3
934 J. Am. Chem. Soc., Vol. 118, No. 16, 1996
Yamamoto et al.
state become very weak and the new broad absorption band in
a range of 450-700 nm becomes stronger. Formation of special
electronic states like polaron and bipolaron states accounts for
the appearance of the new band. Of the two π-π* absorption
bands, the one with a longer wavelength seems to receive a
Table 3. Electrochemical and Electrical Data of Polyquinoxalines
a
peak potential /V
E
pc
E
pa
σb/(S cm-1
(Na-doped)
)
E° c
polymer
P(5,8-Qx)
(n-doping) (n-undoping)
-
-
3
-2.00
-2.45
-2.45
-2.20
-2.43
-2.02
-1.95
-2.08
-1.98
-2.15
-2.10
-1.83
-1.96
-2.16
-2.25
-1.93
-1.93
-1.82
-1.82
-1.87
-1.90
-1.81
-1.98
-1.67
-1.98 1.2 × 10
-2.31 8.0 × 10
+
stronger effect from the n-doping at -1.8 Vs Ag/Ag .
4
P(5,8-Qx(diEt))
P(5,8-Qx(diHep))
P(5,8-Qx(MePh))
P(5,8-Qx(BuPh))
P(5,8-Qx(diPh))
P(5,8-Qx(diTol))
P(5,8-Qx(diAns))
P(5,8-Qx(diBP))
P(5,8-Qx(diPy))
P(5,8-Qx(diFu))
P(2,6-Qx)
On the oxidation, the original UV-vis spectrum of the
nondoped polymer is obtained, and this doping-undoping cycle
can be repeated with good reproducibility. Films of other
polyquinoxalines on the ITO electrodes also show similar
electrochromic behavior.
-2.35
d
-
3
-2.07 d7 .4 × 10
-2.18
-
-
4
4
-1.92 1.2 × 10
-1.89 1.1 × 10
-1.98 1.1 × 10
-1.94 4.6 × 10
-1.98 2.8 × 10
-2.04 2.8 × 10
-1.75 3.0 × 10
-4
-
-
-
-
4
4
3
4
In spite of its facile electrochemical reduction (n-doping),
the film of P(5,8-Qx) is electrochemically intact against oxida-
+
tion up to 1.2 V Vs Ag/Ag , where oxidation of electron-
donating π-conjugated polymers such as polypyrrole (E° ) -0.3
+
12
+
13
a
Versus Ag/Ag+. Measured in an acetonitrile solution of
V Vs Ag/Ag ), polythiophene (0.49 V Vs Ag/Ag ), and poly-
[(C
2
H
5
)
4
N]ClO (0.1 M) or [(C N]BF
4
2 5
H )
4
4
(0.1 M). b Treated with
+
13c,14
(
p-phenylene) (ca. 1.1 V Vs Ag/Ag )
these data reveal the electron-accepting properties of P(5,8-Qx)
due to having two electron-withdrawing imine nitrogens in
the recurring unit.
takes place. All of
sodium naphthalenide. Measured by the two-probe and/or four-probe
method. c The average of Epc and Epa is given. d Not measured due to
1
5
good solubility in THF.
Nondoped polyquinoxalines are essentially insulators with σ
As shown in Figure 2b, P(2,6-Qx) is also electrochemically
active only for the reduction (n-doping) and affords a similar
-10
-1
values of less than 10 S cm . Sodium-doped polyquinoxa-
-
3
-4
-1
lines have σ values of 10 -10 S cm (measured with
compressed powders) as summarized in Table 3. ESR spectra
of Na-doped P(5,8-Qx) and P(2,6-Qx) show symmetrical signals
at g ) 2.0021 and 2.0022 with ∆Hpp (peak-to-peak width) of
CV and color change. The redox potential (E°) of P(2,6-Qx)
+
(
E° ) -1.75 V Vs Ag/Ag ) is less negative than that of P(5,8-
Qx) (E° ) -1.98 V) by 0.23 V, and the difference in E° may
be explained by a longer effective π-conjugation system of
P(2,6-Q) and more direct effect of the electron-withdrawing
imine nitrogens in P(2,6-Qx) compared with that of the imine
nitrogens in P(5,8-Qx), which exist in the side chain and will
not give a direct effect on the π-conjugation system.
0
.31 and 0.26 mT, respectively, at room temperature. Measure-
ment of electrical conductivity of the electrochemically n-doped
polymer was not possible. The red-brown or black sodium-
doped samples are sensitive to air, and exposure to air caused
a rapid decrease in their electrical conductivity with a color
change to yellow.
The above shown electrochemical data were obtained by using
[
(C2H5)4N]ClO4 as the electrolyte. Use of [(n-C4H9)4N]ClO4
in the electrochemical redox reactions of P(5,8-Qx), P(5,8-Qx-
diPh)), and P(2,6-Qx) gives essentially the same data, and an
Light-Emitting Diodes. Light-emitting diodes (LEDs) using
16
fluorescent π-conjugated poly(arylene)s such as PPP, PTh and
(
1
3c,17
9,18
its derivatives,
and PPV and its derivatives
are now
example of the CV curves obtained by using [(n-C4H9)4N]ClO4
is given in an inset in Figure 2b. In this case, scanning can be
carried out to a more negative potential, up to -2.3 V Vs Ag/
actively investigated. Since polyquinoxalines having aromatic
substituents show strong fluorescence, the polymers are expected
to be useful materials to make LEDs. Actually, ITO/polymer/
Mg(Ag) (Figure 4a) simple electric junctions (polymer ) P(5,8-
Qx(diPh)), P(5,8-Qx(diTol)), P(5,8-Qx(diAns)), P(5,8-Qx-
+
Ag , where flow of irreversible electric current due to decom-
position of the solvent starts.
Table 2 compares the E° values of the n-doping of polynaph-
thylenes,11 polyquinolines, and polyquinoxalines, all of which
have naphthalene-1,4-diyl or naphthalene-2,6-diyl type bonding
between the monomer units. As shown in Table 2, introduction
of each imine nitrogen makes the n-doping occur more easily
with a change in the E° value by about 0.35 V per each of the
imine nitrogens, for both types of the polymers.
(
diBP))) emit light on application of an electric field. The peak
3
positions of electroluminescence (EL) spectra of LEDs es-
sentially agree with those of the fluorescence spectra of the
polymers (Table 1). However, the intensity of the light from
the simple type LEDs is relatively weak; for example, the LEDs
using P(5,8-Qx(diPh)) and P(5,8-Qx(diBP)) as the light-emitting
-
2
materials show emission intensities of 1 cd m at 14 V and 10
Other polyquinoxalines are also electrochemically active for
the n-doping and n-undoping, and Table 3 summarizes the
n-doping and n-undoping potentials as well as electrical
conductivity (σ) of sodium-doped polyquinoxalines. As shown
in Table 3, the introduction of electron-donating alkyl group(s)
into P(5,8-Qx) makes the n-doping more difficult with a shift
of the E° value to a more negative side, whereas the introduction
of electron-withdrawing aryl group(s) makes the n-doping easier.
-2
cd m at 18 V, respectively.
(
16) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Synth. Met. 1992,
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(
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(
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