Electropolymerized three-dimensional randomly branched edot-containing copolymers

Article abstract of DOI:10.1021/la403050c

The potential of 2,2′;3,2″-terthiophene (3T) as branching units in 3D copolymers is presented with EDOT as an example comonomer. Branched EDOT/3T polythiophenes were prepared by electropolymerization, and their electrochemical and optical properties are discussed. Two different approaches were employed: (i) the direct electropolymerization of a novel branched thiophene monomer (3TE₃) consisting of a 3T core that contains three outer EDOT end groups and (ii) the electrochemical copolymerization of a EDOT/3T mixture in different ratios from [1:1] to [1:10]. Cyclic voltammetric and vis spectrometric experiments show that the EDOT content within the polymer has a strong influence on the electronic properties of the material: with increasing EDOT content, the HOMO-LUMO gap is decreased. To prove copolymer formation of EDOT and 3T, chemically synthesized reference copolymers of EDOT and 3T were prepared by oxidative coupling using FeCl₃, and their optical and electronic properties were compared to those of the electrodeposited films. In addition, the copolymer formation is indicated by the comparison of the electrochemical and spectroscopic results with those of the homopolymers P3T and PEDOT.

Full text of DOI:10.1021/la403050c

Article
pubs.acs.org/Langmuir
Electropolymerized Three-Dimensional Randomly Branched EDOT-
Containing Copolymers
Steffen M. Link,† Martin Scheuble,† Miriam Goll,† Erna Muks,† Adrian Ruff,† Anke Hoffmann,‡
Thomas V. Richter,† Juan T. Lopez Navarrete,§ M. Carmen Ruiz Delgado,§ and Sabine Ludwigs*,†
†Institut fur Polymerchemie, Universitat Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
̈
̈
̈
‡Institut fur Makromolekulare Chemie, Universitat Freiburg, 79104 Freiburg, Germany
̈
§Department of Physical Chemistry, University of Mal
́ ́
aga, 29071 Malaga, Spain
S
* Supporting Information
ABSTRACT: The potential of 2,2′;3,2″-terthiophene (3T) as branching units in 3D copolymers is presented with EDOT as an
example comonomer. Branched EDOT/3T polythiophenes were prepared by electropolymerization, and their electrochemical
and optical properties are discussed. Two different approaches were employed: (i) the direct electropolymerization of a novel
branched thiophene monomer (3TE3) consisting of a 3T core that contains three outer EDOT end groups and (ii) the
electrochemical copolymerization of a EDOT/3T mixture in different ratios from [1:1] to [1:10]. Cyclic voltammetric and vis
spectrometric experiments show that the EDOT content within the polymer has a strong influence on the electronic properties
of the material: with increasing EDOT content, the HOMO−LUMO gap is decreased. To prove copolymer formation of EDOT
and 3T, chemically synthesized reference copolymers of EDOT and 3T were prepared by oxidative coupling using FeCl3, and
their optical and electronic properties were compared to those of the electrodeposited films. In addition, the copolymer
formation is indicated by the comparison of the electrochemical and spectroscopic results with those of the homopolymers P3T
and PEDOT.
crystalline character that results in anisotropic charge carrier
mobilities, and post-treatment protocols (e.g., temperature
annealing) are typically involved in the sample preparation.8−10
Branched 3D conducting polythiophenes have also attracted
a great deal of attention in organic electronics because of their
capability of isotropic charge transport in these materials.11
First approaches in the literature included oligothiophene
chains that were connected through 3D sp3 carbon atoms,12
silicon atoms,13,14 or triphenylamine15 as the 3D cross-linker.
One elegant way is to combine β- and α-conjugated thiophenes
in one system because this ensures a 3D architecture in the
corresponding polymer. The synthesis of a variety of branched
3D oligomers that consist entirely of thiophene units (including
α- and β-conjugation) led to swivel-cruciform,16 X-shaped,17
INTRODUCTION
■
Since its first publication in 1988, 3,4-ethylenedioxythiophene
(EDOT) has repeatedly opened up entirely new applications
for conductive polymers and therefore has contributed to the
breakthrough of intrinsically conductive polymers in large-scale
applications.1,2 The polymer PEDOT is particularly interesting
because of its high conductivity and stability and its low
oxidation potentials and transparency in the doped state.3,4 The
development of aqueous dispersions of oxidized PEDOT with
poly(styrenesulfonate) (PSS) as a stabilizer established the use
of PEDOT:PSS as a transparent electrode material in various
applications (e.g., in organic solar cells, in sensor applications,
and in electrochromic windows).2,5,6 In addition, other linear
conjugated polythiophenes have also been successfully applied
in organic electronics. For example, poly(3-hexylthiophene)
(P3HT) was used as the electron donor material in organic
solar cells7,8 or in organic field effect transistors.9 One typical
characteristic of these linear systems is their strongly semi-
Received: August 7, 2013
Revised: October 14, 2013
Published: November 22, 2013
© 2013 American Chemical Society
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a
Scheme 1. Synthesis of Branched Copolymers Prepared in This Work
a
(A) Polymerization of a monomer unit with a 3T core and three EDOT end groups (3TE3). (B) Formation of a copolymer from a monomer
mixture of 3T and EDOT.
a
Scheme 2. Stepwise Synthesis of 3TE3
a
Target compound 3TE3 is formed in a coupling reaction between a stannyl compound and 3TBr3 under Stille conditions.42
and spiderlike18 architectures. In addition, Advincula et al.19
chemical polymerization of trifunctional thiophene mono-
mers.28,29 In particular, the electropolymerization of branched
all-thiophene monomer 2,2′;3,2″-terthiophene (3T, Scheme 1)
leads to the formation of 3D polymers (P3T).29
and Bauerle et al.20 used dendrimeric approaches to obtain pure
̈
thiophene dendrimers with up to 90 thiophene units. Such
branched all-thiophene systems were successfully employed as
donor materials in organic solar cells.21,22
As a continuation of our former work, we now report on the
formation of 3D copolymers based on branching unit 3T and
EDOT as the comonomer by a pure electrochemical approach
to combine the properties of the two materials. Looking at the
literature, a large number of publications describe the chemical
and/or electrochemical formation of linear-EDOT-containing
copolymers (e.g., refs 30−38), for application in electrochromic
devices.39,40 The high capability of EDOT as building block in
3D oligomers and polymers was nicely demonstrated and
highlighted by Roncali and co-workers, who synthesized a
multitude of branched β-conjugated EDOT-containing mono-
mers that could be readily electropolymerized.41,42
Using electropolymerization approaches, Visy et al.23,24 and
Tanaka and Kumei25 made films of 3′-thienyl-2,2′:5′,2″-
terthiophene and 3′,3′-bis(2,2′:5′,2″-terthiophene), respec-
tively. Furthermore, Zotti et al. electropolymerized a pentoxy-
modified 3′-thienyl-2,2′:5′,2″-terthiophene monomer (namely,
2,3,5-tris(4-pentoxy-2-thienyl)thiophene).26 One key advantage
of electrochemical polymerization is that polymer films can be
directly deposited onto conducting substrates of various sizes
and geometries.27 This is of particular interest for the
preparation of electrochromic or sensor applications.
We have recently reported the one-step synthesis of
hyperbranched polythiophenes by the chemical and electro-
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Figure 1. (A) Cyclic voltammograms of the electrodeposition of EC-P(3TE3) on ITO (c(3TE3) = 0.6 mM, scan rate = 50 mV s−1) in 0.1 M
NBu4PF6/MeCN. (B) Complete oxidation and reduction voltammograms of EC-P(3TE3) (dashed line) and EC-P3T (solid line) and the oxidation
cycle of EC-PEDOT (dotted line) on ITO in monomer-free solution (0.1 M NBu4PF6/MeCN, scan rate = 20 mV s−1).
Our approach is to directly use pure EDOT with the
branched 3T monomer, which leads to 3D copolymers that
combine the benefits of a branched polythiophene with the
properties of PEDOT (low-lying HOMO level and trans-
parency in the doped state) and additionally allows reduced
synthetic efforts. We show that opto-electronic properties of
the resulting copolymers can be nicely modified by varying the
stoichiometric ratio of the two monomers. The article is
organized as follows: first we describe the electropolymerization
of a precoupled EDOT-modified 3T core (Scheme 1A) and
then report the direct electrochemical copolymerization of
EDOT and 3T mixtures in different monomer ratios (Scheme
1B).
positive than the monomer oxidation occurs that can be
attributed to the oxidation of the newly formed polymer. The
intensity of this peak increases with an increasing number of
cycles, indicating the polymer growth on the electrode surface.
For comparative purposes, the homopolymers of 3T (EC-P3T)
and EDOT (EC-PEDOT) were deposited as well.
The oxidation potential Eox of monomer 3TE3 (+0.4 V on
ITO vs the formal potential of the Fc/Fc+ (Fc = Ferrocene)
potential standard44) is considerably lower than that of 3T (Eox
(vs Fc/Fc+) = 1.1 V on ITO) and EDOT (Eox (vs Fc/Fc+) =
1.0 V on ITO). This demonstrates that because of the
incorporation of three EDOT units the 3TE3 monomer is an
electron-rich species that can be readily oxidized. In addition to
the electron-withdrawing effect of the oxygen atoms in EDOT,
the effective conjugation length in 3TE3 is longer than that in
3T and can therefore contribute to the low oxidation potential
of the 3TE3 monomer compared to that of 3T.
RESULTS AND DISCUSSION
■
Polymerization of 3TE3: EC-P(3TE3). Synthesis of
Monomer 3TE3. Compound 3TE3 was synthesized in a
stepwise reaction sequence starting from 3T (Scheme 2). In
the first step, the 3T core is brominated with N-
bromosuccinimide (NBS) in dimethylformamide (DMF),
which is similar to a protocol reported earlier.43 The resulting
halogenated 3T unit (3TBr3) is then reacted with Stille reagent
2-(tri-n-butylstannyl)-3,4-ethylenedioxythiophene36,37 in a Pd-
catalyzed cross-coupling reaction42 to yield the target
compound as an orange powder in moderate yield.
Cyclic voltammograms of EC-P(3TE3) in monomer-free 0.1
M NBu4PF6/MeCN solutions (Figure 1B, dashed line)
demonstrate that EC-P(3TE3) can be reversibly p and n
ox
doped with oxidation (E
) and reduction onset potentials
onset,CV
red
(E
) of ∼−0.1 and ∼−2.0 V (vs Fc/Fc+), respectively.
onset,CV
From the I−E curves, the electrochemical band gap (HOMO−
LUMO gap) of this polymer could be estimated45 to be ∼1.9
eV. By comparing the voltammograms of EC-P(3TE3) with
that of the electrodeposited homopolymer of 3T on ITO (EC-
P3T, Figure 1B, solid line) and EDOT on ITO (EC-PEDOT,
dotted line), it is obvious that polymers EC-P(3TE3) and EC-
P3T show chemically reversible p- and n-doping processes. For
EC-PEDOT, only the oxidation is reversible. Under our
conditions, we observe only a weak, chemically irreversible
reduction wave (not shown).
The 1H NMR spectrum of 3TE3 in CDCl3 reveals all signals
that would be expected for the structure of the monomer
(Experimental Section). The UV/vis spectrum of 3TE3 in
CH2Cl2 (Figure S1 in the Supporting Information) shows an
absorption maximum at 350 nm with a distinct shoulder at
around 400 nm that is independent of the concentration. The
values are comparable to UV/vis data of similar branched
EDOT-containing thiophene monomers reported by Roncali
and co-workers.42 The same authors state that the presence of
two absorption maxima can be attributed to the coexistence of
two different conjugation pathways in the 3D arrangement of
the monomer.42
Electrochemical Deposition of EC-P(3TE3) on Indium Tin
Oxide. The electropolymerization of the branched 3TE3
monomer on indium tin oxide (ITO) electrodes in acetonitrile
(MeCN) containing tetrabutylammonium hexafluorophosphate
(NBu4PF6, c = 0.1 M) as the supporting electrolyte yields
homopolymer EC-P(3TE3). (In the remainder of the article, all
films that were deposited electrochemically are designated as
EC.) The corresponding multicycle voltammogram is shown in
Figure 1A. After the first cycle (red curve), a new peak less
The oxidation potentials of the polymers differ significantly.
This can be ascribed to the strong electron-donating nature of
the EDOT units in EC-P(3TE3) and pristine PEDOT, which
leads to a lifting of the HOMO level in EC-P(3TE3) and
therefore to a facilitated oxidation of the polymer compared to
that of EC-P3T. This is consistent with DFT calculations
performed for the single monomers: the HOMO level of 3TE3
is 0.93 eV higher in energy than that of 3T, whereas its LUMO
level is moderately stabilized by 0.29 eV (Figure 2).
The vis spectrum of an electropolymerized EC-P(3TE3) film
on ITO-coated glass (Figure 3) reveals a broad absorption with
a maximum at 510 nm that is red-shifted by 50 nm compared to
that for homopolymer EC-P3T (460 nm, on ITO). This red
shift might be ascribed to an increase in the effective
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The strong shifts in the oxidation potential and the
absorption maximum of EC-P(3TE3) compared to those of
EC-P3T demonstrate that the incorporation of EDOT in a
branched thiophene polymer leads to a remarkable change in
the material properties. However, if the 3TE3 unit is used as the
starting monomer, then the EDOT/3T ratio is always fixed at
3:1. Thus, it is not possible to adjust the stoichiometry of the
EDOT and 3T units.
Copolymerization of 3T and EDOT in Different Ratios
on Gold and ITO: EC-P(E-co-3T). The strong change in the
material properties by the inclusion of EDOT into a branched
thiophene polymer motivated us to explore the potential of the
direct copolymerization of monomers EDOT and 3T (Scheme
1B). For the direct electrochemical copolymerization of two
different monomers, the oxidation potentials of the two
comonomer species must be very similar.33,47−49 Indeed, cyclic
voltammetric measurements on gold electrodes in 0.1 M
NBu4PF6/MeCN show that oxidation potentials Eox (vs Fc/
Fc+) of EDOT (+1.0 V on gold) and 3T (+0.9 V on gold) are
almost identical. This is consistent with the calculated HOMO
levels of the comonomers (Figure 2). Thus, the electrochemical
formation of a copolymer (EC-P(E-co-3T)) should be possible.
To prove the influence of stoichiometry, different comonomer
(EDOT/3T) ratios from [1:1] to [1:10] were explored.
Figure 4A shows the electrochemical deposition of an
EDOT/3T mixture [1:5] on a gold electrode under
potentiodynamic conditions. The multicycle voltammograms
of the other monomer ratios show similar behavior to the [1:5]
system. It should be noted that on gold the polymer growth
was highly homogeneous and covered the entire gold surface.
Figure 4B shows cyclic voltammograms of the EC-P(E-co-
3T) copolymer films obtained with monomer ratios of [1:1],
[1:3], and [1:10] on gold in monomer-free solutions (0.1 M
NBu4PF6/MeCN). All polymers show a reversible oxidation
wave. The onset of the oxidation (Eoonxset,CV) shifts to lower
potentials when going from EC-P(E-co-3T)-[1:10] to -[1:1]
(Table S1 in the Supporting Information). This demonstrates
that EC-P(E-co-3T)-[1:1] approaches the properties of EC-
PEDOT, whereas EC-P(E-co-3T)-[1:3] represents a compro-
mise of properties between the two respective homopolymers.
However, values for the onsets of the oxidation potentials of
Figure 2. DFT//B3LYP/6-31G** wave functions of the frontier
molecular orbitals in 3T, EDOT, and 3TE3.
Figure 3. UV/vis spectrum of EC-P3T (dashed line) and EC-
P(3TE3) (solid line) as films on ITO-coated glass.
conjugation length as a result of self-rigidification upon
inclusion of EDOT.37,46 The incorporation of EDOT favors
the planarity of the polymer backbone π-system (better
molecular overlap) and thus increases the overall π-conjugation.
The optical band gaps of EC-P(3TE3) and EC-P3T are 1.8 and
2.0 eV, respectively (determined from the onsets of the
absorption maxima).
Figure 4. (A) Potentiodynamic deposition of EC-P(E-co-3T)-[1:5] on gold in 0.1 M NBu4PF6/MeCN (c(EDOT/3T) = 2 mM, scan rate = 20 mV
s−1). (B) Cyclic voltammograms of EC-P(E-co-3T)-[1:1] (dashed line, oxidation cycle), -[1:3] (solid line), and -[1:10] (dotted line) on gold in
monomer-free solution (0.1 M NBu4PF6/MeCN, scan rate = 20 mV s−1). The films in B were deposited under potentiostatic control at 0.9 V [1:1
and 1:3] or 1.0 V [1:10]; both potentials are vs Fc/Fc+.
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Figure 5. In situ spectroelectrochemistry of EC-P(E-co-3T)-[1:5] deposited on ITO in 0.1 M NBu4PF6/MeCN. (A) Cyclic voltammograms of the
reversible p-doping and n-doping processes of the copolymer. (B) Vis/NIR spectra recorded during the forward scan (oxidation) in the p-doping
process: red line, absorption spectrum measured at E (vs Fc/Fc+) = −0.2 V; dotted line, spectrum of the radical cation species (E (vs Fc/Fc+) = +0.3
V); and dashed line, spectrum of the dication (E (vs Fc/Fc+) = +0.8 V). (C) Vis/NIR spectra recorded during the forward scan (reduction) in the n-
doping process: red line, absorption spectrum recorded at E (vs Fc/Fc+) = −1.9 V; dotted line, spectrum of the radical anion species (E (vs Fc/Fc+)
= −2.3 V). (D) Intensity of the absorption as a function of applied potential for characteristic wavelengths: red line, 470 nm (absorption of the
neutral polymer); black line, 810 nm (absorption of the radical cation).
copolymers EC-P(E-co-3T)-[1:5] and EC-P(E-co-3T)-[1:10]
are close to the onset potential of homopolymer EC-P3T
and C) of the corresponding forward scans. In the forward scan
of the p-doping process (B), the absorption of the neutral
species at 470 nm begins to decrease and two new bands at
higher wavelengths appear that are attributed to the absorption
of the radical cation species. Figure 5D shows the absorption
intensity as a function of applied potential for the absorption
maxima of the neutral polymer (red line) and the radical cation
species (black line). From these graphs, one can deduce that at
E = −0.2 V (vs Fc/Fc+) the absorption of the neutral band is
(Table S1).
red
onset,CV
The onset potentials of the reduction (E
) of
copolymers EC-P(E-co-3T)-[1:3], -[1:5], and -[1:10] and
the value for homopolymer EC-P3T are rather constant at
around −2.0 V (Table S1). The larger amount of EDOT in
EC-P(E-co-3T)-[1:1] seems to prevent a chemically reversible
reduction of this material under our conditions: only a
chemically irreversible signal is obtained. The same effect is
observed for EC-PEDOT, which can be reversibly n-doped
only under highly inert conditions.50,51
Films of the copolymers were also prepared on ITO under
potentiostatic conditions because transparent electrodes also
allow us to probe the absorption behavior. In situ
spectroelectrochemistry measurements of these films give
information on the redox behavior as well as the absorption
characteristics of the neutral and charged species upon
electrochemical cycling. In contrast to the gold substrates, the
polymer growth on ITO is less homogeneous. In some cases,
only a few polymer patches on the ITO substrate were
observed. This is most likely because of the higher resistivity of
the ITO electrodes compared to that of gold substrates and the
inhomogeneity of the ITO layer.
decreasing and the signal of the radical cation appears. We call
ox
onset
this potential the onset potential (E ). The maximum values
for the radical cation are situated at 810 and 1530 nm
corresponding to a potential of +0.3 V (vs Fc/Fc+), with Δ =
+0.5 V relative to the onset potential (Figure 5B, dotted line).
At this potential, clear and pronounced signals are observed.
With further doping, the two maxima of the radical cation
merge into a very broad band centered at ∼1350 nm, which
suggests dication formation (dashed line, E = +0.8 V (vs Fc/
Fc+)). Similar spectroelectrochemical behavior was found for
the EDOT-containing networks shown by Roncali and co-
workers.42 The shape of the vis/NIR spectrum and λmax after
the backward scan (dedoping) correspond to those of the
neutral state.
In the forward scan of the n-doping process (Figure 5C), the
absorption band at 470 nm (red line, E = −1.9 V (vs Fc/Fc+))
also decreases when going toward more negative potentials. At
Figure 5 shows the reversible p- and n-doping (A) of
copolymer EC-P(E-co-3T)-[1:5] and the vis/NIR spectra (B
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Table 1. Absorption and Electrochemical Characteristics of Electrochemically Deposited (Potentiostatic Control) Copolymers
EC-P(E-co-3T)-[1:1], -[1:3], -[1:5], and -[1:10] and Homopolymers EC-PEDOT and EC-P3T Derived from in Situ
a
Spectroelectrochemical Experiments in 0.1 M NBu4PF6/MeCN on ITO Electrodes
b
λ/nm (eV)
d
c
c
ox
red
onset
polymer
EC-PEDOT
neutral state
radical cation
E
/V (HOMO/eV)
E
/V (LUMO/eV)
electrochemical band gap/eV
onset
e
630 (2.0)
530 (2.3)
500 (2.5)
470 (2.6)
440 (2.8)
450 (2.8)
880
860
830
810
810
780
2060
1530
1520
1530
1540
1440
−0.8 (−4.0)
−0.6 (−4.2)
−0.3 (−4.5)
−0.2 (−4.6)
+0.1 (−4.9)
+0.3 (−5.1)
−2.1 (−2.7)
1.3
1.5
1.7
1.7
2.2
2.4
e
EC-P(E-co-3T)-[1:1]
EC-P(E-co-3T)-[1:3]
EC-P(E-co-3T)-[1:5]
EC-P(E-co-3T)-[1:10]
EC-P3T
−2.1 (−2.7)
−2.0 (−2.8)
−1.9 (−2.9)
−2.1 (−2.7)
−2.1 (−2.7)
a
b
Values for the HOMOs, LUMOs, and electrochemical band gaps are estimated according to ref 45. The error is estimated to be ∼ 10 nm.
c
d
e
ox
onset
Determined from spectroelectrochemical experiments, E vs Fc/Fc+. Values determined at E = E
+ 0.5 V. Chemically irreversible reduction.
red
the same time, the signal of the radical anion at ∼850 nm is
increasing (dotted line, E = −2.3 V (vs Fc/Fc+)). After the
reversal scan, the spectrum of the neutral polymer is again fully
restored. Thus, the spectroscopic results confirm the data
obtained by cyclic voltammetry, which indicate chemically
reversible redox behavior for both p- and n-doping of the
copolymers. Spectroelectrochemical data of the other copoly-
mers show similar behavior. (See Figure S2 in the Supporting
Information for the p-doping process of copolymers EC-P(E-
co-3T)-[1:1], -[1:3], and -[1:10] as well as homopolymers EC-
P3T and EC-PEDOT.) However, EC-P(E-co-3T)-[1:1]
exhibits a chemically irreversible reduction. The characteristic
absorption values are listed in Table 1.
E
in Table 1 were determined by analyzing the intensity of
onset
the absorption band of the neutral polymer as a function of
applied potential E. (See Figure 5D for EC-P(E-co-3T)-[1:5]
as an example.) In our opinion, the analysis of the
spectroelectrochemical data allows the determination of E
ox
onset
red
onset
and E
in a more precise way than a conventional
determination from pure voltammetric data.
Values for Eoonxset/HOMO (Table 1) are increasing/decreasing
when going from EC-P(E-co-3T)-[1:1] (−0.6 V/−4.2 eV) to
EC-P(E-co-3T)-[1:10] (+0.1 V/−4.9 eV). The E /HOMO
values for the homopolymers (Table 1) are −0.8 V/−4.0 eV
(EC-PEDOT) and +0.3 V/−5.1 eV (EC-P3T). Obviously,
comonomer ratio [1:1] leads to a PEDOT-like copolymer
whereas the [1:10] copolymer reveals P3T-like behavior.
Polymers EC-P(E-co-3T)-[1:3] and EC-P(E-co-3T)-[1:5]
ox
onset
The absorption of the neutral copolymers changes from 440
to 530 nm when going from EC-P(E-co-3T)-[1:10] to EC-
P(E-co-3T)-[1:1] (Table 1) most likely because of a self-
rigidification effect (see discussion above).37,46
combine the properties of the two comonomer units. Values
red
onset
for E /LUMO are rather constant for all copolymers (∼−2.1
The signals of the radical cation (Table 1) change from 810
to 860 nm and therefore follow the same trend as for the
absorption bands of the neutral state: the values are red-shifted
with increasing EDOT content. The values of the second band
are rather constant. In addition to the influence of the
monomer ratio, the values of the absorption maxima of the
radical cation species are partially shifted and seem to be
affected by the applied potential (Figure 5B,C). For
comparative purposes, values for the absorption maxima of
the radical cation were always determined from spectra that
correspond to a potential Δ = +0.5 V relative to the onset
potential.
The absorption maxima of the neutral homopolymers EC-
P3T and (EC-PEDOT) are located at 450 and 630 nm,
respectively. The bands of the radical cation (Table 1) are
situated at 780 and 1440 nm (EC-P3T) as well as at 880 and
2060 nm (EC-PEDOT).
V/−2.7 eV). Hence, values for the electrochemical band gap
are decreasing with increasing EDOT content.
For both applied electrode materials (gold and ITO), the
values of the oxidation onset and the reduction onset potentials
follow the same trend (cf. Tables S1 and 1).
In summarizing our data, we find that the introduction of
EDOT has a significant influence on both the absorption and
electrochemical properties of the electropolymerized films. To
obtain further information on the chemical composition of the
electrodeposited copolymers, one of the EC-P(E-co-3T)-[1:3]
films coated on ITO was subjected to chlorobenzene (CB).
Visual inspection showed that most of the material could be
dissolved in this solvent. The absorption maximum in the
solution UV/vis spectrum (not shown) of EC-P(E-co-3T)-
[1:3] is 460 nm. This value is considerably red-shifted
compared to that for the CB fraction of a CB-treated EC-
P3T homopolymer film (440 nm). Bearing in mind that P3T
homopolymers exhibit good solubility in organic solvents28 and
PEDOT is insoluble, we find that these data support the
hypothesis that the electropolymerized films are indeed
copolymers and not blends of soluble EC-P3T and insoluble
EC-PEDOT. However, an analysis of the exact chemical
composition is not possible because of the small amount of
material of the electrodeposited films and the large amount of
remaining supporting electrolyte within these films.
The color of the copolymer film upon oxidation changes
from brown to blue. For PEDOT-like copolymer EC-P(E-co-
3T)-[1:1], a slightly blue-colored transparent film was
observed. This again demonstrates the strong influence of
EDOT in this material because the pure EC-PEDOT films are
transparent in the p-doped state, which is consistent with the
literature data.4,52 The potential of these materials for the
fabrication of electrochromic windows is currently explored in
our group.
Chemical Polymerization with FeCl3: Reference Sys-
tem. In the case of 3T, polymers obtained by chemical
oxidative polymerization using FeCl3 exhibit properties
ox
onset
red
onset
Table 1 also contains values for E
and E
for all
28
copolymers and homopolymers. The onset in cyclic voltammo-
grams can be affected by the overall shape of the I−E curves
comparable to those of polymers deposited electrochemically,
as we have recently shown.29 Because chemical polymerization
allows the production of larger amounts of material, a proper
(e.g., multiple peaks, shoulders at the beginning of the main
ox
peak, or high capacitive currents). Thus, all values of E
and
onset
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Figure 6. Characterization of the polymers prepared via chemical polymerization. (A) MALDI-TOF mass spectrum of the CH-P(E-co-3T)-[1:3]-
TCM measured with trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile as the matrix: green circles, 3T1En, n = 5−7; red
circles, 3T2En, n = 3−7; black circles, 3T3En, n = 1−5; green triangles, 3T4En, n = 1−6; red triangles, 3T5En, n = 2−7; black triangles, 3T6En, n = 2−7;
1
green squares, 3T7En1, n = 3−8; red squares, 3T8En, n = 3−8; black squares, 3T9En, n = 4−8; asterisk, 3T3 and 3T4). (B) H NMR spectrum and
contour plot DOSY H NMR spectrum of CH-P(E-co-3T)-[1:3]-TCM in C2D2Cl4, with the solvent residual peak marked with an asterisk. (C)
Solution vis absorption spectra of CH-P3T-TCM (dotted line, recorded in TCM), CH-P(E-co-3T)-[1:3]-TCM (dashed line, recorded in TCM),
and CH-P(E-co-3T)-[1:3]-CB (solid line, recorded in CB). (D) Cyclic voltammograms of CH-P3T-TCM (dashed line), CH-P(E-co-3T)-[1:3]-
TCM (solid line), and CH-P(E-co-3T)-[1:3]-CB (dotted line) deposited (drop casting) on ITO in monomer-free solution (0.1 M NBu4PF6/
MeCN, scan rate = 20 mV s−1); voltammograms are rescaled for better visualization.
polymer characterization becomes possible. Thus, we per-
formed a chemical copolymerization (CH) of EDOT and 3T in
a [1:3] ratio (CH-P(E-co-3T)-[1:3]). After polymerization and
purification, the product was fractionated with chloroform
(TCM) and CB. In the following text, the polymers of the
corresponding TCM and CB fractions are designated as CH-
P(E-co-3T)-[1:3]-TCM and CH-P(E-co-3T)-[1:3]-CB, re-
spectively. The weight-average molecular weight (Mw) of the
polymers in the two different fractions could be determined by
size exclusion chromatography (GPC) as 1900 g mol−1 (PDI =
1.64) and 1700 g mol−1 (PDI = 2.83), respectively. Again,
homopolymer CH-P3T is considered for comparison reasons.
It was fractionated with TCM and CB to obtain CH-P3T-TCM
and CH-P3T-CB. The synthesis28 and electrochemical
characterization29 of CH-P3T were reported recently.
distribution of signals that can be assigned to species consisting
of EDOT and 3T units. These data show that the formation of
soluble EDOT-containing copolymers takes place.
This assumption is further corroborated by NMR experi-
ments on CH-P(E-co-3T)-[1:3]-TCM. The 1H NMR
spectrum of CH-P(E-co-3T)-[1:3]-TCM is shown in Figure
6B (top). The ratio of the aliphatic signals of the EDOT units
around 4.3 ppm and the aromatic signals between 6.8 and 7.6
ppm from 3T units is approximately 4:9. We further performed
1
diffusion-ordered H NMR spectroscopy (DOSY) of CH-P(E-
co-3T)-[1:3]-TCM (Figure 6B). The DOSY analysis of the
diffusion coefficient of both signals in the aromatic region and
in the ethylenedioxy region implies that both signals belong to
species with the same hydrodynamic radius (i.e., a copolymer
from EDOT and 3T).
To prove copolymer formation, NMR spectroscopy and
MALDI-TOF spectrometry were employed. The MALDI-TOF
spectra of the TCM (Figure 6A) and CB fractions show a broad
Figure 6C depicts the solution vis absorption spectra of CH-
P(E-co-3T)-[1:3]-TCM and CH-P(E-co-3T)-[1:3]-CB. With
respect to homopolymer CH-P3T-TCM, the absorption
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or was dried over activitated Al2O3 prior to use. All reactions were
carried out under an argon atmosphere unless otherwise noted. The
syntheses of 3T and CH-P3T have been published elsewhere.28
maximum of CH-P(E-co-3T)-[1:3]-TCM has a pronounced
red shift of 50 nm (i.e., from 440 to 490 nm). CB fraction CH-
P(E-co-3T)-[1:3]-CB even exhibits a maximum of 520 nm;
this is only 26 nm blue-shifted in comparison to that of soluble
PEDOT derivatives presented by the Yamamoto group that
again highlights the significant effect that the incorporation of
the EDOT units imparts to the optical properties of the
copolymers.53
1
Methods. H{250 MHz} and 13C{63 MHz} NMR spectra were
recorded on a Bruker Avance 250 spectrometer. Diffusion-ordered 1H
NMR experiments were performed on a Bruker ARX 300
spectrometer.
MALDI-TOF mass spectrometric measurements were performed
on a Bruker autoflex speed spectrometer. The polymer sample
concentration was set to 3 mg mL−1. A poly(ethylene glycol) standard
was used for internal calibration (3 mg mL−1). Matrix trans-2-[3-(4-
tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB, 10
mg mL−1) containing the ionizer sodium trifluoromethanesulfonate
was used. The polymer samples were mixed in a 1:2 ratio with the
DCTB matrix. The mixed samples were prepared by the dried-droplet
method (1 μL of the mixed solution was allowed to dry on a polished
steel target). EI and ESI mass spectra were measured on a MAT 95
spectrometer (Finnigan) and a micrOTOFq spectrometer (Bruker
Daltonics), respectively.
UV/vis spectra of the solution and of the polymer films were
recorded with a Lambda 35 spectrometer (Bruker) in quartz cuvettes
with d = 1 mm and on ITO-coated glass substrates, respectively.
GPC measurements were performed in THF using a three-column
(PSS-SDV) arangement with pore sizes of 10 000, 1000, and 100 Å,
respectively. For calibration, a polystyrene standard was employed.
The single fractions were detected using a Waters 2487 dual λ
absorbance detector. GPC data were analyzed with PSSWinGPC
software.
Electrochemical Experiments. All electrochemical experiments
were performed with an Autolab PGSTAT101 potentiostat (Metrohm,
Germany) in a three-electrode glass cell under an argon atmosphere at
room temperature. The counter electrode was a Pt plate. The
reference electrode consisted of a AgCl-coated silver wire that was
directly immersed into the electrolyte. As working electrodes, gold (50
nm layer)-coated Si wafers (with a 5 nm Cr layer between the Si wafer
and the Au layer) or ITO-coated glass (≤50 Ω/sq, PGO, Germany)
slides were used (∼2 × 1 cm2). The gold working electrodes were
fabricated by the physical vapor deposition of Cr and Au on rotating Si
wafers. The gold-coated Si wafers and the ITO substrates were
thoroughly washed with acetone prior to use. Additionally, the
electrodes were treated with oxygen plasma for at least 5 min before
measurements were made. NBu4PF6 was used as the electrolyte at a
concentration of 0.1 M. Electrolyte solutions were deaerated by argon
bubbling. All potentials are referenced to the formal potential of the
Fc/Fc+ internal redox standard.44 To avoid charge-trapping effects
during cyclic voltammetric experiments, the oxidation and reduction
cycles were performed separately.
Visible/NIR spectroelectrochemical measurements were made in
situ with transparent ITO electrodes and 0.1 M NBu4PF6/MeCN as
the electrolyte. Samples of chemically synthesized films were prepared
by drop casting 50 μL of 3 mg mL−1 solutions in chloroform on ITO-
coated glass (∼300 Ω/sq). The electrodeposited films were used
directly for the measurements. The vis/NIR spectra were recorded
with a diode array spectrometer from the Zeiss MCS 600 series
(equipped with a Zeiss CLH600 halogen lamp and two MCS 611 NIR
2.2 and MCS 621 VIS II spectrometer cassettes).
Cyclic voltammograms on ITO including the chemically
reversible oxidation and reduction of drop cast films of CH-
P(E-co-3T)-[1:3]-TCM and CH-P(E-co-3T)-[1:3]-CB are
ox
onset,CV
presented in Figure 6D. Oxidation onset potentials E
of copolymers CH-P(E-co-3T)-[1:3]-TCM and CH-P(E-co-
3T)-[1:3]-CB determined from voltammetric data are ∼−0.1 V
(vs Fc/Fc+) and ∼−0.2 V (vs Fc/Fc+). The lower E
value
ox
onset,CV
of the CB fraction compared to that of the TCM fraction
corresponds well to the vis spectroscopic results (red shift of
the CB fraction). Furthermore, both values are located at lower
oxidation potentials than that of homopolymer CH-P3T-TCM
(∼+0.5 V vs Fc/Fc+, onset value determined from the I−E
curve depicted in Figure 6D). This is consistent with the results
obtained from the electrochemically deposited films (Table 1)
and again indicates the strong electron-donating effect of the
EDOT units within the copolymers. It is interesting that the
films of the polymers prepared via chemical polymerization
with FeCl3 exhibit optoelectronic behavior that is similar to that
of electrodeposited films (Supporting Information, Figure S3).
CONCLUSIONS
■
The formation of novel EDOT containing branched poly-
thiophenes was successfully demonstrated by the direct
polymerization of an EDOT containing branched monomer
and by the electrochemical copolymerization of EDOT/3T
mixtures. The latter case allows the variation of the comonomer
ratio and therefore control of the material properties.
Cyclic voltammetric and in situ spectroelectrochemical
experiments show that the copolymers can be reversibly p-
and n-doped. Only the copolymer with the highest EDOT
content exhibits a chemically irreversible electron-transfer
reaction. A comparison with homopolymers 3T and EDOT
shows that the formed polythiophenes are indeed copolymers.
This is supported by MALDI-TOF mass spectrometric and
diffusion-ordered NMR spectroscopic experiments performed
with chemically synthesized reference systems.
Furthermore, the presented study on the systematic variation
of the EDOT/3T ratio during the electropolymerization
demonstrates that the electronic properties (e.g., the
HOMO−LUMO gap) of the formed materials can be readily
adjusted by controlling the stoichiometry of the comonomers.
The use of a branched monomer unit (3T) leads to the
formation of 3D oligothiophene networks. Such architectures
are interesting for electrochemical devices where ion flow in
and out of the structure is required. Further copolymerizations
of 3T with other interesting comonomers (e.g., for low-band-
gap polymers) are currently being conducted in our group.
Electropolymerization of EC-P(3TE3). The electrodeposition of
EC-P(3TE3) on ITO was performed under potentiodynamic control
(18 cycles) using a 0.6 mM solution of 3TE3 in 0.1 M NBu4PF6/
MeCN at a scan rate of 50 mV s−1 and switching potentials of +0.4 and
−1.4 V vs Fc/Fc+. After electropolymerization, the films were washed
with pure MeCN to remove residual NBu4PF6 and monomers.
Electropolymerization of EC-P(E-co-3T)-[1:1], -[1:3], -[1:5], and
-[1:10]. Electropolymerization of the monomer mixtures of 3T and
EDOT on gold or ITO was performed under potentiostatic control
(200 s) with an overall comonomer concentration of 2 mM in 0.1 M
NBu4PF6/MeCN at 0.9−1.1 V vs Fc/Fc+. The deposition potentials
correspond to the peak potential of the comonomer oxidation in the
corresponding electrolyte. The desired concentrations were achieved
by taking appropriate aliquots from stock solutions (c = 10 or 20 mM).
EXPERIMENTAL SECTION
■
Materials. All chemicals and solvents were purchased from Sigma-
Aldrich or Alfa-Aesar. Solvents were at least HPLC grade and were
used as received except otherwise noted. NBu4PF6 (Sigma-Aldrich,
electrochemical grade) was stored in a desiccator over blue gel.
Acetonitrile (Alfa Aesar, supergradient HPLC grade (far-UV), 99.9+
%) was stored over activated neutral Al2O3 under an argon atmosphere
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After deposition, the oxidized films were directly reduced to the
neutral form by applying a potential between −1.3 and −1.4 V (vs Fc/
Fc+) for 205 s. Finally, the deposited films were thoroughly washed
with MeCN.
MHz} NMR (CDCl3): δ/ppm 4.20−4.39 (m, 12 H), 6.22 (s, 2H),
6.26 (s, 1H), 6.98−7.06 (m, 2H), 7.07−7.13 (m, 2H), 7.24 (s, 1H).
UV/vis (CH2Cl2, c = 100 μM): λ/nm 350 (λmax with a shoulder at
∼400). EI-MS (70 eV (m/z)): 668 (M+, 100%), 570, 334. ESI-MS:
(m/z) calculated for [C30H20O6S6]+, 668.9657; found, [M]+ 668.9665.
Chemical Polymerization of CH-P(E-co-3T)-[1:3]. To a vigorously
stirred mixture of 3,4-ethylenedioxythiophene (27 μL, 0.25 mmol, 0.25
equiv) and 3T (190 mg, 0.77 mmol, 0.75 equiv) in deaerated
acetonitrile (25 mL), FeCl3 (502 mg, 3.09 mmol, 3 equiv, dissolved in
25 mL of deaerated acetonitrile) was added within 15 min under an
argon atmosphere. After 22 h, the reaction mixture was filtered and the
residue was washed with acetonitrile (3 × 25 mL). The residue was
dissolved in chlorobenzene and refluxed repeatedly with aqueous
ammonia. The dedoped polymer was successively fractionated by
solubility in acetone, chloroform, and chlorobenzene. For this, the
respective solvent was placed into a flask filled with the raw material,
and after several minutes of vigorous agitation, the supernatant was
centrifuged at 6000 rpm for 15 min to separate insoluble from soluble
material. This procedure was repeated twice for each solvent, and the
insoluble material was subjected to the same treatment with the next
solvent. The acetone fractions were not investigated in detail; the
chloroform fractions were the main fractions whereas the chlor-
obenzene fractions showed good solubility only at elevated temper-
atures. Yield (soluble material): 21% (for spectroscopic and electro-
chemical data, see Results and Discusion section).
Electropolymerization of EC-P3T and EC-PEDOT. Homopolymers
EC-P3T and EC-PEDOT were prepared either potentiostatically
(ITO: analogous to the conditions described above, potential of
deposition = 1.0−1.1 V vs Fc/Fc+; gold: EC-P3T: 200 s; EC-PEDOT:
30 s; potential of deposition = 1.0−1.1 V vs Fc/Fc+; after deposition
the films were reduced to the neutral form by applying potentials
between −1.3 and −1.4 V (vs Fc/Fc+)) or potentiodynamically in 0.1
M NBu4PF6/MeCN at a scan rate of 20 or 50 mV s−1 in 6 to 20 cycles
by potential cycling between −1.4 and +1.0 V or −0.5 and +1.0 V
(EC-P3T) and −1.4 and +1.1 V (EC-PEDOT); all potentials are
given versus Fc/Fc+. Monomer concentrations of 0.8−2.0 mM were
used. The films were washed with pure MeCN after deposition.
Description of Density Functional Theory Calculations. The
molecular geometries of the neutral states of 3T, EDOT, and 3TE3
were calculated at the density functional theory (DFT) level using the
B3LYP functional54,55 and the 6-31G** basis set,56−58 as implemented
in the Gaussian 09 program.59 All geometrical parameters were
allowed to vary independently apart from the planarity of the rings,
and no symmetry constraints were imposed during the optimization
process. For the resulting ground-state optimized geometries,
harmonic frequency calculations were performed to ensure that a
global minimum was reached. Molecular orbital contours were plotted
using Molekel 4.3.60
ASSOCIATED CONTENT
* Supporting Information
■
Synthesis. Synthesis of 3TBr3. To a solution of 3T (6.21 g, 825
mmol) in DMF (300 mL), NBS (16.46 g, 92.5 mmol, 3.7 equiv) in
DMF (300 mL) was slowly added under light exclusion. The reaction
mixture was stirred overnight under ambient conditions and then
diluted with 600 mL of CHCl3. The organic phase was washed with
water (2 × 600 mL). After the separation of the organic layer, the
solvent was removed in vacuum. Recrystallization (two times) of the
crude product from MeOH gave yellow needlelike crystals (8.2 g,
67%). 1H{250 MHz} NMR (CDCl3): δ/ppm 6.79 (d, J = 4.0 Hz, 1H),
6.86 (d, J = 3.8 Hz, 1H), 6.96 (d, J = 3.8 Hz, 1H), 6.99 (d, J = 4.0 Hz,
1H), 7.07 (s, 1H). 13C{1H}{63 MHz} NMR (CDCl3): δ/ppm 112.4,
113.0, 114.4, 127.3, 128.9, 130.0, 131.6, 131.8, 132.1, 134.6, 137.1.
Synthesis of 2-(Tri-n-butylstannyl)-3,4-ethylenedioxythiophene.
Under an argon atmosphere, 1.5 mL (14 mmol) of 3,4-ethyl-
enedioxythiophene was dissolved in dry THF (40 mL) and cooled to
−78 °C. To this solution, n-BuLi (8.75 mL, 1.6 M in n-hexane) was
added dropwise. The resulting mixture was stirred for 15 min at −78
°C, warmed to room temperature, and stirred for another 100 min.
The solution was cooled again to −78 °C, and tributyltin chloride
(4.94 mL, 18.2 mmol, 1.3 equiv) dissolved in dry THF (20 mL) was
added dropwise over a period of 20 min. The mixture was allowed to
warm to room temperature and stirred at this temperature for another
19.5 h. The reaction mixture was poured into cold water (200 mL),
and the aqueous phase was extracted with Et2O (3 × 100 mL). The
combined organic phases were dried over MgSO4 and filtered through
Celite. The solvent and residual starting material were removed under
reduced pressure. The crude product (6.8 g) was used without further
S
Values of the onset potentials determined from pure
voltammetric data, spectroelectrochemical results of the
chemically synthesized films, UV/vis spectrum of monomer
3TE3, and spectroelectrochemical data of the p-doping process
of the copolymers and homopolymers. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sabine.ludwigs@ipoc.uni-stuttgart.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge scientific discussions with Dr. K. Dirnberger.
We thank A. Hasenhindl and B. Omiecienski for 1H NMR and
some of the electrochemical (B.O.) measurements as well as F.
Fischer, D. Freytag, and T. Purckhauer for laboratory
̈
assistance. Funding was provided by the DFG in the framework
of an Emmy Noether grant and a project within SPP-1355. The
work in Mal
Innovacion
́
aga was supported by the Ministerio de Ciencia e
́
(MICINN) through project reference CTQ2009-
1
́
purification in the following step (GC/MS: purity of 96%). H{250
10098 and by the Junta de Andalucia through research project
MHz} NMR (CDCl3): δ/ppm 0.83−0.94 (t, 9H), 1.05−1.15 (m, 6H),
1.25−1.42 (m, 6H), 1.48−1.64 (m, 6H), 4.10−4.24 (m, 4H), 6.58 (s,
1H). Signals are partly overlapped by the signals of remaining starting
material
PO9-FQM-4708. M.C.R.-D. thanks the MICINN for a Ramon
́
y Cajal Research contract.
REFERENCES
Preparation of 3TE3. To a solution containing 485 mg (1 mmol)
3TBr3, and 346 mg (0.30 mmol) Pd(PPh3)4 in 50 mL of dry toluene,
2587 mg (6 mmol) of crude 2-(tri-n-butylstannyl)-3,4-ethylenediox-
ythiophene was added dropwise by means of a syringe. The reaction
mixture was refluxed for 15 h before another 75 mg (0.06 mg) of
Pd(PPh3)4 was added. The mixture was refluxed for another 24 h and
then washed sequentially with H2O, aqueous HCl (6 M), and aqueous
KOH (1 M). The organic phase was separated and dried over MgSO4.
After the removal of the solvent, the crude residue was purified by
column chromatography (three times; Et2O). The title compound was
■
(1) Jonas, F.; Heywang, G.; Schmidtberg, W.; Heinze, J.; Dietrich, M.
Polythiophenes, Process for Their Preparation and Their Use. Eur.
Pat. 0339340, 1989.
(2) Kirchmeyer, S.; Reuter, K. Scientific importance, properties and
growing applications of poly(3,4-ethylenedioxythiophene). J. Mater.
Chem. 2005, 15, 2077−2088.
(3) Heywang, G.; Jonas, F. Poly(alkylenedioxythiophene)s- new, very
stable conducting polymers. Adv. Mater. 1992, 4, 116−118.
(4) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Electrochromic
̈
1
obtained as an orange powder (247 mg, 0.37 mmol, 37%). H{250
and highly stable poly(3,4-ethylenedioxythiophene) switches between
15471
dx.doi.org/10.1021/la403050c | Langmuir 2013, 29, 15463−15473

Langmuir
Article
opaque blue-black and transparent sky blue. Polymer 1994, 35, 1347−
1351.
(26) Zotti, G.; Salmaso, R.; Gallazzi, M. C.; Marin, R. A. In situ
conductivity of a polythiophene from a branched alkoxy-substituted
tetrathiophene. enhancement of conductivity by conjugated cross-
linking of polymer chains. Chem. Mater. 1997, 9, 791−795.
(27) Inzelt, G. Chemical and Electrochemical Syntheses of
Conducting Polymers. Conducting Polymers - A New Era in
Eletrochemistry; Springer: Heidelberg, 2008; Chapter 4.
(5) Yang, S. Y.; Cicoira, F.; Byrne, R.; Benito-Lopez, F.; Diamond,
D.; Owens, R.; Malliaras, G. G. Electrochemical transistors with ionic
liquids for enzymatic sensing. Chem. Commun. 2010, 46, 7972−7974.
(6) Heuer, H. W.; Wehrmann, R.; Kirchmeyer, S. Electrochromic
window based on conducting poly(3,4-ethylenedioxythiophene)-
poly(styrene sulfonate). Adv. Funct. Mater. 2002, 12, 89−94.
(7) Sirringhaus, H. Device physics of solution-processed organic
field-effect transistors. Adv. Mater. 2005, 17, 2411−2425.
(8) Ma, W.; Kim, J. Y.; Lee, K.; Heeger, A. J. Effect of the molecular
weight of poly(3-hexylthiophene) on the morphology and perform-
ance of polymer bulk heterojunction solar cells. Macromol. Rapid
Commun. 2007, 28, 1776−1780.
(9) Reyes-Reyes, M.; Kim, K.; Dewald, J.; Lopez-Sandoval, R.;
Avadhanula, A.; Curran, S.; Carrol, D. Meso-structure formation for
enhanced organic photovoltaic cells. Org. Lett. 2005, 7, 5749−5752.
(10) Crossland, E. J. W.; Rahimi, K.; Reiter, G.; Steiner, U.; Ludwigs,
S. Systematic control of nucleation density in poly(3-hexylthiophene)
thin films. Adv. Funct. Mater. 2011, 21, 518−524.
(28) Richter, T. V.; Link, S.; Hanselmann, R.; Ludwigs, S. Design of
soluble hyperbranched polythiophenes with tailor-made optoelectronic
properties. Macromol. Rapid Commun. 2009, 30, 1323−1327.
(29) Link, S.; Richter, T.; Yurchenko, O.; Heinze, J.; Ludwigs, S.
Electrochemical behavior of electropolymerized and chemically
synthesized hyperbranched polythiophenes. J. Phys. Chem. B 2010,
114, 10703−10708.
́ ́
(30) Donat-Bouillud, A.; Levesque, I.; Tao, Y.; D’Iorio, M.; Beaupre,
S.; Blondin, P.; Ranger, M.; Bouchard, J.; Leclerc, M. Light-emitting
diodes from fluorene-based π-conjugated polymers. Chem. Mater.
2000, 12, 1931−1936.
́
(31) Stephan, O.; Tran-Van, F.; Chevrot, C. New organic materials
for light emitting devices based on dihexylfluorene-co-ethylenediox-
ythiophene copolymers exhibiting improved hole-injecting properties.
Synth. Met. 2002, 131, 31−40.
(11) Roncali, J.; Leriche, P.; Cravino, A. From one- to three-
dimensional organic semiconductors: in search of the organic silicon?
Adv. Mater. 2007, 19, 2045−2060.
(12) Tour, J. M.; Wu, R.; Schumm, J. S. Extended orthogonally fused
conducting oligomers for molecular electronic devices. J. Am. Chem.
Soc. 1991, 113, 7064−7066.
(32) Berlin, A.; Zotti, G.; Zecchin, S.; Schiavon, G.; Cocchi, M.;
Virgili, D.; Sabatini, C. 3,4-Ethylenedioxy-substituted bithiophene-alt-
thiophene-S,S dioxide regular copolymers. Synthesis and conductive,
magnetic and luminescence properties. J. Mater. Chem. 2003, 13, 27−
33.
(13) Roncali, J.; Guy, A.; Lemaire, M.; Garreau, R.; Hoa, H. A.
Tetrathienylsilane as a precursor of highly conducting electrogenerated
polythiophene. J. Electroanal. Chem. 1991, 312, 277−283.
(14) Roncali, J.; Thobie-Gautier, C.; Brisset, H.; Favart, J.-F.; Guy, A.
Electro-oxidation of tetra(terthienyl)silanes: towards 3D electroactive
π-conjugated systems. J. Electroanal. Chem. 1995, 381, 257−260.
(33) Huang, H.; Pickup, P. G. A donor-acceptor conducting
copolymer with a very low band gap and high intrinsic conductivity.
Chem. Mater. 1998, 10, 2212−2216.
(34) Nie, G.; Qu, L.; Xu, J.; Zhang, S. Electrosyntheses and
characterizations of a new soluble conducting copolymer of 5-
cyanoindole and 3,4-ethylenedioxythiophene. Electrochim. Acta 2008,
53, 8351−8358.
(35) Aubert, P.-H.; Knipper, M.; Groenendaal, L.; Lutsen, L.; Manca,
J.; Vanderzande, D. Copolymers of 3,4-ethylenedioxythiophene and of
pyridine alternated with fluorene or phenylene units: synthesis, optical
properties, and devices. Macromolecules 2004, 37, 4087−4098.
́
(36) Turbiez, M.; Frere, P.; Blanchard, P.; Roncali, J. Mixed π-
conjugated oligomers of thiophene and 3,4-ethylenedioxythiophene
́ ́
(15) Cravino, A.; Aleveque, S. R. O.; Leriche, P.; Frere, P.; Roncali, J.
Triphenylamine-oligothiophene conjugated systems as organic semi-
conductors for opto-electronics. Chem. Mater. 2006, 18, 2584−2590.
(16) Bilge, A.; Zen, A.; Foster, M.; Li, H.; Galbrecht, F.; Nehls, B. S.;
Farrell, T.; Neher, D.; Scherf, U. Swivel-cruciform oligothiophene
dimers. J. Mater. Chem. 2006, 16, 3177−3182.
(17) Sun, X. B.; Liu, Y. Q.; Chen, S. Y.; Qiu, W. F.; Yu, G.; Ma, Y. Q.;
Qi, T.; Zhang, H. J.; Xu, X. J.; Zhu, D. B. X-shaped electroactive
molecular materials based on oligothiophene architectures: facile
synthesis and photophysical and electrochemical properties. Adv.
Funct. Mater. 2006, 16, 917−925.
(EDOT). Tetrahedron Lett. 2000, 41, 5521−5525.
(37) Turbiez, M.; Frere, P.; Allain, M.; Videlot, C.; Ackermann, J.;
́
Roncali, J. Design of organic semiconductors: tuning the electronic
properties of π-conjugated oligothiophenes with the 3,4-ethyl-
enedioxythiophene (EDOT) building block. Chem.Eur. J. 2005,
11, 3742−3752.
(18) Benicori, T.; Capaccio, M.; De Angelis, F.; Falciola, L.; Muccini,
M.; Mussini, P.; Ponti, A.; Toffanin, S.; Traldi, P.; Sannicolo, F. Spider-
like oligothiophenes. Chem.Eur. J. 2008, 14, 459−471.
(19) Xia, C.; Fan, X.; Locklin, J.; Advincula, R. C. A first synthesis of
thiophene dendrimers. Org. Lett. 2002, 4, 2067−2070.
(38) Roncali, J.; Blanchard, P.; Frere, P. 3,4-Ethylenedioxythiophene
́
(EDOT) as a versatile building block for advanced functional p-
conjugated systems. J. Mater. Chem. 2005, 15, 1589−1610.
(20) Ma, C.-Q.; Mena-Osteritz, E.; Debaerdemaeker, T.; Wienk, M.
̈
M.; Janssen, R.; Bauerle, P. Functionalized 3D oligothiophene
(39) Algia, M. P.; Oztas, Z.; Tirkes, S.; Cihaner, A.; Algi, F. A new
̈
dendrons and dendrimersnovel macromolecules for organic
electrochromic copolymer based on dithienylpyrrole and EDOT. Org.
Electron. 2013, 14, 1094−1102.
(40) Yua, W.; Chen, J.; Fu, Y.; Xu, J.; Nie, G. Electrochromic
property of a copolymer based on 5-cyanoindole and 3,4-ethyl-
enedioxythiophene and its application in electrochromic devices. J.
Electroanal. Chem. 2013, 700, 17−23.
(41) Piron, F.; Leriche, P.; Mabon, G.; Grosu, I.; Roncali, J.
Electropolymerization of three-dimensional π-conjugated system based
on 3,4-ethylenedioxythiophene (EDOT). Electrochem. Commun. 2008,
10, 1427−1430.
electronics. Angew. Chem., Int. Ed. 2007, 46, 1679−1683.
(21) Mangold, H. S.; Richter, T. V.; Link, S.; Wurfel, U.; Ludwigs, S.
̈
Optoelectronic properties of hyperbranched polythiophenes. J. Phys.
Chem. B 2012, 116, 154−159.
(22) Ma, C.-Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.;
Janssen, R. A. J.; Bauerle, P. Solution-processed bulk-heterojunction
̈
solar cells based on monodisperse dendritic oligothiophenes. Adv.
Funct. Mater. 2008, 18, 3323−3331.
(23) Visy, C.; Lukkari, J.; Kankare, J. Scheme for the anodic and
cathodic transformations in polythiophenes. Macromolecules 1993, 26,
3295−3298.
(42) Piron, F.; Leriche, P.; Grosu, I.; Roncali, J. Electropolymerizable
3D π-conjugated architectures with ethylenedioxythiophene (EDOT)
end-groups as precursors of electroactive conjugated networks. J.
Mater. Chem. 2010, 20, 10260−10268.
(24) Visy, C.; Lukkari, J.; Kankare, J. Electrochemically polymerized
terthiophene derivatives carrying aromatic substituents. Macromolecules
1994, 27, 3322−3329.
(43) Richter, T. V.; Braun, C. H.; Link, S.; Scheuble, M.; Crossland,
(25) Tanaka, S.; Kumei, M. A new polythiophene prepared by the
electropolymerization of a branched sexithienyl. J. Chem. Soc., Chem.
Commun. 1995, 815−816.
E. J. W.; Stelzl, F.; Wurfel, U.; Ludwigs, S. Regioregular
̈
polythiophenes with alkylthiophene side chains. Macromolecules
2012, 45, 5782−5788.
15472
dx.doi.org/10.1021/la403050c | Langmuir 2013, 29, 15463−15473

Langmuir
Article
(44) Gritzner, G.; Kuta, J. Recommendations on reporting electrode
̊
potentials in nonaqueous solvents. Pure Appl. Chem. 1984, 56, 461−
466.
(45) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R.; Bassler,
̈
H.; Porsch, M.; Daub, J. Efficient two layer LEDs on a polymer blend
basis. Adv. Mater. 1995, 7, 551−554.
(46) Perepichka, I. F.; Levillain, E.; Roncali, J. Effect of substitution of
3,4-ethylenedioxythiophene (EDOT) on the electronic properties of
the derived electrogenerated low band gap conjugated polymers. J.
Mater. Chem. 2004, 14, 1679−1681.
(47) Yohannes, T.; Carlberg, J. C.; Inganas, O.; Solomon, T.
̈
Electrochemical and spectroscopic characteristics of copolymers
electrochemically synthesized from 3-methylthiophene and 3,4-ethyl-
enedioxythiophene. Synth. Met. 1997, 88, 15−21.
(48) Nie, G.; Qu, L.; Xu, J.; Zhang, S. Electrosyntheses and
characterizations of a new soluble conducting copolymer of 5-
cyanoindole and 3,4-ethylenedioxythiophene. Electrochim. Acta 2008,
53, 8351−8358.
(49) Latonen, R.; Kvarnstrom, C.; Ivaska, A. Electrochemical
̈
synthesis of a copolymer of poly(3-octylthiophene) and poly-
(paraphenylene). Electrochim. Acta 1999, 44, 1933−1943.
(50) Ahonen, H. J.; Lukkari, J.; Kankare, J. n- and p-Doped poly(3,4-
ethylenedioxythiophene): two electronically conducting states of the
polymer. Macromolecules 2000, 33, 6787−6793.
(51) Fu, Y.; Cheng, H.; Elsenbaumer, R. L. Electron-rich thienylene-
vinylene low bandgap polymers. Chem. Mater. 1997, 9, 1720−1724.
(52) Gustafsson, J. C.; Liedberg, B.; Inganas, O. In situ spectroscopic
̈
investigations of electrochromism and ion transport in a poly(3,4-
ethylenedioxythiophene) electrode in a solid state electrochemical cell.
Solid State Ionics 1994, 69, 145−152.
(53) Shiraishi, K.; Kanbara, T.; Yamamoto, T.; Groenendaal, L.
Preparation of a soluble and neutral alkyl derivative of poly(3,4-
ethylenedioxythiophene) and its optical properties. Polymer 2001, 42,
7229−7232.
(54) Becke, A. D. Density-functional thermochemistry. III. The role
of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(55) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti
correlation-energy formula into a functional of the electron density.
Phys. Rev. B: Condens. Matter 1988, 37, 785−789.
(56) Hariharan, P. C.; Pople, J. A. Influence of polarization functions
on MO hydrogenation energies. Theor. Chim. Acta 1973, 28, 213−222.
(57) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon,
M. S.; DeFrees, D. J.; Pople, J. A. Self consistent molecular orbital
methods. XXIII. A polarizationtype basis set for secondrow elements. J.
Chem. Phys. 1982, 77, 3654−3665.
(58) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-consistent
molecular orbital methods. XII. Further extensions of Gaussian-type
basis sets for use in molecular orbital studies of organic molecules. J.
Chem. Phys. 1972, 56, 2257−2261.
(59) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman,
J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision B.1;
Gaussian, Inc.: Wallingford, CT, 2009.
(60) Flukinger, P.; Luthi, H. P.; Portmann, S.; Weber, J. Molekel 4.3
̈
̈
2000; Swiss Center for Scientific Computing.
15473
dx.doi.org/10.1021/la403050c | Langmuir 2013, 29, 15463−15473