Bipolar heteroleptic green iridium dendrimers containing oligocarbazole and oxadiazole dendrons for bright and efficient nondoped electrophosphorescent devices

Article abstract of DOI:10.1002/asia.201100016

Bipolar heteroleptic green light-emitting iridium (Ir) dendrimers G(OXD) and G(DOXD) have been designed and synthesized under mild conditions in high yields, in which the first C^N and second O^O ligands are functionalized with oligocarbazole- and oxadiazole-based dendrons, respectively. To avoid affecting the optical properties of the emissive iridium core, all the functional moieties are attached to the ligands through a flexible spacer. Compared with the unipolar dendrimer G(acac), dendrimers G(OXD) and G(DOXD) exhibit the close emission maxima of 511-512 nm and photoluminescence quantum yield of 0.39-0.40 in a solution of toluene. Moreover, on going from G(acac) to G(OXD) and G(DOXD), we have found that the introduction of oxadiazole fragments decreases the lowest unoccupied molecular orbital (LUMO) energy levels to facilitate the electron injection and electron transporting, while their highest occupied molecular orbital (HOMO) energy levels remain unchanged. This means that, we can individually tune the HOMO and LUMO energy levels based on the heteroleptic structure to ensure the relative independence between the hole and electron in the emitting layer (EML), which is a favorable feature for bipolar optoelectronic materials. As a result, a bilayer nondoped electrophosphorescent device with G(DOXD) as the EML gives a maximum luminous efficiency of 25.5 cd A^-1 (I·_ext: 7.4 %) and a brightness of 33 880 cd m^-2. In comparison to G(acac) (17.2 cd A^-1, 17 680 cd m^-2), both the efficiency and brightness are improved by about 1.5 and 2 times, respectively. These state-of-the-art performances indicate the potential of these bipolar heteroleptic iridium dendrimers as solution-processible emitting materials for nondoped device applications. Copyright

Full text of DOI:10.1002/asia.201100016

FULL PAPERS
DOI: 10.1002/asia.201100016
Bipolar Heteroleptic Green Iridium Dendrimers Containing Oligocarbazole
and Oxadiazole Dendrons for Bright and Efficient Nondoped
Electrophosphorescent Devices
Lingcheng Chen,[a, b] Junqiao Ding,*[a] Yanxiang Cheng,[a] Zhiyuan Xie,[a]
Lixiang Wang,*[a] Xiabin Jing,[a] and Fosong Wang[a]
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Chem. Asian J. 2011, 6, 1372 – 1380

Abstract: Bipolar heteroleptic green
light-emitting iridium (Ir) dendrimers
going from G
GACHTUNGRNET(NUGN DOXD), we have found that the in-
(acac) to G
E
tween the hole and electron in the
emitting layer (EML), which is a favor-
able feature for bipolar optoelectronic
materials. As a result, a bilayer non-
doped electrophosphorescent device
GG
N
troduction of oxadiazole fragments de-
creases the lowest unoccupied molecu-
lar orbital (LUMO) energy levels to fa-
cilitate the electron injection and elec-
tron transporting, while their highest
occupied molecular orbital (HOMO)
energy levels remain unchanged. This
means that, we can individually tune
the HOMO and LUMO energy levels
based on the heteroleptic structure to
ensure the relative independence be-
designed and synthesized under mild
conditions in high yields, in which the
first C^N and second O^O ligands are
functionalized with oligocarbazole- and
oxadiazole-based dendrons, respective-
ly. To avoid affecting the optical prop-
erties of the emissive iridium core, all
the functional moieties are attached to
the ligands through a flexible spacer.
Compared with the unipolar dendrimer
with GACTHNUTRGNE(NUG DOXD) as the EML gives a
maximum luminous efficiency of
25.5 cdAꢀ1 (hext: 7.4%) and a bright-
ness of 33880 cdmꢀ2. In comparison to
GACTHNUTRGNEUNG
(acac) (17.2 cdAꢀ1, 17680 cdmꢀ2),
both the efficiency and brightness are
improved by about 1.5 and 2 times, re-
spectively. These state-of-the-art per-
formances indicate the potential of
these bipolar heteroleptic iridium den-
drimers as solution-processible emitting
materials for nondoped device applica-
tions.
GN
ACHTUNGTRENUN(NG OXD) and G-
ACHTUNGTRENNUNG
Keywords: bipolar · dendrimers ·
iridium · nondoped devices · heter-
oleptic ligands
maxima of 511–512 nm and photolumi-
nescence quantum yield of 0.39–0.40 in
a solution of toluene. Moreover, on
Introduction
um core as the dendron shell, and it plays the same role as
the host. Through tuning the dendron generation, we ob-
tained a maximum luminous efficiency of 34.7 cdAꢀ1 for the
second generation green dendrimer G2 based on nondoped
device configuration.[7] Moreover, by increasing the number
of dendrons, the efficiency was further improved to
45.7 cdAꢀ1, which is close to that of the doped devices.[9]
However, the synthesis of all the above-mentioned homo-
leptic iridium dendrimers required high reaction tempera-
tures and the yields were very low, especially for higher gen-
eration dendrimers, for example, G2 was obtained in only
8% yield.[7] In contrast, according to the literature,[10] the
heteroleptic dendrimers were easily synthesized under mild
conditions with high yields. By considering the large-scale
preparation and the practical application, it is highly desira-
ble to develop heteroleptic iridium dendrimers suitable for
nondoped devices.
Organic light-emitting diodes (OLEDs) based on phosphor-
escent dyes have entered the commercial stage owing to
their prominent market share in large-area flat-panel-display
technology,[1] as they can harvest both singlet and triplet ex-
citons to realize a theoretical internal quantum efficiency of
100%.[2] Among these dyes, iridium (Ir) complexes are ex-
tensively studied because of their high photoluminescence
quantum yields and appropriate exciton lifetimes.[3] To ach-
ieve high performance, doped electrophosphorescent devi-
ces are fabricated, in which the iridium complexes are dis-
persed into the host matrix.[4] Although this physical blend-
ing technique is usually effective, the inevitable phase segre-
gation could deteriorate the device stability and lifetime.[5]
To solve this problem, nondoped devices are employed, in
which the host is no longer needed in the EML.
With this aim in mind, the iridium complexes are cova-
lently attached to the polymeric host to form electrophos-
phorescent polymers.[6] Unfortunately, the device efficiency
is still much lower than that of the corresponding physical
blending counterparts. As an alternative, bifunctional phos-
phorescent iridium dendrimers with a “self-host” feature
have been recently developed by our research group.[7–9]
Here oligocarbazole is incorporated into the emissive iridi-
In addition, bipolar hosts containing suitable electron-rich
and electron-deficient moieties in one molecule have been
intensively studied by several groups,[11] as they are believed
to be beneficial to maintain the charge balance in the emis-
sive layer so as to improve the device efficiency accompa-
nied by a small roll-off. With the same concept, bipolar
phosphorescent heavy-metal complexes, such as PtII, RuII,
and ReI complexes,[12,13] are also explored. However, until
now few endeavors have been paid to the development of
bipolar iridium dendrimers. In this study, we report the ef-
fective synthesis of novel bipolar heteroleptic green iridium
dendrimers and their application in nondoped electrophos-
[a] L. Chen, Dr. J. Ding, Prof. Y. Cheng, Prof. Z. Xie, Prof. L. Wang,
Prof. X. Jing, Prof. F. Wang
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022 (P. R. China)
Fax : (+86)431-85685653
phorescent devices. For the bipolar dendrimer GACTHNUTRGNEUG(N DOXD), a
promising efficiency as high as 25.5 cdAꢀ1 together with a
high brightness of 33880 cdmꢀ2 is realized. Compared to
that of the corresponding unipolar dendrimer GACHTUNGTRENNUNG(acac)
E-mail: junqiaod@ciac.jl.cn
(17.2 cdAꢀ1, 17680 cdmꢀ2), both the efficiency and the
brightness are improved by about 1.5 and 2 times, respec-
tively.
lixiang@ciac.jl.cn
[b] L. Chen
Graduate School of Chinese Academy of Sciences
Beijing 100039 (P. R. China)
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L. Wang et al.
FULL PAPERS
Results and Discussion
formed and then was successfully treated with the dimer to
afford the bipolar heteroleptic dendrimers. Notably, this re-
action could be performed at room temperature with a yield
over 90%, which is much higher than that of the homoleptic
iridium dendrimers.[18] This made it possible to produce het-
eroleptic iridium dendrimers with higher yields on gram
Synthesis
Scheme 1 depicts the functionalization procedure of the first
C^N and second O^O ligands. As for the first C^N ligand,
the second generation carbazole dendron 2 was linked to 2-
phenyl-1H-benzoimidazole (1) through a flexible spacer,
which was prepared by condensation of ortho-phenylenedia-
mine with benzoic acid.[14] As for the second O^O ligand,
the oxadiazole moiety was introduced to one or both ends
of acetylacetone through a reported modular synthetic
methodology.[15] It is noteworthy that such functional entities
and ligands are combined through a nonconjugated bond so
as to weaken the electronic coupling between them, and
thus retain the optical property of the iridium complex core.
Furthermore, according to the literature,[16] the emission is
mainly from the metal to the first C^N ligand transitions for
the heteroleptic iridium complexes. This means that this het-
eroleptic dendritic skeleton, in which the hole- and electron-
transporting units are individually incorporated into the dif-
ferent ligands, can ensure the relative independence be-
tween the hole and electron transporting in the emitting
layer. This feature is very favorable, especially for the bipo-
lar optoelectronic materials.
scale. For comparison, the model compound GACTHNUTRGENUG(N acac) was
also synthesized. Additionally, the surface of all the den-
drimers was decorated with tert-butyl groups to ensure their
solubility. For instance, the dendrimers were highly soluble
in common organic solvents, such as chloroform, chloroben-
zene, toluene, and tetrahydrofuran at room temperature.
From these solutions, high-quality films could be achieved
by spin-coating. Furthermore, all dendrimers were thermally
stable with the decomposition temperature (Td) as high as
332–3708C. The structures of the dendrimers were verified
by using 1H NMR spectroscopy, elemental analysis, and
matrix-assisted laser desorption-ionization time-of-flight
(MALDI-TOF) mass spectrometry.
Photophysical and Electrochemical Properties
Figure 1 shows the absorption spectra in dichloromethane
and the photoluminescence (PL) spectra of the iridium den-
drimers in toluene, and the related photophysical properties
The synthesis of the bipolar heteroleptic dendrimers G-
ACHTUNGTRENNUNG(OXD) and GACHTUNGTRENNUNG(DOXD) was carried out in a modified two-
step approach, as illustrated in Scheme 2. IrCl3·nH2O was
first treated with an excess of the first ligand LG to form a
chloride-bridged dimer in a solvent mixture of 2-ethoxyetha-
nol, water (H2O), and tetrahydrofuran (THF). In the second
step, we initially tried the typical reaction conditions by
using Na2CO3 as the base,[17] however, we could not obtain
the desired product, as the nitrogen atom in the oxadiazole
ring could affect this ligation reaction. Therefore, we modi-
fied the synthetic route, as illustrated in Scheme 2. After
treatment of the second ligand with one equivalent of
sodium tert-butoxide (tBuONa) in a mixed solvent of etha-
nol and dichloromethane (DCM), the monoanion was
Figure 1. The absorption spectra (10ꢀ5 m in CH2Cl2 solution) and the PL
spectra (10ꢀ5 m in toluene solution) of G
ACTHUNRTGENN(GU acac) (c), GACHTUNGTRENN(UGN OXD) (b),
and GACTHNUTRGNE(NUG DOXD) (g).
Abstract in Chinese:
are displayed in Table 1. All of the dendrimers show two
major absorption bands. The absorption bands below
330 nm are attributed to the spin-allowed ligand-centered
(LC) transitions and the weak absorption shoulders in the
range of 330–500 nm are assigned to the metal-to-ligand
charge-transfer (MLCT) transitions of the iridium com-
plexes. Obviously, all the dendrimers display almost identi-
cal spectral features, except for the bands around 300 nm;
these bands are ascribed to oxadiazole subchromophores,
the intensity of which increases gradually in the order G-
A
ACHUTGTNREN(UNG OXD)<GCAHTUNGTRENN(UGN DOXD).
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Green Iridium Dendrimers
Scheme 1. Synthesis of the dendritic ligands LG, OXDacac, and DOXDacac. PPA=polyphosphoric acid.
cence spectra measured at 77 K are close to the emissive iri-
dium core. These results demonstrate that the introduction
of oligocarbazole and oxadiazole dendrons through a flexi-
ble spacer does not alter the optical property of the emissive
iridium core, as discussed before. In addition, the intensity
emissive iridium cores. However, there is still aggregation to
some degree, because a red shift of 7–9 nm from solution to
film for the emission peak is observed (Table 1). Moreover,
the lifetimes increase gradually with the number of oxadia-
zole units; this indicates the effective tuning of intermolecu-
lar interactions.
of the shoulder around 540 nm for G
ACHTUNGTREN(NUGN DOXD) is lower than
that of G(acac), indicative of a reduced interaction between
ACHTUNGTRENNUNG
Table 1. Photophysical and Electrochemical Properties of the Dendrimers.
[b]
labs [nm] (log e)[a]
lem [nm]/Fp
lem [nm][c] t1 [ms][d] t2 [ms][d] T1 [eV][e] Eg [eV][f] HOMO [eV][g] LUMO [eV][h]
G
G
G
N
239
347
239
348
G
G
N
G
520
520
519
0.06
0.08
0.13
0.32
0.51
0.58
2.44
2.44
2.44
2.43
2.43
2.43
ꢀ5.1
ꢀ5.1
ꢀ5.1
ꢀ1.8
ꢀ1.9
ꢀ2.1
348
[a] Measured in CH2Cl2 at 298 K at a concentration of 10ꢀ5 m. e is the absorption coefficient in the Beer–Lambert equation. [b] Measured in toluene at 298 K
at a concentration of 10ꢀ5 m and an excitation wavelength of 410 nm. [c] Neat-film data measured at 298 K. PL spectra were measured with an excitation
wavelength of 409 nm. [d] Measured in solid films at 298 K in the air and the lifetimes are obtained by biexponential fit of emission decay curves. [e] Esti-
mated from the highest energy peak of the phosphorescence spectra at 77 K. [f] Eg =The optical band gap estimated from the onset of the absorption edge.
[g] HOMO=ꢀe (4.8 V+Eox), where Eox was taken from the onset of the oxidation potential. [h] LUMO=ꢀe (4.8 V+Ered), where Ered was taken from the
onset of the reduction potential.
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L. Wang et al.
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Scheme 2. a) Synthesis of the bipolar heteroleptic dendrimers with G
ACHTUNGTRENNUNG
drimers GACHTUNGTRENNUNG(OXD) and GACHTUNGTRENNUNG(DOXD) together with the model compound GACTUHNGTRENNNUG
The electrochemical behaviors of dendrimers G
N
the second O^O ligand can facilitate efficient electron injec-
tion. Most importantly, it does not affect the hole injection
and transporting at the same time.
A
ACHTUNGTRENNUNG
try (CV), and the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels (Table 1) were calculated from the
onset of the oxidation and reduction potential, respectively.
Upon anodic sweeping in dichloromethane, as shown in
Figure 2, all dendrimers showed multiple oxidation waves.
The first oxidation wave located at the lowest potential was
reversible, and could be assigned to the oxidation of the iri-
dium-phenyl center. The other oxidation waves were irrever-
sible, and were related to the peripheral carbazole-based
dendrons. During the cathodic scan in acetonitrile, only irre-
versible reduction waves were found. In comparison to G-
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
from ꢀ1.8 eV to ꢀ1.9 and ꢀ2.1 eV, respectively, while their
HOMO energy level remained to be ꢀ5.1 eV. This observa-
tion suggests that the incorporation of an oxadiazole into
Figure 2. The electrochemical spectra of G
and G(DOXD) (g). a) Measured in CH2Cl2, and b) in acetonitrile
(scan rate=100 mVsꢀ1).
ACHUTGTNRENN(GU acac) c, GCAHTUNGTRENN(UGN OXD) (b),
AHCTUNGTRENNUNG
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Green Iridium Dendrimers
Electroluminescent Properties
The solution-processibility and bipolar nature of the den-
drimers render them suitable candidates for nondoped elec-
trophosphorescent devices. Therefore, the single layer devi-
ces with the configuration of indium tin oxide (ITO)/
Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)
(PEDOT:PSS) (50 nm)/EML (100 nm)/Ca (10 nm)/Al
(100 nm) (Structure A, Figure 3) were firstly fabricated, and
Figure 4. a) The current density-voltage-luminescence characteristics and
b) current density dependence of luminous efficiency as well as external
quantum efficiency for devices with structure A.
and a maximum brightness of 12,730 cdmꢀ2 for G
are obtained. Compared with
(acac) (2.8 cdAꢀ1,
ACHTUNGTRENNUNG(DOXD)
Figure 3. Schematic diagram of EL device configurations and the molecu-
lar structures of the relevant compounds used in these devices.
GACHTUNGTRENNUG
4810 cdmꢀ2), both the efficiency and brightness are greatly
enhanced due to the bipolar transporting nature. Nonethe-
less, we have noted that the performance is still very poor
relative to the multilayer device, which may result from the
large electron injection barrier (0.8 eV) between the EML
and the calcium cathode (2.9 eV).[19] Therefore, studies on
how to further lower the LUMO energy level of these bipo-
lar dendrimers are under investigation, and it is beyond the
focus of this article.
the device performances are listed in Table 2. Figure 4a and
4b show the current density-voltage-luminescence character-
istics, and the current density dependence of the luminous
efficiency together with the external quantum efficiency
(hext). Owing to the improved electron injection and trans-
porting, the luminous efficiency gradually increases when G-
ACHTUNGTRENNUNG(acac) is replaced with GACHTUNGTERG(NNUN DOXD). Consequently, G-
A
To optimize the nondoped device performance, bilayer
devices with the configuration of ITO/PEDOT:PSS (50 nm)/
dendrimers. A maximum luminous efficiency of 5.5 cdAꢀ1
Table 2. Performance of the devices using the dendrimers.
[c]
[d]
EML
Vturnꢀon [V]
Bmax [cd/m2][c]
hl [cdAꢀ1
]
hext [%][c]
hl [cdAꢀ1
]
hext [%][d]
lem [nm]
CIE [x, y]
G
G
G
G
G
G
N
5.1
5.8
5.7
3.5
4.0
4.1
4810
8740
12730
17680
26140
33880
2.8
3.7
5.5
17.2
19.6
25.5
0.8
1.1
1.6
5.0
5.7
7.4
–
3.7
5.5
13.3
16.4
19.8
–
520
520
520
520
520
520
(0.38, 0.58)
(0.37, 0.59)
(0.35, 0.60)
(0.36,0.60)
(0.34,0.60)
(0.34,0.60)
1.1
1.6
4.0
4.6
5.7
[a] Device data with structure A. [b] Device data with structure B. [c] The maximum values for brightness (Bmax), luminous efficiency (hl), and external
quantum efficiency (hext). [d] Data collected at a brightness of 5000 cdmꢀ2
.
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L. Wang et al.
FULL PAPERS
EML (45 nm)/TPBI (60 nm)/LiF (1 nm)/Al (100 nm) (Struc-
ture B, Figure 2) were prepared, in which 1,3,5-tris(2-N-phe-
nylbenzimidazolyl)-benzene (TBPI) acted as the electron-
transporting and hole-blocking material. As shown in
Figure 5, all these dendrimers exhibit strong green electrolu-
Figure 5. The EL spectra at a driving voltage of 9 V for devices with
structure B.
Figure 6. a) The current density-voltage-luminescence characteristics and
b) current density dependence of luminous efficiency as well as external
quantum efficiency for devices with structure B.
minescence (EL) at about 520 nm with similar Commission
Internationale de L’Eclairage (CIE) coordinates of (0.36,
0.59), (0.34, 0.60), and (0.34, 0.60) for GACHTUTNGERNNUG(acac), GHCATUNGTRNEN(UGN OXD),
and G(DOXD), respectively. The EL spectra are identical
ACHTUNGTRENNUNG
to their PL counterparts, thus indicating that the emission
occurs from the iridium core. Furthermore, their EL spectra
are independent of the applied voltages from 5 V to 14 V,
and no additional emission signals from aggregates or exci-
mers have been observed. This further proves that the “self-
host” feature makes it possible to use these multifunctional
light-emitting dendrimers alone as the EML for nondoped
devices.
As illustrated in Figure 6a, the turn-on voltage (at a
brightness of 1 cdmꢀ2) reduces from 5.1–5.8 V of the single-
layer devices to 3.5–4.1 V of the bilayer ones, and a decrease
of approximately 1.6–1.8 V is achieved because of the inser-
tion of an additional electron-transporting and hole-blocking
layer. Similar to the single-layer devices, both the luminous
efficiency and brightness of the bilayer devices are improved
Conclusions
In summary, we have demonstrated a novel approach, which
is the integration of hole-transporting carbazole-based den-
drons, electron-transporting oxadiazole-based dendrons, and
an iridium complex core to form heteroleptic bipolar green
dendrimers for nondoped electrophosphorescent device ap-
plications. Compared with the unipolar dendrimer G
both the luminous efficiency and brightness of the bipolar
dendrimer G(DOXD) have been significantly improved. In
terms of the mild synthetic conditions, high yields, the state-
of-the-art luminous efficiency together with brightness, we
believe that this strategy would be suitable for the design
and synthesis of novel multifunctional heteroleptic bipolar
iridium dendrimers for bright and efficient nondoped elec-
trophosphorescent devices, which have an emission color
other than green for use in full-color OLED displays.
ACHTUNGTRENNUNG(acac),
AHCTUNGTRENNUNG
when G
luminous efficiency and brightness of G
1.5 and 2 times that of G(acac), respectively. As a conse-
quence, a maximum luminous efficiency of 25.5 cdAꢀ1, and
a maximum brightness of 33880 cdmꢀ2 for G
(DOXD) have
ACHTUNGTRENNUNG(acac) is replaced with GACTHUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
been realized. Considering the mild synthetic conditions as
well as the high yields, the performance of the bipolar den-
drimer GACHTUNGTRENNUNG(DOXD) is very promising. In particular, the effi-
Experimental Section
Synthesis
ciency still remains as high as 19.8 cdAꢀ1 when the bright-
ness increases to 5000 cdmꢀ2, indicative of a gentle roll-off
at high current density. To the best of our knowledge, this is
the first report of heteroleptic bipolar iridium dendrimers
being used for bright and efficient nondoped electrophos-
phorescent devices.
All chemicals and reagents were used as received from commercial sour-
ces without further purification. Solvents for chemical synthesis were pu-
rified according to the standard procedures. All chemical reactions were
carried out under an inert atmosphere. The intermediates 3,6,3’’,6’’-tetra-
kis-tert-butyl-9’H-[9,3’;6’,9’’]tercarbazole,[20]
2-phenyl-1H-benzoimida-
zole,[14] H-OXD, OXDacac, and DOXDacac[15] were prepared according
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Chem. Asian J. 2011, 6, 1372 – 1380

Green Iridium Dendrimers
to the literature procedures. TBPI was synthesized in our laboratory fol-
lowing a literature method.[21]
2H), 6.57 (t, J=7.5 Hz, 2H), 6.46 (d, J=7.5 Hz, 2H), 5.11 (s, 1H), 4.80
(s, 4H), 4.44–4.29 (m, 4H), 2.35–2.17 (m, 8H), 1.65 (s, 6H), 1.45 ppm (s,
72H); Anal. Calc. for C143H147N10O2Ir: C 77.02, H 6.64, N 6.28; found: C
76.98, H 6.69, N 6.12; MALDI-TOF (m/z): 2229.1 [M+].
2-phenyl-1H-benzoimidazole
A mixture of benzoic acid (12.2 g, 0.1 mol), ortho-phenylenediamine
(10.8 g, 0.1 mol), and polyphosphoric acid (PPA, 65 mL) was heated at
2108C for 6 h under argon, then permitted to cool to about 1008C, and
poured into rapidly stirred water (500 mL). The insoluble residue was
collected by filtration, washed with water (3ꢁ20 mL), and reslurried in
an excess of 10% sodium carbonate solution. The alkaline slurry was fil-
tered, and the product was washed thoroughly with water and dried at
608C. The crude product was recrystallized from ethanol and water to
Dendrimer GACTHNUGTRNEUNG(OXD)
A solution of sodium tert-butoxide (34 mg, 0.35 mmol) in ethanol (3 mL)
was added dropwise to the stirred solution of ancillary ligand OXDacac
(157 mg, 0.35 mmol) in DCM (15 mL) and ethanol (10 mL) at 08C. After
the reaction temperature returned to 258C, the reaction was stirred for
another 1 h. Then the chloro-bridged iridium dimer (0.60 g, 0.14 mmol)
in DCM (8 mL) was added dropwise at room temperature. The reaction
was stirred for another 2 h at room temperature before completion. The
mixture was extracted with DCM (3ꢁ10 mL), washed with water, dried
over anhydrous sodium sulfate, and then filtered. After the solvent was
removed, the residue was purified by column chromatography on alka-
line alumina with DCM/methanol (100: 1) as an eluent to afford G-
1
obtain the pure product (14.9 g, 77%). H NMR (300 MHz, [D6]DMSO):
d=12.94 (s, 1H), 8.23–8.15 (m, 2H), 7.76–7.44 (m, 5H), 7.26–7.16 ppm
(m, 2H).
Intermediate 3
AHCTUNGTRENNUNG
(OXD) (686 mg, 95%) 1H NMR (300 MHz, CDCl3): d=8.15 (s, 12H),
A mixture of 3,6,3’’,6’’-tetrakis-tert-butyl-9’H-[9,3’;6’,9’’] tercarbazole (2)
(3.6 g, 5 mmol), 1,4-dibromobutane (10.8 g, 50 mmol), potassium hydrox-
ide (2.8 g, 50 mmol), tetrabutyl ammonium bromide (0.2 g, 0.5 mmol),
toluene (40 mL), and water (6 mL) was refluxed for 24 h under argon.
After cooling to room temperature, the mixture was poured into water
and extracted with DCM. The organic extracts were washed with water
and dried over anhydrous sodium sulfate. After the solvent had been
completely removed, the excess 1,4-dibromobutane was distilled, then
the residue was purified by column chromatography on silica gel with pe-
troleum/ethyl acetate (12:1) as the eluent to give the product (2.5 g,
58%). 1H NMR (300 MHz, CDCl3): d=8.16 (s, 6H), 7.64 (s, 4H), 7.46–
7.43 (m, 4H), 7.31 (d, J=8.7, 4H), 1.46 ppm (s, 36H).
7.98 (t, J=8.4 Hz, 4H), 7.73–7.55 (m, 8H), 7.50–7.24 (m, 28H), 6.81 (d,
J=8.7, 4H), 6.58–6.46 (m, 4H), 5.12 (s, 1H), 4.77 (s, 4H), 4.43–4.28 (m,
4H), 3.57–3.45 (m, 2H), 2.35–2.20 (m, 8H), 1.88 (s, 2H), 1.66 (s, 3H),
1.57 (s, 2H), 1.45 (s, 74H), 1.34 ppm (s, 11H); Anal. Calc. for
C
165H171N12O4Ir: C 76.86, H 6.68, N 6.52; found: C 76.56, H 6.71, N 6.12;
MALDI-TOF (m/z): 2578.6 [M++H]
Dendrimer G(DOXD)
This compound was prepared according to the procedure for the synthe-
sis of G
(OXD) in a yield of 99%. 1H NMR (300 MHz, CDCl3): d=8.14
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(s, 12H), 8.00 (t, J=9.0 Hz, 8H), 7.72–7.64 (m, 4H), 7.57 (d, J=8.7 Hz,
4H), 7.49–7.38 (m, 16H), 7.35–7.20 (m, 14H), 6.86–6.79 (m, 6H), 6.61–
6.51 (m, 4H), 5.13 (s, 1H), 4.79 (s, 4H), 4.39–4.29 (m, 4H), 3.55–3.44 (m,
4H), 2.35 (s, 4H), 2.22 (s, 4H), 1.95–1.86 (m, 4H), 1.58 (s, 4H), 1.44 (s,
76H), 1.34 ppm (s, 22H); Anal. Calc. for C187H195N14O6Ir: C 76.74, H
6.72, N 6.70; found: C 76.66, H 6.78, N 6.40; MALDI-TOF (m/z): 2926.4
[M+].
Ligand LG
A solution of N,N-dimethylformamide (DMF; 40 mL) and tetrahydrofur-
an (THF; 80 mL) was added dropwise over 20 min to a stirred mixture of
2-phenyl-1H-benzoimidazole (1.4 g, 7 mmol) and sodium hydride (1.3 g,
56 mmol). After that, the solution of intermediate 3 (6.0 g, 7 mmol) dis-
solved in DMF (10 mL) and THF (20 mL) was slowly added dropwise.
When the addition was completed, the resulting solution was refluxed for
24 h under argon. After cooling to room temperature, the mixture was
poured into water (125 mL) and extracted with DCM (3ꢁ30 mL). The
organic extracts were washed with water and dried over anhydrous
sodium sulfate. After the solvent had been completely removed, the resi-
due was purified by column chromatography on silica gel with DCM/
ethyl acetate (30:1) as the eluent to give the product (5.2 g, 77%).
1H NMR (300 MHz, CDCl3): d=8.17 (s, 6H), 7.89–7.86 (m, 1H), 7.69–
7.66 (m, 2H), 7.61–7.58 (m, 2H), 7.51–7.40 (m, 10H), 7.38–7.25 (m, 6H),
4.35 (s, 4H), 1.94 (s, 4H), 1.46 ppm (s, 36H).
Measurement and Characterization
1H NMR spectra were recorded with a Bruker Avance 300 NMR spec-
trometer. The elemental analysis was performed by using a Bio-Rad ele-
mental analysis system. MALDI/TOF (matrix assisted laser desorption
ionization/time-of-flight) mass spectra were performed on AXIMA CFR
MS apparatus (COMPACT). Cyclic voltammetry experiments were per-
formed on an EG&G 283 (Princeton Applied Research) potentiostat/gal-
vanostat system. All measurements were carried out with a conventional
three-electrode system consisting of a platinum working electrode, a plat-
inum counter electrode, and an Ag/AgCl reference electrode. The sup-
porting electrolyte was tetrabutylammonium perchlorate (nBu4NClO4;
0.1m). Ferrocene was used as a standard to calibrate the system. With
regard to the energy level of the ferrocene reference (4.8 eV relative to
the vacuum level), the HOMO and LUMO energy levels were calculated
according to the following two equations: HOMO=ꢀe (4.8 V+Eox) and
LUMO=ꢀe (4.8 V+Ered). Here, Eox and Ered were taken from the onset
of the oxidation and reduction potential, respectively.[22] The UV/Vis ab-
Chloro-bridged iridium dimer
A mixture of LG (4.4 g, 4.4 mmol), iridium chloride trihydrate (0.7 g,
2.0 mmol), 2-ethoxyethanol (60 mL), THF (20 mL), and water (20 mL)
was refluxed under argon for 48 h. After cooling to room temperature,
the precipitate was collected by filtration and washed with water and eth-
anol. Then the crude product was purified by column chromatography on
silica gel with DCM as an eluent to give the chloro-bridged iridium
dimer.
sorption and PL spectra were measured by using
a Perkin–Elmer
Lambda 35 UV/Vis spectrometer and a Perkin–Elmer LS 50B spectro-
fluorometer, respectively. Solution spectra were recorded in DCM or tol-
uene with a concentration of 10ꢀ5 m. Thin films on quartz for spectroscop-
ic measurements were prepared by spin-coating. All the above experi-
ments and measurements were carried out at room temperature under
ambient conditions. Solution PL quantum efficiency was measured in ni-
trogen-saturated toluene by a relative method using fac-[Ir
tris(2-phenylpyridyl)iridium
Phosphorescence spectra at 77 K were measured in a mixed solvent of
toluene/ethanol/methanol (5:4:1). The triplet energies were estimated as
the maximum of the first vibronic mode (S0
Dendrimer GACHTUNGTRENNUNG(acac)
The chloro-bridged iridium dimer (480 mg, 0.11 mmol), pentane-2,4-
dione (Hacac; 45 mg, 0.44 mmol), and sodium carbonate (116 mg,
1.1 mmol) in 2-ethoxyethanol (15 mL), and CHCl3 (5 mL) were refluxed
under argon for 48 h. After cooling to room temperature, water was
added. The mixture was extracted with DCM (3ꢁ15 mL), washed with
water, dried over anhydrous sodium sulfate, and then filtered. After the
solvent was removed, the residue was purified by column chromatogra-
phy on alkaline alumina with DCM/petroleum (1:1) as an eluent to
ACHTUNGRTENUN(NG ppy)3] (fac-
AHCTUNGTRENNUNG
!
T1v=0) of the corre-
v=0
sponding phosphorescence spectra at 77 K. The lifetimes of phosphores-
cence from the samples were measured in the air by exciting the samples
with 355 nm light pulses with approximately a 3 ns pulse width from a
Quanty-Ray DCR-2 pulsed Nd:YAG laser.
afford GACHTUNGTRENNUNG
(acac) (456 mg, 93%). 1H NMR (300 MHz, CDCl3): d=8.15 (s,
12H), 7.72 (d, J=8.0 Hz, 2H), 7.65 (d, J=8.2 Hz, 2H), 7.57 (d, J=
8.3 Hz, 4H), 7.45–7.37 (m, 16H), 7.32–7.29 (m, 10H), 6.82 (t, J=6.9 Hz,
Chem. Asian J. 2011, 6, 1372 – 1380
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1379

L. Wang et al.
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Received: January 11, 2011
Published online: April 21, 2011
1380
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2011, 6, 1372 – 1380