Electronic properties of
poly(thiophene-3-methyl acetate)
Alex L. Gomes,1,2 Jordi Casanovas,3 Oscar Bertran,4 João Sinézio
de C. Campos,2 Elaine Armelin,1,5,* and Carlos Alemán1,5,*
1
Departament d’Enginyeria Química, E.T.S d’Enginyers Industrials de Barcelona,
Universitat Politècnica de Catalunya, Diagonal 647, Barcelona E-08028, Spain
2
Departamento de Tecnologia de Polímeros, Faculdade de Engenharia Química,
Universidade Estadual de Campinas, Avenida Albert Einstein nº 500, Barão Geraldo, CEP
13083-852, Campinas-SP, Brazil
3
Departament de Química, Escola Politècnica Superior, Universitat de Lleida, c/Jaume II
n°69, Lleida E-25001, Spain
4
Departament de Física Aplicada, E.U.E.T.I.I., Universitat Politècnica de Catalunya, Pça
Rei 15, Igualada 08700, Spain
5
Center for Research in Nano-Engineering, Universitat Politècnica de Catalunya, Campus
Sud, Edifici C′, C/Pasqual i Vila s/n, Barcelona E-08028, Spain
*
elaine.armelin@upc.edu and carlos.aleman@upc.edu
1
Abstract
The electronic structure of poly(thiophene-3-methyl acetate) has been investigated
using UV-vis absorption spectroscopy and quantum mechanical calculations. Experimental
measures in chloroform solution indicate that the -conjugation length increases with the
polymer concentration, which is reflected by the red shift of the absorbance peak of the
-* transition. On the other hand, the energy required for the -* transition has been
found to decrease with the volatility of the solvent for concentrated polymer solutions, even
though the influence of the solvent is very small for dilute solutions. Quantum mechanical
calculations indicate that the interactions between the -conjugated backbone and the
methyl acetate side groups are very weak. On the other hand, the lowest energy transition
predicted for an infinite polymer chain that adopts the anti-gauche and all-anti
conformations is 2.8 and 1.9 eV, respectively. Finally, measurements on spin-casted
nanofilms reflect that the -* transition energy increases with the thickness, which has
been attributed to the distortion of the molecular conformation. In spite of this, the energy
gap obtained for the thinnest film (1.52 eV) is significantly smaller than that determined for
dilute and concentrated chloroform solutions (2.56 and 2.09 eV, respectively).
Keywords: Conducting polymers; Polythiophene; UV-vis spectroscopy; Quantum
chemistry; Conjugation
2
Introduction
Organic semiconducting polymers have important applications in microelectronics,
electrochemical switching, photovoltaics, light-emitting diodes, field effect transistors,
electrochromic and electromechanical devices, chemical and physical sensing, etc [1-2].
Among these materials, polythiophene (PTh) derivatives are the most commonly used due
to their high charge carrier mobility. In particular, polymers bearing substituents into the 3position of the thiophene are especially important because in some cases they overcome the
solubility and processability problems intrinsically associated to PTh. Thus, the
incorporation of long alkyl side chains increases the solubility in organic solvents [3-5],
while hydrophilic substituents produce PTh derivatives soluble in water or polar solvents
[6].
PTh derivatives bearing carboxylic acid groups have been found to be soluble in a wide
variety of organic solvents [6-13]. Within this field, the work of Heeger and co-workers on
poly(thiophenesulfonate)s was a pioneering contribution to water-soluble semiconducting
polymers [6]. At the end of the nineties, electroactive polymers and copolymers with acetic
acid, propinionic acid and octanoic acid linked to the thiophene ring were also investigated
[8,9]. More recently, we reported the synthesis, structure and electronic characterization of
PTh derivatives bearing malonic acid [10,11] and acrylic acid [12,13] as substituents.
Among all these functionalized, soluble and electroactive PTh derivatives, poly(thiophene3-acetic acid), or PT3AA (Scheme 1), has attracted much attention because of its versatility
to prepare layer-by-layer (LBL) self-assembled systems. Specifically, in the last few years
multilayered LBL thin films obtained by combining PT3AA with organic polycations [e.g.
poly(sodium-2-(4-methyl-3-thiehyloxy)ethane sulfonate) and chitosan] [14,15], small
3
surfactants (e.g. dialkyldimethylammonium) [16], metallic and inorganic particles (e.g.
gold, TiO2 and zeolites) [17,18], other conducting polymers (e.g. polyaniline) [19],
conventional organic polymers [e.g. poly(4-vinylpyridine) and polyimide] [20] or even
peptides [21] have been reported.
CH2-COOH
S
n
PT3AA
Scheme 1
PT3AA presents high surface resistivity as compared with other PTh derivatives. For
example, the surface resistivity reported by Zhang and Srinivasan for PT3AA-polyimide
composites ranged from 106 to 108 ·sq-1 depending on the preparation of the sample [22].
Similarly, the surface resistivity of the undoped forms of both PT3AA and poly(3thiophene-acetic acid-co-3-hexylthiophene) was found to be 7·1010 ·sq-1 [23]. After
doping with iodine, the surface resistivity of the latter copolymer decreases to 105 ·sq-1,
while the homopolymer does not exhibit any change remaining almost identical to the value
measured before doping [23]. On the other hand, the electrical conductivity reported for
PT3AA-poly(ethyleneoxide) blends was also very low, i.e. around 10-6 S·cm-1 [24]. The
high electrical resistivity of PT3AA was attributed to the presence of carboxylic side
groups close to the -conjugated backbone [23]. Thus, this polar substituent was proposed
to affect the electron transport through the -system of the polymer. Moreover, the poor
doping behavior of PT3AA was explained by the dense packing of the chains with 4
CH2COOH pendant groups that interact through hydrogen bonds, precluding the diffusion
of the dopant molecules through the polymeric matrix [23]. Furthermore, the electronic
structure of PT3AA was examined by studying the pH and temperature dependence of the
UV-Vis absorption spectrum [8]. An abrupt reversible increase of max was found when the
pH varies from 5 to 6, which was attributed to a variation in the effective electronic length.
Thus, the electrostatic repulsions between the dissociated carboxylic acid side groups were
suggested to induce conformational changes in the polymer chains from the aggregated
state, which is stabilized by hydrogen bonds, to the extended state, giving rise to an
increased effective conjugation length [25]. On the other hand, the max after the pH
transition (pH= 6.3) decreased with the increase in temperature, whereas that before the
transition (pH= 5.2) increases slightly with increasing temperature. These thermal effects
suggested that at the former pH the extended polymer chains evolve towards a disordered
state as the temperature increases [8]. In opposition, at pH= 5.2 the intramolecular
hydrogen bonds break upon the increase of temperature, PT3AA chains becoming more
conjugated.
In this work we examine the electronic structure of a PT3AA derivative in which both
the ability to form hydrogen bonds and the pH dependence have been eliminated. More
specifically, the electronic properties of poly(thiophene-3-methyl acetate), or PT3MA
(Scheme 2), have been investigated in detail by combining both absorption spectroscopy
and quantum mechanical calculations. Specifically, experimental investigations were
performed considering dilute and concentrated PT3MA solutions in different organic
solvents as well as nanofilms prepared by spin-coating, while theoretical studies were
5
carried out using Density Functional Theory (DFT) calculations in both gas-phase and
solution environments.
Methods
Materials and Polymerization. 3-Thiophene acetic acid (T3AA) and anhydrous ferric
chloride (FeCl3) were purchased from Sigma-Aldrich Química S.A. and were employed
without further purification. All solvents were purchased from Panreac Química S.A. with
ACS grade.
3-Thiophene methyl acetate (T3MA) was prepared and subsequently polymerized by
oxidative coupling following the procedure described by Kim et al. [8], which is
summarized in Scheme 2. The purified T3MA monomer was obtained with 82% of yield,
while the yield of PT3MA after remove the residual oxidant and oligomers was 25%.
Unfortunately, changes in the monomer:oxidant (T3MA:FeCl3) ratio did not improve the
low yield of the oxidative coupling polymerization. The characterization of PT3MA using
FTIR and 1H-NMR was previously reported [8], our results being in complete agreement
with them. Table 1 describes the solubility of the resulting PT3MA in different organic and
polar solvents at room (25 ºC) and high temperatures (60 ºC).
6
CH2-COOH
CH2-COOCH3
FeCl 3
CH3OH
S
T3AA
H2SO4 / 90ºC
CH2-COOCH3
CHCl 3 / 0ºC
S
T3MA
S
n
PT3MA
Scheme 2
Spin-Casting. PT3MA solutions were prepared by dissolving the polymer in
chloroform, concentrations ranging from 0.01 mg/mL to 5.00 mg/mL being considered.
Nanofilms were obtained by spin-coating (WS-400B-NPP Laurell Technology Co.) about
18-20 drops of solution deposited on indium-tin oxide (ITO) glass slides, previously
cleaned with ethanol, at 8000 rpm for 60 s with the acceleration set to be of 15 s. It is well
known that the film thickness depends on the concentration and the speed of spinning.
Previous studies on other polymeric systems reported film thickness ranging from 10 to 80
nm for concentrations similar to those used in this work [26,27]. Due to the poor
mechanical properties of P3TMA, the thickness of the nanofilms was estimated using a
calibration curve for 50:50 mixtures of this PTh derivative and poly(tetramethylene
succinate), the latter being added to impart mechanical consistency. The thickness of the
nanofilms obtained using spinning speeds ranging from 1500 to 12000 rpm for 60 s were
determined by scratch AFM. According to the curve thickness versus speed, the thickness
of the nanofilms obtained at 8000 rpm for 60 s was comprised between 20 and 30 nm.
Absorption Spectroscopy. The absorption spectra were obtained with a Shimadzu 3600
spectrophotometer equipped with a tungsten halogen visible source, a deuterium arc UV
source, a photomultiplier tube UV-vis detector, and a InGaAs photodiode and cooled PbS
photocell NIR detectors. Spectra were recorded in the absorbance mode using the
7
integrating sphere accessory (model ISR-3100), the wavelength range being 200-1200 nm.
The interior of the integrating sphere is coated with a highly diffuse BaO reflectance
standard. Spectra were obtained for both polymer solutions and nanofilms. Single-scan
spectra were obtained at a scan speed of 60 nm/min using the UVProbe 2.31 software.
Quantum Mechanical Calculations. Complete and partial geometry optimization of
oligomers containing n repeating units (n-T3MA) with n ranging from 1 to 10 were
performed using Density Functional Theory (DFT) calculations at the B3LYP [28,29] level
combined with the 6-31+G(d,p) basis set [30]. Calculations on n-T3MA were performed
considering different arrangements for the methyl acetate side groups, which were selected
from previous studies on polythiophene derivatives bearing carboxylic acid substituents
[10,31]. The all-anti conformation, in which all the inter-ring dihedral angles (; defined by
the S-C-C-S sequence) adopt a value of 180.0º, was used as starting structure in all cases.
The Koopman’s theorem [32], which according to Janak’s theorem can be applied to
DFT calculations [33], was used to estimate the ionization potentials (IPs) and electron
affinities (EAs). Accordingly, the IP and the EA were taken as the negative of the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
energies, respectively, i.e. IP= -EHOMO and EA= -ELUMO. The IP and EA indicate if a given
acceptor (p-type dopant) and donor (n-type dopant) is capable of ionizing, at least partially,
the molecules of the compound, respectively. The -* lowest transition energy (Eg) was
approximated as the difference between the HOMO and LUMO energies, i.e. Eg= EHOMO ELUMO. Levy and Nagy evidenced that g can be rightly approximated using this procedure
in DFT calculations [34].
8
Results and Discussion
Dilute Solutions (0.01 mg/mL to 0.05 mg/mL). Dried PT3MA (5 mg) was used to
prepare a chloroform solution with concentration 2.6·10-3 mol·L-1. In order to achieve the
best solubility conditions, chloroform anhydrous than contains 0.5-1.0% ethanol was used
as solvent. This is because some residual oxidant particles coming from synthesis were
found to precipitate when no stabilizer was contained in the solvent, while such residuals
disappear in presence of a small fraction of ethanol (i.e. hereafter, chloroform refers to
chloroform stabilized with ethanol). Exact volumes (1-5 mL) of such solution were
transferred to 10 mL volumetric flasks and diluted with chloroform to prepare dilute
solutions with various known concentrations.
Dilute solutions show an absorbance peak in the UV-vis range at max= 400 nm (Figure
1), which correspond to the -* transition of the thiophene ring of PT3MA [35,36].
Furthermore, the absorbance of the solutions scaled with concentrations linearly. These
UV-vis absorbance (A) data (peak at 400 nm) were used to develop the Beer’s law
calibration curve (Figure 2), for which each measurement was repeated three times and the
average values were recorded. The linear portion was plotted and the molar absorptivity ()
was obtained from the slop of the straight line. The Beer’s law plot for PT3MA was
expressed as the equation: A= 6193.7c + 0.1038. Accordingly, considering A= ·b·c, where
b is the path length of the sample (b= 1 cm for a standard cuvette), the molar absorptivity of
PT3MA (max= 400 nm) in chloroform is = 6193.7 mol·g-1·cm-1, while 0.1038 is typically
attributed to systematic experimental errors (cuvettes, air, solvent, etc).
9
Concentrated Solutions (0.03 mg/mL to 2.5 mg/mL). The evidence of chemical reaction
between the dopant and the polymer comes from the changes observed in the UV-vis
spectra. Figure 3 compares the UV-vis spectra of PT3MA in chloroform solution with
concentrations ranging from 0.03 mg/mL to 2.5 mg/mL. The -* transition of the
thiophene ring shows a red shift with respect to the spectra recorded for dilute solutions
(Figure 1) indicating that the -conjugation length increases with the concentration of
polymer (i.e. the energy required for the -* transition decreases). Thus, the max increases
from 400 nm (dilute solutions) to 496 nm (highest concentration in Figure 3). Furthermore,
the band observed at 744 nm, which is not present in dilute solutions, can be attributed to
the formation of polarons and bipolarons in polymer chains evidencing the influence of the
dopant agent (i.e. FeCl3) [37-39].
Effect of the solvent. Changing from chloroform to more polar and less volatile solvents
in dilute solutions, has a negligible effect in the -* absorption band of PT3MA. Thus, the
max is 400, 407 and 400 nm for a 0.01 mg/mL solution in chloroform, chlorobenzene and
cyclohexanone, respectively (spectra not shown). The optical -* lowest transition energy
(Eg) or band gap energy was determined from the onset wavelength (onset) of the UV-vis
spectra recorded in these environments, i.e. Eg= 1240/onset eV [40], the resulting values
being 2.56, 2.48 and 2.48 eV (Table 2), respectively. These results indicate that the
influence of the solvent in the -electron delocalization is very small (< 0.1 eV) at the
molecular level. Thus, although the dielectric constant of these solvents ranged from 3.7
(chloroform) to 15.6 (cyclohexanone), the nature of the non-specific solute···solvent
10
interactions (i.e. specific hydrogen bonds between these solvents and P3TMA are not
possible) is relatively similar in all cases minimizing the effects.
In opposition, the solvent plays a very important role in the electronic structure of
concentrated solutions, which is reflected in Figure 4 for the 5.0 mg/mL solutions in the
same three solvents. As it can be seen, the solvent affects the peak position of the -*
transition, the max obtained for the -* transition in chloroform, cyclohexanone and
chlorobenzene being 508, 522 and 537 nm, respectively. Thus, red shift occurs when the
volatility of the solvent decreases, which results in a reduction of the energy required for
the -* transition (Table 2). Thus, in chloroform (vapor pressure 160 mmHg at 20º) the
value of Eg is 2.09 eV, decreasing to 1.91 and 1.89 eV in cyclohexanone (vapor pressure
3.4 mmHg at 20ºC) and chlorobenzene (vapor pressure 11.8 mmHg at 25ºC), respectively.
These results indicate that the intermolecular interactions among the polymer molecules are
affected by the molecular mobility of the different solvents in the medium, which in turn
vary with their volatility. Another interesting feature is that the oxidation band observed at
744 nm in chloroform solution is not detected in chorobenzene and cyclohexanone
confirming that this band corresponds to the FeCl3 complex.
Effect produced by the addition of other dopants. Polystyrensulfonic acid (PSSA) and
dodecylbenzenesulphonic acid (DBSA) were dropped in dilute (0.001, 0.005 and 0.01
mg/mL) and concentrated (5.0 mg/mL) chloroform solutions of PT3MA. The spectra
absorption recorded for PT3MA doped with PSSA and DBSA were very similar to those
obtained with the FeCl3 coming from polymerization (data not shown). For example, the
max and Eg obtained for PSSA in chloroform solution (0.01 mg/mL) were 403 nm and 2.53
11
eV, respectively, whereas they were 402 nm and 2.54 eV for DBSA. The poor doping
behavior of PT3MA is fully consistent with that found for PT3AA [23], which was
attributed to the short distance between the conjugated chain and the quenching carboxylate
group.
Modeling of the electronic properties in the gas-phase: influence of the molecular
conformation. Quantum mechanical calculations on 4-T3MA were performed to determine
the conformational preferences of the methyl acetate side group. Thus, different
conformations, which only differ in the disposition of the methyl acetate side groups, were
constructed considering an anti arrangement for all the inter-ring dihedral angles (= 180º).
All these structures were used as starting points for geometry optimizations at the
B3LYP/6-31+G(d,p) level. The two arrangements of lower energy, which are almost
isoenergetic, are depicted in Scheme 3. As it can be seen, in the lowest energy one (I) the
side groups of all repeating units retain the same relative orientation, while in the second
one (II) the side groups show an alternated disposition, the latter being only 0.1 kcal/mol
less stable than the former. In both structures, all the inter-ring dihedral angles evolved
toward an anti-gauche conformation with  ranging from 130º to 132º.
H3COOC
H3COOC
S
H3COOC
S
S
S
S
H3COOC
H3COOC
S
S
H3COOC
S
H3COOC
I
H3COOC
II
12
Scheme 3
In order to evaluate the influence of the side groups in electronic properties (IP, EA and
Eg) of PT3MA, quantum mechanical calculations on n-T3MA oligomers with n ranging
from 2 to 10 were performed considering the arrangements I and II for each value of n. The
most favored arrangement was I in all cases, the maximum destabilization obtained for II
being only 0.3 kcal/mol. Furthermore, the backbone adopted an anti-gauche conformation
in all cases with  varying between 128º and 140º. These results clearly indicate that,
independently of the number of repeating units, the interaction between the methylacetate
side groups and the polythiophene backbone is very weak. On the other hand, the all-anti
arrangement has been also considered for the modeling of the electronic properties. Thus,
each n-T3MA oligomer was optimized considering initially the side groups as in
arrangement I and fixing the inter-ring dihedral angles at 180º (restricted geometry
optimizations). As it was expected, the all-anti conformation was less stable than the antigauche one, this effect increasing with n, i.e. the relative energy increases from 1.9 to 12.8
kcal/mol when n grows from 2 to 10.
Figure 5 displays the variation of the IP, EA and Eg calculated in the gas-phase with the
inverse of the number of repeating units for the anti-gauche (I and II) and all-anti
arrangements. Linear regression analyses, which are also displayed in Figure 5, allowed
extrapolate the IP, EA and Eg values for an infinite chain of PT3MA. As it was expected,
the IP predicted for two anti-gauche arrangements, which only differ in the relative
disposition of the side groups, were similar due to the relatively poor influence of the
backbone – side chains interactions (Figure 5a), i.e. 5.25 and 5.37 eV, respectively.
13
However, oxidation of PT3MA becomes an easier process when the thiophene rings are
obligated to adopt a planar disposition, the IP for the all-anti conformation being 4.74 eV.
The latter value is almost identical to those calculated for the all-anti conformation of
unsubstituted polythiophene (4.75 eV) [41] and poly(2-thiophen-3-yl-malonic acid
dimethyl ester) (PT3MDE, 4.71 eV) [10], a similar polythiophene derivative that contains
two carboxilate groups per repeating unit (Scheme 4), using similar theoretical methods.
CH-(COOCH3)2
S
n
PT3MDE
Scheme 4
Figure 5b, which displays the variation of the EA against 1/n for the three investigated
arrangements, indicates that energy of the LUMO is around 0.4 eV larger for the all-anti
than for I and II. The influence of the backbone conformation on the IP and EA has a
significant impact on the Eg, the value predicted for an infinite of PT3MA being 2.78, 2.93
and 1.89 for the I, II and all-anti conformations, respectively (Figure 5c). As it was
expected, the g value decreases when the planarity of the backbone increases, which must
be attributed to the enhancement of the -conjugation produced when the inter-ring
dihedral angles evolve towards 180º. The Eg value determined experimentally for a dilute
chloroform solution, which is the solvent with the lowest polarity among those used in this
work, is 2.56 eV indicating an overestimation of 0.22 and 0.37 eV for arrangements I and
14
II, respectively. In contrast, the Eg of the all-anti conformation is underestimated by 0.67
eV. Accordingly, the backbone of the PT3MA molecules in dilute chloroform solution
adopts an intermediate conformation between the anti-gauche and the all-anti, even though
it is closer to the former than to latter. In contrast, the Eg determined by UV-vis
spectroscopy for a concentrated chloroform solution (5 mg/mL) is closer to the Eg predicted
for the all-anti conformation than for the anti-gauche one, i.e. the difference is 0.20 and
0.69 eV, respectively. Accordingly, PT3MA chains tend to adopt planar arrangements in
concentrated solutions, which is fully consistent with the red-shift observed for max (i.e.
the -conjugation length increases in polymer chains). Moreover, this planarity increases
when the volatility of the solvent decreases, the Eg predicted for a concentrated
chlorobenzene solution being identical to that calculated for the ideal all-anti conformation.
Figure 6 shows the contour plots HOMO and LUMO computed for n-T3MA with n= 3
and 10. As it can be seen, the HOMO presents a major bonding character delocalized along
the backbone. For the shortest oligomer such delocalization is uniform, whereas for the
largest one this effect involves the eight central repeating units. Furthermore, in both cases
the HOMO resides in the conjugated backbone, no participation of the carbonyl side groups
being detected. This is fully consistent with the weak interaction found between the
methylacetate side groups and the polythiophene backbone. Similar distributions are
predicted for the LUMO of 3-T3MA and 10-T3MA, i.e. complete and partial
delocalization, respectively, which mainly resides in the inter-ring  bonds.
Nanofilms. Figure 7 shows the absorption spectra of ultra-thin films of PT3MA. Films
were prepared by spin-casting from chloroform solutions using the same speed and time of
15
spinning in all cases, their thickness being varied by changing the concentration of polymer
in the solution (see Methods section). Although the absorption profiles recorded in the solid
state looks similar to those obtained in solution, the thickness of the films affects both the
peak position and the absorbance. A blue shift is detected with increasing the concentration
of PT3MA in the solution used for the spin-casting, the max of the films cast using 0.01,
1.00 and 5.00 mg/mL in chloroform being 482, 433 and 419 nm, respectively. Furthermore,
the Eg gap increases with the concentration, growing from 1.52 to 2.17 eV when the
concentration increases from 0.01 to 5.00 mg/mL (Table 2). These results indicate that the
more effective packing of the -conjugated backbone is obtained for the nanofilm samples
prepared using the more dilute solutions. Furthermore, the very low Eg values determined
for nanofilms produced using dilute solutions indicate that intermolecular -conjugation
effects are very important in the solid state, as was recently predicted from sophisticated ab
initio calculations [42]. Thus, the Eg measured for the nanofilms casted from the 0.01
mg/mL solution is 0.37 eV smaller than that predicted for an isolated polymer chain
arranged in an ideal planar conformation.
On the other hand, it is worth noting that the Eg measured for the film cast from the
5.00 mg/mL solution (2.17 eV) is higher than those determined in the solution state for the
same concentration, independently of the solvent (1.89-2.09 eV, in Table 2). Thus, the
reduction of the gap produced by -stacking interactions is not enough to compensate the
negative conformational effects in the molecular -conjugation length. Accordingly, spincasting of concentrated solutions produce distortions in the shape of the polymer chains,
which are not able to readapt their conformations due to the packing of the neighboring
molecules.
16
Conclusions
The electronic properties of PT3MA have been examined in different environments
using UV-vis absorption spectroscopy and quantum mechanical calculations. The
absorption of the thiophene -* transition is not affected by the chemical nature of the
solvent in dilute polymer solutions. In contrast, this transition shows a red-shift for
concentrated solutions, which produces a reduction of the Eg. On the other hand, quantum
mechanical calculations indicate that the anti-gauche arrangement is more stable in the gasphase than the all-anti by 3 kcal/mol per repeating unit. Comparison of the Eg values
calculated for such two conformations with those determined experimentally allows
conclude that in dilute solution PT3MA chains are closer to the anti-gauche than to the allanti, whereas in concentrated solutions polymer chains prefer the latter. The Eg values
measured for nanofilms are significantly smaller than that predicted for an ideally planar
molecule, the more effective packing of the chains being obtained for the films prepared
using the more dilute solutions.
Acknowledgements
This work has been supported by MICINN and FEDER (Grant MAT2009-09138), by
the Generalitat de Catalunya (research group 2009 SGR 925 and XRQTC). Computer
resources were generously provided by the “Centre de Supercomputació de Catalunya”
(CESCA). A.G. acknowledges financial support from the Euro Brasilian Windows agency
(Grant No. 41309-EM-1-2008-PT-ERAMUNDUS-ECW-L16) for his 6-month stay at the
17
UPC. Support for the research of C.A. was received through the prize “ICREA Academia”
for excellence in research funded by the Generalitat de Catalunya.
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20
Table 1. Basic solubility data of PT3MA in different solvents at room and high
temperature.
Solvent
25 ºC
60 ºC
Water (pH= 7.0)


NaOH aq.


Acetone
Δ
Δ
CH2Cl2


CHCl3


THF


DMSO
Δ

DMF
Δ

TFA


Xylene


Methanol


Ethanol


Toluene


Hexane


Acetonitrile


Diethyl ether

-
Ciclohexanone


Chlorobenzene


 = soluble; Δ = partialy soluble;  = insoluble
21
Table 2. Values of max, onset and Eg determined experimentally and theoretically for
PT3MA in different conditions.
max (nm)
onset (nm)
Eg (eV)
Dilute chloroform sol. (0.01 mg/mL)
400
485
2.56
Dilute chlorobenzene sol. (0.01 mg/mL)
407
500
2.48
Dilute cyclohexanone sol. (0.01 mg/mL)
400
499
2.48
Concentrated chloroform sol. (5.00 mg/mL)
508
592
2.09
Concentrated chlorobenzene sol. (5.00 mg/mL)
537
657
1.89
Concentrated cyclohexanone sol. (5.00 mg/mL)
522
647
1.92
DFT calculations, gas-phase, arrangement I
-
-
2.78
DFT calculations, gas-phase arrangement II
-
-
2.93
DFT calculations, gas-phase, all-anti
-
-
1.89
Nanofilm – spin casting (0.01 mg/mL)
482
814
1.52
Nanofilm – spin casting (1.00 mg/mL)
433
626
1.98
Nanofilm – spin casting (5.00 mg/mL)
419
572
2.17
Conditions
22
Captions to Figures
Figure 1. UV-vis absorption spectra of PT3MA in chloroform solutions with different
concentrations used to develop the calibration curve.
Figure 2. Calibration curve used to develop the Beer’s law slope for PT3MA in dilute
chloroform solution.
Figure 3. UV-vis absorption spectra of concentrated PT3MA solutions in chloroform.
Figure 4. UV-vis absorption spectra of concentrated PT3MA solutions (5.0 mg/mL) in
chloroform, cyclohexanone and chlorobenzene.
Figure 5. Variation of the calculated IP (a), EA (b) and Eg (c) against 1/n, where n is
the number of repeat units for PT3MA arranged in the following conformations: antigauche with I (black line, black triangles), anti-gauche with II (grey line, grey squares) and
all-anti (dashed line, empty circles). The lines correspond to the linear regressions used to
obtain the values of these electronic properties for infinite chain systems.
Figure 6. Comparison of the contour plots of the HOMO and LUMO for n-T3MA with
(a) n= 3 and (b) n= 10.
Figure 7. UV-vis absorption spectra of PT3MA nanofilms obtained by the solution
spin-casting process using polymer solutions with concentrations ranging from 0.01 to 5.00
mg/mL. The values of max are indicated for each concentration.
23
2.0
Absorbance
1.8
2.64 x 10-4 mol/L
2.11 x 10-4 mol/L
-4
1.58 x 10 mol/L
1.06 x 10-4 mol/L
-4
0.53 x 10 mol/L
1.5
1.3
1.0
0.8
0.5
0.3
0.0
297
347
397
447
Wavelength (nm)
Figure 1
24
497
547
1.9
Absorbance
1.7
y = 6193.7x + 0.1038
2
R = 0.9921
1.5
1.3
1.1
0.9
0.7
0.5
0.3
0.00004
0.0001
0.00016
0.00022
Concentration (mol/L)
Figure 2
25
0.00028
Absorbance
3.0
2.50 mg/mL
1.25 mg/mL
0.625 mg/mL
0.3125 mg/mL
0.03125 mg/mL
2.0
1.0
0.0
437
562
687
812
Wavelength (nm)
Figure 3
26
937
1062
6.0
Absorbance
5.0
chlorobenzene
4.0
cyclohexanone
3.0
chloroform
2.0
1.0
0.0
437
562
687
812
Wavelength (nm)
Figure 4
27
937
1062
(a)
6.0
IP (eV)
5.8
Arrangement II
IP = 1.40(1/n ) + 5.37
R2 = 0.9800
Arrangement I
IP = 1.62(1/n ) + 5.25
R2 = 0.997
5.6
5.4
All-anti
IP = 2.08(1/n ) + 4.74
2
R = 0.999
5.2
5.0
4.8
4.6
0.0
(b)
0.1
0.2
0.3
0.4
0.5
0.6
0.5
0.6
0.5
0.6
1/n
2.9
EA (eV)
2.7
All-anti
EA = -2.40(1/n ) + 2.85
R2 = 0.999
2.5
2.3
2.1
1.9
1.7
1.5
Arrangement I
EA = -2.13(1/n ) + 2.47
R2 = 0.995
Arrangement II
EA = -2.10(1/n ) + 2.44
2
R = 0.993
1.3
0.0
g (eV)
(c)
0.1
0.2
0.3
0.4
1/n
Arrangement II
eg = 3.51(1/n ) + 2.93
R2 = 0.990
4.6
4.2
3.8
Arrangement I
eg = 3.76(1/n ) + 2.78
R2 = 0.996
3.4
3.0
All-anti
eg = 4.48(1/n ) + 1.89
2
R = 0.999
2.6
2.2
1.8
0.0
0.1
0.2
0.3
1/n
Figure 5
28
0.4
(a)
LUMO
HOMO
(b)
HOMO
LUMO
Figure 6
29
Absorbance (%)
0.30
0.25
5 mg/mL
1mg/mL
0.5 mg/mL
0.1 mg/mL
0.01 mg/mL
419 nm
0.20
0.15
433 nm
432 nm
0.10
471 nm
0.05
0.00
300
482 nm
350
400
450
500
Wavelength (nm)
Figure 7
30
550
600
Text for the Table of Contents
The -conjugation length and the energy required for the -* transition of poly(thiophene3-methyl acetate) have been examined in different environments (i.e. gas-phase, dilute and
concentrated solutions considering solvents with different polarity and volatility, and spincasted nanofilms) using a combination of UV-vis spectroscopy and quantum mechanical
calculations.
Graphic for the Abstract
31