Synthesis and characterization of polyquinolines for light-emitting diodes

Article abstract of DOI:10.1039/a903620b

A novel light-emitting polymer containing both a highly electron- affinitive segment, biquinoline, and a good hole transporting segment, dialkoxyphenylenevinylene, was synthesized and characterized. This polymer possessed excellent film-forming properties

Full text of DOI:10.1039/a903620b

J O U R N A L O F
Synthesis and characterization of polyquinolines for light-emitting
diodes
Michelle S. Liu, Yunqi Liu, R. Craig Urian, Hong Ma and Alex K-Y. Jen*
Department of Chemistry, Northeastern University 360 Huntington Avenue, Boston, MA 02115,
USA
C H E M I S T R Y
Received 6th May 1999, Accepted 19th May 1999
A novel light-emitting polymer containing both a highly electron-affinitive segment, biquinoline, and a good hole
transporting segment, dialkoxyphenylenevinylene, was synthesized and characterized. This polymer possessed
excellent film-forming properties, good thermal stability, charge injection properties and electroluminescence
efficiency. A greenish-yellow light emitting diode was fabricated using this copolymer as an emitter layer.
an electron-deficient biquinoline unit that achieved much
improved light-emitting performance (Chart 1).
Introduction
Polymer light-emitting diodes (LEDs), based on conjugated
polymers or polymers with conjugated segments or pendant
groups, have been extensively studied recently due to their
potential applications in large-area displays.1–12 In principle,
a polymer LED requires the injection of holes and electrons
into the emitter layer. The recombination of the injected
electrons and holes in the polymer layer generates singlet
excitons whose radiative decay produces visible light. There
are a number of factors that influence the performance of
LEDs. Some factors are intrinsic to the device design and the
methods of preparation. Other factors, such as the fluorescence
quantum efficiency, charge carrier mobility, and thermal stab-
ility are inherent properties of the emitting materials. Since
the first report of using poly( p-phenylenevinylene) (PPV) as
an emissive layer in a LED, much effort has been put into the
design and synthesis of suitable materials. It is known that
balanced and efficient charge injection/transport for electrons
and holes are crucial for achieving high device efficiency.
However, most of the electroluminescent (EL) polymers
investigated so far have unipolar character; there are rarely
good conductors for both holes and electrons. Due to imbal-
anced charge injection and transport, the charge recombination
in thin-film polymer LEDs often occurs close to the metal
electrode. Thus, luminescence quenching by the metal usually
results in a lower device efficiency. Bipolar types of EL
polymers13–15 are potential candidates that may achieve bal-
anced charge injection in a single layer LED, since they
combine all three desirable properties, electron transporting,
hole transporting, and light emitting, in a single species.
Polyquinoline (PQ) has been studied as a blue light-emitting
material in polymer LEDs. However, preliminary results from
the literature16 of a single layer LED device based on an
electron-deficient, fluorinated PQ exhibited low levels of light
emission and internal quantum efficiency (4×10−3% using Ca
as cathode) owing to poor hole injection. Recently, we have
demonstrated that, by introducing tetraphenyldiaminobi-
phenyl (TPD) moieties into a PQ backbone, the hole injecting
and transporting properties of the resulting polymer are sig-
nificantly improved. A single layer LED with aluminum as
the cathode showed a bright yellow light emission and had an
external quantum efficiency of 1.8×10−2%. Since TPD is a
much more efficient hole injecting/transporting material com-
pared to biquinoline for electron injecting/transporting, the
overall light-emitting property of the polymer may still be
affected by the imbalance of charge carriers. In this paper, we
report a polymer (PQ-MEH-PPV) which contained a less
electron-rich segment dialkoxyphenylenevinylene moiety and
Chart 1 Structures of the polymers.
Results and discussion
The design and synthesis of the monomers and PQ-MEH-PPV
polymer are outlined in Scheme 1. The bis(o-aminophenyl
ketone) 1 was synthesized (75% yield) according to a literature
procedure.17 Compound 2 was obtained from a Friedel–Crafts
reaction between bromobenzene and octanoyl chloride in
carbon disulfide (80% yield). A condensation reaction between
1 and 2 under acidic conditions afforded compound 3 (71%
yield). Monomer 4 was obtained (48% yield) by lithiation of
3 with n-BuLi (2.1 equiv.) at −78 °C and then quenching with
dimethylformamide. The Wittig–Horner polycondensation
reaction18 between 4 and 5 was carried out in THF using
potassium tert-butoxide as base and afforded polymer 6 as a
yellow powder. The 1H NMR spectrum of the polymer in
THF-d and elemental analysis demonstrated the expected
8
polymer structure. The introduction of dialkoxy and hexyl
side chains helped prevent early precipitation during the
polymerization process. The polymer was soluble in common
organic solvents, such as tetrahydrofuran, chloroform, and
toluene. Pinhole-free, homogeneous, thin films could be formed
by spin-coating from a CHCl solution of the polymer. The
3
weight average molecular weight of this polymer (M =40 200
w
with a polydispersity of 1.4) was determined by gel permeation
chromatography (GPC) using polystyrene standards. The ther-
J. Mater. Chem., 1999, 9, 2201–2204
2201

Fig. 2 UV–VIS spectra of polymer in chloroform solution and as
solid film.
The relatively sharp absorption edge indicated that the forma-
tion of the aggregates was minimal due to the long alkyl
chains attached to the polymer that prevented the polymer
chains from packing. The bandgap of the polymer (2.70 eV)
was determined by extrapolation to its absorption band edge.
Electrochemical measurements were performed using cyclic
voltammetry (CV) at room temperature in a conventional
three-electrode cell with a polymer thin film spin-coated onto
indium tin oxide (ITO) glass as the working electrode
(~3 cm2). Pt gauze was used as the counter-electrode and
Ag/Ag+ was used as the reference electrode, with 0.1 M
tetrabutylammonium perchlorate (TBAP) in acetonitrile as
the electrolyte. Fig. 3 shows the cyclic voltammogram of the
PQ-MEH-PPV. The polymer exhibited a reversible reduction
peak under cathodic sweep. However, the oxidative sweeping
only produced an irreversible peak, which is similar to that of
the MEH-PPV during an anodic sweep. The onset potentials
of the reduction and oxidation were located at −2.08 V and
+0.67 V vs. FOC (E +0.12 V vs. Ag/Ag+ ). Regarding the
FOC
energy level of the ferrocene/ferrocenium reference (−4.8 eV),
the HOMO and LUMO energy levels of PQ-MEH-PPV were
−5.47 eV and −2.72 eV, respectively (Fig. 4). Compared with
the HOMO and LUMO energy levels of TPD-PQ, the larger
energy barrier between the PQ-MEH-PPV/ITO interface made
the injection of holes more difficult, while the smaller energy
gap between the polymer and Al cathode facilitated the
injection of electrons. Therefore, a more balanced charge
injection could be achieved.
PQ-MEH-PPV was used as an emitter layer in a single layer
LED device using aluminum as the cathode. The polymer
layer was spin-coated from a chloroform solution onto an
ITO-coated glass substrate. The aluminum layer was then
deposited under vacuum (10−6 Torr) onto the polymer film as
the cathode. Fig. 5 shows the photoluminescence spectra of a
thin film of PQ-MEH-PPV and the electroluminescence spec-
trum of an ITO/PQ-MEH-PPV/Al device. The two spectra are
Scheme 1 Synthesis of PQ-MEH-PPV.
mal stability of the polymer was measured by thermogravi-
metric analysis (TGA) under a nitrogen atmosphere. Its
decomposition temperature was above 400 °C, as shown in
Fig. 1. The glass-transition temperature (T ) of 147 °C was
g
determined by differential scanning calometry (DSC) analysis.
These results are consistent with the thermal properties of a
typical rigid structure parent polymer with flexible side-chains
The UV–VIS spectra of PQ-MEH-PPV are shown in Fig. 2.
The polymer had an absorption peak (l ) at 406 nm, which
max
was derived from the p–p* transition of the conjugated system.
Fig. 3 Cyclic voltammogram of polymer film on ITO glass (Measured
in acetonitrile solution of TBAP (0.1 M) at 40 mV s−1, referenced
vs. Ag/Ag+).
Fig. 1 Thermogravimetric analysis of PQ-MEH-PPV.
2202
J. Mater. Chem., 1999, 9, 2201–2204

higher than that using MEH-PPV12 as emitting layer with the
same device configuration. This result indicated that a better
match of electron and hole injection from opposite electrodes
was achieved in this polymer. However, further work on
studying the mechanisms of charge transport in bipolar types
of polymers is still needed.
In conclusion, a new polymer composed of regularly
alternating biquinoline and dialkoxyphenylene vinylene moiet-
ies has been prepared by the Wittig–Horner polycondensation.
The polymer possesses excellent thermal stability and solu-
bility, and good charge transport and emissive capabilities. A
single layer LED device with the relatively air-stable aluminum
electrode using this polymer as an emitter layer showed a
respectable external quantum efficiency of 0.06%.
Experimental
NMR spectra were recorded on a Varian WM-300 (300 MHz)
spectrometer using deutero solvent as reference or internal
deuterium lock. The chemical shift data for each signal were
given in units of d relative to tetramethylsilane (TMS) where
d (TMS)=0. Differential scanning calometry (DSC) and
thermogravimetric analysis (TGA) for the polymers were
carried out under nitrogen at a rate of 10 °C per minute using
the Shimadzu instrument. UV–VIS spectra were recorded on
a Perkin-Elmer spectrophotometer (Lambda 9 UV/VIS/NIR).
Gel permeation chromatography (GPC) measurements were
carried out using a Waters Styragel (HR 4E, 7.8×300 mm)
column with polystyrene as the standard, and THF as the
solvent.
Fig. 4 Energy level diagrams of PQ-MEH-PPV and TPD-PQ.
Cyclic voltammograms of the polymer films (spin-coated
onto indium-tin oxide coated glass, ITO glass) were recorded
at room temperature in a typical three electrode cell with a
working electrode (ITO glass), a reference electrode (Ag/Ag+,
externally referenced against Fc/Fc+, 0.12 V), and a counter
electrode (Pt gauze) under a nitrogen atmosphere at a sweeping
rate of 40 mV s−1 (CV-50W Voltammetric Analyzer, BAS). A
solution of tetrabutylammonium perchlorate (TBAP) in
anhydrous acetonitrile was used as the electrolyte.
Fig. 5 Photoluminescence and electroluminescence spectra of the
polymer.
almost identical. This indicates that electron–hole recombina-
tion occurs in the polymer layer, resulting in the excitation of
the polymer. The emission peak was centered at 550 nm,
which was slightly red-shifted when compared to the
photoluminescence of the solution.
The typical current–voltage and light–voltage curves of this
device are shown in Fig. 6. Bright greenish-yellow light emis-
sion can be seen easily under normal light conditions and is
relatively stable at room temperature. Current and light arose
at the same voltage, and the two curves matched very well.
The turn-on voltage (18 V) was almost the same as that of
TPD-PQ (17 V). However, an external quantum efficiency of
0.06% was obtained, which was more than three times higher
than the device made by using the TPD-PQ and thirty times
LEDs fabrication and measurement
For the single layer devices, polymer solutions in chloroform
(8 mg mL−1) were used to spin-cast on top of the anodic
electrode, ITO glass (100 V/%, Delta Techn. Ltd.). The
cathode Al electrode (ca. 200 nm) was deposited by thermal
evaporation under vacuum (2×10−6 Torr). The film thickness
was measured with a Dektak surface profilometer (Model
3030). The active emissive area defined by the cathode was
about 8 mm2.
Photoluminescent spectra of the neat films (coated on quartz
glass) or solutions of the polymers were recorded by a CCD
system (Instaspec IV CCD, Oriel) under UV light excitation
(366 nm). Electroluminescence spectra of LEDs were recorded
with the same CCD system. I–V–L curves of the LEDs were
recorded by a semiconductor analyzer (4155B Semiconductor
Parameter Analyzer, Hewlett Packard) with a calibrated multi-
functional optical meter (model 2835-C, Newport).
Syntheses
Commercially available chemicals (Aldrich) were used without
further purification. Bis(o-aminophenyl ketone) 117 and
diphosphonate 518 were prepared according to the procedures
from the literature. All reactions were performed under a dry
nitrogen atmosphere.
Compound 2. To a 100 ml 3-necked flask with aluminum
chloride (14.66 g, 110 mmol) was added dropwise octanoyl
chloride (17.07 mL, 100 mmol). The resulting solution was
stirred at room temperature for 15 min, to which was added
Fig. 6 Current/light–voltage characteristics of an ITO/PQ-MEH-
PPV/Al device.
J. Mater. Chem., 1999, 9, 2201–2204
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dropwise bromobenzene (34.29 g, 218.4 mmol). The reaction
mixture was stirred at room temperature for another 5.5 h,
and then poured into a mixture of crushed ice (50 g) and
hydrochloric acid (25 mL). The product was filtered, washed
sequentially with 2% hydrochloric acid, 5% sodium hydroxide
solution and water, and dried to afford a white solid (22.6 g,
vacuo. The resulting solid was washed with a solution of
methanol, water, and acetone (35151) to remove any inorganic
by-products. The yellow color polymer was then Soxhlet
extracted with acetone for 12 h to remove any low molecular
weight products. The polymer was collected by filtration and
dried under vacuum for 1 day to give 150 mg of product
80%). 1H NMR (300 MHz, CDCl ) d 0.88 (t, J=8.6 Hz, 3H),
1.29–1.34 (m, 8H), 1.72 (quintet, 2H), 2.92 (t, J=7.6 Hz,
(50%). 1H NMR (300 MHz, THF-d ) d 0.73 (t, J=7.0 Hz,
3
8
12H), 0.87–0.97 (m, 17H), 1.00–1.07 (m, 8H), 2.56 (br, 4H,
2H), 7.59 (d, J=8.6 Hz, 2H), 7.82 (d, J=8.6 Hz, 2H).
a-CH ), 3.96–4.07 (br, 5H), 7.32–7.57 (m, 30H, vinyl and
2
aromatic). Anal. Calcd for C H N O : C, 86.52; H, 7.56;
73 76 2 2
N, 2.76. Found: C, 85.77; H, 7.48; N, 2.55%.
Compound 3. To a stirred solution of 3,3∞-dibenzoylbenzidine
1 (0.78 g, 2 mmol) and 4∞-bromoacetophenone 2 (1.70 g,
6 mmol) in glacial acetic acid (4.0 mL) was added concentrated
sulfuric acid (0.04 mL). The reaction mixture was refluxed for
50 h. During this period of time, additional acetic acid and
sulfuric acid were added to drive the dehydration to com-
pletion. After that, the reaction mixture was cooled down and
poured slowly into ice-cold water. Concentrated ammonium
hydroxide was added to neutralize the mixture. The resultant
pale yellow suspension was suction filtered, then washed with
water, methanol, and methylene chloride to get rid of the
starting materials. The product was a pale yellow powder
Acknowledgment
The authors would like to thank S. Liu for the molecular
weight measurements. Financial support from Office of Naval
Research (ONR) through the MURI Center (CAMP) and Air
Force Office of Scientific Research (AFOSR) through the
DURIP program is gratefully acknowledged.
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ated and the residue was purified through a short silica gel
column with methylene chloride–ethyl acetate=951 as eluent.
The product was a yellow powder (424.56 mg, 48%). 1H NMR
(300 MHz, CDCl ) d 0.69 (t, J=7.1 Hz, 6H), 0.84–1.25 (m,
3
16H), 2.53 (t, J=7.8 Hz, 4H), 7.27 (dd, J=8.8 Hz, J=2.9. Hz
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