Synthesis and characterization of soluble and n-dopable poly(quinoxaline vinylene)s and poly(pyridopyrazine vinylene)s with relatively small band gap

Article abstract of DOI:10.1021/ma0111111

Synthesis and characterization of poly(quinoxaline vinylene)s and poly(pyridopyrazine vinylene)s with linear and branched aliphatic side chains are reported. The electron affinity of the polymers was measured with cyclic voltammetry (CV) and found to be h

Full text of DOI:10.1021/ma0111111

1638
Macromolecules 2002, 35, 1638-1643
Synthesis and Characterization of Soluble and n-Dopable
Poly(quinoxaline vinylene)s and Poly(pyridopyrazine vinylene)s with
Relatively Small Band Gap
Ma r ia J on for sen ,† Tom a s J oh a n sson ,‡ Olle In ga n a1s,‡ a n d Ma ts R. An d er sson *,†
Department of Polymer Technology, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden,
and Biomolecular and Organic Electronics, Department of Physics, Linko¨ping University,
SE-581 83 Linko¨ping, Sweden
Received J une 28, 2001; Revised Manuscript Received October 31, 2001
ABSTRACT: Synthesis and characterization of poly(quinoxaline vinylene)s and poly(pyridopyrazine
vinylene)s with linear and branched aliphatic side chains are reported. The electron affinity of the polymers
was measured with cyclic voltammetry (CV) and found to be highest for the pyridopyrazine vinylene
polymers. Compared to CN-MEH-PPV, the pyridopyrazine vinylene polymers were easier to reduce, while
the quinoxaline derivatives were harder. UV-vis absorption measurements showed that the polymers
have relatively small band gaps.
In tr od u ction
als.13,14 A polymer of this type has later been used as
the electron-transporting material in polymer LEDs.15
More recently, polymers have also been synthesized by
dehalogenation polycondensation using nickel com-
plexes, both where quinoxaline rings are connected in
positions 2 and 616 and where they are bound in the
positions 5 and 8.17,18
Another nitrogen heterocyclic compound is pyridopy-
razine. This heterocycle is expected to be an even better
electron acceptor than quinoxaline due to the additional
nitrogen atom. Few studies of conjugated polymers
containing pyridopyrazine have been reported, although
the characteristics of poly(2,3-diphenylpyrido[3,4-b]-
pyrazine) have been given in comparison with an
alternating copolymer of pyrido[3,4-b]pyrazine and
thiophene.19
In this paper, we describe synthesis and some proper-
ties of PPV-type polymers with quinoxaline and pyri-
dopyrazine moieties in the polymer backbone (see
Scheme 1). To our knowledge, the only previous study
of vinylene polymers with quinoxaline heterocycles used
copolymers with phenylene moieties,20 while the use of
poly(pyridopyrazine vinylene) derivatives in photovol-
taic devices was first reported in 2001.21 The electron-
withdrawing nitrogens give our polymers high electron
affinity, while the fused ring system and planar back-
bone give the polymers absorption at long wavelengths.
We chose to make poly(5,8-quinoxaline vinylene) and
the pyridopyrazine analogue so that we could easily
attach side chains to obtain soluble polymers.
Many different conjugated polymers have been syn-
thesized since the discovery in 1977 that polymers can
conduct electricity when doped.1 In particular, poly-
(phenylene vinylene) (PPV) type polymers have been
thoroughly studied since electroluminescence was re-
ported for this class of polymers in 1990.2 Most conju-
gated polymers synthesized to date have low electron
affinity, despite the advantages of high electron affinity
for many applications. One example is in light-emitting
diodes (LEDs), where high electron affinity allows the
fabrication of LEDs with good electron injection from
stable cathodes such as aluminum, rather than the low
work function metals required for more electron-rich
polymers. Other possible applications are n-type field
effect transistors (FET)3 or as the electron-accepting
material in photodiodes or solar cells. Since the discov-
ery of photoinduced electron transfer from conjugated
polymers to C60,4 fullerene derivatives as the electron-
accepting material in conjugated polymer photovoltaic
devices have been studied quite extensively.5,6 Because
of the lack of suitable conjugated acceptor polymers,
devices with conjugated polymers as acceptor material7,8
have not been studied as thoroughly even though some
of the highest efficiencies for conjugated polymer pho-
tovoltaic devices have been achieved with this type.9
One way to achieve high electron affinity is to attach
electron-withdrawing groups to the polymer, for ex-
ample cyano groups in CN-MEH-PPV.10 Another way
to get a polymer with high electron affinity is to
introduce an electron-deficient heterocyclic unit in the
polymer backbone, for example pyridine, pyrimidine,
quinoline, oxadiazole, or quinoxaline.11,12 Quinoxaline
is one of the most promising heterocycles for this type
of conjugated polymer. Polymers with quinoxaline moi-
eties were first polymerized by a condensation reaction
between aromatic bis(o-diamines) and bis(1,2-dicarbo-
nyls) for use as heat- and chemical-resistant materi-
Exp er im en ta l Section
In str u m en ta tion . NMR spectra were recorded on a Varian
VXR300 spectrometer, and mass spectra were recorded on a
VG ZabSpec (Fison Instruments) in positive FAB/LSIMS mode
with 3-nitrobenzoyl alcohol as matrix material. UV-vis spectra
were recorded on a Perkin-Elmer Lambda 20 UV/vis spec-
trometer. Fourier transform infrared spectroscopy (FT-IR)
spectra were recorded on a Perkin-Elmer FT-IR spectrum 1000
spectrometer. The spectra were taken from films prepared on
BaF2 substrates.
* Corresponding author: e-mail matsa@pol.chalmers.se; phone
+46-31-772 3401; fax +46-31-772 3418.
† Chalmers University of Technology.
The molecular weight of the polymers was determined by
size exclusion chromatography (SEC) on a Waters WISP 712
with three commercial styragel columns and a Waters 410
‡ Linko¨ping University.
10.1021/ma0111111 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/23/2002

Macromolecules, Vol. 35, No. 5, 2002
PPV-Type Polymers 1639
Sch em e 1. P r ep a r ed Qu in oxa lin e Vin ylen e (P 1a a n d
P 1b) a n d P yr id op yr a zin e Vin ylen e (P 2a a n d P 2b)
P olym er s (On e of th e P ossible Con figu r a tion s)
5,8-Dibr om o-2,3-h exyloctylqu in oxalin e (4a). 2,3-Diamino-
1,4-dibromobenzene (0.415 g, 1.57 mmol) was dissolved in 28
mL of ethanol/water (1:0.12) and heated to reflux. 7,8-Hexa-
decandione (0.40 g, 1.57 mmol) dissolved in warm ethanol (5
mL) was added dropwise. After reflux for 40 h the product had
formed as a precipitate. After filtration and recrystallization
in acetone 414 mg (55%) was obtained (Tm ) 50-60 °C). 1H
NMR (300 MHz, CDCl3): δ (ppm) 7.82 (s, 2H); 3.07 (t, J ) 7.5
Hz, 4H); 1.91 (k, J ) 7.5 Hz, 4H); 1.20-1.56 (m, 16 H); 0.91
(t, J ) 7.0 Hz, 3H); 0.89 (t, J ) 7.0 Hz, 3H). HRMS: Calcd for
C
22H33N2Br2: 483.101. Found: 483.094.
5,8-Dibr om o-2-(2′-eth ylh exyl)-3-h exylqu in oxa lin e (4b).
The synthesis was performed in the same way as for 4a . After
reflux for 40 h the solvent was evaporated in vacuo. The
product was separated from unreacted material by column
chromatography with petroleum ether/chloroform (3:1); 681 mg
1
(75%) of yellow oil was obtained. H NMR (300 MHz, CDCl3):
δ (ppm) 7.84 (s, 2H); 3.07 (t, J ) 7.5 Hz, 2H); 3.00 (d, J ) 6.9
Hz, 2H); 2.14 (m, 1H); 1.91 (k, J ) 7.5 Hz, 2H); 1.20-1.56 (m,
14H); 0.94 (t, J ) 7.0 Hz, 3H); 0.92 (t, J ) 7.0 Hz, 3H); 0.88 (t,
J ) 7.0 Hz, 3H). HRMS: Calcd for C22H33N2Br2: 483.101.
Found: 483.082.
5,8-Dibr om o-2,3-h exyloctylp yr id o[3,4-b]p yr a zin e (6a ).
6a was prepared in the same way as 4b. After drying, 226 mg
(62%) of yellow crystals (Tm ) 30-40 °C) was obtained. 1H
NMR (300 MHz, CDCl3): δ (ppm) 8.69 (s, 1H); 3.10 (t, J ) 7.5
Hz, 2H); 3.11 (t, J ) 7.5 Hz, 2H); 1.92 (m, 4H); 1.20-1.56 (m,
16H); 0.91 (t, J ) 7.0 Hz, 3H); 0.89 (t, J ) 7.0 Hz, 3H).
HRMS: Calcd for C21H32N3Br2: 484.096. Found: 484.097.
5,8-Dibr om o-2-(2′-eth ylh exyl)-3-h exylpyr ido[3,4-b]pyr a-
zin e a n d 5,8-Dibr om -2-h exyl-3-(2′-eth ylh exyl)p yr id o[3,4-
b]p yr a zin e (6b). 6b was prepared in the same way as 4b.
After drying, 334 mg (46%) of yellow oil was obtained. In the
NMR spectra it was seen that two different isomers were
refractive index detector. A Wyatt Down DSP laser photometer
was connected on-line to the SEC. The samples were run at
30 °C with a flow rate of 1.0 mL/min in chloroform with
polystyrene standards as references.
1
formed in equal amounts. H NMR (300 MHz, CDCl3) isomer
Electrochemical measurements of films adsorbed on a Pt
wire from a chloroform solution were measured in a single-
compartment electrochemical cell with a Pt counter electrode
and an Ag/AgCl quasi-reference electrode. Tetrabutylammo-
nium perchlorate (0.1 M) in acetonitrile (dried over molecular
3 Å sieves) was used as the supporting electrolyte. The cell
was purged with nitrogen before each measurement. Ferrocene
(half-wave potential 0.326 vs Ag/AgCl)22 was used for calibra-
tion of the reference electrode. The sweep rate was 100 mV/s
unless otherwise indicated.
Ma ter ia ls. All starting materials are commercially avail-
able from Aldrich or Acros. Tetrahydrofuran was distilled over
sodium/benzophenone, and DMF was dried by azeotropic
distillation with benzene, then over BaO for 15 h, and finally
by distillation at reduced pressure. 2,3-Diamino-1,4-dibro-
mobenzene (3) was synthesized from 2,1,3-benzothiadiazole,
which was brominated to 4,7-dibromo-2,1,3-benzothiadiazole23
and then reduced to 2,3-diamino-1,4-dibromobenzene.24 3,4-
Diamino-2,5-dibromopyridine (5) was prepared by bromination
of 3,4-diaminopyridine.19 1,2-Bis(tri-n-butylstannyl)ethylene
(7) was prepared from tributyl(ethynyl)tin and tributyltin
hydride with a catalytic amount of R,R′-azoisobutyronitrile.25,26
Syn th esis. 7-Hexa d ecyn (1a ). 1-Octyne (5 g, 45.5 mmol)
was dissolved in tetrahydrofuran (125 mL) and lithiated by
adding butyllithium solution (28.45 mL, 1.6 M in hexanes)
dropwise at 0 °C under N2(g). The reaction mixture was raised
to room temperature, and 1-bromooctane (8.78 g, 45.5 mmol)
was added dropwise. The mixture was refluxed for 68 h. After
cooling to room temperature, the solution was extracted with
5% sodium sulfite (aqueous) and water and dried over MgSO4.
After distillation (140 °C, 4 mbar) 8.33 g (83%) of colorless oil
was obtained.
1 and 2: δ (ppm) 8.68 (s, 1H); 3.10 (t, J ) 7.5 Hz, 2H); 3.03 (d,
J ) 6.9 Hz, 2H); 2.16 (m, 1H); 1.92 (m, 2H); 1.20-1.56 (m,
14H); 0.84-0.98 (m, 9H) and 8.68 (s, 1H); 3.11 (t, J ) 7.5 Hz,
2H); 3.02 (d, J ) 6.9 Hz, 2H); 2.16 (m, 1H); 1.92 (m, 2H); 1.20-
1.56 (m, 14H); 0.84-0.98 (m, 9H). HRMS: Calcd for C21H32N3-
Br2: 484.096. Found: 484.088.
P olym er iza tion . A typical polymerization was performed
as follows.
P oly(2(2′-et h ylh exyl)-3-h exylq u in oxa lin e vin ylen e)
(P 1b). 5,8-Dibromo-2-(2′-etylhexyl)-3-hexylquinoxaline (150
mg, 0.310 mmol), 1,2-bis(tri-n-butylstannyl)ethylene (188 mg,
0.310 mmol), and tetrakis(triphenylphosphine)palladium(0)
(18 mg, 0.016 mmol) were dissolved in dry DMF under N2(g).
The polymerization was performed at 110 °C for 24 h,
whereafter the polymer was precipitated by addition of ethanol
and purified by Soxhlet extraction with ethanol for 17 h. The
pure polymer was washed out with chloroform. After drying,
110 mg (95%) of dark violet powder was obtained. 1H NMR
(300 MHz, CDCl3): δ (ppm) 8.7-9.1 (2H); 8.2-8.6 (2H); 2.8-
3.4 (4H); 2.1-2.4 (1H); 1.8-2.1 (2H); 1.1-1.8 (14H); 0.6-1.1
(9H). FT-IR (in cm-1): 3053 (m); 2927/2956 (s); 2856/2870 (s);
1615 (m); 1560 (m); 1463/1482 (s); 1378 (m); 1336/1347 (m);
1261 (m); 1143/1161 (m); 1094 (m); 977 (m); 833 (m).
Resu lts a n d Discu ssion
Syn th esis. The monomers (4a , 4b, 6a , and 6b) were
prepared by condensation reaction of the diones and
diamines as described in the experimental part28 (see
Scheme 2). 2,3-Diamino-1,4-dibromobenzene (3) and 3,4-
diamino-2,5-dibromopyridine (5) were prepared as de-
scribed in the literature.19,23,24 The diones (2a and 2b)
were synthesized by oxidation of triple bonds with
potassium permanganate.27 The alkyns used (1a and
1b) were synthesized by lithiating 1-octyne with butyl-
lithium and then reacting it with 2-ethylhexyl bromide
or 1-bromooctane, respectively, as described in the
experimental part.
5-Eth yl-7-tetr a d ecyn e (1b). The synthesis was performed
in the same way as for 7-hexadecyn (1a ), but a longer reaction
time (4 weeks) was necessary to get high yield. After extraction
as described for 1a and distillation (120 °C, 4 mbar) 7.48 g
(73%) of colorless oil was obtained.
7,8-Hexa d eca n d ion e (2a ) a n d 5-Eth yl-7,8-tetr a d eca n -
d ion e (2b). The diones were prepared as described by Srini-
vasan et al.27

1640 J onforsen et al.
Macromolecules, Vol. 35, No. 5, 2002
Sch em e 2. Syn th esis of th e Qu in oxa lin e Vin ylen e a n d P yr id op yr a zin e Vin ylen e P olym er s
Ta ble 1. Molecu la r Weigh ts a n d Yield s for
Ta ble 2. Solu bility for P 1b a n d P 2b a t Room
Tem p er a tu r e
P olym er iza tion a t Differ en t Tim es
polymerization
P 1b
P 2b
a
polymer
time (h)
Mh n
PDa
yield (%)
p-xylene
toluene
chloroform
THF
diethyl ether
DMF
DMSO
partly
partly
yes
partly
no
no
no
no
no
partly
partly
yes
yes
partly
no
no
no
yes
P 1b
P 1b
P 1b
P 1a
P 2b
P 2b
P 2a
15
24
62
62
24
62
62
5800
6100
6800
4900
5800
8700
3100
2.7
2.2
1.9
2.3
1.5
2.4
1.3
65
95
68
75
39
22
40
ethanol
formic acid
a
Measured by size exclusion chromatography relative to poly-
styrene standards with chloroform as eluent.
The polymerization of the pyridopyrazine monomers
(P 2) gave lower yields than the quinoxalines. This can
be explained by losses at workup because of lower
solubility of the poly(pyridopyrazine vinylene)s in chlo-
roform. The additional nitrogen makes the pyridopyra-
zine polymers more polar. See Table 2 for a summary
of the polymers’ solubility. The poly(pyridopyrazine
vinylene)s are about as soluble in THF as in chloroform,
while the poly(quinoxaline vinylene)s are much less
soluble in the more polar THF. The poly(pyridopyrazine
vinylene)s are also likely to be more soluble in the polar
DMF than the poly(quinoxaline vinylene)s. This is
probably the reason for the higher molecular weight for
P 2b polymerized for 62 h. An even more drastic differ-
ence in solubility between the polymers is seen in formic
acid where the poly(pyridopyrazine vinylene)s are about
as soluble as in chloroform or THF, and the poly-
(quinoxaline vinylene)s are insoluble. The solubility in
this case is probably due to hydrogen bonds to the
nitrogen in position 6 in the pyridopyrazine (see the
discussion about protonation in the next section). Even
though the higher polarity of the pyridopyrazines makes
it harder to get good yields, the solubility can be an
advantage for these polymers since they are soluble in
a wide range of solvents. The solubility in formic acid
can, for example, be useful for preparation of double-
layer devices for LEDs or solar cells.
In the condensation reaction, two different isomers
of the pyridopyrazine monomers (6) can be formed. The
two isomers could be distinguished in the 1H NMR
spectra of 6b , where they were seen to be formed in
1
equal amounts. In the H NMR spectra of 6a , the two
isomers cannot be distinguished, but there is no reason
to believe that the monomer only consists of one of the
two isomers. Since the two isomers cannot be separated
by conventional techniques, they were used together in
the polymerization.
The quinoxaline vinylenes and pyridopyrazine vi-
nylenes were polymerized by Stille coupling29 with 1,2-
bis(tri-n-butylstannyl)ethylene (7), which was prepared
from tributyl(ethynyl)tin and tributyltin hydride with
a catalytic amount of R,R′-azoisobutyronitrile.25,26 The
polymerization worked well in DMF at 110 °C. A typical
polymerization is described in the experimental part.
In Table 1, SEC results and yields are shown for
polymers of the four different monomers at different
polymerization times. Even though the number of
experiments is rather small, it seems like the best
polymerization time is about 24 h. At longer times, the
yield decreases, which is probably due to formation of
insoluble polymer that is lost during workup.

Macromolecules, Vol. 35, No. 5, 2002
PPV-Type Polymers 1641
F igu r e 2. UV-vis absorption spectra for P 2b in chloroform
solution before and after protonation with oxalic acid.
F igu r e 1. UV-vis absorption spectra for thin films of P 1b
and P 2b.
Sch em e 3. P la n a r Con for m a tion of Qu in oxa lin e
Vin ylen e P olym er s
The high yields for linear side chains compared with
branched can be explained by precipitation during the
polymerization that prevents polymer of high molecular
weight being formed. The lower molecular weights
measured for these polymers support this theory. Even
though the linear side chains gave higher yields in the
polymerization, we chose to use the polymers with
branched side chains in the following analysis, because
the polymers with linear side chains seem to show a
greater tendency to form aggregates. The strongest
support for this suspicion was seen with a light scat-
tering detector connected on-line to the SEC. With this
detector we could see a fraction of high molecular weight
material that was not visible with the refractive index
detector, which we believe is caused by aggregates.
UV-vis Absor p tion . The UV-vis absorption of films
of P 1b and P 2b, spin-coated from chloroform, are shown
in Figure 1. The onset of the absorption is quite similar
for the two polymers, but the maximum is slightly red-
shifted for the poly(pyridopyrazine vinylene), 558 nm
compared to 540 nm for the poly(quinoxaline vinylene).
The same difference is seen in chloroform solution where
the absorption maxima are at 530 and 517 nm, respec-
tively. A possible explanation for the difference is that
the steric hindrance is smaller in the poly(pyridopyra-
zine vinylene) due to the absence of hydrogen in position
6 on the heterocyclic rings. The low energy for the
absorption indicates that the polymers have extended
conjugation. Computer models of the polymers, made
in Materials Studio version 1.130 (with COMPASS31 as
force field), where the energy in the gas phase has been
minimized, indicate that the polymers can have a very
flat, low-energy conformation with double bonds point-
ing in the same direction on both sides of the rings, as
shown in Scheme 3. This planar conformation is prob-
ably important, especially in the solid state.
F igu r e 3. Enlarged FT-IR spectra of P 1b before and after
photooxidation. The spectra have been offset for clarity.
bases than pyridine (pKa ) 5.25) and are for example
not protonated by formic acid (pKa ) 3.75). The absorp-
tion maximum for P 2b is the same in formic acid as in
chloroform, which indicates that the solubility in formic
acid for P 2b is due to hydrogen bonds. With stronger
acids, like oxalic acid for P 2b and methanesulfonic acid
for P 1b, the UV-vis absorption is red-shifted. The effect
is seen in both film and solution. In Figure 2 the
protonation of P 2b in chloroform is shown. The red shift
is 27 nm for P 2b both in film and in chloroform solution
and 28 nm for P 1b. The red shift is accompanied by a
decrease in absorption, which is recovered along with
the blue shift back to the original absorption by the
addition of base (for example ammonia).
Sta bility. The UV-vis absorption of thin films of all
our polymers decreased rapidly when exposed to light
and air. For example, when a thin film of P 2b was
exposed to ordinary domestic lighting, it lost most of
its color in 2 h. The absorbance, at the absorption
maximum, decreased from 0.30 to about 0.25 in 30 min.
Thicker films are more stable, and the polymers are
much more stable when stored in the dark. No change
in absorbance has been seen for films stored dark for
several months. When the concentration of nitrogen in
air is increased, by repeatedly evacuating to 0.4 mbar
and refilling with N2(g), the degradation is considerably
reduced. FT-IR spectra of relatively thick films of P 1b
and P 2b exposed to light and air for 24 h show
formation of carbonyl peaks at 1690 cm-1 and a decrease
of the peaks at 1615 cm-1 (stretch of conjugated double
bond) (see Figure 3). This suggests a reaction with
oxygen at the double bonds.
The electronic properties and UV-vis spectra of
nitrogen heterocycles are known to be influenced by
protonation. The pyridopyrazine and quinoxaline het-
erocycles have been reported to have quite different base
strengths, with pKa in water of 0.56 for quinoxaline and
2.47 for the pyridopyrazine.32 They are both weaker
1
In the H NMR spectra of degraded polymers we see
a decrease and broadening of the peaks in the aromatic
region. We also see a decrease, broadening, and slight

1642 J onforsen et al.
Macromolecules, Vol. 35, No. 5, 2002
1
F igu r e 4. Enlarged H NMR spectra of P 1b before and after
photooxidation. The spectra have been vertically scaled so that
the peaks at 0.8-1.1 ppm are the same and offset for clarity.
shift of the peak at 2.8-3.4 ppm, a decrease of the peaks
at 2.1-2.4 and 1.8-2.1 ppm, and formation of a new
peak at 1.7 ppm (see Figure 4). This means that
something is happening in the side chains of the
polymers as well as to the conjugated backbone. The
changes in the NMR spectra can be explained by
formation of carbonyl groups in the R position of the side
chains, due to abstraction of the labile hydrogens in this
position, similar to that seen for alkylthiophenes.33
The molecular weight of the polymers, measured with
SEC, is lower for photooxidized polymers, but not to the
extent that the decline in UV-vis absorption might
suggest. There is clearly more than one reaction hap-
pening when the polymers are exposed to light and air.
The decline in absorption for PPV derivatives has been
reported to be due to reaction of the double bond with
singlet oxygen.34-36 This would mean that electron-
withdrawing groups increase the stability of the double
bond toward the electrophilic singlet oxygen.37 The
stability of P 2b, with the more electronegative pyri-
dopyrazine ring, is slightly better than that of the
quinoxaline analogue (P 1b), but both polymers are less
stable than expected. We think it likely that both the
double bonds and the side chains are oxidized in the
degradation process. Possible explanations for the low
stability compared with other PPV polymers are the
labile hydrogens of the side chains33,38 or the extended
conjugation caused by the fused ring.36
F igu r e 5. Cyclic voltammogram of (a) P 1b and (b) P 2b films
on Pt in 0.1 M tetrabutylammonium perchlorate acetonitrile
solution.
mated as the average between the reduction and reoxi-
dation peaks to be -1.52 and -1.19 V vs Ag/AgCl, for
P 1b and P 2b, respectively), and the oxidation peaks are
2.47 eV for P 1b and 2.32 eV for P 2b, which is similar
to the UV-vis absorption peaks (2.30 and 2.23 eV,
respectively).
For comparison, CN-MEH-PPV, synthesized as de-
scribed in the literature,39 was measured under the
same conditions as P 1b and P 2b. The formal reduction
potential was -1.30 V vs Ag/AgCl, which is similar to
previously reported values;40 i.e., P 1b has a lower, and
P 2b a higher, electron affinity than CN-MEH-PPV.
Con clu sion s
Cyclic Volta m m etr y. Cyclic voltammetry of the
polymers with branched side chains (P 1b and P 2b)
showed that the polymers have high electron affinity
(see Figure 5). For the quinoxaline (P 1b) the reduction
peak is seen at -1.56 V vs Ag/AgCl and the reoxidation
at -1.47 V vs Ag/AgCl. The pyridopyrazine polymer
(P 2b) had a reduction peak at -1.23 V vs Ag/AgCl and
a reoxidation peak at -1.14 V vs Ag/AgCl. Earlier
reported values for P 2b differ slightly from these,
because different solvent systems were used.21 The
higher electron affinity for the pyridopyrazine is due to
the extra nitrogen in the heterocyclic ring. The reduction
process is reversible for both polymers and can be
repeated many times without any large degradation
being noticed. For P 2b there is however a new reduction
peak being formed at -1.04 V vs Ag/AgCl on repeated
scanning. The reason for this is unknown, and we do
not see any clear reoxidation of this peak.
We have synthesized poly(quinoxaline vinylene)s and
poly(pyridopyrazine vinylene)s, with linear and branched
aliphatic side chains, at different polymerization times.
The different monomers gave polymers with different
solubility. As a consequence, yields and molecular
weights varied. All prepared polymers were soluble in
common organic solvents such as chloroform. The poly-
(pyridopyrazine vinylene)s were also soluble in formic
acid. All polymers had UV-vis absorption at long
wavelengths due to the fused ring system and planar
polymer backbone. Cyclic voltammetry showed that the
polymers could be reduced with formal reduction po-
tentials, for P 1b and P 2b, of -1.52 and -1.19 V vs Ag/
AgCl, respectively, which means that the poly(quinox-
aline vinylene) has lower, and the poly(pyridopyrazine
vinylene) higher, electron affinity than CN-MEH-PPV.
Unfortunately, the photooxidative stability of all poly-
mers is quite poor. The exact reason for this is unknown.
The oxidation of both polymers is irreversible, with a
peak at 0.96 V vs Ag/AgCl for P 1b and 1.14 V vs Ag/
AgCl for P 2b (sweep rate 5 mV/s). The band gaps
calculated from the formal reduction potential (esti-
Ack n ow led gm en t. Financial support for this work
has been provided by the Swedish Foundation for
Strategic Research. We also thank Gunnar Stenhagen

Macromolecules, Vol. 35, No. 5, 2002
PPV-Type Polymers 1643
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Mikael J ohansson for the synthesis of CN-MEH-PPV.
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