Electrochemistry and ion-sensing properties of calix[4]arene derivatives

Article abstract of DOI:10.1016/j.electacta.2010.06.076

The cyclic voltammetric properties of several substituted calix[4]arenes were examined in acetonitrile and dichloromethane. The compounds that contained one phenolic group in the macrocyclic cavity were able to be electrochemically oxidised at positive potentials. In acetonitrile, cyclic voltammetry experiments indicated that the phenolic compounds were oxidised in a two-electron (one-proton) process over all measured scan rates (up to 50 V s^-1), while in dichloromethane, the oxidation process occurred by one-electron at scan rates ≥5 V s^-1, to most likely form the radical cations. In both solvents, longer timescale (minutes to hours) controlled potential coulometry experiments indicated that the oxidation process occurred by two-electrons per molecule, to form reactive diamagnetic cations that could not be reduced back to the starting materials under electrolysis conditions. The ion-sensing properties of the compounds were investigated in polymer membrane ion-selective electrodes and it was found that they responded reversibly in a Nernstian fashion to Groups 1 and 2 metals and had the highest selectivity to the cesium cation.

Full text of DOI:10.1016/j.electacta.2010.06.076

Electrochimica Acta 55 (2010) 7036–7043
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemistry and ion-sensing properties of calix[4]arene derivatives
Shanshan Chena, Richard D. Webstera,∗, Carmen Talottab, Francesco Troisib,
Carmine Gaetab, Placido Nerib,∗∗
a Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
b Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo, I-84084 Fisciano (Salerno), Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The cyclic voltammetric properties of several substituted calix[4]arenes were examined in acetonitrile
and dichloromethane. The compounds that contained one phenolic group in the macrocyclic cavity were
able to be electrochemically oxidised at positive potentials. In acetonitrile, cyclic voltammetry exper-
iments indicated that the phenolic compounds were oxidised in a two-electron (one-proton) process
over all measured scan rates (up to 50 V s−1), while in dichloromethane, the oxidation process occurred
by one-electron at scan rates ≥5 V s−1, to most likely form the radical cations. In both solvents, longer
timescale (minutes to hours) controlled potential coulometry experiments indicated that the oxidation
process occurred by two-electrons per molecule, to form reactive diamagnetic cations that could not be
reduced back to the starting materials under electrolysis conditions. The ion-sensing properties of the
compounds were investigated in polymer membrane ion-selective electrodes and it was found that they
responded reversibly in a Nernstian fashion to Groups 1 and 2 metals and had the highest selectivity to
the cesium cation.
Received 24 April 2010
Received in revised form 24 June 2010
Accepted 26 June 2010
Keywords:
Calixarenes
Bromodienone
Polymer membrane ion-selective electrode
Radical cation
Cesium cation
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
(Fc/Fc+), with the cyclic voltammograms displaying a forward oxi-
dation peak but no reverse reductive peak when the scan direction
The supramolecular sensing properties of calixarenes towards
ions or neutral molecules have been studied extensively over the
last two decades [1–3]. The weak and reversible interactions with
Groups 1 and 2 metals have made them ideal for use in potentio-
metric chemical sensors and in optical sensors [1–3]. Considerable
attention has focused on new synthetic strategies to modify the
calixarene structures to improve or diversify their recognition
properties [4–7]. In this work we discuss the sensing properties
of some calix[4]arenes recently synthesised via the so-called “p-
bromodienone route” [8,9], where one aromatic group contains a
hydroxyl (phenolic) or quinone [10] based functionality (Scheme 1,
compounds 1a, 1b, 2a, 5a, 5b and 5c).
In addition to the ion-sensing properties of the calix[4]arenes,
the electrochemical properties were also examined. Calixarenes
often display complicated electrochemical properties due to the
presence of several phenolic or ether groups resulting in multiple
electron transfer oxidative processes, and because of adsorption
processes affecting the shape of the voltammograms [11–14].
Usually the calixarenes with phenolic or ether groups display a
multi-electron oxidation process at potentials ≥1 V vs. ferrocene0/+
is switched, indicating that the oxidised compounds undergo cou-
pled chemical reactions after the initial electron transfer step
[11–14]. However, compounds 1a, 5a, 5b and 5c contain one elec-
troactive phenolic group, and so were expected to show simpler
electrochemical properties. Phenolic compounds are known to
undergo a number of proton and electron transfer reactions in an
electrochemical “square-scheme” mechanism, where the lifetimes
of the oxidised species depend on their structures and particu-
larly on the substituents in the 2, 4 and 6 positions [15–18]. Recent
studies have demonstrated that some phenolic compounds can be
oxidised to form relatively long-lived diamagnetic cations (phenox-
onium ions) in a two-electron and one-proton process [19–28]. We
were particularly interested in determining whether the macro-
cyclic structure of the calix[4]arenes, composed by electron-rich
␲-basic rings, could impact improved lifetimes on the oxidised
forms of their phenolic rings.
2. Experimental
2.1. Chemicals and reagents
HPLC grade CH3CN (Tedia) and CH2Cl2 (J.T. Baker) were dried
over 3 Å molecular sieves and HPLC grade tetrahydrofuran (THF)
was used directly from the bottle. Bu4NPF6 was prepared by
reacting equal molar amounts of aqueous solutions of Bu4NOH
∗
Corresponding author. Tel.: +65 6316 8793; fax: +65 6791 1961.
Corresponding author. Tel.: +39 089 969572; fax: +39 089 969603.
E-mail addresses: webster@ntu.edu.sg (R.D. Webster), neri@unisa.it (P. Neri).
∗∗
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.06.076

S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
7037
Scheme 1. Structure of compounds and the synthetic p-bromodienone route.
(40%, Alfa Aesar) and HPF6 (65%, Fluka), washing the precipitate
with hot water, recrystalising 3 times from hot ethanol followed
by drying under vacuum at 160 ◦C for 24 h and storing under vac-
uum. High molecular weight poly(vinyl chloride), 2-nitrophenyl
octyl ether (NPOE) and potassium tetrakis(4-biphenylyl)borate
(KTBPB) were obtained from Fluka. Di-n-octyl phenylphosphonate
(DOPP), di-n-butyl sebacate (DBS), di-n-butyl phthalate (DBP) were
obtained from Alfa Aesar. LiCl, NaCl, KCl, CsCl, CaCl2, NH4Cl, MgSO4
salts were obtained from Goodrich Chemical Enterprise (GCE).
Rb+ standard solution (1000 ␮g/ml in 2% HNO3) was obtained
from Certified standards. All sample solutions for analysis were
prepared with water, with a resistivity ≥18 Mꢀ cm from an ELGA
Purelab Option-Q. The synthesis of compounds 1a, 1b, 2a, 5a, 5b
and 5c has been described previously [8,10].
(GC) disks (Cypress Systems), used in conjunction with a Pt aux-
iliary electrode (Metrohm) and an Ag wire miniature reference
electrode (Cypress Systems) connected to the test solution via a
salt bridge containing 0.5 M Bu4NPF6 in CH3CN. Accurate poten-
tials were obtained using ferrocene as an internal standard, which
was added to the test solution at the end of the measurements.
2.3. Controlled potential electrolysis
Bulk electrolysis experiments were performed in a divided
controlled potential electrolysis (CPE) cell separated with
a
porosity no. 5 (1.0–1.7 ␮m) sintered glass frit [29]. The working
and auxiliary electrodes were identically sized Pt mesh plates
symmetrically arranged with respect to each other with an Ag
wire reference electrode (isolated by a salt bridge) positioned to
within 2 mm of the surface of the working electrode. The volumes
of both the working and auxiliary electrode compartments were
approximately 20 ml each. The solution in the working electrode
compartment was simultaneously deoxygenated and stirred using
bubbles of argon gas. The number of electrons (n) transferred
2.2. Voltammetry
Cyclic voltammetry (CV) experiments were conducted with a
computer controlled Eco Chemie ␮Autolab III potentiostat. Work-
ing electrodes were 1 mm diameter planar Pt and glassy carbon

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S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
during the bulk oxidation process was calculated from
Q
N =
(1)
nF
where N is the no. of moles of starting compound, Q is the charge
(coulombs) and F is Faraday constant (96,485 C mol−1).
2.4. Preparation of polymer membrane ion-selective electrodes
The PVC-based membranes were prepared by mixing the
calix[4]arenes (1 mg), KTBPB (1 mg), plasticizers (65 mg), and PVC
(33 mg) in THF (2 ml). After the mixture had completely dissolved
in the THF, the clear solution was poured into a 5 cm diameter
Petri dish and the THF was allowed to evaporate for approximately
24 h at room temperature. A 5 mm diameter circular piece of the
membrane was cut out and attached to the end of a commercial
electrode (Selectophore Fluka). The inner filling solution of the ISE
electrode contained 0.1 M of the ions that the electrode was tested
with, and the electrode/membrane was conditioned for 2 days in a
0.5 M solution of the particular ion.
2.5. Selectivity coefficient measurements
Selectivity coefficients (Kipjot) were determined using cesium as
the primary ion (i) and other Groups 1 and 2 metals as interfering
ions (j) using both the fixed interference method (FIM) and separate
solution method (SSM) [30–33]. In the SSM, the solution of 0.01 M
NaCl served as the internal filling solution and the electrodes were
conditioned in 0.01 M NaCl solution for at least 48 h before measur-
ing other 0.01 M interfering ions and 0.01 M primary cesium ion.
The selectivity coefficient was calculated by Eq. (2), where Ei is the
cell potential of the solution containing the primary ion, Ej is the
cell potential of the solution containing the interfering ion, Zi and Zj
are the charges on the primary and interfering ions respectively, ai
is the activity of the primary ion calculated by Debye–Hückel the-
ory, R is the gas constant (8.3143 J K−1 mol−1), T is the temperature
(in K) and F is Faraday constant (96,485 C mol−1):
Fig. 1. Voltammetric andcoulometric data of 1 mM 5a inCH3CN with 0.2 MBu4NPF6.
(a) CVs recorded at 22
◦C at variable scan rates with a 1 mm diameter GC
electrode. Current data were scaled by multiplying by ꢁ−0.5. (b) CVs recorded at
◦C at a 1 mm diameter Pt electrode (solid line) prior to the bulk oxida-
tion of 5a, and (dotted line) after the exhaustive oxidation of 5a at an applied
potential of +0.8 V vs. Fc/Fc+. Current data were scaled by multiplying by ꢁ−0.5. (c)
Current/coulometry vs. time data obtained during the exhaustive oxidation of 5a at
+0.8 V vs. Fc/Fc+ at −20 2 ◦C.
2
−20
2
ꢀ
ꢁ
Ej − Ei
(2.303RT)/(ZiF)
Zi
Zj
pot
ij
log K
=
+
1 −
log ai
(2)
transfer step (an EC or ECE process), and not due to a slow hetero-
geneous electron transfer rate. It is possible that if sufficiently fast
scan rates were used in CH3CN, the ipox/ipred-ratio could approach
unity (if the follow up homogeneous reaction was outrun) [34].
In contrast to the data in CH3CN, the CV data obtained in
CH2Cl2 show that the ipox/ipred-ratio for the first oxidation pro-
cess approaches unity as the scan rate is increased to 5 V s−1
(Fig. 2a). Furthermore, the anodic (Epox) to cathodic (Epred) peak-
to-peak potential separation (ꢂEpp = 100 mV) at the fastest scan
rate (5 V s−1) was the same as the value observed for ferrocene (a
model system for fast electron transfer [35]) under the same con-
ditions, indicating that the cyclic voltammograms for 5a were not
For the FIM, the concentration of the interfering ion was kept
at 0.01 M, whereas the Cs+ ion was varied from 1.0 × 10−7 to
1.0 × 10−1 M. Before taking any potential measurements, the mem-
branes were equilibrated in 0.5 M Cs+ solution for 48 h, and the
inner solution consisted of 0.1 M CsCl. The selectivity coefficients
were calculated from Eq. (3), where DL (detection limit) and BG
(background) were obtained graphically from readings of the pri-
mary ion in the presence of the interfering ion [33]:
ai(DL)
pot
ij
log K
= log
(3)
aj(BG)Z /Z
i
j
affected by slow heterogeneous electron transfer rates. The ꢂEpp
-
value of 100 mV at a scan rate of 5 V s−1 is wider than the theoretical
value of 57 mV at 25 ◦C for a one-electron transfer [34], but this
likely originates from uncompensated solution resistance effects
brought about by the low dielectric constant of CH2Cl2 (ε = 9.1).
3. Results and discussion
3.1. Electrochemistry of monophenolic calix[4]arenes 1a, 5a, 5b,
and 5c
When the concentration of 5a was decreased to 0.2 mM, the ꢂEpp
-
value obtained by CV at 5 V s−1 decreased to 75 mV, which supports
the larger than theoretical value of ꢂEpp being due to solution resis-
tance effects. The data in Fig. 2a show that the anodic peak current
(ipox) is proportional to the square root of the scan rate, which is the
expected result for a diffusion controlled process [34].
Fig. 1a shows CVs of p-methoxyphenolic calix[4]arene (5a) in
CH3CN at a variety of scan rates at a GC electrode. The compound
shows an oxidation process with an anodic peak potential (Epox) at
∼+0.6 V vs. Fc/Fc+. The oxidation process remained chemically irre-
versible in the sense that the anodic (ipox) to cathodic (ipred) peak
current ratio (ipox/ipred) was ꢀ1 at all measured scan rates (Fig. 1a).
The reason for the chemical irreversibility in the CVs relates to
a coupled chemical reaction that occurs after the initial electron
The oxidation of phenols (POH) typically occurs by two-
electrons per molecule via an electrochemical ECE mechanism
(Scheme 2) [15–28], or possibly via a disproportionation mecha-

S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
7039
(Fig. 2a) are similar in appearance to what is expected for a one-
electron transfer. Therefore, it is likely that the difference in the CVs
observed in the two solvents is due to a different number of elec-
trons transferred on the short voltammetric timescale (seconds).
It was previously observed that the cation radical of the phenolic
compound ␣-tocopherol had an increased lifetime in CH2Cl2 com-
pared to in CH3CN, due to a change in the equilibrium constant
for the deprotonation step of the cation radical (Keq in Scheme 2)
[20–25,36]. This can be rationalised by the phenolic radical cations
being less acidic in low dielectric constant solvents. It is likely that
the same situation applies to 5a, so that the cation radical formed
by initial one-electron oxidation has a longer lifetime in CH2Cl2
compared to CH3CN. The CVs obtained of 5a in CH2Cl2 display
lower peak currents than those obtained for equivalent concentra-
tions in CH3CN, which supports the assignment of the one-electron
oxidation in CH2Cl2 (assuming similar diffusion coefficient values
between the different solvents).
An alternative mechanism to account for the ipox/ipred-ratio of
unity for the CV of 5a at a scan rate of 5 V s−1 in CH2Cl2 (Fig. 2a),
involves the reversible two-electron/one-proton process given in
Scheme 2 to produce a diamagnetic cation (an ECE mechanism),
as seen for ␣-tocopherol [20–25]. It can be proposed that the for-
ward oxidative CV scan produces the diamagnetic cation (PO+)
and the reverse reductive scan regenerates the starting material
(POH). However, this reversible two-electron process is consid-
ered unlikely for compound 5a. In situations where a diamagnetic
cation has been voltammetrically detected, the Epox to Epred separa-
tions (ꢂEpp) are usually very wide (∼300–400 mV) due to the wide
potential separation between the formal potentials (Efo) of the phe-
nol and phenoxyl ion oxidation steps (E1 and E2 in Scheme 2). The
ꢂEpp-value of 100 mV observed in the CV in Fig. 2a at a scan rate
of 5 V s−1, is instead consistent with a one-electron transfer, taking
into account the resistance effects of the low dielectric constant
solvent.
Although the cyclic voltammograms in CH2Cl2 (Fig. 2a)
show signs of chemical reversibility, the observation that scan
rates > 0.1 V s−1 are needed to obtain ipox/ipred-ratios approaching
unity, indicates that the lifetime of the one-electron oxidised
species is still only a few seconds. Controlled potential electroly-
sis of 5a in both CH3CN (Fig. 1c) and CH2Cl2 (Fig. 2c) occurred by
the transfer of close to two-electrons per molecule. Therefore, the
longer timescale electrolysis experiments in CH2Cl2 allow time for
the intermediate cation radical to undergo deprotonation and then
be further oxidised, resulting in an increase in the number of elec-
trons transferred compared to the faster timescale CV experiments.
It was also observed that in both exhaustively electrolysed solu-
tions (CH2Cl2 and CH3CN), the two-electron oxidised compound
had reacted to form a new product that showed a reduction pro-
cess at approximately −0.4 to −0.6 V vs. Fc/Fc+ (Figs. 1b and 2b),
indicating that the diamagnetic cation (if formed) was not long-
lived under electrolysis conditions. During the electrolysis reaction
the solution was observed to change to a pale yellow-green colour,
possibly due to the appearance of the intermediate radical cation
Fig. 2. Voltammetric and coulometric data of 1 mM 5a in CH2Cl2 with 0.2 M
Bu4NPF6. (a) CVs recorded at 22 2 ◦C at variable scan rates with a 1 mm diame-
ter GC electrode. Current data were scaled by multiplying by ꢁ−0.5. (b) CVs recorded
at −20
2
◦C at a 1 mm diameter Pt electrode (solid line) prior to the bulk oxi-
dation of 5a, and (dotted line) after the exhaustive oxidation of 5a at an applied
potential of +0.8 V vs. Fc/Fc+. Current data were scaled by multiplying by ꢁ−0.5. (c)
Current/coulometry vs. time data obtained during the exhaustive oxidation of 5a at
+0.8 V vs. Fc/Fc+ at −20 2 ◦C.
nism that also involves two-electrons per molecule [26]. The first
electron transfer step produces the cation radical (POH+•), which
loses a proton to form a neutral phenoxyl radical (PO•). Because
the phenoxyl radicals are easier to oxidise than their associated
phenols, they immediately undergo further oxidation to form the
diamagnetic cation (PO+). Normally the diamagnetic cations are
short-lived and undergo rapid hydrolysis reactions with trace water
or other nucleophiles in the solvent [15–28].
The cyclic voltammograms obtained in CH3CN (Fig. 1a) which
show ipox/ipred-ratios ꢀ1 at all scan rates are consistent with what is
expected for most phenols, where the oxidation mechanism occurs
by two-electrons per molecule in a chemically irreversible pro-
cess. However, the voltammograms that are observed in CH2Cl2
Scheme 2. Oxidation mechanism for phenols [15–28].

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S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
Fig. 3. CVs obtained at T = 22 2 ◦C of 1 mM solutions of analytes recorded at a 1 mm diameter GC working electrode at ꢁ = 0.1 V s−1 in CH3CN or CH2Cl2 with 0.2 M Bu4NPF6
as the supporting electrolyte. Current data were scaled by multiplying by ꢁ−0.5. (a) 5b, (b) 5a, (c) 5c and (d) 1a.
present in small amounts. The proton released during the oxida-
tion process was not detected at Pt or GC electrodes, although this
is consistent with the results obtained during the oxidation of other
phenols at Pt surfaces in organic solvents [19–28].
one-electron oxidation were relatively short-lived. Furthermore,
in each case the ipox-values measured in CH3CN were significantly
larger than those measured in CH2Cl2 suggesting a greater num-
ber of electrons were transferred in CH3CN, at least on the short
CV timescale. However, longer timescale electrolysis experiments
on the p-i-propoxy derivative (5b) in CH2Cl2 and CH3CN at −20 ◦C,
indicated in both solvents that the oxidation processes occurred by
two-electrons per molecule to form a final product that could not
be reduced back to the starting material by applying a reductive
potential (similar to the results obtained for 5a).
The anodic peak current observed for 1a (with a t-butyl group)
was greater than the other compounds and the oxidation potential
was shifted to more positive potentials (compared to 5a, 5b and 5c).
If it is assumed that the diffusion coefficient of 1a is the same as the
other compounds, the increase in peak current for 1a is likely due to
an increased number of electrons transferred. It is possible that the
compounds undergo oxidation at multiple sites on the molecule,
such as at the ether groups in addition to the phenolic groups.
Other calixarenes that do not contain phenolic groups have been
reported to undergo chemically irreversible oxidation reactions
The voltammetric data obtained for phenolic calix[4]arenes
para-substituted with an i-propoxy (5b), or phenyloxy (5c) group
were similar to that obtained for 5a, although there were some
differences in the observed potential of the oxidation processes
and the degree of chemical reversibility in CH2Cl2. A compari-
son of the cyclic voltammetry for each compound in CH3CN and
CH2Cl2 is given in Fig. 3. In each case and at the equivalent scan
rate, the ipox/ipred-ratios were closer to unity in CH2Cl2 compared
to in CH3CN, indicating that the homogeneous chemical reactions
coupled to the electron transfer steps occurred at a slower rate
in CH2Cl2 compared to in CH3CN. As the scan rate was increased,
the ipox/ipred-ratios for the least positive process observed by cyclic
voltammetry in CH2Cl2 became very close to unity indicating that
the cation radicals had an observable lifetime. In contrast, in CH3CN,
the CVs displayed ipox/ipred-ratios ꢀ1 at all scan rates measured (up
to 50 V s−1), indicating that the cation radicals formed by the initial

S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
7041
−1.8 V vs. Fc/Fc+ (Fig. 4b). The first reduction process at −1.2 V
vs. Fc/Fc+ had an ipox/ipred-ratio close to unity at slow scan rates
(ꢁ = 0.1 V s−1), indicating that the reduced species survived on the
timescale of the CV experiment. The ꢂEpp-value obtained by CV for
the first reduction process was 65 mV, suggesting a one-electron
process to most likely form an anion radical. Coulometry exper-
iments performed by exhaustively reducing 1b in CH3CN in an
electrolysis cell at −1.5 V vs. Fc/Fc+ confirmed the transfer of one-
electron per molecule over electrolysis timescales. However, the
anion radical did not survive for a sufficiently long time to enable
it to be oxidised back to the starting material, and most likely
underwent further chemical reactions. There is also a pre-peak or
shoulder before the first reduction process in Fig. 4b at approxi-
mately −1.1 V vs. Fc/Fc+ which has not been assigned.
The second reductive electron transfer process at −1.8 V vs.
Fc/Fc+ in Fig. 4b only displayed a forward reduction peak (Epred) with
no detectable oxidation peak (Epox) when the scan direction was
reversed. The ipred-value for the second electron transfer process
was much less than the first process (Fig. 4b). In organic solvents,
quinones are often reduced in two one-electron steps to form
first the anion radical (semiquinone) and then at more negative
potentials the dianion is produced [37–41]. However, it is not clear
whether the second electron transfer step that occurs at −1.8 V vs.
Fc/Fc+ is due to the further reduction of the semiquinone, or due to
a different electron transfer process. It can be observed in Fig. 4a
that compound 2a, which does not contain a quinone group, also
shows a reduction process at −1.6 V vs. Fc/Fc+ which is similar in
appearance to the process observed for 1a.
Fig. 4. CVs recorded at a 1 mm diameter GC working electrode at ꢁ = 0.1 V s−1 and
at 22
◦C. Current data were scaled by multiplying by ꢁ−0.5. (a) 1 mM 2a in CH2Cl2
containing 0.2 M Bu4NPF6 and (b) 1 mM 1b in CH3CN containing 0.2 M Bu4NPF6.
2
The cyclic voltammetric results that were obtained for 2a and
1b (Fig. 4) indicate that the compounds can also be oxidised but at
a greater potential that the other compounds shown in Fig. 3. Fur-
thermore, the oxidation peak currents for 2a and 1b were greater
than the other compounds indicating that multiple electron steps
were occurring. The major difference between 2a/1b and the other
compounds is that they do not contain a phenolic group. Therefore,
it is likely that the oxidation for 2a and 1b is associated with the
ether groups as has been observed for other calixarenes that display
multiple electron transfer processes at potentials >+1 V [11,12].
involving multiple electron transfer steps at positive potentials
[11,12]. The possibility that the molecules undergo oxidation at
more than one site on the molecule is supported by the controlled
potential electrolysis data from 5a (Fig. 2b), where the second oxi-
dation process at approximately +1.1 V vs. Fc/Fc+ was still detected
after the compound had been exhaustively oxidised at +0.8 V vs.
Fc/Fc+, suggesting that the two processes occurred independently
of one another.
3.2. Electrochemistry of calix[4]arene p-bromodienone 2a
3.4. Ion-sensing properties
A cyclic voltammogram of 2a in CH2Cl2 at 20 ◦C is shown in
Fig. 4a, which displays a reduction process at approximately −1.6 V
vs. Fc/Fc+ and a number of oxidation processes at potentials >+0.9 V
vs. Fc/Fc+. The ipox/ipred-ratios obtained by cyclic voltammetry for
both the oxidation and reduction processes did not show any sign
The sensing properties of the compounds were tested for
Groups 1 and 2 ions by incorporating the molecules into poly-
mer membrane ion-selective electrodes (see Section 2) [42–50].
The membrane composition consisted (by mass) of PVC (33%),
several different types of plastizers (65%), potassium tetrakis-(4-
biphenyl)borate as the counter ion (1%) and the calix[4]arene (1%).
A total ionic strength adjustment buffer was not used for the mea-
surements because of strong interference from the Groups 1 and 2
metals commonly used in buffers.
The membranes containing the sensing compounds were all
found to display a potential dependent response except for the
membrane containing p-bromodienone 2a (and 2b), which showed
no change in potential as the concentration of ions were var-
ied. 2a has a bromide and a tert-butyl group bonded to the same
carbon, which could hinder ions entering the calix[4]arene cav-
ity from the upper or wide (or exo) rim, thereby preventing weak
bonding interactions. However, it is more likely that the cations
interact with the O-atoms on the lower rim, therefore; 2a should
offer similar ion-sensing properties to the other compounds. There
are several reports where sensing for Groups 1 and 2 ions occurs
through the ether oxygen atoms in the lower rims of the cal-
ixarenes [42,43,46–50]. Membranes containing compound 1b, with
the para-quinone moiety, were found to reversibly vary in poten-
tial as the concentration of ions were varied, indicating that the
of approaching unity as the scan rate was increased up to 50 V s−1
,
indicating that the oxidised and reduced forms of 2a were chemi-
cally short-lived. According to the p-bromodienone route [8,9], 2a
is thought to convert to compounds 5a, 5b and 5c in an SN1 mecha-
nism involving the diamagnetic cationic intermediate, 3 (Scheme 1)
[8]. Therefore, we were interested in determining whether the
intermediate, 3, could be voltammetrically detected during the oxi-
dation of 2a with AgSbF6 (or AgClO4). 2a is poorly soluble in CH3CN
but dissolves completely in CH2Cl2. Chemical oxidation experi-
ments were performed by adding equal molar amounts of AgSbF6
to solutions of 2a at −25 ◦C in CH2Cl2. Cyclic voltammetry experi-
ments on the chemically oxidised solutions did not show any clear
evidence of the presence of an intermediate or semi-stable cation
(3) that should form via loss of the Br− from the starting material,
suggesting that 3 was short-lived (should it actually exist).
3.3. Electrochemistry of calix[4]arene monoquinone 1b
Cyclic voltammetry experiments on 1b in CH3CN led to the
detection of two reduction processes at approximately −1.2 and

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S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
Table 1
Results from polymer membrane ion-selective electrode experiments with varying membrane compositions and Group 1 ions.
Ion (i)
Membrane composition
Slope of plot of E vs. log ai/mV
Linear concentration range/M
Cs+
Cs+
Cs+
Cs+
K+
1a, KTBPB, PVC/NPOE
1a, KTBPB, PVC/DBS
1a, KTBPB, PVC/DOPP
1a, KTBPB, PVC/DBP
1a, KTBPB, PVC/NPOE
1a, KTBPB, PVC/NPOE
1a, KTBPB, PVC/NPOE
1b, KTBPB, PVC/NPOE
5a, KTBPB, PVC/NPOE
5b, KTBPB, PVC/NPOE
5c, KTBPB, PVC/NPOE
59.3
56.0
52.9
56.0
55.6
46.0
46.0
56.9
56.7
59.9
58.6
4.6 × 10−6 to 3.6 × 10−2
5.5 × 10−6 to 4.0 × 10−2
4.5 × 10−6 to 1.1 × 10−2
3.6 × 10−6 to 4.0 × 10−2
3.2 × 10−6 to 3.3 × 10−2
7.6 × 10−6 to 9.1 × 10−2
1.7 × 10−6 to 7.3 × 10−2
4.6 × 10−6 to 4.0 × 10−2
5.0 × 10−6 to 4.2 × 10−2
5.0 × 10−6 to 4.3 × 10−2
4.9 × 10−6 to 4.1 × 10−2
Na+
Li+
Cs+
Cs+
Cs+
Cs+
presence of a phenolic group was not essential for a reversible
response. Calixarenes containing phenolic groups [44,45] often dis-
play similar sensing properties to those without phenolic groups
that interact with metal ions through the ether oxygen atoms
[42,43,46–50]. Therefore, an explanation for compound 2a not dis-
playing ion-sensing properties may relate to solubility problems
within the PVC membrane making it less mobile. Alternatively, 2a
is known to be not as chemically stable as the other compounds
[10]. Thus, cations may act as Lewis acids and, by coordinating to
and Cs+/Li+ selectivity is comparable with other calix[4]arenes and
calix[6]arenes [42–50].
4. Conclusions
The electrochemical oxidation of calix[4]arenes 5a, 5b and
5c that contain a p-substituted phenolic group in the macro-
cyclic structure occurred by one-electron per molecule in
dichloromethane on short voltammetric timescales (seconds) to
most likely form cation radicals. In acetonitrile, and over the longer
timescales (minutes to hours) of controlled potential electrolysis
experiments, the cation radicals deprotonate to form neutral radi-
cals which are further oxidised to reactive diamagnetic cations in an
ECE mechanism. Compound 1b, containing a quinone group in the
macrocyclic structure displayed electrochemical properties simi-
lar to other quinones, that is, could be electrochemically reduced
to a semiquinone in a one-electron reduction process at negative
potentials.
With the exception of p-bromodienone 2a, all of the compounds
displayed ion-sensing properties for Groups 1 and 2 metals, with
the highest selectivity shown for the Cs+ ion. The reason that 2a did
not potentiometrically respond to the cations most likely relates to
chemical instability of 2a or poor solubility (hence mobility) of 2a
within the PVC membrane. There appeared to be little change to the
sensing properties by changing the quinone (1b) to a phenol (1a, 5a,
5b and 5c) functionality, suggesting that the size of the macrocyclic
structure was the most important factor in determining the sensing
properties. Correspondingly, the order of selectivity of the Groups
1 and 2 ions (and NH4+) calculated according to both the separate
solution and fixed interference methods, were found to correlate
with the order of the hydrated ion size.
C
O, may favour the decomposition (by de-bromination or de-tert-
butylation) of the p-bromodienone system.
It was found that compounds 1a, 1b, 5a, 5b and 5c all showed
the highest selectivity for Cs+ compared to the other metal ions.
There have been a number of reports where calix[4]arenes or
calix[6]arenes have been found to be selective for Cs+ when incor-
porated into the polymer membranes of ion-selective electrodes
[42–50]. Table 1 provides a summary of the sensing results obtained
for the different compounds in the presence of different Group 1
ions as well as with different plasticizers in the membranes. In
most instances the slopes were close to +59 mV (except for Na+
and Li+) and the linear region of the response was between 10−6 M
and 10−2 M. The best result (in terms of the Nernstian slope) was
obtained using NPOE as the plasticizer.
Table 2 gives selectivity coefficient results that were obtained
from p-t-butylphenolic 1a in the presence of a range of Groups
1 and 2 ions (and NH4+) using the SSM and FIM methods [33].
Variations were obtained for the absolute selectivity coefficient val-
ues for the different ions using the different methods (FIM and
SSM), but the trends were similar. It can be observed from the
results in Table 2 that the ISE containing 1a is more selective
in the order Cs+ (0.25) > Rb+ (0.25) > K+ (0.3) > NH4 (0.25) > Na+
+
(0.43) > Li+ (0.6) > Ca2+ (0.6) > Mg2+ (0.8), which closely follows the
order of hydrated ion size given in parenthesis (in nm) [51]. The
selectivity for 1a (and the other compounds) for Cs+ was similar
to the selectivity observed from reports of other calix[4]arenes or
calix[6]arenes, which suffer from the strongest interference from
Acknowledgment
This work was supported by a Singapore Government Ministry
of Education research grant (T208B1222).
Rb+, K+ and NH4 [42–50]. Similarly, the relatively high Cs+/Na+
+
References
Table 2
[1] D. Diamond, M.A. McKervey, Chem. Soc. Rev. 25 (1996) 15.
[2] D. Diamond, K. Nolan, Anal. Chem. 73 (2001) 22A.
[3] R.M. El Nashar, H.A.A. Wagdy, H.Y. Aboul-Enein, Curr. Anal. Chem. 5 (2009) 249.
[4] A. Ikeda, S. Shinkai, Chem. Rev. 97 (1997) 1713.
[5] Z. Asfari, V. Böhmer, J. Harrowfield, J. Vicens (Eds.), Calixarenes 2001, Kluwer,
Dordrecht, 2001.
[6] J. Vicens, J. Harrowfield (Eds.), Calixarenes in the Nanoworld, Springer, Dor-
drecht, 2007.
[7] C.D. Gutsche, Calixarenes: An Introduction, Royal Society of Chemistry, Cam-
bridge, UK, 2008.
[8] F. Troisi, T. Pierro, C. Gaeta, P. Neri, Org. Lett. 11 (2009) 697.
[9] F. Troisi, T. Pierro, C. Gaeta, M. Carratù, P. Neri, Tetrahedron Lett. 50 (2009)
4416.
[10] E. Wanigasekara, C. Gaeta, P. Neri, D.M. Rudkevich, Org. Lett. 10 (2008) 1263.
[11] A. Pailleret, D.W.M. Arrigan, Electrochem. Commun. 3 (2001) 24.
Ion selectivity coefficient measurements for Cs+ as the primary ion (i) in the presence
of interfering ions (j) for a membrane composition of 1a/KTBPB/PVC/NPOE.
Interfering ion (j)
log10 selectivity coefficient (log 10 Kpot
)
Cs,j
SSM (from Eq. (2))
FIM (from Eq. (3))
Rb+
K+
−0.19
−0.86
−1.26
−2.07
−2.64
−4.01
−4.49
−0.58
−1.65
−2.05
−2.65
−2.86
−3.81
−4.12
+
NH4
Na+
Li+
Ca2+
Mg2+

S. Chen et al. / Electrochimica Acta 55 (2010) 7036–7043
7043
[12] A. Pailleret, G. Herzog, D.W.M. Arrigan, Electrochem. Commun. 5 (2003) 68.
[13] R. Vataj, A. Louati, C. Jeunesse, D. Matt, J. Electroanal. Chem. 565 (2004) 295.
[14] G. Diao, J. Gu, Electrochim. Acta 52 (2006) 42.
[15] O. Hammerich, B. Svensmark, in: H. Lund, M.M. Baizer (Eds.), Organic Electro-
chemistry, 3rd ed., Marcel Dekker, New York, 1991 (Chapter 16).
[16] A. Rieker, R. Beisswenger, K. Regier, Tetrahedron 47 (1991) 645.
[17] H. Eickhoff, G. Jung, A. Rieker, Tetrahedron 57 (2001) 353.
[18] R.D. Webster, Electrochem. Commun. 1 (1999) 581.
[35] A.M. Bond, T.L.E. Henderson, D.R. Mann, T.F. Mann, W. Thormann, C.G. Zoski,
Anal. Chem. 60 (1988) 1878.
[36] P. Lehtovuori, H. Joela, Phys. Chem. Chem. Phys. 4 (2002) 1928.
[37] N. Gupta, H. Linschitz, J. Am. Chem. Soc. 119 (1997) 6384.
[38] J.Q. Chambers, in: Z. Rappoport, S. Patai (Eds.), The Chemistry of the Quinonoid
Compounds, vol. II, Wiley, New York, 1988, p. 719 (Chapter 12).
[39] M. Quan, D. Sanchez, M.F. Wasylkiw, D.K. Smith, J. Am. Chem. Soc. 129 (2007)
12847.
[19] U. Svanholm, K. Bechgaard, V.D. Parker, J. Am. Chem. Soc. 96 (1974) 2409.
[20] L.L. Williams, R.D. Webster, J. Am. Chem. Soc. 126 (2004) 12441.
[21] S.B. Lee, C.Y. Lin, P.M.W. Gill, R.D. Webster, J. Org. Chem. 70 (2005) 10466.
[22] G.J. Wilson, C.Y. Lin, R.D. Webster, J. Phys. Chem. B 110 (2006) 11540.
[23] S.B. Lee, A.C. Willis, R.D. Webster, J. Am. Chem. Soc. 128 (2006) 9332.
[24] R.D. Webster, Accounts Chem. Res. 40 (2007) 251.
[40] Y. Hui, E.L.K. Chng, C.Y.L. Chng, H.L. Poh, R.D. Webster, J. Am. Chem. Soc. 131
(2009) 1523.
[41] Y. Hui, E.L.K. Chng, L.P.-L. Chua, W.Z. Liu, R.D. Webster, Anal. Chem. 82 (2010)
1928.
[42] J.S. Kim, A. Ohki, R. Ueki, T. Ishizuka, T. Shimotashiro, S. Maeda, Talanta 48
(1999) 705.
[25] H.M. Peng, R.D. Webster, J. Org. Chem. 73 (2008) 2169.
[43] H. Oh, E.M. Choi, H. Jeong, K.C. Nam, S. Jeon, Talanta 53 (2000) 535.
[44] L. Chen, H. Ju, X. Zeng, X. He, Z. Zhang, Anal. Chim. Acta 447 (2001) 41.
[45] R.K. Mahajan, M. Kumar, V. Sharma, I. Kaur, Talanta 58 (2002) 445.
[46] M. Arvand-Barmchi, M.F. Mousavi, M.A. Zanjanchi, M. Shamsipur, S. Taghvaei,
Anal. Lett. 35 (2002) 7673.
[47] S. Jeon, H. Yeo, H.K. Lee, S.W. Ko, K.C. Nam, Electroanalysis 16 (2004) 472.
[48] Y. Choi, H. Kim, J.K. Lee, S.H. Lee, H.B. Lim, J.S. Kim, Talanta 64 (2004) 975.
[49] R. Bereczki, V. Csokai, A. Grün, I. Bitter, K. Tóth, Anal. Chim. Acta 569 (2006)
42.
[26] W.W. Yao, H.M. Peng, R.D. Webster, J. Phys. Chem. B 112 (2008) 6847.
[27] H.M. Peng, B.F. Choules, W.W. Yao, Z. Zhang, R.D. Webster, P.M.W. Gill, J. Phys.
Chem. B 112 (2008) 10367.
[28] W.W. Yao, H.M. Peng, R.D. Webster, J. Phys. Chem. C 113 (2009) 21805.
[29] R.D. Webster, A.M. Bond, T. Schmidt, J. Chem. Soc. Perkin Trans. 2 (1995) 1365.
[30] E. Bakker, J. Electrochem. Soc. 143 (1996) L83.
[31] E. Bakker, Anal. Chem. 69 (1997) 1061.
[32] F.J.S. de Viteri, D. Diamond, Analyst 119 (1994) 749.
[33] S. Makarychev-Mikhailov, A. Shvarev, E. Bakker, in: X. Zhang, H. Ju, J. Wang
(Eds.), Electrochemical Sensors, Biosensors and Their Biomedical Applications,
Academic Press, New York, 2007 (Chapter 4).
[34] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applica-
tions, 2nd ed., Wiley, New York, 2001.
[50] A. Radu, S. Peper, C. Gonczy, W. Runde, D. Diamond, Electroanalysis 18 (2006)
1379.
[51] D.A. Skoog, D.M. West, F.J. Holler, S.R. Crouch, Fundamentals of Analytical
Chemistry, 8th ed., Thomson Brooks/Cole, Belmont, 2004, p. 274.