Improving the performance of phosphorescent polymer light-emitting diodes using morphology-stable carbazole-based iridium complexes

Article abstract of DOI:10.1039/b705342h

A series of morphology-stable carbazole-based iridium(iii) complexes with green to red emission have been prepared and characterized by elemental analysis, nuclear magnetic resonance, and mass spectroscopy. Their thermal, electrochemical, electronic absor

Full text of DOI:10.1039/b705342h

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Improving the performance of phosphorescent polymer light-emitting diodes
using morphology-stable carbazole-based iridium complexes{
Kai Zhang,a Zhao Chen,b Chuluo Yang,*a Xiaowei Zhang,a Youtian Tao,a Lian Duan,a Liang Chen,a
Linna Zhu,a Jingui Qin*a and Yong Cao*b
Received 10th April 2007, Accepted 6th June 2007
First published as an Advance Article on the web 21st June 2007
DOI: 10.1039/b705342h
A series of morphology-stable carbazole-based iridium(III) complexes with green to red emission
have been prepared and characterized by elemental analysis, nuclear magnetic resonance, and
mass spectroscopy. Their thermal, electrochemical, electronic absorption, and photoluminescent
properties have been studied. Highly efficient polymer light-emitting devices by using these
complexes as dopant emitters, both non-conjugated polymer (PVK) and conjugated polymer,
polyhedral oligomeric silsesquioxane-terminated poly(9,9-dioctylfluorene) [PFO(poss)], as the
host materials, have been achieved. With the device structure of ITO/PEDOT/(PFO(poss) + 30%
PBD)–2 wt.% 1/Ba/Al, a maximum external quantum efficiency of 6.4% and a maximum
luminous efficiency of 6.00 cd A21 with red emission at 608 nm were obtained. With the device
configuration of ITO/PEDOT/(PFO(poss) + 30% PBD)–4 wt.% 4/Ba/Al, a maximum external
quantum efficiency of 9.9% and a maximum luminous efficiency of 22.4 cd A21 with yellow–green
emission at 544 nm were realized. The increased morphology stability of 1 and 2 imparted by the
N-decyl long chains at the N atom of carbazole results in significantly better device performance
than their short chain analogues 1a and 2a under identical device configurations.
bis(2-phenyquinolyl-N,C29) acetylacetonate [PhqIr] into the
Introduction
blends of poly(9,99-dioctylfluorene) (PFO) and PBD.
Phosphorescent heavy-metal complexes as emitters in organic
Several groups have studied the mechanism of the PLEDs
light-emitting diodes (OLEDs) have attracted great attention
based on phosphorescent complexes. They believe that the
because they can fully utilize both singlet and triplet excitons
Fo¨rster energy transfer plays a minor role in achieving high
through the strong spin-orbital coupling of heavy-metal ions.1
device efficiency, instead that direct charge trapping plays the
dominant role in electroluminescence.2b,4 Moreover, a multi-
Phosphorescent OLEDs include vacuum-deposited small-
molecule-based devices and phosphorescent dye-doped poly-
layer architecture consisting of the hole-transporting (HT), the
mer-based devices. The efficiencies of polymer light-emitting
electron-transporting (ET) and the emissive layers is generally
diodes (PLEDs) based on phosphorescent dyes are usually
inferior to those of small-molecule-based devices. However,
the advantage of ease of fabrication of PLEDs by processing
the materials from solution, such as by spin coating or printing
techniques, have made them attractive.2 Most recently,
considerable progress on PLEDs based on phosphorescent
iridium complexes has been made.3 For example, Gong
et al.2b,c reported the high efficiency (external quantum
efficiency (QEext) = 10% ph el21, luminous efficiency (LE) =
32 cd A21) yellow–green phosphorescent PLED by doping
tris[9,9-dihexyl-2-(pyridyl-29)fluorene] iridium(III) [Ir(DPF)3]
into a blend of PVK and PBD. Jiang et al.3d reported the
adopted to attain balanced injection and transport of holes
and electrons, which are key points for a high efficiency OLED
device.5 Therefore, in order to fabricate high efficiency and/or
simple configuration PLEDs, it would be reasonable to have
the phosphorescent complexes contain structural features for
optimizing charge injection and transport across the bulk.
Carbazole-based compounds have played very important
roles in organic/polymeric optoelectronic materials. In organic
light-emitting diodes (OLEDs), carbazole derivatives can
usually be used as host materials for both small-molecule
OLEDs (such as 4,49-N,N9-dicarbazolebiphenyl, CBP) and
polymer OLEDs (such as poly(vinylcarbazole), PVK) because
of their high triplet energy and good hole-transporting
ability.1d,6 In addition, the energy levels of carbazole-based
compounds can be tuned by substitution at the 3/6 or 2/7
positions due to their different electronic density.7 We
previously reported four novel carbazole-based Ir(III) and
Pt(II) complexes, which exhibit emission from blue–green
to red by altering the ligation of metal with carbon atom at
the 2 or 3 position of the carbazole unit. Vacuum-deposited
high efficiency (QEext = 12% ph el21 and LE = 5.2 cd A21
)
red phosphorescent PLED by incorporating iridium(III)
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan, 430072, People’s
Republic of China. E-mail: clyang@whu.edu.cn
bInstitute of Polymer Optoelectronic Materials and Devices, South
China University of Technology, Guangzhou, 510640, People’s Republic
of China. E-mail: poycao@scut.edu.cn
small-molecule-based electroluminescent devices with
a
{ Electronic supplementary information (ESI) available: TGA curves
of 1–4, PL and EL spectra of 1a and 2a, CIE coordinate plot for
PLEDs. See DOI: 10.1039/b705342h
configuration of ITO/NPB/CBP–dopant/BCP/AlQ3/LiF/Al
(NPB
=
4,49-bis[N-(1-naphthyl)-N-phenylamino]biphenyl;
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BCP = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; AlQ3 =
tris(8-hydroxyquinoline)aluminium) perform with very high
efficiencies by using these complexes as dopants.8 However,
the easy crystallization of these complexes due to the short
alkyl chain at the N atom of carbazole is unfavorable for the
fabrication of polymer-based devices because a crystalline
dopant can not disperse well with an amorphous host polymer,
which consequently gives rise to phase separation.9
temperatures ranging from 328 uC to 388 uC. DSC analysis
reveals the morphology of these complexes could be tuned by
the ligand structures. Complex 2a with short alkyl chain at the
N atom of carbazole is a crystalline solid, with no phase-
transition signal from 30 to 300 uC. On the contrary,
complexes 1–4 are amorphous solids, most likely due to the
introduction of the long and flexible n-decyl or 2-ethylhexyl
in the ligand frameworks.9a,12 1 and 2 undergo crystallization
at 206 and 105 uC, followed by melting transitions at 257 and
244 uC, respectively, however, only 1 shows the glass transition
at 110 uC. In contrast, both 3 and 4 with larger ligands all
exhibit glass transition, with significantly higher Tg (140 uC for
3, and 170 uC for 4) than 1. Furthermore, no crystallization
and melting transition are observed for 3 and 4 before
decomposition. This suggests that 3 and 4, with long alkyl
chain and bulky ligand structures, are more resistant to
crystallization.
Aiming to develop solution-processible phosphorescent
PLEDs, we designed and synthesized four new carbazole-
based iridium complexes, in which the long alkyl chain at the N
atom of carbazole was introduced to prevent crystallization
and improve the compatibility between the dopants and the
host polymers, consequently to suppress the phase separation.
In addition, the bulky ligand should also tend to diminish the
aggregation of dopants. The four complexes can be divided
into two types: one type is that the carbazole unit directly
ligates with iridium by the C-2 or C-3 positions of carbazole,
respectively; the other is that the carbazole unit does not
directly bond to iridium. This provides us with access to
evaluate the effect of coordinated or uncoordinated carbazole
on the emission and the charge-transporting ability of these
iridium complexes. By applying both non-conjugated polymer
(PVK) and conjugated polymer (PFO) as host polymers for the
four iridium-complex dopants, we have successfully fabricated
high-performance PLEDs. The thermal, electrochemical and
photophysical properties of these complexes will also be
discussed.
Electrochemistry
The electrochemical behavior of the complexes was examined
using cyclic voltammetry, and the electrochemical data are
given in Table 1. The four complexes all undergo a reversible
one-electron oxidation wave ranging from 0.00 to 0.38 V
during an anodic scan in CH2Cl2 (Fig. 2). These values fall
within those of Ir(ppy)2(acac) (ppy = 2-phenylpyridine) and
other analogues.13 The oxidations appearing at more positive
positions may be assigned to the oxidation of ligand carbazole.
A representative example for 3 is shown in Fig. 2. Cathodic
sweeps in THF exhibit irreversible reduction processes. Based
on the onset potentials of the oxidation and reduction, HOMO
and LUMO energy levels of these complexes were estimated
with regard to the energy level of ferrocene (4.8 eV below
vacuum).14 As shown in Table 1, the 3-position carbon of
carbazole-ligated complex 1 reveals a higher HOMO and lower
LUMO level than its isomeric 2-position-ligated complex 2.
This electrochemical behavior is in agreement with the
description of pyridine-localized LUMO and metal-involved
HOMO.15 The more electron-donating 3-position carbon of
carbazole will destabilize the metal d-orbital when ligating to
the iridium in 1, leading to a higher HOMO than 2 with the
less electronic 2-position carbon bonding. Simultaneously, the
connection of pyridine with carbon at the less electronic
2-position of the carbazole unit stabilizes the LUMO more
than the connection at the 3-position. The two factors work
together to narrow the energy gap of complex 1. Contrasting
to the striking difference in energy levels between 1 and 2, the
complexes 3 and 4 show almost the same HOMO and LUMO
levels because their carbazole parts do not directly bond to
iridium. The oxidation potentials of 3–4 raise significantly
compared to 1–2, attributing to the increasing ligand size. The
higher HOMO levels in 1 (24.71 eV ) and 2 (24.86 eV )
relative to 3 (25.03 eV ) and 4 (25.02 eV ) indicate that 1–2
have lower ionization potentials than 3–4, thus have
better hole-transporting abilities.5e The LUMO levels of 3–4
are similar to that of 1 and significantly lower than 2,
attributing to the strong electron-donating properties of
carbazole units connected at the phenyl ring of the phenyl-
pyridine ligand.
Results and discussion
Synthesis and characterization
As shown in Scheme 1, two ligands, 2-pyridinyl-N-decyl-
carbazole (2-PyDeCz) and 3-pyridinyl-N-decylcarbazole
(3-PyDeCz) were prepared from the corresponding bromo-
substituted carbazole and 2-bromopyridine through the
Negishi cross-coupling reaction. The other two ligands, 2-(49-
(20-phenylpyridinyl))-N-(2-ethylhexyl)carbazole
(2-PhPyCz)
and 3-(49-(20-phenylpyridinyl))-N-(2-ethylhexyl)carbazole (3-
PhPyCz) were prepared from the corresponding carbazole
boronic acid and 2-(4-bromophenyl)pyridine via the Suzuki
cross-coupling reaction. The cyclometalated iridium complexes
were synthesized in two steps:10 the ligands were first reacted
with iridium trichloride hydrate to give the corresponding
cyclometalated m-chloro-bridged dimers, then subsequent
treatment of the dimers with acetylacetone in the presence of
Na2CO3 afforded the desired complexes 1–4.
All the complexes were fully characterized by NMR,
1
elemental analysis and mass spectroscopy. The H NMR data
of the iridium complexes reveal the symmetry in the com-
plexes, with the iridium centers coordinated by the two cyclo-
metalated ligands with cis-C,C and trans-N,N conformation.11
Thermal analysis
The thermal properties of these complexes were investigated
by TGA (thermal gravimetric analysis) and DSC (differential
scanning calorimetry) (Fig. 1 and Table 1). All complexes
exhibit good thermal stability with 5% weight loss
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Scheme 1 Synthesis of the complexes.
Photophysical properties
(extended to ca. 550 nm) than the other three complexes
(extended to ca. 500 nm).
The absorption spectra of 1–4 in dichloromethane solution are
shown in Fig. 3. The intense absorptions in the ultraviolet
region are assigned to transitions of ligand-centered states with
mostly spin-allowed 1p-p* characters because their energies
and extinction coefficients are comparable to those of the
corresponding free ligands. The absorption bands of low
energy that extend to the visible region are conventionally
assigned to metal-to-ligand charge-transfer bands (including
1MLCTand 3MLCT) and 3p-p* transitions.1g,13b It is note-
worthy that 1 exhibits significantly lower energy absorptions
The photoluminescence (PL) spectra of 1–4 in CH2Cl2 are
shown in Fig. 4. Complex 1 emits orange–red light with
emission peak at 594 nm, while 2–4 emits green or yellow–
green light with an emission maximum ranging from 511 to
548 nm. The notable difference of emission wavelengths
between 1 and 2–4 is consistent with the discrepancy of their
low energy absorptions, implying the emissions come from the
triplet excited state phosphorescence. In addition, the long
luminescence decay lifetimes of these complexes dispersed in
PVK film that fall between 1.57 to 3.22 ms (Table 2) indicate
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Fig. 3 Normalized absorption spectra of 1–4 in dichloromethane
solution.
Fig. 1 DSC scans of 1–4. Tg = glass-transition temperature, Tc
crystallization temperature, Tm = melting temperature.
=
Fig. 2 Cyclic voltammogram of 1–4 in dichloromethane solution at
298 K. As an example, the ligand-centered oxidations for 3 are shown.
Fig. 4 Normalized PL spectra of 1–4 in dichloromethane solution.
3MLCT excited state. Differently, the emission spectra of 3–4
show vibrational fine structures, indicating that these com-
plexes emit from a mixed 3MLCT–3p-p* state with more 3p-p*
character than those in 1–2, owing to the increasing ligand
size.1g,16
that their emitting states have triplet characters. The remark-
able tuning (83 nm) is observed between the linkage isomers 1
and 2 where the carbazole unit directly ligates to the iridium.
Contrasting to this, the offset of emission maxima is only 8 nm
between the linkage isomers 3 and 4 where the carbazole unit
does not directly bond to iridium. The above alteration of
emission wavelength is in good correlation with the variation
of energy gap evaluated from the results of cyclic voltammetry
(vide supra). The emission spectra of 1–2 exhibit broad and
featureless characters, suggesting that the lowest excited triplet
states of these complexes are likely to be dominated by the
Polymer light-emitting devices
To evaluate the electroluminescent performance of these
iridium complexes in PLEDs, the single-active-layer
devices using them as dopant emitters were fabricated with
both non-conjugated polymer (PVK) and conjugated polymer,
Table 1 Thermal and electrochemical data for the complexes
Complex
Tda/uC
Phase-transition temperature/uC (Tg/Tc/Tm)b
E1/2ox/Vc
HOMO/eVd
LUMO/eVe
Eg/eVf
1
2
3
4
a
351
328
375
388
110/206/257
—/105/244
140/—/—
167/—/—
b
0.00
0.15
0.34
0.38
24.71
24.86
25.03
25.02
22.31
22.04
22.35
22.38
2.40
2.82
2.68
2.54
Td = Temperature at which 5% weight of sample is lost. Tg = glass-transition temperature; Tc = crystallization temperature; Tm = melting
temperature. Oxidation potential versus Fc/Fc+. Determined from the onset of oxidation potentials. Determined from the onset of
reduction potential. Determined from the difference between HOMO and LUMO levels.
c
d
e
f
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Table 2 Photophysical data for complexes
Complex
l
abs/nm (log e)a
l
em/nma
Wb
t/msc
l
EL/nmd
CIE (x, y)d
1
2
3
4
a
289 (5.0), 344 (5.0), 478 (3.8), 518 (3.9)
323 (5.2), 420 (4.0), 453 (3.9)
300 (1.9), 341 (1.9), 450 (1.0)
269 (3.4), 332 (2.7), 454 (0.9)
b
594
511
548
540
0.24
0.31
0.26
0.32
3.22
1.57
3.04
2.87
608
524
554 (592)
544 (582)
c
0.62, 0.38
0.35, 0.62
0.41, 0.57
0.38, 0.60
Measured in dichloromethane at 298 K. Measured in degassed dichloromethane solution by integrating sphere. Phosphorescence lifetime
d
of 1–4 dispersed in PVK film. Measured from PLEDs.
polyhedral oligomeric silsesquioxane-terminated poly(9,99-
dioctylfluorene) [PFO(poss)], as the host materials.
524 nm with CIE coordinates of (0.35, 0.62). The two emission
peaks display bathochromic shifts of 13 and 14 nm,
respectively, relative to their PL emissions in dichloromethane
solution (Table 2). The emission maxima of EL devices based
on 3 and 4 show a little red-shifts compared to their PL,
however, the intensities of the lower energy shoulders observed
in the solution PL spectra significantly increased in the EL
devices (Fig. 6).
Fig. 5a shows the device configurations in which PVK acted
as a host material for the complexes. When PFO acted as host
material, an additional layer of PVK was used on the top of
PEDOT–PSS as a hole-injection layer because PFO(poss) has
a HOMO level at 25.77 eV (Fig. 5b). In all the devices, PBD
was blended into the host material to increase the electron-
transporting ability with concentration of 40 wt.% and 30 wt.%
in PVK and PFO(poss), respectively. The doping concentra-
tion of iridium complexes varied from 2–4 wt.% in each type of
devices.
Fig. 7 shows the external quantum efficiency (QEext) and
luminance as a function of current density (J) for all the
devices. Table 3 summarizes the operating conditions and the
characteristics of these devices. For red-emitting complex 1,
the devices with 2 wt.% doping concentration display better
performances than those with 4 wt.% doping level where the
PVK or PFO(poss) is applied as the host polymer. The best
performance was achieved in the device based on 2 wt.%
iridium complex doping into PFO–PBD(30 wt.%), in which a
maximum brightness of 6402 cd m22 at J = 224 mA cm22, a
The devices based on 1 exhibit red emissions at 608 nm with
CIE (Commission Internationale de I’Eclairage ) coordinates
of (0.62, 0.38), and those based on 2 show green emission at
maximum external quantum efficiency of 6.4% and
a
luminance efficiency (LE) of 6.00 cd A21 at J = 14 mA cm22
were obtained. For green- or yellow–green-emitting complexes
2–4, the devices with 4 wt.% doping concentration exhibit
significantly better performances than those with 2 wt.%
doping ratio where the PVK or PFO(poss) hosts the
phosphorescent complexes. This phenomenon suggests that
the devices with red-emitting complex as phosphorescent dyes
are easier to get concentration saturation than those with
green- or yellow–green-emitting complexes. The devices with
4 wt.% iridium complexes 2–4 doping exhibited maximum
QEext in the range of 9.4 to 9.9%, maximum LE ranging from
21.2 to 22.4 cd A21, and maximum brightness between 11 845
Fig. 5 The configuration of devices (a) and (b), as well as the
molecular structures of the host.
Fig. 6 EL spectra of PLEDs using the iridium complexes as dopants.
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Fig. 7 The luminance and external quantum efficiency versus current density of PLED devices using complex 1 (a), 2 (b), 3 (c), and 4 (d) as
phosphorescent dopant.
and 18 730 cd m22. These performance data are amongst the
best green- and yellow–green-emitting PLEDs based on
phosphorescent dyes under similar device configura-
tions.2b,3f,17 When evaluating the device with different host
polymers, the devices with PFO(poss) as host material reveal
comparable or even better performance than those with PVK
as host at the same doping concentration. In most work, the
conjugated polymer-hosted phosphorescent PLEDs generally
have lower quantum efficiencies than non-conjugated polymer,
PVK,2 because of phosphorescence quenching by conjugated
polymers with a low-energy triplet state.18 We previously
reported that high-efficiency red-phosphorescent PLEDs can
be achieved with both non-conjugated polymers and con-
jugated polymers as the host materials doped with iridium(III)
bis(2-phenylquinolyl-N,C29) acetylactonate.3d Here we demon-
strated that green-, and yellow–green-emitting iridium com-
plexes could also be compatible with both non-conjugated and
conjugated host polymers for highly efficient PLEDs. This
implies that host quenching is only one of the important
factors limiting emission efficiency if the triplet energy level of
PFO(poss) is not significantly different from that of PFO
homopolymer (y2.1 eV).18 In addition, as proposed by
Sudhakar et al.,18 the phosphorescent quenching rate might
be reduced in the solid state due to reduced intermolecular
interactions between host and guest molecules in the solid film.
For comparison, the devices based on complexes 1a and 2a
with short alkyl chains at the N atom of carbazole were also
fabricated. The PL and EL spectra of 1a and 2a are almost the
same with their analogues 1 and 2 (see ESI,{ Fig. S2 and S3),
respectively, indicating that the lengths of the alkyl chains have
no effect on the emission energy. The external quantum
efficiency and luminance versus current density for the
comparative devices are shown in Fig. 8. It is obvious that
the device performance based on 1 and 2 are significantly
better than their short chain analogues 1a and 2a under the
identical device conditions, respectively. This suggests that the
increased solubility and morphology stability of 1 and 2
imparted by long chains favor the formation of homogeneous
film, and improve the compatibility between the dopants and
the host polymer PVK, resulting in high device performance.
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Table 3 Summary of the fabrication conditions and the characteristics of the PLEDs
1a
2a
1
2
2 wt.%
PVK
2 wt.%
PFO
4 wt.%
PFO
2 wt.%
PVK
4 wt.%
PVK
4 wt.%
PVK
2 wt.%
PFO
4 wt.%
PFO
2 wt.%
PVK
4 wt.%
PVK
J/mA cm22 a
L/cd m22 a
15.2
373
2.40
3.60
149
2088
1.40
2.10
14.3
863
2.10
100
3.20
202
40.0
1451
3.63
3.75
45.1
1633
3.62
3.75
3.90
563
14.3
7.40
134
9899
7.40
3.80
0.28
31.2
11.1
6.43
169.9
9077
5.34
3.03
2.70
578
5.55
473
2.50
534
LEmax/cd A21
QEext,max (%)
J/mA cm22 b
Lmax/cd m22
LE/cd A21 b
QEext (%)b
6.00
6.42
224.3
6402
2.85
3.06
4.76
4.99
74.5
2090
2.80
2.91
6.31
6.59
76.9
3409
4.43
4.59
21.4
9.56
71.7
9367
13.1
5.93
8.52
4.83
88.4
5717
6.47
3.66
21.4
9.60
72.2
11845
16.4
7.44
3
4
2 wt.%
PFO
4 wt.%
PFO
2 wt.%
PVK
4 wt.%
PVK
2 wt.%
PFO
4 wt.%
PFO
2 wt.%
PVK
4 wt.%
PVK
J/mA cm22 a
L/cd m22 a
LEmax/cd A21
QEext,max (%)
J/mA cm22 b
Lmax/cd m22
LE/cd A21 b
QEext (%)b
a
5.30
958
18.1
8.00
197
13644
6.90
3.06
b
10.5
2115
20.1
8.94
178
16539
9.30
4.12
18.5
2503
13.5
5.99
187
11816
6.30
2.78
23.0
4884
21.2
9.41
202
18680
9.20
4.07
9.30
1949
21.0
9.30
196
19692
10.0
4.50
8.10
1812
22.4
9.90
191
18730
9.80
4.30
13.3
2053
15.4
6.80
142
10673
7.50
3.30
25.0
4666
18.6
8.30
198
16734
8.50
3.70
The data at maximum QEext
.
The data at maximum brightness.
display better device performance than those based on their
short chain analogues. We also note that the highly efficient
PLEDs with green to red emission can be achieved where the
non-conjugated PVK or conjugated PFO(poss) was applied as
the host matrixes.
Experimental
General information
2-Bromo-N-decyl-carbazole and 3-bromo-N-decyl-carbazole
were synthesized according to the literature procedures.19 The
preparation of the complexes 1a and 2a was reported in our
previous paper.8 PFO(poss) was kindly supplied by American
Dye Sources Inc. PBD was purchased from Aldrich.
2-Bromopyridine, n-butyllithium, anhydrous zinc chloride and
tetrakis(triphenylphosphine)palladium were purchased from
Acros. Solvents were dried using standard procedures. 1H
NMR spectra were measured on a MECUYR-VX300 spectro-
meter in CDCl3 using tetramethylsilane as an internal reference.
Elemental analyses of carbon, hydrogen, and nitrogen were
performed on a Carlorerba-1106 microanalyzer. Mass spectra
were measured on a ZAB 3F-HF mass spectrophotometer. UV-
Vis absorption spectra were recorded on a Shimadzu 160A
recording spectrophotometer. PL spectra were recorded on a
Hitachi F-4500 fluorescence spectrophotometer. The PL quan-
tum yields were measured from dilute dichloromethane solutions
of complexes 1–4 (ca. 1026 mol L21) by an absoluted method
using the Edinburgh Instruments integrating sphere excitated
with a Xe lamp. The photoluminescence lifetimes was recorded
on a single-photon-counting spectrometer from Edinburgh
Instruments (FLS920) with a hydrogen-filled pulse lamp as the
excitation source. The data were analyzed by iterative convolu-
tion of the luminescence decay profile with the instrument-
response function using the software package provided by
Fig. 8 Comparison of the luminance and external quantum efficiency
versus current density of devices using 1a, 1, 2a, and 2 as dopant.
Conclusion
In conclusion, we have developed a series of morphology-
stable carbazole-based iridium complexes for solution-proces-
sible phosphorescent PLEDs. For the linkage isomeric
complexes with the carbazole carbon directly ligated with
iridium, their energy levels and emissions can be significantly
tuned, whereas these values show only a little difference
between the linkage isomeric complexes with the carbazole
carbon not bonded to iridium. Further more, the former shows
better hole-transporting ability than the latter. By introducing
the long and bulky alkyl chain in the ligand frame, the
morphology stability of the complexes and the compatibility
between the complexes and polymer host can be improved; as
a consequence, the PLEDs using these complexes as emitters
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Edinburgh Instruments. Differential scanning calorimetry
(DSC) was performed on a NETZSCH DSC 200 PC unit at a
heating rate of 10 uC min21 from 30 to 300 uC under argon.
Thermogravimetric analysis (TGA) was undertaken with a
NETZSCH STA 449C instrument. The thermal stability of
the samples under a nitrogen atmosphere was determined
by measuring their weight loss while heating at a rate of
20 uC min21 from 25 to 600 uC.
(0.356 g, 1.52 mmol), Pd(PPh3)4 (3% mmol, 10 mg) and
Na2CO3 (15 mmol, 1.59 g) in 15 ml of toluene and 5 ml of
distilled water was stirred at 70 uC for 24 h. After reaction, the
resulting mixture was poured into water and extracted with
anhydrous ethyl ether. The crude product was purified
by chromatography using dichloromethane–petroleum ether
(1 : 1) as eluent. Yield: 46%. 1H NMR (CDCl3, 300 MHz)
d[ppm]: 8.73 (d, J = 4.8 Hz, 1H), 8.13 (t, J = 8.4 Hz, 4H), 7.85–
7.75 (m, 4H), 7.63 (s, 1H), 7.55–7.39 (m, 3H), 7.21 (m, 2H),
4.23 (d, J = 6.0 Hz, 2H), 2.12 (m, 1H), 1.27–1.43 (m, 8H), 0.85–
0.96 (m, 6H).
Electrochemistry
Cyclic voltammetry (CV) was carried out in nitrogen-purged
anhydrous THF or dichloromethane at room temperature with
a CHI voltammetric analyzer. Tetrabutylammonium hexa-
fluorophosphate (TBAPF6) (0.1 M) was used as the supporting
electrolyte. The conventional three-electrode configuration
consists of a platinum working electrode, a platinum wire
auxiliary electrode, and an Ag wire pseudo-reference electrode
with ferrocenium–ferrocene (Fc+/Fc) as the internal standard.
Cyclic voltammograms were obtained at scan rate of
100 mV s21. Formal potentials are calculated as the average
of cyclic voltammetric anodic and cathodic peaks.
Synthesis of 3-(49-(20-phenylpyridinyl))-N-(2-ethylhexyl)car-
bazole (3-PhPyCz). Ligand of 3-PhPyCz was synthesized
according to the same method as 2-PhPyC except using
3-boronic-N-(2-ethylhexyl)carbazole to replace 2-boronic-N-
(2-ethylhexyl)carbazole. Yield: 73%. 1H NMR(CDCl3,
300 MHz) d[ppm]: 8.73 (d, J = 4.2 Hz, 1H), 8.38 (s, 1H),
8.18 (d, J = 7.8 Hz, 1H), 8.13 (d, J = 8.4 Hz, 2H), 7.86–7.77 (m,
6H ), 7.51–7.40 (m, 2H), 7.28–7.24 (m, 2H), 4.20 (d, J = 5.4 Hz,
2H), 2.10 (m, 1H), 1.30–1.40 (m, 8H), 0.86–0.96 (m, 6H).
Synthesis of complexes. A mixture of ligand (1.42 mmol),
IrCl3?3H2O (0.21 g, 0.59 mmol), 2-ethoxyethanol (12 ml) and
distilled water (4 ml) was stirred under argon at 120 uC for 24 h.
Cooled to room temperature, then the precipitate was collected
by filtration and washed with water, ethanol and hexane
successively, and then dried in vacuum to give m-chloro-
bridged cyclometallated Ir(III) dimer. Then the dimer complex
(0.08 mmol), acetylacetone (0.24 mmol) and Na2CO3 (86 mg,
0.8 mmol) were dissolved in 2-ethoxyethanol (8 ml), and the
mixture was refluxed under argon at 100 uC for 16 h. After
cooling to room temperature, the precipitate was filtered
off and washed with water, ethanol and hexane. The crude
Compounds
Synthesis of 2-pyridinyl-N-decylcarbazole (2-PyDeCz).
2-Bromopyridine (0.47 ml, 4.7 mmol) was added to 7.5 ml of
anhydrous THF in a Schlenk tube filled with argon at 278 uC.
n-BuLi (2.5 M in hexane, 4.0 ml, 9.4 mmol) was added
dropwise. After this mixture had been stirred at 278 uC for
45 min, a solution of anhydrous ZnCl2 (1.28 g, 9.4 mmol) in
15 ml of THF was added slowly and stirred for 1.5 h at room
temperature. Then a solution of 2-bromo-N-decyl-carbazole
(1.79 g, 4.6 mmol) and Pd(PPh3)4 (55.3 mg, 0.05 mmol) in 15 ml
of THF was added, and the reaction mixture was refluxed
under an argon atmosphere for 16 h. After the mixture had
cooled to room temperature, a solution of NH4Cl (2.2 g,
41 mmol) in water (10 ml) was added. Then the mixture was
stirred for 15 min and extracted several times with CH2Cl2,
and dried over anhydrous Na2SO4. The pure product was
obtained after column chromatography on silica gel using
product was flash chromatographed through
a silica
column using CH2Cl2 as eluent to afford the desired iridium
complex.
Ir(2-PyDeCz)2(acac) (1). Yield: 58%. 1H NMR (CDCl3,
300 MHz) d[ppm]: 8.63 (d, J = 5.4 Hz, 2H), 8.03 (d, J = 8.1 Hz,
2H), 7.80 (t, J = 8.1 Hz, 2H), 7.60 (d, J = 7.2 Hz, 4H), 7.20–
7.09 (m, 4H), 6.93–6.85 (m, 6H), 5.23 (s, 1H), 4.65 (t, J =
7.2 Hz, 4H), 1.79 (s, 6H), 1.31–1.22 (m, 32H), 0.85 (t, J =
6.3 Hz, 6H). Anal. Calcd for C59H69IrN4O2 (%): C 66.95, H
6.57, N 5.29; Found: C 66.42, H 6.77, N 4.78. MS (FAB): m/z
1058 (M+).
1
chloroform–petroleum ether (1 : 10) as eluent. Yield: 65%. H
NMR (CDCl3, 300 MHz) d[ppm]: 8.74 (d, J = 4.5 Hz, 1H),
8.18–8.11 (m, 2H), 7.88–7.78 (m, 4H), 7.48–7.25 (m, 2H), 7.25
(t, J = 7.5 Hz, 2H), 4.32 (t, J = 7.5 Hz, 2H), 1.93 (m, 2H), 1.46–
1.25 (m, 14H), 0.89 (t, J = 6.3 Hz, 3H).
Synthesis of 3-pyridinyl-N-decylcarbazole (3-PyDeCz).
Ligand of 3-PyCz was synthesized according to the same
method as 1a except using 3-bromo-N-decyl-carbazole to
replace 2-bromo-N-decylcarbazole. Yield: 32%. 1H NMR
(CDCl3, 300 MHz) d[ppm]: 8.74 (d, J = 4.5 Hz, 1H), 8.19–
8.11 (m, 1.5 Hz, 2H), 7.86 (d, J = 7.8 Hz, 1H), 7.80 (t, J =
7.1 Hz, 1H), 7.51–7.40 (m, 4H), 7.23–7.18 (m, 2H), 4.35 (t,
J = 3.9 Hz, 2H), 1.89 (m, 2H), 1.35–1.23 (m, 14H), 0.88 (t, J =
4.5 Hz, 3H).
Ir(3-PyDeCz)2(acac) (2). Yield: 53%. 1H NMR (CDCl3,
300 MHz) d[ppm]: 8.56 (d, J = 5.7 Hz, 2H), 8.22 (d, J = 6.3 Hz,
2H), 7.99 (d, J = 8.1 Hz, 2H), 7.88 (d, J = 7.8 Hz, 2H), 7.74 (t,
J = 7.5 Hz, 2H), 7.24–7.20 (m, 4H), 7.11–6.99 (m, 6H), 5.25 (s,
1H), 3.83 (t, J = 7.2 Hz, 4H), 1.82 (s, 6H), 1.26–1.16 (m, 32H),
0.90 (t, J = 6.0 Hz, 6H). Anal. Calcd for C59H69IrN4O2 (%): C
66.95, H 6.57, N 5.29; Found: C 66.38, H 6.51, N 4.77. MS
(FAB): m/z 1058 (M+).
Synthesis of 2-(49-(20-phenylpyridinyl))-N-(2-ethylhexyl)car-
bazole (2-PhPyCz). A mixture of 2-boronic-N-(2-ethylhexyl)-
carbazole (0.536 g, 1.67 mmol), 2-(4-bromophenyl)pyridine
Ir(2-PhPyCz)2(acac) (3). Yield: 87%. 1H NMR (CDCl3,
300 MHz) d[ppm]: 8.62 (d, J = 5.4 Hz, 2H), 7.97 y 8.05 (m,
4H), 7.9 (d, J = 5.1 Hz, 2H), 7.75 (t, J = 7.2 Hz, 2H), 7.65 (d,
3458 | J. Mater. Chem., 2007, 17, 3451–3460
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View Article Online
Appl. Phys. Lett., 2001, 79, 156; (g) S. Lamansky, P. Djurovich,
D. Murphy, F. Abdel-Razzaq, H. E. Lee, C. Adachi, P. E. Buttows,
S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 2001, 123,
4304; (h) S.-C. Lo, G. J. Richards, J. P. J. Markham, E. B. Namdas,
S. Sharma, P. L. Burn and I. D. W. Samuel, Adv. Funct. Mater.,
2005, 15, 1451; (i) Y. J. Su, H. L. Huang, C. L. Li, C. H. Chien,
Y. T. Tao, P. T. Chou, S. Datta and R. S. Liu, Adv. Mater., 2003,
15, 884; (j) S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi,
M.-H. Ho, S.-F. Hsu and C. H. Chen, Adv. Mater., 2005, 17, 285.
2 (a) F. C. Chen, Y. Yang, M. E. Thompson and J. Kido, Appl. Phys.
Lett., 2002, 80, 2308; (b) X. Gong, J. C. Ostrowski, M. R. Robinson,
D. Moses, G. C. Bazan and A. J. Heeger, Adv. Mater., 2002, 14,
581; (c) X. Gong, J. C. Ostrowski, G. C. Bazan, D. Moses and
A. J. Heeger, Appl. Phys. Lett., 2003, 81, 3711.
3 (a) Y. Kawamura, S. Yanagida and S. R. Forrest, J. Appl. Phys.,
2002, 92, 87; (b) X. Gong, J. C. Ostrowski, G. C. Bazan, D. Moses,
A. J. Heeger, M. S. Liu and A. K. Y. Jen, Adv. Mater., 2003, 15,
45; (c) P. A. Lane, L. C. Palilis, D. F. O’Brien, C. Giebeler,
A. J. Cadby, D. G. Lidzey, A. J. Campbell, W. Blau and
D. D. C. Bradley, Phys. Rev. B: Condens. Matter, 2001, 68,
235206; (d) C. Y. Jiang, W. Yang, J. B. Peng, S. Xiao and Y. Cao,
Adv. Mater., 2004, 16, 537; (e) M. Suzuki, S. Tokito, F. Sato,
T. Igarashi, K. Kondo, T. Koyama and T. Yamaguchi, Appl. Phys.
Lett., 2005, 86, 103507; (f) X. Yang, D. C. Mu¨ller, D. Neher and
K. Meerholz, Adv. Mater., 2006, 18, 948.
4 R. A. Negres, X. Gong, J. C. Ostrowski, G. C. Bazan, D. Moses
and A. J. Heeger, Phys. Rev. B: Condens. Matter, 2003, 68, 115209.
5 (a) G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94; (b)
C. Wang, M. Kilitziraki, L.-O. Palsson, M. R. Bryce,
A. P. Monkman and I. D. W. Samuel, Adv. Funct. Mater., 2001,
11, 47; (c) S. Bettington, M. Tavasli, M. R. Bryce, A. Beeby, H. Al-
Attar and A. P. Monkman, Chem.–Eur. J., 2007, 13, 1423; (d)
K. T. Kamtekar, C. Wang, S. Bettington, A. S. Batsanov,
I. F. Perepichka, M. R. Bryce, J. H. Ahn, M. Rabinal and
M. C. Petty, J. Mater. Chem., 2006, 16, 3823; (e) R. A. Campos,
I. P. Kovalev, Y. Guo, N. Wakili and T. Skotheim, J. Appl. Phys.,
1996, 80, 7144; (f) M. R. Robinson, M. B. O’Regan and
G. C. Bazan, Chem. Commun., 2000, 1645; (g) W.-Y. Wong,
Z. He, S.-K. So, K.-L. Tong and Z. Lin, Organometallics, 2005, 24,
4079; (h) W.-Y. Wong, G.-J. Zhou, X.-M. Yu, H.-S. Kwok and
B.-Z. Tang, Adv. Funct. Mater., 2006, 16, 838; (i) W.-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; (j) Z. Liu, M. Guan,
Z. Bian, D. Nie, Z. Gong, Z. Li and C. Huang, Adv. Funct. Mater.,
2006, 16, 1441; (k) L. Chen, H. You, C. Yang, X. Zhang, J. Qin
and D. Ma, J. Mater. Chem., 2006, 16, 3332.
6 S. Lamansky, P. I. Djurovich, F. Abdel-Razzaq, S. Garon,
D. L. Murphy and M. E. Thompson, J. Appl. Phys., 2002, 92, 1570.
7 (a) J. F. Ambrose, L. L. Carpenter and R. F. Nelson,
J. Electrochem. Soc., 1975, 122, 876; (b) K. Brunner, A. V. Dijken,
H. Borner, J. J. Bastiaansen, N. M. Kiggen and B. M. Langeveld,
J. Am. Chem. Soc., 2004, 126, 6035; (c) A. Chen, J. Liao, Y. Liang,
M. O. Ahmed, H. Tseng and S. Chen, J. Am. Chem. Soc., 2003,
125, 636; (d) J.-F. Morin, N. Drolet, Y. Tao and M. Leclerc, Chem.
Mater., 2004, 16, 4619.
J = 5.1 Hz, 2H), 7.34–7.42 (m, 6H), 7.14–7.22 (m, 8H), 6.68 (s,
2H), 5.27 (s, 1H), 4.11 (d, J = 3.9 Hz, 4H), 2.03 (m, 2H), 1.85
(s, 6H), 1.26–1.37 (m, 16H), 0.86–0.92 (m, 12H). Anal. Calcd
for C67H69IrN4O2 (%): C 69.70, H 6.02, N 4.85; Found: C
69.22, H 6.35, N 4.93. MS (FAB): m/z 1055 (M+ 2 acac).
Ir(3-PhPyCz)2(acac) (4). Yield: 74%. 1H NMR (CDCl3,
300 MHz) d[ppm]: 8.55 (d, J = 5.4 Hz, 2H ), 8.05–7.99 (m, 4H),
7.83 (d, J = 5.4 Hz, 2H), 7.75 (t, J = 7.8 Hz, 2H), 7.63 (d, J =
8.1 Hz, 2H), 7.40–7.48 (m, 6H), 7.36 (d, J = 8.1 Hz, 2H), 7.15–
7.20 (m, 6H), 6.57 (s, 2H ), 5.19 (s, 1H ), 4.04 (d, J = 3.6 Hz,
4H), 1.97 (m, 2H), 1.76 (s, 6H), 1.18–1.27 (m, 16H), 0.77–0.85
(m, 12H ). Anal. Calcd for C67H69IrN4O2 (%): C 69.70, H 6.02,
N 4.85; Found C 69.79, H 6.41, N 4.98. MS (FAB): m/z 1055
(M+ 2 acac).
PLED Fabrication and measurements
The device configuration was ITO/PEDOT(40 nm)/(PVK +
40% PBD)–Ir-complex (80 nm)/Ba(4 nm)/Al(120 nm) and ITO/
PEDOT(40 nm)/PVK(40 nm)/(PFO(poss) + 30% PBD)–Ir-
complex (80 nm)/Ba(4 nm)/Al(120 nm). The fabrication of
electrophosphorescent devices followed our previous proce-
dure.3d A 40 nm-thick layer of poly(ethylenedioxythiophene)–
poly(styrene sulfonic acid) (PEDOT–PSS) was spin-cast onto
pre-cleaned ITO-glass substrates. A mixture of Ir-complex
with host was spin-cast from a chlorobenzene solution (for
PVK + PBD) and p-xylene–chlorobenzene = 7 : 3 (for
PFO(poss) + PBD). The deposition speed and the thickness
of the barium and aluminium layers were monitored with a
thickness–rate meter model STM-100 (Sycon Instrument,
Inc.). Device fabrication was carried out in a controlled
atmosphere dry-box (Vacuum Atmosphere Co.) under N2
circulation. Current density(I)–voltage(V)–luminance(L) data
were collected using a Keithley 236 source measurement unit
and a calibrated silicon photodiode. External EL quantum
efficiencies ( QEext) were obtained by measuring the total light
output in all directions in an integrating sphere (IS-080,
Labsphere). The luminance (cd m22) and luminous efficiency
(cd A21) were measured by a silicon photodiode and calibrated
using
a PR-705 SpectraScan spectrophotometer (Photo
Research). Electroluminescence spectra were recorded using
a CCD spectrophotometer (Instaspec 4. Oriel).
Acknowledgements
8 (a) X. Zhang, C. Yang, L. Chen, K. Zhang and J. Qin, Chem. Lett.,
2006, 35, 72; (b) C. Yang, X. Zhang, H. You, L.-Y. Zhu, L. Chen,
L.-N. Zhu, Y. Tao, D. Ma, Z. Shuai and J. Qin, Adv. Funct.
Mater., 2007, 17, 651.
9 (a) F.-I. Wu, H.-J. Su, C.-F. Shu, L. Luo, W.-G. Diau,
C.-H. Cheng, J.-P. Duan and G.-H. Lee, J. Mater. Chem., 2005,
15, 1035; (b) A. J. Sandee, C. K. Williams, N. R. Evans,
J. E. Davies, C. E. Boothby, A. Kohler, R. H. Friend and
A. B. Holmes, J. Am. Chem. Soc., 2004, 126, 7041.
10 (a) M. Nonoyama, Bull. Chem. Soc. Jpn., 1974, 47, 767; (b)
M. Tavasli, S. Bettington, M. R. Bryce, A. S. Batsanov and
A. P. Monkman, Synthesis, 2005, 1619.
11 (a) K. A. King, P. J. Spellane and R. J. Watts, J. Am. Chem. Soc.,
1985, 107, 1431; (b) S. Sprouse, K. A. King, P. J. Spellane and
R. J. Watts, J. Am. Chem. Soc., 1984, 106, 6647; (c) L. Chen,
C. Yang, J. Qin, J. Gao, H. You and D. Ma, J. Organomet. Chem.,
2006, 391, 3519.
We thank the National Natural Science Foundation of China
(project no. 20474047 and 20371036) and the Program for New
Century Excellent Talents in University, the Ministry of
Education of China for financial support.
References
1 (a) M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151; (b)
Y. Cao, I. D. Parker and A. J. Heeger, Nature, 1999, 397, 414; (c)
M. Wohlgenannt, K. Tandon, S. Mazumdar, S. Ramasesha and
Z. V. Vardeny, Nature, 2001, 409, 494; (d) M. A. Baldo,
S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest,
Appl. Phys. Lett., 1999, 75, 4; (e) C. Adachi, M. A. Baldo,
S. R. Forrest and M. E. Thompson, Appl. Phys. Lett., 2000, 77,
904; (f) M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki and Y. Taga,
12 J. C. Ostrowski, M. R. Robinson, A. J. Heeger and G. C. Bazan,
Chem. Commun., 2002, 784.
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3451–3460 | 3459

View Article Online
13 (a) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq,
R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau and
M. E. Thompson, Inorg. Chem., 2001, 40, 1704; (b) J. Li,
P. I. Djurovich, B. D. Alleyne, M. Yousufuddin, N. N. Ho,
J. C. Thomas, J. C. Peters, R. Bau and M. E. Thompson, Inorg.
Chem., 2005, 44, 1713.
14 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Ba¨ssler,
M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
15 (a) P. J. Hay, J. Phys. Chem. A, 2002, 106, 1634; (b) J. Brooks,
Y. Babayan, S. Lamansky, P. I. Djurovich, I. Tsyba, R. Bau and
M. E. Thompson, Inorg. Chem., 2002, 41, 3055.
16 (a) 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; (b) P. Spellane, R. J. Watts and A. Vogler, Inorg. Chem.,
1993, 32, 5633.
17 J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing and
F. Wang, Adv. Funct. Mater., 2006, 16, 575.
18 (a) M. Sudhakar, P. I. Djurovich, T. E. Hogen-Esch and
M. E. Thompson, J. Am. Chem. Soc., 2003, 125, 7796; (b)
F. C. Chen, G. He and Y. Yang, Appl. Phys. Lett., 2003, 82, 1006.
19 A. Lu¨tzen and M. Hapke, Eur. J. Org. Chem., 2002, 2292.
3460 | J. Mater. Chem., 2007, 17, 3451–3460
This journal is ß The Royal Society of Chemistry 2007