Self-assembly, stability quantification, controlled molecular switching, and sensing properties of an anthracene-containing dynamic [2]rotaxane

Article abstract of DOI:10.1039/b926568f

The preparation of a novel anthracene-containing dynamic [2]rotaxane by a templating self-assembly process between a diamine and a dialdehyde to form a [24]crown-8 macrocyclic diimine, in the presence of a dumbbell containing a secondary dialkylammonium ion center as the template, which has been exploited for its sensing properties. By appealing to the ability of the anthracene ring system - one of the two stoppers associated with the dumbbell - to act as a fluorescent probe, the fluorescence and fluorescence-quenching nature of the dynamic rotaxane in an equilibrium mixture has been investigated and quantified in the presence of external stimuli such as water, acids, salts, and an amine. The stability, as expressed by the hydrolysis of the dynamic rotaxane has been monitored by following: (i) the anthracene fluorescence and (ii) the movements of the signals in the ¹H NMR spectra. The rate of hydrolysis (t _1/2 = 6.9 min) of the dynamic rotaxane in the presence of a small amount (1 equiv.) of acid was found to be very much faster than when the hydrolysis was carried out with a large amount (>100 equiv.) of water, when t_1/2 > 140 min. Furthermore, it has been established that the anthracene fluorescence of the dynamic rotaxane rises with an increasing amount of acid. Two acid sensors have been identified with different operating modes - namely, logarithmic and linear. The combination of different inputs involving water, acids, salts and an amine leads to different fluorescence outputs from the dynamic rotaxane, hence, producing a prototype for expressing molecular logic.

Full text of DOI:10.1039/b926568f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Self-assembly, stability quantification, controlled molecular switching, and
sensing properties of an anthracene-containing dynamic [2]rotaxane†
Wing-Yan Wong,a Ken Cham-Fai Leung*a and J. Fraser Stoddart*b
Received 22nd December 2009, Accepted 18th February 2010
First published as an Advance Article on the web 19th March 2010
DOI: 10.1039/b926568f
The preparation of a novel anthracene-containing dynamic [2]rotaxane by a templating self-assembly
process between a diamine and a dialdehyde to form a [24]crown-8 macrocyclic diimine, in the presence
of a dumbbell containing a secondary dialkylammonium ion center as the template, which has been
exploited for its sensing properties. By appealing to the ability of the anthracene ring system—one of
the two stoppers associated with the dumbbell—to act as a fluorescent probe, the fluorescence and
fluorescence-quenching nature of the dynamic rotaxane in an equilibrium mixture has been investigated
and quantified in the presence of external stimuli such as water, acids, salts, and an amine. The stability,
as expressed by the hydrolysis of the dynamic rotaxane has been monitored by following: (i) the
anthracene fluorescence and (ii) the movements of the signals in the 1H NMR spectra. The rate of
hydrolysis (t1/2 = 6.9 min) of the dynamic rotaxane in the presence of a small amount (1 equiv.) of acid
was found to be very much faster than when the hydrolysis was carried out with a large amount
(>100 equiv.) of water, when t1/2 > 140 min. Furthermore, it has been established that the anthracene
fluorescence of the dynamic rotaxane rises with an increasing amount of acid. Two acid sensors have
been identified with different operating modes—namely, logarithmic and linear. The combination of
different inputs involving water, acids, salts and an amine leads to different fluorescence outputs from
the dynamic rotaxane, hence, producing a prototype for expressing molecular logic.
imines are relatively unstable and undergo rapid hydrolysis in
Introduction
the presence of water.9 However, attention has been drawn10
Recently, dynamic covalent chemistry (DCC) has attracted1 in-
recently to the fact that imine-containing macrocycles possess
remarkable stabilities in the presence of water. Other systems,11
involving dative coordination bonds between imines and transition
metal ions,12 can also experience increased imine bond stability
in aqueous solutions. The pH control13 over the structure and
properties of some of these compounds, as a result of molecular
switching in aqueous solutions, may offer practical applications
in, for example, microfluidic devices.14
The interaction between light and designed supramolecular sys-
tems exploits the energy and information content of photons.15 In
one particular mode, a photoreaction occurring in a supramolec-
ular system causes some changes in the properties of the system,
which are reflected in a monitorable signal. Moreover, inter-
action between supramolecular self-assemblies of components
affects photoresponses, a property which can be revealed by
some form of excited state manifestation—often luminescence. In
supramolecular systems, the photo-physicochemical properties of
each component can be modified profoundly upon complexation.
Such changes can be useful in obtaining information on the
superstructure of a complex, and also in assisting in the design
of sensors for practical applications.16
creasing attention on account of its usefulness when it comes
to producing compounds, such as molecular Borromean rings,2
some selected dendrimers,3 and other exotic products,4 in a highly
efficient manner using one-pot, multicomponent, templated,
self-assembly processes.5 The nearly quantitative formation of
(super)structures with mechanically intertwined and interlocked
entities indicates that the template effect5b is highly efficient, yield-
ing thermodynamically stable supramolecular intermediates. The
main advantage of the thermodynamic process over the kinetic one
is that the former operates under (reversible) equilibrium control,
such that the formation of undesired or competitive intermediates
will eventually be transformed into the most energetically favoured
products by error checking and proof reading.1h
DCC relies upon the use of covalent bonds that form and
break in a completely reversible manner during a particular
reaction step in a synthesis. Common dynamic equilibrating
(reversible) reactions exploited in such syntheses include olefin
metathesis,6 and disulfide7 and imine8 bond formation. Generally,
aCenter of Novel Functional Molecules, Department of Chemistry, The Chi-
nese University of Hong Kong, Shatin, NT, Hong Kong SAR, P. R. China.
E-mail: cfleung@cuhk.edu.hk; Fax: (+852) 2603-5057
Based on our previous investigations on the high-yielding
synthesis of [2]rotaxanes employing thermodynamic templated
syntheses,8 we report here the synthesis and characterisation of
an anthracene-containing dynamic [2]rotaxane. Our interest in
the development of photochemical molecular and supramolecular
devices, together with studies on photoenergy- and electron-
transfer processes in supramolecular systems,17 has led us to
bDepartment of Chemistry, Northwestern University, 2145 Sheridan
Road, Evanston, Illinois 60208, USA. E-mail: stoddart@northwestern.edu;
Fax: (+1) 847-491-1009
† Electronic supplementary information (ESI) available: Experimental
procedures, analytical and spectral characterisation data, and expanded
discussions of peripheral findings and control experiments. See DOI:
10.1039/b926568f
2332 | Org. Biomol. Chem., 2010, 8, 2332–2343
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Scheme 1 Thermodynamically driven formation of the rotaxane R-H·PF6.
explore: (i) the stability, (ii) exchange dynamics and (iii) switching
in this dynamic [2]rotaxane in the presence of various stimuli, e.g.,
water, salts, acids, and amines.
An equilibrated (5 min) equimolar (60 mM) mixture of 1-H·PF6,
2 and 3 in MeCN yielded a mass spectrum (Fig. S1, ESI†) in which
a singly charged molecular ion base peak (m/z) of 833 reveals the
formation of the R-H+ ion, having lost its PF6 counterion. No
-
significant higher mass signals were observed, indicating that no
other higher-order macrocyclic homo-oligomers or related acyclic
oligomers were formed.
Results and discussion
Scheme
1
summarises the template-directed synthesis18 of
the [2]rotaxane R-H·PF6, incorporating a photosensitising an-
thracene unit as one of the two stoppers on its dumbbell
component. It involves the self-assembly of 2,6-diformylpyridine
(2) and the diamine 3 in MeCN in the presence of the secondary
dialkylammonium salt 1-H·PF6. The resulting [24]crown-8-like
macrocycle is amplified as a result of the highly selective [1 + 1]
condensation between 2 and 3, and becomes interlocked mechan-
ically around the template ion 1-H+. The 3,5-dimethoxyphenyl
group serves as the other bulky stopper and the rotaxane R-
H·PF6, constituted of a macrocyclic diimine surrounding the
dumbbell-shaped component, is the outcome ultimately of the
equilibrium-controlled reaction. The thermodynamic stability of
the rotaxane arises as a result of complementary recognition
associated with [N+–H ◊ ◊ ◊ X] hydrogen bonds and [N+C–H ◊ ◊ ◊ O]
interactions (X = O or N), in addition to electrostatic and C–
H ◊ ◊ ◊ p interactions8c between the [24]crown-8-like macrocycle and
the dumbbell component.
The formation of R-H·PF6 is also supported by 1H NMR
spectroscopy carried out on an equilibrating equimolar mixture of
the three starting materials in CD3CN. The spectrum (Fig. 1) of the
rotaxane shows the emergence8c of new sharp resonances which
are very different from those of the starting materials, namely
1-H·PF6, 2 and 3. The nature of these new resonances suggests that
R-H+ is the thermodynamically most stable product of the reaction
mixture. In particular, the characteristic signal for the benzylic
methylene group protons (Hd/He) adjacent to the NH2+ center are
shifted from d = 4.4 and 5.2 ppm in 1-H+ to d = 5.1 and 5.8 ppm
in R-H+. Moreover, the formyl proton (CHO) resonance (d =
10.1 ppm) in 2 disappears in the spectrum of the product, while
the corresponding imine (Hi) resonance (d = 7.9 ppm) becomes a
prominent and predominant signal. The sharpness of the peaks for
1
the rotaxane in the H NMR spectrum indicates that it is stable
1
kinetically on the H NMR time scale. We conclude from this
experiment that the formation of R-H·PF6 proceeds in >90% yield.
Fig. 1 1H NMR spectra (400 MHz, CD3CN, 295 K) of R-H·PF6 and its free components: dumbbell 1-H·PF6, dialdehyde 2 and diamine 3 (f = free).
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2332–2343 | 2333
©

View Article Online
In order to elucidate the absorption and emission characteristics
of the multicomponent rotaxane, reference has to be made to
the starting materials and a suitable model compound, e.g., the
open-chain analogue 4 of the macrocycle. The UV/Vis absorption
spectrum (Fig. 2) of R-H·PF6 is not all that dissimilar from the sum
of the separate components. In particular, the absorption spectra
of R-H·PF6 and 1-H·PF6 both show the characteristic band at
300–400 nm which originates from the anthracene moiety.17 Upon
excitation at 290 nm, where most of the light is absorbed by the
components, anthracene fluorescence (lmax = 418 nm) is observed
(Fig. 3) in the case of 1-H·PF6, whereas the aminophenolic residues
fluoresce at lmax = 334 nm for 3, 4 and R-H·PF6. By contrast, the
anthracene fluorescence of the rotaxane is completely quenched.
It is well known18 that when anthracene is in close proximity to
an amino group, quenching occurs by electron transfer from that
group. Here, the anthracene fluorescence quenching in R-H·PF6
is presumably being caused by electron transfer from the imine
and pyridine moieties in the macrocycle. Since the solutions are
extremely dilute, intermolecular quenching can be ruled out. The
anthracene fluorescence of the dumbbell 1-H·PF6 is not quenched
Fig. 3 Fluorescence emission spectra (excitation wavelength = 290 nm)
of components (conc. = 0.02 mM in MeCN).
by the physical mixing of it with 2, 3 or 4, separately (Fig. S4,
ESI†).
By monitoring the anthracene fluorescence intensity in the
equilibrium mixture of the reactants and products, we can get
a handle on the relative concentrations of the rotaxane and
its separate components based on the fact that the dumbbell
is fluorescent while the rotaxane is fluorescence quenched. A
dissociation mechanism for the rotaxane in the presence of water
is shown in Scheme 2. The kinetically labile imine bonds in the
rotaxane can be broken and reformed in the presence of water
molecules, affording the intermediates I and II which may also
possess some residual stability as a result of the noncovalent
bonding interactions, until the separate components are further
fragmented and dissolved in the solvent system.
Fig. 2 UV/visible absorption spectra (conc. = 0.02 mM in MeCN) of
components.
Scheme 2 Stability through dissociation of R-H·PF6 and its intermediates I, II and III.
2334 | Org. Biomol. Chem., 2010, 8, 2332–2343 This journal is The Royal Society of Chemistry 2010
©

View Article Online
The fluorescence emissions of the dumbbell and the rotaxane
were monitored after treatment with an excess (~200 equiv.) of
water or acid (1.0 M HCl). Anthracene fluorescence bands are
observed (Fig. 4) for the dumbbell when it is mixed separately
with an excess of water or acid. No fluorescence quenching occurs.
The slight decrease in fluorescence intensity of the dumbbell
on addition of an excess of water compared to the dumbbell
free of water may be attributed to a dilution effect. There is a
significant increase, however, in the fluorescence intensity on the
addition of excess acid compared to the dumbbell free of acid. This
observation may be attributed to the partial protonation of the
methoxy groups of the dumbbell such that intramolecular electron
transfer from these groups to the anthracene is deactivated. On the
other hand, when the rotaxane was treated with an excess of water,
the anthracene fluorescence is completely quenched. By contrast,
anthracene fluorescence is observed when the rotaxane is treated
with an excess of acid.
Finally, addition of base (NaOH, Et3N, etc.) to the rotaxane
leads to the deprotonation of the dumbbell, whereupon its tem-
plating role is lost, leading to its dissociation into its components.
The deprotonated dumbbell is fluorescence quenched because the
covalently linked secondary amine deactivates the lower lying
anthracene fluorescence. Thus, its fluorescence cannot be observed
in the presence of water or base, only in the presence of acid.
Dissociation of rotaxane in the presence of water
Since fluorescence spectroscopy reveals (Fig. 4) that the an-
thracene fluorescence of the rotaxane is quenched in the presence
of excess water, a dissociation mechanism (Scheme 3) can be
drawn in which the equilibrium is displaced towards the rotaxane
and intermediate I, in keeping with their fluorescence quenching
properties. The intermediate III is presumably likely to dissociate
to the dialdehyde 2 and the diamine 3 in the presence of water
because of the loss of the template effect.
The equilibrated mixture (CD3CN, 0.02 mM) in the presence
1
of excess of D2O was probed using H NMR spectroscopy. The
spectrum (Fig. 5) reveals sets of signals which correspond to
all of the components. Integration of these signals, however,
demonstrates that the rotaxane is still the dominant species in
the mixture. The additional signal at d = 10.05 ppm can be
attributed to the monoaldehyde (CHO) proton in the intermediate
I (Scheme 3). The rotaxane shows remarkable stability in water.8c
The hydrolysis of the rotaxane is a reversible process. Removal of
the water from the reaction mixture with drying agents, such as
anhydrous Na2SO4, does not jeopardize the break-down products,
which duly recombine to produce the rotaxane once again when
water is removed.
Fig. 4 Fluorescence emission spectra (excitation wavelength = 290 nm)
of components (conc. = 0.02 mM in MeCN).
Scheme 3 Dissociation mechanism and fluorescence quenching of R-H·PF6 in the presence of water. The equilibrium favours the formation of R-H·PF6
from which the anthracene fluorescence is quenched.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2332–2343 | 2335
©

View Article Online
Fig. 5 Partial 1H NMR spectrum (400 MHz, CD3CN, 295 K) of R-H·PF6 in the presence of excess of D2O (f = free).
Dissociation of rotaxane in the presence of acid/water
the 1H NMR spectrum (Fig. 6b) of the equilibrated mixture in the
presence of excess of HPF6/H2O (0.1 M) reveals that the rotaxane
is dissociated entirely into its components. Slight shifts in the 1H
NMR resonances are observed on account of a counterion effect.
In particular, the weak residual monoaldehyde (CHO) signal
resonates at d = 10.00 ppm.
Fluorescence spectroscopy reveals (Fig. 4) that the anthracene
fluorescence was observed once again when the rotaxane was
treated with an excess of acid (1.0 M HCl). We have found
out—in an experiment designed to compare counterion and
acidity effects—that the use of 0.1 M HPF6 (a weaker acid) for
rotaxane dissociation also results in anthracene fluorescence. It is
believed that the addition of NH4PF6 (a weak acid) would also
trigger dissociation8b of the rotaxane. A dissociation mechanism
(Scheme 4) can be proposed in which the equilibrium favours the
formation of the separate components, namely, 1-H+, 2 and 3,
and so accounts for the observation of anthracene fluorescence.
The equilibrated mixture (in CD3CN) in the presence of 1.0 equiv.
HCl/H2O (0.5 M) was examined by 1H NMR spectroscopy. The
spectrum (Fig. 6a) reveals sets of signals for all of the equilibrating
components. From 1H NMR integrations, the rotaxane turns out
to be the least dominant species in the mixture. The weak signal
resonating at d = 10.12 ppm can be assigned to the monoaldehyde
(CHO) proton in the intermediate III (Scheme 4). By contrast,
The change in the rotaxane concentration resulting from
dissociation using either water or aqueous acid was monitored
(Fig. 7) by 1H NMR spectroscopy. The dissociation of the rotaxane
was confirmed by observing the gradual decrease in the intensity
of the imine signal (d = 7.92 ppm) intensity with the consequent
increase in the intensity of the aldehyde resonance (d = 10.10 ppm)
and the shifts of the N+C–H signals from d = 5.1 (Hd) and 5.8 ppm
(He) to d = 4.4 (Hd(f)) and 5.2 ppm (He(f)), respectively.
Fig. 8 shows a plot of rotaxane concentration versus time while
D2O or HCl/H2O were being added. 1H NMR spectroscopic
signals are tracked. On addition of water or aqueous acid to the
rotaxane in CD3CN solution, equilibria were reached generally
after about 5 h. In particular, it requires the presence of over
200 equiv. of water for the rotaxane to become half dissociated.
Scheme 4 Dissociation mechanism and fluorescence quenching of R-H·PF6 in the presence of acid/water (1.0 M HCl or 0.1 M HPF6). The equilibrium
favours the dissociation of R-H·PF6 to give the dumbbell 1-H·PF6 from which the anthracene fluorescence is restored.
2336 | Org. Biomol. Chem., 2010, 8, 2332–2343
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Fig. 6 Partial 1H NMR spectrum (400 MHz, CD3CN, 295 K) of R-H·PF6 in the presence of (a) 1 equiv. HCl/H2O (0.5 M) and (b) excess of HPF6/H2O
(0.1 M) (f = free).
Fig. 7 Stacked partial 1H NMR (400 MHz, CD3CN, 295 K) plots of R-H·PF6 in the presence of (a) 200 equiv. of D2O and (b) 1 equiv. of HCl/H2O
(0.5 M) with time.
To be more specific, there are 71, 54 and 24% of the rotaxane
remaining at equilibrium in the presence of 100, 200, and 500 equiv.
of water. In the beginning, the dissociation rates are rather similar.
Upon addition of 1 equiv. of HCl/H2O (0.5 M) to the rotaxane
solution, however, the dissociation rate of the rotaxane was much
faster than when employing water alone. The half-lives (t1/2) for
the dissociation were determined to be, in turn, 149, 132, 78 and
6.9 min for 100, 200, 500 equiv. D2O and 1 equiv. HCl/H2O. The
reactions within the first hour were shown to follow second-order
kinetics from a plot of 1/[R-H·PF6] versus time. See Fig. S7 in the
ESI.†8i
Dissociation of rotaxane in the presence of acid/ether
The stability of the rotaxane has also been examined by dissolving
it in anhydrous MeOH, Me2NCHO (DMF) and Et2O,8i wherein
negligible dissociation is observed. The dissociation of the rotax-
ane was then investigated in excess of anhydrous HCl/Et2O (0.1
M). To our surprise, the 1H NMR spectrum (Fig. S8, ESI†) of the
resulting mixture revealed a complete dissociation of the rotaxane
into its separate components. We believe that tiny amounts of
water molecules, inevitably present in the mixture, are responsible
for the dissociation when the acid is present in excess.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2332–2343 | 2337
©

View Article Online
Dissociation of rotaxane by competitive exchange using toluidine
In order to explore the reversible nature of imine exchange,
the rotaxane was treated with an amine–toluidine (Tol-NH2 or
p-MeC6H4NH2). Imine exchange using toluidine during imine
formation is expected to inhibit the production of the rotaxane
by cleaving the [24]crown-8-like macrocycle. When 2.0 equiv.
of toluidine were added to the rotaxane, the 1H NMR spec-
trum (Fig. S9, ESI†) revealed new signals at d = 8.63, 8.21
and 8.01, corresponding to imine (H¢i) and the pyridine aryl
(H¢j and H¢k) protons, respectively. No formyl signal was ob-
served. Signals, however, for the free dumbbell 1-H·PF6 were
observed.
The fluorescence emission spectra (Fig. S15, ESI†) demonstrate
that the anthracene fluorescence is quenched when the rotaxane
is mixed with 10 equiv. of toluidine in MeCN. By contrast, the
anthracene fluorescence can be observed when additional water
or acid is added to the rotaxane in admixture with the toluidine.
In a control experiment, we found that the anthracene fluorescence
of 1-H·PF6 is not quenched on the addition of 1 equiv. of toluidine.
Since the rotaxane–toluidine mixture contains the free dumbbell
1-H·PF6 (Fig. S9, ESI†) and, because of the fact that the an-
thracene fluorescence of 1-H·PF6 is quenched (Fig. S15, ESI†), it is
reasonable to conclude that the newly formed (Scheme 5) diimine 5
with toluidine might have gained some additional supramolecular
interactions with the dumbbell in close proximity20—as in the
intermediates IV or V—leading to anthracene quenching. In
addition, the effect of anthracene fluorescence was investigated
by the addition of KPF6, toluidine, water and acid in turn. The
anthracene fluorescence (Fig. S16, ESI†) is observed only when
acid is present in the system. These results confirm repeatedly that
acid is crucial in order to be able to control the ON/OFF switching
of the anthracene fluorescence in the mixed system containing the
rotaxane.
Fig. 8 A plot of the concentration of R-H·PF6 (mM) in CD3CN versus
time (h) obtained by 1H NMR spectroscopy (400 MHz, CD3CN, 295 K).
Effect of Salt
It occurred to us that dissociation of the rotaxane by competitive
binding with a salt, e.g., KPF6, might offer the possibility of
broadening the range of stimuli.19 The competitive binding would
become feasible if the [24]crown-8-like macrocycle were to complex
preferentially with K+ ions, leading to the displacement of the
anthracene-containing dumbbell 1-H·PF6. The rotaxane and the
dumbbell compound were treated successively with excess of
KPF6, H2O and HCl (1.0 M). The fluorescence spectra show
(Fig. S14, ESI†) that when KPF6 or KPF6/H2O were added to
the rotaxane R-H·PF6, no anthracene emission was observed. By
contrast, the anthracene emission was observed in the presence
of HCl (1.0 M) and KPF6. It follows that KPF6 has no effect
on the rotaxane—that is, the rotaxane does not dissociate to
give the free dumbbell, although it does in the presence of
acid.
Scheme 5 Dissociation mechanism and fluorescence quenching properties of R-H·PF6 in the presence of toluidine upon competitive exchange. The
equilibrium favours formation of R-H·PF6 or intermediates IV and V, wherein the anthracene fluorescence is quenched.
2338 | Org. Biomol. Chem., 2010, 8, 2332–2343
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Fig. 9 (a) Stacked fluorescence emission of R-H·PF6 with different amounts of HCl/H2O (0.1 M). (b) A plot of fluorescence intensity at 418 nm
versus equiv. of HCl/H2O. Error = 2%.
Molecular switching of the rotaxane at different acid
concentrations
HCl/Et2O (0.1 M). From the plot (Fig. 10b) of the anthracene
fluorescence intensity versus the amount of HCl/Et2O in the
rotaxane solution, the anthracene fluorescence is directly pro-
portional (linear) to the concentration of HCl/Et2O. Clearly,
the addition of acid to the rotaxane results in the dissocia-
tion of the anthracene-containing dumbbell, which on its own,
gives an observable anthracene fluorescence signal. The rotaxane
possesses not only an exclusive ON/OFF switch in the presence
of acid, but also a switch with dimmer control, i.e., the fluo-
rescence intensity of anthracene—also the aminophenolic and
3,5-dimethoxybenzyl groups—can be fine-controlled by varying
the concentration of the acid. An increase in acid concentration
leads to an increase in anthracene fluorescence. Essentially,
two acid sensors have been uncovered with different modes of
dimmer control, i.e., logarithmic (for HCl/H2O) and linear (for
HCl/Et2O).
In a further set of experiments, the rotaxane was exposed to
varying amounts of HCl/H2O (1.0 M). To our surprise, the
anthracene fluorescence intensities that are caused by treating the
rotaxane with increasing amounts of HCl/H2O result in a rise
(Fig. 9a) in the fluorescence. This trend was witnessed very clearly
in the range for 0–5 equiv. of HCl/H2O (1.0 M). Only a relatively
small difference in fluorescence intensity is evident, however, in
the range of 5–10 equiv. of acid. Indeed, the increase (Fig. 9b) in
the anthracene fluorescence intensity reaches a plateau, following
a logarithmic curve. The anthracene fluorescence intensities in
relation to the amount of HCl/Et2O in the rotaxane were also
investigated. The outcome (Fig. 10a) is very similar, except
that the trend can be witnessed clearly from 0–10 equiv. of
Fig. 10 (a) Stacked fluorescence emission of R-H·PF6 with different amounts of HCl/Et2O (0.1 M). (b) A plot of fluorescence intensity at 418 nm
versus equiv. of HCl/Et2O. Error = 2%.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2332–2343 | 2339
©

View Article Online
Table 1 Summary of the fluorescence output of the rotaxane R-H·PF6 in the presence of four different additive inputs (H2O, H+, KPF6, and Tol-NH2).
Input: “1” represents the presence of the additive while “0” represents the absence of the additive. Output: “1” represents observable fluorescence while “0”
represents no observable fluorescence (fluorescence quenched). The output entries C, I, J, and O (parentheses) are somewhat uncertain since thoroughly
dry conditions are unable to obtain
Input
H2O
Output
Anthracene fluorescence
(418 nm, excitation = 290 nm)
Fluorescence at 330 nm
(excitation = 290 nm)
Entry
H+
KPF6
Tol-NH2
A
B
C
D
E
F
G
H
I
0
1
0
0
0
1
1
1
0
0
0
1
1
1
0
1
0
0
1
0
0
1
0
0
1
1
0
1
1
0
1
1
0
0
0
1
0
0
1
0
1
0
1
1
0
1
1
1
0
0
0
0
1
0
0
1
0
1
1
0
1
1
1
1
0
0
(1)
0
0
1
0
0
(1)
(1)
0
1
1
0
(1)
1
1
1
(0)
1
1
0
1
1
(0)
(0)
1
0
0
1
(0)
0
J
K
L
M
N
O
P
an Electrothermal 9100 digital melting point apparatus and are
uncorrected. UV/Visible absorption spectra were obtained using a
Cary 5G UV-Vis-NIR spectrophotometer. Excitation and fluores-
cence spectra were recorded using a Hitachi F-4500 Fluorescence
spectrophotometer. All nuclear magnetic resonance (NMR) spec-
tra were recorded on Bruker Avance 400 (1H: 400 MHz; 13C: 101
MHz) spectrometer at 298 K and CDCl3 was used as the solvent
unless otherwise stated. The residual proton resonance signals of
the non-deuterated solvents were used as reference calibration.
Chemical shifts are reported as parts per million (ppm) for both
1H and 13C NMR spectroscopies. Electrospray ionization (ESI)
mass spectra were measured on a Thermo Finnigan MAT95XL
mass spectrometer with CH2Cl2–MeOH (1 : 1) as the mobile phase.
The reported molecular mass (m/z) values correspond to the most
abundant monoisotopic masses. For dissociation experiments, the
initial rotaxane concentration was 0.02 M in MeCN.
Conclusion
A novel anthracene-containing dynamic [2]rotaxane-containing
imine bonds has been synthesised in high yield by a three-
component self-assembly process under templating—and char-
1
acterised by H NMR spectroscopy and mass spectrometry. The
stability of the rotaxane has been investigated after the addition
of water, acid, salt, and amine. The dissociation of the rotaxane
has been monitored by observing the anthracene fluorescence and
the chemical shifts in the 1H NMR spectra. The dissociation rate
for the rotaxane in the presence of acid is much faster than in the
presence of water. Furthermore, the anthracene fluorescence of the
rotaxane rises with increasing acid concentration. Two acid sensors
have been identified with different modes of dimmer control—that
is, logarithmic and linear. The use of a combination of stimuli
for the rotaxane is summarized in Table 1, employing a binary
notation with four different inputs and one output. Noticeably,
the output entries E, F, G, and H are somewhat uncertain since
we are unable to obtain thoroughly dry conditions. If the situation
relating to the residual water molecules present in the solutions is
overlooked, then the outputs can be interpolated (as the bracketed
values). As a result, different molecular logic can be obtained by
rational selections of certain specific input combinations.
Synthesis
The nitro compound 621 was reduced (Scheme 6) to the aniline
derivative 7 in the presence of hydrazine and palladium on charcoal
in 93% yield. Then, two equivalents of 7 were treated with the
˚
dialdehyde 2 in the presence of molecular sieves (4 A) to give the
diimine 4 in 94% yield. Commercially available starting materi-
als 9-anthraldehyde (8) and 3,5-dimethoxybenzylamine (9) were
heated (Scheme 7) under reflux using a Dean–Stark apparatus for
condensation, followed by reduction with NaBH4. This reaction
results in the formation of the amine thread 1 in 88% yield.
Subsequently, 1 was protonated by careful addition of HPF6 in
H2O, leading to the formation of the dumbbell 1-H+ in 75% yield.
Experimental
General methods
All chemicals were purchased from Aldrich or Acros and used
without further purification. All solvents were dried, distilled
and stored with molecular sieves (4 A). Tetrahydrofuran (THF)
˚
was dried with Na–benzophenone. MeCN was dried with CaH2;
PhMe was dried with Na; and CH2Cl2 was dried with NaH.
All reactions were performed under high purity nitrogen. Thin
layer chromatography (TLC) was performed on silica gel 60 F254
(Merck). Column chromatography was performed on silica gel 60F
(Merck 9385, 0.040–0.063 mm). Melting points were measured on
2-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)aniline 7. 1-(2-(2-(2-
Methoxyethoxy)ethoxy)ethoxy)-2-nitrobenzene (6) was prepared
using the literature procedure.21 A solution of the starting material
(0.14 g, 0.50 mmol) in EtOH (10 mL) was added to wet Pd/C
(catalytic amount) and hydrazine hydrate (0.10 mL, 3.21 mmol),
and heated under reflux for 24 h. The resulting mixture was filtered
2340 | Org. Biomol. Chem., 2010, 8, 2332–2343
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
Scheme 6 Synthesis of compound 4
Scheme 7 Synthesis of the dumbbell 1-H·PF6
through a pad of Celite, washed with H2O (5 mL) and extracted
with CH2Cl2 (3 ¥ 10 mL), dried (MgSO4) and evaporated to
dryness to give the product 7 as a faintly yellow liquid (0.12 g,
93%). 1H NMR (CDCl3, 400 MHz, 295 K) d = 3.38 (s, 3 H), 3.54–
3.57 (m, 2 H), 3.65–3.69 (m, 4 H), 3.72–3.74 (m, 2 H), 3.84–3.86
(m, 2 H), 3.87–3.93 (br, 2 H), 4.14–4.17 (m, 2 H), 6.67–6.73 (m, 2
H), 6.78–6.82 (m, 2 H); 13C NMR (CDCl3, 101 MHz, 296 K): d =
58.7, 68.1, 69.5, 70.3, 70.4, 70.5, 71.7, 112.8, 114.9, 117.8, 121.6,
137.1, 146.0; MS (HR-ESI): C13H21NO4 [M+Na]+: calcd 278.1363,
found 278.1364.
were dried (MgSO4) and the resulting solution was evaporated
to dryness. Flash column chromatography with hexane–EtOAc–
Et3N (150 : 50 : 1) on silica gel of the recovered product allowed
isolation of the amine 1 (0.94 g, 88%) as a bright yellow powder.
◦
Rf 0.26 (hexane–EtOAc–Et3N = 150 : 50 : 1); M.p. 85.4–87.1 C;
1H NMR (CDCl3, 400 MHz, 296 K): d = 3.82 (s, 6 H), 3.99 (s, 2
H), 4.69 (s, 2 H), 6.42 (t, J = 2.2 Hz, 1 H), 6.62 (d, J = 2.2 Hz,
2 H), 7.44–7.52 (m, 4 H), 8.01 (d, J = 7.7 Hz, 2 H), 8.25 (d, J =
8.8 Hz, 2 H), 8.41 (s, 1 H); 13C NMR (CDCl3, 101 MHz, 296
K): d = 44.9, 54.5, 55.5, 99.5, 106.1, 124.3, 125.0, 126.1, 127.3,
129.2, 130.4, 131.7, 131.8, 143.1, 161.1; MS (HR-ESI): C24H23NO2
[M+H]+: calcd 358.1802, found 358.1799.
Diimine 4. A solution of 2,6-pyridinedicarboxaldehyde (2)
(0.03 g, 0.23 mmol) and 2-(2-(2-(2-methoxyethoxy)ethoxy)-
ethoxy)aniline (7) (0.12 g, 0.46 mmol) in CH2Cl2 (5 mL) was stirred
Dumbbell 1-H·PF6. A solution of 1 (0.73 g, 2.05 mmol) in
CH2Cl2–MeCN (3 : 1, 40 mL) was treated dropwise with 60 wt.%
HPF6 in H2O (0.60 mL, 7.28 mmol) at 0 ◦C over a period of
5 min and stirred at ambient conditions for 2 h. The solvents were
removed in vacuo to give a yellow solid, which was then dissolved in
CH2Cl2 (40 mL), washed with H2O (20 mL), further extracted with
CH2Cl2 (3 ¥ 10 mL) and dried (Na2SO4). The resulting solution
was evaporated to give the product 1-H·PF6 as a milky yellow
˚
with molecular sieves (4 A) for 24 h. Removal of molecular sieve
1
and solvent gave the diimine 4 (0.14 g, 94%) as a yellow oil. H
NMR (CDCl3, 400 MHz, 295 K): 3.33 (s, 6 H), 3.49 (t, J = 4.6 Hz,
4 H), 3.59–3.64 (m, 8 H), 3.75 (t, J = 4.6 Hz, 4 H), 3.89 (t, J =
4.9 Hz, 4 H), 4.24 (t, J = 4.9 Hz, 4 H), 7.00–7.02 (m, 4 H), 7.13
(d, J = 7.3 Hz, 2 H), 7.20 (t, J = 7.3 Hz, 2 H), 7.92 (t, J = 7.7 Hz,
1 H), 8.31 (d, J = 7.7 Hz, 2 H), 8.70 (s, 2 H); 13C NMR (CDCl3,
101 MHz, 295 K): 59.0, 68.9, 69.7, 70.5, 70.7, 71.0, 71.9, 114.1,
121.0, 121.6, 122.9, 127.5, 137.1, 140.9, 151.6, 154.8, 161.2; MS
(HR-ESI): C33H43N3O8 [M+Na]+: calcd 632.2942, found 632.2940.
◦
1
powder (0.78 g, 75%). M.p. >194 C (decomposed); H NMR
(CD3CN, 400 MHz, 296 K): d = 3.80 (s, 6 H), 4.41 (t, J = 5.2 Hz,
2 H), 5.24 (t, J = 6.0 Hz, 2 H), 6.61 (t, J = 2.2 Hz, 1 H), 6.69
(d, J = 2.2 Hz, 2 H), 7.12–7.50 (br, 2 H), 7.58–7.68 (m, 4 H),
8.10 (d, J = 8.8 Hz, 2 H), 8.16 (d, J = 8.3 Hz, 2 H), 8.74 (s, 1
H); 13C NMR (CD3CN, 101 MHz, 296 K, one aromatic signal
is missing/overlapping): d = 43.9, 52.8, 56.3, 102.3, 109.1, 122.1,
124.1, 126.6, 128.6, 130.5, 131.8, 132.3, 133.4, 162.5; MS (HR-
ESI): C24H24F6NO2P [M-PF6]+: calcd 358.1802, found 358.1795.
Amine 1. A solution of 9-anthraldehyde (8) (0.62 g, 2.99 mmol)
and 3,5-dimethoxybenzylamine (9) (0.55 mL, 3.64 mmol) in PhMe
(50 mL) was heated under reflux for 24 h using a Dean–Stark
apparatus. The resulting solution was evaporated to dryness. The
residue was dissolved in a mixture of MeOH–CH2Cl2 (2 : 1, 30 mL)
and NaBH4 (0.51 g, 13.40 mmol) added at 0 ◦C. After stirring the
reaction mixture under ambient conditions for 24 h, the solvents
were removed in vacuo, and the residue was partitioned between
H2O (50 mL) and CH2Cl2 (25 mL). The aqueous layer was further
extracted with CH2Cl2 (3 ¥ 25 mL). The combined organic extracts
Rotaxane R-H·PF6. A solution of the dialdehyde 2 (0.04 g,
0.28 mmol), the diamine 38c (0.10 g, 0.28 mmol) and the dumbbell
1-H·PF6 (0.14 g, 0.28 mmol) in dry MeCN (0.46 mL, 60 mM)
was stirred for 5 min at ambient temperature. Evaporation of
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2332–2343 | 2341
©

View Article Online
the solvent yielded the rotaxane R-H·PF6 (94%) as a dull yellow
solid. M.p. >163 ◦C (decomposed);1H NMR (CD3CN, 400 MHz,
295 K): 3.32 (s, 6 H), 3.59–3.84 (m, 10 H), 4.03–4.08 (m, 2 H),
4.29–4.31 (m, 4 H), 5.13 (t, J = 6.9 Hz, 2 H), 5.82 (t, J = 6.6 Hz,
2 H), 6.11 (t, J = 2.0 Hz, 1 H), 6.48 (d, J = 7.7 Hz, 2 H), 6.52 (d,
J = 2.0 Hz, 2 H), 6.83 (t, J = 7.7 Hz, 2 H), 7.00–7.07 (m, 4 H),
7.15 (t, J = 7.4 Hz, 2 H), 7.24–7.28 (m, 4 H), 7.71 (t, J = 7.7 Hz,
1 H), 7.79 (d, J = 8.4 Hz, 2 H), 7.92 (s, 2 H), 8.35 (s, 1 H), 8.55
(d, J = 8.9 Hz, 2 H), 9.67–10.05 (br, 2 H); 13C NMR (CD3CN,
101 MHz, 295 K): 45.3, 53.3, 55.6, 69.2, 70.4, 71.8, 72.0, 100.9,
105.3, 112.9, 120.6, 122.2, 124.4, 124.9, 126.0, 127.3, 129.5, 129.6,
130.1, 130.5, 131.8, 132.1, 136.6, 139.5, 140.2, 152.8, 153.0, 160.1,
161.4; MS (HR-ESI): C51H53F6N4O7P [M-PF6]+: calcd 833.3909,
found 833.3882.
(c) K. E. Griffiths and J. F. Stoddart, Pure Appl. Chem., 2008, 80, 485–
506; (d) J. F. Stoddart and H. M. Colquhoun, Tetrahedron, 2008, 64,
8231–8263.
6 For examples of rotaxane syntheses with ring closing metathesis, see:
(a) J. A. Wisner, P. D. Beer, M. G. B. Drew and M. R. Sambrook,
J. Am. Chem. Soc., 2002, 124, 12469–12476; (b) A. F. M. Kilbinger, S. J.
Cantrill, A. W. Waltman, M. W. Day and R. H. Grubbs, Angew. Chem.,
Int. Ed., 2003, 42, 3281–3285; (c) F. Arico´, P. Mobian, J.-M. Kern and
J.-P. Sauvage, Org. Lett., 2003, 11, 1887–1890; (d) J. D. Badjic, S. J.
Cantrill, R. H. Grubbs, E. N. Guidry, R. Orenes and J. F. Stoddart,
Angew. Chem., Int. Ed., 2004, 43, 3273–3278; (e) E. N. Guidry, S. J.
Cantrill, J. F. Stoddart and R. H. Grubbs, Org. Lett., 2005, 7, 2129–
2132; (f) S. J. Cantrill, R. H. Grubbs, D. Lanari, K. C.-F. Leung, A.
Nelson, K. G. Poulin-Kerstien, S. P. Smidt, J. F. Stoddart and D. A.
Tirrell, Org. Lett., 2005, 7, 4213–4216; (g) H. Hou, K. C.-F. Leung,
D. Lanari, A. Nelson, J. F. Stoddart and R. H. Grubbs, J. Am. Chem.
Soc., 2006, 128, 15358–15359; (h) E. N. Guidry, J. Li, S. J. Cantrill,
J. F. Stoddart and R. H. Grubbs, J. Am. Chem. Soc., 2007, 129, 8944–
8945.
7 For rotaxane-containing disulfide units in their dumbbell-shaped
components that are formed by riveting and stoppering approaches, see:
(a) A. G. Kolchinski, N. W. Alcock, R. A. Roesner and D. H. Busch,
Chem. Commun., 1998, 1437–1438; (b) Y. Furusho, T. Hasegawa, A.
Tsuboi, N. Kihara and T. Takata, Chem. Lett., 2000, 18–19; (c) T. Oku,
T. Furush and T. Takata, J. Polym. Sci. Part A, 2003, 41, 119–123; (d) Y.
Furusho, T. Oku, G. A. Rajkumar and T. Takata, Chem. Lett., 2004,
33, 52–53.
Acknowledgements
This work was supported by a Strategic Investments Scheme from
The Chinese University of Hong Kong and a General Research
Fund (CUHK401808) by The Research Grants Council of Hong
Kong.
8 (a) S. J. Cantrill, S. J. Rowan and J. F. Stoddart, Org. Lett., 1999, 1,
1363–1366; (b) S. J. Rowan and J. F. Stoddart, Org. Lett., 1999, 1,
1913–1916; (c) P. T. Glink, A. I. Oliva, J. F. Stoddart, A. J. P. White and
D. J. Williams, Angew. Chem., Int. Ed., 2001, 40, 1870–1875; (d) M.
Horn, J. Ihringer, P. T. Glink and J. F. Stoddart, Chem. Eur. J., 2003,
17, 4046–4054; (e) F. Arico´, T. Chang, S. J. Cantrill, S. I. Khan and
J. F. Stoddart, Chem. Eur. J., 2005, 11, 4655–4666; (f) C. D. Meyer,
C. S. Joiner and J. F. Stoddart, Chem. Soc. Rev., 2007, 36, 1705–1723;
(g) P. C. Haussmann, S. I. Khan and J. F. Stoddart, J. Org. Chem.,
2007, 72, 6708–6713; (h) P. C. Haussmann and J. F. Stoddart, Chem.
Rec., 2009, 9, 136–154; (i) K. C.-F. Leung, W.-Y. Wong, F. Arico´, P. C.
Haussmann and J. F. Stoddart, Org. Biomol. Chem., 2010, 8, 83–89.
9 (a) C. Godoy-Alca´ntar, A. K. Yatsimirsky and J.-M. Lehn, J. Phys. Org.
Chem., 2005, 18, 979–985; (b) W. Lu and T. H. Chang, J. Org. Chem.,
2001, 66, 3467–3473; (c) T. Hirashita, Y. Hayashi, K. Mitsui and S.
Araki, J. Org. Chem., 2003, 68, 1309–1313.
References
1 (a) P. A. Brady, R. P. Bonar-Law, S. J. Rowan, C. J. Suckling and J. K. M.
Sanders, Chem. Commun., 1996, 319–320; (b) P. A. Brady and J. K. M.
Sanders, Chem. Soc. Rev., 1997, 26, 327–336; (c) R. L. E. Furlan, S.
Otto and J. K. M. Sanders, Proc. Natl. Acad. Sci. USA, 2002, 99, 4801–
4804; (d) J. K. M. Sanders, Pure Appl. Chem., 2000, 72, 2265–2274;
(e) P. T. Corbett, J. Leclaire, L. Vial, K. R. West, J.-L. Wietor, J. K. M.
Sanders and S. Otto, Chem. Rev., 2006, 106, 3652–3711; (f) J.-M. Lehn,
Chem. Eur. J., 1999, 5, 2455–2463; (g) J.-M. Lehn, Chem. Soc. Rev.,
2007, 36, 151–160; (h) S. J. Rowan, S. J. Cantrill, G. R. L. Cousins,
J. K. M. Sanders and J. F. Stoddart, Angew. Chem., Int. Ed., 2002,
41, 898–952; (i) L. M. Greig and D. Philp, Chem. Soc. Rev., 2001,
30, 287–302; (j) A. Herrmann, Org. Biomol. Chem., 2009, 7, 3195–
3204.
2 (a) K. S. Chichak, S. J. Cantrill, A. R. Pease, S. H. Chiu, G. W. V. Cave,
J. L. Atwood and J. F. Stoddart, Science, 2004, 304, 1308–1312; (b) S. J.
Cantrill, K. S. Chichak, A. J. Peters and J. F. Stoddart, Acc. Chem.
Res., 2005, 38, 1–9; (c) K. S. Chichak, S. J. Cantrill and J. F. Stoddart,
Chem. Commun., 2005, 27, 3391–3393; (d) A. J. Peters, K. S. Chichak,
S. J. Cantrill and J. F. Stoddart, Chem. Commun., 2005, 27, 3394–3396;
(e) K. S. Chichak, A. J. Peters, S. J. Cantrill and J. F. Stoddart, J. Org.
Chem., 2005, 70, 7956–7962; (f) C. D. Pentecost, A. J. Peters, K. S.
Chichak, G. W. V. Cave, S. J. Cantrill and J. F. Stoddart, Angew. Chem.,
Int. Ed., 2006, 45, 4099–4104; (g) C. D. Pentecost, N. Tangchaivang,
S. J. Cantrill, K. S. Chichak, A. J. Peters and J. F. Stoddart, J. Chem.
Ed., 2007, 84, 855–859.
3 (a) K. C.-F. Leung, F. Arico´, S. J. Cantrill and J. F. Stoddart, J. Am.
Chem. Soc., 2005, 127, 5808–5810; (b) K. C.-F. Leung, F. Arico´, S. J.
Cantrill and J. F. Stoddart, Macromolecules, 2007, 40, 3951–3959;
(c) K. C.-F. Leung, Macromol. Theory Simul., 2009, 18, 328–335;
(d) K. C.-F. Leung, S. Xuan and C.-M. Lo, ACS Appl. Mater. Interfaces,
2009, 1, 2005–2012.
4 (a) A. R. Williams, B. H. Northrop, T. Chang, J. F. Stoddart, A. J. P.
White and D. J. Williams, Angew. Chem., Int. Ed., 2006, 45, 6665–6669;
(b) B. H. Northrop, N. Tangchiavang, J. D. Badjic and J. F. Stoddart,
Org. Lett., 2006, 8, 3899–3902; (c) C. D. Pentecost, K. S. Chichak,
A. J. Peters, G. W. V. Cave, S. J. Cantrill and J. F. Stoddart, Angew.
Chem., Int. Ed., 2007, 46, 218–222; (d) J. Wu, K. C.-F. Leung and
J. F. Stoddart, Proc. Natl. Acad. Sci. USA, 2007, 104, 17266–17271;
(e) L. M. Klivansky, G. Koshkakaryan, D. Cao and Y. Liu, Angew.
Chem., Int. Ed., 2009, 48, 4185–4189.
10 (a) V. Saggiomo and U. Lu¨ning, Eur. J. Org. Chem., 2008, 4329–4333;
(b) V. Saggiomo and U. Lu¨ning, Tetrahedron Lett., 2009, 50, 4663–4665.
11 A. Simion, C. Simion, T. Kanda, S. Nagashima, Y. Mitoma, T. Yamada,
K. Mimura and M. Tashiro, J. Chem. Soc. Perkin Trans. 1, 2001, 2071–
2078.
12 M. Hutin, C. A. Schalley, G. Bernardinelli and J. R. Nitschke, Chem.
Eur. J., 2006, 12, 4069–4076.
13 K. C.-F. Leung, C.-P. Chak, C.-M. Lo, W.-Y. Wong, S. Xuan and
C. H. K. Cheng, Chem. Asian J., 2009, 4, 364–381.
14 (a) S. R. Quake and A. Scherer, Science, 2000, 290, 1536–1540; (b) J.
Atencia and D. J. Beebe, Nature, 2005, 437, 648–655; (c) P. S. Dittrich,
K. Tachikawa and A. Manz, Anal. Chem., 2006, 78, 3887–3907.
15 (a) V. Balzani, M. Clemente-Leo´n, A. Credi, B. Ferrer, M. Venturi,
A. H. Flood and J. F. Stoddart, Proc. Natl. Acad. Sci. USA, 2006,
103, 1178–1183; (b) B. Ferrer, G. Rogez, A. Credi, R. Ballardini, M. T.
Gandolfi, V. Balzani, Y. Liu, H.-R. Tseng and J. F. Stoddart, Proc. Natl.
Acad. Sci. USA, 2006, 103, 18411–18416; (c) S. Saha, K. C.-F. Leung,
T. D. Nguyen, J. F. Stoddart and J. I. Zink, Adv. Funct. Mater., 2007,
17, 685–693; (d) T. D. Nguyen, K. C.-F. Leung, M. Liong, Y. Liu, J. F.
Stoddart and J. I. Zink, Adv. Funct. Mater., 2007, 17, 2101–2110.
16 (a) L. Kovbasyuk and R. Kramer, Chem. Rev., 2004, 104, 3161–3187;
(b) J. F. Callan, A. P. de Silva and D. C. Magri, Tetrahedron, 2005, 61,
8551–8588; (c) W. R. Browne and B. L. Feringa, Nat. Nanotechnol.,
2006, 1, 25–35; (d) A. B. Descalzo, R. Mart´ınez-Ma´n˜ez, F. Sanceno´n,
K. Hoffmann and K. Rurack, Angew. Chem., Int. Ed., 2006, 45, 5924–
5948; (e) E. R. Kay, D. A. Leigh and F. Zerbetto, Angew. Chem., Int.
Ed., 2007, 46, 72–191; (f) T. Cheng, D. I. Rozkiewicz, B. J. Ravoo, E. W.
Meijer and D. N. Reinhoudt, Nano Lett., 2007, 7, 978–980; (g) J. F.
Stoddart, Chem. Soc. Rev., 2009, 38, 1802–1820.
5 (a) D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed., 1996, 35, 1155–
1196; (b) F. Arico´, J. D. Badjic, S. J. Cantrill, A. H. Flood, K. C.-F.
Leung, Y. Liu and J. F. Stoddart, Top. Curr. Chem., 2005, 249, 203–259;
17 (a) P. R. Ashton, R. Ballardini, V. Balzani, M. Go´mez-Lo´pez,
S. E. Lawrence, M. V. Mart´ınez-D´ıaz, M. Montalti, A. Piersanti, L.
Prodi, J. F. Stoddart and D. J. Williams, J. Am. Chem. Soc., 1997, 119,
2342 | Org. Biomol. Chem., 2010, 8, 2332–2343
This journal is
The Royal Society of Chemistry 2010
©

View Article Online
10641–10651; (b) P. R. Ashton, R. Ballardini, V. Balzani, I. Baxter,
A. Credi, M. C. T. Fyfe, M. T. Gandolfi, M. Go´mez-Lo´pez, M. V.
Mart´ınez-D´ıaz, A. Piersanti, N. Spencer, J. F. Stoddart, M. Venturi,
A. J. P. White and D. J. Williams, J. Am. Chem. Soc., 1998, 120, 11932–
11942; (c) R. Ballardini, V. Balzani, W. Dehaen, A. E. Dell’Erba, F. M.
Raymo, J. F. Stoddart and M. Venturi, Eur. J. Org. Chem., 2000, 591–
602; (d) S. Garaude´e, S. Silvi, M. Venturi, A. Credi, A. H. Flood and
J. F. Stoddart, ChemPhysChem, 2005, 6, 2145–2152; (e) S. Saha, L. E.
Johansson, A. H. Flood, H.-R. Tseng, J. I. Zink and J. F. Stoddart,
Small, 2005, 1, 87–90; (f) S. Saha, L. E. Johansson, A. H. Flood, H.-R.
Tseng, J. I. Zink and J. F. Stoddart, Chem. Eur. J., 2005, 11, 6846–6858;
(g) S. Saha and J. F. Stoddart, Chem. Soc. Rev., 2007, 36, 77–92.
18 R. A. Bissell, A. P. de Silva, H. Q. N. Gunaratne, P. L. M. Lynch,
G. E. M. Maguire, C. P. McCoy and K. R. A. S. Sandanayake, Top.
Curr. Chem., 1993, 168, 223–264.
19 (a) K. C.-F. Leung, T. D. Nguyen, J. F. Stoddart and J. I. Zink, Chem.
Mater., 2006, 18, 5919–5928; (b) R. Liu, Y. Zhang and P. Feng, J. Am.
Chem. Soc., 2009, 131, 15128–15129.
20 (a) By NMR titration between the dumbbell 1-H+ and diimine
compound 5, a Job plot has been created (unpublished results). The
plot reveals that the dumbbell 1-H+ and diimine compound 5 formed a
1 : 1 supramolecular complex.
21 M. Kimura, K. Kajita, N. Onoda and S. Morosawa, J. Org. Chem.,
1990, 55, 4887–4892.
This journal is
The Royal Society of Chemistry 2010
Org. Biomol. Chem., 2010, 8, 2332–2343 | 2343
©