Self-assembled monolayers of mono-tetrathiafulvalene calix[4]pyrroles and their electrochemical sensing of chloride

Article abstract of DOI:10.1002/chem.200901394

A study was conducted to investigate the formation self-assembled monolayers (SAM) of mono-tetrathiafulvalene (TTF) calix[4]pyrroles and their electrochemical sensing of chloride. The study demonstrated that the TTF calix[4]pyrroles had the potential to a

Full text of DOI:10.1002/chem.200901394

DOI: 10.1002/chem.200901394
Self-Assembled Monolayers of Mono-Tetrathiafulvalene Calix[4]pyrroles
and Their Electrochemical Sensing of Chloride
Lise G. Jensen,[a] Kent A. Nielsen,[a] Tony Breton,[b] Jonathan L. Sessler,[c]
Jan O. Jeppesen,*[a] Eric Levillain,*[b] and Lionel Sanguinet*[b]
Since the first reports more than a half-century ago,[1] in-
terest in self-assembled monolayers (SAMs) on metal surfa-
ces has increased almost exponentially. At present, such sup-
ported SAMs are used for numerous applications, ranging
from metallurgy—wherein the surface properties, such as
conductivity,[2] wettability,[3] corrosion resistance,[4] and etch-
ing resistance,[5] are modified—to the preparation of sen-
sors[6] and materials for use in separations.[7] In fact, the use
of SAMs is now recognized as offering a convenient, flexi-
ble, and simple way to tailor the interfacial properties of
metals.[8] This is because the spontaneous organization and
absorption of molecules on metal, metal oxide, or semicon-
ducting surfaces often creates highly ordered systems with
well-defined dimensions on the nanoscale.[9]
monitoring can be completed by using impedance spectro-
scopic measurements. Although, cation (generally metallic
ions) recognition using SAMs is well documented, and sev-
eral systems that exhibit both excellent selectivity and sensi-
tivity have been described in the literature,[12,13] much less
work on anion recognition by using SAMs has been report-
ed, perhaps reflecting the fact that anions have variable
sizes and shapes and exhibit strong solvation in most sol-
vents. The first anion recognition SAM systems were report-
ed by Astruc et al. and involved the use of ferrocene deriva-
tives for dihydrogenophosphate anion detection.[14] More re-
cently, Echegoyen et al. described SAMs for use in acetate
anion recognition.[15] Although these systems are very selec-
tive, their sensitivity is relatively low and the limits of detec-
tion do not reach the sub-millimolar scale.
Calix[4]pyrrole and its derivatives have been extensively
studied in the search for chemosensors capable of recogniz-
ing specific chemical species, with a number of optically
active calix[4]pyrrole-based sensors having now been report-
ed.[16] Some of us have recently described[17] the first fully
successful examples of electrochemically active sensors[18]
based on calix[4]pyrroles; these were prepared by attaching
one, two, or four redox-active tetrathiafulvalene[19] (TTF)
units directly to the calix[4]pyrrole platform so as to pro-
duce receptors with enhanced binding affinities toward
anions as compared to the parent meso-octamethyl calix[4]-
pyrrole.[20] In the case of the mono-TTF calix[4]pyrrole, we
observed[17b] selectivity between the chloride anion (Ka =
2900mÀ1) and the bromide anion (Ka =96mÀ1) in CH2Cl2 so-
lution.
During the last decade,[10,11] increasing attention has been
dedicated to the design and elaboration of SAMs that inte-
grate a molecular or ionic receptor unit with a transducer el-
ement so as to create sensing devices for environmental or
biologic purposes. If a redox-active element is incorporated
into the monolayer or the guest, the recognition event can
be followed by cyclic voltammetry (CV). In other cases, the
[a] L. G. Jensen, Dr. K. A. Nielsen, Prof. J. O. Jeppesen
Department of Physics and Chemistry
University of Southern Denmark
Campusvej 55, DK-5230, Odense M (Denmark)
Fax : (+45)6615-8780
E-mail: joj@ifk.sdu.dk
Homepage: http://www.jojgroup.sdu.dk
[b] Dr. T. Breton, Prof. Dr. E. Levillain, Dr. L. Sanguinet
Universitꢀ d’Angers—CNRS
This success has led us to consider that TTF calix[4]pyr-
roles could be used to create anion-sensing SAMs. These
kinds of receptors are attractive in this regard because they
incorporate within one molecular framework both pyrrole-
based anion recognition and TTF-derived transducer sub-
2 boulevard Lavoisier 49045, Angers Cedex (France)
E-mail: eric.levillain@iniv-angers.fr
lionel.sanguinet@univ-angers.fr
[c] Prof. J. L. Sessler
Department of Chemistry and Biochemistry
The University of Texas at Austin
AHCTUNGTREGuNNUN nits. We wish to report here that such SAMs may be pre-
pared and that they are effective for chloride anion recogni-
tion at the sub-millimolar level, thus providing a sensitivity
that differs dramatically from previously reported systems.
1 University Station, A5300, Austin, TX 78712-0165 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901394.
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Chem. Eur. J. 2009, 15, 8128 – 8133

COMMUNICATION
To test whether TTF calix[4]pyrroles could be used to
construct anion-selective SAMs (i.e., solid-supported devices
for which the solution phase recognition properties would
be transferred to the condensed phase provided by a metal
surface), we synthesized the two mono-TTF calix[4]pyrroles
1 and 2 shown in Figure 1. Both derivatives are functional-
ed with 1 equivalent of CsOH·H2O affording the MPTTF
derivatives 4 or 5, in 79 and 87% yield, respectively. Re-
moval of the tosyl group was carried out by heating 4 or 5
carefully in a 3:1 solvent mixture of THF and MeOH, in the
presence of an excess of NaOMe.[24] This gave 6 and 7 in
good yields. Mixing these latter MPTTF derivatives with ap-
proximately two equivalents of the tripyrrane[23] 10 in a mix-
ture of Me2CO and CH2Cl2 in the presence of one equiva-
lent of tetrabutylammonium chloride (TBACl) and excess
trifluoroacetic acid (TFA) provided the mono-TTF calix[4]-
pyrroles 11 or 12 in 23 and 22% yield, respectively. Heating
a solution of these latter precursors under reflux in the pres-
ence of excess potassium thioacetate (KSAc) in anhydrous
THF then afforded the corresponding thioesters 1 or 2 in 88
and 69% yield, respectively. The control thioesters 8 or 9
were obtained in 70 and 95% yield, respectively, by likewise
heating a mixture of the MPTTF derivatives 6 or 7 with an
excess KSAc in an analogous fashion.
Figure 1. Structural formulas of mono-TTF calix[4]pyrroles 1 and 2; as
detailed in the text, these receptors bear an acetyl-protected alkanethiol
anchoring group suitable for formation of SAMs.
Both mono-TTF calix[4]pyrroles 1 and 2 were isolated as
yellow solids and fully characterized (cf. Supporting Infor-
mation) using traditional techniques. Electrochemical char-
acterization of these species was carried out in a glovebox in
CH2Cl2 solution at room temperature using cyclic voltamme-
try (CV). The electrochemical behavior of the MPTTF
model compounds 8 and 9 was also monitored under similar
conditions. As expected, compounds 1, 2, 8, and 9 display
two one-electron oxidation processes (E1/21 =0.010–0.030 V
and E1/22 =0.450–0.465 V vs. Fc/Fc+), which are ascribed to a
first and second one-electron TTF-centered oxidation event,
respectively.
The mono-TTF calix[4]pyrroles 1 and 2 were designed to
allow the complexation of anions to be detected via binding-
induced changes in the electrochemical properties of the
TTF units. CV was used to probe the putative chloride
anion-induced changes in the redox potentials of compound
1 and 2. Chloride anions were chosen for these initial studies
since they are known to bind well to calix[4]pyrroles, at
ized with an alkanethiol anchor group (protected with an
acetyl group) that was expected to allow for attachment to a
gold substrate.[21] As detailed below, the kinetics for the for-
mation of the SAMs were then followed using quartz crystal
microbalance (QCM) measurements; this allowed us to
probe the effects of conformational changes in the mono-
TTF calix[4]pyrrole skeleton and obtain insights into the
molecular events associated with chloride anion complexa-
tion.
The syntheses of the mono-TTF calix[4]pyrroles 1 and 2,
as well as those of two mono-pyrroloTTF (MPTTF) model
compounds 8 and 9, were carried out as outlined in
Scheme 1.
The routes employed are based on the use of two key pre-
cursors already reported in the literature, namely i) the
MPTTF derivative[22] 3 and ii) the tripyrrane[23] 10. A solu-
tion of the MPTTF building block[22] 3 and an excess of
either 1,6-dibromohexane or 1,10-dibromodecane was treat-
Scheme 1. Synthesis of the two mono-TTF calix[4]pyrroles 1 and 2. Also shown is the synthesis of the two mono-pyrroloTTF (MPTTF) model com-
pounds 8 and 9.
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J. O. Jeppesen, E. Levillain, L. Sanguinet et al.
least under non-competitive conditions, and to induce a con-
formational flip between the limiting 1,3-alternate and cone
conformations.[16,25] The progressive addition of chloride
anions to a solution of the mono-TTF calix[4]pyrroles 1 and
2 in CH2Cl2 at room temperature revealed features that are
consistent with chloride anion binding. These studies also
provided evidence for an exchange rate that is slow on the
CV time scale. For instance, the addition of 0.4 equivalent
of chloride anions (as the tetrabutylammonium salt) to a so-
lution of the mono-TTF calix[4]pyrrole 2 in CH2Cl2 was ob-
served (Figure 2) to give rise to a new peak in the region
reveal any observable shifts in the waves ascribed to the
first oxidation process.
The above background studies completed, SAMs derived
from receptors 1 and 2 and model compounds 8 and 9 were
prepared. Towards this end, the acetyl protecting groups
present in compounds 1, 2, 8, and 9 were removed to afford
the corresponding thiols; this was done by treating with
CsOH·H2O in a mixture of THF and MeOH.[27] The SAMs
themselves were then obtained by immersing homemade
gold substrates[28] for 12 h into CH2Cl2 solutions (1 mm) con-
taining the thiol in question under an anhydrous, oxygen-
free (<1 ppm) argon atmosphere. The SAMs prepared from
compounds 1, 2, 8, and 9 were stable as confirmed by CV by
scanning over repetitive cycles.[29] The CVs of the SAMs did
not shown any shift between the oxidation and the reduction
peaks. Moreover, linear dependencies between peak intensi-
ties and scan rates were observed for all four systems. Both
observations are considered characteristic of an immobilized
redox system.[30]
A comparison of the surface coverage obtained for the
SAMs prepared from the two MPTTF model compounds 8
and 9 with those obtained for the SAMs derived from the
TTF calix[4]pyrroles 1 and 2 revealed that the MPTTF
model compounds have a value which is almost three times
higher (i.e., (1.9Æ0.2ꢂ10À10) molcmÀ2 and (0.7Æ0.1ꢂ
10À10) molcmÀ2 for 8 and 1, respectively[31]). Based on Cory–
Pauling–Koltun (CPK) models, the area per molecule for 8
and 1 were estimated to be (49Æ7) and (79Æ11) ꢃ2, respec-
tively. On this basis, it was concluded that the TTF calix[4]-
pyrroles were not organized as close to the monolayer sur-
face as were the control mono-pyrroles.
Figure 2. CVs of receptor 2 (CH2Cl2, 0.5 mm) obtained by adding increas-
ing quantities of a concentrated CH2Cl2 solution of nBu4NCl containing
0.5 mm 2 so as to counter potential dilution effects (Reference electrode
Fc/Fc+ with nBu4NPF6 (0.1m) as the supporting electrolyte; on Pt at
0.2 VsÀ1).
As true in case of the predicative solution phase analyses
(vide supra), CV was used to probe the changes in the
redox potentials of the SAMs prepared from compounds 1,
2, 8, and 9 as a function of chloride anions. Although, it was
expected that addition of chloride anions to the SAMs de-
rived from the two MPTTF model compounds 8 and 9
would lead only to a minor negative shift of the first TTF-
centered oxidation process, it was actually found that addi-
corresponding to the first oxidation process.[26] This new
peak, at E1/21 =À0.13 V (vs. Fc/Fc+), is ascribed to oxidation
of the supramolecular complex formed from this receptor
and chloride anions (i.e., 2·ClÀ). Further addition of chloride
anions caused the intensity of the peak at E1/21 =+0.03 V
(vs. Fc/Fc+), corresponding to oxidation of the uncomplexed
mono-TTF calix[4]pyrrole 2, to decrease along with a con-
tion of a large amount of chloride anions to the SAMs in-
1
1
current increase in the intensity of the peak at E1/2
=
duced
a
small, but significant positive shift (DE1/2
=
À0.13 V (vs. Fc/Fc+) ascribed to 2·ClÀ. After addition of ca.
1 equivalent of chloride anions to the mono-TTF calix[4]-
pyrrole 2, the peak at E1/21 =+0.03 V (vs. Fc/Fc+) could no
longer be observed and only the peak at E1/21 =À0.13 V (vs.
Fc/Fc+) was seen in the voltammogram. From this point for-
ward, further addition of chloride anions to compound 2 re-
sulted in little observable change. This is taken as evidence
that a strong binding interaction exists between the mono-
TTF calix[4]pyrrole 2 and chloride anions under the condi-
tions of the CV experiment. Similar results were obtained
when the mono-TTF calix[4]pyrrole 1 was studied in the
presence of chloride anions, with the maximum displace-
ment of the first oxidation potentials being DE1/21 =À0.16 V
and À0.17 V for compounds 1 and 2, respectively. Control
experiments, involving analogous titrations of the MPTTF
model compounds 8 and 9 with chloride anions, did not
+15 mV) to the corresponding waves. This observation,
which remains somewhat recondite, stands in marked con-
trast to what is seen in the case of the SAMs derived from
the TTF calix[4]pyrroles 1 and 2. Here, the progressive addi-
tion of chloride anions between 0 and 0.5 mm[32] induced
1
(Figure 3) a significant negative shift (DE1/2 ꢀÀ30 mV) in
the first TTF-centered oxidation waves.
A comparison of the maximum negative shifts induced by
the addition of chloride anions to a solution of the TTF cal-
ix[4]pyrrole 2 (DE1/21 =À170 mV) and to the SAM derived
from it (DE1/21 =À30 mV) reveals that the chloride anion in-
duced shift is much smaller in the case of the supported
system than in solution. This observation is rationalized by
the fact that immobilization of the TTF calix[4]pyrrole re-
ceptor renders more difficult the conformational change
from a 1,3-alternate conformation to the cone conformation
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Calix[4]pyrroles and the Electrochemical Sensing of Chloride
COMMUNICATION
to investigate how the use of a TTF calix[4]pyrrole bearing
a pre-coordinated chloride anion (i.e., 1·ClÀ) would affect
SAM formation. Consequently, the deposition kinetics of 1
in the absence and presence of chloride anion were com-
pared. Because the binding constant (Ka =2.9ꢂ103 mÀ1 in
CH2Cl2 at 298 K)[36] for the interaction between 1 and ClÀ is
not large enough to preclude the presence of substantial
amounts of uncomplexed material when mixed at a 1:1 stoi-
chiometry at initial concentrations of 1.0 mm, these studies
were carried out using 20 equivalents of chloride anions
(tetrabutylammonium counter cation; CH2Cl2 solution; [1]=
1.0 mm). Under these conditions, the fraction of 1·ClÀ is ex-
pected to be very large (ꢁ98%) compared with the small
amount of residual free receptor 1. By employing, the Lang-
muir adsorption isotherm model,[37] it proved possible to es-
tablish (Figure 4) that the deposition kinetics are almost six-
Figure 3. CVs in CH2Cl2 of the SAM prepared from the TTF calix[4]pyr-
role 2 (the surface coverage of 2 is (0.7Æ0.1ꢂ10À10) molcmÀ2) recorded
after additions of successive aliquots of nBu4NCl (Reference electrode
Fc/Fc+ with nBu4NPF6 (0.1m) as the supporting electrolyte; at 0.2 VsÀ1).
Insert: CV titration curves showing the negative shift of the first oxida-
tion potential (measured at the maximum intensity) of 2 seen upon addi-
tion of ClÀ ions.
known to favor anion binding.[33,34] In fact, it is possible that
only a fraction of the immobilized TTF calix[4]pyrroles
have enough free space to undergo this crucial conforma-
tional change. Support for this latter notion came from a
close inspection of the CVs recorded in the presence of
chloride anions. The CVs show (Figure 3) that the oxidation
and reduction peaks become broader upon addition of chlo-
ride anions. Such an observation is consistent with a system
that results from the superposition of two different redox
processes, one corresponding to the first redox process asso-
ciated with the uncomplexed TTF calix[4]pyrrole 2 and one
corresponding to the complex (i.e., 2·ClÀ) formed between
the TTF calix[4]pyrrole 2 and chloride anions. Unfortunate-
ly, however, the separation between these two presumed ox-
idation processes proved too small to resolve under the con-
ditions of the experiment.
Figure 4. Frequency variations as a function of time for the gold-coated
quartz crystal observed during the addition of a 1 mm CH2Cl2 solution of
1 into the QCM cell in the presence of 20 mm of ClÀ (top) and without
ClÀ (bottom). The data points have been fitted according to the Lang-
muir adsorption isotherm model.[37]
To test further whether conformational restrictions involv-
ing the appended TTF calix[4]pyrroles were influencing
their binding ability when contained in SAMs, we performed
fold slower in the presence of chloride anions than in their
absence (kobs =(0.077Æ0.004) sÀ1 for uncomplexed 1 and
(0.0134Æ0.008) sÀ1 for complexed 1·ClÀ). On the other
hand, the deposition kinetics for the MPTTF model com-
pound 8 do not change (see the Supporting Information)
when chloride anions are added into a CH2Cl2 solution of 8
prior to adsorption to a metal surface (kobs =(0.035Æ
0.001) sÀ1 with and without chloride anions).
The fact that the kinetics for the formation of SAMs de-
rived from the TTF calix[4]pyrrole 1 are dramatically affect-
ed by the presence of chloride anions, whereas no effect is
observed in the case of the MPTTF model compound 8,
serves to highlight the negative effects that electrostatic re-
pulsion and/or steric obstruction can have on the self-assem-
bly process when the complexed TTF calix[4]pyrrole 1·ClÀ is
present. Furthermore, in accord with what was seen in the
absence of chloride anion (vide supra), it should be noted
that a clear difference in surface coverage was seen for
1·ClÀ ((0.5Æ0.1ꢂ10À10) molcmÀ2; calculated from the QCM
measurements by using
a quartz crystal microbalance
(QCM). For these experiments, a vertical cell setup[35] with a
20 mL cell containing CH2Cl2 was used. The thiol derivatives
(vide supra) were initially dissolved in CH2Cl2 (500 mL) and
subsequently added in one portion to the cell under stirring.
This resulted in a final concentration of 1 mm (in thiols) in
the cell. Both the MPTTF model compound 8 and the TTF
calix[4]pyrrole 1 showed very fast adsorption kinetics with a
maximum coverage being reached within 100 seconds. The
surface coverage calculated from the deposited mass ((1.8Æ
0.2ꢂ10À10) and (0.8Æ0.1ꢂ10À10) molcmÀ2, respectively, for 8
and 1) are in good agreement with those values determined
from the CV experiments described above.
Considering that the mass difference between complexed
(i.e., 1·ClÀ) and uncomplexed TTF calix[4]pyrrole (i.e., 1) is
too small to be measured using our QCM set-up, we decided
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J. O. Jeppesen, E. Levillain, L. Sanguinet et al.
data) vs. 1 ((0.8Æ0.1ꢂ10À10) molcmÀ2). Taken in concert,
these results provide further support for the claim that the
conformational change from the initial 1,3-alternate confor-
mation (as in 1) to the corresponding cone conformation (as
in 1·ClÀ) is restricted in the case of the surface bound TTF
calix[4]pyrroles, and that this has a profound influence on
both the kinetics of chloride recognition by the resulting
SAM, as well as the extent of the surface coverage. In par-
ticular, it is suggested that the cone conformation, 1·ClÀ, is
not able to arrange as close to the supporting surface mono-
layer as the corresponding anion-free, 1,3-alternate confor-
mer (i.e., 1). This decrease in coverage density and reduced
conformational flexibility presumably serves to mask the po-
tential chloride-induced response of the SAMs derived from
1 and 2. Nevertheless, it is important to appreciate that a
clear anion dependence on the electrochemical signature of
these SAMs is seen upon exposure to chloride anions, and
that, in contrast with earlier systems, this response is pro-
duced at sub-micromolar anion concentrations.
In summary, we have prepared two different mono-TTF
calix[4]pyrroles 1 and 2 bearing protected alkanethiol an-
choring groups and demonstrated that, after deprotection,
they may be used to form SAMs on gold surfaces. The re-
sulting systems are chemically stable under conditions of
electrochemical analysis. As such, the effect of immobilizing
redox-active mono-TTF calix[4]pyrroles on a surface could
be analyzed directly using electrochemical methods. It was
found that addition of chloride anions gave rise to a maxi-
mal shift in the first TTF-centered oxidation wave of
(DE1/21=À30 mV). Although this shift is smaller than that
seen in solution (DE1/21 =À170 mV), SAMs built up from
the present mono-TTF calix[4]pyrrole receptors respond to
added chloride anion at a submicromolar sensitivity level.
Further work is aimed at developing mixed SAMs designed
to decrease the problems associated with the lack of confor-
mational freedom inferred from the QCM experiments and
to develop systems that are optimized for other substrates
of interest.
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Acknowledgements
We gratefully acknowledge the Lundbeckfonden for a Post doc scholar-
ship to KAN and financial support provided by the COST D31 Pro-
gramme of the European Science Foundation to LGJ, the Danish Natural
Science Research Council (FNU, projects no. 272-08-0047 to KAN and
no. 272-08-0578 to JOJ), and the Danish Strategic Research Council in
Denmark through the Young Researchers Programme (no. 2117-05-0115
to JOJ) for financial support. The work in Angers was supported by the
Centre National de la Recherche Scientifique (CNRS—France), the
“Agence Nationale de la Recherche” (ANR—France), and the “Rꢀgion
des Pays de la Loire” (France). Support from the U.S. National Science
Foundation (CHE-0749571 to JLS) and Robert A. Welch Foundation
(F-1018 to JLS) is also gratefully acknowledged.
[17] a) K. A. Nielsen, J. O. Jeppesen, E. Levillain, J. Becher, Angew.
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[18] For other examples of redox-active chemosensors, See: a) P. D.
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Keywords: calix[4]pyrroles
assembled monolayers · sensors · tetrathiafulvalenes
· cyclic voltammetry · self-
8132
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Chem. Eur. J. 2009, 15, 8128 – 8133

Calix[4]pyrroles and the Electrochemical Sensing of Chloride
COMMUNICATION
2003, 133–134, 309–315; e) J. O. Jeppesen, J. Becher, Eur. J. Org.
Chem. 2003, 3245–3266; f) TTF Chemistry (Eds.: J.-i. Yamada, T.
Sugimoto), Kodansha, Tokyo, 2004; g) “Special issue on molecular
conductors”: Chem. Rev. 2004, 104, 4887–5782; h) T. Otsubo, K. Ta-
kimiya, Bull. Chem. Soc. Jpn. 2004, 77, 43–58.
electrode cell equipped with a platinum-plate counter electrode and
a silver wire served as the quasi-reference electrode; its potential
was calibrated against the ferrocene/ferricinium couple (Fc/Fc+)
before and after each experiment. The potentials were then recalcu-
lated and given against Fc/Fc+. CVs were recorded in anhydrous
HPLC-grade CH2Cl2 using tetrabutylammonium hexafluorophos-
phate (nBu4NPF6) as the supporting electrolyte. Based on repetitive
measurements, absolute errors in the stated potentials are consid-
ered to be ꢂ5 mV.
[20] Recently, the use of octafluorocalix[4]pyrrole for the creation of mi-
croelectrodes capable of sensing halide anions was reported. See:
a) R. Cui, R. Q. Li, D. E. Gross, X. Meng, B. Li, M. Marquez, R.
Yang, J. L. Sessler, Y. Shao, J. Am. Chem. Soc. 2008, 130, 14364–
14365; b) Calixpyrrole derivatives have also been used to prepare
ion-selective electrodes. See for instance: T. V. Shishkanova, R. Volf,
V. Krꢅl, J. L. Sessler, S. Camiolo, P. A. Gale, Anal. Chim. Acta 2007,
587, 247–253, and references therein.
[30] A. J. Bard, L. R. Faulkner Electrochemical Methods: Fundamentals
and Applications, 2nd ed., Wiley, New York, 2001.
[31] Deduced from an integration of the voltammetric signal carried out
in the absence of chloride anions.
[21] It is well-known that alkanethiols and gold substrates are able to
form well-defined SAMs, see: A. Ulman, Chem. Rev. 1996, 96,
1533–1554.
[22] a) J. O. Jeppesen, K. Takimiya, F. Jensen, T. Brimert, K. Nielsen, N.
Thorup, J. Becher, J. Org. Chem. 2000, 65, 5794–5805; b) J. O. Jep-
pesen, J. Becher, J. F. Stoddart, Org. Lett. 2002, 4, 557–560; c) J. O.
Jeppesen, K. A. Nielsen, J. Perkins, S. A. Vignon, A. Di Fabio, R.
Ballardini, M. T. Gandolfi, M. Venturi, V. Balzani, J. Becher, J. F.
Stoddart, Chem. Eur. J. 2003, 9, 2982–3007,.
[23] C. Bucher, R. S. Zimmerman, V. M. Lynch, C. Krꢅl, J. L. Sessler, J.
Am. Chem. Soc. 2001, 123, 2099–2100.
[24] Prolonged reaction time and high temperature should be avoided, in
order to suppress the competing elimination reaction leading to the
formation a terminal alkene.
[25] J. L. Sessler, D. E. Gross, W.-S. Cho, W. M. Lynch, F. P. Schmidtchen,
G. W. Bates, M. E. Light, P. A. Gale, J. Am. Chem. Soc. 2006, 128,
12281–12288.
[26] As previously observed (See: Ref. [17b]), the oxidation of chloride
anion to chlorine occurs close to the second oxidation process asso-
ciated with the TTF unit. As a consequence of the overlapping
nature of these two oxidation processes, it is not possible to obtain
any information as to whether complexation of chloride anions to
compounds 1 or 2 also affects the second oxidation process associat-
ed with the TTF unit in these two compounds.
[27] The acetyl protecting group in compounds 1, 2, 8, and 9 was cleaved
using the following general procedure: A solution of CsOH·H2O
(2 equiv) was slowly added to a solution of the appropriate com-
pound in anhydrous degassed THF. The reaction mixture was stirred
for at least 3 h, where upon H2O was added. The resulting mixture
was extracted with CH2Cl2 and subsequently filtered though silica
gel. Evaporation of the solvent afforded the corresponding thiol;
these unprotected species were used directly without further purifi-
cation.
[32] 0.5 mm corresponds to ~10 equivalents of the amount of “immobi-
lized” TTF calix[4]pyrrole 2 on gold ((0.7Æ0.1ꢂ10À10) molcmÀ2),
added in 10 mL of 0.1m nBu4NPF6/CH2Cl2 solution. This value de-
pends on the area of the gold substrates. For this reason, the use of
ultra-microelectrode could reach a subpicomolar sensitivity level.
See for instance: C. Amatore, D. Genovese, E. Maisonhaute, N.
Raouafi, B. Schçllhorn, Angew. Chem. 2008, 120, 5289–5292;
Angew. Chem. Int. Ed. 2008, 47, 5211–5214.
[33] M. P. Wintergerst, T. G. Levitskaia, B. A. Moyer, J. L. Sessler, L. H.
Delmau, J. Am. Chem. Soc. 2008, 130, 4129–4139.
[34] It is known that TTF calix[4]pyrroles, exists predominantly in the
1,3-alternate conformation in the absence of anions. However, in
halogenated solvents TTF calix[4]pyrroles binds anions very tightly,
with a binding constant (Ka) of 2.9ꢂ103 mÀ1 (CH2Cl2, 298 K) having
been reported (see ref. [17b]) for the interaction between a mono-
TTF calix[4]pyrrole (which is structural similar to the TTF calix[4]-
pyrrole 2 of this study) and chloride anion; it is known that this
binding favors formation of the cone conformation. See
ref. [16f,g,h].
[35] G. Jerkiewicz, G. Vatankhah, A. Zolfaghari, J. Lessard, Electrochem.
Commun. 1999, 1, 419–424.
[36] The binding constant corresponding to the interaction between chlo-
ride anions and a mono-TTF calix[4]pyrrole—which is structural
similar to 1, but without an alkanethiol anchor group—has been re-
ported (See: Ref. [17b]) to be 2.9ꢂ103 mÀ1 (CH2Cl2, 298 K).
[37] a) D. S. Karpovich, G. J. Blanchard, Langmuir 1994, 10, 3315–3322;
b) It is possible to extract the deposition rate constant (kobs)from the
following integrated equation:
[28] The substrates were prepared by deposition of ꢀ5 nm of chrome
followed by ꢀ50 nm of gold onto a glass substrate using physical
vapor deposition techniques and were made immediately before
use. See: a) L. Sanguinet, O. AlꢀvÞque, P. Blanchard, M. Dias, E.
Levillain, D. Rondeau, J. Mass Spectro. 2006, 41, 830–833; b) M.
Bounichou, L. Sanguinet, K. Elouarzaki, O. AlꢀvÞque, M. Dias, E.
Levillain, D. Rondeau, J. Mass Spectro. 2008, 43, 1618–1626.
[29] Electrochemical experiments were carried out using an EGG PAR
273A potentiostat with positive feedback compensation in a glove
box containing anhydrous, oxygen-free (<1 ppm) argon, at room
temperature. Cyclic voltammetry (CV) was performed in a three-
q is the fraction of surface covered, (1Àq) represents the available
sites for adsorption and C is the analytical concentration. The values
of both ka and kd corresponding to adsorption/desorption rate con-
stants enable the kinetic parameters to be calculated. The values of
K’ and kobs are obtained by fitting the experimental data to the
equation.
Received: May 25, 2009
Published online: July 11, 2009
Chem. Eur. J. 2009, 15, 8128 – 8133
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8133