Vinyl-type polynorbornene with 9,9′-(1,1′-biphenyl)-4,4′- diylbis-9H-carbazole side groups as a host material for highly efficient green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c0jm03755a

The soluble polynorbornene (P1) bearing 9,9′-(1,1′-biphenyl)-4, 4′-diylbis-9H-carbazole (CBP) side groups was investigated as a host material for green emitters in phosphorescent OLED devices. The vinyl addition polymerization of norbornene monomers using Pd(ii) catalyst efficiently produces P1 in combination with 1-octene chain transfer agent. P1 exhibits high thermal stability with high decomposition (T_d5 > 451°C) and glass transition temperatures (T_g > 361°C). The HOMO (ca. -5.5 eV) and LUMO (ca. -2.1 eV) levels with the triplet energy of ca. 2.60 eV suggest that P1 is suitable for a host material for green emitters. The solution-processed devices based on the emissive layers containing P1 host doped with various concentration of fac-Ir(ppy)₃ (1-6 wt%) display stable green emission of fac-Ir(ppy)₃ with high device performances. The external quantum efficiency and power efficiency reach 7.2% and 11 lm/W, respectively, at the optimum doping concentration of fac-Ir(ppy)₃ (2 wt%). The device performances are found to be slightly lower than those of PhOLED with molecular CBP host but higher than those of a PVK-based device. It is shown that in conjunction with the good processability of polynorbornene backbones, the high levels of the effective hole and electron mobilities of P1 (ca. 10^-3 and 10^-5 cm²/Vs, respectively) as well as large triplet energy inherited from CBP side groups are mainly responsible for the high performance of the phosphorescent OLEDs with solution-processed P1 host:emitter layers.

Full text of DOI:10.1039/c0jm03755a

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PAPER
Vinyl-type polynorbornene with 9,90-(1,10-biphenyl)-4,40-diylbis-9H-carbazole
side groups as a host material for highly efficient green phosphorescent
organic light-emitting diodes†
a
Jun Ha Park, Changhun Yun, Tae-Wook Koh, Youngkyu Do, Seunghyup Yoo and Min Hyung Lee
b
b
a
b
c
*
*
*
Received 3rd November 2010, Accepted 5th January 2011
DOI: 10.1039/c0jm03755a
The soluble polynorbornene (P1) bearing 9,90-(1,10-biphenyl)-4,40-diylbis-9H-carbazole (CBP) side
groups was investigated as a host material for green emitters in phosphorescent OLED devices. The
vinyl addition polymerization of norbornene monomers using Pd(II) catalyst efficiently produces P1 in
combination with 1-octene chain transfer agent. P1 exhibits high thermal stability with high
decomposition (Td5 > 451 ꢀC) and glass transition temperatures (Tg > 361 ꢀC). The HOMO (ca. ꢁ5.5
eV) and LUMO (ca. ꢁ2.1 eV) levels with the triplet energy of ca. 2.60 eV suggest that P1 is suitable for
a host material for green emitters. The solution-processed devices based on the emissive layers
containing P1 host doped with various concentration of fac-Ir(ppy)3 (1–6 wt%) display stable green
emission of fac-Ir(ppy)3 with high device performances. The external quantum efficiency and power
efficiency reach 7.2% and 11 lm/W, respectively, at the optimum doping concentration of fac-Ir(ppy)3
(2 wt%). The device performances are found to be slightly lower than those of PhOLED with molecular
CBP host but higher than those of a PVK-based device. It is shown that in conjunction with the good
processability of polynorbornene backbones, the high levels of the effective hole and electron mobilities
of P1 (ca. 10ꢁ3 and 10ꢁ5 cm2/Vs, respectively) as well as large triplet energy inherited from CBP side
groups are mainly responsible for the high performance of the phosphorescent OLEDs with solution-
processed P1 host:emitter layers.
based on carbazole derivatives such as 9,90-(1,10-biphenyl)-4,40-
diylbis-9H-carbazole (CBP) have been well established,6,17
polymeric host materials are also of considerable interest owing
to their good solution processability that can be highly bene-
ficial for volume production of OLEDs using printing tech-
nologies in applications such as electronic signage, lighting, and
displays.18–20 In this regard, many studies have been dedicated
to the development of conjugated polymeric hosts for phos-
phorescent materials. In particular, red PhOLED devices with
high device efficiency comparable to those of vacuum deposited
devices were demonstrated by several groups.21–26 However,
successful polymeric host materials for green emitters have less
been reported partly due to the lower triplet energy of most
conjugated polymers27–31 than those of typical green emitters
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs)
based on heavy metal complexes have attracted great attention
since they can theoretically achieve an internal quantum effi-
ciency of 100% by fully utilizing both singlet and triplet exci-
tons.1–7 To prevent triplet–triplet annihilation and
concentration quenching,8–10 PhOLEDs usually adopt an active
layer in which phosphorescent emitters are doped into suitable
host materials. In general, host materials in PhOLEDs should
have higher triplet energy level (ET) than phosphorescent
emitters for efficient energy transfer from the host into the
guest and for confinement of triplet excitons in guest mate-
rials.11–16 Although studies on small molecular host materials
such as fac-tris(2-phenylpyridine)iridium(III) (fac-Ir(ppy)3, ET
¼
aDepartment of Chemistry, KAIST, Daejeon, 305-701, Republic of Korea.
E-mail: ykdo@kaist.ac.kr; Fax: +82 42 350 2810; Tel: +82 42 350 2869
bDepartment of Electrical Engineering, KAIST, Daejeon, 305-701,
Republic of Korea. E-mail: syoo@ee.kaist.ac.kr; Fax: +82 42 350 8083;
Tel: +82 42 350 3483
cDepartment of Chemistry and Energy Harvest-Storage Research Center,
University of Ulsan, Ulsan, 680-749, Republic of Korea. E-mail: lmh74@
ulsan.ac.kr; Fax: +82 52 259 2348; Tel: +82 52 259 2335
† Electronic supplementary information (ESI) available: NMR spectra of
1, M1, and P1, and TGA, DSC, GPC curves, and AFM image of P1. See
DOI: 10.1039/c0jm03755a
2.41 eV)32 although it has been recently reported that high
efficiency is achievable through the modification of the main-
chain polymer structure leading to the increase of ET.33,34
Alternatively, non-conjugated, side-chain polymers like poly(9-
vinylcarbazole) (PVK) have often been adopted as a polymeric
host for the green emitters due to their relatively high triplet
energy (ET ¼ 2.5 eV).35–38 Nevertheless, the poor charge carrier
transport property of PVK, which causes charge flux imbalance
between electrons and holes, has resulted in relatively low or
5422 | J. Mater. Chem., 2011, 21, 5422–5429
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moderate efficiency of devices based on PVK host doped with
fac-Ir(ppy)3. In order to achieve highly efficient green PhO-
LEDs, it would thus be ideal to develop polymeric host
materials that contain an established small-molecular green
host such as a CBP moiety (ET ¼ 2.56 eV)13,17,39 as pendant side
groups so that the high triplet energy as well as good charge
carrier transport properties of such a moiety can be combined
with the processability of the polymeric backbone.
attach a CBP group to the norbornene moiety, the key starting
material 9-[40-carbazol-9-yl-biphenyl-4-yl]-3-bromo-9H-carba-
zole (1) was prepared via Ullmann-type coupling reaction
between 3-bromocarbazole and 4-iodo-40-(carbazolyl)biphenyl.
Subsequent reaction of 5-norbornene-2-pentyl bromide with the
lithium salt generated from 1 and t-BuLi afforded the M1
monomer in good yield (69%).
Homopolymerization of M1 was carried out in a manner
similar to our published polymerization procedure in chloro-
benzene at room temperature.40 Combined use of cationic Pd(II)
catalyst, derived from the in situ reaction of N-heterocyclic
carbene (NHC; N,N0-bis(2,6-diisopropylphenyl)-imidazol-2-yli-
dene) complex, [(NHC)Pd(h3-allyl)Cl]44 with AgSbF6, and
0.1 mol% of 1-octene chain transfer agent led to 54% monomer
conversion under the given conditions (Table 1). The resulting
polymer (P1) is highly soluble in common organic solvents such
as chloroform, dichloromethane, tetrahydrofuran, and chloro-
benzene at room temperature. Characterization of P1 by 1H and
13C NMR spectroscopy and elemental analysis confirms the
formation of vinyl-type polynorbornene with pendant CBP
groups. Gel permeation chromatography (GPC) measurement
indicates the high molecular weight of P1 (Mw ¼ 413 000 g
molꢁ1) with a narrow polydispersity index of 2.51.
Recently, we reported that vinyl-type polynorbornenes with
triarylamine side groups can work as efficient polymeric hole-
transporting materials in OLEDs. It was revealed that the
hole-transporting properties of pendant triarylamine moieties
are effectively retained in the polymer structure. Moreover, it
was also shown that such polymers have good solubility in
common organic solvents appropriate for solution processes
ꢀ
and high glass transition temperature (Tg > 268 C) important
for thermal stability.40 Accordingly, vinyl-type poly-
norbornenes bearing side-chain CBP moieties could be
considered as promising polymeric host materials in PhO-
LEDs.41–43 Hence vinyl addition polymerization of norbornene
monomer bearing a CBP side group is explored in this work
using a palladium(II) catalyst to provide a new class of highly
efficient polymeric host materials for green PhOLEDs. Details
of the synthesis as well as its use as a host material in actual
PhOLEDs are described to demonstrate that the proposed
approach can afford green PhOLEDs with performances
comparable to that of the vacuum-processed counterpart using
molecular CBP as a host.
Table 1 Polymerization result of M1 with Pd(II) catalysta
d
e
CTA
(mol%)b
Conversion
(%)
10ꢁ3
Mw
Mw/
Mn
Tg
(ꢀC)
Td5
(ꢀC)
c
c
polymer
P1
0.1
54
413
2.51
361
451
a
Conditions: catalyst ¼ [(NHC)Pd(h3-allyl)Cl]/AgSbF6; [Pd]/[AgSbF6]
Results and discussion
¼ 1/1.5; [Pd] ¼ 1.5 mmol; [Mon.]/[Pd] ¼ 1000; solvent: 15 mL of
b
chlorobenzene; Tp ¼ 25 ꢀC; tp ¼ 20 h. Molar ratio of chain transfer
Synthesis and characterization
c
d
agent (1-octene) to monomer. Determined by GPC. Determined by
DSC. e Determined by TGA at 5% weight loss.
Synthesis routes to CBP-functionalized norbornene monomer
M1 and its polymer P1 are described in Scheme 1. In order to
Scheme 1 Reagents and conditions: (i) NBS, CH2Cl2; (ii) 4,40-diiodobiphenyl, Cu, K2CO3, 18-crown-6, 1,2-dichlorobenzene; (iii) Cu, K2CO3, 18-crown-
6, 1,2-dichlorobenzene; (iv) 2 t-BuLi, THF, ꢁ78 ꢀC; (v) 5-norbornene-2-pentyl bromide, THF, 0 ꢀC; [Pd(II)] ¼ [(NHC)Pd(h3-allyl)][SbF6].
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Thermal, photophysical, and electrochemical properties
Electrophosphorescent properties
Thermal properties of P1 were evaluated by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC).
TGA measurement reveals that P1 possesses high thermal
In order to confirm the capability of P1 as a host material in
PhOLEDs, devices based on polymeric emissive layers contain-
ing P1 host doped with various concentration of fac-Ir(ppy)3
(1–6 wt%) were fabricated in the following configuration: ITO/
PEDOT:PSS (40 nm)/P1:fac-Ir(ppy)3 (50 nm)/BCP (12 nm)/
Bphen (35 nm)/LiF (1 nm)/Al (100 nm), where PEDOT:PSS is
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate); BCP
is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline; Bphen is 4,7-
diphenyl-1,10-phenanthroline. The emissive layers were con-
structed on top of a PEDOT:PSS layer by spin casting the
polymer solution containing P1 and fac-Ir(ppy)3. For compar-
ison, reference PhOLEDs with PVK polymer host and molecular
CBP host doped with 4 wt%36 and 7 wt%6 of fac-Ir(ppy)3 were
fabricated in the same batch by spin-coating and vacuum
deposition methods, respectively. Although PhOLEDs based on
molecular CBP host normally use additional hole-transporting
layers between PEDOT:PSS and the emissive layer, all the
devices were prepared with identical structures to investigate the
influence of the host materials themselves on device performance.
Prior to device fabrication, it was confirmed by atomic force
microscopy that spin-coating of the polymer solution containing
P1 and 4 wt% fac-Ir(ppy)3 forms a homogeneous thin film with
a flat surface that is almost free of pinholes or aggregates
(Fig. S9†).
ꢀ
stability with a decomposition temperature (Td5) above 450 C.
In particular, DSC analysis shows a high glass transition
temperature (Tg) of 361 ꢀC, which is comparable to that of NPB-
containing polynorbornene40 and moreover is the highest Tg
value among those of conjugated27 and side-chain45,46 polymeric
host materials reported so far. We also note that the Tg of P1 is
largely increased by the rigid polynorbornene backbone since
molecular CBP itself has a relatively low Tg of 62 C.47
ꢀ
According to the UV-Vis absorption and PL spectra of P1
(Fig. 1), it can be seen that P1 exhibits the major absorption
bands at 295 and 325 nm attributable to the p–p* transitions in
the pendant carbazole groups. The optical band gap (Eg) deter-
mined from the absorption onset wavelength (371 nm) of a thin-
film of P1 was calculated to be ca. 3.34 eV (Table 2). Upon
excitation at 332 nm, the PL spectrum of P1 at 298 K displays
intense emission centered at around 390 nm in solution.
Comparison with the spectra of M1 monomer indicates almost
identical absorption and emission features of P1 and M1 with no
indication of aggregation or excimer formation between the side-
chain CBP moieties in P1. Furthermore, these spectral features
of P1 are found to be well matched with those of molecular CBP,
implying that the polynorbornene backbone does not affect the
photophysical properties of the pendant CBP groups.48 The PL
spectrum obtained at 77 K was utilized to determine the triplet
energy (ET) of P1. By taking the highest-energy phosphorescence
peak as the transition of T1 / S0, the ET of P1 is estimated to be
ca. 2.60 eV, comparable to that of molecular CBP (2.56 eV). The
higher ET value than that of typical green phosphorescent emitter
(fac-Ir(ppy)3, ET ¼ 2.41 eV) also suggests that P1 could be
suitable for a host material for green emitters.
As shown in Fig. 3, all D1–D6 devices with different hosts and
doping concentration of P1 emit nearly identical green phos-
phorescence with an emission maximum lem of 512 nm, indi-
cating that the light emission is solely from the phosphorescent
dopant fac-Ir(ppy)3 by complete energy transfer from the host to
the guest.
The device performance of D1–D6 is summarized in Table 3
and Fig. 4 shows the external quantum efficiency (hEQE, Fig. 4a)
and power efficiency (hPE, Fig. 4b) data for the devices fabricated
in this work. It can be seen that hEQE (7.2%) and hPE (11 lm/W)
that are almost comparable to those of PhOLED with molecular
CBP host (D6) can be achieved with P1-based devices with the
optimum doping concentration (2 wt%). In addition, these effi-
ciencies are 32% higher than those of device (D5) based on PVK,
which is the most common polymeric host material for fac-Ir
(ppy)3. It is noted that the doping concentration of P1 where
hEQE reaches its maximum value (D2, 2 wt%) is much lower than
that of CBP in D6 (ca. 7 wt%). This difference in the optimum
doping concentration might be ascribed to the smaller number of
CBP moieties per unit weight of P1 host, requiring lower
concentrations of fac-Ir(ppy)3 guest molecules for high efficiency
than D6.
The cyclic voltammogram of P1 shows an irreversible oxida-
tion feature as typically observed for the carbazole derivatives
with unprotected 3,6-positions (Fig. 2).49,50 From optical and
electrochemical data, the HOMO and LUMO levels are calcu-
lated to be ca. ꢁ5.5 eV and ꢁ2.1 eV, respectively. The observed
oxidation behavior and the energy levels are also in good
agreement with those reported for molecular CBP.51,52
The lower hPE of devices with P1 host than that of D6 is due to
the high operating voltage as can be seen in the current density–
voltage–luminance (J–V–L) and external quantum efficiency–
luminance (hEQE–L) characteristics of PhOLEDs presented in
Fig. 5. It can be seen that the overall operating voltage including
turn-on voltage in polymeric systems is higher than that found in
the molecular CBP host device. Nevertheless, the turn-on voltage
of devices with P1 host is lower than that of PVK-based D5,
being in parallel with the fact that the HOMO level of P1 (ꢁ5.5
eV) is closer to the work function of ITO (ꢁ4.7 to ꢁ4.8 eV) than
that of PVK (ꢁ5.8 eV).53,54 Furthermore, D2 shows better
Fig. 1 UV-Vis absorption (left) and PL (right) spectra of P1 in dilute
CH2Cl2 solution.
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Table 2 Photophysical and electrochemical data of P1
Polymer
labs (nm) (log3)a
lem (nm)b
lphos. (nm)c
Eg (eV)d
ET (eV)e
Eox (eV)f
HOMOg/LUMO (eV)h
P1
295 (4.52), 325 (4.42)
391
476
3.34
2.60
0.68
ꢁ5.48/ꢁ2.14
a
Absorption measured in CH2Cl2 (2.0 ꢂ 10ꢁ5 M). b Recorded in CH2Cl2 (2.0 ꢂ 10ꢁ6 M) at 298 K. c Recorded in CH2Cl2 glass at 77 K. d Estimated from
e
the solid-state absorption edge. Triplet energy level estimated from the highest-energy phosphorescence peak. The onset of the oxidation wave in
f
cyclic voltammetry curves. g Calculated from Eox
.
Estimated from the HOMO and band-gap (Eg) energies.
h
Fig. 2 Cyclic voltammogram of P1.
Fig. 4 (a) Statistical box plot of external quantum efficiency (hEQE) and
(b) power efficiency (hPE) for D1–D6 with different emissive layers
measured at 1000 cd mꢁ2. Values in parentheses indicate the relative
doping concentration of guest material fac-Ir(ppy)3.
performances than D5 in terms of both luminance and external
quantum efficiency in a wide range of applied voltages.
Fig. 3 EL spectra of devices (D1–D6) with P1, PVK, and molecular CBP
hosts doped with fac-Ir(ppy)3.
In order to elucidate factors affecting device performances of
PhOLEDs based on P1 host, the charge transport properties of
Table 3 Device performance of PhOLEDs based on P1, PVK, and CBP hosts
Device
Host
Conc. of fac-Ir(ppy)3
CIE (x, y)a
Vturn-on (V)b
Lmax (cd mꢁ2
)
EQEmax (%)
PEmax (lm/W)
D1
D2
D3
D4
D5
D6
P1
P1
P1
P1
PVK
CBP
1 wt (%)
2 wt (%)
4 wt (%)
6 wt (%)
4 wt (%)
7 wt (%)
0.289, 0.631
0.287, 0.633
0.287, 0.638
0.298, 0.632
0.289, 0.631
0.269, 0.643
4.7
4.9
5.0
4.9
5.1
3.3
8400
9300
6600
8000
7500
66000
7.1
7.2
6.5
5.8
5.4
9.3c
11
11
10
8
8
14
a
c
. It is noted that the maximum EQE of D6 is lower than that
Measured at 14 mA cmꢁ2 current density. b Applied voltage at the luminance of 1 cd mꢁ2
of the conventional PhOLED based on CBP host (ca. 15–17%), because D6 does not contain hole-transporting layers (HTLs). This is for fair comparison
with D1–D5.
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than but quite comparable to those of the devices based on
molecular CBP. From a space-charge-limited current method,55–57
the lower limit‡ for the effective hole and electron mobilities
of P1 is estimated to be around 10ꢁ3 and 10ꢁ5 cm2/Vs,
respectively, with the dielectric constant of P1 set as that of
CBP.58 Although a clear space-charge-limited region was not
observed in CBP and PVK cases and thus their mobility
values were hard to obtain from this experiment, comparison
with the data from the literature indicates that the observed
hole mobility of P1 is in a similar range with the value
reported for molecular CBP while its electron mobility is one
order of magnitude smaller than that for molecular CBP.59,60
It can be thus suggested that the charge flux balance in the P1
host layer could be poorer than that in the molecular CBP
host, which may explain the slightly lower efficiencies
observed in the experiment. Nonetheless, the carrier mobility
of P1 is still significantly larger than that of PVK,61,62 and at
the same time, the high triplet energy of CBP is well retained
in P1, leading to the overall high device performances. The
lifetime tests for D2 and D5 are also consistent with the
device performances. Under the constant current supply
yielding the initial luminance of 1000 cd mꢁ2 in each case, the
time to reach L of 500 cd mꢁ2 is larger in D2 than that in D5
by about 60% (Fig. S10†).
Conclusions
Fig. 5 (a) Current density–luminance–applied voltage (J–V–L) and (b)
external quantum efficiency–luminance (hEQE–L) curves of devices with
P1 (D2), PVK (D5), and molecular CBP (D6) hosts doped with fac-Ir
(ppy)3.
We have demonstrated that soluble polynorbornene bearing
CBP side groups can be efficiently produced by the vinyl addition
polymerization of norbornene monomers using Pd(II) catalyst in
combination with 1-octene chain transfer agent. It is shown that
the proposed polymer is an excellent host material for typical
green emitters (fac-Ir(ppy)3). Based on the proposed polymer,
high-performance phosphorescent OLEDs were realized with
solution-processed host:emitter layers. A key to the successful
demonstration was to combine the good processability of poly-
norbornene backbones and the desirable electronic properties
inherited from CBP side groups such as the good carrier mobility
and large triplet energy.
P1 were investigated using single carrier (hole- and electron-only)
devices and compared with those of PVK and molecular CBP. As
shown in Fig. 6, the device with P1 exhibits higher current
densities than those of PVK in both hole-only and electron-only
cases. Moreover, the current densities of the P1 device are lower
Experimental
Materials and characterization
All operations were performed under an inert nitrogen atmo-
sphere using standard Schlenk and glove box techniques.
Anhydrous grade solvents (Aldrich) were dried by passing
through an activated alumina column and stored over activated
ꢀ
molecular sieves (5 A). Spectrophotometric-grade dichloro-
methane was used as received from Aldrich. Commercial
‡ M–I–M type organic devices often undergo injection-limited
characteristics before reaching the space-charge-limited-conduction
(SCLC) regime. This injection-limited current density follows ꢃVp (p
[ 2) but cannot be larger than the ‘‘ceiling’’ of SCLC current density
(JSCLC) at a given bias. In the present work, the devices showed SCLC
behavior only in the limited range near the highest bias. (Beyond that,
current density was too high to withstand device break-down.) It is
therefore challenging to obtain the exact values of mobility from these
data. Nevertheless, the method used here should be sufficient to
provide ‘‘the lower limit’’ of the mobility values because the current
Fig. 6 J ꢂ d vs. V/d plots for hole-only devices (left) and electron-only
devices (right) with P1, PVK, and molecular CBP hosts in which d is the
thickness of the organic layer, i.e. 80, 70, and 90 nm, respectively. The
hole- and electron-only device structures are ITO/PEDOT:PSS/host
material/WO3/Al and Al/Ca/host material/Ca/Al, respectively.
density at a given V cannot be larger than JSCLC
.
5426 | J. Mater. Chem., 2011, 21, 5422–5429
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reagents were used without any further purification after
purchasing from Aldrich (carbazole, N-bromosuccinimide, 4,40-
diiodobiphenyl, copper, potassium carbonate, 18-crown-6, tert-
butyllithium (1.7 M solution in pentanes), and Strem (AgSbF6).
3-Bromocarbazole,63 4-iodo-40-(carbazolyl)biphenyl,48 5-nor-
bornene-2-pentyl bromide64,65 (endo:exo ¼ 2 : 1), and allylchloro
[N,N0-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]palladium(II)
([(NHC)Pd(h3-allyl)Cl])44 were analogously synthesized accord-
ing to the published procedures. 1-Octene (Aldrich) was purified
by passing through an activated alumina column and stored over
evaporated to dryness, affording an oily residue. The crude
product was purified by column chromatography on silica
(eluent: CH2Cl2–n-hexane ¼ 1/4), which afforded M1 as white
solid. Yield ¼ 1.79 g (69%, mixture of endo and exo isomers (2/
ꢀ
1
1)). mp ¼ 96–99 C. H NMR (CDCl3): d 0.48–0.54 (m), 1.02–
1.47 (m) (10H), 1.67–2.05 (m, 3H), 2.53 (s), 2.72–2.88 (m) (4H),
5.94 (dd, J ¼ 5.9/2.7 Hz, Hendo), 6.03 (dd, J ¼ 5.9/2.7 Hz, Hexo),
6.09–6.11 (m, Hexo), 6.12 (dd, J ¼ 5.9/3.0 Hz, Hendo) (2H), 7.26–
7.37 (m, 4H), 7.41–7.56 (m, 7H), 7.70 (d, J ¼ 8.2 Hz, 4H), 7.85–
7.93 (m, 4H), 7.98 (s, 1H), 8.16 (d, J ¼ 8.0 Hz, 1H), 8.19 (d, J ¼
8.1 Hz, 2H). 13C NMR (CDCl3): d 28.57, 28.79, 29.60, 32.37,
32.43, 33.10, 34.77, 36.01, 36.57, 38.74, 41.85, 42.51, 45.20, 45.41,
46.34, 49.54, 109.51, 109.74, 109.80, 119.68, 119.86, 120.05,
120.27, 120.35, 123.47, 123.56, 125.80, 125.99, 126.77, 127.28,
127.43, 128.41, 128.44, 132.42, 134.79, 134.82, 136.15, 136.85,
136.91, 137.16, 137.46, 138.97, 139.22, 139.27, 140.79, 140.95. HR
EI-MS: m/z Calcd for C48H42N2, 646.3348; Found, 646.3343.
ꢀ
activated molecular sieves (5 A). CDCl3 from Cambridge Isotope
Laboratories was used after drying over activated molecular
ꢀ
sieves (5 A). NMR spectra of compounds were recorded on
1
a Bruker Avance 400 spectrometer (400.13 MHz for H, 100.62
MHz for 13C) at ambient temperature. Chemical shifts are given
in ppm, and are referenced against external Me4Si (1H, 13C). HR
EI-MS measurement (JEOL JMS700) was carried out at Korea
Basic Science Institute (Daegu). UV-Vis and photoluminescence
spectra were recorded on a Jasco V-530 and a Spex Fluorog-3
Luminescence spectrophotometer, respectively, in CH2Cl2 with
a 1 cm quartz cuvette at ambient temperature. Cyclic voltam-
metry experiment was performed using an AUTOLAB/
PGSTAT12 system. Atomic force microscopy (AFM) measure-
ments were carried out on a NanoMan (Veeco).
Polymerization procedure. An activated catalyst solution
(2.0 mM) was prepared in situ by the addition of chlorobenzene
(6.0 mL) to the mixture of [(NHC)Pd(h3-allyl)Cl] and 1.5 equiv
AgSbF6 followed by stirring for 2 h at room temperature. The
filtered solution of the activated catalyst (1.5 mmol) was intro-
duced into the chlorobenzene solution containing a prescribed
amount of monomer (1.5 mmol, [M1]/[Pd] ¼ 1000) and 1-octene
(0.1 mol% to monomer) to initiate polymerization. The poly-
Synthesis of monomer (M1)
ꢀ
merization was carried out at 25 C for 20 h. The mixture was
9-[40-Carbazol-9-yl-biphenyl-4-yl]-3-bromo-9H-carbazole (1).
To the o-dichlorobenzene (50 mL) slurry containing copper
(0.42 g, 6.6 mmol), 18-crown-6 (0.87 g, 3.3 mmol), and K2CO3
(5.53 g, 40.0 mmol) was added an o-dichlorobenzene solution of
3-bromocarbazole (2.46 g, 10.0 mmol) and 4-iodo-40-(carbazolyl)
biphenyl (5.34 g, 12.0 mmol) at room temperature. The reaction
mixture was heated to reflux and stirred for 30–36 h. After
cooling to room temperature, the reaction mixture was filtered
over a silica bed and the filtrate was evaporated to dryness. The
crude product was washed with acetone (30 mL ꢂ 3). The
washed product was recrystallized in chloroform, which afforded
the product as white powder. Yield ¼ 2.54 g (45%). mp ¼ 253–
255 ꢀC. 1H NMR (CDCl3, 400 MHz): d 7.27–7.36 (m, 4 H), 7.40–
7.53 (m, 7 H), 7.63–7.73 (m, 4 H), 7.88–7.93 (m, 4 H), 8.10 (d, J ¼
poured into a large volume of acidified methanol (5% v/v, 300
mL) to precipitate the polymer. After stirring for 1 h, the
precipitated polymer was collected by filtration and washed with
methanol (3 ꢂ 50 mL). The polymer was purified by dissolving in
CHCl3 and reprecipitating into methanol/acetone (v/v ¼ 4/1),
which was repeated three times. The obtained polymer was
finally dried in a vacuum oven at 70 ꢀC to constant weight. P1: 1H
NMR (CDCl3): d 0.5–2.0 (br, 15H), 2.1–3.1 (br, 4H), 7.21 (bs,
8H), 7.31 (bs, 11H), 7.83 (bs, 2H), 8.07 (bs, 2H). 13C NMR
(CDCl3): d 26.4–49.6 (br), 109.7, 120.0, 120.3, 123.4, 125.9, 126.9,
127.1, 128.1, 134.6, 136.9, 137.1, 138.4, 138.9, 140.6. Anal. Calcd for
P1: C, 89.12; H, 6.54; N, 4.33. Found: C, 89.63; H, 6.61; N, 4.35.
1
Polymer analysis. H and 13C NMR spectra of the polymer
7.7 Hz, 1 H), 8.15 (d, J ¼ 7.4, 2 H), 8.27 (d, J ¼ 1.8 Hz, 1 H). 13
C
were recorded on a Bruker Avance 400 spectrometer at ambient
temperature in CDCl3. The molecular weight (Mw) and mole-
cular weight distribution (Mw/Mn) of the polymer were analyzed
by gel-permeation chromatography (GPC) on a Viscotek T60A
equipped with UV and RI detectors using THF as an eluent at
35 ꢀC and calibrated with narrow polystyrene standards as
a reference. Thermogravimetric analysis (TGA) was performed
under N2 atmosphere using a TA Instrument Q500 at a heating
rate of 20 ꢀC minꢁ1 from 50 ꢀC to 800 ꢀC. Differential scanning
calorimetry (DSC) measurement was performed on a TA
Instrument Q100. Any thermal history in the ꢀpolymer was
eliminated by the first heating the samples at 20 C minꢁ1, and
then recording the second DSC scan at 10 ꢀC minꢁ1 to the
decomposition temperature.
NMR (CDCl3, 100 MHz): d 109.89, 110.15, 111.38, 113.02,
120.21, 120.43, 120.60, 122.64, 123.22, 123.76, 125.48, 126.09,
126.85, 127.58, 127.67, 128.58, 128.70, 128.81, 137.05, 137.69,
139.32, 139.79, 139.90, 141.09, 141.48. HR EI-MS: m/z Calcd for
C36H23BrN2, 562.1045; Found, 562.1041.
Synthesis of M1. To the THF slurry (50 mL) containing 1
(2.35 g, 4.0 mmol) was added slowly 2 equiv of t-BuLi (4.7 mL) at
ꢁ78 ꢀC. The reaction mixture was stirred at this temperature for
2 h and then allowed to warm to room temperature. A THF
solution (10 mL) of 5-norbornene-2-pentyl bromide (1.17 g, 4.8
mmol) was slowly added via cannula at 0 ꢀC. The reaction
mixture was allowed to warm to room temperature and stirred
overnight. After the addition of a saturated aqueous solution of
NH4Cl (30 mL), the organic portion was separated, and the
aqueous layer was further extracted with ether (2 ꢂ 30 mL). The
combined organic portions were dried over MgSO4, filtered, and
Cyclic voltammetry. Cyclic voltammetry measurement was
carried out with a three-electrode cell configuration consisting of
platinum working and counter electrodes and a Ag/AgNO3
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J. Mater. Chem., 2011, 21, 5422–5429 | 5427

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(0.1 M in CH3CN) reference electrode at room temperature. The
solvent was CH2Cl2 and 0.1 M tetrabutylammonium hexa-
fluorophosphate was used as the supporting electrolyte. The
oxidation potentials were recorded at a scan rate of 50 mV sꢁ1
and reported with reference to the ferrocene/ferrocenium (Fc/
Fc+) redox couple.
01001-0 for S. Yoo) and the Priority Research Centers Program
of the NRF (No. 2009-0093818 for M.H. Lee) funded by the
Korea Ministry of Education, Science and Technology is grate-
fully acknowledged.
Device fabrication. Glass substrates pre-coated with 150 nm
thick ITO were cleaned using soapy water, deionized water,
acetone, and isopropyl alcohol in an ultrasonic bath, and treated
by air plasma using a plasma cleaner (PDC-32G, Harrick
Plasma). An aqueous dispersion of PEDOT:PSS (Baytron
AI4083, H.C. Starck) was spun (2500 rpm for 30 s) onto ꢀthe
substrates. The samples were then dried on a hotplate (140 C)
for 10 min. P1 and PVK batches doped with Ir(ppy)3 were spin-
coated onto the substrates from a chlorobenzene solution
(0.9 wt% and 1 wt%, respectively) at 3000 rpm and then dried at
Notes and references
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ꢁ1
ꢁ1
ꢀ
ꢀ
ꢀ
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Financial support from the National Research Foundation of
Korea (No. 2010-0008264 for Y. Do and No. 2010-0007796 for
M.H. Lee), the ERC program of the NRF (No. R11-2007-045-
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