A clicked bistable [2]rotaxane

Article abstract of DOI:10.1021/ol070052u

A universal diazide-terminated polyether, incorporating tetrathiafulvalene (TTF, green) and 1,5-dioxynaphthalene (DNP, red) units, was prepared and subsequently employed in the template-directed synthesis of a switchable donor/acceptor [2]rotaxane. The tr

Full text of DOI:10.1021/ol070052u

ORGANIC
LETTERS
2007
Vol. 9, No. 7
1287-1290
A Clicked Bistable [2]Rotaxane
Ivan Aprahamian,† William R. Dichtel,†,‡ Taichi Ikeda,† James R. Heath,*,‡ 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, and DiVision of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
stoddart@chem.ucla.edu
Received January 8, 2007
ABSTRACT
A universal diazide-terminated polyether, incorporating tetrathiafulvalene (TTF, green) and 1,5-dioxynaphthalene (DNP, red) units, was prepared
and subsequently employed in the template-directed synthesis of a switchable donor/acceptor [2]rotaxane. The triazole rings (magenta), which
are introduced into the rotaxane during requisite click reactions, do not present themselves as competing recognition sites for the tetracationic
cyclophane (blue) as it is induced to switch between the TTF unit, when it becomes dicationic (green adorned with yellow extremities), and
the DNP unit.
Switchable donor/acceptor rotaxanes1 incorporating cyclo-
bis(paraquat-p-phenylene)2 (CBPQT4+) as their ring compo-
nent(s) have found3,4 applications in the fabrication of
Molecular Electronic Devices (MEDs) and as actuating
materials in NanoElectroMechanical Systems (NEMS). Until
recently, the synthesis of donor/acceptor catenanes and
rotaxanes5 has involved6 the “clipping” of the π-accepting
CBPQT4+ ring, using appropriate acyclic precursors around
rings and dumbbells containing π-donating units. Since the
CBPQT4+ ring in these donor/acceptor catenanes and rotax-
anes is destroyed by reducing agents, as well as by bases
and nucleophiles, the “clipping” reaction is almost always
the final step in their synthesis. In practice, these constraints
put severe limitations not only on yields, but also on the
structural complexity of the mechanically interlocked com-
pounds that can be produced.
† University of California, Los Angeles.
‡ California Institute of Technology.
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10.1021/ol070052u CCC: $37.00
© 2007 American Chemical Society
Published on Web 02/28/2007

functional group tolerance, together with the mild reaction
conditions associated with “click” chemistry,8 to overcome
these limitations, and results in the efficient and convergent
preparation of [2]-, [3]-, and [4]rotaxanes, as well as of [2]-
catenanes. This synthetic strategy relies upon the formation
of pseudorotaxanes, wherein the π-electron-deficient CBPQT4+
ring threads linear molecules containing π-electron-rich recog-
nition units and terminated by azide and/or alkyne functions
which can undergo Cu(I)-catalyzed Huisgen9 1,3-dipolar
cycloadditions,10 forming rotaxanes and catenanes incorpo-
rating 1,2,3-triazole units. Herein, we report the template-
directed synthesis11 (Scheme 1) of a redox-switchable
Scheme 1. Synthesis of the Bistable [2]Rotaxane 1‚4PF6
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[2]rotaxane 1‚4PF6, comprised of a CBPQT4+ ring, which can
be electrochemically induced to move between tetrathiaful-
valene (TTF) and 1,5-dioxynaphthalene (DNP) recognition
units12 located along its dumbbell component. This bistable
compound was characterized by (i) electrospray ionization
mass spectrometry (ESI-MS) and (ii) 1H NMR spectroscopy,
and its switching behavior was demonstrated by using (iii)
cyclic voltammetry (CV), (iv) differential pulse voltammetry
(DPV), and (v) UV-vis spectroelectrochemistry (SEC).
The acyclic polyether 4, incorporating both TTF and DNP
units, was prepared in 60% yield by the alkylation (K2CO3/
DMAP/18C6/CH2Cl2) of the known13 THP-protected DNP
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Org. Lett., Vol. 9, No. 7, 2007

derivative 2 with the TTF-containing monotosylate12d 3,
followed by deprotection (HCl/MeOH/CH2Cl2). Tosylation
(TsCl/DMAP/NEt3/CH2Cl2) of 4 afforded a ditosylate, which,
on reaction with NaN3 in DMF at 80 °C for 1 day, gave 5
in 85% yield. When the diazide 5 was mixed with 1.1 equiv
of CBPQT‚4PF6 in DMF, an intense green solution was
obtained immediately, indicating the formation of [5 ⊂
CBPQT]‚4PF6. This pseudorotaxane was stirred for 2 days
in the presence of (i) 2.0 equivs of the propargyl ether
functionalized stopper7a 6 and (ii) a catalytic amount of
CuSO4‚5H2O and ascorbic acid as an in situ reductant.
Chromatographic purification (SiO2: 1% w/v NH4PF6/
Me2CO) of the crude product from the reaction mixture
afforded 1‚4PF6 as a green solid in 60% yield. The ESI-MS
of this solid revealed peaks at m/z 1294, 814, and 574,
corresponding to the loss of two, three, and four PF6- anions,
respectively, from the bistable [2]rotaxane, described by the
structural formula 1‚4PF6 in Scheme 1.
suggesting that the presence of the two triazole rings does
not affect either the thermodynamics or the kinetics of the
switching process. The first oxidation peak for the TTF unit,
which is observed15 around +0.50 V for “free” TTF, was
hardly detectable (Figure 1) in the anodic scan. DPV analysis
revealed (Figure 2a) a small peak at +0.44 V. These
The 1H NMR spectrum of 1‚4PF6 in CD3COCD3 indicated
the presence of predominantly (>95%) a single translational
isomer in solutionsnamely, the one in which the CBPQT4+
ring encircles the TTF unit.10e This assignment follows from
the observation that (i) the signals for the DNP protons are
all to be found at low field and (ii) the fact that no
correlations are observed in the COSY spectrum involving
high-field DNP proton resonancessa characteristic spectro-
scopic signature12 when the DNP unit is encircled by the
CBPQT4+ ring. This observation indicates that the presence
of triazole rings in 1‚4PF6 does not influence the isomeric
distribution between the only two translational isomers that
are populated to any extent. The NOESY spectrum shows
no correlations between the triazole ring proton and the
CBPQT4+ ring protons, indicating that the 1,2,3-triazole rings
do not act as recognition units toward the CBPQT4+ ring.14
The electrochemical behavior of 1‚4PF6, as indicated
(Figure 1) by CV measurements, is similar to those previ-
ously reported12d,g for TTF/DNP two-station [2]rotaxanes,
Figure 2. DPV results of the bistable [2]rotaxane 1‚4PF6: (a)
oxidation region (E > 0 V) and (b) reduction region (E < 0 V).
All the data were recorded in argon-purged MeCN at 298 K.
observations show that the existence of “free” TTF is
associated with a very small amount of the minor transla-
tional isomer of 1‚4PF6 at equilibrium, supporting the
conclusions reached by NMR spectroscopy. The first sub-
stantial TTF oxidation peak is shifted to higher potential and
overlaps (Figure 1) with the second oxidation peak. In the
DPV analysis, the peak maxima for the first and second
1
(14) The H NMR spectrum shows characteristic resonances that arise
from the constitutionally heterotopic methine protons of the CBPQT4+
-
encircled TTF units: pairs of signals resonate at δ 6.14, 6.03 (2H) and
5.87 ppm. These resonances originate from the cis and trans isomers of
such a TTF unit encircled by a CBPQT4+ ring. See: (a) Kreitsberga, Y.
N.; Liepin’sh, EÄ .; Mazheika, I. B.; Neilands, O. Y. Zh. Org. Khim. 1986,
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Fabio, A.; Gandolfi, M. T.; Mattersteig, G.; Nielsen, M. B.; Raymo, F. M.;
Rowan, S. J.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. J. Org. Chem.
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Chem. 2000, 65, 1924-1936. (b) Ballardini, R.; Balzani, V.; Di Fabio, A.;
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Figure 1. Cyclic voltammetry of the bistable [2]rotaxane 1‚4PF6.
The cyclic voltammogram was recorded at 200 mV s-1 in argon-
purged MeCN. The concentrations of the 1‚4PF6 sample and the
supporting electrolyte TBA‚PF6 were 0.5 mM and 0.1 M, respec-
tively.
Org. Lett., Vol. 9, No. 7, 2007
1289

oxidations were to be found (Figure 2a) at +0.66 and
+0.73 V, respectively. The first oxidation potential of the
CBPQT4+-encircled TTF unit is shifted to higher potential
because of the lowering16 of the HOMO energy level caused
by the encapsulation by the CBPQT4+ ring. The second oxi-
dation potential (DPV peak maximum observed15 at +0.73 V)
of the TTF unit in 1‚4PF6 is not influenced by the CBPQT4+
ring, indicating that it has already moved to the DNP unit.
In the cathodic scan, two well-separated CV peaks are
observed, corresponding to the reduction of the oxidized
TTF•+ radical cation and the TTF2+ dication. The positions
of these peaks are consistent with those for “free” TTF units,
an observation which means that the CBPQT4+ ring remains
on the DNP unit during the process.
In the reduction region phase of the CV, a typical12e,2b,17
splitting of the first two-electron reduction peak and no
splitting of the second two-electron reduction peak of the
CBPQT4+ ring are observed (Figure 1). DPV analysis reveals
(Figure 2b) three clear peaks at -0.28, -0.39, and -0.78 V
(vs SCE).
Figure 3. The change in UV-vis spectra during the SEC
measurements. The applied potential changes from E ) 0 to 1.22 V
and then back to E ) 0 V. All the data were recorded at 0.25 mV
s-1 in argon-purged MeCN. The concentrations of the sample and
supporting electrolyte were 0.5 mM and 0.1 M, respectively.
The splitting of the first reduction peak can be attributed
to (i) the electronic mixing from the charge transfer (CT)
interaction16,17 between TTF and CBPQT4+ and/or (ii) the
generation of asymmetry by the back-folding2b,12g,15a of the
aromatic rings in the stoppers. The absence of any splitting
in the second peak indicates that the CT interaction between
TTF and CBPQT2+/•+ is much weaker.
At E ) 1.22 V, a small absorption peak at around 530 nm
associated with the CT band between DNP and CBPQT4+
can be detected. When the applied potential is switched off
(E ) 0 V), the spectrum gradually changes back to its
original form, verifying the full reversibility of the electro-
chemical reaction.
The mechanical switching in 14+ has also been investigated
(Figure 3) by SEC. The band centered around 840 nm in
the ground state (E ) 0 V) originates2b,12e,15a,18 from the CT
interaction between TTF and CBPQT4+. The absorption
spectrum starts to change around +0.50 V (vs Ag wire), and
the peaks (λmax ) 445 and 595 nm) corresponding to the
TTF•+ absorption increase until the applied potential reaches
+0.80 V. This process is accompanied by the bleaching of
the CT band around 840 nm, an observation which indicates
that the CBPQT4+ ring has moved away from the TTF•+
radical cation to the DNP unit. When the applied potential
is increased above +0.80 V, the absorption band of the TTF•+
radical cation starts to bleach and a new peak (λmax ) 380
nm), assignable to the TTF2+ dication, emerges (Figure 3).
It is now clear that a “threading-followed-by-stoppering”
synthetic strategy for the making of redox switchable donor/
acceptor [2]rotaxanes by click chemistry is a viable one, and
promises to gain in popularity during the furtherance of a
much wider range of applications for these mechanically
operated molecular switches.
Acknowledgment. This work was supported by the
Microelectronics Advanced Research Corporation (MARCO)
and its focus center on Functional Engineered NanoArchi-
tectonics (FENA) and the Defense Advanced Research
Projects Agency (DARPA) and the center for Nanoscale
Innovative Defense (CNID).
Supporting Information Available: Experimental details
and spectral characterization data of all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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