Preparation and electrochemical performance of TEMPO-modified polyterthiophene electrode obtained by electropolymerization

Article abstract of DOI:10.1016/j.elecom.2019.106623

A novel monomer of terthiophene with a side chain TEMPO, 4-(2,5-di(thiophen-2-yl)thiophen-3-yl)acetyl-oxy-2,2,6,6-tetramethylpiperidin-1-yloxy (TT-TEMPO) was successfully synthesized. Its corresponding polymer PTT-TEMPO was prepared via electrochemical polymerization on Pt electrode in boron trifluoride diethyl etherate solution. PTT-TEMPO film showed a uniform network structure. Compared with bare Pt electrode, PTT-TEMPO electrode exhibited high electrocatalytic performance for oxidation of benzyl alcohol in the presence of 2,6-lutidine under similar conditions. Benzyl alcohol was selectively oxidized to benzaldehyde based on in situ FTIR analysis.

Full text of DOI:10.1016/j.elecom.2019.106623

Journal Pre-proofs
Preparation and electrochemical performance of TEMPO-modified polyter-
thiophene electrode obtained by electropolymerization
Pengfei Niu, Yanchao Cai, Ming Guo, Zhenlu Shen, Meichao Li
PII:
S1388-2481(19)30286-3
DOI:
https://doi.org/10.1016/j.elecom.2019.106623
ELECOM 106623
Reference:
To appear in:
Electrochemistry Communications
Received Date:
Revised Date:
Accepted Date:
1 November 2019
28 November 2019
28 November 2019
Please cite this article as: P. Niu, Y. Cai, M. Guo, Z. Shen, M. Li, Preparation and electrochemical performance
of TEMPO-modified polyterthiophene electrode obtained by electropolymerization, Electrochemistry
Communications (2019), doi: https://doi.org/10.1016/j.elecom.2019.106623
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Preparation and electrochemical performance of TEMPO-modified
polyterthiophene electrode obtained by electropolymerization
Pengfei Niu Investigation Writing-Original drafta,b, Yanchao Cai Investigationa,b, Ming
Guo Investigationc, Zhenlu Shen Supervision Writing-review & editing Funding
acquisitiona,*,zhenlushen@zjut.edu.cn, Meichao Li Funding acquisitiona,b,*,
limc@zjut.edu.cn
aCollege of Chemical Engineering, Zhejiang University of Technology, Hangzhou
310032, P. R. China
bResearch Center of Analysis and Measurement, Zhejiang University of Technology,
Hangzhou 310032, P. R. China
cSchool of Science, Zhejiang Agricultural & Forestry University, Hangzhou 311300, P.
R. China
*Corresponding author.
Graphical abstract
Highlights
A novel terthiophene monomer with a side chain TEMPO (TT-TEMPO) was
synthesized successfully.
The polymer PTT-TEMPO film displayed a uniform network structure.
The PTT-TEMPO film electrode functioned well for selective oxidation of benzyl
alcohol to benzaldehyde.
Abstract: A novel monomer of terthiophene with a side chain TEMPO,
4-(2,5-di(thiophen-2-yl)thiophen-3-yl)acetyl-oxy-2,2,6,6-tetramethylpiperidin-1-yloxy
(TT-TEMPO) was successfully synthesized. Its corresponding polymer PTT-TEMPO
was prepared via electrochemical polymerization on Pt electrode in boron trifluoride
1

diethyl etherate solution. PTT-TEMPO film showed a uniform network structure.
Compared with bare Pt electrode, PTT-TEMPO electrode exhibited high
electrocatalytic performance for oxidation of benzyl alcohol in the presence of
2,6-lutidine under similar conditions. Benzyl alcohol was selectively oxidized to
benzaldehyde based on in situ FTIR analysis.
Keywords: terthiophene, TEMPO, electrochemical polymerization, benzyl alcohol
Introduction
The stable nitroxyl radical 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO) and its
derivatives have received considerable attentions in the past decades, because of their
low toxicity, high efficiency and selectivity in oxidation reactions [1-4]. It is
well-known that TEMPO can be successfully employed as the catalyst in synthesis of
many important compounds including aldehydes [5, 6], nitriles [7, 8] and imines [9, 10]
by chemical or electrochemical methods. However, the separation of TEMPO from the
reaction solution is difficult, and the TEMPO residue will contaminate the target
product [11].
One feasible strategy for solving this issue was to immobilize TEMPO onto solid
supports, such as silica [12, 13], carbon nanotubes [14, 15] and graphene oxide [16].
These immobilized TEMPO catalysts were often used for selective oxidation of
alcohols, and could be easily separated by centrifugation or filtration. Recently, the
electrochemical approach to immobilize TEMPO on the surface of electrode has
attracted more and more attention [17, 18]. It could be regarded as a plug-and-play
2

electrode and the separation of TEMPO could be well avoided. The substrate electrode
coated with a poly(acrylic acid) film modified with TEMPO was applied for series of
alcohol oxidation [19, 20].
Polythiophene, one of the attractive conducting polymers, has been extensively
investigated and widely applied in various electrical devices such as sensors,
electrochromic devices and solar cells [21-23]. However, the onset oxidation potential
of thiophene was relatively high and electrochemical activities of the polymer were
unsatisfactory in many cases [24]. One way to solve the problem is to extend the main
chain length of thiophene monomer. A terthiophene structure modified with acetic acid
was synthesized by Xu and co-workers [25]. Its corresponding polymer possessed
good electrochromic performance and redox stability. Owing to its virtues,
terthiophene structure might be an ideal backbone for constructing functional polymers
[26].
Herein, a novel terthiophene monomer with a side chain TEMPO,
4-(2,5-di(thiophen-2-yl)thiophen-3-yl)acetyl-oxy-2,2,6,6-tetramethylpiperidin-1-yloxy
(TT-TEMPO) was synthesized for the first time. The binding force of terthiophene
with the side chain TEMPO was quite strong via ester linkage. The corresponding
polymer PTT-TEMPO was prepared via electrochemical polymerization and the
thickness of the film could be easily controlled with the cycling number [27-29]. The
electrocatalytic performance for oxidation of benzyl alcohol was investigated and
discussed.
1. Material and methods
3

1.1 Synthesis of the monomer TT-TEMPO
TT-TEMPO was synthesized by the route displayed in Scheme 1.
3-Thiopheneacetic acid was treated with thionyl chloride (SOCl2), and then reacted
with methanol to afford methyl 2-(thiophene-3-yl)acetate. The compound methyl
2-(2,5-di(thiophene-2-yl)thiophene-3-yl)acetate was obtained by the electrophilic
bromination procedure using N-bromosuccinimide(NBS) as the bromination reagent.
Then, 2-(2,5-di(thiophene-2-yl)thiophene-3-yl)acetic acid was prepared via the Suzuki
coupling and hydrolysis reaction. The monomer TT-TEMPO was synthesized through
esterification reaction of 2-(2,5-di(thiophene-2-yl)thiophene-3-yl)acetic acid with
4-OH-TEMPO using 4-dimethylaminopyridine (DMAP) as the catalyst and
dicyclohexylcarbodiimide (DCC) as the dehydrating agent at room temperature. The
monomer TT-TEMPO was analyzed in electrospray positive ion mode on a GCT
premier (Waters) GC-MS/TOF (time-of-flight).
To characterize the structure of TT-TEMPO by NMR spectroscopy, TT-TEMPO
was reduced to TT-TEMPOH using isoascorbic acid according to the reported
1
literature [30]. H NMR (500 M, CDCl3) δ: 7.33-7.01 (m, 7H), 5.08 (s, 1H), 3.70 (s,
13
2H), 1.91 (d, J=10.0 Hz, 2H), 1.58 (t, J=11.2 Hz, 2H), 1.19 (d, J=8.8 Hz, 12H). C
NMR (125 M, CDCl3) δ: 168.8, 135.3, 134.4, 133.3, 130.5, 129.2, 126.4, 126.3, 125.3,
4

125.0, 124.8, 123.3, 122.4, 66.1, 57.8, 42.2, 33.7, 30.3. MS (ESI+), m/z, 462.1
[M+H+].
1.2 Preparation, electrocatalytic performance and characterization of PTT-TEMPO
Electropolymerization experiment was carried out in an undivided cell using a
CHI 600e electrochemical analyzer (CH Instrument Inc., USA). A conventional
three-electrode system was employed with an “L” type platinum disk electrode
(diameter: 3 mm) as the working electrode, a platinum sheet electrode (2.25 cm2) as
the counter electrode, and Ag/Ag+ electrode (0.1 M AgNO3 in acetonitrile (MeCN)
solution) as the reference electrode. PTT-TEMPO was prepared in boron trifluoride
diethyl etherate solution (15 mL) containing monomer (0.1 mmol) with anhydrous
sodium perchlorate (1.0 mmol) as the supporting electrolyte. The scanning potential
was set between 0 and 1.1 V at the scan rate of 25 mV/s. After polymerization, the
electrode was washed with MeCN and dried at room temperature. Unless otherwise
specified, all electrode potentials reported in this work were quoted to the Ag/Ag+
electrode.
The electrocatalytic activity of PTT-TEMPO electrode for oxidation of benzyl
alcohol was investigated in 15 mL of 0.1 M NaClO4/MeCN solution containing benzyl
alcohol (0.5 mmol) and 2,6-lutidine (1.0 mmol). Cyclic voltammetry was carried out
between 0 and 0.8 V at the scan rate of 50 mV/s.
PTT-TEMPO was characterized by FTIR using a Nicolet 6700 spectrometer
(ThermoFisher, America). The surface morphology of PTT-TEMPO film was
characterized using scanning electron microscopy (Zeiss Gemini500).
5

1.3 In situ FTIR spectroscopic experiment
In situ FTIR spectroscopy experiments were performed on 670 FTIR spectrometer
(ThermoFisher, America) equipped with a MCT-A detector cooled by liquid nitrogen.
A CaF2 disk (32 mm * 2 mm) was chosen as the IR window. The Pt disk electrode
(d=6 mm) was modified with PTT-TEMPO, and used as the working electrode. Two
hundred interferograms were collected at each sample potential, and the spectral
resolution was 8 cm-1.
2. Results and discussion
2.1 Characterization of PTT-TEMPO
Fig. 1(A) showed the FTIR spectra of TT-TEMPO and PTT-TEMPO. The peaks
appeared at 2974, 2936 and 2858 cm-1 in the spectrum (a) were assigned to the methyl
and methylene stretching vibration of the monomer TT-TEMPO [31]. The strong
intense band appeared at 1731 cm-1 was attributed to the C=O stretching vibration and
the peak at 1362 cm-1 was related to the stretching vibration of N-O• [32, 33]. The
peaks at 1236 and 1161 cm-1 were attributed to the stretching vibration of acetate
C(=O)-O-C [34]. The band at 1460 cm-1 was associated with thiophene ring [35].
When the polymer PTT-TEMPO was formed, wavelength shift of some peaks could be
observed. Compared with the spectrum of TT-TEMPO, the intensive vibration bands in
spectrum (b) at 3102 and 692 cm-1 as a result of α-H vibration of thiophene were nearly
disappeared [36]. Whereas, the peaks at 3069 and 835 cm-1 assigned to β-H vibration
were still remained [37]. The new band at 795 cm-1 attributed to β-H of thiophene ring
6

appeared in PTT-TEMPO, indicating that α,α’-bonding predominated during
electrochemical polymerization of TT-TEMPO [38, 39], as shown in Scheme 2. In
-
addition, the wide peak at 1111 cm-1 was possibly caused by doping of ClO4 ions into
the film during the polymerization process [40]. The band at 1385 cm-1 was assigned to
the C-C stretching vibration of polymer backbone [35]. With the interference from the
bands nearby, the peaks corresponded to the stretching vibration of N-O• and acetate
C(=O)-O-C in spectrum (b) were not obviously observed in the spectrum of
PTT-TEMPO.
The morphological characterization of PTT-TEMPO film was also studied as
shown in Fig. 1 (B). The surface of PTT-TEMPO film displayed a uniform and
homogeneous network structure, which may be beneficial to improve its performance.
2.2 Electrochemical performance of PTT-TEMPO electrode
The electrochemical polymerization of TT-TEMPO was performed by cyclic
voltammetry. The effect of cycling number on PTT-TEMPO performance was
investigated and shown in Fig. 2 (A). Cycle numbers 1, 4, 10 and 15 were chosen as
7

the representatives. The reversible redox peaks centered at about 0.37 V were observed
on PTT-TEMPO in 0.1 M NaClO4/CH3CN solution. It was corresponded to the one
electron transfer process between TEMPO and TEMPO+ [41]. With the increasing of
cycling number, the peak current increased gradually, implying the growth of
PTT-TEMPO on the surface of working electrode [42].
The electrocatalytic performance of PTT-TEMPO electrode (with cycling number 10)
for oxidation of benzyl alcohol in the presence of 2,6-lutidine as the base was
performed in 0.1 M NaClO4/CH3CN solution at the scan rate of 50 mV/s. As shown in
Fig. 2 (B), the typical redox peaks were observed in the blank solution with the
oxidation to reduction peak current ratio of about 1.0 (curve a). When benzyl alcohol
(0.5 mmol) was added to the solution, only a slight alteration could be observed in
curve b. So the electrocatalytic activity of PTT-TEMPO for oxidation of benzyl
alcohol was quite low. With the further addition of 2,6-lutidine (1.0 mmol) to the
solution, the oxidation peak current increased obviously, while the corresponding
current of reduction peak decreased. It showed that the base 2,6-lutidine could
accelerate the reaction between TEMPO+ and benzyl alcohol.
For comparison, the voltammetric study of bare Pt electrode for oxidation of
benzyl alcohol in the presence of 2,6-lutidine was carried out as shown in curve (d) Fig.
2 (B). On the bare Pt electrode, no redox peaks were observed over the potential range
between 0 and 0.8 V. It demonstrated that the oxidation of benzyl alcohol occurs at the
surface of PTT-TEMPO electrode instead of the substrate electrode Pt [43].
8

In order to further demonstrate the efficiency of the PTT-TEMPO electrode, the
electrolysis experiment was performed in an undivided cell containing 0.1 M
anhydrous NaClO4/CH3CN solution (15 mL), benzyl alcohol (0.1 mmol) and
2,6-lutidine (0.2 mmol) at the constant current 6 mA. After being drop-casted with 60
μL ethanol containing 10 v% Nafion, the PTT-TEMPO electrode (2.25 cm2) was used
as the working electrode [17]. The electrolytic reaction was monitored by gas
chromatography with peak area normalization method. It was found that benzyl
alcohol was selectively converted into benzaldehyde with 91% conversion in 1.5 h.
2.3 In situ FTIR investigation
To further investigation the electrochemical behavior of PTT-TEMPO electrode
for oxidation of benzyl alcohol, in situ FTIR spectroscopy was used and the electrode
potential was varied from 100 to 800 mV with an interval of 100 mV. The reference
potential was 0 mV, and the background noise was subtracted from all the FTIR
spectra. When the potentials were more positive than 200 mV, some characteristic
peaks appeared and strengthened gradually as shown in Fig. 3 (A). Three
negative-going bands at 1647, 1630 and 1586 cm-1 were assigned to ring stretching
vibration of 2,6-lutidinium cation [44]. The bands at 1280 and 1175 cm-1 were related
to ring stretching modes and C-H in-plane bending of 2,6-lutidinium cation [45]. It
confirmed that 2,6-lutidine received a hydrogen proton to generate 2,6-lutidinium
cation. Meanwhile, three important bands at 1702, 1312 and 1204 cm-1 were observed
clearly, which were attributed to C=O stretching vibration, CHO in-plane bending and
C-O stretching vibration of benzaldehyde, respectively [46, 47]. The weak upward
9

band of N-O • stretching vibration was observed at 1362 cm-1 [48]. But the
corresponding band of N+=O stretching vibration was not observed with the
interference from the bands nearby. Moreover, the band located at 1130 cm-1 was
-
assigned to ClO4 ions of supporting electrolyte [49].
Another experiment has been carried out to investigate the spectral changes with
increasing time at 400 mV. As shown in Fig. 3 (B), the corresponding characteristic
bands which were related to benzaldehyde and 2,6-lutidinium cation, were also
observed obviously. In addition, the intensity of these bands increased significantly and
no new bands emerged with the increasing of time. It showed that the PTT-TEMPO
electrode acted as an effective catalyst with high selectively for oxidation of benzyl
alcohol in the presence of 2,6-lutidine.
3. Conclusions
We successfully synthesized a novel TEMPO-modified terthiophene monomer
TT-TEMPO which was analyzed using FTIR, NMR and MS. The PTT-TEMPO film
was obtained by electropolymerization and showed a uniform network structure. It
functioned well as the active catalyst for selective oxidation of benzyl alcohol to
benzaldehyde in the presence of 2,6-lutidine by the techniques of cyclic voltammetry
and in situ FTIR spectroscopy. These basic and explored investigations will lead to
further boost this conducting polymer material for other applications such as solar cells
and sensors.
10

Acknowledgments
This work was supported by the National Natural Science Foundations of China
(21773211 and 21776260), and the Natural Science Foundation of Zhejiang Province
(LY17B060007).
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Fig. 1. (A) FTIR spectra of TT-TEMPO (a) and PTT-TEMPO (b). (B) SEM image of PTT-TEMPO
film.
Fig. 2. (A) Cyclic voltammograms of PTT-TEMPO electrode (prepared with different cycling
numbers 1, 4, 10 and 15) in 0.1 M NaClO4/CH3CN solution. (B) Cyclic voltammograms of
PTT-TEMPO electrode (with cycling number 10) in 0.1 M NaClO4/CH3CN solution (a); with
benzyl alcohol (0.5 mmol) (b); with benzyl alcohol (0.5 mmol) + 2,6-lutidine (1.0 mmol) (c);
Cyclic voltammogram of bare Pt electrode in 0.1 M NaClO4/CH3CN solution with benzyl alcohol
(0.5 mmol) + 2,6-lutidine (1.0 mmol) (d).
Fig. 3. (A) In situ FTIR spectra collected on PTT-TEMPO electrode during the oxidation of benzyl
alcohol in the presence of 2,6-lutidine in 0.1 M NaClO4/CH3CN solution at potentials varied from
100 to 800 mV. (B) In situ time-resolved FTIR spectra collected on PTT-TEMPO electrode during
the oxidation of benzyl alcohol in the presence of 2,6-lutidine in 0.1 M NaClO4/CH3CN solution at
400 mV.
Scheme 1. Synthesis of the monomer TT-TEMPO.
Scheme 2. Electrochemical polymerization of the monomer TT-TEMPO.
15