Bistable [c2] Daisy Chain Rotaxanes as Reversible Muscle-like Actuators in Mechanically Active Gels

Article abstract of DOI:10.1021/jacs.7b06710

The implementation of molecular machines in polymer science is of high interest to transfer mechanical motions from nanoscale to macroscale in order to access new kinds of active devices and materials. Toward this objective, thermodynamic and topological

Full text of DOI:10.1021/jacs.7b06710

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Communication
Bistable [c2]daisy chain rotaxanes as reversible
muscle-like actuators in mechanically active gels
Antoine Goujon, Thomas Lang, Giacomo Mariani, Emilie Moulin,
Gad Fuks, Jesus Raya, Eric Buhler, and Nicolas Giuseppone
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06710 • Publication Date (Web): 12 Oct 2017
Downloaded from http://pubs.acs.org on October 12, 2017
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Antoine Goujon,[a]† Thomas Lang,[a] Giacomo Mariani,[b] Emilie Moulin,[a] Gad Fuks,[a] Jesus Raya,[c]
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Eric Buhler,[b] and Nicolas Giuseppone[a]*
[a] SAMS research group – University of Strasbourg – Institut Charles Sadron, CNRS – 23 rue du Loess, BP 84047, 67034
Strasbourg Cedex 2, France.
[b] Matière et Systèmes Complexes (MSC) Laboratory, UMR CNRS 7057, Sorbonne Paris Cité, University of Paris Diderot-
Paris VII, Bâtiment Condorcet, 75205 Paris Cedex 13, France.
[c] Institut de Chimie, UMR 7177 CNRS, Université de Strasbourg, 1 rue Blaise Pascal, BP 296 R8, 67008 Strasbourg Cedex.
Supporting Information Placeholder
a)
b)
d)
[c2] daisy chain monomers
c) Cross-linked polymer
ABSTRACT: The implementation of molecular machines in
polymer science is of high interest to transfer mechanical motions
from nanoscale to macroscale in order to access new kinds of
active devices and materials. Towards this objective, thermody-
namic and topological aspects need to be explored for reaching
efficient systems capable of producing a useful work. In this paper
we describe the branched polymerization of pH-sensitive bistable
[c2]daisy chain rotaxanes by using copper (I) catalyzed Huisgen
1,3-dipolar cycloaddition (“click chemistry”). With this cross-
linked topology, the corresponding materials in the form of chem-
ical gels can be contracted and expanded over a large variation of
volume (≈ 50%) by changing the protonation state of the system.
HR-MAS 1H NMR and neutron scattering experiments reveal that
this macroscopic response of the gels result from the synchronized
actuation of the mechanical bonds at the molecular level.
Contracted form
Contracted form
Extended form
Main chain polymer
n
Contracted form
Chemical structures of the [c2] daisy chain units
-H+
+H+
= Supramolecularor covalent linkage
Figure 1. a) Schematic representation of contracted and extented
[c2] daisy chain monomers; b) Schematic representation of a main
chain polymer based on [c2] daisy chain monomers; c) Schematic
representation of a cross-linked polymer network based on [c2]
daisy chain monomers; d) Detailed chemical structure of the [c2]
daisy chain unit used in the present study.
In living systems, the integration in space and time of protein
machineries with the dynamic structuring of the cytoskeleton is
central to many functions such as growth, motility, contraction,
and mechanotransduction.1,2 These important biological responses
can be modelled as a mechanical continuum going from subcellu-
lar components to tissues and organs.3,4 Partly inspired by bio-
molecular machines,5,6 chemists have synthesized a number of
artificial molecules capable of relative internal translations or
rotations.7–9 Their individual functioning can be actuated at ther-
modynamic equilibrium (molecular switches)10,11 or out-of-
equilibrium (molecular motors).12–16 However, to make use of the
work produced by these nanomachines, one should link them to
other elements which can amplify and/or transduce their mechani-
cal motion in something useful.17–20 Following these lines, a few
recent works have demonstrated the possible integration of large
numbers of artificial molecular machines in order to connect them
with the outside world.21–25
oligomers based on [c2]daisy chain rotaxanes have been described
independently by the groups of Stoddart31 and Grubbs.32 Our
group recently introduced coordination33 and hydrogen-bonded34–
36
main chain supramolecular polymers involving pH-responsive
bistable [c2]daisy chain rotaxanes (Figure 1b). In these examples,
high degrees of polymerization were demonstrated, leading for
instance to i) microscopic extension / contraction of single chain
polymers;33 ii) supramolecular polymerization / depolymerization
process;36 iii) changes of mesophase morphologies in bundles of
supramolecular polymers;35 and iv) macroscopic sol-gel transi-
tions in supramolecular polymer networks.34 Another important
advance was made by the group of Harada who synthesized a
covalent 3D polymer network integrating [c2]daisy chains in
which cyclodextrines (rings) bind to photoswitchable azobenzene
groups (axles).37 Of peculiar interest, in the dry state, the corre-
sponding xerogel is able to perform a fast pseudo-reversible de-
formation induced by an alternation of UV and visible lights.
A particularly interesting class of artificial nanomachines to be
further coupled in space and time are bistable [c2]daisy chain
rotaxanes (so-called “molecular muscles”).26 Their complex mo-
lecular structure involves two rings linked to two axles that are
double threaded (Figure 1a). By introducing different binding
stations for the macrocycles on the axles, the controlled gliding of
their subunits can lead to internal contraction and extension based
on the dynamic of their mechanical bond.27–30 The first covalent
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In the present article, we now explore a branched covalent pol-
were again rinsed and swollen back in acetonitrile before measur-
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ymer involving pH-sensitive bistable [c2]daisy chain rotaxanes as
monomers (Figures 1c,d). We show that in the chemically cross-
linked network topology, the pH-actuation of the mechanical bond
at the molecular level translates to large and fully reversible mac-
roscopic contraction/expansion of the swollen chemical gel by ≈
50% of its volume.
ing their volumes (Figure 3a-c). We used the software ImageJ to
evaluate the size change of the main surface area of the gel after
each step. As the contraction and expansion are isotropic in this
random network configuration, the following equation was used
to estimate the volume change: Vrel = (Af/Ai)3/2, where Vrel being
the relative volume in %, Af the surface area after protonation or
deprotonation, and Ai the surface area before starting the experi-
ment. As shown in Figure 3d, the volume of Gel4AM changed
from 100% to 68% after deprotonation, and went back to 110%
after protonation. The volume of Gel4BM decreased from 100% to
61% after deprotonation and went back to 98% after protonation.
Repetitive sequences of protonation and deprotonation were
performed on gel Gel4AM, showing the successful cycling of
contraction and expansion of the material at least three times
without degradation (Figure 3f). The fact that the sizes slightly
exceed the original sizes after the first cycle can be explained by
the partial exchange of counter anions of the [c2]daisy chains
during the experiments, modifying their swelling ability. In both
cases, the volume variations measured are in good agreement with
the expected response of the [c2]daisy chain units, which strongly
suggest their successful incorporation in the materials and their
dynamic integration from nano- to macroscopic scales. Moreover,
control experiments with non-methylated gels Gel4A and Gel4B
showed only negligible variations of their volumes after using the
same protocol of deprotonation/protonation: 100-95-101% for
Gel4A and 100-99-100% for Gel4B (Figure 3e). Importantly, this
result confirms that the presence of a second station for the rotax-
ane (i.e. the triazolium binding site) is mandatory to observe
contraction and expansion of the material. It also indicates that the
protonation / deprotonation of the secondary amine does not lead
to a measurable change in the swelling properties of the gel.39
The synthetic design of the present chemical network is depict-
ed in Figure 2a. It involves the cross-linking by click chemistry of
three components: pseudo rotaxane bis-alkyne 1,33,38 bis-azide 2,
and tris-azide 3 (see SI for the detailed syntheses and characteri-
zation of compounds 2 and 3). We used three different propor-
tions of cross-linking agent 3 (Figure 2b) and the reactions were
performed directly into a metallic mold (Figure S1). This protocol
led to the formation within 30 minutes of plastic-like materials
(Gel4A-C) which swell upon addition of acetonitrile. Gel4A and
Gel4B were robust enough to be easily manipulated, while Gel4C
was too soft and fragile to be further studied. Traces of remaining
starting materials were removed by several rinsing of Gel4A and
Gel4B in dichloromethane, and copper salts were then extracted
using an aqueous solution of ethylenediaminetetraacetic acid
(EDTA) at a pH of 8. The triazole functions in Gel4A and Gel4B
were finally methylated to furnish Gel4AM and Gel4BM by simple
immersion of the gels in a solution of acetonitrile and methyl
iodide (1:1) followed by anion exchange with ammonium hex-
afluorophosphate. After rinsing and evaporation of the solvents,
the plastic materials can be re-swollen in acetonitrile up to 1900%
of their dry volume (Figure S2).
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a)
1
Finally, we performed a HR-MAS H NMR experiment40 which
allowed us to directly probe the motion of the mechanical bond in
the gel state for Gel4AM (Figure S7). The typical 0.5 ppm down-
field shift of the triazolium proton resonance signal in the con-
tracted form (vs. upfield shift in the extended form) unambiguous-
ly proved the expected actuation in the gel state. Altogether, these
experiments illustrate the relationship existing between the vol-
ume variations at the macroscopic scale and the controlled dy-
namic of the [c2]daisy chains at the molecular level.
To probe further the conformational dynamic at the levels of the
mechanical bond and of the polymer chain, we performed small
angle neutron scattering (SANS) experiments. Figure 4a shows
variations in absolute scattered intensity, I, as a function of the
magnitude of the scattering wave vector, q, for both expanded and
contracted Gel4AM. In the experimental scattering vector regime
1
1
1
1
2
3
b)
Gel4AM
Gel4BM
Gel4CM
0.33
0.5
0.44
0.33
0.22
0.66
푞 > 푅퐺−1, where RG is the radius of gyration of the objects, the
−퐷푓
( )
scattered intensity of a fractal particle is given by 퐼 푞 ≈ 푞
.
Figure 2. a) Chemical structures of monomers 1-3 and experi-
mental conditions leading to the formation of chemical gels as
illustrated by the picture on the right; b) Compositions and labels
of the gels obtained using the conditions described in panel (a)
and for different ratios of monomers.
Here, Df is the so-called fractal dimension of the particle. Both
scattering curves in Figure 4a exhibit an overall behavior charac-
terized by the following sequence: (i) a power law regime at low q
giving a fractal exponent Df = 3 and indicating the presence of
large assemblies; (ii) another power law regime in the mid q
region with Df = 1 and characteristic of the rod-like behavior of
the chains for distances smaller than the persistence length; and
finally (iii) a Guinier regime at high q associated to the cross-
section of the chains. The signal measured at mid and high q
corresponds to the scattering of the polymer chains that compose
the network. The two curves decay at the same q in the high q
region. This means that the cross-section remains almost un-
changed in the extended and contracted state and that there is no
bundling of the polymer chains when switching the system.
We then probed the contraction/expansion of the material by
direct macroscopic observation. The process was performed step-
wise in order to swell the gel in the same conditions when con-
tracted or expanded, and thus to avoid solvation effects. In their
initial state, the gels were swollen in acetonitrile and the [c2]daisy
chains were in their protonated (extended) state. The initial pieces
of Gel4AM and Gel4BM were measured before being immersed in
a 1 M solution of sodium hydroxide to deprotonate the [c2]daisy
chains units. The resulting gels were rinsed and swollen back in
acetonitrile to measure their volumes. The contracted pieces of
gels were then immersed in an aqueous solution of ammonium
hexafluorophosphate to protonate the [c2]daisy chains, and they
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ities in the network with size larger than the investigated q range;
i.e., larger than 40 nm.
a)
d)
e)
f)
8Gel1
Gel4A8MGel2
1
2
3
4
5
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7
8
Gel4BM
120
110
100
90
The work reported here shows the integration of pH switchable
[c2]daisy chain rotaxanes in “clicked” polymer networks. In this
topology, the resulting chemical gels can be swollen by 20 times
of their volume and produce cyclic muscle-like contraction and
expansion upon protonation and deprotonation. Such a macro-
scopic response was not accessible to date by using this type of
bistable [c2]daisy chain, neither by incorporating them in linear
polymers31,33,35,42 or in physical gels which could not sustain their
topological integrity upon mechanical work.34 Importantly, HR-
MAS 1H NMR in the gel state and control experiments performed
by removing the triazolium station in the [c2]daisy chain show
that the macroscopic change in volume is directly related to the
actuation of the mechanical bond at the molecular level. SANS
experiments confirmed this relationship by showing a 50% in-
crease of the linear mass density of the polymer chains in the
network upon contraction, while their cross-section remains con-
stant. The understanding of such a large amplification of the
conformational dynamic of polymer chains in network topologies
up to the macroscopic response of a gel is of high interest for the
implementation of molecular machines in materials science.
80
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Initial
Base
Acid
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Gel4A
Gel4B
b)
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 Gel4AM Extended
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Initial Base Acid Base Acid Base Acid
Figure 3. a-c) Pictures of methylated gels 4GelAM (a), after treat-
ment with NaOH (b) and after being subsequently treated with
NH4PF6 (c); d) Variation of volume Vrel upon contrac-
tion/expansion for methylated gels 4GelAM and 4GelBM; e) Variation
of volume Vrel upon contraction/expansion for non-methylated
gels 4GelA and 4GelB; f) Variation of volume Vrel upon three con-
b)
 Gel4AM Contracted
 Gel4AM Extended
traction/expansion cycles for methylated gels 4GelAM
.
The Guinier approximation for elongated objects yields a cross-
section radius of gyration of Rc = 0.47 and 0.50 ± 0.03 nm for
extended and contracted Gel4AM, respectively. In the mid q re-
gion, both curves increase proportionally to q-1 but, their intensity
is markedly different (Figure 4b). This observation is extremely
important since the signal intensity is only proportional to the
linear mass density ML of the polymer chain in this region (see
more details in SI section 5), and a higher intensity corresponds to
a higher linear mass density. Hence, the contracted system has a
higher linear mass density as compared to the extended one and
this clearly proves the contraction of the [c2]daisy chains compos-
ing the polymer network. This behavior is in full agreement with
previously reported literature on single chain polymers.33 The data
in this regime of q range can be fitted satisfactorily using a rigid-
rod model41 giving a ML = 325 g mol-1 nm-1 for the extended
polymer, and a new ML = 480 g mol-1 nm-1 for the contracted one
(+50%) (Figure 4b). This compares well with the theoretical
linear mass densities ML of 365 and 456 g mol-1 nm-1 for the
extended and the contracted, respectively (see SI section 5). For
the present system we were not able to discriminate the persis-
tence length of the two polymers because the transition between
the q-1 rod-like and the q-2 coil-like characteristic regimes, ob-
served for polymer chains,41 is here masked by the q-3 upturn at
low q. However, the transition between the q-1 and the q-3 regimes
occurring at q* can give a lower bound for the persistence length
that is lP  1.91/q*~13 ± 2 nm.41 The reason for which, at low q,
the signal rises proportionally to q-3 is the presence of heterogene-
Figure 4. a) SANS curves for Gel4AM contracted and Gel4AM
extended polymers. The high q data have been fitted by a Guinier
expression for the form factor of the polymer cross-section (see
continuous line and eq 4 in SI). The arrow indicates the transition
q* between the q-1 and q-3 domains. b) Enlargement of the q-range
demonstrating the difference in linear mass density between the
two polymers. The continuous lines represent the best fits of the
data using a rigid rod model (see SI, eq 5).
Synthetic protocols and characterization of compounds 2 and 3,
synthetic protocols for the formation of gels, procedures for the
contraction and extension experiments, SANS experimental pro-
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The authors declare no competing financial interests.
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This work was supported by the Agence Nationale pour la Re-
cherche (Grant number ANR-14-CE06-0021, fellowships to A.G.,
T.L. and G.M.) and the French Ministry of Research (fellowship
to A.G.). We wish to thank the Laboratory of Excellence for
Complex System Chemistry (LabEx CSC), the international cen-
ter for Frontier Research in Chemistry (icFRC), the Centre Na-
tional de la Recherche Scientifique (CNRS), the University of
Strasbourg (Unistra), the University of Paris Diderot (Sorbonne
Paris Cité) and the Institut Universitaire de France (N.G.) for
financial supports. We also thank the Laboratoire Léon Brilloin
(LLB, CEA, Saclay, France) for beamtime allocation and Dr.
Jean-Marc Strub for HRMS experiments.
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