Highly efficient red electroluminescent polymers with dopant/host system and molecular dispersion feature: Polyfluorene as the host and 2,1,3-benzothiadiazole derivatives as the red dopant

Article abstract of DOI:10.1039/b712562c

By selecting polyfluorene as the polymer host, choosing 2,1,3-benzothiadiazole derivative moieties as the red dopant units and covalently attaching 0.3 mol% of the dopant units to the side chain of the polymer host, we developed a novel series of red electroluminescent polymers of dopant/host system with molecular dispersion feature. Their EL spectra exhibited predominant red emission from the dopant units because of the energy transfer and charge trapping from the polymer backbone to the dopant units. The emission wavelength of the polymers could be tuned by modifying the chemical structures of the dopant units. Single-layer devices (device configuration: ITO/PEDOT: PSS/polymer/Ca/Al) of these polymers emitted red light with a peak at 615 nm, a luminous efficiency of 5.04 cd A^-1 and an external quantum efficiency of 3.47%, or emitted deep-red light with a peak at 650 nm, a luminous efficiency of 1.70 cd A^-1 and an external quantum efficiency of 2.75%. Their high EL efficiencies were due to the energy transfer and charge trapping from the host to the dopant units as well as the molecular dispersion of the dopant units in the host. Increase of the dopant unit content led to increased turn-on voltages and decreased EL efficiencies of the resulting devices. The Royal Society of Chemistry.

Full text of DOI:10.1039/b712562c

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly efficient red electroluminescent polymers with dopant/host system
and molecular dispersion feature: polyfluorene as the host and
2,1,3-benzothiadiazole derivatives as the red dopant{
Jun Liu,ab Lei Chen,ab Shiyang Shao,ab Zhiyuan Xie,a Yanxiang Cheng,a Yanhou Geng,a Lixiang Wang,*a
Xiabin Jinga and Fosong Wanga
Received 15th August 2007, Accepted 5th November 2007
First published as an Advance Article on the web 23rd November 2007
DOI: 10.1039/b712562c
By selecting polyfluorene as the polymer host, choosing 2,1,3-benzothiadiazole derivative moieties
as the red dopant units and covalently attaching 0.3 mol% of the dopant units to the side chain of
the polymer host, we developed a novel series of red electroluminescent polymers of dopant/host
system with molecular dispersion feature. Their EL spectra exhibited predominant red emission
from the dopant units because of the energy transfer and charge trapping from the polymer
backbone to the dopant units. The emission wavelength of the polymers could be tuned by
modifying the chemical structures of the dopant units. Single-layer devices (device configuration:
ITO/PEDOT : PSS/polymer/Ca/Al) of these polymers emitted red light with a peak at 615 nm, a
luminous efficiency of 5.04 cd A21 and an external quantum efficiency of 3.47%, or emitted
deep-red light with a peak at 650 nm, a luminous efficiency of 1.70 cd A21 and an external
quantum efficiency of 2.75%. Their high EL efficiencies were due to the energy transfer and
charge trapping from the host to the dopant units as well as the molecular dispersion of the
dopant units in the host. Increase of the dopant unit content led to increased turn-on voltages and
decreased EL efficiencies of the resulting devices.
units18 all exhibit low-bandgap p-conjugation and the resulting
Introduction
copolymers all show red emission. However, red fluorescent
Since the initial discovery of electroluminescence (EL) from
polymers based on low-bandgap-conjugated fluorene copoly-
conjugated polymers,1 electroluminescent polymers have been
mers seldom show a luminous efficiency exceeding 1.5 cd A21
the subject of intense academic and industrial research because
because red chromophores are prone to aggregation in the
of their potential application in flat-panel displays. For full
solid state and are highly susceptible to concentration
color displays, blue, green and red light emitting polymers
quenching. Hence, it remains a challenging task to develop
are required. Among them, red light emitting polymers with
another approach to design efficient and bright red electro-
high EL efficiencies are scarce when compared with the other
luminescent polymers.
two colors.
Our group has proposed a novel approach to design light-
Polyfluorene and its derivatives have recently emerged as
emitting polymers of dopant/host systems with high EL
the most promising electroluminescent polymers due to
efficiencies and tunable emission colors by covalently attach-
their high photoluminescence (PL) and EL efficiencies, good
ing a small amount of dopant unit to the side chain of a
thermal and chemical stability.2–5 Normally, polyfluorene
polymer host (polyfluorene).20–29 Blue, green and white
itself emits blue light with a large bandgap. The common
light emitting polymers have been demonstrated with this
approach to realize red emission from polyfluorene derivatives
strategy. In this article, we further adopt the strategy of the
is to incorporate a narrow bandgap unit into the main chain of
dopant/host system to design a red light emitting polymer by
polyfluorene.3–19 For example, 4,7-dithienyl-2,1,3-benzothia-
covalently attaching a very small amount of red emissive
diazole,8–10 2,1,3-naphthaselenadiazole,11,12 39,49-dihexyl-
dopant unit to the side chain of a polymer host. The resulting
2,29;59,20-terthiophene-19,19-dioxide,13 cyanovinyl-containing
polymer can be considered as a system with a low content
units,14,15 3,4-diphenylmaleimide derivative units,16 2-pyran-
of red dopant unit molecularly dispersed in a polymer host,
4-ylidene-malononitrile17 unit and phenothiazine derivative
in which the concentration quenching of the red dopant
unit can be effectively suppressed. In the EL process,
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
since the emission comes from the red dopant unit with
Institute of Applied Chemistry, Chinese Academy of Sciences,
suppressed concentration quenching, remarkable EL efficien-
Changchun, (130022) P. R. China. E-mail: lixiang@ciac.jl.cn.;
cies can be expected. As the result, single-layer devices of the
resulting polymers emit red light with an emission maximum
at 615 nm, a luminous efficiency of 5.04 cd A21 and an
external quantum efficiency of 3.47%, or deep-red light with
an emission maximum wavelength of 650 nm, a luminous
Fax: +86-431-5685653.
bThe Graduate School, Chinese Academy of Sciences, Beijing, (100039)
P. R. China
{ This work is supported by the National Natural Science Foundation
of China (No. 20574067 and No. 50633040), Science Fund for Creative
Research Groups (No. 20621401) and 973 Project (2002CB613402).
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efficiency of 1.70 cd A21 and an external quantum efficiency
of 2.75%.
high yields. The three model compounds were de-methyled
by BBr3 and then reacted with 2,7-dibromo-9-hexyl-9-(6-
bromohexyl)fluorene (12) to afford the three red-dopant-
unit-containing monomers, Monomer R1, Monomer R2
and Monomer R3. All five red light emitting polymers
were prepared by the Suzuki polycondensation32 of 2,7-
Results and discussion
Design consideration
dibromo-9,9-dioctyl-fluorene
(13),
2,7-bis(trimethylene-
Scheme 1 shows the chemical structures of the target red
electroluminescent polymers of the dopant/host system, P-R1,
P-R2, P-R3, P-R2-50 and P-R2-100. Polyfluorene was selected
as the polymer host because of its large bandgap and good
film-forming properties.2–5 Three red emissive moieties based
on the electron-withdrawing 2,1,3-benzothiadiazole unit30 and
the electron-donating triphenylamine unit were designed and
selected as the dopant units (their model compounds are
MC-R1, MC-R2 and MC-R3, see Scheme 2). One or two
thiophene units were inserted between the electron donor and
the electron acceptor to tune the emission wavelength of the
dopant unit and consequently the emission wavelength of the
resulting polymers. All the three model compounds can form
dopant/host systems with polyfluorene in terms of spectral
overlapping and energy levels. The three dopant units were
covalently attached to the side chain of polyfluorene (host)
with alkyl spacers in order to realize the molecular dispersion
of the dopant units in the host.31 The contents of the dopant
units in P-R1, P-R2 and P-R3 were controlled to be 0.3 mol%
to achieve dominant emission from the red dopant units and
to avoid their concentration quenching effect. In order to
investigate the effect of the dopant unit content on the EL
performance of the resulting polymers, the contents of the
dopant unit in P-R2, P-R2-50 and P-R2-100 were tuned within
the range of 0.3–1.0 mol%.
borate)-9,9-dioctyl-fluorene (14) and the corresponding
red-dopant-unit-containing monomers with corresponding
feed ratios. Polyfluorene homopolymer (PF) was also synthe-
sized for comparison. All these polymers are readily soluble in
common organic solvents (e.g. toluene, chloroform, tetra-
hydrofuran) and can form high-quality thin films by spin-
coating. The number-average molecular weights (Mn) of P-R1,
P-R2, P-R3, P-R2-50 and P-R2-100, as determined by gel
permeation chromatography (GPC) with polystyrene as
standards, range from 21 100 to 26 300 with polydispersities
(PDI) ranging from 2.00 to 2.14. All these polymers exhibit
good thermal stability with thermal degradation temperature
(Td) higher than 420 uC.
Electrochemical properties
In order to investigate the LUMO and HOMO energy levels of
the three dopant units, we performed cyclic voltammetry mea-
surements of the three model compounds, MC-R1, MC-R2
and MC-R3, in CH2Cl2 solution. The results are shown in
Fig. 1a. Their onset oxidation voltages and onset reduction
voltages are listed in Table 1. According to the formulae
EHOMO = 2(Eox + 4.34) eV and ELUMO= 2(Ered + 4.34) eV,33
the LUMO and HOMO energy levels of the three model
compounds were calculated and are listed in Table 1. It is very
obvious that the insertion of thiophene units in these model
compounds leads to the increase of the HOMO energy levels as
well as the decrease of the LUMO energy levels. Despite their
difference, the LUMO and HOMO energy levels of all the
three model compounds all lie between those of PF (see
Fig. 1b), indicating that charge trapping of the red dopant
units in the EL process is energetically favorable.34,35
Synthesis
The synthetic routes of the model compounds, monomers
and polymers are outlined in Scheme 2. The three model
compounds, MC-R1, MC-R2 and MC-R3, were all synthesized
by the two-step Stille coupling of 4,7-dibromo-2,1,3-benzothia-
diazole and the corresponding tributylstannyl derivatives with
The electrochemical properties of the polymers were also
investigated by cyclic voltammetry with their thin films on a
glassy-carbon working electrode. P-R1, P-R2, P-R3, P-R2-50
and P-R2-100 all exhibit the same redox behavior as PF. The
incorporation of the small amount of red dopant units leads to
negligible changes in the electrochemical properties and energy
levels of the resulting polymers.
Photophysical properties
Fig. 2 shows the absorption spectra and photoluminescence
(PL) spectra of the three model compounds in toluene. The
corresponding absorption and PL maxima are listed in Table 1.
It is very interesting that the insertion of each thiophene unit
leads to a redshift by 25 nm in the absorption spectra and a
redshift by 35 nm in the PL spectra. Table 1 also lists the PL
quantum efficiencies (WPL) of the three model compounds in
toluene.36 MC-R1 and MC-R2 both exhibit high WPL values of
0.74 and 0.59, respectively. However, the WPL of MC-R3 is as
low as 0.24. It is obvious that the insertion of thiophene units
leads to decreased PL quantum efficiencies. As shown in
Scheme 1 Chemical structures of the red light emitting polymers of
the dopant/host system.
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Scheme 2 Chemical structures and synthetic routes of the model compounds, monomers and polymers. (Reagents and conditions: i) NBS, CCl4,
60 uC; ii) a) n-BuLi, THF, 278 uC, b) (n-Bu)3SnCl; iii) CuCl, 1,10-phenanthroline monohydrate, KOH, toluene, reflux; iv) Pd(PPh3)4, toluene,
100 uC; v) a) n-BuLi, THF, 278 uC, b) n-C6H13Br, c) Br2, CHCl3; vi) Br(CH2)6Br, NaOH (aq. 50 wt%), toluene, Bu4NBr, 80 uC; vii) BBr3, CH2Cl2,
278 uC; viii) K2CO3, ethanol, reflux; ix) Pd(PPh3)4, K2CO3 (aq. 2 M), Aliquat 336, toluene, 90 uC.)
Fig. 2a, the absorption spectra of MC-R1, MC-R2 and MC-R3
all overlap with the emission spectrum of PF, favoring Fo¨rster
energy transfer from the polyfluorene backbone to the red
dopant units in the three red polymers. In the case of MC-R3,
the spectral overlap is poor and inefficient energy transfer is
expected.
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Fig. 2 (a) Absorption spectra of MC-R1, MC-R2 and MC-R3 as well
Fig. 1 Cyclic voltammetry (a) and energy levels (b) of MC-R1,
MC-R2, MC-R3 and PF.
as PL spectrum of PF. (b) PL spectra of MC-R1, MC-R2 and MC-R3
in toluene.
Table 1 The photophysical properties, electrochemical properties
and energy levels of MC-R1, MC-R2 and MC-R3
excitation wavelength of 380 nm. The PL spectra of P-R1,
P-R2 and P-R3 (see Fig. 3c) all exhibit both blue emission
from the polyfluorene backbone and orange/red emission from
the three dopant units. Since the absorbance of the excitation
light at 380 nm by the red dopant units is negligible, we
attribute the strong orange/red emission band to the Fo¨rster
energy transfer from the polymer backbone to the three
dopant units. This is also supported by the aforementioned
spectral overlap. The red emission band in the PL spectrum of
P-R3 is relatively weak. This is probably caused by the low
WPL of the red dopant unit as well as the poor energy transfer
in the case of P-R3. For the PL spectra of P-R2, P-R2-50
and P-R2-100 (see Fig. 3d), with the increase of the dopant
unit content, the relative intensity of the red emission band
increases due to the more complete energy transfer.
labs
/
lPL
nm nm WPL
/
Eonset,ox
V
/
HOMO/ Eonset,red
eV
/
LUMO/
eV
V
MC-R1 464 595 0.74 0.83
MC-R2 499 620 0.59 0.78
MC-R3 534 645 0.24 0.75
25.17
25.12
25.09
21.29
21.17
20.97
23.05
23.17
23.37
Fig. 3a and b show the absorption spectra of the polymers in
solid films. P-R1, P-R2, P-R3, P-R2-50 and P-R2-100 all have
similar absorption spectra to that of PF with an absorption
peak at 393 nm attributed to the p–p* transition of the
polymer backbone. The absorption of the three dopant units at
464 nm/499 nm/534 nm cannot be clearly detected. Fig. 3c and
d shows the PL spectra of the polymers in solid films with an
Fig. 3 Absorption spectra (a, b) and PL spectra (c, d) of P-R1, P-R2, P-R3, P-R2-50 and P-R2-100 in thin films.
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Fig. 5 Voltage–current density–brightness characteristics of the
devices based on P-R2 (a) and P-R3 (b).
Fig. 4 EL spectra of the devices of P-R1, P-R2, P-R3, P-R2-50 and
P-R2-100.
backbone. This difference in the EL and PL spectra is
attributed to the charge trapping effect of the red dopant
units.21,37 In the EL process, both energy transfer and charge
trapping contribute to the red dopant unit’s emission.
However, in the PL process, charge trapping does not occur
and only energy transfer is involved. Hence, the relative
intensity of the red emission band is much larger in the EL
spectra compared to in the PL spectra. As shown in Fig. 4b,
the blue emission bands in the EL spectra of P-R2-50 and
P-R2-100 are more negligible than that in the EL spectrum of
P-R2 because of the more complete energy transfer and charge
trapping at higher dopant unit contents.
Electroluminescent properties
In order to investigate the EL properties of the polymers, we
fabricated single-layer devices with the structure of indium–tin
oxide (ITO)/poly(styrene sulfonic acid) doped poly(ethylene-
dioxythiophene) (PEDOT : PSS) (40 nm)/polymer (90 nm)/Ca
(10 nm)/Al (100 nm). Fig. 4 shows the EL spectra of these
devices at the current densities of about 2 mA cm22. The
corresponding emission maxima and CIE coordinates are
listed in Table 2. The EL spectra of P-R1, P-R2 and P-R3 all
exhibit predominant orange/red emission from the dopant
units and negligible blue emission from the polyfluorene
backbone. The device of P-R1 emits orange light with the
emission maximum at 580 nm and CIE coordinates of (0.50,
0.47). P-R2 is a red electroluminescent polymer with the
emission maximum at 615 nm and CIE coordinates of (0.59,
0.39). The device of P-R3 emits deep-red light with the
emission maximum at 650 nm and CIE coordinates of (0.63,
0.32). Comparison of an EL spectrum with the corresponding
PL spectrum shows that the contribution of the blue emission
band is much smaller in the EL spectra compared to in the PL
spectra. For example, the PL spectrum of P-R3 exhibits
dominant blue emission from the polyfluorene backbone and
only weak red emission from the dopant unit; in contrast, the
EL spectrum of P-R3 exhibits predominant red emission from
the dopant unit and negligible blue emission from the polymer
Table 2 summarizes the EL performance of the single-layer
devices of P-R1, P-R2 and P-R3. All these devices exhibit very
high EL efficiencies. For example, the device of P-R2 emits red
light with the CIE coordinates of (0.59, 0.39), turn-on voltage
of 5.0 V and maximum brightness of 12170 cd m2. At a current
density of 8.12 mA cm22, it exhibits the maximum luminance
efficiency of 5.04 cd A21, power efficiency of 1.76 lm W21 and
external quantum efficiency of 3.47%. Fig. 5a shows its voltage–
current density–brightness curve. The device of P-R3 emits deep
red light with the CIE coordinates of (0.63, 0.32), turn-on voltage
of 5.0 V and maximum brightness of 3240 cd m22. At a current
density of 10.68 mA cm22, it shows the maximum luminance
efficiency of 1.70 cd A21, power efficiency of 0.68 lm W21 and
external quantum efficiency of 2.75%. The voltage–current
density–brightness characteristics of this device are shown in
Table 2 EL performance of the devices of P-R1, P-R2, P-R3, P-R2-50 and P-R2-100
Turn-on
voltage/V
Luminous
efficiency/cd A21
Power
efficiency/lm W21
External quantum
efficiency (%)
Maximum
brightness/cd m22
CIE coordinates
(x, y)
lmax/nm
P-R1
P-R2
P-R3
P-R2-50
P-R2-100
5.0
5.0
5.0
5.5
6.0
10.46
5.04
1.70
4.13
2.40
5.30
1.76
0.73
1.73
0.78
3.96
3.47
2.75
2.86
1.69
14570
12170
3240
9197
7810
580
615
650
615
617
(0.50, 0.47)
(0.59, 0.39)
(0.63, 0.32)
(0.60, 0.39)
(0.61, 0.38)
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(2,1,3-benzothiadiazole derivative units) to the side chain of a
polymer host (polyfluorene). Their EL spectra exhibit pre-
dominant red emission from the dopant units due to the energy
transfer and charge trapping from the polymer host to the
dopant units. Single-layer devices (ITO/PEDOT/polymer/Ca/
Al) of these polymers emit red light with a maximum at 615 nm
and an external quantum efficiency of 3.47%, or with a maxi-
mum at 650 nm and an external quantum efficiency of 2.75%.
The remarkable EL performance is attributed to the energy
transfer and charge trapping from the host to the red dopant
unit as well as the molecular dispersion of the small amount of
red dopant units in the host. Our results indicate that intra-
molecular dopant/host systems are an efficient strategy to
develop light emitting polymers with very high EL efficiencies
and different emission colors (blue, green, red and white).
Further investigation of deep red electroluminescent polymers
of dopant/host systems with enhanced PL quantum efficiency
of deep-red dopant unit is in progress in our laboratory.
Experimental
27
The synthesis of 4-diphenylamino-1-bromobenzene (1),
tributyl(4-(diphenylamino)phenyl)stanne (2), 27 2-(4-(diphenyl-
27
Fig. 6 Dependence of the external quantum efficiencies on the current
densities of the devices of P-R1, P-R2, P-R3, P-R2-50 and P-R2-100.
amino)phenyl)thiophene (8),
tributyl(5-(4-(diphenylamino)
phenyl)thienyl-2)stannane (9) 27, 4,7-dibromo-2,1,3-benzothia-
diazole,38 9,9-dioctyl-2,7-dibromofluorene (13)39 and 9,9-dioc-
tyl-2,7-bis(trimethyleneborate)fluorene (14)39 have been
reported previously.
Fig. 5b. The dependence of the external quantum efficiencies on
the current densities of the three devices is shown in Fig. 6a. The
high EL efficiencies of the red electroluminescent polymers of the
dopant/host system are attributed to the energy transfer and
charge trapping from the polyfluorene backbone to the three
dopant units.21,37
4-(N-Phenyl-N-(4-methoxy)phenylamino)-1-bromobenzene (3)
A mixture of 4-bromo-diphenylamine (4.50 g, 20.0 mmol),
4-methoxy-1-iodobenzene (5.76 g, 24.5 mmol), CuCl (0.20 g,
2.0 mmol), 1,10-phenanthroline monohydrate (0.40 g,
2.0 mmol), KOH (11.2 g, 200 mmol) and toluene (45 mL)
was heated to reflux and stirred for 36 hours. After workup,
the reaction mixture was poured into brine and extracted with
CH2Cl2. The organic layer was washed with brine and then
dried with Na2SO4. The solvent was removed and the residue
was purified by column chromatography on silica gel to afford
the title compound as a white solid. Yield: 4.75 g (67%).
Table 2 also lists the EL performances of the devices of
P-R2, P-R2-50 and P-R2-100. It can be seen that the increase
of the dopant unit contents leads to the decrease of the EL
efficiencies (see Fig. 6b) and the increase of the turn-on
voltages of the resulting devices. The former is attributed to
the exciton quenching of the dopant unit at high content and
the latter is due to the charge trapping effect of the dopant
unit. At low driving voltage, the electrons/holes injected from
the cathode/anode are trapped by the dopant units near the
cathode/anode and cannot migrate to recombine with opposite
charge carriers. Therefore, high turn-on voltage is required for
the device to emit light. The higher the dopant unit content,
the more charge carrier traps, the higher the turn-on voltage.
In comparison, we fabricated a control device with a blend
of PF and 0.3 mol% MC-R3 as the emissive layer. Its EL
spectrum shows dominant red emission from MC-R3 and
negligible blue emission from PF. This device emits deep red
light with the maximum luminous efficiency of 1.25 cd A21
and power efficiency of 0.64 lm W21. This performance is
somewhat inferior to that of the device based on P-R3,
indicating that covalently attaching the dopant to the host and
molecular dispersion of the dopant in the host are favorable
for the excellent EL performance of the resulting polymers.
Tributyl(4-(N-phenyl-N-(4-methoxy)phenylamino)phenyl)-
stannane (4)
To a solution of 3 (1.77 g, 5.0 mmol) in THF (20 mL) at
278 uC was added n-BuLi (1.6 M solution in hexane, 3.2 mL,
5.2 mmol). After stirring for 1 hour, the mixture was added to
(n-Bu)3SnCl (1.5 mL, 5.5 mmol). The resulting mixture was
slowly warmed to room temperature and stirred overnight.
After workup, the mixture was poured into aqueous NaHCO3
and extracted with hexane. The organic layer was washed with
water and then dried with Na2SO4. Removal of the solvent
afforded the title compound as a light-yellow liquid. Yield:
2.83 g (100%).
Conclusion
4-(4-(Diphenylamino)phenyl)-7-bromo-2,1,3-benzothiadiazole
(5)
In summary, we have developed a series of red electro-
luminescent polymers of dopant/host systems by covalently
attaching very small amounts of red emissive dopant units
A mixture of 4,7-dibromo-2,1,3-benzothiadiazole (2.95 g,
10.0 mmol), 2 (1.60 g, 3.0 mmol), Pd(PPh3)4 (0.034 g,
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0.3 mmol) and toluene (50 mL) was heated to 100 uC and
stirred in the dark for 24 hours. After workup, the mixture
was poured into aqueous KF and extracted with CH2Cl2.
The organic layer was washed with water and then dried
with Na2SO4. The solvent was removed and the residue was
purified by column chromatography on silica gel to afford the
title compound as an orange-yellow solid. Yield: 0.92 g
(67%). 1H NMR (CDCl3, 300 MHz) d (ppm): 7.88 (d, 1H),
7.80 (dd, 2H), 7.53 (d, 1H), 7.21–7.16 (m, 4H), 7.32–7.25 (m,
6H), 7.06 (t, 2H).
4-(5-(4-(N-Phenyl-N-(4-methoxy)phenylamino)phenyl)thienyl-
2)-7-(5-(4-(diphenylamino)phenyl)thienyl-2)-2,1,3-
benzothiadiazole (MC-R3)
Compound MC-R3 was synthesized following the same
procedure for MC-R1 using 9 and 10 instead of 5 and 4.
Yield: 86%. 1H NMR (CDCl3, 300 MHz) d (ppm): 8.11 (d,
2H), 7.87 (s, 2H), 7.56 (t, 4H), 7.32–7.25 (m, 8H), 7.16–7.03
(m, 15H), 6.87 (d, 2H), 3.82 (s, 3H). Anal. calcd: C, 74.97; H,
4.44; N, 6.86. Found: C, 74.63; H, 5.08; N, 6.23%.
9-Hexyl-2,7-dibromofluorene (11)
4-(4-(Diphenylamino)phenyl)-7-(4-(N-phenyl-N-(4-methoxy)-
To a solution of fluorene (8.30 g, 50.0 mmol) in dry THF
(150 mL) at 278 uC was added n-C4H9Li (1.6 M solution in
hexane, 31.2 mL, 49.9 mmol). After stirring for one hour, the
mixture was added to n-C6H13Br (8.3 mL, 58.4 mmol). The
resulting mixture was slowly warmed to room temperature
and stirred overnight, followed by pouring into water and
extraction with CH2Cl2. The organic layer was washed with
water and then dried with Na2SO4. Removal of the solvent
afforded a colorless liquid. The liquid was mixed with CHCl3
(100 mL) and anhydrous FeCl3 (0.51 g, 0.31 mmol). Bromine
(5.7 mL, 110 mmol) was added dropwise to the mixture in the
dark and the resulting mixture was stirred at room tempera-
ture in the dark for 24 hours. After workup, the mixture was
washed subsequently with aqueous NaHSO3 and water,
followed by drying with Na2SO4. Removal of the solvent
afforded the crude product, which was further purified by
recrystallizaiton in ethanol to give the title compound as a
white solid. Yield: 13.68 g (67%). 1H NMR (CDCl3, 300 MHz)
d (ppm): 7.65 (s, 2H), 7.60 (d, 2H), 7.52 (d, 2H), 4.00–3.97 (m,
1H), 2.04–1.96 (m, 2H), 1.25–1.15 (m, 8H), 0.88 (t, 3H).
phenylamino)phenyl)-2,1,3-benzothiadiazole (MC-R1)
A mixture of 5 (0.91 g, 2.0 mmol), 4 (1.70 g, 3.0 mmol),
Pd(PPh3)4 (0.034 g, 0.3 mmol) and toluene (30 mL) was heated
to 100 uC and stirred in the dark for 24 hours. After workup,
the mixture was poured into aqueous KF and extracted with
CH2Cl2. The organic layer was washed with water and then
dried with Na2SO4. The solvent was removed and the residue
was purified by column chromatography on silica gel to afford
the title compound as an orange-red solid. Yield: 1.11 g (84%).
1H NMR (CDCl3, 300 MHz) d (ppm): 7.90 (dd, 3H), 7.77 (s,
2H), 7.38–7.18 (m, 19H), 7.12–7.02 (m, 3H), 6.91 (dd, 2H),
3.86 (s, 3H). Anal. calcd: C, 79.11; H, 4.94; N, 8.58. Found: C,
79.01; H, 4.46; N, 8.67%.
2-(4-(N-Phenyl-N-(4-methoxy)phenylamino)phenyl)thiophene
(6)
Compound 6 was synthesized following the same procedure
for MC-R1 using 3 and tributyl(thienyl-2)stannane instead of 5
and 4. Yield: 56%. 1H NMR (CDCl3, 300 MHz) d (ppm): 7.48
(d, 2H), 7.27 (m, 4H), 7.12–7.04 (m, 8H), 6.89 (d, 2H), 3.85
(s, 3H).
9-Hexyl-9-(6-bromohexyl)-2,7-dibromofluorene (12)
A mixture of 11 (8.1 g, 20 mmol), (n-C4H9)4NBr (0.32 g,
1.0 mmol), 1,6-dibromohexane (15.0 mL, 100 mmol), toluene
(30 mL) and aqueous NaOH (50 wt%, 50 mL) was stirred at
80 uC for 18 hours. After workup, the organic layer was
separated and washed subsequently with dilute chloric
acid, aqueous NaHCO3 and water, followed by drying with
Na2SO4. Removal of the solvent and the excessive 1,6-
dibromohexane at reduced pressure afforded the crude
product, which was further purified by column chromatogra-
phy to give the title compound as a white solid. Yield: 9.80 g
(86%). 1H NMR (CDCl3, 300 MHz) d (ppm): 7.54 (d, 2H),
7.46 (m, 4H), 3.31 (t, 2H), 1.97–1.90 (m, 4H), 1.70 (m, 2H),
1.22–1.07 (m, 12H), 0.80 (t, 3H), 0.62 (br, 4H).
Tributyl(5-(4-(N-phenyl-N-(4-methoxy)phenylamino)phenyl)-
thienyl-2)stannane (7)
Compound 7 was synthesized following the same procedure
for 4 using 6 instead of 3. Yield: 99%.
4-(5-(4-(N-Phenyl-N-(4-methoxy)phenylamino)phenyl)thienyl-
2)-7-bromo-2,1,3-benzothiadiazole (10)
Compound 10 was synthesized following the same procedure
for 5 using 7 instead of 2. Yield: 37%. 1H NMR (CDCl3,
300 MHz) d (ppm): 8.12 (d, 1H), 7.88 (d, 1H), 7.73 (d, 1H),
7.56 (d, 4h), 7.29 (m, 4H), 7.15–7.05 (m, 7H), 6.93 (d, 2H), 3.86
(s, 3H).
Synthesis of Monomer R1
4-(5-(4-(N-Phenyl-N-(4-methoxy)phenylamino)phenyl)-
thienyl-2)-7-(4-(diphenylamino)phenyl)-2,1,3-benzothiadiazole
(MC-R2)
To a solution of MC-R1 (0.33 g, 0.5 mmol) in CH2Cl2 (4 mL)
at 278 uC was added dropwise BBr3 (1.0 M solution in
CH2Cl2, 4.0 mL, 4.0 mmol) and the mixture was stirred at this
temperature for 2 hours. The reaction mixture was slowly
warmed to room temperature and stirred for 24 hours, followed
by the addition of several drops of water. After removal of the
solvent with reduced pressure at room temperature, the residual
was mixed with K2CO3 (2.07 g, 15 mmol), 2,7-dibromo-9-hexyl-
9-(6-bromohexyl)fluorene (0.40 g, 0.7 mmol) and ethanol
Compound MC-R2 was synthesized following the same
procedure for MC-R1 using 2 and 10 instead of 5 and 4.
Yield: 78%. 1H NMR (CDCl3, 300 MHz) d (ppm): 8.15 (d,
1H), 7.91 (d, 2H), 7.74 (d, 1H), 7.58 (d, 2H), 7.36–7.08 (m,
23H), 6.91 (d, 2H), 3.86 (s, 3H). Anal. calcd: C, 76.81; H, 4.66;
N, 7.62. Found: C, 77.34; H, 4.26; N, 7.09%.
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 319–327 | 325

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(15 mL) and then was heated to reflux for 24 hours. After
workup, the mixture was poured into water and extracted
with CH2Cl2. The organic layer was washed with water and
then dried with Na2SO4. After removal of the solvent, the
residue was purified by silica column chromatography to
give the title compound as an orange-red solid. Yield: 0.085 g
(15%). 1H NMR (CDCl3, 300 MHz) d (ppm): 7.90 (t, 4H),
7.67 (s, 2H), 7.56 (d, 2H), 7.51–7.48 (m, 4H), 7.36–7.04 (m,
21H), 6.86 (d, 2H), 3.88 (t, 2H), 2.01–1.93 (m, 4H), 1.64 (m,
2H), 1.29–1.07 (m, 10H), 0.82 (t, 3H), 0.65 (br, 4H). Anal.
calcd: C, 71.27; H, 5.36; N, 4.96. Found: C, 72.34; H, 4.78;
N, 4.46%.
0.497 mmol), 14 (0.2792 g, 0.500 mmol) and Monomer R1
(0.0033 g, 0.003 mmol) were used in the polymerization. GPC:
Mn = 2.16 6 104, PDI = 2.07. P-R2: red fibre. 13 (0.2726g,
0.497 mmol), 14 (0.2792 g, 0.500 mmol) and Monomer R2
(0.0036 g, 0.003 mmol) were used in the polymerization. GPC:
Mn = 2.63 6 104, PDI = 2.03. P-R3: violet-red fibre. 13
(0.2726 g, 0.497 mmol), 14 (0.2792 g, 0.500 mmol) and
Monomer R3 (0.0040 g, 0.003 mmol) were used in the
polymerization. GPC: Mn = 2.24 6 104, PDI = 2.09. P-R2-
50: red fibre. 13 (0.2713 g, 0.495 mmol), 14 (0.2792 g,
0.500 mmol) and Monomer R2 (0.0060 g, 0.005 mmol) were
used in the polymerization. GPC: Mn = 2.43 6 104, PDI =
2.14. P-R2-100: red fibre. 13 (0.2687 g, 0.490 mmol), 14
(0.2792 g, 0.500 mmol) and Monomer R2 (0.0123 g,
Synthesis of Monomer R2
0.010 mmol) were used in the polymerization. GPC: Mn
=
Monomer R2 was synthesized following the same procedure
for Monomer R1 using MC-R2 instead of MC-R1. Yield: 17%.
1H NMR (CDCl3, 300 MHz) d (ppm): 8.10 (d, 1H), 7.87 (d,
2H), 7.70 (d, 1H), 7.55–7.50 (m, 4H), 7.47–7.44 (m, 4H), 7.36–
7.08 (m, 23H), 6.80 (d, 2H), 3.84 (br, 2H), 1.95–1.89 (m, 4H),
1.60 (m, 2H), 1.25–1.05 (m, 10H), 0.78 (t, 3H), 0.61 (br, 4H).
Anal. calcd: C, 70.41; H, 5.16; N, 4.63. Found: C, 71.63; H,
4.98; N, 4.71%.
2.11 6 104, PDI = 2.00. P-R1, P-R2, P-R3, P-R2-50 and
P-R2-100 all exhibited similar 1H NMR and elemental analysis
results as those of PF.
Instruments
1H NMR spectra were recorded with a Bruker Avance 300
NMR spectrometer. The elemental analyses were performed
using a Bio-Rad elemental analysis system. The number- and
weight-average molecular weights of the polymers were
determined by gel permeation chromatography (GPC) using
Synthesis of Monomer R3
Monomer R3 was synthesized following the same procedure
for Monomer R1 using MC-R3 instead of MC-R1. Yield: 14%.
1H NMR (CDCl3, 300 MHz) d (ppm): 8.10 (br, 2H), 7.16
(br, 2H), 7.61–7.51 (m, 6H), 7.46–7.44 (m, 4H), 7.32–7.05 (m,
23H), 6.81 (t, 2H), 3.84 (br, 2H), 1.95–1.89 (m, 4H), 1.61 (m,
2H), 1.26–1.05 (m, 10H), 0.79 (t, 3H), 0.62 (br, 4H). Anal.
calcd: C, 69.65; H, 4.99; N, 4.33. Found: C, 70.03; H, 5.26;
N, 4.56%.
a
Waters 410 instrument with polystyrene as standard
and THF as eluent. UV-Vis absorption spectra were
measured by a Perkin-Elmer Lambda 35 UV/Vis spectrometer.
Photoluminescence spectra were recorded by a Perkin-Elmer
LS50B spectrofluorometer. Cyclic voltammograms of polymer
films on glassy carbon electrodes were recorded on an EG&G
283 (Princeton Applied Research) potentiostat/galvanostat
system at room temperature in a solution of n-Bu4NClO4
(0.10 M) in fresh acetonitrile at a scan rate of 100 mV s21. A Pt
wire and an Ag/AgCl electrode were used as the counter
electrode and the reference electrode, respectively. Cyclic
voltammograms of small organic molecules were carried out
with their solution in fresh dichloromethane. Other conditions
were the same as for polymer films except that a Pt disc was
used as the working electrode. The EL spectra and current–
voltage and brightness–voltage characteristics of devices were
measured with a Spectrascan PR650 spectrophotometer in the
forward direction and a computer-controlled Keithley 2400
instrument under ambient conditions.
General procedure of the polymers
A mixture of 2,7-dibromo-9,9-dioctylfluorene (13), 2,7-bis-
(trimethyleneborate)-9,9-dioctylfluorene (14), corresponding
red-unit-containing comonomer, Aliquat 336 (0.10 g,
0.25 mmol), tetrakis(triphenylphosphine)palladium (11.0 mg,
0.01 mmol) under argon was added 2.5 mL degassed 2 M
aqueous potassium carbonate and 6 mL degassed toluene. The
resulting mixture was stirred in the dark at 90 uC for 48 hours
and then poured into methanol. The precipitate was collected
by filtration, dried and then dissolved in dichloromethane. The
solution was washed with water and dried over anhydrous
Na2SO4. After most of the solvent had been removed, the
residue was poured into stirred methanol to give a fiber-like
solid. The polymer was further purified by extraction with
acetone for 24 hours. The reprecipitation procedure in
dichloromethane–methanol was then repeated several times.
The final product was obtained after drying in vacuum with a
yield of 45–60%.
Device fabrication
ITO glass plates were degreased in an ultrasonic solvent bath
and then dried in a heating chamber at 120 uC. The PEDOT :
PSS was spin-coated on the cleaned ITO at 3000 rpm for 60 s
and then baked for 15 min at 120 uC to give an approximate
thickness of 40 nm. The polymer layer (approximately 90 nm)
was then spin-coated onto the PEDOT/ITO coated glass
substrate in fresh toluene solution (15 mg ml21) under ambient
atmosphere. Finally, a thin layer of calcium (10 nm) followed
by a layer of aluminium (100 nm) was deposited in a vacuum
thermal evaporator through a shadow mask at a pressure
of 3 6 1023–5 6 1023 Pa. The active area of the diodes
was 10 mm2.
PF: light yellow fibre. 13 (0.2742 g, 0.500 mmol) and 14
(0.2742 g, 0.500 mmol) were used in the polymerization. GPC:
Mn = 2.47 6 104, PDI = 2.09. 1H NMR (300 MHz, CDCl3) d
(ppm): 7.87 (d, 2H), 7.72 (br, 4H), 2.10 (br, 4H), 1.14 (br,
24H), 0.81 (t, 6H). Anal. Calcd: C, 89.69; H, 10.31. Found:
C, 89.08; H, 10.02%. P-R1: orange-red fibre. 13 (0.2726g,
326 | J. Mater. Chem., 2008, 18, 319–327
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