STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2001
SPECTROSCOPIC INVESTIGATION OF THE ELECTRON
DELOCALIZATION AND MOLECULAR CONFORMATIONAL
CHANGES IN POLYPYRROLE INDUCED BY DIFFERENT
DOPING IONS
R. TURCU*, R. GRECU**, M. BRIE**, I. PETER*,
A. BOT*, W. GRAUPNER***
* National Institute of R & D for Isotopic and Molecular Technologies,
P.O. Box 700, 3400 Cluj-Napoca, Romania
** Institute of Chemistry "Raluca Ripan", 3400 Cluj-Napoca, Romania
*** Institut fur Festkorperphysik, Technische University Graz, A-8010
Graz, Austria
ABSTRACT. We report a comparative study of the optical properties of polypyrrole (PPY)
synthesized by two methods: electrochemical and chemical oxidation of the monomer
(pyrrole) in the presence of different doping ions (p-toluensulfonate, dodecylsulfate,
dodecylbenzenesulfonate, perchlorate, tetrafluoroborate). By using FTIR and UV-Vis
spectroscopy we investigated the changes of the molecular and electronic structure of
polypyrrole induced by the preparation conditions. Our main goal was to find out the
relevant synthesis parameters, which have a strong influence on the electron delocalization
length and the molecular architecture of polypyrrole.
1. Introduction
Conducting polymers are new materials that combine the mechanical and chemical
properties of isolator polymers with the electrical and optical properties of inorganic
semiconductors and metals.
Polypyrrole is one of the most used conducting polymers for applications due to
the good stability of his properties and because it can be easy synthesised as a
homopolymer and also as a composite. The success of PPY applications depends on
the improvement of the properties and the processability of this material. Therefore one
of the main research goal is the correlation between the synthesis parameters and the
molecular architecture of PPY in order to obtain the required properties for a specific
application [1].
In this paper we report a comparative study of the optical properties of
polypyrrole (PPY) synthesized by two methods: electrochemical and chemical oxidation of
the monomer (pyrrole) in the presence of different doping ions (p-toluensulfonate,
dodecylbenzenesulfonate, perchlorate, tetrafluoroborate). By using FTIR, UV-Vis
spectroscopy we investigated the changes of the molecular and electronic structure of
polypyrrole induced by the preparation conditions. Our main goal was to find out the
relevant synthesis parameters, which have a strong influence on the electron delocalization
length and the molecular architecture of polypyrrole.
SPECTROSCOPIC INVESTIGATION OF THE ELECTRON DELOCALIZATION …
2. Experimental
2.1 Samples preparation
a) Electrochemical synthesis of polypyrrole
The electrochemical synthesis of polypyrrole was carried out with different
electrolyte types: p-toluenesulfonic acid (HTSO), sodium toluenesulfonate (NaTSO),
lithium perchlorate (LiClO4) and tetrabutyl-ammonium-tetrafluoroborat (Bu4 NBF4),
sodium dodecylbenzenesulfonate (DBSNa). The electrolyte concentration, ce was varied in
the range 0.01-0.15 M. The polymerization was carried out in galvanostatic conditions
at room temperature using different current densities: 0.25 j  1.25 mA/cm2.
Polypyrrole films with thickness h in the range 10-25 m were obtained.
b) Chemical synthesis of polypyrrole
The monomer pyrrole and the dopant DBSNa were mixed vigorously with
distilled water. The water solution of the oxidant ammonium persulfate (APS) was
added by dropping to the pyrrole-DBSNa solution. The polymerization was performed
by using the oxidant/monomer (O/M) molar ratio 0.2 and dopant/monomer (D/M)
molar ratio 0.5 or 0.8. The mixture was stirred out at 15C for 24 hours and adding
excess methanol to the reaction flask terminated the polymerisation of pyrrole. PPY
was formed as a black powder, which was filtered and washed successively with
distilled water, methanol and acetone, followed by filtering and drying in a vacuum
oven at 30C for 14 hours. PPY chemically synthesised using large-size protonic acid
dopants such as DBS is soluble in organic solvents like m-cresol and chloroform.
The polypyrrole fine black powder (100 mg) was dissolved in 2 ml of
chloroform or m-cresol. As was previously reported by J.Y. Lee et al., PPY(DBS) has a
lower solubility in chloroform than in m-cresol [2]. To improve the solubility of PPY
powder DBSNa (100 mg) was added to the solution. The solutions were stirred for 18
hours and then filtered through a Millipore filter paper. Thin films of polypyrrole were
cast from these solutions on glass or ITO (indium tin oxide) covered polycarbonate
supports for UV-Vis transmission measurements.
2.2 Optical measurements
Specular reflectance measurements in the range 200 - 6000 cm-1 were
performed on free-standing PPY films by FTIR spectroscopy on a BOMEM MB-102
spectrophotometer. The absolute value of sample reflectivity was determined relative
to the reflectivity of an aluminium mirror. The optical constants for PPY films were
calculated from the reflectance spectra with Kramers-Kronig (K-K) technique [3].
Infrared transmission spectra in the range 400-4000 cm-1 were recorded by a
spectrophotometer JASCO FTIR-610, on pressed pellet prepared from the chemically
prepared PPY(DBS) powder embedded in KBr or on thin film cast on KBr crystal from
the filtered solution of PPY in chloroform.
UV-Vis transmission spectra of the as-prepared thin polymer films on glass
support were recorded by a Lambda 9 spectrophotometer.
217
R. TURCU, R. GRECU, M. BRIE, I. PETER, A. BOT, W. GRAUPNER
3. Results and discussion
3.1 Reflectance spectra of the electrochemically prepared PPY
The reflection spectra of polypyrrole samples, shown in Fig.1 are sensitive to
the nature of the electrolyte and to the d.c. conductivity values. The higher reflectivity
was obtained for TSO doped PPY samples. Significantly lower reflectivity was
obtained for sample 4, with the lower dc. These results demonstrate that the surface
morphology of the sample has a significant contribution to the reflectance. We consider
that the roughness of the film surface depends on the nature and concentration of the
electrolyte. The surface roughness is lower for TSO- doped film as compared with
ClO4- doped one and decreases with the decrease of the electrolyte concentration.
70
(1)
(2)
(3)
(4)
Reflection (%)
60
50
 = 14 S/cm
 = 77 S/cm
 = 22 S/cm
 = 11 S/cm
40
30
(1)
20
(2)
(3)
10
(4)
0
0
1000
2000
3000
4000
5000
6000
-1
Wavenumber [ cm ]
Fig.1 Reflection spectra of PPY films electrochemically prepared with different electrolytes:
1- NaTSO ; 2-HTSO ; 3- LiClO4; 4-NaDBS
The increase of the reflectance at lower frequencies indicates a metal-like
behavior. But to find out if the electrons are indeed delocalized in our samples we
have to determine the behavior of the dielectric function in the investigated spectral
range.
The real and imaginary parts of the refractive index, n() and k() were
calculated from the reflection spectra with the Kramers-Kronig technique [3], where
first the phase angle  is determined using the following relationship:
  0   

    0  d log R 
1
log
d


2 0     0 
d
Then we can determine n and k :
218
(1)
SPECTROSCOPIC INVESTIGATION OF THE ELECTRON DELOCALIZATION …
n
1 R
1  R  2 R cos
k
2 R sin 
1  R  2 R cos
(2)
For this purpose we have to extrapolate the experimental data to zero and infinite
energies (wavenumbers). For the low energies a Hagens-Rubens function was used [3, 4],
as the reflectance increases to low energies for all films investigated. To high energies
the data were extrapolated by a decay function (-s).
The spectral dependencies of n and k for TSO- doped polypyrrole sample 2 are
shown in Fig.2. Similar behaviour for n and k was obtained for the other polypyrrole
samples investigated [4]. Several peaks located at 850, 1018, 1150, 1300, 1450 and
1530 cm-1, which correspond to the characteristic absorption peaks of the pyrrole ring,
can be observed in the spectral dependence of k (Fig.2). The values of k() increase
with the increasing of the d.c. conductivity values of the samples. The band located at
1530 cm-1 is attributed to the collective vibration of C=C/C-C inter and intra-rings. The
characteristic frequency of this band is related to the conjugation length on the chains:
the lower is the frequency, the higher is the conjugation length. We can estimate a
conjugation length of 9-10 rings for our electrochemically prepared PPY [1,4].
6
Refractive index
5
4
3
n
2
k
1
400
600
800
1000
1200
1400
1600
1800
2000
-1
Wavenumber (cm )
Fig.2 Real part (n) and imaginary part (k) of the refractive index for PPY (sample 2 from Fig.1)
The complex dielectric function,  is defined by:
  1  i2  1  i

0
(3)
The real and imaginary part of the dielectric function were calculated from n
and k values, using the relations:
219
R. TURCU, R. GRECU, M. BRIE, I. PETER, A. BOT, W. GRAUPNER
 1  n 2  k 2 ;  2  2 nk 

 0
(4)
From Fig.3 one can observe that the real part of the dielectric function 1 is
positive in all the spectral range. This behavior indicates that most charge carriers are
localized in our PPY samples.
25
200
20

150
100
15

50
10
0
0
1000 2000 3000 4000 5000 6000
-1
Wavenumber (cm )
5
0
0
1000
2000
3000
4000
5000
6000
-1
Wavenumber (cm )
Fig.3 Real and imaginary parts of the complex dielectric function for PPY (sample 2 from Fig.1)
3.2 Optical properties of the chemically prepared PPY
UV-vis absorption spectra
The absorption spectra of PPY(DBS) thin films cast from chloroform and mcresol filtered solutions (100 mg PPY1 (DBS) powder in 2 ml of solvent) are presented
in the Fig 4. It is well know that two characteristic bipolaronic absorption bands
located at ~ 1 eV and ~ 2.6 eV appears in the optical spectra of doped PPY [6]. A shift
of these bipolaronic bands could appear as a function of the doping level.
From the Fig. 4 one can see that the bipolaronic absorption band located at
2.6eV for PPY(DBS) obtained from chloroform solution is shifted to higher energies
for the sample obtained from m-cresol solution. The shift of the absorption band could
be attributed to the polymer-solvent interaction. The nature of the solvent molecule
influences the polymer chains conformation and the length of the conjugated segments
leading to a difference in the energy gaps. We consider that PPY(DBS) cast from mcresol solution contain shorter conjugated segments as compared with PPY(DBS) cast
from chloroform solution [7].
220
SPECTROSCOPIC INVESTIGATION OF THE ELECTRON DELOCALIZATION …
1.1
Normalized absorbance
1.0
0.9
a
0.8
0.7
b
0.6
0.5
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
E(eV)
Fig.4 Normalized absorbance of PPY(DBS) films cast from: a- chloroform solution;
b- m-cresol solution
IR spectra
In Fig. 5 we can compare the infrared spectra for the as-synthesised PPY(DBS),
spectrum A and for the soluble fraction, spectrum B obtained by the filtration of
PPY(DBS) chloroform solution. Both spectra show an increase of the absorbance in the
range 1700-4000 cm-1 , which is attributed to the tail of the electronic transition located
in the near infrared. The absorbance increases more slowly in this region for the
soluble fraction PPY(DBS) as compared to the as-synthesised sample. This behaviour
could be due to a lower oxidation degree or to a decrease of the  electrons delocalization
along the chains of the soluble fraction. The lower contribution of the electronic
transitions involving charge carriers make a clear evidence possible in the spectrum B
of the absorption bands located near 2900 cm-1 attributed to the vibrations of C-H
groups in the dopant DBS. Moreover, the stretching vibrations of the N-H group in
PPY can be observed as a shoulder near 3300 cm-1 in the spectrum B.
The significant differences between the spectra A and B appear for the bands
located around 900 cm-1, 1200 cm-1 and 1550 cm-1, see the inset in the Fig. 5. These
absorption bands characteristic for the oxidized PPY have been associated by Zerbi
with the effective conjugation coordination and show sensitivity to the oxidation level
and to the conjugation length of the chain [9].
The band located at 1550 cm-1 in the spectrum A is attributed to C=C/C-C
stretching vibrations of the PPY chain [9]. The shift of this band to a higher frequency,
1569 cm-1 in the spectrum B indicate that soluble PPY(DBS) contain mainly shorter
conjugated chains as compared with the as-prepared PPY .
The absorption band located at 1198 cm-1 in the spectrum A are ascribed to
ring breathing vibrations and its shift to higher frequency in the spectrum B support the
idea of the lower oxidation level for the soluble fraction as compared to the assynthesised PPY.
221
R. TURCU, R. GRECU, M. BRIE, I. PETER, A. BOT, W. GRAUPNER
From the inset in the Fig. 5 one can observe that the intense band located at
914 cm-1 in the spectrum A has a significant shift to higher frequency (by 26 cm-1) in
the spectrum B. This band could be attributed to a PPY ring vibration mode in the
oxidation state. This assignment is based on the fact that ring vibrations are much more
sensitive to the oxidation level and to the conjugation length of the chains as compared
to C-H vibrations [9].
2.2
1.8
1.6
Absorbance
1.2
2.0
A
1.0
0.8
B
0.6
0.4
0.2
900
Absorbance
1000
1100
1200
1300
1400
1500
1600
-1
1.4
wavenumber (cm )
A
1.2
1.0
0.8
0.6
B
0.4
0.2
500
1000
1500
2000
2500
3000
3500
4000
-1
wavenumber (cm )
Fig.5 IR spectra of PPY(DBS): A-as-synthesised sample; B-soluble fraction in chloroform
The comparison of the IR spectra for the chemically prepared soluble PPY(DBS)
and the electrochemically prepared PPY doped with different ions (TSO, ClO4, BF4, DBS)
shows that the former contains mainly shorter conjugation lengths and has a lower degree
of oxidation.
R E FER EN CE S
1.
2.
3.
4.
5.
6.
7.
8.
222
J. Rodriquez, H. J. Grande, T.F. Otero, "Polypyrroles: from Basic Research to Technological
Applications", in: Handbook of Organic Conductive Molecules and Polymers, ed. H.S. Nalwa,
John Wiley & Sons, 1997, Vol.2, p.415
J.Y. Lee, D.Y. Kim, C.Y. Kim, Synth. Met. 74, (1995), 103
G. Leising, in "Organic Materials for Photonics", ed.G.Zerbi, Elsevier, Amsterdam 1993.p.173
R.Turcu, M.Brie, G.Leising, V.Tosa, A.Mihut, A.Niko, A.Bot, Appl. Phys. A67, (1998), 283
R.S. Kohlman, T.Ishiguro, H.Kaneko, A.J. Epstein, Synth. Met. 69, (1995), 325
O. Chauvet, S. Paschen, L. Forro, L. Zuppiroli, Synth. Met. (1994) 63, 115
R.Turcu, W.Graupner, C.Filip, A.Bot, M.Brie, R.Grecu, Adv.Mater. Opt. Electron.(1999), 9, 157
G.Zerbi, M. Gussoni, C. Castiglioni, Conjugated Polymers; Ed. J.L.Bredas, R.Silbey, Kluwer
Academic Publishers, 1991; 435-507