The foundation of a light driven molecular muscle based on stilbene and α-cyclodextrin

Article abstract of DOI:10.1039/b809014a

The rotaxane 3(E,E) serves as the basis of a light driven molecular muscle, where reversible photoisomerisation of the stilbene units causes the cyclodextrins to move off and on the stilbene units, contracting and extending the distance between the blocking groups. The Royal Society of Chemistry.

Full text of DOI:10.1039/b809014a

Chemical Communications
www.rsc.org/chemcomm
Number 34 | 14 September 2008 | Pages 3941–4080
ISSN 1359-7345
COMMUNICATION
Ryan E. Dawson, Stephen F. Lincoln and Christopher J. Easton
The foundation of a light driven molecular muscle based on stilbene and
α-cyclodextrin

COMMUNICATION
www.rsc.org/chemcomm | ChemComm
The foundation of a light driven molecular muscle based on stilbene
and a-cyclodextrinw
Ryan E. Dawson,a Stephen F. Lincolnb and Christopher J. Easton*a
Received (in Cambridge, UK) 28th May 2008, Accepted 3rd July 2008
First published as an Advance Article on the web 17th July 2008
DOI: 10.1039/b809014a
The rotaxane 3(E,E) serves as the basis of a light driven molecular
muscle, where reversible photoisomerisation of the stilbene units
causes the cyclodextrins to move off and on the stilbene units,
contracting and extending the distance between the blocking groups.
illustrated in Scheme 1. Kaneda and co-workers18 have
reported the synthesis of oligomers of an azobenzene based
rotaxane, where the movement resulting from photoisomerisa-
tion was not demonstrated but only predicted using molecular
simulations. In our case, each of the extended, intermediate
and contracted states of the rotaxane 3(E,E), 3(E,Z) and
3(Z,Z) has been isolated and fully characterised using 2D
NMR spectroscopy, thus directly establishing the changes in
geometry and function akin to that of a muscle fibre.
In nature, biological machines perform tasks that enable life to
proceed.1–4 For example, the contraction and expansion of
muscle fibres, caused by the simultaneous sliding of the stacked
filaments of the myosin and actin upon chemical stimulation by
ATP hydrolysis, enables our controlled movement.3,4 In the
pursuit of nanoscale machines, chemists are inspired to mimic
nature, to produce synthetic structures that perform tasks at
our discretion.5–9 Recently, examples of molecular mimics of
muscle fibres have appeared in the literature.10–18 Methods for
synthesising these have centred on the formation of [2]-
rotaxanes, which are well-suited for the construction of mole-
cular machines as the relative motions of their mechanically
interlocked components can be controlled.19,20 Previous exam-
ples of molecular muscle fibre mimics have primarily depended
on transition metal complexation10–15 and redox chemistry16,17
to operate. In this communication we describe the use of
a-cyclodextrin (aCD) and a stilbene in the construction of a
reversible molecular mimic of a muscle fibre, that instead relies
on hydrophobic forces and size constraints, and photochemical
isomerisation of the stilbene units, to function.
The trans,trans-isomer of the muscle 3(E,E) was synthesised as
shown in Scheme 2 (see ESI for detailsw). In the final step, the
hermaphroditic cyclodextrin 1 reacted with the dimethoxychloro-
triazine 2 to give the rotaxane 3(E,E) in 30% yield, after
purification using HPLC. The symmetrical dimeric nature of
this material was determined from the ESI mass spectrum and
1
the simplicity of the 1D H and 13C NMR spectra, while the
trans-geometry was confirmed by the coupling constant of
16.5 Hz between the olefinic protons in the 1H NMR spectrum.
Irradiation at 350 nm of an aqueous solution of the rotaxane
3(E,E) gave the cis,trans- and cis,cis-isomers 3(E,Z) and 3(Z,Z),
and recovered starting material, in yields of 21, 11 and 36%,
respectively, again after purification through HPLC (Scheme 1).
The 1D 1H and 13C NMR spectra of the cis,trans-isomer 3(E,Z)
In aqueous solutions, aCD forms host–guest complexes with
hydrophobic guests having the appropriate size to fit inside the
CD annulus.21–25 Hermaphroditic CDs, where the guest is
bonded to the CD, form complexes with a doubly threaded
topology, termed Janus complexes. Capping these with bulky
blocking groups, to prevent dissociation of the components,
affords [2]-rotaxanes called c2-daisy chains.10,26–29 trans-
Stilbenes are readily complexed by aCD, much more so than
the cis-isomers, and the complexes have been capped to give
rotaxanes.30–33 Accordingly, we report the construction of the
molecular muscle fibre mimic 3 having a ditopic axle compris-
ing both a stilbene and an aliphatic group for CD binding.
Interconversion between the trans- and cis-stilbene isomers
causes the CDs to move between the binding sites, expanding
and contracting the distance between the capping groups, as
a ARC Centre of Excellence for Free Radical Chemistry and
Biotechnology, Research School of Chemistry, The Australian
National University, Canberra, ACT 0200, Australia.
E-mail: easton@rsc.anu.edu.au; Fax: +61 2 6125 8114
b School of Chemistry and Physics, The University of Adelaide,
Adelaide, SA 5005, Australia
w Electronic supplementary information (ESI) available: Synthetic
details, compound characterisation, Fig. S1 and S2. See DOI:
10.1039/b809014a
Scheme 1 Contraction and expansion of the molecular muscle 3.
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3980 | Chem. Commun., 2008, 3980–3982

by analysing each compound using 2D NMR spectroscopy. A
section of the ROESY contour plot of the trans,trans-rotaxane
3(E,E) is shown in Fig. 1. The resonances of the stilbene
protons designated B, C and D show NOE interactions with
the signals of the CD-H5 protons, while those of the stilbene
protons E and F show interactions with the resonances of the
CD-H3 protons. The signal of the stilbene protons A shows
relatively weak interactions with what could be the resonances
of the CD-H6 protons of either the same or the other mono-
meric unit of the dimer. In any event, the other interactions
show that the stilbenes are included within the CD cavities and
the structure of the rotaxane 3(E,E) is as illustrated in Fig. 1.
For the cis,trans-rotaxane 3(E,Z), the corresponding section of
the ROESY spectrum is shown in Fig. 2. All of the resonances of
the trans-stilbene moiety show NOE interactions with CD proton
signals, whereas the resonances of protons B, C and D of the cis-
stilbene group do not. cis-Stilbene protons A, E and F interact
with CD protons and, in the absence of interactions involving
protons B, C and D, this indicates that cis-stilbene protons A
interact with protons of the CD of the same monomeric unit,
while E and F interact with protons of the other CD. The structure
of the rotaxane 3(E,Z) is therefore as illustrated in Fig. 2, where
the trans-stilbene is included within a CD and the other CD is
located over the far end of the cis-stilbene and the propyl group.
NOE interactions between CD and propyl group protons are
indistinguishable from those between different CD protons.
With the cis,cis-rotaxane 3(Z,Z), for which the most relevant
section of the ROESY spectrum is shown in Fig. S2 of the ESI,w
Scheme 2 Synthesis of the hermaphroditic [2]-rotaxane 3(E,E).
show two sets of stilbene resonances, including two pairs of
olefinic proton resonances with coupling constants of 12 and
16.5 Hz, while the corresponding spectra of the cis,cis-rotaxane
3(Z,Z) are again characteristic of a symmetric dimer but in this
case the 1H NMR coupling constant between the olefinic protons
is 12.5 Hz. In aqueous solution, the ultraviolet–visible spectra of
the rotaxanes 3(E,E) and 3(Z,Z) show lmax 334 nm (emax 105 100)
and lmax 305 nm (emax 10 300), typical of the trans- and cis-
stilbene moieties, respectively. For the cis,trans-isomer 3(E,Z),
lmax 330 nm (emax 40 000) with a shoulder at lower wavelength is
also characteristic.
When the irradiation of the trans,trans-rotaxane 3(E,E) at
350 nm was monitored using ultraviolet–visible spectroscopy and
HPLC, some decomposition was observed but a photostationary
mixture of the isomers 3(E,E), 3(E,Z) and 3(Z,Z) formed, in the
ratio ca. 2 : 2 : 1. This wavelength was chosen based on the
ultraviolet–visible spectra, to selectively excite the trans-stilbene
moieties. Subsequent irradiation with 254 nm light, to target cis-
stilbene residues, afforded a photostationary mixture of the
isomers 3(E,E) and 3(E,Z) in the ratio ca. 6 : 1. These ratios
are typical of stilbene photoisomerisations34 and do not appear
to be substantially affected by the presence of a cyclodextrin. The
reversibility of the photoinduced interconversion between the
rotaxanes 3(E,E), 3(E,Z) and 3(Z,Z) was studied by preparing a
6.4 Â 10À6 M solution of the trans,trans-isomer 3(E,E) in H2O,
and sequentially and repeatedly irradiating it with light of
wavelength 350 nm for 3 min, followed by light of wavelength
254 nm for 3 min. Under these conditions, approximately 2–3%
decomposition accompanied each cycle (see Fig. S1 in the ESIw).
The structural changes associated with interconversion be-
tween the isomers 3(E,E), 3(E,Z) and 3(Z,Z) were examined
Fig. 1 A section of the 500 MHz ROESY NMR spectrum of the
hermaphroditic [2]-rotaxane 3(E,E) in D2O at 25 1C, of the region
where crosspeaks are observed between aromatic and cyclodextrin
proton signals.
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Chem. Commun., 2008, 3980–3982 | 3981

off the stilbene shortens the length of the rotaxane 3 and the
distance between the blocking groups, so the isomers 3(E,E),
3(E,Z) and 3(Z,Z) represent the extended, intermediate and
contracted states of a reversible, light driven molecular mimic
of a muscle fibre (Scheme 1).
The authors acknowledge support received for this work
from the Australian Research Council.
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3982 | Chem. Commun., 2008, 3980–3982
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