Phenylene-functionalized polythiophene derivatives for light-emitting diodes: Their synthesis, characterization and properties

Article abstract of DOI:10.1039/b103717j

The design, synthesis and characterization of a new series of conjugated polymers, poly[(3-(4′-n-butylphenyl)thiophene-2,5-diyl)(2,5-dialkoxy-1,4- phenylene)(4-(4′-n-butylphenyl)thiophene-2,5-diyl)] are described in this contribution. Three polymers modif

Full text of DOI:10.1039/b103717j

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Phenylene-functionalized polythiophene derivatives for light-
emitting diodes: their synthesis, characterization and properties{
Ai-Lin Ding, Jian Pei, Yee-Hing Lai and Wei Huang*
Institute of Materials Research and Engineering and Department of Chemistry, National
University of Singapore, 3 Research Link, Singapore 117602, Republic of Singapore.
E-mail: wei-huang@imre.org.sg
Received 25th April 2001, Accepted 26th June 2001
First published as an Advance Article on the web 31st October 2001
The design, synthesis and characterization of a new series of conjugated polymers, poly[(3-(4’-n-
butylphenyl)thiophene-2,5-diyl)(2,5-dialkoxy-1,4-phenylene)(4-(4’-n-butylphenyl)thiophene-2,5-diyl)] are
described in this contribution. Three polymers modified by phenyl groups have been successfully synthesized via
FeCl3-oxiditive polymerization. The well-defined structure of the polymers is fully verified by elemental
1
analysis, FT-IR, and H and 13C NMR spectroscopy. All polymers show good thermal stability and solubility
in common organic solvents. The absolute photoluminescence (PL) efficiencies of the polymers in the neat film
can be up to 11%. The electrochemical properties of the polymers indicate that their HOMO and LUMO
energy levels can be adjusted by means of the aromatic groups both in the side chain and the backbone
structure. Yellowish green electroluminescence is achieved from single-layered polymer light-emitting diodes
(PLEDs) with the configuration ITO/polymer/Ca or Al.
In this work, we present the design, synthesis, character-
Introduction
ization and properties of a series of conjugated polymers, i.e.,
symmetrically substituted polymers, poly[(3-(4’-n-butylphenyl)-
thiophene-2,5-diyl)(2,5-bis(decyloxy)-1,4-phenylene)[(4-(4’-
Conjugated polymers exhibit great potential for electronic and
photonic applications because they combine the processing and
mechanical advantages of polymers with the electronic, optical
and physical properties of metals or semiconductors.1 In
particular, enormous efforts have been made toward the
development of conjugated polymers as light-emitting materi-
als since the first discovery of electroluminescence (EL) from
poly(p-phenylenevinylene) (PPV).2–5 So far, a wide range of
conjugated polymer systems, including poly(p-phenylene-
vinylene) (PPV),4–7 poly(p-phenylene) (PPP),8 polyfluorene
(PF),9–12 polythiophene (PT)13–15 and their derivatives, have
been investigated as active materials in PLEDs.
During the past two decades, intensive studies have been
devoted to the synthetic methodology, structure characteriza-
tion, electrochemical properties, stability, and electrical con-
ductivity of polythiophene derivatives (PTs).13–18 However, the
low PL quantum efficiency in the solid state (typically 1–3%)
and poor EL performance of processable polythiophenes (PTs)
have limited their application as emissive materials in PLEDs.
The low PL efficiency of PTs could be attributed to the
intersystem crossing of excitons from the singlet to the triplet
state and the intrinsic features of the electronic structure of
PTs.19,20 Therefore, there is still considerable interest in
introducing some special functional groups to modify proces-
sable polythiophenes to achieve high PL quantum efficiency
and balance of charge transporting.
n-butylphenyl)thiophene-2,5-diyl)]
(PPTDOPPT,
4a),
poly[(3-(4’-n-butylphenyl)thiophene-2,5-diyl)(2,5-bis(2-ethyl-
hexyloxy)-1,4-phenylene)(4-(4’-n-butylphenyl)thiophene-2,5-
diyl)] (PPTEHOPPT, 4c),27 and asymmetrically substituted
polymer,
poly[(3-(4’-n-butylphenyl)thiophene-2,5-diyl)(2-(2-
ethylhexyloxy)-5-methoxy-1,4-phenylene)(4-(4’-n-butylphenyl)-
thiophene-2,5-diyl)] (PPTEHMOPPT, 4b), which are composed
of phenylene moieties and phenylene-substituted thiophene
moieties along the backbone, to examine the effect of sub-
stitution on polymer structure and ultimately their physical,
optical, and electrochemical properties.
The pendant phenylene ring at the 3-position of the
thiophene ring is expected to act as a molecular bumper to
separate the polymer chains and thus reduce the intersystem
crossing of excitons. It also allows us to control the connec-
tion manner between two adjacent thiophene rings as a
defined head-to-head pattern. The head-to-head conformation
makes the polymer chains highly twisted. Furthermore, the
4-butylphenyl group on the thiophene ring can activate the
thiophene ring more effectively than an alkyl group to yield
polymers with higher molecular weights.28 We also investigate
the effect of the phenylene groups on the backbone on
improvement of the PL quantum efficiency and the adjustment
of the optical properties and the HOMO and LUMO energy
levels of the resulting polymers.
Through symmetrically and asymmetrically incorporating
long flexible alkoxy side chains onto the phenylene moiety of
the polymer backbone, we could examine the effect of the side
chain, as well as the symmetry of the repeat unit on the phy-
sical and electrical properties of the resulting polymers. Other
important effects of these alkoxy side chains are not only to
give good solubility to the polymers, which is essential for
electroluminescence (EL) materials, but also to improve the PL
quantum yields of the desired polymers.23,25
Most recently, it has been demonstrated that the PL effi-
ciency could be significantly increased from a few percent to
over 30% by modifying the backbone structure or/and the side
chain of thiophene-containing oligomers and polymers.19–27
The aromatic groups modifications are efficient in improving
the PL efficiency, tuning the optical properties and adjusting
the HOMO and LUMO energy levels of the resulting polymers.
{Electronic supplementary information (ESI) available: experimental
details for the preparation of compounds 2, 3a–c and 4a–c. See http://
www.rsc.org/suppdata/jm/b1/b103717j/
3082
J. Mater. Chem., 2001, 11, 3082–3086
This journal is # The Royal Society of Chemistry 2001
DOI: 10.1039/b103717j

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transmetallation reaction with anhydrous zinc chloride in
THF. The coupling reaction between 4-(4-n-butylphenyl)-2-
thienylzinc chloride and 1,4-dialkoxy-2,5-dibromobenzene (1)
in the presence of tetrakis(triphenylphosphine)palladium(0)
gives the monomers (3).
The oxidative polymerizations of the monomers using ferric
chloride were carried out in dry chloroform. However, the crude
polymers should be fully dedoped by ammonium hydroxide in
order to avoid the negative effect of FeCl3 residues on the
performance of the polymers as emissive materials.30,31 After
fully dedoping and precipitating, the solids collected by fil-
tration were washed with acetone for 2 days and extracted with
chloroform in a Soxhlet. After the solvent was evaporated, the
polymers were obtained as yellow solids. All the polymers
showed good solubility in common organic solvents such as
chloroform, THF, toluene and xylene. The structure and the
purity of both monomers and polymers were confirmed by 1H
and 13C NMR spectroscopes, FT-IR and elemental analyses.
The molecular weights were measured by means of gel per-
meation chromatography (GPC) using THF as eluent and
polystyrene as standard. A summary of the molecular weights
is listed in Table 1. In comparison with our previous reports,23,25
this series of polymers has much higher molecular weights,
which shows that the phenylene group at the 3-position can
activate the thiophene ring more effectively than alkyl groups.
Basic characterization
A comparison between the 1H NMR spectra of monomer
3c and polymer 4c is shown in Fig. 1 as representative. In the
1H NMR spectrum of monomer 3c, the singlet peak at
7.29 ppm is assigned to two protons at the phenylene ring of
the backbone. Two other doublet peaks at 7.57 and 7.23 ppm
with a coupling constant of 8.07 Hz are consistent with the
resonance of the adjacent protons at the phenylene rings of the
side chain. In the aromatic region, the remaining two doublet
peaks centered at 7.81 and 7.42 ppm with a coupling constant
of 1.3 Hz are attributed to the protons on the thiophene rings.
Since the coupling constant is much lower than that of the
adjacent protons (normally about 5.2 Hz) at the thiophene
ring, it demonstrated that the two doublet peaks result from
the resonance of the protons at the 3 and 5-positions of the
Scheme 1 The synthetic route to the polymers. Reagents and conditions:
(i) Mg, Ni(dppp)Cl2, THF; (ii) bromine, I2; (iii) n-BuLi, diisopropyl-
amine, ZnCl2, Pd(PPh3)4, THF; (iv) FeCl3, CHCl3, 0 uC–rt.
Table 1 Basic properties of three polymers
Polymers Mn 6 104 Mw 6 104 Polydispersity Td/uC) Tg/uC
4a
4b
4c
8.23
6.85
4.36
18.0
10.4
7.34
2.19
1.51
1.68
350
400
350
85
135
90
Results and discussion
Preparation of polymers
The approach to poly[(3-(4’-n-butylphenyl)thiophene-2,5-diyl)-
(2,5-dialkoxy-1,4-phenylene)(4-(4’-n-butylphenyl)thiophene-2,5-
diyl)] is outlined in Scheme 1.
The starting material, 3-(4-n-butylphenyl)thiophene (2), is
prepared by the Grignard coupling reaction between 4-n-
butylphenylmagnesium bromide and 3-bromothiophene em-
ploying Ni(dppp)Cl2 as the catalyst. Alkoxy substituents are
introduced onto the benzene ring through the use of the
Williamson ether route from hydroquinone for symmetric
disubstitution and 4-methoxyphenol for asymmetric substitu-
tion.29 The derivatives with long alkoxy side chains were liable
to be selectively brominated with bromine at the 2 and 5
positions of the phenyl ring.
The coupling reaction between an organic halide and excess
organic zinc chloride compound is employed to give the
monomers according to our previous papers.23–27 4-(4-n-
Butylphenyl)-2-thienylzinc chloride is prepared by the lithia-
tion of the 3-(4-n-butylphenyl)thiophene and following the
Fig. 1 Comparison of 1H NMR spectra between monomer 3c and
polymer 4c.
J. Mater. Chem., 2001, 11, 3082–3086
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thiophene ring. The low coupling constant is due to the far
distant coupling resonance between these two protons. These
results prove that we have successfully confined the coupling
reaction between the 3-(4-n-butylphenyl)thiophene and the
substituted 1,4-dibromobenzenes at the 5-positions of the
thiophene rings with high selectivity ensuring the high
regioregularity of the resulting polymers.
After polymerization, besides the two doublet peaks assigned
to the protons on the side chain phenylene, only two singlet
peaks are observed, which are attributed to the resonance of the
protons at the 3 position of the thiophene ring and on the back-
bone phenylene respectively. The chemical shift of the proton
on the thiophene ring is increased by 0.12 ppm after the poly-
merization because of the enhancement of p-delocalization.
On the contrary, the chemical shifts of the two protons of the
n-butylphenyl ring decrease. The reason for the phenomenon is
that after the polymerization, the thiophene rings have a larger
electron donating effect towards the 4-n-butylphenyl group.
Thus the electron density of the phenyl ring increases and the
resonances of these two protons on the n-butylphenyl ring shift
to higher field. Moreover, because the C3-H is in an ortho
position relative to the thiophene rings, it is more affected than
the C5-H, which is meta relative to the thiophene ring. Hence,
the chemical shift of C3-H decreases 0.60 ppm, which is larger
than that of C5-H (0.33 ppm).
Compared with the spectrum of the monomer 3c, the signal
corresponding to C2-H in the monomer completely disappears
after the polymerization, which is caused by proton elimination
from the 5 and 5’ monomer position during polymerization.
The absence of any residue of C2-H demonstrates that all the
polymers have high regioregularity of head-to-head linkage
between two adjacent thiophene rings.
The thermal stability of polymers was studied using
thermogravimetric analyses (TGA). All the polymers exhibited
the onset of degradation above 300 uC with no weight loss at
lower temperature. Table 1 also shows the onsets of decom-
position (Td) and glass transition temperatures (Tg) studied by
differential scanning calorimetry (DSC). All the polymers have
glass transition temperatures higher than 80 uC. The glass
transition temperature of polymer 4b is much higher than that
of polymer 4a and 4c, which agrees with the fact that the
temperature of glass transition decreases as the length of the
side chain increases.
Fig. 2 Spectra of UV-Vis absorption and photoluminescence spectra of
the polymer films.
spectra of polymers 4 are quite similar to those of poly-
[(4-hexylthiophene-2,5-diyl)(2,5-bis(decyloxy)-1,4-phenylene)(4-
hexylthiophene-2,5-diyl).23,25 It seems that the phenylene group
at the side chain might not enhance the conjugation length of
the resulting polymers compared with the alkyl groups.
The absorption spectra between the film and the solution of
every polymer are quite similar. It is indicative of the lack of
polymer interchain interaction even in the film states. The
results prove that the insertion of a 4-n-butylphenyl group onto
the 3-postion of the thiophene ring could obviously separate
the polymer chains. It might reduce the opportunities for
exciton transfer between chains and thus enhance the PL
efficiency of the polymers.
The onsets of absorption, which correspond to an approxi-
mation of the band gaps, are calculated to be 2.36 eV for
polymer 4a, 2.35 eV for polymer 4b and 2.38 eV for 4c. It is
shown that the band gaps of the three polymers are almost the
same. The pendant phenyl groups influence the coplanarity
of the polymer backbone due to the steric hindrance of the pen-
dant groups. In addition, the phenyl groups conjugated with
the polymer backbone may decrease the band gap. These two
effects are opposite on the band gap of the polymer. By com-
parison with the bandgap of poly[(4-hexylthiophene-2,5-diyl)-
(2,5-bis(decyloxy)-1,4-phenylene)(4-hexylthiophene-2,5-diyl)],
2.36 eV, and that of poly[(4-cyclohexylthiophene-2,5-diyl)(2,5-
bis(decyloxy)-1,4-phenylene)(4-cyclohexylthiophene-2,5-diyl)],
2.48 eV, it indicates that the phenylene group on the side chain
does not enhance the conjugation of the whole molecule.
However, the phenylene group in the backbone still plays the
most important role in the conjugation length.
Optical properties
The optical properties of the polymers were investigated by
absorption and photoluminescence (PL) spectroscopy. Films of
the polymers were prepared onto quartz plates or onto micro
slides by spin-casting from their solutions in xylene (1.5% w/v).
Under ultraviolet light irradiation, the films emitted intense
fluorescence of a yellowish green colour. The absorption and
photoluminescence spectra of the polymers measured from the
films are shown in Fig. 2. There are two peaks in the absorption
spectra of the solutions and the films of the polymers. One is
due to the p – p* transition of delocalized p electrons along the
backbone and the other is due to the conjugation of the side
chain. The peak wavelengths of the spectra and the p – p* band
gaps of the polymers estimated from the absorption onset
wavelengths are summarized in Table 2. Fig. 2 shows that the
absorption maxima of polymer films are about 430–440 nm.
According to Andersson et al., the maximal absorption of the
film of regioregular poly[3-(4-octylphenyl)thiophene] occurs at
493 nm.28 Compared with poly[3-(4-octylphenyl)thiophene]
the maxima of polymers 4 are blue-shifted up to 50–60 nm.
This shift is attributed to the insertion of the phenylene ring
into the polythiophene backbone since it induces a larger
torsion angle between the thiophene and the phenyl rings and
between the two adjacent head-to-head thiophene rings, and
therefore lowers the conjugated length. The absorption and PL
The absolute PL quantum efficiencies of the polymers in neat
films are measured in an integrated sphere at room temperature
in air following the procedure described by Greenham et al.
Table 2 Optical and electrochemical properties of the polymer films
Polymers
4a
4b
4c
Abs. lmax/nm
442, 292
426, 286
429, 287
Abs. lonset/nm (band gap/eV)
PL lmax/nm
PL quantum yield (%)
525 (2.36) 528 (2.35) 522 (2.38)
532 538 533
11 ¡ 2 12 ¡ 2 11 ¡ 2
p-doping
Eonset/V
Epa/V
Epc/V
0.66
0.79
0.62
0.64
0.79
0.67
0.73
0.81
0.74
n-doping
Eonset/V
Epc/V
Epa/V
HOMO
LUMO
21.65
21.73
21.64
5.06
2.75
2.31
21.64
21.79
—
5.04
2.76
21.64
22.09
—
5.13
2.79
Energy levels/eV
Band gap/eV
2.28
2.34
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using an argon ion laser line of 358 nm as the excitation
source.32 The results are also shown in Table 2. It shows that
the absolute PL quantum efficiencies of the polymer films are
over 10%, which are higher than those of normal conjugated
polymers based on thiophenes. Although it proves that
introducing the phenylene ring onto the polythiophene is a
useful approach in increasing the PL quantum efficiency of
thiophene-based polymers, the PL quantum efficiencies of
polymers 4 are about 60–70% lower than that of poly[(4-
hexylthiophene-2,5-diyl)(2,5-bis(decyloxy)-1,4-phenylene)(4-hexyl-
thiophene-2,5-diyl)], and 30% lower than that of
poly[(4-cyclohexylthiophene-2,5-diyl)(2,5-bis(decyloxy)-1,4-phe-
nylene)(4-cyclohexylthiophene-2,5-diyl)]. It could be attributed
to the change in the supramolecular organization effected by
the aromatic groups at the side chain of the thiophene ring.
Fabrication of PLEDs
Typical single-layered polymer light-emitting diodes were
fabricated using these new polymers as the active materials.
Indium-tin-oxide (ITO) coated glass (y20 V/%) was used as
the substrate. Polymer films were spin-cast on the substrates
from their solutions in xylene (2% w/v, 1500 rpm) onto indium-
tin-oxide (ITO)-coated glass substrates. The thickness of the
films was around 100 nm. A calcium or aluminium layer (100–
200 nm) was then deposited on the top of the polymer films
under a pressure around 1026 Torr at evaporation rates of
5–8 A s21. Diode area was 15 mm2 defined by the cathode.
˚
Device characteristics were measured with a calibrated Si
photodiode. All the processes and measurements were carried
out in a dry box filled with nitrogen. In forward bias, the ITO
electrode was wired as the anode.
The diodes emitted visible light above sufficient voltage under
the forward bias. The current–voltage and light intensity–
voltage characteristics of a device with the ITO/polymer 4c/Ca
configuration are given in Fig. 4 as a representative example.
For polymer 4c, the turn-on voltage is around 7 V using Ca as
the cathode. The current and the light emission both increase
rapidly almost at the same voltage. The device of polymer 4c
turns on around 9 V by using Al as the cathode. The fact that
changing the cathode material from Al to Ca decreases the turn
on voltage may be attributed to the lower work function of Ca
and thus the lower energy barrier for electron-injection at the
cathode contact. However, a relatively high external EL
quantum efficiency of 0.1% is achieved whether using Ca or
using Al as cathode. Polymer 4a and polymer 4b exhibit similar
properties. These results demonstrate the potential of these new
polymers as emissive materials in polymer LEDs, although the
optimization of polymer chemical structure and of device
structure is necessary to improve the performance of PLED
devices based on the new series of polymers.
Electrochemical properties
The redox behavior as well as the HOMO and LUMO energy
levels of the polymers were investigated by cyclic voltammetry
(CV) in an electrolyte consisting of a solution of 0.10 M
tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) in
acetonitrile. The cyclic voltammograms of the polymers are
displayed in Fig. 3. In the cyclic volatmmogram of polymer 4a,
for example, it shows a reversible p-doping process and a
partially reversible n-doping process. On sweeping the polymer
anodically, the oxidation process of polymer 4a onsets at
0.66 V and gives a sharp oxidation peak at 0.79 V. The re-
reduction peak appears at 0.62 V. In the cathodic scan, the
reduction onsets at 21.57 V. The cathodic peak is at 21.65 V
with a corresponding re-oxidation peak at 21.64 V. The
electrochemical data of the polymers obtained from their cyclic
voltammograms are summarized in Table 2. The p-doping
onset potential of polymer 4c (0.73 V) is 0.13 V higher than that
of poly[(4-hexylthiophene-2,5-diyl)(2,5-bis(decyloxy)-1,4-phenyl-
ene)(4-hexylthiophene-2,5-diyl)] (0.60 V) reported earlier.23
The increase of the oxidative potential may be attributed to
the electronegativity of the polymer main chain resulting from
the phenylene ring attached to the thiophene rings.
Experimental
See Electronic Supplementary Information{ for detailed syn-
thesis procedures, spectroscopic and analytical data for the
monomers and polymers prepared in this work.
The onset potentials of n-doping and p-doping processes can
be used to estimate the HOMO and LUMO values of a
conjugated polymer. According to the equation reported by
Li et al. and deLeeuw et al.,34,35
4.39 eV] and
HOMO ~ [Eox
E
LUMO ~ [Ered(onset vs. SCE) z
SCE) z 4.39 eV], the
E
Conclusions
(onset vs.
LUMO and HOMO energy of the polymers are thus deter-
mined and also listed in Table 2. Compared to polymers 4a and
4b, the band gap of polymer 4c is a little larger due to the larger
steric hindrance of the two 2-ethylhexyloxy substituents, which
is consistent with the conclusion from the absorption and
PL spectra.
A series of phenyl-functionalized polythiophene derivatives,
poly[3-(4’-n-butylphenyl)thiophene-2,5-diyl)(2,5-dialkoxy-1,4-
phenylene)(4-(4’-n-butylphenyl)thiophene-2,5-diyl)], have been
synthesized by oxidative polymerization using ferric chloride as
oxidant. The structures of the polymers were verified by
Fig. 4 Current–voltage and luminescence–voltage characteristics of an
ITO/polymer 4c/Ca device.
Fig. 3 Cyclic voltammograms for the polymers.
J. Mater. Chem., 2001, 11, 3082–3086
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elemental analysis, FT-IR, 1H and 13C NMR spectroscopy.
The results reveal that the linkages between the adjacent
thiophene rings are predominantly at the 5 and 5’ position
because of the high stereo-hindrance of the n-butylphenyl
group at the 4-position of the thiophene ring. All polymers
retain some important advantages of substituted polythio-
phenes, such as good solubility in common organic solvents;
good thermal and environmental stability, while the PL
quantum efficiency is increased compared with those of
conventional polythiophene materials. The results of the
optical and electrochemical investigations demonstrate that
inserting a phenyl ring onto the side chain and the backbone of
polythiophenes is still an efficient structural modification
approach for improving the PL quantum efficiency and
adjusting the HOMO and LUMO energy levels of the resulting
thiophene-based conjugated polymers although it does not
dramatically increase the PL efficiency of the new polymers.
13 J. Roncali, Chem. Rev., 1992, 92, 711.
14 J. Roncali, Chem. Rev., 1997, 97, 173.
15 R. D. McCullough, Adv. Mater., 1998, 10, 93.
16 M. R. Andersson, M. Berggren, O. Inganas, G. Gustafsson, J. C.
Gustafsson-Carlberg, D. Selse, T. Hjertberg and O. Wennerstrom,
Macromolecules, 1995, 28, 7525.
17 G. Schopf and G. Kossmehl, Adv. Polym. Sci., 1997, 129, 1.
18 M. Leclerc and K. Faid, Adv. Mater., 1997, 9, 1087.
19 O. Ingana¨s, T. Granlund, M. Theander, M. Berggren, M. R.
Andersson, A. Ruseckas and V. Sundstro¨m, Opt. Mater., 1998, 9,
104.
20 D. Beljonne, J. Cornil, R. H. Friend, R. A. J. Janssen and
J. L. Bredas, J. Am Chem Soc., 1996, 118, 6453.
21 A. W. Gigli, G. Barbarella, L. Favaretto, F. Cacialli and
R. Cingolani, Appl. Phys. Lett., 1999, 75, 439.
22 T. Granlund, M. Theander, M. Berggren, M. R. Andersson,
A. Ruzeckas, V. Sundstro¨m, G. Bjo¨rk, M. Granstro¨m and
O. Ingana¨s, Chem. Phys. Lett., 1998, 288, 879.
23 J. Pei, W.-L. Yu, W. Huang and A. J. Heeger, Macromolecules,
2000, 33, 2462.
24 J. Pei, W.-L. Yu, W. Huang and A. J. Heeger, Synth. Met., 1999,
105, 43.
25 J. Pei, W.-L. Yu, W. Huang and A. J. Heeger, Acta Polym., 1999,
50, 327.
26 J. Pei, W.-L. Yu, W. Huang and A. J. Heeger, Chem. Commun.,
2000, 1631.
27 A. L. Ding, J. Pei, Z. K. Chen, Y. H. Lai and W. Huang, Thin
Solid Films, 2000, 363, 114.
28 M. R. Andersson, D. Selse, M. Berggren, H. Jarvinen,
T. Hjertberg, O. Inganas, O. Wennerstrom and J. E. Osterholm,
Macromolecules, 1994, 27, 6503.
29 J. P. Ruiz, J. R. Dharia and J. R. Reynolds, Macromolecules, 1992,
25, 849.
References
1
Handbook of Conductive Polymers, eds. T. A. Skotheim,
R. L. Elsenbaumer and J. R. Reynolds, Dekker, New York, 1998.
R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Buuroughes,
R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. dos Santos,
J. L. Bre´das, M. Lo¨gdlund and W. R. Salaneck, Nature, 1999, 397,
121.
2
3
4
A. J. Heeger, Solid State Commun., 1998, 107, 673.
J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks,
K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature,
1990, 347, 539.
5
6
7
G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri
and A. J. Heeger, Nature, 1992, 357, 477.
N. C. Greenham, S. C. Moratti, D. D. C. Bradley, R. H. Friend,
P. L. Burn and A. B. Holmes, Nature, 1993, 365, 628.
S. Son, A. Dodabalapur, A. J. Lovinger and M. E. Galvin,
Science, 1995, 269, 376.
30 M. S. A. Abdou, X. Lu, Z. W. Xie, F. Orfino, M. J. Deen and
S. Holderoft, Chem. Mater., 1995, 7, 631.
31 F. Chen, P. G. Mehta, L. Takiff and R. D. McCullough, J. Mater.
Chem., 1996, 6, 1763.
32 N. C. Greenham, I. D. W. Samuel, G. R. Hayes, R. T. Phillips,
Y. A. R. R. Kessener, S. C. Moratti, A. B. Holmes and
R. H. Friend, Chem. Phys. Lett., 1995, 241, 89.
33 P. Barta, F. Cacialli, R. H. Friend and M. Zago´rska, J. Appl.
Phys., 1998, 84, 6279.
34 Y. Li, Y. Cao, J. Gao, D. Wang, G. Yu and A. J. Heeger, Synth.
Met., 1999, 99, 243.
35 D. M. deLeeuw, M. M. J. Simenon, A. R. Brown and
R. E. F. Einerhand, Synth. Met., 1997, 87, 53.
8
9
Y. Yang, Q. Pei and A. J. Heeger, J. Appl. Phys., 1996, 79, 934.
Q. Pei and Y. Yang, J. Am. Chem. Soc., 1996, 118, 7416.
10 A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Indasekaran,
W. W. Wu and E. P. Woo, Appl. Phys. Lett., 1998, 73, 629.
11 S. Janietz, D. D. C. bradley, M. Grell, C. Giebeler, M. Inbasekaran
and E. P. Woo, Appl. Phys. Lett., 1998, 73, 2453.
12 M. Kreyenschmidt, G. Klaerner, T. Fuhrer, J. Ashenhurst,
S. Karg, W. D. Chen, V. Y. Lee, J. C. Scott and R. D. Miller,
Macromolecules, 1998, 31, 1099.
3086
J. Mater. Chem., 2001, 11, 3082–3086