![UV−vis−NIR absorption spectroscopic and single crystal XRD evidence that CBPQT 2(+@BULLET) threads across the Coulombic PY + barrier. (a) Spectra of DB0 3+@BULLET (black trace), CBPQT 2(+@BULLET) (blue trace), and a 1:1 mixture of DB0 3+@BULLET and CBPQT 2(+@BULLET) (purple trace). The broad peak around 1100 nm indicates formation of the trisradical inclusion complex DB0 3+@BULLET ⊂CBPQT 2(+@BULLET) . [DB 4+ ] = 1.0 × 10 −4 M, [CBPQT 4+ ] = 1.0 × 10 −4 M. Solvent, MeCN; T = 298 K. (b) A tubular representation of the solid-state superstructure of (DB0⊂CBPQT)·6PF 6 . 14,15 Solvent molecules, hydrogen atoms, and counterions are omitted for the sake of clarity.](https://www.researchgate.net/profile/Chuyang-Cheng/publication/266153137/figure/fig1/AS:392214640381956@1470522665418/UV-vis-NIR-absorption-spectroscopic-and-single-crystal-XRD-evidence-that-CBPQT_Q320.jpg)
![1 H NMR spectroscopic analysis of [2]pseudorotaxane formation and the kinetic barrier to disocciation. Partial 1 H NMR spectra (600 MHz, CD 3 CN, 279 K) of (a) the dumbbell DB1 3+ , (b) a reaction mixture containing DB1 3+ ⊂CBPQT 4+ after performing a cycle of reduction and oxidation, (c) after standing at 279 K for 4 h, and (d) CBPQT 4+ . Peaks are highlighted in red and blue for DB1 3+ and CBPQT 4+ , respectively. The changes in chemical shifts (blue and black dashed lines) are indicative of [2]pseudorotaxane formation. (e) Plot of the change in molar fraction over time at 279 K based on integration of the 1 H NMR spectra. Inset: linear relationship between −ln(c/c 0 ) and time, indicating that the dethreading process obeys firstorder kinetics. The activation barrier ΔG ⧧ was calculated using the Eyring equation.](https://www.researchgate.net/profile/Chuyang-Cheng/publication/266153137/figure/fig2/AS:392214640381957@1470522665545/1-H-NMR-spectroscopic-analysis-of-2pseudorotaxane-formation-and-the-kinetic-barrier-to_Q320.jpg)
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Energetically Demanding Transport in a Supramolecular Assembly
Chuyang Cheng,
†
Paul R. McGonigal,
†
Wei-Guang Liu,
‡
Hao Li,
†
Nicolaas A. Vermeulen,
†
Chenfeng Ke,
†
Marco Frasconi,
†
Charlotte L. Stern,
†
William A. Goddard III,
‡
and J. Fraser Stoddart*
,†
†
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
‡
Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, United States
*
SSupporting Information
ABSTRACT: A challenge in contemporary chemistry is
the realization of artificial molecular machines that can
perform work in solution on their environments. Here, we
report on the design and production of a supramolecular
flashing energy ratchet capable of processing chemical fuel
generated by redox changes to drive a ring in one direction
relative to a dumbbell toward an energetically uphill state.
The kinetics of the reaction pathway juxtapose a low
energy [2]pseudorotaxane that forms under equilibrium
conditions with a high energy, metastable [2]-
pseudorotaxane which resides away from equilibrium.
The active transport of ions and small molecules across cell
membranes, driven by bespoke biochemical machinery,
plays a pivotal role in the operation of cells.
1
Nature’s
transmembrane protein pumps create concentration gradients
of ions, such as Na+,K
+and Ca2+, in addition to protons,
2
whose
stored potential may then be used as a secondary energy
resource
3
in metabolic processes, e.g., ATP synthesis.
4
During
billions of years of evolution, these proteins have developed
finely tuned secondary and tertiary structures capable of
harnessing external fuel to exert precise control over the
noncovalent forcesthe potential energy landscape of energy
barriers and wellsexperienced by their cargos in order to drive
them energetically uphill, temporarily away from thermodynamic
equilibrium.
5
Over the past 30 years, chemists have taken advantage of the
restricted degrees of freedom available to the components of
rotaxanes
6
and pseudorotaxanes
7
to construct and control
increasingly sophisticated artificial molecular switches that
exhibit large amplitude relative motions between their
constituent parts.
8
In the majority of cases, however, the
application of a stimulus to these artificial systems induces
molecular motions toward a low-energy equilibrium state. The
development of artificial molecular assemblies
9
that can drive a
cargo away from its equilibrium position by manipulating kinetic
pathways−thereby mimicking the function of their biochemical
counterparts−is still very much in its infancy.
10
Here, we report a
family of synthetic supramolecular pumps that exploit chemical
energy to drive a ring away from its initial state in solution to form
metastable, energetically demanding products that can then
gradually decay back toward equilibrium. Control of this
supramolecular transport is exercised by influencing the kinetics
of the reaction pathway using only reversible noncovalent
bonding interactions and without the need to make or break
covalent bonds. The end result is that the ring is ensnared on an
oligomethylene chain
11
for which it has little to no binding
affinity at any stage of the pumping process.
We designed (Figure 1) a homologous series of dumbbells
DB1−93+ bearing 4,4′-bipyridinium (BIPY2+) units as radical
recognition sites
12
for cyclobis(paraquat-p-phenylene)
13
(CBPQT4+). A 3,5-dimethylpyridinium (PY+) unit is attached
to one side of the BIPY2+ unit by a short oligomethylene chain,
while a bulky stopper is attached to the other side by a longer
chain. Before testing the dumbbells, a symmetrical derivative
DB04+ (Figure 1) was prepared as a model compound to confirm
that CBPQT2(+•)can thread across the PY+barrier in order to
form the trisradical complex DB03+•⊂CBPQT2(+•)under the
reducing conditions that can bring about radical−radical
interactions. The absorption spectra of the radical cation
DB03+•and diradical dication CBPQT2(+•)were first of all
recorded separately in MeCN at concentrations of 10−4M. No
absorption indicative of BIPY+•dimerization was observed in the
near-IR (NIR) region under these conditions. By contrast, when
Received: August 21, 2014
Published: September 25, 2014
Figure 1. Structural formulas and simplified graphical representations of
the CBPQT4+ ring, the symmetrical dumbbell DB04+ and the nine ‘one-
stroke supramolecular pumps’DB1−93+ employed in this structure−
function investigation. Charges are balanced by PF6
−counterions which
are omitted for the sake of clarity.
Communication
pubs.acs.org/JACS
© 2014 American Chemical Society 14702 dx.doi.org/10.1021/ja508615f |J. Am. Chem. Soc. 2014, 136, 14702−14705
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DB04+ and CBPQT4+ were reduced together, a distinctive purple
solution was obtained within seconds and UV−vis−NIR
spectroscopy revealed (Figure 2a) features characteristic of
BIPY+•radical−radical interactions, including a broad NIR band
around 1100 nm that is indicative
12
of the formation of the
trisradical inclusion complex DB03+•⊂CBPQT2(+•).Single
crystals of (DB0⊂CBPQT)·6PF6suitable for X-ray diffraction
(XRD)
14
were grown in a glovebox.
15
The solid-state super-
structure confirmed (Figure 2b) that the ring encircles the
dumbbell.
With the viability of complex formation between CBPQT2(+•)
and DB03+•verified, we hypothesized that subjecting a mixture of
CBPQT4+ and a dumbbell DB1−93+ (Figure 1) to a cycle of
reduction and oxidation would result in the formation of a high-
energy [2]pseudorotaxane. To test our hypothesis, activated zinc
dust was added to a 1:1 mixture of DB13+and CBPQT4+ in
CD3CN, giving a purple solution indicative of trisradical complex
formation within a few seconds. After filtration to remove the
excess of zinc, tris(4-bromophenyl)aminium hexachloridoantim-
onate (magic blue) was added at 0 °C to reoxidize the radical
species to their fully charged states, and a 1H NMR spectrum was
recorded. Comparison of the 1H NMR spectra of the dumbbell,
the reaction mixture, and the ring revealed (Figure 3a,b,d)
formation of a product species in ∼80% yield that exhibited
broadening (Hα/β) and splitting (HCH2) of resonances associated
with the ring as well as significant upfield shifts of the methylene
proton resonances H1−4of the dumbbell component, indicating
the formation of DB13+⊂CBPQT4+ in which the ring encircles
the hexamethylene chain.
12b,d
This [2]pseudorotaxane was
found to be metastable and to dissociate entirely within half an
hour at room temperature or within 4 h at 279 K (Figure 3c).
Changes in the relative proportions of DB13+⊂CBPQT4+ and its
separated components were monitored (Figure S4)
15
by 1H
NMR spectroscopy at a constant temperature (279 K) which
revealed (Figure 3e) that the dethreading displays first-order
kinetics with a rate constant k= (8.4 ±0.6) ×10−5s−1. Based
upon these data, the transition-state energy barrier (ΔG⧧) for the
CBPQT4+ ring to dissociate from DB13+ by slipping over its
positively charged BIPY2+ and PY+units was found to be 21.5
kcal mol−1.
It appears, therefore, that the design of dumbbell DB14+ allows
it to “pump”a ring actively onto its hexamethylene chain
following potential energy surfaces resembling those represented
as curves in Scheme 1. Initially, the tetracationic CBPQT4+ ring is
repelled (Scheme 1a) by both the PY+and BIPY2+ units.
Reduction, however, simultaneously lowers the kinetic barrier to
threading
7d
that comes from Coulombic repulsion between the
PY+unit and the ring, while also bringing radical−radical
interactions into play to create (Scheme 1b) a new global
minimum in which the dumbbell is encircled by the ring. As a
result, the ring slips over the PY+unit to form (Scheme 1c) a
thermodynamically stable trisradical inclusion complex
12
BIPY+•⊂CBPQT2(+•). A subsequent oxidation step to restore
the repulsion between the highly charged ring and dumbbell
components
12d
then imparts a driving force for the CBPQT4+
ring to undergo translational motion. In thermodynamic terms,
the global energy minimum would be reached if the ring were to
Figure 2. UV−vis−NIR absorption spectroscopic and single crystal
XRD evidence that CBPQT2(+•)threads across the Coulombic PY+
barrier. (a) Spectra of DB03+•(black trace), CBPQT2(+•)(blue trace),
and a 1:1 mixture of DB03+•and CBPQT2(+•)(purple trace). The broad
peak around 1100 nm indicates formation of the trisradical inclusion
complex DB03+•⊂CBPQT2(+•).[DB4+] = 1.0 ×10−4M, [CBPQT4+]=
1.0 ×10−4M. Solvent, MeCN; T= 298 K. (b) A tubular representation
of the solid-state superstructure of (DB0⊂CBPQT)·6PF6.
14,15
Solvent
molecules, hydrogen atoms, and counterions are omitted for the sake of
clarity.
Figure 3. 1H NMR spectroscopic analysis of [2]pseudorotaxane formation and the kinetic barrier to disocciation. Partial 1H NMR spectra (600 MHz,
CD3CN, 279 K) of (a) the dumbbell DB13+, (b) a reaction mixture containing DB13+⊂CBPQT4+ after performing a cycle of reduction and oxidation,
(c) after standing at 279 K for 4 h, and (d) CBPQT4+. Peaks are highlighted in red and blue for DB13+ and CBPQT4+, respectively. The changes in
chemical shifts (blue and black dashed lines) are indicative of [2]pseudorotaxane formation. (e) Plot of the change in molar fraction over time at 279 K
based on integration of the 1H NMR spectra. Inset: linear relationship between −ln(c/c0) and time, indicating that the dethreading process obeys first-
order kinetics. The activation barrier ΔG⧧was calculated using the Eyring equation.
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja508615f |J. Am. Chem. Soc. 2014, 136, 14702−1470514703
retrace its path and dissociate from the dumbbell. The
electrostatic barrier to translation originating from the PY+
group, however, is restored upon oxidation, kinetically
disfavoring the backward pathway and forcing the ring to shuttle
away (Scheme 1d) in the opposite direction. Consequently, the
CBPQT4+ ring is ensnared (Scheme 1e) as part of a metastable
[2]pseudorotaxane that does not benefitfromstabilizing
noncovalent bonding interactions. Indeed, the enforced
proximity of the three charged viologen units dictates that the
[2]pseudorotaxane resides in an energetically demanding state
compared to its separated components.
By performing the same reduction and oxidation protocol with
the entire homologous series of dumbbells DB2−83+, we were
able to gain some insight into the relationship between the
structure and energetics of the supramolecular pumps. Extension
of the oligomethylene chain between the BIPY2+ and stopper
units resulted in two pseudorotaxanes, namely
DB23+⊂CBPQT4+ and DB33+⊂CBPQT4+, that exhibited
prolonged half-lives compared to DB13+⊂CBPQT4+.The
dissociations of DB23+⊂CBPQT4+ and DB33+⊂CBPQT4+
were monitored by 1H NMR spectroscopy at 331 and 365 K,
respectively, which allowed the kinetic barriers to dethreading to
be determined (Figures S5 and S6)
15
as 25.3 and 29.0 kcal mol−1.
Although the intrinsic barrier to association ΔG⧧0(Scheme 1a),
which stems mainly from Coulombic repulsion between
positively charged BIPY2+/PY+units and the CBPQT4+ ring, is
similar for dumbbells DB1−33+, the large variation in the kinetic
barriers to dethreading can be rationalized by considering the
difference in the Gibbs free energy ΔGof the metastable
[2]pseudorotaxanes. A shorter oligomethylene chain confines
the like-charged CBPQT4+ ring and BIPY2+ unit in closer
proximity to one another, destabilizing the pseudorotaxane and
resulting in a higher ΔG.
12d
Density functional theory (DFT)
calculations support this reasoning. We calculated (Table S3)
15
increases in enthalpy (ΔH) of 14.9, 8.8, and 7.0 kcal mol−1for the
[2]pseudorotaxanes based on DB13+,DB23+,andDB33+,
respectively, compared to their separated components.
The oligomethylene portion of the dumbbell can be replaced
by an oligoethylene glycol chain without diminishing the
efficiency of the pumping process. Dumbbells DB43+ and
DB53+, bearing triethylene glycol and tetraethylene glycol tails,
respectively, gave rise to [2]pseudorotaxanes in ∼85% yield, as
estimated by 1H NMR spectrocopy.
15
While the dissociation of
DB43+⊂CBPQT4+ follows (Figure S7)
15
first-order kinetics in a
manner consistent with the oligomethylene derivatives, our
observations indicate that the dissociation (Figure S8)
15
of
DB53+⊂CBPQT4+ is autocatalytic. The most probable explan-
ation for this phenomenon is that hydrogen bonding between the
tetraethylene glycol chain and CBPQT4+ ring, which is a well-
established interaction,
16
stabilizes the transition state to
dethreading.
7d,17
Indeed, the addition of 1.0 equiv of dumbbell
DB53+ to a solution of DB53+⊂CBPQT4+ accelerated the
dissociation of the [2]pseudorotaxane.
15
In order to avoid this
complication in subsequent investigations, we decided to limit
dumbbell design to those containing oligomethylene chains.
Having explored the effect of altering the length and nature of
the chain between the BIPY2+ unit and the stopper, we turned
our attention to assessing the influence of the short oligo-
methylene linker connecting the Coulombic barrier
18
PY+to the
rest of the dumbbell. Taking DB23+ as a point of reference, two
dumbbells were synthesized bearing a butylene (DB63+) and a
bismethylene (DB73+) linker in place of the propylene one in
DB23+. Surprisingly, this seemingly minor alteration in molecular
structure was found to have a dramatic effect on the outcomes of
redox cycling. Subjecting a mixture of CBPQT4+ and DB63+ to
the standard protocol gave only a physical mixture after
reduction and oxidation (Figure S9),
15
suggesting that
elongating the linker diminishes considerably the ability of the
BIPY2+ and PY+units to act together in creating a highly charged
end group that retains the ring on the dumbbell on account of
Coloumbic repulsion. In contrast, when CBPQT4+ and DB73+
were reduced and oxidized under the same conditions, a bench
stable [2]rotaxane was produced that could be isolated (Figure
S1)
15
and fully characterized by NMR spectroscopy and mass
spectrometry. Indeed, the stability of the mechanically
interlocked product precluded measurement of any barrier to
dethreading; no dissociation could be observed (Figure S10),
15
even after refluxing in MeCN for 18 h! In order to quantify the
change in the energy barrier that occurs upon shortening the
propylene linker, we prepared DB83+ with a bismethylene linker
and a hexamethylene chain on which to collect the ring, rather
than the octamethylene one in DB73+. The [2]pseudorotaxane
DB83+⊂CBPQT4+ dissociates at 355 K with a rate constant k=
(2.2 ±0.1) ×10−5s−1, corresponding to a dethreading barrier of
28.5 kcal mol−1(Figure S11).
15
It is quite remarkable that, with
the removal of a single methylene group from DB13+ (propylene
linker) to give DB83+ (bismethylene linker), we observe an
increase of 7.0 kcal mol−1in the barrier to dissociation, from 21.5
to 28.5 kcal mol−1. This phenomenon can be accounted for to a
certain extent by considering that the magnitude of Coloumbic
forces are inversely proportional to the square of the distance
between two charged species and that the longer the linker, the
further the ring resides away from the PY+unit. DFT calculations
indicate, however, that the interplay between the PY+and
BIPY2+ units also plays a role. The calculated potential energy
profile for dissociation of DB83+⊂CBPQT4+ is characterized
(Figure S12)
15
by one major energy barrier of 18.7 kcal mol−1.
The profile of a model [2]pseudorotaxane analog that contains a
butylene linker, DB93+⊂CBPQT4+, exhibits (Figure S13)
15
two
smaller barriers, the highest of which is only 12.4 kcal/mol. It
Scheme 1. Graphical Representation of the Pumping
Mechanism and Corresponding Energy Profiles
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja508615f |J. Am. Chem. Soc. 2014, 136, 14702−1470514704
appears, therefore, that the dense buildup of charge between
closely tethered BIPY2+ and PY+units (e.g., in DB7−83+) acts in
an additive manner in repelling the slippage of the CBPQT4+
ring, although this effect is diminished if a longer spacer is
employed.
19
In summary, we have designed, synthesized, and operated a
family of artificial supramolecular pumps which couple the
dissipation of redox chemical energy to the formation of a non-
equilibrium state. The alternation between the asymmetric
potential energy surfaces that they create shapes the kinetically
favored pathway followed by a positively charged ring, leading to
a metastable [2]pseudorotaxane in which the ring is trapped on a
dumbbell in the absence of any stabilizing interactions. By
studying a homologous series of supramolecular pumps and their
respective [2]pseudorotaxanes, we have elucidated the delicate
balance between molecular structure and the energetics of the
pumping process. Judicious choice of molecular structure allows
for both high-efficiency [2]pseudorotaxane formation and the
regulation of its subsequent dissociation over a Coloumbic
barrier, which can be tuned within a range >10 kcal mol−1. The
supramolecular pumps we have described exhibit some of the
qualities of the active transport
1,2
of ions and small molecules
that takes place in Nature−i.e., they harness an energy input in
order to oscillate between asymmetric potential energy surfaces
and, thus, transport a species to a high-energy state. In their
current form, however, these synthetic supramolecular pumps
are unable to act in the repetitive manner reminiscent of their
natural counterparts in order to push a system incrementally
further and further from equilibrium, as occurs, e.g., when carrier
proteins create concentration gradients. By bringing the
knowledge gained in this structure−function study to bear on
the design of synthetic supramolecular pumps, it may be possible
to devise more sophisticated systems capable of this kind of
repetitive and progressive action.
■ASSOCIATED CONTENT
*
SSupporting Information
Detailed synthesis procedures and characterization data (NMR
spectroscopy and mass spectrometry) for all compounds. UV−
vis−NIR spectroscopic and single crystal XRD characterization
of the [2]pseudorotaxane (DB0⊂CBPQT)·6PF6. This material
is available free of charge via the Internet at http://pubs.acs.org.
■AUTHOR INFORMATION
Corresponding Author
stoddart@northwestern.edu
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
This material is based on work supported by the National Science
Foundation (NSF) under CHE-1308107. W.-G.L. and W.A.G.
thank NSF (CMMI-1120890 and EFRI-1332411) for financial
support and to ONR-DURIP and NSF-CSEM for computing
resources.
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Chem. Soc. 2014,136, 6373.
(18) (a) Li, H.; Zhao, Y. L.; Fahrenbach, A. C.; Kim, S. Y.; Paxton, W.
F.; Stoddart, J. F. Org. Biomol. Chem. 2011,9, 2240. (b) Hmadeh, M.;
Fahrenbach, A. C.; Basu, S.; Trabolsi, A.; Benítez, D.; Li, H.; Albrecht-
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(19) Taking all of the calculated and experimentally measured
thermodynamic data together (Table S18), the fraction of the redox
energy input that is stored in the high-energy pseudorotaxanes can be
calculated to be between 5 and 10%.
Journal of the American Chemical Society Communication
dx.doi.org/10.1021/ja508615f |J. Am. Chem. Soc. 2014, 136, 14702−1470514705
