Versatile self-complexing compounds based on covalently linked donor-acceptor cyclophanes

Article abstract of DOI:10.1002/chem.200400614

A range of covalently linked donor-acceptor compounds which contain 1) a hydroquinone (HQ) unit, 2) a 1,5-dioxynaphthalene (DNP) ring system, or 3) a tetrathiafulvalene (TTF) unit as the π-donor, and 4) cyclobis(paraquat-p- phenylene) (CBPQT⁴⁺)

Full text of DOI:10.1002/chem.200400614

FULL PAPER
Versatile Self-Complexing Compounds Based on Covalently Linked
Donor–Acceptor Cyclophanes
[a]
Yi Liu,Amar H. Flood,Ross M. Moskowitz,and J. Fraser Stoddart*
Abstract: A range of covalently linked
donor–acceptor compounds which con-
tain 1) a hydroquinone (HQ) unit, 2) a
ated with the equilibria have been un-
raveled by using variable-temperature
1H NMR spectroscopy.The negative
DH8 and DS8 values account for the
fact that the “uncomplexed” conforma-
tion becomes the dominant species,
since the entropy gain associated with
the decomplexation process overcomes
the enthalpy loss resulting from the
breaking of the donor–acceptor inter-
actions.The armꢀs in-and-out move-
ments with respect to the linked cyclo-
phanes can be arrested by installing a
bulky substituent at the end of the
arm.In the case of compounds carrying
a DNP ring system in their side arm,
two diastereoisomeric, self-complexing
conformations are observed below
272 K in hexadeuterioacetone.By con-
trast, control over the TTF-containing
armꢀs movement is more or less inef-
fective through the thermally sensitive
equilibrium although it can be realized
by chemical and electrochemical ways
as a result of TTFꢀs excellent redox
properties.Such self-complexing com-
pounds could find applications as
thermo- and electroswitches.In addi-
tion, the thermochromism associated
with the armꢀs movement could lead to
some of the compounds finding uses as
imaging and sensing materials.
1,5-dioxynaphthalene
system, or 3)
(DNP)
tetrathiafulvalene
ring
a
(TTF) unit as the p-donor, and 4)
cyclobis(paraquat-p-phenylene)
(CBPQT4+) as the p-accepting tetraca-
tionic cyclophane were prepared and
shown to operate as simple molecular
machines.The p-donating arms can be
included inside the cyclophane in an in-
tramolecular fashion by virtue of stabi-
lizing noncovalent bonding interac-
tions.What amounts to self-complex-
ing/decomplexing
equilibria
were
Keywords: donor–acceptor
sys-
shown to be highly temperature de-
pendent when the p-donating arm con-
tains either an HQ or DNP moiety.
The thermodynamic parameters associ-
tems molecular switches
·
·
self-complexing · thermochromism ·
thermodynamics
Introduction
actions in response to a given stimulus associated with the
[4]
generation of distinctive signals.Recent examples
of such
As early as in 1959, Feynman addressed the importance of
materials functioning at the molecular level in his lecture
entitled “There is Plenty of Room at the Bottom”.[1] Since
then, the design and synthesis of functional molecules[2]
have become one of the most important branches of chemis-
systems (Scheme 1a and b) include bistable switchable
[2]catenanes[5,6] and [2]rotaxanes,[7,8] which contain two ring
[3]
try.In particular, a molecular machine refers to a molecule
that contains multiple movable components, the locations
and motions of which can be controlled by external stimuli.
An efficient molecular machine requires fast and reversible
[a] Dr.Y.Liu, Dr.A.H.Flood, R.M.Moskowitz, 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
Scheme 1.Graphical representations of switchable molecular machines,
a) a bistable [2]catenane, b) a bistable [2]rotaxane and c) a covalently
linked donor–acceptor macrocyle.S and S ’ indicate external stimuli.
Chem. Eur. J. 2005, 11, 369 – 385
DOI: 10.1002/chem.200400614
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369

components and one ring and one dumbbell component, re-
spectively, that are interlocked with each other by mechani-
cal bonds.The relative ring-to-ring or ring-to-dumbbell loca-
tions are dictated by noncovalent interactions.By switching
“off” and “on” these mutual interactions using chemical,[9]
electrochemical,[10] or photochemical[11] means, the relative
locations of the components can be changed accordingly to
generate machinelike molecular motions.
Following an analogous yet different approach, controlla-
ble molecular motions can exist in a self-complexing system
(Scheme 1c), in which an arm that is covalently linked to a
ring component has sufficient flexibility such that the arm
can be included inside the ringꢀs cavity by virtue of the pre-
programmed, stabilizing, noncovalent bonding interactions.
Several self-complexing systems[12–14] have been constructed
by the attachment of an arm component to an already pre-
formed ring, for example, a b-cyclodextrin,[12] a crown
ether,[13] or a cyclam.[14] Controlling the armꢀs movement
takes advantage in these different systems of 1) hydrophobic
interactions,[12] 2) hydrogen bonding,[13a] and 3) ionic[13b,14] in-
teractions.
Self-complexing compounds—such as 1·4PF6 and 2·4PF6
shown in Scheme 2—have also been obtained[15,16] by carry-
ing out macrocyclizations mainly with the aid of donor–ac-
ceptor interactions[17] around an aromatic template (usually
p-donating) that is covalently linked to one of the ringꢀs pre-
cursors.In the case of the cyclophane 1·4PF6, it was found
(Scheme 2a) that the self-complexing conformation in which
the 1,5-dioxynaphthalene (DNP) ring system resides inside
the tetracationic cyclophane, cyclobis(paraquat-p-phenyl-
ene) (CBPQT4+), is strongly favored[15] over the “uncom-
plexed” conformation in solution.However, in the case of
the cyclophane 2·4PF6, the “uncomplexed” conformation in
which the substituted tetrathiafulvalene (TTF) unit is pres-
ent in a dangling arm (Scheme 2b) was observed to be much
more favored[16] in a very slow equilibrium process.In the
case of both of these examples, temperature is a less than
successful handle for controlling the armꢀs position, thus
limiting the usage of these systems as thermoswitches.For
the purpose of building more efficient machines that can
generate quick and reversible molecular movements, the
molecular structures need to be fine-tuned in order to adjust
their self-complexing abilities.
In this paper, we demonstrate that the in-and-out move-
ments of the arms can be made highly sensitive to tempera-
ture and applied voltages after an all-important structural
modification, thus rendering these systems potential thermo-
sensors and electroswitches.In contrast with the benzoyl
linkage in 1·4PF6, a p-donor-containing arm is attached[18]
by a spacer to a diimide that is fused symmetrically onto
the CBPQT4+ cyclophane in the molecular structures
(Scheme 2c) of compounds 3–7·4PF6.The diimide moiety
serves to increase the distances between the donors and the
acceptors, as well as simplifying the spectroscopic analyses.
We describe here 1) the template-directed synthesis of five
self-complexing compounds 3–7·4PF6, 2) their quick and
reversible thermally and electrochemically controllable
switching behavior together with 3) a description of switch-
ing hysteresis, 4) a novel diastereoisomerism observed in the
DNP-containing compounds in addtion to their chromo-
Scheme 2.The complexation/decomplexation equilibria for a) 1·4PF6 and b) 2·4PF6 in solution showing that 1·4PF6 favors a self-complexed conforma-
tion, while 2·4PF6 favors an “uncomplexed” one, and c) molecular structures of self-complexing compounds 3–7·4PF6.
370
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Chem. Eur. J. 2005, 11, 369 – 385

Self-Complexing Compounds
FULL PAPER
phoric
receptor
behavior
toward TTF, and 5) the fixing
of the threading/dethreading
motion of the arm in one case
by attachment of a bulky stop-
per to the end of its arm.The
potential of the self-complexing
molecules to undergo reversible
movements of their arm into
and out of their ringꢀs cavities
in response to a particular stim-
ulus renders them viable candi-
dates for the construction of
nanoscale machinery.[19] In addi-
tion, interesting properties asso-
ciated with the thermo-switch-
ing behavior could lead to their
future applications as imaging
and sensing materials.
Results and Discussion
Synthesis: The synthesis of
3·4PF6 is outlined in Scheme 3.
The alcohol 10 was obtained by
reaction of the tosylate 8[7b]
with phenol (9) in the presence
of K2CO3 in 95% yield.Esteri-
fication of 10 with the carboxyl-
ic acid derivative 11[18] with 1,3-
Scheme 3.The synthesis of the HQ-containing compound 3·4PF6 and the equilibrium (I) between the “uncom-
plexed” and self-complexing conformations UC-3·4PF6 and SC-3·4PF6, respectively.
dicyclohexyl
carbodiimide
(DCC) as the coupling agent
gave the desired dibromide 12
in 71% yield.The cyclophane
3·4PF6 was isolated in a yield
of 9% by reaction of the di-
bromide
12
with
the
bis(hexafluorophosphate) salt
13·2PF6,[15] followed by counter-
ion exchange and column chro-
matography, by using a mixture
of MeOH/NH4Cl (2m)/MeNO2
(7:2:1) as the eluent.The equili-
brium between “uncomplexed”
(UC) and self-complexed (SC)
conformations in solution are
described in Equilibrium (I)
(Scheme 3).The equilibrium
constant K refers to the molar
ratio of the SC to the UC con-
formation present in the so-
lution.The
synthesis
of
DNP-containing
compounds
[18]
4·4PF6
and 5·4PF6 are out-
lined in Scheme 4.The alcohols
16 and 17 were obtained by re- Scheme 4.The synthesis of DNP-containing compounds 4·4PF6 and 5·4PF6.
Chem. Eur. J. 2005, 11, 369 – 385
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371

J.F.Stoddart et al.
action of the tosylate 14 with phenol (9) or 4-hydroxybenzy-
laldehyde (15) in the presence of K2CO3.Esterification of
16 or 17 with the carboxylic acid derivative 11, with DCC as
the coupling agent, gave the desired dibromide, either 18 or
19.The cyclophanes 4·4PF6 and 5·4PF6 were isolated after
reaction of either the dibromide 18 or 19 with the bis(hexa-
fluorophosphate) salt 13·2PF6, followed by counterion ex-
change and column chromatography, by using a mixture of
MeOH/NH4Cl (2m)/MeNO2 (7:2:1) as the eluent.Following
a similar synthetic protocol (Scheme 5), the TTF-containing
A and the concentration c0 over the range 3.6”10ꢀ3–2.8”
10ꢀ5 m is observed, suggesting the presence of a unimolecular
equilibrium and the absence of any high order equilibria.
1
Moreover, since the H NMR spectra of 4·4PF6 recorded at
243 K in CD3COCD3 show only marginal concentration de-
pendences in a range 3.11”10ꢀ2–0.97” 10ꢀ3 m, it seems un-
likely that supramolecular oligomers or polymers are being
formed to any significant extent.Similarly, the absorption
UV-visible spectra of 6·4PF6, recorded in MeCN at 258C,
show a strong CT band in the visible region (lmax =859 nm,
emax =3342mꢀ1 cmꢀ1), which is
characteristic for the donor–ac-
ceptor interactions between its
two bipyridium units and its
TTF unit.[9f] A linear correla-
tion between absorption A and
concentration c0 over the range
of 1.91”10ꢀ4 to 5.35”10ꢀ5 m is
also observed, suggesting the
absence of high order equili-
bria.The same conclusions
were reached for the other tet-
racationic cyclophanes 3·4PF6,
5·4PF6, and 7·4PF6.
1H NMR spectroscopy of the
HQ- and DNP-containing cy-
clophanes: Both SC and UC
conformations were observed in
CD3COCD3 by 1H NMR spec-
troscopy in the case of 3·4PF6,
4·4PF6, and 5·4PF6.Resonances
Scheme 5.The synthesis of TTF-containing compounds 6·4PF6 and 7·4PF6.
associated with either the UC
or SC conformation can be dif-
ferentiated as a result of the
[18]
self-complexing compounds 6·4PF6 and 7·4PF6 were ob-
following two properties: 1) the complexing/decomplexing
1
tained.The alcohols 21 and 22 were obtained separately by
the reactions of the tosylate 20[20] with either phenol (9) or
4-hydroxybenzylaldehyde (15) in the presence of K2CO3.Es-
terification of 21 or with the carboxylic acid derivative 11,
with DCC as the coupling agent, gave the desired dibro-
mide, either 23 or 24.The cyclophanes 6·4PF6 and 7·4PF6
were isolated after the reactions of either the dibromide 23
or 24 with the dicationic salt 13·2PF6, followed by counter-
ion exchange and column chromatography, by using a mix-
ture of MeOH/NH4Cl (2m)/MeNO2 (7:2:1) as the eluent.
equilibria in CD3COCD3 are slow on the H NMR timescale
within a certain temperature range, and 2) the molecular
structures of the SC conformations possess Cs symmetry,
while the UC conformations have averaged C2v symmetry
1
on the H NMR timescale, as a result of the fast rotation
of the bipyridium and xylene units.Consequently, equilibri-
um constants (K) for the self-complexing equilibria at dif-
ferent temperatures can be obtained by measuring the ratio
of these coexisting conformations from 1H NMR integra-
tion.
1
In the H NMR spectrum of 3·4PF6 (CD3COCD3, 196 K),
Establishing the intramolecular self-complexing behavior:
The intramolecular self-complexing behavior of 3–7·4PF6
was measured by titration experiments by employing
1H NMR and UV-visible spectroscopic methods.For exam-
ple, the absorption spectrum of 4·4PF6, recorded in MeCN
at 258C, shows a broad charge-transfer (CT) band in the
visible region (lmax =521 nm, emax =900mꢀ1 cmꢀ1), which is
characteristic of the donor–acceptor interactions between its
two bipyridium units and the complementary p-electron-rich
DNP ring system.[15] Linear correlations between absorption
four resonances were observed (Figure 1) for both the a-
and the b-bipyridinium protons, indicating a structure for
SC-3·4PF6 with Cs symmetry.The two doublets at very high
field (d=1.99 and 2.45 ppm), together with two doublets at
d=5.62 and 5.70 ppm, correspond to the four HQ ring pro-
tons, indicating that this ring is located inside the cavity of
the cyclophane.[21] Additionally, no resonances were evident
for protons from the “uncomplexed” HQ ring.As the tem-
perature was increased in the range between 196 and
260 K,[22] resonances for the UC-3·4PF6 conformation in-
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Self-Complexing Compounds
FULL PAPER
Changing the p-electron donor from an HQ ring to a
1
DNP ring system introduces more complexity into H NMR
spectra, because of the DNPꢀs additional element of planar
chirality,[24] a property which gives rise to two self-complex-
ing conformations for each of the DNP-containing cyclo-
phanes 4·4PF6 and 5·4PF6.One of the two possible self-
complexing conformations, in which H-4 on the DNP ring is
pointing away from diimidophenylene ring and towards the
xylene ring of the tetracationic cyclophane, is denoted as
exo-SC (Scheme 6).The other conformation, in which H-4
on the DNP ring system is pointing towards the diimidophe-
nylene ring of the tetrcationic cyclophane, is denoted as
endo-SC.Both of the SC conformations are in equilibrium
with a UC conformation.The equilibria demonstrated by
both 4·4PF6 and 5·4PF6 in CD3COCD3 solutions were char-
Figure 1.The 1H NMR spectrum of SC-3·4PF6 recorded in CD3COCD3
at 196 K corresponding to a self-complexing conformation.
creased in intensity.Equilibrium constants, as well as the de-
rived free energies (DG8) at different temperatures were ob-
tained (Table 1).Thermodynamic parameters, such as DH8
and DS8 values, were deduced by plotting lnK against 1/T.
The vanꢀt Hoff plot[23] of Equilibrium (I) (Figure 2) gives
DH8 and DS8 values of ꢀ6.95 kcalmolꢀ1 and ꢀ26.3 cal
molꢀ1 Kꢀ1, respectively.
[a]
Table 1.Thermodynamic data
at different temperatures.
for Equilibrium (I) of 3·4PF6 in MeCN
T [K][b]
K
DG8 [kcalmolꢀ1
]
207
218
228
237
249
260
35.7
16.8
9.1
4.8
2.1
-3.6
ꢀ2.8
ꢀ2.2
ꢀ1.6
ꢀ0.74
ꢀ0.18
1.2
[a] Determined by 1H NMR spectroscopy.[b] Calibrated by using a neat
MeOH sample.
Scheme 6.The equilibria between the UC-conformation and the exo-SC
conformation and the endo-SC conformation for the DNP-containing
compounds 4·4PF6 and 5·4PF6.The descriptor exo refers to the SC con-
formation in which H-4 on the DNP ring is pointing away from diimido-
phenylene ring and towards the xylene ring, while the descriptor endo
refers to the SC conformation in which H-4 on the DNP ring is pointing
towards the diimidophenylene ring.
Figure 2.The vanꢀt Hoff plot for the equilibrium between the UC and SC
conformations of 3·4PF6.The following equation for the equilibrium was
obtained: (lnK/T)=(ꢀ6.95 kcalmolꢀ1)(1/RT)+(ꢀ26.3 calmolꢀ1 Kꢀ1)/R.
Chem. Eur. J. 2005, 11, 369 – 385
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373

J.F.Stoddart et al.
acterized by variable-temperature 1H NMR spectroscopy.
Although the resonances (Figure 3) from the three confor-
mations overlap each other partially, three species are clear-
On the basis of a 2D TROESY spectrum of 5·4PF6 re-
corded at 227 K (Figure 4), the exo-SC conformation can be
assigned as the major self-complexing isomer present in the
solution.The ratio of the exo-SC to the endo-SC conforma-
tion is 3.7:1. A triplet, centered on d=6.47 ppm, correlates
to a doublet (H-8 of the DNP ring) at d=2.64 ppm and thus
can be assigned to H-7 of the DNP ring.This triplet, in turn,
exhibits through-space correlation to the singlet for the di-
imidophenylene protons (H-a) at d=8.82 ppm (Figure 4).
This through-space correlation is the key to the assignment,
because it is only in the exo-SC conformation that such a
correlation between DNP protons and diimidophenylene
protons could be present.By contrast, in the endo-SC con-
formation, no such correlation should be observed, since the
DNP and diimidophenylene protons are far apart, as illus-
trated in Figure 4.Thus, the major isomer is exo-SC-5·4PF6
and the minor one is endo-SC-5·4PF6.
Equilibrium constants [Kexo for Equilibrium (II) and Kendo
for Equilibrium (III); Scheme 6] and the derived free ener-
gies DG8 (Table 2) for self-complexing in 5·4PF6 were mea-
sured at different temperatures.The vanꢀt Hoff plots
(Figure 5) give DH8 and DS8 values of ꢀ11.2, ꢀ7.45 kcal
molꢀ1 and ꢀ44.1, ꢀ29.6 calmolꢀ1 Kꢀ1 for the Equilibria (II)
and (III), respectively.A similar treatment was applied to
4·4PF6, from which equilibrium constants [Kexo for Equilibri-
um (II) and Kendo for Equilibrium (III)] and derived free en-
Figure 3.Partial 1H NMR spectra of 5·4PF6 recorded in CD3COCD3 at
different temperatures, showing the a-bipyridinium proton region (d=
8.7–9.5 ppm) and H-4 and H-8 of DNP ring system (d=2.6–2.8 ppm) of
&
*
*
the exo-SC ( ), endo-SC ( ) and the UC-conformations ( ).
ly discernable.In particular, the
location of the a-H resonances
of the bipyridinium units at low
field, and the resonances for H-
4 and H-8 on the DNP ring
system at high field, are evident
for all three conformations.The
UC conformation of 5·4PF6, as
indicated (Figure 3a) by the
two doublets (full circles), was
the major one in CD3COCD3
solution at 272 K.As the tem-
perature was lowered, however,
the equilibria become displaced
towards the formation of the
SC conformations, which are in-
dicated (Figure 3b–e) by the
emergence of two sets (empty
circles and full squares) of four
doublets in the region of d=
Figure 4.The partial 2D TROESY spectrum of 5·4PF6 recorded in CD3COCD3 at 227 K.The correlation be-
tween the DNP proton (H-7) and the diimidophenylene protons (H-a) in the major conformational isomer
8.7–9.6 ppm. Formation of the
SC conformations were also
exo-SC-5·4PF6 is highlighted (Box).The diastereotopic CH protons (H-b and H-b’) can also be assigned as a
2
evidenced by the appearance of result of a correlation of H-b (at d=6.32 ppm) with H-a.
two sets of doublets with un-
equal intensities, resonating at
very high field between d=2.5
and 3.0 ppm and corresponding to the shielded H-4 and H-8
protons on the DNP ring system in the two SC conforma-
tions.[25]
ergies DG8 (Table 2) for its self-complexation at different
temperatures were obtained.The vanꢀt Hoff plots (Figure 6)
give DH8 and DS8 values as ꢀ9.67, ꢀ6.72 kcalmolꢀ1 and
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Self-Complexing Compounds
FULL PAPER
[a]
Table 2.Thermodynamic data
for Equilibria (II) and (III) of both
enthalpically favored process as a result of favorable donor–
acceptor interactions, while the negative DS8 values suggest
that SC conformations have more constrained structures
and are less flexible than the UC conformations.As temper-
ature increases, the entropy gain associated with the decom-
plexation processes overcomes the enthalpy loss from dimin-
ishing donor–acceptor stabilization, pointing the equilibrium
towards the formation of UC conformations.
4·4PF6 and 5·4PF6 in MeCN at different temperatures.
T
Kexo
Kendo
DGoexo
DGoendo
[K][b]
[kcalmolꢀ1
]
[kcalmolꢀ1
]
4·4PF6
5·4PF6
227
238
250
261
272
227
237
248
259
271
18.2
7.0
2.5
1.1
0.55
18.2
7.7
2.9
0.95
0.33
3.8
2.0
1.0
0.59
0.31
4.9
2.5
1.3
ꢀ1.3
ꢀ0.60
ꢀ0.33
0.00
0.28
0.63
ꢀ0.72
ꢀ0.44
ꢀ0.14
0.22
ꢀ0.92
ꢀ0.46
ꢀ0.031
0.32
ꢀ1.3
ꢀ0.96
ꢀ0.53
0.026
0.60
Thermochromism: One interesting property associated with
the temperature-dependent complexing/decomplexing be-
havior is thermochromism.Since the UC conformations are
colorless, while the SC conformations are colored, a ther-
moreversible color change can be displayed by the heat-in-
duced conformational changes.Typically, variable-tempera-
ture UV-visible absorption spectroscopic studies of a so-
lution of 4·4PF6 in MeCN (Figure 7) reveal that, as the so-
0.65
0.34
0.58
[a] Determined by 1H NMR spectroscopy.[b] Calibrated by using a neat
MeOH sample.
Figure 5.The vanꢀt Hoff plots for the equilibria between the UC confor-
mation and the exo-SC or the endo-SC conformations of 5·4PF6.The fol-
lowing equations for the two equilibria were obtained: (lnK/T)=
(ꢀ11.2 kcalmolꢀ1)(1/RT)+(ꢀ44.1 calmolꢀ1 Kꢀ1)/R
and
(lnK/T)=
(ꢀ7.45 kcalmolꢀ1)(1/RT)+(ꢀ29.6 calmolꢀ1 Kꢀ1)/R.
Figure 7.UV-visible spectra of 4·4PF6 recorded in MeCN in the tempera-
ture range 0–508C, showing the intensity decrease of the charge-transfer
band at around 520 nm.
lution is heated from 0 to 508C, the intensity of the CT
band around 520 nm decreases accordingly, correlating with
the decomplexation of the SC-4·4PF6.The temperature-de-
pendent color change is completely reversible and can be re-
cycled many times.This thermochromic behavior occurs as a
result of thermally induced molecular motions.By contrast,
the TTF-bearing 6·4PF6 and 7·4PF6 compounds display very
small temperature-dependent spectroscopic changes in the
temperature range +10 to +508C, with an intense green
color persisting throughout.The stronger donor–acceptor in-
teractions[9f] between TTF and CBPQT4+ is believed to be
responsible for the weaker temperature dependence shown
by 6·4PF6 and 7·4PF6.
Figure 6.The vanꢀt Hoff plots for the equilibria between the UC confor-
mation and the exo-SC or the endo-SC conformations of 4·4PF6.The fol-
lowing equations for the two equilibria were obtained: (lnK/T)=
(ꢀ9.67 kcalmolꢀ1)(1/RT)+(ꢀ36.8 calmolꢀ1 Kꢀ1)/R
and
(lnK/T)=
1H NMR spectroscopy of the TTF-containing cyclophanes:
(ꢀ6.72 kcalmolꢀ1)(1/RT)+(ꢀ26.9 calmolꢀ1 Kꢀ1)/R.
1
No UC conformation is observed in the H NMR spectra of
ꢀ36.8, ꢀ26.9 calmolꢀ1 Kꢀ1 for Equilibria (II) and (III), re-
spectively.
The negative DH8 values associated with Equilibria (I)–
(III) indicate that the formation of SC conformations is an
the TTF-containing compounds 6·4PF6 and 7·4PF6, an ob-
servation that is presumably the result of the strong donor–
acceptor interactions between the TTF unit and the
CBPQT4+ cyclophane.In addition, each compound exists as
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J.F.Stoddart et al.
a mixture of cis/trans-TTF isomers.These isomers can inter-
convert between each other under different conditions (vide
infra).
almost imperceptible under ambient light conditions.The
trans-to-cis isomerization can also be triggered by the addi-
tion of acid.When a trace amount of trifluoroacetic acid
(TFA) is added into a solution of trans-6·4PF6 in Me2CO,
the same equilibrium state (cis/trans=2:3) is attained after
two weeks.The all- cis self-complex could not be prepared.
1
The H NMR spectra of both 6·4PF6 and 7·4PF6 recorded
in CD3COCD3 show that two species with Cs symmetry are
present such that the ratio varies when they experience dif-
ferent conditions, for example, acid,[26] electricity,[27] and
light.[28] The interconversion suggests that the two species
are indeed the cis/trans-TTF isomers that can interconvert
between each other when conditions favorable to cis/trans
Chromophoric receptor behavior of self-complexing cyclo-
phanes toward TTF: The self-complexing cyclophanes
3·4PF6, 4·4PF6, and 5·4PF6 also behave as chromophoric
probes for the receptor binding of TTF.For instance, addi-
tion of one molar equivalent of TTF to the solution of
5·4PF6 in MeCN results in a dramatic color change of the
solution from purple to green, consistent with the inclusion
of TTF inside its cavity with the concomitant exclusion of
the DNP ring system.The color change reflects a shift in the
absorbance maximum, corresponding to the CT band[30] as-
isomerization are established.The
1H NMR spectrum
(CD3COCD3, 253 K) of 6·4PF6, recorded immediately after
workup, shows that the cis/trans isomers are present in the
ratio of about 2:3.However, when a solid mixture is stored
in the dark for two months only one of the isomers is fa-
vored (Figure 8a)—probably the trans one.[29] Four doublets
sociated with donor–acceptor interactions, from lmax
521 nm (when the DNP ring system is the donor) to lmax
=
=
850 nm (when the TTF unit is the donor).The 1H NMR
spectrum of this complex, recorded in CD3CN, displays sig-
nals at d=6.84 (1H), 7.11 (1H), 7.34 (2H), and 7.79 ppm
(2H) for the unbound DNP ring protons, indicating that the
DNP ring system is indeed expelled from the cavity.By con-
trast, the TTF ring protons resonate in the spectrum as a
singlet at d=5.69 ppm, reflecting the fact that they are
shielded within the cavity of the tetracationic cyclophane.[30]
A UV-visible dilution method was employed to determine
the binding constant (Ka) between TTF and 5·4PF6 in
MeCN, and a Ka value of 3250mꢀ1 was obtained.Such a che-
mosensitive color change, based on a supramolecular switch-
ing action, is characterized by its efficiency and simplicity.
Stoppering the arm movement of a DNP-containing cyclo-
phane: The in-and-out movement of the arm with respect to
the linked cyclophane can be arrested by installing a bulky
substituent at the end of the arm (Scheme 7).3,5-Di- tert-bu-
tylaniline (25) was chosen for 1) its bulk and 2) its reactivity
towards the formyl group in 5·4PF6.Both the SC conforma-
tions and the UC conformation of 5·4PF6 should react with
25 to generate stoppered version of all three conformations.
While the self-complexing equilibrium of 5·4PF6 is tempera-
ture sensitive, such behavior is expected to be prohibited
after the condensation reaction, since the bulky stopper will
prevent the arm from passing through the linked cyclo-
phane.A mixture of 25, 5·4PF6, and MgSO4 in CD3CN were
kept in an NMR tube at room temperature and the conden-
sation reaction was monitored by 1H NMR spectroscopy.
The completion of the reaction was verified by the disap-
pearance of the resonance at d=9.95 ppm, corresponding to
the aldehyde proton in 5·4PF6.The electrospray ionization
mass spectrometry (ESI-MS) of the reaction mixture
Figure 8.Partial 1H NMR spectra for the self-complexing cyclophane
6·4PF6 recorded in CD3COCD3 after irradiation with visible light
(500 W) for a) 0 min, b) 20 min, and c) 50 min, indicating the formation
of presumably the cis-TTF-containing isomer from the trans one.
resonating between d=10.0 and 9.0 ppm, together with four
doublets resonating between d=7.0 and 6.0 ppm, can be as-
signed to the eight a-protons of the bipyridinium units and
the eight methylene protons of the cyclophane, respectively.
The occurrence of four sets of signals is consistent with the
location of the TTF unit residing inside the cavity of the cy-
clophane.In addition, the two protons of the disubstituted
TTF units experience different shielding effects and are well
separated.When a solution of the all- trans self-complex, ob-
tained after dark storage, in CD3COCD3 is irradiated with
broad-band visible light (500 W) at room temperature, the
trans-to-cis isomerization is triggered, as evidenced by the
emergence of signals associated with the cis self-complex of
6·4PF6 (Figure 8b and 8c).An equilibrium (photostationary
state) is reached after 50 min when the cis/trans ratio is ap-
proximately 2:3.By comparison, such an isomerization is
showed
a singly charged peak at m/z value 1691.6
([MꢀPF6]+), together with doubly and triply charged peaks
at m/z values 773.3 ([Mꢀ2PF6]2+) and 467.2 ([Mꢀ3PF6]3+),
an observation which corresponds to the consecutive loss of
one, two, and three hexafluorophosphate ions (mol.wt. =
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Self-Complexing Compounds
FULL PAPER
Scheme 7.The “in” and “out” motion of the DNP-containing arm of 5·4PF6 is prevented when it is treated with a sterically hindered amine 25, which un-
dergoes a condensation reaction with 5·4PF6 to give 26·4PF6.
145 Da each) from the mixture
of products which we will refer
to as 26·4PF6.
The 1H NMR spectrum of a
solution recorded at 298 K indi-
cates (Figure 9a) the presence
of a major species correspond-
ing to UC-26a·4PF6, together
with small amount of another
species, which is presumably a
mixture of exo-SC-26b·4PF6
and endo-SC-26b·4PF6.All
these forms of 26·4PF6 are the
stoppered versions of the three
different
conformations
of
5·4PF6.By comparison, the
spectrum of 5·4PF6, taken
under the same condition (Fig-
ure 9b) is similar except that 1)
it is broader and 2) the signals
of SC conformations are barely
evident, suggesting that the
complexation/decomplexation
Figure 9.Partial 1H NMR spectra of a) 26·4PF6 at 298 K, b) 5·4PF6 at 298 K, c) 26·4PF6 at 233 K, and
equilibrium is fast on the d) 5·4PF6 at 233 K.The peaks with asterisks in a) and c) represent an excess of 3,5-di- tert-butylanaline (25).
1H NMR timescale at this tem-
perature.As the temperature is lowered to 233 K, UC-
26a·4PF6 remains the major species in solution (Figure 9c),
while the resonances for SC-26b·4PF6 become sharper.By
contrast, lowering the temperature of a solution of 5·4PF6 to
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J.F.Stoddart et al.
233 K displaces the equilibria towards the formation of SC
conformations (Figure 9d).Concurrently, there is no visual
color change when the solution of 26·4PF6 is cooled down
from room temperature to ꢀ408C, in sharp contrast to the
behavior of unstoppered 5·4PF6.All these observations sug-
gest that, by attaching a bulky stopper to the end of the self-
complexing system, the threading action of the side-arm is
passivated to generate an approximate “snapshot” of the
fixed equilibrium state.This stoppering strategy provides a
delicate way of controlling molecular motions through dy-
namic covalent-bond formation.[31]
TTF2+ dication no longer resides inside the CBPQT4+
cavity: its protons resonate at d=9.83 ppm. The remainder
of the spectrum of the hexacationic species is commensurate
with the UC-66+ conformation with averaged C2v symmetry.
When Zn dust is added to the NMR tube, the original spec-
trum (Figure 10c) is regenerated, indicating a return to the
SC-64+ conformation with the neutral TTF unit back inside
the cavity of the CBPQT4+ cyclophane.
Electrochemical switching behavior of a TTF-containing cy-
clophane: The cyclic voltammograms (CVs) of the self-com-
plexing macrocycle 6·4PF6, and of the dumbbell analogue
21, were recorded in order to reveal its electrochemical
switching behavior.In addition, the dynamic processes re-
sponsible for the oxidatively initiated switching between the
self-complexing and uncomplexed forms were investigated.
The decomplexation/complexation and oxidative electro-
chemical processes can be considered with reference to a re-
action scheme involving five species (Scheme 8).In the
Chemical switching behavior of a TTF-containing cyclo-
phane: Although temperature is more or less ineffective in
controlling the movement of the TTF arm in SC-64+ and
SC-74+, both chemical and electrochemical methods work
well.The reason is that the molecular recognition between a
TTF donor and a CBPQT4+ acceptor can be turned “off” by
the oxidation of the TTF unit to a charged species—either
+
TTFC or TTF2+—and “on” by their reduction back to the
starting state, an excess of SC-64+ exists relative to UC-64+
,
neutral form.The chemical redox cycle of 6·4PF6 has been
monitored by 1H NMR spectroscopy at 253 K in
CD3COCD3 (Figure 10).Prior to its oxidation, the TTF unit
in SC-6·4PF6 is observed to be a 2:3 mixture (d=6.0–
7.0 ppm) of cis/trans-isomers (Figure 10a).Upon addition of
two equivalents of the chemical oxidant, tris(p-bromophe-
nyl)aminium hexachloroantimonate,[9f] into the solution of
6·4PF6 in CD3COCD3, a much simpler spectrum (Fig-
ure 10b) is observed, indicating that, upon oxidation, the
with the relative amounts of each controlled by a dynamic
equilibrium pro-
cess, reaction A (Ka).The singly oxidized TTF C+ states of
the two forms of 6·4PF6 can be generated by applying posi-
tive potentials.Oxidation of the SC conformation by reac-
tion B results in the generation of a single positive charge
on the TTF unit residing within the tetracationic cyclophane
and consequently its oxidation potential, E’(SC-6)5+/4+ is ex-
pected to lie at more positive potentials than that for the
Figure 10.Partial 1H NMR spectra of 6·4PF6 recorded in CD3COCD3 at 253 K a) before oxidation, b) after addition of two eqeuivalents of tris(p-bromo-
phenyl)aminium hexachloroantimonate, and c) after addition of Zn dust as a reductant.The resonances indicated by the asterisk arise from the oxidan t.
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Self-Complexing Compounds
FULL PAPER
plexed TTF unit and is assigned
to two reversible TTF-based
oxidations concomitant with
molecular motion and a scan-
rate-dependent behavior.At
the faster scan rate of
1000 mVsꢀ1, the CV displays
only one two-electron oxidation
peak at +0.91 V versus SCE,
together with two single-elec-
tron reduction peaks on the
return sweep (Figure 11a).At
10 mVsꢀ1, two separate one-
electron oxidation processes are
observed
at
+0.53
and
+0.73 V versus SCE with their
corresponding reduction pro-
cesses along the return sweep
(Figure 11b).This scan-rate-de-
pendent observation differs
from that of the dumbbell,
which displays two reversible
and well-separated, one-elec-
tron oxidation processes, E1/2
-
Scheme 8.Parametric reaction scheme illustrating how 6·4PF6 interconverts between SC and UC conforma-
tions (parameter 1) as a function of the TTF oxidation state (parameter 2).Process A is a reversible thermody-
namic equilibrium; processes B, C, and E are reversible electrochemical equilibria; and D is an irreversible re-
action.
(21)+/0 =+0.36 and E1/2(21)2+/+
=
+0.70 V versus SCE, both of
which are independent of the
scan rate.Furthermore, CV res-
dumbbell 21.The monocationic state, initially in the SC con-
formation, is electrostatically unfavored—equivalent to a
transition state (°)—and so, rapidly decomplexes by reac-
tion D (kUC) to the thermodynamically favored UC confor-
mation.The small amount of UC-6·4PF6, which will also be
present in the starting state on account of equilibrium A, re-
sembles the free dumbbell and
olution of the cis and trans isomer forms in the 2:3 mixture
was not observed by comparison with the CVs of the all-
trans-SC-6·4PF6 compound.
The scan-rate dependence of the CV can be rationalized
ꢀ1
from Scheme 8.At 10 mVs
the starting state will contain
both UC and SC conformations.The initially small concen-
has a formal reduction poten-
tial, associated with reaction C
(E’(UC-6)5+/4+), which is pre-
dicted to lie at a similar poten-
tial
as
that
for
the dumbbell 21.The UC con-
formation is the only stable
form of the monocation.It dis-
plays one further single-elec-
tron oxidation process, reac-
tion E (E’(UC-6)6+/5+), which is
predicted to lie at the same po-
tential as that for the dumbbell.
The SC conformation of the di-
cation TTF2+ (not shown in
Scheme 8) is predicted to be
highly unfavored and thus, is
not considered an important
species when considering the
switching behavior of 6·4PF6.
The CV of 6·4PF6 reveals a
Figure 11.Cyclic voltammograms of 1”10 ꢀ3 m solutions of 6·4PF6 and 21 in MeCN at a) 1000 mVsꢀ1 and
b) 10 mVsꢀ1, and the proposed mechanistic pathways along which the oxidation process (thin arrows) and re-
typical profile[5b] for
a com- duction process (bold arrows) occurs at the c) fast and d) slow scan rates, respectively.
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J.F.Stoddart et al.
tration of the UC starting state will continually decrease by
electrochemical oxidation (reaction C).However, it will also
be continually replenished by the decomplexation equili-
brium A.The rate of replenishment by equilibrium A is suf-
ficiently fast to convert all of the SC conformation in the
starting state into the UC conformation, ready for subse-
quent oxidation at applied potentials that are greater than
+0.53 V, that is, all of the starting state becomes oxidized
along the reaction pathway C before it can be oxidized by
reaction B.Subsequently, the oxidative process E, which is
responsible for interconverting between the mono- and di-
cationic states, is dominated by the UC conformation, as
evidenced by the redox coupleꢀs reversibility (DE=70 mV)
and its similarity to the dumbbell.Returning the monocat-
ionic state of the UC conformation to the SC starting state
is, again, along a pathway that is dominated by coupling be-
tween the electrochemical reaction C and the dynamic equi-
librium A.The coupling of these two reactions generates
the appearance of a quasi-reversible (DE=120 mV) oxida-
tion process at +0.53 V that is anodically shifted by 170 mV
(Figure 11a) relative to the dumbbell.Anodic shifts of
140 mV have been reported for TTF-based macrocyclic
hosts in the presence of the ionic Pb2+ guests,[32] and a simi-
lar rationalization for the observed shift was proposed.In
contrast, at 1000 mVsꢀ1, the rate of the decomplexation
equilibrium A is slow by comparison with the sweep rate of
the applied potential.Therefore, the singly-oxidized state of
the SC conformation is produced by reaction B at or near
the same potential (ꢁ+0.9 V) as reaction E, which produces
the dicationic state of the decomplexed form.Consequently,
E1/2(SC-6)5+/4+ =E1/2(UC-6)6+/5+ and the two peaks coalesce
to give the appearance of a 2eꢀ process in the CV.There-
fore, the UC conformation of the dication is produced along
a different pathway at a faster scan rate than at a slower
scan rate.However, the starting state of the SC conforma-
tion is regenerated along the same pathway at both slow
It is informative to consider how the two mechanisms of
switching can be rationalized in terms of two explanatory
models[33] for biomolecular machinery.[34] The oxidatively
driven process is akin to the “power-stroke” mechanism,[33]
postulated to account for the rotary motions in ATPase.In
this instance, chemical oxidation of the TTF unit, rather
than binding and hydrolysis of ATP, generates an instanta-
neous high-energy conformational state that relaxes by mo-
lecular mechanical movements.Furthermore, given that the
cycle of switching at fast scan rates can be considered as a
hysteresis (Figure 11c) in two parameters—TTF oxidation
state and conformational state—then these molecular ma-
chines display a parametric trajectory that has been postu-
lated[35] to produce unidirectional motion at these nanoscop-
ic length scales.Alternatively, the oxidatively trapped pro-
cess reflects the utilization of the TTFꢀs oxidation, rather
than ATPꢀs hydrolysis, as a switch[36] that captures the mole-
cule in its “uncomplexed” conformation that is itself gener-
ated by stochastic Brownian motions.Although a thorough
analysis of the self-complexing systems, together with other
molecular switches is the objective of future work, this brief
discussion suggests that technological applications with arti-
ficial molecular machinery may continue to profit[36b,37] by
the transfer of concepts from the life sciences to materials
science.
In addition to its oxidative activity, 6·4PF6 displays rever-
sible reduction electrochemistry.The first two-electron re-
duction is split into two closely spaced one-electron process-
es at ꢀ0.25 and ꢀ0.35 V versus SCE. This splitting can be
assigned to the presence of chemical inequivalence between
the two paraquat units.[5b,38] The second two-electron reduc-
tion process, observed at ꢀ0.71 V versus SCE, occurs at the
same position as the free cyclophane[5b,38] and is therefore
consistent with the formation of the “uncomplexed” form
following the first two single-electron reduction processes.
The UV-visible spectroelectrochemistry of the oxidation
cycle of 6·4PF6 (Figure 12), obtained by using a controlled-
electrolysis protocol in an optically-transparent thin-layer
electrochemical (OTTLE) cell, was investigated in order to
confirm the site of redox activity and the slow-scan-rate
electrochemically driven decomplexation and complexation
reactions.Two oxidation and two reduction processes are
observed; this is consistent with the CV recorded at
10 mVsꢀ1, included as insets in Figure 12.The first oxidation
process C is initiated at +0.4 V and continues as the applied
potential is increased in 50 mV steps up to +0.65 V. The
TTF!CBPQT4+ band at 853 mm is bleached concomitant
with the formation of the TTF+-based chromophore in the
visible region.[39] The second oxidation process E displays a
bleach of the TTF+ chromophore as the TTF2+-based band
at 388 nm is observed to emerge.These changes are initiated
directly after the completion of the first series of changes at
+0.65 V in a manner that is consistent with the two closely
spaced oxidation waves observed in the CV (10 mVsꢀ1).
The reduction processes display the reverse spectroscopic
changes over a range of applied voltages that are less posi-
tive than those required for the oxidation, an observation
+
and fast scan rates, because the TTFC -based UC conforma-
tion is not in dynamic equilibrium with any other species.
The scan-rate dependent behavior distinguishes between
two different mechanisms that are available for electro-
chemically switching the self-complexed 6·4PF6 between
two states.At fast scan rates, the switching is oxidatively
driven and results in a hysteresis cycle; that is, when the
monocationic state of the SC conformation is generated by
oxidation B, it rapidly proceeds by the irreversible decom-
plexation reaction D at potentials that are sufficient to pro-
duce simultaneously the dicationic state by oxidation pro-
cess E.At slower scan rates, the UC conformation of the
starting state is oxidatively trapped.In this case, the inter-
conversion between SC-64+ and UC-65+ is along the same
pathway.Consequently, the applied potential needs only to
sit at intermediary voltages for oxidation reaction C to be
initiated in order to drive decomplexation reaction A to
completion.In summary, when the switching reaction is
driven quickly, the dicationic form TTF2+ is generated; how-
ever, when it is driven slowly, it is the monocationic TTFC+
form that is produced.
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Self-Complexing Compounds
FULL PAPER
1,5-dioxynaphthalene ring system are purple in their self-
complexed form and consequently display thermochromism.
On the other hand, the compounds containing tetrathiaful-
valene can be switched using chemical or electrochemical
means.In this manner, the initial self-complexed state can
be converted completely and almost instantaneously to the
“uncomplexed” conformation by electrochemical oxidation
of the tetrathiafulvalene unit to its dication or more slowly
via its monocation.Of the systems studied, the thermochro-
mic self-complexing compound is the most promising switch
for imaging and sensing materials.
Experimental Section
General methods: Reagents were purchased from Aldrich or synthesized
as described.The tosylates 8,[7b] 14,[18] and 20,[20] the carboxylic acid deriv-
[15]
ative 11,[18] and the bis(hexafluorophosphate) 13·2PF6
were all pre-
pared according to literature procedures.Solvents were purified accord-
ing to literature procedures.[40] Thin-layer chromatography (TLC) was
carried out by using aluminum sheets, precoated with silica gel 60F
(Merck 5554).The plates were inspected by UV-light prior to develop-
ment with iodine vapor.Melting points were determined on an Electro-
thermal 9200 apparatus and are uncorrected.UV-visible spectra were ob-
1
tained using a Varian Cary 100 Bio spectrophotometer. H and 13C NMR
spectra were recorded on a Bruker Avance 500, Avance 600, or ARX500
spectrometers, using the deuterated solvent as lock and the residual pro-
tiated solvent as internal standard.All chemical shifts are quoted using
the d scale, and all coupling constants (J) are expressed in Hertz (Hz).
Electrospray mass spectra (ESI-MS) were measured on a VG ProSpec
triple focusing mass spectrometer.Elemental analyses were performed
by Quantitative Technologies Inc.
Figure 12.UV-visible spectroelectrochemistry of a 1”10 ꢀ3 m MeCN (0.1m
TBAPF6) solution of 6·4PF6 at a) 0.40–0.65, b) 0.65–0.95, c) 0.75–0.65,
and d) 0.65–0.00 V. Insets: 10 mVsꢀ1 CV, with the potential ranges that
correspond to each redox process highlighted in bold.
General procedure for the preparation of the alcohols 10,16,17,21,and
22: A mixture of the appropriate monotosylate (1.0 mmol), the phenol 9
or 15 (1.2 mmol), K2CO3 (2.0 mmol), LiBr (catalytic amount), and
[18]crown-6 (catalytic amount) in MeCN (30 mL) was heated under
reflux for 24 h.The resulting suspension was filtered and the solid was
washed with Me2CO until the filtrate was colorless.The combined organ-
ic solution was evaporated and subjected to column chromatography
(SiO2: hexanes/EtOAc 1:4) to give the product.
that is consistent with the CV.The charge transfer band at
853 nm of the self-complexed neutral TTF state grows back
with the bleaching of the monocationꢀs spectrum.The pres-
ence of TTFC+ and TTF2+ chromophores in the spectroelec-
trochemistry is consistent with oxidation processes centered
on the TTF unit.The reformation of the spectrum of the
ground state is consistent with a switch that can be reversi-
bly operated under electrochemical control.
Alcohol 10: This alcohol was obtained as a white solid (0.34 g, 95%).
1
M.p. 67.0–68.58C; H NMR (CDCl3, 500 MHz, 298 K): d=2.94 (brs, 1H),
3.65 (t, J=4.2 Hz, 2H), 3.75 (m, 2H), 3.82 (t, J=4.2 Hz, 2H), 3.91 (m,
4H), 4.07 (t, J=4.2 Hz, 2H), 4.11 (t, J=4.2 Hz, 2H), 4.15 (m, 2H), 6.87
(s, 4H), 6.95 (m, 3H), 7.30 ppm (m, 2H); 13C NMR (CD3COCD3,
125 MHz, 298 K): d=61.5, 67.2, 67.9, 68.0, 69.6, 69.8, 69.9, 72.6, 114.5,
115.5, 115.6, 120.8, 129.3, 152.9, 153.1, 158.6 ppm; HRMS (MALDI): m/z
calcd for C20H26O6Na+ [M+Na]+: 385.1622; found: 385.1605; elemental
analysis calcd (%) for C20H26O6: C 66.28, H 7.23; found: C 66.12, H 7.23.
Alcohol 16: This alcohol was obtained as a white solid (0.35 g, 84%).
1H NMR (CD3OD, 500 MHz, 298 K): d=7.88, 7.85 (2d, J=3.7 Hz, 2H),
7.42–7.24 (m, 4H), 6.96–6.92 (m, 5H), 4.33 (m, 4H), 4.18 (t, J=4.5 Hz,
2H), 4.06 (t, J=4.5 Hz, 2H), 4.00 (m, 4H), 3.76 ppm (m, 4H); 13C NMR
(CD3OD, 125 MHz, 298 K): d=159.0, 154.2, 154.2, 128.9, 126.6, 126.6,
124.6, 124.6, 120.3, 114.1, 114.0, 114.0, 105.3, 105.2, 72.4, 69.6, 69.5, 69.3,
67.6, 67.0, 60.8, 60.8 ppm; MS (EI): m/z (%): 412.2 (90) [M]+.
Conclusion
A series of molecular switches in the form of covalently
linked donor–acceptor cyclophanes have been prepared by
fine-tuning the molecular structures so that they display re-
versible thermally or redox-activated molecular motions.
The donor-bearing arms in the former class, containing
either hydroquinone or 1,5-dioxynaphthalene ring systems,
are capable of rapidly complexing and decomplexing into
and out of the cyclophaneꢀs cavity, respectively, under ther-
mal control over a range of ambient temperatures from ꢀ40
up to +508C.Furthermore, the cyclophanes bearing the
Alcohol 17: This alcohol was obtained as a white solid (0.39 g, 88%).
M.p. 81.0–82.58C; 1H NMR (CD3COCD3, 500 MHz, 298 K): d=9.86 (s,
1H), 7.82 (m, 4H), 7.34 (m, 4H), 7.10 (d, J=7.6 Hz, 2H), 6.95 (t, J=
7.6 Hz, 2H), 4.30 (m, 6H), 4.04 (t, J=4.5 Hz, 2H), 3.99 (t, J=4.5 Hz,
2H), 3.95 (t, J=4.5 Hz, 2H), 3.65 ppm (m, 4H); 13C NMR (CD3COCD3,
125 MHz, 298 K): d=163.8, 154.3, 154.3, 131.5, 130.2, 126.6, 125.0, 114.8,
114.1, 114.1, 105.6, 105.5, 72.8, 72.8, 69.5, 69.3, 67.9, 67.8, 61.0. 60.9 ppm;
HRMS (MALDI): m/z calcd for C25H28O7Na+ [M+Na]+: 463.1727;
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J.F.Stoddart et al.
found: 463.1754; elemental analysis calcd (%) for C25H28O7: C 68.17, H
6.41; found: C 68.22, H 6.38.
9.89 (s, 1H), 7.83 (m, 4H), 7.75 (s, 2H), 7.36 (t, J=8.2 Hz, 1H), 7.33 (t,
J=8.2 Hz, 1H), 7.11 (d, J=8.2 Hz, 2H), 6.92 (dd, J=7.7 Hz, 2.7 Hz,
2H), 4.98 (s, 4H), 4.49 (s, 2H), 4.40 (t, J=4.5 Hz, 2H), 4.30 (m, 4H),
4.26 (t, J=4.5 Hz, 2H), 4.05 (t, J=4.5 Hz, 2H), 4.01 (t, J=4.5 Hz, 2H),
3.97 (t, J=4.5 Hz, 2H), 3.87 ppm (t, J=4.5 Hz, 2H); HRMS (MALDI):
m/z calcd for C37H35Br2NO10Na+ [M+Na]+: 834.0520; found: 834.0568.
Alcohol 21: This alcohol was obtained as a yellow wax (0.41 g, 79%).
1H NMR (CD3COCD3, 500 MHz, 298 K): d=7.29 (m, 2H), 6.96 (m, 3H),
6.57 (s, 1H), 6.55 (s, 1H), 4.36 (s, 4H), 4.15 (t, J=4.5 Hz, 2H), 3.85 (t,
J=4.5 Hz, 2H), 3.70–3.56 (m, 10H), 3.54 ppm (t, J=4.5 Hz, 2H);
13C NMR (CD3COCD3, 125 MHz, 298 K): d=158.9, 134.7, 129.2, 120.4,
116.6 (2), 116.5 (2), 114.4, 109.9, 72.6, 70, 3, 70.0, 69.4, 69.1 (2), 67.6, 67.5,
67.1, 61.0 ppm; MS (FAB): m/z (%): 516.05 (75) [M]+.
Dibromide 23: This dibromide was obtained as a yellow wax (0.15 g,
33%). 1H NMR (CD3COCD3, 500 MHz, 298 K): d=7.95 (s, 2H), 7.29
(m, 2H), 6.96 (m, 3H), 6.57 (s, 1H), 6.55 (s, 1H), 5.12 (s, 4H), 4.52 (s,
2H), 4.36 (s, 4H), 4.18 (t, J=4.5 Hz, 2H), 3.85 (t, J=4.5 Hz, 2H), 3.74–
3.62 ppm (m, 12H); 13C NMR (CD3COCD3, 125 MHz, 298 K): d=167.1,
166.3, 158.9, 137.3, 136.8, 134.7, 129.3, 128.4, 120.4, 116.6, 114.3, 70.3 (2),
69.4, 69.1, 69.0, 68.5, 67.2, 64.7, 38.5, 25.9 ppm; MS (FAB): m/z (%):
886.96 (20) [M]+.
Alcohol 22: This alcohol was obtained as a yellow wax (0.45 g, 83%).
1H NMR (CD3COCD3, 500 MHz, 298 K): d=9.92 (S, 1H), 7.90 (dd, J1 =
8.8 Hz, J2 =1.7 Hz, 2H), 7.16 (dd, J1 =8.8 Hz, J2 =1.7 Hz, 2H), 6.57 (s,
1H), 6.55 (s, 1H), 4.36 (s, 4H), 4.30 (t, J=4.5 Hz, 2H), 3.90 (t, J=4.5 Hz,
2H), 3.73 (m, 2H), 3.67–3.59 (m, 8H), 3.55 ppm (t, J=4.5 Hz, 2H);
13C NMR (CD3COCD3, 125 MHz, 298 K): d=190.2, 163.8, 134.8, 134.7,
134.7, 131.5, 130.2, 116.6, 116.5, 116.5 114.8, 72.6, 70, 3, 70.1, 69.2, 69.2
69.1, 67.8, 67.6, 67.5, 61.0 ppm; HRMS (MALDI): m/z calcd for
Dibromide 24: This dibromide was obtained as a yellow wax (0.32 g,
71%). 1H NMR (CD3COCD3, 500 MHz, 298 K): d=9.91 (s, 2H), 7.92 (s,
2H), 7.88 (t, J=8.8 Hz, 2H), 7.15 (t, J=8.8 Hz, 2H), 6.56 (s, 1H), 6.54 (s,
1H), 5.10 (s, 4H), 4.52 (s, 4H), 4.35 (m, 2H), 4.30 (t, J=4.5 Hz, 2H),
3.90 (t, J=4.5 Hz, 2H), 3.76–3.61 ppm (m, 10H); 13C NMR (CD3COCD3,
125 MHz, 298 K): d=190.2, 167.1, 166.3, 163.8, 137.3, 136.7, 134.7, 134.7,
130.5, 130.1, 128.4, 116.6, 116.5, 114.8, 70.3, 70.3, 69.2, 69.1, 69.1, 69.1,
68.5, 67.9, 64.7, 38.5, 25.9 ppm; MS (FAB): m/z (%): 914.95 (25) [M]+.
+
C23H28O7S4 [M]+: 544.0712; found: 544.0695.
Synthesis of the carboxylic acid derivative 11:[18] A mixture of 3,6-dime-
thylphthalimide (1.57 g, 8.97 mmol), tert-butyl 2-bromoacetate (1.92 g,
9.90 mmol), and K2CO3 (2.48 g, 17.90 mmol) in MeCN (50 mL) was
heated under reflux for 3 h.The resulting suspension was filtered and the
filtrate was evaporated to give a pale yellow solid, which was recrystal-
lized from EtOAc and hexanes (1:1) as colorless crystals (2.48 g, 95%).
Subsequently, a mixture of the obtained cyrstal (1.90 g, 6.57 mmol), NBS
(2.34 g, 13.15 mmol), and AIBN (20 mg, cat. amount) in CH2Cl2 (20 mL)
was heated under reflux for 4 h.The resulting suspension was filtered,
and the filtrate was evaporated and subjected to column chromatography
(SiO2: hexanes/EtOAc, 8:1) to give a white solid (1.40 g). The solid was
dissolved in CH2Cl2 (10 mL) and placed in an ice/water bath, and HNO3
(1.5 mL, 90%) was added dropwise into the solution. The mixture was
then stirred at room temperature for three more hours.The solvent was
evaporated under reduced pressure and the remaining solid was washed
with hexanes to give 11 as a white solid (1.09 g, 41% for three steps).
General procedure for the preparation of the self-complexing compounds
3–7·4PF6: A solution of the appropriate dibromide (0.36 mmol) and the
bipyridium salt 13·2PF6 (0.36 mmol) in DMF (5 mL) was stirred at room
temperature for eight days.In order to ensure full precipitation of the
purple salt, Et2O (50 mL) was added to the reaction mixture.The precipi-
tate was filtered off under reduced pressure and subjected to column
chromatography (SiO2: MeOH/NH4Cl (2m)/MeNO2 7:2:1).The fractions
containing the product were combined and concentrated.NH 4 PF6 was
added to precipitate the product as a solid.
Cyclophane 3·4PF6: This compound was obtained as a red solid (47 mg,
1
9%).Mp. .190 8C (decomp); H NMR of the SC-conformer (CD3COCD3,
500 MHz, 196 K): d=9.65 (d, J=5.4 Hz, 2H), 9.52 (d, J=5.4 Hz, 2H),
9.42 (d, J=5.4 Hz, 2H), 9.06 (brs, 2H), 8.75 (d, J=5.4, 2H), 8.58 (s, 2H),
8.52 (d, J=5.4, 2H), 8.50 (d, J=5.4, 2H), 8.50 (s, 2H), 8.18 (brs, 2H),
8.01 (s, 2H), 7.35 (t, J=8.4 Hz, 2H), 7.15 (d, J=8.4 Hz, 2H), 7.05 (d, J=
12.8 Hz, 2H), 7.01 (t, J=8.4 Hz, 1H), 6.41 (d, J=12.8 Hz, 2H), 6.22 (d,
J=12.8 Hz, 2H), 5.97 (d, J=12.8 Hz, 2H), 5.77 (d, J=7.4 Hz, 1H), 5.67
(d, J=7.4 Hz, 1H), 5.12 (brs, 2H), 4.59 (br, 2H), 4.51 (br, 2H), 4.32 (br,
2H), 4.11 (br, 2H), 3.92 (br, 4H), 3.82 (br, 4H), 2.53 (br, 1H), 2.02 ppm
(d, J=7.4 Hz, 1H); HRMS (ESI): m/z calcd for C60H57F24N5O9P4S4
[MꢀPF6]+: 1554.1606; found: 1554.1638.
1
M.p. 228.0–230.58C; H NMR (CDCl3, 500 MHz, 298 K): d=7.90 (s, 2H),
5.07 (s, 4H), 4.44 ppm (s, 2H); 13C NMR (CDCl3, 125 MHz, 298 K): d=
167.7, 166.3, 137.3, 136.7, 128.5, 38.2, 25.8 ppm; MS (EI): m/z (%): 389.0
(25) [M]+, 309.0 (100) [MꢀBr]+; elemental analysis calcd (%) for
C12H9Br2NO4: C 36.86, H 2.32, N 3.58; found: C 37.07, H 2.21, N 3.58.
General procedure for the preparation of the dibromides 12,18,19,23,
and 24: A mixture of the appropriate alcohol (0.50 mmol), the carboxylic
acid derivative 11 (0.55 mmol), 1,3-dicyclohexylcarbodiimide (1.0 mmol),
and 4-dimethylaminopyridine (catalytic amount) in CH2Cl2 (20 mL) was
stirred overnight at room temperature.The resulting suspension was fil-
tered, and the filtrate was evaporated and subjected to column chroma-
tography (SiO2: hexanes/EtOAc 1:1) to give the dibomide.
Cyclophane 4·4PF6: This compound was obtained as a purple solid
(96 mg, 17%).Mp..220
8C (decomp); 1H NMR of the major isomer
(CD3COCD3, 500 MHz, 253 K): d=9.69 (d, J=5.4 Hz, 2H), 9.63 (d, J=
5.4 Hz, 2H), 9.52 (d, J=5.4 Hz, 2H), 9.23 (d, J=5.4 Hz, 2H), 8.65 (brd,
2H), 8.54 (s, 2H), 8.45 (brd, 2H), 8.36 (m, 4H), 7.98 (s, 2H), 7.88 (s,
2H), 7.10 (t, J=8.4 Hz, 2H), 6.82 (t, J=8.4 Hz, 1H), 6.75 (d, J=12.8 Hz,
2H), 6.74 (s, 1H), 6.61 (d, J=8.4 Hz, 2H), 6.20 (s, 1H), 6.13 (d, J=
12.8 Hz, 2H), 6.02 (d, J=12.8 Hz, 2H), 5.94 (d, J=12.8 Hz, 2H), 4.72 (s,
2H), 4.62 (brd, 2H), 4.30 (m, 4H), 4.23 (s, 2H), 4.15 (brt, 2H), 4.05 (br.
t, 2H), 3.97 (brt, 2H), 3.75 (brt, 2H), 3.68 (brt, 2H), 3.59 (brt, 2H), 2.77
(d, J=8.4 Hz, 1H), 2.67 ppm (d, J=8.4 Hz, 1H); HRMS (ESI): m/z calcd
for C64H59F24N5O9P4 [MꢀPF6]+: 1476.2869; found: 1476.3189; HRMS
(ESI): m/z calcd for C64H59F24N5O9P4 [Mꢀ2PF6]2+: 665.6793; found:
665.6796.
Dibromide 12: This dibromide was obtained as a white wax (0.26 g,
71%). 1H NMR (CD3COCD3, 500 MHz, 298 K): d=7.92 (s, 2H), 7.30 (t,
J=8.5 Hz„ 2H), 6.96 (m, 3H), 6.90 (s, 4H), 5.09 (s, 4H), 4.51 (s, 2H),
4.37 (t, J=4.5 Hz, 2H), 4.17 (t, J=4.5 Hz, 2H), 4.12 (t, J=4.5 Hz, 2H),
4.07 (t, J=4.5 Hz, 2H), 3.90, (m, 4H), 3.81 ppm (m, 4H); 13C NMR
(CD3COCD3, 125 MHz, 298 K): d=167.1, 166.2, 158.9, 153.1, 137.3,
136.7, 129.2, 128.4, 120.4, 115.4, 115.3, 114.3, 69.6, 69.5, 69.4, 68.5, 67.8,
67.2, 64.7, 38.4, 25.8 ppm; HRMS (MALDI): m/z calcd for C32H33Br2NO9-
Na+ [M+Na]+: 756.0414; found: 756.0411.
Dibromide 18: This dibromide was obtained as
a yellow semisolid
(0.30 g, 77%). 1H NMR (CDCl3, 500 MHz, 298 K): d=7.91, 7.88 (2d, J=
8.5 Hz, 2H), 7.64 (s, 2H), 7.40–7.29 (m, 4H), 7.00–6.97 (m, 3H), 6.87 6.86
(2d, J=7.5 Hz, 2H), 4.94 (s, 4H), 4.49 (s, 2H), 4.44 (t, J=4.5 Hz, 2H),
4.33 (t, J=4.5 Hz, 2H), 4.30 (t, J=4.5 Hz, 2H), 4.21 (t, J=4.5 Hz, 2H),
4.10 (t, J=4.5 Hz, 2H), 4.04 (t, J=4.5 Hz, 2H), 4.00 (t, J=4.5 Hz, 2H),
3.90 ppm (t, J=4.5 Hz, 2H); 13C NMR (CDCl3, 125 MHz, 298 K): d=
167.0, 166.3, 158.7, 154.2, 154.1, 137.0, 136.4, 129.3, 128.2, 126.6, 125.1,
125.0, 120.8, 114.6, 114.6, 114.5, 105.7, 69.9, 69.9, 69.7, 69.0, 67.9, 67.8,
67.4, 64.9, 38.7, 25.7 ppm; MS (EI): m/z (%): 785.1 (10) [M]+; HRMS
(MALDI): m/z calcd for C36H35Br2NO9 [M]+: 783.0679; found: 783.0685.
Cyclophane 5·4PF6: This compound was obtained as a purple solid
(77 mg, 13%).Mp..214
8C (decomp); 1H NMR of the major isomer
(CD3COCD3, 500 MHz, 227 K): d=9.82 (s, 1H), 9.46 (d, J=6.2 Hz, 2H),
9.37 (d, J=6.2 Hz, 2H), 9.15 (d, J=6.2 Hz, 2H), 8.92 (d, J=6.2 Hz, 2H),
8.83 (s, 2H), 8.47 (s, 2H), 8.22 (s, 2H), 8.00 (d, J=6.2 Hz, 2H)), 7.93 (d,
J=6.2 Hz, 2H), 7.88 (d, J=6.2 Hz, 2H), 7.80 (d, J=8.4 Hz, 2H), 7.60
(br, 2H), 7.15 (d, J=8.4 Hz, 2H), 7.10 (d, J=12.8 Hz, 2H), 6.54 (d, J=
8.4 Hz, 2H), 6.47 (t, J=8.4 Hz, 1H), 6.35 (d, J=8.4 Hz, 1H), 6.31 (d, J=
12.8 Hz, 2H), 6.19 (t, J=8.4 Hz, 1H), 6.16 (d, J=12.8 Hz, 2H), 6.00 (d,
J=12.8 Hz, 2H), 5.28 (s, 2H), 4.65–4.38 (m, 12H), 4.19 (br, 2H), 4.03
(br, 2H), 2.71 (d, J=8.4 Hz, 1H), 2.64 ppm (d, J=8.4 Hz, 1H); HRMS
Dibromide 19: This dibromide was obtained as a yellow solid (0.39 g,
95%). M.p. 65.0–67.08C; 1H NMR (CD3COCD3, 600 MHz, 298 K): d=
382
ꢁ 2005 Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 369 – 385

Self-Complexing Compounds
FULL PAPER
(ESI): m/z calcd for C65H59F24N5O10P4 [MꢀPF6]+: 1504.3182; found:
J. Am. Chem. Soc. 1992, 114, 5303–5311; f) S.Anderson, H.L.An-
derson, J.K.M. Sanders, Acc. Chem. Res. 1993, 26, 469–475;
1504.3624; HRMS (ESI): m/z calcd for C65H59F24N5O10P4 [Mꢀ2PF6]2+
:
679.6767; found: 679.6905.
g) AP..Bisson, FJ..Carver, CA. .Hunter, JP..Waltho,
J. Am.
Chem. Soc. 1994, 116, 10292–10293; h) C.A. Hunter, Angew.
Chem. 1995, 107, 1181–1183; Angew. Chem. Int. Ed. Engl. 1995, 34,
1079–1081; i) J.P.Schneider, J.W.Kelly, Chem. Rev. 1995, 95, 2169–
Cyclophane 6·4PF6: This self-complexing compound was obtained as a
green solid (0.23 g, 36%). M.p. 2508C (decomp); 1H NMR spectroscopy
of the major isomer (CD3COCD3, 500 MHz, 253 K): d=9.69 (d, J=
5.4 Hz, 2H), 9.63 (d, J=5.4 Hz, 2H), 9.52 (d, J=5.4 Hz, 2H), 9.23 (d, J=
5.4 Hz, 2H), 8.65 (brd, 2H), 8.54 (s, 2H), 8.45 (brd, 2H), 8.36 (m, 4H),
7.98 (s, 2H), 7.88 (s, 2H), 7.10 (t, J=8.4 Hz, 2H), 6.82 (t, J=8.4 Hz, 1H),
6.75 (d, J=12.8 Hz, 2H), 6.74 (s, 1H), 6.61 (d, J=8.4 Hz, 2H), 6.20 (s,
1H), 6.13 (d, J=12.8 Hz, 2H), 6.02 (d, J=12.8 Hz, 2H), 5.94 (d, J=
12.8 Hz, 2H), 4.72 (s, 2H), 4.62 (brd, 2H), 4.30 (m, 4H), 4.23 (s, 2H),
4.15 (brt, 2H), 4.05 (brt, 2H), 3.97 (brt, 2H), 3.75 (brt, 2H), 3.68 (brt,
2H), 3.59 ppm (brt, 2H); HRMS (ESI): m/z calcd for C62H59F24N5O9P4S4
[MꢀPF6]+: 1580.2121; found: 1580.2232.
2187; j) F.M. Raymo, J.F. Stoddart,
Pure Appl. Chem. 1996, 68,
313–322; k) A.G.Kolchinski, N.W.Alcock, R.A.Roesner, D.H.
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278–286; n) Templated Organic Synthesis (Eds.: F. Diederich, P. J.
Stang), Wiley-VCH, Weinheim, 1999; o) M.Nakash, Z.C.Watson,
N.Feeder, S.J.Teat, J.K.M.Sanders,
2119; p) J.K.M. Sanders, Pure Appl. Chem. 2000, 72, 2265–2274;
q) DT. .Bong, TD. .Clark, JR. .Granja, MR. .Ghadiri, Angew.
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1011; r) L.J.Prins, D.N.Reinhoudt, P.Timmerman, Angew. Chem.
Chem. Eur. J. 2000, 6, 2112–
Cyclophane 7·4PF6: This self-complexing compound was obtained as a
green solid (0.22 g, 35%). M.p. 2458C (decomp); 1H NMR spectroscopy
of the major isomer (CD3COCD3, 500 MHz, 253 K): d=9.77 (s, 1H), 9.67
(d, J=5.4 Hz, 2H), 9.61 (d, J=5.4 Hz, 2H), 9.52 (d, J=5.4 Hz, 2H), 9.25
(d, J=5.4 Hz, 2H), 8.58–8.52 (m, 4H), 8.45 (d, J=5.4 Hz, 2H), 8.40–8.35
(m, 4H), 7.98 (s, 2H), 7.88 (s, 2H), 7.68 (d, J=5.6 Hz, 2H), 6.89 (d, J=
5.6 Hz, 2H), 6.80 (d, J=13.7 Hz, 2H), 6.73 (s, 1H), 6.22 (s, 1H), 6.13 (d,
J=13.7 Hz, 2H), 6.02 (d, J=13.7 Hz, 2H), 5.92 (d, J=13.7 Hz, 2H), 4.72
(s, 2H), 4.46 (brs, 4H), 4.30 (s, 4H), 4.23 (s, 2H), 4.07 (brt, 2H), 3.97
(brt, 2H), 3.75 (brt, 2H), 3.66 (brt, 2H), 3.61 ppm (brt, 2H); HRMS
(ESI): m/z calcd for C63H59F24N5O10P4S4 [MꢀPF6]+: 1608.2065; found:
2001, 113, 2446–2492; Angew. Chem. Int. Ed. 2001, 40, 2382–2426;
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Stoddart, H.-R. Tseng, Proc. Natl. Acad. Sci. USA 2002, 99, 4797–
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1524–1528; Angew. Chem. Int. Ed. 2003, 42, 1486–1490; w) L.
Hogg, D.A.Leigh, P.J.Lusby, A.Morelli, S.Parsons, J.K.Y.Wong,
Angew. Chem. 2004, 116, 1238–1241; Angew. Chem. Int. Ed. 2004,
43, 1218–1221; x) X.Zheng, M.E.Mulcahy, D.Horinek, F.Galeotti,
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Gómez-López, J.A. Preece, J.F. Stoddart, Nanotechnology 1996, 7,
1608.2435; HRMS (ESI): m/z calcd for C63H59F24N5O10P4S4 [Mꢀ2PF6]2+
:
731.6209; found: 731.6221.
Electrochemistry and spectroelectrochemistry: Electrochemical and spec-
troelectrochemical experiments were carried out at room temperature in
argon-purged solutions of the compounds in MeCN, with a Princeton Ap-
plied Research 263A Multipurpose instrument interfaced to a PC.Cyclic
voltammetric experiments were performed by using a glassy carbon
working electrode (0.018 cm2, Cypress Systems); its surface was polished
routinely with a 0.05 mm alumina–water slurry on a felt surface immedi-
ately before use.The counter electrode was a Pt wire and the reference
electrode was a Ag/AgCl electrode (“No Leak” Ag/AgCl Reference
Electrode, Cypress Systems).Tetrabutylammonium hexafluorophosphate
(0.1m) was added as supporting electrolyte.Cyclic voltammograms were
obtained at scan rates of 10, 100, 500 and 1000 mVsꢀ1.For reversible
processes, E1/2 was calculated from an average of the cathodic and anodic
cyclic voltammetric peaks.To establish the reversibility of a process, we
used the critieria of 1) 60 mV between cathodic and anodic peaks, and 2)
close to unity ratio of the intensities of the cathodic and anodic currents.
[Ru(bpy)3]2+ [E1/2(Ru3+/Ru2+)=+1.290 V][41] was present in the solution
as an internal reference.Spectroelectrochemical experiments were made
183–192; c) V.Balzani, M.Gómez-López, J.F.Stoddart,
Acc. Chem.
Res. 1998, 31, 405–414; d) V.Balzani, A.Credi, F.M.Raymo, J.F.
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456–464; f) C.A. Schalley, K. Beizai, F. Vçgtle,
2001, 34, 465–476; g) J-.P.Collin, C.Dietrich-Buchecker, P.GaviÇa,
M.C.Jimꢂnez-Molero, J-.P.Sauvage, Acc. Chem. Res. 2001, 34, 477–
Acc. Chem. Res.
487; h) R.Ballardini, V.Balzani, A.Credi, M.T.Gandolfi, M.Ven-
turi, Struct. Bonding 2001, 99, 163–188; i) L.Raehm, J-.P.Sauvage,
Struct. Bonding 2001, 99, 55–78; j) C.A. Stainer, S.J. Alderman,
T.D.W. Claridge, H.L. Anderson,
Angew. Chem. 2002, 114, 1847–
1850; Angew. Chem. Int. Ed. 2002, 41, 1769–1772; k) V.Balzani, A.
Credi, M.Venturi, Chem. Eur. J. 2002, 8, 5524–5532; l) H.-R. Tseng,
J.F. Stoddart in Modern Arene Chemistry (Ed.: D. Astruc), Wiley-
VCH, Weinheim, 2002, pp5.74–599; m) Y.Chen, G-Y. .Jung,
D.A.A.Ohlberg, X.Li, D.R.Stewart, J.O.Jeppesen, K.A.Niel-
sen, J.F. Stoddart, R.S. Williams,
468; n) J.R.Heath, M.A.Ratner, Physics Today 2003, May, 43–49;
o) V.Balzani, A.Credi, M.Venturi, Molecular Devices and Ma-
Nanotechnology 2003, 14, 462–
in
a custom built optically-transparent thin layer electrochemical
(OTTLE) cell[42] with an optical path of 1 mm, using a Pt grid as working
electrode, a Pt wire as counter electrode, and a Ag wire pseudo-reference
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This research was supported by the Defense Advanced Research Projects
Agency (DARPA) and the Institute of Cell Mimietic Space Exploration
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Received: June 18, 2004
Please note: Minor changes have been made to this manuscript since its
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Published online: November 24, 2004
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