Dual stimulus switching of a [2]catenane in water

Article abstract of DOI:10.1002/anie.201006362

The controllable mechanical movement of either the polyether macrocycle or the tetracationic cyclophane in a doubly bistable water-soluble [2]catenane can be accomplished by using orthogonal external stimulia-redox and amine/acid, respectivelya-in aqueous media .

Full text of DOI:10.1002/anie.201006362

DOI: 10.1002/anie.201006362
Molecular Machines
Dual Stimulus Switching of a [2]Catenane in Water**
Lei Fang, Cheng Wang, Albert C. Fahrenbach, Ali Trabolsi, Youssry Y. Botros, and
J. Fraser Stoddart*
The most sophisticated and efficient nanoscale machines are
those which reside in living cells in order to perform complex
mechanical tasks, such as cell division and intracellular
transport.[1] These motor molecules transduce[2] one form of
energy (typically chemical, both covalent and noncovalent in
nature) into another form (typically mechanical) for the most
part in an aqueous environment. The rapid development of
artificial molecular machinery[3] during the past two decades
is rendering it possible to mimic and infiltrate biomolecular
machines using wholly synthetic systems—even if the com-
plexity and efficiency of the nonnatural counterparts are in no
way comparable to their naturally occurring cousins. One of
the most important, yet challenging, issues in the field is the
development of artificial molecular machinery that is able to
become part of an integrated system and still perform
molecularly controlled work in aqueous environments, open-
ing up opportunities for developing applications in nano-
prosthetics to aid and abet in combating disease at the
molecular level. Cyclodextrins[4] and cucurbiturils[5] are
amongst some of the most highly investigated examples of
building blocks suitable for the construction of water-friendly
artificial molecular machinery, principally because both these
receptors boast controllable molecular recognition towards
many different substrates dissolved in water.
Tetracationic cyclophanes[6,7] (TCs4+) are an important
class of water-soluble synthetic receptors that demonstrate[8]
excellent binding affinities towards p-electron-rich substrates
in both organic solvents (when the counteranions are soft,
ꢀ
e.g., PF6ꢀ, BF4 ) and in aqueous solutions (when the counter-
ions are hard, e.g., CF3CO2ꢀ, Clꢀ). The synthetic versatil-
ity[9,10] surrounding TCs4+ has enabled the elaboration of
different, carefully designed molecular machines/switches[3]
on the basis of these TCs4+ which have been studied
extensively and applied to the development of nanoelectro-
mechanical systems,[11] mechanized nanoparticles[12] for drug
delivery, and nanoelectronic devices.[13] Surprisingly, despite
their ability to exhibit molecular recognition in water, TCs4+
have seldom been used in building molecular switches/
machines which work in aqueous media. Here, we report
the design of a water-soluble, donor–acceptor [2]catenane in
which the switching of both macrocyclic components can be
addressed individually and orthogonally by external stimuli
working in aqueous media.
In order to effect the circumrotation of both rings in a
donor–acceptor [2]catenane independently, two nondegener-
ate mechanically interlocked rings, which can be addressed by
[*] L. Fang, C. Wang, A. C. Fahrenbach, A. Trabolsi, Prof. J. F. Stoddart
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
Fax: (+1)847-491-1009
E-mail: stoddart@northwestern.edu
Homepage: http://stoddart.northwestern.edu
Dr. Y. Y. Botros
Department of Materials Science and Engineering
Northwestern University, Evanston, IL (USA)
and
Intel Labs, Building RNB-6-61, Santa Clara, CA (USA)
and
National Center for Nano Technology Research
King Abdulaziz City for Science and Technology (KACST)
P.O. Box 6086, Riyadh, 11442 (Saudi Arabia)
[**] The research described here was sponsored in part by the National
Center for Nano Technology Research at the King Abdulaziz City for
Science and Technology (KACST) in Saudi Arabia. We thank Dr.
Turki S. Al-Saud and Dr. Soliman H. Alkhowaiter at KACST for their
generous support of this program of research at Northwestern
University (NU). We also acknowledge support from the Air Force
Office of Scientific Research (AFOSR) under the Multidisciplinary
Research Program of the University Research Initiative (MURI),
Award FA9550-07-1-0534 entitled “Bioinspired Supramolecular
Enzymatic Systems”. L.F. acknowledges the support of a Ryan
Fellowship and a Non-Equilibrium Research Center (NERC) Fel-
lowship from NU. A.C.F. thanks the National Science Foundation
(NSF) for the award of a NSF Graduate Research Fellowship.
Figure 1. Formulas and schematic representations of the ground-state
co-conformation of [2]catenane C4+ and the circumrotational move-
ments of both the tetracationic cyclophane (TC4+) as well as the
polyether macrocycle (PM).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201006362.
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The [2]catenane C·4CF3CO2 is readily soluble in water,
forming an emerald green aqueous solution in the process.
1H NMR spectra (Figure 2) of C·4CF3CO2 in D2O were
recorded at a range of temperatures from 277 up to 343 K.
Based on H-1H-g-DQF-COSY and H-1H-ROESY spectra
(see the Supporting Information), all the resonances can be
assigned with some precision. Although the existence of cis-
and trans-isomers of the STTFS unit lead to all the 1H NMR
signals separating into two sets of peaks, the otherwise simple
spectrum suggests that only one of the four possible transla-
tional isomers is populated in aqueous solution. The major co-
conformation of C·4CF3CO2 in water was deduced from the
nuclear Overhauser effects observed (see the Supporting
Information) in the ROESY spectrum. The through-space
interactions between the STTFS, HQ, BIPY2+, and DAP2+
units indicate that the TC4+ ring encircles the STTFS unit,
1
1
Figure 2. 1H NMR spectrum of C·4CF3CO2 in D2O at 277 K with all the
resonance peaks assigned. The intercomponent nuclear Overhauser
1
effects observed in H-1H ROESY experiment are shown as purple
arrows.
two different external stimuli, are a requirement. In this
context, a p-electron-rich polyether macrocycle (PM) and a
p-electron-deficient TC4+ ring—both containing two different
molecular recognition sites—need to be employed (Figure 1)
as the two catenated components. On the one hand, we have
arranged for the PM to contain two p-electron-rich recog-
nition units—namely,
a
4,4’(5’)-bisthiotetrathiafulvalene
(STTFS) unit and a hydroquinone (HQ) unit, both of which
interact to some degree with TC4+. On the other hand, we
have arranged for TC4+ to contain two p-electron-deficient
recognition units—that is, a bipyridinium (BIPY2+) unit and a
diazapyrenium[7,14,15] (DAP2+) unit—which will both interact,
to some extent, with the PM. Once assembled into the
[2]catenane C4+, these two ring components will [p···p] stack
in unison with each other and form a “donor–acceptor–
donor–acceptor” array, thus stabilizing[10] the [2]catenane on
account of [p···p] charge transfer (CT) and hydrophobic
interactions. Since TC4+ binds the STTFS unit more strongly
than it does the HQ unit, while the PM has a higher affinity
for the DAP2+ unit than it has for the BIPY2+ unit, the
ground-state co-conformation (GSCC) is expected to corre-
spond to the catenane geometry illustrated in Figure 1. The
controlled switching of each individual ring can be addressed
using different stimuli. While the STTFS can be oxidized,[16]
either chemically or electrochemically, and hence induce the
circumrotation of the PM ring, the DAP2+ unit forms[14] an
adduct with amines that should lead to the circumrotation of
the TC4+ ring.[15]
On the basis of this blueprint, the designed [2]catenane
C·4CF3CO2, containing two bistable and complementary
rings, was synthesized (see the Experimental Section and
the Supporting Information) by employing a template-
directed protocol.[17] The pure product, a dark green solid,
was isolated using preparative HPLC following the formation
of the TC4+ ring under the templation of the PM ring.
Analytical HPLC of C·4CF3CO2 revealed (Supporting Infor-
mation) two peaks, which eluted close to each other,
corresponding to the [2]catenanes containing a mixture of
cis- and trans-STTFS units.
Figure 3. a) UV/Vis spectroelectrochemistry of C4+ recorded at 0 V
(green line) and 1.2 V (orange line) in water. [C4+]=2.34ꢀ10ꢀ4 molLꢀ1
electrolyte=tetrabutylammonium chloride (TBACl) (0.1 molLꢀ1);
T=298 K; l=1 mm. b) Cyclic voltammetry measurements of C4+ at
different scan rates (the current intensities are normalized).
;
[C4+]=1ꢀ10ꢀ3 molLꢀ1; electrolyte=TBACl (0.1 molLꢀ1). 1000 mVsꢀ1
the first scan is black, the second scan is gray; 200 mVsꢀ1: the first
scan is blue, the second scan is purple.
:
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while the PM ring encircles the DAP2+ unit, leaving the HQ
and BIPY2+ units to occupy alongside orientations. This
observation matches our expectation as to the preferred co-
conformation (Figure 1) based on the relative binding affin-
ities of the four different units, i.e., STTFS > HQ in binding to
TC4+ and DAP2+ > BIPY2+ in binding to PM. No significant
provides evidence for the circumrotation of the TC4+ ring
from the STTFS unit to the HQ unit on the PM ring.
The movement of the TC4+ ring was achieved using an
amine as the actuator. On adding 1,4-diazabicyclo[2.2.2]-
octane (DABCO) to an aqueous solution of C·4CF3CO2, a
new absorption band centered on 562 nm (an [n···p] CT band
originating in the DABCO–DAP complex)[22] appeared and
the CT band at 800 nm decreased ever so slightly (Figure 4a).
The retention of the 800 nm CT band indicates that the
STTFS unit was still encircled by the TC4+ ring after the
formation of DABCO–DAP complex.
1
changes in the H NMR spectrum in D2O were observed on
heating the sample up to 343 K or cooling it down to 277 K in
keeping with the belief[18] that C4+ exists, to all intents and
purposes, as a single co-conformation. This property of the
[2]catenane in aqueous solution renders switching much more
precise.[19]
This complex formation between DABCO and the DAP2+
unit is a dynamic process which can be reversed on the
addition of acid. On the basis of a UV/Vis absorption titration
experiment (Supporting Information), the binding constant
The UV/Vis absorption spectrum of C·4CF3CO2 in water
reveals (Figure 3a) a strong broad absorption band centered
on 800 nm. This peak is characteristic[16c] of a CT interaction
formed between STTFS and BIPY2+/DAP2+ units when the
STTFS unit is encircled by the TC4+ ring (STTFSꢁTC4+).
Since the STTFS unit can be oxidized electrochemically to its
STTFS2+ dication, cyclic voltammetry (CV) of the [2]cate-
nane in water (with 0.1 molLꢀ1 TBACl as the electolyte) was
recorded. The first scan showed (Figure 3b) only one major
oxidation peak at + 655 mV, an observation which provides
yet another piece of strong evidence that the STTFS unit is all
but located inside the TC4+ ring. This conclusion is reflected in
the fact that both the first and second oxidation processes
appear in combination[20]—an indication that the STTFS is
firmly ensconced inside the TC4+ ring. It is well known that,
upon oxidation, Coulombic repulsion between the dicationic
STTFS2+ unit and the TC4+ ring will force the PM ring to
circumrotate into a co-conformation in which the HQ unit
resides inside the TC4+ ring. On reduction,[21] the STTFS2+
unit undergoes two one-electron reduction processes to its
neutral state while the STTFS2+ unit is located alongside the
TC4+ ring with the HQ unit inside.
Such a metastable co-conformation (MSCC) eventually
relaxes back to the GSCC as a result of the PM ring
circumrotating around the TC4+ ring such that the STTFS unit
moves back inside the TC4+ ring. This process can be
confirmed (Figure 3b) by monitoring the oxidation process
associated with the STTFS unit in the second cycle of the CV
experiment while varying the scan rates. If the oxidation-
reduction cycle is fast enough, then the characteristic
oxidation peak of the MSCC will be observed at around
+ 300 mV during the second cycle. Indeed, when the scan rate
was increased to l000 mVsꢀ1, the second CV scan revealed an
oxidation process occurring at + 310 mV where the integrated
peak area equaled that of the peak observed at + 650 mV. By
varying the scan rate, the kinetics of the relaxation process
(MSCC!GSCC) could be obtained (see the Supporting
Information). The half-life of the MSCC was found to be
1.21 ꢂ 0.12 s at 298 K, corresponding to a free energy of
activation of 17.5 ꢂ 0.2 kcalmolꢀ1 for the process in water.
The electrochemically driven circumrotation of the PM
ring can also be monitored (Figure 3a) by UV/Vis spectro-
electrochemistry. On setting the voltage of the mesh-shaped
working electrode to + 1.2 V, the CT band at 800 nm
disappeared and a new absorption band—which can be
assigned to the CT band formed between the HQ unit and
the TC4+ ring—emerged at ca. 500 nm.[6c] This blue shift
Figure 4. a) Normalized UV/Vis absorption spectrum of C·4CF3CO2
recorded in water (black line), as well as the spectrum upon the
addition of excess of DABCO (blue line), and further addition of
excess of trifluoroacetic acid (TFA) (red line).
[C·4CF3CO2]=2.34ꢀ10ꢀ4 molLꢀ1, [DABCO]=0.03 molLꢀ1; [TFA]=0.03
molLꢀ1. (b) Differential pulse voltammetry measurements of C4+
(black line), upon the addition of excess of DABCO (blue line) and
after further addition of excess of HCl (red line).
[C4+]=2ꢀ10ꢀ4 molLꢀ1; electrolyte=TBACl (0.1 molLꢀ1); [DABCO]=
0.02 molLꢀ1; [HCl]=0.02 molLꢀ1. The inset is a diagram of potential
values of the first reduction processes of BIPY2+ and DAP2+ units
before (black dots) and after (blue dots) the addition of DABCO, as
well as that after further addition of HCl (red dots).
Angew. Chem. Int. Ed. 2011, 50, 1805 –1809
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Communications
H2O–MeCN/0–100% in 30 min, with 0.1% TFA), to afford pure
C·4CF3CO2 as a dark green solid (66 mg, 38%). 1H NMR (D2O,
600 MHz, 258C): d = 3.12–3.17 (m, 4H), 3.63–3.98 (m, 32H), 5.65–
5.77 (m, 6H), 6.10–6.29 (m, 8H), 7.61–7.90 (m, 8H), 7.95–8.04 (m,
4H), 9.13–9.17 (m, 4H), 10.21–10.28 ppm (m, 4H). 13C NMR (D2O,
125 MHz, 258C): d = 34.1, 34.3, 63.2, 64.7(m), 66.4(m), 67.4, 67.9, 68.5,
69.2, 69.3, 69.6, 69.7, 69.9, 106.1, 106.2, 114.5, 114.8, 116.2 (q, J =
290 Hz), 116.4, 116.7, 125.0, 125.2, 125.4, 126.2, 126.5, 127.0, 128.3,
129.2, 129.3, 129.5, 129.6, 130.5 (m), 135.9 (m), 139.4, 139.6, 139.9,
144.0, 144.2, 144.4, 145.1, 145.2, 145.3, 151.6, 151.9, 163.9 ppm (q, J =
(Ka) between DABCO and C·4CF3CO2 was found to be 174 ꢂ
8 Lmolꢀ1 in water. This Ka value is comparable with that for
DABCO with dibenzyl-diazapyrenium (264 ꢂ 28 Lmolꢀ1) in
water (Supporting Information). In this equilibrium involving
the [2]catenane C4+, the formation of the DABCO!DAP2+
adduct forces the TC4+ ring to undergo circumrotation with
respect to the PM ring, such that the DAP2+ unit resides
alongside, while the BIPY2+ unit occupies the inside of the
PM ring. The process is a result of 1) the steric hindrance of
the DABCO!DAP2+ adduct and also 2) its decreased
electron deficiency. The UV/Vis absorption spectrum is fully
regenerated on neutralizing the DABCO by addition of an
equivalent of an acid, e.g., CF3CO2H.
35 Hz).
ESI-MS:
m/z = 1601.33
[MꢀCF3CO2]+;
1487.33
687.16
[MꢀCF3CO2ꢀCF3CO2H]+;
744.15
[Mꢀ2CF3CO2]2+
;
[Mꢀ2CF3CO2ꢀCF3CO2H]2+. Analytical HPLC chromatogram of
C·4CF3CO2 shows (Supporting Information, Figure S1) two closely
eluting peaks (12.3 min and 12.8 min) as a result of the existence of
both cis- and trans-STTFS units in the catenane C·4CF3CO2. The ratio
of the peak integrals is 1.08:1 (12.3 min : 12.8 min).
The circumrotation of the TC4+ ring with respect to the
PM ring has been confirmed by carrying out electrochemical
measurements on C·4CF3CO2 in water before and after
addition of DABCO. Differential pulse voltammetry (DPV)
of the [2]catenane (2 ꢀ 10ꢀ4 molLꢀ1) from 0 to ꢀ800 mV in
water reveals (Figure 4) two reduction peaks which could be
assigned[15] to the first reduction processes of the alongside
BIPY2+ unit (ꢀ567 mV) and the inside DAP2+ unit
(ꢀ671 mV), respectively. The shift of the DAP2+ reduction
peak is caused by the formation of the DABCO!DAP2+
adduct. The 5 mV shift of the peak at ꢀ567 mV indicates[15]
that the BIPY2+ unit has located itself inside the PM ring after
the addition of the DABCO—i.e., circumrotation of the TC4+
ring through the PM ring has occurred. On addition of
100 equiv of HCl,[23] the DPV trace was restored completely
as a consequence of the complete recovery to the GSCC
following the neutralization of DABCO.
We have demonstrated that the controllable mechanical
movement of either the polyether macrocycle or the tetra-
cationic cyclophane in a doubly bistable water-soluble
[2]catenane can be effected using orthogonal external stimuli
(redox and amine/acid). These reversible molecular motions
can be controlled in aqueous media—it would appear, with
even more precision than in organic solvents[19]—bringing us
just a little closer to what nature can do and affording us a
better chance to modify natureꢁs catalysts (enzymes) and
motor molecules with artificial molecular switches/machines.
The organization of such molecular actuators onto polymer
scaffolds,[24] the surfaces of nanostructures,[25] and into the
struts of metal–organic frameworks[26] could provide the
stepping stone for the creation of integrated complex systems.
Water-compatible artificial molecular switches/machines
could also act as molecular prosthetics[27] in living systems,
providing us with an opportunity to regulate biological
processes using correlated molecular motions at the nano-
scale level.
Received: October 11, 2010
Published online: January 20, 2011
Keywords: artificial molecular machines · electrochemistry ·
.
translational isomerism · water-soluble catenane
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Experimental Section
Synthesis of C·4CF3CO2: The polyether macrocyle PM (76 mg,
0.107 mmol), diazapyrene (44 mg, 0.214 mmol), and bis(p-bromo-
methylbenzyl)-4,4’-bipyridinium bishexafluorophosphate (174 mg,
0.214 mmol) was dissolved in DMF and the mixture was stirred at
room temperature for 4 days. After removing the solvent under
reduced pressure, the resulting dark green residue was subjected to
column chromatography (SiO2: NH4PF6/CH3COCH3:5 gLꢀ1). The
green band was collected and further purified by RP-HPLC (C18;
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[19] The [2]catenane C4+ does not exhibit the same kind of precision
in organic solvents. In CD3CN, for example, the 1H NMR
spectrum (see the Supporting Information) of C·4CF3CO2
reveals that the PM ring distributes itself between the STTFS
unit being located inside the TC4+ ring in contradistinction to the
HQ ring being located inside the TC4+ ring with a ratio of 1:1.8.
[20] It is well known that the first oxidation process of STTFS unit
shifts drastically to more positive potential as a result of the TC4+
encirclement.
[21] It should be noted that the oxidation of STTFS unit becomes
irreversible when it is exposed to oxidant for a relatively long
ˇ
´
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)
ˇ
´
revealed that the integrals of the cathodic peaks are slightly
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ˇ
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identified, probably on account of the low binding constant
between DAP2+ and DABCO. On the other hand, the resonance
signals corresponding to a-protons in the DAP2+ unit were
shifted upfield and became broad. These phenomena lead us to
the conclusion that the electron lone pair on DABCO interacts
primarily with the aforementioned a-protons while the equilib-
rium of the formation and dissociation of the [n···p] complex
occurs at a rate slower than the 1H NMR timescale. See the
1
Supporting Information for the H NMR spectrum.
[23] HCl was used in the DPV measurement, instead of TFA, in
order to prevent counterion exchange in the TBACl electrolyte
solution, so that the dominant counteranion in the solution
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