Redox-driven switching in pseudorotaxanes

Article abstract of DOI:10.1039/b819466a

Two donor-acceptor thread-like compounds incorporating viologen (V ²⁺) units and 1,5-dihydroxynaphthalene (DNP) stations have been prepared. Their ability to form self-assembled charge-transfer (CT) complexes with cucurbit[8]uril (CB[8]) is evi

Full text of DOI:10.1039/b819466a

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New Journal of Chemistry
An international journal of the chemical sciences
Volume 33 | Number 2 | February 2009 | Pages 213–440
www.rsc.org/njc
Themed Issue: Dedicated to Professor Jean-Pierre Sauvage
ISSN 1144-0546
PAPER
J. Fraser Stoddart et al.
Redox-driven switching in
pseudorotaxanes
1144-0546(2009)33:2;1-1

PAPER
www.rsc.org/njc | New Journal of Chemistry
Redox-driven switching in pseudorotaxaneswz
Ali Trabolsi,a Mohamad Hmadeh,b Niveen M. Khashab,a Douglas C. Friedman,a
Matthew E. Belowich,a Nicolas Humbert,b Mourad Elhabiri,b Hussam A. Khatib,a
Anne-Marie Albrecht-Gary*b and J. Fraser Stoddart*a
Received (in Montpellier, France) 3rd November 2008, Accepted 15th December 2008
First published as an Advance Article on the web 15th January 2009
DOI: 10.1039/b819466a
Two donor–acceptor thread-like compounds incorporating viologen (V2+) units and
1,5-dihydroxynaphthalene (DNP) stations have been prepared. Their ability to form
self-assembled charge-transfer (CT) complexes with cucurbit[8]uril (CB[8]) is evidenced by
UV-Vis and NMR spectroscopies, as well as by mass spectrometry. Binding studies show the
formation of 1 : 1 and 2 : 1 complexes between CB[8] and a thread-like compound containing two
viologen units, while only a 1 : 1 inclusion complex was observed between CB[8] and a thread-like
compound containing only a single viologen unit. The switching behavior of the threads within
their pseudorotaxane frameworks was investigated by using cyclic voltammetry (CV) and UV-Vis
spectroscopy.
reported14,15a,b,d,f previously, quantitative data for these inter-
Introduction
actions has yet to be obtained and assessed. Virtually all
previous studies14,15 of similar compounds have resulted
in qualitative analyses of the resulting supramolecular com-
plexes. The determination of thermodynamic parameters
associated with these switchable complexes is a critical next
step in the development of useful molecular switches. Herein,
detailed studies on both switches and model systems have been
performed.
Artificial molecular switches have received much attention1,2
in recent years because of their response to external stimuli,
such as pH, redox change or light. A wide variety of molecular
switches based on rotaxanes,2 catenanes,2 molecular muscles,3
scissors,4 and elevators5 have been reported, illustrating these
phenomena. Moreover, the switching behavior of these mole-
cules has been harnessed in a range of nanoelectromechanical
devices, such as molecular muscle-activated cantilevers,6,3a
macroscopic liquid transport,7 molecular electronic devices,8
and mesoporous silica-mounted nanovalves.9 Cucurbit[n]uril10
(CB[n], n = 5–10), a family of macrocycles comprising n
glycoluril units, has been employed, not only in the context
of molecular recognition, but also in the construction of a wide
range of self-assembled entities including mechanically inter-
locked molecules11 and molecular switches.12,13 In particular,
CB[8], with a cavity size comparable to that of g-cyclodextrin,
can include two identical guest molecules to form10b a binary
1 : 2 complex, or two different guest molecules to form a ternary
1 : 1 : 1 complex.14 Kim et al.15 have exploited this redox-
coupled, guest-exchange process16 in designing a molecular
machine reminiscent of a loop lock. Although preliminary
studies into the use of DNP and viologen as complementary
pairs for binding within the cavity of CB[8] have been
We report the synthesis, characterization and switching
behavior of two different [2]pseudorotaxane-based complexes
which undergo reversible conversion between inter- and
intramolecular motion triggered by electrochemical stimuli.
Results and discussion
Synthesis
Compounds 1ꢀ4Cl and 2ꢀ2Cl were conveniently prepared
(Scheme 1) from the appropriate diazide 6 and monoazide 7,
respectively. Tosylation of 5,17 followed by reaction with
NaN3, afforded 6 and 7 in 80% and 48% yields, respectively.
The highly efficient and functional group tolerant ‘‘Cu(I)
catalyzed azide/alkyne cycloaddition’’18 was utilized to obtain
the final thread-like compounds. Compound Vꢀ2PF6 was
prepared following a previously reported procedure.19 Two
equivalents of Vꢀ2PF6 were allowed to react with diazide 6
(1 equiv.) in Me2CO in the presence of Cu(I) to afford 1ꢀ4PF6
in 73% yield; similarly, Vꢀ2PF6 (1 equiv.) was reacted with the
monoazide 7 (2 equiv.) to give 2ꢀ4PF6 in 74% yield. Anion
exchange to afford the chloride salts was achieved by dis-
a Department of Chemistry, Northwestern University, 2145 Sheridan
Road, Evanston, Il, 60208, USA.
E-mail: stoddart@northwestern.edu; Fax: (+1)-847-491-1009;
Tel: (+1)-847-491-3793
b Laboratoire de Physico-ChimieBioinorganique, ULP-CNRS
(UMR 7177), Institut de Chimie, ECPM, 25 rue Becquerel, 67200
Strasbourg, France. E-mail: amalbre@chimie.u-strasbg.fr;
Fax: +33 (0) 3 90 24 26 39; Tel: +33 (0) 3 90 24 26 38
w Dedicated to Prof. Jean-Pierre Sauvage on the occasion of his 65th
birthday.
ꢁ
solving the PF6 salts in cold MeOH (10 1C) followed by the
dropwise addition of concentrated HCl. After 1 h, THF and
Et2O were added to precipitate the product. The precipitates
were washed several times with ether before being dried in
vacuum to afford 1ꢀ4Cl and 2ꢀ2Cl in 91 and 88% yields,
respectively.
z Electronic supplementary information (ESI) available: Further
UV-Vis and fluorescence spectroscopic studies, switching properties
and NMR spectroscopy studies. See DOI: 10.1039/b819466a
ꢂc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
254 | New J. Chem., 2009, 33, 254–263

Scheme 1 Synthesis of 1ꢀ4Cl and 2ꢀ2Cl
by summing the electronic spectra of their individual
Spectroscopic properties
components, Vꢀ2Cl and N-1. In addition, the electronic spectra
of 1ꢀ4Cl and 2ꢀ2Cl are characterized by broad and weak
absorption bands (lmax = 460 nm; e460 = 390 Mꢁ1 cmꢁ1
for 1ꢀ4Cl and lmax = 475 nm; e475 = 390 Mꢁ1 cmꢁ1 for 2ꢀ2Cl)
in the visible region.
The electronic properties of the two donor–acceptor systems,
1ꢀ4Cl and 2ꢀ2Cl, as well as the reference compounds (Fig. 1)
Vꢀ2Cl, N-1 and N-2, were considered and are listed in Table 1.
It is also noteworthy that cucurbit[8]uril (CB[8]) is not an
absorbing species that Vꢀ2Cl is a not an emitting species
and that 1ꢀ4Cl and 2ꢀ2Cl are far less emitting than their N-1
(N-2) counterparts, within the spectral window considered.
Assuming that the tetraethyleneglycol (TEG) chains con-
necting the chromophores are poor electronic relays, 1ꢀ4Cl
and 2ꢀ2Cl can be characterized (Fig. 2) by significant
hypochromic and moderate bathochromic shifts with respect
to their recalculated electronic spectra, which were obtained
These bands can be assigned to charge transfer (CT) inter-
actions between the electron-donating 1,5-dioxynaphthalene
ring system and the electron-accepting 4,40-bipyridinium units
present in the two compounds. These CT interactions are also
responsible for the large decrease of the emission intensity
centered on the electron-rich subunit of the two compounds.
These spectroscopic data (Table 1) demonstrate that 1ꢀ4Cl and
Fig. 1 Structural formulas and graphical representations of the model compounds relevant to the research in this paper.
Table 1 Spectroscopic properties (absorption and emission) of the chromophoric systems in 1ꢀ4Cl, 2ꢀ2Cl, Vꢀ2Cl, N-1 and N-2a
Emission
Absorption
l
ꢁ1
System
max/nm (10ꢁ4el /M cmꢁ1
)
lmax/nm
Fabs (%)
max
b
b
Vꢀ2Cl
N-1
N-2
263 (2.10)
223 (5.53)/283 (0.74)/294 (0.93)/310 (0.68)/324 (0.47)
250 (0.36)/259 (0.42)/269 (0.44)/279 (0.25)/343 (0.27)
225 (3.71)/266 (2.88)/313 (0.57)/326 (0.37)/460 (0.039)
224 (8.40)/274 (2.30)/282 (2.28)/294 (1.91)/311 (1.20)/325 (0.80)/475 (0.039)
344/357
374/428
359/424
354
11.8
37.0
0.4
1ꢀ4Cl
2ꢀ2Cl
0.2
Solvent: H2O, pH 7.0 (phosphate buffer 0.1 M), T = 25.0(2) 1C. The errors in lmax, on el
respectively. Non-emitting species.
and in Fabs are ꢃ 1 nm, 5% and 10%,
a
max
b
ꢂc
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New J. Chem., 2009, 33, 254–263 | 255

(Fig. S4 in ESIw). The two components of V2+CCB[8] are
held together by ion–dipole interactions between the positive
charges borne by the bipyridinium unit and the polar oxygens
of the macrocyclic CB[8]. The subsequent high binding affinity
of this supramolecule was found to be in excellent agreement
with that reported in the literature.21 Although CB[8] has a
cavity large enough10a to encapsulate two Vꢀ2Cl molecules,
only a single dicationic viologen unit is hosted by the macro-
cyclic ligand. The formation of the 2 : 1 complex is most likely
precluded by the strong electrostatic repulsions between the
two Vꢀ2Cl dications. However, these recognition properties
can be tuned by simple redox chemistry and CB[8] was shown
to be able to accommodate reduced viologen radical cations.21
Having established the binding stoichiometry and strength
of the complex between CB[8] and Vꢀ2Cl, it was thus of
interest to investigate the recognition properties in the pre-
sence of a p-electron donating unit such as N-1 or N-2.
Cucurbit[8]uril was gradually added to an equimolar solution
of Vꢀ2Cl and N-1 (or N-2) and the UV-Visible spectro-
photometric variations were monitored (Fig. S5 and S7 in
ESIz). For the statistical treatment of the data, the electronic
spectra and stability constant of V2+CCB[8] were fixed. It is
important to note that Vꢀ2Cl could interact with N-1 or N-2 in
the absence of CB[8], as was shown for the two compounds,
1ꢀ4Cl and 2ꢀ2Cl, as well as for closely related systems.14,22
However, their intermolecular associations lead to rather
low stability constants that could not be determined with
Fig. 2 Electronic spectra of 1ꢀ4Cl (solid blue line) and 2ꢀ2Cl (solid
black line) compared to their calculated spectra (dashed lines)
obtained by summing the electronic spectra of the individual com-
ponents N-1 and Vꢀ2Cl. Solvent: H2O, pH 7.0 (phosphate buffer
0.1 M), T = 25.0(2) 1C.
2ꢀ2Cl can undergo spontaneous self-folding in solution under
the experimental conditions (Fig. S1 and S2 in ESIz).
Binding properties
The thorough study of the inclusion complexes 1ꢀ4Cl and
2ꢀ2Cl within CB[8] first requires an examination of the recog-
nition properties of the macrocyclic host towards the mono-
topic dicationic Vꢀ2Cl guest in the absence and in the presence
of the p-electron donating counterpart, N-1 or N-2. Fig. 3
shows the spectral variation of the viologen-centered electronic
transitions upon gradual addition of CB[8]. Significant and
concomitant hypochromic and bathochromic shifts of p–p*
transitions as well as the presence of isosbestic points supports
that a 1 : 1 complex is exclusively formed from Vꢀ2Cl and
CB[8]. This 1 : 1 complex was further confirmed by a Job’s
plot20 (Fig. S3 in ESIz). The spectrophotometric data were
processed by statistical methods20 allowing us to calculate
great accuracy. Their logarithmic values (log KV2+
and
ꢀN-1
log KV2+ꢀN-2) were, however, estimated to be lower than 2.
Therefore, under the experimental conditions employed,
V2+ꢀN-1 or V2+ꢀN-2 associations represent only minor
species, which were consequently neglected. Regardless, the
nature of the p-electron donating unit (N-1 or N-2), ternary
1 : 1 : 1 complexes, involving one CB[8] host and two
hetero-guests, namely Vꢀ2Cl and N-1 (or N-2) were characterized
and quantified. Their global stability constants, as well as
some relevant spectrophotometric parameters, are given in
Tables 1 and 2 (Fig. S6 and S8 in ESIz). The binding con-
stants given in Table 2 are defined by the following equilibria
(N = N-1 or N-2):
the association constant of the
(log KV2+ = 4.8(2)) as well as its electronic spectrum
V
2+CCB[8] species
KV2þ
ꢁ! V
ꢁ
ꢅCB½8ꢄ
CCB[8]
V2þþ CB½8ꢄ
2þꢅCB½8ꢄ;
½V2þꢅCB½8ꢄꢄ
KV
¼
2þꢅCB½8ꢄ
½V2þꢄ½CB½8ꢄꢄ
bV2þ
ꢁ! V
ꢁ
ꢀNꢅCB½8ꢄ
N þ V2þþCB½8ꢄ
2þ ꢀ NꢅCB½8ꢄ;
½V2þ ꢀ NꢅCB½8ꢄꢄ
bV
¼
2þꢀNꢅCB½8ꢄ
½Nꢄ½V2þꢄ½CB½8ꢄꢄ
KV2þ
ꢁ! V
ꢁ
ꢀNꢅCB½8ꢄ
N þ V2þ ꢅ CB½8ꢄ
2þ ꢀ NꢅCB½8ꢄ;
½V2þ ꢀ NꢅCB½8ꢄꢄ
KV
¼
2þꢀNꢅCB½8ꢄ
½Nꢄ½V2þ ꢅ CB½8ꢄꢄ
Fig. 3 UV-Visible absorption spectrophotometric titration of the
bisalkyneviologen Vꢀ2Cl by CB[8]. Solvent: H2O, pH 7.0 (phosphate
buffer 0.1 M), T = 25.0(2) 1C, l = 1 cm, (1) [Vꢀ2Cl]tot = 2.15 ꢆ 10ꢁ5 M,
(2) [CB[8]]tot/[Vꢀ2Cl]tot = 3.6.
bV2+
= KV2+
ꢆ KV2+
ꢀNCCB[8]
ꢀNCCB[8]
CCB[8]
ꢂc
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256 | New J. Chem., 2009, 33, 254–263

Table 2 Spectroscopic properties and stability constants of the inclusion complexes with cucurbit[8]urila
ꢁ1
lmax/nm (10ꢁ4el /M cmꢁ1
)
max
Species
log b (ꢃ3s)
V
2+CCB[8]
4.8(2)b
9.8(9)b
8.7(2)c
9.5(6)b
9.4(3)c
5.8(4)b
9.2(5)b
6.1(2)b
271 (1.41)
272 (2.26)/310 (0.81)/325 (0.53)/445 (0.043)
V2+ꢀN-1CCB[8]
V2+ꢀN-2CCB[8]
254 (0.72)/263 (0.75)/275 (0.71)/285 (0.67)/298 (0.73)/417 (0.078)/566 (0.077)
1
1
2
4+CCB[8]
4+C(CB[8])2
2+CCB[8]
230 (2.98)/271 (2.55)/580 (0.044)
231 (3.13)/276 (2.26)/565 (0.07)
283 (2.09)/293 (1.98)/310 (1.31)/325 (0.79)/577 (0.05)
a
Solvent: water, pH 7.0 (phosphate buffer 0.1 M), T = 25.0(2) 1C. The reported errors are given as 3s with s = standard deviation. The errors on
lmax and on elmax are equal to ꢃ 1 nm and 5%, respectively. Determined from absorption titration. Determined from emission titration.
b
c
The electron-rich systems, N-1 or N-2, themselves do not
bind CB[8] in the absence of the dicationic bisalkyneviologen
Vꢀ2Cl. As for 1ꢀ4Cl and 2ꢀ2Cl, the ternary complexes
V2+ꢀN-1CCB[8] and V2+ꢀN-2CCB[8] are also characterized
by CT absorption bands in the visible region (Table 2), which
are clear signatures of the inclusion of the electroactive hetero-
pairs in the CB[8] host cavity (Fig. 4). It is noteworthy that
the position of the CT absorption band of V2+ꢀN-2CCB[8]
(lmax = 566 nm; e566 = 770 Mꢁ1 cmꢁ1) is in agreement
with that previously measured14 for a closely related ternary
complex with methylviologen dication (lmax = 580 nm).
Fig.
5
Fluorescence spectrophotometric titration of Vꢀ2Cl and
N-2 with CB[8]. Solvent: H2O, pH 7.0 (phosphate buffer 0.1 M),
T = 25.0(2) 1C, lex = 327 nm, emission and excitation slit widths
both 6 nm, respectively, [Vꢀ2Cl]tot = [N-2]tot = 1.73 ꢆ 10ꢁ5 M;
(1) [CB[8]]tot = 0 M, (2) [CB[8]]tot = 5.03 ꢆ 10ꢁ5 M. Inset: Stern–Volmer
plot at l = 427 nm (F0/F = 1 + KV2+
ꢆ [CB[8]]tot) .
ꢀN-2CCB[8]
are amplified in aqueous solution. Although the formation of
a 1 : 1 : 1 complex is entropically unfavorable, the enthalpic
stabilization gained by the hydrophobic interactions between
CB[8] and its guests yield a net stabilizing effect for the
complex.
Fig.
4
Schematic representation of the inclusion complexes
V
2+CCB[8], V2+ꢀN-1CCB[8] and V2+ꢀN-2CCB[8].
We also took advantage of the emission properties of N-1,
N-2 and Vꢀ2Cl to assess the binding affinities of the ternary
complexes by spectrofluorimetric titrations. The two p-electron
donating compounds display emission signals in the visible
region, while the dicationic p-electron accepting compound is
not an emitting species and can act as a fluorescence emission
inhibitor. Spectrofluorimetric titrations were carried out and are
shown in Fig. 5 (Fig. S9 in ESIz). These titrations indicate that,
upon addition of CB[8] to an equimolar mixture of Vꢀ2Cl and N-1
(or N-2), a significant quenching of the N-1 (or N-2) centered
fluorescence emission is observed. As the V2+ꢀN-1 or V2+ꢀN-2
complexes are minor species in solution, these quenching pro-
cesses result from CT interactions between the two building
blocks V2+ and N-1 (or N-2) within the CB[8] cavity, thus leading
to stable ternary species. A Stern–Volmer approach23 was used to
calculate the binding constants (log KV2+ꢀN-1CCB[8] = 3.88(3) and
log KV2+ꢀN-2CCB[8] = 4.57(2)). These values are in close agreement
with those obtained (Table 2) from absorption titrations, and
clearly emphasize a strong stabilization of the CT complexes by
more than two orders of magnitude when CB[8] is used. The
inclusion of the electron-rich guest is most likely driven by the van
der Waals interactions in the hydrophobic cavity of CB[8] which
The recognition properties of cucurbit[8]uril towards
p-electron rich (N-1 and N-2) and deficient (Vꢀ2Cl) models
being clearly established, we then examined the host–guest
complexes with 1ꢀ4Cl and 2ꢀ2Cl and CB[8]. Electrospray
ionization mass spectrometry (ESI-MS) was used to probe
the nature of the host–guest complexes with CB[8] (Fig. 6).
We characterized a single complex 22+CCB[8] with 2ꢀ2Cl,
while two species containing one (14+CCB[8]) and two CB[8]
(14+C(CB[8])2) units were observed (Fig. 7) with 1ꢀ4Cl. All the
ESI-MS signals (m/z = 604.0 for [14+CCB[8] ꢁ 4Cl]4+, 936.5 for
[14+C(CB[8])2 ꢁ 4Cl]4+ and 1333.5 for [22+CCB[8] ꢁ 2Cl]2+
)
correspond to multicharged species after losing their chloride
counterions.
We have therefore examined, by spectrophotometric means,
the formation of the host–guest complexes with CB[8] and
1ꢀ4Cl and 2ꢀ2Cl. Absorption spectrophotometric titrations
(Fig. S10 and S11, ESIz) of 1ꢀ4Cl and 2ꢀ2Cl by CB[8] were
carried out and the corresponding data were processed
statistically.20 The electronic spectra are presented in Fig. 8
and the stability constants are listed in Table 2. Although the
electronic spectrum of 22+CCB[8] is characterized by the
ꢂc
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New J. Chem., 2009, 33, 254–263 | 257

same behavior is also valid for the 14+CCB[8] complex. Its
electronic spectrum is characterized by a hypochromic shift
of the viologen-centered transitions, as well as a significant
bathochromic shift of the CT absorption band (Table 2 and
Fig. 8). The complex 14+CCB[8] most likely adopts (Fig. 7)
the same ‘‘end-to-interior loop’’ arrangement as 22+CCB[8], as
shown (Table 2) by their comparable association constants. In
contrast to 22+CCB[8], complex 14+CCB[8] contains a non-
hosted viologen residue, which can behave similarly to the
Vꢀ2Cl reference compound. In the presence of excess CB[8], a
second complex 14+C(CB[8])2 (Fig. 7) has indeed been iden-
tified from both spectrophotometric and mass spectrometric
studies. The electronic spectrum of 14+C(CB[8])2 is charac-
terized by a further hypochromic effect with respect to
1
4+CCB[8], and the maximum of the CT absorption band
remains unchanged. These spectrophotometric data indicate
that a second CB[8] macrocycle interacts with the free viologen
unit of
4+C(CB[8])2. The respective stability constants have been
determined (log K14+ = 5.8(4) and log K14+
1
4+CCB[8] to afford the ternary complex,
1
=
C(CB[8])2
CCB[8]
3.4(4), Table 2). The value of log K14+
can be directly
= 4.8(2))
C(CB[8])2
compared to that of V2+CCB[8] (log KV2+
CCB[8]
and emphasizes that the formation of 14+C(CB[8])2 is
destabilized by more than one order of magnitude with respect
Fig. 6 ESI mass spectra of the host–guest complexes (a) 1ꢀ4Cl and
(b) 2ꢀ2Cl, with CB[8]. Solvent H2O–MeOH (80/20 by weight), ESMS+,
Skim1 = 40 V, Top: [1ꢀ4Cl]tot = 7.5 ꢆ 10ꢁ6 M, [CB[8]]tot = 8.5 ꢆ 10ꢁ6 M,
Capillary exit
[CB[8]]tot = 2.8 ꢆ 10ꢁ5 M, Capillary exit = 270 V.
=
190 V, Bottom: [2ꢀ2Cl]tot
=
1.4
ꢆ
10ꢁ5 M,
same set of absorption bands as free 2ꢀ2Cl, it is, however,
significantly shifted. Indeed, the absorption band of the
viologen unit undergoes a significant hypochromic shift, which
clearly points to its inclusion within the CB[8] cavity. In
addition, the CT absorption band is preserved but significantly
red-shifted (Table 2), thus indicating that stronger CT inter-
actions take place between the p-electron accepting bipyridi-
nium unit and one of the two p-electron donating
dioxynaphthalene groups, located within the CB[8] cavity.
Therefore,
2
2+CCB[8] most likely adopts (Fig. 7) an
‘‘end-to-interior loop’’24 superstructure in which an intra-
molecular CT complex is included in the CB[8] cavity, leaving
one 1,5-dioxynaphthalene unit free. The stability constant of
2
2+CCB[8] (Table 2) is more than one order of magnitude
higher than that measured for V2+CCB[8], thus supporting
the formation of a stable intramolecular CT complex. The
Fig. 8 Electronic spectra of the host–guest complexes. (a) 1ꢀ4Cl,
Fig. 7 Schematic representation of the inclusion complexes of guests
14+ and 22+ with the host CB[8].
1
4+CCB[8], and 14+C(CB[8])2, (b) 2ꢀ2Cl and 22+CCB[8]. Solvent:
H2O, pH 7.0 (phosphate buffer 0.1 M), T = 25.0(2) 1C.
ꢂc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
258 | New J. Chem., 2009, 33, 254–263

to V2+CCB[8], most likely as a consequence of strong steric
interactions that inhibit the binding of a second CB[8] from
one side of 14+CCB[8].25 In addition, the UV-Vis data are
supported by 1H NMR spectroscopy (Fig. S13 and S14 in
ESIz).
Switching properties
The mechanical switching of the two [2]pseudorotaxane mole-
cular machines (Fig. 9) were studied by cyclic voltammetry
(CV) and UV-Vis spectroscopy. Typical cyclic voltammo-
grams of each thread 1ꢀ4Cl and 2ꢀ2Cl and its relative
Fig. 9 Chemically-triggered interconversions in host–guest com-
plexes: (a) reduction of 14+CCB[8] with ‘‘end-to-interior loop’’ super-
structure to form an ‘‘end-to-end loop’’ superstructure and (b)
reduction of 22+CCB[8] with ‘‘end-to-interior loop’’ superstructure
leads to the formation of a [3]pseudorotaxane superstructure.
[2]pseudorotaxane
1
4+CCB[8] (1
:
1,
1
mM) and
2
2+CCB[8] (1 : 1, 1 mM) are shown in Fig. 10. The two
threads 1ꢀ4Cl (2ꢀ2Cl) undergo two consecutive two-electron
(one-electron) processes with a sharp second peak that indi-
cates strong adsorption of the fully reduced species 10 and 20
on the electrode. The 1 : 1 complex is present in 495%
in solution.26 Compared to 1ꢀ4Cl, however, 14+CCB[8]
exhibits a splitting of the first reduction peak (ꢁ0.42 V)
into two different peaks (ꢁ0.44 and ꢁ0.82 V), corresponding
to two electrochemically different viologen units. One is
involved in the CT complex formation with the DNP unit
(at ꢁ0.82 V), of 1ꢀ4Cl, inside CB[8], while the other is free
(at ꢁ0.44 V). The second reduction peak of compound 1ꢀ4Cl
(ꢁ0.78 V) shows a very large shift in the presence of CB[8]
(ꢁ1.21 V).
Thus, the cyclic voltammetric behavior of 14+CCB[8]
suggests that the two-electron reduction of the [2]pseudo-
rotaxane 14+CCB[8] with an end-to interior loop structure
results in the generation of a species containing two terminal
viologen radical cation units, which then undergo a rapid
intramolecular pairing process inside CB[8] to form the stable
[2]pseudorotaxane, 12(+ )CCB[8], with an end-to-end loop
ꢇ
structure. On the other hand, the [2]pseudorotaxane
2+CCB[8] exhibits a large shift for the first reduction
2
peak (ꢁ0.67 V) compared to the monoviologen thread 2ꢀ2Cl
(ꢁ0.45 V). This large shift indicates the formation of a CT
inclusion complex in the cavity of the CB[8]. The second
reduction peak of the monoviologen compound, 2ꢀ2Cl
(ꢁ0.78 V), is split into two peaks in the presence of CB[8]
(ꢁ0.78 and ꢁ1.20 V) corresponding to two electrochemically
different viologen units. The peak at ꢁ0.78 V corresponds to
the viologen radical cation (V+ꢇ) threaded with CB[8], but not
dimerized. The second peak at ꢁ1.20 V corresponds to the
formation of the [3]pseudorotaxane (2+ꢇ)2CCB[8] and four
terminal DNP units. The peak at ꢁ1.20 V is more visible at
50 mV sꢁ1 than at 200 mV sꢁ1 (Fig. S12, ESIz), reflecting a
slower dimerization of the viologen radical cation in the case
of 2ꢀ2Cl than in 1ꢀ4Cl. The formation of these complexes was
confirmed by UV-Vis spectroscopy (Fig. 11). The treatment of
Fig. 10 Cyclic voltammograms of 1 : 1 mixture of (a) 1 mM of 1ꢀ4Cl
(red) and 14+CCB[8] (blue), (b) 1 mM of each 2ꢀ2Cl (red) and
solution containing 1 2
4+CCB[8] or 2+CCB[8] with
2
2+CCB[8] (blue). Supporting electrolyte: 0.1 M phosphate buffer
a
(pH 7.0). Scan rate: 200 mV sꢁ1
.
Na2S2O4 results in the appearance of new absorption bands
at l E 380, 550 and 4 900 nm which reflect the dimerization
of the viologen radical cation units in the cavity of the
CB[8]. Upon exposure to air, the viologen radical cations
were oxidized back to their original dicationic state. The
‘‘end-to-interior loop’’ structures, as evidenced by the CT band,
were thus regenerated (Fig. 11).
Conclusions
Two donor–acceptor thread-like compounds, incorporating
viologen units and 1,5-dihydroxynaphthalene ring systems
have been prepared. Their ability to form an end-to-interior
ꢂc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
New J. Chem., 2009, 33, 254–263 | 259

Fig. 11 Absorption spectra of (a) 14+CCB[8] (1 mM) and (b) 22+CCB[8] (1 mM), before (blue), after (red) reduction with Na2S2O4, and
following (black) reoxidation with air (10 min), in phosphate buffer 0.1 M (pH = 7.0), l = 1 cm.
superstructure with cucurbit[8]uril has been demonstrated
by UV-Vis spectroscopic studies. The appearance of a
charge transfer band is a phenomenon which is characteristic
of an intramolecular interaction between a viologen and
a dihydroxynaphthalene ring system in the cavity of the
cucurbituril. In the case of the bisviologen thread, the reduction
of the viologen units results in an intramolecular motion
(Fig. 9(a)), which can be contrasted with the intermolecular
motion observed (Fig. 9(b)) in the case of the monoviologen
thread.
UV-Vis absorption spectra were recorded on a Varian Cary-300
spectrophotometer.
Synthesis
6. The ditosylate derivative of 5 (500 mg, 0.54 mM) and
NaN3 (105 mg, 1.5 mM) were heated in DMF (20 mL) at 90 1C
overnight. After the removal of the solvent by evaporation, the
residue was dissolved in CH2Cl2, and the insoluble material
was removed by filtration. Concentration under reduced
pressure gave 6 (90%) as an orange oil.
1ꢀ4Cl. A solution (10 mL) of the DNP-diazide 6 (1.0 g,
1.5 mM) in Me2CO was added dropwise at room temperature
over 2 h to a solution of the bisalkyneviologen (Vꢀ2PF6)
(4.14 g, 7.5 mM) and a catalytic amount of tetrakis(acetonitrile)-
copper(I) hexafluorophosphate and tris(benzyltriazolylmethyl)-
amine (TBTA). The reaction mixture was left to stand overnight.
After the removal of the solvent by evaporation, the residue was
purified by column chromatography (SiO2, Me2CO plus 2%
ammonium hexafluorophosphate). Concentration under vacuum
afforded 1ꢀ4PF6 (73%) as a red, glassy product. The thread 1ꢀ4Cl
was obtained in 91% yield as a red, glassy product as described in
Experimental
General
All reagents and starting materials were purchased from
Aldrich and used without further purification. Compounds
7
and Vꢀ2PF6 were prepared according to literature
procedures.15 Thin-layer chromatography (TLC) was performed
on silica gel 60 F245 (E. Merck). Column chromatography
was carried out on silica gel 60F (Merck 9385, 0.040-0.063).
Deuteurated solvents (Cambridge Isotope Laboratories) for
NMR spectroscopic analyses were used as received. NMR
spectra were recorded on Varian 500 spectrometer, with
working frequencies of 500.13 MHz for 1H nuclei, and
100 MHz for 13C nuclei, respectively. Chemical shifts are quoted
in ppm relative to tetramethylsilane with the residual solvent
peak as a reference standard. High-resolution mass spectra were
measured either on an Applied Biosystems Voyager DE-PRO
MALDI TOF mass spectrometer (HR-TOF), or on a Finnigan
LCQ ion-trap mass spectrometer (HR-ESI). Electrochemical
experiments were carried out at room temperature in argon-
purged 100 mM phosphate buffered solutions (pH 7.0) with a
Princeton Applied Research 263 A Multipurpose instrument
interfaced to a PC. Cyclic voltammetry experiments were per-
formed by using a glassy carbon working electrode (0.018 cm2,
Cypress system). The electrode surface was polished routinely
with 0.05 mm alumina/water slurry on a felt surface immediately
before use. The counter electrode was a Pt coil and the reference
electrode was a Ag/AgCl electrode. The concentration of the
1
the results and discussion section. H NMR (500 MHz, D2O)
d 2.55 (t, 1H, J = 2.5 Hz), 2.98 (td, 2H, J = 6.5, 2.5 Hz), 3.31
(t, 2H, J = 6.5 Hz), 3.56 (m, 4H), 3.64–3.68 (m, 2H), 3.73–3.80
(m, 5H), 3.95 (br m, 2H), 4.14 (br m, 1H), 4.43 (m, 2H),
4.74 (t, 2H, J = 7 Hz), 4.72 (d, 1H, J = 7.5 Hz), 7.08 (t, 1H,
J = 8 Hz), 7.25 (d, 1H, J = 8.5 Hz), 7.75 (s, 1H), 8.08 (d, 2H,
J = 7 Hz), 8.25 (d, 2H, J = 7 Hz), 8.58 (d, 2H, J = 7 Hz); 13
C
NMR (100 MHz, D2O) d 20.1, 26.7, 49.9, 60.0, 61.3, 67.8, 68.6,
69.3, 69.4, 69.7, 69.8, 70.3, 74.4, 78.5, 106.4, 114.2, 124.9, 125.4,
126.2, 126.4, 141.7, 145.0, 145.6, 148.9, 149.1, 153.4. HRMS
(HR-ESI): m/z calc. for C62H74N10O8: 271.6; found: 271.8.
2ꢀ2Cl. Compounds 7 (400 mg, 0.744 mM) and Vꢀ2PF6
(205 mg, 0.372 mM) were mixed in Me2CO (50 mL). A
catalytic amount of tetrakis(acetonitrile)copper(I) hexafluoro-
phosphate and TBTA were added at room temperature. The
mixture was left to stand overnight. After the removal of the
solvent by evaporation, the residue was purified by column
chromatography (SiO2, Me2CO plus 2% ammonium
samples were 1 mM and the scan rate was set to 200 mV sꢁ1
.
ꢂc
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260 | New J. Chem., 2009, 33, 254–263

hexafluorophosphate). Concentration in vacuo afforded
2ꢀ2PF6 (74%). The thread 2ꢀ2Cl was obtained in 88% yield
as described in the results and discussion section. 1H NMR
(500 MHz, D2O) d 3.34 (t, 2H, J = 6 Hz), 4.46 (m, 3H),
3.5–3.6 (m, 16H), 3.65 (br m, 4H), 3.75 (t, 3H, J = 4 Hz), 3.79
(br s, 3H), 3.82 (br s, 2H), 4.00 (br s, 2H), 4.07 (br s, 2H),
4.24 (t, 2H, J = 4 Hz), 4.73 (t, 2H, J = 6 Hz), 6.65 (d, 1H,
J = 8 Hz), 6.69 (d, 1H, J = 8 Hz), 7.05 (t, 1H, J = 8 Hz), 7.13
(t, 1H, J = 8 Hz), 7.35 (d, 1H, J = 9 Hz), 7.45 (d, 1H,
J = 9 Hz), 7.80 (s, 1H), 7.91 (d, 2H, J = 6.5 Hz), 8.55 (d, 2H,
J = 6.5 Hz); 13C NMR (100 MHz, D2O) d 26.6, 49.9, 60.3,
61.1, 67.7, 68.7, 69.2, 69.3, 69.5, 69.6, 69.7, 70.0, 70.2, 71.7,
106.4, 106.6, 114.2, 114.3, 124.9, 125.8, 125.8, 125.9, 125.9,
141.8, 144.9, 148.0, 153.6, 153.7. HRMS (HR-ESI): m/z calc.
for C70H96N8O18: 668.4; found: 668.5.
into the mass spectrometer source with a syringe pump
(Cole–Parmer Instrument Company, Vernon Hills, IL) at a
flow rate of 3.33 mL minꢁ1. For electrospray ionization, the
drying gas was heated at 300 1C (14+) or 350 1C (22+). Its flow
was set at 5 L minꢁ1, with 43.5 psi nebulizer pressure. The
capillary and skimmer voltage were set at 4000 and 40 V,
respectively. The capillary exit was adjusted at 125 V (14+),
230 V (22+), 190 V (14+/CB[8]) or 270 V (22+/CB[8]).
Scanning was performed from m/z = 200 to 2000.
Spectrophotometric titrations. The spectrophotometric titra-
tions of V2+ (2.15 ꢆ 10ꢁ5 M) and of 22+ (1.32 ꢆ 10ꢁ5 M) with
cucurbit[8]uril (CB[8]) were carried out in a Hellma quartz
optical cell (l = 1 cm). It is noteworthy that CB[8] has a low
solubility in H2O at pH 7.0 (B2 ꢆ 10ꢁ4 M). Microvolumes
of a concentrated solution of CB[8] (1.9 ꢆ 10ꢁ4 M) were added
to 2 mL of V2+ or 22+with the help of a microburette
Physico-chemical measurements. 2ꢀ2Cl (C70H96Cl2N8O18,
MW = 1408.46 g molꢁ1), 1ꢀ4Cl (C62H74Cl4N10O8, M. W. =
1229.13 g molꢁ1), the bisalkyneviologen Vꢀ2Cl (C16H14Cl2N2,
MW = 305.22 g molꢁ1) and 1,5-bis[2-(2-hydroxyethoxy)-
(Eppendorf). The [CB[8]]tot/[V2+
]
and [CB[8]]tot/[22+
]
tot tot
ratios were varied from 0 to 3.6 and from 0 to 6.7, respectively.
Special care was taken to ensure that complete equilibration
was attained. After each addition, a UV-Vis spectrum was
recorded from 230 to 800 nm on a Cary 300 (Varian) spectro-
photometer maintained at 25.0(2) 1C by the flow of a Haake
NB 22 thermostat. In order to determine the association
constants of the 1 : 1 : 1 ternary complexes formed with the
electron-donor/electron-acceptor pair V2+ꢀN-1CCB[8] or
V2+ꢀN-2CCB[8], aliquots (2 mL) of an equimolar mixture
of N-1 (or N-2) and V2+ were titrated by adding microvolumes
of a concentrated solution of CB[8]. Special care was taken to
ensure that complete equilibration was attained and UV-Vis
spectra were recorded. For 14+, a batch titration was required
to ensure that all the sample did reach the equilibrium and
thus favored the formation of the 14+C(CB[8])2 complex. The
concentration of 14+ was fixed at 9.36 ꢆ 10ꢁ6 M and the ratio
ethoxy]naphthalene (N-1) (C18H24O6, MW = 336.38 g molꢁ1
)
were prepared and purified as described above. Naphthalene-
2,6-diol (N-2) (C10H8O2, MW = 160.17 g molꢁ1) (Alfa Aesar)
and cucurbit[8]uril (CB[8]) (C48H48N32O16, MW = 1329 g molꢁ1
)
(Aldrich) are commercial products, which were used without
further purification. All solutions were prepared in distilled
H2O which was further purified by passing it through a mixed
bed of ion-exchanger (Bioblock Scientific R3-83002, M3-83006)
and activated carbon (Bioblock Scientific ORC-83005). It was
then boiled and de-oxygenated using CO2 and O2 free argon prior
to use (Sigma Oxiclear cartridge). All stock solutions were
prepared using an AG 245 Mettler Toledo analytical balance
(precision 0.01 mg), and complete dissolution in phosphate buffer
was achieved using an ultrasonic bath. The experiments were
carried out at 25.0(2) 1C maintained with the help of Haake
FJ thermostats. In all the solutions, the pH was maintained at
7.00 ꢃ 0.05 by the use of a 0.1 M phosphate buffer, which
was prepared by mixing 30.5 mL of Na2HPO4ꢀ2H2O (0.2 M)
(Prolabo) with 19.5 ml of NaH2PO4 (0.2 M) (Prolabo) and
diluting to 100 mL. The final pH of the solution was then adjusted
to the required value by using phosphoric acid (85%, Labosi).
The pH was measured with an Ag/AgCl combined glass electrode
(Metrohm 6.0234.500, long life) filled with 0.1 M NaCl (Fluka,
p.a.) in H2O. Standardization of the millivoltmeter and the
verification of the linearity of the electrode response were
performed using a set of commercial Merck buffered solutions
(pH 1.68, 4.00, 6.86, 7.41 and 9.18).
[CB[8]]tot/[14+
]
was varied from 0 to 18.6. UV-Vis spectra
tot
were recorded from 230 to 800 nm on a Cary 300 (Varian)
spectrophotometer maintained at 25.0(2) 1C by the flow
of a Haake NB 22 thermostat. The spectrophotometric data
were processed using the Specfit program27 which adjusts
the stability constants and the corresponding extinction
coefficients of the species formed at equilibrium. Specfit uses
factor analyses to reduce the absorbance matrix and to extract
the eigenvalues prior to the multi-wavelength fit of the reduced
data set according to the Marquardt algorithm.28
Spectrofluorimetric titrations. For the spectrofluorimetric
titrations, solutions were prepared in such concentrations to
get absorbances smaller than 0.1 at wavelengths Z lexc in
order to avoid any errors due to the inner filter effect. More-
over, the excitation wavelengths correspond to isosbestic
points, where both complexed and uncomplexed species
exhibit the same molar absorption coefficient. The excitation
wavelengths were set at 325(1) nm and 327(1) nm for N-1 and
N-2, respectively. An aliquot (2 mL) of an equimolar mixture
of N-1 (or N-2) and V2+ ([N-1] = [V2+] = 1.51 ꢆ 10ꢁ5 M and
[N-2] = [V2+] = 1.73 ꢆ 10ꢁ5 M) was introduced into a 1 cm
Hellma quartz optical cell. Microvolumes of a concentrated
Electrospray ionization mass spectrometric measurements.
ESI Mass spectra of 1ꢀ4Cl, 2ꢀ2Cl and their respective com-
plexes formed with CB[8] were obtained with an ion-trap
instrument (Bruker Esquire 300plus, Bruker Daltonic,
Bremen, Germany), equipped with an Agilent electrospray
(ESI) ion source (Agilent Headquarters, Palo Alto, CA). The
solutions of 1ꢀ4Cl (1.5 ꢆ 10ꢁ5 M) and 2ꢀ2Cl (1.5 ꢆ 10ꢁ5 M)
and their complexes with CB[8] ([14+] = 1.4 ꢆ 10ꢁ5 M;
[CB[8]] = 2.8 ꢆ 10ꢁ5 M); ([22+] = 7.5 ꢆ 10ꢁ6 M;
[CB[8]] = 8.5 ꢆ 10ꢁ6 M), prepared in the mixed solvent
H2O–MeOH (80/20 by weight), were continuously introduced
solution of CB[8] were added, and the [CB[8]]tot/[N/V2+
]
tot
ratios were varied from 0 to 5 for N-1 (from 0 to 2.9 for N-2).
ꢂc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2009
New J. Chem., 2009, 33, 254–263 | 261

The formation of the ternary complexes was evidenced by the
quenching of the fluorescence centered on N-1 or N-2. After
each addition, special care was taken to ensure that complete
equilibration had been attained. The luminescence spectra
were recorded from 300 to 600 nm on a Perkin-Elmer
LS-50B instrument maintained at 25.0(2) 1C by the flow of a
Haake FJ thermostat. The excitation and emission band-
widths were set at 5 nm for the ternary complex with
N-1 and at 6 nm for that with N-2. The source was a pulsed
xenon flash lamp with a pulse width at half peak height
o10 ms and power equivalent to 20 kW. Fluorescence quantum
yields were determined relative to fluorescent standard quinine
sulfate (Fabs = 0.546 in 0.5 M H2SO4) with the possibility of
correcting for differences between the refractive index of the
reference nr, and the sample solutions ns using the expression:
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R
IsðlÞDrn2s
R
ff ðsÞ ¼ ff ðrÞ
IsðlÞDsn2r
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The indices s and r denote sample and reference, respectively.
The integrals over I represent areas of the corrected emission
spectra, D is the optical density at the wavelength of excitation.
The spectrofluorimetric data were analysed using the Microcal
Origin program.29
Acknowledgements
This work was supported by the Microelectronics Advanced
Research Corporation (MARCO) and its focus center on
Functional Engineered NanoArchitectonics (FENA) in the
United States, and by the Centre Nationale de la Recherche
Scientifique (UMR 7177 and CNRS-ULP) in France, as well
as the French Ministry of Research and Education (Doctoral
Fellowship to M. H.).
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