Synthesis and characterization of monomeric and polymeric Cu(II) complexes of 3,4-ethylenedioxythiophene-functionalized with cyclam ligand

Article abstract of DOI:10.1016/j.electacta.2010.08.075

A functionalized EDOT derivative with 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand pendant to the ethylene bridge (4) and its complexes [M(4)(BF₄)₂], where M(II) = Cu(II), was prepared and characterized. Their electrochemical copolymerization with EDOT was studied. The electro-co-polymerized films were characterized by electrochemical methods, X-ray photoelectron spectroscopy and by X-ray fluorescence spectroscopy. The co-polymerization method was found to afford a good control of the metal concentration in the polymer matrix and represents a good technique for preparing electronically conductive polymers containing redox-active metal complexes.

Full text of DOI:10.1016/j.electacta.2010.08.075

Electrochimica Acta 56 (2010) 326–332
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Synthesis and characterization of monomeric and polymeric Cu(II) complexes of
3
,4-ethylenedioxythiophene-functionalized with cyclam ligand
Kuhamoorthy Velauthamurtya,b, Simon J. Higgins , R.M. Gamini Rajapakse
H.M.N. Bandara , Masaru Shimomura
a
a
b,c,∗
,
b
c
Department of Chemistry, University of Liverpool, Crown Street, Liverpool L69 7ZD, UK
Department of Chemistry, University of Peradeniya, Peradeniya, Sri Lanka
Research Institute of Electronics, Shizuoka University, 3-5-1 Jyouhoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 June 2010
Received in revised form 12 August 2010
Accepted 17 August 2010
Available online 16 September 2010
A functionalized EDOT derivative with 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand pendant to the
ethylene bridge (4) and its complexes [M(4)(BF4)2], where M(II) = Cu(II), was prepared and characterized.
Their electrochemical copolymerization with EDOT was studied. The electro-co-polymerized films were
characterized by electrochemical methods, X-ray photoelectron spectroscopy and by X-ray fluorescence
spectroscopy. The co-polymerization method was found to afford a good control of the metal concen-
tration in the polymer matrix and represents a good technique for preparing electronically conductive
polymers containing redox-active metal complexes.
Keywords:
Cyclam
©
2010 Elsevier Ltd. All rights reserved.
3
,4-Ethylenedioxythiophene
Copolymer
Polymeric Cu(II) complexes of
3
,4-ethylenedioxythiophene
1
. Introduction
ble intermediate containing a nickel–carbon bond [8,9]. Provided
that the Ni-macrocycle entity remains intact throughout the reac-
tion sequence, the complex would have the role of a catalyst in
the reduction of alkyl halides to radicals or carbanions. This rich
chemistry of nickel and its importance in catalytic processes have
prompted a number of studies which involve the reduction of alkyl
halides by nickel complexes [10–12]. Further, Ni-cyclam modified
polyaniline electrodes were used as electrocatalysts for methanol
oxidation in basic medium [13]. Our interest thus aroused due to
the possibility of synthesizing EDOT functionalized with cyclam
to achieve the goal of preparing robust polythiophene materi-
als containing covalently attached copper–cyclam complexes. This
could afford a route to electropolymerized, catalytically active poly-
mer films for the copper coupling reactions. Similar complexes of
tetraaza macrocycles bearing pendant thiophenes with nickel have
been prepared and successfully electropolymerized [14].
Poly(3,4-ethylenedioxythiophene) (PEDOT) is a particularly
stable polythiophene derivative [2,15]. The electron rich oxygen-
bridge in EDOT unit greatly lowers the redox potentials of both
the monomer and the polymer compared to bare thiophene [1].
The main advantage of (co)polymerization of metal complexes
with pendant thiophene units is the less likelihood for oxidative
degradation of the complex [16]. Also the successful polymer-
ization of copper(II) cyclam based complexes has been achieved
and their possible use in various applications such as in radio
pharmaceutical area has been disclosed [17]. We have therefore,
Among electronically conductive polymers, poly (3,4-
ethylenedioxy thiophene (PEDOT) has fascinated specific attention
due to its excellent environmental stability, high conductivity,
low band gap and attractive electrochromic properties [1–3]. The
monomer EDOT has a lower oxidation potential compared to
other thiophenes and hence it could be electropolymerized from
aqueous solutions. Consequently, the chemistry of the monomer
3
,4-ethylenedioxythiophene (EDOT) has been investigated thor-
oughly in recent years with the aim of developing synthetic routes
to prepare tailor-made functionalized EDOT derivatives. We have
shown recently that functionalized EDOT derivatives can be used
as building blocks for the design and synthesis of biosensors. One
such specific example is the preparation of biotin-functionalized
poly(3,4-ethylenedioxythiophene) coated microelectrode that
responds electrochemically to avidin binding [4].
In the present study, we have chosen cyclam, a square planer
macrocyclic system, because it forms complexes of high thermody-
namic and kinetic stability with transition metals [5–7]. The Ni(I)
complexes can react rapidly with alkyl halides to form an unsta-
∗ Corresponding author at: Department of Chemistry, University of Peradeniya,
Peradeniya, Sri Lanka. Tel.: +94 81 2389129; fax: +94 81 2389129.
E-mail address: rmgr@pdn.ac.lk (R.M.G. Rajapakse).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.08.075

K. Velauthamurty et al. / Electrochimica Acta 56 (2010) 326–332
327
developed synthetic routes to prepare cyclam functionalized EDOT
2.3.1. Synthesis of 4-((6-bromohexyloxy)methyl)-
2,2-dimethyl-1,3-dioxolane (1)
The compound 1 was prepared by the method as reported earlier
[4]. The product obtained was sufficiently pure to be used in the
derivatives and their Cu(II) complexes and the materials thus pre-
pared were fully characterized. Subsequently, the materials were
co-electro-polymerized with EDOT in non-aqueous acetonitrile
medium. We have chosen a non-aqueous medium due to the fact
that although PEDOT itself is aqueous-compatible, but the redox
processes of the metal complexes which we have appended to
PEDOT do not all occur within the potential ‘window’ afforded
by aqueous electrolytes. The polymers obtained were also fully
characterized. In this publication we reveal the details of these
studies.
+
next step. Yield 16.23 g, 73%. MS (ES+), m/z: calc. for [C1 H O Br] :
2
24
3
1
295.09088. Found: 295.090154. H NMR (400 MHz): ı 1.35, 1.41 (s,
6H, 2CH ), 1.35–1.50 (m, 4H, –CH CH –), 1.55 (m, 2H, J 7.4, –CH –),
3
2
2
2
9
1.88 (q, J 7.4, –CH –), 3.41 (t, 2H, J 6.5, H ), 3.41–3.50 (overlapping
multiplets, 4H, H ), 3.72 (m, 1H, J1,1 8.1, J1,2 6.4, H ), 4.05 (dd, 1H,
J 6.45, J1,1 8.1, H1 ), 4.25 (m, 1H, H ). C{ H} NMR (100.6 MHz):
ı 25.8, 27.2, 28.3, 29.75, 32.9, 34.1, 67.3, 71.9, 72.3, 75.2,109.7.
2
3
,4
ꢀ
1
ꢀ
ꢀ
2
13
1
2
. Experimental
2.3.2. Synthesis of 3-(6-bromohexyloxy)propane-1,2-diol (2)
The compound 2 was prepared by the method reported
elsewhere [4]. Yield 0.78 g, 91%. Anal. calcd. for C H O Br: C,
Polymer preparation and characterization: For all electrochem-
9
19
3
istry experiments, the electrolyte used was tertabutylammonium
tetrafluoroborate (Bu NBF ) which was recrystallised and dried
under high vacuum for several hours prior to use. The solvent used
42.33; H,7.51%. Found C, 42.55; H, 7.59%. MS (ES+) m/z: calc. for
+
1
4
4
[C H O BrNa] : 277.0415. Found 277.0419. H NMR (400 MHz): ı
9
19
3
9
1.35, 1.45, 1.61, 1.88 (m’s, 2H each, –CH ), 3.41 (t, 2H, J 6.64, H ),
2
3
,4
was CH CN (99.97% electrochemistry grade). Solvent and the elec-
3.49–3.41 (overlapping m’s, 4H, H ), 3.68 (m, 2H, J
ꢀ
11.4, J1,2 7.8,
1,1
3
ꢀ
1,1
2
13
1
trolyte were handled under Schlenk conditions. A three-electrode
electrochemical cell having separate compartments for the work-
ing (Pt or glassy carbon, 0.1 mm diameter except for XPS, XRF
samples), auxiliary (Pt gauze), and a quasi-reference (polypyrrole-
coated Pt wire) electrode [18,19] were employed. The potentiostat
used was an Autolab PG-30 operated by GPES software (EcoChimie,
Netherlands). Potentials are quoted with respect to the Pt/PPy QRE,
which has the potential +0.225 V vs saturated calomel electrode in
this electrolyte.
H
), 3.85 (m, 1H, J2,3, J2,1 5.7, H ). C{ H} NMR (100.6 MHz): ı
25.7, 28.3, 29.7, 32.9, 34.1, 64.5, 71.0, 71.9, 72.7.
2
.3.3. Synthesis of 2,3,4,5-tetrabromothiophene
3
To a vigorously stirred solution of Thiophene (0.3 mol, 24 cm ) in
2 cm3 CHCl3 a solution of Br2 (69 cm , 1.35 mol) was added drop-
3
1
wise. The reaction mixture was kept at room temperature for 17 h
with stirring and then refluxed for further 2 h. After allowing to cool
3
to room temperature, KOH (33 g) in ethanol (180 cm ) solution was
added. The mixture was refluxed for 4 h, poured into an ice/water
3
2.1. X-ray photoelectron spectroscopy
mixture (400 cm ), and extracted with CHCl . Colourless crystals
3
of the desired product were obtained. This was then recrystallised
For XPS experiments, polymer samples were electrodeposited
from a CHCl /EtOH mixture.
3
on the 1.1 cm × 1.1 cm square gold on Cr-primed glass slides. A gold
wire was attached to one corner of the slide to act as a contact.
The gold slides were annealed in a butane flame prior to polymer
electrodeposition. Spectra were acquired using a Scienta ESCA 300
Yield: 97.44 g, 81.2%. Analysis: calc. for C Br S: C, 12.12%. Found:
4
4
C, 12.02%. MS (ES+) m/z: calc. for [C Br S]+: 395.6454. Found
4
4
395.6440. 13C{ H} NMR (100.6 MHz): ı 116.4, 110.2.
1
◦
spectrometer (NCESS, Daresbury Laboratory, U.K.) at 10 take-off
2
.3.4. Synthesis of 3,4-dibromothiophene
angle. The Al K␣ X-ray line (1486.6 eV) was used for photoelectron
excitation. The spectra were referenced to the Au 4d5/2 line posi-
tioned at 335.0 eV. In all cases, a survey scan (0–1300 eV) was run,
followed by high-resolution scans at characteristic peak positions
for all metals and N. For the polymer the XPS studies were carried
out at the Research Institute for Electronics, Shizuoka University,
Japan using ESCA-LAB Mk II (VG) with an Al/Mg twin anode X-ray
source.
To a 500 cm3 round-bottom flask containing a mixture of
3
acetic acid and water (1:2, 180 cm ), purified powdered zinc (60 g,
9
18 mmol) and 2,3,4,5-tetrabromo thiophene (113 g, 283 mmol)
were added alternatively in small portions. Then the resultant mix-
ture was kept stirring at room temperature for 4 h. It was then
filtered through a celite bed and the product from the filtrate
was extracted with diethyl ether, dried with Na SO , the solvent
2
4
removed and the product obtained distilled to separate the pure
product from the residue to result in a colourless liquid.
2.2. X-ray fluorescence spectroscopy
Yield: 42.31 g, 61.89%. Analysis: Calc. for C H Br : C, 20.0%; H,
4
2
2
+
0
.83%. Found C, 20.03%; H 0.94%. MS (ES+) m/z: calc. for [C H Br ] :
239.8244. Found 239.8250. H NMR (400 MHz): ı 7.29 (s, 2H).
4 2 2
1
For XRF experiments, the polymer samples were electrode-
13
1
posited on 1.0 cm × 0.5 cm FTO (fluorine doped tin oxide) coated
glass slides. A crocodile clip was attached to one corner of the
slide to act as an electrical contact. The FTO slides were sonicated
with acetone and dried in a N2 gas stream prior to polymer elec-
trodeposition. Subsequent to electrodeposition, FTO plates were
washed thoroughly with the electrolyte and their XRF spectra were
acquired using a Fisherscope X-Ray XAn Spectrometer (Fischer Sci-
entific) at an anodic current of 900 ␮A and maximum voltage of
C{ H} NMR (100.6 MHz): ı 124.18, 114.41.
2
.3.5. Synthesis of 3,4-dimethoxythiophene
Sodium (12.6 g) and 80 cm3 of dry methanol were added to
a 250 cm3 round-bottom flask fitted with a magnetic stirrer and
a reflux condenser with a calcium chloride dry tube. This was
then allowed to stir at room temperature for 15 min, potassium
iodide (173 mg), 3,4-dibromothiophene (23.31 g, 96.32 mmol), and
cupric oxide (7.83 g) were added and refluxed for 3 days. Potassium
iodide (173 mg) was then again added and the mixture refluxed
for another day, allowed to cool to room temperature and filtered.
The filtrate was diluted with two volumes of cold water and the
5
0 kV using a Ni primary filter.
2.3. Electrochemical impedance spectroscopy
The EIS data were recorded at selected DC bias potentials, each
of which was superimposed with a 10 mV AC potential in the fre-
quency range between 10 MHz to 0.1 Hz in the single sine wave
3
product extracted three times with 200 cm of ether. The collected
ether portions were washed with 200 cm3 distilled water, dried
with MgSO4 and evaporated. The crude product was purified by
mode in the 0.1 M Bu NBF /CH CN solution.
◦
means of low-pressure distillation, b.p. 95–101 C at 12 mbar.
4
4
3

3
28
K. Velauthamurty et al. / Electrochimica Acta 56 (2010) 326–332
Yield: 10.60 g, 76.45%. Analysis: Calc. for C H O S: C, 49.98%:
Yield: 1.24 g, 54.4%. Analysis: calc. for C23H42N O B F SCu: C,
4 3 2 8
6
8
2
H, 5.59%. Found: C, 50.05%: H, 5.66%. MS (ES+) m/z: calc. for
39.93%: H, 6.12%: N, 8.10%. Found C, 40.42%: H, 6.38%. N, 7.77%.
+
1
2+
[
(
C H O S] : 145.0323. Found 145.0321. H NMR (400 MHz): ı 6.19
MS (ES+) m/z: calc. for [C23H42N O BrCu] : 258.6131. Found
6
9
2
4
3
13
1
s, 2H, CH -thienyl), 3.85 (s, 6H, 2CH ). C{ H} NMR (100.6 MHz):
258.6136.
.4. Electrochemical (co)polymerization
Repetitive scan cyclic voltammetry at 100 mV s 1 was used to
2
3
ı 147.91, 96.36, 57.57.
2
2
2
.3.6. Synthesis of 2-((6-bromohexyloxy)methyl)-
−
,3-dihydrothieno[3,4-b][1,4]dioxine (3)
3
grow the polymer films and performed in a solution of EDOT
(0.01 M) or a solution containing the required copper complex
which was synthesized as described above and EDOT (1:5 mole
ratio; 0.01 M total monomer) in deoxygentated (N2 purged) 0.1 M
Bu4NBF4 in CH3CN. Ten cycles between −0.1 V and +1.5 V were used
to grow all polymer films. After the film growth, the electrode was
held at 0 V for 10 min and removed from the organic electrolyte,
To a 100 cm three-necked flask fitted with a condenser were
3
added dry toluene (20 cm ) followed by 3,4-dimethoxythiophene
(
(
(
1.00 g, 6.94 mmol), the 3-(6-bromohexyloxy) propane-1,2-diol
2.20 g, 8.66 mmol), p-toluenesulfonic acid (0.69 mmol) and water
◦
50 ␮L). The mixture was refluxed using an oil bath at 170 C for
2
0 min, the lid of the condenser was removed for 5 s while being
refluxed, and refluxing was continued for 16 h under a continuous
flow of nitrogen. Solvent was removed in vacuo and the residue
washed with CH CN, dried and transferred to fresh deoxygentated
3
3
3
(N2 purged) 0.1 M Bu4NBF4 in CH3CN. Cyclic voltammetry of the
polymer films was then undertaken in the absence of the respective
monomers.
was extracted into CH Cl (50 cm ), washed with water (5 × 5 cm ),
2
2
dried over MgSO , and filtered. The solvent was then removed and
4
the yellow oil was then adsorbed onto neutral alumina and eluted
with CH Cl /hexane (1:1) to separate the pure product as yellow oil.
2
2
3. Results
Yield 1.51 g, 69.7%. Anal. calcd. for C13H19O BrS: C, 46.57%; H, 5.71%.
3
+
Found C, 47.22; H, 5.78%. MS (ES+) m/z: calcd. for [C13H20O BrS] :
35.0317. Found: 335.0313. H NMR (400 MHz): ı 1.37, 1.44 (over-
3
1
3.1. Synthesis and characterization of EDOT-functionalized
ligands and complexes
3
lapping m, 4H, –CH –), 1.60 (m, 2H, –CH –), 1.86 (m, 2H, –CH –),
2
2
2
9
4
3
4
6
2
.41 (t, 2H, J 6.83, H ), 3.50 (t, 2H, J 6.5, H ), 3.65 (m, 2H, J 10.4,
.9, H ), 4.05 and 4.22 (m, 2H, J 11.7, 7.6, H ), 4.29 (m, 1H, H ),
ꢀ
ꢀ
3,3
1,1
2
The synthesis of cyclam-funcationalized EDOT monomer 4 is
shown in Scheme 1. In doing so the required chemicals were also
pre-synthesized and for the synthesis of 3,4-dibromothiophene the
literature procedure [20,21] was modified by slowly adding 2,3,4,5-
tetrabromothiophene and zinc powder alternatively at room tem-
perature for 4 h instead of refluxing and the modification afforded a
higher yield (89%). We have also modified the synthetic procedure
13
1
.335, 6.337 (AB, JAB 1.4, thienyl H’s). C{ H} NMR (100.6 MHz): ı
5.7, 28.3, 29.7, 33.1, 34.3, 66.6, 69.6, 72.2, 73.0, 99.9, 100.1, 141.9.
2
2
.3.7. Synthesis of (6-(2,3-dihydrothieno[3,4-b][1,4]dioxin-
-yl)methoxy)hexyl)-1,4,8,11-tetraazacyclotetradecane (4)
To 1,4,8,11-tetraazacyclotetradecane (cyclam) (1.8 g, 9.3 mmol)
[
4] for the preparation of 2-((6-bromohexyloxy)methyl)-2,3-
3
3
in refluxing benzene (50 cm ), pyridine (15 cm ) followed
by 4-(dimethyl amino)pyridine (catalytic amount) and 2-((6-
bromohexyloxy)methyl)-2,3-dihydrothieno [3,4-b][1,4]dioxine 3
dihydrothieno[3,4-b][1,4]dioxine, 3 where acid catalysed ether
exchange between 3,4-dimethoxythiophene and 3-(6-bromo
hexyloxy)propane-1,2-diol in the presence of a small amount of
water was adopted in order to prevented the Pinacol rearrange-
ment of the diol. This modification has enhanced the yield by 11%.
(
1.05 g, 3.1 mmol) were added dropwise while stirring. The solu-
tion was then refluxed for 12 h, after which it was allowed to cool
to room temperature and the solvent was removed. The residue
3
-(6-Bromohexyloxy)propane-1,2-diol was synthesized from reac-
tion of glycerine acetone ketal with excess 1,6-dibromohexane
followed by acid hydrolysis. Treatment of with 1,4,8,11-
3
was dissolved in 0.2 M aqueous NaOH (60 cm ) and extracted 10
times with diethyl ether (400 cm3 each). The ether extracts were
3
dried over MgSO , filtered and the solvent removed to yield an
4
tetraazacyclotetradecane (cyclam) in benzene/pyridine in the
presence of catalytic amount of 4-(dimethylamino)pyridine gave a
brown coloured semisolid which has been identified to be function-
alized EDOT-cyclam 4. Spectroscopic and analytical data indicated
that it was sufficiently pure to use as prepared. Reaction of 4
with [Cu(BF ) ·2H O] in MeOH gave sparingly soluble complexes,
orange/brown waxy oil. Yield: 0.92 g, 64.6%. Analysis: calc. for
C23H42N O S: C, 60.79%: H, 9.25%. Found C, 61.45%: H, 9.32%. MS
4
3
+
1
(
ES+) m/z: calc. for [C23H43N O S] : 455.3073. Found 455.3056.
H
4
3
1
NMR (400 MHz): ı 6.32 (m, 2H, thienyl H), 4.33–4.22 (m, 2H, H ,
H ), 4.05 (dd, 1H, J 7.59, J1,2, J 11.59, J1,1
J3,1, J 4.94, J3,3
1
2
ꢀ
ꢀ
, H1 ), 3.63 (dq, 2H, J 10.44,
H ,H ), 3.47(t, 2H, J 6.65 Hz, J , H ), 2.75–2.40 (m,
8H, cyclam-CH2 and H ), 1.75 (m, 9H, cyclam-CH and H ), 1.60
ꢀ
4
2
2
3
3
4
ꢀ
4,5
5
(M = Cu) respectively. The paramagnetic nature of the metal com-
9
5
2
plex 5 prohibited its characterization by NMR. Due to the presence
of long carbon chain of spacer group the single crystal growth of
this compound was also made impossible. However, microanalyt-
ical and mass spectrometric data are consistent with the expected
formulae, [M(BF ) (4) ].
9
6
7
8
13
1
(
(
q, 2H, J 6.64, J9,8, H ), 1.35–1.20 (m, 6H, H , H , H ). C{ H} NMR
2 1 3
100.6 MHz): ı 141.94, 100.0, 73.4 (C ), 72.2 (C ), 69.53 (C ), 66.68
4
9
5
8
(
C ), 52.45–44.23 (10C, C and 8 cyclam NC), 34.4 (C ), 33.6 (C ),
7
6
2
9.4 (C ), 25.6 (C ).
4
2
2
2
.3.8. Synthesis of {(6-((2,3-dihydrothieno[3,4-b][1,4]dioxin-2-
3.2. XPS characterization of the copolymer of EDOT and 5
yl)methoxy)hexyl)-1,4,8,11-tetraazacyclotetradecane}copper(II)
tetrafluroborate [Cu(4)(BF ) ] (5)
XPS spectrum of Cu 2p for the copolymer of EDOT and 5 is shown
in Fig. 1. The copolymer of EDOT and 5 shows a single and symmet-
rical peak for Cu 2p3/2 at the binding energy of 934.0 eV. The binding
energy of Cu 2p3/2 of the copolymer approximately agrees with that
of Cu(II) in CuO [22]. Existence of satellite lines (so-called shake up
lines) that are never present in the spectrum of Cu(I) and metal-
lic Cu evidently [22] indicates that the copolymer of EDOT and 5
contains Cu(II) ion. As a result of quantitative XPS analysis of the
copolymer of EDOT and 5, content percentages of Cu, N, S, and F
in the copolymer are estimated to be 0.9, 3.0, 4.7, and 7.6 atomic%,
4
2
3
To a 100 cm round-bottom flask fitted with a magnetic stir-
3
rer and a reflux condenser MeOH (20 cm ) and ligand 7 (1.33 g,
3
.1 mmol) were added and stirred and refluxed for 10 min.
3
[
Cu(BF ) ·2H O] (0.735 g, 3.1 mmol) in hot MeOH (30 cm ) was
4
2
2
then added dropwise. The reaction mixture was left stirring for
h for its completion and the pink solution obtained was filtered
1
and allowed to cool to room temperature. The oil was washed
with diethyl ether (10 cm ) and recrystallised three times from hot
MeOH–EtOH mixture.
3

K. Velauthamurty et al. / Electrochimica Acta 56 (2010) 326–332
329
Scheme 1. Schematic illustration of the synthetic route of 4.
respectively. Fluorine is detected in the copolymer of EDOT and 5,
as they may be located as a counter anion. Copper(II) fluoride whose
binding energy of Cu 2p3/2 is ∼937 eV [23] is excluded as the copper
compound in the copolymer. The ratio of N to Cu is 3.3. Taking into
account of the fact that the Cu 2p3/2 line appears as a single peak
and the quantitative analysis contains error of ∼1 atomic%, nearly
all Cu atoms may be located in the Cyclam. EDOT is functionalized
in every 5–6 units estimated from the ratio of S to N and Cu.
polymerization with unsubstituted monomer is a strategy that has
often been successful in cases where a functionalized monomer
does not electropolymerize alone [25–28] and we, therefore,
attempted to grow polymer films from electrolyte solutions con-
taining 5 and EDOT (5:1 mole ratio; 0.01 M total monomer
concentration) using repetitive scan cyclic voltammetry as shown
in Fig. 3 and the redox-active polymer films were deposited on the
electrode surfaces.
Fig. 3B shows a typical cyclic voltammogram for redox active
film containing the copolymer of EDOT and 5, the latter contains
cyclam coordinated to Cu(II) which is covalently bonded to EDOT
through the same spacer group. As expected for a surface localized
redox process the linear variation of peak current variation with
the scan rate is clearly obtained [29].
3
.3. XRF characterization of the copolymer of EDOT and 5
The XRF spectrum of copolymer of EDOT and 5 is shown in
Fig. 2 where Cu K␣ and S K␣ appear as single peaks at 8.05 keV
and 2.30 keV respectively and Cu K␤ and S K␤ can be observed at
The cyclic voltammograms obtained during electrodeposition
of the copolymer of EDOT and 5 on Platinum electrode (Pt) in
the non-aqueous medium are shown in Fig. 3A. The peak currents
8
.9 eV and 2.5 eV respectively. These values are in excellent agree-
ment with the theoretical values [24] and confirm the presence of
Cu and S in the polymer matrix.
increase due to polymerization of the monomers. In this CH CN
3
solvent, the cyclic voltammogram (CV) shows (Fig. 3B) a chem-
3
.4. Electrochemical synthesis of
ically reversible one electron oxidation peak centred at −0.03 V
poly(3,4-ethylenedioxythiophene) containing cyclam–copper
complexes
(vs Pt/PPy QRE) due to the Cu(I)/Cu(II) couple and this one elec-
tron oxidation peak in the CV is similar to that reported earlier for
copper complexes with poly-aza macrocycles [30]. However, on
repetitive scans, when more and more polymer is deposited on the
electrode surface, the electrochemistry of the polymer dominates
over the electrochemistry of Cu(I)/Cu(II) couple and the latter is,
The electrochemical oxidation of 5 alone did not result in
the formation of polymer films on the electrode surface. Co-
Fig. 2. XRF data for complex copolymer of EDOT and 5.
Fig. 1. XPS spectra for copolymer of EDOT and 5 showing the Cu 2p region.

3
30
K. Velauthamurty et al. / Electrochimica Acta 56 (2010) 326–332
A
-
3
3
4
1
1
5
.5x10
C
6
4
2
.0x10-4
.0x10-4
.0x10-4
-
.0x10
-
.0x10
0.0
0.0
-4
-
5.0x10
-
2.0x10-4
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
-1.0 -0.5 0.0
0.5
1.0
1.5
2.0
Potential / V Vs Pt/PPy QRE
Potential/ V Vs Pt/PPy QRE
-
3
4
4
4
4
8.0x10-5
6.0x10-5
4.0x10-5
1
8
6
4
2
.0x10
B
D
-
.0x10
-
.0x10
-
2.0x10-5
0.0
.0x10
-
.0x10
0.0
2.0x10-5
-
-
4
-4.0x10-5
-6.0x10-5
-
2.0x10
-4
-
4.0x10
-1.0 -0.5 0.0
0.5
1.0
1.5
2.0
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential / V Vs Pt/PPy QRE
Potential / V vs Pt/PPy QRE
Fig. 3. Repetitive scan cyclic voltammetry experiments to grow copolymer of EDOT and 5 (A), cyclic voltammogram of copolymer film of EDOT and 5 in the bare electrolyte
B), repetitive scan cyclic voltammetry experiments to grow PEDOT (C) and cyclic voltammogram of PEDOT film in the bare electrolyte (D). Conditions used are scan rate
(
−1
of 100 mV s , N2 purged 0.1 M Bu4NBF4/CH3CN electrolyte and 0.01 M total monomer concentration on Pt working electrode, Pt/PPy QRE reference and Pt gauze counter
electrode.
therefore, hidden with the oxidation wave of the polymer from its
polaronic conducting state to bipolaronic state. On the first scan of
the polymerization of EDOT and 5 mixture (in the positive direc-
tion), there are three prominent anodic peaks appearing at +0.94 V,
has taken place or not as we have clearly explained in our previ-
ous publication also [18]. We have, therefore, used XPS and XRF to
characterize the copolymer.
+1.35 V and +1.60 V respectively as shown in Fig. 3A. For compar-
3.5. EIS characterization of the copolymer of EDOT and 5
ison, the CVs for the electropolymerization of EDOT alone under
identical conditions are given in Fig. 3C while the CV of PEDOT
in background electrolyte is depicted in Fig. 3D. The correspond-
ing peaks in PEDOT appear at relatively lower potentials for the
first two (+0.75 V and +1.25 V) but at a higher potential for the
third one (+1.82 V). The irreversible peak obtained at +1.60 V for
the copolymer and at +1.82 V for the bare PEDOT could be due
to irreversible oxidation of the bipolaron form of the polymers.
In the repetitive scans, these peaks grow consecutively indicat-
ing the growth of polymers in both cases. McAuley et al. [31] have
From the electrochemical impedance spectra of copolymer of
EDOT and 5 it is apparent that at positive potentials (Fig. 4) the
data obtained could be analyzed using the dual rail transmission
line circuit model proposed for the analysis of impedance behavior
100
0
V
+0.3 V
+0.6 V
ꢀ
prepared and characterized Ni(II) and Cu(II) complexes of 6,6 -C-
80
60
40
spirobi(cyclam) (cyclam = 1,4,8,11-tetraazacyclotetradecane), L1,
where they have clearly explained the syntheses and redox chem-
istry of [MH (L )]X (M = Cu2+, Ni , [Cu (L )]X , and [CuNi(L )]X ,
2+
2
1
4
2
1
4
1
4
−
−
X
= ClO4 . They have reported an irreversible oxidation of Cu(II)
+
to Cu(III) in 1 M KCl(aq) containing 0.1 M H (aq) electrolyte
with an E1/2 of +0.79 vs NHE in Cu(II)H cyclamX4 complex.
2
They have also reported the redox chemistry of Cu(II) com-
ꢀ
plexes of 6,6 -C-spirobi(cyclam) in 0.1 M tetraethylammonium
perchlorate in acetonitrile and shown that the redox couples
Cu(II)–Cu(II)/Cu(III)–Cu(III) and Cu(II)–Cu(III)/Cu(III)–Cu(III) show
irreversible waveswithE1/2 values at +1.120 Vand+1.430 V, respec-
tively, with respect to NHE. In our copolymer, however, it is
extremely difficult to locate these redox waves due to the rich
electrochemistry of the polymer masking the electrochemistry of
isolated pendant Cu(II) sites. Therefore, cyclic voltammetry alone
is insufficient to confirm whether copolymerization of EDOT and 5
2
0
0
0
20
40
60
80
100
Z'(Ohms)
Fig. 4. Nyquist plot of copolymer of EDOT and 5 at positive DC potentials.

K. Velauthamurty et al. / Electrochimica Acta 56 (2010) 326–332
200
331
of electronically conducting polymers [32–36]. This dual rail trans-
mission line model proposed originally by Albery et al. [32,33] is a
very useful circuit to analyze impedance behavior of electronically
conductive polymers. The two resistive lines represent the resis-
tance for electronic motion in the polymer and the resistance for
the ionic motion within pores of the polymer film. The two resistive
lines are connected parallelly by a series of capacitors which rep-
resents the three capacitances in series that contribute to the net
capacitance Cꢀ in the transmission line. The three capacitances,
namely, the Nernst capacitance, CN, Donnan capacitance, CD, and
the double layer capacitance, C, are related to the net capacitance
by the reciprocal addition.
-
-
-
0.4 V
0.5 V
0.1 V
1
1
50
00
5
0
Another important feature of ac impedance plots of these mate-
rials is the high-frequency semicircle. This has been attributed
to the parallel connection of the charge transfer resistance Rct
obtainable from the Butler–Volmer kinetics and the double layer
capacitance at either the electrode/polymer or polymer/electrolyte
interface. The overall circuit then includes the serial combination
of this circuit element to the dual rail transmission line as clearly
explained by Albery and Mount [32]. By considering one-electron
transfer reaction of the form A ꢀ B + e, Albery and Mount [32] have
developed theory for the ac impedance due to small ac perturbation
to a DC potential applied to the polymer coat and found that the
charge transfer resistance is negligible when the applied potential
is at the potentials at which the polymer is electronically conduct-
ing but is significant at either extreme ends. As they have pointed
out the semicircle appears when the polymer coat is reduced, i.e.,
at low applied potentials.
0
0
50
100
150
200
Z'(Ohms)
Fig. 5. Nyquist plot of copolymer of EDOT and 5 at negative DC potentials
tial drastically increases the width of the semicircle, because the
copolymer then becomes non-conducting.
Following the analysis given by Albery and Mount [32], the solu-
tion resistance, Rs, electronic resistance, Re, ionic resistance RI, and
the double layer capacitance, C , can be calculated and the cal-
d
culated values are shown in Table 1. Interestingly, the electronic
resistance calculated at any given potential for the copolymer is
always higher than the corresponding ionic resistance even when
the applied potential is such that the copolymer is in its fully con-
ducting state. As expected the electronic resistance decreases as the
applied DC potential is increased from 0 V to +0.8 V as the polymer
is progressively becoming more and more electronically conduct-
ing. However, the magnitude of increase in Re for the copolymer
is considerably small when compared to that usually obtained for
bare PEDOT or other conducting homo-polymers. For the latter, at
potentials where they are electronically conducting their Re vales
can be considered to be virtually zero. Another very interesting fea-
ture observed for this copolymer is that the ionic resistance, RI,
increases as the applied DC potential is increased from 0 V to +0.8 V
from ca. 18.15 ꢁ at 0 V to 38.79 ꢁ at +0.8 V. RI also increases from ca.
For electronically conducting homo-polymers at potentials
where the polymer is in its electronically conducting state, the
electronic resistance is negligible compared to the ionic resis-
tance as the polymer is more conducting than the pores. Then
at high frequency, the charging of the polymer takes place at
◦
the polymer/electrolyte interface. The high frequency 45 straight
line which can be attributed to ionic diffusion, and a low fre-
quency almost vertical line representing pure capacitive behavior
are prominent at DC potentials at which the polymer is in its fully
electronically conducting form.
However, an important feature to notice in our co-polymer
is that even when the applied potential is sufficiently positive
(
such as +600 mV or +700 mV) to make the polymer electroni-
cally fully conducting, a small semicircle at high frequency still
appears. Such a behavior is not present in pure PEDOT polymer
as reported by many authors ([32] and references therein). When
pure PEDOT (or polyaniline or polypyrrole) is in its fully conducting
2
8.89 ꢁ to 44.01 ꢁ as the potential increased from −0.1 V to −0.5 V
in the negative direction. The increase in ionic resistance is a man-
ifestation of the difficulty for counter-ion transport in the pores of
the polymer again due to the steric hindrance imposed by bulky
pendant groups attached to the polymer backbone. As expected,
◦
state, the impedance spectra consists of only 45 inclined War-
berg impedance appearing at high frequency and almost vertical
pure capacitive line appearing at low frequency. The fact that,
although small, semicircle present in the impedance spectra of the
co-polymer can be attributed to the steric hindrance imposed by
the bulkier nature of pendant cyclam moiety present in every sixth
monomer unit of the polymer, as observed in XPS analysis, making
it difficult for counter-ion transport that is required for efficient oxi-
dation of the polymer backbone for fast charge transport. Thus even
at potentials where the polymer is in its conducting state, the circuit
element formed from the parallel connection of the charge transfer
resistance with the double layer capacitance is still playing a signif-
icant role in the ac impedance characteristics of the co-polymer as
opposed to the same in PEDOT homopolymer. We have observed a
similar behavior in PEDOT modified with pendant biotin moieties
where the charge transfer kinetics is too slow even at sufficiently
positive potentials due to the steric hindrance imposed by the bulky
biotin moieties [35]. At lower potentials, a high frequency semicir-
cle can be clearly observed (Fig. 5) indicating the influence of the
charge transfer resistance in parallel with double layer capacitance
becoming important. As shown in Fig. 5 increasing negative poten-
Table 1
Resistance and capacitance data extracted from the EIS data for the copolymer of
◦
EDOT and 5; Re stand for electronic resistance, R1 point at which 45 line intersects
ꢀ
ꢀ
the z axis, R2 is from the point at which vertical line intersects the z axis. RI is the
ionic resistance and Rs is the solvent resistance.
E/V
Rs/ꢁ
Re/ꢁ
78.45
R1/ꢁ
R2/ꢁ
RI(R2 − R1) × 3
Ce/␮F
0
50.65
48.53
47.62
46.85
46.43
45.99
45.69
45.31
45.66
48.50
52.62
56.33
66.25
79.77
81.07
69.46
65.03
61.27
59.08
57.34
55.83
54.76
55.11
59.42
63.69
74.43
97.69
142.23
87.12
79.48
75.09
72.81
71.07
69.19
68.47
68.25
68.04
69.05
72.07
82.18
105.00
156.90
18.15
30.06
30.18
34.62
35.97
35.55
37.92
40.47
38.79
28.89
25.14
23.25
21.93
44.01
5.52
6.67
8.21
+
+
+
+
0.1
66.77
62.24
58.83
56.59
54.59
53.60
53.38
53.85
57.05
61.43
72.40
94.69
139.14
0.2
0.3
0.4
9.89
11.09
10.00
13.68
27.06
20.00
8.97
3.59
10.17
9.17
+0.5
+0.6
+
+
0.7
0.8
0.1
0.2
−
−
−0.3
−
−
0.4
0.5
14.35

3
32
K. Velauthamurty et al. / Electrochimica Acta 56 (2010) 326–332
the double layer capacitance determined by the charge density of
the polymer layer and its thickness also increases as the applied
potential is increased.
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The significant differences of the AC impedance behavior of
the copolymer of EDOT and 5 to that of PEDOT homopolymer is
a clear indication of the formation of the copolymer on the elec-
trode surface during the electropolymerization of EDOT and 5 in
the non-aqueous system. The cyclic voltammetric data also gives
some insight into this. The XPS and XRF data clearly indicate the
presence of Cu(II).
[6] D.P. Rillema, J.F. Endicott, E. Papaconstantinou, Inorg. Chem. 10 (1971) 1739.
[
[
[
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[
18] K. Velauthamurty, S.J. Higgins, R.M.G. Rajapakse, J. Bacsa, H. van Zalinge, R.J.
Nichols, W. Haiss, J. Mater. Chem. 19 (2008) 1.
derivatives can be synthesized and can be effectively co-
polymerized electrochemically with bare EDOT to form redox
active conjugated copolymer films on the working electrodes. The
presence of Cu(II) in the copolymers has been confirmed by X-ray
photoelectron spectroscopy, X-ray fluorescence spectroscopy and
by electrochemical methods. This successful co-polymerization of
[
19] J. Ghilane, P. Hapiot, A.J. Bard, Anal. Chem. 78 (2006) 6868.
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2
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[
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[
[
[
[
[
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5
with EDOT further opens up the possibility of using this as elec-
trocatalyst in the copper coupling reactions.
Acknowledgements
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We wish to thank the University of Jaffna, Sri Lanka (QEF grant-
Batch 2, IRQUE Project) for KV’s financial support and the U.K. EPSRC
for partial funding. We also thank U.K. NCESS and UK Chemical
Database Services and Dr. Graham Beamson (NCESS, Daresbury)
for experimental assistance.
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