Molecular fibers and wires in solid-state and solution self-assemblies of cyclodextrin [2]rotaxanes

Article abstract of DOI:10.1021/ol8002145

Cyclodextrin [2]rotaxanes have been prepared by coupling dimethylanilines with dicarboxylic acids using DMT-MM. in aqueous solutions of a-cyclodextrin, and the example illustrated shows unusual fluorescence emission and other spectroscopic behavior characteristic of the formation of molecular wires in solution, similar to the fibers observed in the solid state.

Full text of DOI:10.1021/ol8002145

ORGANIC
LETTERS
2008
Vol. 10, No. 10
1885-1888
Molecular Fibers and Wires in
Solid-State and Solution Self-Assemblies
of Cyclodextrin [2]Rotaxanes
Subashani Maniam,† Marta M. Cieslinski,† Stephen F. Lincoln,‡ Hideki Onagi,†
Peter J. Steel,§ Anthony C. Willis,† and Christopher J. Easton*,†,
Research School of Chemistry, Australian National UniVersity, Canberra ACT 0200,
Australia, School of Chemistry and Physics, UniVersity of Adelaide, Adelaide SA 5005,
Australia, Department of Chemistry, UniVersity of Canterbury, PriVate Bag 4800,
Christchurch, New Zealand, and ARC Centre of Excellence for Free Radical Chemistry
and Biotechnology, Australia
easton@rsc.anu.edu.au
Received February 6, 2008
ABSTRACT
Cyclodextrin [2]rotaxanes have been prepared by coupling dimethylanilines with dicarboxylic acids using DMT-MM, in aqueous solutions of
r-cyclodextrin, and the example illustrated shows unusual fluorescence emission and other spectroscopic behavior characteristic of the
formation of molecular wires in solution, similar to the fibers observed in the solid state.
Rotaxanes are supramolecular species consisting of macrocycles
encompassing axles that are bonded to bulky blocking groups
in order to prevent the macrocycles and axles from dissociating.1
Cyclodextrins are well-suited for use as the macrocycles in
rotaxane synthesis since, in aqueous solution, they readily form
host-guest complexes with hydrophobic species that can be
exploited as the axles.2–5 Accordingly, a range of cyclodextrin-
based rotaxanes have been prepared and used as molecular
shuttles and other devices.4–16 Previously, we had prepared
the cyclodextrin [2]rotaxanes 1a-d and found, by X-ray
crystallographic analysis, that they all form remarkably
similar networks of aligned molecular fibers in the solid
state.6,7 Analogous strands were found by the Anderson
group8,9 with the two cyclodextrin [2]rotaxanes that they have
† Australian National University.
(6) Onagi, H.; Carrozzini, B.; Cascarano, G. L.; Easton, C. J.; Edwards,
‡ University of Adelaide.
A. J.; Lincoln, S. F.; Rae, A. D. Chem. Eur. J. 2003, 9, 5971–5977
(7) Cieslinski, M. M.; Steel, P. J.; Lincoln, S. F.; Easton, C. J. Supramol.
Chem. 2006, 18, 529–536
(8) Terao, J.; Tang, A.; Michels, J. J.; Krivokapic, A.; Anderson, H. L.
Chem. Commun. 2004, 56–57
(9) Stanier, C. A.; O’Connell, M. J.; Clegg, W.; Anderson, H. L. Chem.
Commun. 2001, 493–494
(10) Easton, C. J.; Lincoln, S. F.; Meyer, A. G.; Onagi, H. J. Chem.
Soc., Perkin Trans. 1 1999, 2501–2506
(11) Kawaguchi, Y.; Harada, A. J. Am. Chem. Soc. 2000, 122, 3797–
3798.
(12) Onagi, H.; Easton, C. J.; Lincoln, S. F. Org. Lett. 2001, 3, 1041–
.
§ University of Canterbury.
ARC Centre of Excellence for Free Radical Chemistry and Biotech-
nology.
.
(1) Schill, G. Catenanes, Rotaxanes and Knots; Academic Press: New
York, 1971.
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Eds.; Wiley-VCH: Weinheim, 1999
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Templates for Supramolecular Chemistry; Imperial College Press: London,
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1028–1064
.
10.1021/ol8002145 CCC: $40.75
Published on Web 04/15/2008
2008 American Chemical Society

with R-cyclodextrin 3 (8 equiv) in carbonate buffer at pH 10
to allow the corresponding inclusion complex 4 to form. DMT-
MM (4 equiv) and 3,5-dimethylaniline 5 (4 equiv) were then
added, and the mixture was stirred for a further 10 h before the
product 6 was isolated in 27% yield through chromatography
on a Diaion HP-20 column. Similar methods were used to
prepare the rotaxanes 719 and 819 in yields of 25% and 5%,
respectively. For comparison, the diamides 9 and 10 corre-
sponding to the dumbbells of the rotaxanes 6 and 7 were also
synthesized.19
crystallographically characterized. While the trinitrophenyl
groups of the rotaxanes 1a-d are twisted at an angle of
approximately 90° to the stilbene units, the blocking groups
in Anderson’s rotaxanes are coplanar and conjugated with
those axles, with π-π stacking arrangements between the
blocking groups of the type that are thought to be important
in the semiconductivity of insulated molecular wires of
related conjugated polyrotaxanes.5 Here we report the
synthesis of a new group of rotaxanes, by coupling dim-
ethylanilines with dicarboxylic acids in aqueous solutions
of R-cyclodextrin 3, using the water-soluble and compatible
reagent 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmor-
pholinium chloride (DMT-MM).17,18 Through the choice of
either 2,6- or 3,5-dimethylaniline 5 as the blocking reagent,
this approach allows us to manipulate the extent of conjuga-
tion of the blocking groups with the axles, as well as the
formation of molecular fibers through solid-state self-
assembly of the rotaxanes. Analogous behavior is observed
in solution, where spontaneous self-assembly of the rotaxane
6 leads to the formation of molecular fibers similar to those
observed in the solid-state. The unusual fluorescence emis-
sion and other spectroscopic evidence characteristic of this
aggregation in solution reflects intermolecular electronic
interactions, which demonstrate, for the first time, that the
fibers are clearly behaving as molecular wires.
Attempts to obtain crystals of the rotaxane 8 and the diamide
10 appropriate for X-ray analysis were not successful, but
suitable samples were obtained in the cases of the rotaxanes 6
and 7, and the diamide 9, through slow evaporation of methanol/
water solvent over a period of several weeks for the former
pair, and by recrystallization from pyridine with the latter
material (see the Supporting Information). In the solid state,
the diamide 9 forms a complex lattice of molecules displaying
a variety of interactions and oriented on several different axes
(Figure 1a). By contrast, the dumbbells of the rotaxane 6
assemble as molecular fibers, linearly aligned along a single
axis and insulated by the cyclodextrins, and they only come
into contact with adjacent dumbbells through their blocking
groups (Figure 1b). The blocking groups are π-π-stacked with
the rings coplanar and their mean planes separated by a distance
of 3.461 Å. Their relative alignment is characteristic of the most
common type of π-π stacking, with the centroid of one ring
lying over one carbon of the other (Figure 2).20–22 The blocking
groups of the dumbbells are almost coplanar with the axles,
so there is extended conjugation along the length of each
fiber.
Scheme 1. Synthesis of the Rotaxane 6
(16) Easton, C. J.; Lincoln, S. F.; Barr, L.; Onagi, H. Chem. Eur. J.
2004, 10, 3120–3128.
(17) Kunishima, M.; Kawachi, C.; Morita, J.; Terao, K.; Iwasaki, F.;
Tani, S. Tetrahedron 1999, 55, 13159–13170.
(18) Kunishima, M.; Kawachi, C.; Hioki, K.; Terao, K.; Tani, S.
Tetrahedron 2001, 57, 1551–1558.
The rotaxane 6 was prepared as outlined in Scheme 1.19 The
dicarboxylic acid 2 was stirred at room temperature for 2 h
(19) See the Supporting Information for experimental details.
(20) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
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(22) Cockroft, S. L.; Perkins, J.; Zonta, C.; Adams, H.; Spey, S. E.;
Low, C. M. R.; Vinter, J. G.; Lawson, K. R.; Urch, C. J.; Hunter, C. A.
(15) Onagi, H.; Blake, C. J.; Easton, C. J.; Lincoln, S. F. Chem. Eur. J.
2003, 9, 5978–5988.
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.
1886
Org. Lett., Vol. 10, No. 10, 2008

Figure 3. Illustration of steric interactions preventing coplanarity of
the blocking groups with the axle in the rotaxane 7 (left) and their
absence in the rotaxane 6 (right).
for the trinitrophenyl-capped rotaxanes 1a-d,6,7 the length
required to accommodate each successive dumbbell in the
fiber is twice the depth of the cyclodextrin, and the extent
of overlap of adjacent blocking groups must correspond to
this. Further, the choice of either 2,6- or 3,5-dimethylaniline
5 as the blocking reagent can be used to manipulate this
overlap and therefore to control both the formation of axially
aligned and separated molecular fibers of the axles through
self-assembly of rotaxanes as well as the extent of conjuga-
tion of the axles with the blocking groups.
In solution, ultraviolet-visible spectra of the rotaxane 6 in
either methanol or DMSO obey Beer’s law; that is, there is a
direct correlation between the concentration of the rotaxane 6
and the absorbance of the solution, with λmax 350 nm in each
solvent and ꢀmax 32000 and 28000 in methanol and DMSO,
respectively (Figures 4a,b). By contrast, solutions in water show
substantial deviations from this correlation, particularly at
concentrations of the rotaxane 6 above 10 µM (Figure 4c). For
example, at 350 nm, the absorbance of a 100 µM solution is
less than 5 times that of a 10 µM one, instead of the typical 10.
At the higher concentrations and as the deviations become
greater, the spectra also exhibit an absorbance of increasing
intensity around 430 nm, ranging up to 600 nm, and excitation
of these solutions at 480 nm results in fluorescence emission at
520 nm (Figure 5). The absorbance at 350 nm is characteristic
of an azobenzene, while those at higher wavelengths, and the
deviations as well as the fluorescence behavior referred to above,
are typical of aggregation through π-π-stacking.20,23–28 In the
absence of aggregation, the azobenzene chromophore is
generally nonfluorescent in solution at room temperature,
because radiative deactivation of photoexcited azobenzenes
is not competitive with their photoisomerization.29–31
The ultraviolet-visible and fluorescence spectra show that
the aggregates form at concentrations above about 15 µM. The
structure of the rotaxane 6 limits the possible modes of
aggregation. The cyclodextrin insulates the axle, so only the
blocking groups are accessible for π-π-interactions. These must
Figure 1. Solid-state structures of (a) the diamide 9, (b) the rotaxane
6 with the cyclodextrins (green) aligned along a single axis, and (c)
the rotaxane 7 with the cyclodextrins oriented in one direction (green)
interspersed by others aligned at right angles (red).
With the rotaxane 7, coplanarity of each blocking group with
the axle is prevented by nonbonding steric interactions between
the amide oxygen and the methyl substituents at the 2- and
6-positions of the blocking group (Figure 3). This stops
conjugation through the dumbbell. The out-of-plane amide
moiety also precludes the extent of overlap of adjacent blocking
groups required for π-π-stacking of the type seen with the
rotaxane 6 (Figure 2). Other interactions between the blocking
groups can be envisaged, including that actually observed, but
then there is less overlap of adjacent dumbbells. As a conse-
quence, the cyclodextrins are forced too far apart for a close-
packed arrangement along one axis and, instead, the cyclodex-
trins oriented in one direction are interspersed by others aligned
at right angles (Figure 1c).
Thus, as is illustrated in Figure 1, this study shows that for
the alignment of the rotaxanes into fibers along a single axis,
of the type described here for the rotaxane 6 and reported earlier
(23) Miyamoto, N.; Kuroda, K.; Ogawa, M. J. Am. Chem. Soc. 2001,
123, 6949–6950
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Figure 2. Schematic representation of π-π stacking between
2742
.
blocking groups observed in the solid-state structure of the rotaxane
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Org. Lett., Vol. 10, No. 10, 2008
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of the azobenzene moiety, so the axles of the dumbbells must
be fully conjugated with the blocking groups. Thus, there are
striking similarities between the solution and solid-state self-
assemblies of the rotaxane 6.
The spectra indicate that there is no aggregation of the
rotaxane 6 in either methanol or DMSO, presumably because
these solvents disrupt π-π-stacking. There was no evidence
of abnormal spectroscopic behavior with solutions of either the
diamide 9 in DMSO, or the rotaxane 7 in either methanol or
water (see the Supporting Information). Due to its low solubility,
it was not practical to examine the properties of the diamide 9
in aqueous solution. In regard to the rotaxane 7, aggregation
may still be occurring yet not be reflected in the spectra because
of the lack of conjugation of the dumbbell in that case.
The fibrous structure adopted by the dumbbells of the
rotaxane 6 in the solid state (Figure 1b) is suggestive of a
network of aligned molecular wires. There is extended conjuga-
tion along the length of each fiber, comprising π-π-interactions
between the blocking groups of sequential dumbbells as well
as coplanarity of the blocking groups of the dumbbells with
their axles. However, there is no property of the solid that
directly reflects this structure or shows behavior characteristic
of a wire. By contrast, the spectroscopic evidence of the
formation of the molecular fibers of the rotaxane 6 in solution
reflects intermolecular electronic interactions along the exten-
sively conjugated system and demonstrates that in this phase
the fibers are indeed performing as molecular wires.
In summary, cyclodextrin [2]rotaxanes have been prepared
by coupling dimethylanilines with dicarboxylic acids using
DMT-MM in aqueous solutions of R-cyclodextrin 3. The choice
of either 2,6- or 3,5-dimethylaniline 5 can be exploited to
manipulate both the extent of conjugation of the axles with the
blocking groups and the formation of molecular fibers through
self-assembly of the rotaxanes in the solid state. There are
striking similarities between the solid-state behavior of the
rotaxane 6 and that observed in solution, where spontaneous
self-assembly leads to the formation of fibers displaying
spectroscopic properties characteristic of their performance as
molecular wires. Such fibers could possibly form interesting
lyotropic liquid crystalline phases.32
Figure 4. Absorption spectra of the rotaxane 6 in (a) methanol, (b)
DMSO, and (c) water at concentrations ranging from 2.5 to 100 µM,
recorded at room temperature (see the Supporting Information for
corresponding plots of absorbance at 350 nm vs concentration of the
rotaxane 6).
be intermolecular such that the rotaxane 6 is forming fibers.
Nevertheless, these π-π-interactions between the blocking
groups of adjacent dumbbells affect the spectral characteristics
Acknowledgment. We acknowledge support received for
this research from the Australian Research Council.
Supporting Information Available: Details of the syn-
thesis of compounds 6-10, X-ray crystallographic analysis
of compounds 6, 7, and 9, ultraviolet/visible spectroscopy
of compounds 7 and 9, graphs of ultraviolet-visible absor-
bance and fluorescence emission intensity vs concentration
1
for aqueous solutions of the rotaxane 6, and H and 13C
spectra of compounds 6-10. This material is available free
of charge via the Internet at http://pubs.acs.org.
OL8002145
Figure 5. Fluorescence emission spectra of the rotaxane 6 in water at
concentrations ranging from 10 to 100 µM, recorded at room
temperature (λex ) 480 nm, see the Supporting Information for the
corresponding plot of fluorescence emission intensity vs concentration
of the rotaxane 6).
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