Vibrational Spectroscopy of Conducting Polymers
Yukio Furukawa

Reproduced from:

Handbook of Vibrational Spectroscopy
John M. Chalmers and Peter R. Griffiths (Editors)
 John Wiley & Sons Ltd, Chichester, 2002

Vibrational Spectroscopy of Conducting Polymers
Yukio Furukawa
Waseda University, Tokyo, Japan

1 INTRODUCTION
A new field of materials science resulted from the discovery of organic polymers having conjugated p-electrons:polyacetylene, polythiophene, poly(p-phenylene), poly(pphenylenevinylene), etc.1 – 3 The chemical structures of the
conjugated polymers are shown in Figure 1. These conjugated polymers are organic semiconductors whose bandgaps are typically in the range from 1 to 3 eV. A conjugated polymer can be doped chemically with an electron
acceptor such as halogens, AsF5 , and FeCl3 or an electron
donor such as alkali metals. The chemical doping is performed by exposing a polymer film in various ways to an
acceptor or a donor. The main process of doping is redox
reaction between polymer chains and acceptors or donors.
Upon acceptor doping (p-type doping), an ionic complex
consisting of multiply oxidized (positively charged) polymer chains and counter anions (e.g. I3  , AsF6  ) is formed.
Counter anions are generated by reduction of acceptors. In
the case of donor doping (n-type doping), an ionic complex
consisting of multiply reduced (negatively charged) polymer chains and counter cations (e.g. NaC , KC ) is formed.
Counter cations are generated by oxidation of donors.
The electrical conductivity of the polymer increases withincreasing content of the dopant. In a heavily doped state,
the polymer shows metallic properties. Since the electricalconductivity (i.e. the Fermi level) of a conjugated polymer
can be controlled by the content of a dopant, conjugatedpolymers can be used as organic semiconductors or metals. Doping also can be performed electrochemically. It has
been demonstrated that conjugated polymers can be used forfield-effect transistors,4,5 photovoltaic cells (solar cells),6
and light-emitting diodes,7,8 as organic semiconductors.
 John Wiley & Sons Ltd, 2002.

The properties of conjugated polymers have been interpreted by new concepts of elementary electronic excitations
such as solitons,9 polarons,10 – 12 and bipolarons.11 – 13 Since
they can move along a polymer chain or hop between
chains, they are also called quasi-particles. Charged excitations such as charged solitons, polarons, and bipolarons
are considered to be charge carriers. Charge carriers can begenerated by light irradiation and injection in junction structures. These new techniques in carrier generation are used
for transistors, light-emitting diodes, photovoltaic cells, etc.
Conjugated polymers can be classified into degenerate
and nondegenerate polymers, according to the degeneracyof the ground states. The prototype of degenerate polymers is trans-polyacetylene, which has a simple structurewith alternating CDC and C–C bonds. The energy curve oftrans-polyacetylene has two equal minima, where the alternating CDC and C–C bonds are reversed. However, a nondegenerate polymer has no two identical structures in the
ground state. Most conjugated polymers belong to this class.
The types of elementary excitations depend on the degeneracy of ground states. When an electron is removed froma nondegenerate polymer such as polythiophene, charge Ceand spin 1/2 are localized over several repeating units withstructural changes, as shown schematically in Figure 2(a).
This is called a positive polaron. Since a positive polaronhas charge Ce and spin 1/2, it corresponds to a radical
cation in chemical terminology. When another electron isremoved from the positive polaron, charge C2e is localized
over several units, as shown in Figure 2(b). This species iscalled a positive bipolaron, which has charge C2e and no
spin. Bipolarons are spinless carriers. A positive bipolaroncorresponds to a dication in chemical terminology. If abipolaron is unstable, two polarons are formed, as shownin Figure 2(c). In the case of donor doping, a negative
polaron and a negative bipolaron can be formed. In a

2

Analysis and Characterization of Polymers and Rubbers

S
n

n

n

n

R
(b)

(a)

(c)

R

(d)

OCH2CH(C2H5)(CH2)3CH3

n

n

R′

OCH3
(e)

(f)

H
N

H
N

S
n

n

n

R
(g)

(h)

(i)

Figure 1. Chemical structures of conjugated polymers: (a) transpolyacetylene; (b) cis-polyacetylene; (c) poly(p-phenylene); (d)polythiophenes (R: H, C6 H13 , C8 H17 , etc.); (e) poly(pphenylenevinylene)s (R, R0 : H, OCH3 , OC6 H13 , C8 H17 , etc.);(f) poly(2-methoxy-5-(20 -ethylhexyloxy)-p-phenylenevinylene)(MEH-PPV); (g) polypyrrole; (h) poly(2,5-thienylenevinylene)s(R, R0 : OCH3 , OC6 H13 , etc.); (i) polyaniline (emeraldine baseform).

degenerate polymer such as trans-polyacetylene, solitons
can be formed. Solitons are classified into neutral, positive,
and negative types according to their charges. A neutral
soliton has no charge and spin 1/2 (Figure 2d). A positive
soliton has charge Ce and no spin (Figure 2e). A negative

(a)

(b)

(c)

S

S

S
S

+

S
S

S

•

S

S

+

S

S

S
S

S
S

+

S

•

S

S

S

S

S

S

S
+

S

S

S
S

S

S

•

S

S

soliton has charge e and no spin. Charged solitons are
spinless carriers. A neutral soliton, a positive soliton, anda negative soliton correspond to the neutral radical, thecation, and the anion of a linear trans-oligoene with an odd
number of carbon atoms, respectively. Although the charge
and/or spin are depicted as being localized on one carbon
atom in Figure 2(a–e), it should be noted that they extend
over several rings with structural changes in real polymers. Elementary excitations in conjugated polymers arelisted in Table 1, together with the corresponding chemicalterms. Since these excitations are associated with structuralchanges, they are also called self-localized excitations.
Actual samples of conjugated polymers have more orless sp3 defects and a distribution of conjugation length.
The structures of neutral conjugated polymers (such asconformations, configurations, conjugation length, defects,etc.) can be obtained by infrared (IR) and Raman spectroscopy. Spectroscopic studies of well-defined oligomers
are useful for a better understanding of the Raman and
IR spectra of pristine polymers. Studies on vibrational
spectra of pristine conjugated polymers and doping- andphoto-induced IR absorption spectra have been reviewed
previously.14 – 18 Recently, electronic and vibrational spectra associated with the charge carriers generated by doping
have been discussed.19 – 21 In this article, we will focus ourattention on the Raman and IR studies of charge carriers.
The charged species of oligomers are models of the charge
carriers, polarons, bipolarons, and charged solitons, in conjugated polymers. Thus the studies of the charged speciesof oligomers are useful for explaining the Raman and IR
spectra originating from carriers. In this article, we will
mention the vibrational spectra of pristine polymers briefly,

S

+

S
S

S

S

(d)
•
(e)
+

Figure 2. Schematic structures of elementary excitations: (a) a positive polaron; (b) a positive bipolaron; (c) two positive polarons;(d) a neutral soliton; (e) a positive soliton.

881

1400

1200

1000

661
634

886
860

959

1193

1171

798

624

700

1045

1219

1368

1282
1222

1600

800

600

Raman shift /cm−1

1600

1200

466

582

696

838

509

752
700
800

429

1000

560
556

837

965

1001

804

923

1068
1029
1111

1400

784

1800

1013

1596

(d)

1340
1270
1210
1179
1107

1518

(c)

1601

(b)

1226

1490
1440

788

891

1078

1292
1251

1630

(a)

739

1012

Figure 3. Raman spectra of conjugated polymers: (a) trans-polyacetylene; (b) polythiophene; (c) poly(p-phenylene); (d) poly(pphenylenevinylene). Excitation wavelength is 1064 nm.

1719

Totally symmetric bands are dominant in the observed
Raman spectra of pristine conjugated polymers in mostcases, and they reflect effective conjugation length of thepolymer chains. In the IR spectra of pristine conjugatedpolymers, the CH out-of-plane bending vibrations, which
are markers of geometrical isomerism or substituent
positions of benzene rings or heterocycles, are strongly
observed. The Raman and IR spectra of several representative polymers are shown in Figures 3 and 4, respectively.
The vibrational spectra of trans-polyacetylene, cispolyacetylene, polythiophene, poly(p-phenylene), poly(pphenylenevinylene), etc. have been assigned on the basisof infinite planar structures for the first approximation. The
point groups and vibrational irreducible representation at
the zone center (k D 0) for each conjugated polymer are
listed in Table 2.
The Raman and IR spectra of trans-polyacetylene areshown in Figures 3(a) and 4(a), respectively. The Raman
spectrum is taken with the 1064-nm laser line. A planar
infinite trans-polyacetylene chain is isomorphous to thepoint group C2h . There are five Raman-active modes including one CH stretching and three IR-active modes including one CH stretching. The assignments of the observed
bands have been made on the basis of the wavenumber shifts on isotope substitutions and normal coordinate
calculations.22 – 27 The Raman bands observed at 1458,
1294, and 1068 cm1 are assigned to the n2 ag , n3 ag ,and n4 ag  modes. The 881-cm1 Raman band is assigned
to the n6 bg  mode. The IR band observed at 1251 cm1is assigned to the n8 bu  mode. The intense IR band at
1012 cm1 is characteristic of the CH out-of-plane wagging
n5 au  associated with trans-CHDCH groups, whereas

1800

Absorbance2 INFRARED AND RAMAN SPECTRAOF PRISTINE POLYMERS

1014

1294
1457
1624

(d)

1328
1301

(c)

1415

and describe the Raman and IR studies on carriers generated
by chemical doping, light irradiation, and injection.

1493

(b)

1468

1/2
1/2
0
0
1/2
0
0
0
1

1481

Ce
e
C2e
2e
0
Ce
e
0
0

1392

Radical cation
Radical anion
Closed-shell dication
Closed-shell dianion
Neutral radical
Cation
Anion
S1
T1

1424

Positive polaron
Negative polaron
Positive bipolaron
Negative bipolaron
Neutral soliton
Positive soliton
Negative soliton
Singlet exciton
Triplet exciton

1595

Spin

1581

Charge1546

Chemical term(a)Raman intensity

Elementary excitation

1458

Table 1. Elementary excitations in conjugated polymers andchemical terminology.

1068

Vibrational Spectroscopy of Conducting Polymers 3

600

400

Wavenumber / cm−1

Figure 4. IR spectra of conjugated polymers: (a) transpolyacetylene; (b) polythiophene; (c) poly(p-phenylene); (d) poly(p-phenylenevinylene).

the corresponding band of cis-CHDCH groups is observed
at 740 cm1 . The assignments of weak bands are given
in Takeuchi et al.26 and Hirata et al.27 The Raman spectra of trans-polyacetylene have been explained by theeffective conjugation coordinate (ECC) theory proposed

4

Analysis and Characterization of Polymers and Rubbers

Table 2. Point groups and vibrational representations.
Polymertrans-Polyacetylene
Polythiopheneb
Poly(p-phenylene)b
Poly(p-phenylenevinylene)

Point group

Representationa

C2h
D2h
D2h
C2h

4ag R C au IR C bg R C 2bu IR
7ag R C 3au C 7b1g R C 3b1u IR C 3b2g R C 6b2u IR C 3b3g R C 6b3u IR
5ag R C 2au C 5b1g R C 2b1u IR C 3b2g R C 4b2u IR C b3g R C 4b3u IR
14ag R C 6au IR C 6bg R C 12bu IRa R and IR denote Raman- and IR-active vibrations, respectively. The ag and bu modes are in-plane vibrations, and the au and bg modes are out-of-planevibrations. The ag , b1g , b2u and b3u modes are in-plane vibrations, and the au , b1u , b2g and b3g modes are out-of-plane vibrations.
b The xy plane is taken in the polymer plane (x-axis along the polymer chain) and the z axis perpendicular to the polymer plane.

by Castiglioni et al.28 Their theory has been successfully
applied to other conjugated polymers.16,18 Their theory isa molecular spectroscopy version of the amplitude mode
theory.29,30 According to these theories, the vibrationalmodes that induce oscillations in bond alternation havestrong Raman intensities. The n2 ag  and n4 ag  vibrations
modulate strongly bond alternation: antisymmetric stretching of CDC and C–C bonds. It is well known that the n2 andn4 Raman bands of trans-polyacetylene in Figure 3(a) showchanges in wavenumber and/or bandshape with the excitation of various wavelengths. These observations reflect
the coexistence of the segments having various conjugation
lengths in a trans-polyacetylene film.
An important problem remaining to be solved in thevibrational spectra of trans-polyacetylene is the assignmentof the Raman band observed at 1014 cm1 . This bandhas been assigned to the CH out-of-plane wagging modethat is IR active and Raman inactive on the basis of thewavenumber shift on 13 C-substitution.26 The appearance of
the Raman-inactive mode in the Raman spectrum can be
explained by the reduction of symmetry due to the distortionof polyacetylene chains. Recently, helical polyacetylene has
been synthesized by using chiral nematic liquid crystal
solvents.31 Probably, trans-polyacetylene films prepared by
the previous method are the racemic mixtures of helicalchains.
The Raman and IR spectra of polythiophene are shownin Figures 3(b) and 4(b), respectively. The observed bands
have been explained by assuming a coplanar s-trans infinite structure (D2h ).32 – 37 In reference 37, the Raman bands
observed at 1457, 1368, 1219, 1045, and 700 cm1 areassigned to the n2 ag , n3 ag , n4 ag , n5 ag , and n6 ag modes, respectively. The 1493-cm1 band is assigned to
the n12 b1g  mode. However, different assignments areproposed for the Raman bands observed at 1493 and1368 cm1 . Navarrete and Zerbi35 assigned the 1493-cm1band to a totally symmetric fundamental band insteadof the 1368-cm1 band. The marker bands of distorted
structures, conjugation length, etc. are reviewed in Haradaand Furukawa.15 The 786-cm1 IR band is assigned to
the CH out-of-plane wagging n18 b1u  of 2,5-disubstituted

thiophene rings and the 695-cm1 band to the CH outof-plane wagging of terminal 2-monosubstituted thiophenerings. Thus the intensity ratio of these bands can be used asa marker of the degree of polymerization. The 466-cm1 IRband is also assigned to the out-of-plane bending vibrationn19 b1u . The 1490- and 1440-cm1 IR bands are assigned
to the CDC antisymmetric stretching n34 b3u  and symmetric stretching n25 b2u , respectively. The complete assignments of the observed IR bands have not been made yet.
The Raman and IR spectra of poly(p-phenylene) are
shown in Figures 3(c) and 4(c), respectively. These spectra
have been analyzed by a coplanar infinite polymer structure(D2h ). Major Raman and IR bands have been assigned on
the basis of the wavenumber shifts on isotope substitutions
and normal coordinate calculations.21,38 – 41 The Raman
bands observed at 1595, 1282, 1222, 798, and 624 cm1
are assigned to n2 ag , n3 ag , n4 ag , n5 ag , and n11 b1g ,
respectively. The IR band observed at 804 cm1 is assigned
to the CH out-of-plane wagging of para-disubstituted benzene rings n13 b1u . The IR bands at 752 and 700 cm1 aredue to CH out-of-plane wagging of terminal phenyl rings.
The observation of these two bands suggests that the degreeof polymerization is rather small.
The Raman and IR spectra of poly(p-phenylenevinylene)
are shown in Figures 3(d) and 4(d), respectively. These
spectra have been analyzed by a coplanar infinite polymer
structure (C2h ).42,43 There are fourteen ag modes including three CH stretchings. The Raman bands observed at
1624, 1581, 1546, 1328, 1301, 1193, 1171, 886, 661, 634,
and 328 cm1 are assigned to the in-plane ag fundamental
modes. There are twelve bu modes including three CH
stretchings. The IR bands observed at 1518, 1424, 1340,
1270, 1179, 1107, 1013, 784, and 429 cm1 are assigned
to the in-plane bu fundamental modes. The 965-cm1 IRband is assigned to the CH out-of-plane wagging of thetrans vinylene groups. No band due to cis vinylene groupis observed. The 837-cm1 band is assigned to the inphase CH out-of-plane wagging of para-disubstituted benzene rings. The 556-cm1 band is also assigned to an
out-of-plane vibration. The weak Raman band observed
at 959 cm1 is probably assigned to the CH out-of-planeVibrational Spectroscopy of Conducting Polymers 5
wagging that is IR active and Raman inactive under local
C2h symmetry of the trans planar structure. The observationof this mode in the Raman spectrum possibly reflects the
distortion of the vinylene group from the trans planar form.

3 RAMAN SPECTRA OF DOPEDPOLYMERS
Most pristine conjugated polymers show the ppŁ electronic absorptions in the region from ultraviolet to visible.
Upon doping, new absorption bands associated with the
sub-gap bands of elementary excitations appear in theregion from visible to IR. Thus, the structures of doped
polymers (elementary excitations generated by doping) can
be studied by resonance Raman spectroscopy with a wide
range of excitation wavelength from visible to IR.44 The
longest excitation wavelength used so far is 1320 nm. Clear
conclusions were not obtained in previous Raman studies on
doped states, because it is difficult to assign the observed
Raman bands. A positive polaron (charge Ce, spin 1/2)
and a negative polaron (charge e, spin 1/2) correspondto the radical cation and the radical anion of an oligomer,
respectively. A positive bipolaron (charge C2e, spin 0) anda negative bipolaron (charge 2e, spin 0) correspond tothe closed-shell dication and the closed-shell dianion of anoligomer, respectively. Thus, marker bands for identifying
polarons and bipolarons can be obtained from the studiesof the radical ions and the divalent ions of oligomers. Thus
studies of charged oligomers are very important. Recently,
the Raman spectra of the charged species of oligomers havebeen studied extensively. The chemical structures of theoligomers described in this article are shown in Figure 5.
Before going to the Raman studies of doped polymers,
it is useful to describe the electronic states of charge
carriers in conjugated polymers.20,45 The schematic electronic structures of carriers are shown in Figure 6. Although

excitonic effect or electron correlation plays an importantrole in the optical properties of conjugated polymers andoligomers, we will use a one-electron picture because of its
simplicity. The band structure of a neutral polymer is shownin Figure 6(a). For a polaron or a bipolaron, two localized electronic levels, bonding and antibonding, are formed
symmetrically with respect to the gap center at !0 and
C!0 , respectively, as shown in Figure 6(b–e). The positions of the localized electronic levels, i.e. š!0 , dependon the extent of the structural changes associated with thepolarons and the bipolarons. When a positive polaron isformed, one electron is removed from the !0 level. Thusa positive polaron has two sub-gap transitions, P1 and P2(Figure 6b). When a negative polaron is formed, one electron is added to the C!0 level. Thus a negative polaron also
has two sub-gap transitions, P1 and P2 (Figure 6c). Whena positive bipolaron is formed, two electrons are removed
from the !0 level. Thus a positive bipolaron has a single
sub-gap transition BP1 (Figure 6d). When a negative bipolaron is formed, two electrons are added to the C!0 level.
Thus a negative bipolaron has a single sub-gap transition
BP1 (Figure 6e). For a positive and a negative soliton, S,
a nonbonding level is formed in the center of the bandgap. Thus, a single sub-gap absorption is expected to beobserved at the middle of the band edge of the neutralpolymer as shown in Figure 6(g) and (h).

3.1 Doped polyacetylene
The Raman spectra of doped polyacetylene have beenmeasured with excitation of various wavelengths.23,46 – 50
The Raman spectra of trans-polyacetylene and heavily Nadoped trans-polyacetylene with 1320-nm excitation50 are
shown in Figure 7(a) and (b), respectively. The electronic
absorption band of heavily Na-doped trans-polyacetylene
appears from visible to IR, whereas a band that has peaked

(a)

(b)

S

S

S
S

S

S

(c)

Figure 5. Chemical structures of (a) 19,190 ,20,200 -tetranor-b,b-carotene (TNBC), (b) a-sexithiophene, and (c) p-quinquephenyl.

6Analysis and Characterization of Polymers and Rubbers

CB

(a)

P1

+ω 0
−ω 0

S

S

P1

(b)

VB
(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 6. Schematic electronic structures of elementary excitations: (a) neutral polymer; (b) positive polaron; (c) negative
polaron; (d) positive bipolaron; (e) negative bipolaron; (f) neutral
soliton; (g) positive soliton; (h) negative soliton.

1551
(c)

1496

1159 1130
1275 1190

1537
(d)
1275

1068

1469
1400

1010

1200

1400

1118
1176

1200

1000

Raman shift /cm−1

Figure 8. Raman spectra of the various oxidation states of
TNBC: (a) neutral species; (b) and (c) radical anion; (d) dianion.
Excitation wavelengths used in (a), (b), (c), and (d) are 1064,
1064, 1320, and 1064 nm, respectively. (a) Solid; (b), (c), and(d), tetrahydrofuran (THF) solutions. The bands of THF and background are subtracted in (b), (c), and (d).

953

1151
1105

1172

1290
1255

(b)

1600

1130
1159
1275

1551

1600

1493

Raman intensity(a)1290 1174

1496

S

BP1

1128

1597

P2

Raman intensity

P2

BP1

1522

1000

800

Raman shift /cm−1

Figure 7. Raman spectra of (a) trans-polyacetylene and (b) heavily Na-doped trans-polyacetylene (CHNa0.14 ). Excitation wavelengths are 1320 nm.

at 1.83 eV, which is due to neutral trans-polyacetylene,
disappears completely. Since the excitation wavelength
1320 nm locates within the doping-induced absorption, the1320-nm excited Raman spectrum mainly arises from new
structures generated by Na doping. It is expected that negative polarons or negative solitons can be formed by Na doping. The n2 band of trans-polyacetylene shows an upward
shift from 1469 to 1493 cm1 upon Na doping. The n3 bandshows a downward shift from 1290 to 1255 cm1 upon
Na doping. The n4 band also shows an upward shift from1068 to 1105 cm1 upon Na doping. A broad strong band
is observed at 1151 cm1 , which corresponds to the 1172cm1 band of pristine trans-polyacetylene. These spectralchanges will be compared with those of model compounds.
The Raman spectra of the neutral species, the radical
anion, and the dianion of TNBC are shown in Figure 8(a).51
The radical anion corresponds to a negative polaron,
whereas the dianion corresponds to two negative solitons.
The radical anion of TNBC has an intense absorption at
1.46 eV (849 nm) and a weak band at 1.00 eV (1240 nm),whereas the neutral TNBC has a band peaked at

2.82 eV (440 nm). An intense fluorescence backgroundhas prevented us from observing the rigorous resonanceRaman spectra of the radical anion excited at either 753 nm(1.65 eV) or 781 nm (1.59 eV) within the 1.46-eV electronic
absorption band. However, the Raman spectra of the radical
anion have been obtained with excitation at either 1064 nm(1.17 eV) or 1320 nm (0.94 eV). The 1551- and 1496-cm1bands (CDC stretchings) of the radical anion correspondto the 1597- and 1522-cm1 bands of neutral TNBC,
respectively. The lower wavenumber shift of each band
reflects a decrease in the CDC bond order. The 1290cm1 band of TNBC downshifts to 1275 cm1 for theradical anion. The 1128-cm1 band upshifts to 1130 cm1 .
This small upshift seems to be the result of strong mixing
between the C–C and CDC stretches. The intensity of eachof the 1551-, 1275-, 1190-, and 1159-cm1 bands in the1320-nm excited spectrum is stronger than that in the 1064nm excited spectrum.
The dianion of TNBC has an intense absorption at
1.59 eV (780 nm). It was not possible to observe the resonance Raman spectrum of the dianion with the 632.8- or
753-nm excitation line, because of the presence of an
intense fluorescence background. However, a pre-resonant
Raman spectrum of the dianion has been observed withthe 1064-nm excitation line (Figure 8d). The 1537-cm1
broad band of the dianion corresponds to the 1522-cm1band of neutral TNBC. This 15-cm1 upshift observed forthe dianion cannot be explained by a simple consideration

1600

1062

1156

1400

1057

1151

1220

1473

(b)

1359

1415

1224

1473

1354

(a)

Raman intensity

1200

1000

Raman shift /cm−1

Figure 9. Raman spectra of a BF4  -doped polythiophene film.
Excitation wavelengths are 753 and 1064 nm for (a) and (b),respectively.
1459

of the bond order of the CDC bonds. This upshift may
originate from large structural changes on going from theneutral TNBC to the anion. According to the ab initio
molecular orbital calculations of 1,3,5,7,9-decapentaene,52bond alternation is reversed in the middle of the chain. The
1275-cm1 band of the dianion corresponds to the 1290cm1 band of the neutral species. The low wavenumber
position of the n3 band is a marker of negatively charged
polyenes. The 1118-cm1 band of the dianion corresponds
to the 1128-cm1 band of the neutral species. This 10cm1 downshift also may reflect the inversion of the bondalternation.
It seems that the spectral changes observed for transpolyacetylene upon Na doping are similar to those observed
for TNBC on going from the neutral species to the dianion(two negative solitons). However, further experimental andtheoretical studies on the charged species of polyenes are
requisite for a complete analysis of the Raman spectra of
Na-doped trans-polyacetylene.

1419

Vibrational Spectroscopy of Conducting Polymers 7

1600

1400

1300

1051
1067
1055

1149

1160

1200

(c)
1083

1223

1355 1356

1234

(b)

1483
1500

1220

1368

1440 1426

(a)

1525
1502
1487
1448
1419
1405

Raman intensity

The as-polymerized BF4  -doped polythiophene preparedby an electrochemical method has two broad absorption
bands peaked at 1.68 and 0.73 eV (738 and 1700 nm).53
In a nondegenerate conjugated polymer such as polythiophene, polarons or bipolarons are expected to be formedby doping. The observed two bands are attributable topositive polarons, because a polaron gives rise to the two
sub-gap electronic absorption bands, whereas a positive
bipolaron gives rise to a single band. The Raman spectra of an as-polymerized BF4  -doped polythiophene film
excited at 753 and 1064 nm are shown in Figure 9(a) and(b), respectively.53 The excitation lines, 753 and 1065 nm,
locate within the doping-induced electronic absorptions.
The observed Raman spectra are different from that ofpristine polythiophene. These spectra are attributable to thecharge carriers generated by doping.
The radical cation and the dication of a-sexithiophenecorrespond to a positive polaron and a positive bipolaron,
respectively. Thus, the Raman spectra of the radical cationand the dication are useful for determining the types of
elementary excitations generated by doping. The Raman
spectra of the neutral species, the radical cation, and the
dication of a-sexithiophene are shown in Figure 10.53 The
753- and 1064-nm excited Raman spectra are quite similar
to the spectrum of the radical cation, but different fromthat of the dication. This indicates that positive polarons
are generated by doping in the BF4  -doped polythiophenefilm, which is consistent with the conclusion drawn by
the electronic absorption study. The normal coordinate

1505
1472

3.2 Doped polythiophene

1100

1000

900

Raman shift /cm−1

Figure 10. Raman spectra of the various oxidation states of asexithiophene: (a) neutral species; (b) radical cation; (c) dication.
Excitation wavelengths used in (a), (b), and (c) are 1064, 781, and781 nm, respectively. (a) Solid; (b) CH2 Cl2 solution; (c) CD2 Cl2solution. The bands of the solvents and backgrounds are subtractedin (b) and (c).

calculations of the radical cations and the dications ofoligothiophenes have been made by means of the ab initio
molecular orbital method and density functional theory
method.54 – 56 The strongest band at 1440 cm1 is assigned
to the mixture of the CDC and C–C stretchings.
In the as-polymerized BF4  -doped polythiophene, only
polarons exist. However, stabilities of polarons and bipolarons depend on the dopant concentration. Apperloo et al.57
have shown that bipolarons are formed for heavily dopedpoly(3-alkylthiophene) in a solution by using electronic
absorption spectroscopy.

Analysis and Characterization of Polymers and Rubbers

8

3.3

Doped poly( p-phenylene)

1400

980

1221
1179

992
974
956

1200

(b)

2.

1000

800

Raman shift /cm−1

1042
1004
994
974

1008
992

1222
1215

(b)

1400

1200

1000

The wavenumbers of the inter-ring CC stretchings
reflect the structures of polarons (radical anions) andbipolarons (dianions). The 1282-cm1 Raman band ofpristine poly(p-phenylene) (Figure 3c), the 1315-cm1band due to polarons, and the 1344-cm1 band due tobipolarons are assigned to the inter-ring CC stretchings.
The wavenumber of the inter-ring CC stretching shows
the 33-cm1 upshift on going from the neutral polymer
to polarons and the 62-cm1 upshift on going from theneutral polymer to bipolarons. These observations are
explained by changes in the order of the inter-ring CC
bonds; that is, the length of the inter-ring CC bonds inthe order: neutral species > polaron > bipolaron.
The spectral features of the 1064-nm excited Raman
spectrum attributed to negative bipolarons are quite
different from those of the 514.5-nm excited spectrum attributed to negative polarons. The 1591-, 1315-,
and 1221-cm1 bands due to negative polarons can becorrelated to the 1595-, 1282-, and 1222-cm1 bands(ag modes) of the neutral polymer, respectively. Similarly, the 1586-, 1344-, and 1220-cm1 Raman bands
due to negative bipolarons can be correlated to thesame three bands of the neutral polymer, respectively.
The other observed bands of bipolarons originate from
Raman-inactive modes of the neutral polymer with D2hsymmetry. The appearance of these Raman-inactivemodes may be explained by considering that the formation of bipolarons breaks translational symmetry;bipolarons are localized on the several rings of thepolymer chain.

(c)

951
*

1346

(a)

1214
1184

1279
1320

1491

1592

1601
1571
1517
1506
1495
1482
1468

Raman intensity

1595

Figure 11. Raman spectra of Na-doped poly(p-phenylene). Excitation wavelengths are (a) 514.5 and (b) 1064 nm, respectively.

1600

1.

(a)

1220
1190

1290

1315
1600

1344

1493
1471

1586

Raman intensity

1591

The Raman spectra of Na-doped poly(p-phenylene) dependon the excitation wavelength.58 The Raman spectra excited
at 514.5 and 1064 nm are shown in Figure 11(a) and (b),respectively. These spectra have been analyzed on the basisof the Raman markers of polarons and bipolarons obtained
from the Raman spectra of the radical anions and the dianions of p-oligophenyls.58 For example, the Raman spectraof the neutral species, the radical anion, and the dianionof p-quinquephenyl are shown in Figure 12. The 514.5-nm
excited Raman spectrum of Na-doped poly(p-phenylene)
is attributed to negative polarons, because this spectrum issimilar to those of the radical anions of p-oligophenyls(Figure 12b). On the other hand, the 1064-nm excitedspectrum of Na-doped poly(p-phenylene) is ascribed tonegative bipolarons, because this spectrum is similar to
those of the dianions of p-oligophenyls (Figure 12c). Thus,
the Raman bands of polarons and bipolarons have been
selectively observed by changing the excitation wavelengthfrom 514.5 to 1064 nm. Raman bands arising from thepolarons and the bipolarons have been assigned on thebasis of the data of 13 C-substituted poly(p-phenylene).41
Two significant features in the Raman spectra of polarons
and bipolarons are as follows.

3.4 Doped poly( p-phenylenevinylene)
800

Raman shift /cm−1

Figure 12. Raman spectra of various oxidation states of pquinquephenyl: (a) neutral species; (b) radical anion; (c) dianion.
Excitation wavelengths used in (a), (b), and (c) are 514.5, 514.5,
and 1064 nm, respectively. (a) Solid; (b) and (c), THF solutions.
The bands of THF are subtracted in (b). The band marked withan asterisk is due to THF. Background due to fluorescence has
been subtracted in (c).

Upon doping with sulfuric acid (H2 SO4 ), the electronic
absorption band of neutral polymer disappears and newabsorption bands appear at 2.25 and 1.00 eV (551 and1240 nm).59 These two absorptions are due to positive
polarons generated by H2 SO4 doping. Figure 13 shows theRaman spectra of a H2 SO4 -doped poly(p-phenylenevinylene) film with various excitation wavelengths (441.6,
514.5, 632.8, 711.0, 740.0, and 1064 nm). The observed

1169

960
1169
1165

1316
1265

1007

1247

1413
1375
1324

1542

1316
1270
1258

(e)

(f)

1500

966

Raman spectra show only small changes with the exciting
laser wavelengths 441.6, 514.5, 632.8, 711.0, and 740.0 nm,
which locate within the broad band observed at 2.25 eV.
The Raman spectra obtained with excitations at 441.6,
514.5, 632.8, 711.0, and 740.0 nm are essentially the sameas the Raman spectrum of the radical cation of an oligomer
having two vinylene groups and three benzene rings. The
radical cation corresponds to a positive polaron. Thus
these spectra originate from positive polarons generatedby H2 SO4 doping. The 1064-nm excited Raman spectrum
is different from those obtained with other wavelengths,
as shown in Figure 13. It has been concluded that thisspectrum is contributed by the inter-chain polaron dimers,

(d)

1165

1415

1623
1623

Figure 13. Raman spectra of a H2 SO4 -treated poly(p-phenylenevinylene) film. Excitation wavelengths are 441.6, 514.5, 632.8,
711.0, 740.0, and 1064 nm for (a)–(f), respectively. Asterisks indicate the bands of H2 SO4 . Fluorescence backgrounds are subtracted
in all the spectra. [Reproduced by permission of American Chemical Society from A. Sakamoto, Y. Furukawa and M. Tasumi, J.
Phys. Chem., 98, 4635 (1994).]

1571
1545

1000

Raman shift / cm−1

(c)

1571
1546

1500

1326
1300
1247

1582

(b)

1564

1176
1331
1282
1263

(f)

1595
1557
1520
1437

1582
1550

1179
1330
1294

1434

1523

(e)

(a)

1314
1271
1171
1163

1178
1179

1330
1296
1331
1295

1436

1523

(d)

1313
1288
1238
1211 1167
1153

1179
1331
1295

1434
1435

1566

1523

(c)

1566

Raman intensity

1566

1523

(b)

1583
1559
1502

1565

*

1414

*

on the basis of the Raman spectra of the dimer of the radical
cations of the model oligomer.60
A single electronic absorption band appears at 1.55 eV(800 nm) upon Na doping,19 which is in contrast to theresult of H2 SO4 doping. This band is attributable to bipolarons, because a bipolaron gives rise to a single sub-gap
absorption band. However, different results have beenobtained from a Raman study.61 Figure 14 shows theRaman spectra of a Na-doped poly(p-phenylenevinylene)
film with various excitation wavelengths (488.0, 514.5,
632.8, 711.0, 740.0, and 1064 nm). The observed Raman
spectra strongly depend on excitation wavelength. These
spectra have been analyzed on the basis of the Raman spectra of the radical anions and the dianions of three oligomers.

1547

1332
1295

1179

(a)

Raman intensity

1625
1591
1567

Vibrational Spectroscopy of Conducting Polymers 9

1000

Raman shift /cm−1

Figure 14. Raman spectra of a Na-doped poly(p-phenylenevinylene) film. Excitation wavelengths are 488.0, 514.5, 632.8,
711.0, 740.0, and 1064 nm for (a)–(f), respectively. Fluorescencebackgrounds are subtracted in (b), (d), (e), and (f). [Reproducedby permission of American Chemical Society from A. Sakamoto,Y. Furukawa and M. Tasumi, J. Phys. Chem., 96, 3870 (1992).]

886
785
725
688
675
640

1026

1109

1600

1200

949

1030

1223

1400

1000

800

600

Wavenumber / cm−1

(a)

0.0

1600

1400

1200

902
1043

1107
1119
1092

0.5

1354
1323
1273

1473

(d)

0.0

1313
1275

(c)

1473

0.5

1414

1545

0.0

1186

1198
1164

(b)
0.2

1001
958
937

0.4

1108

1315

0.0

1009

1286

0.5

1369

Figure 15. IR absorption spectra of iodine-doped conjugatedpolymers: (a) trans-polyacetylene; (b) polythiophene; (c) poly(pphenylene); (d) MEH-PPV.

1621

The charge carriers in conjugated polymers can be generated by chemical doping and light irradiation. The formation of carriers such as polarons, bipolarons, and charged
solitons are associated with structural changes and localized vibrational modes as well as localized electronic states
within the band gap. Doping- and photo-induced Infrared
active vibrational (IRAV) modes can be observed in the IR
region.62 – 64 The iodine-doping-induced IR absorption spectra of trans-polyacetylene, polythiophene, poly(p-phenylene), and MEH-PPV are shown in Figure 15. Photoinduced
IR absorption spectra of trans-polyacetylene, polythiophene, poly(p-phenylene), and poly(p-phenylenevinylene)
are shown in Figure 16. These spectra were measured at
78 K by the Fourier transform infrared (FT-IR) differencespectrum method.65 The absorptivities of the doping- andphoto-induced IR bands are far larger than those of pristine polymers. The generated charged species give rise to
giant IR intensities. Thus, even if the concentration ofcarriers is very low, the carriers can be detected by IR
spectroscopy. The doping- and photo-induced IR bands
as well as the Raman spectra of pristine polymers havebeen explained successfully in terms of the ECC theory proposed by Zerbi et al.16,18 In their model, charge
oscillation along the polymer chain induces strong IR intensities. The vibrational modes corresponding to the totallysymmetric modes inducing oscillation in bond alternation for a neutral polymer are strongly observed in the
IR spectra due to carriers generated by chemical doping and light irradiation. Thus, the doping- and photoinduced IR bands correspond to the totally symmetricRaman bands of its neutral polymer. However, the number of doping- or photo-induced IR bands is in generallarger than that of the totally symmetric modes of thepristine polymer. For example, in the IR spectrum ofiodine-doped polythiophene 11 doping-induced bands areobserved, although there are 7 ag modes. It should be notedthat the ECC theory is a good approximation method,

1610

DOPING- AND PHOTO-INDUCEDINFRARED ABSORPTION∆ A (×10−3)

4

1111

*

*

(d)

1315
1265
1201

(c)

1485
1464

(b)

1200
1169
1113

1381
1327

1290

(a)

1566
1522
1481

As a result, the observed Raman spectra of Na-dopedpoly(p-phenylenevinylene) have been explained by thecoexistence of polarons and bipolarons having various
localization lengths. Since each species has its electronic
absorption, the Raman bands due to the species is resonantly enhanced by choosing the excitation wavelength
within the broad doping-induced absorption band. The
major species generated by Na doping is negative bipolarons, because the electronic absorption maximum isobserved at 1.55 eV.

1398

Analysis and Characterization of Polymers and Rubbers

∆A

10

1000

Wavenumber / cm−1

Figure 16. Photoinduced IR absorption spectra of conjugatedpolymers: (a) trans-polyacetylene; (b) polythiophene; (c) poly(pphenylene); (d) poly(p-phenylenevinylene). Temperature, 78 K.

but is not complete. It is expected that the normal coordinate calculations based on ab initio molecular orbital
methods of the charged oligomers is useful in analyzing
doping- and photo-induced IR spectra. We can identify thetypes of carriers by sub-gap electronic transitions appearing
in the region from visible to IR. In the lightly iodinedoped polythiophene, poly(p-phenylene), and MEH-PPV,Vibrational Spectroscopy of Conducting Polymers 11
two sub-gap absorption bands are observed. These twobands are attributable to positive polarons. Therefore,
the doping-induced IR bands are ascribed to positive
polarons. Similarly, photoinduced IR spectra of polythiophene, poly(p-phenylene), and poly(p-phenylenevinylene)
are also ascribed to polarons generated by light irradiation.
On the other hand, in the case of trans-polyacetylene having a degenerate ground state, doping- and photo-induced
IR absorption spectra are assigned to charged solitons.62 – 64
The decay mechanism of photogenerated carriers in the
micro- to millisecond region has been studied by means of
the phase-modulation technique.66,67 The observed results
have been explained on the basis of the second-orderkinetics involving a neutralization recombination process
between the positive and negative polarons. The mechanismof carrier generation induced by light irradiation has beenstudied by picosecond time-resolved IR spectroscopy.68,69
Moses et al.68 have concluded that carriers are photoexcited
directly, and are not generated via excitons. However, carrier generation via excitons has been proposed by Köhler
et al.70
In the composite of a conjugated polymer and C60 ,electrons are transferred from the conjugated polymer to
C60 by light irradiation.71 These composites are promising
candidates for photovoltaic cells and solar cells. Ultrafast spectroscopy has shown that the charge separation
occurs within a few hundred femtoseconds, with a very
slow back transfer. The photoinduced IR absorption spectraof poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV)and DOO-PPV/C60 composite measured at 77 K are shownin Figure 17(a) and (b), respectively.72 Photoinduced spectra are attributed to polarons generated by light irradiation.
The intensities of the bands due to polarons are enhanced
more than ten times by the addition of C60 . The carrier
generation has been studied by picosecond time-resolved
IR absorption spectroscopy.73

951

1084
1030

1489
1460

(b)

6 CONCLUDING REMARKS

Polymer

Anode

Al-Li alloy

Cathode

ITO

1216

0.5

1610

∆ A (×10−3)

1.0

The polymer light-emitting diodes fabricated with poly(pphenylenevinylene) derivatives, poly(fluorene) derivatives,
etc., show high power efficiencies. The schematic structure of a single-layer polymer light-emitting diode isshown in Figure 18. In the light-emitting diodes fabricatedwith a conjugated polymer, it is considered that carriers are injected from electrodes to the polymer layer by
the application of voltage. The electroluminescence resultsfrom the radiative decay of the singlet excitons that areformed by the recombination of injected positive and negative carriers. The information about carriers and/or excitons can be obtained by in situ IR reflective absorption
spectroscopy.74,75 The polymer light-emitting diode wasplaced on an external reflection apparatus of an FT-IR spectrophotometer. IR light was incident on the BaF2 plate andreflected off the metal electrode. IR light passed through thepolymer layer twice. The reflected IR light was returned tothe spectrophotometer. The voltage-induced IR absorption
spectra were measured by the FT-IR difference-spectrum
method. The voltage-induced IR absorption spectra of thelight-emitting diodes fabricated with DOO-PPV and MEHPPV are shown in Figure 19.74,75 Since the observed spectrum is similar to that of iodine-doped polymer for eachpolymer, the observed spectrum probably is attributed to
injected positive carriers (polarons); negative carriers are
not observed. The observation of the positive carriers is
probably related to the predominance of injected positive
carriers in these polymer light-emitting diodes.75 In orderto make more efficient light-emitting diodes, the injectionof more negative carriers is required.

It has been demonstrated that IR and Raman spectroscopy isa powerful tool for studying the structures of doped as wellas pristine conjugated polymers. The charge carriers such as
charged solitons, polarons, and bipolarons can be generated

2.0

1.5

5 INFRARED SPECTRA OF INJECTEDCARRIERS

(a)
BaF2

0.0
1600

1400

1200

1000

800

Wavenumber / cm−1

Figure 17. Photoinduced IR absorption spectra of (a) DOO-PPV
and (b) DOO-PPV/C60 composite. Temperature, 78 K.

IR beamFigure 18. Schematic structure of a single-layer light-emitting
diode fabricated with a conjugated polymer.

0
1800

1600

1400

926

1024

1103

1200

7. 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, 347, 539 (1990).

1000

8. R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes,
R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos,
J.L. Brédas, M. Lögdlund and W.R. Salaneck, Nature, 397,
121 (1999).

903

2

1610

(b)

1099

4

1427

1487

0

6. G. Yu, J. Gao, J.C. Hummelen, F. Wudl and A.J. Heeger,
Science, 270, 1789 (1995).

1018

(a)

1606

∆A (×10 −5)

1

1514
1468
1441

2

1215

Analysis and Characterization of Polymers and Rubbers

1219

12

9. W.P. Su, J.R. Schrieffer and A.J. Heeger, Phys. Rev. B, 22,
2099 (1980).
800

Wavenumber / cm −1

Figure 19. Voltage-induced IR reflective absorption spectra of
the light-emitting diodes fabricated with (a) DOO-PPV and(b) MEH-PPV at room temperature.

by doping. Polarons and bipolarons generated by chemical
doping have been identified by Raman spectroscopy. IR
spectroscopy is a powerful tool for detecting the photogenerated and injected carriers in polymer optoelectronic
devices, because the carriers give rise to giant IR intensities. In situ IR spectroscopy will be useful in studies oflight-emitting diodes, solar cells, and field-effect transistors
fabricated with conjugated polymers.

10. W.P. Su and J.R. Schrieffer, Proc. Natl. Acad. Sci. USA, 77,
5626 (1980).
11. S.A. Brazovskiı́ and N.N. Kirova, Sov. Phys. JETP Lett., 33,
4 (1981).
12. A.R. Bishop, D.K. Campbell and K. Fesser, Mol. Cryst. Liq.
Cryst., 77, 253 (1981).
13. J.L. Brédas, R.R. Chance and R. Silbey, Mol. Cryst. Liq.
Cryst., 77, 319 (1981).
14. H. Kuzmany, Makromol. Chem., Makromol. Symp., 37, 81(1990).
15. I. Harada and Y. Furukawa, ‘Vibrational Spectra and Structure of Conjugated and Conducting Polymers’, in “Vibrational
Spectra and Structure”, ed. J.R. Durig, Elsevier, Amsterdam,
369–469, Vol. 19 (1991).

ABBREVIATIONS AND ACRONYMS

16. M. Gussoni, C. Castiglioni and G. Zerbi, ‘Vibrational Spectroscopy of Polyconjugated Materials: Polyacetylene andPolyenes’, in “Spectroscopy of Advanced Materials”, eds
R.J.H. Clark and R.E. Hester, John Wiley & Sons, Chichester, 251–353 (1991).

DOO-PPV
ECC
IRAV
MEH-PPV

17. A.J. Epstein, R.P. McCall, J.M. Ginder and A.G. MacDiarmid, ‘Spectroscopy and Photoexcitation Spectroscopy ofPolyaniline: A Model System for New Phenomena’, in“Spectroscopy of Advanced Materials”, eds R.J.H. Clark
and R.E. Hester, John Wiley & Sons, Chichester, 355–396(1991).

THF
TNBC

Poly(2,5-dioctyloxy-p-phenylenevinylene)
Effective Conjugation Coordinate
Infrared Active Vibrational
Poly(2-methoxy-5-(20 -ethylhexyloxy)p-phenylenevinylene)
Tetrahydrofuran
19,190 ,20,200 -Tetranor-b,b-carotene

REFERENCES
1. T.A. Skotheim, R.L. Elsenbaumer and J.R. Reynolds (eds),‘Handbook of Conducting Polymers’, Marcel Dekker, NewYork (1997).
2. N.S. Sariciftci (ed.), ‘Primary Photoexcitations in Conjugated
Polymers: Molecular Exciton versus Semiconductor Band
Model’, World Scientific, Singapore (1997).

18. G. Zerbi, M. Gussoni and C. Castiglioni, ‘Vibrational Spectroscopy of Polyconjugated Aromatic Materials with Electrical and Nonlinear Optical Properties. A Guided Tour’,in “Conjugated Polymers”, eds J.L. Brédas and R. Silbey,
Kluwer Academic, Dordrecht, 435–507 (1991).
19. Y. Furukawa, A. Sakamoto and M. Tasumi, Macromol. Symp.,
101, 95 (1996).
20. Y. Furukawa, J. Phys. Chem., 100, 15 644 (1996).
21. Y. Furukawa and M. Tasumi, ‘Vibrational Spectroscopy ofIntact and Doped Conjugated Polymers and Their Models’, in“Modern Polymer Spectroscopy”, ed. G. Zerbi, Wiley-VCH,
Weinheim, 207–237 (1991).
22. H. Shirakawa and S. Ikeda, Polymer J., 2, 231 (1971).

3. H. Kiess (ed.), ‘Conjugated Conducting Polymers’, SpringerVerlag, Berlin (1992).

23. H. Kuzmany, Phys. Status Solidi B, 97, 521 (1980).

4. A. Tsumura, H. Koezuka and T. Ando, Appl. Phys. Lett., 49,
1210 (1986).

25. D. Jumeau, S. Lefrant, E. Faulques and J.P. Buisson, J. Physique, 44, 819 (1983).

5. J.H. Burroughes, C.A. Jones and R.H. Friend, Nature, 335,
137 (1988).

26. H. Takeuchi, T. Arakawa, Y. Furukawa, I. Harada andH. Shirakawa, J. Mol. Struct., 158, 179 (1987).

24. G. Zannoni and G. Zerbi, J. Mol. Struct., 100, 485 (1983).

Vibrational Spectroscopy of Conducting Polymers 13
27. S. Hirata, H. Torii and M. Tasumi, J. Chem. Phys., 103, 8964
(1995).

53. N. Yokonuma, Y. Furukawa, M. Tasumi, M. Kuroda and
J. Nakayama, Chem. Phys. Lett., 255, 431 (1996).

28. C. Castiglioni, J.T.L. Navarrete, G. Zerbi and M. Gussoni,
Solid State Commun., 65, 625 (1988).

54. Ch. Ehrendorfer and A. Karpfen, Vib. Spectrosc., 8, 293(1995).

29. B. Horovitz, Solid State Commun., 41, 729 (1982).

55. Ch. Ehrendorfer and A. Karpfen, J. Phys. Chem., 99, 5341
(1995).

30. E. Ehrenfreund, Z. Vardeny, O. Brafman and B. Horovitz,
Phys. Rev. B, 36, 1535 (1987).
31. K. Akagi, G. Piao, S. Kaneko, K. Sakamaki, H. Shirakawa
and M. Kyotani, Science, 282, 1683 (1998).
32. G. Poussigue and C. Benoit, J. Phys. Condens. Matter, 1,9547 (1989).
33. E. Faulques, W. Wallnöfer and H. Kuzmany, J. Chem. Phys.,90, 7585 (1989).
34. C.X. Cui, M. Kertesz and H. Eckhardt, Synth. Met., 43, 3491
(1991).
35. J.T.L. Navarrete and G. Zerbi, J. Chem. Phys., 94, 957(1991).
36. M. Kofranek, T. Kovár, H. Lischka and A. Karpfen, J. Mol.
Struct. (Theochem.), 259, 181 (1992).
37. G. Louarn, J.-Y. Mevellec, J.P. Buisson and S. Lefrant, Synth.
Met., 55, 587 (1993).

56. J. Casado, V. Hernández, S. Hotta and J.T.L. Navarrete, J.
Chem. Phys., 109, 10 419 (1998).
57. J.J. Apperloo, J.A.E.H. van Haare and R.A.J. Janssen, Synth.
Met., 101, 417 (1999).
58. Y. Furukawa, H. Ohtsuka and M. Tasumi, Synth. Met., 55,
516 (1993).
59. A. Sakamoto, Y. Furukawa and M. Tasumi, J. Phys. Chem.,
98, 4635 (1994).
60. A. Sakamoto, Y. Furukawa and M. Tasumi, J. Phys. Chem.,
101, 1726 (1997).
61. A. Sakamoto, Y. Furukawa and M. Tasumi, J. Phys. Chem.,
96, 3870 (1992).
62. C.R. Fincher, Jr, M. Ozaki, A.J. Heeger and A.G. MacDiarmid, Phys. Rev. B, 19, 4140 (1979).

38. G. Zannoni and G. Zerbi, J. Chem. Phys., 82, 31 (1985).

63. Z. Vardeny, J. Orenstein and G.L. Baker, Phys. Rev. Lett., 50,
2032 (1983).

39. K. Krichene, J.P. Buisson and S. Lefrant, Synth. Met., 17, 589(1987).

64. G.B. Blanchet, C.R. Fincher, T.C. Chung and A.J. Heeger,
Phys. Rev. Lett., 50, 1938 (1983).

40. L. Cuff and M. Kertesz, Macromolecules, 27, 762 (1994).

65. Y.-H. Cha, Y. Furukawa, M. Tasumi, T. Noguchi and
T. Ohnishi, Chem. Phys. Lett., 273, 159 (1997).

41. Y. Furukawa, H. Ohtsuka, M. Tasumi, I. Wataru, T. Kanbara
and T. Yamamoto, J. Raman Spectrosc., 24, 551 (1993).
42. B. Tian and G. Zerbi, J. Chem. Phys., 95, 3198 (1991).
43. I. Orion, J.P. Buisson and S. Lefrant, Phys. Rev. B, 57, 7050
(1998).
44. Y. Furukawa, A. Sakamoto, H. Ohta and M. Tasumi, Synth.
Met., 49, 335 (1992).
45. Y. Furukawa, Synth. Met., 69, 629 (1995).

66. Y. Furukawa, Appl. Spectrosc., 47, 1405 (1993).
67. Y. Furukawa, Y.-H. Cha, T. Noguchi, T. Ohnishi and
M. Tasumi, J. Mol. Struct., 521, 211 (2000).
68. D. Moses, A. Dogariu and A.J. Heeger, Chem. Phys. Lett.,
316, 356 (2000).
69. A. Sakamoto, O. Nakamura, G. Yoshimoto and M. Tasumi,
J. Phys. Chem. A, 104, 4198 (2000).

46. I. Harada, Y. Furukawa, M. Tasumi and H. Shirakawa, J.
Chem. Phys., 73, 4746 (1980).

70. A. Köhler, D.A. dos Santos, D. Beljonne, Z. Shuai, J.-L. Brédas, A.B. Holmes, A. Kraus, K. Müllen and R.H. Friend,
Nature, 392, 903 (1998).

47. J. Tanaka, Y. Saito, M. Shimizu, C. Tanaka and M. Tanaka,
Bull. Chem. Soc. Jpn., 60, 1595 (1987).

71. N.S. Sariciftci, Prog. Quant. Electr., 19, 131 (1995).

48. Y. Furukawa, H. Ohta, A. Sakamoto and M. Tasumi, Spectrochim. Acta, 47A, 1367 (1991).
49. S. Lefrant, E. Mulazzi and C. Mathis, Phys. Rev. B, 49,
13 400 (1994).

72. K. Kudo and Y. Furukawa, Personal communication.
73. U. Mizrahi, I. Shtrichman, D. Gershoni, E. Ehrenfreund and
Z.V. Vardeny, Synth. Met., 102, 1182 (1999).

51. J.-Y. Kim, Y. Furukawa and M. Tasumi, Chem. Phys. Lett.,
276, 418 (1997).

74. M. Ishima, Y. Furukawa, T. Noguchi and T. Ohnishi,‘Infrared Spectra of the Electroluminescent Devices Based on
Poly(2,5-dioctyloxy-p-phenylenevinylene’, in “Proceedingsof the Twelfth International Conference on Fourier
Transform Spectroscopy”, eds K. Itoh and M. Tasumi,
Waseda University Press, Tokyo, 423–424 (1999).

52. Y. Kawashima, K. Nakayama, H. Nakano and K. Hirao,
Chem. Phys. Lett., 267, 82 (1997).

75. Y. Furukawa, M. Ishima, T. Noguchi and T. Ohnishi, Synth.
Met., in press.

50. J.-Y. Kim, S. Ando, A. Sakamoto, Y. Furukawa and
M. Tasumi, Synth. Met., 89, 149 (1997).