Functionally rigid bistable [2]rotaxanes

Article abstract of DOI:10.1021/ja0663529

Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are

Full text of DOI:10.1021/ja0663529

Published on Web 01/05/2007
Functionally Rigid Bistable [2]Rotaxanes
Sune Nygaard,† Ken C.-F. Leung,‡ Ivan Aprahamian,‡ Taichi Ikeda,‡ Sourav Saha,‡
Bo W. Laursen,§ Soo-Young Kim,‡ Stinne W. Hansen,† Paul C. Stein,†
Amar H. Flood,‡ J. Fraser Stoddart,*,‡ and Jan O. Jeppesen*,†
Contribution from the Department of Physics and Chemistry, UniVersity of Southern Denmark
(Odense UniVersity), CampusVej 55, DK-5230, Odense M, Denmark, California NanoSystems
Institute and Department of Chemistry and Biochemistry, The UniVersity of California, Los
Angeles,405 Hilgard AVenue, Los Angeles, California 90095, and Department of Chemistry,
UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100, KøbenhaVn Ø, Denmark
Received September 1, 2006; E-mail: joj@ifk.sdu.dk, stoddart@chem.ucla.edu
Abstract: Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a
nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized
and characterized in solution. Their dumbbell-shaped components are composed of polyether chains
interrupted along their lengths by (i) two π-electron-rich stationsstwo NP moieties or a MPTTF unit and a
NP moietyswith (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated
by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was
investigated as a means to simplify both molecular structure and switching function previously observed in
related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis-
(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-
free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the
tetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed
(> 95 < 5), surprisingly across a wide temperature range (198-323 K), meaning that the CBPQT4+ ring
component resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. As
a consequence of these two effects, the assignment of NMR and UV-vis data is more simplified as
compared to previous donor-acceptor bistable [2]rotaxanes. This development has not only allowed for
much better control over the position of the ring component in the ground state but also for control over the
location of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically
(1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines an
unambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. The
degenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMR
spectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from these
measurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+
ring from one station to another as compared with previous systemssa finding that is thought to be a
result of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and a
relatively unimpeded path between the two NP stations. This example augers well for exploiting rigidity
during the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excesses
of structural conformations that have characterized the first generations of molecular switches based on
the donor-acceptor recognition motif.
Introduction
dumbbell component contains two different recognition sites
(stations) for the macrocyclic ring to reside upon at equilibrium.
Since their first inception, bistable [2]rotaxanes,1 comprising
two different recognition sites, have attracted considerable
attention as molecular switches2 and as potential candidates for
applications in molecular electronics.3,4 Rotaxanes are mechani-
cally interlocked molecules, which consist of a linear rod
component, stoppered at each end by bulky substituents (dumb-
bell) and encircled by a macrocyclic ring component. Most
bistable [2]rotaxanes are constructed in such a way that the
This design creates two different translational isomers, which
are populated in a particular ratio depending mainly on the
relative strengths of the weak interactions stabilizing the ring
component when it encircles either one of the two stations.2i A
well-known class of bistable [2]rotaxanes have been designed
such that the dumbbell component incorporates the π-electron-
rich unit, tetrathiafulvalene2d,e,f (TTF), or one of its deriva-
tivesseither monopyrrolotetrathiafulvalene2b,c,i (MPTTF) or
bispyrrolotetrathiafulvalene3i,5 (BPTTF)stogether with a second,
much less π-electron-rich naphthalene (NP) moiety. The bista-
† University of Southern Denmark (Odense University).
‡ University of California, Los Angeles.
§ University of Copenhagen.
9
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J. AM. CHEM. SOC. 2007, 129, 960-970
10.1021/ja0663529 CCC: $37.00 © 2007 American Chemical Society

Functionally Rigid [2]Rotaxanes
A R T I C L E S
bility is achieved as a result of the dumbbell being encircled
by a π-electron-deficient tetracationic macrocyclic ring6s
cyclobis(paraquat-p-phenylene) (CBPQT4+). It has been shown7
that the CBPQT4+ ring interacts much more strongly with the
TTF unit than it does with the NP moiety. The reversible
oxidation of TTF8 to a mono- and dication, sequentially, can
be used to create a positive charge on the TTF unit in the bistable
[2]rotaxane, producing a Columbic charge-charge repulsion
between the oxidized TTF+•/2+ unit and the CBPQT4+ ring,
thereby inducing the ring component to move to and reside on
the only remaining π-electron-rich station of the dumbbell,
namely, the NP moiety. In an ideal system, the CBPQT4+ ring
would only encircle the strongest π-electron donor, that is the
TTF unit, thereby creating the possibility for perfect control
over the location of the tetracationic cyclophane (ring) using
electrical impulses. The use of the core TTF unit in the dumbbell
component, however, suffers from the major drawback that the
facile cis/trans isomerization9 of TTF creates a dumbbell that
exists as two inseparable isomers,10 thereby significantly
complicating the NMR spectroscopic assignments of both the
dumbbells and the [2]rotaxanes. To overcome this shortcoming,
the isomer-free MPTTF unit11 has already found widespread
use in the construction of bistable [2]rotaxanes, although it is
known experimentally2b,c,i that, in MPTTF-based bistable [2]-
rotaxanes, the CBPQT4+ ring does not encircle the MPTTF unit
exclusively but also resides on the NP station to some significant
extent. This observation has been attributed mainly to the fact
that π-π interactions, and hence the relative donor strengths
of the TTF and MPTTF stations, are not the only interactions
to consider when determining the translational isomeric distribu-
tion. Previous investigations and the analysis of experimental
data have revealed that a wide range of weak interactions, such
as [C-H · · · π] ones and hydrogen bonding, influence the
position the tetracationic ring occupies along the dumbbell
component in solution. Moreover, the use of polyethyleneglycols
(PEGs) as spacers between the stations in the bistable [2]-
rotaxanes introduces additional weak noncovalent interactions
between them and the CBPQT4+ ring, a situation that affects
the isomeric distribution in donor-acceptor bistable [2]rotaxanes
in a unpredictable manner. Such a situation leaves the molecular
structures and hence the desired switching properties less well-
defined. It follows that a greater level of structural control is
required to remedy these issues, and for this reason, we decided
to explore of introducing rigidity into the backbones of bistable
[2]rotaxanes.
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A R T I C L E S
Nygaard et al.
Figure 1. Structural formulas of the bistable MPTTF/NP-containing [2]rotaxane 1‚4PF6 and the degenerate NP/NP-containing [2]rotaxane 2‚4PF6.
structures are not at all well-defined. For instance, back-folding
phenomena13 can occur when the CBPQT4+ ring encircles the
MPTTF or NP station and the dumbbell component of the
bistable [2]rotaxane folds back in such a way that the ring can
interact in an alongside manner with the unoccupied station.
This back-folding could be prevented if the PEG linkers between
the MPTTF and NP units were replaced with a linear and rigid
linker. Such a modification might well favor a situation where
the π-electron-deficient CBPQT4+ ring encircles only the more
π-electron-rich MPTTF unit, to all intents and purposes, rather
than the NP moiety, since a rigid system would minimize the
influence of the weak interactions caused by the PEG linkers.
Thus, creating rigidified MPTTF- and NP-containing [2]-
rotaxanes constitutes the first step toward bistable [2]rotaxane
systems with precisely defined structures.
Notwithstanding the advantages of structural rigidity in the
context of molecular switches, the replacement of the PEG
spacers for rigid ones has obvious rewards in the design of
machine-like actuators.14 In addition, it is undoubtedly less
efficient to harvest3g,h the mechanical energy of the moVement
of the CBPQT4+ ring along the dumbbell of bistable [2]-
rotaxanes from station to station when using the PEG linkers
because some energy must inevitably be spent on the loss of
conformational freedom when the CBQPT4+ ring passes across
the PEG spacers. Moreover, the evaluation of such an entropic
contribution to the switching rates would be challenging to
determine both experimentally3f,i,4b,15 and computationally.16 For
physical understanding17 and mechanical applications,3g the
distance over which a force is exerted in order to perform work
would be much better defined with a linear rigid spacer present
in the dumbbell component of such rigidified bistable [2]-
rotaxanes. The mechanochemical analyses would be greatly
simplified and consequently offer a far greater opportunity to
harness the performance of molecular machines by a rational
design approach rather than being at the mercy of rampant
disorder when highly flexible bistable [2]rotaxanes are bound
to surfaces for technological applications.18 It is likely that the
linear rigid spacers have potential returns as functional units as
well as the structural ones. Linear-rigid bistable [2]rotaxanes
could herald a significant advance over previous systems by
providing the bistable [2]rotaxanes with well-defined conforma-
tions and motions.
In the first part of this paper, we report on the preparation of
two [2]rotaxanes (1‚4PF6 and 2‚4PF6, in Figure 1) having rigid
spacers in between their two stationssMPTTF and NP or
both NP.
We employed the arylethynyl and butadiynyl units as the rigid
spacers to act as bridges between the two stations on account
of the ease of their preparation and the fact that their lengths
are both similar to those of the PEG chains frequently used in
previous2 bistable [2]rotaxanes. In the present case, however,
the rigidity dictates inter-station distances of 1.5 and 1.0 nm.
The second part of the paper focuses on the characterization of
1‚4PF6 and 2‚4PF6 by (i) 1H NMR spectroscopy in order to (a)
investigate the effect of the spacer on the isomeric distribution
of the bistable [2]rotaxane 1‚4PF6, and (b) to analyze magnitude
of the shuttling barrier of the CBPQT4+ ring in the degenerate
[2]rotaxane 2‚4PF6, and by (ii) UV-vis absorption spectroscopy
and (iii) electrochemistry to determine the switching of the
MPTTF-NP [2]rotaxane 1‚4PF6 as a result of applying either
chemical or electrochemical stimuli.
Results and Discussions
Design and Synthetic Strategy. Retrosynthetic analysis of
the bistable MPTTF/NP-containing [2]rotaxane 1‚4PF6 and the
(17) Astumian, R. D. Sci. Am. 2001, 285, 5-64.
(18) (a) Chia, S.; Cao, J.; Stoddart, J. F.; Zink, J. I. Angew. Chem., Int. Ed.
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B.-D.; Celestre, P. C.; Chang, J. P.; Stoddart, J. F.; Ho, C.-M. Nano Lett.
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H. R.; Zink, J. I.; Stoddart, J. F. Chem. Eur. J. 2005, 11, 6846-6858. (e)
Nguyen, T. D.; Tseng, H.-R.; Celestre, P. C.; Flood, A. H.; Liu, Y.; Stoddart,
J. F.; Zink, J. I. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10029-10034.
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Functionally Rigid [2]Rotaxanes
A R T I C L E S
Scheme 1. Synthesis of the MPTTF-Containing (Green) Half-Dumbbell 10
degenerate NP/NP-containing [2]rotaxane 2‚4PF6 revealed that
a modular strategy, similar to those employed in the past,2,3i,5
would be the most sensible and efficient. This feeling is evident
from the fact that the two dumbbell compounds 18 and 19 can
be prepared from a combination of two building blocks, namely,
the MPTTF unit 10 and the NP unit 17. These two units can be
cross-coupled using a Pd-catalyzed Sonogashira19 type reaction,
which ensures that the linkage between the two π-electron-rich
moieties is a rigid and linear one. Common to preparation of
both 1‚4PF6 and 2‚4PF6 is the template-directed clipping reaction
at ultrahigh pressure to form the CBPQT4+ ring around the
respective dumbbell-shaped compounds 18 and 19. The modular
approach to the syntheses of the dumbbell compounds relies
on the progressive assembly of four different fragments, namely
(i) the iodine-terminated hydrophobic tetraarylmethane stopper20
9, which is employed as the stopper on (ii) the new MPTTF
unit 10, and (iii) the hydrophobic diisopropylaryl stopper 14,
which is subsequently used on (iv) the new NP unit 17.
Synthesis. The synthetic routes employed in the fabrication
of the rigid, two-station [2]rotaxane 1‚4PF6 as well as the
degenerate [2]rotaxane 2‚4PF6 are outlined in Schemes 1-4.
The preparation of the MPTTF-containing half-dumbbell 10
began with the reaction of dibromide21 3 with p-iodoaniline (4)
in the presence of K2CO3 in MeCN, affording the crude ring-
closed product 5, which was resuspended in anhydrous THF
and subsequently oxidized with activated MnO2 to yield the
desired thione building block 6 in an overall yield of 55% for
the two steps.
A cross-coupling reaction in neat (EtO)3P between the thione
building block 6 and the ketone building block22 7 gave the
MPTTF derivative 8 in 47% yield. The iodide-terminated
tetraarylmethane stopper20 9 was coupled to the MPTTF unit
8sfollowing the in situ deprotection of the cyanoethyl protection
group to form the S-nucleophile with 1.1 equiv of CsOH‚H2Os
to give the MPTTF-containing half-dumbbell 10 in 52% yield.
Similarly, a modular approach was employed in the synthesis
of half-dumbbell 17. Initially, the amino functionality of
5-aminonaphthalen-1-ol (11) was converted into an iodo group
to afford 5-iodonaphthalen-1-ol (12), employing the interme-
diacy of diazonium salt formation, in an acceptable 40% yield.
Second, 2,6-diisopropylphenol (13) was treated with 2-(2-
chloroethoxy)ethanol in the presence of K2CO3 and LiBr to give
the alcohol 14 in 65% yield.
Subsequently, the naphthol 12 was coupled with the alcohol
14 to afford 15 in 73% yield by employing a Mitsunobu reaction
in the presence of diisopropylazodicarboxylate (DIAD) and
triphenylphosphine (Ph3P). Finally, a Pd-catalyzed Sonogashira
cross-coupling reaction of the iodide 15 with trimethylsilyl
(TMS) acetylene gave the TMS-protected compound 16 in 90%
yield, and subsequent deprotection of the TMS group with
K2CO3/MeOH provided the acetylene-teminated NP half-
dumbbell compound 17 in 60% yield.
In order to accomplish the synthesis of the desired rigid
MPTTF/NP- and NP/NP-containing dumbbell compounds 18
and 19, two different coupling reactions were carried out. A
Pd-catalyzed cross-coupling reaction with the two-half-dumbbell
compounds 10 and 17, afforded the desired rigid MPTTF/NP-
containing dumbbell compound 18 in 94% yield, whereas the
rigid, degenerate NP/NP-containing dumbbell compound 19 was
obtained in 76% yield by a homo-coupling reaction of the NP-
(19) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 50,
4467-4470. (b) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara,
N. Synthesis 1980, 627-630.
(20) The iodide-stoppered compound 9 was prepared by a Finkelstein reaction
(see Supporting Information) of the bromide-stoppered compound 9a (see
Supporting Information), previously reported by Raehm, L.; Kern, J.-M.;
Sauvage, J.-P. Chem. Eur. J. 1999, 5, 3310-3317.
(21) (a) Jeppesen, J. O.; Takimiya, K.; Jensen, F.; Becher, J. Org. Lett. 1999, 1,
1291-1294. (b) Jeppesen, J. O.; Takimiya, K.; Jensen, F.; Brimert, T.;
Nielsen, K.; Thorup, N.; Becher, J. J. Org. Chem. 2000, 65, 5794-5805.
(22) Jia, C.; Zhang, D.; Guo, K.; Wan, S.; Zhu, D. Synthesis 2002, 2177-2182.
9
J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 963

A R T I C L E S
Nygaard et al.
Scheme 2. Synthesis of the NP-Containing (Red) Half-Dumbbell 17
Scheme 3. Synthesis of the Rigid MPTTF/NP-Containing Two-Station Dumbbell Compound 18 and the Degenerate NP/NP-Containing
Dumbbell Compound 19
containing half-dumbbell compound 17 using Cu(I)-promoted
Hay oxidative coupling conditions.23
pressure reaction in DMF from the dumbbell compound 19, with
the precursors 20‚2PF6 and 21. The [2]rotaxane 2‚4PF6 was
isolated as a purple solid in 8% yield after column chromatog-
raphy on silica gel using Me2CO/NH4PF6 (100:1 v/w) as the
eluent: once again, a [3]rotaxane was a byproduct in 6% yield.
Structural Characterization of [2]Rotaxanes by Mass
Spectrometry. The [2]rotaxane structures were characterized
unambiguously by electrospray ionization mass spectrometry
(ESI-MS). A set of peaks (Table 1) corresponding to the
consecutive loss of two, three, and four PF6- counterions were
observed for both [2]rotaxanes 1‚4PF6 and 2‚4PF6. The ESI-
MS of 1‚4PF6 reveals a doubly charged peak at m/z of 1075.0
Generally, the two dumbbell compounds 18 and 19 underwent
template-directed clipping reactions (Scheme 4) at high pres-
sure24 (10 kBar) with the dicationic precursor6a 20‚2PF6 and
1,4-bis(bromomethyl)benzene (21) to give the corresponding
[2]rotaxanes 1‚4PF6 and 2‚4PF6, respectively. The rigid MPTTF/
NP-containing [2]rotaxane 1‚4PF6 was obtained after counterion
exchange from the template-directed high-pressure reaction in
DMF, where the dumbbell compound 18 functions as a template
around which the CBPQT4+ ring forms from precursors
20‚2PF6 and 21. The [2]rotaxane 1‚4PF6 was isolated in 23%
yield as a green solid with a [3]rotaxane as a byproduct in 21%
yield by preparative thin-layer chromatography (PTLC) using
Me2CO/NH4PF6 (100:1 v/w) as the eluent. Similarly, the
degenerate NP/NP-containing [2]rotaxane 2‚4PF6 was obtained
following counterion exchange from the template-directed high-
([M - 2PF6]2+), a triply positively charged ([M - 3PF6]3+
)
signal at m/z of 668.3, and a quadruply positively charged ion
(23) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000,
39, 2632-2657.
(24) Kla¨rner, F.-G.; Wurche, F. J. Prakt. Chem. 2000, 7, 609-636.
9
964 J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007

Functionally Rigid [2]Rotaxanes
A R T I C L E S
Table 1. ESI-MS Data of the [2]Rotaxanes 1‚4PF6 and 2‚4PF6
m/z
a
+
+
+
[2]rotaxane
M+
A
[M
−
PF6]+
[M
−
2PF6]2
[M
−
3PF6]3
[M −
4PF6]4
1‚4PF6
2‚4PF6
(2440.7)
(1930.6)
(2295.7)
(1785.6)
1075.0
820.2
668.3
498.5
465.0
337.6
a Peaks corresponding to the molecular ions (M+) and the loss of one PF6 counterion ([M - PF6]+) were not observed. These molecular weights are
-
shown in parentheses however.
Figure 2. 1H NMR spectra (600 MHz, (CD3)2CO) of 1‚4PF6: (a) at 273 K, the solution is green; (b) after the addition of 2.0-2.5 equiv of the oxidant, the
solution becomes red; (c) after the addition of Zn powder, the solution is green.
Scheme 4. Synthesis of [2]Rotaxanes 1‚4PF6 and 2‚4PF6
([M - 4PF6]4+) at m/z of 465.0. Besides these ions originating
from the loss of PF6- counterions from the bistable [2]rotaxane,
a singly charged positively signal at m/z of 561.2 is also
observed, corresponding to the fragmentation of CBPQT4+ ring
into the 20‚2PF6 fragment minus a PF6- counterion. The same
pattern was observed in the ESI-MS of the degenerate [2]-
rotaxane 2‚4PF6, where doubly, triply, and quadruply positively
charged peaks were observed at m/z of 820.2, 498.5, and 337.6,
respectively.
Structural Characterization of [2]Rotaxanes by 1H NMR
Spectroscopy. An analysis of the 1H NMR spectrum of 1‚4PF6
recorded in (CD3)2CO at various temperatures (198-298 K),
reveals that the CBPQT4+ ring predominantly encircles the
MPTTF unit. This observation is quite surprising, as previous
bistable [2]rotaxanes in which the two recognition unitss
MPTTF and (D)NPsare connected by flexible PEG linkers have
been isolated as mixtures of the two possible translational
1
isomers at room temperature.2i An inspection of the H NMR
(25) This statement is only true, of course, if the equilibration between the two
translational isomers is slow on the 1H NMR timescale in (CD3)2CO at
room temperature. Since there was no evidence for a second set of signals
at low temperatures, we are prepared to accept that equilibration is fast on
the 1H NMR timescale at room temperature and, if it does slow down at
lower temperatures, then the extent to which the minor isomer is populated
must be miniscule. If we assume fast exchange between the signals at room
temperature for major and minor translational isomers, then the chemical
shifts we observe for the signals associated with the six NP protons should
differ in the bistable [2]rotaxane 1‚4PF6 from those observed for its
dumbbell precursor 18 where there is no CBPQT4+ ring present. A
comparison of the chemical shifts of these six protons in 18 (δ ) 6.94,
7.46, 7.50, 7.80, 8.01, 8.35 in CDCl3) with the chemical shifts for the same
six protons in 1‚4PF6 (δ ) 7.14, 7.57, 7.62, 7.89, 8.09, 8.39 in (CD3)2CO)
is good evidence, despite the running of the two spectra in different solvents,
for the NP unit in 1‚4PF6 being “free” from encirclement by CBPQT4+
rings at room temperature in solution.
spectrum (Figure 2a) reveals that all resonances are present only
once in the case of 1‚4PF6, whereas all the signals would be
present twice and in various ratios, if the isolated [2]rotaxane
1‚4PF6 had been a mixture of the two possible translational
isomers.25 To the limits of 1H NMR spectroscopy, this represents
a > 95 < 5 Boltzmann-weighted ratio, in which 1‚4PF6 exists
preferentially as a single translational isomer at room temper-
ature in (CD3)2CO.
The most diagnostic evidence in favor of the assignment that
1‚4PF6 exists as a single translational isomer predominantly,
and hence that the CBPQT4+ ring does not encircle the NP unit,
9
J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 965

A R T I C L E S
Nygaard et al.
can be found by observing the NP proton resonances H4 and
H8. Previous H NMR studies on NP containing [2]rotaxanes
as a consequence of the NP moiety being encircled by the
CBPQT4+ ring. These observations support the proposed
switching mechanism of the MPTTF/NP-containing [2]rotaxane.
The addition of a small amount of Zn powder followed by
vigorous shaking returns the MPTTF2+ to its neutral form. The
1
as well as those performed on the degenerate NP-based
[2]rotaxane 2‚4PF6 reveal that the encirclement by the CBPQT4+
ring of the NP moiety results in a very significant upfield shift
of the resonances of the NP H4 and H8 protons in the order of
∼5 ppm. This upfield shift is not observed in the case of
1‚4PF6 at either 198 K (see Supporting Information) or 298 K
as the NP H4 and H8 protons resonate in the aromatic part of
the spectrum supporting the interpretation that only a single-
translational isomer is found primarily in solution over the
sampled temperature range. This assignment is substantiated
further by the 1H DQ-COSY (double quantum correlation
spectroscopy) spectrum for 1‚4PF6, where all the scalar cou-
plings for the NP protons are clearly evident in the low-field
part of the spectrum (see Supporting Information). If an
appreciable amount of NP units had been encircled, an easily
recognizable scalar coupling from the low-field region to the
high-field region would have been present.
Cooling down the sample of 1‚4PF6 to 198 K in (CD3)2CO
does not significantly change the translational isomeric distribu-
tion. This inherent lack of temperature dependence has, to the
best of our knowledge, never been observed before in any TTF/
MPTTF-containing two-station [2]rotaxanes. Previously, it has
been noted2c,i that lowering the temperature has a considerable
influence on which station CBPQT4+ encircles. More specifi-
cally all previously isolated systems of this type have shown
that lowering the temperature favors the situation where the NP
station is encircled. It is thus evident that a temperature-
independent “all-or-nothing” two-station [2]rotaxane system
based on a TTF derivative has been created that presents the
appearance of a single isomer > 95 < 5 in solution.
1
resulting H NMR spectrum (Figure 2c) correlates well with
the original spectrum (Figure 2a) before oxidation. One notable
difference is the presence of two large signals at δ ) 7.50 and
7.04 ppm, corresponding to tris(p-bromophenyl)amine, the
reduced form of the oxidant. Thus, the chemically induced
switching of this bistable [2]rotaxane was shown to adopt the
switched state in which the NP station is encircled by the
CBPQT4+ ring upon oxidation and return to the stable ground
state upon reduction.
Dynamic 1H NMR Spectroscopy of Degenerate [2]Rotax-
ane. Since the switching rates of bistable [2]rotaxanes play an
important role in relation to their performance in device settings,
it was crucial for us to study the effect of rigidifying the linker
between the recognition sites on the rate of return from the
metastable state to the ground state in 2‚4PF6. For this purpose
variable temperature (VT) NMR spectroscopic studies were
performed on the structurally related and degenerate [2]rotaxane
2‚4PF6.
As both recognition sites are identical in 2‚4PF6, the shuttling
process will be between two isoenergetic co-conformers. In the
absence of the ring, the dumbbell component of this rotaxane
possesses an axis of symmetry normal to its long axis that bisects
it affording two equivalent halves. Localization of the CBPQT4+
ring on one of the two possible recognition sites breaks this
symmetry, resulting in two nonequivalent halves. If the ring
shuttles between the two sites rapidly on the 1H NMR timescale,
then it will appear as though the axial symmetry has been
restored as is the case for 2‚4PF6 at high temperatures (Figure
3). As the temperature is decreased, the shuttling rate slows
down and eventually, at a low enough temperature, the
symmetry will be broken and two sets of signals will be
observed for the protons of the two stoppers. One set corre-
sponds to the stopper close to the recognition site where the
CBPQT4+ ring resides and the other for the more distant stopper.
The temperature dependence of the 1H NMR spectrum (Figure
3) of 2‚4PF6 was studied by varying the temperature of the
sample from 270 down to 185 K.
Chemical Switching of the Bistable [2]Rotaxane Moni-
tored by 1H NMR Spectroscopy. The switching properties of
1‚4PF6 in solution have been invesitgated using 1H NMR
spectroscopy. Chemical switching can be achieved by oxidizing
the MPTTF derivative using tris(p-bromophenyl)ammonium
hexafluoroantimonate to its dicationic state (MPTTF2+) and then
reducing the MPTTF2+ dication back to its neutral state with
Zn powder.2e,f,i,3h
1
The H NMR spectrum (Figure 2a) of 1‚4PF6 in (CD3)2CO
was recorded at 273 K first of all for comparison and then the
MPTTF was oxidized to MPTTF2+ by adding 2.0-2.5 equiv
An estimation of the symmetry-averaging shuttling process
was obtained using the coalescence method28 and it was found29
1
of the oxidant, followed by running the H NMR spectrum
q
that the free energy of activation (∆Gc ) for this process is 9.6
(Figure 2b). As a result of the oxidation process, the color of
the solution changes from green to red as is expected from such
a system on account of the generation of the MPTTF dication.
( 0.1 kcal/mol at 199 K. This activation barrier is relatively
low compared with those obtained from previous studies2g on
similar degenerate [2]rotaxanes (∆Gcq ≈ 15.0 kcal/mol at ∼240
K), which have more flexible PEG linkers between the two NP
stations. The difference most likely stems from two factors,
namely, (i) the interaction between the NP units and the
CPBQT4+ ring in 2‚4PF6 is substantially weaker in this system
since the stabilizing intramolecular [C-H‚‚‚O] interactions are
severely curtailed by the absence of enough and appropriate
oxygen atoms and (ii) the rigid hydrocarbon linker does not
1
A comparison between the H NMR spectra26 of the neutral
and oxidized bistable [2]rotaxane reveals that the resonances
associated with the HTTF, HPyr, and HSCH2 protons are all shifted
to lower fields (δ ) 9.62, 8.80, and 3.88 ppm, respectively)
relative to the neutral system (δ ) 6.44, 7.37, and 3.31 ppm,
respectively). Previously investigations2i,27 have shown that this
behavior is entirely consistent with the formation of a MPTTF2+
dication. Moreover, the resonances associated with the NP
protons, H4 and H8, are shifted to a higher field (δ ) 3.28 ppm)
(28) In the coalescence method, the rate of exchange at the point of coalescence
is calculated from the separation, ∆υ, between the two peaks at minimum
exchange (low temperature) using the equation, kex ) (π∆υ)/x2. The rate
constant, kex, is then entered into the Eyring equation, ∆Gcq ) -RTc ln-
(kexh/kbTc), along with the coalescence temperature, Tc, in order to calculate
the free energy of activation, ∆Gcq. See Dynamic NMR Spectroscopy;
Sandstrom, J., Ed.; Academic Press: New York, 1982.
(26) The 1H NMR spectrum of oxidized 1‚4PF6 was recorded at various
temperatures and since the best spectrum was obtained at 265 K, this
spectrum was used as a comparison.
(27) Vignon, S. A.; Stoddart, J. F. Collect. Czech. Chem. Commun. 2005, 10,
1493-1576.
9
966 J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007

Functionally Rigid [2]Rotaxanes
A R T I C L E S
observed continuous absorption stretching from 335 nm to
approximately 530 nm in 18 might result from the internal
transition of the conjugated backbone, linking the MPTTF and
the NP stations. As expected, 19 has an intense absorption
situated around 370 nm arising from the two NP units in the
dumbbells backbone.
Figure 4. Comparison of the absorption spectra of 18 and 19 as well as
10 and 17. The spectra for 17 and 18 were recorded in MeCN at 298 K,
whereas those for 10 and 19 were recorded in Me2CO at 298 K.
(b) Rotaxanes. A comparison of the UV-vis spectra recorded
for 1‚4PF6 and 2‚4PF6 reveals, as expected, major differences.
A broad and relatively intense charge transfer (CT) band is
observed (Figure 5) at 810 nm for 1‚4PF6. This absorption band
reflects the encirclement of the MPTTF unit by the CBPQT4+
ring in 1‚4PF6, which gives the solution its green color. On the
other hand, the CT band in 2‚4PF6 is observed at 470 nm,
corresponding to the encirclement of the NP unit by the
CBPQT4+ ring, which gives the solution its red color. The
absorption spectrum of 1‚4PF6 in MeCN indicates that the CT
band is fully symmetric and centered around 810 nm, whereas
1
Figure 3. Section of the temperature-dependent H NMR spectrum (600
MHz, (CD3)2CO) of 2‚4PF6. The splitting of the resonances associated with
the methine protons of the isopropyl groups was used to probe the shuttling
q
process and the coalescence data were used to calculate ∆Gc .
interact with the CBPQT4+ ring in the same manner as do the
PEG chains. It is expected that entropic and enthalpic contribu-
tions to the barrier will be simpler to interpret in the case of
the rigid [2]rotaxanes and will lead us to the view that the
shuttling process has less friction and consequently is faster.
This investigation shows that the shuttling rates in [2]rotaxanes
can be tuned by using appropriate linkers between the recogni-
tion sites.
Photophysical Properties. In order to understand the be-
havior and physical properties of the ground state systems, as
well as the oxidized systems, a series of UV-visible (UV-
vis) spectroscopic investigations have been carried out.
(a) Half-Dumbbells and Dumbbells. The two half-dumbbell
compounds 10 and 17 serve as reference compounds for the
chromophore units present in the full dumbbell compounds 18
and 19. The UV-vis spectrum (Figure 4) of the MPTTF-
containing half-dumbbell derivative 10 can be attributed to the
absorption of the MPTTF unit, whereas in 17, the spectrum
results from the incorporated NP unit. The UV-vis spectrum
of the two-station dumbbell compound 18 is very similar to
the sum of the spectra of the model compounds 10 and 17, as
indicated by the broad absorption around 335 nm. Notably, the
Figure 5. Comparison of the absorption spectra of 1‚4PF6 and 2‚4PF6
recorded in MeCN at 298 K.
there is no CT band observed around 470 nm as in 2‚4PF6.
Such symmetric CT bands have also been observed2c in
previously studied two-station (pseudo) [2]rotaxanes. In the case
of two-station MPTTF-containing [2]rotaxanes, two different
observations regarding the UV-vis behavior have been made
in the past. First of all, the CT band has not been fully symmetric
around the 800 nm band originating from the MPTTF-CBPQT4+
interaction, as a consequence of either the fact that the NP unit
is encircled by the CBPQT4+ ring, giving rise to a 470 nm CT
band. There has been some evidence of the existence of
alongside CT interactions between the CBPQT4+ ring, which
is positioned on the NP station, and the MPTTF unit giving
(29) The methine proton in the isopropyl group on the stoppers was used in
variable temperature NMR spectroscopy to probe the dynamic process. The
investigation was performed on a 600 MHz NMR spectrometer as this high
field instrument enabled the observation of the unsymmetrical [2]rotaxane.
9
J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 967

A R T I C L E S
Nygaard et al.
rise to an asymmetry in the CT band.2c Therefore, the preserved
symmetry of the MPTTF/CBPQT4+ CT band in this case
supports the assignment that the CBPQT4+ ring encircles the
MPTTF unit predominantly, as no back-folding effect is possible
in this molecule on account of the inherent rigidity of the linker
between the NP and MPTTF units.
transition is red-shifted to 390 nm because of the solvent’s
polarities. Thus, it appears that in CHCl3 the entire spectrum is
red-shifted by approximately 135 nm relative to the more polar
solvents MeCN and Me2CO.
The assignment of the exact position of the CT band,
originating from the interaction between the NP unit and the
CBPQT4+ ring in the degenerate [2]rotaxane 2‚4PF6, correlates
1
with the H NMR spectroscopy results, which show that the
CBPQT4+ ring predominantly encircles the MPTTF unit in the
two-station [2]rotaxane 1‚4PF6. A series of experiments were
carried out to investigate whether or not the translational
isomeric distribution is affected by varying the temperature. The
temperature dependence that is observed (see Supporting
Information) on heating a solution of 1‚4PF6 in MeCN, arises
solely from the trivial effect of thermal dilution, meaning that
the CT band situated at 810 nm dominates at all temperatures
ranging from 283 to 323 K, but a decrease is observed in the
CT band intensity at higher temperatures as a result of the
expansion of the solvent. No significant effect of the fluctuating
temperature is observed in the region around 470 nm, again
indicating that the CBPQT4+ ring predominantly encircles the
MPTTF unit from 283 to 323 K (see Supporting Information).
The lack of temperature dependence on the translational iso-
meric distribution in 1‚4PF6 confirms the assumption that the
CBPQT4+ ring encircles the MPTTF unit and that this confor-
mation constitutes the energetic ground-state at a wide range
of temperatures, not only from 198 to 298 K as determined by
1H NMR but also up to 323 K. This finding suggests that the
PEG spacer has played a significant role in previous bistable
[2]rotaxanes, wherein significant temperature effects have been
the basis for a thorough review of the underlying thermodynam-
ics controlling translational isomerism,30 as well as for a
correlation3i with the temperature effects of molecular switch
tunnel junction devices that arise from the concomitant effect
of temperature on the translational isomerism of bistable [2]-
rotaxanes.
The effect of the solvent on the absorption spectrum (Figure
6) of 1‚4PF6 was investigated in Me2CO, CHCl3, and MeCN.
Since the CT bands represent a charge redistribution going from
the ground state to the excited state, one would expect that
changing the solvent would greatly affect the chromophoric
properties of the system as well as possibly changing the
translational isomeric distribution. Comparatively, the variation
of absorption spectra, recorded in Me2CO and MeCN is small,
as both solvents yields clear green solutions with a distinct CT
band at approximately 800 nm. Another feature, assumed to be
the local transition of the MPTTF unit, is also observed around
365 nm. Changing the solvent from Me2CO to the less polar
solvent CHCl3, influences the absorption spectra, as the CT
interaction is red-shifted by 135 nm out of the visible area and
into the near-infrared (NIR) area, yielding an orange solution,
which at first might be taken as an indication of a change in
the isomeric distribution. However, inspection of the absorption
spectra show that the observed color change is caused by the
CT band being red-shifted by 135 nm out of the visible area
and into the near-infrared (NIR) region. Also the local MPTTF
Figure 6. Absorption spectra of 1‚4PF6 recorded in Me2CO, CHCl3, and
MeCN displaying the significant red-shift in the absorption maxima in going
from Me2CO/MeCN to CHCl3. Inserted is a magnification of the CT band
in the visible to NIR area.
Chemical Switching of the Bistable [2]Rotaxane Moni-
tored by Absorption Spectroscopy. The chemical oxidant
Fe(ClO4)3 has been employed to oxidize the MPTTF unit of
the [2]rotaxane 1‚4PF6 and the associated spectroscopic changes
were followed by absorption spectroscopy. The titration of 1.0
equiv of Fe(ClO4)3 with 1‚4PF6 (see Supporting Information)
resulted in the formation of three dominant absorptions centered
around 725, 441, and 350 nm. These absorptions, originating
from the mono-oxidized MPTTF•+ unit are very intense,
compared to the initial CT band at 810 nm; hence, it is not
possible to follow the position of the CBPQT4+ ring along the
dumbbell. Addition of another equivalent of Fe(ClO4)3 results
in a bleach of the three absorption bands attributed to the
MPTTF•+ and the appearance of a new broad absorption
centered around 575 nm, resulting from the formation of the
doubly oxidized MPTTF2+ unit. Again, the much more intense
absorption from the MPTTF2+ unit dominates the CT band
originating from the NP unit encircled by the CBPQT4+ ring,
making it impossible to speculate about the position of the
CBPQT4+ ring from this experiment.
Electrochemical Switching Investigated by CV, DPV, and
SEC. The electrochemically induced switching behavior of the
rigid rotaxane 1‚4PF6 was investigated using cyclic voltammetry
(CV), differential pulse voltammetry (DPV), and UV-vis
spectroelectrochemistry (SEC). The half-dumbbell compounds
10 and 17, the rigid MPTTF/NP-containing dumbbell compound
18, and the free cyclophane CBPQT4+ were studied as refer-
ences. The CV traces for these compounds are shown in Figure
7. The half-dumbbell compound 10 shows (Figure 7a) two
reversible and monoelectronic oxidization waves, which are the
signatures of the stepwise oxidation process of the MPTTF
unit.11 The half-wave potentials (see Supporting Information)
of the first and second oxidization of MPTTF half-dumbbell
compound 10 at +0.55 and +0.75 V (vs SCE) were determined
by the DPV measurement. The oxidation (Figure 7b) of the NP
half-dumbbell compound 17 is irreversible, and its anodic peak
lies at +1.47 V (vs SCE). The rigid dumbbell compound 18
showed two reversible and one irreversible oxidation processes
associated with the MPTTF and NP unit, respectively (Figure
7c). The half-wave potentials of the MPTTF unit in the rigid
(30) Moonen, N. N. P.; Flood, A. H.; Fernandez, J. M.; Stoddart, J. F. Top.
Curr. Chem. 2005, 262, 99-132.
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Functionally Rigid [2]Rotaxanes
A R T I C L E S
the MPTTF to NP unit after the oxidation of the MPTTF unit
and thus hinders the oxidation of the NP unit through CT
interaction. The cathodic peaks for the first and second reduction
of the MPTTF unit (MPTTF2+ f MPTTF•+ f MPTTF) are
not separated well in the CV measurement.
The anodic sweep of the CV indicates that the MPTTF station
is oxidized 250 mV more anodic than the dumbbell component,
suggesting that it is tightly encircled by the CBPQT4+ ring3h
(i.e., it is not in equilibrium with the NP station). At ∼800 mV,
the monocation is formed and the CBPQT4+ ring is typically
repelled away, allowing the second oxidation generating the
dication MPTTF2+ to occur at the same potential as that
observed in the dumbbell. In the present case, however, the
dication is formed at a potential greater than that for the
dumbbell by ∼100 mV. This observation is consistent with the
movement of the ring to the NP unit but that the electrostatic
effect13 of the tetracationic CBPQT4+ ring on the cationic
MPTTF+•/2+ unit is still being felt through space across the
length of the rigid spacer. The return sweep of the CV shows
the anodic shift for the dication’s reduction to the monocation.
The anodic shift for the reformation of the neutral MPTTF unit
is larger than that observed in previous systems and is consistent
with the rapid and unhindered return of the CBPQT4+ ring to
the neutral MPTTF unit. In summary, the electrochemistry data
confirm that the CBPQT4+ is switched reversibly between the
MPTTF and NP units during a cycle of two-electron oxidation
and reduction and potentially only after one-electron oxidation.
Figure 7. Cyclic voltammetry of the (a) MPTTF half-dumbbell compound
10, (b) NP half-dumbbell compound 17, (c) rigid dumbbell component 18,
(d) CBPQT4+, and (e) rigid rotaxane 1‚4PF6. The data have been scaled
for ease of comparison of the CVs, and the scale bars at 0.0 V correspond
to 5 µA. All data presented were recorded at 200 mV s-1 in argon-purged
DMF. 1.0 mM of the sample in 0.1 M TBA·PF6 solution, 298 K, working
electrode: glassy carbon electrode (0.0178 cm2).
The reduction behavior of the CBPQT4+ ring in the rigid
rotaxane 1‚4PF6 is similar to that in the simple one-station [2]-
rotaxanes. The half-wave potentials of the CBPQT4+ ring were
determined to be -0.25 and -0.70 V (vs SCE). These values
are both more negative than those for the free CBPQT4+
,
dumbbell compound 18 (+0.55 and +0.76 V vs SCE) are almost
the same as those of the precursor half-dumbbell compound
10. Although redox potentials are strictly thermodynamic
properties that relate to the stabilities of both neutral reactant
and oxidized product, the fact that the oxidation (+1.41 V vs
SCE) of the NP unit in 18 occurs at a slightly lower potential
than that for 17 (+1.47 V vs SCE) is consistent with the
extension of π-conjugation observed in the UV-vis spectro-
scopic data. The cyclophane CBPQT4+ shows the two character-
istic,6a reversible two-electron reduction processes at -0.29 V
and -0.72 V (vs SCE) (Figure 7d).
The electrochemical properties of the rigid [2]rotaxane
1‚4PF6 are distinctly different from (Figure 7e) that of its rigid
dumbbell component 18. In the oxidation region (V > 0 V),
two close anodic peaks were observed between +0.80 and
+0.90 V (vs SCE). The integral of the whole process corre-
sponds to a two-electron oxidation, suggesting that the first and
second oxidation of the MPTTF unit occur at almost the same
potential. The more positive first oxidation potential of the
MPTTF (or TTF) unitswhen encircled by the CBPQT4+ rings
has been reported previous studies on bistable [2]rotaxanes2,3i
and [2]catenanes.31 This phenomenon suggests that CBPQT4+
ring encircles MPTTF in the ground state. The oxidation of the
NP unit does not occur until +1.6 V (vs SCE), a potential that
is higher than is required for the oxidation of the bare NP unit.
This result suggests that the CBPQT4+ ring is translocated from
suggesting that the effect of the dumbbell component on the
electrochemical behavior of the CBPQT4+ is consistent with a
MPTTF-CBPQT4+ CT interaction. The cathodic shift of the
second reduction peak has not been observed all that often. It
suggests that there is an interaction between the reduced forms
of the ring and dumbbell component. The first reduction peak
of the CBPQT4+ ring in rotaxanes and catenanes often shows
separations2d,g,i, 27, 31 on account of the nonequivalent environment
for the two bipyridinium units. In the case of rotaxanes, the
back-folding phenomena may be the cause of why the two
bipyridinium units have nonequivalent electrochemical environ-
ments. Although there is an absence of direct separation in the
first reduction peak of CBPQT4+ in the rigid rotaxane 1‚4PF6
there is a peak broadening when compared to CBPQT4+ as
illustrated in Figure 7, panels e and d, respectively. This result
suggests that back-folding phenomena is much reduced and that
its contribution to previous peak separations may be larger than
has been otherwise considered.
The UV-vis spectroscopic changes associated with the
electrochemical oxidation process of the rigid rotaxane 1‚4PF6
were recorded in order to shed some light on the mechanical
movement of the CBPQT4+ ring between the two electron-rich
units MPTTF and NP (Figure 8).
The ground state UV-vis spectrum of 1‚4PF6 displays
(Figure 8a) the characteristic CT band at 820 nm, corresponding
to the MPTTF-CBPQT4+ CT interaction. By applying a
potential of +0.75 V (Vap), the ground-state bands bleach and
are replaced by bands in the visible region at 441 and 725 nm,
(31) Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Raymo, F. M.;
Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Org. Chem.
2000, 65, 1924-1936.
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J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007 969

A R T I C L E S
Nygaard et al.
role of the spacer in modulating the effect of temperature on
each isomer. It is believed that the rigid backbone linking the
two stations in 1‚4PF6 effectively minimizes the back-folding
effect associated with the dumbbell, thereby favoring the
translational isomer where the CBPQT4+ ring encircles the
stronger donor, namely, the MPTTF unit. By employing
dynamic 1H NMR measurements on the degenerate [2]rotaxane
2‚4PF6, it was found that the shuttling barrier over the 1.0 nm
distance is significantly lowersindeed, by more than 5 kcal/
molsthan that found previously in other non-rigid rotaxanes,
implying that this barrier can be fine-tuned by changing the
molecular architecture. In order for these molecules to work as
molecular switches, one has to be able to switch the position
of the CBPQT4+ ring from the ground-state position around the
MPTTF unit to the NP station by applying an appropriate
stimulus. We have shown here that the two-station [2]rotaxane
1‚4PF6 is addressable in solution by electrochemical, as well
as by chemical stimuli. Cyclic voltammetry suggests that
oxidation to the dication and reduction back to the neutral form
results in a complete switch of 1.5 nm between two close to
“all-or-nothing” states. The range of favorable properties
introduced with the rigid linker make this molecular design an
interesting candidate for the construction of molecular switches
that could be expected to display strong on-off ratios that are
temperature-independent and exhibit rapid kinetics in suitably
designed devices. Finally, these functional rigid bistable [2]-
rotaxanes will serve as an important starting point for the
utilization of rigidity when well-defined structures and functions
are of the outmost importance, such as in molecular pistons
and simple motor molecules.
Figure 8. UV-vis spectroelectrochemistry of the rigid rotaxane 1‚4PF6
(1.0 mM 1‚4PF6 in 0.1 M TBA·PF6/MeCN) recorded at applied voltage
(Vap): (a) ground-state Vap ) 0 V; (b) Vap ) +0.75 V (vs Ag) to generate
MPTTF•+ radical cation; (c) Vap ) +0.95 V (vs Ag) to generate MPTTF2+
dication; and (d) Vap ) 0 V to regenerate the ground state.
which are associated with the absorption of the MPTTF•+ radical
cation (Figure 8b). After switching the applied potential from
+0.75 to +0.95 V, the spectrum displays a bleaching of the
MPTTF•+ bands, which is concomitant with the formation of a
large broad band at 555 nm, characteristic of the MPTTF2+
dication absorption (Figure 8c). These observations mirror those
obtained for the chemical oxidation of 1‚4PF6 in which the
chromophores of the MPTTF cations dominate the visible
region. The original spectrum at the ground state was fully
reproduced by leaving the solution at room temperature for
overnight at 0 V (Figure 8d). The electrochemical reaction is
fully reversible.
Acknowledgment. This work was funded in part by a Ph.D.
scholarship from the University of Southern Denmark to S.N.,
by the Danish Natural Science Research Council (SNF, Project
21-03-0317), and by the Danish Strategic Research Council in
Denmark through the Young Researchers Programme (2117-
05-0115) in Denmark and by the Materials Structures and
Devices, the Moletronics Program of the Defense Advanced
Research Projects Agency (DARPA) in the United States.
Conclusion
In conclusion, the syntheses and characterization of a pair of
rigid, two-station [2]rotaxanes, the bistable MPTTF/NP-contain-
ing [2]rotaxane 1‚4PF6 and the degenerate NP/NP-containing
[2]rotaxane 2‚4PF6, have been reported. The incorporation of a
rigid spacer into the two-station [2]rotaxane 1‚4PF6 results in a
very favorable and temperature-independent isomeric distribu-
tion of the isolated compound, > 95 < 5, in favor of the
translational isomer where the CBPQT4+ ring encircles the
MPTTF unit. This finding is quite surprising as all other
MPTTF-containing two-station [2]rotaxanes investigated so far
have been mixtures of the two possible translational isomers
whose ratios vary significantly with temperature implicating the
Supporting Information Available: Synthetic procedure of
all compounds as well as the relevant NMR and UV-vis
spectroscopic data. Complete ref 6a. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA0663529
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970 J. AM. CHEM. SOC. VOL. 129, NO. 4, 2007