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Infrared spectra of iodine doped polyazines
~o,~ Solid State Communications, Vol. 68, No. 3, pp.291-293, 1988. ~ Prlnted in Great Britain.
INFRARED
SPECTRA
OF
IODINE
William Department
The of
mid-IR
spectra
doping
new
level.
bands
remain
as
midgap
state
second
new
This of
band the
is
RI
6
of
doped
iodine the
A
the
broad
assigned
1505 to
a
associated
Euler
of
Chemistry
Rhode
1988
cm a
USA
by
J.
polyazines
other
POLYAZINES
Island
02881
Tauc)
is
reported
increases
peaks
in
centered
transition
intensity
as
B.
content
peak
probably at
June
iodine
electronic
is band
relative
conductivity vibration
an
Kingston,
while
unchanged.
assigned
of
(Received
As
develop
university
DOPED
to
0038-1098/88 $3.00 + .00 Pergamon Press plc
a
in
at
vibrational -1 4000 cm or
low
lying
midgap
bipolaron
C=C
bond
or
N=N
double -1 cm
the
1505
function
of
iodine
with
band
this
-N=C-C=N-N=C-C=N-
polyacetylene
polyazine
is
of
peak
content
originates
from
above
is This
formation.
The
stretch.
origin.
Correlation
the
suggests the
two
spectrum
state.
vibrational
with
function
charge
electrical that
the
carrier.
All samples were prepared as reported previously. Polyazine ( -[N=C(CH~)-C(CH~)=N]) was • D J X . synthesized from an acld catalyzed eondensatlon of 2,3-butanedione and 2,3-butanedionedihydrazone, giving the polymer in the all ~rans conformation as previously determined . Iodine doping was accomplished by slurrying the polymer with chloroform solutions of the dopant. For very high iodine levels (y>0.75) it was necessary to initiate the doping by allowing the polymer to react with free iodine gas at room temperature before the solvent was added. All IR spectra were measured at room temperature as KBr pellets on a Perkin-Elmer 281B_~nfrared spectrometer between 600 and 4000 cm ~. Pressed pellet conductivities were determined using the VanderPauw technique. Figure 1 shows the IR spectra of the iodine doped polyazine as a function of iodine concentration for the entire spectral region measured. Two features are to be noted. First, the vibrational signature of the polymer skeleton is unchanged by the oxidation, although somewhat washed out at high doping levels. This implies that the polymer chain remains intact upon doping with iodine and that this mild oxidant does not decompose the material. Second, as the dopant level increases, a broadland featureless absorption centered at 4000 cm- or above (!0.5 eV) grows in intensity. This ty~e~of band is often seen in conducting polymer0'~'l and is assigned as an electronic absorption, probably from the valence band to a low lying midgap state. In comparison, in polythiophene and polypyrrole the lowest lying electronic absorptions are observed at 0.65 ev and 0.7 eV, respectively, consistent with the assignment made here. Further support for this assignment is found from previously reported Extended Huckel calculations: depending on which atoms (nitrogen or carbon) the oxidation is centered around, it was found that the lowest lying midgap state in polyazine ranqed from 0.26 eV to 0.60 eV above
The study of conducting polymers has been fruitfullin leading to new materials and to new physics. Th~ prototype of these polymers is polyacetylene , which is a simple linear chain of carbon atoms with alternating single and double bonds. Polyazine is also a linear chain of atoms with alternating single and double bonds, but with pairs of nitrogen atoms substituted for pairs of carbon atoms in the polyacetylene chain: -C=C-C=C-C=C-C=C-
a
polymer,
the
associated with -I is narrow and
of
as
this
The effeGt of the nitrogen atoms is twofold. First, unlike polyac~tylene, polyazine is environmentally stable. Second, the nitrogen atoms change the topology of polyazine relative to polyacetylene so that the ground state of polyazine cannot be degenerate. This has bearing on the nature of the excitations available to polyazine: solitons, such as found in polyacetylene, cannot exist in polyazine; rather, midgap states must be polarons or bipolarons. Like polyacetylene, polyazine can be doped with iodine and other oxidants to give a highly conducting material. Charge storage in bipolaron states has been found in other conducting polymers containing heter~cyclic monomers,_ most notably polythiophene ~ and polypyrrole b. Oxidation from the pi system of polyazine is also e~pected to give bipolaron type midgap states , but several structures are possible depending on which atoms the charge is centered on. In polyazine, oxidation could put charge on nitrogen atoms, carbon atoms, or a combination of both.6Previously reported Extended Huckel calculations show that any of these combinations is posssible, with comparable energies within the accuracy of the method. 291
INFRARED SPECTRA OF IODINE DOPED POLYAZINES
292
Vol. 68, No. 3
r=o.o ~r=o.22
Y=o.38 w
(D z
Y=o.42 Y=(xso
<
I,I-
w (D Z
Y=o.57 o9 z
r=o.67
rr I--
'=o.8o '=
J
I
I
~ooo
I
I
~
L
,~
doo
<
i.o9
6O f <~ Cr
C4N6N2Iy)x
I
,~
N
WAVENUMBER Figure i. IR spectra of (C~H.N~I) as a func: .~ z x -i tion of iodin~ content, y, in ~he ~ange 600 cm to 4000 cm . All spectra I were taken as KBr pellets. The peak at 2300 cm- is due to CO 2.
l~)x 18 O0
1400
1000
WAVENUMBER the top of the valence edge. Further spectroscopy in the near-IR is needed to determine the absorption maximum of this transition as well as to find the companion peaks predicted for a bipolaron type structure. Closer inspection of Figure 1 also shows a narrow vibrational peak~growing with increasing iodine level at 1505 cm . Thls is shown zn more detail in Figure 2. This peak is assigned to either a C=C or N=N double bond stretch. Either of these geometries is consistent with bipolaron formation upon oxidation: a bipolaron of the form - i
.
R
.
.
R
k
h
+
/
C=N-N-C=C-N-N=C
+
/
k
\ R
Figure 2. IR spectra of (C~HrN.I) as a func= , . .q z x -± tlon of iodine content, y, in ~he ~ange 800 cm to 1800 cm- . All spectra were taken as KBr pellets. charge on adjacent (even in a somewhat delocalized bipolaron) atoms should be strongly disinclined for coulombic reasons. Second, the offchain effect of putting the counterions (presumably triiodides) in close proximity, would be opposed on both steric and coulombic grounds. The conductivity of the iodine doped materials is also affiliated with the number and mobility of the bipolaron-associated defect, thu~ suggesting a correlation between the 1505 cm peak and the conductivity. Support for this contention is shown in Figure ~I where the rela-
R
would store charge on nitrogen atoms with a C=C double bond while a bipolaron of the form R
(C4H6N22y) x
0.9
R
\
\+
0
/
N=C-C-N=N-C-C=N
+\
/
R
R
would store charge on carbon atoms with a N=N double bond. Either of these interpretations implies oxidation from the polymer pi system. An alternate possibility is oxidation from the nitrogen lone pairs giving a structure of the form R R
h
/
+\
0.7 O9 m .<
\+ R
\
b 0.5 0
which also gives a N=N double bond. However, this structure is not favored for two reasons. First, the on-chain effect of concentrating
•
0.3
ib
--4
m .<
-5 0.I I
0.0 R
-2 rO
~..~e
N=C-C=N=N=C-C=N
/
-I
00
\
I
I
I
Q8 Y
I
I
!
1.6
Figure ~I Plot of the relative intensity of the 1505 cm peak and the room temperature conductivity as a function of iodine content, y.
Vol. 68, No. 3
INFRARED SPECTRA OF IODINE DOPED POLYAZINES
tive intensity of the 1505 _c~ peak (using the strong and invariant 1355 cm peak as an internal reference) and the log of the room temperature pressed pellet conductivity are plotted as a function of iodine content. The parallel between the two curves is striking. Both curves increase rapidly from y=0 to y=0.8 and then flatten out for y> 0.8. Assuming the iodine takes the form of triiodide, then y=0.8 suggests 0.27 charges per monomer or 0.07 charges per atom along the chain, as compared to polyacetylene which undergoes a similar transition st about 0.02 charges per atom along the chain . Although the two quantities plotted in Figure 3 are clearly parallel, the correspondence between the relative absorbance of the charge carrier, which is proportional to the number of carriers, and the logarithm of the conductivity is puzzling. To a first approximation, the conductiv-
293
ity shoul~0also be proportional to the number of carrlers; however, both the carrier mobility and the Fermi level may depend on the extent of oxidation thus accounting for the observed behavior. In conclusion, the mid-IR behavior of iodine doped polyazine is characterized by two featurea: a broad electronic band centered at 4000 cm or _~bove and a narrow vibrational band at 1505 cm . The electronic band is consistent with a transition to a low lying midgap state associated with bipolaron formation, as seen in other conducting polymers. The vibrational band is assigned to a double bond stretch, either N=N or C=C. Correlation of the intensity of this band with the room temperature conductivity strongly suggests that the bipolaron charge carrier is associated with the double bond formed upon oxidation by iodine.
REFERENCES i. J. E. Frommer, Acc. Chem. Res., 1986, 19, 2. 2. S.R. 385.
Roth, H.
Bleier, Adv. Phys., 1987, 36,
3. C. R. Hauer, G. S. King, E. L. McCool, W. B. Euler, J. D. Ferrara, W. J. Youngs, J. Am. Chem. Soc., 1987, 109, 5760. 4. T. C. Chung, J. H. Kaufman, A. J. Heeger, F. Wudl, Phys. Rev. B, 1984, 30, 702. 5. J. L. Br~das, J. C. Scott, K. Yakushi, G. B. Street, phys. Rev. B, 1984, 30, 1023. 6. W. B. Euler, J. Phys. Chem., 1987, 9_/1, 5795.
7. B. N. Diel, T. Inabe, J . W . Lyding, K. F. Schoch, Jr., C . R . Kannewurf, T. J. Marks, J. Am. Chem. Soc., 1983, 105, 1551. 8. MNDO and VEH calculations suggest this type of structure. L.A. Burke, R. N. Butler, private communication. 9. Y.-W. Park, A. J. Heeger, M. A. Druy, A. G. MacDiarmid, J. Chem. Phys., 1980, 73, 946. i0. J. H. Perlstein, in "Solid State Chemistry and Physics, An Introduction," P. F. Weller, ed., Marcel Dekker, Inc., New York, 1973, Vol. i, 189.
INFRARED
SPECTRA
OF
IODINE
William Department
The of
mid-IR
spectra
doping
new
level.
bands
remain
as
midgap
state
second
new
This of
band the
is
RI
6
of
doped
iodine the
A
the
broad
assigned
1505 to
a
associated
Euler
of
Chemistry
Rhode
1988
cm a
USA
by
J.
polyazines
other
POLYAZINES
Island
02881
Tauc)
is
reported
increases
peaks
in
centered
transition
intensity
as
B.
content
peak
probably at
June
iodine
electronic
is band
relative
conductivity vibration
an
Kingston,
while
unchanged.
assigned
of
(Received
As
develop
university
DOPED
to
0038-1098/88 $3.00 + .00 Pergamon Press plc
a
in
at
vibrational -1 4000 cm or
low
lying
midgap
bipolaron
C=C
bond
or
N=N
double -1 cm
the
1505
function
of
iodine
with
band
this
-N=C-C=N-N=C-C=N-
polyacetylene
polyazine
is
of
peak
content
originates
from
above
is This
formation.
The
stretch.
origin.
Correlation
the
suggests the
two
spectrum
state.
vibrational
with
function
charge
electrical that
the
carrier.
All samples were prepared as reported previously. Polyazine ( -[N=C(CH~)-C(CH~)=N]) was • D J X . synthesized from an acld catalyzed eondensatlon of 2,3-butanedione and 2,3-butanedionedihydrazone, giving the polymer in the all ~rans conformation as previously determined . Iodine doping was accomplished by slurrying the polymer with chloroform solutions of the dopant. For very high iodine levels (y>0.75) it was necessary to initiate the doping by allowing the polymer to react with free iodine gas at room temperature before the solvent was added. All IR spectra were measured at room temperature as KBr pellets on a Perkin-Elmer 281B_~nfrared spectrometer between 600 and 4000 cm ~. Pressed pellet conductivities were determined using the VanderPauw technique. Figure 1 shows the IR spectra of the iodine doped polyazine as a function of iodine concentration for the entire spectral region measured. Two features are to be noted. First, the vibrational signature of the polymer skeleton is unchanged by the oxidation, although somewhat washed out at high doping levels. This implies that the polymer chain remains intact upon doping with iodine and that this mild oxidant does not decompose the material. Second, as the dopant level increases, a broadland featureless absorption centered at 4000 cm- or above (!0.5 eV) grows in intensity. This ty~e~of band is often seen in conducting polymer0'~'l and is assigned as an electronic absorption, probably from the valence band to a low lying midgap state. In comparison, in polythiophene and polypyrrole the lowest lying electronic absorptions are observed at 0.65 ev and 0.7 eV, respectively, consistent with the assignment made here. Further support for this assignment is found from previously reported Extended Huckel calculations: depending on which atoms (nitrogen or carbon) the oxidation is centered around, it was found that the lowest lying midgap state in polyazine ranqed from 0.26 eV to 0.60 eV above
The study of conducting polymers has been fruitfullin leading to new materials and to new physics. Th~ prototype of these polymers is polyacetylene , which is a simple linear chain of carbon atoms with alternating single and double bonds. Polyazine is also a linear chain of atoms with alternating single and double bonds, but with pairs of nitrogen atoms substituted for pairs of carbon atoms in the polyacetylene chain: -C=C-C=C-C=C-C=C-
a
polymer,
the
associated with -I is narrow and
of
as
this
The effeGt of the nitrogen atoms is twofold. First, unlike polyac~tylene, polyazine is environmentally stable. Second, the nitrogen atoms change the topology of polyazine relative to polyacetylene so that the ground state of polyazine cannot be degenerate. This has bearing on the nature of the excitations available to polyazine: solitons, such as found in polyacetylene, cannot exist in polyazine; rather, midgap states must be polarons or bipolarons. Like polyacetylene, polyazine can be doped with iodine and other oxidants to give a highly conducting material. Charge storage in bipolaron states has been found in other conducting polymers containing heter~cyclic monomers,_ most notably polythiophene ~ and polypyrrole b. Oxidation from the pi system of polyazine is also e~pected to give bipolaron type midgap states , but several structures are possible depending on which atoms the charge is centered on. In polyazine, oxidation could put charge on nitrogen atoms, carbon atoms, or a combination of both.6Previously reported Extended Huckel calculations show that any of these combinations is posssible, with comparable energies within the accuracy of the method. 291
INFRARED SPECTRA OF IODINE DOPED POLYAZINES
292
Vol. 68, No. 3
r=o.o ~r=o.22
Y=o.38 w
(D z
Y=o.42 Y=(xso
<
I,I-
w (D Z
Y=o.57 o9 z
r=o.67
rr I--
'=o.8o '=
J
I
I
~ooo
I
I
~
L
,~
doo
<
i.o9
6O f <~ Cr
C4N6N2Iy)x
I
,~
N
WAVENUMBER Figure i. IR spectra of (C~H.N~I) as a func: .~ z x -i tion of iodin~ content, y, in ~he ~ange 600 cm to 4000 cm . All spectra I were taken as KBr pellets. The peak at 2300 cm- is due to CO 2.
l~)x 18 O0
1400
1000
WAVENUMBER the top of the valence edge. Further spectroscopy in the near-IR is needed to determine the absorption maximum of this transition as well as to find the companion peaks predicted for a bipolaron type structure. Closer inspection of Figure 1 also shows a narrow vibrational peak~growing with increasing iodine level at 1505 cm . Thls is shown zn more detail in Figure 2. This peak is assigned to either a C=C or N=N double bond stretch. Either of these geometries is consistent with bipolaron formation upon oxidation: a bipolaron of the form - i
.
R
.
.
R
k
h
+
/
C=N-N-C=C-N-N=C
+
/
k
\ R
Figure 2. IR spectra of (C~HrN.I) as a func= , . .q z x -± tlon of iodine content, y, in ~he ~ange 800 cm to 1800 cm- . All spectra were taken as KBr pellets. charge on adjacent (even in a somewhat delocalized bipolaron) atoms should be strongly disinclined for coulombic reasons. Second, the offchain effect of putting the counterions (presumably triiodides) in close proximity, would be opposed on both steric and coulombic grounds. The conductivity of the iodine doped materials is also affiliated with the number and mobility of the bipolaron-associated defect, thu~ suggesting a correlation between the 1505 cm peak and the conductivity. Support for this contention is shown in Figure ~I where the rela-
R
would store charge on nitrogen atoms with a C=C double bond while a bipolaron of the form R
(C4H6N22y) x
0.9
R
\
\+
0
/
N=C-C-N=N-C-C=N
+\
/
R
R
would store charge on carbon atoms with a N=N double bond. Either of these interpretations implies oxidation from the polymer pi system. An alternate possibility is oxidation from the nitrogen lone pairs giving a structure of the form R R
h
/
+\
0.7 O9 m .<
\+ R
\
b 0.5 0
which also gives a N=N double bond. However, this structure is not favored for two reasons. First, the on-chain effect of concentrating
•
0.3
ib
--4
m .<
-5 0.I I
0.0 R
-2 rO
~..~e
N=C-C=N=N=C-C=N
/
-I
00
\
I
I
I
Q8 Y
I
I
!
1.6
Figure ~I Plot of the relative intensity of the 1505 cm peak and the room temperature conductivity as a function of iodine content, y.
Vol. 68, No. 3
INFRARED SPECTRA OF IODINE DOPED POLYAZINES
tive intensity of the 1505 _c~ peak (using the strong and invariant 1355 cm peak as an internal reference) and the log of the room temperature pressed pellet conductivity are plotted as a function of iodine content. The parallel between the two curves is striking. Both curves increase rapidly from y=0 to y=0.8 and then flatten out for y> 0.8. Assuming the iodine takes the form of triiodide, then y=0.8 suggests 0.27 charges per monomer or 0.07 charges per atom along the chain, as compared to polyacetylene which undergoes a similar transition st about 0.02 charges per atom along the chain . Although the two quantities plotted in Figure 3 are clearly parallel, the correspondence between the relative absorbance of the charge carrier, which is proportional to the number of carriers, and the logarithm of the conductivity is puzzling. To a first approximation, the conductiv-
293
ity shoul~0also be proportional to the number of carrlers; however, both the carrier mobility and the Fermi level may depend on the extent of oxidation thus accounting for the observed behavior. In conclusion, the mid-IR behavior of iodine doped polyazine is characterized by two featurea: a broad electronic band centered at 4000 cm or _~bove and a narrow vibrational band at 1505 cm . The electronic band is consistent with a transition to a low lying midgap state associated with bipolaron formation, as seen in other conducting polymers. The vibrational band is assigned to a double bond stretch, either N=N or C=C. Correlation of the intensity of this band with the room temperature conductivity strongly suggests that the bipolaron charge carrier is associated with the double bond formed upon oxidation by iodine.
REFERENCES i. J. E. Frommer, Acc. Chem. Res., 1986, 19, 2. 2. S.R. 385.
Roth, H.
Bleier, Adv. Phys., 1987, 36,
3. C. R. Hauer, G. S. King, E. L. McCool, W. B. Euler, J. D. Ferrara, W. J. Youngs, J. Am. Chem. Soc., 1987, 109, 5760. 4. T. C. Chung, J. H. Kaufman, A. J. Heeger, F. Wudl, Phys. Rev. B, 1984, 30, 702. 5. J. L. Br~das, J. C. Scott, K. Yakushi, G. B. Street, phys. Rev. B, 1984, 30, 1023. 6. W. B. Euler, J. Phys. Chem., 1987, 9_/1, 5795.
7. B. N. Diel, T. Inabe, J . W . Lyding, K. F. Schoch, Jr., C . R . Kannewurf, T. J. Marks, J. Am. Chem. Soc., 1983, 105, 1551. 8. MNDO and VEH calculations suggest this type of structure. L.A. Burke, R. N. Butler, private communication. 9. Y.-W. Park, A. J. Heeger, M. A. Druy, A. G. MacDiarmid, J. Chem. Phys., 1980, 73, 946. i0. J. H. Perlstein, in "Solid State Chemistry and Physics, An Introduction," P. F. Weller, ed., Marcel Dekker, Inc., New York, 1973, Vol. i, 189.