A study on the electro-oxidation and electropolymerization of a new OPE linear molecule by EQCM and in situ FTIR spectroelectrochemistry

Article abstract of DOI:10.1016/j.electacta.2010.09.026

A novel symmetric conjugated oligo(phenylene-ethynylene) (OPE) linear molecule (1,4-bis(4-aminophenylethynyl)benzene); BAB) was synthesized by Sonogashira cross-coupling reactions. The structure and purity of the compound were confirmed by ¹H N

Full text of DOI:10.1016/j.electacta.2010.09.026

Electrochimica Acta 56 (2010) 454–462
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
A study on the electro-oxidation and electropolymerization of a new
OPE linear molecule by EQCM and in situ FTIR spectroelectrochemistry
Jiao Luo , Meiling Liu , Qiangqin Zhao , Jie Zhao , Youyu Zhanga,∗, Liang Tana,
a
a
a
a
a
a
a,b,∗∗
a
Hao Tang , Qingji Xie , Haitao Li
Key Laboratory of Chemical Biology & Traditional Chinese Medicine Research (Ministry of Education, China),
College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081, PR China
, Shouzhuo Yao
a
b
Hunan Vocational Institute of Safety Technology, Changsha 410151, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 June 2010
Received in revised form 2 September 2010
Accepted 9 September 2010
Available online 16 September 2010
A
novel symmetric conjugated oligo(phenylene-ethynylene) (OPE) linear molecule (1,4-bis(4-
aminophenylethynyl)benzene); BAB) was synthesized by Sonogashira cross-coupling reactions. The
1
13
structure and purity of the compound were confirmed by H NMR, C NMR and infrared (IR) and mass
spectrometry (MS). The electrochemical oxidation process and mechanism of BAB were investigated
via in situ Fourier transform infrared (FTIR) spectroelectrochemistry and electrochemical quartz crys-
tal microbalance (EQCM). The electrochemical oxidation mechanism of BAB was proposed. The studies
revealed that the BAB concentration and oxidation potential had a significant influence on the growth of
the polymer film. A densely packed polymer film, which exhibited nonelectroactivity, was formed when a
high monomer concentration and a high oxidation potential were used. When the electropolymerization
of BAB was conducted at a lower concentration, a new pair of redox peaks appeared, and the resultant
thin film had better electroactivity. The in situ FTIR studies confirmed that BAB could be electro-oxidized
into radical cations and then electropolymerized via para (N-N) and/or ortho (N-C) coupling reactions to
form polymers with a larger conjugated ␲-electron system. The surface morphology of the poly-BAB was
also investigated with atomic force microscopy (AFM) and scanning electron microscopy (SEM).
Keywords:
In situ FTIR spectroelectrochemistry
EQCM
Sonogashira reaction
Oxidation mechanism
1
,4-Bis(4-aminophenylethynyl)benzene)
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
conductivity. With proper modification, OPEs can behave as molec-
ular resonant tunneling diodes (MRTDs) [8] or molecular memory
devices [9]. OPEs display a reversible negative differential resis-
tance (NDR) [10]. Most research has focused on the synthesis
of OPE-based functionalized hybrid oligomers with diverse and
tunable optical, electronic and photophysical properties [11–14].
The electronic behaviors of nitro-OPEs have been investigated
extensively [15–17]. However, the study of electro-oxidative poly-
merization of OPEs has not received the same attention. Real-time
information on the electro-oxidative polymerization of OPEs,
which is essential for understanding the properties of OPEs, has
not been reported.
Conventional electrochemical methods, which can only pro-
vide electronic parameters, are not sufficient to fully examine
the electrochemical oxidation and properties of a material. Simul-
taneous measurements with an electrochemical quartz crystal
microbalance (EQCM) and in situ Fourier transform infrared (FTIR)
spectroelectrochemistry can provide more complete and real-time
information, leading to a better description of a practical electro-
chemical process. EQCM, using a piezoelectric quartz crystal (PQC)
as its central sensing element, is a powerful tool. It is capable of
detecting an electrode mass change down to the monolayer or sub-
monolayer level, the viscoelasticity of a foreign film modified on
The continuous miniaturization of conventional silicon-based
electronic devices has encountered insurmountable problems due
to quantum effects, such as diffraction, tunneling and interference
of electrons [1]. Since the possibility of using organic molecules
as the functional units in nanometric electronic devices was
first theoretically demonstrated by Aviram and Ratner in 1974
[
2], researchers have focused on the construction of devices at
the molecular level, such as molecular wires, switches/storage
devices, diodes and field-effect transistors [3,4]. Conjugated organic
molecules have been investigated extensively for their potential
application in optoelectronic devices [5–7]. Among these con-
jugated compounds, oligo(phenylene-ethynylene) (OPE) and its
derivatives are of great interest because of their rigidity and ideal
∗
Corresponding author. Tel.: +86 731 88872046; fax: +86 731 88872046.
Corresponding author at: Key Laboratory of Chemical Biology & Traditional Chi-
∗
∗
nese Medicine Research (Ministry of Education, China), College of Chemistry and
Chemical Engineering, Hunan Normal University, Changsha 410081, PR China.
Tel.: +86 731 88872046; fax: +86 731 88872046.
E-mail addresses: zhangyy@hunnu.edu.cn (Y. Zhang), haitao-li@hunnu.edu.cn
(
H. Li).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.09.026

J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
455
TMSA, a
b
H N
I
TMSA
NH2
H
NH2
2
1
2
c
H
NH + I
I
H N
NH2
2
2
2
3
a: Pd(pph )Cl CuI, THF:i-Pr NH=4:1, reflux; b: K CO , THF:CH OH=1:1, r.t.;
3
2,
2
2
3
3
c: Pd(pph )Cl CuI, THF:i-Pr NH=5:1, reflux;
.
3
2,
2
Scheme 1. Synthesis of 1,4-bis(4-aminophenylethynyl)benzene.
an electrode and the viscodensity of a solution near the electrode
during an electrochemical process [18–20]. In situ FTIR spectro-
electrochemistry has been well established as a powerful tool to
investigate the orientation of chemical functional groups and the
bonding mode of the species on electrode surfaces [21].
The conjugated oligo(phenylene-ethynylene), 1,4-bis(4-
aminophenylethynyl) benzene (BAB, Compound 3, Scheme 1),
was synthesized via the Sonogahsira reactions in this work. The
electro-oxidation and electropolymerization processes of BAB
were monitored by EQCM and in situ FTIR spectroelectrochem-
istry. The mechanisms of these electrochemical processes were
investigated.
and the reference potential (0 V), respectively [23,24]. The upward
and downward bands in the ꢀS/S spectra were due to the loss and
gain of the absorbing species at the corresponding potentials.
A JSM-6360 scanning electron microscope (SEM) (JEOL Ltd.,
Japan) was used to study the surfaces of polymer films. The
atomic force microscope (AFM) measurements were performed
on a PicoPlus microscope (Molecular imaging Co., USA) with a n*-
silicon cantilever (type ppp-NCLR-50) with a resonant frequency
−
1
of 146–236 kHz, a force constant of 21–98 N m and a scan speed
−
1
of 0.8 lines s . All AFM images were taken under tapping mode
at ambient temperature in air. The structure and purity of BAB
were determined by NMR (Bruker Co., Ltd., Germany) and IR (Nico-
let Instrument Co., USA) and mass spectrometry (Shimadzu Co.,
Japan).
2
. Experimental procedures
Lithium perchlorate trihydrate (Sinopharm Chemical Reagent
◦
Co., Ltd. China) was dried in an oven under vacuum at 60 C for
2.1. Instruments and reagents
2
days and kept in an N2 atmosphere until use. After being thor-
oughly dried by the sodium and benzophenone, reagent grade
tetrahydrofuran (THF) and diisopropylamine were distilled under
a slightly positive pressure of nitrogen. All other reagents were
of analytical grade and were used as received without further
purification. Deionized water (Millipore Trading Co., Ltd., USA)
and freshly prepared solutions were used throughout the exper-
iments. Prior to the electrochemical experiments, the solution was
bubbled with ultra high-purity nitrogen gas for 15 min for deoxy-
genation. All electrochemical experiments were carried out at room
temperature.
The electrochemical experiments were performed on a CHI660C
electrochemical workstation (CH Instruments Co., USA) controlled
by the CHI660C software. The quartz crystal microbalance (RQCM,
Maxtek Inc., USA) was controlled using the Maxtek RQCM software.
The RQCM recorded the resonant frequency (f) and the motional
resistance (R) of the crystal via its high-performance phase-lock
oscillator circuit. The electrochemical measurements were con-
ducted on a conventional three-electrode cell. A 9-MHz AT-cut PQC
(
(
Beijing Chenjing Electronics Co. Ltd., China) with gold electrodes
6 mm in diameter) on both sides was used. One side of the PQC gold
electrode was in contact with the solution as the working electrode,
while the other side was located in a waterproof air compartment
as the sense electrode. A Pt foil and a KCl-saturated calomel elec-
trode with a salt bridge containing acetonitrile and 0.1 M LiClO4
were used as the counter electrode and the reference electrode,
respectively. All the potentials reported in the paper were related
to this reference electrode.
2.2. Procedure
2.2.1. Synthesis of BAB
BAB was prepared by palladium-catalyzed Sonogashira coupling
reactions, shown in Scheme 1.
Synthesis of compound 1. 4-Iodophenyl aniline (3.44 g,
In situ FTIR spectroelectrochemical measurements were per-
formed on a Nicolet Nexus 670 FTIR spectrometer (Nicolet
Instrument Co., USA) equipped with a liquid nitrogen-cooled
mercury–cadmium–telluride (MCT) detector. In situ electrochem-
ical experiments were carried out in a thin-layer spectroelectro-
chemical cell with a CaF2 window as the IR transparent window
with an incidence angle of 60 . A series of IR spectra was
synchronously collected in Fourier transform infrared reflection
absorption spectroscopy (FTIR-RAS) mode [22] during one cycle
15.7 mmol), Pd(PPh ) Cl
0.78 mmol), trimethyl silyl acetylene (TMSA; 2.4 mL, 16.5 mmol),
(0.55 g, 0.78 mmol), CuI (0.14 g,
3
2
2
diisopropylamine (i-Pr NH; 10 mL) and THF (40 mL) were added
2
◦
into a 100-mL flask. After being stirred at 60 C for 5 h under a N2
atmosphere, the reaction solution was poured into a saturated
solution of NH Cl and then extracted with ethyl acetate (EtOAc;
4
◦
3×). The organic layer was washed with brine (3×) and dried with
anhydrous MgSO , and the solvent was removed in vacuum. Using
4
petroleum ether and acetoacetate as the eluent and silica gel as the
adsorbent, the residue was purified by column chromatography
(Petroleum ether:EtOAc = 4:1). A brown solid product (Compound
potential, scanning in a 0.1 M LiClO -acetonitrile solution that con-
4
−
1
tained 2 mM monomer from 0 V to 1.0 V at a scan rate of 5 mV s
start potential: 0 V, vertex potential: 1.0 V). Each spectrum was
obtained by the co-addition of 38 interferograms at every 100 mV
with a resolution of 8 cm . Ominic software, version 6.0a (Nico-
let Instrument Co., USA), was used for processing the spectral data.
The measured spectra were subtractively normalized based on the
reference spectrum, according to ꢀS/S = (Ss − Sr)/Sr, where Ss and Sr
are the single beam spectra obtained at the investigation potential
(
1) was obtained (2.62 g, 88%). 1H NMR (500 MHz, CDCl ): ıH = 7.27
3
(2H, d, J = 8.8 Hz), 6.57 (2H, d, J = 8.8 Hz), 3.80 (2H, s), 0.23 (9H, s).
Synthesis of compound 2. Compound 1 (2.5 g, 13.2 mmol) and
K CO (18.3 g, 132 mmol) were dissolved in CH OH (30 mL) and
−1
2
3
3
THF (30 mL). After being stirred for 2 h at room temperature, the
reaction was stopped. The solution was poured into an NH Cl
aqueous solution and extracted with EtOAc (3×). The extract
4

4
56
J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
EE // VV v s. SS CC EE
EE/ V/ VVS v. sS SC CE E
0.4 0.6
VS
0
.0
0.2
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
0
0
150
300
-
800
-
-
Δ
-1600
2400
-
1
8
4
0
2
6
3
0
.2
.1
.0
Δ
0.24
0.16
0.08
0.00
0
0
0
.034
.017
.000
B
A
-0.017
0
.0
0.2
0.4
EE // VV V
0.6
s. SS CC EE
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
v
S
EE/ /VV vs. SS CC EE
VS
Fig. 1. Typical current (i), frequency shift (ꢀf) and resistance (ꢀR) responses for the PQC gold electrode during the 10-cycle potential sweeping between 0 V and 1.0 V in an
−
1
acetonitrile solution containing 2 mM (A) or 0.2 mM (B) BAB monomer with 0.1 M LiClO4 as the supporting electrolyte. Scan rate: 50 mV s
.
was then washed, dried, desolvated and purified in a treatment
3. Results and discussion
similar for Compound 1, and a brown-yellow solid product (Com-
pound 2) was obtained (1.43 g, 92%). 1H NMR (500 MHz, CDCl ):
3.1. Electro-oxidation and electropolymerization of BAB
3
ıH = 7.30 (2H, d, J = 8.8 Hz), 6.59 (2H, d, J = 8.8 Hz), 3.82 (2H, s), 2.96
(
1H, s).
Synthesis of compound 3. Compound 2 (330 mg, 2.82 mmol),
,4-diiodbenzene (443 mg, 1.34 mmol), Pd(PPh ) Cl (98 mg,
Fig. 1 shows the typical responses for the current (i), frequency
(ꢀf) and motional resistance (ꢀR) of the PQC gold electrode dur-
ing cyclic voltammetric scanning in solutions containing 2 mM
(A) or 0.2 mM (B) BAB monomer in a conventional electrochem-
ical cell. In Fig. 1A, the CVs exhibit a large and broad oxidation
peak beginning at 0.58 V during the first cycle, which is similar
to the oxidation of aniline in the first cycle [26,27]. The oxida-
tion peak could be assigned to the oxidation of amine groups. In
the first positive potential scan, a significant decrease in the ꢀf
and a simultaneously increase in the ꢀR were observed at 0.80 V,
which indicated the occurrence of a polymerization reaction and
the deposition of a substance on the electrode. Before the oxidation
peak appeared in the first positive potential scan, a slow and dura-
tive decrease in the frequency was found in Fig. 1A and B. Because
the polymerization and substance deposition onto the electrode
did not occur at this stage, this effect could be attributed to the
adsorption of BAB molecules onto the electrode. This deduction is
reasonable when considering the conjugated ␲-electron system of
the BAB molecules and the increasing positive charge on the elec-
trode surface with the increasing scanning potential. In the reversal
potential scan for the first cycle (Fig. 1A), the frequency only slightly
increased as the scanning potential became more negative. After
the first cycle, ꢀf was more than 800 Hz, and a densely compact
orange-brown film was visible on the electrode surface. During
the subsequent potential scan, the current and the polymerization
rate (expressed as ꢀf/ꢀt) decreased gradually as a result of the
block in the electron transfer by the formation of polymer film.
After 10 cycles in the solution containing 2.0 mM BAB monomer,
1
0
3
2
2
.14 mmol) and CuI (27 mg, 0.14 mmol) were dissolved in diiso-
◦
propylamine (6 mL) and THF (30 mL). Stirred overnight at 60 C
under a N atmosphere, the solvent was evaporated in vacuum. The
crude product was purified by column chromatography (Petroleum
ether:EtOAc = 3:1 and 2:1) and yielded an orange-yellow solid
2
+
+
(
330 mg, 80%, Compound 3). MS (EI): 309 (M +1, 100), 308 (M +1,
1
3
7). H NMR (500 MHz, CDCl ): ıH 7.43 (4H, s), 7.39 (4H, d,
3
13
J = 8.2 Hz), 6.64 (4H, d, J = 8.2 Hz), 3.84 (4H, s). C NMR (100 MHz,
CDCl ): ıC 147 (2C), 133 (4CH), 131 (4CH), 123 (2C), 115 (4CH), 112
3
(
1
2C), 92.0 (2C≡), 87.3 (2C≡). IR: 3437, 3368, 2208, 1617, 1521, 1284,
−1
178, 1143, 834 cm
.
2
.2.2. Electrochemical measurements
To remove possible surface contaminations, the PQC gold elec-
trode was treated with fresh piranha solution (H SO :H O = 3:1,
v/v) and then underwent cyclic scanning between 0 V and 1.5 V
in 0.2 M HClO4 solution until reproducible cyclic voltammograms
2
4
2
2
(
CVs) were obtained [25]. The poly-BAB films were prepared on
the pre-treated PQC gold electrode by cyclic voltammetry in the
−
1
potential range of 0–1.0 V at a scan rate of 50 mV s in an acetoni-
trile solution with 0.1 M LiClO4 as the supporting electrolyte in the
presence of 2 mM or 0.2 mM monomer (start potential: 0 V, vertex
potential: 1.0 V). The poly-BAB film-modified electrodes were pre-
pared, then thoroughly rinsed with copious deionized water and
dried in a N2 stream for further measurements.

J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
457
0
0
.1 mM
0
0 V-0.6 V
V-0.7 V
0
0
.2 mM
-
800
-
800
0 V-0.8 V
1
mM
Δ
-
-
1600
2400
-
-
1600
2400
A
B
0 V-1.0 V
2
mM
0
100
200
300
400
500
0
100
200
t / s
300
400
500
t / s
Fig. 2. The frequency changes during the 10-cycle potential sweeping in the potential range of 0–1.0 V in an acetonitrile solution containing the BAB monomer at different
concentrations: 0.1 mM, 0.2 mM, 1 mM and 2 mM from top to bottom (A). The frequency changes in a 1.5 mM monomer solution at different potential scan ranges: 0–0.6 V,
−
1
0
–0.7 V, 0–0.8 V, 0–1.0 V from top to bottom (B). Scan rate: 50 mV s
.
the ratio of ꢀf/ꢀR, calculated from the EQCM measurement, was
ferent chain lengths [37,38]. Therefore, the pair of redox peaks in
Fig. 1B could be attributed to dimers produced after the radical
cation coupling reaction.
−
279 ± 3 Hz/ꢁ. The absolute value of this ratio is much larger than
that obtained from the net density/viscosity change of the solu-
tion based on the Martin equation (∼10 Hz/ꢁ) [19]. It could be
inferred that the mass effect predominates the EQCM frequency
response during the BAB electro-oxidation and electropolymeriza-
tion [19,28]. The mass of the deposited polymer was calculated to
be 1.37 ± 0.01 ␮g by Sauerbrey equation [29].
The EQCM responses for the ꢀf and ꢀR shown in Fig. 1B
are similar to that in Fig. 1A. The frequency declined during
the oxidative polymerization, and the main decrease in the fre-
quency occurred during the reverse sweep for the case of lower
monomer concentration. The ꢀf was −341 ± 5 Hz and the ꢀf/ꢀR
was −50.20 ± 0.74 Hz/ꢁ after ten cycles. The absolute value of the
ꢀf/ꢀR ratio was larger than ∼10 Hz/ꢁ, and the EQCM response
remained predominated by the mass effect during the oxidative
polymerization. As a result, the mass of the polymer deposited
on the electrode was calculated to be 169 ± 2.49 ng by Sauerbery
equation.
To further investigate the influence of the concentration and
the potential range, the polymerization reactions were conducted
at different monomer concentrations across the potential range
0–1.0 V (Fig. 2A) and at different potential ranges while maintaining
a monomer concentration of 1.5 mM (Fig. 2B). With the increase in
the concentration of the monomer, the change in the PQC frequency
(ꢀf) became significant, and the highest ꢀf value was obtained in
the first cycle at each monomer concentration (Fig. 2A). A signif-
icant decrease in the PQC frequency was observed only when the
monomer concentration was at least 1 mM and the scanning poten-
tial was higher than 0.8 V (Fig. 2A and B). The compact polymer
film deposited onto the electrode could be produced only in a high
concentration monomer solution and at a high scanning poten-
tial. Combined with the previous results, we concluded that the
BAB molecules were first adsorbed onto the PQC surface. Then the
adsorbed molecules underwent oxidization reactions to generate
the corresponding radical cations, were further polymerized and
were deposited onto the PQC electrode surface at a higher potential.
The polymer film deposited under the conditions shown in
Fig. 1A was strongly adhered onto the gold electrode surface and
could not be easily removed even upon rinsing with fresh piranha
solution. In addition, the frequency remained almost unchanged
during the potentiostatic degradation in 0.2 M H SO or NaOH solu-
2
4
tion, indicating that the resultant compact polymer was difficult
to be degraded under the tested conditions. This thick poly-BAB-
modified electrode exhibited nonelectroactivity in 0.2 M HClO4
and in blank acetonitrile solution. This suggests that the transfer
of electrons was blocked by the polymer film formed from the
electropolymerization of the BAB linear conductive molecules. To
obtain information about the electro-oxidation and electropoly-
merization of BAB, we needed to sustain the conductivity of the
poly-BAB-film-modified electrode at a certain level. To achieve this,
the formation rate and the compactness of the deposited poly-
BAB film were reduced by lowering the monomer concentration
to 0.2 mM BAB.
As shown in Fig. 1B, the onset of a large current response began
at 0.80 V during the first positive potential scan, corresponding to
the oxidation of BAB in the lower concentration solution (0.2 mM).
During the subsequent potential scan, a new pair of redox peaks
appeared at 0.69 and 0.63 V, which differed from that in Fig. 1A.
By increasing the number of scans, the current intensity and peak-
to-peak separation gradually increased, suggesting an increase in
the electroactive species on the PQC electrode. These peaks are
clearly different from the oxidation peak of the monomer. Gener-
ally, the electro-oxidation of aniline, benzene and their derivatives
are reported to occur via the initial formation of radical cations at
the first step [30–32]. In non-aqueous media, the deprotonation
step of these radical cations is always followed by a dimerization
reaction either in the presence of the parent compound or another
base, and the resultant dimers are more easily oxidized [33–36]. It
has been reported that the dimers generally predominate at rela-
tively lower concentrations, and the additional coupling reactions
may occur at higher concentrations, producing oligomers with dif-
3.2. The electrochemical properties of poly-BAB
Fig. 3 shows the CVs of the thin poly-BAB-film-modified elec-
trode prepared under the conditions shown in Fig. 1B at different
scan rates in monomer-free acetonitrile solution. The anodic and
cathodic current intensities and the peak-to-peak potential sepa-
ration (ꢀEp) increased with an increase in the scan rates. However,
the peak current ratio (ipa/ipc) and the conditional potential were
independent of the scan rates. In Fig. 3, the oxidation peak cur-
rent increased linearly with the scan rates within the range from

4
58
J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
at 0.52 and 0.47 V was observed in the CVs. The reduction peak cur-
rent increased linearly with the scan rates from 40 to 400 mV s ,
which demonstrated a surface-controlled electrochemical process
was being observed. Fig. 4B shows the CVs of the thin film-modified
1
1
1
8
5
2
9
6
3
−1
0
0
0
0
.12
.08
.04
.00
-1.0142142857
.0413757143
.99976289389
0
0
electrode in an HClO -NaClO4 solution with different pHs in the
4
−1
potential range of 0–0.7 V at a scan rate of 50 mV s . In Fig. 4B,
one pair of redox peaks was observed with the thin poly-BAB-
film-modified electrode in a strong acid solution. The redox peaks
currents decreased with the increase in the solution pH, and the
peaks almost disappeared as the pH approached 5.1. It was inferred
from this result that the modified electrode exhibited little elec-
troactivity when the pH was higher than 5, suggesting that the thin
poly-BAB film exhibited better electrochemical activity in a strong
acid solution.
1
00 200 300 400
-1
v / mV s
Ferrocene carboxylic acid, potassium ferricyanide and ruthe-
nium trichloride hexahydrate were chosen as electrochemical
probes to investigate the behavior of poly-BAB-modified electrodes
in aqueous and non-aqueous solutions. If the electroactive probe is
able to diffuse to the underlying electrode surface through the poly-
mer defect site, then a Faradic current will be observed. The shape
of the CVs can provide valuable information about the surface of
the film-modified electrode. Prior to the test, different poly-BAB-
film-modified electrodes were prepared by varying the deposited
polymer mass, represented by the ꢀf value in the PQC frequency.
Fig. 5 shows the cyclic voltammograms of the pre-prepared (bare)
electrodes and the 500 Hz and 1000 Hz deposited polymer-film-
modified electrodes in different solutions with the electrochemical
probes. The current responses decreased while the poly-BAB film
formed on the electrode surface (Fig. 5). As the degree of poly-
merization increased, a thicker film was gradually formed, and the
current responses decreased rapidly. The electron transfer between
the electrochemical probes and the electrode surface were inhib-
ited to a certain extent whether for a thin poly-BAB film (500 Hz)
or relatively thick poly-BAB film (1000 Hz) in aqueous and non-
aqueous solutions. Good current responses were still obtained on
-
0.04
0
.0
0.2
0.4
0.6
0.8
1.0
EE// VV S.sS SC CE E
V
v
Fig. 3. CVs of the poly-BAB-modified electrode in a monomer-free acetonitrile solu-
tion at scan rates from 100 to 400 mV s . The inset shows the relationship between
−
1
ipa and v.
00 mV s-1 to 400 mV s-1, which indicates that it was a typical
surface-controlled electrochemical process. A pair of oxidation-
1
reduction peaks at 0.69 and 0.63 V was observed with an |ipa/ipc|
−
1
ratio approximately equal to 1. At a scan rate of 100 mV s
,
the ꢀEp was 60 mV, which was near that for a mono-electron
reversible redox reaction [38]. Therefore, the electrochemical pro-
cess of the poly-BAB in an acetonitrile solution was a mono-electron
reversible redox process. The observed peak pair was similar to
the pair that appeared during the electro-oxidization of 0.2 mM
BAB in the acetonitrile solution (Fig. 1B). According to our previ-
ous analysis, the oxidation–reduction peaks at 0.69 V and 0.63 V
have already been defined and are ascribed to dimers produced
by the coupling of radical cations. Hence, the electrochemical
responses of the poly-BAB-modified electrode can be attributed to
the redox of the monomer and dimer embedded in the polymer
film.
5
00-Hz-polymer-modified electrode in both the PBS and the ace-
tonitrile solutions containing ferrocene carboxylic acid. A large
capacitance current was also obtained in the presence of ruthenium
chloride as a positive-charged electrochemical probe. However,
when using Fe(CN)63 as the electroactive probe, only a small
Faradic current was observed with the same poly-BAB electrodes.
This suggests that the poly-BAB film could be a negatively charged
film, which may result from the large ␲-conjugated electron system
of poly-BAB.
−
Fig. 4A shows the current responses of the thin poly-BAB-film-
modified electrode prepared under the conditions shown in Fig. 1B
in a 0.2 M HClO solution at various scan rates. A pair of redox peaks
4
0
0
0
0
.045
.030
.015
.000
pH=0.02
pH=1.04
pH=2.07
1
0
.0866512516
0. 0. 02 28 81 12 29 92 26 68 81 1
.9 09 . 89 79 8 07 38 90 03 29 02
1
2
9
6
3
0
0
0.016
pH=3.01
pH=4.02
pH=5.01
0
0
.008
.000
0
100 200 300 400
-
1
v / mV s
-
0.008
A
B
-
0.015
0
.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
EE // VV vs. SSCCE E
EE // VV vs. SS CC EE
VS
VS
−
1
Fig. 4. CVs of the poly-BAB-modified electrode in a 0.2 M HClO4 solution at scan rates from 40 to 400 mV s with an inset showing ipc as a function of v (A). The CVs for the
−
1
electrode in 0.2 M NaClO4 with different pHs in the potential range of 0–0.7 V vs. SCE at a scan rate of 50 mV s (B).

J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
459
bare gold ee lec tt rode
a 500-Hz pp olymer
a 100 00 -- HH zz pp oo ll yy mm ee rr
bare go ll dd ee ll ee cc tt rr oo dd ee
a 500-Hz pp oo ll yy mm ee rr
a 1000-Hz pp olymer
0
0
0
.02
.01
.00
0
0
0
.06
.03
.00
-
0.03
-
0.01
A
B
0
.2
0.4
0.6
0.8
1.0
0.0
0.2
0.4
0.6
EE/ V/ VVS vv .s SSS CCC EEE
EE /// VVV VvS ss . SS CC EEE
0.015
bare gol dd electrode
a 50 00 -- HH zz pp oo ll yy mm er
a 10 00 00 -- HH zz pp oo ll yy mm ee rr
bare gold electrode
a 500-Hz polymer
a 1000-Hz polymer
0
0.010
0
.005
.000
0
0
-
-
0.005
-
0.08
C
D
0.010
-
0.2
0.0
0 .. 22
00 .. 44
00 .. 66
0.1
0.2
0.3
0.4
0.5
E E/ V/ VVS vv. sS SC CE E
EE // VV vVsS .S SC CE E
Fig. 5. CVs of the different poly-BAB-modified electrodes in a 0.1 M LiClO4 acetonitrile solution containing 2 mM ferrocene carboxylic acid (A) and in a 0.2 M PBS + 0.1 M
−
1
NaClO4 (pH 7.4) containing 0.5 mM ferrocene carboxylic acid (B), 2 mM potassium ferricyanide (C) or 2 mM ruthenium chloride (D). Scan rate: 50 mV s
.
3
.3. Polymer film morphology
trochemistry was conducted in a thin-layer spectroelectrochemical
cell. Similar CV responses were obtained in the thin-layer cell as
that in the conventional cell. Fig. 7 contains a sequence of simul-
SEM images for the 300-Hz, 2000-Hz poly-BAB films are dis-
−
1
played in Fig. 6A and B, respectively. Fig. 6C and D are the related
AFM images. The SEM and AFM images show the surface morphol-
ogy of the deposited poly-BAB-polymer film. The peak-to-valley
height, which reflects the thickness of the film to a certain extent,
was obtained from the AFM image with the color palette. In
Fig. 6A and C, the 300-Hz poly-BAB film appears to be composed
of close-packed, approximately spherical polymers, which seem to
be coalesced with their neighbors. However, for the 2000-Hz poly-
BAB film, more compact and homogeneous surface images were
observed (Fig. 6B and D). The peak-to-valley values of the 300-Hz
poly-BAB film and the 2000-Hz poly-BAB film were 275 ± 5 nm and
taneously collected FTIR-RAS spectra in the 800–2300 cm region
during the first potential cycle in an acetonitrile solution containing
−
1
2 mM monomer (scan rate: 5 mV s ). The downward bands refer
to newly formed and/or an increase in the species, and the upward
bands refer to a decrease or disappearance of the species in the thin
layer cell. The IR transmission spectrum of the monomer and the
reflectance spectrum of the resultant polymer were also collected
(Fig. 8).
In Fig. 7, the spectrum at the potential of 0.05 V refers to the
unoxidized adsorbed BAB. When the potential was 0.65 V, a broad
−
1
band at approximately 1650 cm appeared, and it was assigned
to free carriers (unpaired electrons and positive charges) [39],
which shift to a lower wave number in large conjugated sys-
tems. This indicates the formation of radical cations during the
first stage of oxidation process. When the potential was increased
to 0.85 V, several upward peaks at 2210, 1609, 1526, 1300, 1180
8
25 ± 5 nm, respectively. These observations clearly indicate that
the deposited poly-BAB film became more compact, smoother and
thicker as the polymerization degree increased. The difference in
packing may have resulted from the difference in the degree of
polymerization.
−
−
1
and 1146 cm
and 1121 cm
and downward peaks at 1620, 1597, 1380, 1221
appeared. These peaks did not disappear during
1
3
.4. Mechanism studies by in situ FTIR spectroelectrochemistry
the reverse scan, and the intensity of the IR bands increased with
the scan cycles, which suggest that the thickness of poly-BAB film
increased.
To clarify the electro-oxidation mechanism and to obtain
real-time structural change information, in situ FTIR spectroelec-

4
60
J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
Fig. 6. SEM (A and B) and AFM (C and D) images of the 300-Hz (A and C), 2000-Hz (B and D) poly-BAB-film-modified electrodes.
Accompanied with the decrease of the C–N bond (1300 cm 1), the
C and C N double bonds increased, indicating the transforma-
tion from C–N to C C and C N. In addition, a broad downward peak
−
Compared with the IR transmission spectrum of the BAB
monomer, the upward peaks at 2210, 1609, 1526 and 1300 cm
−
1
C
were assigned to the stretch vibration of C C, the N–H defor-
mation vibration of a primary amine, the aromatic C C and the
stretch vibration of the C–N bond [40], which corresponds to the
disappearance of the monomer. The upward peaks at 1146 and
−
1
below 1000 cm
appeared, which corresponds to the in-plane
−1
−1
1
180 cm and the downward peaks at 1221 and 1120 cm could
110
also be attributed to a change in the conjugative structure of the
−
1
benzene ring in the OPE [41]. The downward peaks at 1620 cm
monomer
−1
100
and 1597 cm that arose at a higher potential were assigned to
the characteristic stretch vibration of C C and C N bonds [42,43].
1
143
9
0
2208
1
178
1284
0
0
0
0
0
0
.00
.02
.04
.06
.08
.10
80
70
8
34
1
521
0
0
.05 V
.65 V
3437
3368
1
617
9
6
1
609
1
300
polymer
ΔS/S
2210
0.85 V
0.85 V
1
526
1
180 1146
95
1
620
1
380
1
597
1
221
1
121
1
650
2205
1400
0
0
.65 V
.05 V
9
9
4
3
1178
1
515
839
3376
1600
2
000
1500
Wave numbers (cm )
1000
4
000
3000
2000
1000
-
1
-
1
Wavenumbers (cm )
Fig. 7. In situ FTIR subtractive spectra from the first electro-oxidation cycle of BAB
in an acetonitrile solution containing 2 mM monomer in a thin-layer spectroelec-
trochemical cell. Scan rate: 5 mV s , potential range: 0–1.0 V.
Fig. 8. IR transmission spectrum of the monomer and the reflectance spectrum of
the resultant polymer.
−1

J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
461
H N
NH2
2
e-
H N
NH2
2
+
mon
-
o
m
+
2H
para
c
ortho coupling
( b )
(
(
a )
N
N
H N
NH
2
2
H
H
N
H
N
H
N
H
N
H
N
H
e-
N
H
N
H
N
H
+
-
(
2H / e )
N
H
N
N
N
H
(
b ))
H N
NH2
2
N
NH2
H
H
N
H
NH2
N
N
H
+
H / 4e-
4
N
N
H
N
N
H
Scheme 2. Suggested reaction pathways for the oxidative polymerization of BAB at the PQC gold electrode in an acetonitrile solution.
bending vibration of 1,2,4-substituted rings [44] and indicates that
some coupling with carbon may have occurred at position 2 of
the aromatic ring. It is likely that the electrochemical polymeriza-
tion proceeds via ortho (C–N) coupling reactions, and a new ring is
formed. However, N-N coupling cannot be discarded because the
tra and the reflectance spectrum of the resultant dimer or polymer.
−
1
Meanwhile, the IR adsorption peak near 1650 cm , assigned to
the characteristic C C stretch vibration in the monomer, shifted
−
1
−1
from 1617 cm to 1600 cm when the monomer was polymer-
ized (Fig. 8). This observation refers to the formation of a larger
conjugated ␲-electron system and confirms the occurrence of
−1
N
N bond at 1380 cm [44] is present in both the in situ FTIR spec-

4
62
J. Luo et al. / Electrochimica Acta 56 (2010) 454–462
BAB polymerization. In summary, the electro-oxidation and elec-
tropolymerization mechanism can be interpreted as follows: BAB
is initially electro-oxidized to form radical cations and then under-
goes para (N-N) or ortho (N-C) coupling reactions to give a linear
chain-like polymer (Scheme 2).
[10] I. Kratochvilova, M. Kocirik, A. Zambova, J. Mbindyo, T.E. Mollouk, T.S. Mayer, J.
Mater. Chem. 12 (2002) 2927.
[
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563.
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7.
3
9
[
[
13] K.L. Chandra, S. Zhang, C.B. Gorman, Tetrahedron 63 (2007) 7120.
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4
. Conclusion
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[
16] K.H. Jung, E. Hase, Y. Yasutake, H.K. Shin, Y.S. Kwon, Y. Majima, Jpn. J. Appl. Phys.
5 (2006) 840.
A novel symmetric linear molecule, (1,4-bis(4-aminophenyl-
4
ethynyl)benzene) conjugated molecule (BAB), was electro-
oxidized into radical cations and then electropolymerized via
para (N-N) or ortho (N-C) coupling reactions to form a polymer
with a larger conjugated ␲-electron system. The concentration
and oxidation potential influenced the growth of the polymer
film. The thick poly-BAB film formed immediately and exhibited
nonelectroactivity when BAB was electro-oxidized in solutions
with a relatively high monomer concentration, and the dimer
was produced in solutions with a relatively low monomer con-
centration. The resultant thin poly-BAB-film-modified electrode
exhibited better electroactivity and a specific repulsion effect
[17] S.U. Kim, B.S. Kim, J.C. Park, K. Shin, Y.S.K. Current, J. Appl. Phys. 6 (2006) 8.
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[
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[
[
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3−
for Fe(CN)6 . The AFM and SEM images also revealed that the
poly-BAB film appeared to be more compact and smoother with
an increase in degree of polymerization.
[
27] M.L. Liu, M. Ye, Q. Yang, Y.Y. Zhang, Q.J. Xie, S.Z. Yao, Electrochim. Acta 52 (2006)
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This work was supported by the National Natural Science Foun-
dation of China (21075037, 20975037, 20905025) and the Scientific
Research Fund of Hunan Provincial Science and Technology Depart-
ments (09JJ3019, 07JJ3024).
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