An acid-base-controllable [c2]daisy chain

Article abstract of DOI:10.1002/anie.200803036

(Figure Presented) The long and the short of it: A bistable molecular muscle, based on a doubly threaded Janus-type [2]rotaxane, is assembled using a template-directed self-assembly process. Under the influence of changes in the pH value, the two identical matching components glide along one another through terminal crown ether moieties, bringing about expansion and contraction of the molecule.

Full text of DOI:10.1002/anie.200803036

Communications
DOI: 10.1002/anie.200803036
Interlocked Molecules
An Acid–Base-Controllable [c2]Daisy Chain**
Jishan Wu, Ken Cham-Fai Leung, Diego Benítez, Ja-Young Han, Stuart J. Cantrill, Lei Fang, and
J. Fraser Stoddart*
Artificial molecular-based muscles, which can convert chem-
ical, electrochemical, or photochemical energy into mechan-
ical motion, have attracted attention as a result of their
potential for spawning nanoelectromechanical systems
(NEMS).[1] Several materials, such as conducting polymers,[2]
single-walled carbon nanotubes,[3] and dielectric elastomers,[4]
have been developed which exhibit muscle-like behavior at
the nanoscale level. However, all these systems rely upon the
response of a bulk substance, rather than on the behavior of
individual molecules. Recently, artificial muscles have been
designed on a molecular scale by taking advantage of
conformational changes exerted by electrochemical stim-
uli.[5–8] For example, oligothiophene-calix[4]arene copoly-
mers[5] and thiophene-fused annulenes[6] exhibit molecular
actuating behavior under redox control while crown-ether-
annelated oligothiophenes[7] and polyheterocyclic strands[8]
have ion-triggered muscle-like properties. Nanoscale molec-
ular motions, based on artificial molecular machines,[9] offer
alternative opportunities to design artificial muscle-like
materials.
Bistable rotaxanes are a promising component for such
materials, because relative linear mechanical translocation of
the ring and dumbbell components can be achieved upon
activation by chemical,[10] electrical,[11] or light irradiation[12]
stimuli. Converting such internal molecular motions into
practical actuating materials requires relocating these internal
motions into components, which, when taken together,
exhibit linear expansion and contraction. Sauvage et al.[13]
have reported a linear molecular muscle, based on a
transition-metal templated, doubly threaded rotaxane,
which can undergo the required expansion and contraction
motions on the addition or removal of metal ions. On the
other hand, we have reported a switchable, palindromically
constituted, doubly bistable [3]rotaxane[14] which can be self-
assembled onto gold-coated microcantilevers with disulfide-
terminated tethers emanating from its two rings in such a
manner that they can be moved towards and away from each
other under redox control. Controllable and reversible
deflection of the microcantilevers can be achieved when the
integrated system is exposed to the addition of oxidants or
reductants (or subjected to oxidizing or reducing electro-
chemical potentials).
Acid–base controllable, bistable, rotaxane-based molec-
ular shuttles have been reported[10b,c] in which a dibenzo[24]-
crown-8 (DB24C8) ring switches under acid–base control
between two different recognition sites on a dumbbell
component, where one of the sites is a secondary dialkylam-
+
monium (R2NH2 ) center and the other site, an N,N’-
dialkylated-4,4’-bipyridinium (Bpym2+) unit. Although this
switching has subsequently been employed[10d,e] in the design
of more complex machines, such as nanoscale elevators, the
mechanical motions are still only relative internal movements,
that is to say, there is no contraction/expansion in their overall
molecular dimensions. Herein, a bistable molecular architec-
ture incorporating a two-component [c2]daisy chain top-
ology[15] is designed and synthesized (Scheme 1), wherein two
mechanically interlocked filaments glide along one another
through the terminal crown ether rings and in which the end
of each filament is attached to bulky stoppers to prevent
[*] L. Fang, Prof. J. F. Stoddart
Department of Chemistry
Northwestern University
2145 Sheridan Road, Evanston, IL 60208-3113 (USA)
Fax: (+1)847-491-1009
E-mail: stoddart@northwestern.edu
Dr. J. Wu, Dr. K. C.-F. Leung, Dr. D. Benítez, Dr. J.-Y. Han,
Dr. S. J. Cantrill, Prof. J. F. Stoddart
The California NanoSystems Institute and Department of Chemistry
and Biochemistry, University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095-1569 (USA)
[**] We gratefully acknowledge the National Science Foundation (NSF)
for funding support.
Scheme 1. Schematic representation of the anticipated mechanical
motions within an acid–base-controlled molecular muscle system
based on a [c2]daisy chain.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803036.
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dethreading of the rings. In this Janus-type molecule, the two
DB24C8 rings move between the two different recognition
4bPF6 in overall yields exceeding 80%. The 1H NMR spectra
of 4aPF6 and 4bPF6 revealed that the molecules exist as
monomers in polar solvents, such as Me2SO. In less polar
solvents, such as CHCl3 and MeCN, the self-assembled dimers
(4a)2(PF6)2 and (4b)2(PF6)2 and higher oligomers constitute
the major components. The solvent-dependent aggregation
was further confirmed by electrospray ionization mass
spectrometry (ESI-MS) measurements of 4aPF6 in protic
and aprotic solvents. A peak at m/z 660.22, corresponding to
the single positively charged ion [MÀPF6]+, was detected
when a protic solvent, such as methanol, was used as the
solvent. On the contrary, a peak at m/z 660.40, corresponding
to the doubly positively charged ion [M2À2PF6]2+ (differ-
entiated by the isotopic mass profile), was observed when
using a less polar and aprotic solvent, such as MeCN,
indicating the formation of dimeric species (see Figure S1 in
the Supporting Information).[15b,16] Finally, solutions of (4a)2-
sites—R2NH2 and Bpym2+—under acid–base control, con-
+
ferring upon the molecule contraction and expansion, rem-
iniscent of the action of a muscle.
The template-directed synthesis[15b,c] of this kind of
[c2]daisy chain relies on the self-assembly of two benzylam-
monium-terminated DB24C8 rings to give a thermodynami-
cally stable, dimeric pseudorotaxane,[16] in solvents such as
MeCN. The incorporation of the second recognition site
(Bpym2+) equipped with a bulky stopper affords a kinetically
stable, [c2]daisy chain. Employing this synthetic strategy, both
1a(PF6)6 and 1b(PF6)6 have been prepared (Scheme 2) in
which the latter compound (1b(PF6)6) contains tetradeuter-
ated catechol rings.
The intermediate compounds 4aPF6 and 4bPF6 with
benzylic bromide groups self-assemble (Scheme 2) into the
dimeric superstructures (4a)2(PF6)2 and (4b)2(PF6)2, respec-
tively. Prior to this self-assembly step, the synthesis involves
the condensation of the formyl-substituted DB24C8 deriva-
tives 2a and 2b[15b] with methyl 4-(aminomethyl)benzoate to
give the corresponding imines. Simultaneous reduction
(LiAlH4) of the imine bonds and the ester functions gave
compounds 3a and 3b, respectively, with overall yields in
excess of 88%. The hydroxymethyl groups in 3a and 3b were
then brominated by reaction with PBr3 in CH2Cl2, followed by
protonation of the amino groups with HBr and counterion
exchange with NH4PF6 to afford the compounds 4aPF6 and
[10]
(PF6)2 and (4b)2(PF6)2 were treated with 5PF6 in MeCN/
CHCl3 (1:1 v/v) at 608C for 10 days, after which the products
were purified by column chromatography on silica, using
MeOH/2m aqueous NH4Cl/MeNO2 (7:2:1 v/v/v) as the eluent
before being subjected to counterion exchange with NH4PF6,
yielding 1a(PF6)6 and 1b(PF6)6 in 15–40% yields. The low
yields can be attributed to the propensity of the dimeric
superstructures, (4a)2(PF6)2 and (4b)2(PF6)2, to disassemble
during the stoppering reaction at elevated temperatures. High
resolution ESI mass spectra of 1a(PF6)6 and 1b(PF6)6 showed
major peaks at m/z 1230.996 and 1234.986, respectively, which
Scheme 2. The template-directed syntheses of the molecular muscles 1a(PF6)6 and 1b(PF6)6.
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can be assigned to doubly threaded species after the loss of
currents of the DB24C8 and the p–p interaction the catechol
rings, the signals (box in Figure 1) for Ha, Hb, and Hc are
shifted upfield. In addition, two sets of signals (see Figure 1,
red for the major isomer and pink for the minor isomer) for
the undeuterated catechol rings were indentified in the region
d = 6.1–7.0 ppm, indicating that there are two diastereoiso-
mers present in the solution. This phenomenon was also
detected in a previously reported[15b] self-assembling [c2]daisy
chain involving DB24C8 rings and -NH2+- center. On account
of the unsymmetrical substitution of the DB24C8 ring,
dimerization gives two C2-symmetrical (chiral) enantiomers
and a Ci symmetrical (meso) diastereoisomer (see Supporting
Information). The two sets of signals for the catechol ring
protons in the 1H NMR spectrum of 1b(PF6)6 can be assigned
to the racemic modification and the meso isomer.
two PF6À counterions (see Supporting Information). 1H NMR
and COSY-NMR spectroscopies were also used to character-
ize these doubly interlocked structures. Although compound
1
1a(PF6)6 shows well-resolved peaks in its H NMR spectrum
in CD3CN, the resonances originating from the aromatic
protons of both the simple and substituted rings overlap (see
Supporting Information). This drawback can be overcome by
1
recording the H NMR spectrum of the deuterated analogue
1b(PF6)6.
1
The H NMR spectrum of 1b(PF6)6 in CD3CN revealed
well-resolved peaks, all of which could be assigned (Fig-
ure 1a) with the help of COSY-NMR spectroscopic measure-
ments. The spectrum indicates that the DB24C8 rings exhibit
an overwhelming selectivity for the encirclement of the
+
R2NH2 recognition sites. The peaks arising from the
To initiate muscle-like behavior at the molecular level,
-OCH2CH2O- repeating unit appear in the region d = 3.2–
4.5 ppm and are not individually resolved on account of the
association of the DB24C8 rings with the -NH2+- centers. The
2.1 equivalents of phosphazene base P1–tBu[17] were added to
1
a solution of 1b(PF6)6 in CD3CN. The H NMR spectrum of
the resulting mixture revealed that the deprotonation of the
+
+
resonance signal for the NH2 protons was observed at d =
-NH2 - centers brought about the migration of the DB24C8
7.21 ppm. The protons on the uncomplexed Bypm2+ units
appear as clearly resolved peaks, resonating downfield in the
region d = 8.3–9.0 ppm. As a result of the diamagnetic ring
rings to the Bypm2+ recognition sites (Figure 1b). This
interpretation was supported by the following: 1) the peaks
for the Bypm2+ protons are shifted downfield on association
with the DB24C8 rings;
2) the peaks for the protons
(Hf and Hg) on the phenyl-
ene spacers are shifted
upfield because of the
mutually
p-overlapping
nature of the para-
phenylene ring systems in
the contracted geometry;
3) the separate peaks for
Ha, Hb, and Hc in the dia-
stereoisomers are less ani-
sochronous and give only
one set of accidentally
equivalent signals because
of the lack of p–p interac-
tions of the catechol ring;
4) the signal for proton Hh
is shifted upfield when the
DB24C8 rings migrate to
the Bypm2+ sites. Re-
+
protonation of the -NH2 -
centers following the addi-
tion of 2.1 equivalents of
CF3CO2H resulted in the
return of the DB24C8 rings
+
to the -NH2 - centers as
evidenced by the regener-
ation of the original
1H NMR spectrum (Fig-
ure 1c). Thus, 1H NMR
spectroscopic
measure-
ments demonstrate that
molecules 1a6+ and 1b6+
perform reversible muscle-
like contraction and expan-
Figure 1. 1H NMR spectra of compound 1b(PF6)6 in CD3CN: a) the original spectrum; b) after addition of
2.1 equivalents P1–tBu base; and c) further addition of 2.1 equivalents CF3CO2H.
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The versatile synthetic strategy we have employed to
make this acid–base controllable [c2]daisy chain is readily
amenable to modification with carefully chosen functional
groups, leading to the production of functionalized muscle
molecules, which could, for example, be incorporated into
liquid crystalline[21] or polymeric[22] systems. The material
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Received: June 24, 2008
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Published online: August 26, 2008
Keywords: molecular devices · nanostructures ·
.
NMR spectroscopy · rotaxanes · template synthesis
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[19] It should be pointed out that, after the deprotonation/reproto-
nation cycle, waste salts are produced in solution. Moreover,
À
À
some PF6 counterions are partially replaced by CF3CO2
.
Therefore, there are slight shifts in the NMR spectrum with
respect to the original spectrum. A similar phenomenon is seen
in the UV/Vis absorption spectra for the same reasons.
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