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Molecular Recognition at an Interface Binding of Monolayer-Anchored Ferrocenyl Groups by an Amphiphilic Calixarene Host.

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superstructure at room temperature has been confirmed by
Weissenberg photographs of the likO and hkl layers. and there
is no sign of ;I transition in the magnetic data > 6 K.
Received: July 16. 1994 [Z 7133 IE]
German version: An,q,ii-. Chivn. 1995. 107. 134
Keywords: halogen compounds. lanthanide compounds. metallic conductors . semiempirical calculations . solid-state structures
[I] G. Meyer. ( ' h n RIY. 1988, H X , 93.
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16,294: c) H. Mattausch. J. B. Hendricks. R. Eger. J. D. Corbett.
A. Simon. rbid 1980. IY, 2128: d) H. Mattausch. A. Simon, N. Holzer. R. Eger.
%. 4no,x. , 4 / / , ~ Chon. 1980. 466. 7: e ) R. Araujo. J. D Corbett. Ii70rg. Cheni.
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[3] i i ) D. G. Adolphson. J. D. Corhett. fnorg. C/zrnr. 1976. 15. 1820: b) R. L.
D'iakc. J. D. Corbett. ihid. 1977. 16. 2029.
[4] a ) 11. Matt:iusch. W. Schrdmm, R. Eger. A. Simon. Z . Anorg. A / / . C/ien?.1985.
530. 43: h) G. Meyer. S.-J. Hwu. S. Wijeyeskerd. J. D. Corbett. h u q . Cl7rm.
. 4x11. c ) A. Simon. H . Mattausch. R. Eger. Z . Anorg. A//g, Chwn.
0 . 50: d ) H. Mattausch, R. Eger. J. D. Corbett, A. Simon. &id. 1992.
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(51 F Ueno. K. R . Ziebeck, H. Mattausch. A. Simon. Rev. C h i . Miner. 1984,21.
[6] a ) .I. D Corhett. R. A. Sallach, D. A. Lokken. A h . Clieni. Ser. 1967, 71. 56:
bl F. Wxkentiii. H. Birnighausen, Z. Anorg. AUg. Ch~wi.1979, 4SY. 1x7.
[7] Ci-ystal structure analysis of LaI: Lattice constants, u = 3 9297(4). c =
9.710(1) A were obtained from Guinier po\ider patterns ( 2 =1.540562 A).
I ' = I?9.X6(4) A'. Z = 2. hexagonal space group Pb,,mnic (no. 194). Data
were collected on a Rigaku AFC6 diffractometer at 22 ^C with mouochromated Mo,, r'idiation ( h . k . + / ) . The structure was solved by direct methods
(SHELXS). The residuals were R ( F ) = 0.028, R, = 0.029 for 49 independent
iretlecttons ( 2 0 < 50'. f > 30(1)) and 7 variables. The platelike crystal habit
(0.05 x 0.15 x 0.6 mm) caused significant absorption problems ( p =
27X Y c m - ' ) ( R = 0.054) which were corrected by the DIFABS method (N.
Walker. D. Stuart. A c f a Crj:s~u//ogr.Secr. A 1983, 3Y. 158; transmission range
0 5x1 1.175). This removed the anisotropy in thermal parameters of both
atoms m d the corresponding "ghost peaks" (3.0-2.1 e A - 9 in the difference
Fourier map. An inverse NiAs structure can clearly be ruled out. since i t pave
a poorer refinement and disparate temperature factors (B-VdltleS). Further
details or the crystnl structure investigation may be obtained from the Fachinformations7entrum Karlsruhe. D-76344 Eggenstein-Leopoldshafen (Germany) on quoting the depository number CSD-58 680.
J. Nakahnra. H. Franzen. D. K. Misemer. J. C/7<,m.Phm. 1982. 76, 4080.
a ) D. S. Dudir. J. D. Corbett, S.-J. Hwu. fi70rp. C/imi. 1986.15. 3434: b) S. M.
K a u h r i c h . T. Hughbanks, J. D. Corbett, P. Klavins. R. N. Shelton. ibid. 1988.
27. 1791.
R. P. Shannon. .4cra ( ' r j m d l u ~ r Secr.
A 1976. 37. 751.
E. Warkentin. H. BBrnighausen. T/iirJ Europmn Cr~~ru/logruphic
Zurich. Switzcrlmd, September. 1976. p. 356.
A Simon. H Mattausch. G . J. Miller. W. Bauhofer. R. K. Kremer. Hundb.
Pbi..~i%rmRuw Eurrhs. 1991, 15. 191.
C . Michaelis. H. Mattausch. H . Borrmann. A. Simon. J. K . Cockcroft. Z.
.4lloI'g. Alip. C'/i(,i77. 1992, 607. 29.
F. Bijttcher. '4.Simon, R. K. Kremer. H. Buchkremer-Hermanns. J. K . Cockcrofc. %. A n o r x . 4//,q Ch~wi.1991. SY8i59Y. 25.
S. Wi.ieyesekera, J. D. Corbett. Solid Srure Cummun. 1985, 54. 657.
M.-H. Whangho. R. Hoffmann, J. A m . Chrm. Soc. 1978, 100. 6093.
C. Rovira. M.-H. Whangbo. Inorg. Chr,m. 1993, 32. 4094
Molecular Recognition at an Interface : Binding
of Monolayer-Anchored Ferrocenyl Groups by
an Amphiphilic Calixarene Host **
Litao Zhang, Luis A. Godinez, Tianbao Lu,
George W. Gokel, and Angel E. Kaifer"
Self-assembly of alkanethiols on gold surfaces constitutes one
the most useful methods for the preparation of supported
monolayers exhibiting a high degree of molecular organization."' The preparation of similar monolayers incorporating
molecular recognition sites is of fundamental as well as practical
importance. However, the literature contains only a few reports
of binding phenomena in supported monolayer systenis.l2I We
demonstrate here that ferrocenyl groups covalently attached to
a supported alkanethiol monolayer can be complexed effectively
by an amphiphilic receptor in the contacting aqueous solution.
The major finding of this work is that the receptor must have
substantial amphiphilic character in order to bind the guest
residues anchored to the monolayer assembly.
Coassembly of the thiolated ferrocene derivative 1 with an
alkanethiol yields mixed monolayers in which. by selecting the
length of the alkanethiol chain, one can easily vary the degree of
accessibility of the ferrocenyl groups to the conSH
tacting solution.[31 Exposure of a gold bead elec1
trode to an ethanolic solution containing 0.25 mM 1 and 0.75 mM decanethiol (C,,H,,SH)
resulted in the coassembly of a monolayer in which the ferrocenyl groups protrude from the hydrophobic core. The voltammetric response of this supported monolayer in 1.0 M HCIO,
(Fig. 1) is characterized by the reversible oxidation of the surface-confined ferrocenyl groups (Fc+ Fc' + e - ) . The formal
oxidation potential measured from the cyclic voltammogram
was +0.51 V vs a sodium chloride saturated calomel electrode
(SSCE). and the potential difference between the anodic and
cathodic peak potentials (AE,) was typically smaller than 10 mV
at scan rates of 0.5 V s-' or less. The surface coverage of ferrocenyl groups, as determined from the integration of the anodic
wave, was 1.2 x lo-" mol cm-2, while the maximum coverage
obtained in our experiments from self-assembly of pure 1 (no
alkanethiol) was 4.3 x lo-" mol cm-2. All these electrochemical parameters are in agreement with published values for this
and similar ferrocenyl-containing, self-assembled monolayer
We have recently reported that the sulfonated calix[6]arene 2
is an excellent host for several ferrocene derivatives in aqueous
media.[51Thus, we were interested in probing the interactions of
monolayer-anchored ferrocenyl groups with this host. However, we could not find any evidence for binding. as the E" value
for the oxidation of the ferrocenyl groups in the monolayer did
not change appreciably when the contacting solution contained
Prof. A. E. Kaifer, L. Zhang, L. A. Godinez
Department of Chemistry. University of Miami
Coral Gables. F L 33124 (USA)
Telcfax: Int. code +(305)662-4007
Prof. G . W. Gokel. Dr. T. Lu
Department of Molecular Biology and Pharmacology
Washington University School of Medicine
Campus Box 8020. 660 S . Euclid Avenue. St. Louis. M O 631 10 (USA)
This work was supported by the National Science Foundation (grant nos.
CHE-9000532 iind CHE-9304262 to A. E. K.) and the National Institutes of
Health (grant nos. GM-36262 and A1-27179 to Ci. W. G.).
L. Z. thanks the
University of Miami for a Maytag Fellowship and L. A. G . [hanks the Universidad Nacional Autonoma de Mexico for a graduate fellowship.
10.2 pA
Fig. 1. Voltammetric response of a mixed monolayer formed by the coassembly of
1 and C,,H,,SH on a gold bead electrode. The voltammogram was recorded with
the monolayer-modified electrode immersed in 1.O M HCIO,. Initial potential: 0.0 V
vs SSCE; scan rate: 0.5 V s - ' .
the calixarene host, even at concentrations as high as 1-2 mM.
This highly charged, anionic host must be strongly solvated by
water molecules,[61which seems to prevent its effective approach
to the ferrocenyl groups residing in the monolayer/solution interface.
Thus we turned our attention to an amphiphilic analogue of
2, the dodecyl derivative 3, which is prepared by 0-alkylation of
the parent calixarene."] Very low concentrations of 3 have a
profound effect on the voltammetric response of the C,,H,,SH/
1 monolayer. For instance, in the presence of0.4 p~ 3 the anodic
peak for ferrocene oxidation splits into two well-resolved peaks
(Fig. 2 top). The first one remains at approximately the same
potential as that observed in the absence of 3, while the second
one is shifted in the positive direction by about 90 mV. The latter
peak increases with the concentration of 3 at the expense of the
original peak. In the presence of 2 ~ L M3, only the calixareneinduced peak is observed. We thus assign this new redox couple
to the oxidation of the monolayer-anchored ferrocenyl groups
complexed by the amphiphilic calixarene hosts. Figure 2 bottom
shows a comparison of the initial (no 3 added) and final (in the
presence of 2.0 p~ 3) voltammetric responses. Further additions
of 3 d o not affect the voltammetric response of the monolayer.
Potential cycling of a gold electrode derivatized with a
Cl0H,,SH/1 monolayer results in a gradual loss of ferrocene
coverage due to the slow decomposition of the oxidized ferrocenyl sites."] This decomposition reaction is responsible for the
decreased peak currents observed in the presence of 2 p~ 3 (see
Fig. 2 bottom), as this voltammogram was recorded after sever-
V C H ~ ~ r l u g s ~ e s ~ ~ l l smc h H
u f, i0-69451 Weinheim. 1995
Fig.a 2in
. Top:
1 in the presence
(0.4 p ~ in) the contacting solution.
Bottom: Comparison of the voltammograms obtained in the absence
(continuous line) and in the presence
(dotted lined) of3 (2.0 p ~in)the contacting solution. All other conditions
were as those used in Figure 1 .
a1 additions of calixarene and several potential cycles to monitor
their effects. As a control experiment, a gold electrode derivatized with a C,,H,,SH/l monolayer was submitted to prolonged potential cycling between 0.0 and 0.9 V vs SSCE in a
1.O M HCIO, solution (no calixarene). As expected, the currents
associated with the ferrocene redox couple continuously decreased with cycling time, but the oxidation potential remained
unaffected at 0.51 V vs SSCE. Therefore, the changes in oxidation potential observed upon addition of calixarene are due to
changes in ferrocene's microenvironment, not to variations in
the surface density of ferrocenyl sites in the monolayer.
Recently, Craeger et al. have shown that the presence of longchain alcohols in the solution phase causes substantial anodic
shifts in the oxidation potential of monolayer-anchored ferrocenyl residues.rg1They interpreted these findings as the result of
aggregation of the long-chain alcohol at the monolayer/solution
interface which increases the hydrophobic character of the ferrocene microenvironment, thus favoring the reduced neutral
form over the oxidized cationic form and shifting the oxidation
potential to more positive values. Our results with 3 cannot be
explained simply by the aggregation of the amphiphilic calixarene at the monolayer/solution interface because of two key
experimental findings: 1) The presence of 3 in the solution causes the split of the ferrocene redox couple into two couples that
can be clearly assigned to the oxidation of free and bound ferrocenyl sites. 2) The splitting of the ferrocene redox couple is only
detected in the concentration range 0.05-2 p~ 3. At higher concentrations of 3 the voltammetric behavior remains constant,
indicating that all the monolayer ferrocenyl groups are bound so
that further additions of host have n o effect in the observed
E" value. These observations are fully consistent with the proposed interfacial binding of the ferrocenyl sites by the amphiphilic calixarene host. We performed several control experiments exposing C,,H,,SH/l monolayers to solutions contain-
0570-0833:9510202-0236 $ 10.00+ ,2510
Angrw. Chem. In:. Ed.
Engi. 1995, 34, N o . 2
ing increasing concentrations of amphiphiles having only one
alkyl chain. The results are shown in Figure 3. In agreement
with Craeger's observations with long-chain alcohols,1g1both
amphiphiles surveyed (dodecyl sulfate and dodecyltrimethylammonium) caused anodic shifts in the oxidation potential of the
monolayer-anchored ferrocenyl groups. However, the redox
couple never splits. and the magnitude of the observed potential
shifts was smaller than the saturation shift observed with 3.
These findings, as well as the insensitivity of the E" shifts to the
amphiphile's positive or negative charge, are consistent with the
aggregation of the amphiphiles on the monolayerlsolution interface. The differences between these data and those obtained
with the amphiphilic calixarene clearly suggest a more specific
mechanism for the interaction between the ferrocene groups and
these host molecules.
containing monolayer. In this case, the alkanethiol's chains are
long enough to surround the ferrocenyl sites in the hydrophobic
core of the monolayer, preventing their interaction with the
calixarene host in the contacting solution. Therefore, the location of the ferrocenyl groups in the monolayer assembly plays
an important role in their binding by amphiphilic hosts.
In summary, we provide evidence in this work for the complexation of monolayer-anchored ferrocenyl groups at an interface by the sparingly water-soluble, amphiphilic calixarene host
3. The binding was detected electrochemically from the profound effects that very low concentrations (0.05-2.0 p ~ of) the
host exert on the voltammetric response of the ferrocenyl sites
at the interface. The parent calixarene 2 lacks the amphiphilic
character that seems to be necessary to show effective receptor
properties in interfacial environments. The results reported here
combined with recently published data" establish calixarene 3
as a versatile interfacial host.
Experinwnlnl Procedure
Thiol 1 was synthesized according to a reported procedure [la]. The calixarene
hosts were prepared as their corresponding salts ( N q - 2 and Na,-3) according to the
methods described by Shinkai and co-workers [7]. Sodium dodecyl sulfate and
dodecyltrimethylammonium bromide were obtained from Fluka and Kodak. respectively, and used without further purification. Gold wire (99.999"%) was purchased from Johnson Matthey. All other chemicals and solvents were of the best
commercial grade available. Distilled water was further purified by passage through
a Barnstead Nanopure four-cartridge system.
c / vmol L-'
Fig. 3. Oxidation potential shift of the monolayer-anchored ferrocenyl groups induced by the addition of micromolar solutions of 3 (*), sodium dodecyl sullite (.),
and dodecyltrimethylammonium bromide (+).
The calixarene-induced E" shift of + 90 mV is very similar to
that observed in solution for the binding of ferrocene derivatives
by /l-cyclodextrin.llol This value contrasts with the larger negative shifts observed for the binding of ferrocene derivatives by
compound 2 in solution.[51In the latter case, the calixarene acts
as an anionic host. In this work, host 3 probably interacts with
the ferrocenyl group primarily by including it "octopus-like" in
its aliphatic "tentacles" (Fig. 4). The change in the microenvironment of the ferrocenyl groups upon binding by 3 is thus close
to that suffered upon inclusion by cyclodextrin in solution experiments, which explains the similarity in the observed oxidation potentials shifts.
Coassembly of 1 and octadecanethiol (C,,H,,SH) leads to a
monolayer in which the formal oxidation potential of the ferrocenyl groups is quite insensitive to the presence of host 3 in the
hydrophobic surface
Fig. 4 Schematic representation of the proposed mode of interactlon between the
monolayer-anchored ferrocenyl groups and the amphiphilic calixarene host 3.
The gold bead working electrodes were made as reported elsewhere 1121. The geometric and true surface areas of these electrodes were measured by published methods [12]. Ths voltammetric response of these electrodes was checked in 1.0 M
HCIO,. Flar hxkground responses in the potential range 0.0 to +0.90 V vs SSCE
were commonly found. Otherwise. the electrode was discarded.
For monolayer preparation the gold bead electrode was immersed overnight in
deoxygenated ethanolic solutions containing mixtures of 1 and the appropriate
alkanethiol. The total concentration of thiol was always maintained at 1.0 mM.
Before the cyclic voltammetric experiments, the gold bead electrode was rinsed with
copious amounts of pure ethanol and then purified water Cyclic voltammetry
(single compartment cell, Pt flag counter electrode, homemade SSCE) was performed with the monolayer-covered gold bead working electrode immersed in 1.0 M
HCIO, (cell volume 10 mLj. As required by the binding studies, the hosts were
introduced to this solution by addition ofmicroliter aliquots by syringe from appropriate stock solutions.
Received: July 20, 1994
Revised version: September 13. 1994 [ Z 7144IEl
German version: Angew. Che~ii.1995. 107. 236
Keywords: calixarenes cyclic voltammetry . thin films . sandwich complexes . supramolecular chemistry
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[2] For a recent example. see: E. U. T. van Velzen. J. F. J. Engbersen. D. N. Reinhoudt, J. Am. Chem. Soc. 1994. 1 / 6 , 3597.
[3] G . K . Rowe. S. E. Creager. Lungmuir 1991, 7. 2307.
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[ 5 ] L. Zhang. A. Macias, T. Lu, J. I. Gordon, G. W. Gokel. A . E. Kaifer, J. Chem.
Soc. Chem. Commim. 1993.1017. In solution at pH 7 calixarene 2 is an octaanion owing to the deprotonation of two of its six phenolic OH groups. In the
acidic media utilized in this work all the phenolic OH groups are protonated
and both calixarene hosts (compounds 2 and 3) are likely to be present as
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[XI a) D. D. Popenoe. R. S. Deinhammer. M. D. Porter. Lmrgmurr 1992. 8, 2521 ;
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Ckem. Soc. Chem. Commun. 1994. 529.
(121 M. Gomez. J. Li. A. E. Kaifer, Lungmuir 1991. 7, 17Y7
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amphiphilic, molecular, calixarenep, group, monolayer, recognition, binding, interface, host, anchored, ferrocenyl
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