Switchable neutral bistable rotaxanes

Article abstract of DOI:10.1021/ja048080k

Two switchable neutral bistable [2]rotaxanes have been synthesized, and their chemically induced mechanical switching has been studied in solution by ¹H NMR spectroscopy. One of the rotaxanes was prepared by a thermodynamically controlled slipp

Full text of DOI:10.1021/ja048080k

Published on Web 07/24/2004
Switchable Neutral Bistable Rotaxanes
Scott A. Vignon,† Thibaut Jarrosson,‡ Takahiro Iijima,† Hsian-Rong Tseng,†
Jeremy K. M. Sanders,*,‡ and J. Fraser Stoddart*,†
California NanoSystems Institute and Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095-1569, and UniVersity Chemical Laboratory,
UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
Received April 2, 2004; E-mail: stoddart@chem.ucla.edu; jkms@cam.ac.uk
The study of mechanically interlocked molecules1 in the form
of bistable [2]catenanes2 and [2]rotaxanes3 and their incorporation
Scheme 1 Final Steps in the Syntheses of the Switchable Bistable
Neutral [2]Rotaxanes 1 and 2a
ultimately into molecular machines4 and switches5 has been
advancing apace in the past few years. Examples of the attachment
of such molecules to surfaces have already been reported,6 and the
development of molecular actuators4,5,7 and switches4,5,8 in devices
is well underway. Not only is the structural complexity9 of these
molecules on the increase, but also the degree of control is becoming
more precise and predictable, whether light,3,10 electricity,11 or
chemicals9,12,13 are employed as the stimulus.
One of the most common methods of controlling switching in
bistable catenanes and rotaxanes is by ion-induced co-conforma-
tional changes.14 Sauvage15 has demonstrated remarkable control
of mechanical movement in exquisitely designed molecules using
Cu+/Cu2+ ions. While the UCLA group has employed variations
in pH (base/H+) to switch off and on the binding of dialkylam-
monium centers to crown ethers in bistable interlocked molecules,
the Cambridge group has used16 Li+ ions to control mechanical
motions in neutral pseudorotaxanes. Here, we report the template-
directed syntheses17 of two neutral bistable [2]rotaxanes and the
use of Li+ ions and [12]crown-4 to effect their switching between
two states in solution.
Utilizing the neutral donor-acceptor recognition system18 de-
veloped by the Cambridge group, two bistable [2]rotaxanes 1 and
2 were designed and made. They both incorporate 1/5DNP38C10
(Scheme 1), with its two π-electron rich 1,5-dioxynaphthalene ring
systems, around their dumbbell components, wherein pyromellitic
a Two different template-directed approaches were used in the final steps,
diimide (PmI) and naphtho-diimide (NpI) are present as the two
i.e., namely slippage (A) or dynamic bond formation (B).
competing π-electron deficient units for interaction with the crown
ether. Recognition is achieved by a combination19 of [π‚‚‚π]
generation Grubbs’ catalyst24c in CHCl3/MeOH (98:2), also in the
presence of an excess of LiBr. Both bistable [2]rotaxanes were fully
stacking and [C-H‚‚‚O] interactions and is expected20 to be
expressed in the form of a preference for 1/5DNP38C10 to encircle
characterized following purification by column chromatography on
the NpI unit in both rotaxanes. Addition of Li+ ions should lead to
SiO2 using CHCl3/EtOAc (8:2) as the eluent.
1H NMR spectroscopic studies were carried out in solution on
both 1 and 2 to ascertain if the preferred translational isomers were
the formation of strong 2:1 complexes with the translational isomers
in which the 1/5DNP38C10 ring encircles the PmI units, ensuring
that the initial preference is reversed. In the knowledge21 that [12]-
indeed those with the 1/5DNP38C10 ring encircling the NpI units.
crown-4 is a very strong sequestering agent for Li+ ions, we have
Within the limits of detection (∼3%) afforded by 1H NMR
all the ingredients at hand to render both 1 and 2 chemically
switchable in organic solvents.
The template-directed syntheses17 of 1 and 2 from their immedi-
spectroscopy, the anticipated outcome was observed for 1 in CD2-
Cl2/CD3COCD3 (4:1) and 2 in CDCl3/CD3OD (98:2). The 1H NMR
spectrum of 1 reveals (Figure 1a) that the signals corresponding to
ate precursors were both achieved (Supporting Information) under
the NpI unit (δ ) 8.17 ppm), and the PmI unit (δ ) 8.21 ppm) are
thermodynamic controls1 by slippage22 in CHCl3/MeOH (95:5)
present: no signals could be identified for free NpI and encircled
PmI units. This situation was reversed completely (Figure 1b) on
addition of at least 2 equiv of LiClO4 to the NMR tube: the signals
at δ 8.17 and 8.21 ppm disappear and a new pair25 of signals (δ )
8.66 and 8.69 ppm) corresponding to a free NpI unit are observed,
leaving the signal for the PmI unit now encircled by 1/5DNP38C10
at 60 °C of 1/5DNP38C10 over the appropriately chosen stoppers
on the preformed dumbbell compound in the presence of an excess
of LiBr and 2 by dynamic covalent chemistry,23 employing olefin
metathesis24 performed on appropriate half-dumbbell compounds
carrying terminal alkene functions for reaction with the second-
obscured by other aromatic signals somewhere in the region δ )
6.5-7.5 ppm. Qualitatively, the original H NMR spectrum was
† University of California, Los Angeles.
‡ University of Cambridge.
1
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J. AM. CHEM. SOC. 2004, 126, 9884-9885
10.1021/ja048080k CCC: $27.50 © 2004 American Chemical Society

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1
Figure 1. Partial H NMR spectrum recorded at 600 MHz of 1 in CD2-
Cl2/CD3COCD3 (80:20) at 273 K in (a) the neutral ground state, after (b)
addition of excess of LiClO4, and after (c) addition of excess of [12]crown-
4. Signals corresponding to the free and encircled PmI and NpI units are
labeled to show that the crown ether macrocycle moves from the NpI unit
to the PmI unit in the presence of Li+ ions and back to the NpI unit after
the Li+ ions are removed with [12]crown-4. Peak labels are defined in
Scheme 1.
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8.16 ppm) and the free PmI unit (δ ) 8.20 ppm) were slightly
shifted with respect to their chemical shifts in the starting spectrum
(Figure 1a). Similar results were obtained (Supporting Information)
for 2 using LiBr as the Li+ ion source and aqueous extraction of
the sample to return this chemical switch to its original state.
We have demonstrated the reversible chemical switching of a
pair of neutral bistable rotaxanes. Currently, we are investigating
the ability of similar compounds to undergo electrochemical
switching in solution with every intention of introducing them into
solid-state molecule electronic devices5,8 at a later date.
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(25) The signal corresponding to the NpI unit is separated into two signals
because of the asymmetry of the dumbbell, causing the two ends of the
NpI unit to be nonequivalent. This separation is not observed in the neutral
ground state because it is small, but the separation is much larger and
visible in the 2Li+ co-complex, probably as a result of the presence of
the Li+ ions in the dumbbell increasing the difference in environments
for the two ends of the NpI unit.
Acknowledgment. This research was supported by an equip-
ment grant (CHE-0116853) from the NSF, DARPA, EPSRC, and
the JSR Corporation.
Supporting Information Available: Experimental preparations for
2 and 3 and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
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