Synthesis and characterization of a new conjugated polymer containing cyano substituents for light-emitting diodes

Article abstract of DOI:10.1039/b008521i

A new luminescent conjugated polymer (CN-P3PV) containing cyano groups as electron transporting units was synthesized by a Suzuki coupling reaction. CN-P3PV was characterized by ¹H NMR, FT-IR, GPC, DSC, and TGA. The synthesized polymer possessed high thermal stability (T_d = 360 °C, T_g= 151 °C), high electron affinity, and good thin-film forming ability. Electrical characterization of a double-layer organic LED based on the structure of ITO/CuPc/CN-P3PV/Ca/Ag showed high electron transporting ability and good electroluminescence performance with the emission of bright orange light.

Full text of DOI:10.1039/b008521i

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Synthesis and characterization of a new conjugated polymer
containing cyano substituents for light-emitting diodes
Xia Wu, Yunqi Liu* and Daoben Zhu*
Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100080, China. E-mail: liuyq@infoc3.icas.ac.cn.
Received 23rd October 2000, Accepted 7th February 2001
First published as an Advance Article on the web 22nd March 2001
A new luminescent conjugated polymer (CN-P3PV) containing cyano groups as electron transporting units was
1
synthesized by a Suzuki coupling reaction. CN-P3PV was characterized by H NMR, FT-IR, GPC, DSC, and
TGA. The synthesized polymer possessed high thermal stability (Td~360 ‡C, Tg~151 ‡C), high electron
affinity, and good thin-film forming ability. Electrical characterization of a double-layer organic LED based on
the structure of ITO/CuPc/CN-P3PV/Ca/Ag showed high electron transporting ability and good
electroluminescence performance with the emission of bright orange light.
We report a new cyano-containing polymer, CN-P3PV,
Introduction
which can be recognized as a CN-PPV derivative,18 with two
less vinylene linkages for every four vinylene groups. The
number of CN substituents and vinylene groups is half that of
the CN-PPV reported by the Cambridge group.18 The cyano
group,19 an excellent electron-transporting moiety, is intro-
duced into the conjugated polymer to achieve desirable
properties of luminescence and good electron injection/
transporting ability. The hexyloxy side groups in the repeat
unit can enhance the solubility of the resulting polymer, and
therefore enhance the processability and quality of the thin film
in LED devices. The solubilizing side groups on the phenyl
groups twist the replaced phenylene rings considerably out of
plane and confine the rotation to the bonds that link the
aromatic ring systems. This drastically decreases the interac-
tion of the aromatic p-electron system, so that the polymer
should have a higher glass-transition temperature (Tg). In this
paper, we present our detailed results on synthesis, optoelec-
tronic properties, and electroluminescence of the polymer.
Since the first report by Cambridge group in 1990 on
electroluminescence (EL) in a polymer light-emitting diode
(LED),1 a great variety of polymers with different color
emission,2–4 electron affinities,5,6 ionization potentials7,8 and
photoluminescence (PL) efficiencies have been synthesized, and
their EL properties have also been extensively studied. Light
emission from a polymer LED is produced in the luminescent
polymer layer via recombination of electrons and holes from
two electrodes. The basic requirements for the organic layer are
that it should luminesce efficiently and be a good charge
transporter, and it must be capable of undergoing efficient
charge transfer to an electrode.9 The EL emission efficiency is
high on the balanced injection of the charges from the
respective electrodes.10,11 However, for the majority of
conjugated polymers investigated, electron injection has
proved to be more difficult than hole injection, because organic
materials and polymers tend to have low electron affinities.
This has been largely remedied by the use of metals with a low
work-function (especially Ca) as the cathode material. How-
ever, calcium is highly susceptible to atmospheric degrada-
tion.12 To circumvent these problems, several strategies have
been utilized, such as, using multilayer structures with one or
more charge transporting layers,13,14 and blending small
molecules into a polymer matrix.15 These methods have been
very successful.
Because electrical and optical properties (e.g. color and
emission efficiency) of conjugated polymers can be tailored by
manipulation of their chemical structures, the improvement of
EL devices requires optimization of not only the structure of
the device but also the chemical structure of the electro-
luminescent polymer, and recently synthetic design has become
a vital component in the optimization of polymer LEDs.16,17
We have been investigating polymers with balanced charge
injection by increasing the electron affinity of the polymer. The
introduction of strong electron affinity groups,18,19 e.g. cyano,
oxadiazole, halide, methylsulfinyl or trifluoromethyl groups,
onto a poly(p-phenylenevinylene) (PPV) backbone has proved
to be an effective way to lower the LUMO (to shift away from
the vacuum level) of a polymer and therefore enhance the
electron injection ability. With the enhancement of electron
injection of luminescent polymers, air-stable metals such as
aluminium can be employed without the loss of electrolumi-
nescent efficiency.
Results and discussion
Synthesis and characterization
A copolymer containing cyano groups was prepared with the
Suzuki coupling reaction,20 from the dibromo-terminated
phenylenevinylene monomer (1) and the diboronic acid (2)
followed by hydrolysis in the presence of aqueous HCl. The
synthetic route to this copolymer is illustrated in Scheme 1.
The reaction of 4-bromophenylacetonitrile and terephtha-
laldehyde easily occurred to produce 1,4-bis(b-cyano-p-bro-
mostyryl)benzene, monomer 1, in over 95% yield using NaOH
as the base.
2,5-Dibromo-1,4-bis(hexyloxy)benzene was synthesized
from 1,4-benzoquinone by a two-step reaction according to
the literature method.21 2,5-Dibromo-1,4-bis(hexyloxy)ben-
zene could be converted to 2,5-bis(hexyloxy)phenyl-1,4-
diboronic acid monomer 2, in 40–50% yield by reaction with
magnesium to form a bifunctional Grignard reagent followed
by treatment with trimethyl borate and acidic hydrolysis.20–22
The Suzuki coupling of monomer 1 with monomer 2 in the
presence of Pd(PPh3)4 and K2CO3 (aq.) generated polymer
CN-P3PV in 85% yield. CN-P3PV was purified by addition of
its chloroform solution to methanol and precipitation three
DOI: 10.1039/b008521i
J. Mater. Chem., 2001, 11, 1327–1331
This journal is # The Royal Society of Chemistry 2001
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Scheme 1 Synthetic route of CV-P3PV via the Suzuki coupling reaction.
times. Some insoluble solid was also obtained, which may be
the cross-linking by-products.
thermal decomposition. These results imply that the polymer is
a good candidate as an EL material.
CN-P3PV is an orange powder and can be dissolved easily in
conventional organic solvents such as methylene chloride,
chloroform, toluene and tetrahydrofuran. The good solubility
can be partially attributed to the two n-hexyloxy side-chains
that are attached on the repeat units. The 1H NMR spectrum of
CN-P3PV was well resolved and indicated a well-defined
polymer structure identical with the expected structure. Its
molecular weight was revealed by gel permeation chromato-
graphy (GPC) using THF as eluent and polystyrene as the
calibration standard. The weight-average (Mw) and the
number-average (Mn) molecular weights of the polymer were
3000 and 1600 (PDI~1.94), respectively. On the basis of the
modestly low molecular weight, CN-P3PV is more strictly an
oligomer than a polymer. From the information of elemental
analysis, we can deduce that the Br atoms remain as the main
terminal groups in the polymer. A pinhole free thin film was
obtained readily by the spin-coating technique from its CHCl3
solution.
Electrochemical and photophysical properties of CN-P3PV
In order to gain information on the charge injection,
electrochemical measurements were performed. Fig. 2 shows
a full scan cyclic voltammogram (CV) for CN-P3PV. It exhibits
irreversible waves under cathodic sweep and the main reaction
took place at 21.55 V (onset potential), which was higher than
its parent polymer, PPV (21.99 V).23a The reduction process
for a conjugated polymer in an electrochemical cell is related to
the electron capture ability (or electron injection ability).
Therefore it is reasonable to infer that the polymer CN-P3PV
has a higher electron affinity than PPV, owing to the
introduction of electron-withdrawing cyano groups. The
oxidation exhibits an irreversible wave when swept anodically
and the onset potential of oxidation for CN-P3PV is located at
0.82 V. This value is relatively high but lower than that for CN-
PPV (1.0 V). Since the oxidation process for a conjugated
polymer in an electrochemical cell is closely related to the
removal of electrons (or the injection of the holes) from the
HOMO of the material, a high oxidation potential of CN-
P3PV (0.82 V) and an irreversible anodic wave present a barrier
to hole injection. This may lead to an imbalance of charge
The thermal properties of CN-P3PV were analyzed by
thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) under a nitrogen atmosphere. A typical
TGA curve demonstrated a high thermal stability, up to
360 ‡C, with a heating rate of 20 ‡C min21 (Fig. 1). DSC
measurements revealed a glass transition temperature of 151 ‡C
and no melting points were observed for the polymer before
Fig. 2 Cyclic voltammogram of the CN-P3PV film spin-coated on ITO
glass. (Measured in an acetonitrile solution of TBAP (0.1 M) at a scan
rate of 40 mV s21, referenced vs. Ag/Agz and calibrated externally vs.
Fc/Fcz 0.12 V).
Fig. 1 Thermal gravimetric analysis (TGA) curve of CN-P3PV (under
a nitrogen atmosphere at 20 ‡C).
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Table 1 Electrochemical and optical parameters for polymer CN-
P3PVa
l
max/ nm EOg PT/ eV Eox/V Ered/V HOMO/eV LUMO/eV
CN-P3PV 520
2.38
0.82 21.55 25.50
23.13
aEFOC~0.12 V vs. Ag/Agz
.
injection and charge transport for a single layer LED made by
this polymer.
The redox potentials obtained from CV measurement for
CN-P3PV are listed in Table 1. The HOMO and LUMO values
were calculated by estimating the energy level of ferrocence
(EFOC) at 24.8 eV23b below the vacuum level. The formal
potential of FOC was measured as 0.12 V against Ag/Agz. The
low LUMO energy level (23.13 eV) will greatly facilitate the
electron injection from the cathode of the LED, indicating that
an air stable metal, such as Al, can be used as a cathode. The
band gap can be derived from the difference between the onset
of the first oxidation potential and the first reduction potential
(EgEC~2.37 eV).
Fig. 3 shows the UV and photoluminescent (PL) spectra of
CN-P3PV in CHCl3 solution and as a thin solid film. In the
absorption spectra of the polymer in CHCl3 solution, the p–p*
transition maximum and the edge of absorption appear at
414.5 nm and 520 nm, respectively. However, the absorption
spectrum of the polymer film was red shifted by 20 nm
compared with that of the polymer in solution. On the basis of
the absorption band edge, the optical band gap of CN-P3PV is
2.38 eV (lonset~520 nm, EOg PT~2.38 eV). It is obvious that this
value is in good agreement with the electrochemical value
obtained from the CV data (EEg C~2.37 eV). As compared to
the CN-PPV measured by Holmes and co-workers,18 the band
gap for CN-P3PV is slightly larger (2.16–2.10 eV) due to the
decrease of cyano-substituent content in the polymer chain.
The polymer fluoresced orange under the irradiation of UV
light and the emission maxima at 538 nm in CHCl3 solution
(excited at 365 nm) and 590 nm as a solid film (excited at
380 nm). As a film on a quartz substrate, the absorption bands
and PL spectra became broad and shifted toward the longer
wavelength region. This is due to the ground state energy
increasing with increasing intermolecular interaction as a result
of closer packing of the molecules in the film state. The
fluorescence peak of the polymer was dramatically blue-shifted
compared with that of CN-PPV (710 nm) owing to the
decreased number of CN substituents and vinylene groups in
CN-P3PV.
Fig. 4 I–V–L curve for the device of ITO/CuPc/CN-P3PV/Ca/Ag.
CN-P3PV may possess very good electron transporting
properties and it needs the CuPc layer to balance the overall
charge injection/transporting properties in order to optimize
the performance of the multilayer LED. The typical current–
voltage and light–voltage curves of this device are shown in
Fig. 4. Bright orange light emission could be seen easily under
normal light conditions. The current–voltage–brightness (I–V–
L) characteristics of the device revealed an excellent diode
behavior (Fig. 4), i.e., under the forward bias, the current and
the intensity of the emitting light increased superlinearly with
the increase of applied voltage after surpassing the turn-on
voltage (v7 V). Moreover, under reverse bias, no obvious
increase of current was observed when the applied voltage was
increased. The highest luminescence, before breaking the
device, was 213 cd m22 with an external quantum efficiency
of 0.011% at 10.8 V and a current density of 9.7 mA mm22
.
The electroluminescence spectra of the LED under different
drive voltages and the photoluminescence spectra of a thin film
of CN-P3PV are shown in Fig. 5. The PL spectra of CN-P3PV
film have an emission peak at 590 nm and a shoulder emission
at 555 nm when it was excited at 380 nm. The EL spectrum
from the device was almost identical with the PL spectrum
(film) of the polymer, indicating that the same excitation state
was involved. At low drive voltage (6 V), a peak at 730 nm was
noticeable, which could be observed from the PL spectrum.
This emission is due to the formation of an excimer, a result of
strong p electron repulsion as the separation distance is reduced
between molecules. The EL intensity was increased with an
increasing in applied voltages. Bright orange light emission was
observed in daylight under forward bias for the LED, implying
a high EL efficiency for the polymer. Considering that the
device structure has not been optimized in this study, this result
could be improved after the structural optimization.
Electroluminescence properties
To investigate the EL properties of the polymer, a two-layer
LED of ITO/CuPc/CN-P3PV/Ca/Ag was fabricated and
studied, where CuPc was used as a hole-transporting layer.
Fig. 3 UV-Vis and PL spectra for CN-P3PV in CHCl3 solution (dashed
line) and in solid film (solid line).
Fig. 5 Electroluminescent spectra of ITO/CuPc/CN-P3PV/Ca/Ag.
J. Mater. Chem., 2001, 11, 1327–1331
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spectrophotometer. 1H NMR spectral data are expressed in
ppm relative to internal standard tetramethylsilane and were
obtained on a Bruker dmx (300 NMR) and Varian (200 NMR)
spectrometers. Elemental analysis were taken on a Carlo Erba
1106 elemental analyzer, and bromine atom was analyzed by
mercurimetric determination
TGA and DSC measurements were performed on a Perkin-
Elmer Series 7 thermal analysis system under N2 at a heating
rate of 20 ‡C min21. The weight average molecular weight
(Mw) and polydispersity index of polymer were measured on
PL-GPC Model 210 chromatograph at 35 ‡C, using THF as
eluent and standard polystyrene as reference. Electrochemical
measurements were performed on a voltammetric analyzer
using cyclic voltammetry at room temperature in a conven-
tional three-electrode cell with a polymer thin film spin-coating
onto indium tin oxide (ITO) glass as the working electrode
(3 cm2). Pt gauze was used as the counter electrode and Ag/
Agz was used as the reference electrode, with 0.1 M
tetrabutylammonium perchlorate (TBAP) in acetonitrile as
the electrolyte.
The EL device was constructed using a double-layer
structure with copper phthalocyanine as the hole transporting
layer and CN-P3PV as the emitting layer. Prior to the
deposition of these organic films, the ITO-coated glass
substrate was cleaned by sequential ultrasonic washing in
detergent solution, deionized water, methanol, and chloroform
for 30 min, respectively, and finally rinsed with acetone for 2 h
in a Soxhlet extractor. CuPc, Ca and Ag layers were evaporated
under vacuum. A thin film of CN-P3PV was spin-coated from
CHCl3 solution at 2000 rpm for 20 s. The thicknesses of CuPc
and CN-P3PV were 50 nm, and the active area of the resulting
device was 0.07 cm2. All device testing was carried out at room
temperature under an ambient atmosphere. Photometric units
(cd m22) were calculated using the forward output power and
the EL spectra of the devices, assuming Lambertian distribu-
tion of the EL emission.25
Conclusion
In conclusion, a high performance light-emitting polymer
containing the electron-affinity moiety of cyano group was
synthesized via a Suzuki coupling reaction. This facile method
provides the synthesis of light-emitting polymers with a broad
variation of polymer backbones. The resulting polymer
possesses high thermal stability, high electron affinity, good
solubility and thin film forming properties. In addition, we
have shown the EL properties of the PPV derivative in organic
LEDs, and bright orange light emission was achieved for the
LED fabricated with this polymer. From these results we
believe that CN-P3PV is a new candidate for a light emitting
material in organic LEDs.
Experimental
Materials
All the solvents used in this study were purified according to
standard methods prior to use, and all other chemicals were of
reagent grade and were used as purchased without further
purification. 2,5-Dibromohydroquinone, 2,5-dibromo-1,4-
bis(hexyloxy)benzene, and monomer 2 were synthesized by
reactions according to the literature.20–22 The catalyst tetra-
kis(triphenylphosphine)palladium(0) was synthesized by a
literature method.24
Synthesis
1,4-Bis(b-cyano-p-bromostyryl)benzene (1). 4-Bromophenyl-
acetonitrile (1.96 g, 10 mmol) and terephthalaldehyde (0.62 g,
5 mmol) were dissolved in 50 mL of dry ethanol under a
nitrogen atmosphere in 100 mL three-necked round-bottomed
flask. A mixture of 50 mg of sodium hydroxide and 30 mL of
dry ethanol was added slowly, and then the crude product was
precipitated in the reaction mixture. The reaction mixture was
stirred for 1 h at room temperature, and the precipitate was
filtered out and washed with water. A yellow powder was
obtained.
1H NMR (200 MHz, d6-DMSO, ppm): d 8.1 (2H, s, vinyl H),
8.0 (2H, d, arom. H), 7.6–7.8 (8H, m, arom. H), 7.3 (2H, d,
arom. H). Anal. Calcd. for C24H14N2Br2: C, 58.81; H, 2.88; N,
5.71. Found: C, 58.45; H, 3.18; N, 5.51%.
Acknowledgements
The author would like to thank the National Natural Science
Foundation of China, a Key Program of Chinese Academy of
Sciences.
Polymer. Under N2, a mixture of 1 (0.490 g, 1 mmol), 2
(0.366 g, 1 mmol) and Pd(PPh3)4 (53 mg, 0.05 mmol) in THF
(7.5 mL) and K2CO3 (7.5 mL, 1 M aqueous solution) was
refluxed for 48 h. CHCl3 (100 mL) was added to this reaction
mixture, and the organic layer was separated. The solution was
washed with brine and dried over Na2SO4. The solution was
concentrated under vacuum, methanol was added to precipi-
tate out the polymer. The precipitation process was repeated 3
times. After drying under vacuum, a bright orange powder was
obtained.
1H NMR (300 MHz, CDCl3, ppm): d 0.90 (6H, m, CH3),
1.31–1.44 (12H, m, CH2), 1.73–1.88 (4H, m, CH2), 3.99 (4H, m,
OCH2), 7.05 (2H, m, arom. H), 7.81–7.56 (12H, m, arom. H),
8.05 (4H, m, arom. H and vinyl H). FT-IR (KBr, cm21):
3033.24 (w), 2955.26 (s), 2929.99 (s), 2858.89 (s), 2214.40 (m),
1605.08 (m), 1523.37 (w), 1492.10 (s), 1468.72 (s), 1423.35 (w),
1386.33 (s), 1261.55 (s), 1209.40 (s), 1053.23 (s), 1017.72 (s),
943.5 (w), 905.21 (w), 835.03 (s), 803.40 (s), 726.82 (w). Anal.
Found for CN-P3PV: [(C42H42N2O2)nBr2, n~3–5]: C, 77.53;
H, 6.79; N, 3.97; Br, 5.90%.
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