Download Electronic properties of the conducting form of

Synthetic Metals 116 (2001) 157±161
Electronic properties of the conducting form of polyaniline
from electroabsorption measurements
Lavanya Premvardhana, Linda A. Peteanua,*, Pen-Cheng Wangb, Alan G. MacDiarmidb
a
Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA
Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA
b
Abstract
The polaron band at 1.4 eV of d,l-camphorsulfonic acid-doped polyaniline (()-HCSA-PANI) in a poly(methyl methacrylate) (PMMA)
matrix was studied using electroabsorption (Stark-effect) spectroscopy at 298 K. A very small change in dipole moment on excitation (jDmj)
Ê 3. The electroabsorption
in the order of 1:4 0:2 D was measured as well as an average change in polarizability (Da) of only ÿ1:3 0:4 A
signal of ()-HCSA-PANI in this region showed evidence for heterogeneity in the absorption band. # 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Electroabsorption; Acid-doped polyaniline; Polaron; Dipole; Polarizability
1. Introduction
Electroabsorption (Stark-effect) spectroscopy has proven
to be a very fruitful source of information regarding the
electronic properties of conjugated polymers. For some
applications see for example [1±5]. This technique provides
a convenient means to measure both the degree of chargetransfer and the extent of electron delocalization in states
that are optically coupled to the ground state. The chargetransfer properties of a given electronic state are re¯ected in
the change in dipole moment following excitation (Dm). The
extent of electron delocalization is correlated to the change
in polarizability (Da). With input from electronic structure
calculations, Da can also be analyzed to give the transition
matrix elements between the electronic state being probed
and the other states of the molecule. Equally importantly,
comparison of the measured and calculated values of Dm and
Da for a given electronic transition can frequently aid in
assigning its orbital nature in cases where agreement does
not exist in the literature.
The polymer we have chosen for study here, polyaniline,
exists in three states at various levels of oxidation: the fullyreduced leucoemeraldine base form, the half-oxidized emeraldine base (EB) form and the fully-oxidized pernigraniline
base form. In general, the protonation of EB produces the
emeraldine salt, which contains a stable delocalized polysemiquinone radical cation [6] with a half-®lled polaron
*
Corresponding author.
E-mail address: [email protected] (L.A. Peteanu).
conduction band [7] and can typically exhibit a 108 factor
increase in conductivity over EB.1 Therefore, protonation of
EB is considered to be a unique form of doping, which
neither increases nor reduces the number of electrons associated with the polymer backbone.
In this study, a localized polaron transition band at
1.4 eV exhibited by a doped PANI sample is investigated
by electroabsorption spectroscopy. One unique aspect of this
work is that we are studying the conductive doped PANI.
This experiment is made possible by the fact that the doped
PANI sample is dispersed within a matrix of poly(methyl
methacrylate) (PMMA) in which the sample as a whole is
suf®ciently insulating to allow a ®eld of 3 105 V/cm to
be applied. As a result, the electric-®eld response of electronic transitions that are characteristic of the conducting
form of our doped PANI may be measured.
2. Experimental
2.1. Preparation of d,l-HCSA-doped polyaniline (()HCSA-PANI)
128 mg of ()-HCSA (d,l-camphorsulfonic acid) was
®rst dissolved in 20 ml of chloroform. Then 100 mg of
1
The conductivity of EB is 10ÿ10 S/cm. Up to the time this paper was
submitted, the conductivity of an emeraldine salt reported in the literature
typically ranges from 10ÿ2 to 103 S/cm, depending on the protonic acid
used and/or processing method.
0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 4 7 7 - X
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L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161
EB [8] was slowly added to the previous ()-HCSA chloroform solution. This new solution was stirred for 21 h to
make a ()-HCSA-PANI solution.
2.2. Sample preparation
A small quantity (0.5 ml) of the above ()-HCSA-PANI
solution and 0.6 g of PMMA was added to 20 ml of
chloroform. The solution as-made was then poured into an
aluminum dish containing two pieces of ITO-coated quartz
slides. After the solvent had evaporated, the ITO slides, on
which a thin ®lm of the ()-HCSA-PANI/PMMA blend was
deposited, were clamped together and heated in an oven at
1508C for 10±15 min. The blend sample cell thus prepared
held off a suf®ciently high ®eld (1000±1500 V over a 50 mm
sample thickness) for an electroabsorption signal to be
measured. In order to obtain the absorption spectra, blanks
consisting of PMMA alone were made by the same procedures outlined above.
To determine the magnitude of the ®eld applied across the
sample, the thickness of the sample is determined from the
interference patterns in the near IR (900±2000 nm) obtained
using an absorption spectrometer (Perkin-Elmer l900). The
refractive index of PMMA (1.49) was used to determine the
thickness of the PMMA ®lms. These were typically 50 mm
thick with an error of 10 mm.
2.3. Instrumentation
The instrumentation has been previously described in
great detail [9]. The experiments were performed at a
spectral resolution of 1.25 nm.
2.4. Theory of electroabsorption and data analysis
procedures
Our analysis [9] of the electroabsorption data follows the
standard procedure as outlined by Liptay [10]. Essentially,
the change in absorption due to the application of an external
electric ®eld is ®t to the weighted sum of zeroth, ®rst and
second derivatives of the zero-®eld absorption spectrum.
The overall change in transmitted light intensity caused by
the application of an electric ®eld is described by the
following equation:
@ A…n†
@ 2 A…n†
‡ cw n 2
DA…n† ˆ F 2 aw …A…n†† ‡ bw n
@n
n
n
@n
(1)
The unperturbed absorption spectrum as a function of
frequency (n) is designated A(n), and the change induced
by the field is denoted DA(n). The effective field at the
sample, F, includes the enhancement of the applied field due
to the cavity field of the matrix. All spectra were obtained
with an angle of 54.78 between the direction of the applied
electric field and the electric field vector of the polarized
light.
Each of the coef®cients in Eq. (1), aw, bw, and cw, represent
the magnitude of the contributions of the zeroth, ®rst, and
second derivative of the absorption spectrum, respectively.
These are related to molecular parameters in the following
fashion. The aw term is proportional to the change in the
oscillator strength or transition dipole of the absorption band
due to the applied ®eld. In a rigid, isotropic sample, the bw
coef®cient is related primarily to the average change in
polarizability. The polarizability for a state i is given as
X jmij j2
E ÿ Ej
i6ˆj i
where mij represents the transition moment between electronic state i and all other states j and E represents the energy of
the respective states. Here the average value of Da, denoted
Da, is reported (i.e. Da ˆ 13 Tr…Da†). Finally, the coefficient
cw represents the contribution to the signal due to broadening
of the absorption spectrum by the applied field. This broadening, which results from interaction between the isotropically oriented ground and excited state dipole moments of
the molecule and the field, is proportional to …Dm F†2 where
Dm is the change in dipole moment on excitation. Because
the molecules in the sample are not oriented, only the
absolute value of Dm (denoted jDmj) is measured. For a
more detailed discussion of these effects, see [10,11].
The contributions made by the zeroth, ®rst, and second
derivatives of the absorption spectrum to the overall lineshape of the electroabsorption spectrum are extracted by
means of a linear least-squares (LLSQ) ®t of the electroabsorption signal to the sum of the derivatives of A(n). If the
resultant ®t to the absorption lineshape is not of high quality,
the following factors may be responsible. There may be
more than one transition (electronic or vibronic) underlying
the absorption band, each of which has different electrooptical properties (jDmj and/or Da) and/or multiple groundstate conformers with differing properties may be present in
the sample. As will be shown below, the electroabsorption
spectrum of ()-HCSA-PANI shows evidence for heterogeneity of this type. There are a number of strategies
available for ®tting such spectra [2,9,12]. These require
deconvolving the absorption band into its constituent transitions and independently varying the ®tting parameters of
each. At present, however, we do not have a unique method
for deconvolving the absorption spectrum of ()-HCSAPANI so a ®t of this type is not reported.
3. Results
3.1. Electronic transitions of doped polyaniline
The absorption spectrum of the samples of ()-HCSAPANI in PMMA used in this study exhibits several prominent transitions. There are two bands in the near UV, one of
which has been assigned to a p±p transition (3.6 eV), and
L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161
Fig. 1. (a) The absorption band of d,l-camphorsulfonic acid-doped
polyaniline (()-HCSA-PANI) in the region of the polaron transition;
(b) the electroabsorption spectrum of the polaron band (solid line) and the
fit to the spectrum (dashed line); (c) the individual contributions of the first
derivative (dotted) and second derivative (dashed) of the absorption
spectrum to the lineshape of the electroabsorption spectrum; (d) scaling of
the first derivative (dotted) and second derivative (dashed) contributions
for comparison to the electroabsorption lineshape. This comparison
indicates that the second derivative of the absorption band makes the
predominant contribution to the fit of the electroabsorption spectrum. The
y-axis units of the electroabsorption spectra, normalized by the applied
field, are reported in (cm/V)2.
the second of which is identi®ed as a polaron band (2.9 eV).
Another transition at 1.4 eV has been assigned to a localized polaron that results from the positive charge being
localized on the amine nitrogen (Fig. 1a) [7]. In addition, the
absorption contains a small low-energy charge-carrier `tail'
that begins at 1200 nm and extends beyond 2000 nm [13].
The absorption spectrum of the samples of ()-HCSA-PANI
in PMMA used in these experiments closely resembles those
of ()-HCSA-PANI with less expanded molecular conformation [14], indicating that the polymer is in a ``coil-like''
form in the blend sample [13].
3.2. Electroabsorption spectra of ()-HCSA-PANI
Electroabsorption data obtained for the region of the
polaron band are shown in Fig. 1b (solid lines) with the
®t to this band shown in dashed lines. The ®t is comprised of
zeroth, ®rst and second derivatives of the experimentally
159
obtained absorption spectrum (see Analysis section above).
The ®rst (dotted line) and second (dashed line) derivative
components are reproduced in Fig. 1c. The zeroth derivative component is not shown because it makes a very small
contribution to the ®t. The overall magnitude and shape of
the spectrum are fairly well reproduced using this ®tting
method. Therefore, we expect that the corresponding
results provide a reasonable measure of the jDmj and of
the Da of the polaron transition. The values thus obtained
for ()-HCSA-PANI in PMMA are jDmj ˆ 1:4 D and
Ê 3. 2
Da ˆ ÿ1:3 A
In Fig. 1d, the ®rst (dotted line) and second (dashed line)
derivatives of the absorption spectrum have been scaled up
so that a direct comparison between both derivatives and the
lineshape of the electroabsorption spectrum (solid line) can
be made. The scaling factors used are given in the ®gure
legend. This ®gure demonstrates that the dominant contribution to the electroabsorption spectrum arises from the
second derivative of the absorption spectrum (jDmj). If we
were to ®t the electroabsorption spectrum to the second
derivative of the absorption spectrum alone, the value of
jDmj extracted from this ®t would only be 10% larger than
the result reported above, as jDmj increases by the square
root of the scaling factor. However, if we compare the
quality of such a ®t (Fig. 1d, dashed line) to one that is
obtained by including a ®rst derivative component as well
(Fig. 1b dashed line), we see that the latter is a better match
to the experimental spectrum. Therefore, we conclude that
the contribution of Da to the ®eld response of this transition,
though small, is not negligible.
Though the ®t to the electroabsorption spectrum of ()HCSA-PANI in PMMA in this spectral region is of fairly
high quality, systematic and reproducible deviations
between the experiment and the ®t (Fig. 1b) are seen. These
deviations indicate that the absorption band consists of
multiple underlying transitions that vary in their characteristic values of jDmj and/or Da. A ®t that is essentially
indistinguishable from the experimental electroabsorption
spectrum can be obtained by deconvolving the absorption
band into two gaussians that model the underlying transitions and allowing the ®eld response of each (aw, bw, and cw
for each) to vary independently in the LLSQ ®tting procedure. However, the results from this ®t (not shown) are very
similar to those reported here.
4. Discussion
We ®nd that there have been few experiments or calculations published to date with which we can directly compare
2
A ground-state dipole moment for a charged molecule such as ()HCSA-PANI is not uniquely defined because the moment due to the
charge will depend on the origin chosen. However, the change in dipole
moment upon excitation for such a molecule is defined because the
difference measurement removes this dependence.
160
L. Premvardhan et al. / Synthetic Metals 116 (2001) 157±161
our results for ()-HCSA-PANI. Not surprisingly, comparison to the results of Monkman et al. for the EB form [15] as
well as our own for EB in PMMA [16] demonstrates that
there is a substantial change in the electronic properties of
the material upon doping. Moreover, all of these materials
demonstrate heterogeneity in their electronic properties as
evidenced by deviations between the electroabsorption spectra and the corresponding ®ts [15,17]. In future work, we
intend to examine the role of conformational heterogeneity
in the electroabsorption spectrum of ()-HCSA-PANI by
exposing the ®lms to m-cresol which will favor the formation of a more expanded polymer chain [13] as well as by
obtaining spectra of shorter oligomers.
In contrast to the electroabsorption results for the 1Bu
exciton states of numerous conjugated systems [1±4], both
the jDmj and the Da of the polaron band of ()-HCSA-PANI
are quite small, indicating little change in either the spatial
extent of charge-transfer or of electron delocalization upon
excitation. The negative sign of Da implies that, if anything,
electron delocalization is slightly decreased in the polaron
excited state relative to the ground state. From another
perspective, a negative Da implies that the transition
moments between the excited state being probed and
higher-lying electronic states of the system are small compared to that to the ground state. Alternatively, it may
indicate the presence of a low-lying state with a large
transition moment to the polaron excited state.
One possible reason why the values of jDmj and Da for the
polaron band of ()-HCSA-PANI are measured to be small
for our sample is that this molecule already exhibits considerable charge separation and/or electron delocalization in
its ground state as a result of the doping process. The results
are particularly striking when compared to those derived
from electroabsorption measurements on the absorption
bands of photogenerated polarons in samples of C60-doped
poly(p-phenylene). For these systems, changes in polarizÊ 3 were measured as well as
ability in the order of 107 A
Ê 3 [17].
ground state polarizabilities of 104 A
Having measured a relatively weak electroabsorption
signals for ()-HCSA-PANI, particularly in comparison
to those that have been observed for both EB and for the
other conjugated polymers, we considered the possibility
that the doped polymer itself sets up a ®eld that opposes the
applied ®eld, such that the net ®eld strength on the sample is
substantially diminished [17]. This opposing ®eld would
arise by virtue of the fact that doped PANIs are conductors in
their ground electronic states.
We have begun to investigate this possibility using the
following approach. We prepared a sample in which a dye
(nile red) was incorporated into the PMMA matrix along
with the ()-HCSA-PANI polymer [18]. Nile red was
chosen because it exhibits a fairly strong electro-optic
response and its lowest energy absorption bands are clearly
separable from those due to ()-HCSA-PANI. The electroabsorption spectrum of nile red in the ()-HCSA-PANI/
PMMA blend appears to be approximately a factor of two
smaller than that of nile red in PMMA alone (data not
shown) [16]. From this observation we may conclude that,
due to the presence of the conducting polymer, the strength
of the applied ®eld may be diminished by approximately
factor of two but it is not zero, at least within the region of the
sample matrix accessible to the dye.
5. Conclusions
Here, we have presented the electroabsorption spectrum
of a conducting polymer in its doped form, ()-HCSA-PANI
in a PMMA matrix. The electronic properties measured for
the polaron band at 1.4 eV are jDmj ˆ 1:4 0:2 D and
Ê 3. Future work is in progress to underDa ˆ ÿ1:3 0:4 A
stand the effects of ground state conformation and conjugation length on these properties as well as to examine other
electronic transitions in the spectrum.
Acknowledgements
We would like to thank Drs. David Yaron, Andrew Shreve,
and Ian Norris for helpful discussions, the NSF-CAREER
and POWRE programs for funding (LAP), and the Center
for Molecular Analysis at Carnegie Mellon University for
the use of the absorption spectrometer. A.G.M. would like to
thank ONR for ®nancial support.
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