)-
benzene (TPBI) was used as hole-blocking/electron-transporting
material.
Table 2
Device performance of the complexesa.
Dopant
concentration Von Lmax
(wt.-%)
LEmax QEmax lmax CIEb
(V) (cd/m2) (cd/A) (%)
(nm)
(PPOHC)2Ir(acac)
2
4
6
2
4
6
7.0 3793
7.0 6354
7.5 4020
5.6 4079
6.0 6323
6.3 6178
7.6 3.0
11.1 4.4
7.6 3.1
10.3 4.1
16.4 6.6
15.8 6.3
564 (0.51, 0.46)
564 (0.52, 0.47)
566 (0.52, 0.47)
588 (0.50, 0.47)
560 (0.50, 0.49)
590 (0.51, 0.48)
Ir(PPOHC)3
a
Device configuration: ITO/PEDOT: PSS/PVK: PBD: complex/TPBI/CsF/Al.
Fig. 6. Energy diagram of the materials used in the devices. Device configuration: ITO/
PEDOT: PSS/PVK:PBD:complex (70:30:x)/TPBI/CsF/Al.
b
at 12 mA/m2.

H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
419
Table 3
7000
6000
5000
4000
3000
2000
1000
0
a
Device performances of the FIrpic and Ir(PPOHC)3
.
Firpic:Ir(PPOHC)3
(mass ratio)
Von
(V)
Lmax
LEmax
(cd/A)
QEmax
(%)
CIEb
(x, y)
(cd/m2)
5:1
4.2
4.4
4.5
4.3
7873
5954
6275
5047
13.7
12.2
15.3
10.5
5.3
4.71
5.5
0.41, 0.48
0.37, 0.46
0.39, 0.44
0.29, 0.44
7.5:1
8.5:1
10:1
3.4
a
Device configuration: ITO/PEDOT: PSS/PVK: OXD-7: FIrpic: Ir(PPOHC)3/CsF/Al.
b
at 12 mA/m2.
(PPOHC) Ir(acac)
Ir(PPOHC)
a
(PPOHC)2Ir(acac), higher luminance and current density can be
observed at the same voltage for Ir(PPOHC)3 due to more efficient
and balanced charge injection. It can be understood from the energy
level diagrams in Fig. 6 that the closer match of the HOMO level of
Ir(PPOHC)3 with PEDOTand the LUMO level of Ir(PPOHC)3 with TPBI
can decrease the hole and electron migrating barrier. The current
efficiency of Ir(PPOHC)3 declines relatively faster with the increase
of current density at low current density (<20 mA/cm2), this may be
attributed to the faster increase of tripletetriplet (TeT) annihilation
and field-induced quenching effects [14], however, both Ir(PPOHC)3
and (PPOHC)2Ir(acac) declines slowly with the increase of current
density at high current efficiency. The current efficiency of Ir(P-
POHC)3 is higher than that of (PPOHC)2Ir(acac) even at a high
current density near 100 mA/cm2, so Ir(PPOHC)3 is a better elec-
troluminescent phosphor for OLEDs.
The white polymeric light-emitting diodes were fabricated by
doping of Ir(PPOHC)3 and blue light-emitting FIrpic in PVK matrix.
The configuration of the devices are ITO/PEDOT: PSS (40 nm)/
emitting layer (80 nm)/CsF (1.5 nm)/Al (120 nm), emitting layer is
PVK (63 wt.-%): OXD-7 (27 wt.-%): dopants (FIrpic and Ir(PPOHC)3,
10 wt.-%). OXD-7 was electron-transporting material. The FIrpic
and Ir(PPOHC)3 were doped at different mass ratios of 5:1, 7.5:1,
8.5:1 and 10:1.
The EL spectra are shown in Fig. 8 and the performances are list
in Table 3. The emission in the region of 450e530 nm and
530e700 nm originate from FIrpic and Ir(PPOHC)3, respectively
[47]. The relative EL intensity of FIrpic increases with the increase of
its doping concentration, at the same time, the intensity of Ir(P-
POHC)3 decreases. When the mass ratio of FIrpic:Ir(PPOHC)3 is
8.5:1, both emission of FIrpic and Ir(PPOHC)3 are sufficient in
magnitude and balanced, so this mass ratio is an optimal ratio for
obtaining white-light. The turn-on voltage of the device at this
mass ratio is 4.5 V, the maximum luminance and efficiency are
6275 cd/m2 (at 13 V) and 15.3 cd/A respectively, with the CIE
coordinates of (0.39, 0.44). The luminance, current density vs.
voltage and current efficiency vs. current density characteristics of
-2
0
2
4
6
8
10 12 14 16 18
Voltage (V)
20
15
10
5
(PPOHC) Ir(acac)
Ir(PPOHC)
0
-5
b
-10
0
20
Current density
40
60
80
mA/cm
100
)
120
2
(
Fig. 7. Luminance and current density (inset) vs. voltage (a), and current efficiency vs.
current density (b) characteristics of the devices at 4 wt.-% doping concentrations.
Device configuration: ITO/PEDOT: PSS/PVK: PBD: complex (70: 30: 4)/TPBI/CsF/Al.
HOMO level of Ir(PPOHC)3 with PEDOT (w5.20 eV), and the LUMO
level of Ir(PPOHC)3 with TPBI (w2.70 eV) (as shown in Fig. 6), which
are beneficial to hole and electron injection. By comparing the
performance of different dopant concentrations, it is clearly seen
that 4 wt.-% is an optimal doping concentration because of the
maximum efficiencies and luminances being obtained, the
maximum efficiencies of the devices using (PPOHC)2Ir(acac) and
Ir(PPOHC)3 are 11.1 cd/A and 16.4 cd/A, and the maximum lumi-
nances are 6354 cd/m2 and 6323 cd/m2, with CIE coordinates of
(0.52, 0.47) and (0.50, 0.49) respectively.
The luminance and current density vs. voltage, and current
efficiency vs. current density characteristics of the devices at 4 wt.-%
doping concentrations are shown in Fig. 7. Compared with
5:1
1.0
7.5:1
8.5:1
0.8
10:1
0.6
0.4
0.2
0.0
350 400 450 500 550 600 650 700 750 800
Wavelength (nm)
Fig. 9. Luminance and current density vs. voltage, and efficiency vs. current density
(inset) characteristics of the white-light device. Device configuration: ITO/PEDOT: PSS/
PVK: OXD-7: FIrpic: Ir(PPOHC)3 (8.5: 1)/CsF/Al.
Fig. 8. EL spectra of the white-light devices based on the FIrpic and Ir(PPOHC)3. Device
configuration: ITO/PEDOT: PSS/PVK: OXD-7: FIrpic: Ir(PPOHC)3/CsF/Al.

420
H. Tang et al. / Dyes and Pigments 91 (2011) 413e421
properties of their red- and green-light-emitting Ir(III) complexes. J Org Chem
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the device at this optimal mass ratio are shown in Fig. 9. The current
efficiency declines slowly with increasing current density nearly
linear, the current efficiency remains 10 cd/A at near 50 mA/cm2,
and still remains 6 cd/A at high current density near 100 mA/cm2.
By comparison with the EL efficiencies declining quickly with
increasing current density in a decurved line of some OLEDs
[42,48], the results indicate that the device possesses good stability.
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The ligand 3-(5-(4-(pyridin-2-yl)phenyl)-1,3,4-oxadiazol-2-yl)-
9-hexyl-9H-carbazole (PPOHC) and its iridium(III) complexes
(PPOHC)2Ir(acac) and Ir(PPOHC)3 were synthesized. The (PPOH-
C)2Ir(acac) and Ir(PPOHC)3 complexes have high thermal stability
with 5% weight-reduction temperatures (D
T5%) of 343 ꢀC and
370 ꢀC, and glass-transition temperatures (Tg) of 188 ꢀC and 201 ꢀC,
respectively. The polymeric light-emitting diodes using (PPOH-
C)2Ir(acac) or Ir(PPOHC)3 as the dopant emitters showed maximum
efficiencies of 11.1 cd/A and 16.4 cd/A respectively. The mass ratio of
FIrpic:Ir(PPOHC)3 (8.5:1) was an optimal ratio for the white-light
device, and a maximum efficiency of 15.3 cd/A was obtained with
CIE coordinates of (0.39, 0.44).
Acknowledgments
This work was supported by China Postdoctoral Science Foun-
dation (No. 20090450858), the National Natural Science Foundation
of China (Nos.U0634003 and 21074038), and State Key Basical
Research Project of China (Nos. 2009CB623602 and 2009CB930604).
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