Acid-base controllable molecular shuttles

Article abstract of DOI:10.1021/ja982167m

Two novel [2]rotaxanes, comprised of a dibenzo[24]crown-8 (DB24C8) macroring bound mechanically to a chemical 'dumbbell' possessing two different recognition sites - viz., secondary dialkylammonium (NH₂⁺) and 4,4'-bipyridinium (Bpym²⁺) units - have been synthesized by using the supramolecular assistance to synthesis provided by, inter alia, hydrogen bonding interactions. One of these rotaxanes bears a fluorescent and redox- active anthracene (Anth) stopper unit. NMR spectroscopy and X-ray crystallography have demonstrated that the DB24C8 macroring exhibits complete selectivity for the NH₂⁺ recognition sites, i.e., that the [2]rotaxanes exist as only one of two possible translational isomers. Deprotonation of the rotaxanes' NH₂⁺ centers effects a quantitative displacement of the DB24C8 macroring to the Bpym²⁺ recognition site, an outcome that can be reversed by acid treatment. The switching processes have been investigated by ¹H NMR spectroscopy and, for the Anth-bearing rotaxane, by electrochemical and photophysical measurements. Furthermore, it is possible to drive the DB24C8 macroring from the dumbbell's Bpym²⁺ unit, in the deprotonated form of the Anth-bearing rotaxane, by destroying the stabilizing DB24C8-Bpym²⁺ charge- transfer interactions via electrochemical reduction. The photochemical and photophysical properties of this rotaxane (in both its protonated and deprotonated states) have also been investigated.

Full text of DOI:10.1021/ja982167m

11932
J. Am. Chem. Soc. 1998, 120, 11932-11942
Acid-Base Controllable Molecular Shuttles†
Peter R. Ashton,‡ Roberto Ballardini,§ Vincenzo Balzani,*,# Ian Baxter, Alberto Credi,#
Matthew C. T. Fyfe,‡ Maria Teresa Gandolfi,# Marcos Go´mez-Lo´pez,‡
M.-Victoria Mart´ınez-D´ıaz,‡ Arianna Piersanti,‡ Neil Spencer,‡ J. Fraser Stoddart,*,‡,¶
Margherita Venturi,# Andrew J. P. White, and David J. Williams*,
Contribution from The School of Chemistry, The UniVersity of Birmingham, Edgbaston, Birmingham
B15 2TT, UK, Istituto FRAE-CNR, Via Gobetti 101, 40129, Bologna, Italy, Dipartimento di Chimica
“G. Ciamician” dell’UniVersita` di Bologna, Via Selmi 2, 40126, Bologna, Italy, and The Department of
Chemistry, Imperial College, South Kensington, London SW7 2AY, UK
ReceiVed June 22, 1998
Abstract: Two novel [2]rotaxanes, comprised of a dibenzo[24]crown-8 (DB24C8) macroring bound mechani-
cally to a chemical “dumbbell” possessing two different recognition sitessviz., secondary dialkylammonium
(NH2+) and 4,4′-bipyridinium (Bpym2+) unitsshave been synthesized by using the supramolecular assistance
to synthesis provided by, inter alia, hydrogen bonding interactions. One of these rotaxanes bears a fluorescent
and redox-active anthracene (Anth) stopper unit. NMR spectroscopy and X-ray crystallography have
+
demonstrated that the DB24C8 macroring exhibits complete selectivity for the NH2 recognition sites, i.e.,
that the [2]rotaxanes exist as only one of two possible translational isomers. Deprotonation of the rotaxanes’
NH2+ centers effects a quantitative displacement of the DB24C8 macroring to the Bpym2+ recognition site, an
outcome that can be reversed by acid treatment. The switching processes have been investigated by 1H NMR
spectroscopy and, for the Anth-bearing rotaxane, by electrochemical and photophysical measurements.
Furthermore, it is possible to drive the DB24C8 macroring from the dumbbell’s Bpym2+ unit, in the deprotonated
form of the Anth-bearing rotaxane, by destroying the stabilizing DB24C8-Bpym2+ charge-transfer interactions
via electrochemical reduction. The photochemical and photophysical properties of this rotaxane (in both its
protonated and deprotonated states) have also been investigated.
Introduction
of these parts can be induced by chemical6 or electrical7 energies,
as well as by light,8 and can be monitored by a variety of
techniques, such as NMR, absorption and emission spec-
troscopies, along with electrochemistry.
Molecular information processing, which exploits the switch-
ing of molecular and supramolecular systems between clearly
identifiable states, is a topic of rapidly increasing interest.1 Aside
from the continuing attention being paid to photochemically
active and electrochromic compounds,2,3 along with chemical
sensors,4 much focus is being centered on molecular and
supramolecular systems in which the relative positions of their
component parts can be altered by an external stimulus.
Appealing examples of chemical species that have mechanically
movable component parts are suitably designed catenanes,
rotaxanes, and pseudorotaxanes.5 The relative displacements
Rotaxanes are fashioned from dumbbell-shaped and macro-
cyclic components that are bound to one another mechanically.
In the past few years, a large number of these molecular species,
based on bipyridinium-type (Bpym2+) π-electron deficient and
aromatic π-electron rich units, have been synthesized, using the
supramolecular assistance to synthesis provided by a variety of
(3) (a) Kawai, S. H.; Gilat, S. L.; Posinet, R.; Lehn, J.-M. Chem. Eur. J.
1995, 1, 285-293. (b) Matsushima, R.; Mizuno, H.; Itoh, H. Photochem.
Photobiol. A Chem. 1995, 89, 251-256. (c) Tsivgoulis, G. M.; Lehn, J.-
M. Chem. Eur. J. 1996, 2, 1399-1406. (d) Pina, F.; Melo, M. J.; Maestri,
M.; Ballardini, R.; Balzani, V. J. Am. Chem. Soc. 1997, 119, 5556-5561.
(e) Kollmannsberger, M.; Gareis, T.; Heinl, S.; Breu, J.; Daub. J. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1333-1335. (f) Asakawa, M.; Ashton, P.
R.; Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Montalti, M.;
Spencer, N.; Stoddart, J. F.; Venturi, M. Chem. Eur. J. 1997, 3, 1992-
1996.
(4) (a) Fluorescent Chemosensors for Ion and Molecule Recognition;
ACS Symp. Ser. No. 538; Czarnik, A. W., Ed.; American Chemical
Society: Washington, DC, 1992. (b) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. ReV. 1997, 97, 1515-1566.
(5) (a) Benniston, A. C. Chem. Soc. ReV. 1996, 25, 427-435. (b) Balzani,
V.; Go´mez-Lo´pez, M.; Stoddart, J. F. Acc. Chem. Res. 1998, 31, 405-
414. (c) Sauvage, J.-P. Acc. Chem. Res. 1998, 31, 611-619.
(6) Recent prominent articles: (a) Bissell, R. A.; Co´rdova, E.; Kaifer,
A. E.; Stoddart, J. F. Nature 1994, 369, 133-137. (b) Amabilino, D. B.;
Dietrich-Buchecker, C. O.; Livoreil, A.; Pe´rez-Garc´ıa, L.; Sauvage, J.-P.;
Stoddart, J. F. J. Am. Chem. Soc. 1996, 118, 3905-3913. (c) Credi, A.;
Balzani, V.; Langford, S. J.; Stoddart, J. F. J. Am. Chem. Soc. 1997, 119,
2679-2681.
† Molecular Meccano, Part 46. For Part 45, see: Bravo, J. A.; Raymo,
F. M.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. Eur. J. Org. Chem.
1998, 2565-2571.
‡ University of Birmingham.
§ Istituto FRAE-CNR.
# Universita` di Bologna.
Imperial College.
¶ Current address: Department of Chemistry and Biochemistry, Univer-
sity of California, Los Angeles, CA 90095.
(1) (a) Molecular Electronic DeVices; Carter, F. L., Siatkowsky, R. E.,
Woltjien, H., Eds.; Elsevier: Amsterdam, 1988. (b) Kuhn, H. In Molecular
Electronics; Lazarev, P. I., Ed.; Kluwer: Dordrecht, 1991; Chapter 12. (c)
Lehn, J.-M. Supramolecular ChemistrysConcepts and PerspectiVes;
VCH: Weinheim, 1995. (d) Verdaguer, M. Science 1996, 272, 698-699.
(e) Goldhaber-Gordon, D.; Montemerlo, M. S.; Love, J. C.; Opiteck, G. J.;
Ellenbogen, J. C. Proc. IEEE 1997, 85, 521-540. (f) Astruc, D. Acc. Chem.
Res. 1997, 30, 383-391.
(2) For reviews, see: (a) PhotochromismsMolecules and Systems; Du¨rr,
H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990. (b) Feringa, L.
B.; Jager, W. F.; de Lange, B. Tetrahedron 1993, 49, 8267-8310. (c)
Balzani, V.; Scandola, F. In ComprehensiVe Supramolecular Chemistry;
Reinhoudt, D. N., Ed.; Pergamon: Oxford, 1996; Vol. 10, pp 687-746.
10.1021/ja982167m CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/10/1998

Acid-Base Controllable Molecular Shuttles
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11933
Figure 1. (a) Cartoon showing an acid-base controllable molecular shuttle. Initially (State 0), the macrocyclic component resides solely on the
NH2+ station. After treatment with a nonnucleophilic base, deprotonation coerces the macroring to move to the Bpym2+ station (State 1). Addition
of acid regenerates the NH2+ center, moving the macrocyclic component back to bind its original station and rendering the switching cycle reversible.
(b) Structural formulas of the switchable [2]rotaxanes described here.
noncovalent interactions.9 More recently, the formation of
pseudorotaxane inclusion complexes between secondary dialkyl-
ammonium ions (e.g., the dibenzylammonium ion (DBA+)) and
macrocyclic polyethers (e.g., dibenzo[24]crown-8 (DB24C8) and
bis-p-phenylene[34]crown-10 (BPP34C10)) as a result of, inter
alia, [N+-H‚‚‚O] and [C-H‚‚‚O] hydrogen bonds has been
reported.10,11 Here, we describe12 the novel switchable [2]-
rotaxanes 1a-H3+ and 1b-H3+ (Figure 1), based upon a
dumbbell-shaped componentspossessing both secondary di-
alkylammonium (NH2+) and Bpym2+ units, i.e., two different
recognition sitessthat is threaded through the DB24C8 mac-
roring. In rotaxanes of this type, it is possible to switch the
macroring’s position between the two different recognition sites
by the action of external stimuli. In [2]rotaxane 1b-H3+, we
have used an anthracene (Anth) moiety as a stopper, as we
thought that its absorption, luminescence, and redox properties
would be useful for (1) monitoring the system’s state and (2)
propelling the macroring by photochemical and electrochemical
stimulation.
(7) For recent leading papers, see refs 3f, 6a, and the following: (a)
Livoreil, A.; Dietrich-Buchecker, C. O.; Sauvage, J.-P. J. Am. Chem. Soc.
1994, 116, 9399-9400. (b) Ashton, P. R.; Ballardini, R.; Balzani, V.; Credi,
A.; Gandolfi, M. T.; Menzer, S.; Pe´rez-Garc´ıa, L.; Prodi, L.; Stoddart, J.
F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1995,
117, 11171-11197. (c) Ashton, P. R.; Ballardini, R.; Balzani, V.; Boyd, S.
E.; Credi, A.; Gandolfi, M. T.; Go´mez-Lo´pez, M.; Iqbal, S.; Philp, D.;
Preece, J. A.; Prodi, L.; Ricketts, H. G.; Stoddart, J. F.; Tolley, M. S.;
Venturi, M.; White, A. J. P.; Williams, D. J. Chem Eur. J. 1997, 3, 152-
170. (d) Livoreil, A.; Sauvage, J.-P.; Armaroli, N.; Balzani, V.; Flamigni,
L.; Ventura, B. J. Am. Chem. Soc. 1997, 119, 12114-12124. (e) Asakawa,
M.; Ashton, P. R.; Balzani, V.; Credi, A.; Hamers, C.; Mattersteig, G.;
Montalti, M.; Shipway, A. N.; Spencer, N.; Stoddart, J. F.; Tolley, M. S.;
Venturi, M.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1998,
37, 333-337.
(8) For recent leading papers, see refs 7c, 7d, and the following: (a)
Bauer, M.; Mu¨ller, W. M.; Mu¨ller, U.; Rissanen, K.; Vo¨gtle, F. Liebigs
Ann. 1995, 649-656. (b) Benniston, A. C.; Harriman, A.; Lynch, V. M. J.
Am. Chem. Soc. 1995, 117, 5275-5291.
(9) (a) Amabilino, D. B.; Stoddart, J. F. Chem. ReV. 1995, 95, 2725-
2828. (b) Philp, D.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1996, 35,
1154-1196.
For both 1a-H3+ and 1b-H3+, the DB24C8 macroring should
+
exhibit complete selectivity for the NH2 recognition site so
that the [2]rotaxanes exist as only one of two possible
+
translational isomers. Deprotonation of the NH2 center can
be expected to induce the displacement of the macroring to the
Bpym2+ station, a displacement that should be reversed (Figure
1) upon reprotonation. These switching processes have been
investigated, for both 1a-H3+ and 1b-H3+, by 1H NMR
spectroscopy and, in the case of 1b-H3+, by electrochemical
and photophysical measurements. We also conjectured that it
should be possible to displace the crown ether from the Bpym2+
station by destroying the catechol-Bpym2+ charge transfer (CT)
interactions in the deprotonated [2]rotaxanes 1a2+ and 1b2+
through either electrochemical (1) reduction of the Bpym2+
station or (2) oxidation of the DB24C8 macroring. The
photochemical and photophysical properties of 1b2+ have also
been studied because of the presence of the potentially photo-
reactive and fluorescent Anth unit.
(10) (a) Kolchinski, A. G.; Busch, D. H.; Alcock, N. W. J. Chem. Soc.,
Chem. Commun. 1995, 1289-1291. (b) Ashton, P. R.; Chrystal, E. J. T.;
Glink, P. T.; Menzer, S.; Schiavo, C.; Spencer, N.; Stoddart, J. F.; Tasker,
P. A.; White, A. J. P.; Williams, D. J. Chem. Eur. J. 1996, 2, 709-728. (c)
Montalti, M.; Ballardini, R.; Prodi, L.; Balzani, V. Chem. Commun. 1996,
2011-2012. (d) Ashton, P. R.; Ballardini, R.; Balzani, V.; Go´mez-Lo´pez,
M.; Lawrence, S. E.; Mart´ınez-D´ıaz, M.-V.; Montalti, M.; Piersanti, A.;
Prodi, L.; Stoddart, J. F.; Williams, D. J. J. Am. Chem. Soc. 1997, 119,
10641-10651.
Results and Discussion
Synthesis.13 Since DB24C8 shows14 very little binding
affinity toward Bpym2+ dications and is able10 to form pseu-
(12) Preliminary communication on part of this research: Mart´ınez-D´ıaz,
M.-V.; Spencer, N.; Stoddart, J. F. Angew. Chem., Int. Ed. Engl. 1997, 36,
1904-1907.
(13) Full experimental procedures for all synthetic steps can be found
in the Supporting Information.
(11) The papers listed in ref 10 describe systems that can undergo acid-
base driven threading/dethreading of amines from crown ethers and can be
viewed, therefore, as chemically driven machines.

11934 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Ashton et al.
Scheme 1. Synthetic Protocol Employed To Prepare
1a-H‚3PF6 and 1b-H‚3PF6 (X ) Cl for 2a-H‚PF6 and Br for
2b-H‚PF6)a
Figure 2. The solid-state superstructure of the [2]pseudorotaxane
[DB24C8‚3b-H]2+. Hydrogen bonding distances and angles {[X‚‚‚O],
[H‚‚‚O] distances (Å), [X-H‚‚‚O] angles (deg)}: (a) 3.03, 2.14, 166;
(b) 3.03, 2.14, 164; (c) 3.28, 2.36, 161. [C-H‚‚‚π] interaction: (d)
[H‚‚‚π] 2.85 Å, [H‚‚‚π] vector inclined by 74° to the p-Xy ring plane.
pared by reacting either 3a-H‚2PF6 or 3b-H‚2PF6 with 4 in the
absence of DB24C8.
a Yields (%) for compounds in a series [b series]: Step 1, 84 [60];
Step 2, 66 [25]; Step 3, 38 [30].
dorotaxanes with NH2+-containing salts, the [2]rotaxanes 1a-
H
3+ and 1b-H3+ seemed to be appropriate targets for switchable
compounds. They were prepared using an approach that relies15-
on the initial solution-phase formation of pseudorotaxanes
between DB24C8 and rodlike components (which already
+
contain an NH2 center and one of the stoppers that prevents
macroring loss in the final [2]rotaxanes) followed by stoppering
reactions that lead to simultaneous Bpym2+/dumbbell formation.
The rotaxanes’ rodlike precursorssviz., 3a-H‚2PF6 and 3b-
H‚2PF6swere prepared by using the route shown in Scheme 1.
Since these compounds are only partially soluble in CHCl3 (an
ideal solvent for studying hydrogen bonding interactions10b),
the binding constants (Ka) of their DB24C8 complexes were
X-ray Crystallography. The X-ray analysis of [DB24C8‚
3b-H][PF6]2 shows the dication to be threaded through DB24C8’s
center to generate (Figure 2) a pseudorotaxane with a very
similar10b geometry to that of closely related complexes. The
Anth-CH2NH2+CH2-p-Xy (p-Xy ) p-xylyl) portion of the
backbone adopts a near planar, all-anti geometry with mean
torsional twists about the C-Anth and C-p-Xy bonds of 85
and 14°, respectively. The pyridinium (Pym+) ring is aligned
almost orthogonally with the p-Xy ring, the associated C-C
and C-N+ torsional angles being 82 and 21°, respectively. The
twist angle about the bond linking the pyridyl and Pym+ rings
is ca. 27°. The DB24C8 macrocycle adopts a V-shaped
conformation, its two catechol rings being inclined by ca. 79°
to each other. Complexation is achieved by [N+-H‚‚‚O] and
[C-H‚‚‚O] hydrogen bonds, as well as by [C-H‚‚‚π] and π-π
stacking interactions that occur between the dication’s Anth unit
and the DB24C8 catechol ring positioned over its center (mean
interplanar separation 3.43 Å, centroid‚‚‚centroid distance 3.63
Å, ring systems inclined by ca. 6°).
1
measured by H NMR spectroscopy in CDCl3-CD3CN mix-
tures. By applying the single-point method,10b similar Ka values
(ca. 2300 M-1) were calculated for the 1:1 complexes
[DB24C8‚3a-H][PF6]2 and [DB24C8‚3b-H][PF6]2 in CDCl3-
CD3CN (1:1).16 These relatively strong binding constants
allowed the production of the [2]rotaxanes in reasonable yields
when the appropriate precursors were reacted with 3,5-di-tert-
butylbenzyl bromide (4) in the presence of an excess of
DB24C8. The synthesis of 1b-H‚3PF6 was also performed by
an alternative route, specifically, via reaction of 2b-H‚PF6 with
5‚PF6 in the presence of DB24C8. Both of the [2]rotaxanes
prepared survive chromatography, indicating that both the Anth
and 3,5-di-tert-butylphenyl (Dtbp) stoppers are large enough
to ensure macroring-dumbbell interlocking. The free dumb-
bell-shaped compounds 6a-H‚3PF6 and 6b-H‚3PF6 were pre-
(14) Ashton, P. R.; Glink, P. T.; Mart´ınez-D´ıaz, M.-V.; Stoddart, J. F.;
White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1996, 35,
1930-1933.
(15) Ashton, P. R.; Glink, P. T.; Stoddart, J. F.; Tasker, P. A.; White,
A. J. P.; Williams, D. J. Chem. Eur. J. 1996, 2, 729-736.
(16) A Ka of 5120 M-1 was calculated for the complex [DB24C8‚2a-
H][PF6] in CDCl3-CD3CN (4:1) by the same method.
An inspection of the packing of the [2]pseudorotaxanes
reveals (Figure 3) the presence of helical π-π stacked chains
of 1:1 complexes, the Pym+ ring of one complex being stacked
with the Anth unit of its 21 screw-related neighbor. The Pym+
ring of one dication is inclined by ca. 7° to the Anth plane of

Acid-Base Controllable Molecular Shuttles
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11935
Figure 5. Part of the π-stacked supramolecular array composed of
pairs of independent lattice-translated 1b-H3+ molecules.
Figure 3. A segment of the π-stacked helical chains {[DB24C8‚3b-
H]2+}n.
by π-π stacking interactions between one of DB24C8’s
catechol rings and one of the trication’s Anth rings.17
The packing (Figure 5) of the [2]rotaxanes is similar to that
observed for the [2]pseudorotaxane [DB24C8‚3b-H]2+. In 1b-
H‚3PF6, however, one of the Pym+ rings of one independent
molecule overlays the Anth ring of another, a pattern that is
then repeated for lattice-translated crystallographically inde-
pendent pairs of molecules.18
The crystal data, data collection, and refinement parameters
for the structures are given in Table 1.
NMR/Mass Spectra. The 1H NMR spectrum of 1a-H‚3PF6
is consistent (Figure 6a) with [2]rotaxane formation. Its data,
along with the data for the noninterlocked dumbbell 6a-H‚3PF6,
are listed in Table 2. The spectrum demonstrates that DB24C8
binds selectively with the NH2+ center, since the characteristic10b
+
Figure 4. The crystal structure of [2]rotaxane 1b-H3+. Hydrogen
bonding distances and angles with values for the second independent
molecule in square brackets {[X‚‚‚O], [H‚‚‚O] distances (Å), [X-H‚
‚‚O] angles (deg)}: (a) 3.05, 2.16, 171 [3.12, 2.25, 163]; (b) 3.04, 2.15,
169 [3.09, 2.21, 165]; (c) 3.41, 2.50, 158 [3.22, 2.49, 133]; (d) 3.08,
2.35, 132 [3.36, 2.47, 156]. In the second independent molecule, the
[C-H‚‚‚O] hydrogen bond (d) is formed to the polyether linkage’s
third oxygen atom, rather than to the second as shown. [C-H‚‚‚π]
interaction (e) [H‚‚‚π] 2.85 Å [3.10 Å], [H‚‚‚π] vectors inclined by
81° [77°] to the p-Xy ring plane.
resonance for the four CH2 protons adjacent to a “bound” NH2
center, at δ 4.85-4.87, is observable and experiences a
downfield shift of ca. 0.2 ppm with respect to the complex
formed between DB24C8 and 6a-H‚3PF6. This selective
1
binding was also confirmed by H NMR 1D-NOE doubled-
pulsed field-gradient spin-echo experiments (GOESY).19 Se-
lective irradiation of the protons C showed that intercomponent
NOEs were detected (Figure 6b) with OCH2 protons on only
one of the crown ether’s two heterotopic faces. The same effect
was noted when the protons A were irradiated selectively.
Additionally, we would expect to see another set of signals for
a second translational isomer if the DB24C8 macroring were
the next and overlays two of this unit’s rings with centroid‚‚‚
centroid distances (to the central and outer rings, respectively)
of 3.88 and 3.79 Å and a mean interplanar separation of
3.58 Å.
+
to reside anywhere else, other than on the NH2 center, as the
movement of the crown ether over the thread’s benzenoid units
is anticipated10b to be slow on the H NMR time scale. Since
1
The solid-state structure of 1b-H‚3PF6 reveals (Figure 4) that
the addition of the Dtbp stopper, to create the [2]rotaxane,
produces little change in the remainder of the system’s geometry.
The structure contains two crystallographically independent [2]-
rotaxane molecules in the asymmetric unit, but with only slight
geometrical differences (the parameters of the second indepen-
dent molecule are given in square brackets). The geometry of
the Anth-CH2NH2+CH2-p-Xy portion of the backbone is once
again all-anti with the Anth unit oriented approximately
orthogonally (ca. 78° [86°]); the p-Xy ring is much less steeply
inclined with C-p-Xy torsional angles of 31° [41°]. The p-Xy
and nearby Pym+ rings adopt an “open-book” conformation,
the torsion angles about the adjoining C-C and C-N+ bonds
being 76 and 72° [74 and 64°], respectively. The mean torsional
twist around the bond linking the two Pym+ rings is 30° [26°].
The Dtbp terminus assumes a skewed geometry with respect to
the proximal Pym+ ring, the respective torsion angles about the
connecting N+-C and C-C bonds being 90 and 51° [89 and
79°]. The DB24C8 macroring has a V-shaped conformation,
the planes of the catechol rings being inclined by 69° [60°], in
a slightly more closed geometry than that observed for the [2]-
pseudorotaxane [DB24C8‚3b-H]2+. The [2]rotaxane molecules
are stabilized by the same combination of [N+-H‚‚‚O], [C-H‚
‚‚O] hydrogen bonds and [C-H‚‚‚π] interactions, as well as
no new sets of resonances, for any additional species, could be
detected in the low temperature 1H NMR spectrum (400 MHz,
CD3COCD3, 193 K), it was concluded that the DB24C8
+
macroring is bound to the NH2 center with at least 98%
selectivity from 193 to 304 K. A detailed analysis of the
chemical shift differences experienced by protons C and D
indicates that the crown ether’s catechol rings are probably, to
some extent, sandwiching the p-Xy spacer so as to benefit from
supplementary stabilization by means of weak π-π stacking
interactions.
The 1H NMR spectrum of the [2]rotaxane 1b-H‚3PF6 shows
that the dumbbell and ring components are present in equimolar
(17) The catechol ring is positioned over the central ring of the Anth
unit; it is inclined by only ca. 2° [2°], with a ring centroid‚‚‚ring centroid
distance of 3.60 Å [3.50 Å] and a mean interplanar separation of 3.42 Å
[3.37 Å].
(18) In one of the two independent π-π stacking interactions, the Pym+
ring is positioned more above the Anth unit’s central ring with an interplanar
separation of 3.60 Å and centroid‚‚‚centroid distances (to the central and
outer rings, respectively) of 3.69 and 4.08 Å, the Pym+ and Anth rings
being inclined by ca. 12°. For the other π-π stacking interaction, the
interacting Pym+ ring is inclined by 8° and is centered more toward one of
the Anth unit’s outer rings. The associated ring centroid‚‚‚ring centroid
separations are 3.67 (to the outer ring) and 3.83 Å (to the central ring), the
mean interplanar separation being 3.44 Å.
(19) Stonehouse, J.; Adell, P.; Keeler, J.; Shaka, A. J. J. Am. Chem.
Soc. 1994, 116, 6037-6038.

11936 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Ashton et al.
Table 2. 1H NMR Data (400 MHz, CD3COCD3, 304 K)a,b
1a-H‚3PF6 1a‚2PF6 6a-H‚3PF6 6a‚2PF6
Table 1. Crystal Data, Data Collection, and Refinement
Parametersa
[DB24C8‚3b-H][PF6]2
1b-H‚3PF6
HR′
HR
Hâ′
Hâ
A′
B′
A
B
C
D
9.56
9.23
8.79-8.80 8.96
8.79-8.80 8.77
7.61
7.61
7.46
7.51
7.41
7.21
9.44
9.29
9.53
9.42
8.78
8.76
7.61
7.61
7.43
7.56
7.74
7.70
6.20
6.14
4.70
4.63
9.49 (br)
9.49 (br)
8.78 (br)
8.78 (br)
7.61
7.61
7.22
7.34
7.61
7.52
formula
solvent
C57H61N3O8‚2PF6
MeCN
C72H84N3O8‚3PF6
4Me2CO
formula weight
color, habit
crystal size (mm)
lattice type
space group symbol, no. P21/c, 14
cell dimensions
1247.1
yellow needles
1786.6
orange/red rhombs
7.63
7.63
7.22
7.34
7.37
7.37
5.96
5.79
0.50 × 0.13 × 0.12 0.73 × 0.70 × 0.53
monoclinic
triclinic
P1h, 2
a (Å)
b (Å)
c (Å)
11.551(1)
26.371(1)
18.911(1)
20.138(3)
21.696(5)
23.916(4)
115.28(2)
101.73(2)
97.52(2)
8967(3)
4b
1.323
3744
1.45
2.1-57.5
Bpym2+-CH2-Dtbp 6.14
6.13 (br)
6.13 (br)
3.82
Bpym2+-CH2-p-Xy 5.88
NH2+-CH2-Dtbp
NH2+-CH2-p-Xy
C(CH3)3
4.85-4.87 3.73
4.85-4.87 3.73
R (deg)
â (deg)
γ (deg)
97.31(1)
3.75
1.23, 1.30 1.30, 1.33 1.28, 1.30 1.30, 1.31
6.83-6.86 6.74-6.81
V (Å3)
5713.8(5)
4
1.450
2592
1.56
2.9-60.0
catechol
OCH2
Z
3.58-4.22 3.09-4.04
Dc (g cm-3
F(000)
)
a The proton descriptors are explained in Figure 6. b All chemical
shift values are reported on the δ scale.
µ (mm-1
)
θ range (deg)
no. of unique reflcns
measd
8387
21069
obsd, |Fo| > 4σ(|Fo|)
c
5771
768
0.105
0.263
0.217, 1.242
0.57, -0.70
12294
2094
0.101
0.252
0.164, 25.900
0.85, -0.67
no. of variables
R1
d
wR2
weighting factors a, be
largest diff peak,
hole (eÅ-3
)
a Details in common: graphite monochromated Cu KR radiation,
ω-scans, Siemens P4/RA diffractometer, 203 K, refinement based on
F2. b There are two crystallographically independent molecules in the
asymmetric unit. c R1 ) ∑||Fo| - |Fc||/∑|Fo|. d wR2 ) {∑[w(Fo
-
2
Fc2)2]/∑[w(Fo )2]}1/2
.
e w-1 ) σ2(Fo ) + (aP)2 + bP.
2
2
Figure 7. Logical progression from [2]rotaxane 1b-H3+, through its
constituents, to model compounds for the chromophoric and redox-
active units of dumbbell 6b-H3+
.
+
the complexed NH2 ion, it may be presumed that the crown
ether resides exclusively on this site, at least on the NMR time
scale.
LSI mass spectroscopy also provides evidence for the
formation of the [2]rotaxanes. Base peaks were observed, at
m/z 1420 and 1408, corresponding to the species [1a-H‚2PF6]+
and [1b-H‚2PF6]+.
Photophysical, Photochemical, and Electrochemical Prop-
erties. The photophysical, photochemical and electrochemical
investigations have been performed on 1b-H3+ because it
possesses an additional chromophoric and electroactive units
namely, the Anth stoppersin comparison to 1a-H3+
. To
elucidate the behavior of a multicomponent species, reference
has to be made to the properties of its molecular components
and of suitable model compounds for the single chromophoric
and electroactive units. Suitable model compounds for the
1
Figure 6. (a) H NMR spectrum (400 MHz, CD3COCD3, 304 K) of
1a-H‚3PF6 and (b) the GOESY spectrum following irradiation of the
protons C.
+
Anth-bearing NH2 center and Bpym2+ unit of the dumbbell
6b-H3+ are (Figure 7) the 9-(benzylammoniummethyl)an-
thracene cation (7-H+) and the dibenzylviologen dication
(DBV2+), respectively.
amounts and are also interlocked with one another. The
DB24C8 macrocycle binds selectively with the NH2 center,
+
the characteristic resonances for the four CH2 protons adjacent
to this binding site experiencing a downfield shift of ca. 0.5
ppm upon rotaxane formation. As signals are observed only for
(a) Absorption and Emission Spectra. Figure 8 shows the
absorption spectra of the dumbbell component 6b-H3+ and the
model compounds, viz., 7-H+ and DBV2+, for its chromophoric

Acid-Base Controllable Molecular Shuttles
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11937
Figure 8. Absorption spectra (MeCN, 293 K) for 6b-H3+ (solid line)
and its chromophoric units, viz., 7-H+ (dashed line) and DBV2+ (dotted
line). Inset: fluorescence spectra of 6b-H3+ and 7-H+ upon excitation
in the low energy Anth band.
Figure 9. Absorption and (inset) fluorescence spectra (MeCN, 293
K) of 1b-H3+ and its components, viz., DB24C8 and 6b-H3+. Excitation
was performed in the low-energy Anth band for both 1b-H3+ and 6b-
H
3+, as well as at 277 nm for DB24C8.
Table 3. Fluorescence and Photochemical Dataa
excited state (energy ca. 4 eV) can be ascribed to energy transfer
to the nearby Anth moiety, as previously observed10c in a related
system, and/or to strongly exoergonic electron transfer to the
Bpym2+ unit.21 The fluorescent behavior of the Anth-based unit
will be discussed more fully in the section dealing with the
switching processes.
c
compd
λmaxb (nm)
Iem
Φrd
6b-H3+
1b-H3+
6b2+
417
417e
411
411e
1.0
2.1
0.5
1.1
0.8
1.0
1.8
2.6
1b2+
a Air equilibrated MeCN solutions, 293 K. b Wavelength of the
emission maximum. c Relative emission intensity upon excitation in
the low-energy anthracene band. In this scale, the relative emission
intensity of model compound 7-H+ is 170 (corresponding to a quantum
yield of 0.3). d Relative photoreaction quantum yield upon irradiation
at 365 nm. In this scale, the relative photoreaction quantum yield of
7-H+ is 12 (corresponding to a quantum yield of 6 × 10-4). e Upon
excitation at 277 nm, no emission can be seen in the 310 nm region
where DB24C8 exhibits an intense fluorescence.
(b) Photochemical Behavior. It is well-known22 that the
triplet excited state of Anth derivatives sensitizes the formation
of singlet oxygen (1∆)O2 in aerated solutions. This species
reacts with the central Anth ring to provide an 1,10-endoper-
oxide that then evolves toward other products. Such a photo-
reaction, which can be followed easily by the disappearance of
the characteristic Anth-type absorption and fluorescence bands,
is moderately efficient in the case of 7-H+. However, it is much
less efficient (Table 3) for both 6b-H3+ and 1b-H3+. This strong
photoreactivity decrease can be attributed to the quenching of
the Anth moiety’s singlet excited state by the Bpym2+ unit,
preventing formation of the Anth triplet excited state.
units. The most intense absorption features, arising from the
Anth unit, are essentially unperturbed on going from 7-H+ to
6b-H3+. In particular, it is only the Anth moiety that absorbs
in the spectral region above 350 nm, a property that allows us
to perform selective excitation of this unit when necessary. The
cation 7-H+ shows a strong and structured fluorescence band
in the visible region that is characteristic20 of the Anth unit,
whereas DBV2+ is not emissive. As shown in the inset of Figure
8, such a fluorescence band is still present in 6b-H3+, but with
(Table 3) a 170-fold reduction in intensity. This strong
quenching can be assigned to a photoinduced electron transfer
from the fluorescent excited state of the Anth moiety to the
Bpym2+ unit (vide infra).
(c) Electrochemical Behavior. The data for the electro-
chemical experiments are listed in Table 4. All the reduction
processes are fully reversible, while the oxidation processes are
not. Figure 10 shows a correlation of the potential values
obtained for 1b-H3+, its components, viz., 6b-H3+ and DB24C8,
and three reference complexes. The dumbbell 6b-H3+ exhibits
three monoelectronic reduction processes and one oxidation
process. This behavior is exactly that predicted from the
electrochemical properties associated with the electroactive units
of the model compounds DBV2+ and 7-H+. The [2]rotaxane
1b-H3+ shows three reduction processes that, by comparison
with the behavior of 6b-H3+, can be assigned straightforwardly
to the first and second one-electron reductions of the Bpym2+
unit and to the one-electron reduction of the Anth unit. The
fact that the reduction of the Bpym2+ unit takes place at the
same potentials in 6b-H3+ and 1b-H3+ provides further evidence
Figure 9 compares the absorption spectra of 1b-H3+ and its
constituents, viz., 6b-H3+ and DB24C8. Both the dominant
Anth-based absorption features, i.e., the strong band around 255
nm and the weaker structured band in the 320-400 nm region,
are perturbed. As shown in the inset, the intense crown ether
fluorescence band of the [2]rotaxane (λmax ) 308 nm),
assigned7b,21 to the lowest singlet excited state of the dioxy-
benzene units, is completely quenched. Conversely, the fluo-
rescence band of the Anth unit, which is strongly quenched in
6b-H3+ (vide supra), partially regains (Table 3) its intensity.
The quenching of the crown ether’s fluorescent dioxybenzene
+
that the DB24C8 macroring is located on the dumbbell’s NH2
station. On oxidation, 1b-H3+ shows two processes whose
assignment is less simple. From the behavior of its separate
dumbbell and macroring components, three oxidation processes
would, in fact, be expectedsone localized on the Anth unit and
two on the crown ether. Since the crown ether’s oxidation
(20) Berlman, I. B. Handbook of Fluorescence Spectra of Aromatic
Compounds; Academic: London, 1965.
(21) Anelli, P.-L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado,
M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszk-
iewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.;
Stoddart, J. F.; Vicent, C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114,
193-218.
+
processes are likely to be affected by the NH2 ion within its
(22) Turro, N. J. Modern Molecular Photochemistry; Benjamin: Menlo
Park, CA, 1978.

11938 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Table 4. Electrochemical Dataa
Ashton et al.
ance changed from clear to yellow, indicating24 the creation of
CT interactions between DB24C8’s catechol rings and the
compd
reduction
oxidationb
Bpym2+ dication. The H NMR spectrum shows (Figure 11)
1
DB24C8
c
+1.32, +1.47
that, on base addition, deprotonation of the NH2+ center occurs,
while leaving the Bpym2+ unit intact. Resonances for the CH2-
NH2+CH2 protons disappear and new signals, which can be
assigned to CH2NHCH2 protons, become visible. Moreover,
+1.32, +1.47
+1.31, +1.45
d
DBV2+
c
-0.37, -0.79
-0.37, -0.79
d
-0.37, -0.79
1
the H NMR spectrum of 1a‚2PF6 shows dramatic chemical
7-H+
7
-1.98
+1.43
shift changes compared to that of 1a-H‚3PF6, especially (Table
2) for the resonances associated with the Bpym2+ and p-Xy
protons, a feature that suggests that the DB24C8 macroring
interacts asymmetrically with the Bpym2+ unit.
-1.98
+1.09,e +1.43f
+1.41
7-H+ d
6b-H3+
6b2+
6b-H3+ d
1b-H3+
1b2+
1b-H3+ d
-1.99
-0.35,g -0.78,g -1.98f
-0.36,g -0.78,g -1.98f
-0.35,g -0.78,g -1.98f
-0.35,g -0.77,g -2.02f
-0.59,g,j -0.77,g -2.03f
-0.35,g -0.78,g -2.02f
+1.43
+1.09,e +1.43f
+1.41
1H NMR studies on the deprotonated [2]rotaxane 1a‚2PF6
(400 MHz, CD3COCD3, 193 to 304 K) reveal that the DB24C8
macroring resides exclusiVely on the Bpym2+ recognition site.
Although the spectrum broadens below 233 K, no supplementary
sets of resonances, for any other translational isomers, are
evident. Upon addition of CF3CO2H, the NH2+CH2 protons
resonate at δ 4.85-4.87, suggesting that the DB24C8 component
shuttles back completely to the NH2+ recognition site following
reprotonation. In other words, the [2]rotaxane 1a-H‚3PF6
exhibits fully reversible switching (Figure 1) of its crown ether
macroring between the dumbbell’s two stations via de-/
reprotonation.
+1.39,f +1.47h,i
+1.12,e +1.39,f +1.50h,i
+1.38,f +1.47h,i
a Ar-purged MeCN solutions, 293 K; halfwave potential values in
V vs SCE; reversible and monoelectronic processes, unless noted
otherwise. b All oxidation processes are not fully reversiblespotential
values estimated from DPV peaks. c After addition of 1 equiv of Bu3N.
d After addition of 1 equiv of Bu3N and subsequent treatment with 1
f
equiv of TfOH. e NH2+ unit. Anth unit. g Bpym2+ unit. h Crown ether.
j
i More than one electron is involved. Value obtained by simulation.
See text for details.
cavity, we have examined23 the electrochemical behavior of the
[2]pseudorotaxane model10b [DB24C8‚DBA]+. This complex
displays only one oxidation process, at +1.46 V, which, as
foreseen, is displaced toward more positive potentials with
respect to the first oxidation process of DB24C8. This
irreversible process seems to involve more than one electron
and can be assigned to the simultaneous oxidation of the crown
ether’s noninteracting dioxybenzene units. An alternative
possibility is that the second one-electron oxidation of the crown
ether occurs at the potential of the first because the first
oxidation process causes the displacement of the positively
The Anth-containing [2]rotaxane 1b-H‚3PF6 behaves simi-
larly, as deprotonation of the NH2+ center occurs when a slight
molar excess of i-Pr2NEt is added to a CD3COCD3 solution of
1b-H‚3PF6. The resonances (δ 5.45 and 5.73) for the four CH2-
NH2+CH2 protons disappear and new resonances, at δ 4.00 and
4.70, become visible for the CH2NHCH2 moieties. Furthermore,
the macrocycle moves to interact with the Bpym2+ unit, resulting
in changes to the R- and â-Bpym2+ protons’ resonances (δ [∆δ]
values for HR′, HR, Hâ′, and Hâ (ppm): 9.42 [-0.12], 9.31
[+0.05], 8.94 [+0.19], and 8.74 [-0.01]). On addition of CF3-
+
1
charged macroring away from the NH2 center. In any case,
CO2H, the original H NMR spectrum is regenerated, demon-
strating, once again, that the switching process is reversible.
the oxidation behavior of [2]pseudorotaxane [DB24C8‚DBA]+
allows us to assign the first and second oxidation processes of
the [2]rotaxane 1b-H3+ to the Anth and crown ether units,
respectively.
(b) Absorption and Emission Experiments. Addition of 1
equiv of Bu3N to MeCN solutions of the [2]rotaxane 1b-H3+
,
dumbbell 6b-H3+, or model compound 7-H+ furnishes their
deprotonated forms. In the case of 6b-H3+ and 7-H+, depro-
tonation effects only very small changes in the Anth-based
structured band (300-400 nm). Similarly, the stronger band,
centered at ca. 255 nm, shows only a small intensity decrease.
Indeed, this is the behavior expected20 for a small change in
the nature of an 9-Anth substituent. For 1b-H3+, deprotonation
is accompanied by a small blue shift and an increase in the
absorbance of both the high (Figure 12a) and low energy Anth-
based bands. This behavior indicates20 a change in the
perturbation of the Anth chromophoric group along both of its
symmetry axes and is consistent with the displacement of
DB24C8’s aromatic moieties from the Anth unit. Even from a
purely empirical viewpoint, it can be noticed that, upon
deprotonation, the intensity of the Anth absorption bands
decreases for 6b-H3+, while it increases for 1b-H3+. Conse-
quently, the spectral changes observed upon deprotonation of
The [2]Rotaxanes’ Switching Processes. We have already
established that the [2]rotaxanes’ DB24C8 component resides
+
exclusively on the NH2 recognition center. Bases that can
deprotonate this center could act as chemical inputs that promote
(Figure 1) the movement of the crown ether to the Bpym2+ unit.
The choice of base to be used was not easy, since the dumbbell’s
Bpym2+ unit is very sensitive to nucleophilic bases. It was
found that diisopropylethylamine12 (i-Pr2NEt) and tributylamine
(Bu3N) are ideal bases to drive the switching process because
+
they are strong enough to deprotonate the NH2 center while
behaving concurrently as very weak (i.e., unreactive) nucleo-
philes toward Bpym2+ units. Reprotonation can be performed
by the addition of a suitable acid (e.g., trifluoroacetic (CF3CO2H)
or triflic (TfOH) acids). The acid-base switching processes
1
have been monitored by H NMR spectroscopy for both [2]-
rotaxanes. The switching process has also been investigated
by photophysical and electrochemical techniques for 1b-H3+
(a) 1H NMR Experiments. A slight molar excess of
i-Pr2NEt or Bu3N was added to a solution of 1a-H‚3PF6 (ca.
4.7 mM) in CD3COCD3. Immediately, the solution’s appear-
1b-H3+ cannot be due simply to proton removal from the NH2
+
.
centersthey must be assigned to a change in the interaction
between the Anth unit and DB24C8. As the macroring situated
on the NH2+ recognition site affects the Anth bands (Figure 9),
accompanied by the fact that the deprotonated (6b2+ and 1b2+
)
species exhibit very similar absorption spectra, we can conclude
(23) The electrochemical experiments were performed on an Ar-purged
MeCN solution containing 1.2 × 10-3 M DB24C8 and 5.0 × 10-2 M DBA‚
PF6. Under these conditions, DB24C8 is complexed almost quantitatively
by DBA+ to generate the [2]pseudorotaxane [DB24C8‚DBA]+.
(24) Ashton, P. R.; Langford, S. J.; Spencer, N.; Stoddart, J. F.; White,
A. J. P.; Williams, D. J. Chem. Commun. 1996, 1387-1388.

Acid-Base Controllable Molecular Shuttles
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11939
Figure 10. Correlation of the potentials (V vs SCE) for the oxidation/reduction processes of 1b-H3+, its constituents (6b-H3+ and DB24C8), and
some model compounds/complexes.
Figure 12. Spectral changes (MeCN, 293 K) for 1b-H3+ on addition
of Bu3N: (a) Anth-based high-energy absorption band; (b) Anth-based
fluorescence (excitation at the isosbestic point (353 nm)). The solid,
dotted, and dashed lines refer, respectively, to [Bu3N]/[1b-H3+] ratios
of 0, 0.5, and 1. Inset (c) shows the titration curves for absorbance at
255 nm (b) and emission intensity at 417 nm (O). The changes can be
reversed completely by addition of TfOH (absorbance (2) and emission
intensity (4)).
1
Figure 11. Partial H NMR spectrum (400 MHz, CD3COCD3, 304
K) of 1a‚2PF6. The proton descriptors are defined in Figure 6.
after addition of 1 equiv of base (Bu3N), and can be completely
reversed (Figure 12c) by addition of a stoichiometric amount
of TfOH.
that deprotonation of the NH2+ site of 1b-H3+ drives the crown
ether away from the Anth moiety.
The Anth’s fluorescence quenching processes, which can
Deprotonation also produces (Figure 12b) an intensity
decrease for the Anth emission bands of both 6b-H3+ (and its
model compound 7-H+) and 1b-H3+. For 1b-H3+, both the
changes caused by deprotonation, viz., in the absorbance at 255
nm and in the emission intensity at 417 nm, reach a plateau
occur in the protonated/deprotonated species, are depicted
schematically in Figure 13. Intermolecular quenching processes
can be neglected, since the lifetime of the excited Anth unit is
very short (8.0 ns for model compound 7-H+) and the solutions
used are very dilute (2 × 10-5 M). The energy of the Anth

11940 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Ashton et al.
Figure 13. Photoinduced processes in the (a) dumbbell components 6b-H3+/6b2+ and (b) [2]rotaxanes 1b-H3+/1b2+ that involve the Anth chromophoric
group.
moiety’s fluorescent excited state is ca. 3.3 eV, the potentials
for the oxidation and reduction processes of the Anth unit in
6b-H3+ (and also in 7-H+) being +1.43 and -1.98 V vs SCE,
respectively. Hence, the intensity decrease (Table 3) for the
Anth emission band of 6b-H3+, with respect to 7-H+, can be
ascribed to oxidative quenching of the fluorescent excited state
by the Bpym2+ unit (reduction potential -0.35 V). The further
decrease observed upon deprotonation of 6b-H3+ can be
attributed to the slightly exoergonic reductive quenching of the
Anth fluorescent excited state by the newly created amino group
(which is oxidized at +1.09 V). The emission intensity increase
on going from 6b-H3+ to 1b-H3+ can be credited to the
interposition of the crown ether macroring between the Anth
moiety and the Bpym2+ quencher. The emission intensity
decrease upon deprotonation of 1b-H3+ can be attributed to
further quenching by the amino group that is generated. Finally,
the fact that the emission intensity of 1b2+ is larger than that of
6b2+ can, once again, be considered to arise from the protection
offered by DB24C8 toward the interaction between the excited
state of the Anth moiety and the Bpym2+ quencher.
(vide supra). The potential values (Table 4) demonstrate that
the processes exhibited by 6b-H3+, along with those of 7-H+
and DBV2+, are not affected by deprotonation. The only
difference between the electrochemical behavior of the dumb-
bell’s protonated/deprotonated forms (and also of 7-H+) is the
presence of a characteristic irreversible process, at ca. +1.1 V
vs SCE, resulting from the oxidation of the amine unit formed
+
by the deprotonation of the NH2 center.
The deprotonated [2]rotaxane 1b2+ displays three separate
reduction and oxidation processes. On the reduction side, a
comparison between the behavior of 1b-H3+ and 1b2+ shows
(Figure 14a) that deprotonation shifts the Bpym2+ unit’s first
reduction process strongly toward more negative potentials. On
the other hand, deprotonation does not affect this unit’s second
reduction or the reduction of the Anth moiety. The displacement
of the first reduction process toward more negative potentials
indicates,21 along with the NMR and photophysical results, that
the Bpym2+ unit is engaged in donor-acceptor interactions in
1b2+, a fact that is consistent with the switching of the DB24C8
macroring from the NH2+ to the Bpym2+ recognition site upon
deprotonation of 1b-H3+. It can also be noticed that the first
reduction process of 1b2+ is characterized by a very large
separation between the cathodic and anodic peaks and by some
other features that will be discussed below in greater detail.
For the present discussion, it is important to note that the
cathodic and anodic peaks move toward more and less negative
potentials, respectively, when the scan rate is increased. This
trend attests to the fact that reduction is followed by a
rearrangement process that occurs on the time scale of the
electrochemical experiments. The nature of such a process is
elucidated from the characteristics of the Bpym2+ unit’s second
reduction, whose potential value practically coincides with that
of the dumbbell component. This property means that the
monoreduced Bpym2+ unit of 1b2+ is no longer engaged in
donor-acceptor interactions, since even the second reduction
potential is displaced21,25 toward more negative values when a
Bpym2+ unit is forced to remain within a crown ether’s cavity.
In addition, the complete reversibility of the second reduction
process indicates that the system does not undergo any further
rearrangements after one-electron reduction. Therefore, the first
one-electron reduction process promotes the displacement of
The deprotonated form of the [2]rotaxane does not show any
fluorescence from the DB24C8 component, as is21,25 the norm
for aromatic crown ethers engaged in CT interactions.
(c) Photochemical Experiments. In aerated solutions, both
6b-H3+ and 1b-H3+ undergo a low efficiency photochemical
reaction that can be followed (vide supra) through the changes
in the absorption and emission spectra. The photoreactions’
quantum yields (Table 3) depend on both protonation state and
macroring position. However, any interpretation of the data
dealing with the switching process is precluded by the large
number of possible intervening factors. The photoreaction
occurs22 in the Anth unit’s lowest triplet excited state, so that
the protonation state and macroring position can now participate
at three different levels involving the (1) relative concentration
of excited singlets, (2) efficiency of intersystem crossing, and
(3) intrinsic triplet reactivity.
(d) Electrochemical Experiments. The electrochemical
behavior of deprotonated forms 6b2+ and 1b2+ has been
investigated and compared with that of their protonated forms
(25) (a) Ashton, P. R.; Ballardini, R.; Balzani, V.; Belohradsky, M.;
Gandolfi, M. T.; Philp, D.; Prodi, L.; Raymo, F. M.; Reddington, M. V.;
Spencer, N.; Stoddart, J. F.; Venturi, M.; Williams, D. J. J. Am. Chem.
Soc. 1996, 118, 4931-4951. (b) Asakawa, M.; Ashton, P. R.; Ballardini,
R.; Balzani, V.; Belohradsky, M.; Gandolfi, M. T.; Kocian, O.; Prodi, L.;
Raymo, F. M.; Stoddart, J. F.; Venturi, M. J. Am. Chem. Soc. 1997, 119,
302-310.
DB24C8 back toward the NHCH2-Anth moiety of 1b2+
.
Furthermore, it is reasonable to expect that oxidation of the
crown ether also leads to its displacement from the Bpym2+
unit. Unfortunately, the fact that the oxidation process is not

Acid-Base Controllable Molecular Shuttles
J. Am. Chem. Soc., Vol. 120, No. 46, 1998 11941
-0.38 V, respectively, tend to disappear as the rate increases.
According to the square scheme shown in Figure 14b, the wave
at more negative potential is assigned to the redox couple A/B,
while that at less negative potential is ascribed to D/C. The
results obtained suggest that dethreading of A and rethreading
of C and D occur in the time scale of the electrochemical
experiment (10-1000 mV s-1), dethreading of monoreduced
B occurring at a faster rate. A simple digital simulation of the
cyclic voltammetric patterns shows that E°A/B ) -0.59 V, E°D/C
) -0.34 V, kAD ≈ 0.15 s-1, kDA e 2.5 s-1, kBC g 100 s-1, and
kCB ≈ 1 s-1
.
It can be noticed that the Bpym2+ unit’s first reduction
potential shifts to a much greater extent when it is engaged with
DB24C8 in 1b2+ with respect to rotaxanes and catenanes in
which a Bpym2+ unit undergoes donor-acceptor interactions
with crown ethers such as BPP34C107b,21,25 and 1,5-dinaphtho-
[38]crown-10 (1/5DN38C10).7b,e,25b Since DB24C8 is a poorer
electron donor than both of these crown ethers, the effect cannot
be accredited to a stronger donor-acceptor interaction. NMR
experiments on solutions of 1b2+ indicate (vide supra) that
DB24C8 surrounds predominantly one Pym+ ring of the
Bpym2+ unit on account of its (relatively) small cavity. Such
an asymmetric interaction could influence the twist angle
between the two Pym+ rings, which is known27 to play a major
role in shifting the value of the reduction potential. Structural
effects and steric crowding can account for the fact that
macroring dethreading from the reduced Bpym2+ unit (process
B f C) is fast, while rethreading (process D f A) is very slow.
To test the reversibility of the switching process, we have
added an equivalent of TfOH to the 1b2+ solution, obtained by
deprotonation of 1b-H3+, before electrochemical analyses. The
results obtained match those exhibited by the original 1b-H3+
species exactly, indicating that protonation of the amine group
propels the crown ether from the Bpym2+ unit back to the
regenerated NH2+ recognition center, as also evidenced by NMR
and photophysical experiments. The switching of the macroring
between recognition centers can be repeated by successive acid-
base treatments without any electrochemical evidence of side
reactions.
Figure 14. (a) Cyclic voltammetric behavior on reduction of the
dumbbell components 6b2+/6b-H3+ and [2]rotaxanes 1b-H3+/1b2+ (Ar-
purged MeCN solution, scan rate 50 mV s-1). The current intensity
has been corrected to account for differences in diffusion coefficients.
(b) Dual-pathway square-scheme mechanism for the 1b2+/+ system.
Only the Bpym2+ station and macroring are shown (schematically) to
represent the mechanical movements involved. Species A and C
represent the stable co-conformations of 1b2+ and 1b+, respectively,
whereas B and D are metastable intermediates.
Conclusions
The syntheses of two [2]rotaxanes, viz., 1a-H3+ and 1b-H3+
,
completely reversible prevents an investigation of its kinetic
implications. Additional evidence that deprotonation of 1b-
composed of a DB24C8 macroring bound mechanically to a
dumbbell-shaped component endowed with an NH2+ center and
a Bpym2+ unit, have been achieved by using the principles of
synthetic supramolecular chemistry.28 Acid-base switching
experiments, monitored by 1H NMR spectroscopy, demonstrated
that, upon addition of an appropriate base to a solution of either
of the [2]rotaxanes, the crown ether switches from the NH2+ to
H
3+ promotes the movement of DB24C8 to the Bpym2+ unit is
obtained by comparing the oxidation behavior of 1b-H3+ and
1b2+. Whereas the Anth unit’s oxidation process occurs at the
same potential in both rotaxanes, the crown ether’s oxidation
is shifted toward more positive potentials, by 30 mV, in 1b2+
,
indicating21,25 that the DB24C8 macroring becomes engaged in
π-donor-π-acceptor interactions after deprotonation.
the Bpym2+ station. Treatment with acid restores the NH2
+
It will be recalled that the cyclic voltammetric pattern of the
first reduction process of 1b2+ is indicative of kinetic complica-
tions, a result that is not unexpected for systems in which redox
processes can induce mechanical motions.7 The behavior
observed can be discussed on the basis of a square scheme26
and can be elucidated by experiments performed at different
scan rates. On increasing the scan rate from 10 to 1000 mV
s-1, the cathodic peak moves toward more negative potentials,
reaching a value of -0.62 V, while the anodic peak moves
toward less negative potentials, up to -0.30 V. Furthermore,
the small oxidation and reduction waves, located at -0.53 and
center and reverses the process. In the case of anthracene-
bearing 1b-H3+, the reversible switching process has also been
monitored completely by means of electrochemical and pho-
tophysical techniques. This particular controllable molecular
shuttle symbolizes a major performance enhancement over other
previously reported switching systems.
Acknowledgment. This research was supported by the
Engineering and Physical Sciences Research Council (UK), the
University of Bologna (Funds for Selected Research Topics),
and the EU (TMR Grant No. FMRX-CT96-0076). We thank
(26) For a recent example, see: Villeneuve, N. M.; Schroeder, R. R.;
Ochrymowycz, L. A.; Rorabacher, D. B. Inorg. Chem. 1997, 36, 4475-
4483.
(27) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637-1755.
(28) Fyfe, M. C. T.; Stoddart, J. F. Acc. Chem. Res. 1997, 30, 393-
401.

11942 J. Am. Chem. Soc., Vol. 120, No. 46, 1998
Ashton et al.
the Ministerio de Educacio´n y Ciencia (Spain) for a Postdoctoral
Fellowship to M.-V.M.-D.
patterns for 1b2+ (40 pages, print/PDF). See any current
masthead page for ordering information and Web access
instructions.
Supporting Information Available: Experimental proce-
JA982167M
dures, X-ray crystallographic data, and cyclic voltammetric