High-throughput screening method to detect amphiphilic counterions able to solubilize conducting polymers

Full text of DOI:10.1021/cc1001428

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J. Comb. Chem. 2010, 12, 814–817
High-Throughput Screening Method to
Detect Amphiphilic Counterions Able to
Solubilize Conducting Polymers
such as camphor sulfonic acid (CSA) and dodecyl-benzene-
-
sulfonic acid (DBSA). The charged (SO
3
) headgroup of the
surfactant associates with the positively charged backbone
through Coulomb attraction, providing a solubilizing side
chain compatible with organic solvents.
Natalia E. Monge, Mar ´ı a C. Miras, and
C e´ sar A. Barbero*
On the other hand, the demand for diverse compound
libraries for screening in materials science and drug discovery
is the driving force behind the development of new technolo-
Departamento de Qu ´ı mica, UniVersidad Nacional de R ´ı o
Cuarto, 5800-R ´ı o Cuarto, Argentina
12
gies for rapid parallel and combinatorial synthesis. Then,
to find a better dopant to improve PANI solubility in common
solvents and conductivity, combinatorial synthesis of the
amphiphilic counterions could be an efficient technique.
However, if counterions are easily synthesized, the bottleneck
of the discovery process becomes the screening of the ability
of the counterion to solubilize PANI. To do that, we used
two properties of polyaniline. One is its strong optical
absorption in the visible which is due to the extended
conjugation and, therefore, common with other conducting
polymers. The other is the ability to deposit thin (<1 µm)
films on solid subtrates making easier to manipulate the
ReceiVed July 24, 2010
Inherently conducting polymers have attracted considerable
interest since the discovery of doped polyacetylene in 1977.
Since then, a large variety of conjugated polymers have been
developed, such as polythiophene, polyaniline, poly(phe-
nylenevinylene), and polypyrrol. However, severe practical
problems in developing materials were encountered: the
stability of the materials is insufficient and they are insoluble
in common solvents and not miscible with common polymers
1
2
(
i.e., poorly processable). Among conducting polymers,
1
4
PANI has received greater attention due to its advantages
over other conducting polymers. The simplicity of its
preparation from cheap materials, good environmental stabil-
ity, its conductivity that can be reversibly controlled either
electrochemically (by oxidation/reduction) or chemically (by
protonation/deprotonation), make it very useful in preparing
lightweight batteries, electrochromic devices, sensors and
electro-luminescent devices.
conducting polymer, as it was shown before.
We report a new method of high-throughput screening
designed to find amphiphilic counterions to solubilize PANI
3
3
in CHCl . It could be extrapolated to other polymers, like
polypyrrole, other solvents or other targets. Also, it is possible
to use it to find counterions/solvent pairs that dope the
polymer without dissolution.
4
5
,6
Materials. Polyaniline (emeraldine base form) was de-
However, the major problem related to its successful
utilization lays in its poor mechanical properties and pro-
cessability due to its insolubility in common organic sol-
13
posited on a film of LDPE, as described previously. Octyl-
2
-sulfobenzoate (A3O6) (Scheme 1) was synthesized by
Fischer esterification from 2-sulfobenzoic acid cyclic anhy-
dride and octanol. The synthetic procedure was performed
using a domestic microwave oven (700 W, 2.45 GHz) and
solvent-free in standard glass reaction tubes.
7
vents. The insulating emeraldine base (EB) form of PANI
is soluble in some hydrogen bonding solvents, such as
sulphuric acid, 80% acetic acid, 60%-88% formic acid, and
N-methylpyrrolidone. The protonated PANI, electrically
conducting, is very much less soluble than EB in these
Characterization. The samples were analyzed by FTIR
and UV-vis. Fourier transform infrared spectra (FTIR) were
taken by a FTIR Impact 400 (Nicolet) Spectrometer and
UV-vis spectras were recorded using a HP 8452 UV-vis
spectrophotometer.
The method consists of depositing polyaniline (emeraldine
base form) on a film of LDPE, which acts as solid support.
Then, portions of the films are immersed in a solution of
8
solvents. For this reason, the topic of many publications is
focused on the improvement of its processability in the past
several years. This can be done by substitution of aromatic
ring of polyaniline with different functional groups leads to
9
higher solubility in organic solvents and even in water.
However, while the addition of side groups has enhanced
its solubility and processability, the electrical conductivity
is reduced because of steric hindrance, increase of interchain
3
the amphiphilic counterion in ClCH . In this case, A3O6 was
1
0
used as amphiphilic counterion as proof of concept. If the
experiment is positive, the LDPE film will be clean and PANI
will be doped (green color) and solubilized in the solution.
length and decrease of conjugation length.
Instead of modification, if bulkier counterions are used as
doping agents, the solubility and the electrical conductivity
increased. Moreover, the solubility in weakly polar solvents
is achieved when the counterion protonates the polymer and
Scheme 1. A3O6 Chemical Structure
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also acts as a surfactant for the solvent, both simultaneously.
1
1
Cao et al. have shown that PANI can be solubilized in
a number of common organic solvents in the metallic
emeraldine salt form by the addition of protonic surfactants
*
To whom correspondence should be addressed. E-mail: cbarbero@
exa.unrc.edu.ar.
1
0.1021/cc1001428  2010 American Chemical Society
Published on Web 09/23/2010

Reports
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 6 815
Scheme 2. HTS Procedure
If the experiment is negative, PANI will remain supported
onto LDPE film (Scheme 2).
In Figure 1 a photographic record of the use of the
procedure can be seen.
In the left box, the tubes containing solutions of the
counterion (A3O6), together with the PANI (EB) films on
PE are shown before testing. Specifically, the solution of
To verify further this method, the FTIR and UV-vis
spectra of the PANI/PE films and of the solution of PANI
were measured.
LDPE has few bands in the mid-IR region (Figure 2d): at
-
1
2920 cm (asymmetrical stretching mode C-H of the
-
1
methyl group), at 2850 cm (symmetrical stretching mode
-
1
C-H of the methyl group), at 1463 cm (C-H bending
-1
A3O6 in CHCl
3 3
, CH OH, and H
2
O are shown, revealing the
vibrations of the methyl group), and at 710 cm (rocking
vibration of the methylene group). Then it is possible to sense
the bands of the polymer deposited on LDPE and the
modifications on it, like the presence of dopants.
absence of color in the counterion solutions.
In the right box, the tubes containing solutions of the
counterion (A3O6), together with the PANI (EB) films on
PE are shown after testing. As it can be seen the counterion
Figure 2a shows the FTIR spectrum of a PANI (emeraldine
base) film deposited on LDPE. The spectrum of PANI shows
all characteristic bands of LDPE and those corresponding
to itself: 1588 (CdC stretching of the quinoid rings), 1496
(CdC stretching of benzenoid rings), 1306 (C-N stretching
3
solution in CHCl (Figure 1d) has changed color to a deep
green, characteristic of the solution of doped PANI. Ac-
cordingly, the PE film that was immersed in the tube shows
no traces of the EB film, indicating that the PANI has
-
1
solubilized in the CHCl
solutions of A3O6 in water (Figure 1e) and A3O6 in CH
Figure 1f) remain unchanged, suggesting that polyaniline
3
solution. On the other hand, the
mode), and 1164 cm (NdQdN, where Q represents the
3
OH
(
is not soluble in those counterion/solvents pairs. However,
while the PANI/PE film remain unchanged (blue color) when
immersed in A3O6/H
film immersed in A3O6/CH
2
O solution (Figure 1e), the PANI/PE
OH solution has a green color
3
because of the doping of the PANI film by the counterion.
It seems that methanol is not able to solubilize PANI but
wets the film enough to allow the protonation of the polymer
backbone by the acid.
While the result in water is negative, it also reveals that
water is unable to wet the polymer enough to effect the
doping/dedoping. Accordingly, the doped film remains doped
upon immersion in water. This is an important result toward
the use of doped PANI films in wet environmental conditions.
Figure 1. HTS proceeding: Before film dipping in each solution: (a)
A3O6 solution in Cl3CH and PANI-EB film, (b) A3O6 solution in
water and PANI-EB film, and (c) A3O6 solution in methanol and
PANI-EB film. After film dipping in each solution: (d) PANI doped
with A3O6 solution in Cl3CH and PE film clean (POSITIVE), (e)
A3O6 solution in water and PANI-EB film (NEGATIVE), and (f)
A3O6 solution in methanol and PANI doped with A3O6 film
Figure 2. FTIR spectra of the materials produced during the HTS
procedure.
(POSITIVE for doping, NEGATIVE for solubilization).

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16 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 6
Reports
The UV-vis spectrum of doped PANI solution (Figure
c) presents three characteristic bands: at 349 nm, attributed
3
to the π f π* transition of the aniline ring, 413 nm, assigned
to the n f π* transition of the localized radical cation, and
the band assigned to polarons/bipolarons in protonated PANI
1
7
at 725 nm for PANI/A3O6 solution.
In summary, the UV-vis and FTIR bands due to PANI-
EB/PE can be seen in the original films. When the films are
immersed in the methanol solution of amphiphilic counterion,
PANI becomes doped. It can be seen visually, by the change
of color of the film from blue to green (Figure 1f), by FTIR
(Figure 2b), where the bands corresponding to the am-
phiphilic counterion (A3O6 in this case (Figure 2c) appears
in the spectra of PANI EB on LDPE, and by UV-vis (Figure
3
b), where a typical UV-vis spectra of PANI doped is
presented.
When the dissolution of PANI is performed, the color of
the solution changes from transparent to green and the color
of the film changes from green to transparent (Figure 1d).
Accordingly, the UV-vis spectra of doped PANI solution
is observed, while the FTIR and UV-vis spectra of the film
correspond to clean PE.
A new method of high-throughput screening has been
developed to find an amphiphilic counterion to solubilize
PANI in chloroform. It is simple and flexible, so it can be
used with other polymers, other solvents and other targets.
Using this procedure, it was found that octyl-2-sulfobenzoate
Figure 3. UV-vis spectra of the different parts of the proceeding.
1
4
quinoid ring). Figure 2c shows the FTIR spectrum of
A3O6, which presents the following main bands: 1670 (CdO
stretching), 2924 (asymmetrical stretching mode C-H of the
methyl group), 2855 (symmetrical stretching mode C-H of
the methyl group), 1185 (symmetric stretching of the
3
(A3O6) solubilizes PANI in CHCl , while a solution of the
same counterion in methanol is able to dope PANI. On the
other hand, a solution of the counterion in water does not
solubilize nor doped polyaniline.
-
1
S(dO)
S(dO)
2
), and 1375 cm (asymmetric stretching of the
1
5
2
).
Acknowledgment. The present work was financed by
FONCYT and SECYT-UNRC. N.M. thanks to CONICET
for a graduate fellowship. C.B. is a permanent research fellow
of CONICET.
Figure 2b shows the FTIR spectrum of PANI doped with
A3O6, deposited on LDPE. The presence of the doping agent
is detected by the occurrence of the characteristic vibrational
bands of the counterion. It is observed that the absortion band
of carbonyle group rise in the stretching frequency from 1670
CdO of A3O6) to 1723 cm- (CdO of PANI/A3O6
1
(
References and Notes
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CC1001428