Radical-cation dimerization overwhelms inclusion in [N]pseudorotaxanes

Article abstract of DOI:10.1002/chem.201400069

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a well-known phenomenon. Herein, two counter-examples are presented. Two viologen-containing thread molecules were designed, synthesized, and thoroughly ch

Full text of DOI:10.1002/chem.201400069

DOI: 10.1002/chem.201400069
Full Paper
&
Pseudorotaxanes
Radical-Cation Dimerization Overwhelms Inclusion in
[n]Pseudorotaxanes
Katia Nchimi-Nono,[a] Parastoo Dalvand,[b, c] Kuldeep Wadhwa,[a] Selbi Nuryyeva,[a]
Shaikha Alneyadi,[d] Thirumurugan Prakasam,[a] Albert C. Fahrenbach,[e] John-Carl Olsen,[f]
Zouhair Asfari,[c] Carlos Platas-Iglesias,[g] Mourad Elhabiri,*[b] and Ali Trabolsi*[a]
Abstract: Suppression of the dimerization of the viologen
radical cation by cucurbit[7]uril (CB7) in water is a well-
known phenomenon. Herein, two counter-examples are pre-
sented. Two viologen-containing thread molecules were de-
signed, synthesized, and thoroughly characterized by 1H
DOSY NMR spectrometry, UV/Vis absorption spectrophotom-
etry, square-wave voltammetry, and chronocoulometry:
rotaxane complexes with CB7 (MV2+ꢀCB7 and BMV2+ꢀCB7)
were also investigated. As expected, the control pseudoro-
taxanes remained intact after one-electron reduction of their
viologen-recognition stations. In contrast, analogous reduc-
tion of BV4+ꢀ(CB7)2 and HV12+ꢀ(CB7)6 led to host–guest de-
complexation and release of the free threads BV2( +) and
C
1
HV6( +), respectively. H DOSY NMR spectrometric and chro-
C
BV4+, which contains two viologen subunits, and HV12+
,
nocoulometric measurements showed that BV2( +) and
HV6( +) have larger diffusion coefficients than the corre-
C
C
which contains six. In both threads, the viologen subunits
are covalently bonded to a hexavalent phosphazene core.
The corresponding [3]- and [7]pseudorotaxanes that form on
complexation with CB7, that is, BV4+ꢀ(CB7)2 and HV12+
ꢀ(CB7)6, were also analyzed. The properties of two mono-
meric control threads, namely, methyl viologen (MV2+) and
benzyl methyl viologen (BMV2+), as well as their [2]pseudo-
sponding [3]- and [7]pseudorotaxanes, and UV/Vis absorp-
tion studies provided evidence for intramolecular radical-
cation dimerization. These results demonstrate that radical-
cation dimerization, a relatively weak interaction, can be
used as a driving force in novel molecular switches.
[a] Dr. K. Nchimi-Nono,+ Dr. K. Wadhwa, S. Nuryyeva, Dr. T. Prakasam,
Prof. A. Trabolsi
Introduction
Centre for Science and Engineering
New York University Abu Dhabi (NYUAD), Abu Dhabi (UAE)
E-mail: ali.trabolsi@nyu.edu
[b] P. Dalvand,+ Dr. M. Elhabiri
Because of its many anticipated applications, the ability to
induce fast and reversible movements of large amplitude
within molecular bodies has become a major scientific goal. To
that end, many nanometric mimics of mechanical tools and de-
vices have been synthesized. These include nanoscale twee-
zers,[1]gears,[2] rotors,[3–7] shuttles,[8] actuators,[9–11] and eleva-
tors,[12] the movements of which respond to a variety of exter-
nal stimuli including changes in pH,[13–15] redox potential,[1,16]
temperature,[17] and light.[18,19]
Laboratoire de Chimie Bioorganique et Mꢀdicinale
UMR 7509 CNRS, Universitꢀ de Strasbourg
ECPM, 25 rue Becquerel, 67200 Strasbourg (France)
E-mail: elhabiri@unistra.fr
[c] P. Dalvand,+ Dr. Z. Asfari
Laboratoire d’Ingꢀnierie Molꢀculaire Appliquꢀe ꢁ l’Analyse
IPHC, UMR 7178 CNRS, Universitꢀ de Strasbourg
ECPM, 25 rue Becquerel, 67200 Strasbourg (France)
Here we describe two novel nanomechanical systems:
a [3]pseudorotaxane and a [7]pseudorotaxane, each composed
of a multimeric viologen-based thread molecule and the mac-
rocycle cucurbit[7]uril (CB7). In aqueous solution, these sys-
tems can be electrochemically switched between complexed
states, defined by the pseudorotaxanes themselves, and un-
complexed states comprised of their separate components.
The driving force for disassembly (i.e., “dethreading”) is intra-
molecular dimerization of the viologen radical cation. An anal-
ogy can be made between these artificial systems and voltage-
gated membrane channels, the protein components of which
undergo large conformational changes under the influence of
an action potential.[20]
[d] S. Alneyadi
Department of Chemistry, Faculty of Science
UAEU University, Al Ain (UAE)
[e] Dr. A. C. Fahrenbach
Department of Chemistry, Northwestern University
2145 Sheridan Road, Evanston, Illinois 60208-3113 (USA)
[f] Prof. J.-C. Olsen
School of Sciences
Indiana University Kokomo, Kokomo, IN 46904 (USA)
[g] C. Platas-Iglesias+
Departamanto de Quimꢂca Fundamental
Universidade da CoruÇa, Campus da Zapateira–Rffla da Fraga 10
15008 A CoruÇa (Spain)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400069. It contains full descriptions of
the syntheses of threads HV12+, BV4+, and BMV2+
Cucurbit[n]urils (CBn) are a class of macrocycles[21] that are
synthesized by the acid-catalyzed condensation of formalde-
.
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+
C
hyde with 5, 6, 7, 8, or 10 glycoluril molecules. They have
a pumpkinlike shape and two identical carbonyl-rimmed por-
tals, as well as a hydrophobic cavity that can accommodate
a variety of guest molecules. The host–guest chemistry of CBn
has attracted tremendous interest[21,22] and has many potential
applications in nanotechnology and materials science.[23–24] CBn
hosts have been used for the synthesis of a range of self-as-
sembled compounds including precursors to mechanically in-
terlocked molecules and molecular switches.[21,22] CB7, in par-
ticular, has drawn much attention because of its high solubility
in water.[21] CB7 has a hydrophobic internal cavity that meas-
ures 7.3 ꢁ in diameter and 9.1 ꢁ in height[25] and can accom-
modate guest substrates[21,26–28] such as imidazoles,[22,29,30] stil-
benes,[31–32] viologens,[32–38] acridiziniums,[39] pyridine/pyridini-
ums,[40–41] adamantanes,[28] naphthalenes,[42] and ferrocenes,[43–44]
to name but a few.
and 0, respectively.[56] Furthermore, BIPY radical cations spon-
taneously dimerize[53,57] to give (BIPY +)2 in aqueous solution[58]
C
in a process driven by radical–radical interactions that favor
formation of the singlet state. This phenomenon, which is also
known as p dimerization or pimerization, has been used to
generate translation and rotation in mechanically interlocked
molecules.[59]
+
C
The CB7 macrocycle is well known to suppress MV dimeri-
zation by sequestering MV monomers. CB7 forms 1:1 inclu-
+
C
sion complexes in aqueous solution with two different oxida-
tion states of methyl viologen: MV2+ꢀCB7 and MV
+
C
C
ꢀCB7.[22,26] Typical binding constants K reported for the MV
+
ꢀCB7 complex are in the range of 104–106 mꢁ1 and are usually
+
C
larger than those associated with the dimerization of MV
(Kꢂ102–105 mꢁ1, Table 1). This comparison indicates that inclu-
sion of the radical cations is thermodynamically favorable
enough to prevent dimerization.[20] In contrast, we have found
two special cases in which dimerization outcompetes inclusion
and forces host–guest dissociation. We synthesized two multi-
meric pseudorotaxane threads: a bis-adduct and a hexakis-
adduct of viologen, BV4+ and HV12+, respectively (Figure 1). In
both ligands, the viologen subunits are covalently attached to
a cyclophosphazene core. Pseudorotaxanes composed of these
threads and CB7 dissociate on reduction as the result of intra-
molecular dimerization of viologen radical cations. Decomplex-
ation and dimerization do not occur in experiments involving
CB7 and either of the two monomeric guest viologens MV2+
Substituted viologens V2+ (Table 1) are a class of com-
pounds that contain the dicationic 1,1’-dialkyl-4,4’-bipyridinium
(BIPY2+) core, formed by diquaternization of 4,4’-bipyridine.[44]
The prototype viologen, 1,1’-dimethyl-4,4’-bipyridinium (MV2+),
is known as methyl viologen or paraquat, while the other
simple (un)symmetrical bipyridinium analogues are referred as
substituted viologens. These compounds have rich redox
chemistry[46] and exhibit many physicochemical properties that
make them attractive for use in electrochromic,[47] catalytic,[48]
and bioanalytical systems.[49] For example, BIPY2+ forms stable
+
monoradical cation BIPY after one-electron reduction.[50–55] In
C
contrast to the virtually colorless BIPY2+ dications, viologen
and BMV2+
.
+
C
radical monocations BIPY are intensely blue. Their high
Herein, we present a detailed physico-electrochemical study
of four viologen/CB7 pseudorotaxanes. We investigated the
formation of the multimeric pseudorotaxanes BV4+ꢀ(CB7)2
molar absorption coefficient results from charge transfer be-
tween the nitrogen atoms, which carry formal charges of +1
Table 1. Thermodynamic and spectroscopic parameters of viologen radical cations in monomeric and dimeric states.
Radical cation
Anion
Clꢁ
lgKDim
l
m [nm] (em [104 mꢁ1 cmꢁ1])
ld [nm] (ed [104 mꢁ1 cmꢁ1
]
Refs.
+
+
C
C
C1C1V (MV
)
2.70[c]
396 (4.2)/
602 (1.37)
362 (5.0)
518 (2.1)
[3,62]
[57]
n.d.
n.d.
CH3SO4
Clꢁ
2.58[e]
2.86[f]
5.10[a]
2.75[c]
2.89
–
–
675 (0.38)/604 (1.69)/396 (3.49)/600
598 (1.37)/606 (1.37)
–
–
537 (3.24)/369 (8.97)
480/565/670
–
–
[64]
[61]
[62,63]
[62]
[63]
[62]
[63]
[63]
[63]
[61]
[60]
[63]
[61]
[61]
[61]
ꢁ
+
C
C
C1C6V
C1C7V
+
+
+
Clꢁ
Clꢁ
2.81
2.93
–
–
C
C1C8V
Clꢁ
396 (4.2)/602 (1.37)
–
–
362 (5.0)/518 (2.10)
–
–
Clꢁ
2.89[c]
4.04[c]
4.86[c]
5.03[a]
4.68[a]
2.74[c]
4.33[a]
3.85[a]
3.85[a]
4.00[d]
3.29[a]
5.33[d]
4.0
C
C1C9V
C1C10
Clꢁ
Clꢁ
–
–
+
C
V
+
+
+
+
+
+
C
C
C
C
C
C
C2C2V
C3C3V
C4C4V
C5C5V
C6C6V
C7C7V
Brꢁ
Brꢁ
Clꢁ
597 (1.22)
597 (1.06)
–
564 (0.890)
550 (0.887)
–
563 (0.869)
558 (0.870)
515 (0.856)
600 (0.4)
504 (0.855)
600 (0.4)
560 (1.7), 604 (2.44)
540 (2.03), 900 (0.7)
Brꢁ
Brꢁ
Brꢁ
Brꢁ
Brꢁ
Brꢁ
Clꢁ
602 (1.04)
600 (1.025)
571 (1.016)
600 (1.38)
548 (0.892)
600 (1.38)
560 (1.72), 604 (1.22)
600 (1.23)
+
C
C8C8V
[61]
[65]
[66]
this work
+
+
C
C
C
BzBzV (BBV
)
)
+
+
Clꢁ/Iꢁ
3.46[b]
C
C1BzV (BMV
[a] Water, I=0. [b] Water, pH 7.0 (0.1m sodium phosphate buffer). [c] 0.1m KCl. [d] pH 8.1 (0.1m tris/H2SO4 buffer). [e] 1m salt concentration; [f] pH 10.0
(0.1m glycine buffer). n.d.: not defined. KDim =[Dim]/[Mon]2
Chem. Eur. J. 2014, 20, 7334 – 7344
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a
fresh O2-free aqueous solution of sodium dithionite
(Na2S2O4(aq)).
+
C
UV/Vis/NIR absorption spectra of samples with BMV con-
centrations in the range of 0.01 to 1 mm were recorded (Sup-
porting Information). The total absorbance at each wavelength
was correlated to the absorptivities of both the monomer and
the dimer, the concentrations of which are related by the equi-
[67,68]
librium constant for dimerization KDim
.
From the absorb-
+
C
ance data and concentrations, we determined that BMV self-
associates in solution with a lgKDim value of 3.46(5), which is in
the same range as lgKDim values determined for the methyl vi-
+
ologen radical cation MV (ꢂ2.5–2.9[63,64]) and the bis-benzyl
C
viologen radical cation BBV (4.0[65]). The fact that these
+
C
+
+
+
C
C
C
values increase in the order MV <BMV <BBV indicates
that the benzyl substituents reinforce the strength of binding
+
C
between the two BIPY units, likely by means of hydrophobic
interactions. Figure 2 shows the electronic spectra and distribu-
+
C
tion diagram of the BMV monomer and dimer. The spectrum
Figure 1. Structural formulas and 3D representations of phosphazene deriva-
tives bearing six 4,4’-bipyridinium subunits (HV12+) and two 4,4’-bipyridini-
um subunits and two [1,1’-biphenyl]-2,2’-diol protective groups (BV4+).
+
C
of BMV is characterized by an intense and structured absorp-
tion in the visible region (lmax ꢂ600 nm,
e
600 =1.23ꢂ
Methyl viologen (MV2+) and 1-benzyl-1’-methyl-4,4’-bipyridinium (BMV2+
)
104 mꢁ1 cmꢁ1), in agreement with the spectroscopic parameters
were used as reference compounds.
+
+
C
C
determined for MV and BBV (Table 1). Formation of the
dimer (BMV )2 induces a significant hypsochromic shift of the
+
C
and HV12+ꢀ(CB7)6 by 1H NMR and UV/Vis spectroscopy and
measured their switching properties using electrochemistry
and UV/Vis absorption spectroscopy. For comparison, we also
fully characterized two monomeric control threads, that is,
methyl viologen (MV2+) and 1-benzyl-1’-methyl-4,4’-bipyridini-
um (BMV2+), as well as their corresponding [2]pseudorotax-
anes MV2+ꢀCB7 and BMV2+ꢀCB7, the switching properties of
which we also measured.
absorption around 600 nm and also gives rise to an intense ab-
sorption in the near-IR region (lmax ꢂ900 nm,
e
900 =7.0ꢂ
103 mꢁ1 cmꢁ1). This new signal is the result of an allowed radi-
cal–radical transition that occurs in the dimer[57] and serves as
a valuable and discrete indicator of the presence of the dimer
in solution.
Results and Discussion
Dimerization of viologen threads
+
In aqueous solution, monomers V and dimers (V +)2 of violo-
gen radical cations coexist in equilibrium. Table 1 lists thermo-
dynamic and spectroscopic data for the dimerization of a varie-
ty of viologens. Differences between the corresponding values
reflect different experimental conditions. Ionic strength, pH,
and polarity of the solvent, as well as temperature and nature
of the counteranion all influence the position of the equilibri-
um. Factors such as high solvent polarity that diminish electro-
static repulsion between the cationic monomers tend to favor
dimerization,[45,61] as do attractive hydrophobic interactions be-
tween alkyl substituents borne by the electroactive
chromophores.
C
C
+
C
Intermolecular dimerization of BMV
Structurally, 1-benzyl-1’-methyl-4,4’-bipyridinium (BMV2+
analogous to one free arm of the multimeric threads. Dimeriza-
) is
+
C
Figure 2. a) Electronic UV/Vis/NIR absorption spectra of BMV (blue), its
+
C
tion of its reduced form BMV was examined by absorption
dimer (BMV +)2 (magenta), and HV6( (red). b) Distribution diagrams of
BMV and (BMV +)2 in water. Solvent: H2O buffered at pH 7.0 with 0.1m
+)
C
C
+
C
+
spectrophotometry. The BMV radical cation was generated in
C
C
freshly degassed water at pH 7.0 by adding a large excess of
Na2HPO4/NaH2PO4. T=25.0(1)8C.
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Intramolecular pimerization of BV2( +) and HV6( +)
C
C
We recently reported the intramolecular pimerization process
that occurs on electrochemical reduction of HV12+ [47]
A six-
.
electron reduction forms HV6( +), a hexaradical hexacation,
which immediately pimerizes intramolecularly to form three
pairs of viologen radical-cation dimers. We refer to the fully pi-
C
merized species as HVD6( +). We have determined overall equi-
C
6( +)
librium constants bDim for the formation of HVD
in several solvents.[47] The high bDim values in all of the solvents
from HV6( +)
C
C
indicate nearly complete dimerization of all six viologen sub-
units. For example, in water bDim is 1.69ꢂ1014. In the present
6( +)
C
study, we generated HVD
in water with Na2S2O4(aq) and mea-
sured the UV/Vis/NIR absorption spectrum of the pimerized
complex. As shown in Figure 2a, two intense absorption
bands, one at 530 (e530 =3.69ꢂ104 mꢁ1 cmꢁ1) and another,
a charge-transfer (CT) absorption band, at 944 nm (e944 =1.61ꢂ
104 mꢁ1 cmꢁ1), characterize the spectrum. For comparison, we
Figure 3. Electronic spectra of MV2+ (a), BMV2+ (b), BV4+ (c), and HV12+ (d)
and their corresponding complexes with CB7. Solvent: H2O buffered at
pH 7.0 with 0.1m Na2HPO4/NaH2PO4. T=25.0(1)8C.
synthesized and analyzed BV2( +), which contains only one pair
C
+
C
of firmly pimerized BIPY subunits. In the spectrum of this
species (see Supporting Information), the CT absorption band
at 933 nm (e933 =2.87ꢂ103 mꢁ1 cmꢁ1) indicates pimerization and
corroborates the same in HV6( +). Moreover, it indicates that the
that is well tailored for binding the BIPY2+ moiety. The report-
ed 1H NMR studies on aqueous MV2+ꢀCB7 also support a bind-
ing model in which the bipyridinium moiety sits within the
cavity of CB7 and the two methyl groups project outward.[69]
DFT calculations are also in agreement. The geometry of the
MV2+ꢀCB7 system optimized at the B3LYP/6-31G(d) level (see
Supporting Information) indicates that the MV2+ unit is posi-
tioned within the CB7 cavity with the methyl groups pointing
outward. Four CH···O hydrogen-bonding interactions with C···O
distances of 3.16–3.17 ꢁ, CH···O distances of 2.20–2.33 ꢁ, and
CH···O angles of 133–1468 appear to be the main cause of
stabilization.
C
dimerization can occur between viologen units that are at-
tached to the same phosphorus atom of the phosphazene
core. Discrepancies are present between the electronic spec-
6( +)
trum of HVD
(e944 =1.62ꢂ104 mꢁ1 cmꢁ1, see Supporting Infor-
C
6(·+)
mation) and that deduced from the BMV dimer (eHV
, see Supporting Information) and suggest that
ground-state interactions[58] occur between (BIPY )2 dimers
=
+
C
·+
)
3e(BMV
2
+
C
within the dendrimeric structure, but not between dimerized
+
C
monomers of BMV . A comparison of the electronic spectrum
2(·+)
6( +)
C
C
of HVD
and that deduced from BV2( +) (=3eBV , see Sup-
porting Information) reveals similar discrepancies.
Thread/CB7 [n]pseudorotaxanes
Recognition of MV2+ by CB7
Recognition of BMV2+ by CB7
To elucidate the interactions between CB7 and the BIPY2+
units of the hexaviologen dendrimer HV12+, we first analyzed
the asymmetrical benzyl methyl viologen BMV2+ as a model
compound. Viologens MV2+ and BMV2+ readily form stable 1:1
inclusion complexes with CB7 in water, as evidenced by
1H NMR spectroscopy (Figure 4 and Job plot in the Supporting
Information). In D2O the singlet that corresponds to the methyl
group of BMV2+ remains unchanged on addition of CB7,
whereas the singlet that corresponds to the aromatic protons
(Hc) of the benzyl ring is split into one doublet and two trip-
lets. This differentiation is due to CB7’s restricting the rotation
of the benzyl ring. The multiplets are shifted upfield because
of the increased shielding the protons experience inside the
macrocycle. The singlet associated with the methylene bridge
(Hb) is shifted upfield by more than 0.7 ppm, and the signals of
the a’ aromatic protons of the viologen are shifted upfield by
about 0.6 ppm. Those of the a aromatic protons are nearly un-
changed. The signals of both b and b’ aromatic protons experi-
ence a small downfield shift. Together, these results are consis-
tent with complexation of the benzyl ring by CB7 through hy-
The complexation of MV2+ with CB7 has been extensively
studied, in particular by Kim et al.[21,25] and Kaifer et al.,[33,37,41]
who reported that MV2+ forms a thermodynamically stable
(lgKMV2+ꢀCB7 =5.01 at 0.2m NaCl[33] and lgKMV2+ꢀCB7 =5.3 at 0.1m
phosphate buffer, pH 7.0[21]) and relatively kinetically inert 1:1
inclusion species with CB7 (kdiss =1.9ꢂ102 sꢁ1 [26] in aqueous so-
)
lution. We performed further absorption binding titrations of
MV2+ 2Iꢁ with CB7 in water at pH 7.0 to confirm this 1:1 stoi-
chiometry (for a Job plot determined from absorption spectro-
photometric data, see the Supporting Information) and repro-
duced the previously determined binding constants (lgKMV2+
ꢀCB7 =5.30(2) at 0.1m phosphate buffer, pH 7.0) as a benchmark
for our own studies. The formation of MV2+ꢀCB7 is character-
ized by a significant hypochromic shift of the signal that corre-
sponds to the p–p* transition centered on the BIPY2+ core
(Figure 3a). This shift allows detection of the complex in solu-
tion. Taken together, the spectroscopic data point to an inclu-
sion complex strongly stabilized by hydrophobic and charge–
dipole interactions and suggest that the CB7 host has a cavity
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Figure 5. Calculated B3LYP/6-311G** electrostatic potential [Hartree] of
+
BMV2+ (a) and BMV (b) on the molecular surface defined by the
C
0.001 ebohrꢁ3 contour of the electronic density and a view of the SOMO of
+
C
BMV (c) calculated in aqueous solution at the same computational level.
d) Capped-stick representation of the minimum-energy geometry of
HV12+ꢀ(CB7)6 obtained with PM6 calculations.
tween BMV2+ and CB7 in the gas-phase involves the bipyridi-
nium unit rather than the benzyl moiety. When solvent effects
were accounted for with the aid of the PCM model, the free-
energy difference between the two binding modes decreased
from 18.0 to 7.3 kcalmolꢁ1. Because the NMR spectral data pro-
vide strong evidence of binding to the benzyl unit, it is clear
that, at least in aqueous solution, our DFT calculations overesti-
mate the stability of the complex in which the viologen unit
sits inside the cavity of the macrocycle. This discrepancy is
likely due to the frequent inadequacy of continuum dielectric
solvent models for investigating ionic solutes that have con-
centrated charge densities with strong local solute–solvent in-
teractions.[74–76] The calculated 10.7 kcalmolꢁ1 stabilization of
the complex that has the benzyl group inside CB7 can be ex-
plained in terms of its more efficient solvation, as this binding
mode leaves the bipyridinium unit (with its positive electro-
static potential) exposed to the solvent.
1
Figure 4. Partial H NMR spectra of MV2+ (a), BMV2+ (b) HV12+ (c), and BV4+
(d) in the absence and in the presence of increasing ratios (numbers in
bold) of CB7. For MV2+, BMV2+, and HV12+, measurements were carried out
in D2O at room temperature. For solubility reasons, the titration of BV4+ was
performed in [D6]DMSO at room temperature. For full assignments, see Sup-
porting Information.
drophobic and dipole–charge attractions, and with interaction
(see Supporting Information) between CB7 and only part of
the viologen moiety.[70–71]
The binding of BMV2+ by CB7 was also examined by ab-
sorption spectrophotometry (see Supporting Information).
Unlike recognition of MV2+ by CB7, the recognition of BMV2+
by CB7 has little influence on the BIPY2+ p–p transitions,
which indicates, as do the 1H NMR data, that the CB7 host
does not fully encircle this chromophore but is instead posi-
tioned on the benzyl moiety.
Nevertheless, for the BMV2+ꢀCB7 system, two different
minimum-energy conformations were deduced from DFT calcu-
lations (see Supporting Information): one in which the violo-
gen unit resides inside the CB7 host, and a second isomer in
which the benzyl group sits inside. The free-energy difference
between these two forms, calculated in the gas phase, sug-
gests, in contrast to the previous data, that the conformation
with the viologen unit inside the cavity is more stable by
18.0 kcalmolꢁ1. However, this is not surprising considering the
electrostatic potential on the surface of BMV2+. This property
(as defined by the 0.001 eBohrꢁ3 contour of the electron densi-
ty following the suggestion of Bader et al.[72]) was calculated at
the B3LYP/6-311G** level (Figure 5). Due to the net positive
charge on the molecule, the surface is completely positive,
with the most positive region located on the hydrogen atoms
of the bipyridinium unit, as is observed for related viologens.[73]
Such a distribution suggests that the primary interaction be-
The binding constant of the BMV2+ꢀCB7 complex in water
was determined by absorption spectrophotometry to be
lgKBMV2+ꢀCB7 ꢂ6.9(8). Although this value is only a rough esti-
mate (spectral changes observed during the titration were
weak), it indicates that BMV2+ forms a significantly more
stable [2]pseudorotaxane than MV2+. Moreover, the value is
similar in magnitude to those determined for other cations
(ammonium,[77,78] pyridinium, or 4,4’-bipyridinium[71]) that have
the hydrophobic benzyl substituent attached to a cationic core
+
(e.g., lgK=8.398(4) for (PhCH2)N(CH3)3 versus 5.079(2) for
+
N(CH3)4
and lgK=8.613(2) for (PhCH2)(CH3)2N+(CH2)2OH
versus 5.813(2) for (CH3)3N+(CH2)2OH).
Recognition of BV4+ and HV12+ by CB7
1
The solubility of BV4+ in water is low. Thus, H NMR monitoring
of the interaction of BV4+ with CB7 was carried out in a 3 mm
solution of BV4+ in [D6]DMSO. The spectra (Figure 4c) were re-
corded for additions of up to three equivalents of CB7. As ob-
served with HV12+ (Figure 4d), the signals for the b and b’ aro-
matic protons are the most affected by CB7, with both types
shifted significantly upfield. The a and a’ proton signals are
shifted upfield as well. All other signals, including those corre-
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sponding to the methylene groups, remain nearly unchanged.
These results confirm the formation of a BV4+ꢀ(CB7)2 inclusion
complex in which the bipyridinium chromophores are posi-
tioned inside the hydrophobic cavities of the two CB7 macro-
cycles, and the methyl groups and hindered benzyl moieties
are outside. In this complex, the dioxybiphenyl groups increase
steric hindrance near the phosphazene core and prevent inclu-
sion of the benzyl group.
phenyl protecting groups, which do not act as binding sites
for CB7 but do absorb UV light strongly. Owing to their pres-
ence, the electronic spectrum of BV4+ (e249 =6.34ꢂ
104 mꢁ1 cmꢁ1) was significantly different than those of HV12+
,
MV2+, and BMV2+, and complexation by CB7 led to relatively
modest spectral changes.
The HV12+ꢀ(CB7)6 system was also characterized by means
of theoretical calculations based on the semiempirical PM6
model. The optimized geometry of this system (Figure 5d) ap-
proaches D3 symmetry with each of the bipyridinium units of
HV12+ residing inside a CB7 host. The six CB7 units are quite
far apart from each other; the shortest C···C distance between
carbon atoms of different macrocycles is about 7.9 ꢁ. This
ample spacing suggests that simultaneous binding of six CB7
units to HV12+ is feasible.
The titration results also suggest that conformational re-
arrangements of the dioxybiphenyl protecting groups and the
phosphazene core occur on binding. For example, the signals
between 7.3 and 8 ppm associated with the protecting groups
are clearly shifted upfield, which is consistent with steric
effects.
Absorption titration of BV4+ in 0.1m phosphate buffer at
pH 7.0 (see Supporting Information) allowed us to detect the
Interestingly, only an apparent association constant[81,82]
formation of
a
[2]pseudorotaxane [BV4+ꢀCB7, lgKBV4+
(lgK*HV12+
=4.23(7) at 0.1m phosphate buffer pH 7.0) could
ꢀ(CB7)6
ꢀCB7
ꢂ5.9(2)] and a [3]pseudorotaxane (BV4+ꢀ(CB7)2, lgKBV4+
be deduced from the spectrophotometric titrations (see Sup-
porting Information). This reflects the fact that the BIPY2+ sub-
units in HV12+ behave as independent binding sites. In addi-
tion, the apparent stability constant value is lower than that
ꢀ(CB7)2
ꢂ4.7(4), see Supporting Information for Job plot). The stability
constant of the [2]pseudorotaxane BV4+ꢀCB7 is similar to that
measured for MV2+ꢀCB7 [lgKMV2+ꢀCB7 =5.30(2)], which sug-
gests that CB7 prefers the bipyridinium moiety in BV4+. A
comparison of the stabilities of the [2]pseudorotaxane BV4+
*
expected for a system with statistical interactions (lgKcalcd
values of 4.8 and 5.1 can be estimated assuming statistical in-
ꢀCB7 and the [3]pseudorotaxane BV4+ꢀ(CB7)2 (KBV4+
/
teractions and using lgKMV2+ꢀCB7 =5.30(2) and lgKBV4+
=
ꢀCB7
ꢀ(CB7)2
KBV4+ꢀCB7 =0.03 !0.25) indicates that the binding of the first
CB7 sterically inhibits binding of the second;[79,80] in other
words, the system exhibits negative cooperativity as a conse-
quence of the bulky protecting groups, which decrease the
flexibility of the phosphazene core and limit its ability to re-
arrange to accommodate the second CB7 macrocycle.
5.9(2), respectively), which also suggests negative steric inter-
actions[79,80] due to the bulkiness of the CB7 macrocycles and
HV12+. Thus, the apparent stability constant of HV12+ꢀ(CB7)6
K*HV12+
is more than an order of magnitude lower than
ꢀ(CB7)6
those calculated (see Supporting Information) for MV2+ꢀCB7
[lgKMV2+ꢀCB7 =5.30(2)] and BV4+ꢀCB7 [lgKMV2+ꢀCB7 =5.9(2)].
Lastly, significant hypochromic shifts of the p–p* transitions of
the BIPY2+ chromophores of HV12+ were observed (Figure 3d
and Supporting Information) on complexation with CB7 mac-
rocycles, which substantiates the 1H NMR data and the pro-
posed BIPY2+/CB7 binding mode.
Evidence for the formation of the [7]pseudorotaxane HV12+
ꢀ(CB7)6 was observed in 1H NMR titration experiments in
which 1–12 equivalents of CB7 were successively added to
a 1 mm solution of HV12+. As illustrated in Figure 4c, the most
dramatic change in the spectra of HV12+ is observed for the
b and b’ aromatic protons, which exhibit a large upfield shift
of more than 1.5 ppm. This large shift is consistent with that
observed for the b protons of MV2+. The smaller shifts of other
corresponding signals in HV12+ and MV2+, such as those of
the methyl groups, are also similar. These data indicate that
the mode of interaction between CB7 and BIPY2+ in HV12+ is
very similar to that observed in MV2+ꢀCB7, namely, that in
which the bipyridinium chromophore resides within the cavity
of CB7. Although benzyl spacers in HV12+ have been used to
anchor the BIPY2+ groups on the phosphazene core, strong
steric interactions near the core likely prevent the type of
benzyl-group inclusion seen in BMV2+ꢀCB7.
Reduction of the [n]pseudorotaxanes
+
+
C
C
Monocationic monoradicals MV and BMV
Cyclic voltammetric (CV, see Supporting Information) and
square-wave voltammetric (SWV, Figure 6) experiments in
phosphate-buffered solutions at pH 7 were performed on the
reference compounds MV2+ and BMV2+ in the absence and
presence of CB7. MV2+ (Figure 6a) is characterized by two suc-
cessive one-electron reversible redox waves: E11=2(MV /MV )=
2+
+
C
ꢁ0.66 V and E12=2(MV /MV )=ꢁ0.96 V). Addition of CB7 mark-
edly alters its electrochemical properties. In the presence of
three equivalents of CB7, the first redox wave shifts slightly to
a more negative potential (DE11=2 =30 mV) and retains its rever-
sible shape, while the second reduction wave shifts to a much
more negative potential (DE12=2 =60 mV). These shifts reflect
the relative affinities of CB7 for the different redox states of
+
0
C
The existence in solution of the [7]pseudorotaxane HV12+
ꢀ(CB7)6 was further confirmed by a Job plot (see Supporting
Information), which supports the HV12+:CB7 binding stoichi-
ometry of 1:6. The electronic spectrum of HV12+ (e259 =12.29ꢂ
104 mꢁ1 cmꢁ1) was found to be similar to the sum of six spectra
12+
of MV2+ (e256 =2.06ꢂ104 mꢁ1 cmꢁ1 ꢂ(1/6)ꢂeHV ) or six spectra
12+
of BMV2+ (e259 =2.29ꢂ104 mꢁ1 cmꢁ1 ꢂ(1/6)ꢂeHV ). For both
methyl viologen (lgKMV2+ꢀCB7 =5.3, lgKMV·+ꢀCB7 =4.79 and log
HV12+ and MV2+, recognition by CB7 resulted in a significant
decrease in the intensity of the electronic transition at l=
259 nm. On the other hand, BV4+ contains two 2,2’-dioxybi-
K
MV0ꢀCB7 =3.77)[26,83] and indicate that MV remains firmly fixed
+
C
inside CB7, whereas the neutral MV0 is more loosely bound.
The relatively high affinity of CB7 for MV is consistent with
+
C
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binding affinity for BMV2+ than for MV2+ but a similar affinity
+
for both radical cationic forms BMV and MV +. This is consis-
tent with CB7 having two redox-state-dependent binding
modes for the BMV species, and one for the MV species. Thus,
C
C
we infer that on reduction of BMV2+ to BMV +, translocation
C
of CB7 from the benzyl unit to the bipyridinium moiety
occurs. Furthermore, we have estimated the stability constants
+
of BMV ꢀCB7 and BMV0ꢀCB7 (lgKBMV·+ꢀCB7 ꢂ4.53 and
C
lgKBMV0ꢀCB7 ꢂ3.86) using the measured potential shifts and the
estimated binding constant for BMV2+ [lgKBMV2+
=
ꢀCB7
6.9(8)].[33,83] The order of stability of the three CB7/BMV com-
+
plexes is BMV2+ꢀCB7>BMV ꢀCB7>BMV0ꢀCB7, which con-
C
firms that CB7 preferentially binds cationic species over neutral
ones.[83]
We also undertook a UV/Vis/NIR absorption titration of
+
+
C
C
BMV with CB7 in water at pH 7.0 (see SI). BMV was gener-
ated with an excess of freshly prepared Na2S2O4(aq) under O2-
free argon. Statistical processing of the corresponding spectral
data allowed us to evaluate the KBMV·+
value [lgKBMV·+
=
ꢀCB7
ꢀCB7
4.9(1)] which is in excellent agreement with that determined
by the electrochemical approach (lgKBMV·+ꢀCB7 ꢂ4.6). As is usu-
+
C
ally observed for viologen/CB7 complexes, BMV ꢀCB7 is
characterized by significant hypochromic shifts of the absorp-
tion signals associated with the radical cation, and such shifts
provide additional evidence for inclusion of the radical cation
within CB7 (Figure 7a). Also the disappearance of the CT ab-
sorption band at 900 nm and the vanishing of the p–p* transi-
+
+
C
C
tions of BMV are clear signatures of BMV ꢀCB7 formation
(Figure 7a). The same inferences apply to the MV +/CB7
C
Figure 6. SWVs of 0.06 mm MV2+ (a), 0.08 mm BMV2+ (b), 0.024 mm HV12+
(c), and 0.065 mm BV4+ (d) in the absence and the presence of, respectively,
3, 3, 18 and 4 equiv of CB7. All voltammograms were recorded in argon-
purged phosphate buffer solutions at pH 7 and 298 K (E vs. Ag/AgCl).
+
C
suppression of MV dimerization by the macrocycle in aque-
ous solutions (Table 1 and Supporting Information).
As anticipated, BMV2+ is also characterized by two reversible
one-electron redox processes (Figure 6b): E11=2 (BMV2+
/
BMV +)=ꢁ0.55 and E1=2 (BMV +/BMV0)=ꢁ0.87 V, and the ad-
dition of three equivalents of CB7 to BMV2+ also induced
major changes in the SWV of the viologen, including a marked
2
C
C
Figure 7. a) Absorption spectra of BMV2+ (black) and its inclusion complex
shift of the first peak to a more negative potential: DE11=2
=
+
C
with CB7 (magenta) as well as that of BMV (red, mixture of monomer and
140 mV for BMV2+ as compared to only 30 mV for MV2+. The
second reduction wave for BMV2+ is also shifted to a more
negative potential, but, in this case, by a similar amount as
was observed for MV2+: DE12=2 =40 mV for BMV2+ versus
60 mV for MV2+. These results confirm that CB7 has a greater
dimer). Addition of CB7 (blue) suppresses the dimerization of BMV +. The
C
+
C
formation of MV ꢀCB7 is characterized by a hypochromic shift of the ab-
sorption at about 550 nm. b) Distribution diagram of the inclusion complex
+
+
C
C
of BMV with CB7 and of the BMV dimer showing that the inclusion spe-
cies BMV ꢀCB7, which competes with pimerization, is the thermodynami-
+
C
cally favored species.
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system (see Supporting Information). However, in this case, the
electrochemical study on HV12+ in different media (H2O,
CH3CN, DMF, Me2CO) and demonstrated that this intramolecu-
lar dimerization is associated with large bDim values (where bDim
represents the overall equilibrium constant for dimerization of
smaller dimerization constant (lgKDim =2.70[64]) necessitates
+
C
much higher concentrations of MV for self-association to be
easily measured (see Supporting Information).
In the absence of CB7 and on one-electron reduction of
all three pairs of viologen radical cations in HVD6( +)). Absorp-
C
+
BMV2+ with Na2S2O4, BMV was found to self-associate with
tion spectra of HV6( +) in the presence and absence of CB7 pro-
C
C
a dimerization constant of lgKDim =3.46. As expected, the pres-
vided additional evidence for radical-induced dissociation of
ence of CB7 suppressed dimerization (Figure 7b) by forming
CB7 (Figure 8). The addition of Na2S2O4 to a solution of the
+
C
1:1 inclusion complex BMV ꢀCB7, which has a higher stability
constant [lgKBMV·+ꢀCB7 =4.9(1)] than the dimer [lgK(BMV·+
=
)
2
3.4(6)].
Hexacationic hexaradical HV6( +)
C
The cyclic voltammogram recorded for the hexaviologen den-
drimer, HV12+, (see Figure S8c in Supporting Information) clear-
ly shows two distinct and reversible redox waves. The relative
amplitudes and shapes of the peaks for each wave at the
anode (oxidation) are similar to those observed at the cathode
(reduction). Such a pattern is a strong indication of the redox
reversibility of the system. Quantitatively, the difference be-
tween the potential at the peak of the reduction wave and the
potential of the corresponding oxidation were calculated and
were found to be 40 and 60 mV respectively for the first and
second reduction processes, which represent an excellent
proof for reversibility of the two redox processes.
The SWVs of HV12+ (Figure 6c) show two reversible six-elec-
1
2
tron processes (E 1/2 (HV12+/HV6( +))=ꢁ0.46 V and E1=2 (HV6( +)
C
C
/
HV0)=ꢁ0.82 V). Chronocoulometric experiments[47] confirmed
that the first reversible redox process HV12+/HV6( +) involves six
C
electrons, while those associated with the monomeric violo-
gens (MV2+/MV and BMV2+/BMV +) involve only one. The
voltammogram of HV12+ has a shape that is similar to that of
BV4+, which has two electroactive centers, as well as to that of
BMV2+, which has one. This observation is consistent with the
inference that each pair of geminal viologen subunits is inde-
pendent of the others and that, overall, the six viologen sub-
units in HV12+ are identical, non-interacting electroactive cen-
ters.[84] This result is consistent with the spectroscopic and
thermodynamic analyses of the HV12+/CB7 pseudorotaxanes.
On the other hand, the difference in the magnitude of the
peak-to-peak separations in the voltammograms of BMV2+
+
C
C
Figure 8. a) Absorption spectra of HV12+ and HV12+ꢀ(CB7)6. Addition of
Na2S2O4 to HV12+ꢀ(CB7)6 induces dissociation of CB7, as evidenced by the
6(
spectroscopic signatures of the fully dimerized HVD +). b) Absorption spec-
C
6(
tra of HV12+ and the reduced species HVD +). Addition of 6 or 12 equiv of
C
CB7 has negligible influence on the absorption spectrum of the hexaradical,
which confirms that intramolecular dimerization is thermodynamically fa-
vored with respect to inclusion of the viologen radicals by CB7.
[7]pseudorotaxane HV12+ꢀ(CB7)6 resulted in an absorption
spectrum identical to that obtained for HV6( +) in the absence
C
of CB7. This result also suggests that six-electron reduction of
+
and HV12+ (DE1/2 =320 mV and 360 mV for BMV2+ and HV12+
,
the HV12+ thread within the [7]pseudorotaxane forms BIPY
radicals and, in turn, leads to dimerization of geminal BIPY
C
C
+
respectively) is a strong indication that intramolecular dimeri-
zation occurs in HV6( +). It has been shown previously that the
units and ultimately formation of a tertiary dimer HVD6( +), in
C
C
+
+
C
C
thermodynamic stability of BIPY p dimers induces positive
which three pairs of geminal BIPY are firmly stacked. One
shifts in the first redox potential, whereas the second reduc-
tion process leading to BIPY0 is negatively shifted.[82]
consequence of intramolecular dimerization is the prevention
of measurements to determine the binding constant for HV6( +)
C
Interestingly, for HV12+ꢀ(CB7)6, no shift (DE12=2 =0 mV) was
and CB7.
detected for the second reduction wave, which would normal-
ly be attributed to the transformation HV6( +)ꢀ(CB7)6!
C
Dicationic diradical BV2( +)
C
HV0ꢀ(CB7)6. This result indicates that, after the first six-electron
reduction, the thread behaves like free HV6( +). In other words,
To corroborate the reduction-induced pimerization mechanism
of HV12+ꢀ(CB7)6, SWVs of BV4+ in the absence and in the
presence of CB7 were recorded in phosphate buffer (Fig-
ure 6d). Their shapes can be ascribed to two reversible reduc-
tion processes. Assuming that, in the absence of CB7, the elec-
C
the first reduction of HV12+ꢀ(CB7)6 induces complete dissocia-
tion of CB7 and is followed by immediate intramolecular dime-
+
C
rization of the three pairs of geminal BIPY groups to form
the tertiary dimer HVD6( +). We have previously reported[47] an
7341
C
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Full Paper
trochemical properties of BV4+ are analogous to those of
HV12+, we infer that each reduction process of the former in-
Table 2. Diffusion constants [cm2 sꢁ1] calculated from chronocoulometric
experiments in H2O (1 mm TBACl). Reference electrode: Ag/AgCl.
[HV12+]=0.04 mm, [BV4+]=0.065 mm, and [BMV2+]=0.13 mm, with re-
spectively 12, 3, and 5 equiv of CB7 were added.
volves two electrons. This deduction is confirmed by the
1
values of the reduction potentials of BV4+ (E1=2(BV4+/BV2( +))=
C
ꢁ0.49 V, and E12=2(BV2( +)/BV0)=ꢁ0.85 V) and the peak-to-peak
C
ꢁCB7
+CB7
separation DE1/2 =360 mV, which are almost identical to the
corresponding values determined for HV12+. Furthermore, on
addition of CB7 to BV4+, a shift of 40 mV to a more negative
potential was observed for the first reduction peak, whereas
no shift was detected for the second reduction process, in
agreement with the results obtained with HV12+. Thus, forma-
tion of radical cations on two-electron reduction of BV4+ indu-
ces dissociation of CB7 and subsequent pimerization. The neg-
ative shifts of DE11=2 observed for BV4+ (40 mV) and HV12+
(50 mV) in the presence of CB7 are similar in magnitude to the
shift observed for MV2+ (30 mV) after addition of CB7, which is
an indication that the same binding mode (CB7/bipyridinium)
is present in BV4+ꢀ(CB7)2, HV12+ꢀ(CB7)6, and MV2+ꢀCB7.
HV12+
HV6(
HV12+
HV6(
C
+)
+)
C
2.32(4)ꢂ10ꢁ7
BV4+
5.87(3)ꢂ10ꢁ7
2.06(6)ꢂ10ꢁ7
BV4+
7.00(9)ꢂ10ꢁ7
+)
+)
BV2(
BV2(
C
C
9.80(1)ꢂ10ꢁ7
BMV2+
1.78(1)ꢂ10ꢁ6
8.63(1)ꢂ10ꢁ7
BMV2+
2.90(1)ꢂ10ꢁ6
+
+
C
C
BMV
BMV
5.66(3)ꢂ10ꢁ6
7.86(4)ꢂ10ꢁ6
4.75(5)ꢂ10ꢁ6
4.21(5)ꢂ10ꢁ6
oxidized forms (Table 2). This is consistent with the decrease in
hydrodynamic size that occurs on reduction and subsequent
pimerization. The slight increase in D value observed for reduc-
tion of BMV2+ may arise from increased hydrophobic character
and consequent decreases in solvation and effective hydrody-
namic size.[85]
In the presence of CB7, the diffusion coefficients determined
during the reduction process were much smaller as a conse-
quence of inclusion-complex formation. Moreover, large differ-
ences in the D coefficient were observed for the oxidized
versus reduced forms of BV and HV, with the D value for the
radical-cationic form being significantly larger in each case and
comparable to the value determined in the absence of CB7.
These results provide clear evidence for reduction-induced de-
complexation of CB7 and subsequent dimerization of geminal
1H NMR DOSY and chronocoulometry
Diffusion-ordered spectroscopy (DOSY) and chronocoulometry
(see Supporting Information for experimental details) were
used to examine the constitution of the different pseudorotax-
anes that arise from the interaction of CB7 with the viologen-
1
containing threads. The H DOSY experiments reveal that the
magnitude of the diffusion coefficients in the absence of CB7
decreases with increasing hydrodynamic size of the viologen
substrate in the order DMV2+ (5.84ꢂ10ꢁ6)>DBMV2+ (1.71ꢂ10ꢁ6)>
DBV4+ (9.7ꢂ10ꢁ7)>DHV12+ (7.27ꢂ10ꢁ7 cm2 sꢁ1). For solubility rea-
sons, the diffusion coefficients for BV4+ and HV12+ were calcu-
lated in [D6]DMSO, whereas those of MV2+ and BMV2+ were
determined in D2O. As anticipated, diffusion coefficients de-
creased for all four viologen substrates on addition of CB7.
viologen radical cations in both BV2( +) and HV6( +). Changes in
C
C
D value were not pronounced in the case of BMV, because re-
duction of BMV2+ to BMV does not induce CB7 dissociation.
+
C
The volumes (inside a contour of 0.001 eBohrꢁ3) of MV2+
,
BMV2+, BV4+, MV2+ꢀCB7 and BMV2+ꢀCB7 were calculated
by DFT and found to be 239.9, 342.54, 1153.6, 1325.3, and
1364.4 ꢁ3, respectively. The corresponding radii of these spe-
cies, evaluated by considering spheres having the same vol-
umes, were then used to estimate diffusion coefficients with
the aid of the Stokes–Einstein equation.[86,87] The calculated dif-
fusion coefficients for MV2+, BMV2+, BV4+, MV2+ꢀCB7, and
BMV2+ꢀCB7 were 5.70ꢂ10ꢁ6, 5.06ꢂ10ꢁ6, 3.38ꢂ10ꢁ6, 3.22ꢂ
10ꢁ6, and 3.19ꢂ10ꢁ6 cm2 sꢁ1, respectively, and are in reasonably
1
These results are consistent with the H NMR and UV/Vis spec-
troscopic data and indicate strong interactions between the
threads and CB7. After addition of CB7, the diffusion coeffi-
cients were found to be 2.52ꢂ10ꢁ6, 7.55ꢂ10ꢁ7, 8.6ꢂ10ꢁ7, and
5.02ꢂ10ꢁ7 cm2 sꢁ1 for MV2+ꢀCB7, BMV2+ꢀCB7, BV4+ꢀ(CB7)2,
and HV12+ꢀ(CB7)6, respectively.
We used chronocoulometry to determine the diffusion coef-
ficients of the fully oxidized and radical-cationic states of the
different species in aqueous solution (H2O, 0.1 mm tetrabutyl-
ammonium chloride (TBACl) as electrolyte). During the reduc-
tion process (0!ꢁ0.7 V) the fully oxidized species are predom-
inant and diffuse to the electrode surface, where they are re-
duced. By measuring reduction rates, we calculated (see Sup-
porting Information and Table 2) the diffusion constants for
HV12+, BV4+, and BMV2+ in the absence and in the presence
of CB7. The values are comparable to those determined by
DOSY and, in general, follow the same decreasing sequence of
DBMV2+ >DBV4+ >DHV12+. Furthermore, by setting the voltage to
ꢁ0.7 V and performing the oxidation process, we measured
the rates of oxidation of the corresponding radical-cationic
species and calculated their diffusion coefficients.
good agreement with the data obtained from DOSY and chro-
+
nocoulometry. The volume calculated for BMV (316.79 ꢁ3)
C
was somewhat smaller than that obtained for BMV2+
(342.54 ꢁ3), which is also consistent with the larger diffusion
coefficient determined for the radical cation by chronocoulom-
etry (Table 2). The diffusion coefficients obtained experimental-
ly from chronocoulometric and DOSY measurements were
used to estimate the corresponding molecular volumes with
the aid of the Stokes–Einstein equation (see Supporting Infor-
mation). As expected, the molecular volumes increase with in-
creasing molecular weight and are generally larger for the fully
oxidized forms than for the corresponding radical-cationic
states. Furthermore, the molecular volumes obtained from dif-
fusion coefficients agree reasonably well with those estimated
with DFT for MV2+, BMV2+, and MV2+ꢀCB7. However, the mo-
lecular volume obtained from measurements of diffusion coef-
In absence of CB7, larger diffusion coefficients were ob-
served for HV6( +) and BV2( +) than for their corresponding fully
C
C
Chem. Eur. J. 2014, 20, 7334 – 7344
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7342
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2( +)
ficient for BV4+ is much larger than that estimated with DFT,
which suggests an important ordering effect of the solvent
around the highly charged substrate.
ized species BVD
and HVD6( +), respectively. In fact, no evi-
C
C
dence for interactions between CB7 and the multimeric violo-
gen radical cations could be detected. H NMR DOSY and chro-
1
nocoulometric measurements of diffusion coefficients were
consistent with the calculated sizes of the various species pro-
posed and, along with the electrochemical and UV/Vis/NIR
spectroscopic results, support the proposed redox-induced
switching mechanisms. More generally, the data lend credence
to the idea that radical-cation dimerization (a relatively weak
interaction) can be enhanced intramolecularly and exploited
for the design of novel molecular switches.[88]
Conclusion
UV/Vis and electrochemical analyses have demonstrated that
MV2+ and MV bind to CB7 in a similar way, with a viologen
+
C
unit inside the hydrophobic cavity of the macrocycle. There-
fore, when the pseudorotaxane MV2+ꢀCB7 is reduced to MV
+
C
C
+
ꢀCB7, no major conformational changes occur, although MV
ꢀCB7 is a weaker complex (Scheme 1A). In contrast, BMV2+
Acknowledgements
The research described here was sponsored by New York Uni-
versity Abu Dhabi in the UAE. K.N.-N., K.W., S.N., T.P., and A.T.
thank NYUAD for generous support. A.C.F. thanks the National
Science Foundation (NSF) for the award of a NSF Graduate Re-
search Fellowship. M.E., Z.A., and P.D. thank the Centre Nation-
al de la Recherche Scientifique and the University of Stras-
bourg (UMR 7509 and UMR 7178) in France for financial sup-
port. C.P.-I. thanks the Centro de Supercomputaciꢃn de Galicia
(CESGA) for providing the computer facilities.
Keywords: electrochemistry · host–guest systems · molecular
recognition · pseudorotaxanes · radical ions
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