A bistable pretzelane

Article abstract of DOI:10.1039/b910510g

A switchable donor-acceptor pretzelane composed of a crown ether containing tetrathiafulvalene and 1,5-dihydroxynaphthalene recognition units and a covalently tethered cyclobis(paraquat-p-phenylene) ring exhibits unidirectional motion on both oxidation (f

Full text of DOI:10.1039/b910510g

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A bistable pretzelanew
Yan-Li Zhao,ab Ali Trabolsia and J. Fraser Stoddart*a
Received (in Austin, TX, USA) 28th May 2009, Accepted 20th June 2009
First published as an Advance Article on the web 16th July 2009
DOI: 10.1039/b910510g
A switchable donor–acceptor pretzelane composed of a crown
ether containing tetrathiafulvalene and 1,5-dihydroxynaphthalene
recognition units and a covalently tethered cyclobis(paraquat-p-
phenylene) ring exhibits unidirectional motion on both oxidation
(forwards) and reduction (backwards).
synthetic precursors occurs intramolecularly to generate a
bistable pretzelane. Based on this synthetic protocol, we
describe here (Scheme 1) the template-directed synthesis
and redox-active switching properties of the bistable pretzelane
1ꢁ4PF6 containing a CBPQT4+ ring and TTF–DNP units
encircled by a tethered crown ether component. Particular
attention has been given to determining the redox-responsive
switching behavior of the CBPQT4+ ring in 1ꢁ4PF6 and it is
also discussed therein.
The widespread use of controlled molecular-level motion in
biological systems1 suggests to us that artificial molecular
machines can carry out specific tasks by relying mainly on
physical and chemical stimuli in order to mimic the motion
found in the molecular machinery present in living cells.
Thus, the preparation and application of artificial molecular
machines2,3 have attracted the attention of chemists,
physicists, material scientists and engineers in recent times.
In the realm of synthetic molecular machinery, stimuli-responsive
molecular switches2a,2g,3a,3c,3g are highly desirable prototypes
in the fabrication of artificial machines.
The synthesis of 1ꢁ4PF6 is outlined in Scheme S1 (ESIw) and
the bistable pretzelane was studied by cyclic voltammetry
(CV), differential pulse voltammetry (DPV), UV-Vis spectro-
electrochemistry (SEC), and 1H NMR spectroscopy, following
initial probing by UV-Vis spectroscopy.
The UV-Vis spectrum of 1ꢁ4PF6 recorded in MeCN shows a
broad charge transfer (CT) absorption band centered on
840 nm which is characteristic4 of the translational isomer in
which the TTF unit is encircled by the CBPQT4+ ring. No
absorption band is observed in the 450–600 nm region for a CT
band which would result from the other translational isomer
wherein the DNP unit is encircled by the CBPQT4+ ring, a
Electrochemically-driven molecular mechanical switches,
such as bistable donor–acceptor [2]catenanes and [2]rotaxanes,
have been the subject2a,2g,3a,3c of a number of forays into solid-
state devices, as well as in solution. We have developed4
a
template-directed protocol (Scheme 1) for the synthesis of a bistable
[2]catenane incorporating a crown ether containing p-electron
rich tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene
(DNP) recognition units interlocked with a tetracationic
cyclobis(paraquat-p-phenylene) (CBPQT4+) ring comprising
two p-electron deficient bipyridinium units. The bistability of
this switchable [2]catenane relies on the ability of the
CBPQT4+ ring to encircle much more strongly the TTF unit
than it does the considerably less p-electron rich DNP unit in
the crown ether. Switching is achieved by the reversible
oxidation of the TTF unit, firstly to the radical cation
(TTF+ꢀ) and then subsequently to its dication (TTF2+), thus
producing Coulombic charge–charge repulsion between the
oxidized TTF+ or TTF2+ species and the CBPQT4+ ring,
ꢀ
leading to the movement of the CBPQT4+ ring to the DNP
unit. After reduction of the TTF+ radical cation or TTF2+
ꢀ
dication to the neutral form, the CBPQT4+ ring migrates back
to the TTF recognition unit, indicating that the redox-switchable
process in the [2]catenane is fully reversible. When the crown
ether is tethered covalently to a preformed part of the
CBPQT4+ ring, the resulting macrocyclization5 from its
a Department of Chemistry, Northwestern University,
2145 Sheridan Road, Evanston, IL 60208, USA.
E-mail: stoddart@northwestern.edu; Fax: +1 847 491 1009;
Tel: +1 847 491 3793
b Department of Chemistry and Biochemistry, University of California,
Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, USA
w Electronic supplementary information (ESI) available: Synthesis
and characterization. See DOI: 10.1039/b910510g
Scheme 1 Bistable [2]catenane and pretzelane 1ꢁ4PF6 containing the
TTF and DNP recognition units and the CBPQT4+ ring.
ꢂc
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4844 | Chem. Commun., 2009, 4844–4846

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distinguished from each other by CV since the oxidation
potential of the ‘free’ TTF unit on the MSCC is approximately
200 mV3c less positive than that for the GSCC. The return of
the MSCC to the GSCC constitutes a recovery of the
equilibrium distribution and is activated thermally. By varying
the time between the first and second CV scans, the kinetic
parameters for relaxation from the MSCC to the GSCC can be
found. Fig. S2 (ESIw) displays a fitted first-order decay profile
for this relaxation process obtained from the CV data of
1ꢁ4PF6 in Ar-purged MeCN at 298 K with scan rates in the
range of 50 to 700 mV sꢃ1. From these experiments, we have
calculated3c,6 the kinetic parameters that describe the recovery
of the equilibrium distribution. The values for the decay time
(t298), kinetic constant (k298), and free energy of activation
(DGz298) are 0.65 ꢄ 0.03 s, 1.52 ꢄ 0.07 sꢃ1, and 17.0 ꢄ
0.8 kcal molꢃ1, respectively. These kinetic parameters are
Fig. 1 Cyclic voltammetric curves of 1ꢁ4PF6. (a) First (black) and
second (green) cyclic voltammetries of 1ꢁ4PF6 displaying the
characteristic metastable peak (MSCC, +0.43 V) in the second cycle
recorded in Ar-purged MeCN at 298 K with a scan rate of 200 mV sꢃ1
.
(b) First (black) and second (red) cyclic voltammetries of 1ꢁ4PF6
displaying the reduction region recorded in Ar-purged MeCN at
298 K with a scan rate of 200 mV sꢃ1. The concentrations of the
sample and supporting electrolyte are 1.0 mM and 0.1 M, respectively.
similar to those (0.6 s, 1.7 sꢃ1, and 17 ꢄ 0.4 kcal molꢃ1 3c
)
observed in the case of [2]rotaxane, indicating the fact that the
CBPQT4+ ring is tethered covalently to the crown ether ring
in the pretzelane, and does not affect significantly the
switching behavior of the CBPQT4+ ring between the TTF
and DNP recognition sites upon redox activation. Unlike the
random-directed circling of the CBPQT4+ ring around the
crown ether in the [2]catenane,3c a unidirectional reversible
circling of the CBPQT4+ ring between the TTF and DNP
recognition sites is preferable in the pretzelane upon redox
stimulation since the CBPQT4+ ring cannot pass through the
3,5-dihydroxybenzoate unit in the crown ether.
fact which suggests that 1ꢁ4PF6 exists as a single translational
isomer in solution—that is, the one in which the CBPQT4+
ring encircles the TTF unit. Hence, it was clear that the
opportunity presented itself for us to investigate the switching
behavior of the CBPQT4+ ring between the TTF and DNP
units in 1ꢁ4PF6 by addressing the redox states of the TTF unit.
In the CV experiments (Fig. 1(a)) of 1ꢁ4PF6, the first
oxidation peak for the TTF unit which is discernable around
+0.43 V for ‘free’ TTF3c,3h is hardly detectable at all in the
anodic scan. DPV analysis shows (Fig. S1, ESIw) a small peak
at +0.48 V. These results indicate the existence of some ‘free’
TTF in association with a small amount of the minor translational
isomer present in 1ꢁ4PF6 at equilibrium. The first substantial
TTF oxidation peak is shifted to higher potential and overlaps
with the second oxidation peak at +0.98 V. In the DPV
analysis, the first and second TTF oxidation peaks are found
around +0.81 V, potentials which correspond to the first
oxidation of the TTF unit encircled by the CBPQT4+ ring
and/or the second oxidation of the TTF unit once the ring has
migrated from the TTF unit to the DNP unit. In the cathodic
scan of 1ꢁ4PF6, two CV peaks, corresponding to the reduction
of the oxidized TTF+ꢀ radical cation and the TTF2+ dication,
are observed at +0.39 and +0.89 V, respectively. The
cathodic scan of the DPV analysis reveals the related peaks
at +0.45 and +0.78 V. The fact that the CBPQT4+ ring
encircles the DNP unit does not affect the reduction potentials
In order to shed further light on the mechanical movement
of the CBPQT4+ ring between the TTF and DNP units, the
switching of the bistable pretzelane 1ꢁ4PF6 was investigated
(Fig. 2) in solution by SEC. The ground-state (E = 0 V)
UV-Vis spectrum of 1ꢁ4PF6 displays the characteristic CT
band at 840 nm, corresponding to the CT interaction between
the TTF unit and the CBPQT4+ ring. The absorption spectrum
starts to change on applying a potential of +0.6 V, and the
ground-state bands bleach and are replaced by bands in the
visible region at 445 and 595 nm associated with absorptions
of the TTF+ radical cation. These observations indiꢀcate that
ꢀ
the CBPQT4+ ring migrates away from the TTF+ radical
cation to the DNP unit during this process. When the applied
of the TTF+ radical cation and TTF2+ dication. In the
ꢀ
reducing portion (Fig. 1(b)) of the CV of 1ꢁ4PF6, a typical
first two-electron reduction peak and a second two-electron
reduction peak of the CBPQT4+ ring are observed at
ꢃ0.29 and ꢃ0.85 V, respectively.
There is an equilibrium3c,6 established between the
CBPQT4+ ring encircling the TTF unit—called the ground-
state co-conformer (GSCC)—and the DNP unit—called the
metastable state co-conformer (MSCC). The first oxidation
state of the GSCC corresponds to the removal of an electron
from the TTF unit, and is accompanied by a rapidly driven
swingiꢀng of the CBPQT4+ ring to the DNP unit while the
TTF+ radical cation is further oxidized to its TTF2+ dication.
Upon reduction of this dication back to its neutral state,
the MSCC is formed. The MSCC and GSCC are readily
Fig. 2 The changes in the UV-Vis spectra during the SEC measurements
conducted on 1ꢁ4PF6. The applied potential changes from E = 0 (green),
0.6 (yellow) to 1.1 V (red). All data were recorded in Ar-purged MeCN at
298 K. The concentrations of the sample and supporting electrolyte were
1.0 mM and 0.1 M, respectively.
ꢂc
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Chem. Commun., 2009, 4844–4846 | 4845

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potential is increased above +1.1 V, the TTF+ radical cation
One additional feature of the redox switching in the bistable
pretzelane is worthy of note. Since the CBPQT4+ ring is
tethered covalently to the crown ether ring in the pretzelane,
it exhibits, of necessity, unidirectional reversible switching
behavior between its TTF and DNP recognition sites. In this
respect, it displays a fundamental characteristic exhibited by
very few mechanically interlocked molecules investigated to
date3b with different recognition sites.
ꢀ
is oxidized to the TTF2+ dication. The absorption bands for
the TTF+ radical cation are bleached and a small absorption
ꢀ
peak at 525 nm associated with the CT band between the DNP
unit and the CBPQT4+ ring can be detected. The spectrum
gradually reverts back to its original format when the applied
potential is switched off (E = 0 V) and the sample is allowed
to stand at room temperature for 12 h, indicating that all the
electrochemical redox processes undergone by 1ꢁ4PF6 are fully
reversible.
This research was supported by the Microelectronics
Advanced Research Corporation (MARCO) and its Focus
Center Research Program (FCRP) on Functional Engineered
NanoArchitectonics (FENA).
A series of 1H NMR spectra were recorded in order to
investigate the redox-controllable switching of the CBPQT4+
ring in the bistable pretzelane 1ꢁ4PF6. In the 1H NMR
spectrum (Fig. 3(a)) of 1ꢁ4PF6 recorded in CD3CN at 298 K,
the signals for the heterotopic protons on the TTF unit were
observed at 5.92, 5.98, 6.19, and 6.20 ppm, indicating that the
TTF unit in 1ꢁ4PF6 exists in cis and trans isomeric forms, even
when it is encircled by the CBPQT4+ ring. After addition of
2 equiv. of the oxidant, tris(p-bromophenyl)aminium hexa-
chloroantimonate,7 to the NMR tube, the 1H NMR spectrum
was recorded again (Fig. 3(b)) at 243 K in order to ensure the
stability of the oxidized pretzelane. The signals for the TTF
unit moved downfield to 9.28 ppm, the chemical shift
characteristic of the free TTF2+ dication in the oxidized
version of the pretzelane. Further evidence for the movement
of the CBPQT4+ ring from the oxidized TTF unit to the DNP
unit can be found in the dramatic changes in the chemical
shifts for the DNP protons. The resonances of the DNP
protons around 7–8 ppm experienced large up-field shifts to
2.29 and 2.31 (H-4/8), 5.94 and 5.97 (H-3/7), and 6.24 and
6.27 (H-2/6) ppm, respectively, on account of the shielding
effect of the encircling CBPQT4+ ring. Addition of zinc
powder to the NMR tube led to the reduction of the TTF2+
dication to its neutral form, resulting in the movement of the
CBPQT4+ ring away from the DNP and back to the TTF unit
and the restoration (Fig. 3(c)) of the original 1H NMR
spectrum.
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Fig. 3 Partial 1H NMR spectra of 1ꢁ4PF6 (1.0 mM) recorded in
CD3CN. (a) 1ꢁ4PF6 at 298 K, (b) oxidized 1ꢁ4PF6 at 243 K, and
(c) reduced 1ꢁ4PF6 at 298 K.
ꢂc
This journal is The Royal Society of Chemistry 2009
4846 | Chem. Commun., 2009, 4844–4846