Synthesis, electropolymerization and characterization of a cross-linked PEDOT derivative

Article abstract of DOI:10.1039/c2jm14909e

The synthesis of a novel electropolymerizable monomer, 2,2′,3, 3′-tetrahydro-2,2′-bithieno[3,4-b][1,4]dioxine (THBTD), based on two 3,4-ethylenedioxythiophene (EDOT) moieties connected through the ethylenedioxy bridge is reported. The new monomer paves the way for the development of cross-linked networks based on the EDOT moiety. Polymer films were electrogenerated from monomeric solutions by consecutive potential sweeps or by flow of constant anodic currents and the polymer structure was studied using FTIR. The relationship between the mass of the electrogenerated polymer (after reduction) and the polymerization charge gives the productivity of the consumed charge (mg C^-1) while the charge stored and delivered by the films was determined by cyclic voltammetry getting the specific charge (C g ^-1). Simultaneously with the electroinitiated polymerization a chemical polymerization occurs around the electrode by monomer protonation giving protonated and no electroactive polymer chains. This chemical polymerization was followed in solution by UV-Vis spectroscopy using different conditions. Productivities and specific charges change with the conditions of synthesis in opposite directions. The voltammetric control presents a main redox couple (0.54/0.50 V) and a strong reduction process at -2.89 V that only can be reoxidized at more anodic potentials than 0.3 V as usual for charge trapping effects. EQCM studies indicate a remarkable difference between PEDOT and poly(THBTD). While poly(THBTD) shows a predominantly reversible anionic exchange during oxidation from the neutral state, the parent pristine PEDOT presents a mixed anionic and cationic exchange. Electrochromic color changes from an intense blue color of the oxidized film to a clear orange color in the reduced films as observed by in situ UV-Vis spectroscopy. Interestingly, the electrochromic changes in poly(THBTD) are opposed to those of PEDOT. Both thiophene derivative polymers present a similar thermal degradation at 305 and 314 °C, respectively.

Full text of DOI:10.1039/c2jm14909e

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PAPER
Synthesis, electropolymerization and characterization of a cross-linked
PEDOT derivative
a
a
b
b
Joaquin Arias-Pardilla, Pablo A. Gim ꢀe nez-G ꢀo mez, Alejandro de la Pe ~n a, Jos ꢀe L. Segura*
and Toribio F. Otero*a
Received 30th September 2011, Accepted 22nd December 2011
DOI: 10.1039/c2jm14909e
0
0
0
The synthesis of a novel electropolymerizable monomer, 2,2 ,3,3 -tetrahydro-2,2 -bithieno[3,4-b][1,4]
dioxine (THBTD), based on two 3,4-ethylenedioxythiophene (EDOT) moieties connected through the
ethylenedioxy bridge is reported. The new monomer paves the way for the development of cross-linked
networks based on the EDOT moiety. Polymer films were electrogenerated from monomeric solutions
by consecutive potential sweeps or by flow of constant anodic currents and the polymer structure was
studied using FTIR. The relationship between the mass of the electrogenerated polymer (after
ꢀ1
reduction) and the polymerization charge gives the productivity of the consumed charge (mg C ) while
the charge stored and delivered by the films was determined by cyclic voltammetry getting the specific
ꢀ1
charge (C g ). Simultaneously with the electroinitiated polymerization a chemical polymerization
occurs around the electrode by monomer protonation giving protonated and no electroactive polymer
chains. This chemical polymerization was followed in solution by UV-Vis spectroscopy using different
conditions. Productivities and specific charges change with the conditions of synthesis in opposite
directions. The voltammetric control presents a main redox couple (0.54/0.50 V) and a strong reduction
process at ꢀ2.89 V that only can be reoxidized at more anodic potentials than 0.3 V as usual for charge
trapping effects. EQCM studies indicate a remarkable difference between PEDOT and poly(THBTD).
While poly(THBTD) shows a predominantly reversible anionic exchange during oxidation from the
neutral state, the parent pristine PEDOT presents a mixed anionic and cationic exchange.
Electrochromic color changes from an intense blue color of the oxidized film to a clear orange color in
the reduced films as observed by in situ UV-Vis spectroscopy. Interestingly, the electrochromic changes
in poly(THBTD) are opposed to those of PEDOT. Both thiophene derivative polymers present
ꢁ
a similar thermal degradation at 305 and 314 C, respectively.
1
. Introduction
employed as a building block for the synthesis of various classes
6
of molecular p-conjugated systems including fluorophores,
In recent years 3,4-ethylenedioxythiophene (EDOT) has become
a powerful building block for the synthesis of functional
1–5
p-conjugated systems.
7,8
nonlinear optical (NLO)-phores, electroactive moieties, and
9,10
11–15
p-conjugated oligomers.
On the other hand, the concept of cross-linked networks based
The unique combination of strong
electron donor properties and self-structuring effects related to
intramolecular non-covalent interactions between oxygen and
sulfur is the most significant characteristic of this system. Thus,
the functionalization of the EDOT moiety has a direct impact on
the potential properties of EDOT-containing derivatives as
molecular functional p-conjugated systems. In this respect
EDOT has found extensive application in the design of low-
band-gap functional conducting polymers and has also been
16–18
19–23
on polythiophenes
and other conjugated polymers
has
recently received a great deal of attention because of the potential
24
of this strategy to tune the charge transport characteristics of
the network and to provide nanoporous organic materials with
applications in areas such as gas storage, molecular separations
25
and catalysis.
Here we describe the synthesis of a novel EDOT derivative,
namely THBTD (Scheme 1), which, as a consequence of its four
reactive positions, allows for the synthesis of cross-linked
networks based on the EDOT moiety. Polymer films can be
electrogenerated from the new monomer by consecutive poten-
tial sweeps and applying constant anodic currents. The influence
of the cross-linked network in the material on the type of ionic
exchange during p-doping is investigated.
a
Group of Electrochemistry, Intelligent Materials and Devices, Universidad
Polit eꢀ cnica de Cartagena, ETSII, E-30203 Cartagena, Spain. E-mail:
toribio.fotero@upct.es; Fax: +34 968 325915; Tel: +34 968 325519
b
Departamento de Qu ꢀı mica Org aꢀ nica, Facultad de Qu ꢀı mica, Universidad
Complutense de Madrid, E-28040 Madrid, Spain. E-mail: segura@quim.
ucm.es; Fax: +34 91 3944103; Tel: +34 91 3945142
4
944 | J. Mater. Chem., 2012, 22, 4944–4952
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A Thermo Nicolet 5700 spectrometer equipped with a
DTGS/KBr detector and Smart Orbit accessory with a diamond
crystal was employed for FTIR-ATR measurements. The spectra
were obtained from monomer powder or from a platinum
electrode coated with polymer.
For the electrochemical quartz microbalance (EQCM)
measurements an EG&G PAR model QCA917 microbalance was
used. The electrodes used were AT-cut 9 MHz piezoelectric
crystals, coated with Pt (0.3 mm thick) deposited over a Ti adhe-
2
sion layer (50 nm thick), with an area of 0.196 cm . A capacitance
of 1 nF and an inductor of 100 mH were used to isolate the
potentiostat from the microbalance. Linearity of the relation
between the resonant frequency and change of mass was verified
by platinum and silver deposition and the Sauerbrey equation.
This equation was used to calculate mass changes from frequency
8
ꢀ2 ꢀ1
changes with an integral sensitivity of 1.8 ꢂ 10 Hz cm g .
The ‘‘in situ’’ spectroelectrochemical experiments were per-
formed with a DINKO 8500 UV-Vis spectrophotometer,
working in the 370–1100 nm wavelength range. The polymer was
electrogenerated on ITO electrodes. The coated electrodes were
Scheme 1 Synthesis of the THBTD monomer.
2
. Experimental
Melting points were measured with an electrothermal melting
point apparatus and are uncorrected. NMR spectra were
recorded on a Bruker AC-200 apparatus and the chemical shifts
6
studied in 0.10 M TBAPF acetonitrile solution. The absorption
spectra were obtained at different oxidation states after polari-
zation for 20 s at each of the studied potentials.
are reported relative to tetramethylsilane (TMS) at 0.0 ppm
1
for H-NMR). For the assignment of the NMR, the acronym Th
(
Thermogravimetric (TG) experiments of the electrogenerated
polymer samples on Pt wires were conducted in a Mettler Tole-
do TGA/DSC 1HT equipment in the presence of an inert
has been used to indicate the thiophene ring. Mass spectrometry
was performed with a Varian Saturn 2000 GC-MS spectrometer.
Elemental analyses were performed on a Perkin-Elmer EA 2400.
ꢁ
atmosphere from room temperature to 900 C with a heating rate
ꢀ1
2
6
27
ꢁ
Tetrabromothiophene,
3,4-dibromothiophene
and 3,4-
of 10 C min .
26
dimethoxythiophene
were prepared following previously
reported synthetic procedures. All other chemicals were
purchased from commercial sources and used as received without
further purification.
2
.1 Synthesis of the THBTD
The novel THBTD has been synthesized from dimethoxy-
26
Dichloromethane (Fluka) and acetonitrile (Sigma Aldrich)
were used after drying using a molecular sieve UOP Type 3A
thiophene by reaction with (2R,3S)-butane-1,2,3,4-tetrol in
toluene in the presence of p-toluenesulfonic acid by means of
28
(
(
Fluka). Tetrabutylammonium hexafluorophosphate (Fluka)
TBAPF ) was dried and stored in desiccators before the prep-
a transetherification reaction. The synthesis of dimethoxy-
thiophene (4) has been carried out from thiophene (1) by
following the three-step reaction procedure previously described
that involves bromination to afford perbrominated thiophene
(2), selective dehalogenation to afford 3,4-dibromothiophene (3)
and substitution of the bromine atoms for methoxy groups by
reaction with sodium in methanol in the presence of potassium
iodide and copper(I) oxide as depicted in Scheme 1.
6
aration of the solutions.
All electrochemical studies were performed using an Autolab
PGSTAT-100 potentiostat/galvanostat controlled by a personal
computer using the GPES electrochemical software, in a three-
electrodes cell, under nitrogen atmosphere at ambient tempera-
2
ture. A platinum plate of 1 cm surface area was used as the
2
working electrode and two stainless steel plates having 4 cm
1
The H-NMR spectrum of THBTD shows common features
surface area were used as the counter electrode during electro-
polymerization. A silver wire was used as the reference electrode.
Before and after each experiment, the silver reference was cali-
brated vs. the ferrocene redox couple, and then adjusted to match
the Ag/AgCl reference potential (0.353 V vs. Ag/AgCl).
arising from the EDOT unit. Due to the asymmetry of the
substituted 2,3-dihydrothieno[3,4-b][1,4]dioxine fragments,
thiophene protons appear as an AB system centered at 6.67 ppm
with a coupling constant of 4.3 Hz. The protons at position 2 of
the dioxine ring appear as a complex multiplet at 4.22–4.13 ppm,
whereas H-3 protons are deshielded to 4.41–4.34 ppm. The
structure of THBTD is further confirmed by the other standard
spectroscopic and analytical techniques. Thus, mass spectrom-
etry of THBTD generated ions at the expected m/z values of
282 Da and evidence concerning its purity are given by elemental
analysis, which is in accordance with the expected values.
The electrochemical oxidation of the monomer and its poly-
merization were studied in 0.001 M THBTD and 0.1 M TBAPF
6
dichloromethane solutions. Dichloromethane is a good solvent
of thiophene derivative monomers. The electrochemical control
and characterization of the generated films were conducted in
0
.1 M TBAPF acetonitrile solution. Acetonitrile was used to
6
avoid possible dissolution of the generated material. The mass of
the electrogenerated films was determined using a Sartorius SC2
0
0
0
2.1.1 2,2 ,3,3 -Tetrahydro-2,2 -bithieno[3,4-b][1,4]dioxine.
212 mg (1.75 mmol) of (2R,3S)-butane-1,2,3,4-tetrol were added
to a solution of 0.5 g (3.5 mmol) of 3,4-dimethoxythiophene in
ꢀ
7
ultramicrobalance, precision of 10 g, by mass difference
between that of the clean Pt and the coated electrode after rinsing
with acetonitrile and drying.
ꢁ
6 mL of toluene. The solution was heated to 90 C under argon
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atmosphere and, once this temperature is reached, 66 mg
0.35 mmol) of p-toluenesulfonic acid monohydrate were added.
rising anodic currents flowing through the electrode at increasing
anodic potentials present a shoulder at 1.43 V and a maximum at
1.84 V, which can be related to the polymer degradation by
(
Similar amounts of (2R,3S)-butane-1,2,3,4-tetraol and p-tolue-
nesulfonic acid monohydrate were added every three hours for
nine hours and the reaction was then allowed to stay for 48 h. In
this time other six additions with similar amounts were done.
After this time the reaction was allowed to cool down, the
mixture was filtered, washed with hot toluene and the solvent of
the filtrate was finally removed under vacuum. The crude
product thus obtained was purified by flash chromatography
29,30
overoxidation.
During the cathodic potential sweep a reduc-
tion peak is present at 0.31 V probably related to reduction of
short oligomers formed during oxidation.
3
.2 Electropolymerization by potential cycling
In order to restrict the polymeric overoxidation the polymer was
generated by consecutive potential sweeps using a fresh mono-
meric solution and clean electrode, until an anodic potential limit
of 1.25 V. On the clean Pt electrode the oxidation/polymerization
starts at 1.10 V. During the second potential cycle, the presence
of some polymer shifts the beginning of the oxidation–poly-
merization to 1.00 V. After 12 consecutive potential cycles, vol-
tammetric responses of which are shown in Fig. 2, three
oxidation processes are present: a maximum at 0.43 V, a shoulder
at 0.69 V and the fast increase of current after 1.10 V. The two
peaks are due to the polymer film oxidation and the final current
(
silica gel, hexane/dichloromethane 7 : 3) to yield 94 mg (19%) of
THBTD as a white solid.
1
H-NMR (DMSO-d , 200 MHz): d [ppm] ¼ 6.67 (ABsystem,
6
4
4
H, J ¼ 4.3 Hz, Th), 4.41–4.34 (m, 4H,–CH–O–C ), 4.22–
AB
Th
.13(m, 2H,–CH
2
–O–CTh).
13
C-NMR (DMSO-d
6
, 50 MHz): d [ppm] ¼ 141.1 (CTh–O),
1
40.4(CTh–O), 100.4 (CTh), 100.3 (CTh), 71.1 0(Cbridge), 64.4
–O).
MS (GC) (m/z): 282 (M ).
(
CH
2
+
ꢀ1
FTIR (ATR, cm ): 3110, 2923, 2855, 1739, 1484, 1369, 1188,
702, 982, 914, 858, 759.
1
2
Calculated analysis for C H O S : C, 51.05%; H, 3.57%; S,
4 2
1
2
10
2.71%; found C, 51.25%; H, 3.48%; S, 22.81%.
ꢁ
Mp (hexane/dichloromethane): 243–244 C.
3
. Results and discussion
3
.1 Voltammetric characterization of THBTD
ꢀ1
Voltammetric responses performed at 20 mV s using a bare
platinum electrode are depicted in Fig. 1. The dotted line shows
the voltammetric response of the background electrolyte (0.1 M
6
TBAPF dichloromethane solution). The background electrolyte
oxidation starts on a clean Pt electrode at 1.75 V (dotted line).
The solid line was obtained from 1 mM THBTD plus 0.1 M
6
TBAPF dichloromethane solution. On the anodic sweep an
irreversible monomeric oxidation starts at 1.20 V with simulta-
neous electrogeneration of a blue film on the Pt electrode. The
Fig. 2 (a) Cyclic voltammograms (25 cycles) obtained in a 1 mM
2
THBTD and 0.1 M TBAPF + dichloromethane solution using a 1 cm
6
ꢀ
1
Fig. 1 Cyclic voltammograms obtained in 1 mM THBTD and 0.1 M
electrode, switching potential from ꢀ0.50 to 1.25 V. Scan rate, 20 mV s
;
ꢁ
TBAPF
6
dichloromethane (full line) and in the background electrolyte
temperature, 20 C. Inset: enlargement of monomer oxidation during the
first (continuous line) and second (dotted line) potential cycles. (b)
Charge evolution during potential cycling.
2
dashed line) using a 1 cm Pt electrode; initial potential, 0.5 V; scan rate,
(
ꢀ
1
0 mV s ; temperature, 25 C.
ꢁ
2
4
946 | J. Mater. Chem., 2012, 22, 4944–4952
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rise is related to the monomer oxidation–polymerization.
During the cathodic sweep only a wide reduction peak is present
at 0.54 V.
every new monomeric unit, just the value expected from the
ideal polycondensation of the radical cations mechanism. The
ꢀ
1
productivity of the electropolymerization charge (mg C ) is
ꢀ1
Then on the consecutive cycles the potential gap between the
oxidation peak and the shoulder becomes shorter, due to the fast
anodic shift of the maximum (from 0.40 to 0.76 V). Finally both
processes overlap into one anodic maximum at 0.76 V. The
voltammetric charge, obtained from the voltammetric area under
those maxima, increases with the number of cycles indicating the
growth of the polymer by electrogeneration of a new polymer
during every anodic sweep. Fig. 2b shows the evolution of the
charge with consecutive potential cycles indicating that polymer
generation is getting slower as potential cycles are performed.
The presence, for consecutive cycles, of a thicker film of a semi-
conducting material between the metal and the film–solution
interface, where the monomer oxidation occurs, constitutes
a rising resistance for the current flow; the monomer oxidation
starts at rising anodic potentials with flow of lower currents at
the same overpotential on the consecutive cycles. In parallel to
the polymer growth electrochromic changes are observed: blue
colour during oxidation to clear orange colour during reduction.
The colour contrast decreases with the film thickness. In situ
spectroelectrochemical studies will be presented later on.
After 110 potential cycles, the polymerization charge was
lower now but the stored specific charge (C g ) in the film
is higher.
3
.3 Galvanostatic electropolymerization
The electropolymerization also was initiated by monomer
oxidation under flow of constant anodic current densities
ꢀ2
ranging from 0.01 to 0.2 mA cm kept for the time required to
consume 100 mC every time. The evolution of the electrodic
potential was simultaneously followed by recording the chro-
nopotentiometric responses (Fig. 3). The average oxidation–
polymerization process occurs at increasing potentials for
increasing anodic currents, ranging from 1.17 V using 0.01 mA
ꢀ
2
ꢀ2
cm to 1.40 V under flow of 0.2 mA cm . The electrogenerated
films were controlled by cyclic voltammetry in the background
solution in order to obtain the stored charge. Then the mass
of the reduced polymer, after rinsing and drying the coated
electrode, was obtained (Table 1).
For a constant polymerization charge the electrogenerated
mass of the reduced polymer increases (Table 1) for increasing
1
00 mC (calculated as charges from the anodic voltammetric
sweeps minus charge from the cathodic voltammetric sweeps).
The charge stored in the film is obtained by integration of the
characterization voltammogram performed in the background
solution (Table 1). The generation procedure is stopped every
time at the cathodic potential limit in order to get a reduced
polymer. The coated electrode was rinsed and dried. The mass of
the reduced and dried polymer was obtained by ‘‘ex situ’’
microgravimetry. The productivity of the consumed charge is the
mass of the reduced polymer electrogenerated per consumed
ꢀ1
charge unit (mg C ). The number of electrons consumed per
monomeric molecule incorporated into the polymer, calculated
from the value of the productivity, was 1.6 (Table 1), quite far
away from 2.25, as required for a typical polycondensation of
monomeric radical cations with the subsequent loss of two
protons per monomeric molecule incorporated into the polymer
Fig.
3
Electropolymerization chronopotentiograms obtained from
dichloromethane solution using
1
mM THBTD and 0.1 M TBAPF
6
31
chains.
2
a clean 1 cm Pt electrode, a fresh solution and by flow of a constant
By repeating the polymerization up to a higher anodic
potential limit of 1.45 V, 2.5 electrons are required to incorporate
anodic current density: (a) 0.01, (b) 0.05, (c) 0.1 and (d) 0.2 mA cmꢀ2
ꢁ
every time. Temperature 20 C.
Table 1 Deposited mass, anodic charge obtained from the characterization voltammogram and calculated productivity, charge storage and electron per
monomer consumed during electropolymerization using different values of potential or current from a 1 mM THBTD and 0.1 M TBAPF
dichloromethane solution extracting 100 mC of charge
6
+
Applied potential
limit (V vs. Ag/AgCl)
Productivity/
ꢀ
Specific charge/
ꢀ1
Electrons per
monomer unit
1
Deposited mass/mg
Anodic charge/mC
mg C
C g
1
1
.25
.45
0.1839
0.1157
3.60
4.87
1.8
1.2
19.6
42.1
1.6
2.5
Current applied/
ꢀ
Productivity/
ꢀ1
Specific charge/
ꢀ1
Electrons per
monomer unit
2
mA cm
Deposited mass/mg
Anodic charge/mC
mg C
C g
0
0
0
0
.01
.05
.10
.20
0.1055
0.1308
0.1507
0.1277
5.08
4.41
4.37
3.91
1.1
1.3
1.5
1.3
48.1
33.7
29.0
30.6
2.8
2.2
1.9
2.3
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ꢀ2
current densities up to 0.1 mA cm , then decreases. The
productivity of the consumed charge follows a parallel evolution,
while both specific stored charges and the number of electrons
consumed to incorporate a monomeric unit follow an inverse
ꢀ2
evolution, decreasing up to 0.1 mA cm and then increasing.
3
.4 Complex electropolymerization mechanism
Opposed variations on productivities and specific stored
charges in films electrogenerated under different experimental
conditions, and very low number of consumed electrons per
monomeric unit incorporated into the chains of the polymer film
(
Table 1) suggest the presence of parallel processes generating
a polymer through a chemically initiated polymerization not
consuming charge. In an organic solvent having a very low
protophylic nature the initiation reaction can be the monomer
Fig. 4 UV-Vis spectra, obtained at different times indicated, corre-
6
sponding to 0.001 M THBTD and 0.1 M TBAPF dichloromethane
32,33
protonation.
This hypothesis is corroborated by the corre-
solution where (a) acid was added (0.17 M HClO ) and (b) not.
4
lation between decreasing productivities (Table 1) of the
ꢀ1
consumed charges, from 1.8 to 1.1 (mg C ), with increasing
number of electrons consumed to incorporate every monomeric
unit into the film polymer chains, from 1.6 to 2.8 electrons per
monomer, and increasing specific electroactivity of the films able
ꢀ1
to store the rising specific charge, from 19.6 to 48.1 (C g ).
The maximum electroactivity for the poly(THBTD) films of
ꢀ
1
4
8.1 (C g ), measured as the stored specific charges, is lower
ꢀ
1
than that obtained for PEDOT films, 73.0 C g , generated using
the same conditions.
By accepting the polycondensation of radical cations as a basic
mechanism for the electropolymerization, the incorporation of
every new monomeric molecule into the polymer chains liberates
two protons from the electrode and decreases the monomer
concentration at the electrode surface. A fast increase of the
proton concentration near the electrode occurs. The high
concentration gradient promotes a diffusion flow of protons
towards the electrolyte bulk just against the monomeric diffusion
flow from the solution to the electrode. The high protophylic
nature of the monomer, compared with that of the solvent or
ions, will promote its protonation and possible polymerization
by chemical initiation.
Fig. 5 FTIR-ATR spectra obtained for poly(THBTD) and monomer
THBTD.
In order to check this hypothesis, two monomeric solutions
were put in two UV-Vis cells, one used as a reference and in the
ꢀ3
second HClO
protons. Spectra were recorded after 0, 3, 8, 13, 18, 23 and
8 min. Experimental results are presented in Fig. 4. Increasing
concentrations of short neutral chains, giving a narrow peak, are
4
was added to get a 10 M concentration of
Table 2 Assignation of infrared vibration in the THBTD monomer and
poly(THBTD) spectra
2
34
Wavenumber/
ꢀ
observed from 300 to 500 nm. The experiment was repeated in
the absence of acid, Fig. 4b, in order to corroborate the absence
of any other process. In this way experimental variations of
productivities, stored charges and the number of electrons per
monomeric molecule under different conditions of synthesis can
be understood as a result of the parallel chemical polymerization
initiated by monomeric protonation.
1
cm
Assignments
3
5,36
5
1
628
1484
C]C aromatic stretching
polymer
3
C]C aromatic stretching monomer
(C–C)ring stretching, (C]C)ring stretching
36,37
1435
1369–1365
1211
polymer
29,37
(C–C)ring stretching
Doping induced band polymer
both
29
29,37
1188
(C]C)ring stretching, (–COROC–) stretching
monomer
29,37
(–COROC–) stretching
1
072
982
14
858
both
Doping-induced band polymer
29
3
.5 Polymer structural characterization
36,37
(–COROC–) stretching
9
monomer
29,38
Fig. 5 shows the FTIR spectra of poly(THBTD) electrogenerated
in the presence of TBAPF and dichloromethane compared with
the spectrum of the monomer THBTD powder, in the range
(C–S)ring stretching
monomer
ꢀ
6
P–F stretching (PF ) polymer
39
849
759
694
6
29,38
CH out of plane deformation monomer
(C–S–C)ring in plane deformation polymer
29
ꢀ1
1
900–600 cm . In Table 2 are listed the main vibrations
4
948 | J. Mater. Chem., 2012, 22, 4944–4952
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observed. The proposed assignments reported in the literature
based on similar substituted polythiophenes (mainly PEDOT)
oxidation maxima at 0.07 V and 0.54 V and only a reduction
maximum at 0.50 V. The last two processes form a very reversible
29,36,37
0
are given.
Vibrations at 1628, 1435 and 1365 in the polymer and 1484,
redox couple with E ¼ 0.52 V, corresponding to a p-doping
process (oxidation). This value is higher than that observed for
0
ꢀ
1
1
369 and 1188 cm from the monomer are attributed to C]C
and C–C stretchings in the thiophene ring. Further vibrations
a PEDOT film produced under the same conditions E ¼ ꢀ0.25 V
(Fig. 6a), but with a higher degree of irreversibility. The higher
potential value of the poly(THBTD) redox process (0.75 V
compared to PEDOT) can be interesting as a battery cathode,
obtaining a higher battery potential.
ꢀ1
from the C–S bond in the thiophene ring can be seen at 694 cm
ꢀ1
in the polymer and at 858 cm in the monomer. Vibrations at
ꢀ1
1
072 from both samples and at 1188 and 914 cm in the
monomer are assigned to stretching in the alkylenedioxy group.
Finally, due to the oxidized state of the polymer sample vibration
By changing the cathodic potential limit voltammograms
shown in Fig. 6b were obtained. Starting the potential sweep
from ꢀ0.50 V and sweeping towards cathodic potentials
a current maximum is observed at ꢀ2.89 V during the first cycle.
During the anodic potential sweep two oxidation maxima are
present at 0.52 and 1.39 V. The first maximum corresponds to the
same process observed at 0.54 V (Fig. 6a), now showing a higher
current density. Similar results were related to the charge trap-
ꢀ
ꢀ1
P–F from PF6 counterion is also observed at 849 cm .
3
.6 Electrochemical control of the electrogenerated
poly(THBTD) films
A polymer film was generated by flow of a constant anodic
ꢀ2
current density of 0.05 mA cm for 2000 s through a fresh
monomeric solution, with consumption of a charge of 100 mC.
Fig. 6 shows the voltammetric responses of this film in 0.1 M
TBAPF6 acetonitrile solution using different potential limits.
Fig. 6a shows, in the potential range from ꢀ1.00 to 0.75 V, two
40,41
42
ping effect in different polymers: PEDOT,
polypyrrole or
43
thiophene. The charge stored by polymer reduction (n-doping)
during the maximum at ꢀ2.86 V only is reoxidized here, with
a great overpotential. The maximum at 1.39 V can be related
to polymer degradation by overoxidation or reticulation as
indicated by the fact that over 80% of the redox charge decreases
between the first and the second (dotted line) consecutive
voltammetric responses.
3
.7 Electron microscopy characterization of poly(THBTD)
Fig. 7 shows the SEM image of poly(THBTD) electrodeposited
on a platinum electrode. The polymer presents a smooth and
dense aspect.
3
.8 Mass variation during potential cycling
In order to investigate the ionic exchange during potential
ꢀ2
cycling, thin polymeric films (only a charge of 50 mC cm was
consumed during electropolymerization) were electrogenerated
on the quartz crystal microbalance electrode by flow of
ꢀ2
a constant current density of 0.05 mA cm through a fresh
monomeric solution. In the background solution the experiment
was started, Fig. 8a, from the oxidized state of the material at
0
.75 V, going down to ꢀ1.00 V and back to the initial potential.
During the cathodic potential sweep the mass of the polymer film
Fig. 6 Cyclic voltammograms of the poly(THBTD) film in 0.1 M
ꢁ
ꢀ
1
TBAPF
6
acetonitrile solution at 20 mV s and at 20 C. (a) From ꢀ1.00
V to 0.75 V (discontinuous line is a PEDOT film generated in the same
conditions and characterized in the same medium). (b) First (—) and
second (---) cycles using initial potential ꢀ0.50 V, first limit potential
ꢀ
3.0 V and second limit potential 2.0 V.
Fig. 7 SEM image of poly(THBTD) coated on a platinum electrode.
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Fig. 8 (a) Mass change obtained from a film electrogenerated on
Fig. 9 (a) UV-Vis spectra of the poly(THBTD) film deposited in ITO at
different indicated potentials. The colour of the polymer in reduced and
oxidized states is shown in the pictures. (b) Absorbance changes at three
characteristic wavelength values indicated of the poly(THBTD) during
the redox transformation in the potential range between ꢀ0.50 and 1.0 V.
a quartz crystal microbalance electrode during potential cycling in the
ꢀ
1
background electrolyte at 50 mV s between 0.75 V and ꢀ1.00 V. (b) Plot
of mass vs. charge to obtain, from the slope, the apparent molar mass of
the species multiplied by the Faraday constant.
decreases until ꢀ0.20 V, remains constant later on, and is
recovered by oxidation back to 0.75 V. This variation suggests
the expulsion, or entrance, of balancing anions during reduction,
or oxidation, required to compensate the positive charges
eliminated, or generated, respectively, along the polymer
structure of the polymer network in comparison with the
linear PEDOT may produce shorter conjugation lengths
independently of the polymer length. By oxidation of the film at
rising anodic potentials the intensity of the band at 481 nm
decreases and new bands showing increasing absorption are
formed at 750 nm and around 1000 nm. The colour of the film
changes to deep blue (inset picture). So, electrochromic changes
in poly(THBTD) films are opposed to those observed in PEDOT
films, deep blue for oxidised poly(THBTD) and for reduced
PEDOT.
ꢀ1
chains. Apparent molar masses of 89 g mol during p-dedoping
ꢀ1
and 90 g mol for p-doping processes were obtained (Fig. 8b).
ꢀ
ꢀ1
Both values are smaller than 145 g mol of the PF
6
anion,
requiring the movement of at least one solvent molecule
ꢀ1
ꢀ1
(
41 g mol ) in the opposite direction to the anion’s (104 g mol
of apparent molar mass). Similar results were obtained using
Fig. 9b shows the evolution of the three absorption bands with
the applied potential. The increased absorbance at 750 nm is
generally attributed to radical-cations (polarons), while the
optical density increasing at larger wavelengths (ꢃ1000 nm) is
widely connected with the di-cationic (bipolaronic) form, also
observed in the parent PEDOT polymer at similar wave-
44
45
PEDOT in the same solvent or in propylene carbonate but in
a shorter potential range. In the same potential range used here
a mixed ionic exchange has been observed, with predominant
4
6
47
cationic exchange, for PEDOT and other modified PEDOT.
This difference in the ionic exchange can be the origin of the
different voltammograms and lower specific charges stored in
poly(THBTD), compared to those from PEDOT films. Exclusive
anion exchange can be useful for some applications as artificial
muscles giving a larger bending amplitude of the movement by
consumption of the same charge.
49,50
lengths.
The polaronic band, goes through a maximum at
0.65 V, while the absorbance of the bipolaronic band shows
a continuous increase with the anodic potential up to 1.0 V.
3
.9 Spectroelectrochemical characterization
Fig. 9a shows the electronic spectra for the poly(THBTD) film on
ITO for different oxidation states. The material, after reduction
at ꢀ0.50 V for 20 seconds, presents a strong interband p–p*
transition at 481 nm, attributed to the polythiophene backbone,
and a clear orange colour (inset picture). This maximum is
shifted to a lower wavelength related to that of reduced PEDOT
48
films (590 nm) indicating a lower effective conjugation
compared with PEDOT, probably due to a less flat structure of
THBTD compared with that of EDOT. By comparing
the absorption spectra from linear oligoEDOT derivatives
3
4
(
monomer, dimer, trimer and tetramer) conjugation lengths
longer than 4 monomeric units (with an absorption maximum at
51 nm) may be present in the polymer chains. Nevertheless, the
Fig. 10 (a) UV-Vis spectra of a poly(THBTD) film (deposited on ITO)
after polarization at different indicated potentials inside the n-doping
range. (b) Poly(THBTD) absorbance evolution at three wavelengths
during the redox transformations between ꢀ0.30 and ꢀ1.95 V.
4
polymer length and the effective conjugation length in this kind
of polymer networks are not directly related. The less flat
4
950 | J. Mater. Chem., 2012, 22, 4944–4952
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observed for PEDOT films. This effect can be related to a most
cross-linked poly(THBTD) network promoting a higher oxida-
tion potential. Poly(THBTD) films present electrochromic
changes from orange in the neutral state to deep blue when
oxidized. Poly(THBTD) and PEDOT films show similar thermal
stability.
Acknowledgements
We acknowledge financial support from Spanish Government
(
MCI) Projects MAT2008-06702 and CTQ2010-14982, Seneca
Foundation Project 08684/PI/08, Consejer ꢀı a de Educaci oꢀ n de
Murcia and Plan Regional de Ciencia y Tecnolog ꢀı a 2007–2010,
Comunidad Aut oꢀ noma de Madrid (S2009/MAT-1467) and
a UCM-BSCH joint project (GR58/08). J.A.P. thanks Ministerio
de Ciencia e Innovaci oꢀ n of Spain for Juan de la Cierva grant JCI-
Fig. 11 Thermogravimetric analysis curves of (—) poly(THBTD) and
(
/) PEDOT.
2
008-02022. A.P. thanks Universidad Complutese de Madrid for
As shown in Fig. 10, lower absorbance variations were
a predoctoral fellowship.
observed from ꢀ0.3 to ꢀ1.95 V, that means during the expected
polymeric reduction or n-doping. Starting from ꢀ0.30 up to
Notes and references
ꢀ
1.80 V a small increase of p–p* transition at 481 nm is
observed but no polaron or bipolaron bands are observed. When
a reduction potential of ꢀ1.95 V is applied a strong absorbance
related to p–p* transition at 481 nm is observed, together with
the development of a polaronic band at 750 nm. The absorbance
at 1000 nm presents a small increase. A similar behavior was also
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.10 Thermal stability of poly(THBTD)
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52
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. Conclusions
1
1
0
0
0
A new modified EDOT monomer, 2,2 ,3,3 -tetrahydro-2,2 -
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