Angewandte
Communications
Chemie
Molecular Machines
Hot Paper
Hierarchical Self-Assembly of Supramolecular Muscle-Like Fibers
Antoine Goujon+, Guangyan Du+, Emilie Moulin, Gad Fuks, Mounir Maaloum, Eric Buhler,
and Nicolas Giuseppone*
Abstract: An acid–base switchable [c2]daisy chain rotaxane
terminated with two 2,6-diacetylamino pyridine units has been
self-assembled with a bis(uracil) linker. The complementary
hydrogen-bond recognition patterns, together with lateral van
der Waals aggregations, result in the hierarchical formation of
unidimensional supramolecular polymers associated in bun-
dles of muscle-like fibers. Microscopic and scattering tech-
niques reveal that the mesoscopic structure of these bundles
depends on the extended or contracted states that the rotaxanes
show within individual polymer chains. The observed local
dynamics span over several length scales because of a combi-
nation of supramolecular and mechanical bonds. This work
illustrates the possibility to modify the hierarchical mesoscopic
structuring of large polymeric systems by the integrated
actuation of individual molecular machines.
motions and for their implementation in nanotechnology
and materials science.[4–6]
The group of Sauvage reported in 2000 the first bioin-
spired molecular muscle based on a bistable [c2]daisy chain
rotaxane incorporating two coordination stations on its axle.
This individual machine can achieve a switchable contraction/
extension motion with an amplitude of 1.8 nm depending on
the nature of the coordinated metal ions (CuI or ZnII).[7]
Although a number of rotaxane-based molecular muscles
was subsequently developed,[4a] their integration within
oligomers and polymers was only recently envisioned to
access artificial muscle-like materials.[8–11] Our group de-
scribed the first amplification of such molecular motions up to
the microscopic scale by linking thousands of bistable
rotaxanes within single-chain polymers.[10] However, in
order to build contractile materials from molecular machines,
further hierarchical organization of these single-chain poly-
mers into higher-scale structures is required, as myofibrils do
when laterally packed in bundles of muscular fibers.[4b] Here
we show that such a hierarchical structuring is possible within
a system incorporating supramolecular polymers based on
hydrogen-bonded [c2]daisy chain rotaxanes.
Multiple hydrogen-bond motifs have been described for
the preparation of supramolecular polymers because of their
high directionality, kinetic lability, and ease of synthesis.[12,13]
For instance, Lehn and co-workers implemented the
uracil:diaminopyridine recognition pattern to produce chiral
triple helices at the micrometric scale with liquid-crystalline
properties.[14,15] This heterocomplementary motif was then
used to produce a variety of morphologies (spheres, rods,
fibers) with a controlled degree of polymerization.[16]
Inspired by this hydrogen-bond polymerization unit, and
in a continuation of our initial reports,[10,17] we targeted the
synthesis of [c2]daisy chain rotaxane (12) with two diacetyl-
amino pyridine units as stoppers, and a complementary
ditopic linker (5) incorporating two 1-hexyluracil moieties
at its extremities (Scheme 1). Molecule 5 was also decorated
with branched alkyl chains to ensure solubility in organic
solvents[18,19] and to provide additional van der Waals
interactions for stabilizing the primary hydrogen-bond pat-
tern in a cooperative self-assembly. We also envisioned that
a mismatch between the polar character of rotaxane unit 12
(presence of crown ethers and ion pairs) and the apolar
character of linker 5, would favor lateral aggregations of the
single-chain supramolecular polymers driven by microphase
separation, thus reinforcing the cooperative mechanism of the
supramolecular polymerization.
B
iomolecular machines are key elements of living organisms
that perform essential functions such as replication, synthesis,
transport, and motion.[1,2] Among several important charac-
teristics, some of these processes involve the integration of
molecular machines in order to amplify their motions at
a scale larger than their typical individual size. A well-known
example is related to the collective molecular motion
produced in muscular tissues because of their hierarchical
organization. Within sarcomeres, the coordinated movements
of thousands of myosin heads lead to the gliding of thick
myosin filaments along thin actin filaments. By polymerizing
these contractile sarcomere units longitudinally in myofibrils
and by associating these myofibrils laterally in bundled fibers,
macroscopic motions can be reached.[3] It thus appears very
attractive to take inspiration from these biological processes
in order to design artificial systems displaying such a hier-
archical structuring for the amplification of molecular
[*] A. Goujon,[+] Dr. G. Du,[+] Dr. E. Moulin, Dr. G. Fuks,
Prof. Dr. M. Maaloum, Prof. Dr. N. Giuseppone
SAMS research group, Institut Charles Sadron, CNRS
University of Strasbourg
23 rue du Loess, BP 84047, 67034 Strasbourg Cedex 2 (France)
E-mail: giuseppone@unistra.fr
Prof. Dr. E. Buhler
Matire et Systmes Complexes (MSC) Laboratory
University of Paris Diderot—Paris VII, UMR 7057
Bâtiment Condorcet, 75205 Paris Cedex 13 (France)
Dr. G. Du[+]
Current address:
Southwest Petroleum University, No.8 Xindu Avenue
Bis(uracil) linker 5 was synthesized in five steps using
Sonogashira couplings as key reactions (Scheme 1a). Initially,
branched alkyl bromide 1, prepared from commercially
available 2-hexyl-1-decanol using the Appel reaction, was
Xindu District, Chengdu City, Sichuan Province 610500 (P.R. China)
[+] These authors contributed equally to the work.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2016, 55, 703 –707
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
703