A redox-driven multicomponent molecular shuttle

Article abstract of DOI:10.1021/ja0724590

A multicomponent [2]rotaxane designed to operate as a molecular shuttle driven by light energy has been constructed, and its properties have been investigated. The system is composed of (1) a light-fueled power station, capable of using the photon energy

Full text of DOI:10.1021/ja0724590

Published on Web 09/19/2007
A Redox-Driven Multicomponent Molecular Shuttle
Sourav Saha,† Amar H. Flood,‡ J. Fraser Stoddart,*,† Stefania Impellizzeri,§
Serena Silvi,§ Margherita Venturi,§ and Alberto Credi*,§
Contribution from the California NanoSystems Institute and the Department of Chemistry and
Biochemistry, UniVersity of California, Los Angeles, California 90095, Department of Chemistry,
Indiana UniVersity, Bloomington, Indiana 47405, and Dipartimento di Chimica “G. Ciamician”,
UniVersita` di Bologna, Via Selmi 2, 40126 Bologna, Italy
Received April 9, 2007; E-mail: stoddart@chem.ucla.edu; alberto.credi@unibo.it
Abstract: A multicomponent [2]rotaxane designed to operate as a molecular shuttle driven by light energy
has been constructed, and its properties have been investigated. The system is composed of (1) a light-
fueled power station, capable of using the photon energy to create a charge-separated state, and (2) a
mechanical switch, capable of utilizing such a photochemically generated driving force to bring about
controllable molecular shuttling motions. The light-fueled power station is, in turn, a dyad comprising (i) a
π-electron-accepting fullerene (C60) component and (ii) a light-harvesting porphyrin (P) unit which acts as
an electron donor in the excited state. The mechanical switch is a redox-active bistable [2]rotaxane moiety
that consists of (i) a tetrathiafulvalene (TTF) unit as an efficient π-electron-donor station, (ii) a dioxynaph-
thalene (DNP) unit as a second π-electron-rich station, and (iii) a tetracationic cyclobis(paraquat-p-phenylene)
(CBPQT4+) π-electron-acceptor cyclophane, which encapsulates the better π-electron-donating TTF station.
Diethylene glycol spacers were conveniently introduced between the electroactive components in the
dumbbell-shaped thread to facilitate the template-directed synthesis of the [2]rotaxane. A modular synthetic
approach was undertaken for the overall synthesis of this multicomponent bistable [2]rotaxane, beginning
with the syntheses of the P-C60 dyad unit and the two-station TTF-DNP-based [2]rotaxane separately,
using conventional synthetic methodologies. These two components were finally stitched together by an
1
esterification to afford the target rotaxane. Its structure was characterized by H NMR spectroscopy and
mass spectrometry as well as by UV-vis-NIR absorption spectroscopy and voltammetry. The observations
reflect remarkable electronic interactions between the various units, pointing to the existence of folded
conformations in solution. The redox-driven shuttling process of the CBPQT4+ ring between the two
competitive electron-rich recognition units, namely, TTF and DNP, was investigated by electrochemistry
and spectroelectrochemistry as a means to verify its operational behavior prior to the photophysical studies
related to light-driven operation. The oxidation process of the TTF unit is dramatically hampered in the
rotaxane, thereby reducing the efficiency of the shuttling motion. These results confirm that, as the structural
complexity increases, the overall function of the system no longer depends simply on its “primary” structure
but also on higher-level effects which are reminiscent of the secondary and tertiary structures of
biomolecules.
Introduction
Just like their macroscopic counterparts, molecular machines
need a power supply that can fuel mechanical movements. While
macroscopic machines do not move or work until energy is
supplied to it for a specific function, machines on the molecular
level are in perpetual random (Brownian) motion at ambient
temperature.4b,5 Hence, the energy supplied to molecular
The construction of mechanical machines at the ultimate level
of miniaturization, i.e., that of molecules, is a formidable
challenge for research in nanoscience and nanotechnology.1,2
The scientific basis for this idea is rooted in the existence of
natural molecular machines3 that carry out many essential
functions in living organisms, including human beings. In fact,
these biological nanomachines constitute the premier, proven
examples of the utility and feasibility of a nanotechnology based
on soft materials.4
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† University of California.
‡ Indiana University.
§ Universita` di Bologna.
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J. AM. CHEM. SOC. 2007, 129, 12159-12171
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A R T I C L E S
Saha et al.
machines, in the form of chemical,6 electrochemical,7 or
photochemical8 inputs, is usually employed to bias the random
thermal motion in order to obtain directed movements of the
machine’s components, although molecular systems wherein the
energy input can be converted directly into controlled motion
are available.9 Another important requirement is the ability to
detect and control the specific molecular actuations in response
to the applied stimulation by monitoring suitable output signals.
Mechanically interlocked molecules, such as rotaxanes and
catenanes,10 are one of the most suitable candidates for
molecular machines because (i) the mechanical bond allows a
large variety of mutual arrangements of the molecular compo-
nents, while conferring stability on the system; (ii) the inter-
locked architecture limits the amplitude of the intercomponent
motion in the three dimensions; (iii) the stability of a specific
arrangement (co-conformation11) is determined by the strength
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virtue of their electrical properties and bi- or multistable
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(NEMS).
In a bistable [2]rotaxane, the competitive affinity of a
macrocyclic ring with two distinct recognition sites on the thread
component allows the macrocycle to translate between them,
generating a nanoscale mechanical motion. Systems of this type,
termed molecular shuttles,16 constitute the most common
implementation of the molecular machine concept with artificial
molecules.17 The primary requirement for generating such a
controllable motion of the ring component and the detection of
two separate translational isomers of a bistable [2]rotaxane is a
large difference in the affinity of the macrocycle between the
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A Redox-Driven Multicomponent Molecular Shuttle
A R T I C L E S
Figure 1. Structural formula and graphical representation of the multicomponent rotaxane 14+
.
competing stations on the thread. A free energy difference
greater than 1.3 kcal mol-1 between two translational isomers
ensures7e that, at room temperature, in more than 90% of
rotaxane molecules the ring component resides around one
recognition site over the other. In order to generate significant
translational motion of the ring within the interlocked molecules,
a variety of external signals, such as chemical, electrochemical,
and/or photoelectrochemical stimuli6-8 have been successfully
employed.
For a variety of reasons, light is one of the most important
means of supplying energy to nanodevices, including molecular
machines.8,18 Important directions for the design of light-
powered molecular machines are provided by research in
artificial photosynthesis.19 For instance, donor-chromophore-
acceptor molecular triads comprising a π-electron donor (e.g.,
tetrathiafulvalene (TTF), π-extended TTF, ferrocene, dihydro-
pyrene, carotene), a chromophoric porphyrin unit, and a
π-electron acceptor (e.g., C60, quinones) have been shown to
(i) generate photocurrent,20 (ii) perform active transportation of
H+ and Ca2+ ions across artificial lipid membranes,21,22 and (iii)
synthesize ATP,21b,c all by harnessing light energy to generate
an electron flow which builds up an electrochemical gradient
at the nanoscale. In our early studies, we have utilized23 a TTF-
P-C60-based photoactive triad to verify that light energy
generates the charge-separated state with a singly oxidized TTF
-
component and singly reduced C60 component, TTF+-P-C60
.
The ensuing photocurrent was used as a power supply to drive
a supramolecular machine in the form of a pseudorotaxane.
Integration of such a light-harvesting moiety into an interlocked
molecular system, such as a two-station TTF-DNP-based
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(21) (a) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust,
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Figure 2. Progeny of the multicomponent rotaxane. Structural formulas of the multicomponent [2]rotaxane 14+, the tetrad 2, the dyad 3, the bistable
[2]rotaxane 44+, and the triad 16 as well as their retrosynthetic scheme involving dumbbell-shaped compound 17, the DNP derivative 18, the TTF derivative
19, the free-base tetraarylporphyrin 20, and the fulleropyrrolidine 21.
bistable [2]rotaxane,14,17d-g may lead to the photochemical
control of the shuttling motion on account of photoinduced
electron transfer (PET) processes within the multicomponent
rotaxane.
most common way to power molecular machines is through
the addition of a fuel capable of causing an appropriate chemical
reaction that eventually causes the motion, as it happens in the
ATP-powered biological molecular motors. Such an approach,
which is strongly related to operation in solution, requires the
continuous supply of fuel molecules and implies the formation
of waste products. Nature also shows, however, that the energy
needed to sustain the machinery of life is ultimately provided
by sunlight via a charge-separation process, i.e., the generation
of an electrochemical potential.24 Photon inputs can indeed cause
the occurrence of endergonic chemical reactions in suitably
designed artificial molecular systems that can make a machine
work without the generation of waste products. As pointed out
elsewhere,1,2,13,18 light inputs have other advantages over a
chemical or electrochemical energy supply.
In this paper we report the design, modular synthesis, and
properties of the photoactive bistable [2]rotaxane 14+ (Figure
1), its dumbbell-shaped thread component 2, and model
compounds (Figure 2) for the functional units incorporated into
its structure, with the primary objective of establishing whether
the mechanical switching properties of the TTF-DNP-based
rotaxane motif are retained in this multicomponent system. UV-
vis-NIR Absorption spectroscopy, electrochemical methods,
and the coupling of the two (spectroelectrochemistry) have been
employed to characterize the intercomponent interactions in the
rotaxane and to investigate the redox-induced shuttling. The
light-driven operation of 14+ as a molecular shuttle requires
extensive photophysical studies and will be investigated in the
course of time.
Taking advantage of our past experience on photochemically
driven molecular machines,25 culminated by the construction26
of an autonomous molecular shuttle powered by sunlight,8l,m
we came up with the design (Figure 1) of the multicomponent
rotaxane 14+. This system entails the integration of (1) a light-
fueled power generator, capable of converting the photon energy
Design
Molecular machines are structurally organized, functionally
integrated multicomponent systems designed to perform control-
lable mechanical-like motions of their components when fed
by an appropriate energy source.1,2 The energy supply for
molecular machines is a problem of primary importance. The
(24) (a) Oxygenic Photosynthesis: the Light Reactions; Ort, D. R., Yocum, C.
F., Eds.; Kluwer: Dordrech, 1996. (b) Rhee, K.-H.; Morriss, E. P.; Barber,
J.; Ku¨hlbrandt, W. Nature 1998, 396, 283-286. (c) Renger, T.; Marcus,
R. A. J. Phys. Chem. B 2002, 106, 1809-1819.
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A Redox-Driven Multicomponent Molecular Shuttle
A R T I C L E S
into an electrochemical potential, and (2) a mechanical switch,
capable of utilizing such a photochemically generated driving
force to bring about controllable molecular shuttling motions.
The light-fueled power generator is a dyad comprising (i) a
fullerene (C60) component and (ii) a tetraarylporphyrin (P) unit.
between the TTF station and the ring can be turned off by
oxidation of the TTF component, at which stage the second
π-electron-donating DNP station provides a higher affinity to
the CBPQT4+ ring. Thus, a combination of the electrostatic
repulsion of the CBPQT4+ ring away from the oxidized TTF
and the stabilizing interactions afforded by the DNP station
induces a molecular movement of the ring from its initial TTF
station to the DNP station, according to a sort of “push-pull”
mechanism possibly assisted by the ambient thermal energy.
Regeneration of the neutral TTF unit by a reduction process
gives rise to a metastable co-conformation which may relax back
thermally to the rest state through the return of the ring to the
original TTF station. The switching processes of the CBPQT4+
ring in TTF-DNP-based bistable interlocked molecules in
response to chemical and electrochemical inputs are now well-
understood phenomena.14,17d-g,32,33
C60 was selected because of its electron-accepting character and
for the low electronic reorganization energy27 on account of its
three-dimensional spherical geometry. Taken together, these two
features make the C60 unit a remarkably efficient acceptor in
electron-transfer systems and a most attractive component in
artificial photosynthetic devices.19-23 Porphyrins are the prin-
cipal chromophores chosen by Nature for harvesting sunlight.28
Free-base porphyrins are widely used in charge-separation
molecular devices19,23a-e,29 because they can act as electron
donors in the excited state. Furthermore, their absorption
spectrum,30 while distributed across most of the visible spectral
region, shows a sharp and very intense peak (Soret band, ca.
420 nm) which is isolated conveniently from the absorption
bands of the other components present in 14+, thereby allowing
selective excitation of the porphyrin unit in the assembly. The
light harvesting dyad moiety 3 (Figure 2) of the multicomponent
rotaxane was prepared23 by attaching the C60 unit covalently
with the porphyrin unit.
Since it is known34 that visible light excitation of the
porphyrin unit in TTF-P-C60 triads leads to a TTF+-P-C60
-
charge-separated state, we envisaged that the oxidation of the
TTF unit in the rotaxane 14+ could be obtained as a result of a
sequence of electron-transfer processes initiated by light.
The reduction of TTF+ back to neutral TTF will then be
-
accomplished by back electron transfer from the C60 unit.
Therefore, visible light excitation of 14+ is expected to trigger
a “power stroke” leading to ring displacement from the TTF to
the DNP station. Subsequent charge recombination, by restoring
the starting electronic distribution, triggers the “recovery stroke”
which leads to shuttling of the ring back to the TTF station and
overall reset of the machine. Thus, the light-driven operation
of 14+ as a molecular shuttle with autonomous behavior relies
on a complex sequence of electronic and nuclear motions that
must be carefully synchronized, similarly to what happens for
the four strokes of an internal combustion engine. In this paper,
we will not discuss in detail the requirements and features of
the light-driven mechanism. Rather, we will focus our discussion
on a key process for the operation of multicomponent rotaxane
The mechanical switch is a redox-active bistable [2]rotaxane
moiety that consists of a molecular thread containing (i)
tetrathiafulvalene (TTF) and (ii) dioxynaphthalene (DNP) units
as π-electron-donating stations, separated by a poly(ethylene
glycol) spacer, and of (iii) a tetracationic cyclobis(paraquat-p-
phenylene) (CBPQT4+) π-electron-accepting tetracationinc cy-
clophane (hereafter called the ring). To prevent the CBPQT4+
ring from slipping off and from approaching the porphyrin unit,
bulky 2,6-diisopropyl aryl stoppers were placed at both ends
of the thread. The model compound for such a mechanical
switch is the bistable rotaxane 44+ (Figure 2). The competitive
noncovalent-binding of the CBPQT4+ ring with two recognition
sites, namely TTF and DNP, on the thread component can be
exploited to induce its shuttling between these sites.14,17d-g In
the starting, rest state of the multicomponent rotaxane, a strong
π-π charge-transfer (CT) interaction between the CBPQT4+
ring and the electron-rich TTF unit, reinforced by C-H‚‚‚O
interactions,31 provides a thermodynamic advantage for the ring
to reside32 on the TTF station. The stabilizing interactions
1
4+, namely, the oxidation of the TTF unit by chemical and
electrochemical means.
Results and Discussions
Syntheses. The multicomponent rotaxane 14+ and the cor-
responding dumbbell compound 2 were synthesized in a modular
fashion, starting with the syntheses of the active components,
and by putting them together to obtain these target molecules
as illustrated in Schemes 1-3. The P-C60 dyad 323 and
compounds 5,35 7,36 10,7g 13,37 and 152+38 were prepared
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S. Chem.sEur. J. 2000, 6, 3558-3574.
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Scheme 1. Preparation of the [2]Rotaxane 4‚4PF6
according to literature procedures. The bistable [2]rotaxane 44+
was synthesized (Scheme 1) by a template-directed macrocy-
clization to form the CBPQT4+ ring around the templating TTF
component. Finally, the P-C60 dyad 3 with a terminal benzylic
alcohol functionality and the rotaxane 44+ bearing a carboxylic
acid group were stitched together (Scheme 2) by an esterification
then tosylated to obtain compound 12 in 84% yield. Compound
12 was etherified with 2,6-diisopropyl-4-hydroxymethyl phenol
13 to furnish the dumbbell-shaped compound 14 in 80%
yield. Finally, a macrocyclization of 15‚2PF6 with R,R′-dibromo-
p-xylene, templated by 14, followed by a counteranion ex-
change with NH4PF6 afforded the bistable [2]rotaxane 4‚4PF6
in 45% yield. The benzyl alcohol functionality at the TTF end
of the [2]rotaxane 4‚4PF6 is available for its further derivati-
zation.
to furnish the multicomponent [2]rotaxane 14+
.
Synthesis of Rotaxane 4‚4PF6: The synthesis of the rotaxane
4
4+ is outlined in Scheme 1. The bis-tosylate 535 was etherified
with 1 equiv of 2,6-diisopropyl phenol in the presence of K2-
CO3 to obtain the monotosylate 6 in 85% yield. Etherification
of the monobenzyl protected 1,5-dioxynaphthanlene derivative
736 with the tosylate 6 in the presence of K2CO3 and 18-crown-6
as a phase-transfer catalyst afforded compound 9 in 79% yield.
A subsequent hydrogenolysis of 8 gave the dioxynaphthalene
derivative 8 in 78% yield. Another etherification of the naphthol
derivative 9 with the TTF-monotosylate 107g under the same
conditions provided the alcohol 11 in 60% yield, which was
Syntheses of [2]Rotaxane 1‚4PF6 and Tetrad 2. The
synthesis of the multicomponent rotaxane 14+ is illustrated in
Scheme 2. The P-C60 dyad 3,23 containing a reactive carboxylic
acid group, was esterified with the rotaxane 4‚4PF6 using
2-chloro-4,6-dimethoxy-1,3,5-triazine, 4-methylmorpholine, and
dimethyl-4-aminopyridine (DMAP) as the activating agents to
afford the title rotaxane 14+ in 90% yield. Similarly, an
esterification of the dyad 323 with the alcohol 11 using the same
conditions furnished the tetrad 2 in a near quantitative yield as
described in Scheme 3.
UV-vis-NIR Spectroscopy. The [2]rotaxane 14+ is a
multicomponent species, which contains a large number of
chromophoric units (Figures 1 and 2). Its absorption spectrum
in DMF solution (Figure 3) shows multiple peaks throughout
(38) (a) Anelli, P.-L., et al. J. Am. Chem. Soc. 1992, 114, 193-218. (b) Ashton,
P. R.; Brown, G. R.; Isaacs, N. S.; Giuffrida, D.; Kohnke, F. H.; Mathias,
J. P.; Slawin, A. M. Z.; Smith, D. R.; Stoddart, J. F.; Williams, D. J. J.
Am. Chem. Soc. 1992, 114, 6330-6353. (c) Asakawa, M.; Ashton, P. P.;
Ballardini, R.; Balzani, V.; Belohradsky, M.; Gandolfi, M. T.; Kocian, O.;
Prodi, L.; Raymo, F. M.; Stoddart, J. F.; Venturi, M. J. Am. Chem. Soc.
1997, 119, 302-310.
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A Redox-Driven Multicomponent Molecular Shuttle
A R T I C L E S
Scheme 2. Synthesis of the Multicomponent [2]Rotaxane 1‚4PF6
Scheme 3. Synthesis of the Tetrad 2
the near UV and visible regions and a broad and weak band
centered around 840 nm, which extends into the near-infrared.
The absorption features in the UV-vis region can be attributed
to the corresponding chromophoric unitssnamely, C60, por-
phyrin, TTF, DNP, and 4,4′-bipyridinium dicationsby com-
parison with model compounds.39 Specifically, the band peaking
at 267 nm is ascribed to C60 and the 4,4′-bipyridinium units of
CBPQT4+; the broad absorption between 300 and 400 nm is
assigned to the C60, TTF, and DNP units; the sharp band at
λmax ) 419 nm is the porphyrin Soret band; and the features
from 500 through 650 nm are attributed to the porphyrin Q
bands.30 The broad band at 840 nm (Figure 3b) is characteristic31
of a CT interaction between the electron-rich TTF component
and the electron-deficient CBPQT4+ ring.
(39) For solubility reasons, it was not possible to obtain the absorption
spectra of all the compounds shown in Figure 2 in the same solvent.
Hence, the spectra of rotaxanes 14+ and 44+ were measured in DMF and
the spectra of all other compounds, except CBPQT4+, were measured in
THF. The spectrum of CBPQT4+ was recorded in MeCN because its
absorption bands fall in a spectral region below the cutoff wavelength of
DMF.
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Saha et al.
The absorption bands of triad 16 (Figure 4) are noticeably
perturbed compared to those of its constituents, taken as the
dyad 3 and the TTF derivative 19. The C60 absorption around
260 nm becomes less intense, the Soret band of the P unit is
weaker and broader, and a weak tail is observed at λ > 600
nm. To our knowledge, there are no reports in the literature on
ground-state electronic interactions in TTF-porphyrin systems,42
while intramolecular donor-acceptor interactions in covalently
linked TTF-C60 have been observed43 to cause a decrease in
the fullerene UV absorption and the appearance of a weak
absorption tail at λ > 600 nm. In summary, our observations
suggest that the ground state of the triad 16 is characterized by
non-negligible through-space electronic interactions involving
the electron-accepting C60 unit and the electron-donating P and
TTF units.
Figure 3. (a) UV-visible absorption spectrum of 14+ in DMF (black trace),
sum of the spectra of its separated molecular components, 2 in THF and
CBPQT4+ in MeCN (red trace), and sum of the spectra for compounds 18,
19, 20, and 21 in THF and CBPQT4+ in MeCN (blue trace) which serve as
models for the chromophoric units contained in 14+. The spectrum of tetrad
2 in DMF is nearly identical to that in THF. (b) Detail of the visible-NIR
region for the spectra described in part (a). The green trace shows the
difference between the absorption spectrum of 14+ and that of 2 in the 700-
1100 nm region.
Figure 4. UV-visible absorption spectrum in THF of the triad 16 (black
trace) and sum of the spectra of the dyad 3 and the TTF derivative 19
(red trace).
The absorption spectrum of the tetrad 2 is very similar to
that of the parent triad 16 at wavelengths longer than 350 nm
(see Supporting Information). The fact that the sharp absorption
features of the DNP unit at λ < 350 nm are no longer observed
in 2 indicates25b,38a that such a unit is also engaged in CT
interactions with the other chromophoric components. Inspection
of CPK molecular models indicates that the tetrad 2 is a very
flexible molecule. Therefore, we reason that in solutions tetrad
2 occurs as folded conformationssakin to foldamers44 which
form folded secondary structures, stabilized by noncovalent
interactions, namely, π-π-stacking, hydrogen bonding interac-
tions, as well as solvophobic forces. In the triad 16 and tetrad
2 a preponderance of folded conformations are made possible
by the presence of the flexible ethylene glycol linkers among
the P, TTF, and DNP units which allow the molecules to fold
up in such a way that the electron-donating (P, TTF, and DNP)
and -accepting (C60) units can interact with each other electroni-
cally (Figure 5) on account of favorable π-π-stacking interac-
A careful analysis of the absorption spectra of all the
compounds examined (Figure 2) shows, however, that the
absorption features of multicomponent species 2, 3, and 16
cannot be obtained simply by linear combinations of the bands
of their chromophoric units. The absorption spectrum of dyad
3 is slightly different from the sum of the spectra of its
components 20 and 21 (see Supporting Information). A decrease
in the intensity of the porphyrin peaks and an increase of
absorption at around 300 nm indicates the occurrence of
electronic interactions between the C60 and P units in the ground
state. It is more likely that such interactions occur through
spacesmolecular models indicate that the two units can easily
approach each other in a face-to-face fashionsrather than
through bonds, because the linker between the units is not fully
conjugated. The presence of through space electronic interactions
in covalently linked P-C60 species is well documented for
flexible systems,40 whereas it is not observed in dyads with rigid
bridges.34,41
(42) Covalently linked TTF-porphyrin species: Sadaike, S.; Takimiya, K.; Aso,
Y.; Otsubo, T. Tetrahedron Lett. 2003, 44, 161-165.
(43) Allard, E.; Cousseau, J.; Ordu´na, J.; Gar´ın, J.; Luo, H.; Araki, Y.; Ito, O.
Phys. Chem. Chem. Phys. 2002, 4, 5944-5951.
(44) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180. (b) Hill, D. J.;
Mio, M. J.; Prince, R. B.; Hughes, T.; Moore, J. S. Chem. ReV. 2001, 101,
3893-4011. (c) Schmitt, J.-L.; Stadler, A.-M.; Kyritsakas, N.; Lehn, J.-
M. HelV. Chim. Acta 2003, 86, 1598-1624. (d) Kelley, R. F.; Rybtchinski,
B.; Stone, M. T.; Moore, J. S.; Wasielewski, M. R. J. Am. Chem. Soc.
2007, 129, 4114-4115.
(40) Armaroli, N.; Accorsi, G.; Song, F.; Palkar, A.; Echegoyen, L.; Bonifazi,
D.; Diederich, F. ChemPhysChem 2005, 6, 732-743 and references therein.
(41) Lembo, A.; Tagliatesta, P.; Guldi, D. M. J. Phys. Chem. A 2006, 110,
11424-11434.
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12166 J. AM. CHEM. SOC. VOL. 129, NO. 40, 2007

A Redox-Driven Multicomponent Molecular Shuttle
A R T I C L E S
tions, a situation which is further supported by the electro-
chemical data (vide infra).
Interestingly, on going from the tetrad 2 to the multicompo-
nent [2]rotaxane 14+ the absorption spectrum becomes much
more similar to that obtained by summing up the spectra of the
single chromophoric units (Figure 3). This change is most
evident from the intensity recovery of the porphyrin absorption
bands. Presumably, the presence of CBPQT4+ around the TTF
station prevents the occurrence of the intercomponent electronic
interactions observed in tetrad 2.
Figure 6. Cyclic voltammetry of 14+ in DMF (1.0 × 10-3 M, 0.1 M
TBAPF6) recorded at 200 mV s-1 with each of the peaks assigned to a
particular electroactive unit of the rotaxane.
of 14+, it is necessary to describe the results obtained for the
molecular components and some model compounds.
The electrochemical behaviors of the tetrad 2 and its model
constituents, namely, TTF (19), the porphyrin 20, the fullerene
21, the dyad 3, and the triad 16, were investigated in THF (Table
1). The redox processes recorded for the dyad 3 fall at potential
values expected23b,45,46 for its porphyrin and C60 moieties. This
observation indicates that the electronic interactions between
such units in the dyad, which are revealed from the spectroscopic
data, have a negligible effect on the redox potentials. For the
triad 16 the oxidation of the porphyrin unit cannot be observed
within the potential window explored and, therefore, occurs most
likely at potentials higher than +1.4 V vs SCE. Moreover, the
processes for the TTF oxidation (+0.53 and +0.80 V) are
positively shifted compared to the same processes in model
compound 19 (+0.42 and +0.71 V) and exhibit electrochemical
irreversibility as well as a current intensity ca. 30% weaker than
that expected for monoelectronic processes (Supporting Infor-
mation). These observations are in agreement with the UV-
vis spectroscopic data and suggest that the porphyrin and TTF
units of 16 are involved in intramolecular electronic interactions
in which they presumably act as electron donors and, as a
consequence, become more difficult to oxidize. The lack of
electrochemical reversibility and the decrease in the current
intensity for TTF oxidation could be explained by the presence
of folded conformations in which such a unit is shielded and
cannot approach the electrode surface.47 This hypothesis is
consistent with the fact that the electrochemical reversibility
and current intensity of the TTF oxidation processes increase
by adding 15% of MeCN to the THF solution (see Supporting
Information) in order to improve solvation. The electron-
accepting counterpart is expected to be the fullerene unit;
however, little or no change in the potential values for reduction
of the C60 moiety is observed compared (Table 1) to the dyad
Figure 5. Schematic representation of the chemical equilibrium between
four possible folded conformations for the tetrad 2 in THF solution, in which
the electron-donating (P, TTF, and DNP) and -accepting (C60) units can
interact electronically with each other.
A quantitative comparison of the CT bands of 14+ and the
model [2]rotaxane 44+ (Supporting Information) indicates that
the CBPQT4+ ring is engaged in CT interactions with the TTF
station in about 75% of the 14+ molecules. This behavior could
be caused by a competition between CBPQT4+ and C60 electron
acceptors for the electron-donating TTF unit. In such a case, it
is to be expected that in 25% of the 14+ molecules the CBPQT4+
surrounds the DNP station; however, this hypothesis cannot be
confirmed because the weak DNP-CBPQT4+ CT band (λmax
) 515 nm, ꢀ ) 650 M-1 cm-1 in MeCN)25b would be buried
under the intense bands of the porphyrin chromophore and,
therefore, undetectable.
Electrochemistry. The potentials for the redox processes
observed in voltammetric experiments for the multicomponent
rotaxane 14+, its dumbbell-shaped tetrad component 2, and some
model compounds (Figure 2) are listed in Table 1. These data
are important for (i) applying suitable electrochemical stimuli
to 14+ in order to trigger specific molecular movements and
(ii) identifying the electron-transfer processes that can be
expected to occur upon light absorption in the rotaxane and in
its model compounds.
The cyclic voltammetric curve obtained for 14+ in DMF
(Figure 6) shows as many as nine redox waves in the potential
window examined, emphasizing once again the multicomponent
nature of this rotaxane. Assignment of each redox wave to the
corresponding electroactive unit is possible from a comparison
with the behavior of appropriate reference species (Figure 2 and
Table 1). Before discussing in detail the voltammetric behavior
(45) (a) Carano, M.; Da Ros, T.; Fanti, M.; Kordatos, K.; Marcaccio, M.;
Paolucci, F.; Prato, M.; Roffia, S.; Zerbetto, F. J. Am. Chem. Soc. 2003,
125, 7139-7144. (b) Sandanayaka, A. S. D.; Araki, Y.; Ito, O.; Deviprasad,
G. R.; Smith, P. M.; Rogers, L. M.; Zandler, M. E.; D’Souza, F. Chem.
Phys. 2006, 325, 452-460.
(46) Kadish, K. M.; Royal, G.; van Caemelbecke, E.; Gueletti, L. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, 2000; Vol. 9, pp 1-219.
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A R T I C L E S
Saha et al.
Table 1. Reduction Potential Values (in V vs SCE) for the Compounds Examineda
assignment
compound
(solvent)b
0
0
+/+
TTF2
0
+/
3+
+/
2
+
+/
0
0/-1
-1/-
2
/-
P0
1
1
P-
/-2
-2/-3
DNP+/
P+/
TTF+/
CBPQT4
CBPQT3
CBPQT2
C60
C60
C60
rot. 14+
(DMF)
tetrad 2
(THF)
triad 16
(THF)
dyad 3
(THF)
porphyrin 20
+1.37c
+1.11c
+1.35c
f
+0.91c,d
+0.75c,d
+0.80c,d
+0.51c,d
+0.46c,d
+0.53c,d
-0.28
-0.39
-0.73e
-0.84
-0.50
-0.51
-0.53
-0.99
-1.05c
-1.07
-1.07
-1.39c
-1.71c
-1.23
-1.26
-1.23
-1.48
-1.82
+1.23
f
-1.55
-1.56
-1.56
f
+1.34c
+1.31c
-1.78
(THF)
fullerene 21
(THF)
-0.53
-1.20c
-1.82c
TTF 19
+0.71
+0.74
+0.42
+0.55
(THF)
rot. 44+
(DMF)
f
-0.29
-0.23e
-0.36
-0.73e
-0.67e
CBPQT4+
(DMF)
dumbbell 17
(DMF)
+1.34c
+0.72
+0.46
a Halfwave potential values referring to reversible and monoelectronic processes, unless otherwise noted. b For solubility reasons, it was not possible to
perform the electrochemical experiments for all the compounds shown in Figure 2 in the same solvent. Hence, the experiments for compounds 14+, 44+, and
CBPQT4+ were carried out in DMF, and those of all other compounds were carried out in THF. In all cases 0.1 M tetrabutylammonium hexafluorophosphate
was the supporting electrolyte. c Irreversible or poorly reversible process; potential estimated from the DPV peak. d The intensity of the CV wave is much
weaker than that expected for a monoelectronic process. e Bielectronic process. f Not observed.
3. It is possible that these processes are not much affected by
through-space electronic interactions involving the C60 moiety
owing to the strong delocalization of the additional electrons
in the reduced fullerene unit.
The CV profile for the tetrad 2 is similar to that of the triad
16, although a comparison of the redox potentials indicates
weaker intercomponent interactions compared to those in the
triad. The current intensities associated with the TTF oxidations
are again considerably weaker than those expected for mono-
electronic processes, suggesting the presence of folded confor-
mations (Figure 5) which in this case are not affected by the
addition of up to 20% MeCN to the THF solution.
The voltammetric data obtained in DMF for the rotaxane
44+swhich is a model for the mechanical switching part of
14+sthe dumbbell-shaped component 17, and the CBPQT4+
ring are also gathered in Table 1 (cyclic voltammograms are
reported as Supporting Information). CBPQT4+ exhibits the
usual two reversible bielectronic reduction processes (-0.23 and
-0.67 V) assigned to the consecutive one-electron reductions
of the two equivalent bipyridinium units, both slightly shifted
to more negative potentials compared to those observed in
MeCN.38a No oxidation process is detected. Compound 17
shows the two reversible monoelectronic oxidation processes
of the electron-rich TTF unit, generating the TTF+ and TTF2+
species, respectively, and, at more positive potentials, the irre-
versible oxidation of the DNP unit. No reduction process is
observed.
In the rotaxane 44+ the first two reduction processes (-0.29
and -0.36 V vs SCE, Table 1) are assigned to the reversible
one-electron reduction of each of the two bipyridinium units
present in the CBPQT4+ ring. These potential values are more
negative than the corresponding first (two-electron) process,
measured for CBPQT4+ alone because of the CT interaction
with the TTF unit. The small albeit detectable splitting of the
CBPQT4+/2+ reduction into two monoelectronic waves shows
that, in 44+, the two bipyridinium units of the ring, which are
expected to be topologically equivalent, are not electrochemi-
cally equivalent. Such an effect was previously observed in
related rotaxanes and attributed7g,17d,48 to the fact that the
CBPQT4+ ring, while encircling a given electron-donating
station, is involved in alongside interactions with the other
electron-rich units. Such a behavior implies the presence of
folded conformations wherein the two bipyridinium sides of the
ring experience chemically different environments. The bielec-
tronic process observed at -0.73 V is assigned to the reduction
of CBPQT2+ to its neutral form (CBPQT0). The fact that this
process is also negatively shifted compared to the same process
in the free cyclophane indicates that CBPQT2+ is still involved
in electronic interactions with the dumbbell-shaped component.
The bielectronic nature of the process indicates that, in the
doubly reduced 42+ rotaxane, the two bipyridinium units of the
ring gain topological equivalence.
(47) The hindrance of electron-transfer processes for redox sites deeply enclosed
within a molecular structure, e.g., in dendrimers, is a well-documented
phenomenon. For example, see: (a) Cardona, C. M.; Mendoza, S.; Kaifer,
A. E. Chem. Soc. ReV. 2000, 29, 37-42. (b) Gorman, C. B.; Smith, J. C.
Acc. Chem. Res. 2001, 34, 60-71. (c) Hong, Y. R.; Gorman, C. B.
Langmuir 2006, 22, 10506-10509. The lack of expected redox processes
has also been observed in dendrimers containing multiple electroactive units
both in the core and in the periphery and was attributed to the presence of
conformations in which some of the redox sites are not accessible. See:
(d) Baker, W. S.; Lemon, B. I., III; Crooks, R. M. J. Phys. Chem. B 2001,
105, 8885-8894. (e) Marchioni, F.; Venturi, M; Ceroni, P.; Balzani, V.;
Belohradsky, M.; Elizarov, A. M.; Tseng, H.-R.; Stoddart, J. F. Chem.s
Eur. J. 2004, 10, 6361-6368.
Considering only the region of positive potentials, the first
oxidation process of the TTF unit is anodically shifted (+90
mV) compared to the same process in 17, whereas the second
TTF oxidation occurs at a very similar potential. This behavior
indicates that (i) the CBPQT4+ ring is located initially on the
(48) Yamamoto, T.; Tseng, H.-R.; Stoddart, J. F.; Balzani, V.; Credi, A.;
Marchioni, F.; Venturi, M. Collect. Czech. Chem. Commun. 2003, 68,
1488-1514.
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A Redox-Driven Multicomponent Molecular Shuttle
A R T I C L E S
TTF station and (ii) after the first oxidation of this unit the ring
moves away, leaving the second oxidation process unaltered
compared to that in the dumbbell-shaped component. The
oxidation of the DNP unit is not observed within the potential
window examined, most likely because this process falls at
potentials higher than +1.5 V vs SCE on account of the fact
that it becomes surrounded by the CBPQT4+ ring after the
oxidation of the TTF station. This hypothesis is in fact confir-
med by spectroelectrochemical experiments (see Supporting
Information).
In the multicomponent rotaxane 14+ the first three reduction
processes are very similar to those of 44+ and are assigned to
the CBPQT4+ component. The splitting of the CBPQT4+/2+
reduction into two monoelectronic waves (110 mV) is more
evident than that in 44+ (70 mV) and shows that also 14+ exists
as folded conformations7g,17d,48 in which the electrochemical
equivalence of the two bipyridinium units of the ring is lost.
The third process, observed at -0.73 V, is bielectronic and
therefore assigned to the CBPQT2+/0 reduction. The successive
process at -0.84 V is assigned45 to the first monoelectronic
reduction of the electron-accepting C60 unit, whereas the
following four processes that are observed through the region
from -0.9 to -1.9 V are attributed23b,45,46 to reductions
involving the C60 and porphyrin units (Table 1).
the TTF unit are concerned, provided the motivation to
undertake spectroelectrochemical investigations. In the present
studies, the isolation of the intense Soret band (λmax ) 419 nm,
ꢀ ) 410 000 M-1 cm-1) allows us to monitor the co-
conformational changes within the molecule in response to redox
stimulation by visualizing the diagnostic, albeit weaker, absorp-
tion features in the 475-900 nm spectroscopic window, such
as (a) the TTF+ absorption peak at 600 nm, (ii) the TTF f
CBPQT4+ CT band at 840 nm, and (iii) the CBPQT2+ absorption
peak at 600 nm.31 The differential absorption spectra (∆A) were
derived from the original spectra recorded during electrolysis
in an attempt to distinguish the appearance and disappearance
of certain peaks from the overlapping Q-bands of the porphyrin
chromophore in the 450-650 nm region. Spectroelectrochemical
results for the TTF model 19 and the rotaxane 44+ can be found
in the Supporting Information.
The spectroelectrochemical behavior of the tetrad 2 was
investigated in DMF solution (Figure 7). The straight line
passing through zero in Figure 7b represents the initial dif-
ferential spectrum of the tetrad 2. The onset of the TTF+ radical
cation’s absorption was first observed at Eap ) +0.45 V (vs
Ag pseudoreference electrode), a situation which is consistent
with the first oxidation potential of the TTF unit. The TTF+
radical cation’s absorption bands continued to grow and
ultimately reached their maximum (Figure 7, green trace) at
Eap ) +0.60 V. The absorption features (green trace) of the
On the oxidation side, the two poorly reversible processes at
+0.51 and +0.91 V vs SCE are assigned to the two monoelec-
tronic oxidations of the TTF unit. The first one occurs at a
potential similar to that found for the corresponding process in
4
4+, whereas the second TTF-centered oxidation is considerably
more difficult in 14+ than in 44+. This observation may suggest
that the CBPQT4+ ring is still encircling the mono-oxidized TTF
station but would also be consistent with a stabilization of the
TTF+ species and/or destabilization of the TTF2+ species, no
longer surrounded by the ring, because of electronic interactions
with the other units present in the rotaxane structure. Spectro-
electrochemical data (vide infra) are also in good agreement
with this latter interpretation. Two further irreversible oxidation
processes observed at +1.11 and +1.37 V are attributed to the
porphyrin23b,46 and DNP14,17d-g units, respectively.
It is important to note that the current intensity associated
with the TTF oxidation processes in 14+ is considerably weaker
than that expected for one-electron processes. A lack of current
intensity in the TTF oxidation waves is observed also for the
triad 16 and the tetrad 2 in THF. This issue is an important one
because the shuttling operation of 14+ relies on the reversible
oxidation of its TTF unit. The UV-vis experiments suggest
that in 14+ the intercomponent interactions involving the TTF,
P, and C60 moieties, observed for 16 and 2, are impeded by the
presence of the CBPQT4+ ring encircling the TTF unit. The
reduction behavior of the CBPQT4+ ring shows, on the other
hand, that the molecules of 14+ (and 44+) occur as folded
conformations. Nevertheless, the current intensities for the TTF
oxidations in 44+ are as expected for monoelectronic processes.
Taken together, these observations indicate that the shielding
of the TTF unit toward electrochemical oxidation is a nontrivial
behavior of the rotaxane 14+ and has to be related to its complex
multicomponent structure.
Figure 7. UV-visible spectroelectrochemical changes for the oxidation
of tetrad 2 (1 mM DMF, 0.1 M TBAPF6, optical path length 1 mm). (a)
Absorption spectra; (b) differential absorption spectra (∆A). Black traces,
original spectra; green traces, spectra obtained after oxidation of the TTF
unit to the TTF+ form at an applied potential of +0.60 V; red traces, spectra
obtained after regeneration of the neutral TTF unit at an applied potential
of -0.10 V.
Spectroelectrochemistry. The complex voltammetric be-
haviors of the multicomponent [2]rotaxane 14+, as well as the
corresponding tetrad 2, particularly as far as the oxidations of
9
J. AM. CHEM. SOC. VOL. 129, NO. 40, 2007 12169

A R T I C L E S
Saha et al.
oxidized form are similar but not identical to those of the TTF+
radical cation (Supporting Information) generated in the TTF-
diol 19. On the basis of the absorption coefficients for the TTF+
species at 600 nm one can estimate, however, that about 15%
of the TTF units have been oxidized. Subsequently, reducing
the TTF+ radical cation back to the neutral TTF (Eap ) -0.10
V) reproduced (Figure 7, red trace) the original spectrum. Thus,
the electrochemical response of the tetrad 2 in DMF is reversible
upon first one-electron oxidation of the TTF unit that, however,
can be achieved only for a modest fraction of the tetrad
molecules. These observations help to identify the spectroscopic
changes that are indicative of the oxidatively induced switching
process in the more complicated [2]rotaxane 14+
.
We first investigated the oxidation of 14+ in DMF by applying
a positive potential. The onset of spectroscopic changes (Figure
8) is observed when the applied potential is increased to +0.85
V (relative to a Ag pseudo-reference electrode). The charac-
teristic absorption band of the TTF+ species, centered at 600
nm, emerges (Figure 8) over a period of 25 min at a constant
applied potential (+0.85 V). On the basis of the absorption
coefficients for the TTF+ species at 600 nm one can estimate,
however, that ca. 30% of the TTF units have been oxidized.
This change is concomitant with a decrease of the characteristic
TTF f CBPQT4+ CT band at 840 nm until complete disap-
pearance (Figure 8), defining together an isosbestic point at 660
nm. These changes are similar but not identical to those recorded
for the control rotaxane 44+, for which full monoelectronic
oxidation of the TTF unit and complete disappearance of the
CT band at 830 nm are observed (see Supporting Information).
On the other hand, the absorption bands for the TTF+ species
Figure 9. UV-visible spectroelectrochemical changes for the reduction
of the CBPQT4+ ring of 14+ (1 mM DMF, 0.1 M TBAPF6, optical path
length 1 mm). (a) Absorption spectra; (b) differential absorption spectra
(∆A). Black traces, original spectra; purple traces, spectra obtained after
complete reduction of the CBPQT4+ ring to the CBPQT2+ form upon
application of a potential from -0.25 to -0.40 V; red traces, spectra
obtained after regeneration of the CBPQT4+ ring at an applied potential of
0 V for 14+. The blue trace in panel b shows the absorption features of the
CBPQT2+ ring obtained upon electrochemical reduction of the [2]rotaxane
44+ in DMF under the same conditions employed for 14+
.
are very similar to those obtained upon electrolysis of tetrad 2
(Figure 7). Had the ring remained encircling the TTF+ com-
ponent, the 600-nm band would have been significantly af-
fected.49 The above observations on 14+ are consistent with a
TTF unit encircled by a CBPQT4+ ring (the major oxidative
peak in related simpler rotaxanes is observed14g,31 at around +0.8
V) and with the movement of the tetracationic CBPQT4+ ring
away from the singly oxidized TTF+ unit. Based on our exten-
sive studies on simpler TTF-DNP-based bistable rotax-
anes14,17d-g,31,32 and the results gathered for 44+, we believe that
electrochemical one-electron oxidation of the TTF unit in 14+
causes the displacement of the CBPQT4+ ring onto the now-
better electron-donating DNP station. Conclusive proof of the
position of the tetracationic cyclophane, at this point, would
have been the emergence of a characteristic weak DNP f
CBPQT4+ CT band at ca. 500 nm.25b,31c Unfortunately, we were
unable to identify any increase in absorbance over and above
the very intense porphyrin absorption peaks in this wavelength
region.
The successive application of a potential of 0 V causes the
disappearance of the TTF+ component’s absorption peak at 600
Figure 8. UV-visible spectroelectrochemical changes for the oxidation
of 14+ (1 mM DMF, 0.1 M TBAPF6, optical path length 1 mm) recorded
over a period of 25 min at an applied potential of +0.85 V. (a) Absorption
spectra; (b) differential absorption spectra (∆A).
(49) Nygaard, S.; Laursen, B. W.; Flood, A. H.; Hansen, C. N.; Jeppesen, J.
O.; Stoddart, J. F. Chem. Commun. 2006, 144-146.
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A Redox-Driven Multicomponent Molecular Shuttle
A R T I C L E S
nm, suggesting that reduction of the TTF+ species back to the
neutral TTF has occurred. However, the original spectrum of
1
structures in solution. The TTF unit can be electrochemically
oxidized only in a limited fraction of the 14+ molecules. In these
species, removal of one electron from the TTF unit causes the
shuttling of the CBPQT4+ ring away from this station. The lack
of complete electrochemical oxidation of the TTF unit is
observed also for the dumbbell-shaped component of 14+ and
for a molecular triad comprising TTF, porphyrin, and C60
modules. Most likely, these systems occur as conformations in
which the TTF unit is buried inside a complex equilibrium
mixture of conformations and co-conformations and is therefore
protected against oxidation performed by an electric potential
applied externally. Such a behavior limits the efficiency for the
operation of rotaxane 14+ as a redox-driven molecular shuttle.
The possibility of achieving TTF oxidation by an electric
potential generated internally through intramolecular photoin-
duced electron transfer is currently under investigation.
4+ in the 475-700 nm region is not fully recovered, indicating
that some decomposition takes place on the time scale of
electrolysis (minutes) upon oxidation/reduction of the TTF
station in the rotaxane. The decrease of the band at 600 nm is
accompanied by a recovery of the TTF f CBPQT4+ CT band
at 840 nm to 75% of its original intensity. Kinetic effects can
be ruled out because the intensity of this band does not increase
with time.
To summarize, the spectroelectrochemical data confirm that
the electrochemical oxidation of the TTF moiety in 14+ is
hampered, possibly because of the presence of folded conforma-
tions in which this moiety cannot reach the electrode surface.47
However, electrolytic oxidation of the TTF station can be
partially achieved, causing the displacement of the CBPQT4+
ring away from the TTF+ species. Subsequent electrolysis at 0
V regenerates the neutral TTF unit, but only 75% of the
CBPQT4+ rings return to encircle it, as shown by the incomplete
recovery of the CT band at 840 nm.
In principle, the movement of the CBPQT4+ ring away from
the TTF station could be achieved also by monoelectronic
reduction of both its bipyridinium moieties to give the CBPQT2+
form. This process is expected to destabilize the CT interactions
with either the TTF or DNP donor units, thereby leaving the
ring to move “freely” along the dumbbell, i.e., without being
locked at either station. In order to explore this possibility we
performed spectroelectrochemical experiments on the reduction
of 14+. At an applied potential of -0.40 V, a structured band
centered at 600 nm (Figure 9), typical of monoreduced 4,4′-
bipyridinium units,50 appears. On the basis of the absorption
coefficients for the bipyridinium radical cation at 600 nm one
can estimate that more than 90% of the total bipyridinium units
have been reduced. A clear decay of the TTF f CBPQT4+ CT
band at 840 nm could not be observed because of the absorption
tail of the CBPQT2+ component in the 750-900 nm region.
Back electrolysis at 0 V causes the disappearance of the bands
characteristic of the reduced bipyridinium units, and the original
spectrum is restored.
In general terms, our results indicate that as the structural
complexity increases, the overall properties of the system cannot
be easily rationalized solely on the basis of the type and
sequence of the functional units incorporated in the molecular
framework, that is, its “primary” structure. Although some
aspects of the behavior of the investigated compounds are not
fully understood, it seems that higher-level conformational and
co-conformational effects, which are reminiscent of those related
to the secondary and tertiary structure44 of biomolecules, have
to be taken into consideration. The comprehension of these
effects constitutes a stimulating scientific problem and a
necessary step for the design of novel artificial molecular devices
and machines.
Acknowledgment. We thank Professor Vincenzo Balzani and
Dr. Nicolle Moonen for valuable discussions, and Dr. Alberto
Di Fabio for his help in some experiments. This research was
supported in the U.S. by a National Science Foundation’s (NSF)
Nanoscale Interdisciplinary Research Team (NIRT ECS-
0103559) fund, an Office of Naval Research (ONR Contract
number N00014-00-1-0216) grant, an NSF Equipment Grant
(CHE-9974928), and the Center for Cell Mimetic Space
Exploration (CMISE) - a NASA University Research, Engi-
neering and Technology Institute (URETI), the award number
being NCC2-1364. In Italy, the research was supported by the
EU Biomach project (NMP2-CT-2003-505487), Ministero
dell’Universita` e della Ricerca (PRIN 2006034123), Regione
Emilia-Romagna (NANOFABER), and Universita` di Bologna
(Progetto strategico CompReNDe).
Conclusions
By following a modular strategy, we have synthesized and
characterized a multicomponent [2]rotaxane designed to perform
as an autonomous molecular shuttle powered by light. UV-
vis-NIR absorption spectroscopy, together with electrochemical
and chemical oxidation, studies have been employed to inves-
tigate the redox-controlled shuttling of an electron-accepting
ring between two competitive electron-donating stations. The
experimental results show that remarkable electronic interactions
between the various units located on the dumbbell-shaped
component take place, suggesting the existence of folded
Supporting Information Available: Experimental section,
synthetic procedures, additional spectroscopic, voltammetric, and
spectroelectrochemical data, as well as complete refs 17a, 25b,
and 38a. This material is available free of charge via the Internet
at http://pubs.acs.org.
(50) Ballardini, R.; Credi, A.; Gandolfi, M. T.; Giansante, C.; Marconi, G.; Silvi,
S.; Venturi, M. Inorg. Chim. Acta 2007, 360, 1072-1082.
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