Electrochemical Response of the Threading/de-threading Process of Calix[6]arene-based Pseudorotaxanes Anchored on Glassy Carbon Electrodes

Article abstract of DOI:10.1016/j.electacta.2016.12.169

The development of functional materials and smart surfaces has received tremendous attention in recent years. The anchoring of molecular machines and the transfer of their properties on to solid substrates may facilitate the design of new smart devices. H

Full text of DOI:10.1016/j.electacta.2016.12.169

Electrochimica Acta 227 (2017) 391–400
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journal homepage: www.elsevier.com/locate/electacta
Electrochemical Response of the Threading/de-threading Process of
Calix[6]arene-based Pseudorotaxanes Anchored on Glassy Carbon
Electrodes
a
b,
a
a,
c
Guido Orlandini , Jessica Groppi *, Andrea Secchi , Arturo Arduini *, Jeremy D. Kilburn
a
Dipartimento di Chimica Università degli Studi di Parma, Parco Area delle Scienze 17/a, 43124 Parma, Italy
Dipartimento di Scienze e Tecnologie Agro–alimentari, Universita` di Bologna, viale Fanin 44, 40129 Bologna, Italy
King’s College, The University of Aberdeen, Aberdeen AB24 3FX, UK
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 September 2016
Received in revised form 21 November 2016
Accepted 26 December 2016
Available online 7 January 2017
The development of functional materials and smart surfaces has received tremendous attention in recent
years. The anchoring of molecular machines and the transfer of their properties on to solid substrates
may facilitate the design of new smart devices. Herein, we report an approach to the covalent
modification of glassy carbon electrodes with electrochemically responsive tris(N-phenylureido)calix[6]
arene-based pseudorotaxanes. The main focus of this study has been the electrochemically driven
threading/de-threading process of the redox active dialkylviologen axial component of the
supramolecular assemble. After the creation of an amine monolayer on the electrode, pseudorotaxane
systems were generated at the surface using different approaches. It was revealed, through
electrochemical investigations, that the reversibility of the threading/de-threading process of the axle
depended on the nature of the alkyl substituents on the axle.
Keywords:
calixarene
glassy carbon electrode
pseudorotaxane and rotaxane
viologen
electrochemically switchable Host-Guest
system
©
2016 Elsevier Ltd. All rights reserved.
1
. Introduction
contrast, Beer et al. reported that redox active bis-ferrocenyl
rotaxanes, exploited in solution for chemical sensing, maintain
their properties when organized in SAMs on gold electrodes [9]. In
related work, the same authors reported that a [2]catenane, able to
bind chloride ions in solution selectively, lost its binding properties
when anchored to gold electrodes [10]. Some of us demonstrated
that the ability of tris(N-phenylureido)calix[6]arene derivatives,
such as 1 (Fig. 1), to act as three-dimensional heteroditopic
receptors, able to form oriented pseudorotaxanes and rotaxanes
with viologen salts [11–13], is substantially maintained when
anchored on Si(100) [14] and polycrystalline Cu [15] surfaces, and
that the electrochemically driven threading/de-threading was
unaffected by the nature of the inorganic surface. The formation of
these pseudorotaxane-based systems was later exploited for the
assembly of gold nanoparticles on functionalized Si(100) surfaces
[16].
Glassy carbon (GC) electrodes have been extensively studied as
anchoring supports for monolayers aimed at analytical applica-
tions [17,18]. Yet, despite its high electrical conductivity, the
stability in a wide potential window, the possibility to directly
monitor the extent of the surfaces functionalization with redox
active species and the electrochemical properties conferred by the
monolayer, to the best of our knowledge, no studies have been so
far reported on the role of GC as a supporting matrix for prototypes
During the last decades, the tremendous advance in the
comprehension of molecular recognition processes has allowed
the transfer of the principles and methods of Supramolecular
Chemistry [1,2] to the construction of new nanodevices and
functional materials and in particular to supporting Host-Guest
systems on solid surfaces [3–5]. Relatively less explored is how the
functioning mode of a prototype of molecular machine [6,7] is
affected by its anchoring on a solid support or by the nature of the
solid support. This is particularly true in those cases where these
systems should operate under the action of electrochemical
stimuli and the nature of the interaction between the anchored
active coating and the solid support could modify the response
unpredictably. Cooke and co-workers [8] reported that the
reversibility of electrochemically responsive pseudorotaxanes,
obtained by anchoring tetrathiofulvalene derivatives on gold in
a self-assembled monolayer (SAM), is significantly ꢀ and adversely
ꢀ
modified compared to the behaviour observed in solution. In
*
Corresponding authors.
E-mail addresses: jessica.groppi@unibo.it (J. Groppi), arturo.arduini@unipr.it
(
A. Arduini).
http://dx.doi.org/10.1016/j.electacta.2016.12.169
013-4686/© 2016 Elsevier Ltd. All rights reserved.
0

3
92
G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
Fig. 1. Calix[6]arene derivatives focus of this study.
of molecular machines based on pseudorotaxanes and rotaxanes
and their working mode. Herein, we present a study aimed at
evaluating the electrochemical response of calix[6]arene-based
pseudorotaxanes and rotaxanes supported on GC electrodes.
and the resulting organic phase washed with a 10% w/v solution of
HCl (80 mL) and twice with water (2ꢂ80 mL). The separated
2 4
organic phase was dried over anhydrous Na SO , filtered to remove
the drying agent and the solvent evaporated to dryness under
reduced pressure. The residue was purified by column chroma-
tography on silica gel (hexane/ethyl acetate 8:2) to yield 0.92 g of 5
2. Experimental
ꢁ
1
as a pale yellow solid (54%). m.p. = 78–79 C; H NMR (CDCl
400 MHz) = 7.69 (s, 6H), 7.23 (s, 6H), 4.4 (broad signal, 6H), 3.8 (m,
H), 3.6 (broad signal, 6H), 2.88 (s, 9H), 2.21 (t, 6H, J = 7.6 Hz),
3
,
2.1. Chemicals and Synthesis
d
6
13
All solvents were dried using standard procedures; all other
3
1.7-1.3 (multiple signals, 96H); C NMR (CDCl ,100 MHz) d= 173.3,
reagents were of reagent grade quality obtained from commercial
suppliers and were used without further purification. NMR spectra
were recorded at 400 MHz for 1H and 100 MHz for C. Melting
159.9, 154.3, 146.8, 143.6, 135.6, 132.2, 127.4, 123.1, 79.9, 74.0, 60.0,
35.6, 34.3, 31.5, 31.3, 30.9, 30.3, 29.5, 29.3, 29.1, 28.1 26.0, 25.1; MS
13
+
+
(ESI): m/z: 1725.6 (86) [M+Na] , 1726.6 (100) [M+Na] , 1727.6 (64)
+
points are uncorrected. Chemical shifts are expressed in ppm (
using the residual solvent signal as internal reference (7.26 ppm for
CHCl , 3.31 for CH OH and 2.50 for DMSO). Mass spectra were
recorded in ESI mode. The synthesis of Calix[6]arenes 1 [19] and 4
20], compound 8 [21] and 1-octadecyl tosylate 10 [14] has been
d
)
[M+Na]
Calix[6]arene (6). In a 100 mL round bottom flask kept under
nitrogen atmosphere, a tip of spatula of Pd/C catalyst was
3
3
cautiously added to
a suspension of compound 5 (0.52 g,
[
0.31 mmol) in methanol (50 mL), then hydrazine monohydrate
(1.53 g, 30 mmol) was added dropwise. The resulting heteroge-
neous mixture was refluxed for 6 h, cooled to room temperature
and then filtered, under nitrogen atmosphere, through a celite
pad to remove the catalyst. The filtered solution was evaporated
to dryness under reduced pressure to yield 0.48 g of 6 as a white
solid (98%). Because of its instability to air, compound 6 was
employed in the following synthetic step without any further
purification.
reported in previous publications.
tert-butyl 11-bromoundecanoate(3). In
bottom flask, kept under nitrogen atmosphere and at 0 C through
a
250 mL round
ꢁ
an external ice bath, to a solution of 11-bromoundecanoic acid
(
5 g, 19 mmol) in freshly distilled tetrahydrofuran (100 mL),
trifluoroacetic anhydride (7.8 g, 38 mmol) was added dropwise.
ꢁ
The reaction mixture was stirred for 2 hours at 0 C, then
tert-butanol was added (7 g, 95 mmol). After stirring the resulting
reaction mixture for 16 hours at room temperature, the solvent
was removed under reduced pressure and the residue taken up
with ethyl acetate (100 mL). The organic phase was washed with a
Calix[6]arene (7). In a 50 mL round bottom flask kept under
nitrogen atmosphere, to a solution of 6 (0.48 g, 0.3 mmol) in
dichloromethane (20 mL), phenyl isocyanate (0.17 g, 1.44 mmol)
was added dropwise. The reaction mixture was stirred for 2 hours
at room temperature, then the solvent was evaporated to dryness
under reduced pressure and the residue was purified by column
chromatography on silica gel (hexane/ethyl acetate 7:3) to yield
0
(
.1 M solution of NaHCO
2ꢂ100 mL). The separated organic phase was dried over anhy-
drous Na SO , filtered to remove the drying agent and the solvent
removed under reduced pressure to dryness to yield 5.2 g of 3 as a
3
(100 mL) and twice with water
2
4
1
ꢁ
1
colorless oil (87%). H NMR (CDCl
J = 6.9 Hz), 2.20 (t, 2H, J = 7.5 Hz), 1.85 (m, 2H, J = 7.2 Hz), 1.58 (m, 2H,
J = 7.2 Hz), 1.45 (s, 9H), 1.29 (m, 14H); 13C NMR (CDCl
, 100 MHz)
= 173.3, 79.9, 35.6, 34.0, 32.8, 29.3, 29.2, 29.0, 28.7, 28.1, 25.1; MS
3
, 400 MHz)
d
= 3.41 (t, 2H,
0.42 g of 7 as a pale yellow solid (71%). m.p. = 111–112 C; H NMR
(CDCl , 400 MHz) = 7.22 (s, 6H), 7.12 (s, 6H), 6.94 (s, 6H), 6.31
3
d
3
(s, 6H), 4.42 (d, 6H, J = 13.2 Hz), 3.93 (broad signal, 6H), 3.58 (d, 6H,
d
14.4 Hz), 2.85 (s, 9H), 2.22 (broad signal, 6H), 1.91 (s, 6H), 1.59
+
+
13
(
ESI): m/z: 343.2 (100) [M+Na] , 344.2 (97) [M+Na] .
Calix[6]arene (5). In a 100 mL sealed glass autoclave, a
3
(s, 12H), 1.47 (s, 54H), 1.32 (s, 36H); C NMR (CDCl , 100 MHz)
d
= 173.4, 155.0, 154.6, 152.3, 146.8, 138.3, 135.7, 133.1, 132.4, 128.9,
heterogeneous mixture of calixarene
4
(1 g, 1.1 mmol),
3
127.7, 123.4, 123.1, 120.5, 79.9, 73.1, 60.2, 35.6, 34.2, 31.5, 31.0, 30.6,
29.6, 29.5, 29.3, 29.1, 28.1, 26.3, 25.1; MS (ESI): m/z: 1971.0 (71)
[M+H] , 1972.0 (100) [M+Na] , 1973.0 (71) [M+Na] .
Calix[6]arene (2). In a 10 mL round bottom flask kept under
nitrogen atmosphere, to a solution of 7 (0.02 g, 0.01 mmol) in
(
2 g, 6.2 mmol) and K CO (0.7 g, 5.1 mmol) in CH
2
3
3
CN (60 mL)
+
+
+
was refluxed under vigorous stirring for 72 h. After cooling to room
temperature, the solvent was evaporated to dryness under reduced
pressure. The solid residue was taken up with ethyl acetate (80 mL)

G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
393
dichloromethane (2 mL), trifluoroacetic acid (0.3 g, 1 mmol) was
2.3. General Procedure for the Modification of GC Electrodes with
added dropwise. The reaction mixture was stirred for 1 hour at
Pseudorotaxanes 1ꢃ9 and 1ꢃ12
room temperature, then the solvent was removed under reduced
1
pressure to yield 0.015 g of 2 as a pale yellow solid (83%).
NMR (CDCl , 400 Hz)
s, 6H), 6.31 (s, 6H), 4.39 (d, 6H, J = 13.2 Hz), 3.93 (t, 6H,
H
In a 10 mL flask, to a solution of calix[6]arene 1 (0.07 g,
0.05 mmol) in CH Cl (5 mL) a molar excess of the appropriate axle
2 2
3
d
= 7.22 (s, 6H), 7.12 (s, 6H), 7.11 (s,9H), 6.9
(
(9 and 12, 0.07 mmol) was added. The solution was stirred at r.t. for
1 hour and then filtered to remove the excess of the salt; afterwards
EDC (0.009 g, 0.07 mmol) and NHS (0.008 g, 0.07 mmol) were
added to the solution, which was stirred for 1 h and then the
electrodes were dipped in the solution. After 4 h, the electrodes
were rinsed with water (5 ml), acetonitrile (5 ml) and left to dry in
air for five minutes.
J = 6.4 Hz), 3.58 (d, 6H, 14.4 Hz), 2.89 (s, 9H), 2.36 (t, 6H,
J = 7.2 Hz), 1.9- 1.8 (m, 12H), 1.64 (s, 12H), 1.47 (s, 36H), 1.32
1
3
(
1
1
2
s, 54H); C NMR (CDCl
52.3, 146.8, 138.3, 135.7, 133.8, 133.1, 132.2, 128.9, 127.7, 123.4,
23.1, 120.5, 73.0, 60.3, 34.2, 31.5, 31.0, 30.6, 29.6, 29.5, 29.3, 29.1,
3
, 100 MHz) d= 179.1, 168.5, 155.0, 154.6,
+
8.1, 26.3, 25.1; MS (ESI): m/z: 1824.7 (80) [M+Na] , 1825.7 (100)
M+Na] , 1826.7 (70) [M+Na] .
+
+
[
Axle 9. In a sealed 100 mL glass autoclave, a solution of 8
0.5 g, 0.8 mmol) and 11-bromoundecanoic acid (0.7 g, 2.6 mmol)
2.4. Electrochemical Experiments
(
in CH
solution was evaporated to dryness under reduced pressure. The
solid residue was triturated with CH CN to afford 0.4 g of 9 as a
3
CN (40 mL) was refluxed for 4 days. Afterwards, the
Electrochemical experiments were performed in a conventional
three electrode cell using an Autolab PGSTAT30 Potentiostat/
Galvanostat and data were analysed with Origin 7.0 software. A
3
ꢁ
1
+
2
yellow solid (50%). m.p. = 240–243 C;
00 MHz) = 12.0 (br.s, 1H) 9.5.9.4 (m, 4H), 8.83 (d, 4H,
J = 6.8 Hz), 7.49 (d, 0.5H, J = 8.0 Hz), 7.3-7.1 (m, 10H), 7.11
d, 0.5H, J = 8.0 Hz), 5.18 (s, 1H), 4.7–4.6 (m, 4H), 4.11 (t, 2H,
J = 6.8 Hz), 2.20 (s, 1H), 2.18 (t, 2H, J = 7.6 Hz), 2.0–1.9 (m, 4H),
.6-1.4 (m, 4H), 1.3–1.2 (m, 16H); 13C NMR (DMSO-d
, 100 MHz)
= 174.9, 172.4, 149.0, 146.2, 146.1, 139.5, 138.1, 129.0, 128.9, 128.5,
27.5, 127.1, 125.9, 64.9, 61.3, 61.2, 56.3, 34.1, 31.2, 31.0, 29.2, 29.1,
H
NMR (DMSO-d
6
,
Ag/Ag electrode was used as the reference electrode, and a 1 cm
4
d
platinum gauze as the counter electrode. Ferrocene was used as
internal standard. The working electrode was a 3 mm diameter
2
(
(0.071 cm ) glassy carbon disc (HTW, Hochtemperatur-Werkstoffe
GmbH, Germany) sealed in glass tube and wired up with copper
wire by using melted indium (Aldrich). Prior to modification, the
GC working electrodes were dry polished with silicon carbide
polishing paper (grade 1200) and rinsed with deionized water
followed by sonication for 5 min in acetonitrile. Electrochemical
Impedance Spectroscopy (EIS) response was analysed by means of
a classic Randles equivalent circuit and fitted using the Autolab FRA
software. The modification of glassy carbon surfaces with amine
monolayers was performed as previously reported [18].
1
6
d
1
2
9.0, 28.8, 28.2, 25.9, 25.3, 25.0, 24.9, 21.2; MS (ES): m/z: 635.5
+
+
(
100) [MꢀH] , 636.5 (86) [MꢀH] .
Salt 11. In a 100 mL round-bottomed flask, 1-octadecyl tosylate
0
10 (1 g, 2.4 mmol) and 4,4 -dipyridyl (1.1 g, 7.1 mmol) were
3
dissolved in CH CN (50 mL) and the solution was refluxed for
24 h. Then the solvent was evaporated at reduced pressure and the
oily residue obtained was triturated with EtOAc (3 ꢂ 20 mL) until of
3. Results and discussion
1
1 precipitated as a white solid recovered by suction filtration (1 g,
ꢁ
1
6
4%). m.p. = 97–99 C; H NMR (CD
3
OD, 400 MHz)
d
= 9.12 (d, 2H,
3.1. Design and synthesis of calix[6]arene derivatives suitable for GC
J = 6.0 Hz), 8.85 (d, 2H, J = 6.8 Hz), 8.52 (d, 2H, J = 5.6 Hz), 7.99 (d, 2H,
J = 6.4 Hz), 7.72 (d, 2H, J = 7.6 Hz), 7.24 (d, 2H, J = 8.0 Hz), 4.68 (t, 2H,
J = 7.6 Hz), 2.37 (s, 3H), 2.18 (t, 2H, J = 6.8 Hz), 1.4–1.3 (m, 30H), 0.92
electrodes modification
Since in calix[6]arene derivatives such as 1 (Fig. 1) the two
domains able to bind viologen salts are at the upper rim, namely
the three phenylureas and the electron-rich aromatic cavity, the
potential grafting points for the functionalisation of GC electrodes
13
(
t, 3H, J = 6.0 Hz); C NMR (CD
3
OD,100 MHz)
d= 153.9,150.4,142.3,
1
2
42.2, 140.2, 128.4, 125.7, 125.6, 122.1, 61.4, 31.7, 31.1, 29.4, 29.3,
9.2, 29.1, 28.7, 25.8, 22.3, 19.9, 13.0; MS (ESI): m/z: 409.3 (100)
+
+
[
MꢀTsO] , 410.3 (33) [MꢀTsO] .
were placed at the lower rim, in the form of three v-undecanoic
Axle 12. In a sealed 100 mL glass autoclave, a solution of salt 11
0.5 g, 0.9 mmol) and 11-bromoundecanoic acid (0.6 g, 2.7 mmol) in
chains (vide infra). Such an approach to the anchoring of the
macrocycle on the electrode allows its recognition sites to be
exposed toward the solution, favouring the threading of a
viologen-based “axle” into the calix[6]arene “wheel” in low
polarity solvents, as previously shown in solution [13,22].
Furthermore, long undecyl alkyl chains were chosen as spacers
between the electron-rich cavity and the electrode in order to
minimise any possible effect of the GC surface on the formation/
decomplexation processes of the pseudorotaxanes.
The novel calix[6]arene-based wheel 2 was obtained as
illustrated in Scheme 1. Trinitro calix[6]arene (5) was synthesised
2 3
in 54% yield by reacting, in refluxing acetonitrile using K CO as
base, the known calix[6]arene precursor 4 with an excess of ester 3.
The nitro groups of 5 were converted in high yields to amines with
hydrazine monohydrate using Pd/C as catalyst. Reaction of the
resulting triamino 6 with phenylisocyanate in dry dichloro-
methane calix[6]arene 7 in 71% yield. Finally, 2 was obtained, in
27% overall yield, by removal of the tert-butyl protecting groups of
7 with trifluoroacetic acid (TFA) in dichloromethane.
(
3
CH CN (40 mL) was refluxed for 4 days. Afterwards, the solution
was evaporated to dryness under reduced pressure. The solid
residue was triturated with CH
solid (62%). m.p. = 250–253 C; H NMR (DMSO-d6, 400 MHz)
3
CN to afford 0.6 g of 12 as a yellow
ꢁ
1
d
= 12.0 (broad signal, 1H) 9.39 (d, 4H, J = 6.2 Hz), 8.79 (d, 4H,
J = 6.2 Hz), 7.49 (d, 1H, J = 8.0 Hz), 7.12 (d, 1H, J = 8.0 Hz), 4.69 (t, 2H,
J = 7.2 Hz), 2.38 (s, 2H), 2.18 (t, 2H, J = 7.6 Hz), 2.0 (br.s, 4H), 1.5 (br.s,
2
H), 1.3–1.2 (m, 42H), 0.85 (t, 3 H, J = 6.0 Hz); 13C NMR (DMSO-d6,
1
6
2
00 MHz)
1.4, 40.5, 40.3, 40.1, 39.9, 39.7, 39.5, 39.3, 34.1, 31.7, 31.2, 29.5, 29.4,
9.1, 29.0, 28.8, 25.9, 24.9, 22.6, 21.2, 14.4; MS (ESI): m/z: 593.6
d= 175.0, 149.1, 146.2, 146.0, 138.2, 128.5, 127.1, 125.9,
+
+
(
100) [MꢀH] , 594.6 (81) [MꢀH] .
2.2. Modification of GC Electrodes with calix[6]arene 2
In a 10 mL flask, to a solution of calixarene 2 (0.09 g, 0.05 mmol)
in DMF (5 mL), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
(
(
EDC) (0.027 g, 0.21 mmol) and N-hydroxysuccinimide (NHS)
0.024 g, 0.21 mmol) were added. The solution was stirred for
The structure of wheel 2 was confirmed through MS and NMR
analysis. As expected, the latter measurements, run in CDCl ,
3
1
h and then the electrodes were dipped for at least 4 h. The
showed that the calix[6]arene macrocycle adopts, on the NMR
time-scale, a pseudo-cone conformation in which the three
alternate N-phenylureido aromatic rings define a trigonal prism,
electrodes were then rinsed with water (5 mL), acetonitrile (5 mL)
and left to dry in air.

3
94
G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
ꢁ
2ꢄ
2
H
Scheme 1. Reagents and Conditions: i) TFAA, t-butanol, THF, 0 C to r.t., 16 h, 87%; ii) K
2
CO
3
, CH
3
CN, reflux, 72 h, 54%; iii) NH
2
NH
O, Pd/C, MeOH, reflux, 6 h, 98%; iv)
6 5 2 2 2 2
C H NCO, CH Cl , r.t., 2 h, 71%; v) TFA, CH Cl , r.t., 1 h, 83%.
while the remaining phenolic units are tilted, orienting their
relative methoxy groups inside the electron-rich cavity of the
macrocycle [11,22]. For this reason, in the H-NMR spectrum of 2,
using the meniscus setup described previously (See SI) [25]. After
the removal of the Boc group in acidic conditions (HCl 4 M), the
electrode was available for further covalent functionalization using
the diamine linker as anchoring point. Among the possible
covalent bonds that an amine function could form, the amide
bond is very stable, and can be readily obtained by reacting the
mono-grafted EDA linker with a molecule characterised by a
1
the signal associated to the methoxy protons is characterised by
significant broadness and is up-field shifted
compared to the expected chemical shift (ca. 3.8 ppm). Further
diagnostic signals are the two doublets at = 4.4 and 3.6 ppm
J = 14.4 Hz) assigned to the bridging methylene units of the
aromatic rings.
(d= 2.9 ppm)
d
(
v-carboxylic acid alkyl chain. The covalent attachment of the
tricarboxy-modified calix[6]arene 2 onto GC electrodes was
therefore achieved by dipping a series of the previously described
amino-coated electrodes in a solution of 2 in DMF with ethyl
3.2. Modification of the GC electrodes
(
dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysucci-
Within the techniques available for the grafting of organic
nimide (NHS) (coupling agents frequently used for solid-phase
synthesis) (Scheme 2). Since 2 is a non-redox active compound, the
characterisation and quantification of the species at the surface by
direct cyclic voltammetry was not possible. As a consequence, we
recorded a cyclic voltammogram (CV) in a 0.1 M KCl solution
monolayers at GC surfaces, the oxidation of primary amines at
carbon electrodes represents one of the most efficient and versatile
approaches, as it allows functionalising the carbon surface with
stable and uniform layers. It must be noted that, although the
electrochemical methodologies for creating such monolayers have
reached quite advanced control over the modification process,
imperfections and formation of multilayers are still possible.
Nonetheless, the generated monolayers allow further modification
by introducing suitable linking groups through solid phase
synthesis techniques (SPPS) [23,24]. Thus GC electrodes were
modified with a uniform monolayer of N-tert-butyloxycarbonyl
ethylenediamine (Boc-EDA) by sweeping the potential of the
electrode in the 0.4–1.95 V range (vs. Ag/AgCl), for 5 times, in a
3 6
containing 5 mM K [Fe(CN) ] after each synthetic step to monitor
the interfacial changes occurring at the GC surface. All the CVs
recorded are presented in Fig. 2. The intensity of the redox peaks at
+
2+
3+
E
mp = 0.2 V vs. Ag/Ag , associated with the Fe /Fe pair, is highly
affected by the changes in thickness of the organic layer covering
the electrode surface. The decrease of intensity of this peak in the
CV corresponding to the electrode modified with 2 (dot-dashed
line in Fig. 2), confirmed that the coupling reaction had occurred
and a monolayer of calix[6]arenes was formed at the electrode. A
further verification of the successful electrode modification was
obtained by EIS measurements [26]. Modifying the nature of the
2
0 mM solution of Boc-EDA in acetonitrile with tetrabutylammo-
nium tetrafluoroborate (TBATFB) 0.1 M as supporting electrolyte,

G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
395
Scheme 2. Schematic representation of GC surface functionalization with EDA-Boc, followed by Boc deprotection and coupling with calix[6]arene 2.
Fig. 3. Nyquist plots of electrochemical impedance spectra obtained at glassy
carbon electrode in presence of 1 mM K Fe(CN) electrochemical probe + 0.1 M KCl
ꢀ
1
Fig. 2. Cyclic voltammograms recorded at a scan rate of 100 mV s in 0.1 M KCl
solution containing 5 mM K [Fe(CN) ]/K [Fe(CN) ] (1:1) before (solid) and after
3
6
3
6
4
6
aqueous solution for: bare (diamonds), EDA-Boc functionalized (triangles), ꢀNH
2
electrochemical modification with EDA-Boc (dotted), deprotection with HCl 4 M in
dioxane (dashed) and grafting of calixarene 2 (dot-dashed).
modified (squares) and calixarene 2 modified (dots) electrode. Fixed potential of
220 mV ꢅ 10 mV within the frequency range of 0.1 to 106 Hz.
coating layer, a substantial alteration of the resistance inferred to
the electrode was observed (Fig. 3). Starting with a resistance value
process, were calculated as the average of anodic potential (E
cathodic potential (E ) using Eq. (1):
c
a
) and
(1)
of 258
tion of the surface, we measured an increased resistance value of
39 for the modified electrode. The small variation of the
V
for a bare GC electrode, after the complete functionaliza-
ꢁ
’
E = (E
a
+ E
c
)/2
3
V
Consistent with our previous results, obtained with similar
resistance value recorded upon derivatization of the CG electrode
with 2 is probably due to the bulkiness of this compound that
prohibits the constitution of a dense organic monolayer on the
electrode.
calix[6]arene derivatives [13], axle DOVꢂ2TsO, when threaded in
ꢁ
’
the cavity of 1 (1ꢃDOVꢂ2TsO) shows a shift of the E of its first
1
redox peak of ca. 160 mV to lower potentials (dashed line, Fig. 4)
compared to the free axle (dotted line, Fig. 4). This shift has been
explained as being due to the stabilization induced by the electron-
rich cavity of the macrocycle on the viologen core of DOV. The peak
3.3. Pseudorotaxane Formation at Chemically Modified GC Electrodes
ꢁ
’
associated to the second redox process presents the same E
2
for
Before studying the formation of possible pseudorotaxane
both the free and the complexed axle, suggesting that after the first
ꢄ
complexes at the liquid-solid interphase between the calix[6]
arene-modified GC electrodes (2/EDA/GC) and the N,N-dioctyl
viologen ditosylate (DOVꢂ2TsO), taken as a typical prototype of a
N,N-dialkylviologen-based axle, we initially evaluated the electro-
chemical behaviour of the free salt and of its pseudorotaxane
complex with calix[6]arene 1 (1ꢃDOVꢂ2TsO) in low polarity
reduction step the radical cation specie formed (DOV ꢂTsO)
dethreads from the cavity of 1. The results obtained for the GC
electrodes modified with 2 were comparable with what had been
ꢁ
’
seen for 1ꢃDOVꢂ2TsO in solution. The E measured was shifted to
1
lower potentials compared to the free DOVꢂ2TsO (solid line,
Fig. 4), although to a lesser extent (ca. 100 mV). More importantly,
after the first reduction step, the reduced viologen cation
ꢀ
1
solvents. The CVs, recorded at a scan rate of 100 mV s
solution of 0.1 M TBAPF in DCM, are reported in Fig. 4 as dashed
line. The formal standard potential (E ), associated with the redox
in a
ꢁ
’
6
generated apparently dethreads from the cavity, since its E
2
ꢁ
’
value is comparable to the one obtained performing the

3
96
G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
ꢀ
1
Fig. 4. (left) CVs recorded at a scan rate of 100 mV s in a 0.1 M TBAPF
6
solution in DCM of a 5 mM solution of DOV ꢂ 2TsO with (dotted line) or without (dashed line) wheel 1
and of DOV ꢂ 2TsO in presence of the calix[6]arene-modified GC electrodes 2/EDA/GC (solid line); (right) schematic representation of the corresponding complexation
processes in solution and at the solid/liquid interphase of the electrode.
measurement on the free DOVꢂ2TsO (dotted line, Fig. 4). These
results suggested that N,N-dialkylviologen-based axles like
DOVꢂ2TsO can easily have access to the aromatic cavity of tris
surface coverage (
in Eq. (2):
G
) of the electrode using Faraday’s law described
G
= Q/nFA
r
(2)
(
N-phenylureido)calix[6]arene derivatives even when the latter
are covalently attached on the glassy carbon surface.
where Q is the charge, obtained from integration of the baseline-
corrected area under the oxidation/reduction peaks, F the Faraday
constant, A the geometric area of the electrode (0.071 cm ), n = 2
The threading/de-threading process of the pseudorotaxane is
only partially reversible, as suggested by the clear changes in the
redox peaks, which occurred by increasing the scans number
2
the number of electrons transferred, and
of the surface. According to literature [27], r
r
is the roughness factor
values could vary
(
Fig. 5). After four scans, the dissociation of the complex on the
surface is complete, as demonstrated by the disappearance of its
relative reduction peak in the CV, while the only reduction peaks
present are relative to the free DOVꢂ2TsO in solution. Such non-
reversibility of the pseudorotaxane formation could be explained
by considering the diffusion processes occurring: the guest, after
reduction and the consequent de-threading, is dispersed in the
bulk solution and, once oxidised, the re-complexation process is
not favoured. Calculating the current associated with the redox
processes of the pseudorotaxane covalently bound to the GC
surface, (DOVꢂ2TsO)ꢆ2/EDA/GC, it was possible to determine the
between 4 and 8 for the polishing technique adopted and 4 was
used in these calculations. The calculated surface coverage,
ꢀ
2
G
= 139 pmol cm , is about one order of magnitude smaller
compared to previous results reported in literature for thick
EDA-Boc monolayer electrografted to GC electrodes [18].
Several explanations may justify the lower amount of
molecules attached to the surface: a) ideally, each molecule of 2
covalently binds three amine moieties on the surface, reducing
considerably the theoretical number of possible pseudorotaxanes
on the surface; b) the three carboxylic groups of the calixarene may
ꢀ1
Fig. 5. Cyclic voltammograms recorded at a scan rate of 100 mV s in 0.1 M TBAPF
6
solution in dichloromethane GC electrodes modified by calixarene derivative
2
ꢃDOVꢂ2TsO. Second (solid), third (dotted) and fourth (dashed) scan are represented.

G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
397
bind three not closely located amine functions, reducing with the
3.4. Creation of a Rotaxane-like System at the GC Electrode
steric hindrance of its cavity the effective number of available sites
on the electrode surface; c) the redox active centre, represented by
the viologen-based guest, is not covalently attached to the
electrode: it is plausible that certain host molecules appended
to the electrode refuse to form pseudorotaxanes due to their wrong
orientation toward the solution; d) since the calculations were
performed on the second scan of the CV measurement, they do not
consider the amount of pseudorotaxane irreversibly dissociated
during the first scan.
The good results obtained for the formation of calix[6]arene-
based pseudorotaxanes at the GC surface prompted us to
investigate the possibility to build-up [2]rotaxane systems using
a similar strategy. Compared to pseudorotaxanes, [2]rotaxanes are
in fact two-component interlocked systems in which the axial (or
threaded) component is confined inside the circular one (wheel)
because of the bulkiness of two stoppers placed at its termini
which prevent the wheel from slipping off the axle. In this context,
we thought the carbon surface of the GC electrode could serve both
3 2 2
Scheme 3. Reagents and conditions: i) 11-bromoundecanoic acid, CH CN, reflux, 4 days, 50%; ii) CH Cl , r.t., 1 h; iii) EDC, NHS, DCM, 5 h, r.t.

3
98
G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
complexes grafted on the surface through the axle can be now
considered as real rotaxane systems in which the calix[6]arene
wheel cannot escape from the surface in the solution. Proof of the
stability of the 1ꢃ(9ꢂ2X)/EDA/GC electrode was provided by the
intensity of the redox peaks relative to 9 which remained
unchanged even after 30 scans of the potential. The potential
values of the reduction and oxidation peaks confirm the inclusion
of the viologen unit of 9 inside the wheel 1, showing a shift of the
ꢁ
0
’
E
by ca. 200 mV to lower potentials compared to DOVꢂ2TsO in
the same solvent. Although the resulting peak was slightly shifted,
the correct determination of the shift of the second redox peak
ꢁ
’
(
E
2
) was impossible, given the interference of molecular oxygen,
whose reduction occurred in the same potential window.
Nonetheless, as expected, the results make clear that the
diphenylacetyl stopper present in axle 9 prevented its slipping
from 1 and confined the redox-active viologen unit in the
macrocycle cavity even in its reduced form.
Calculating the charge obtained from integration of the
baseline-corrected area under the oxidation or reduction peak,
we were able to estimate the amount of rotaxane units linked to
ꢀ
1
ꢀ2
Fig. 6. CVs recorded at a scan rate of 100 mV s in a 0.1 M TBAPF
of (solid) a 5 mM solution of DOV ꢂ 2TsO and (dotted) modified GC electrodes 1ꢆ
9ꢂ2X)/EDA/GC.
6
solution in DCM
the electrode. Using Eq. (2): a value of
G
= 79 pmol cm
was
ꢀ2
obtained. This value is about half the value of 139 pmol cm
calculated in the previous experiment, where calix[6]arene 2 was
directly covalently anchored to the electrode surface. The lower
amount of redox active units on the surface is probably due to the
presence of the complex in solution, being the process governed by
an equilibrium, the yield of the reaction was noticeably reduced.
With the objective of comparing the electrochemical behaviour
of a dialkylviologen salt covalently linked to the surface without
the presence of wheel 1 to the experimental results obtained with
the anchoring of 1ꢃ9ꢂ2X, axle 9ꢂ2X was coupled to a free amine
monolayer using DMF as solvent [25]. However the attempted
formation of the activated ester with EDC was unsuccessful: after
the addition of EDC, the colour of the solution immediately
changed fromyellow to blue, suggesting the formation of a reduced
form of the viologen core of the axle. We speculate that, EDC reacts
more readily with the viologen core instead of forming the
activated ester needed for the coupling with the modified
electrode [28]. Such behaviour was not detected upon activation
of the carboxylic acid group after the pseudorotaxane was formed:
we supposed that the electron-rich cavity of the macrocyclic wheel
shields the viologen unit from reaction with the coupling agent.
(
as a supporting material and as a stopper for the [2]rotaxane axial
component as depicted in Scheme 3. To this end, we designed the
asymmetric viologen axle 9ꢂ2X, where X can be either tosylate or
bromide, bearing a diphenylacetyl group (the stopper) on a C6 alkyl
chain and a
v-carboxylic function on C11 alkyl chain on the
opposite side, suitable for the coupling to an amine monolayer.
This axle provides all the necessary features to: a) perform
unidirectional threading into wheel 1 only from the macrocyle
upper rim; b) its carboxy terminal group may react with the
deprotected EDA-Boc functions grafted on the GC surface.
Axle 9ꢂ2X was synthesised by refluxing the known mono-
alkylated bispyridinium salt 8ꢂTsO with excess of 11-bromoun-
decanoic acid in acetonitrile. The axle was mixed with wheel 2 to
form the pseudorotaxane 1ꢃ9ꢂ2X before its attachment to the GC
electrode surface. The preparation was carried out using a
convergent approach by simply mixing in dichloromethane
1
equimolar amounts of wheel 1 and axle 9ꢂ2X (Scheme 3). The
modification of the electrode was achieved with a solid-phase
synthesis approach similar to the that previously discussed
(
Scheme 3). In particular, a slight excess of axle 9ꢂ2X was added
3
.5. Modification of GC electrode with a pseudorotaxane assembly
to a solution of wheel 1 in dichloromethane. After stirring for one
hour, the solution went from colourless to deep red indicating the
complete formation of the pseudorotaxane 1ꢃ9ꢂ2X. The red
solution was filtered off to remove the uncomplexed and not
soluble viologen axle 9 and the combination of coupling agents
were then added to the filtered solution. Following the formation
of the activated ester, an EDA modified GC electrode (EDA/GC) was
dipped into the above solution for four hours under magnetic
stirring. As usual, the modified electrodes 1ꢃ(9ꢂ2X)/EDA/GC were
Having optimised the conditions for the modification of GC
electrodes with our calix[6]arene-based pseudorotaxanes and
rotaxanes, we decided to study the effect that a long alkyl chain on
the viologen unit could have on the threading/de-threading
process under electrochemical stimuli. As demonstrated in our
previous solution studies, [28] a long C18 alkyl chain acts as a very
effective kinetic stopper in solution so we wanted to determine if it
could similarly prevent the de-threading of the wheel once the
pseudorotaxane was anchored at the surface. Indeed, it could be
supposed that, due to its length, the loss of conformational
freedom needed for the C18 alkyl chain to allow the slipping of the
calix[6]arene macrocycle through its annulus, would be a very slow
and disfavoured process even after the reduction of the viologen
unit. It is reasonable to assume that the monitoring of this process
by cyclic voltammetry will be strongly dependent on the scan rate
employed. To this end, we designed a pseudorotaxane comprising
wheel 1 threaded by axle 12ꢂ2X, where X can be either bromide or
tosylate. Such an axle required, as substituents on the bispyr-
characterised by cyclic voltammetry in a 0.1 M solution of TBAPF
6
in DCM. The CV (Fig. 6) confirmed the successful anchoring of the
pseudorotaxane systems 1ꢃ9ꢂ2X on the GC electrode. As
previously discussed, the surface of the electrode hinders the
slipping of the wheel from the axle, and thus the supramolecular
1
Differently from dichloromethane, axle 9ꢂ2X experiences good solubility in
DMF. In the latter, very polar, solvent the formation of pseudorotaxane species
between calix[6]arenes and viologen-based is greatly reduced. Most important, the
directionality of the threading process, that in low polar solvent such as DCM would
idinium redox active core, a
v-carboxy C11 alkyl chain, whose
occur with the unstoppered (v-carboxy) alkyl chain of 9 preferentially, if not
terminal carboxylic group would be used for the anchoring of the
axle on the EDA/GC surface, and on the other side a C18 alkyl chain
exclusively, from the upper rim of 1, in DMF would occur from both rims of the
2
1
macrocycle yielding a mixture of two orientational pseudorotaxane isomers
.

G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
399
pseudorotaxane formation, its immobilisation on the EDA/GC
electrodes and final characterisation of the modified surfaces 1ꢃ
(
12ꢂ2X)/EDA/GC was accomplished with the procedures previ-
ously described for pseudorotaxanes 1ꢃ9ꢂ2X (Scheme 3). The CVs
ꢀ
1
recorded at 100 mV s
showed the typical pattern of signals
relative to a rotaxane system, [13,29] in which both the reduction
potential peaks are shifted to lower potential compared to a free
viologen derivative in solution (Fig. 7). The decreasing intensity of
the current associated with the redox peaks of the specie at the
surface by scanning the potential window several times, indicates
that, despite the C18 chain presence, the wheel partially de-threads
ꢀ1
form the axle. Lowering the experiments scan rate to 50 mV s , the
de-complexation process was complete after six scans. The result
suggests that, upon reduction of the guest, the wheel slips from the
axle and within the time interval needed to re-oxidise the viologen
unit, the calix[6]arene host diffuses away from the electrode
surface, thus rendering the regeneration of the pseudorotaxane
ꢀ
1
impossible. Nonetheless, at
a scan rate of 100 mV s , the
threading/de-threading process resulted reversible. We speculated
that, the fast reduction/reoxidation process of the viologen,
together with the steric effect of the octadecyl alkyl chain, avoided
the complete decomplexation of the calix[6]arene wheel, favour-
ing the re-inclusion of the viologen unit in the calixarene cavity
once re-oxidised over diffusion processes (Fig. 7a).
0
Scheme 4. Reagents and Conditions: i) 4,4 -bipyridyl, CH
3
CN, reflux, 24 h, 64%; ii)
4
. Conclusions
1
1-bromoundecanoic acid, CH CN, reflux, 4 days, 62%.
3
We presented here a new approach to the covalent modification
of GC electrodes with calix[6]arene and dialkylviologen
based pseudorotaxanes and rotaxanes using solid-phase synthesis
substituent. This dialkylviologen salt was synthesized following
the synthetic route depicted in Scheme 4: the known 1-octadecyl
tosylate 10 was refluxed in acetonitrile with excess of
0
techniques. Initially, the surfaces were modified with a
4
,4 -bipyridyl to obtain the monolakylated bispyridine derivative
tris(N-phenylureido)calix[6]arene derivative: such functionaliza-
tion conferred to the electrode the ability to take up
N,N-dialkylviologen salts in low polarity media such as DCM
1
1
1
1ꢂTsO, which, in turn, was refluxed in acetonitrile with
1-bromoundecanoic acid to yield the target dicationic salt
2ꢂ2X as a yellow powder in 40% of overall yield. The 1ꢃ12ꢂ2X
Fig. 7. (left) pseudorotaxane 1ꢃ12ꢂ2X immobilised on the EDA/GC surface and its electrochemical switch; (right) cyclic voltammograms (10 scans) recorded in a 0.1 M
TBAPF
6
solution in ACN of the modified 1ꢃ(12ꢂ2X)/EDA/GC electrodes at a scan rate of: a) 100 mV sꢀ1; b) 50 mV sꢀ1
.

4
00
G. Orlandini et al. / Electrochimica Acta 227 (2017) 391–400
and to assemble pseudorotaxane systems. In these conditions, the
threading/de-threading processes of the viologen-based axle into
the calix[6]arene-based wheel could be controlled by electro-
chemical stimuli generated by the same electrode to which the
pseudorotaxane was anchored. Next, we designed two solution
generated pseudorotaxane-like structures which could be an-
chored at the surface through the axle, showing different
behaviour under electrochemical stimuli. In the first example, a
bulky group present at one extremity of the axle inhibited its de-
threading from the wheel after the reduction of the viologen unit,
generating a rotaxane, while in the second, the axle was
characterised by a long C18 alkyl chain whose ability to act as a
kinetic stopper was strongly dependant on the scan rate of the
electrochemical experiment. These studies gave further insight on
the behaviour and stability of calix[6]arenes based pseudorotax-
anes and rotaxanes covalently bound to electrode surfaces. Our
results represent a step further toward the creation of electro-
chemically controlled molecular machines confined on GC surfaces
for the development of smarter devices. Future studies could
involve the insertion of a secondary recognition site on the axle,
able to stabilise the mono-reduced form of the viologen axle: using
this work as inspiration it should be possible to design molecular
elevators, shuttles or other switchable Host-Guest systems.
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Acknowledgements
This work was partly supported by the Italian MIUR
10.1039/B809040H.
(
PRIN 2010CX2TLM) and University of Parma. We thank Centro
[
19] J.J. González, R. Ferdani, E. Albertini, J.M. Blasco, A. Arduini, A. Pochini, P.
Prados, J. de Mendoza, Dimeric Capsules by the Self-Assembly of Triureidocalix
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dx.doi.org/10.1002/(SICI)1521-3765(20000103)6:1<73:AID-CHEM73>3.0.
CO;2-#.
Interdipartimentale di Misure “G. Casnati” for NMR and Mass
measurements. We are grateful to Dr. Marco Giannetto of the
Department of Chemistry of the University of Parma for the
electrochemical impedance measurements.
[
20] A. Casnati, L. Domiano, A. Pochini, R. Ungaro, M. Carramolino, J.O. Magrans, P.
M. Nieto, J. Lopezprados, P. Prados, J. Demendoza, R.G. Janssen, W. Verboom, D.
N. Reinhoudt, Synthesis of calix[6] arenes partially functionalized at the upper
rim, Tetrahedron 51 (1995) 12699–12720, doi:http://dx.doi.org/10.1016/0040-
Appendix A. Supplementary data
4
020(95)00826-t.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.2016.
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Constitutionally Isomeric Oriented Calix[6] arene-Based Rotaxanes, Angew.
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