10050 Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 This journal is c the Owner Societies 2012 Cite this: Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 Properties of poly(3-halidethiophene)s Jordi Casanovas, a David Aradilla, bc Jordi Poater, d Miquel Sola`, d Francesc Estrany ce and Carlos Alema´n* bc Received 13th February 2012, Accepted 8th May 2012 DOI: 10.1039/c2cp40436b The influence of the halogen atom on the intrinsic properties of poly(3-halidethiophene)s has been investigated using experimental and theoretical methodologies. Specifically, the electrochemical, electrical, electronic and morphological properties of poly(3-bromothiophene) have been determined and compared with those recently reported for poly(3-chlorothiophene) [Aradilla et al., Polym. Chem., 2012, 3, 436.]. The electrochemical stability and porosity are smaller for poly(3-bromothiophene) than for poly(3-chlorothiophene) while the p–p* lowest transition energy is higher for the former than for the latter. Moreover, quantum mechanical calculations on model oligomers have evidenced that the conformational properties of poly(3-halidethiophene)s, where the halogen is fluorine, chloride or bromine, are dominated by steric interactions and, therefore, are significantly influenced by the size of the halogen atoms. Both the ionization potential and the p–p* lowest transition energy have been predicted to increase slightly when the p-donor character of the halogen atom decreases, in agreement with experimental observations. Introduction Polythiophenes (PThs) are one of the most important classes of conducting polymers (CPs). These materials, which can be prepared by chemical or electrochemical methods, exhibit good electrical and optical properties, environmental and thermal stabilities in conducting (doped) form, rapid response time, and easy functionalization. These unique advantages result in tremendous interest for their application as conducting films, electrochromic and nonlinear optical devices, field-effect transistors, organic condensers, light-emitting diodes, sensors, etc. 1–12 In recent years we have been particularly interested in the development of PThs with electron-withdrawing groups at the 3-position of the thiophene ring, efforts being essentially focused on the incorporation of carboxylic acid groups (e.g. acetic acid, acrylic acid and malonic acid hydrophilic substituents). 13–18 Poly(3-halidethiophene)s is another interesting family of PThs substituted with electron-withdrawing groups (Scheme 1). In these materials halogen atoms are directly attached to the polyconjugated main chain, which may produce a reduction in the electronic density of the thiophene rings. In spite of that characterization of poly(3-halidethiophene)s (where the halogen is fluorine, chlorine or bromine) is crucial for a complete understanding of the effects produced by such electron-withdrawing groups in the PTh backbone, only few works have been reported in the literature. Poly(3-chlorothiophene), hereafter denoted PClTh, is the most known of the three poly(3-halidethiophene)s. This material has been electrochemically synthesized in ionic liquids (i.e. boron trifluoride diethyl etherate and 1-butyl-3-methyl- imidazolium hexafluorophosphate, abbreviated BFEE and [BMIM]PF 6 , respectively) using platinum and nanoporous TiO 2 as substrates, its electrochemical, electrochromic and optical properties being characterized using cyclic voltammetry (CV), spectroelectrochemistry and UV-vis spectroscopy. 19–22 Moreover, the influence of the concentration of electrolyte in the generation medium and the thickness of the films on the Scheme 1 a Departament de Quı´mica, Escola Polite `cnica Superior, Universitat de Lleida, c/Jaume II n o 69, Lleida E-25001, Spain b Departament d’Enginyeria Quı´mica, E. T. S. d’Enginyeria Industrial de Barcelona, Universitat Polite `cnica de Catalunya, Diagonal 647, Barcelona E-08028, Spain. E-mail: [email protected]c 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 d Institut de Quı´mica Computacional and Departament de Quı´mica, Universitat de Girona, Campus de Montilivi, Girona E-17071, Spain e Departament d’Enginyeria Quı´mica, Escola Universita `ria d’Enginyeria Te`cnica Industrial de Barcelona, Universitat Polite `cnica de Catalunya, Comte d’Urgell 187, 08036 Barcelona, Spain PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by Universitat Politecnica de Catalunya on 29 June 2012 Published on 14 June 2012 on http://pubs.rsc.org | doi:10.1039/C2CP40436B View Online / Journal Homepage / Table of Contents for this issue
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10050 Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 This journal is c the Owner Societies 2012
In recent years we have been particularly interested in the
development of PThs with electron-withdrawing groups at the
3-position of the thiophene ring, efforts being essentially
focused on the incorporation of carboxylic acid groups
(e.g. acetic acid, acrylic acid and malonic acid hydrophilic
substituents).13–18 Poly(3-halidethiophene)s is another interesting
family of PThs substituted with electron-withdrawing
groups (Scheme 1). In these materials halogen atoms are
directly attached to the polyconjugated main chain, which may
produce a reduction in the electronic density of the thiophene
rings. In spite of that characterization of poly(3-halidethiophene)s
(where the halogen is fluorine, chlorine or bromine) is crucial
for a complete understanding of the effects produced by such
electron-withdrawing groups in the PTh backbone, only few
works have been reported in the literature.
Poly(3-chlorothiophene), hereafter denoted PClTh, is the
most known of the three poly(3-halidethiophene)s. This
material has been electrochemically synthesized in ionic liquids
(i.e. boron trifluoride diethyl etherate and 1-butyl-3-methyl-
imidazolium hexafluorophosphate, abbreviated BFEE and
[BMIM]PF6, respectively) using platinum and nanoporous
TiO2 as substrates, its electrochemical, electrochromic and
optical properties being characterized using cyclic voltammetry
(CV), spectroelectrochemistry and UV-vis spectroscopy.19–22
Moreover, the influence of the concentration of electrolyte in
the generation medium and the thickness of the films on the
Scheme 1
aDepartament de Quımica, Escola Politecnica Superior,Universitat de Lleida, c/Jaume II no 69, Lleida E-25001, Spain
bDepartament d’Enginyeria Quımica, E. T. S. d’Enginyeria Industrialde Barcelona, Universitat Politecnica de Catalunya, Diagonal 647,Barcelona E-08028, Spain. E-mail: [email protected]
c Center for Research in Nano-Engineering, Universitat Politecnica deCatalunya, Campus Sud, Edifici C’, C/Pasqual i Vila s/n,Barcelona E-08028, Spain
d Institut de Quımica Computacional and Departament de Quımica,Universitat de Girona, Campus de Montilivi, Girona E-17071, Spain
eDepartament d’Enginyeria Quımica, Escola Universitariad’Enginyeria Tecnica Industrial de Barcelona, Universitat Politecnicade Catalunya, Comte d’Urgell 187, 08036 Barcelona, Spain
PCCP Dynamic Article Links
www.rsc.org/pccp PAPER
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(b) Successive cyclic voltammograms (50 scans) of 0.01 M 3-bromo-
thiophene in BFEE with 0.1 M (Bu)4NBF4 on a steel electrode. Initial
and final potentials: 0.10 V; reversal potential: 1.90 V. Scan rate:
50 mV s�1. Arrows indicate the increasing number of cycles.
Table 1 Comparison of the electroactivity (i.e. ability to store charge)and specific capacitance (SC, eqn (1)) of PBrTh and PClTh, respectively,produced by CA in BFEE with 0.1 M (Bu)4NBF4 applying differentpotentials and using a polymerization time of 150 s. Properties for thematerial produced using potentiodynamic methodsa (0.1–1.9 V) arealso displayed for comparison. Data for PClTh were taken from ref. 25
a Films were generated from 25 consecutive oxidation–reduction
cycles in a solution containing monomer in BFEE with 0.1 M
(Bu)4NBF4 at 50 mV s�1.
Table 2 Current efficiency of polymerization (Z; eqn (3)), doping level(dl; eqn (2)), loss of electroactivity (LES; eqn (5)) and electricalconductivity of PBrTh and PClTh films produced by CA in BFEEwith 0.1 M (Bu)4NBF4 applying a constant potential of 1.70 V andusing a polymerization time of 150 s
Polymer Z (in %) dl LES (in %)a s (S cm�1)
PBrTh 88 0.42 47 0.96PClThb 85 0.41 29 0.59
a Determined by considering 100 consecutive oxidation–reduction
cycles. b From ref. 25.
Fig. 2 SEM (left) and AFM (right) high resolution images of:
(a) PBrTh films generated by CA at 1.70 V in BFEE with 0.1 M
(Bu)4NBF4 using a polymerization time of 150 s; and (b) PBrTh films
generated as in (a) and subsequently dedoped in acetonitrile with
0.1 M (Bu)4NBF4 applying a potential of �1.00 V during 50 s.
This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 10057
Larger n-XTh (X= F, Cl and Br) oligomers, with n ranging
from 5 to 16, were constructed using a repetitive sequence of
head-to-tail linkages. It should be emphasized that considera-
tion of a regular chemical structure, which in addition should
be identical for the three families of oligomers, is essential to
compare the influence of the different halogen atoms on the
electronic properties of the polymers without interference of
undesirable substitutional effects. Thus, a repetition of head-
to-tail linkages has been found to be the most consistent with
the results obtained for compounds with n = 2 and 3,
independently of the halogen. Specifically, although the
energies of 3-FTh(3,30–4,40) and 3-FTh(3,40–3,40) differ by less
than 0.1 kcal mol�1, the 2-FTh(3,40) isomer is clearly favored
with respect to the 2-FTh(3,30) one. On the other hand,
repetition of the most favored isomer of 3-ClTh, which is the
4,40–3,40 one, produces a high concentration of head-to-head
linkages that are the least favored, as was evidenced in Fig. 4b.
However, the next isomer in terms of relative energies corres-
ponds to the 3-ClTh(3,40–3,40), which is formed by two adjacent
head-to-tail linkages. Finally, the 3,40–3,40 is the 3-BrTh isomer
that upon repetition produces the lowest concentration of head-
to-head linkages and maintains the inter-ring dihedral angles
close to the anti conformation. It should be mentioned that the
high stability of the head-to-tail sequence in large oligomers
(n > 3) was corroborated in previous theoretical studies invol-
ving other 3-substituted thiophene units.13–16
n-XTh (X = F, Cl and Br) oligomers with n = 5, 7, 9, 11,
13, 15 and 16 were constructed according to these principles
and considering all the inter-ring dihedral angles initially
arranged at 180.01. These structures were used as starting
points for complete geometry optimizations at the B3LYP/
6-31+G(d,p) level. The optimized oligomers were employed
for the analyses of the electronic properties of PFTh, PClTh
and PBrTh, which will be discussed in next sections.
Scheme 3
Fig. 6 y1–y2 maps (y1 and y2 refer to the inter-ring dihedral angles; in
degrees), indicating the location of the minimum energy conformations
found for 3-XTh(3,30–3,40) (squares), 3-XTh(3,30–4,40) (triangles),
3-XTh(3,40–3,40) (diamonds) and 3-XTh(4,40–3,40) (circles) where X =
F, Cl and Br, as is indicated in Scheme 3. Results for X=Cl have been
taken from ref. 25. For each compound the minima of the four isomers
has been categorized in three groups: (i) minima with relative energies
lower than 1.0 kcal mol�1 (largest size symbols); (ii) minima with
relative energies comprised between 1.0 and 2.0 kcal mol�1 (medium
size symbols); and (iii) minima with relative energies larger than
2.0 kcal mol�1 (smallest size symbols). For each compound, the
relative energies have been computed with respect to the lowest energy
minimum of the most stable isomer.
Table 3 Energies (in kcal mol�1) and inter-ring dihedral angles (y) for the minimum energy conformations (plain numbers) of the 2-XTh dimersunder study. The barriers (in kcal mol�1) are also indicated (italic numbers)
syn (y = 01) syn–gauche gauche–gauche (y = 901) anti–gauche anti (y = 1801)
10058 Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 This journal is c the Owner Societies 2012
Prediction of the electronic properties
Fig. 7 represents the linear variation of the IP and EA against
1/n for n-XTh (X=F, Cl and Br) oligomers. Linear regression
analyses, which are included in Fig. 7, allowed extrapolation
of the IP and EA values for infinite chains of PFTh, PClTh
and PBrTh (Table 4). As it can be seen, the IP values predicted
for PClTh and PBrTh are practically identical (B5.2 eV) and
are in excellent agreement with the electrochemical measures
for PClTh and PBrTh prepared by CA using a fixed potential
of 1.70 V (5.38 and 5.43 eV for PClTh25 and PBrTh, respectively).
On the other hand, the electronegative fluorine substituents
induce a slight reduction (B0.2 eV) in the IP calculated for
PFTh. DFT calculations predict an EA of B3.3 eV for the
three polymers.
As was mentioned in the Methods section, in this work the
IPs and EAs have been estimated using the DFT extension of
the Koopmans theorem,36 or more precisely of the Janak
theorem.37 However, this procedure may be affected by the
so-called self-interaction (SI) error, which has been identified when
some early DFT approaches are applied to some compounds.62–65
The SI arises from the interaction of an electron with itself,
and it is related to the Coulomb energy of the Kohn–Sham
Hamiltonian, which is not totally cancelled by the exchange
contribution. Previous studies reported that the DFT HOMO
energy is too small with respect to experimental values, which
was attributed to the SI. However, the good concordance
between the calculated and the experimental IP values for
the compounds studied in this work suggests that the SI error
is small for this family of compounds, probably because of a
fortuitous cancellation of errors.
The variation of the eg values calculated using DFT and
TD-DFT calculations against 1/n for n-XTh (X = F, Cl
and Br) oligomers is displayed in Fig. 8, values extrapolated
for infinite polymer chains being included in Table 4. As it can
be seen, the DFT value is overestimated with respect to the
TD-DFT one by 0.11, 0.16 and 0.18 eV for PFTh, PClTh and
PBrTh, respectively. The relative order predicted by the two
strategies for eg is PFTh o PClTh E PBrTh, even though the
difference between the three CPs predicted by DFT and
TD-DFT calculations is lower than 0.15 and 0.08 eV, respectively.
This is consistent with the increase of the p-donor character ofthe halogen substituent in the order F > Cl E Br.66 The
discrepancy between eg determined experimentally using UV-vis
Fig. 7 Variation of the (a) IP and (b) EA against 1/n, where n is the
number of repeat units in n-XTh oligomers with X = F, Cl and Br
(data for X = Cl taken from ref. 25). The solid lines correspond to the
linear regressions used to extrapolate these electronic properties
towards infinite polymer systems.
Table 4 Ionization potential (IP), electron affinity (EA) and band gap (eg) reported in the literature and determined in this work for PFTh, PClThand PBrTh
IP (eV) EA (eV) eg (eV) Ref. Description
PFTh 5.05 3.33 1.72 This work DFT calculations— — 1.61 This work TD-DFT calculations
PClTh 5.38a/5.29b — — 25 Electrochemical (CV)— — 1.58 25 UV-vis spectroscopy— — 1.74 20 Spectroelectrochemistry— — 1.80 21 Spectroelectrochemistry— — 2.14 22 UV-vis spectroscopy5.18 3.35 1.83 This work DFT calculations— — 1.67 This work TD-DFT calculations
PBrTh 5.43a/5.62b — — This work Electrochemical (CV)— — 1.93a/1.97b This work UV-vis spectroscopy— — 1.90 26 Spectroelectrochemistry— — 2.55 27 UV-vis spectroscopy— — B2.0c 28 UV-vis spectroscopy5.17 3.31 1.86 This work DFT calculations— — 1.68 This work TD-DFT calculations
a Samples prepared by potentiostatic methods (CA). b Samples prepared by potentiodynamic methods (CV). c Extrapolated from Eg = (1240/lonset).
10060 Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 This journal is c the Owner Societies 2012
these conjugated compounds. In addition, it has been reported
that S� � �F interactions contribute to rigidify oligomers in which
the thiophene ring is associated with 3,4-difluorothiophene74,75
or fluorophenylene units.74,76,77 The repulsive nature found for
the S� � �O interactions, which were also previously postulated as
attractive,67,68 and the important role played by aromaticity in
EDOT systems, has motivated a detailed analysis of the S� � �X(where X = F, Cl and Br) interactions in 2-XTh.
Aromaticity analyses have been carried out on the 3-halide-
thiophene monomeric units and the 2-XTh(3,30) dimers. The
minimum energy conformations listed in Table 3 were used for
calculations on the dimers. For each compound the following
aromaticity indices have been calculated for the thiophene rings:
electronic based FLU and MCI, magnetic based NICS(0) and
NICS(1) and geometric-based HOMA aromaticity descriptors.
The aromaticity parameters predicted for the three 3-halide-
thiophene units are compared in Table 5 with those calculated
for the unsubstituted thiophene ring. Most of the aromaticity
measures indicate that the substituents cause a slight decrease
in the aromaticity of the thiophene ring. Thus, the aromaticity
of the unsubstituted thiophene is slightly higher than that of
the 3-halidethiophenes. Table 5 also lists the aromaticity
parameters obtained for the minimum energy conformations
of the 2-XTh(3,30) series of dimers, as well as the unsubstituted
bithiophene (2-Th). The values indicate that the effect of
introducing a substituent into 2-Th, or the replacement of F
by Cl or Br is almost unnoticeable. Therefore, at difference
with the previous series of 2-EDOT and PEDOT, in this case
the preference for a given conformation is not determined by
changes in aromaticity since it keeps almost constant for all
substituents and conformations. To assign an attractive or
repulsive character to the S� � �X bond, the relative energies in
Table 3 may help. First, it must be pointed out that the anti
conformation of the 2-XTh(3,30) system presents two S� � �Xinteractions, whereas that of the 2-XTh(3,40) isomer presents
one, and that of the 2-XTh(4,40) species does not present any
S� � �X interaction. In all cases the (4,40) isomers are the most
stable anti conformations, those with no S� � �X interaction,
especially compared to (3,30) ones. On the other hand, the
energy difference between (3,30) and (4,40) increases from F to
Cl and to Br. If they would be attractive, the S� � �X interactions
should be stronger for Br and energy differences between (3,30)
and (4,40) smaller or even in favor of the (3,30) isomer. Thus,
energy values reveal that S� � �Br interactions are the most
repulsive. Moreover, for the 2-XTh(3,30) isomer, the anti
conformation is the most stable only for X = F. In this case
the repulsive character of the S� � �F interaction is compensated
by the favorable conjugation along the two thiophene rings in
this planar conformer. This does not occur for X = Cl and Br
due to the larger repulsive character of the S� � �X interactions.
All these observations point out to the repulsive character of
the S� � �X interactions for these systems (lone pair Pauli
repulsion). Thus, not unexpectedly, the most stable isomers
are in all cases the 2-XTh(4,40) either in the anti conformation
(X = Br) or in the anti–gauche one (X = F and Cl).
Conclusions
The doping level and electrical conductivity of PBrTh and
PClTh films prepared under identical conditions have been
found to be very similar. In spite of this, the electrochemical
stability of PBrTh is significantly smaller than that of PClTh,
which has been attributed to the lower porosity of the former
with respect to the latter. Thus, the access and escape of
dopant ions in oxidation and reduction processes, respectively,
are easier in the material with the most porous structure than
in that with the most compact one.
Quantum mechanical calculations indicate that the halogen
atom has a significant impact on the conformational prefer-
ences of 2-XTh and 3-XTh oligomers. Thus, the conforma-
tional freedom of oligomers with X = Br is restricted by
strong steric repulsive interactions, especially when repeating
units are associated by head-to-head linkages, while oligomers
with X = F show a significant conformational flexibility.
Compounds with X = Cl present an intermediate behavior.
The eg order predicted for poly(3-halidethiophene)s by DFT
calculations is: PFTh o PClTh r PBrTh. For the latter two
polymers, this relative order is fully consistent with the egvalues determined in this work using UV-vis spectroscopy and
spectroelectrochemistry. On the other hand, the IPs estimated
by CV are in very good agreement with theoretical predictions.
Table 5 Aromaticity parameters (see the text) calculated for the minimum energy conformations of 2-XTh(3,30) with X= F, Cl and Br, as well asthe corresponding monomeric units. For each compound the minimum energy conformations are identified by the inter-ring dihedral angle (y)a,b
This journal is c the Owner Societies 2012 Phys. Chem. Chem. Phys., 2012, 14, 10050–10062 10061
The aromaticity slightly varies from unsubstituted 2-Th to
2-XTh(3,30), independently of the halogen atom. Moreover,
within each series the effect of the halogen atom is very small.
The repulsive character of the S� � �X interaction explains the
higher stability of the 2-XTh(4,40) isomer compared to
2-XTh(3,30) in the anti conformation. Finally, at variance with
previously analyzed 2-EDOT and PEDOT systems, in this
case aromaticity and favorable conjugation along the two
thiophene rings in planar conformers do not play a key role
in determining the conformational preferences.
Acknowledgements
Financial support from the MICINN and FEDER
(MAT2009-09138, CTQ2008-06532/BQU, CTQ2008-03077/
BQU, CTQ2011-23156 and CTQ2011-25086) and Generalitat
de Catalunya (research group 2009 SGR 925, 2009 SGR 637
and XRQTC) is gratefully acknowledged. Support for the
research of C.A. and M.S. was received through the prize
‘‘ICREA Academia’’ for excellence in research funded by the
Generalitat de Catalunya. D.A. thanks the financial support
through a FPU-UPC grant. J.P. thanks the MICINN for the
Ramon y Cajal contract. The authors are indebted to
the Centre de Supercomputacio de Catalunya (CESCA) for
the computational resources provided.
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