Three state redox-active molecular shuttle that switches in solution and on a surface

Article abstract of DOI:10.1021/ja077223a

Although the desirability of developing synthetic molecular machine systems that can function on surfaces is widely recognized, to date the only well-characterized examples of electrochemically switchable rotaxane-based molecular shuttles which can do so

Full text of DOI:10.1021/ja077223a

Published on Web 02/06/2008
Three State Redox-Active Molecular Shuttle That Switches in
Solution and on a Surface
Giulia Fioravanti,‡ Natalia Haraszkiewicz,| Euan R. Kay,† Sandra M. Mendoza,§
Carlo Bruno,‡ Massimo Marcaccio,‡ Piet G. Wiering,| Francesco Paolucci,*,‡
Petra Rudolf,*,§ Albert M. Brouwer,*,| and David A. Leigh*,†
Dipartimento di Chimica “G. Ciamician”, UniVersita` degli Studi di Bologna, V. F. Selmi 2,
40126, Bologna, Italy, School of Chemistry, UniVersity of Edinburgh, The King’s Buildings,
West Mains Road, Edinburgh EH9 3JJ, United Kingdom, Zernike Institute for AdVanced
Materials, UniVersity of Groningen, Nijenborgh 4, 9747AG Groningen, The Netherlands, and
Van’t Hoff Institute for Molecular Sciences, UniVersity of Amsterdam, Nieuwe Achtergracht 129,
NL-1018 WS Amsterdam, The Netherlands
Received September 18, 2007; E-mail: David.Leigh@ed.ac.uk; Francesco.Paolucci@unibo.it; P.Rudolf@rug.nl; A.M.Brouwer@uva.nl
Abstract: Although the desirability of developing synthetic molecular machine systems that can function
on surfaces is widely recognized, to date the only well-characterized examples of electrochemically
switchable rotaxane-based molecular shuttles which can do so are based on the tetracationic viologen
macrocycle pioneered by Stoddart. Here, we report on a [2]rotaxane which features succinamide and
naphthalene diimide hydrogen-bonding stations for a benzylic amide macrocycle that can shuttle and switch
its net position both in solution and in a monolayer. Three oxidation states of the naphthalene diimide unit
can be accessed electrochemically in solution, each one with a different binding affinity for the macrocycle
and, hence, corresponding to a different distribution of the rings between the two stations in the molecular
shuttle. Cyclic voltammetry experiments show the switching to be both reversible and cyclable and allow
quantification of the translational isomer ratios (thermodynamics) and shuttling dynamics (kinetics) for their
interconversion in each state. Overall, the binding affinity of the naphthalene diimide station can be changed
by 6 orders of magnitude over the three states. Unlike previous electrochemically active amide-based
molecular shuttles, the reduction potential of the naphthalene diimide unit is sufficiently positive (-0.68 V)
for the process to be compatible with operation in self-assembled monolayers on gold. Incorporating pyridine
units into the macrocycle allowed attachment of the shuttles to an acid-terminated self-assembled monolayer
of alkane thiols on gold. The molecular shuttle monolayers were characterized by X-ray photoelectron
spectroscopy and their electrochemical behavior probed by electrochemical impedance spectroscopy and
double-potential step chronoamperometry, which demonstrated that the redox-switched shuttling was
maintained in this environment, occurring on the millisecond time scale.
Introduction
is a rotaxane in which the thread contains two (or more)
macrocycle-binding sites (“stations”) connected by a traversable
pathway.3 Provided sufficient thermal energy is available to
break the intercomponent interactions at each site, the ring
moves randomly between the two stations. On average, however,
the macrocycle spends more time on the station with the higher
binding affinity (i.e., the molecule tends to adopt a particular
preferred co-conformation4). In a stimuli-responsive molecular
shuttle, chemically altering one of the stations is used to switch
the preferred location of the ring, while reversing that change
returns the system to the starting distribution. One particularly
attractive stimulus for the operation of such devices is electro-
chemistry, as it allows remote, reagent-free, and waste-free
Synthetic systems in which submolecular motions can be
controlled by the application of external stimuli have been
vigorously pursued in recent years.1 Such “mechanical” mo-
lecular devices are viewed both as putative components for
future functional nanotechnologies and as much simplified
models of complex biological machines. One of the most
versatile designs for a simple switch-like molecular machine is
the stimuli-responsive molecular shuttle.1,2 A molecular shuttle
† University of Edinburgh.
‡ Universita` degli Studi di Bologna.
§ University of Groningen.
| University of Amsterdam.
(1) (a) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F. Angew. Chem.,
Int. Ed. 2000, 39, 3349-3391. (b) Balzani, V.; Venturi, M.; Credi, A.
Molecular DeVices and Machines. A Journey into the Nanoworld; Wiley-
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5131-5133. For the first example of an amide-based molecular shuttle
see: (b) Lane, A. S.; Leigh, D. A.; Murphy, A. J. Am. Chem. Soc. 1997,
119, 11092-11093.
(4) “Co-conformation” refers to the relative positions of the mechanically
interlocked components with respect to each other, see: Fyfe, M. C. T.;
Glink, P. T.; Menzer, S.; Stoddart, J. F.; White, A. J. P.; Williams, D. J.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2068-2070.
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9
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J. AM. CHEM. SOC. 2008, 130, 2593-2601
2593

A R T I C L E S
Fioravanti et al.
control, while providing the opportunity to interface the mo-
lecular devices with existing electronic technologies.5
station shuttles in solid-state molecular electronic devices,5
successfully correlating several aspects of their solution-phase,
surface and solid-state behavior.13
Clearly, a key step in the development of electronic (and
many other) applications of synthetic molecular machines will
be transposing working systems from solution phase onto
surfaces and into the solid state.6 Already molecular shuttles
have been used to construct solid-state and surface-based devices
that exhibit switchable conductance,5,7 optical properties,8
wettability,9 porosity,10 and shape.11 Yet, although stimuli-
induced shuttling is strongly implicated in the operation of each
of these systems, reliable methods for anchoring shuttles to solid
supports and strategies for directly observing and fully char-
acterizing their stimuli-induced motions in such environments
have proven particularly challenging over nearly a decade of
study.5-11
We have previously reported14 on another structural type of
electrochemically switchable molecular shuttle; an amide-
based15-19 rotaxane in which net translocation of the macrocycle
can be induced by photochemical or electrochemical reduction
of a 1,8-naphthalimide station. The solution-phase shuttling was
characterized in detail and both states were shown to exhibit
unprecedented positional integrity. Furthermore, the shuttling
motion is reversible and occurs over a rather large distance of
∼1.5 nm on a microsecond time scale (the precise rate is
dependent on solvent and experimental conditions).14 The
relatively large, negative potentials required to cause switching,
however, prevented the operation of such devices on surfaces.20
In fact, despite the sophistication of modern synthetic routes
toward interlocked molecules,12 few classes of rotaxanes have
proved amenable to the construction of molecular shuttles. Still
fewer shuttles operate using electrochemical stimuli, and two
of these are among the only types of shuttle to be studied in
any detail on surfaces. Willner and co-workers have character-
ized the shuttling kinetics of a cationic ring on a single-station
rotaxane in which a cathode acts simultaneously as a redox
reagent, a stopper, and a station.7d,e However, the integral role
of the solid substrate in producing the bistability prevents
comparisons with solution-phase experiments. An extensive
program by Stoddart, Heath, and co-workers has studied the
shuttling behavior of a particular class of redox-switched two-
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2594 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

Three State Redox-Active Molecular Shuttle
A R T I C L E S
Chart 1. Chemical Structures of Molecular Shuttles 1 and 2, and
Thread 3a
Here, we report on a second member of this series, in which
a naphthalene-1,4,5,8-diimide station can be switched between
three different oxidation states. At each oxidation level, the
naphthalene diimide exhibits a different binding affinity for the
macrocycle, which leads to a different distribution of rings
between two stations in a molecular shuttle. Conveniently,
solution-phase cyclic voltammetry experiments allow the redox
reactions to be carried out, the shuttling kinetics assessed, and
the strongly biased co-conformational ratios quantified, all in
the same experimental setup. The ratio of station occupancies
can be altered over 6 orders of magnitude, from strongly
favoring one station to strongly favoring the other, via an
intermediate state in which the discrimination between the
different binding sites is relatively modest. Furthermore, the
switching conditions are sufficiently mild to allow access to
two of the states when the rotaxanes are confined to an
alkanethiol-based self-assembled monolayer (SAM) on gold. We
report the characterization of molecular shuttle monolayers
constructed through noncovalent interactions between the shuttle
and the SAM. Unlike most previous examples of surface-
confined shuttles, this arrangement results in alignment of the
shuttle threads parallel to the substrate. Evidence for electro-
chemically controlled shuttling at a surface is demonstrated for
the first time for a monolayer of amide-based molecular shuttles.
Design and Synthesis
In order to optimize the shuttling characteristics of the
previously reported14 shuttle 1a (Chart 1), we investigated the
effect of replacing the single imide group by a naphthalene-
1,4,5,8-diimide. The more extensively delocalized aromatic
system could be expected to have a less negative reduction
potential for formation of the radical anion, but should also
exhibit a second reduction process to give the dianionic state.21
Simple AM1 calculations gave an indication of the charge
density distribution in the neutral and reduced forms of the
naphthalene monoimide and diimide (see Figure S1 in Sup-
porting Information). In both cases, the extra electron density
resulting from reduction is localized mainly on the electrone-
gative oxygen atoms. As expected, adding one electron to the
diimide system results in a smaller increase in electron density
at each of the four oxygens compared to the effect of one
additional electron on the two oxygens of the monoimide. The
doubly reduced state of the diimide, however, exhibits a slightly
higher charge density on the oxygens compared to the monoim-
ide radical anion. Therefore, although a poor hydrogen-bond
acceptor in the neutral state, reduction of the naphthalene diimide
should increase the hydrogen-bond basicity of the oxygens (as
previously demonstrated for 1,8-naphthalimide groups14,22 and
other imide systems23,24). Aromatic diimides have previously
been used as neutral π-electron-poor templates for the formation
of catenanes and rotaxanes.25,26 Reduction of the diimide unit25a,c
or addition of alkali metal salts25b,c have both been shown to
disrupt the aromatic charge-transfer interactions and produce a
a Rotaxane structures are shown in the succ-co-conformation which is
strongly preferred for the neutral molecules in all but the most polar of
solvents (e.g., DMSO, formamide).8b Italicized lettering indicates non-
equivalent proton environments.
co-conformational change in examples from this series but, to
the best of our knowledge, the use of naphthalene diimide
moieties as redox-switchable hydrogen-bond acceptors has not
been investigated for the construction of molecular machines.24
In rotaxane 2 (Chart 1), a naphthalene-1,4,5,8-diimide (ndi)27
station is separated from an electrochemically inert succinamide
(succ) station by a C12 aliphatic spacer. The thread component
(24) In some supramolecular complexes where the oxidation state of an imide
derivative has been used to control the binding constant, although hydrogen
bonding to the imide carbonyls is involved, the overall association constant
actually decreased on increasing imide charge as a consequence of stacking
interactions with π-electron-rich aromatics. See: (a) Goodman, A.; Brein-
linger, E.; Ober, M.; Rotello, V. M. J. Am. Chem. Soc. 2001, 123, 6213-
6214. (b) Gray, M.; Goodman, A. J.; Carroll, J. B.; Bardon, K.; Markey,
M.; Cooke, G.; Rotello, V. M. Org. Lett. 2004, 6, 385-388.
(25) See, for example: (a) Hamilton, D. G.; Montalti, M.; Prodi, L.; Fontani,
M.; Zanello, P.; Sanders, J. K. M. Chem. Eur. J. 2000, 6, 608-617. (b)
Vignon, S. A.; Jarrosson, T.; Iijima, T.; Tseng, H.-R.; Sanders, J. K. M.;
Stoddart, J. F. J. Am. Chem. Soc. 2004, 126, 9884-9885. (c) Iijima, T.;
Vignon, S. A.; Tseng, H.-R.; Jarrosson, T.; Sanders, J. K. M.; Marchioni,
F.; Venturi, M.; Apostoli, E.; Balzani, V.; Stoddart, J. F. Chem. Eur. J.
2004, 10, 6375-6392, and references therein.
(26) Imides have also been used as fluorescent reporter units in rotaxane-based
shuttles while playing no apparent role in interactions with the macrocycle.
See, for example: Qu, D.-H.; Ji, F.-Y.; Wang, Q.-C.; Tian, H. AdV. Mater.
2006, 18, 2035-2038, and references therein.
(21) Viehbeck, A.; Goldberg, M. J.; Kovac, C. A. J. Electrochem. Soc. 1990,
137, 1460-1466.
(22) (a) Ge, Y.; Lilienthal, R. R.; Smith, D. K. J. Am. Chem. Soc. 1996, 118,
3976-3977. (b) Niemz, A.; Rotello, V. M. J. Am. Chem. Soc. 1997, 119,
6833-6836. (c) Deans, R.; Niemz, A.; Breinlinger, E. C.; Rotello, V. M.
J. Am. Chem. Soc. 1997, 119, 10863-10864. (d) Gray, M.; Cuello, A. O.;
Cooke, G.; Rotello, V. M. J. Am. Chem. Soc. 2003, 125, 7882-7888.
(23) (a) Niemz, A.; Rotello, V. M. Acc. Chem. Res. 1999, 32, 44-52. (b) Carroll,
J. B.; Gray, M.; McMenimen, K. A.; Hamilton, D. G.; Rotello, V. M. Org.
Lett. 2003, 5, 3177-3180.
(27) The nomenclature used to denote individual rotaxane states and isomers
follows conventions adopted in previous papers (e.g., ref 14b). Namely, a
bolded compound number refers to a given structural formula; the oxidation
state is denoted by a superscript suffix (the absence of which indicates the
neutral form); a prefix succ or ndi denotes the position of the macrocycle
in a particular translational isomer.
9
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A R T I C L E S
Fioravanti et al.
Figure 1. 1H NMR spectra (400 MHz, CDCl3, 298 K) of thread 3 (upper)
and rotaxane 2a (lower). The 1H NMR assignments and coloring correspond
to the labeling in Chart 1. Residual water peaks are shown in gray.
Figure 2. CV curves of a 0.5 mM solution of 2a (solid lines) or 3 (broken
lines) in THF (supporting electrolyte: 0.05 M tetrabutyl ammonium
tetrafluoroborate). Working electrode: platinum disc (125 µm diameter).
Potentials measured with respect to SCE. (A) V ) 1 V s-1, T ) 25 °C; (B)
V ) 1 V s-1, T ) -55 °C; (C) V ) 100 V s-1, T ) 25 °C; (D) V ) 1000
V s-1, T ) 25 °C. Starred peaks (shoulders) indicate the presence of succ-
2a-•/2- intermediates as described in the text and Scheme 1.
bears two bulky diphenylethyl groups adjacent to each station
in order to mechanically trap a benzylic amide macrocycle sa
strong hydrogen-bond donor through the NH groupssonto the
thread. The affinity of the succinamide station for this macro-
cycle is well established.14 Rotaxane 2a bears an isophthalamide-
derived macrocycle, which is replaced by the analogous
pyridine-3,5-dicarboxamide cycle in 2b. This change does not
significantly affect the macrocycle-thread interactions, but is
required for the surface immobilization of the shuttles (vide
infra). The rotaxanes were each obtained in a five-component
clipping reaction starting from thread 3. Full synthetic details
are provided in the Supporting Information.
increasingabilitytoformhydrogenbondswiththemacrocycle.14b,22,23
Previously, we have shown that, in 1a,14b such stabilization
occurs due to shuttling of the macrocycle from the succinamide
station (succ-2a co-conformer), where it is preferentially located
in the neutral state, onto the (reduced) ndi station (ndi-2a-•/2-
co-conformer).28
Co-Conformation in the Neutral State. Comparing the 1H
NMR spectrum of 2a in CDCl3 with that of the thread molecule
3 (Figure 1) confirms that the preferred co-conformation of the
rotaxane in solution is succ-2a (as shown in Chart 1). In
particular, the succinamide methylene protons (Hd and He in
Chart 1 and Figure 1) show an upfield shift of ∆δ ) -1.43
ppm in the rotaxane compared to the thread, characteristic of
shielding caused by aromatic ring currents in the p-xylylene
rings of the macrocycle. Smaller shifts are also observed for
the protons flanking the succ station (Ha, Hb, and Hg).
Conversely, no shifts can be distinguished for any other protons
in the thread, including those of the naphthalene diimide,
indicating that the macrocycle resides almost exclusively over
the succinamide station. A similar situation is observed with
rotaxane 2b (see Figure S2 of the Supporting Information).
Redox-Switched Shuttling. The dynamics of the electro-
chemically induced shuttling in 2a were studied in detail using
cyclic voltammetry (CV) at various scan rates (V) and temper-
atures (Figure 2). At 298 K and V e 50 V s-1 (Figure 2A), the
voltammogram of 2a in THF (Bu4NBF4 as supporting electro-
lyte) displays two reversible peaks associated with the consecu-
tive reductions of the ndi station.21 Compared to the CV of the
thread (3) both redox processes are shifted to less negative
potentials, and the positive shift is notably larger in the case of
the second reduction than the first (compare E1/2 ) -0.64 and
-0.98 V for 2a, and -0.68 and -1.21 V for 3). Consistent
with the electrostatic nature of hydrogen bonding, the observed
changes in the CV pattern of 2a reflect the stabilization of the
mono and dianion of the ndi station as a consequence of their
The response of the CV curves to changing scan rate and
temperature (Figure 2, parts B-D) is also consistent with
electrochemically induced shuttling. As the temperature is
lowered and/or the scan rate increased, the shuttling process
becomes slow on the time scale of the experiment. This is
illustrated in Figure 2, parts C and D, by the presence of cathodic
and anodic shoulders (denoted by asterisks) associated with
processes involving the un-shuttled succ-2a co-conformation.
Furthermore, the broadening of the second reduction peak at
low temperature (Figure 2B) also indicates the presence of both
co-conformers in the diffusion layer.
Further evidence for shuttling was obtained by performing
wider CV scans so as to induce reduction of the macrocycle
(Figure S3 of the Supporting Information). Because of the fast
and irreversible protonation of isophthaloyl-based dianions,29
the amide groups in the reduced macrocycle can no longer form
hydrogen bonds with the thread, so that the features of the
reverse scan resemble the reoxidation of thread 3sthere is now
no interaction between the macrocycle and the reduced ndi
station.
The CV data can be fully rationalized by considering the
extended square scheme (ladder) mechanism set out in Scheme
(28) When a group that is too bulky for the macrocycle to pass over was placed
in between the two stations in 1, the CV of the rotaxane mirrored that of
the corresponding thread; i.e., the shift in the redox potential in the rotaxane
must arise from shuttling of the macrocycle to the reduced station. See ref
14b.
(29) Ceroni, P.; Leigh, D. A.; Mottier, L.; Paolucci, F.; Roffia, S.; Tetard, D.;
Zerbetto, F. J. Phys. Chem. B 1999, 103, 10171-10179.
9
2596 J. AM. CHEM. SOC. VOL. 130, NO. 8, 2008

Three State Redox-Active Molecular Shuttle
A R T I C L E S
Scheme 1. Extended Square Scheme (Ladder Mechanism) for the Redox-Switched Operation of Molecular Shuttle 2aa
a Electrochemical reactions are shown as vertical transitions and co-conformational equilibria in each oxidation state as horizontal ‘rungs’. Thermodynamic
and kinetic parameters shown here were calculated by simulation and fitting of the CV curves obtained in THF (supporting electrolyte: 0.05 M
tetrabutylammonium tetrafluoroborate); working electrode: platinum disc (125 µm diameter); T ) 25 °C.
1.30 The six chemical species are interrelated by a series of
heterogeneous electron-transfer reactions (vertical transitions)
and homogeneous translational isomerizations (horizontal transi-
tions). Under the conditions of Figure 2A (i.e., relatively slow
scan rates and high temperatures), the two different co-
conformations both contribute to a single reversible voltam-
metric peak for each redox reaction, with an apparent E1/2 value
that is related to the true value for the individual translational
isomers and the equilibrium constant for their interconversion.31
Alternatively, when experimental conditions are chosen so that
the chemical intermediates cannot reach their equilibrium
concentrations (as a consequence, for instance, of reducing the
time scale of the experiment or decreasing the temperature,
Figure 2, parts B-D), a less reversible CV pattern is observed
and features associated with each individual co-conformation
become visible in the curve.31 The shape and position of the
CV curves is therefore very sensitive to the relative values of
the thermodynamic and kinetic constants for the different
homogeneous and heterogeneous equilibria shown in Scheme
1, as well as the experimental conditions. The parameters may
be calculated by fitting the shape and the position of the
voltammetric peaks using digital simulation techniques.31,32 The
CV curves in Figure 2 were successfully reproduced (Figure
S5 of the Supporting Information) using the relevant kinetic
and thermodynamic parameters reported in Scheme 1 and Table
S1 (see the Supporting Information).
(32) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York,
(30) Evans, D. H. Chem. ReV. 1990, 90, 739-751.
(31) Lerke, S. A.; Evans, D. H;. Feldberg, S. W. J. Electroanal. Chem. 1990,
296, 299-315.
2001.
(33) Chatterjee, M. N.; Kay, E. R.; Leigh, D. A. J. Am. Chem. Soc. 2006, 128,
4058-4073.
9
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A R T I C L E S
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As observed by 1H NMR (vide supra), the succ-2a co-
conformation dominates in the neutral state (Kn ) (5.0 ( 1) ×
10-3). Reduction to the radical anion 2a-• increases the
hydrogen-bond basicity of the ndi station such that the equi-
librium ratio of co-conformers is now Kred-• ) 2.0 ( 0.4. The
poorer discrimination between the two stations compared to that
observed for the radical anion of 1a (ref 14b) is consistent with
the greater delocalization in the larger diimide system and the
AM1 results (see Figure S1 of the Supporting Information).
However, a second reduction gives the dianionic state 2a2-
where the hydrogen-bonding ability of the ndi unit is further
increased to the extent that the ndi-2a2- co-conformation is now
strongly preferred (Kred ) (4.0 ( 0.8) × 103).
2-
The reduction potentials for the succ-2a co-conformations
were set identical to those of the thread 3 during the simulation
process, whereas it can be seen that when the macrocycle is on
the ndi station, both redox processes occur at less negative
potentials due to stabilization of the reduced species by the
electron-withdrawing effect of forming hydrogen bonds through
the imide carbonyls. The “forward” shuttling (succ f ndi) rates
(kf) in all states were assumed to be the same (kf ) kf-• ) kf2-
) (10 ( 2) × 103 s-1) and similar to the value previously
determined for kf-• in 1a under the same conditions.14b This is
consistent with a shuttling mechanism in which the rate-
determining step is cleavage of the noncovalent interactions
between the macrocycle and the electrochemically inert succ
station, with little or no involvement of the ndi station,
irrespective of its oxidation state. The increased stability of the
ndi co-conformation with increasing charge is reflected in the
decreasing “backward” shuttling (ndi f succ) rates (kb), as
obtained by the simulations.
The same experiments performed on 2b revealed quantita-
tively identical behavior (within the accuracy of the simulation
procedure, see Figure S4 and Table S1 of the Supporting
Information). In both of these shuttles, therefore, controlling
the redox state of the ndi station changes the strength of
hydrogen bonds between the ndi unit and the macrocycle so
that the equilibrium constant between the two co-conformations
(i.e., the average position of the macrocycle on the thread) can
be effectively modulated over 6 orders of magnitude in two,
roughly evenly spaced, intervals (Figure 3). Rotaxane 2
represents the first molecular shuttle in which the affinity of
one station for the macrocycle can be varied between three well-
defined states using a single external stimulus.
Molecular Shuttle Monolayers. With the aim of achieving
redox-induced shuttling in a molecular monolayer, the electro-
chemical addressability of 2b attached to a conducting surface
was investigated. The majority of studies on immobilized
molecular shuttles to date have involved the attachment of a
rotaxane thread to surfaces or incorporation into a polymeric
matrix.5-8,10,34,35 Recently, however, we have investigated the
attachment of the macrocycle of the rotaxane to a surface as an
Figure 3. Machine-performance representation33 of electrochemically
switchable three-state molecular shuttle 2a in THF (supporting electrolyte:
0.05 M tetrabutylammonium tetrafluoroborate); working electrode: platinum
disc (125 µm diameter); T ) 25 °C. The population trace shows the
statistical distribution in the relative position of the macrocycle with respect
to the thread.
alternative approach to creating functional shuttle-based de-
vices.9,20,36 In this procedure, pyridine groups in the macrocycle
are used to attach the rotaxane to a self-assembled monolayer
(SAM) of 11-mercaptoundecanoic acid (11-MUA) by forming
hydrogen bonds with the 11-MUA carboxylates. The relatively
minor structural modification required to incorporate the pyri-
dine moiety, together with the simple deposition procedure,
(34) Investigation of molecular-level motions in drop-casted, vacuum-evaporated
or spin-coated thin films or polycrystalline samples of catenanes and
rotaxanes is another emerging area of interest. See, for example: (a)
Cavallini, M.; Biscarini, F.; Leo´n, S.; Zerbetto, F.; Bottari, G.; Leigh, D.
A. Science 2003, 299, 531-531. (b) Moulin, J. F.; Kengne, J. C.;
Kshirsagar, R.; Cavallini, M.; Biscarini, F.; Leon, S.; Zerbetto, F.; Bottari,
G.; Leigh, D. A. J. Am. Chem. Soc. 2006, 128, 526-532. (c) Biscarini, F.;
Cavallini, M.; Kshirsagar, R.; Bottari, G.; Leigh, D. A.; Leon, S.; Zerbetto,
F. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17650-17654. (d) Farrell, A.
A.; Kay, E. R.; Bottari, G.; Leigh, D. A.; Jarvis, S. P. Appl. Surf. Sci.
2007, 253, 6090-6095.
(35) Polyrotaxane architectures also allow preparation of solid-state interlocked
species. For examples of electroactive polyrotaxanes, see: (a) Zhu, S. S.;
Carroll, P. J.; Swager, T. M. J. Am. Chem. Soc. 1996, 118, 8713-8714.
(b) Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997, 119, 12568-12577.
(c) Buey, J.; Swager, T. M. Angew. Chem., Int. Ed. 2000, 39, 608-612.
(d) Kwan, P. H.; Swager, T. M. Chem. Commun. 2005, 5211-5213.
(36) Cecchet, F.; Pilling, M.; Hevesi, L.; Schergna, S.; Wong, J. K. Y.; Clarkson,
G. J.; Leigh, D. A.; Rudolf, P. J. Phys. Chem. B 2003, 107, 10863-10872.
9
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Three State Redox-Active Molecular Shuttle
A R T I C L E S
represents carbon atoms bound to electronegative atoms. Finally,
the well-defined peak at 288.1 eV is the signature of carbonyl
groups.38 Each of these peaks from 2b directly on gold are also
present in the C 1s spectrum of 2b‚11-MUA‚Au (Figure 4d).
However, there are also new features arising from 11-MUA. In
particular, the peak at 285.1 eV in Figure 4d is more intense
than the corresponding signal in Figure 4b, as a consequence
of the contribution from 11-MUA alkyl-chain carbons, while
the intensity and position of the component at 286.2 eV, which
represents carbon atoms attached to electronegative atoms, is
also influenced by the contribution of 11-MUA and interactions
between the rotaxane and 11-MUA SAM. The signal at 289.1
eV originates from carboxylic acid groups.39
Quantitative analysis of the XPS spectra allows us to
determine the amount of C, N, O, and S on the surface
(hydrogen is not detected by XPS). The coverage (the proportion
of 11-MUA carboxylic groups that are functionalized with
rotaxanes) can be estimated by comparing the experimentally
determined atomic percentages, derived from the intensity of
the photoemission line, with those calculated from the stoichi-
ometry of the molecules at various surface coverages. For the
calculation, we considered a model surface of 100 alkanethiol
chains functionalized with two, six or ten rotaxane molecules,
corresponding to a functionalization yield of 2, 6, or 10%. Table
S2 of the Supporting Information presents the experimental and
theoretical results obtained. The signal of electrons coming from
atoms closer to the Au substrate is attenuated more than the
signal from the topmost layer. Therefore, in the quantification
procedure we corrected the intensity of the S 2p area taking
into account the attenuation40 experienced by electrons traveling
though an ∼17 Å layer thickness and assuming that the
attenuation is only due to the presence of 11-MUA.41 The
experimental values are in good agreement with a coverage of
6%. Additionally, it should be noted that carbon and especially
oxygen signals always contain a contribution of atmospheric
contaminants, which affects all percentages.
This apparently small number of rotaxane-derivatized car-
boxylates results from the orientation adopted by the molecular
shuttles and actually corresponds to virtually complete coverage
of the SAM surface area. The rationale for this is, as previously
shown for 1b‚11-MUA‚Au,20 the macrocycle is anchored
perpendicular to the surface with the thread parallel to the
surface plane such that each rotaxane molecule occupies a large
surface area.
Redox-Switched Shuttling on a Surface. A different amide-
based molecular shuttle (which switches on the basis of alkene
photoisomerization) has previously been grafted onto a SAM
of 11-MUA in a similar fashion, creating surfaces which exhibit
switchable wettability and across which a liquid droplet may
be transported on application of the shuttling stimulus.9 Direct
observation of the rotaxane shuttling motion on the surface,
however, has yet to be achieved. Reversible shuttling obviously
Figure 4. X-ray photoemission spectra of N 1s (a; c) and C 1s (b; d) core
levels of rotaxane 2b deposited on gold (a; b) and grafted onto 11-MUA
SAM (c; d). Spectra (a) and (b) were collected with resolution 1.0 eV,
while (c) and (d) were collected with resolution 1.2 eV. Raw data (9) and
fit to the experimental line (s) are shown.
makes this a particularly convenient method for anchoring
molecular structures to surfaces.
Rotaxane 2b was grafted to the surface of a monolayer of
11-MUA self-assembled on a gold electrode, according to the
previously employed procedure.9,20,36 The resulting 2b‚11-MUA‚
Au films were characterized by X-ray photoelectron spectros-
copy (XPS). Rotaxane 2b and 11-MUA molecules present X-ray
photoemission signals that can overlap, such as the different
components in the C 1s core level signal. We therefore initially
analyzed a reference sample of 2b directly deposited on gold
to identify all of the characteristic spectral features of the
rotaxane molecule before carrying out measurements on the 2b‚
11-MUA‚Au sample.
Figure 4a shows the N 1s core level photoemission spectrum
of rotaxane 2b on gold, which is composed of a main peak at
399.9 eV, resulting from the amide and pyridine groups, and a
shoulder at 400.4 eV, which corresponds to the nitrogen atoms
of the imide groups. The relative area of the two components
is in agreement with the stoichiometry of the rotaxane. Exactly
the same features can be seen in Figure 4c which shows the N
1s spectral region of 2b‚11-MUA‚Ausclear proof that the
functionalization of the SAM with the rotaxane was successful.
The C 1s core level spectrum of rotaxane 2b on gold (Figure
4b) contains contributions from carbon atoms in several different
chemical environments. In practice, XPS may not distinguish
between all the various types of carbon, and therefore, the
standard fitting procedure consists of reconstructing the spectrum
with a minimum number of peaks consistent with the raw data,
the experimental resolution and the molecular structure, with
carbon atoms under very similar chemical environments (i.e.,
very close in binding energy) considered equivalent and
represented by a single peak. Following this criterion, the C 1s
spectrum in Figure 4b was fitted by five peaks. The component
at 284.5 eV can be unambiguously assigned to aromatic carbons
and is associated with the π-π* shakeup structure at 291.7 eV
that represents 5% of the aromatic signal.37 The peak at 285.1
eV is due to aliphatic carbons, while the one at 285.7 eV
(38) (a) Whelan, C. M.; Cecchet, F.; Clarkson, G. J.; Leigh, D. A.; Caudano,
R.; Rudolf, P. Surf. Sci. 2001, 474, 71-80. (b) Mendoza, S. M.; Whelan,
C. M.; Jalkanen, J.-P.; Zerbetto, F.; Gatti, F. G.; Kay, E. R.; Leigh, D. A.;
Lubomska, M.; Rudolf, P. J. Chem. Phys. 2005, 123, 244708.
(39) Mendoza, S. M.; Arfaoui, I. E.; Zanarini, S.; Paolucci, F.; Rudolf, P.
Langmuir 2007, 23, 582-588.
(40) (a) Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1989, 93, 1670-1673.
(b) Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991,
95, 7017-7021.
(41) (a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am.
Chem. Soc. 1987, 109, 3559-3568. (b) Bindu, V.; Pradeep, T. Vacuum
1998, 49, 63-66.
(37) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The
Scienta ESCA300 Database; John Wiley & Sons: Chichester, 1992.
9
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A R T I C L E S
Fioravanti et al.
requires the surface itself to be stable to the stimulus employed.
Unfortunately, relatively large negative potentials are incompat-
ible with gold–thiolate SAMs, which are unstable at potentials
below ca. -1.0 V.42 This prevented study of electrochemical
shuttling in previously constructed films of 1b‚11-MUA‚Au,20
but we reasoned that the less negative first reduction potential
of the naphthalene diimide redox center in 2b could allow access
to at least the monoanion, and consequently electrochemically
induced shuttling in the monolayer.
Electrochemical Impedance Spectroscopy (EIS) and double-
potential step chronoamperometry (CA) were used to investigate
the reduction of 2b‚11-MUA‚Au. Typically, both bare and
rotaxane-modified SAMs possessed rather low double-layer
capacitance values. Furthermore, in both cases, the film
capacitance was nearly independent of the applied potential,
thus excluding any significant rearrangement of the 11-MUA
SAM structure upon the application of potential. Additionally,
in the case of 2b‚11-MUA‚Au, the EIS spectra (Figure S6 of
the Supporting Information) displayed the typical potential-
dependent semicircle associated with slow electron transfer (ET)
processes that would involve the ndi station. Simulation and
fitting of the EIS spectra obtained at various potentials allowed
calculation of the relevant parameters associated with the
electronic response of the interface. In particular, a value of
∼103 s-1 was obtained for the heterogeneous ET rate constant.
Such a low value, together with the rather small time constant
of the electrode (∼8 µs, also obtained by fitting of the EIS data),
permitted investigation of the heterogeneous ET kinetics to and
from immobilized 2b by double potential step chronoamper-
ometry.43
In such experiments, the electrode potential is stepped back
and forth between two limiting values while the current is
monitored continuously during the experiment. The potential
switching frequency is chosen so as to fit with the time scale
of the relevant electrochemical process.32 An example of two
subsequent CA experiments is shown in Figure 5A. At very
short times, double-layer charging dominates the current decay
while, at relatively longer timescales, faradaic currents involving
reduction or oxidation of the redox centers on the surface are
observed. By choosing initial and final potential levels that
correspond to the fully oxidized and monoanion reduced states
of the immobilized species, respectively, information concerning
the dynamics of the ET processes is obtained from the
exponential decay of the CA currents.43 In particular, if
significant rearrangements or chemical modifications take place
in the film following the ET event, such as the shuttling of the
Figure 5. CA transients recorded for the SAM 2b‚11-MUA‚Au. (A)
Forward step potential: -0.4 V; backward step and initial potential: 0 V;
step duration: 0.25 s. (B) Forward step potential: -0.9 V; backward step
and initial potential: 0 V; step duration: 10 ms. Solution: aqueous 0.1 M
KCl, T ) 25 °C. For the sake of comparison, in (B) absolute values of the
current are displayed.
macrocycle along the thread, then the backward and forward
current transients will be non-symmetrical. Electron transfer
involving adsorbed redox species through non-defective SAMs
occurs via non-resonant through-bond tunneling (i.e., the rate
decreases exponentially with the chain length) with decay
constants ranging from 0.8-1.5 Å-1 for saturated chains to 0.2-
0.6 Å-1 for unsaturated ones.44 Any factor affecting the
electronic coupling between an attached redox moiety and the
electrode (e.g., the number and type of covalent bonds, bond
conformations, noncovalent interactions, direct versus through-
bond coupling) causes the ET rate to change markedly. Double
potential step CA has proven to be a sensitive method for
detecting the existence of structurally different binding modes
of redox moieties to electrodes and to detect electrochemically
induced phenomena on films such as bending or translocation
of redox-active units tethered to the electrode.6g Recently, CA
was successfully used to follow the electrochemically induced
motion of a threaded cyclophane along a molecular string
associated with the electrodes.7d,e
(42) In alkaline aqueous systems, this has been shown to be the result of partial
reversible reductive desorption of the thiolates. The exact potential at which
this occurs varies with certain structural features, and consequently this
process has been widely studied as a method for probing the molecular
structure of monolayers. See, for example: (a) Widrig, C. A.; Chung, C.;
Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (b) Walczak,
M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C. K.;
Porter, M. D. Langmuir 1991, 7, 2687-2693. (c) Kakiuchi, T.; Usui, H.;
Hobara, D.; Yamamoto, M. Langmuir 2002, 18, 5231-5238. Even under
neutral conditions, or in organic solvents where reductive desorption is
not observed within the solvent window, alkanethiolate SAMs are still
destroyed at negative potentials. The details of this complex, kinetically
controlled, process are less well understood. See, for example: (d) Everett,
W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67,
292-298. (e) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995,
307, 253-268. (f) Beulen, M. W. J.; Kastenberg, M. I.; van Veggel, F.;
Reinhoudt, D. N. Langmuir 1998, 14, 7463-7467. See also: (g) Finklea,
H. O. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker:
New York, 1996; Vol. 19, p 109.
The CA transients recorded for 2b‚11-MUA‚Au when the
potential was stepped between 0 V and -0.4 V (i.e., remaining
(44) Paddon-Row, M. N. In Stimulating Concepts in Chemistry; Vo¨gtle, F.;
Stoddart, J. F.; Shibasaki, M., Eds.; Wiley-VCH: Weinheim, 2000; pp.
267-292.
(43) Chidsey, C. E. D. Science 1991, 251, 919-922.
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Three State Redox-Active Molecular Shuttle
A R T I C L E S
more positive than the reduction potential of 2b, Figure 5A)
show symmetric backward and forward transients indicating that
no structural change occurs under these conditions. Non-
symmetric backward-to-forward transients were, however,
observed when the potential was stepped from 0 V (correspond-
ing to neutral 2b) to -0.9 V (sufficiently negative to reduce
the ndi stations), and then back to 0 V (Figure 5B). All
transients, recorded during either the forward or the backward
steps, displayed a short-lived (<0.1 ms) component associated
with double-layer charging in agreement with the very small
time constant of the electrode. A slower component (with τ ≈
3 ms) was then observed in both transients, compatible with
the reduction (in the forward step) and reoxidation (in the
backward one) of the ndi station. A rate constant of ∼300 s-1
is in fact typical of ET processes involving redox probes
immobilized onto the surface of SAMs formed from alkanethiols
with alkyl chains of 12-14 methylene units.45
A third component with an intermediate time scale of ∼0.2
ms was also observed in the backward step but not the forward
step. Once the double-layer charging current was subtracted,
this component amounted to ∼2/3 of the overall current decay.
This is an indication that the injection of electrons to the ndi
station of 2b and their subsequent removal occurs along different
molecular pathways. The co-conformational changes associated
with the shuttling process that follows the reduction of the ndi
station in solution would explain the ET behavior of im-
mobilized rotaxanes: in the succ-2b co-conformation (i.e.,
during the forward step), the shortest way for electrons to get
from the electrode surface to the redox site (ndi) is via weak
interactions between the diimide carbonyls and the terminal
carboxylate groups of the SAM. Such a process would cor-
respond to the ∼3 ms component of the transients. Upon
reduction, however, the rotaxane undergoes shuttling that would
make the redox-active station more efficiently electronically
wired (via hydrogen bonds) to the macrocycle and therefore
with the SAM, generating the faster transient (∼0.2 ms)
observed in the oxidation step. In fact, unbound reduced
(negatively charged) ndi stations should be repelled from the
SAM surface, which carries a negative charge under the
experimental conditions (pH 7).46 So, in the absence of a
shuttling process, it might be expected that the reoxidation of
the ndi station would be a slower process than reduction. The
experimentally observed 10-fold acceleration of the ET rate for
the oxidation step is equivalent to shortening of the alkane bridge
by two methylene units.47 The high reproducibility of the CA
transients in subsequent experiments suggests that, following
reoxidation, backward shuttling of the macrocycle to restore
the succ-2b co-conformation is efficient. Simulations of the
shuttling mechanism in the monolayer were performed (Figure
S8 of the Supporting Information) and suggest that the equi-
librium concentrations of the two co-conformations would in
fact be reached on the surface within the 10 ms reduction step.
This explains the presence, in the backward transient, of both
slow and fast components. The fact that the fast transient,
corresponding to oxidation of ndi-2b-•‚11-MUA‚Au(111), ac-
counts for ∼2/3 of the overall decay is in-line with the solution-
phase results (Figure 3) where a 1:2 succ-2a-•:ndi-2a-• co-
conformational ratio was observed.
Conclusions
Perhaps the best way to appreciate the technological potential
of controlled molecular-level motion is to recognize that
molecular-level machines lie at the heart of every significant
biological process. Nature has not repeatedly chosen this solution
for achieving complex task performance without good reason.
In terms of artificial molecular machines, electrochemistry is a
particularly attractive operating stimulus from the viewpoint of
integration with current technologies and also allows for a
powerful set of analytical tools to characterize the molecular-
level motion across a range of different environments. In this
second generation of redox-active imide molecular shuttles, the
positional discrimination and rapid shuttling kinetics of the
original system have been improved upon in a shuttle that now
exhibits three distinct macrocycle distributions, all accessible
using only electrochemistry. Crucially from the perspective of
future applications, the electrochemically induced shuttling
between two of these states has been demonstrated for a SAM
of amide-based molecular shuttles for the first time. The
shuttling is fast (millisecond time scale), reversible, and the
kinetics and thermodynamics can be probed with a range of
experimental techniques. These systems represent a significant
new addition to the few classes of artificial molecular machines
that have been shown to operate on surfaces to date.
Acknowledgment. This work was supported by the European
Union Future and Emerging Technology program Hy3M, the
University of Bologna, the Italian MIUR, The Netherlands
Organization for the Advancement of Research (NWO) and the
Carnegie Trust for the Universities of Scotland. D.A.L. is an
EPSRC Senior Research Fellow and holds a Royal Society-
Wolfson Research Merit Award.
Supporting Information Available: Synthetic procedures and
spectroscopic data for rotaxanes 2a and 2b. AM1 calculations
of imide and diimide charge densities. Details of CV simulation
parameters, supplementary CV experiments and EIS experi-
ments. Details of monolayer preparation and characterization.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(45) Smalley, J. F.; Feldberg, S. W.; Chidsey, C. E. D.; Linford, M. R.; Newton,
M. D.; Liu, Y.-P. J. Phys. Chem. 1995, 99, 13141-13149.
(46) (a) Molinero, V.; Calvo, E. J. J. Electroanal. Chem. 1998, 445, 17-25.
(b) Azzaroni, O.; Vela, M. E.; Martin, H.; Creus, A. H.; Andreasen, G.;
Salvarezza, R. C. Langmuir 2001, 17, 6647-6654.
(47) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657-2668.
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