Poly(2-thiophen-3-yl-malonic acid), a polythiophene with two carboxylic acids per repeating unit

Article abstract of DOI:10.1021/jp1006796

A new substituted polythiophene derivative bearing malonic acid, poly(2-thiophen-3-yl-malonic acid), has been prepared and characterized using a strategy that combines both experimental and theoretical methodologies. The chemical structure of this material has been investigated using FTIR and ¹H NMR, and its molecular conformation has been determined using quantum mechanical calculations. Interestingly, the arrangement of the inter-ring dihedral angles was found to depend on the ionization degree of the material, that is, on the pH, which has been found completely soluble in aqueous base solution. Thus, the preferred anti-gauche conformation changes to syn-gauche when the negatively charged carboxylate groups transforms into neutral carboxylic acid. UV-vis experiments and quantum mechanical calculations on model systems with a head-to-tail regiochemistry showed that the lowest ??-??* transition energy is 2.25 and 2.39 eV for the negatively charged and the neutral polymer, respectively. These values are slightly larger than those previously reported for other polythiophenes with bulky polar side groups. The polymer presents a good thermal stability with a decomposition temperature above 215 ?°C and an electrical conductivity of 10^-5 S/cm, which is characteristic of semiconductor materials. Scanning electron microscopy micrographs showed that, after doping, the surface of this material displays regular distribution pores with irregular sizes. This surface suggests that poly(2-thiophen-3-yl-malonic acid) is a candidate for potential applications such as selective membranes for electrodialysis, wastewater treatment, or ion-selective membranes for biomedical uses. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp1006796

J. Phys. Chem. B 2010, 114, 6281–6290
6281
Poly(2-thiophen-3-yl-malonic acid), a Polythiophene with Two Carboxylic Acids Per
Repeating Unit
Oscar Bertran,*,¶,† Elaine Armelin,*,¶,‡,§ Francesc Estrany,§,| Alex Gomes,‡,⊥ Juan Torras,# and
Carlos Alema´n*,‡,§
Departament de F´ısica Aplicada, EUETII, UniVersitat Polite`cnica de Catalunya, Pc¸a Rei 15, Igualada 08700, Spain,
Departament d’Enginyeria Qu´ımica, ETS d’Enginyers Industrials de Barcelona, UniVersitat Polite`cnica de
Catalunya, Diagonal 647, Barcelona E-08028, Spain, Center for Research in Nano-Engineering, UniVersitat
Polite`cnica de Catalunya, Campus Sud, Edifici C′, C/Pasqual i Vila s/n, Barcelona E-08028, Spain, Unitat de
Qu´ımica Industrial, EUETIB, UniVersitat Polite`cnica de Catalunya, Comte d’Urgell 187, Barcelona 08036, Spain,
Departamento de Tecnologia de Pol´ımeros, Faculdade de Engenharia Qu´ımica, UniVersidade Estadual de
Campinas, Cidade UniVersita´ria Zeferino Vaz, Caixa Postal 6066, CEP 13081-970, Campinas-SP, Brazil,
Departament d’Enginyeria Qu´ımica, EUETII, UniVersitat Polite`cnica de Catalunya, Pc¸a Rei 15,
Igualada 08700, Spain
ReceiVed: January 24, 2010; ReVised Manuscript ReceiVed: March 10, 2010
A new substituted polythiophene derivative bearing malonic acid, poly(2-thiophen-3-yl-malonic acid), has
been prepared and characterized using a strategy that combines both experimental and theoretical methodologies.
1
The chemical structure of this material has been investigated using FTIR and H NMR, and its molecular
conformation has been determined using quantum mechanical calculations. Interestingly, the arrangement of
the inter-ring dihedral angles was found to depend on the ionization degree of the material, that is, on the pH,
which has been found completely soluble in aqueous base solution. Thus, the preferred anti-gauche conformation
changes to syn-gauche when the negatively charged carboxylate groups transforms into neutral carboxylic
acid. UV-vis experiments and quantum mechanical calculations on model systems with a head-to-tail
regiochemistry showed that the lowest π-π* transition energy is 2.25 and 2.39 eV for the negatively charged
and the neutral polymer, respectively. These values are slightly larger than those previously reported for
other polythiophenes with bulky polar side groups. The polymer presents a good thermal stability with a
decomposition temperature above 215 °C and an electrical conductivity of 10-5 S/cm, which is characteristic
of semiconductor materials. Scanning electron microscopy micrographs showed that, after doping, the surface
of this material displays regular distribution pores with irregular sizes. This surface suggests that poly(2-
thiophen-3-yl-malonic acid) is a candidate for potential applications such as selective membranes for
electrodialysis, wastewater treatment, or ion-selective membranes for biomedical uses.
Introduction
of the prepared materials.6-23 Specifically, it was found that the
introduction of long alkyl side chains increases the solubility
Conducting polymers (CPs) are materials with an enormous
projection because their optical and electronic properties are use-
ful for a wide number of potential technological applications.1-4
Among them, polythiophene derivatives are particularly impor-
tant, since much progress has been made in the past two decades
to solve their serious problems of solubility and processability.1,4,5
A widely accepted strategy to overcome such problems consists
of the incorporation of substituents into the 3-position of the
thiophene ring, which produced not only processable CPs but
also allowed the complete chemical and physical characterization
in organic solvents,9-12 whereas hydrophilic substituents produce
poly(thiophene)s soluble in water or polar solvents.13-23
In recent years, we have particularly focused in the develop-
ment of poly(thiophene)s with electron-withdrawing carboxylic
acid groups in the 3-position of the thiophene ring.21-23 Within
this field, the work of Heeger and co-workers13 on poly(thiophe-
nesulfonate)s was a pioneering contribution to water-soluble CPs
influencing subsequent investigations in very similar systems.14,15
Electroactive polymers and copolymers with acetic acid, pro-
pionic acid, and octanoic acid linked to the thiophene ring have
also been investigated.16-20 More recently, we reported the
synthesis and structural and electronic characterization of poly(3-
thiophen-3-yl-acrylic acid methyl ester), poly(3-thiophene-3-yl
acrylic acid), and poly(2-thiophene-3-yl-malonic acid dimethyl
ester), which are soluble in polar solvents.21-23
* Address correspondence to either author. E-mails: (O.B.) oscar@
euetii.upc.es, (E.A.) elaine.armelin@upc.edu, (C.A.) carlos.aleman@
upc.edu.
† Departament de F´ısica Aplicada, EUETII, Universitat Polite`cnica de
Catalunya.
‡ Departament d’Enginyeria Qu´ımica, ETS d’Enginyers Industrials de
Barcelona, Universitat Polite`cnica de Catalunya.
§ Center for Research in Nano-Engineering, Universitat Polite`cnica de
Catalunya.
In this work, we extend our previous investigations to poly(2-
thiophen-3-yl-malonic acid), hereafter abbreviated PT3MA
(Scheme 1), a new CP bearing two carboxylate units per
thiophene ring. This CP has been derived from the alkaline
hydrolysis of the poly(2-thiophene-3-yl-malonic acid dimethyl
ester) (PT3MDE in Scheme 1), which was obtained by chemical
| Unitat de Qu´ımica Industrial, EUETIB, Universitat Polite`cnica de
Catalunya.
⊥ Universidade Estadual de Campinas.
# Departament d’Enginyeria Qu´ımica, EUETII, Universitat Polite`cnica
de Catalunya.
¶ Both authors contributed equally to this work.
10.1021/jp1006796  2010 American Chemical Society
Published on Web 04/22/2010

6282 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Bertran et al.
SCHEME 1
SCHEME 3
SCHEME 2
using tetramethylsilane as the internal standard. The thermal
stability was examined by TGA using a Perkin-Elmer TGA-6
thermobalance at a heating rate of 20 °C·min-1 under a nitrogen
atmosphere. UV-vis optical spectra of PT3MA were obtained
from its 1 mg/mL solution in aqueous base using a Shimadzu
UV-240 Graphtcord UV-vis recording spectrophotometer. SEM
photographs of the polymers surface were carried out with a
JEOL JSM-6400 scanning electron microscope equipped with
an EDS Oxford analyzer.
Theoretical Methods. Molecular Models. Molecular geom-
etries of model oligomers containing n chemical repeating units
with n ranging from 1 to 6 were fully optimized using the
Hartree-Fock (HF) methodology. Because PT3MA is com-
pletely soluble in aqueous base solution (see the Results section),
calculations were performed considering both the ionized form
(n-T3MA), which presents two negative charges per repeating
unit, and the deionized (n-T3MA-H) form of each oligomer
(Scheme 3). The HF method was combined with the 6-31+G(d,p)
and 6-31G(d,p) basis sets24,25 for calculations on n-T3MA and
n-T3MA-H, respectively, the extradiffuse function in the
treatment of n-T3MA being necessary to improve the description
of the electron density. The unrestricted HF formalism (UHF)
was considered for negatively charged oligomers; for deionized
systems, the restricted formalism was used. The abbreviation
used to denote the theoretical level applied to n-T3MA and
n-T3MA-H oligomers is UHF/6-31+G(d,p) and HF/6-31G(d,p),
respectively. Previous studies indicated that these QM methods
provide a very satisfactory description of both the molecular
geometry and the energy of heterocyclic oligomers, such as those
studied in this work.26,27
Starting geometries for the calculated oligomers were gener-
ated considering that two consecutive rings may adopt an anti-
gauche (θ ≈ (145°) or a syn-gauche (θ ≈ (40°) arrangement,
where θ refers to the inter-ring dihedral angle defined by the
S-C-C-S sequence. Thus, the anti-gauche and syn-gauche
dispositions correspond to the global and local energy minima,
respectively, typically found in disubstituted 2,2′-bithiophene
derivatives, their energy difference being intimately related to
both the position and the size of the substituents.28,29
Electronic Properties. The first ionization potentials (IPs)
were estimated using the Koopmans’ theorem.30 Accordingly,
IPs were taken as the negative of the highest occupied molecular
orbital (HOMO) energy; that is, IP) -εHOMO. The IP indicates
if a given acceptor (p-type dopant) is capable of ionizing, at
least partially, the molecules of the compound. The π-π* lowest
transition energy (εg) was approximated as the difference
between the HOMO and lowest unoccupied molecular orbital
(LUMO) energies; that is, εg) εLUMO - εHOMO. Although the
HF method provides a satisfactory qualitative description of
the electronic properties of polyheterocyclic molecules, we are
aware that this theoretical procedure tends to overestimate the
values of the IP and εg.31,32 Accordingly, the electronic properties
presented in this work have been estimated performing single
oxidative coupling polymerization,23 and subsequently studied
using both experimental and theoretical methods.
It should be noted that the properties of PT3MA have been
investigated using both experimental and theoretical methods
that have been used complementarily. Thus, current theoretical
methods, and especially the quantum mechanical (QM) ones,
are able to describe very satisfactorily the structural and
electronic properties of polyconjugated molecules, and nowa-
days, they should be considered as additional characterization
tools. Specifically, for PT3MA, we have characterized the
chemical structure by FTIR and NMR, the molecular conforma-
tion by QM calculations, the thermal properties by thermo-
gravimetric analyses (TGA), the electronic properties by both
UV-vis spectroscopy and QM calculations, and the morphology
by scanning electron microscopic (SEM). In addition, the
solubility and electrical conductivity of this material have been
determined. The resulting properties have been compared with
those reported for other polythiophenes bearing substituents with
polar groups.
Methods
Experimental Methods. Synthesis. PT3MA was prepared
by oxidative coupling polymerization (Scheme 2). For this
purpose, the 2-thiophen-3-yl-malonic acid monomer (T3MA)
was initially transformed into 2-thiophen-3-yl-malonic acid
dimethyl ester (T3MDE) by refluxing (3 g) in dry methanol
(15 mL) with 1-2 drops of concentrated sulphuric acid for 24 h
(yield: 90%). It should be noted that protection of T3MA is
essential to prevent this monomer from oxidative acid decom-
position during the polymerization process. More details about
this procedure were given in our previous work.23 The trans-
formation of PT3MDE into PT3MA was performed by alkaline
hydrolysis (Scheme 2). For this purpose, the former polymer
(∼1 g) was stirred in 100 mL of 2 M NaOH solution for 24 h
at 100 °C. The resulting mixture was filtered to remove the
insoluble part and was poured into 1 M HCl aqueous solution
to precipitate PT3MA. This material was repeatedly washed with
deionized water and dried under vacuum for 3 days (yield:
82-85%).
Equipment. FTIR spectra were recorded on a 4100 Jasco
spectrophotometer equipped with an ATR MKII Golden Gate
Heated Single Reflection Diamond Specac accessory. Proton
NMR spectra of the monomers (T3MA and T3MDE) and
polymers (PT3MA and PT3MDE) were recorded on a Varian
Inova 300 spectrometer operating at 300.1 MHz. For this
purpose, they were dissolved in deuterated chloroform (CDCl3)
or dimethyl sulfoxide (DMSO). Chemical shifts were calibrated

Poly(2-thiophen-3-yl-malonic acid)
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6283
(1657 cm-1), are clearly separated in the monomer. Furthermore,
the O-H stretching vibration associated with intermolecular
hydrogen bond is detected as a strong and very broad band at
3400-2700 cm-1. In contrast and as expected, the CdO
absorption band, related to the intermolecular hydrogen bond
of the carbonyl, is not detected in the polymer, and the CdO
stretching vibration changes to 1705 cm-1. Figure 1b, which
shows the evolution of the carbonyl absorption bands in T3MA
when the temperature increases from 25 to 160 °C, indicates
that the absorption band at ∼1706 cm-1 corresponds to
intramolecular interactions of carbonyl groups forming another
hydrogen bond arrangement in the liquid state. Accordingly,
the CdO acid group is expected to form intramolecular
hydrogen bonds in the PT3MA chain. This assumption is
supported by the O-H stretching vibration in PT3MA, which
changes to a broad and relatively weak absorption band at
3400-3200 cm-1; that is, this band is characteristic of polymers
with hydrogen bonding interactions. Unfortunately, the tem-
perature-dependent FTIR study of the polymer did not provide
any additional evidence because of the broadness of the CdO
absorption bands (Figure 1a). The CdO stretching vibration of
the ester in T3MDE and PT3DME were detected at 1712 and
1740 cm-1, respectively, and the C-O stretching and the CH3
asymmetric bending were identified at around 1200 and 1438
cm-1, respectively (Table 1 and reference23).
The aromatic C-H out-of-plane deformation, which appeared
at 785 cm-1 in the monomer, is absent in PT3MA, proving that
the polymerization occurs at the R-R′ position of the thiophene
rings. Although the bands are significantly broader in the
polymer than in the monomer, a shoulder (pointed out with an
arrow in Figure 1a) can be seen at 1625 cm-1 in the spectrum
of PT3MA. This band should be attributed to the CdC bonds
of the conjugated structure, which is influenced by the electron-
withdrawing group [-CH(COOH)2].
Figure 1. (a) FTIR-ATR spectra of the monomer (T3MA) and
polymer (PT3MA). (b) Detail of the carbonyl absorption bands of
T3MA in the solid and melted states (from 25 to 160 °C).
point density functional theory (DFT) calculations with the
(U)B3PW9133,34 method, combined with the 6-31+G(d,p) and
6-31G(d,p) basis sets,24,25 on the molecular geometries optimized
at the UHF/6-31+G(d,p) and HF/6-31G(d,p) levels, respectively.
Electronic properties predicted by this methodological combina-
tion and experimental values are expected to be quantitatively
comparable. Thus, previous studies on π-polyconjugated systems
indicated that the B3PW91 functional is able to reproduce very
satisfactorily a wide number of electronic properties.21-23,27,35,36
According to the Janak’s theorem,37 the approximation men-
tioned above for the calculation of the IP can be applied to DFT
calculations, whereas Levy and Nagy evidenced that εg can be
rightly approximated as the difference between εLUMO and εHOMO
in DFT calculations.38
The molecular structures of T3MA and P3TMA were
determined by 1H NMR spectroscopy, the results being in
agreement with their expected chemical structures (Figure 2):
T3MA (CDCl3) δ 12.52 ppm (s, -COOH, 1H), 7.26-7.11 ppm
(m, thiophene protons, 3H), 4.86 ppm (s, aliphatic -CH-, 1H);
PT3MA (DMSO) δ 12.51 ppm (s, -COOH, 1H), 7.26-7.06
ppm (thiophene protons, 1H), 4.85 ppm (aliphatic -CH-, 1H).
Comparison of the spectra displayed in Figure 2 with those
previously reported for T3MDE and PT3MDE allows confirma-
tion of the esterification and hydrolysis steps, since the
-COOCH3 and -OH signals appeared and disappeared as
corresponding to each process.23 Thus, the ester groups of
T3MDE were not deteriorated during the oxidative polymeri-
zation (peak at 3.7-3.6 ppm on PT3MDE spectrum).23 As
expected, general spectral broadening was observed as a result
of the polymerization process. Although the absence of any new
peak after the polymerization step suggests the expected R-R
coupling of the thiophene units, the presence of shoulders should
be due to the remaining R-H atom at the ring proton multiple
region. This is usually associated with relatively low molecu-
lar weight polymer molecules. The low molecular weight of
the polymer is also evidenced by the peak area of the thiophene
proton (peak c in Figure 2), which is larger than that of the
aliphatic proton (peak d in Figure 2). In our previous work, we
determined the ratio of head-to-tail (HT) and head-to-head (HH)
diads arising from polymerization for PT3MDE using the
splitting of the -COOCH3 signal. Specifically, the peak areas
of the protons in the 3.7-3.0 ppm region indicated a 75:25
distribution of HT:HH diads, which must be retained in PT3MA.
All the QM calculations presented in this work were
performed using the Gaussian 03 computer program.39
Results and Discussion
Structural Characterization of PT3MA. Figure 1a com-
pares the FTIR spectra of T3MA and PT3MA. The main
absorption bands are listed in Table 1, a comparison with other
related CPs such as poly(3-thiophene acetic acid) (P3TAA)16
and poly(3-hexylthiophene) (PHT)40 being also included. Both
the success of the chemical oxidative reaction with FeCl3 and
the structure of PT3MDE were previously detailed,23 and
therefore, we concentrate our discussion in the hydrolysis
process and the identification of the main absorption bands of
PT3MA. The presence of the acid group in both T3MA and
PT3MA is clearly detected by the carbonyl absorption region.
Two bands, which correspond to the free CdO stretching (1751
cm-1) and CdO associated with intermolecular hydrogen bonds

6284 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Bertran et al.
TABLE 1: Main Infrared Absorption Bands (cm-1) for PT3MA As Well As for the Monomers and Intermediate Compoundsc
aromatic C-H
in-plane deformation
aromatic C-H
out-of-plane deformation
compd abreviation
(see text)
aliphatic C-H
CdO acid
or ester
CH3 asymmetric
bending
C-O
stretches
R
ꢂ
stretch
R
ꢂ
T3MA
1751, 1657
1740
1736
1705
1700
1183, 1244
1261, 1214
1234, 1153
1212, 1048
∼ 1200
785
798
840
896
839
T3MDE
PT3MDE
PT3MA
P3TAA a
PHT b
2923, 2853
2952, 2850
2980, 2900
3000-2800
2959, 2858
1438
1437
3105
3055
∼3000
1410
1377
836
825
a From ref 16. b From ref 44. c Data reported in the literature for related conducting polymers have been included for comparison.
Figure 2. 1H NMR spectra of T3MA and PT3MA obtained from
CDCl3 and DMSO solutions, respectively.
Figure 3. Minimum energy conformations derived for the 1-T3MA
monomer from a systematic conformational search. Calculations
were performed at the UHF/6-31+G(d,p) level. The dihedral angles
of the side group and the relative energy (∆E) of each minimum
Conformational Analysis of the Monomer. QM calculations
on 1-T3MA and 1-T3MA-H were performed to determine the
conformational preferences of the malonic acid side group. The
three-dimensional disposition of the substituent in these mono-
mers is defined by the flexible dihedral angles ꢀ, ꢁ, and ꢁ′
(Scheme 3). Because each of these dihedral angles is expected
to exhibit three different minima (i.e., trans (180°), gauche+
(60°), and gauche- (-60°)), 33 ) 27 theoretical minima can be
anticipated for the potential energy hypersurface, E ) E(ꢀ, ꢁ,
ꢁ′), of each monomer. All these structures were used as starting
points of 1-T3MA and 1-T3MA-H for full geometry optimiza-
tions at the UHF/6-31+G(d,p) and HF/6-31G(d,p) levels,
respectively.
are also displayed. Dihedral angles are defined as:
ꢀ )
CR(Th)-Cꢂ(Th)-C-C(dO) (the two complementary values are
shown); ꢁ and ꢁ′ ) Cꢂ(Th)-C-C(dO)-O for the branch located
above and below the ring, respectively. Th refers to the thiophene
ring.
Calculations led to four and six minimum energy conforma-
tions for 1-T3MA and 1-T3MA-H, respectively, which were
distributed within a relative energy interval of 1.8 and 2.9 kcal/
mol. These optimized structures are depicted in Figures 3 and
4, which also display the conformational parameters and rela-
tive energy (∆E). In all cases, the driving force that defines
the conformation of the substituent is the minimization of the
repulsive interactions between the four oxygen atoms. The
tendency of the T3MA monomer to form complexes stabilized
by intermolecular hydrogen bonds, as it is found by FTIR
(Figure 1), was examined in a previous study.41 Since the energy
differences were relatively small, all the minimum energy
conformations of both 1-T3MA and 1-T3MA-H were considered
in the modeling of the corresponding dimers, that is, 2-T3MA
and 2-T3MA-H, respectively.
Figure 4. Minimum energy conformations derived for the 1-T3MA-H
monomer from a systematic conformational search. Calculations were
performed at the HF/6-31G(d,p) level. The dihedral angles of the side
group and the relative energy (∆E) of each minimum are also displayed.
Dihedral angles are defined as in Figure 3.
were selected for the negatively charged and the neutral dimers:
4,4′ (2-T3MA4,4′ and 2-T3MA-H4,4′), 3,3′ (2-T3MA3,3′ and
2-T3MA-H3,3′), and 3,4′ (2-T3MA3,4′ and 2-T3MA-H3,4′), which
represent the tail-to-tail (TT), head-to-head (HH), and head-to-
tail (HT) polymer linkages, respectively. It should be noted that
the regiochemistry of polymer chains depends on the attractive
or repulsive interactions between monomeric units directly
Conformational Analysis of Dimers: A Molecular Model
for Polymer Chains. Calculations on 2-T3MA and 2-T3MA-H
were carried out to determine the conformational preferences
in terms of both the three-dimensional substituent disposition
and the inter-ring dihedral angle (θ). For this purpose, three
possible isomers, which differ in the position of the substituents,

Poly(2-thiophen-3-yl-malonic acid)
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6285
Figure 5. Relative energy (∆E), calculated with respect to the lowest-
energy arrangement of the most stable isomer, against the inter-ring
dihedral angle (θ) of all the minima energy conformations found for
(a) 2-T3MA4,4′, 2-T3MA3,3′, and 2-T3MA3,4′ and (b) 2-T3MA-H4,4′
2-T3MA-H3,3′, and 2-T3MA-H3,4′
,
.
bonded. Accordingly, regiochemistry can be predicted using
simple model molecules formed by two monomeric units rather
than large polymer chains.
Figure 6. Conformation of lowest energy obtained for (a)
2-T3MA4,4′, (b) 2-T3MA3,4′, and (c) 2-T3MA3,3′. The inter-ring and
side chain dihedral angles are listed for each minimum. The dihedral
Results obtained for 1-T3MA and 1-T3MA-H showed that
each substituent in the corresponding dimer can adopt 4 and 6
different arrangements (Figures 3 and 4), respectively, which
are expected to influence not only the inter-ring dihedral angle
but also the relative stability of the different isomers. Consid-
eration of all these arrangements led to construction of 2 minima
for the internal rotation of θ, anti-gauche and syn-gauche] ×
angles have been defined as follows: θ ) S-C-C-S; ꢀi
)
CR(Th)-Cꢂ(Th)-C-C(dO) (the two complementary values are
displayed); ꢁi and ꢁi′ ) Cꢂ(Th)-C-C(dO)-O for the branch located
above and below the ring, respectively. Th refers to the thiophene
ring; i ) 1 or 2 indicates to the first or second Th in the dimer,
respectively.
m
m2 -
arrangements of the two side chains, where m )
{
[
( )
]
minimum energy conformations of the three isomers being
distributed in groups that are separated by around 20 kcal/mol.
The lowest energy conformation corresponds to the 2-T3MA4,4′
isomer (Figure 6a), which, as expected, shows the lowest
electrostatic repulsion. The inter-ring dihedral angle corresponds
to a conventional anti-gauche arrangement in 6 of the 8 minima
identified for the 4,4′ isomer, whereas the other two are syn-
gauche conformations destabilized by 1.8 and 1.9 kcal/mol.
Regarding 2-T3MA3,4′, all the minima show an anti-gauche
conformation with the exception of one that is arranged per-
pendicularly, that is, gauche-gauche. These minima are unfa-
vored by about 21-25 kcal/mol with respect to the most stable
minimum of 2-T3MA4,4′. Finally, the three minima obtained for
2-T3MA3,3′ adopt an anti-gauche arrangement with energies
ranging from 44.1 to 44.8 kcal/mol.
2
4 and 6 for negatively charged and neutral systems, respectively}
) 20 and 42 starting geometries for the 2-T3MA4,4′ and
2-T3MA-H4,4′, respectively. The same number was obtained for
2-T3MA3,3′ and 2-T3MA-H3,3′ isomers, whereas the number of
starting structures increased to 2 × 42 ) 32 and 2 × 62 ) 72
for the 2-T3MA3,4′ and 2-T3MA-H3,4′, respectively, since the
substitutions at positions 3, 4′ and 4, 3′ are not identical.
Accordingly, a total of 72 and 156 starting geometries were
considered for 2-T3MA and 2-T3MA-H, respectively.
Complete geometry optimizations led to 8, 3, and 7 minimum
energy conformations for 2-T3MA4,4′
, 2-T3MA3,3′, and
2-T3MA3,4′, respectively; that is, 18 minima from an initial set
of 72 structures, and the number of minima found for 2-T3MA-
H
4,4′, 2-T3MA-H3,3′, and 2-T3MA-H3,4′ was 41, 10, and 47,
respectively; that is, 98 minima from an initial set of 156
structures. The distributions of the inter-ring dihedral angle θ
against the relative energy for the isomers of two dimers are
shown in Figure 5. For each dimer, these distributions have been
calculated with respect to the lowest-energy minimum of the
most stable isomer. Figures 6 and 7 depict the lowest-energy
conformation of each isomer for 2-T3MA and 3-T3MA-H,
respectively.
On the other hand, 21 of the 98 minima identified for the
three 2-T3MA-H isomers are within a relative energy interval
of 1.5 kcal/mol (Figure 5b), the inter-ring dihedral angle being
arranged in syn-gauche (θ ranges from 44.1° to 45.8°) and anti-
gauche (θ ranges from 145.4° to 148.5°) in 7 and 14 of them,
respectively. Interestingly, the main difference between the two
minima of lowest energy found for 2-T3MA-H4,4′ (Figure 7a
and b), which differ by only 0.3 kcal/mol, corresponds to the
dihedral angle θ, the conformations adopted by the side groups
being in both cases very similar to that displayed by the
Figure 5a shows that the energies of the 2-T3MA dimer are
affected by the electrostatic repulsion between side groups, the

6286 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Bertran et al.
Figure 8. Potential energy curves obtained for the internal rotation of
2-T3MA3,4′ and 2-T3MA-H3,4′ as a function of the inter-ring dihedral
angle θ. Calculations were performed at the UHF/6-31+G(d,p) and
HF/6-31G(d,p) levels, respectively. Energies are relative to the lowest-
energy minimum of each dimer.
TABLE 2: Relative Energy (∆E; in kcal/mol) and
Inter-Ring Dihedral Angles (θ1 and θ2; in degrees) Obtained
for 3-T3MA-H (see text) at the HF/6-31G(d,p) Level
A
B
C
∆E
θ1
3.1
115.5
116.6
1.6
55.3
115.6
0.0
53.8
54.3
Figure 7. Selected conformations of 2-T3MA-H: (a) lowest energy
minimum and (b) first local minimum of 2-T3MA-H4,4′ and lowest
energy minimum of (c) 2-T3MA-H3,4′ and (d) 2-T3MA-H3,3′. The inter-
ring and side chain dihedral angles are listed for each minimum.
Dihedral angles have been defined as in Figure 6.
θ2
repulsions are also responsible for the annihilation of the syn-
gauche conformations as local energy minima. In contrast, for
the 2-T3MA-H3,4′, the only minimum energy conformations
were the syn-gauche+ (θ ) 55.3°) and syn-gauche- (θ )
-60.1°), the latter being unfavored by 1.2 kcal/mol with respect
to the former. Thus, the annihilation of the anti-gauche minima
should be attributed to the steric interactions between the
susbtituent and the sulfur atom of the adjacent ring. The syn-
gauche minima are separated by two barriers located at the syn
(θ ) 0°) and anti (θ ≈ 180°) arrangements of 6.7 and 5.5 kcal/
mol, respectively.
To ascertain if the stability of the syn-gauche arrangements
found for the neutral system is retained when the polymer chain
grows, three trimers, 3-T3MA-H, were constructed. In all three,
the positions of the substituents, which were arranged as in the
most stable conformation of 2-T3MA3,4′ (Figure 7c), were 3,
4′, 3′′. Initially, the inter-ring dihedral angles θ1/θ2 were anti-
gauche+/anti-gauche+ (A), anti-gauche+/syn-gauche+ (B) and
syn-gauche+/syn-gauche+ (C). Table 2 lists the relative energy
and the inter-ring dihedral angles after geometry optimization
at the HF/6-31G(d,p) level. As can be seen, θ1 and θ2 remained
close to the initial values in the three cases, and in addition,
the substituents also retained the initial arrangements (data not
shown). Furthermore, C was the most stable structure, A and B
being unfavored by 3.1 and 1.6 kcal/mol.
It should be noted that polymerization of 2-T3MA4,4′ units,
which is the most stable isomer of the charged dimer, produces
an alternation of head-to-head and tail-to-tail linkages. The large
energy penalty associated with the tail-to-tail linkages, that is,
the 2-T3MA3,3′ isomer, is destabilized by more than 44 kcal/
mol with respect to the 2-T3MA4,4′ (Figure 5a), which allows
one to discard this structural model for PT3MA. In contrast,
polymerization of the 3, 4′ isomer provides a repetitive sequence
of head-to-tail linkages, which is expected to be more stable.
This feature was corroborated in previous theoretical studies
involving other substituted poly(thiophene)s.21,22 The same
physical principles can be applied to the 2-T3MA-H isomers,
even though the energy differences among the different isomers
of the neutral dimer are significantly smaller than those found
1-T3MA-H monomer (Figure 4a). Moreover, the lowest energy
conformation of 2-T3MA-H3,4′ (Figure 7c), which corresponds
to a syn-gauche arrangement and is destabilized by only 0.1
kcal/mol, also shows the same conformation for the substituents.
Results displayed in Figure 5b reveal that no minimum with
a conventional anti-gauche conformation, that is, θ ≈ 150°, was
detected for 2-T3MA-H3,4′. This should be attributed to the
repulsive interaction between the substituent attached to the C3-
position and the sulfur atom of the neighboring ring, which
induces a distortion of ∼30-40° in the inter-ring dihedral angle,
that is, θ ranges from 108.2° to 118.6°. Furthermore, 2-T3MA-
H3,4′ presents five minima within a relative energy interval of
1.5 kcal/mol with a syn-gauche conformation; that is, θ ranges
from 55.3° to 58.8°. Finally, 2-T3MA-H3,3′ is destabilized by
0.5 kcal/mol (Figure 7d) because of the steric interactions
induced by the substituents. All the minima identified for this
isomer adopt a gauche-gauche conformation, three of them
being within a relative energy interval 1.5 kcal/mol.
The rotational energy profiles of 2-T3MA3,4′ and 2-T3MA-
H3,4′ were calculated by scanning the inter-ring dihedral angle
θ in steps of 15° between 0° and 360° and considering that the
side groups were arranged as in the respective lowest-energy
minimum (Figures 6b and 7c, respectively). Calculations were
performed considering a flexible rotor approximation: each point
of the path was obtained from geometry optimization of the
molecule at a fixed value of θ. The resulting potential energy
curves, which are displayed in Figure 8, provide more insights
about the significant stability of the syn-gauche conformation
predicted for the 3, 4′ isomers.
The potential energy curve calculated for 2-T3MA3,4′ shows
two minima at θ ) 131.2° and -138.0°, which are almost
isoenergetic. In these conformations, which are separated by
the syn (θ ) 0°) and anti (θ ) 180°) barriers (11.4 and 1.97
kcal/mol, respectively), the inter-ring dihedral angle deviates
∼10-20° with respect to the conventional anti-gauche arrange-
ment because of the strong repulsions between the carboxylate
groups of the adjacent rings. Moreover, such electrostatic

Poly(2-thiophen-3-yl-malonic acid)
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6287
Figure 10. Variation of the energy difference between (a) models d-I
(syn-gauche conformation) and d-II (fixed anti conformation) against
n for n-T3MA-H oligomers; (b) models i-I after complete (anti-gauche
conformation) and partial (fixed anti conformation) geometry optimiza-
tion against n for n-T3MA oligomers.
Figure 9. TGA curve (recorded at 20 °C/min of heating rate and from
30 to 700 °C) of PT3MA.
for 2-T3MA. A molecular model in which head-to-tail polymer
linkages are repeated along the chain is consistent not only with
1
the H NMR data reported in this work but also with the
molecular model previously predicted for PT3MDE.23
Solubility and Thermal Properties. PT3MA is completely
insoluble not only in common organic solvents, such as
chloroform, dichloromethane, acetone, and toluene, but also in
neutral water. However, it dissolves completely in aqueous base
solutions, pH 10-12; almost entirely in dimethyl sulfoxide; and
partially in both tetrahydrofurane and dichloroacetic acid.
Furthermore, PT3MA also becomes partially soluble in ethanol
when the temperature is increased. We note that the solubility
properties changed with time (becoming more insoluble), which
should be attributed to aggregation phenomena induced by the
rearrangement of the polymer chains. The solubility of PT3MA
is worse than that of PT3MDE, the latter being completely
soluble in acetone, dichloromethane, chloroform, tetrahydrofu-
ran, dimethyl sulfoxide, dichloroacetic acid, and toluene.23 These
results are consistent with our previous results on poly(3-
thiophene-3-yl acrylic acid),22 which was much less soluble than
the corresponding ester derivatives.21
TGA was performed, scanning from 30 to 700 at 20 °C/min.
In Figure 9, PT3MA shows an onset of weight loss at around
100°, which should be attributed to the loss of the water
molecules trapped in the polymeric matrix. After such a water
loss, PT3MA presents only one decomposition process at 250
°C, which has been attributed to the polymer degradation. This
represents a significant difference with respect to PT3MDE,
which showed a two-step decomposition process.23 In addition,
the remaining weight percentage at 700 °C is considerably
higher for PT3MA than for PT3MDE (53% and 18%, respec-
tively), indicating that the content of inorganic residues in the
former is significant.
Electronic Properties. Model molecules for negatively
charged and neutral PT3MA were built using n-T3MA and
n-T3MA-H oligomers, respectively, with head-to-tail polymer
linkages and with n ranging from 2 to 6. More specifically, for
ionized PT3MA one model (i-I) with all the inter-ring dihedral
angles initially arranged in the anti-gauche conformation was
constructed, whereas two models that differ in the syn-gauche
(d-I) and anti (d-II) conformation of the inter-ring dihedral
angles were considered for deionized PT3MA. The following
calculations were performed for the n-oligomers of each model.
Complete geometry optimizations were carried out for d-I;
partial geometry optimizations were carried out for d-II; and
finally, i-I was subjected to both complete and partial geometry
optimizations. In all cases, partial geometry optimizations were
performed by fixing the inter-ring dihedral angles at 180°.
Previous studies suggested that, to extrapolate the electronic
properties calculated for oligomers to infinite chain polymers,
Figure 11. Molecular model with intramolecular hydrogen bonds
proposed for neutral PT3MA.
conformational defects should be avoided by imposing an all-
anti conformation.42 However, the latter conformation is strongly
destabilized in the case of n-T3MA-H oligomers with head-to-
tail linkages due to the large size of the substituent. Figure 10
shows the variation of the energy difference between the
structures of n-T3MA-H derived from geometry optimization
of d-I and d-II against n. The stability of the former model
increases with the molecular size; that is, 3.1 kcal/mol per
repeating unit. Figure 10 also includes the energy difference
between the structures obtained for i-I after complete and partial
geometry optimizations against n. As can be seen, an energy
penalty of ∼9.6 kcal/mol per repeating unit is produced in the
negatively charged system when the inter-ring dihedral angles
are fixed at 180°.
The side chains of model d-I can be easily rearranged to form
a network of intramolecular hydrogen bonds. It should be noted
that the formation of these interactions is not possible when
the molecular model of the polymer is generated by repeating
systematically the lowest energy conformation obtained for the
2-T3MA-H3,4′ dimer. Thus, these interactions, which involve
not only the carboxylic acid groups of neighboring repeating
units but also of the same repeating unit, are possible only when
the conformation of the side groups located at adjacent dimers
is different. The formation of intramolecular hydrogen bonds,
which are illustrated in Figure 11 for 6-T3MA-H, is fully
consistent with the FTIR spectrum displayed in Figure 1 for
PT3MA. Furthermore, comparison of the model d-I with and
without an intramolecular hydrogen bond indicates that the
former is favored by about 3-4 kcal/mol per repeating unit,
that is, the exact value depends on the number of hydrogen
bonding interactions as well as their intra- or inter-residue nature.
Figure 12 shows the UV spectrum recorded for PT3MA in
aqueous base solution. The first absorption band on the low
energy side of the spectra arises from the π-π* transition in
the delocalized electron system along the chain. Thus, the energy

6288 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Bertran et al.
of 3.84/5.70 and 2.39/4.95 eV for an infinite chain of PT3MA,
which adopts an all-syn-gauche (d-I) and an all-anti (d-II)
conformation, respectively. The dependence of these two
electronic properties on the polymer conformation is very strong.
Thus, the variation of the relative arrangement between con-
secutive thiophene rings from ideal anti to the syn-gauche and,
consequently, the loss of planarity, increases the εg and the IP
by 1.45 and 0.75 eV, respectively.
Comparison of the εg predicted for the PT3MA chain with
that calculated for related CPs using a similar theoretical level
and an idealized all-anti conformation (e.g., poly(3-thiophene-
3-yl acrylic acid methyl ester), 2.28 eV;,21 poly(3-thiophene-
3-yl acrylic acid), 2.12 eV;22 and polythiophene (PT), 1.82 eV35)
indicates that the bulky side group is responsible not only for
the drastic geometrical distortions showed above but also for
the significant increase in the lowest transition energy, which
should be related to a diminution of the optical properties. The
εg and IP values calculated for ionized n-T3MA did not show
linear behavior compared to 1/n (data not shown), which should
be attributed to the large number of negative charges in each
of such oligomers. Thus, the omission of counterions seems to
be a handicap for the prediction of the electronic properties of
PT3MA.
Figure 12. UV-vis spectrum of PT3MA in aqueous base solution.
The HOMA index43 has been calculated for all the n-T3MA
and n-T3MA-H oligomers. This parameter, which provides
information about the aromatic character, is based on the C-C
and C-S bond length alternation pattern along the π-system:
l
R
l
2
HOMA ) 1 -
R
- R
(1)
(
)
∑
opt
i
i)1
where l is the number of bonds, R is an empirical constant (R
) 257.7 and 94.09 for C-C and C-S bonds, respectively), Ri
is the bond length, and Ropt is the optimal bond length (Ropt)
1.388 and 1.677 Å for C-C and C-S bonds, respectively). The
HOMA is equal to zero for a Kekule´ structure formed by a
typical aromatic system with single and double bonds arranged
alternatively and is equal to 1 for a system with all bonds equal
to the optimal value (Ri ) Ropt).
Figure 14 shows the evolution of the HOMA index and the
averaged inter-ring distance against the number of repeating
units, n, determined for model i-I of n-T3MA (after both
complete and partial geometry optimizations) and models d-I
and d-II of n-T3MA-H. The HOMA index presents linear
behavior, even although there is a significant difference between
negatively charged and neutral oligomers (Figure 14a). For
n-T3MA-H oligomers, the HOMA index increases when n
displays a positive convergence, whereas a negative slope is
obtained for the n-T3MA systems. Thus, the aromatic character
of n-T3MA decreases when n increases, the same feature being
also reflected by the positive slope of the average inter-ring
distance (Figure 14b). In contrast, the averaged inter-ring
distance remains constant at 1.470 Å ( 0.001 Å for both d-I
and d-II when n increases from 2 to 6 (data not shown). The
behavior of the ionized system must be attributed to the
Coulombic repulsion between adjacent repeating units, which
acts against the electronic charge delocalization. A similar
behavior was previously detected for poly(thiaheterohelicene)
CPs.44
Figure 13. Variation of (a) εg (in eV) and (b) IP (in eV) against 1/n,
where n is the number of repeat units, for n-T3MA-H oligomers. Models
d-I (triangles) and d-II (squares) with the inter-ring dihedral angles
arranged in syn-gauche and anti conformations, respectively, have been
considered. The gray lines correspond to the linear regressions used to
extrapolate these electronic properties toward infinite polymer systems.
of the band gap of the conduction can be estimated from the
intersection of the tangent placed over the inflection point of
this curve and the wavelength axes. The εg value estimated from
the UV-vis spectrum in aqueous base is 2.25 eV. This value
is considerably lower than that recently obtained for PT3MDE
in acetone (2.75 eV).23 This should be attributed to the effect
of the counterions associated with carboxylate groups, which
are probably increases in the doping level of PT3MA.
To evaluate the εg and IP of PT3MA, single point calculations
at B3PW91/6-31G(d,p) were performed on the optimized
geometries of the n-T3MA-H oligomers, respectively. Unfor-
tunately, UB3PW91/6-31+G(d,p) single point calculations on
the UHF/6-31+G(d,p) geometries of n-T3MA oligomers did
not converge due to the large number of negative charges.
Accordingly, the εg and IP of PT3MA have been calculated
considering the neutral oligomers only. Figure 13 shows the
linear variation of εg and IP against 1/n obtained for the d-I
and d-II models of P3TMA. Linear regression analyses, which
are included in Figure 13, allowed extrapolation of εg/IP values
Morphology and Electrical Conductivity. Because no
morphological investigation was reported in our previous study
on poly(3-thiophene-3-yl acrylic acid), a polythiophene deriva-
tive with the carboxylic acid group separated from the polymer

Poly(2-thiophen-3-yl-malonic acid)
J. Phys. Chem. B, Vol. 114, No. 19, 2010 6289
Figure 15. SEM micrographs of (a) PT3MA before doping; (b)
PT3MA doped with 25% of FeCl3; (c) PT3AA before doping; and (d)
PT3AA doped with 25% FeCl3; Scale bar: 40 µm (a, c), 80 µm (b, d).
applications; for example, membranes for separation, matrixes
for the immobilization of enzymes, and drug delivery systems.
Doping-induced volume changes were previously reported for
other polythiophene derivatives.40
Figure 14. Variation of (a) the aromaticity index HOMA against the
number of monomer units (n) for model i-I of n-T3MA after complete
and partial geometry optimizations (inter-ring dihedral angles in anti-
gauche and anti, respectively) and for models d-I and d-II of
n-T3MA-H (inter-ring dihedral angles in syn-gauche and anti, respec-
tively); and (b) the inter-ring distance (in Å) against the number of
monomer units (n) for model i-I of n-T3MA after complete and partial
geometry optimizations.
The electrical conductivity was determined after doping with
FeCl3 using the standard four-probe technique. Measurements
were performed on pellets of the doped polymer, which were
prepared by applying a pressure of 6 tons. The resulting
conductivities were very low, ∼10-5 S/cm, which is in agree-
ment with results previously obtained for PT3AA.22 Unfortu-
nately, it was not possible to prepare samples of PT3MA using
a modification of the layer-by-layer polymerization-doping
technique,45 because it was previously used to produce PT3MDE
samples on glass plates.23 Thus, the resulting PT3MA films were
very porous, and in addition, it was difficult to remove from
the glass plates. The difficulties found in the preparation of the
PT3MA samples, which resulted in a poor alignment of the
polymer chains, should be attributed to the large content of
inorganic ions (dopant ions and counterions for the carboxylate
groups). It should be noted that the electrical conductivity
determined for PT3DME applying the four-probe technique on
these samples was 6 S/cm,23 which is significantly higher than
that of the carboxylic acid derivative.
SCHEME 4
backbone by a double bond (PT3AA in Scheme 4), in this
section we study the surface of both PT3MA and PT3AA before
and after doping with FeCl3. Representative SEM micrographs
of these four systems are provided in Figure 15).
PT3MA displays a small globular surface as compared with
the densely packed irregular surface morphology of PT3AA.
Interestingly, the surface of the two polymers becomes signifi-
cantly different after doping. More specifically, cellular struc-
tures with uniformly distributed pores and irregularly shaped
pores were found for doped PT3AA and doped PT3MA,
respectively. It is well-known that polymer chains are signifi-
cantly affected by the doping process, which enhances the
planarity of the molecules. Thus, the cauliflower-like structures
observed in electron micrographs of typical heterocyclic CPs
(polypyrrole, polythiophene, etc.) was attributed to the planarity
of the chains.3,4 However, here we present CPs with porous
structure in the doped state. In addition, in the case of doped
PT3AA, the distribution and size of the pores was very regular
(Figure 15d), the average diameter being 4 µm. It should be
noted that the doping-induced porosity cannot be attributed to
the vacuum-drying process because of the homogeneous and
narrow size distribution obtained. Results displayed in Figure
15 suggest that some CPs bearing carboxylic acid substituents
relatively close to the π-system, such as PT3MA and especially
PT3AA, are very promising material for selected and specific
Conclusions
PT3MA has been prepared by oxidative coupling polymer-
ization. This material, which is completely soluble in aqueous
base solution and partially soluble in a number of polar solvents,
has been characterized using a complementary combination of
experimental and theoretical techniques. Specifically, we have
determined the chemical structure (FTIR and NMR spectros-
copy), the molecular conformation (quantum mechanical cal-
culations), the electronic (UV-vis spectroscopy and quantum
mechanical calculations) and thermal (TGA) properties, the
morphology (SEM), and the electrical conductivity (four probe
technique). We concluded that the most stable conformation,
which is obtained by repeating head-to-tail linkages, depends
on the pH. Thus, calculations on n-T3MA indicate that inter-
ring dihedral angles prefer an anti-gauche arrangement, whereas
the preferred conformation is the syn-gauche when the car-
boxylic acid side groups are protonated. This represents a
significant difference with respect to typical substituted poly-
thiophenes, in which the anti-gauche conformation is always

6290 J. Phys. Chem. B, Vol. 114, No. 19, 2010
Bertran et al.
(17) Liaw, D. J.; Liaw, B. Y.; Gong, J. P.; Osada, Y. Synth. Met. 1999,
preferred. The lowest π-π* transition energy found for PT3MA
is slightly larger than typically found for other polythiophenes
with bulky polar side groups. This must be attributed to the
repulsion between adjacent repeating units, which is detrimental
to the charge delocalization and the aromatic character. Analyses
of the morphology at the PT3MA and PT3AA surfaces show
that the compact structures of these materials transform into
cellular structures with a uniform distribution of pores upon
doping with FeCl3. It is particularly remarkable that in the case
of doped PT3AA, the pores show a regular shape and size. These
porous structures are significantly different from that typically
found in doped heterocyclic CPs, suggesting a number of
potential applications; for example, selective membranes and
drug-delivery systems.
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Acknowledgment. This work has been supported by MI-
CINN and FEDER funds (Project No. MAT2009-09138) and
by the DIUE of the Generalitat de Catalunya (Contract Nos.
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