Foldamer-tuned switching kinetics and metastability of [2]rotaxanes

Article abstract of DOI:10.1002/anie.201104099

Slip sliding away: Foldamers (see picture, red) can function as modular stoppers to regulate the slippage and de-slippage of pseudorotaxanes and the switching kinetics and metastability of bistable rotaxanes. By simply changing the solvent or the length o

Full text of DOI:10.1002/anie.201104099

Communications
DOI: 10.1002/anie.201104099
Molecular Devices
Foldamer-Tuned Switching Kinetics and Metastability of
[2]Rotaxanes**
Kang-Da Zhang, Xin Zhao,* Gui-Tao Wang, Yi Liu,* Ying Zhang, Hao-Jie Lu,* Xi-Kui Jiang,
and Zhan-Ting Li*
Molecular switches and devices based on rotaxanes have been
the topic of extensive investigations in supramolecular
chemistry.[1,2] In particular, bistable [2]rotaxanes consisting
of a linear component incorporaing electron-rich tetrathiafu-
valene (TTF) and 1,5-dioxynathpthalene (DNP) binding sites
encircled by a cyclobis(paraquat-p-phenylene) (CBPQT4+)
ring have been investigated as devices for high-density
molecular memories.[3] One issue associated with this type
of devices is the short lifetime of the metastable state co-
conformation (MSCC), which arises from the quick relaxa-
tion of these systems to the ground-state co-conformation
(GSCC).[4] Introducing a bulky or electrostatic barrier
between the TTF and DNP units can slow down the back
shuttling of the CBPQT4+ ring from the DNP unit to the
neutral TTF unit and generate long-lived MSCC even after
removal of the bias by rapid reduction of the pre-oxidized
TTF unit.[5] However, it is still desirable to develop novel
strategies for the control of the interconversion between the
GSCC and the MSCC in a modular manner.
Herein, we describe that hydrogen-bonding-mediated
arylamide foldamers can be utilized to effectively tune this
interconversion.[6] Foldamers are synthetic oligomers with
folded structures stabilized by intramolecular noncovalent
forces.[7] The folded states have apparent sizes larger than
those of the extended ones. We conjectured that, if the
apparent diameter of the extended state of a foldamer is
smaller than the internal diameter of a macrocycle while the
diameter of its folded state is larger, it would allow the
macrocycle to slip over the extended state. However, the
process should be slower than that over a similar but flexible
molecule, because it needs to, at least partially, break the
noncovalent bonds existing in the folded state.[8] Since the
length of foldamers can be readily modulated by simply
changing the number of their repeating units, foldamer
segments could be developed as modular stoppers or spaces
for regulating the dynamic behavior of pseudorotaxanes or
rotaxanes.
Pseudorotaxanes 1a and 1b were first prepared as pure
species from dumbbells 2a and 2b and CBPQT4+(PF6 )4 as
ꢀ
models to investigate the extrusion kinetics of the CBPQT4+
ring over the foldamer segment of the dumbbells
(Figure 1).[9,10] The 2-methoxy-1,3-benzamide-based folda-
ꢀ
=
mers were chosen because their intramolecular N H···O C
hydrogen bonding works in solvents of varying polarity and
short oligomers can cause large conformational change upon
unfolding.[11,12] The large Frꢀchet-type G-3 dendron provides
them with good solubility in both less polar and polar
solvents.[13] The 1H NMR spectrum of complex 1b in
[D3]acetonitrile and [D6]DMSO displayed one set of signals.
After standing for 15 minutes, the resonances of the free 2b
and CBPQT4+(PF6 )4 appeared and those of the complex
ꢀ
diminished, which reached equilibrium after approximately
1
five hours. The H NMR spectrum of 1a in both solvents
recorded upon dissolution already exhibited the signals of
both free and complexed 2a and CBPQT(PF6)4. Based on the
relative intensity of the pyridinium b-H signals of the free and
complexed CBPQT4+ ring, we determined the association
constants (Ka values) of 1a and 1b as complexes to be 752 and
[*] K.-D. Zhang, Prof. X. Zhao, Dr. G.-T. Wang, Prof. X.-K. Jiang,
Prof. Z.-T. Li
State Key Laboratory of Bioorganic and Natural Products Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Lu, Shanghai 200032 (China)
E-mail: xzhao@mail.sioc.ac.cn
868mꢀ1, respectively, in acetonitrile and 157 and 152mꢀ1
,
respectively, in DMSO. These values are higher than those
reported for the pseudorotaxane formed by the TTF itself and
the CBPQT4+ ring,[9a] which might be attributed to the
ztli@mail.sioc.ac.cn
ꢀ
formation of the intermolecular C H···O hydrogen bonds
Prof. Y. Liu
between the pyridinium protons and the neighboring ether
oxygen atoms. The fact that the values in acetonitrile were
higher than those in more polar DMSO also suggests that
The Molecular Foundry, Lawrence Berkeley National Laboratory
One Cyclotron Road, Berkeley, CA 94720 (USA)
E-mail: yliu@lbl.gov
ꢀ
Y. Zhang, Prof. H.-J. Lu, Prof. Z.-T. Li
Chemistry of Department, Fudan University
220 Handan Road, Shanghai 200433 (China)
E-mail: luhaojie@fudan.edu.cn
DMSO inhibited the above C H···O hydrogen bonds more
efficiently than it strengthened the TTF–bipyridinium donor–
acceptor interaction.
The kinetics of extrusion of the CBPQT4+ ring from the
linear components of pseudorotaxanes 1a and 1b was then
investigated using the TTF-CBPQT4+ charge-transfer (CT)
absorption, centered at about 805 nm in the UV/Vis spectra,
as the probe. By monitoring the decrease of this CT
absorption with time, we obtained the rate constants (koff)
of extrusion in eight solvents of low and high polarity by
[**] X.Z. and Z.T.L. thank MOST (2007CB808001), NSFC (20732007,
20921091, 20872167, 20974118), and STCSM (10PJ1412200 and
09XD1405300) for financial support. Y.L. was supported by the
Office of Science, Office of Basic Energy Sciences, of the U.S.
Department of Energy under contract No. DE-AC02-05 CH11231.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104099.
9866
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9866 –9870

assuming first-order kinetics. The data
and the associated change of free
energy are listed in Table 1. Compared
with those of 1a in the same solvents,
the koff values of 1b decreased by 5 to
37 times. This observation indicated
that all the hydrogen-bonded aryla-
mide units in the foldamer segments
contributed to delaying the extrusion
of CBPQT4+. The largest koff value was
displayed by 1a in DMF, which corre-
sponded to a half-life of 301 s, while
the slowest one was displayed by 1b in
chloroform, corresponding to a half-
life of 67 days, with a difference of
19167 times. Thus, by choosing dis-
crete foldamer segments and media,
we could control the passing of the
CBPQT4+ ring from the foldamer seg-
ments in a remarkably long span of
time. It has been established that the
donor–acceptor interactions between
TTF and bipyridinium units become
weakened in less polar solvents owing
to the decreased strength of the p-
stacking interaction.[14] The decom-
plexation of 1a and 1b was substan-
tially slower in less polar solvents than
in polar solvents, suggesting that the
increase of the free energy of the CT
state from a polar solvent to a less
polar solvent was considerably smaller
than the increase of the free energy of
activation for the extrusion of the
CBPQT4+ ring over the foldamer seg-
ment caused by the enhancement of
their intramolecular hydrogen bonds
in less polar solvent.
Cyclic voltammetry (CV) mea-
surements were carried out to inves-
tigate the redox properties of the TTF
unit in pseudorotaxanes 1a and 1b in
less polar chloroform and polar aceto-
Table 1: The kinetic data for the extrusion of the CBPQT4+ ring over the
foldamer segments in pseudorotaxanes 1a and 1b at 258C in different
solvents.
1a
1b
¼
¼
Solvent
e[a]
koff
DG
koff
DG
[sꢀ1
]
[kJmolꢀ1
]
[sꢀ1
]
[kJmolꢀ1
]
DMSO
DMF
MeCN
47.2
36.7
38.3
25.7
21.0
10.4
7.5
1.0ꢀ10ꢀ3
2.3ꢀ10ꢀ3
5.2ꢀ10ꢀ4
2.8ꢀ10ꢀ4
8.4ꢀ10ꢀ4
7.5ꢀ10ꢀ6
2.6ꢀ10ꢀ4
1.9ꢀ10ꢀ6
90
88
92
93
91
102
94
1.1ꢀ10ꢀ4
4.7ꢀ10ꢀ4
4.0ꢀ10ꢀ5
2.0ꢀ10ꢀ5
1.3ꢀ10ꢀ4
2.0ꢀ10ꢀ7
8.3ꢀ10ꢀ6
1.2ꢀ10ꢀ7
96
92
98
100
95
111
102
113
Figure 1. Schematic representation of the foldamer-modulated extru-
sion (pseudorotaxane decomplexation) or slippage (pseudorotaxane
formation) of the CBPQT4+ ring (blue) from or onto a TTF-containing
thread (green). The foldamer segment (red) in its folded state has a
large apparent size and must unfold into an extended conformation to
allow the CBPQT4+ ring to pass.
PhCN
acetone
Cl(CH2)2Cl
THF
chloroform
4.8
106
[a] Dielectric constant of the solvent.
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www.angewandte.org 9867

Communications
nitrile. In chloroform, for both samples, the first oxidation
potential was 0.65 V, which was larger than that of control 2a
(0.53 V), reflecting the decreased electron-donating ability of
the complexed TTF unit with respect to the free one. The
second oxidation potential was 0.84 V for all the three
samples, implying that the formation of radical cation TTF+
had forced the CBPQT4+ ring of 1a and 1b to slip off the TTF
unit; hence the CBPQT4+ ring did not affect the formation of
the TTF2+ ion. The electrochemical behavior is different in
acetonitrile. Both 1a and 1b exhibited only one oxidation
process at 0.77 V, while 2a still gave rise to two peaks at 0.44
and 0.78 V. The voltammogram of 1a showed an additional
weak broad peak at 0.44 V, assignable to the oxidation of the
free TTF unit. Such a peak was not observed for 1b, thus
indicating that within the time scale of the measurement, 1a
started to decomplex, while 1b did not, consistent with the
above UV/Vis kinetic experiments, reflecting the higher
stability of 1b.
The redox-activated dynamics of pseudorotaxanes 1a and
1b were further investigated using chemical oxidation and
reduction methods. Their koff values in both the mono- (TTF+)
and dioxidized (TTF2+) states in acetonitrile were determined
to be 0.11 and 0.02 sꢀ1, respectively (see the Supporting
Information for the method). Since the electrostatic repulsion
between dication TTF2+ and CBPQT4+ should be larger than
that between radical cation TTF+ and CBPQT4+, the fact that
both pseudorotaxanes in the two different oxidation states
displayed an identical rate of decomplexation suggested that
the energy barrier for the extrusion of the CBPQT4+ ring from
the linear component, after the first oxidation of the TTF unit,
was predominantly controlled by its de-slippage over the
bulky foldamer segment. This result may be rationalized by
considering that the linker between the TTF and foldamer
units is long enough to allow the CBPQT4+ ring to rapidly
escape far away from the oxidized TTF unit. The correspond-
ing charge repulsion decayed exponentially to an insignificant
level compared with the energy cost for the CBPQT4+ ring
slipping over the foldamer segment. On the other hand, the
koff values of the mono- and dioxidized 1a and 1b in
acetonitrile were 212 and 500 times higher than the values
of the corresponding un-oxidized samples in the same solvent.
Since the dimeric and trimeric foldamer segments in
pseudorotaxanes 1a and 1b were capable of providing an
energy barrier for the CBPQT4+ ring to extrude at a rate of
10ꢀ3 ꢁ 10ꢀ7 sꢀ1, we then prepared bistable [2]rotaxanes 3a and
3b to study the foldamer-tuned switching of the CBPQT4+
ring between the TTF and 1,5-dioxynapthalene (DNP) sites.
Since the TTF unit is more electron-rich than the DNP unit, in
a two-state model (Figure 2), the equilibrium should be
shifted toward the CBPQT4+ ring encircling the TTF site to
form the GSCC, rather than toward the CBPQT4+ ring
encircling the DNP site to form the less stable MSCC. The
1H NMR spectra of [2]rotaxanes 3a and 3b in [D3]acetonitrile
and [D6]acetone displayed one set of signals (see the
Supporting Information), thus indicating that their
CBPQT4+ ring predominantly encircled the TTF unit to
form the GSCC (Figure 2A).
Figure 2. Schematic representation of foldamer-tuned switching of the
CBPQT4+ ring between the TTF and DNP sites in bistable [2]rotaxanes
3a and 3b, with the TTF unit being neutral or oxidized to TTF+· or
TTF2+
.
CBPQT4+ ring in bistable [2]rotaxanes 3a and 3b from the
GSCC to the MSCC should be comparable to the corre-
sponding koff values of pseudorotaxanes 1a and 1b in the
same solvent. Upon oxidation of the TTF unit to TTF+· or
TTF2+, the CBPQT4+ ring of the [2]rotaxanes would be
repelled to slip more rapidly over the foldamer segment to
encircle the DNP unit (Figure 2C!D), and the shuttling
rates should be similar to the rates of decomplexation of the
corresponding mono- and dioxidized pseudorotaxanes 1a and
1b in the same solvent. Reduction of the TTF+· or TTF2+
cation of the preoxidized [2]rotaxanes to TTF would lead to
the formation of the MSCC (Figure 2D!B), and the
CBPQT4+ ring would slip back to the TTF side to form the
GSCC (Figure 2B!A). This last process is directly related to
the lifetime of the MSCC and is critical for nonvolatile
molecular memory with high metastability.
CV measurements on the two bistable [2]rotaxanes were
then performed in both acetonitrile and chloroform at varying
scan rates to investigate the above processes. For the second
scans, for 3a in acetonitrile (Figure 3), at the slow scan rates of
5 and 10 mVsꢀ1, no peak was displayed at about 0.44 V, which
is typically formed by the free TTF and, for bistable
[2]rotaxane 3a, associated with its MSCC. Within the range
of scan rates from 25 to 75 mVsꢀ1, this peak was formed and
Figure 3. Cyclic voltammagrams of bistable [2]rotaxane 3a (0.1 mm) in
MeCN at 258C: a) the first cycle at 5 mVsꢀ1 and subsequent cycles at
5, 10, 25, 50, and 75 mVsꢀ1. b) The subsequent cycles at 75, 100, 150,
200, 300, and 400 mVsꢀ1. Pt button and coil and Ag/AgCl electrodes
were used as working, counter, and reference electrodes. [nBu4N][PF6]
(0.1m) was used as the electrolyte.
Considering that the TTF sides of their threads have
identical structures, the rate constant of the shuttling of the
9868 www.angewandte.org
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Angew. Chem. Int. Ed. 2011, 50, 9866 –9870

intensified pronouncedly with the increase of the scan rate.
This result indicated that even in solution the MSCC of 3a
had a relatively long lifetime at ambient temperature, which is
comparable with that observed in the CBPQT4+-TTF-DNP-
based rotaxanes entrapped in a solid-state polymer.[4c] With
further increase of the scan rate from 100 to 400 mVsꢀ1, this
peak still survived, but weakened, which might be attributed
to the decreased conversion of C to D (Figure 2) at the rapid
scan rates. For 3a in chloroform and 3b in both chloroform
and acetonitrile, no peaks corresponding to the free TTF unit
were observed at different scan rates, which might be
rationalized by considering the decreased C!D conversion
as well as the increased stability of the GSCC of the
[2]rotaxanes.
controlling the lifetime of the MSCC of the [2]rotaxanes in a
wide range of timescale.
In conclusion, we have demonstrated for the first time that
hydrogen-bonding-induced arylamide foldamers can serve as
a new, deformable moiety in switchable pseudorotaxanes and
rotaxanes to modulate the switching kinetics and metastabil-
ity. The noncovalent nature of the folding conformation in
foldamers endows structural flexibility, making them versatile
structural units to modulate the mechanical movements of the
CBPQT4+ ring along the dumbbell component. In bistable
[2]rotaxanes where foldamer segments are introduced to
bridge a TTF unit and a DNP unit in the dumbbell, the
deformable sizes of the foldamers effectively serve as a steric
barrier to the relaxation from the MSCC to the GSCC. The
lifetime of the post-oxidation MSCC is thus dramatically
increased. The generation of the long-lived MSCC in the
[2]rotaxanes from minutes to days holds great promise for the
application of these mechanically interlocked molecules in
nonvolatile molecular memories. Thus, the marriage of
deformable foldamers and interlocked molecular switches
opens up the possibility of exploring more responsive
dynamic molecular materials.
The 1H NMR spectra in [D3]acetonitrile showed that
adding 2.0 equivalents Magic Blue ((4-BrC6H4)4N+SbCl6 ) to
ꢀ
the solution of the rotaxanes resulted in the appearance of a
new set of signals (see the Supporting Information). For 3a,
the two signals (at 5.78 and 5.89 ppm) of the protons of the
TTF unit encapsulated in the CBPQT4+ ring disappeared,
while the signals of the H2/6 (at 6.16 and 6.20 ppm) and H3/7
(at 5.89 ppm) protons of the DNP unit encapsulated in the
CBPQT4+ ring emerged (see the structure for numbering).[15]
These results supported that the CBPQT4+ ring had shuttled
from the TTF side to the DNP side. Similar results are also
observed for 3b. In separate UV/Vis experiments, both 3a
and 3b were oxidized with 2.0 equivalents Fe(ClO4)3 in
acetonitrile to produce the doubly oxidized TTF2+ ion.
After standing for 50 min, the solutions were quickly treated
with an excess of zinc powder to reduce TTF2+ ion to TTF.
The time-dependent UV/Vis spectra of the solutions were
then recorded, which showed that the DNP–CBPQT4+ CT
absorption at 550 nm gradually decreased in intensity, and the
TTF–CBPQT4+ CT absorption at 805 nm appeared and
increased in intensity (see the Supporting Information). By
assuming first-order kinetics, we determined the rate con-
stants of the shuttling of the CBPQT4+ ring of 3a and 3b from
the MSCC to the GSCC to be 1.1 ꢁ 10ꢀ2 and 7.4 ꢁ 10ꢀ4 sꢀ1,
respectively, which corresponded to half-lives of 66 and 930 s.
Using the same method, the rate constant of the same process
of 3a in chloroform was determined to be 9.9 ꢁ 10ꢀ6 sꢀ1,
corresponding to a half-life of 19.5 h, 1064 times longer than
that in acetonitrile. These rate constants were all higher than
the related koff values of pseudorotaxanes 1a and 1b
(Table 1), which was consistent with the fact that the donor–
acceptor interaction between DNP and CBPQT4+ is weaker
than that between the TTF unit and the CBPQT4+ ring. Since
the foldamer segments were not symmetric, the energy
barrier experienced by the CBPQT4+ ring from the two
sides might be slightly different, which also contributed to the
difference. Remarkably, the solution of the MSCC of 3b in
chloroform, obtained using a similar method, did not exhibit a
perceptible TTF–CBPQT4+ CT band in the UV/Vis spectrum
even after three days, thus indicating that the foldamer
segment efficiently blocked the conversion of the MSCC to
the GSCC. Because both bistable [2]rotaxanes are soluble in
organic solvents of both low and high polarity, the results
suggest that, by changing the length of the foldamer segments
and the polarity of the media, we are actually capable of
Received: June 15, 2011
Published online: September 2, 2011
Keywords: donor–acceptor systems · foldamers ·
.
hydrogen bonds · molecular devices · rotaxanes
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