Molecular shuttles based on tetrathiafulvalene units and 1,5-dioxynaphthalene ring systems

Article abstract of DOI:10.1002/chem.200305725

Six different degenerate [2]rotaxanes were synthesized and characterized. The rotaxanes contained either two tetrathiafulvalene (TTF) units or two 1,5-dioxynaphthalene (DNP) ring systems, both of which serve as recognition sites for a cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) ring. Three different spacer units were incorporated into the dumbbell components of the [2]rotaxanes between the recognition sites. They include a polyether chain, a terphenyl unit, and a diphenyl ether linker, all of which were investigated in order to probe the effect of the spacers on the rate of the shuttling process. Data from dynamic ¹H NMR spectroscopy revealed a relatively small difference in the ΔG^? values for the shuttling process in the [2]rotaxanes containing the three different spacers, in contrast to a large difference between the TTF-containing rotaxanes (18 kcalmol^-1) and the DNP-containing rotaxanes (15 kcalmol^-1). This 3 kcal mol ^-1 difference is predominantly a result of a ground-state effect, reflecting the much stronger binding of TTF units to the CBPQT⁴⁺ ring in comparison with DNP ring systems. An examination of the enthalpic (ΔH^?) and entropic (ΔS^?) components for the shuttling process in the DNP-containing rotaxanes revealed significant differences between the three spacers, a property which could be important in designing new molecules for incorporation into molecular electronic and nanoelectromechanical (NEMs) devices.

Full text of DOI:10.1002/chem.200305725

FULL PAPER
Molecular Shuttles Based on Tetrathiafulvalene Units and 1,5-
Dioxynaphthalene Ring Systems
Seogshin Kang, Scott A. Vignon, Hsian-Rong Tseng, and J. Fraser Stoddart*[a]
Abstract: Six different degenerate
[2]rotaxanes were synthesized and
characterized. The rotaxanes contained
either two tetrathiafulvalene (TTF)
units or two 1,5-dioxynaphthalene
(DNP) ring systems, both of which
serve as recognition sites for a cyclo-
shuttling process. Data from dynamic
1H NMR spectroscopy revealed a rela-
tively small difference in the DG°
values for the shuttling process in the
[2]rotaxanes containing the three dif-
ferent spacers, in contrast to a large
difference between the TTF-containing
rotaxanes (18 kcalmolÀ1) and the DNP-
containing rotaxanes (15 kcalmolÀ1).
This 3 kcalmolÀ1 difference is predomi-
nantly a result of a ground-state effect,
reflecting the much stronger binding of
TTF units to the CBPQT4+ ring in
comparison with DNP ring systems. An
examination of the enthalpic (DH°)
and entropic (DS°) components for the
shuttling process in the DNP-contain-
ing rotaxanes revealed significant dif-
ferences between the three spacers, a
property which could be important in
designing new molecules for incorpora-
tion into molecular electronic and
nanoelectromechanical (NEMs) devi-
ces.
bis(paraquat-p-phenylene) (CBPQT4+
)
ring. Three different spacer units were
incorporated into the dumbbell compo-
nents of the [2]rotaxanes between the
recognition sites. They include a poly-
ether chain, a terphenyl unit, and a di-
phenyl ether linker, all of which were
investigated in order to probe the
effect of the spacers on the rate of the
Keywords: molecular shuttles
¥
nanotechnology ¥ NMR spectros-
copy ¥ rotaxanes ¥ self-assembly ¥
template synthesis
Introduction
mechanism of operation of full, two-terminal devices. One
of the main questions surrounding the full devices relates to
the precise mechanism operating within the single monolay-
er of mechanically-interlocked bistable catenanes[7] or rotax-
anes[8] sandwiched between the two electrodes.
In recent years, the convergence of two areas of scientific
endeavor, namely the design and synthesis of mechanically
interlocked compounds[1,2] and solid-state molecular elec-
tronic devices,[3] has led to many significant and unexpected
discoveries. The incorporation of these compounds, that is,
catenanes and rotaxanes containing mechanical bonds, into
solid-state devices has led to the demonstration[4] of high-
density molecular memory and logic devices. Despite some
skepticism, a number of such devices incorporating different
electrodes[5] and molecular switches[6] have been demon-
strated to operate by mechanisms that are believed to be
similar to those observed in solution[7,8] and in half-devices.[9]
In spite of these numerous examples, a number of questions
still remain to be answered with regard to the detailed
The collaboration between our group at UCLA and the
Heath group at Caltech has resulted[4a d] in the incorporation
of numerous bistable [2]catenanes and [2]rotaxanes into mo-
lecular-switch tunnel junctions (MSTJs). Such devices are
constructed by depositing a Langmuir Blodgett (LB) mono-
layer[10] of molecules onto a patterned bottom electrode,
c]
which has been either polysilicon[4a or a semiconducting
carbon nanotube,[4d] but never a metal. This step is then fol-
lowed by the electron beam (e-beam) deposition of a top
electrode, composed of a layer of Ti followed by a layer of
Al. A schematic of such a device, composed of a polysilicon
bottom electrode and a monolayer of amphiphilic bistable
[2]rotaxanes, and its proposed mode of operation is shown
in Figure 1. The fabricated device can be switched between
high and low conductivity states by applying Æ2 V. If the
higher conducting, or metastable (™ON∫) state of the switch
is left unperturbed, it will undergo a thermal relaxation
back to the lower conducting, ground (™OFF∫) state of the
switch. Also, in keeping with a thermally activated process,
it has been observed that, if the device is cooled down to
[a] S. Kang, S. A. Vignon, Dr. H.-R. Tseng, Prof. J. F. Stoddart
California Nanosystems Institute and
Department of Chemistry and Biochemistry
University of California, Los Angeles
405 Hilgard Avenue, Los Angeles, CA 90095 1569 (USA)
Fax : (+1)310-206-1843
E-mail: stoddart@chem.ucla.edu
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FULL PAPER
Figure 1. Schematic idealized representation of a molecular switch tunnel junction (MSTJ) composed of a bottom polysilicon electrode, a monolayer of
amphiphilic bistable [2]rotaxanes, and a top electrode composed of Ti and Al. The proposed operating mechanism of the device is also shown. Applica-
tion of +2 V oxidizes the TTF unit and causes movement of the CBPQT4+ ring to the DNP ring system. After the voltage is removed, the TTF unit re-
turns to its neutral form, but the ring remains on the DNP ring system in a metastable state. This state is a higher conducting one relative to the ground
state, which is reached when the ring returns to the TTF unit through thermal relaxation or application of À2 V.
sufficiently low temperatures (~200 K), it can no longer be
switched between these two states.[4a] These observations, to-
gether with results from many control experiments, suggest
that an activated process, most likely an electromechanical
one, is responsible for the switching ™ON∫ and ™OFF∫ of
the device.[11]
The experimental results were certainly not unexpected,
since these mechanically interlocked, switchable molecules
had been designed specifically to undergo electrochemical
switching. The bistable [2]catenanes[7] and [2]rotaxanes[8] are
composed of two components, a tetracationic cyclophane,
cyclobis(paraquat-p-phenylene) (CBPQT4+), and either a
two-station crown ether macrocycle in the case of the bista-
ble [2]catenanes,[7] or a two-station dumbbell component in
the case of the bistable [2]rotaxanes.[8] Within the crown
ether and dumbbell components, the two stations must serve
as very different recognition sites for the CBPQT4+ ring.
Examples of such recognition sites are a tetrathiafulvalene
(TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system.
Being the much stronger p-electron donor, the TTF unit re-
sides inside the CBPQT4+ ring with greater than 99:1 pref-
erence over the DNP ring system (Figure 2a). However, if
the TTF unit is oxidized to its mono- or dicationic form,
then the molecules undergo a circumrotation or translation-
al process so as to position the DNP ring system inside the
CBPQT4+ ring (Figure 2b). This mechanical movement can
then be reversed by reducing the oxidized TTF unit (TTF+
or TTF2+) back to its neutral form (TTF). However, a barri-
er (DG°DNP) exists to this returning motion (Figure 2c)–one
which, in solution, is overcome rapidly so that an equilibri-
um is established whereby, once again, the lower energy
translational isomer dominates. In more restricted environ-
ments, however, the time it takes for the higher energy
translational isomer (the metastable state) to return to the
lower energy translational isomer (the ground state) can be
expected to increase and does.[12]
Figure 2. Schematic descriptions of the potential-energy surfaces for bistable [2]catenanes and [2]rotaxanes, a) in the neutral ground state, in which the
lower energy isomer with the TTF unit inside the CBPQT4+ ring constitutes the highly preferred translational isomer; b) in the oxidized state, in which
the DNP ring system now resides exclusively within the CBPQT4+ ring; and c) in the metastable state, in which the DNP ring system still resides within
the CBPQT4+ as a result of the barrier, DG° , to its return to the much lower energy translational isomer where the TTF unit is located inside the
DNP
CBPQT4+ ring.
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Molecular Shuttles
2555 2564
possible for us to incorporate
different spacer units into the
degenerate [2]rotaxanes be-
tween the two recognition sites.
Since the CBPQT4+ ring must
pass over these spacer units to
move from one site to the
other, by changing the size and/
or functionality associated with
these spacer units, it should be
possible to influence the shut-
tling speed. Three different
spacer units were incorporated
Figure 3. Schematic descriptions of the potential-energy surfaces for degenerate [2]rotaxanes that contain two
identical recognition sites for the CBPQT4+ ring, either a) both DNP ring systems or b) both TTF units.
Although this (electro)chemical switching mechanism has
also been observed in solution employing both UV-visible[8c]
into the range of molecular shuttles in order to determine
their influence. Here we will describe 1) the template-direct-
ed synthesis[16] of the six degenerate [2]rotaxanes, 1¥4PF6
6¥4PF6, shown in Figure 4, and their corresponding dumbbell
components, 7--12; 2) the complete characterization of all
1
and H NMR[8c,13] spectroscopy, the kinetics associated with
the switching process cannot be measured by using these
techniques, because it occurs too rapidly. It would be possi-
ble to determine DG°TTF and
DG°DNP (Figure 2) for these non-
degenerate, two-station cate-
nanes[14] and rotaxanes[15] by
studying the circumrotation or
shuttling exchange processes,
respectively, that occur within
them, except that this possibili-
ty is precluded by the fact that
far too little of the higher
energy translational isomer
with the DNP ring system
inside the CBPQT4+ ring is
present to detect in these spec-
troscopic experiments. One way
to avoid this problem is to
design and synthesize model
systems that are similar enough
to the nondegenerate systems,
such that the energy barriers
for the circumrotation and shut-
tling processes will be very sim-
ilar in the two systems. Here,
we present one approach to this
problem that involves synthe-
sizing degenerate, two-station
rotaxanes whereby both recog-
nition sites positioned along the
dumbbell component of the
[2]rotaxane are identical, that
is, either both TTF units or
both DNP ring systems, so that
the exchange process (shut-
tling[15]
)
between two iso-
energetic forms (Figure 3) can
be easily observed by dynamic
1H NMR spectroscopy.
In addition to comparing the
shuttling rates in molecular
shuttles containing dual TTF
Figure 4. The structural formulas for the six degenerate [2]rotaxanes, 1¥4PF6 6¥4PF6, containing either two
TTF units or two DNP ring systems. Also shown are the labels for the protons used to study the shuttling
and DNP stations, it was also process.
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J. F. Stoddart et al.
FULL PAPER
1
the molecular shuttles; and 3) a dynamic H NMR spectro-
scopic study of the shuttling rates in all of these compounds.
respectively, are outlined in Scheme 1. The dumbbell-shaped
compound 7 was obtained in 87% yield by alkylating the al-
cohol[18] 13 with the tosylate 14 in the presence of NaH as
base. The dumbbell-shaped compound 7 was then used as
the template in a template-directed synthesis[16] of the [2]ro-
taxane 1¥4PF6. A CBPQT4+ ring was clipped onto the dumb-
bell-shaped template from its dicationic precursor[19] 18¥2PF6
and a,a’-dibromoxylene (17) to give the [2]rotaxane 1¥4PF6
in 54% yield, as an analytical pure, purple solid after chro-
matography on silica gel using a 1% NH4PF6 solution in
Me2CO as eluent. Similarly, the TTF-containing [2]rotaxane
2¥4PF6 and its dumbbell-shaped precursor 8 were prepared
from the alcohol[8c,13] 15 with the tosylate[8c,13] 16, in 82%
and 51%, respectively, by using the reaction and isolation
conditions similar to those employed in the preparations of
7 and 1¥4PF6.
The preparation of the dumbbell-shaped compounds 9 12
and their corresponding [2]rotaxanes 3¥4PF6 6¥4PF6, are
summarized in Scheme 2. In pathway I, the dumbbell-
shaped compounds 9 and 10 were obtained in 95% and
97% yields by a diesterfication of 4,4’-oxybis(benzoic acid)
(19) with two equivalent of the alcohols 13 and 15, respec-
tively. In pathway II, dialkylation of 4,4’’-dihydroxy-p-ter-
phenyl[20] (20) with the tosylates 14 and 16, in the presence
of K2CO3, LiBr, and [18]crown-6 in DMF, gave the dumb-
bell-shaped compounds 11 and 12 in 90% and 63% yields,
respectively. Once again, the [2]rotaxanes 3¥4PF6 6¥4PF6
were obtained in 25%, 11%, 25%, and 51% yields, respec-
tively, by applying a template-directed protocol[16] in which
Results and Discussion
Design and synthetic strategy: Retrosynthetic analyses of
the degenerate [2]rotaxanes 1¥4PF6 6¥4PF6, and their corre-
sponding precursor dumbbell-shaped compounds 7 12 led to
the following considerations and strategies. Common, of
course, to all six syntheses is the final clipping reaction to
form (Schemes 1 and 2) the CBPQT4+ ring around each of
the six dumbbell-shaped compounds. It is a template-direct-
ed procedure.[16] The conventional syntheses (Schemes 1 and
2) of the DNP-containing dumbbell-shaped compounds 7, 9,
and 11 rely upon two key starting materials, namely, the al-
cohol 3 and its tosylate[17] 14, which are co-joined directly by
nucleophilic substitution to give 7 or to one or two spacers,
namely 19 and 20 by esterification and nucleophilic substitu-
tion, respectively, to afford 9 and 11. Under essentially simi-
lar conditions, the syntheses of the three TTF-containing
dumbbell-shaped compounds 8, 10, and 12 have been ach-
ieved (Schemes 1 and 2) starting from precursors which in-
clude the alcohol 15 and its tosylate 16, as well as the two
spacers 19 and 20 where appropriate.
Synthesis: The routes employed in the syntheses of the de-
generate, two-station [2]rotaxanes 1¥4PF6 and 2¥4PF6, and
their corresponding dumbbell-shaped compounds 7 and 8,
Scheme 1. The syntheses of the [2]rotaxanes 1¥4PF6 and 2¥4PF6.
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Molecular Shuttles
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Table 1. 1H NMR chemical shift values for select protons of the degener-
ate 1,5-dioxynaphthalene-containing rotaxanes 1¥4PF6, 3¥4PF6, and
5¥4PF6 at high and low temperature in CD3COCD3 at 500 MHz.
the dumbbell-shaped compounds were treated with the dica-
tionic precursor 18¥2PF6 and a,a’-dibromoxylene (17) in
DMF at room temperature.
Assignment[a]
Limiting
T
[K]
Limiting
T
[K]
d values[b]
d values[c]
1
Dynamic H NMR spectroscopy: The shuttling processes in
1¥4PF6
3¥4PF6
5¥4PF6
HtBu
HEt
HMe
1.20, 1.27
2.52, 2.58
1.11, 1.17
236
236
236
1.30
2.61
1.21
326
315
326
the six degenerate [2]rotaxanes, namely 1¥4PF6 6¥4PF6,
1
were investigated by acquiring H NMR spectra of each ro-
taxane at a variety of different temperatures in CD3COCD3.
In these rotaxanes, the CBPQT4+ ring is located on one of
two identical stations and shuttles back and forth between
these two degenerate forms. At lower temperatures, this
shuttling process becomes slow on the 1H NMR timescale
HtBu
HEt
HMe
1.25, 1.28
2.56, 2.59
1.15, 1.19
232
232
232
1.32
2.63
1.23
327
317
327
HtBu
HEt
HMe
1.22, 1.25
2.54, 2.57
1.12, 1.15
236
236
236
1.29
2.60
1.20
316
316
316
1
and the signals in the H NMR spectrum separate into pairs
of equal intensity signals. This separation is most evident
1
[a] Assignments for these protons are given in Figure 4. [b] Low tempera-
ture. [c] High temperature.
when observing the H NMR signals (Figure 5) for the alkyl
protons on the hydrophobic stoppers. The chemical shifts
for these protons in the dioxynaphthalene-containing rotax-
anes 1¥4PF6, 3¥4PF6 and 5¥4PF6 at high and low temperature
are listed in Table 1. Quantitative analysis of this shuttling
process was accomplished by using two methods. Spin satu-
ration transfer[21] (SST) was used to determine the rate of
shuttling in the slow-exchange regime and line-shape analy-
sis[22] (LSA) (see Figure 6 for example spectra) was used
over the entire range. The data are summarized in Tables 2
4. From this data, it is apparent that the DG° values for
shuttling in all of these three rotaxanes are all within ~
0.5 kcalmolÀ1 of each other. Thus, very little difference is
observed between the shuttling processes in rotaxanes con-
Scheme 2. The syntheses of the [2]rotaxanes 3¥4PF6 6¥4PF6.
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Table 2. Kinetic and thermodynamic data for shuttling in the [2]rotaxane
1¥4PF6 in CD3COCD3.
[b]
T [K][a]
kex [sÀ1
]
DG° [kcalmolÀ1
]
247
255
0.2[c]
0.7[c]
15.2
15.3
259
271
282
293
305
0.8[d]
3[d]
15.2
15.2
15.3
15.4
15.5
8[d]
20[d]
47[d]
[a] Calibrated using neat MeOH sample. [b] Æ0.1 kcalmolÀ1. [c] These
data were obtained by the spin saturation transfer[21] (SST) method. Ex-
change was observed between the resonances for HMe (Figure 4) at d=
1.15/1.20 ppm. [d] These data were obtained by the line shape analysis[22]
(LSA) method. The signals corresponding to HEt (Figure 4) at d=2.52/
2.58 ppm were simulated.
Table 3. Kinetic and thermodynamic data for shuttling in the [2]rotaxane
3¥4PF6 in CD3COCD3.
[b]
T [K][a]
kex [sÀ1
]
DG° [kcalmolÀ1
]
248
259
270
282
0.2[c]
0.7[c]
1.8[c]
4.7[c]
15.3
15.3
15.4
15.6
270
282
294
304
317
2[d]
6[d]
15.4
15.5
15.5
15.4
15.6
19[d]
55[d]
120[d]
Figure 5. Partial 1H NMR spectra for 5¥4PF6 in CD3COCD3 recorded at
a) 316 K, b) 282 K, and c) 226 K. The resonances corresponding to the
protons on the hydrophobic stopper are labeled. Structural assignments
for these protons can be found in Figure 4. Note that at lower tempera-
tures, the signals for Me, Et (CH2CH3) and tBu separate out into two
triplets, two quartets, and two singlets, respectively: at higher tempera-
tures one triplet, one quartet, and one singlet are observed as a result of
coalescence of the respective signals.
[a] Calibrated using neat MeOH sample. [b] Æ0.1 kcalmolÀ1. [c] These
data were obtained by the spin saturation transfer[21] (SST) method. Ex-
change was observed between the resonances for HtBu (Figure 4) at d=
1.27/1.30 ppm. [d] These data were obtained by the line shape analysis[22]
(LSA) method. The signals corresponding to HEt (Figure 4) at d=2.56/
2.59 ppm were simulated.
Table 4. Kinetic and thermodynamic data for shuttling in the [2]rotaxane
5¥4PF6 in CD3COCD3.
[a]
[b]
T [K]
kex [sÀ1
]
DG° [kcalmolÀ1
]
236
247
259
0.1[c]
0.4[c]
1.5[c]
14.8
14.9
14.9
247
258
270
282
293
305
0.4[d]
1.4[d]
4.4[d]
13[d]
14.8
14.9
15.0
15.0
15.1
15.2
33[d]
85[d]
[a] Calibrated using neat MeOH sample. [b] Æ0.1 kcalmolÀ1. [c] These
data were obtained by the spin saturation transfer[21] (SST) method. Ex-
change was observed between the resonances for H2/6 (Figure 4) at d=
2.26/7.33 ppm. [d] These data were obtained by the line shape analysis[22]
(LSA) method. The signals corresponding to HEt (Figure 4) at d=2.54/
2.57 ppm were simulated.
taining the three different spacers considering DG° values
alone.
Quantitative analyses of the shuttling processes in rotax-
anes 2¥4PF6, 4¥4PF6 and 6¥4PF6 were precluded by the pres-
ence of multiple isomers arising from the different possible
Figure 6. The simulated (top) and experimental (bottom) partial
1H NMR spectra for the resonances of the HEt protons (Figure 4) of
5¥4PF6 at a) 305 K, b) 293 K, c) 282 K, d) 270 K, e) 258 K, and f) 247 K.
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Molecular Shuttles
2555 2564
Table 5. Activation parameters for shuttling in the [2]rotaxanes 1¥4PF6,
3¥4PF6, and 5¥4PF6 in CD3COCD3.
substitution patterns on the TTF moiety (referred to here as
cis and trans). Thus, there are four possible exchange proc-
esses that can occur, with the CBPQT4+ ring moving from
cis-to-trans, trans-to-cis, cis-to-cis, or trans-to-trans. Although
it would be interesting to study the different rates of shut-
tling between these isomers and the relative preference of
the ring for one isomer over the other, it was not possible to
DH° [kcalmolÀ1
]
DS° [cal molKÀ1
]
1¥4PF6
3¥4PF6
5¥4PF6
12.7
14.5
13.6
À9.3
À3.2
À4.8
1
identify these isomers in the H NMR spectrum well enough
to analyze the shuttling process. It was, however, possible to
estimate[23] that the DG° value for shuttling between the
two TTF sites is ~17 18 kcalmolÀ1 by observing the coales-
cences of the many peaks corresponding to the different iso-
mers.
In order to investigate more deeply the different effects
of the three spacers on the shuttling process, the DH° and
DS° values were deduced from the graphs shown in
Figure 7. An examination of this data (Table 5) reveals sig-
nificant differences for the three different spacers. The
glycol chain spacer has the largest entropic penalty (large
negative DS°) associated with a reduction in the conforma-
tional flexibility in the transition state. On the other hand,
the enthalpic penalty for shuttling over the glycol chain
spacer is the smallest of the three, presumably because of fa-
vorable electrostatic interactions between the partially nega-
tive oxygen atoms and the CBPQT4+ ring, and also the
smaller size relative to the aromatic spacers. The terphenyl
and diester spacers have similar entropic penalties and are
smaller than that for the glycol spacer. This result is not su-
prising on account of the more rigid nature of these aromat-
ic-ring-containing spacers. The largest enthalpic penalty of
the three spacers is found to be for the diester-containing
spacer, perhaps resulting from the ™bent∫ nature of the
spacer resulting from the bisphenyloxy linkage, compared to
the linear terphenyl spacer. Enthalpic versus entropic con-
siderations will be important when designing molecules to
operate as molecular machines and switches in the solid
state. The operating temperatures will also have to be con-
sidered when choosing the optimal spacer to incorporate
into the molecular design. In order to construct devices
where the shuttling or switching barrier does not change
dramatically with temperature, more rigid aromatic spacers
will need to be used.
Conclusion
In this paper, kinetic and thermodynamic data have been
presented for shuttling of a CBPQT4+ ring component in a
series of degenerate [2]rotaxanes containing either two TTF
units or two DNP ring systems in their dumbbell compo-
nents as recognition sites for the ring. These degenerate mo-
lecular shuttles serve as models to study the barriers to ring
movement in the nondegenerate bistable [2]rotaxanes used
in molecular electronic devices.[4] It has been found that the
barrier for shuttling between two DNP ring systems is
~15 kcalmolÀ1, while in the case of two TTF units the barri-
er increases to ~17 18 kcalmolÀ1. The difference observed
between these two molecular shuttles arises primarily from
a ground-state effect, reflecting the difference in binding
constants of the TTF unit (Ka =500000mÀ1 in CD3CN,
DG2o98 =À8 kcalmolÀ1)[24] versus the DNP ring system (Ka =
25000mÀ1 in CD3CN, DG2o98 =À6 kcalmolÀ1),[25] each carry-
ing two diethyleneglycol substituents, with a CBPQT4+ ring.
Additionally, the effect of the spacers on the DG° values
was shown to be minimal. When the enthalpic and entropic
activation parameters, namely DH° and DS°, were deter-
mined, large differences were observed. In an upcoming
review article, this thermodynamic data will be compared to
data for similar processes in more restricted environments,
such as half-devices and full-devices, to provide further evi-
dence that the mechanism operating across all of these re-
gimes is a common one, namely a mechanical ring move-
ment.
Figure 7. Eyring plots created from the data in Tables 2 4 for the degen-
erate rotaxanes a) 1¥4PF6, b) 3¥4PF6, and c) 5¥4PF6. The values for DH°
and DS° were obtained from the slopes and intercepts of these plots, re-
spectively, and are given in Table 5.
2561
Chem. Eur. J. 2004, 10, 2555 2564
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

J. F. Stoddart et al.
FULL PAPER
2H); 13C NMR (CD3COCD3, 125 MHz): d=14.8, 19.8, 27.8, 30.7, 33.8,
63.0, 67.3, 67.8, 69.3, 69.4, 69.6, 70.4, 70.6, 105.5, 105.6, 113.1, 114.1, 114.2,
124.0, 124.0, 125.0, 125.0, 126.6, 130.4, 130.7, 131.8, 139.3, 141.3, 144.3,
144.6, 148.1, 154.3, 154.3, 156.8, 169.9 ppm; MS (FAB): m/z (%): 1572
(100) [M+1]+ ; elemental analysis calcd(%) for C106H122O11: C 80.98, H
7.82; found: C 80.78, H 7.86.
Compound 8: Yield: 82%; 1H NMR (CD3COCD3, 500 MHz): d=1.18 (t,
J=7.6 Hz, 6H), 1.27 (s, 36H), 2.58 (q, J=7.6 Hz, 4H), 3.56 3.60 (m,
20H), 3.64 3.66 (m, 4H), 3.77 3.79 (m, 4H), 4.08 4.10 (m, 4H), 4.28
4.29 (m, 8H), 6.47 6.50 (m, 4H), 6.80 6.82 (m, 4H), 7.05 7.10 (m, 20H),
7.27 7.29 ppm (m, 8H); 13C NMR (CD3COCD3, 125 MHz): d=14.9, 27.8,
29.1, 30.7, 33.8, 63.0, 67.2, 67.5, 67.5, 67.6, 69.1, 69.1, 69.1, 69.4, 69.4, 70.2,
70.3, 70.3, 70.3, 109.7, 113.1, 116.4, 116.5, 116.5, 124.1, 126.7, 130.4, 130.7,
131.8, 134.7, 134.7, 134.8, 134.8, 139.3, 141.3, 144.3, 144.6, 148.1,
156.8 ppm; MS(FAB): m/z (%): 1780 (100) [M+1]+ ; elemental analysis
calcd(%) for C102H122O11S8: C 68.80, H 6.91; found: C 68.72, H 6.86.
Experimental Section
General methods: Chemicals were purchased from Aldrich and used as
received. The alcohols 13[16] and 15,[8c,13] the tosylate 16,[8c,13] a,a’-[1,4-
phenylenebis (methylene)]bis-(4,4’-bipyridium) bis(hexa-fluorophos-
phate) (18¥2PF6),[19] and 4,4’-dihydroxy-p-terphenylene (20)[20] were all
prepared according to procedures reported in the literature. Solvents
were dried following methods described in the literature. All reactions
were carried out under an anhydrous argon atmosphere. Thin-layer chro-
matography (TLC) was performed on aluminum sheets coated with silica
gel 60F (Merck 5554). The plates were inspected by UV light and, if re-
quired, developed in I2 vapor. Column chromatography was carried out
by using silica gel 60 (Merck 9385, 230 400 mesh). Melting points were
determined on an Electrothermal 9100 melting point apparatus and are
uncorrected. All 1H and 13C NMR spectra were recorded on either 1) a
Bruker ARX400 (400 MHz and 100 MHz, respectively), 2) a Bruker
General procedure for the preparation of the dumbbell-shaped com-
pounds
ARX500 (500 MHz and 125 MHz, respectively), or 3)
a Bruker
9 and 10: A
suspension of the alcohols 13[16] or 15[8c,13]
Avance 500 (500 MHz and 125 MHz, respectively), using residual solvent
as the internal standard. Samples were prepared by using CDCl3,
CD3COCD3, or CD3CN purchased from Cambridge Isotope Laborato-
ries. All chemical shifts are quoted using the d scale, and all coupling
constants (J) are expressed in Hertz (Hz). Electron impact ionization
mass spectrometry (EIMS) was performed on a AUTO-SPEC instru-
ment. Fast atom bombardment (FAB) mass spectra were obtained using
a ZAB-SE mass spectrometer, equipped with a krypton primary atom
beam, utilizing a m-nitrobenzyl alcohol matrix. Cesium iodide or poly(-
ethylene glycol) were employed as reference compounds. Electrospray
mass spectra (ESMS) were measured on a VG ProSpec triple focusing
mass spectrometer with MeCN as the mobile phase. Microanalyses were
performed by Quantitative Technologies, Inc.
(0.33 mmol), 4,4’-oxybis(benzoic acid) (19) (39 mg, 0.15 mmol), DCC
(129 mg, 0.60 mmol), and DMAP (73 mg, 0.60 mmol) in a solvent mixture
containing CH2Cl2 (7 mL) and THF (20 mL) was stirred at room temper-
ature for 24 h. After removal of solvent, the residue was purified by
column chromatography (SiO2, EtOAc/hexane 1:2) to give the dumbbell-
shaped compounds.
Compound 9: Yield: 95%; 1H NMR (CDCl3, 500 MHz): d=1.22 (t, J=
7.6 Hz, 6H), 1.29 (s, 36H), 2.61 (q, J=7.6 Hz, 4H), 3.96 3.99 (m, 8H),
4.03 (t, J=4.9 Hz, 4H), 4.04 (t, J=4.9 Hz, 4H), 4.14 (t, J=4.7 Hz, 4H),
4.29 (t, J=4.9 Hz, 4H), 4.30 (t, J=4.9 Hz, 4H), 4.52 (t, J=4.7 Hz, 4H),
6.77 6.79 (m, 4H), 6.80 (d, J=7.7 Hz, 2H), 6.81 (d, J=7.7 Hz, 2H), 6.98
(d, J=8.8 Hz, 4H), 7.03 7.09 (m, 20H), 7.20 7.23 (m, 8H), 7.28 (dd, J=
8.5, 7.7 Hz, 2H), 7.29 (d, J=8.5, 7.7 Hz, 2H), 7.84 (d, J=8.5 Hz, 2H),
7.85 (d, J=8.5 Hz, 2H), 8.04 ppm (d, J=8.8 Hz, 4H); 13C NMR (CDCl3,
125 MHz): d=15.2, 28.2, 31.3, 34.2, 63.1, 64.1, 67.2, 67.8, 67.9, 69.4, 69.7,
69.9, 70.0, 105.6, 105.6, 113.1, 114.5, 114.7, 118.5, 124.0, 125.0, 125.0,
125.5, 126.6, 126.6, 126.7, 130.6, 131.0, 131.9, 132.1, 139.7, 141.3, 144.1,
144.5, 148.2, 154.2, 154.2, 156.7, 160.1, 165.8 ppm; MS(FAB): m/z (%):
1812 (88) [M+1]+ ; elemental analysis calcd(%) for C120H130O15: C 79.53,
H 7.23; found: C, 79.07, H, 7.23.
Compound 10: Yield: 97%; 1H NMR (CD3COCD3, 500 MHz): d=1.19
(t, J=7.5 Hz, 6H), 1.30 (s, 36H), 2.61 (q, J=7.5 Hz, 4H), 3.60 3.64 (m,
8H), 3.66 3.71 (m, 8H), 3.80 3.84 (m, 8H), 4.09 4.10 (m, 4H), 4.29 4.31
(m, 8H), 4.43 4.45 (m, 4H), 6.43 6.47 (3îs, 4H), 6.81 6.83 (m, 4H),
7.09 7.16 (m, 20H), 7.30 (d, J=8.4 Hz, 8H), 8.09 ppm (d, J=8.5 Hz,
4H); 13C NMR (CD3COCD3, 125 MHz): d=14.9, 27.8, 29.3, 30.7, 33.8,
63.0, 63.8, 67.2, 67.6, 67.6, 68.8, 69.1, 69.4, 70.3, 70.3, 113.1, 116.3, 116.4,
116.5, 118.6, 124.0, 126.6, 130.4, 130.7, 131.7, 131.8, 139.3, 141.3, 144.3,
144.6, 148.1, 156.8, 160.1, 165.1, 204.3 ppm; MS (FAB): m/z (%): 2021
(97) [M]+.
Tosylate 14: A suspension of 4-[4-ethylphenyl-bis(4-tert-butylphenyl)me-
thyl]phenol[26] (477 mg, 1.00 mmol), the ditosylate[27] of 1,5-bis[2(hydrox-
yethoxy)ethoxy]naphthalene (1.09 g, 1.69 mmol), K2CO3 (276 mg,
2.00 mmol), LiBr (10 mg, cat. amount), and [18]crown-6 (10 mg, cat.
amount) in anhydrous MeCN (150 mL) was heated under reflux for two
days. After cooling to room temperature, the mixture was filtered and
the solid was washed with CH2Cl2. The combined organic phase was
dried in vacuo and the residue was purified by column chromatography
(SiO2, EtOAc/hexane 1:3) to give the tosylate 14 (763 mg, 80%) as a col-
orless oil. 1H NMR (CDCl3, 500 MHz): d=1.23 (t, J=7.6 Hz, 3H), 1.30
(s, 18H), 2.35 (s, 3H), 2.62 (q, J=7.6 Hz, 2H), 3.82 (t, J=4.7 Hz, 2H),
3.91 (t, J=4.7 Hz, 2H), 3.99 (t, J=4.8 Hz, 2H), 4.07 (t, J=4.8 Hz, 2H),
4.16 (t, J=4.9 Hz, 2H), 4.19 (t, J=4.7 Hz, 2H), 4.22 (t, J=4.7 Hz, 2H),
4.32 (t, J=4.9 Hz, 2H), 6.79 (d, J=7.5 Hz, 1H), 6.80 6.82 (m, 2H), 6.86
(d, J=7.5 Hz, 1H), 7.04 7.10 (m, 10H), 7.21 7.24 (m, 6H), 7.31 (dd, J=
8.5, 7.5 Hz, 1H), 7.34 (dd, J=8.5, 7.5 Hz, 1H), 7.76 7.79 (m, 2H), 7.80
(d, J=8.5 Hz, 1H), 7.88 ppm (d, J=8.5 Hz, 1H); 13C NMR (CDCl3,
125 MHz): d=15.3, 21.5, 28.2, 31.3, 34.2, 60.3, 63.1, 67.3, 67.8, 67.9, 68.9,
69.3, 69.8, 70.0, 105.6, 105.7, 113.1, 114.5, 114.7, 124.0, 125.0, 125.1, 126.5,
126.6, 126.7, 127.8, 129.7, 130.6, 131.0, 132.1, 132.9, 139.7, 141.3, 144.1,
144.5, 144.7, 148.2, 154.1, 154.3, 156.5 ppm; MS(FAB): m/z (%): 948 (20)
[M]+ ; elemental analysis calcd(%) for C60H68O8S: C 75.92, H 7.22;
found: C 75.93, H 7.28.
General procedure for the preparation of the dumbbell-shaped com-
pounds 11 and 12: A solution of tosylates 14 or 16[8c,13] (0.321 mmol), the
terphenyl diol 20 (38 mg, 0.146 mmol), K2CO3 (61 mg, 0.437 mmol), LiBr
(10 mg, cat. amount), and [18]crown-6 (10 mg, cat. amount) in anhydrous
DMF (10 mL) was heated at 1008C for 10 h. After cooling down to room
temperature and removal of solvent, the mixture was extracted with
CH2Cl2 (3î50 mL). The combined organic layers were dried (MgSO4)
and evaporated. The residue was purified by column chromatography
(SiO2, EtOAc/hexane/CH2Cl2 1:2:5) to give the dumbbell-shaped com-
pound.
Compound 11: Yield: 90%; 1H NMR (CDCl3, 500 MHz): d=1.25 (t, J=
7.6 Hz, 6H), 1.31 (s, 36H), 2.64 (q, J=7.6 Hz, 4H), 3.99 (t, J=4.8 Hz,
4H), 4.04 (t, J=4.8 Hz, 4H), 4.07 (t, J=4.8 Hz, 4H), 4.10 (t, J=4.8 Hz,
4H), 4.16 (t, J=4.8 Hz, 4H), 4.24 (t, J=4.8 Hz, 4H), 4.31 4.35 (m, 8H),
6.81 6.83 (m, 4H), 6.85 (d, J=7.5 Hz, 2H), 6.86 (d, J=7.5 Hz, 2H), 7.03
(d, J=8.7 Hz, 4H), 7.06 7.12 (m, 20H), 7.23 7.26 (m, 8H), 7.33 (dd, J=
8.4, 7.5 Hz, 2H), 7.34 (dd, J=8.4, 7.5 Hz, 2H), 7.56 (d, J=8.7 Hz, 4H),
7.61 (s, 4H), 7.89 (d, J=8.4 Hz, 2H), 7.90 ppm (d, J=8.4 Hz, 2H);
13C NMR (CDCl3, 125 MHz): d=15.2, 28.1, 29.6, 31.3, 34.2, 63.1, 67.3,
67.6, 67.9, 67.9, 69.9, 70.0, 70.0, 105.7, 113.1, 114.6, 114.6, 114.9, 124.0,
125.0, 126.6, 126.7, 126.7, 126.9, 127.9, 130.6, 131.0, 132.1, 133.4, 139.0,
139.7, 141.3, 144.1, 144.5, 148.2, 154.2, 154.2, 156.5, 158.3 ppm; MS(FAB):
General procedure for the preparation of the dumbbell-shaped com-
pounds 7 and 8: A mixture of the alcohols 13[16] or 15[8c,13] (0.600 mmol)
and NaH (4.8 mmol) in dry THF (20 mL) was heated under reflux for
30 min, and a solution of the tosylate 14 or 16[8c,13] (639 mg, 0.673 mmol),
respectively, in THF (5 mL) was then added into the mixture. The reac-
tion mixture was heated and stirred under reflux for 16 h. After cooling
down to room temperature, H2O was added to the mixture. The mixture
was then extracted with CH2Cl2 (3î50 mL) and brine, and dried
(MgSO4). The solvent was removed and the residue was subjected to
column chromatography (SiO2, EtOAc/Hexane: 1/2) to give the dumb-
bell-shaped compounds.
Compound 7: Yield: 87%; 1H NMR (CD3COCD3, 500 MHz): d=1.16 (t,
J=7.5 Hz, 6H), 1.25 (s, 36H), 2.57 (q, J=7.5 Hz, 4H), 3.61 (t, J=4.5 Hz,
4H), 3.67 (t, J=4.4 Hz, 4H), 3.86 3.91 (m, 8H), 3.93 3.97 (m, 4H), 4.06
4.11 (m, 4H), 4.16 4.20 (m, 4H), 4.20 4.24 (m, 4H), 6.75 6.80 (m, 4H),
6.83 (d, J=6.2 Hz, 2H), 6.85 (d, J=6.2 Hz, 2H), 7.03 7.09 (m, 20H),
7.23 7.29 (m, 12H), 7.80 (d, J=8.4 Hz, 2H), 7.81 ppm (d, J=8.4 Hz,
2562
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 2555 2564

Molecular Shuttles
2555 2564
m/z (%): 1816 (100) [M+1]+
C124H134O12: C 81.99, H 7.44; found: C 81.07, H 7.53.
;
elemental analysis calcd(%) for
2771
(10)
[MÀPF6]+
;
elemental
analysis
calcd
(%)
for
C160H165N4O12P4F24: C 65.90, H 5.70, N 1.92; found: C 64.16, H 5.76, N
1.66.
Compound 12: Yield: 63%; 1H NMR (CD3COCD3, 500 MHz): d=1.17
(t, J=7.6 Hz, 6H), 1.26 (s, 36H), 2.58 (q, J=7.6 Hz, 4H), 3.56 3.64 (m,
12H), 3.64 3.68 (m, 4H), 3.75 3.80 (m, 4H), 3.81 3.83 (m, 4H), 4.06
4.09 (m, 4H), 4.15 4.18 (m, 4H), 4.28 4.30 (m, 8H), 6.46, 6.48, 6.49, 6.49
(4 î s, 4H), 6.79 6.82 (m, 4H), 7.01 (d, J=8.6 Hz, 2H), 7.02 (d, J=
8.6 Hz, 2H), 7.05 7.11 (m, 20H), 7.26 7.28 (m, 8H), 7.59 (d, J=8.6 Hz,
2H), 7.60 (d, J=8.6 Hz, 2H), 7.63 ppm (s, 4H); 13C NMR (CD3COCD3,
125 MHz): d=14.8, 26.7, 27.8, 30.6, 33.8, 63.0, 67.2, 67.5, 67.6, 69.1, 69.2,
69.4, 69.4, 70.3, 70.4, 113.1, 114.9, 116.3, 116.4, 116.4, 116.5, 124.0, 126.6,
126.6, 127.6, 130.4, 130.7, 131.8, 134.7, 134.7, 134.8, 139.2, 139.2, 141.3,
144.3, 144.6, 148.1, 156.8, 158.6 ppm; MS(FAB): m/z (%): 2024 (100)
[M+1]+ ; elemental analysis calcd (%) for C120H134O12S8: C 71.18, H 6.67;
found: C 70.81, H 6.64.
Compound 6¥4PF6: Yield: 51%; m.p. 2288C (decomp); UV/Vis (MeCN):
lmax =846 nm; 1H NMR (CD3COCD3, 500 MHz): d=1.11 1.19 (m, 6H),
1.20 1.30 (m, 36H), 2.51 2.61 (m, 4H), 3.51 4.36 (m, 40H), 5.82 5.95 (m,
4H), 5.96 6.08 (m, 4H), 6.20, 6.25, 6.34, 6.38 (4 ? s, 4H), 6.48 (m, 1H),
6.61 6.84 (m, 7H), 6.95 7.11 (m, 20H), 7.19 7.30 (m, 8H), 7.35 7.62 (m,
8H), 7.82 7.99 (m, 8H), 8.10 8.38 (m, 8H), 9.33 9.50 ppm (m, 8H); MS
(ESI): m/z (%): 1418 [MÀ2PF6]2+, 897 [MÀ3PF6]3+ ; elemental analysis
calcd (%) for C156H166N4O12S8P4F24: C 59.95, H 5.35, N 1.79; found: C
59.99, H 5.37, N 1.75.
General procedure for the preparation of the [2]rotaxanes: A solution of
the dumbbell-shaped compounds 7, 8, 9, 10, 11, or 12 (0.035 mmol), the
dicationic salt 18¥2PF6 (74 mg, 0.105 mmol) and the dibromide 17 (28 mg,
0.105 mmol) in anhydrous DMF (10 mL) was stirred at room tempera-
ture for 10 d (after approximately 2 d the color changed to dark purple
(for DNP) or dark green (for TTF) and a white precipitate formed).
After removal of the solvent, the residue was subjected to column chro-
matography (SiO2) and unreacted dumbbell-shaped compounds was
eluted with Me2CO, whereupon the eluent was changed to Me2CO/
NH4PF6 (1.0 g NH4PF6 in 100 mL Me2CO) and the colored band contain-
ing the [2]rotaxanes was collected. Most of the solvent was removed
under vacuum. After adding H2O (50 mL) to the residue, the precipitate
was collected by filtration, washed with Et2O (30 mL), and dried in
vacuo to afford the [2]rotaxanes.
Acknowledgment
This research was supported by the Defense Advanced Research Projects
Agency (DARPA). Part of this work is based upon research supported
by the National Science Foundation under equipment grant CHE-
9974928.
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Compound 1¥4PF6: Yield: 54%; m.p. 256 2588C; UV/Vis (MeCN):
lmax =514 nm; 1H NMR (CD3CN, 500 MHz): d=1.06 1.15 (m, 6H), 1.20,
1.26 (2 î s, 36H), 2.18 2.23 (m, 1H), 2.24 2.31 (m, 1H), 2.47 2.63 (m,
4H), 3.54 4.41 (m, 32H), 5.53 5.81 (m, 8H), 5.81 5.89 (m, 1H), 6.03
6.10 (m, 1H), 6.11 6.18 (m, 1H), 6.31 6.38 (m, 1H), 6.57 6.74 (m, 2H),
6.77 7.33 (m, 42H), 7.78 8.04 (m, 10H), 8.29 8.45 (m, 4H), 8.63 8.81 (m,
2H), 8.86 9.01 ppm (m, 2H); MS (FAB): m/z (%): 2526 (15) [MÀPF6]+,
2381 (30) [MÀ2PF6]+, 2236 (10) [MÀ3PF6]+ ; elemental analysis calcd
(%) for C142H154N4O11P4F24: C 63.81, H 5.81, N 2.10; found: C 62.90, H
5.98, N 2.00.
Compound 2¥4PF6: Yield: 51%; m.p. 106 1088C (decomp); UV/Vis
(MeCN): lmax =846 nm; 1H NMR (CD3CN, 500 MHz): d=1.17 (t, J=
7.6 Hz, 6H), 1.26 (s, 36H), 2.57 (m, 4H), 3.20 4.28 (m, 40H), 5.47 5.83
(m, 8H), 6.01 6.36 (m, 4H), 6.49 6.64 (m, 2H), 6.66 6.82 (m, 2H), 6.96
7.17 (m, 20H), 7.19 7.33 (m, 8H), 7.51 7.95 (m, 16H), 8.75 9.15 ppm (m,
8H); MS (FAB): m/z (%): 2734 (5) [MÀPF6]+, 2589 (4) [MÀ2PF6]+ ; ele-
mental analysis calcd (%) for C138H154N4O11P4S8F24: C 57.53, H 5.39, N
1.94; found: C 59.85, H 5.23, N 1.99.
Compound 3¥4PF6: Yield: 25%; m.p. 251 2538C; UV/Vis (MeCN):
1
lmax =516 nm; H NMR (CD3CN, 500 MHz): d=1.11 1.18 (m, 6H), 1.19
1.28 (br, 36H), 2.31 2.39 (m, 2H), 2.51 2.60 (m, 4H), 3.80 4.00 (m, 8H),
4.04 4.12 (m, 2H), 4.13 4.34 (m, 16H), 4.36 4.48 (m, 4H), 4.71 4.79 (m,
2H), 5.55 5.74 (m, 8H), 5.86 5.98 (m, 2H), 6.17 6.28 (m, 2H), 6.71 6.85
(m, 6H), 6.86 6.93 (m, 2H), 6.94 7.00 (m, 2H), 7.01 7.14 (m, 24H),
7.15 7.35 (m, 14H), 7.62 7.72 (m, 2H), 7.80 8.10 (m, 12H), 8.46 8.71 (m,
4H), 8.71 8.99 ppm (m, 2H); MS(FAB): m/z (%): 2767 (12) [MÀPF6]+,
2621 (8) [MÀ2PF6]+, 2476 (7) [MÀ3PF6]+ ; elemental analysis calcd (%)
for C156H162N4O15P4F24: C 64.32, H 5.61, N 1.92; found: C 62.71, H 5.46,
N 1.85.
Compound 4¥4PF6: Yield: 11%; m.p. 221 2218C; 1H NMR (CD3CN,
500 MHz): d=1.17 (2ît, J=7.5 Hz, 6H), 1.23 1.27 (m, 36H), 2.57 (q,
J=7.5 Hz, 4H), 3.52 4.58 (m, 20H), 5.49 5.68 (m, 8H), 6.00, 6.04, 6.20,
6.21 (4îs, 2H), 6.52, 6.53, 6.56, 6.58 (4îs, 2H), 7.09 7.16 (m, 20H), 7.30
(d, J=8.4 Hz, 8H), 8.09 (d, J=8.5 Hz, 4H), 6.76 9.00 ppm (m, 52H); MS
(FAB): m/z (%): 2974 (10) [MÀPF6]+, 2831 (9) [MÀ2PF6]+.
[5] Both semiconductors (Heath group at Caltech) and metals (Wil-
liams group at Hewlett Packard) have been used as the bottom elec-
trode for molecular electronic devices incorporating bistable [2]cate-
nanes and [2]rotaxanes. Recently, it has been established that the
devices incorporating metals (See: Y. Chen, D. A. A. Ohlberg, X.
Li, D. R. Stewart, R. S. Williams, J. O. Jeppesen, K. A. Nielsen, J. F.
Stoddart, D. L. Olyniek, E. Anderson, Appl. Phys. Lett. 2003, 82,
1610 1612; Y. Chen, G.-Y. Jung, D. A. A. Ohlberg, X. Li, D. R.
Stewart, J. O. Jeppesen, K. A. Nielsen, J. F. Stoddart, R. S. Williams,
Nanotechnology 2003, 14, 462 468; D. R. Stewart, D. A A. Ohlberg,
P. A. Beck, Y. Chen, R. S. Williams, J. O. Jeppesen, K. A. Nielsen,
J. F. Stoddart, Nano Lett. 2004, 4, 133 136) as the bottom electrode
Compound 5¥4PF6: Yield: 25%; m.p. 252 2548C; UV/Vis (MeCN):
lmax =519 nm; 1H NMR (CD3COCD3, 500 MHz): d=1.13 1.21 (m, 6H),
1.21 1.34 (s, 36H), 2.53 2.61 (m, 4H), 2.72 2.80 (m, 2H), 3.87 4.68 (m,
32H), 5.93 6.15 (m, 8H), 6.20 6.38 (m, 2H), 6.39 6.60 (m, 2H), 6.77
7.16 (m, 32H), 7.19 7.37 (m, 10H), 7.45 7.66 (m, 6H), 7.69 7.96 (m,
10H), 8.15 8.52 (m, 8H), 9.10 9.51 ppm (m, 8H); MS (FAB): m/z (%):
2563
Chem. Eur. J. 2004, 10, 2555 2564
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

J. F. Stoddart et al.
FULL PAPER
do not switch as a result of a molecular-based process. Rather, the
Bissell, G. Clavier, R. GÛrski, A. E. Kaifer, S. J. Langford, G. Mat-
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J. F. Stoddart, Org. Lett. 2002, 4, 3565 3568.
observed bistability is probably
a result of reversible filament
growth between the two electrodes. This result does not pertain,
however, to devices incorporating semiconductors (polysilicon, for
example) as the bottom electrode. To date, all of the available ex-
perimental evidence suggests that the devices incorporating semi-
conducting bottom electrodes switch as a consequence of a molecu-
larly based electromechanical mechanism at low bias voltages (Æ
2 V).
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[11] For a discussion of first-principles computational methods which in-
dicate that, when the CBPQT4+ ring surrounds the DNP ring
system, the switch is ON, whereas, when it surrounds the TTF unit,
the switch is OFF, see: A. H. Flood, R. J. A. Ramirez, W. Deng,
R. P. Muller, W. A. Goddard III, J. F. Stoddart, Aust. J. Chem., in
press results. The difference in conductivity between these two
states is considered to be due to a change in the degree of delocali-
zation of the molecular orbitals when the ring is located around the
TTF unit, as opposed to when it encircles the DNP ring system. In
the case where the CBPQT4+ ring is located around the TTF unit,
the HOMO is localized on the TTF unit, while the LUMO is local-
ized on the DNP ring system. In the metastable state, where the
ring encircles the DNP ring system, the HOMO and LUMO are
both delocalized across both recognition sites. See also a report on
™Computational Nanotechnology∫ by E. K. Wilson, Chem. Eng.
News 2003, April 28, 27 29.
[23] The rate of shuttling in the TTF-containing rotaxanes was estimated
by determining an average value (Dnex ~7 sÀ1) of the peak separa-
tions for the many signals arising from tert-butyl groups in their hy-
drophobic stoppers at low temperatures and coming up with an esti-
mate (320 K) for the coalescence temperature, Tc. This information
was then inserted into the equation, kc =(pDnex)/21/2, to determine
the rate of exchange, kc, which was estimated to be ~16 sÀ1 at the Tc
of ~320 K. By using the Eyring equation, DGc° =RTcln(kch/kbTc), in
which R is the gas constant, h is Planck×s constant, and kb is Boltz-
mann×s constant, this rate constant (kc) corresponds to a DGc° of ~
17 kcalmolÀ1 at Tc. The rate constants at the appropriate coales-
cence temperatures for all three [2]rotaxanes, containing two TTF
[12] The electromechanical switching mechanism has been investigated
for similar molecules in a half-device (ref. [9]) and in a polymer
electrolyte thin film (unpublished results). The barrier for relaxation
of the metastable state to the ground state was determined by per-
forming cyclic voltammetry experiments at a series of temperatures
in these devices, and found to be ~18 and ~21 kcalmolÀ1, respec-
tively. This result can be compared to the barrier for shuttling in the
degenerate DNP-containing rotaxanes reported here, where the bar-
recognition sites, gave DGc° values ranging from 17 18 kcalmolÀ1
.
These results should be compared with those reported previously
for a TTF-based [2]pseudorotaxane for which the ™shuttling∫ mech-
anism is probably bimolecular, see: M. R. Bryce, G. Cooke, W. Dev-
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Chem. 2000, 65, 1924 1936.
rier is ~15 kcalmolÀ1
.
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3905 3913.
Received: November 17, 2003 [F5725]
[15] a) P.-L. Anelli, N. Spencer, J. F. Stoddart, J. Am. Chem. Soc. 1991,
113, 5131 5133; b) P.-L. Anelli, M. Asakawa, P. R. Ashton, R. A.
Published online: March 23, 2004
2564
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2004, 10, 2555 2564