Probing donor-acceptor interactions and co-conformational changes in redox active desymmetrized [2]catenanes

Article abstract of DOI:10.1021/ja909041g

We describe the synthesis and characterization of a series of desymmetrized donor-acceptor [2]catenanes where different donor and acceptor units are assembled within a confined catenated geometry. Remarkable translational selectivity is maintained in all cases, including two fully desymmetrized [2]catenanes where both donors and acceptors are different, as revealed by X-ray crystallography in the solid state, and by ¹H NMR spectroscopy and electrochemistry in solution. In all desymmetrized [2]catenanes the co-conformation is dominated by the strongest donor and acceptor pairs, whose charge-transfer interactions also determine the visible absorption properties. Voltammetric and spectroelectrochemical experiments show that the catenanes can be reversibly switched among as many as seven states, characterized by distinct electronic and optical properties, by electrochemical stimulation in a relatively narrow and easily accessible potential window. Moreover in some of these compounds the oxidation of the electron donor units or the reduction of the electron acceptor ones causes the circumrotation of one molecular ring with respect to the other. These features make these compounds appealing for the development of molecular electronic devices and mechanical machines.

Full text of DOI:10.1021/ja909041g

Published on Web 12/31/2009
Probing Donor-Acceptor Interactions and Co-Conformational
Changes in Redox Active Desymmetrized [2]Catenanes
Dennis Cao,† Matteo Amelia,‡ Liana M. Klivansky,† Gayane Koshkakaryan,†
Saeed I. Khan,§ Monica Semeraro,‡ Serena Silvi,‡ Margherita Venturi,‡
Alberto Credi,*,‡ and Yi Liu*,†
The Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road,
Berkeley, California 94720, Department of Chemistry, UniVersity of California, Berkeley,
California 94720, Dipartimento di Chimica “G. Ciamician”, UniVersita` degli Studi di Bologna,
Via Selmi 2, 40126 Bologna, Italy, and Department of Chemistry and Biochemistry, UniVersity of
California, Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095
Received October 23, 2009; E-mail: alberto.credi@unibo.it; yliu@lbl.gov
Abstract: We describe the synthesis and characterization of a series of desymmetrized donor-acceptor
[2]catenanes where different donor and acceptor units are assembled within a confined catenated geometry.
Remarkable translational selectivity is maintained in all cases, including two fully desymmetrized [2]catenanes
where both donors and acceptors are different, as revealed by X-ray crystallography in the solid state, and
by 1H NMR spectroscopy and electrochemistry in solution. In all desymmetrized [2]catenanes the
co-conformation is dominated by the strongest donor and acceptor pairs, whose charge-transfer interactions
also determine the visible absorption properties. Voltammetric and spectroelectrochemical experiments
show that the catenanes can be reversibly switched among as many as seven states, characterized by
distinct electronic and optical properties, by electrochemical stimulation in a relatively narrow and easily
accessible potential window. Moreover in some of these compounds the oxidation of the electron donor
units or the reduction of the electron acceptor ones causes the circumrotation of one molecular ring with
respect to the other. These features make these compounds appealing for the development of molecular
electronic devices and mechanical machines.
Introduction
isomerism is achieved. The synthesis of such compounds
remains challenging as it requires the selection of pairs of
Since the first templated synthesis1 of [2]catenanes, inter-
locked molecular systems2 can now be obtained efficiently
relying on supramolecular assistance to covalent synthesis. With
the aid of preprogrammed interactions, such as hydrogen
bonding,3 π-stacking,4 anion templating,5 and metal-ligand
coordination,6 many assembly strategies have been developed
for the synthesis of functional interlocked molecules. In
particular, donor-acceptor interaction has become a favorable
driving force in the synthesis of electro- and photoactive
molecular machines on account of the rich electrochemical and
photophysical properties of π-electron donors and acceptors.
Mechanical motions generated from intercomponent movements
render these nanoscale objects ideal candidates for molecular
level switches. Indeed, molecular machines7-9 of such kinds
have been utilized to construct several devices,10 including
molecular memories,11 smart surfaces,12 nanovalves,13 and
molecular muscles.14
complementary recognition units with distinctive binding strength
and the proper assembly of such pairs into one molecular
structure. Stoddart et al. reported a switchable [2]catenane15f
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Desymmetrized [2]catenanes15 serve as a role model of
controllable molecular motors.7,8d Efficient rotary movements
can be implemented only if precise control of translational
† Lawrence Berkeley National Laboratory and University of California,
Berkeley.
‡ Universita` di Bologna.
§ University of California, Los Angeles.
9
1110 J. AM. CHEM. SOC. 2010, 132, 1110–1122
10.1021/ja909041g  2010 American Chemical Society

Redox Active Desymmetrized [2]Catenanes
A R T I C L E S
Scheme 1. Synthetic Scheme for the Desymmetrized [2]Catenanes
with a nearly “all-or-nothing” translational selectivity where two
different π-donors, namely tetrathiafulvalene (TTF) and 1,5-
dioxynaphthalene (DNP) moieties, were incorporated in a crown
ether macrocyclic component that selectively holds the π-de-
ficient tetracationic cyclophane around its TTF station as
opposed to the DNP station. Another yet less explored way to
implement control over the translational isomerism is to include
two π-deficient units with considerably different π-accepting
ability.15c,g Other than the well-known π-acceptor 4,4′-bipyri-
dinium dication (BPY2+), neutral naphthalene diimide (NDI)
and pyromellitic diimide (PMI) units are readily available
π-acceptors that have been incorporated in a variety of su-
pramolecular systems.16 Similar to BPY2+ derivatives, they are
able to form inclusion complexes with π-electron rich crown
ethers. Their π-accepting ability, however, differs significantly.
For example, the typical binding constants of a PMI and NDI
derivative with 1,5-dinaphtho-38-crown-10 (DNP38C10) was
only around 20 and 100 M-1, respectively, while the binding
the primary. In combination with a desymmetrized crown ether
containing TTF and DNP as two different π-donors, [2]cat-
enanes with four different donor and acceptor units are obtained.
In all the [2]catenanes, the crown ether component is shown to
encircle the BPY2+ unit in both solution and the solid state, as
1
revealed by H NMR spectroscopic methods and X-ray single
crystal structures. In the case of fully desymmetrized [2]cat-
enanes, TTF is exclusively included in the dicationic cyclophane
while the DNP unit lays alongside with the BPY2+ unit. The
set of [2]catenanes poise as an interesting collection of machine-
like molecules with potential multistability that is endowed by
mutual donor-acceptor interactions in a confined geometry.
Extensive optical and electrochemical measurements have been
conducted to gain more insight into the nature of the intercom-
ponent interactions in these donor-acceptor systems. The
interrogation of photophysical and electrochemical properties
can also provide in-depth information of how to control co-
conformational units by selectively modulating the electron
density of the donors and acceptors.
constant between 1,1′-dibenzyl-4,4′-bipyridinium and DNP38C10
15c,16j
was around 20 000 M-1
.
The gradient in the π-accepting
ability of these electron deficient units makes them attractive
candidates as π-acceptors in desymmetrized [2]catenanes.
In a preliminary communication,17 we demonstrated the
feasibility of incorporating both BPY2+ and PMI units in a
π-deficient cyclophane component that interlocks with donor-
containing crown ethers to give desymmetrized [2]catenanes.
We intend to investigate these donor-acceptor based [2]cat-
enanes (6-11•2PF6, Scheme 1), with the scope of expanding
[2]catenane synthesis by incorporating various donor-acceptor
pairs. NDI units are successfully incorporated into a series of
[2]catenanes as the secondary π-acceptors with BPY2+ being
Experimental Section
Following a typical clipping protocol for [2]catenane synthesis,
the desymmetrized [2]catenanes can be synthesized by a ring closure
reaction to cyclize the π-electron deficient dicationic cyclophane
around a templating π-electron rich crown ether. In consideration
of the chemical sensitivity of BPY2+, we decided to adapt a
synthetic scheme such that this functionality is introduced at the
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A R T I C L E S
Cao et al.
very end of the synthesis (Scheme 1). The reaction starts from an
equilibrium involving an inclusion complex formed between a
crown ether host and a reactive dibromide containing either PMI
or NDI unit. After the addition of 4,4′-bipyridine into the reaction
mixture, the bromides will be displaced upon nucleophilic attack
of the pyridine to generate two N+-C bonds, commensurate with
the formation of BPY2+ unit. Only intramolecular N+-C bond
formation is desirable for the formation of [2]catenanes while
intermolecular reactions will lead to polymeric species. Thanks to
the similar length (N-N distance around 7 Å) of BPY2+, NDI,
and PMI units, no significant geometrical restraints will impede
the macrocyclization of [2]catenanes. On the other hand, the
formation of BPY2+ is facilitated by the donor-acceptor interactions
reinforced in a fixed alternating geometry within a [2]catenane.
The synthesis is first carried out by reacting a PMI containing
dibromide 1, 4,4′-bipyridine, and a hydroquinone (HQ) containing
crown ether, bis-p-phenylene[34]crown-10 (BPP34C10, 3) in DMF
(Scheme 1). After counterion exchange using an acetone/NH4PF6
mixture, the [2]catenane 6•2PF6 is isolated as a red solid in 30%
yield.Whenthemoreelectronrichcrownether1,5-dinaphtho[38]crown-
10 (DNP38C10, 4) is used, the [2]catenane 7•2PF6 is isolated as a
purple solid in 52% yield. NDI-containing [2]catenanes 9•2PF6 and
10•2PF6 are obtained in a similar fashion when dibromide 2 is
employed.18 The asymmetrical crown ether 5 containing TTF and
DNP units as two different electron donors is prepared15f and
subjected to clipping reactions to give fully desymmetrized [2]ca-
tenanes 8•2PF6 and 11•2PF6 as green solids.
X-Ray Crystal Structural Analysis. Although the BPY2+ unit
is known as a better electron acceptor than the PMI and NDI units
when interacting with crown ethers, the co-conformation selectivity
is to be tested when both π-acceptors are covalently linked in a
[2]catenane setting. X-ray structural analysis provide clear evidence
about the preference of BPY2+ unit being included in the crown’s
cavity in the solid state in all [2]catenanes.19 The crystal structure
of 6•2PF6 and 7•2PF6 (Figure 1, a and b) indicates that the dicationic
cyclophane is interlocked with the crown ethers such that (1) the
HQ and DNP moieties are sandwiched between the nearly parallel-
aligned BPY2+ unit and PMI unit with (2) the BPY2+ unit
sandwiched between the parallel-aligned HQ and DNP moieties.
The mean interplanar separations are 3.3 Å, in keeping with
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(j) Durola, F.; Lux, J.; Sauvage, J.-P. Chem.sEur. J. 2009, 15, 4124–
4134.
(10) Silvi, S.; Venturi, M.; Credi, A. J. Mater. Chem. 2009, 19, 2279–
2294.
(11) (a) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly,
K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science
2000, 289, 1172–1175. (b) Luo, Y.; Collier, C. P.; Jeppesen, J. O.;
Nielsen, K. A.; DeIonno, E.; Ho, G.; Perkins, J.; Tseng, H.-R.;
Yamamoto, T.; Stoddart, J. F.; Heath, J. R. ChemPhysChem 2002, 3,
519–525. (c) Flood, A. H.; Stoddart, J. F.; Steuerman, D. W.; Heath,
J. F. Science 2004, 306, 2055–2056. (d) Green, J. E.; Choi, J. W.;
Boukai, A.; Bunimovich, Y.; Johnston-Halperin, E.; DeIonno, E.; Luo,
Y.; Sheriff, B. A.; Xu, K.; Shin, Y. S.; Tseng, H.-R.; Stoddart, J. F.;
Heath, J. R. Nature 2007, 445, 414–417.
9
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Redox Active Desymmetrized [2]Catenanes
A R T I C L E S
Figure 1. Stick representation of the solid-state structures and the structural formulas of (a) 6•2PF6, (b) 7•2PF6, and (c) 9•2PF6. Solvent molecules and
anions are omitted for clarity.
stabilizing [π-π] stacking interactions. In both [2]catenanes, the
two pyridinium rings are twisted with an angle of around 10° and
the distance between the two quaternary N+ centers is 7.0 Å. The
PMI units are not strictly coplanar, with a dihedral angle between
the mean planes of the diimide rings of 8 and 5°, and a distance
between the two N atoms of PMI units as 6.70 and 6.75 Å,
respectively. Various [C-H· · ·O] and [C-H· · · π] interactions are
present in both [2]catenanes. Intermolecular packings along the axes
of the donor-acceptor stacks are present in both cases.
The solid-state structure of NDI-containing [2]catenane 9•2PF6
implies (Figure 1c) the same common features as the PMI-
containing counterpart where the electronically complementary
π-systems are nearly parallel-aligned within two interlocking
macrocycles with an interplanar distance around 3.4 Å. Only one
translational isomer is observed, in which the BPY2+ unit is
sandwiched between the two electron-rich moieties of the crown
ether. The NDI unit is almost perfectly flat, stacking alongside of
the inside HQ unit. The pyridinium rings are twisted with a dihedral
angle of 15°. Various [C-H· · · O] and [C-H· · · π] interactions, as
well as intermolecular packing along the axes of the donor-acceptor
stacks, are observed.
X-ray structural analyses of 8•2PF6 and 11•2PF6 reveal the
presence of only one translational co-conformation out of four
possible ones in the solid state. Figure 2 shows that the donors and
acceptors arrange alternatively in 8•2PF6 and 11•2PF6 in the PMI-
TTF-BPY2+-DNP and NDI-TTF-BPY2+-DNP stacks, respectively,
the mean plane of which are parallel with one another with an
interplane distance around 3.4 Å. The TTF unit in 8•2PF6 is bent
significantly, with a dihedral angle as high as 25° between the two
planes defined by the two thione five-membered rings. In com-
(16) (a) Hamilton, D. G.; Sanders, J. K. M.; Davies, J. E.; Clegg, W.; Teat,
S. J. Chem. Commun. 1997, 897–898. (b) Hamilton, D. G.; Davies,
J. E.; Prodi, L.; Sanders, J. K. M. Chem.sEur. J. 1998, 4, 608–617.
(c) Hamilton, D. G.; Montalti, M.; Prodi, L.; Fontani, M.; Zanello,
P.; Sanders, J. K. M. Chem.sEur. J. 2000, 6, 608–617. (d) Hansen,
J. G.; Feeder, N.; Hamilton, D. G.; Gunter, M. J.; Becher, J.; Sanders,
J. K. M. Org. Lett. 2000, 2, 449–452. (e) Iijima, T.; Vignon, S. A.;
Tseng, H.-R.; Jarrossson, T.; Sanders, J. K. M; Marchioni, F.; Venturi,
M.; Apostoli, E.; Balzani, V.; Stoddart, J. F. Chem.sEur. J. 2004,
10, 6375–6392. (f) Kaiser, G.; Jarrosson, T.; Otto, S.; Ng, Y.-F.; Bond,
A. D.; Sanders, J. K. M. Angew. Chem., Int. Ed. 2004, 116, 1993–
1996. (g) Vignon, S. A.; Jarrosson, T.; Iijima, T.; Tseng, H.-R.;
Sanders, J. K. M.; Stoddart, J. F. J. Am. Chem. Soc. 2004, 126, 9884–
9885. (h) Pascu, S. I.; Jarrosson, T.; Naumann, C.; Otto, S.; Kaiser,
G.; Sanders, J. K. M. New J. Chem. 2005, 29, 80–89. (i) Pascu, S. I.;
Naumann, C.; Kaiser, G.; Bond, A. D.; Sanders, J. K. M.; Jarrosson,
T. Dalton Trans. 2007, 35, 3874–3884. (j) Koshkakaryan, G.; Parimal,
K.; He, J.; Zhang, X.; Abliz, Z.; Flood, A. H.; Liu, Y. Chem.sEur. J.
2008, 14, 10211–10218. (k) Bhosale, S. V.; Jani, C. H.; Langford,
S. J. Chem. Soc. ReV. 2008, 37, 331–342. (l) Au-Yeung, H. Y.; Pantos,
D. G.; Sanders, J. K. M. Proc. Natl. Acad. Sci. U.S.A. 2009, 106,
10466–10470. (m) Koshkakaryan, G.; Klivansky, L. M.; Cao, D.;
Snauko, M.; Teat, S. J.; Struppe, J. O.; Liu, Y. J. Am. Chem. Soc.
2009, 131, 2078–2079.
(18) It should be noted that the NDI dibromide used for clipping reaction
is mixed with impurities resulted from decomposition under the acidic
bromination conditions and cannot be purified due to limited solubility.
These impurities, together with the poorer solubility of NDI dibromide
2, are probably responsible for lower yields than the corresponding
PMI derivatives.
(19) Crystals suitable for X-ray single crystal analysis were obtained after
diffusion of ether into a Me2CO solution or from a MeCN solution
after diffusion of diisopropyl ether. For more crystallographic informa-
tion, see the crystallographic information files in Supporting Informa-
tion.
(17) Liu, Y.; Klivansky, L. M.; Zhang, X. Org. Lett. 2007, 9, 2577–2580.
9
J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010 1113

A R T I C L E S
Cao et al.
Figure 2. Stick representation of the solid-state structures and the structural formulas of (a) 8•2PF6 and (b) 11•2PF6. Solvent molecules and anions are
omitted for clarity.
parison, the bending of the TTF unit in 11•2PF6 is only 8°. Only
trans-TTF groups are observed in the solid structure in both cases.
The PMI and NDI units in individual [2]catenane are slightly bent,
with a dihedral angle around 8° and 10°, respectively. In addition,
various [C-H· · ·O] and [C-H· · ·π] interactions as well as extended
packing along the stacking direction are present in the solid state
structures.
is dominant at this temperature, but it cannot be excluded that the
two isomers undergo fast exchange such that only an averaged
spectrum is observed. To verify that, the temperature is further
lowered to 193 K, at which point the site exchange is slow enough20
to capture possible translational isomers in equilibrium. As shown
in Figure 3b and d, all the peaks of 6•2PF6 and 7•2PF6 remain
sharp in CD3COCD3 and no new peaks are observed, suggesting
that the single resonance is not a result of site exchange and indeed
the translational selectivity is maintained throughout. On the basis
of the detection limit of 1H NMR spectroscopy, a selectivity greater
than 97:3 can be estimated for both [2]catenanes.
Translational Selectivity by 1H NMR Spectroscopy. 1H NMR
spectroscopy provided insightful information for the interlocked
structures of these [2]catenanes as well as the co-conformation
selectivity. For the PMI containing [2]catenanes, the resonances
of spectra of 6•2PF6 or 7•2PF6 are assignable to one single
translational isomer. The “inside” and “outside” HQ or DNP
moieties in 6•2PF6 or 7•2PF6 are (Figure 3) clearly discernible,
suggesting that the site exchange of the two HQ or DNP units is
Upon replacing the PMI unit with a more electron deficient
π-acceptor, the translational selectivity is maintained, as revealed
1
by the presence of only one single isomer in the H NMR spectra
of both NDI-containing [2]catenanes 9•2PF6 and 10•2PF6 (Figure
4). The symmetry of the crown ethers is broken owing to catenation
and slow site exchange, resulting in two sets of HQ or DNP protons
in 9•2PF6 or 10•2PF6. The “inside” and “outside” HQ protons of
9•2PF6 resonate as two singlets at δ 6.25 and 3.55, respectively. In
the case of 10•2PF6, two groups of doublet-triplet-doublet are
assignable to the resonances of the two DNP units. A characteristic
doublet at δ 2.98 ppm is observed, corresponding to the H4/8 of the
inside DNP moiety. The NDI protons of the dicationic cyclophane
in 9•2PF6 appear as a singlet at 8.43 ppm. In the case of 10•2PF6,
ortho coupling of the aromatic protons of the NDI unit results in
two doublets at δ 8.04 and 7.78 ppm. This splitting, together with
the diastereotopic splitting of all other protons of the dicationic
cyclophane in 10•2PF6, is consistent with the local c2 symmetry
imposed by the DNP unit. Variable temperature studies indicate21
that no new resonances corresponding to other isomers are observed
throughout the temperature range from 193 to 298 K, consistent
with the crown ether ring component located selectively on the
BPY2+ unit. The translational selectivity is thus preserved in these
NDI derived [2]catenanes.
1
slow at the H NMR time scale. Two singlets at δ 6.26 and 3.80
ppm in the spectrum of 6•2PF6 can be assigned to the “inside” and
“outside” HQ, respectively. In the case of 7•2PF6, the resonances
of the two DNP units are expressed in the usual doublet-triplet-
doublet coupling pattern. Characteristically, the H4/8 of the inside
DNP moiety appears as a doublet at δ 2.95 ppm (inset in Figure
3c) as a result of shielding effect from the nearby dicationic
cyclophane. In contrast to 6•2PF6, all except the PMI protons of
the dicationic cyclophane in 7•2PF6 become diastereotopic. The
chemical unequivalence occurs as a result of the local c2 symmetry
imposed by the DNP unit on the dicationic cyclophane.
The observation of only one single set of resonances in both
6•2PF6 and 7•2PF6 at 297 K suggests that one translational isomer
(20) Two mechanisms are possible for the exchange between the two
translational isomers. Either the dicationic cyclophane rotates through
the center of the crown ether component, or the outer DNP unit can
sweep round the dicationic cyclophane by pirouetting about the
included DNP moiety. According to studies by the Stoddart group on
structurally similar [2]catenanes, (see: Stoddart, J. F; et al. J. Am.
Chem. Soc. 1992, 114, 193–218. ) the latter one is the lower energy
process as it requires least energy for the breakdown of donor-acceptor
interactions. The typical associated activation barrier for Stoddart’s
systems is around 12-14 kcal mol-1. Correspondingly, those site
exchange processes can be efficiently “frozen out” below 243 K. For
compounds 6•2PF6 and 7•2PF6, such activation barrier is difficult to
measure on account of the absence of other isomers. However, they
should be very similar to those reported systems, both of which involve
the breaking of donor-acceptor interactions between BPY2+ and the
outer aromatic units.
The combination of two different donors and two different
acceptors in [2]catenanes 8•2PF6 and 11•2PF6 leads to four possible
translational isomers. This situation can be further complicated when
taking account of cis/trans positional isomers of the TTF group,
which coexist in the starting crown ether 5 in a ratio of 2:3. Despite
the complexity of the systems, the well-resolved 1H NMR spectra
indicate the presence of only one isomer, suggesting a remarkable
(21) See Supporting Information.
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1114 J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010

Redox Active Desymmetrized [2]Catenanes
A R T I C L E S
1
Figure 3. Partial H NMR spectra of (a) 6•2PF6 at 297 K, (b) 6•2PF6 at 193 K, (c) 7•2PF6 at 297 K, and (d) 7•2PF6 at 193 K. The inset in c shows the
doublet corresponding to H4/8 of the inside DNP unit.
1
Figure 4. Partial H NMR spectra of (a) 9•2PF6 and (b) 10•2PF6.
9
J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010 1115

A R T I C L E S
Cao et al.
Figure 5. 1H NMR spectra of [2]catenane 11•2PF6 (273 K, CD3CN, 500 MHz) (a) before the addition of oxidant, (b) after the addition of 2.2 equiv oxidant,
and (c) after the addition of Zn powder.
1
Table 1. Comparison of the H NMR Chemical Shift Data of 72+ vs
the Oxidized 84+, and 102+ vs the Oxidized 114+ (500 MHz, 273 K,
CD3CN)a
translational selectivity and the absence of cis/trans TTF isomer-
ization. The only TTF positional isomer can be assigned to the
trans one, which is more electronically and/or sterically favored in
the catenated structure, as is confirmed by the X-ray structures. As
indicated in the 1H NMR spectrum of 11•2PF6 (Figure 5a), the TTF
protons resonate at 5.95 ppm as a singlet while the DNP protons
display the usual doublet-triplet-doublet coupling pattern at the
aromatic region. The lack of a doublet at the high field region
around δ 3.0 ppm confirms the location of DNP being outside of
the cyclophane. The protons of the cyclophane component became
diastereotopic, in agreement with the symmetry breakdown by the
DNP unit. The translational selectivity is preserved as verified by
compound
HPMI
HNDI
72+
6.76
-
84+
6.88 (0.12)
-
102+
114+
-
-
8.03, 7.77
8.11 (0.08), 7.83 (0.06)
a Number in parentheses indicates chemical shift changes.
translocation of the dicationic cyclophane occurs in response to
the oxidative switching of 112+ and the NDI unit remains outside
of the electron-rich crown ether, despite the increased charge
repulsion between BPY2+ and the TTF2+. Presumably the poly-
ethylene glycol chains in the crown ether provide sufficient
flexibility for the TTF2+ group to oscillate around to keep a distance
from the BPY2+ unit to minimize charge-charge repulsion.
The oxidation-reduction experiment on 8•2PF6 disclosed similar
switching behavior, as evidenced by 1H NMR spectroscopy.21 The
characteristic chemical shift changes associated with the TTF and
DNP units indicated that upon two-electron oxidation, the TTF2+
is dislocated outside of the cavity of the dicationic cyclophane while
21
1
variable temperature H NMR spectroscopic studies. Similarly,
the 1H NMR spectra of 8•2PF6 displays spectroscopic features
consistent with the exclusive translational and positional selectiv-
ity.21
1
Chemical Switching by H NMR Spectroscopy. The translo-
cation of crown ether component in the TTF-containing [2]catenanes
8•2PF6 and 11•2PF6 can be driven chemically, as monitored by 1H
NMR spectroscopy in an oxidation-reduction cycle. During the
oxidation of 11•2PF6 using 2.2 equiv. oxidant, tris(4-bromophe-
nyl)iminium hexachloroantimonate, the TTF singlet shifts (Figure
5b) downfield by more than 3 ppm, in accordance with the
formation of TTF2+. Commensurate with the oxidation process,
pronounced chemical shift changes are observed for the DNP
protons. A doublet emerges at δ 2.95 ppm for 114+, which suggests
that the DNP unit is now included in the cyclophane cavity. This
translocation of the crown ether component is completely reversible.
After the addition of Zn powder and vigorous shaking, the original
spectrum is fully restored (Figure 5c).
One remaining question about the oxidative switching is whether
the dicationic cyclophanes also undergo circumrotation on account
of the increased electrostatic repulsion between BPY2+ and the
newly formed TTF2+ (Scheme 2). An analysis of the spectral
changes of NDI protons in 11•2PF6 reveals (Figure 5a and b) a
larger peak separation upon oxidation, indicating a dramatic change
of its surrounding chemical environment that can be accounted for
by the switch of the stacking partner from TTF to DNP. A
comparison of the 1H NMR spectra of 102+ and the oxidized 114+
indicates (Table 1) that the NDI protons display very similar
chemical shifts and splitting pattern, suggesting a similar chemical
environment in both molecules. This similarity implies that no
DNP is now encircled. By comparing the H NMR spectra of 72+
1
and the oxidized 84+ (Table 1), it is concluded that the PMI stays
outside of the crown ether cavity in response to the double oxidation
of the TTF unit.
The successful oxidative switching of the electron donors prompts
us to conduct a chemical switching by reduction of the electron
acceptors. Sodium hydrosulfite is used as the reducing agent, which
is shown to effectively reduce the BPY2+ unit in [2]catenane 6•2PF6
to give a blue color as a result of the formation of BPY+ radical
cation. The paramagnetic nature of the generated radical cation,
however, leads to a very broad spectrum which prevents a
1
meaningful H NMR spectroscopic analysis.
Photophysical Properties. The photophysical properties of
[2]catenanes 62+-112+ are investigated and compared against a
number of model compounds (Scheme 3). Compounds 12-13 are
synthesized21 and studied under similar conditions as for the
[2]catenanes. The physicochemical data for the previously studied
[2]catenanes 154+-174+ and 18-1915c,16b,c are reported for com-
parison purposes. Compounds 12 and 13 exhibit the absorption
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1116 J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010

Redox Active Desymmetrized [2]Catenanes
A R T I C L E S
Scheme 2. Illustration of Circumrotation of the Molecular Components during the Oxidative Switch of 112+
Scheme 3. Structural Formulas of Model Compounds
bands expected for their PMI22 and NDI23 moieties, respectively,
and no luminescence. Compound 142+ shows the typical absorption
band of the BPY2+ unit24 and no luminescence.25 The crown ether
rings 3 and 4 exhibit an absorption band in the near-UV region
and an intense fluorescence emission (λmax ) 324 and 345 nm,
respectively) ascribed to their HQ or DNP units, respectively, in
agreement with previous investigations.15c The absorption spectrum
of macrocycle 5 is that expected on the basis of its TTF and DNP
chromophoric units.15f,26 The DNP emission in 5 is strongly
quenched (>100 times weaker compared with that of 4), presumably
because of an electron-transfer process from the TTF moiety to
the excited DNP unit.
The absorption spectra of [2]catenanes 62+-112+ are different
from the sum of the spectra of the respective chromophoric
components (Table 2). As shown for 102+ (Figure 6), a considerable
decrease in the intensity of the UV absorption of its chromophoric
units and a blurring of the vibrational structure of the DNP and
NDI bands are observed. Additionally, all [2]catenanes exhibit
(Table 2 and Figure 7) a weak and broad band in the visible region
assigned to the charge-transfer (CT) interaction between the
π-acceptor and π-donor units incorporated in the molecular
components.
The discussion of the CT bands in these [2]catenanes is
particularly interesting because, in principle, different interactions
can occur, depending on the types of donor and acceptor units
engaged. Only one CT band is observed in the absorption spectrum
of either 62+ or 72+, which is very similar to that shown by the
respective tetracationic cyclobisparaquat-based model compounds
154+ or 164+. This result is consistent with the co-conformation of
62+ and 72+, in which the BPP34C10 or DNP38C10 macrocyclic
component encircles the better electron accepting BPY2+ unit. On
the other hand, the CT band arising from the interaction between
the electron donors and the alongside electron acceptors, such as
DNP and PMI pair in 72+, expected at around 460 nm on the basis
of the results obtained for the model compound 18 (λmax ) 455
nm and ε ) 730 M-1 cm-1 in CH2Cl2),16b is not observed (Figure
7). For 82+ in its stable co-conformation three different CT
interactions can be expected, involving (i) TTF-BPY2+, (ii)
TTF-PMI, and (iii) DNP-BPY2+ units, respectively. Also in this
case, however, only one CT band with λmax ) 800 nm and ε )
3600 M-1 cm-1 (Figure 7) is observed, which is consistent with
(22) Wiederrecht, G. P.; Niemczyk, M. P.; Svec, W. A.; Wasielewski, M. R.
J. Am. Chem. Soc. 1996, 118, 81–88.
(23) (a) Sazanovich, I. V.; Alamiry, M. H.; Best, J.; Bennett, R. D.;
Bouganov, O. V.; Davies, E. S.; Grivin, V. P.; Meijer, A. J. H. M.;
Plyusnin, V. F.; Ronayne, K. L.; Shelton, A. H.; Tikhomirov, S. A.;
Towrie, M.; Weinstein, J. A. Inorg. Chem. 2008, 47, 10432–10445.
(b) Rogers, J. E.; Kelly, L. A. J. Am. Chem. Soc. 1999, 121, 3854–
3861. (c) Demeter, A.; Brces, T.; Biczk, L.; Wintgens, V.; Valat, P.;
Kossanyi, J. J. Phys. Chem. 1996, 100, 2001–2011.
the TTF group interacting with the BPY2+ unit of the cyclophane
4+
(for 17
λ
) 835 nm and ε ) 3500 M-1 cm-1).15f,27,28 The CT
max
band for the DNP-BPY2+ interaction, expected at around 530 nm
as seen in the model compound 164+ (λmax ) 529 nm and ε )
1100 M-1 cm-1),15c is absent, in agreement with previous observa-
tions on related [2]catenanes.26 No CT band in the region between
450 and 800 nm is observed, suggesting the lack of meaningful
(24) Monk, P. M. S. The Viologens: Physicochemical Properties, Synthesis
and Applications of the Salts of 4,4′-Bipyridine; Wiley: New York,
1998.
(25) Ballardini, R.; Credi, A.; Gandolfi, M. T.; Giansante, C.; Marconi,
G.; Silvi, S.; Venturi, M. Inorg. Chim. Acta 2007, 360, 1072–1082.
(26) Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Raymo, F. M.;
Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Org.
Chem. 2000, 65, 1924–1936.
(27) Witlicki, E. H.; Hanses, S. W.; Christensen, M.; Hansen, T. S.;
Nygaard, S. D.; Jeppesen, J. O.; Wong, E. W.; Jensen, L.; Flood, A. H.
J. Phys. Chem. A 2009, 113, 9450–9457.
9
J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010 1117

A R T I C L E S
Cao et al.
Table 2. UV-vis Absorption and Luminescence Properties of the Investigated Catenanes and Model Compounds (Air Equilibrated CH3CN,
Room Temperature)
absorption
)
luminescence
a
)
b
)
compound
λmax (nm)
εmax (L mol-1 cm-1
∆ν1/2CT(cm-1
HDA (cm-1
λmax (nm)
Φc
τ (ns)
62+
72+
82+
92+
267d
461
275
507
283
800
269
382
480
274
367
526
285
369
815
291
296
297
319
379
260
19000
530
19000
800
25000
3600
20000
19000
700
23000
12000
900
23000
15000
3200
7450
6710
3930
1830
2030
2630
6110
5770
3850
1860
1960
2430
102+
112+
3
4
5
5100
324
345
0.10
0.35
3.0
7.1
16100
16700
2600
27000
20000
12 (PMI control)e
13 (NDI control)e
142+ (BPY control)
a Half-width (fwhm) of the CT absorption band. b Donor-acceptor electronic coupling element; see text for details. c Luminescence quantum yield.
d Shoulder. e For solubility reasons, the experiments were carried out in CH3CN/CHCl3 98:2 (v/v).
Figure 6. Absorption spectrum of [2]catenane 102+ (red) compared with
the sum of the spectra of 13 and 142+, as models for the chromophoric
units contained in the BPY2+-NDI cyclophane, and of macrocycle 4 (black).
TTF-PMI interactions. In summary, for the [2]catenanes containing
the PMI-BPY2+ cyclophane only the charge-transfer interaction
involving the better π-acceptor unit (BPY2+)sand, in case of a
choice as for 82+, the better π-donor unitscan be evidenced in the
absorption spectra.
The analysis for the [2]catenanes containing the NDI-BPY2+
cyclophane is further complicated by the fact that the CT absorption
band expected when the NDI unit is involved as the electron
acceptor, as revealed in the model compound 19 (λmax ) 523 nm,
ε ) 350 M-1 cm-1 in CH2Cl2),16b occurs at a wavelength very
similar to that of the CT band involving the BPY2+ unit.15c In any
Figure 7. CT absorption bands of the [2]catenanes containing the PMI
instance, the CT bands of 92+ and 102+ (Figure 7) are very similar
unit (top) and the NDI unit (bottom).
to those shown by 62+ and 72+ (or model compound 154+ and 164+),
(28) It can be noticed that the TTF-BPY2+ CT band of catenanes 82+ and
112+ has almost the same molar absorption coefficient as that of model
compound 174+, although the former catenanes possess only one
BPY2+ unit. Indeed, it was recently shown (ref 4g) that the alongside
CT interaction between a TTF and a BPY2+ units can give rise to a
band with an absorption coefficient similar to that of the present
catenanes. As observed in previous investigations on related catenanes
(refs 15c, g), the CT band intensities in this type of catenanes do not
usually scale linearly with the number of donor and acceptor units
involved.
respectively, in agreement with a structure in which BPP34C10 or
DNP38C10 encircles the BPY2+ electron-accepting unit. The visible
absorption spectrum of 112+ is dominated by a band assigned to
the CT interaction involving the BPY2+ and TTF units (λmax ) 815
nm, ε ) 3200 M-1 cm-1);15f,27,28 a shoulder in the region between
450 and 500 nm (ε ca. 700 M-1 cm-1 at 470 nm) is also observed.
On the basis of the co-conformation of this catenane, and in view
of the poorer electron-accepting ability of the NDI unit with respect
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Redox Active Desymmetrized [2]Catenanes
A R T I C L E S
Table 3. Electrochemical Data for the [2]Catenanes and Model Compounds (CH3CN, Room Temperature)a
compound
reduction
oxidation
|∆Ered| + |∆Eox| (V)
62+
72+
82+
92+
102+
112+
3
-0.42, -0.85, -0.98, -1.53
-0.54, -0.95,c -1.50
-0.50, -0.92,c -1.41
-0.39, -0.67, -0.96, -1.10
-0.55, -0.73, -0.96, -1.09
-0.51, -0.70, -0.98, -1.10
-
+1.40,b +1.58b
+1.33,b +1.50b
+0.57,d +0.76, +1.49b
+1.39,b +1.56b
+1.32,b +1.46b
+0.59d, +0.78, +1.49b
+1.24,b +1.37b
+1.05,b +1.15b
+0.29, +0.72, +1.28b
-
0.23
0.47
0.43
0.19
0.47
0.46
4
5
-
-
12 (PMI control)e
13 (NDI control)e
142+ (BPY2+ control)f
-0.82, -1.39
-0.58, -1.01
-
-0.35, -0.78
-
f, g
154+
-0.31, -0.44, -0.84c
-0.35, -0.56, -0.81, -0.89
-0.33, -0.49, -0.87c
-0.94, -1.14, -1.57c
-0.70, -0.98, -1.18c
+1.42,b +1.72b
+1.30,b, g +1.55b, h
+0.80c, +1.60b
f
164+
h, i
174+
18j
19j
a Halfwave potential values of reversible and monoelectronic processes in V vs SCE, unless otherwise noted. Conditions: glassy carbon electrode,
tetraethylammonium hexafluorophosphate as supporting electrolyte, ferrocene (E1/2 ) +0.395 V vs SCE) or decamethylferrocene (E1/2 ) -0.110 V vs
SCE) employed as an internal reference. b Irreversible or poorly reversible process; redox potential estimated from the DPV peak recorded at 20 mV/s.
c Two-electron process. d Process characterized by a large separation between the cathodic and anodic peaks; potential taken from DPV measurements at
20 mV/s. e For solubility reasons the experiments were carried out in CH3CN/CHCl3 9:1 (V/V). f Data from ref 15c. g Data from ref 33. h Data from ref
15f. i Data from ref 26. j Data obtained in CH2Cl2, from ref 16b.
to BPY2+, this shoulder can tentatively be ascribed to the CT
interaction between the NDI and TTF units.
is likely that the deactivation to the lower-lying, nonemissive CT
states provides an efficient route for the nonradiative decay of the
upper-lying excited states, in line with previous observations on
similar systems.15c,f-h,16a-c,26
Electrochemical Experiments. In the [2]catenanes, the uptake
and removal of electrons by the acceptors and from the donors,
respectively, can trigger co-conformational rearrangements involv-
ing the exchange of inside and alongside units through the
circumrotation of the molecular components.15c,f,26,32 This behavior
is interesting for the potential utilization of [2]catenanes as
electrochemically driven molecular machines.15a,c-f Comprehensive
electrochemical studies of [2]catenanes 62+-112+ as well as the
model compounds are carried out to probe the co-conformational
changes in response to electrochemical stimuli.
The features of the CT absorption bands can be related to the
electronic coupling element HDA between the donor and acceptor
units involved in the charge-transfer transition. Hush showed that
in the case of a Gaussian-shaped CT band the donor-acceptor
electronic coupling element can be calculated by the following
equation:29,30
ν
√
maxεmax∆ν1/2
HDA ) 2.06 × 10-2
(1)
RDA
where νmax and ∆ν1/2 are the band maximum and half-width in cm-1
max is the molar absorption coefficient at the band maximum in L
,
ε
The electrochemical data for the examined compounds in argon
purged CH3CN solution are gathered in Table 3. All donors and
acceptors embedded within the [2]catenane structures are electro-
active in the examined potential window (from +2 to -2 V vs
saturated calomel electrode, SCE). Crown ethers 3, 4, and 5 exhibit
oxidation processes and no reduction, as expected on the basis of
their HQ, DNP, and TTF units.15c,f,33 The potential values for the
first oxidation (Table 3) suggest that the electron donating ability
should increase in the order HQ < DNP , TTF. Compounds 12,
13, and 142+, that can be taken as models for the PMI, NDI, and
BPY2+ units, respectively, show two consecutive monoelectronic
and reversible reduction processes and no oxidation. On the basis
of the first reduction potential (Table 3), it can be estimated that
the electron accepting ability should increase in the order PMI <
mol-1 cm-1, and RDA is the distance separating the donor and
acceptor moieties in Å. In these [2]catenanes, the donor-acceptor
distance can be unambiguously estimated considering that the
electron donating unit is sandwiched into a rigid electron accepting
macrocycle; X-ray structural investigations show that RDA is about
3.3 Å (see above). The electronic coupling elements for the
investigated [2]catenanes, calculated from eq 1, are compiled in
Table 2. It can be noted that [2]catenanes containing the same
electron donating ring exhibit nearly identical HDA values, in spite
of the fact that they contain different units (PMI or NDI) as the
secondary electron acceptor. This observation supports the previous
discussion, that is, the CT band originates from an interaction in
which BPY2+ unit is the prevailing electron acceptor. The value of
HDA for [2]catenanes containing different donor moieties increases
in the order HQ < DNP , TTF, thereby reflecting the increasing
π-electron donating ability of such units.
NDI < BPY2+
.
The cyclic voltammograms (CVs) of all [2]catenanes exhibit the
expected redox processes associated with their electroactive units.
As a consequence of intercomponent CT interactions in the
catenated structures, the potential values for these processes are
shifted (Table 3) in comparison with the model compounds. The
discussion of the electrochemical properties of the present desym-
metrized [2]catenanes is greatly facilitated by the use of “genetic”
diagrams that correlate the potential values observed for the
None of the examined [2]catenanes exhibited bona fide emission
upon excitation either in the UV region or in the visible CT band.31
Hence, we conclude that the potentially fluorescent singlet excited
states localized on the HQ and DNP units undergo radiationless
deactivation when these units are contained in the [2]catenanes. It
(29) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391–444.
(30) Brunschwig, B. S.; Sutin, N. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, 2001; Vol. II, p 600.
(31) A very weak HQ- or DNP-type fluorescence, whose intensity was from
two to three orders of magnitude smaller than that of the crown ethers
3 or 4, respectively, was detected upon UV excitation. Fluorescence
lifetime measurements indicate that such emissions originate from
small amounts of strongly emissive impurities.
(32) Kaifer, A. E.; Gomez-Kaifer, M. Supramolecular Electrochemistry;
Wiley-VCH: Weinheim, 2000.
(33) Ballardini, R.; Balzani, V.; Credi, A.; Brown, C. L.; Gillard, R. E.;
Montalti, M.; Philp, D.; Stoddart, J. F.; Venturi, M.; White, A. J. P.;
Williams, B. J.; Williams, D. J. J. Am. Chem. Soc. 1997, 119, 12503–
12513.
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J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010 1119

A R T I C L E S
Cao et al.
Figure 8. Correlation diagram of the redox potentials of the [2]catenanes
containing the PMI electron accepting unit (filled symbols), and of their
molecular components and model compounds (empty symbols). Two-
electron processes are labeled with the number 2.
[2]catenane with those of appropriate parent compounds. The
assignment of each process to the corresponding electroactive unit
is thus made by comparison with the behavior of model systems
and assisted by spectroelectrochemical experiments. Only reversible
processes will be discussed in detail.
[2]Catenanes Containing the PMI Unit. CVs of [2]catenanes
62+ and 72+ (Figure 8) show two irreversible oxidative processes
associated with the oxidation of the two HQ or DNP units. These
processes are positively shifted with respect to the crown ethers as
a result of stabilizing CT interactions. The incorporation of two
different donors in the [2]catenane 82+ endows rich electrochemical
information. Three oxidative processes are observed for 82+ (Figure
9a): the first two (+0.57 V and +0.76 V) are assigned to the two
consecutive monoelectronic TTF oxidations,34 (Figure 9b) while
the third one (+1.49 V) is ascribed to the oxidation of the DNP
unit. The first TTF oxidation is substantially displaced (280 mV)
toward more positive potentials as compared to the free crown ether
5, in agreement with the fact that TTF occupies the inside position
and is strongly involved in CT interactions. This process is
characterized by a large separation between the anodic and the
cathodic cyclic voltammetric peaks, which are also scan-rate
dependentson increasing the scan rate, the former moves toward
more positive potentials and the latter toward less positive potentials.
The TTF+/2+ oxidation is reversible and occurs at a similar potential
compared with the same process in the crown ether 5, while the
oxidation of the DNP unit is strongly displaced toward positive
potentials. The scan-rate dependence of the separation between the
anodic and cathodic peaks of the TTF0/+ process in 82+ indicates
that oxidation is followed by a rearrangement taking place on the
time scale of the electrochemical experiment, as previously observed
Figure 9. Voltammetric and spectroelectrochemical response of catenane
8
2+ in CH3CN at room temperature. (a) Cyclic voltammetric curve (4.9 ×
10-4 M, TEAPF6 7.3 × 10-2 M, 200 mV/s, glassy carbon working
electrode). (b) Absorption spectra observed before (black line) and after
exhaustive oxidation at +0.60 V (green) and +1.00 V (pink) versus an Ag
pseudoreference electrode. (c) Absorption spectra observed before (black
line) and after exhaustive reduction at -0.60 V (blue) and -0.90 V (orange)
versus an Ag pseudoreference electrode. Conditions for (b) and (c): 7.5 ×
10-4 M, TEAPF6 9.1 × 10-2 M, platinum grid working electrode, optical
path length 180 µm. The colored arrows in (a) mark the potential values at
which the curves in (b) and (c) were recorded in the spectroelectrochemical
experiments.
On the reduction side, the CV of 62+ reveals four reversible
monoelectronic processes (Figure 8). The first one occurs at -0.42
V and can be assigned to the BPY2+/+ reduction,24 as confirmed
by spectroelectrochemistry. The comparison with 154+ indicates15c
that such a potential value is consistent with the reduction of a
BPY2+ unit engaged in CT interactions inside the cavity of 3, in
agreement with the dominant co-conformation of this [2]catenane.
The second reduction takes place at -0.85 V and corresponds to
the BPY+/0 process according to spectroelectrochemical experi-
ments. Although a unequivocal conclusion can hardly be drawn
from our data, it is likely that the monoreduced BPY+ unit remains
inside the cavity of the crown ether, as is consistently observed in
all other [2]catenanes (see sections below). The third (-0.98 V)
and fourth (-1.53 V) processes are straightforwardly attributed to
the PMI0/- and PMI-/2- reductions, respectively. The PMI0/- process
in 62+ is negatively shifted by 160 mV with respect to 12. In the
related symmetric [2]catenane 18 that contains DNP moieties as
the electron donors, the alongside PMI unit becomes harder to
monoreduce by 120 mV with respect to a model species in
CH2Cl2.16b Considering that HQ is a weaker electron donor than
DNP, the rather large potential shift observed for the PMI0/- process
in 62+ is not compatible with a PMI unit occupying an alongside
position at the moment of its reduction. Hence, it is proposed that
the process observed at -0.98 V is the reduction of the inside PMI
unit. The translocation probably occurs upon reduction of the BPY+
unit to its neutral form while the PMI unit becomes a better acceptor.
for the related [2]catenane 174+ 15f,26 The nature of this rearrange-
.
ment can be elucidated by the following observations: (i) the second
oxidation of the TTF unit occurs at a potential value similar to that
of the crown ether 5, indicating that after the first oxidation the
TTF+• is no longer involved in strong CT interactions; (ii) the TTF+/
2+ wave is reversible, which shows that the system does not undergo
any further rearrangement after one-electron oxidation; and (iii)
the strongly positive potential value for the oxidation of the DNP
unit indicates that, when this process occurs, the DNP moiety is
encircled by the electron accepting cyclophane. These features show
that, after the TTF0/+ oxidation, the crown ether component
circumrotates with respect to the electron accepting cyclophane,
delivering the DNP unit into its cavity.
(34) Ashton, P. R.; Balzani, V.; Becher, J.; Credi, A.; Fyfe, M. C. T.;
Mattersteig, G.; Menzer, S.; Nielsen, M. B.; Raymo, F. M.; Stoddart,
J. F.; Venturi, M.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 3951–
3957.
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Redox Active Desymmetrized [2]Catenanes
A R T I C L E S
Figure 11. Correlation diagram of the redox potentials of the [2]catenanes
containing the NDI acceptor (filled symbols), and of their molecular
components and model compounds (empty symbols).
remains encircling the BPY+ radical cation after the first reduction,
in keeping with the fact that BPY+ is a better electron acceptor
than the PMI unit. In the case of HQ-containing 62+, a site-specific
co-conformational change accompanies the BPY+/0 reduction
process, leading to the translocation of the crown ether component
onto the PMI unit. The successive PMI0/- reduction process
increases the electron density such that donor-acceptor interactions
become ineffective, possibly resulting in non site-specific shuffling
of the crown ether component. In the case of [2]catenanes 72+ and
Figure 10. Co-conformation changes associated with the four reduction
processes for [2]catenanes 62+-82+. Horizontal and vertical processes
represent the BPY-centered and PMI-centered reductions, respectively. In
the upper right part of the scheme, red arrows refer to the behavior of 62+
,
while green arrows describe the behavior of 72+ and 82+. The dotted gray
ellipses indicate that the interactions are turned off and there is little
information about the macrocycle location.
8
2+, the presence of more electron rich donors in the crown ether
Our data are not sufficient to examine the positioning of the PMI-
unit relatively to the crown ether ring. It is reasonable to consider,
however, that the reduction of the PMI unit renders itself sufficiently
electron-rich such that the stabilizing CT interactions are essentially
turned off, and the crown ether component has no translational
preference over either BPY0 or PMI-.
components further shifts the reduction potential of the BPY+
radical cation such that it overlaps with the first reduction potential
of the PMI unit. After this two-electron reduction, it is likely that
the CT interactions are turned off, resulting in non site-specific co-
conformational changes. It is notable that the site-specific co-
conformational change associated with translocation of the crown
ether component can be resolved in [2]catenane 62+, while in the
case of 72+ and 82+ the coincidence of the BPY+/0 and PMI0/- redox
processes prevents an unambiguous co-conformational assignment.
[2]Catenanes Containing the NDI Unit. [2]Catenanes 92+ and
102+ (Figure 11) show two irreversible oxidative processes, assigned
to the oxidation of the two HQ or DNP units of the crown ether
component involved in CT interactions with the electron accepting
cyclophane. The behavior of [2]catenane 112+ upon oxidation
(Figure 11) is very similar to that exhibited by 82+, which indicates
that the TTF0/+ oxidation causes the circumrotation of the crown
ether component with respect to the cyclophane to position the DNP
moiety into its cavity.
The cyclic voltammogram obtained upon reduction of 72+
consists of three reversible waves (Figure 8). Spectroelectrochemical
experiments confirm that the first one (-0.54 V), which is
monoelectronic, concerns the BPY2+/+ process. A comparison with
the behavior of [2]catenane 164+ indicates15c that the BPY2+ unit
occupies the inside position, as expected on the basis of the stable
co-conformation of 72+. The second reduction wave (-0.95 V)
involves the exchange of two electrons, which can be unambigu-
ously assigned by spectroelectrochemistry to the overlapping BPY+/0
and PMI0/-1 processes.35 A comparison with the behavior of
[2]catenanes 164+ and 18 indicates that the BPY+ and PMI units
are inside and alongside, respectively. The third process (-1.50
V) is monoelectronic and corresponds to the PMI-/2- reduction.
The behavior of 82+ upon reduction (Figures 8 and 9) is
qualitatively similar to that of 72+. Three reversible processes are
observed. The first one (-0.50 V) is attributed to the first reduction
of the inside BPY2+ unit as evidenced by spectroelectrochemical
The reduction pattern for all three [2]catenanes comprises four
reversible monoelectronic processes (Figure 11). With the help of
spectroelectrochemistry and by comparison with the model com-
pounds, such processes can be assigned to BPY2+/+, NDI0/-, BPY+/
0, and NDI-/2-, respectively, in the order of increasing reduction
potential. The proposed co-conformational changes coupled with
the four one-electron reduction processes for these [2]catenanes
are also very similar (Figure 12). Taking 92+ as the example, the
electron-rich component 3 remains around the BPY+ radical cation
after the first reduction. The second reduction (-0.67 V), which is
negatively shifted by 90 mV with respect to model 13, is indicated
by spectroelectrochemical experiment to be the NDI0/- process.35
The experiments on the model [2]catenane 19 show that in CH2Cl2
the alongside and inside NDI units become harder to reduce by
100 and 380 mV, respectively, with respect to the NDI model
compound.16b Hence, even considering that the HQ unit is a weaker
donor than DNP, the potential value found for the NDI0/- process
in 92+ is more consistent with the reduction of the NDI moiety in
the alongside position. Accordingly, the crown ether component
still encircles the monoreduced BPY+ unit. This interpretation is
measurements (Figure 9c) and by comparison with the potential
15c
values found for 174+
.
The second process (-0.92 V) is
bielectronic and, for the same reasons discussed for 72+, is ascribed
to the simultaneous reduction of the inside BPY+ and alongside
PMI units (Figure 9c). The third process (-1.41 V) is monoelec-
tronic and is assigned to the PMI-/2- reduction. Overall, [2]catenane
82+ can be switched reversibly among as many as six states by
both oxidation and reduction processes, but only the former ones
can cause site-specific co-conformational changes.
The co-conformation changes coupled with the four-electron
reduction processes for the PMI-containing [2]catenanes are sum-
marized in Figure 10. In all cases, the electron rich crown ether
(35) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski,
M. R. J. Phys. Chem. A 2000, 104, 6545–6551.
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A R T I C L E S
Cao et al.
species are characterized by a prevailing CT interaction, which takes
place between the best donor and acceptor units. The most
interesting observation, however, is that the shift in the redox
potentials exhibited by the [2]catenanes containing crown ether 5
is smaller than that shown by the [2]catenanes of macrocycle 4, in
spite of the larger electronic coupling of the BPY2+ unit with TTF
compared with DNP. Presumably, other factors besides the CT
interaction (e.g., steric and conformational effects) affect the redox
potentials of the electroactive units in the [2]catenanes.
Conclusion
Wehavesynthesizedaseriesofdesymmetrizeddonor-acceptor
[2]catenanes where different donor and acceptor units are
assembled within a confined catenated geometry. Remarkable
translational selectivity is maintained in all cases, including two
fully desymmetrized [2]catenanes where both donors and
1
acceptors are different, as revealed by H NMR spectroscopy
and X-ray single crystal structures. It is consistent that in all
desymmetrized [2]catenanes the co-conformation is dominated
by the strongest donor and acceptor pairs, whose charge-transfer
interactions also determine the visible absorption properties The
co-conformations can be switched by chemically or electro-
chemically altering the redox state of these donors and acceptors.
As shown by electrochemical studies, the uptake of electrons
frustrates the π-accepting properties of the bipyridinium and
bis-imide moieties. The inside/alongside topological preference,
dominated by the intercomponent CT interactions, can be
modulated reversibly upon reduction of the electron acceptors,
or oxidation of the electron donors in the cases of fully
desymmetrized 8•2PF6 and 11•2PF6. Such a feature makes these
catenanes appealing structures for the construction of molecular
machines and, in a perspective, rotary motors. It should also be
noted that these interlocked molecules can be reversibly
switched among several states, characterized by distinct elec-
tronic and optical properties, by electrochemical stimulation in
a relatively narrow and easily accessible potential window. Such
a possibility could open interesting routes for the development
of molecular electronic devices that go beyond binary logic.
Figure 12. Co-conformational changes associated with the four reduction
processes for [2]catenanes 92+-112+. Horizontal and vertical processes
represent the BPY-centered and NDI-centered reductions, respectively. The
dotted gray ellipses indicate that the interactions are turned off and there is
little information about the macrocycle location.
further supported by the remarkably negative potential value (-0.96
V) found for the third reduction wave, that can be assigned to the
BPY+/0 process. The fourth process (-1.10 V) is the NDI-/2-
reduction. As noted previously for the PMI-based catenanes, our
results do not enable to discuss the positioning of the NDI- unit
relatively to the crown ether ring. However, one can reasonably
assume that upon reduction the NDI unit loses its electron-accepting
properties and is no longer involved in CT interactions with the
electron donor ring, which in turn should not preferentially encircle
either BPY0 or NDI-. A similar analysis applies to the reduction
of [2]catenanes 102+ and 112+. It is noteworthy that [2]catenane
112+ can be switched reversibly among as many as seven states
both by reduction (Figure 12) and oxidation processes, but only
the latter ones can cause site-specific macrocycle circumrotation.
To gain more insight into the nature of the intercomponent
interactions in these systems, a comparison between the spectro-
scopic and the electrochemical data can be attempted. Specifically,
if the dominant interaction between the molecular units is the
charge-transfer one, it may be expected to observe a correlation
between the electronic coupling elements estimated from the CT
absorption bands (HDA, Table 2) and the shifts in the potentials for
the first reduction and oxidation processes experienced by the
Acknowledgment. Dedicated to Professor Jean-Pierre Sauvage,
pioneer of molecular catenation, for his 65th birthday. D. Cao, L.
Klivansky, G. Koshkakaryan, and Y. Liu and the materials synthesis
were supported by the Office of Science, Office of Basic Energy
Sciences, of the U.S. Department of Energy under contract No.
DE-AC02-05 CH11231, M. Amelia, M. Semeraro, S. Silvi, M.
Venturi, and A. Credi and the detailed characterization were
supported by Fondazione Cassa di Risparmio in Bologna, Italy.
Supporting Information Available: Experimental details and
characterization data of all new compounds. Crystallographic
information files (CIF) of 8•2PF6, 9•2PF6 and 11•2PF6. Variable
molecular components when they are mutually interlocked (|∆Ered
|
+ |∆Eox|, Table 3). It should be noticed, however, that the solvation
effects associated with the CT optical transition are different from
those associated with the individual (reduction or oxidation)
electrochemical processes.
1
1
temperature H NMR spectra of the [2]catenanes. H NMR
spectra showing the chemical switching process of 8•2PF6. This
material is available free of charge via the Internet at http://
pubs.acs.org.
First of all, the values of HDA and potential shift for the interaction
involving a given donor-acceptor pair do not depend on the specific
[2]catenane structure (for example, 82+ or 112+ in the case of the
TTF-BPY2+ interaction). This result supports the idea that these
JA909041G
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1122 J. AM. CHEM. SOC. VOL. 132, NO. 3, 2010