Jacketed homopolymer with bipolar dendritic side groups and its applications in electroluminescent devices

Article abstract of DOI:10.1002/pola.25067

A dendritic monomer with bipolar side groups containing dendritic carbazole and oxadiazole structures was synthesized by a convergent strategy. The homopolymer was synthesized through a conventional radical polymerization. The number-average molecular wei

Full text of DOI:10.1002/pola.25067

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Jacketed Homopolymer with Bipolar Dendritic Side Groups and its
Applications in Electroluminescent Devices
Wei Zhang, Hao Jin, Dan Wang, Zengze Chu, Zhihao Shen, Dechun Zou, Xinghe Fan
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
Correspondence to: Z. Shen (E-mail: zshen@pku.edu.cn) or X. Fan (E-mail: fanxh@pku.edu.cn)
Received 30 September 2011; accepted 20 October 2011; published online 16 November 2011
DOI: 10.1002/pola.25067
ABSTRACT: A dendritic monomer with bipolar side groups con-
molecular orbital (LUMO) levels calculated from cyclic voltam-
metry data were -5.55 and -2.52 eV, respectively, and the band
gap was 3.03 eV, which suggested that the polymer had both
hole- and electron-transporting capabilities. The efficiencies of
the single-layer device based on this homopolymer were much
higher than those of the same-generation homopolymer with-
taining dendritic carbazole and oxadiazole structures was syn-
thesized by
a convergent strategy. The homopolymer was
synthesized through a conventional radical polymerization. The
number-average molecular weight determined by gel permea-
tion chromatography was 40,000 g/mol. Its 5% weight loss
temperature was 358 ꢀC. Its photophysical properties were
studied in solution and in film. The photoluminescent emission
peak of the film was at 408 nm, which had a blue shift of 9 nm
compared with that of the tetrahydrofuran solution. And there
was an energy transfer from oxadiazole to carbazole. The high-
est occupied molecular orbital (HOMO) and lower unoccupied
C
V
out the oxadiazole moiety.
2011 Wiley Periodicals, Inc. J
Polym Sci Part A: Polym Chem 50: 581–589, 2012
KEYWORDS: liquid-crystalline polymers (LCP); phase behavior;
supramolecular structures
INTRODUCTION Organic light-emitting diodes (OLEDs) have
been intensely researched because of their advantages in appli-
cations as flat panel displays and lighting. Organic light-emit-
ting materials can be classified into small molecules, polymers,
and dendrimers. Since the first report of high-efficiency small
molecular OLED (SMOLED),1 OLEDs began to attract more and
more attention. Compared with SMOLEDs, polymer light-emit-
ting diodes have better thermal stabilities and can be readily
processed using various methods, such as spin-coating and
inkjet-printing, to achieve flexible and large-area displays.
bility, low viscosity, and very large shell surface with many
functional groups.11 Unlike common polymers, dendrimers
always have exact molecular weights, monodispersity, and def-
inite structures. They can also be processed by solution-based
techniques for polymers. Recently, dendronized polymers, the
combination of linear polymers and dendrimers, have also
attracted intensive interests.12–14 Some dendronized polymers
with conjugated15,16 or unconjugated17–19 backbone have
been synthesized. They can self-assemable into different inter-
esting architectures.
Polymers dendronized with twin-dendritic side groups can
self-organize into liquid crystalline phases.20–22 Recently, we
reported a new class of electroluminescent dendronized
polymer with twin-dendritic carbazole side groups.23 The
dendritic side groups force the main chain to take a rigid,
extended conformation, and the first- and second-generation
polymers assemble into columnar phases. This kind of struc-
tures can suppress the intramolecular interactions and for-
mation of excimers, which benefit the optoelectronic
properties.24,25
Polymer light-emitting materials can be divided into the main-
chain type and the side-chain type. For the main-chain poly-
mers, the conjugated main chains are synthesized by conden-
sation polymerization, and they are good for carrier injection
and transportation.2 This kind of rigid structures can suppress
the intramolecular interactions. However, the intermolecular
aggregation may lead to undesirable changes of emitting col-
ors.3,4 The side-chain polymers can be synthesized by conven-
tional radical polymerization or living/controlled radical poly-
merizations.5–10 The molecular weight can be controlled, and
the solubility is good. However, the unconjugated backbones,
which are nonfunctional and usually flexible, result in rela-
tively poor carrier-transporting ability and device efficiencies.
Dendrimers have well-defined core-shell structures, good solu-
For an efficient OLED, the balanced hole and electron trans-
porting capabilities are important. However, common mole-
cules usually have single transporting ability, holes or elec-
trons. There are several ways to balance the transportation
C
V
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of holes and electrons. The first way is to build up multilayer
devices. The disadvantage is the tedious device fabrication.
The second way is doping. Although the manufacturing is
relatively easy, it may suffer from several problems such as
phase separation and crystallization. The third way is to
introduce both hole- and electron-transporting moieties into
the same molecule, which has the so-called bipolar struc-
ture.26,27 Carbazole units are well known for their hole-
transporting properties,23,28 while oxadiazole derivatives are
efficient electron-transporting and hole-blocking materi-
als.29,30 The combination of these two moieties can prospec-
tively lead to the improvement of electroluminescent proper-
ties. Copolymers and homopolymers can both be synthesized
as bipolar polymers. Bipolar copolymers from N-vinylcarba-
zole and oxadiazole-containing monomers have been synthe-
sized.24,31 However, the reactivities of different monomers
vary, and their contents in the polymers are different from
those expected. As a result, the compositions and structures
are not well controllable. For a homopolymer, the ratio of
the functional groups can be precisely controlled because
each repeat unit has the same structure. Thus, we expect to
obtain a homopolymer with a bipolar structure. We can also
change the diameter of the rod-like polymer chain by chang-
ing the generation of the dendritic side groups. In addition,
we can obtain polymer chains with different lengths by con-
trolling the degree of polymerization. Furthermore, this kind
of Janus dendritic monomer makes it possible to control the
structures and properties by changing its dendrons.32,33 By
adjusting the generations of different carrier moieties, the
carrier-transporting balance will be improved.
mamide (DMF) was added. The reaction mixture was
refluxed for 3 h. After cooled to ambient temperature, the
excess SOCl2 was removed by a rotary evaporator to give a
white powder 1, which was then dissolved in CH2Cl2 (100
mL) and added dropwise to a solution of 4-(octyloxy)benzo-
hydrazide (8.0 g, 30.3 mmol) and triethylamine (30 mL) in
CH2Cl2 (100 mL) in an ice/water bath. The yellow solution
was subsequently stirred at ambient temperature for 8 h,
and the mixture was condensed by a rotary evaporator at
50 ꢀC to give a crude product of 2, which was dissolved in
POCl3 (150 mL) directly and heated to reflux for 12 h. The
color of the reaction solution changed from yellow to
orange-red gradually and finally to dark brown. After cooled
to ambient temperature, it was poured into stirring ice/
water (2000 mL) slowly. The brown precipitate was filtered
and washed with water until the filtrate was neutral. The fil-
trate cake was dissolved in a small amount of CH2Cl2 and
subjected to silica gel column chromatography (CH2Cl2:EtOAc
¼ 20:1) to give a white solid (6.0 g, 8.2 mmol) in 60% yield.
1H NMR (400 MHz, CDCl3, d, ppm): 0.88–0.92 (t, 6H, CH3),
1.31–1.35 (m, 16H, CH2), 1.45–1.49 (m,4H, OCH2CH2CH2),
1.79–1.86 (m, 4H, OCH2CH2), 4.02–4.06 (t, 4H, OCH2), 5.25
(s, 2H, OCH2), 7.02–7.04 (d, 4H, ArAH), 7.35–7.39 (t, 1H,
ArAH), 7.42–7.46 (t, 2H, ArAH), 7.51–7.53 (d, 2H, ArAH),
7.88–7.90 (d, 2H, ArAH), and 8.08–8.10 (d, 4H, ArAH). MS
(LR-ESI): m/z 728.4 [MþH]þ. Anal. Calcd. for C45H52N4O5: C,
74.15; H, 7.19; N, 7.69. Found: C, 74.20; H, 7.19; N, 7.68.
Synthesis of HC3OXDG1 (4)
BnOXD-G1 (6.0 g, 8.2 mmol) was dissolved in tetrahydrofu-
ran (THF) (250 mL), and Pa/C catalyst (2.0 g) was added.
The mixture wasꢀ stirred under a hydrogen atmosphere in
water bath at 40 C for 8 h. 3-Bromopropan-1-ol (2.3 g, 16.5
mmol), anhydrous potassium carbonate (2.3 g, 16.5 mmol),
potassium iodide (0.3 g, 2.0 mmol), and acetonitrile (150
mL) was added in the reaction mixture simultaneously. The
mixture was heated to reflux overnight. After cooled to ambi-
ent temperature, the mixture was filtered through celite. The
filtrate was concentrated. The residue was first washed with
acetone, and the pure product was obtained by silica gel col-
umn chromatography (CH2Cl2:EtOAc ¼ 4:1) as a white solid
In this contribution, we report the synthesis of a homopoly-
mer from a dendritic monomer with bipolar side groups con-
taining dendritic carbazole and oxadiazole structures. Its
application in electroluminescent diode is also discussed.
EXPERIMENTAL
Device Fabrication and Characterizations
The device fabrication process used and the characterization
methods such as gel permeation chromatography (GPC), 1H
NMR spectroscopy, thermal gravimetric analysis (TGA), dif-
ferential scanning calorimetry (DSC), UV–vis spectrum, pho-
toluminescent (PL) spectrum, and cyclic voltammetry (CV)
were similar to those we used previously.23
1
(5.0 g, 7.5 mmol) in 90% yield. H NMR (400 MHz, CDCl3, d,
ppm): 0.89–0.92 (t, 6H, CH3), 1.31–1.36 (m, 16H, CH2), 1.44–
1.52 (m,4H, OCH2CH2CH2), 1.75–1.82 (m, 4H, OCH2CH2),
2.12–2.17 (m, 2H, OCH2CH2), 3.95–4.01 (m. 6H, OCH2), 4.27–
4.30 (t, 2H, HOCH2), 6.97–6.99 (d, 4H, ArAH), 7.61 (s, 2H,
ArAH), 7.99–8.02 (d, 4H, ArAH), and 8.32 (s, 1H, ArAH). MS
(LR-ESI): m/z 697.4 [MþH]þ, 719.4 [MþNa]þ. Anal. Calcd.
For C41H52N4O6: C, 70.66; H, 7.52; N, 8.04. Found: C, 70.64;
H, 7.54; N, 8.02.
Materials
2,20-Azobis(isobutyronitrile) (AIBN) was recrystallized from
ethanol. Chlorobenzene was washed with sulfuric acid and
then distilled under reduced pressure. Bis[3-(9H-carbazol-9-
yl)propyl]-5-(3-hydroxypropoxy)-isophthalate
(HC3CbzG1),
5-(benzyloxy)isophthalic acid, and 4-(octyloxy)benzohydra-
zide were obtained as reported before.23,34 All the other
reagents were used as received.
Synthesis of BnCbzCOOH (5)
5-(Benzyloxy)isophthalic acid (4.5 g, 16.5 mmol) was dis-
solved in SOCl2 (80 mL), and then five drops of DMF was
added. The reaction mixture was refluxed for 3 h. After
cooled to ambient temperature, the excess SOCl2 was
removed by a rotary evaporator. The white residue was
dissolved in dry CH2Cl2 (100 mL), which was added with a
solution of HC3CbzG1 (4.4 g, 6.7 mmol) and triethylamine
Synthesis
The synthetic routes are outlined in Scheme 1.
Synthesis of BnOXDG1 (3)
5-(Benzyloxy)isophthalic acid (3.8 g, 13.9 mmol) was dis-
solved in SOCl2 (50 mL), then five drops of N,N0-dimethylfor-
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SCHEME 1 Synthetic routes of the monomer and the polymer.
(30 mL) in CH2Cl2 (100 mL) in an ice/water bath. The mix-
ture was stirred at ambient temperature for 8 h and then for
an additional 30 min after water (100 mL) was added. The
organic layer was separated, dried with anhydrous magne-
sium sulfate, and then concentrated with a rotary evaporator.
The crude product was purified by silica gel column chroma-
tography (CH2Cl2:EtOH ¼ 20:1) to give a white solid (5.8 g,
1
6.4 mmol) in 95% yield. H NMR (400 MHz, DMSO, d, ppm):
2.24–2.29 (m, 6H, COO CH2CH2CH2O, COO CH2CH2CH2N),
4.21–4.27 (m, 6H, CH2O, CH2N), 4.51–4.57 (m, 6H, COO CH2),
5.16 (s, 2H, OCH2), 7.12–7.16 (t, 4H, ArAH), 7.29–7.42 (m,
9H, ArAH), 7.51 (s, 2H, ArAH), 7.57–7.59 (d, 4H, ArAH),
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7.73 (s, 2H, ArAH), 7.94 (s, 1H, ArAH), and 8.08–8.11 (m,
5H, ArAH). MS (LR-ESI): m/z 909.3 [MþH]þ, 931.3
[MþNa]þ. Anal. Calcd. For C56H48N2O10: C, 73.99; H, 5.32; N,
3.08. Found: C, 73.55; H, 5.42; N, 3.13.
Synthesis of VC6CbzOXD (8)
2-Vinylterephthalic acid (0.3 g, 1.6 mmol), HC6CbzOXD (7,
5.5 g, 3.4 mmol), EDCꢁHCl (1.3 g, 6.8 mmol), and DMAP (0.1
g, 0.8 mmol) were added into CHCl3 (150 mL), and the mix-
ture was stirred at ambient temperature for 5 h. The reac-
tion solution was washed with saturated sodium chloride so-
lution (3 ꢂ 100 mL) and dried with anhydrous magnesium
sulfate. Then, CHCl3 was removed by a rotary evaporator.
The pure product was obtained through silica gel column
chromatography (CH2Cl2:EtOAc ¼ 15:1) as a white solid
(0.7g, 0.2 mmol) in 15% yield. 1H NMR (400 MHz, CDCl3, d,
ppm): 0.88–0.98 (t, 12H, CH3), 1.26–1.35 (m, 32H, CH2),
1.35–1.54 (m, 16H, HOCH2CH2), 1.76–1.83 (m, 16H,
HOCH2CH2), 3.95–4.03 (m, 12H, OCH2), 2.33–2.37 (m, 16H,
COOCH2CH2CH2O, COO CH2CH2CH2N), 4.18–4.21 (t, 4H,
OCH2), 4.27–4.34 (m, 16H, NCH2), 4.45–4.49 (t, 8H, COOCH2),
4.55–4.57 (t, 8H, COOCH2), 5.37–5.40 (d, 1H, ¼¼CH2), 5.70–
5.74 (d, 1H, ¼¼CH2), 7.00–7.02 (d, 8H, ArAH), 7.15–7.19 (m,
8H, ArAH), 7.38–7.43 (m, 17H, ArAH, ¼¼CH), 7.68–7.70
(m, 8H, ArAH), 7.75–7.76 (d, 4H, ArAH), 7.85–7.93 (m, 2H,
ArAH), 8.03–8.07 (m, 16H, ArAH), 8.2 (d, 1H, ArAH), 8.27–
8.28 (m,4H, ArAH), and 8.34 (s, 2H, ArAH). MS (MALDI-
Synthesis of BnCbzOXD (6)
BnCbzCOOH (5.6 g, 6.2 mmol), HC3OXDG1 (4, 4.3 g, 6.2
mmol),
1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (EDCꢁHCl, 1.7 g, 8.9 mmol), and 4-dimethyla-
mino pyridine (DMAP, 0.2 g, 1.6 mmol) were added into
CH2Cl2 (200 mL), and the mixture was stirred at ambient
temperature for 5 h. The solvent was removed by a rotary
evaporator. The residue was dissolved in EtOAc (300 mL),
washed with saturated sodium chloride solution (3 ꢂ 100
mL), and dried with anhydrous magnesium sulfate. Then, the
solvent was removed by a rotary evaporator, and the residue
was purified through a silica gel column (CH2Cl2:EtOAc ¼
20:1) to give a pale yellow solid (8.4 g, 5.3 mmol) in 85%
yield. 1H NMR (400 MHz, CDCl3, d, ppm): 0.88–0.91 (t, 6H,
CH3), 1.25–1.40 (m, 16H, CH2), 1.46–1.48 (m,4H,
OCH2CH2CH2), 1.78–1.85 (m, 4H, OCH2CH2), 2.33–2.38 (m,
8H, COO CH2CH2CH2O, COO CH2CH2CH2N), 4.01–4.03 (t, 4H,
OCH2), 4.19–4.21 (t, 2H, OCH2), 4.26–4.29 (t, 2H, OCH2),
4.31–4.34 (t, 4H, NCH2), 4.46–4.49 (t, 4H, COOCH2), 4.55–
4.59 (t, 4H, COOCH2), 5.07 (s, 2H, OCH2), 7.00–7.02 (d, 4H,
ArAH), 7.16–7.20 (m, 4H, ArAH), 7.29–7.41 (m, 13H, ArAH),
7.69–7.70 (d, 2H, ArAH), 7.70–7.81 (m, 4H, ArAH), 8.04–
8.08 (m, 8H, ArAH), 8.29–8.30 (m, 2H, ArAH), and 8.36
(s, 1H, ArAH). MS (LR-ESI): m/z 1588.7 [MþH]þ, 1610.7
TOF):
m/z
3375.5
[MþNa]þ.
Anal.
Calcd.
For
C202H212N12O34: C, 72.38; H, 6.37; N, 5.01. Found: C, 72.47;
H, 6.56; N, 4.85.
Polymerization
VC6CbzOXD (500 mg), chlorobenzene (1.89 g), and AIBN (98
lL, 5 mg/mL in chlorobenzene) were transferred into a glass
tube. After three freeze-pump-thaw cycles, the tube was
sealed off under vacuum. Polymerization was performed at
60 ꢀC for 24 h, and the mixture was quenched in liquid
nitrogen. Then, the tube was opened. The white polymer
was obtained through silica gel column chromatography in
20% yield using CH2Cl2 as eluent. H NMR (400 MHz, CDCl3,
d, ppm): 0.75–2.26 (m, CH2CH2, CH3), 3.36–4.51 (m, OCH2,
NCH2), and 6.31–8.12 (m, ArAH).
[MþNa]þ, 1626.7 [MþK]þ. Anal. Calcd. For C97H98N6O15
:
C, 73.37; H, 6.22; N, 5.29. Found: C, 73.21; H, 6.32; N, 5.23.
Synthesis of HC6CbzOXD (7)
BnCbzOXD (6, 8.4 g, 5.3 mmol) was dissolved in THF (250
mL), and Pa/C catalyst (1.5 g) was added. The mixture wꢀas
stirred under a hydrogen atmosphere in water bath at 40 C
for 8 h. 6-Bromohexan-1-ol (3.0 g, 16.6 mmol), anhydrous
potassium carbonate (3.0 g, 21.7 mmol), potassium iodide
(0.2 g, 1.2 mmol), and acetonitrile (150 mL) was added in
the reaction mixture, which was heated to reflux overnight.
After cooled to ambient temperature, the mixture was fil-
tered through celite. The filtrate was vacuumed to remove
the solvent. The residue was dissolved in a small amount of
CH2Cl2 and purified through a silica gel column (CH2Cl2:
EtOAc ¼ 5:1) to give a white solid (6.3 g, 3.9 mmol) in 74%
yield. 1H NMR (400 MHz, CDCl3, d, ppm): 0.88–0.91 (t, 6H,
CH3), 1.30–1.48 (m, 24H, CH2), 1.55–1.58 (m, 2H,
HOCH2CH2), 1.74–1.85 (m, 6H, OCH2CH2), 2.32–2.38 (m,
8H, COO CH2CH2CH2O, COO CH2CH2CH2N), 3.61–3.64 (t, 2H,
HOCH2), 3.95–3.98 (t, 2H, OCH2), 4.01–4.04 (t, 4H, OCH2),
4.19–4.22 (t, 2H, OCH2), 4.27–4.30 (t, 2H, OCH2), 4.32–4.35
(t, 4H, NCH2), 4.46–4.50 (t, 4H, COOCH2), 4.56–4.59 (t,
4H, COOCH2), 7.00–7.03 (d, 4H, ArAH), 7.16–7.20 (m, 4H,
ArAH), 7.37–7.42 (m, 8H, ArAH), 7.69–7.70 (m, 8H, ArAH),
7.76–7.77 (d, 2H, ArAH), 8.04–8.08 (m, 8H, ArAH), 8.27–
8.29 (m, 2H, ArAH), and 8.35 (s, 1H, ArAH). MS (LR-ESI):
m/z 1598.8 [MþH]þ. Anal. Calcd. For C96H104N6O16: C,
72.16; H, 6.56; N, 5.26. Found: C, 71.96; H, 6.59; N, 5.22.
1
RESULTS AND DISCUSSION
Synthesis
There are three strategies to synthesize a dendronized poly-
mer, ‘‘graft-through,’’ ‘‘graft-onto,’’ and ‘‘graft-from.’’ The ‘‘graft-
through’’ strategy is to polymerize a dendritic macromono-
mer which was synthesized first.35,36 The ‘‘graft-onto’’ strat-
egy is to perform a reaction between a dendrimer and a
polymer chain.37,38 The ‘‘graft-from’’ strategy is to synthesize
a polymer chain with initiating sites and then initiate the
growth of the dendrimer generation by generation.39,40 We
chose the ‘‘graft-through’’ strategy to synthesize the target
dendronized polymer because the structure of the polymer
would be precise with a 100% grafting density. The core of
the target dendritic monomer contained a vinyl, which would
easily undergo some side reactions during a long synthetic
route. Therefore, we chose a convergent method to synthe-
size the dendritic shell first and then introduce the vinyl into
the molecule in the last step.
To obtain a dendritic monomer with a high yield, a high-effi-
ciency ‘‘protection–deprotection’’ reaction is of great
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TABLE 1 Molecular Weight Information and Thermal Properties
of the Polymer
Yield (%)
Mn (g/mol)a
PDIa
1.33
Td (ꢀC)b
Tg (ꢀC)c
30
40,000
358
59
a
Determined by GPC using polystyrene standards.
b
5% weight loss temperature determined by TGA under a nitrogen
atmosphere at a heating rate of 20 ꢀC/min.
c
Determined by DSC in the second heating process at a heating rate of
20 ꢀC/min.
importance. In the present work, a hydrogenation reaction of
benzyl in the presence of Pd/C catalyst was chosen. In the
hydrogenation reaction of BnOXDG1, the reaction mixture
obtained was gel-like, leading to a tedious purification pro-
cess. This phenomenon was probably caused by the strong
hydrogen-bonding interaction between the oxadiazole moiety
and the phenolic hydroxyl. When the mixture was heated,
the hydrogen bond was destroyed, and HOXDG1 became
soluble in THF. Therefore, HOXDG1 was not purified, and the
reaction mixture was used directly in the following etherifi-
cation reaction. A small amount of CH3CN was added into
the mixture to increase the polarity of the solution, which
was good for the nucleophilic substitution. The similar pro-
cess was performed when hydrogenating BnCbzOXD.
FIGURE 1 GPC curve using polystyrene as standards.
resulted in almost no reduction during hydrogenation. The
polymer had good solubility in organic solvents, such as
CH2Cl2, THF, and chlorobenzene.
Photophysical Properties
The absorption and emission data are summarized in Table 2.
The UV–vis absorption of the polymer in THF solution and
To obtain BnCbzCOOH (5) with a high yield, 5-(benzyloxy)-
isophthalic acid should be much excess compared with
HC3CbzG1. However, in an esterification reaction using N,N0-
dicyclohexylcarbodiimide as dehydrating agent, the poor sol-
ubility of 5-(benzyloxy)isophthalic acid led to a very low
concentration of carboxyl group in solution and resulted in a
heterogeneous reaction. Both of the two carboxyl groups
would react with the hydroxyl group. Because 5-(benzyloxy)-
isophthaloyl dichloride (1) could dissolve readily in CH2Cl2
and THF, with the excess amount of 1, 5 with the asymmet-
ric structure could be synthesized, and the high yield of over
90% demonstrated that this method was highly efficient.
The number-average molecular weight (Mn), polydispersity
index (PDI), and thermal properties of the polymer are sum-
marized in Table 1. The GPC curve is shown in Figure 1, and
the TGA and DSC traces are shown in Figure 2. The molar
ratio of monomer to initiator in the polymerization was
50:1. However, the degree of polymerization calculated from
Mn was about 12. The difference between the designed
degree of polymerization and the real one was probably due
to the inaccurate molecular weight determined by GPC. A
dendronized polymer usually has a smaller hydrodynamic
volume than that of a linear polymer with the same molecu-
lar weight. Therefore, the GPC method might underestimate
the molecular weight of the dendronized polymer.
The relatively high PDI could be attributed to the conven-
tional polymerization method. The high Td indicated that the
polymer had a good thermal stability. However, the Tg was
relatively low, probably caused by the flexible alkyl tails and
spacers in the molecule. We chose octyls as the tails because
the very bad solubility of BnOXDG1 with shorter tails
FIGURE 2 TGA (a: at a heating rate of 20 ꢀC/min) and DSC
(b: at a heating rate of 20 ꢀC/min) traces of the polymer.
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TABLE 2 UV–vis and PL Spectra Data in Solution and in Film
In THF In film
UV (nm)a
PL (nm)b,c
UV (nm)d
PL (nm)c,e
261, 295, 310
417
261, 296, 311
408
a
10ꢃ5 mol/L.
b
0.1 mg/mL.
c
Excitation wavelength was 310 nm.
The film was spin-coated from a THF solution (10 mg/mL) and dried
d
at ambient temperature.
e
The film was cast from a THF solution (10 mg/mL) and dried at ambi-
ent temperature.
in film was investigated. The concentration of the solution
was about 10ꢃ5 mol/L, and the film was fabricated on a
quartz substrate by spin-coating from a THF solution (10
mg/mL). The spectra are shown in Figure 3. The absorption
peaks at about 260 and 295 nm were attributed to the car-
bazole segments.23 The shoulder peaks at about 310 nm was
caused by the oxadiazole segments.24 The onset absorptions
at 353 nm in solution and 359 nm in film gave the calcu-
lated band gaps of 3.51 and 3.46 eV, respectively.
The PL emission spectra excited at 310 nm both in solution
and in film were also measured. The concentration of the so-
lution was 0.1 mg/mL, and the film was cast from a THF so-
lution (10 mg/mL). The spectra are shown in Figure 4. The
emission peak in THF was 417 nm. In film, the emission
peak was 408 nm, a blue shift of 9 nm compared with that
in THF. This phenomenon might be attributed to the rear-
rangement of the polymer chains in film.23 To verify which
part of the polymer emitted the light, we investigated the PL
emissions of the intermediates, HC3CbzG1 and HC3OXDG1,
in film. For HC3OXDG1, the main emission peak was at 375
nm, and there was an obvious aggregation peak at about
540 nm. For HC3CbzG1, the main emission peaks were at
414 and 434 nm, and there was a shoulder peak at about
480 nm. In the polymer, there was only one peak at 408 nm,
which was very close to that of HC3CbzG1. This indicated
FIGURE 4 Normalized PL spectra of PCbzOXD in THF and in
film (a) and those of HC3CbzG1 and HC3OXDG1 in film (b).
that an energy transfer from the oxadiazole moiety to carba-
zole moiety occurred. Additionally, the long-wavelength emis-
sions in both HC3CbzG1 and HC3OXDG1 were probably
caused by the aggregation of the molecules. This phenom-
enon was more clearly observed in HC3OXDG1 because it
FIGURE 3 Normalized UV–vis absorption spectra in THF and in
FIGURE 5 Cyclic voltammogram of the polymer film coated on
film.
the carbon electrode.
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TABLE 3 Electroluminescent Properties of PCbzOXD and PCbzG2
Device
Polymer
Uonset (V)a
Lmax (cd/m2)b
gImax (cd/A)c
gLmax (lm/W)d
gextmax (%)e
CIE Coordinatef
a
a
b
b
c
c
a
PCbzG2
7.2
9.0
5.0
7.2
6.4
9.1
33
12
0.007
0.042
0.093
0.100
0.167
0.115
d
0.003
0.015
0.043
0.032
0.063
0.039
0.005
0.030
0.068
0.069
0.122
0.086
0.18, 0.19
0.18, 0.21
0.18, 0.19
0.18, 0.21
0.18, 0.19
0.18, 0.21
PCbzOXD
PCbzG2
763
184
1182
214
PCbzOXD
PCbzG2
PCbzOXD
Turn-on voltage at 1 cd/m2.
Maximum luminance.
Maximum luminescence efficiency.
Maximum external quantum efficiency.
1931 CIE coordinate.
b
c
e
f
Maximum current efficiency.
was a planar molecule and could easily stack through p–p
interaction. In contrast, no undesirable long-wavelength
emission in the polymer film was observed, which suggested
that the huge steric hindrance of the dendritic side group
could effectively suppress the aggregation of the functional
moieties.
were blue. Compared with the polymer PCbzG2 we reported
previously,23 which only contained carbazole side groups,
the EL spectrum of PCbzOXD red-shifted about 15 nm. This
was probably caused by the reduction of the band gap
because of the reduced LUMO level.
The most important advantage of bipolar molecules is that
they perform better in single-layer devices. Therefore, we
fabricated devices a with PCbzG2 and PCbzOXD. The current
efficiency, energy efficiency, and external quantum efficiency
of PCbzOXD were one order of magnitude higher than those
of PCbzG2. The performance of the device with PCbzOXD
was better because its carrier transportation was more bal-
anced than that of PCbzG2.
Electrochemical Properties
CV was used to investigate the electrochemical properties of
the homopolymer. The result is illustrated in Figure 5. On the
anodic sweep, the onset potential was 1.20 V, which was
attributed to the oxidation of the carbazole moiety. On the ca-
thodic sweep, the onset potential was ꢃ1.83 V, which was
attributed to the reduction of the oxadiazole moiety. According
To optimize the device structure, TPBI was added (device b).
The device performances were greatly improved, with the
corresponding differences between PCbzG2 and PCbzOXD
reduced, because TPBI acted as a hole-blocking and electron-
transporting layer. On the one hand, it made the recombina-
tion of electrons and holes confined in the emitting layer; on
the other hand, the electron-transporting capability of the
electroluminescent polymers was improved, especially for
PCbzG2.
to the formulae EHOMO ¼ ꢃe[Uonset(ox) þ (4.8 V ꢃ U1/2,FOC
)
and ELUMO ¼ ꢃe[Uonset(red) þ (4.8 V ꢃ U1/2,FOC),23 the HOMO
and LUMO energy levels were calculated to be ꢃ5.55 and
ꢃ2.52 eV, respectively. By introducing the oxadiazole moieties
into the polymer, no significant difference was found in the
HOMO level compared with the result we reported previ-
ously.23 However, the LUMO level decreased dramatically,
owing to the electron affinity of the oxadiazole moiety. This
result indicated that the homopolymer had a potential for
transporting electrons. The band gap calculated from HOMO
and LUMO levels was 3.03 eV, which was significantly differ-
ent from that calculated from the onset absorption in film
(3.46 eV). This large deviation might result from the different
testing methods and mechanisms. The onset oxidation or
reduction started from part of the side groups, while the UV–
vis absorption method considered the side groups as a whole.
Therefore, the optical band gap would be larger than the elec-
trochemical one.9
In the further optimization of the device structure, electron-
transporting layer AlQ was added (device c). The
Electroluminescent Properties
The light emitting devices were fabricated in three configura-
tions [device a, ITO/PEDOT:PSS/Polymer/LiF (1 nm)/Al (90
nm); device b, ITO/PEDOT:PSS/Polymer/1,3,5-tri(1-phenyl-
1H-benzo[d]imidazol-2-yl)phenyl (TPBI; 15 nm)/Ca (20
nm)/Al (90 nm); device c, ITO/PEDOT:PSS/Polymer /TPBI
(15 nm)/8-hydroxyquinoline aluminum (AlQ; 30 nm)/Ca (20
nm)/Al (90 nm)]. The results are shown in Table 3.
Figure 6 shows the EL spectrum of device c at 16 V. Its
emission peaks were at 450 and 486 nm, both of which
FIGURE 6 EL spectrum of device c.
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10 Zorn, M.; Bae, W. K.; Kwak, J.; Lee, H.; Lee, C.; Zentel, R.;
Char, K. ACS Nano 2009, 3, 1063–1068.
performance of the device with PCbzG2 was greatly
improved, while that with PCbzOXD showed less improve-
ment. The maximum current efficiency and energy efficiency
were 0.115 cd/A and 0.039 lm/W, respectively. In this device
structure, the efficiencies of PCbzOXD were even slightly
lower than those of PCbzG2. The addition of AlQ could
improve the electron-transporting capability of the device,
which was good for PCbzG2. For PCbzOXD, because it al-
ready had the electron-transporting moiety oxadiazole, AlQ
might break the balance of carrier transportation, resulting
in less improvement of device performance.
11 Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.;
Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17,
117–132.
12 Guo, Y. F.; van Beek, J. D.; Zhang, B. Z.; Colussi, M.; Walde,
P.; Zhang, A.; Kroger, M.; Halperin, A.; Schluter, A. D. J. Am.
Chem. Soc. 2009, 131, 11841–11854.
13 Rosen, B. M.; Wilson, C. J.; Wilson, D. A.; Peterca, M.;
Imam, M. R.; Percec, V. Chem. Rev. 2009, 109, 6275–6540.
14 Percec, V.; Obata, M.; Rudick, J. G.; De, B. B.; Glodde, M.;
Bera, T. K.; Magonov, S. N.; Balagurusamy, V. S. K.; Heiney,
P. A. J. Polym. Sci. Part A: Polym. Chem. 2002, 40,
3509–3533.
CONCLUSIONS
15 Percec, V.; Rudick, J. G.; Peterca, M.; Wagner, M.; Obata,
M.; Mitchell, C. M.; Cho, W.-D.; Balagurusamy, V. S. K.; Heiney,
P. A. J. Am. Chem. Soc. 2005, 127, 15257–15264.
A dendritic monomer containing carbazole and oxadiazole
moieties was synthesized. And conventional radical polymer-
ization was conducted to obtain a homopolymer with good
thermal stability. However, the flexible parts of the molecule
led to a low Tg. Its UV–vis absorption peaks in film were at
261, 296, and 311 nm. The PL emission peak of the film was
at 408 nm. And an energy transfer from oxadiazole to carba-
zole occurred. The PL emission peak in film showed a blue
shift of 9 nm compared with that in THF. No undesirable
long-wavelength emission was observed because the huge
steric hindrance of the dendritic side groups could suppress
the aggregation of the molecules. The HOMO and LUMO lev-
els estimated by electrochemistry were ꢃ5.55 and ꢃ2.52 eV,
respectively, with a band gap of 3.03 eV. The single-layer de-
vice of the homopolymer PCbzOXD showed much higher effi-
ciencies than those of the second-generation homopolymer
PCbzG2 reported previously, which had the same generation
as PCbzOXD, indicating the more balanced carrier transporta-
tion of the polymer. Device c showed the best performance
with the maximum luminance of 214 cd/m2, maximum
external efficiency of 0.086%, and maximum current effi-
ciency of 0.115 cd/A.
16 Rudick, J. G.; Percec, V. Acc. Chem. Res. 2008, 41,
1641–1652.
17 Percec, V.; Ahn, C. H.; Cho, W. D.; Jamieson, A. M.; Kim, J.;
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19 Prokhorova, S. A.; Sheiko, S. S.; Moller, M.; Ahn, C.-H.; Per-
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21 Percec, V.; Bera, T. K.; Glodde, M.; Fu, Q. Y.; Balagurusamy,
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22 Percec, V.; Rudick, J. G.; Peterca, M.; Yurchenko, M. E.;
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23 Jin, H.; Xu, Y.; Shen, Z.; Zou, D.; Wang, D.; Zhang, W.; Fan,
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24 Wang, P.; Chai, C.; Wang, F.; Chuai, Y.; Chen, X.; Fan, X.;
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Financial support from the National Natural Science Founda-
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