Formation of supramolecular isomers; Poly[2]rotaxane and supramolecular assembly

Article abstract of DOI:10.1039/b713588b

Poly[2]rotaxane and supramolecular assembly have been prepared by modified cyclodextrins bearing an adamantyl group in an aqueous medium. The Royal Society of Chemistry.

Full text of DOI:10.1039/b713588b

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Formation of supramolecular isomers; poly[2]rotaxane and
supramolecular assembly{
Atsuhisa Miyawaki, Masahiko Miyauchi, Yoshinori Takashima, Hiroyasu Yamaguchi and Akira Harada*
Received (in Cambridge, UK) 5th September 2007, Accepted 16th November 2007
First published as an Advance Article on the web 26th November 2007
DOI: 10.1039/b713588b
Poly[2]rotaxane and supramolecular assembly have been
prepared by modified cyclodextrins bearing an adamantyl
group in an aqueous medium.
Cooperative interaction plays an important role in controlling the
process of molecular recognition and the formation of supramo-
lecular structures.1,2 Controlling the process of molecular recogni-
tion and the formation of supramolecular structures provides
useful applications for developing unique functions. There are
many reports on controlling the formation of supramolecular
structures using external stimuli such as pH,3 redox,4 light5 etc., by
adjusting the space between recognition moieties,6 stoichiometry7
or by chemical modification.8 Cyclodextrins (CDs) are known to
form complexes with organic compounds in aqueous solutions.9
Especially, adamantane derivatives form 2 : 1 inclusion complexes
with a-CD10 and are often utilized as steric stoppers for rotaxanes
having a-CD.11 To control the formation of supramolecular
structures between supramolecular assembly and poly[2]rotaxane,
adamantane derivatives are suitable for a-CD derivatives as a
guest and a sterically hindered group, respectively.
Scheme 1
Previously, formation of supramolecular complexes using host–
guest interactions of CDs was reported.12 However, to the best of
our knowledge, there are no reports on the formation of CD based
self-assembled supramolecular complexes and mechanically locked
supramolecules consisting of the same components. Herein, we
report our successful efforts to control the formation of
poly[2]rotaxane and novel supramolecular assembly from the
same components under different reaction conditions.
corresponds to the molecular weight of 1 with the adamantyl
group as a monomer. On the other hand, the compound 2
prepared by Method 2 did not show polymeric species. These
results indicate that poly[2]rotaxane forms a stable supramolecular
complex due to the adamantyl groups.
As previously mentioned, although the adamantane derivatives
are sterically hindered groups for the cavity of a-CD, they form
inclusion complexes with a-CD. Therefore, compound 2 may form
a supramolecular assembly in aqueous solutions. To determine the
properties and structure of the supramolecular assembly of 2,
circular dichroism (cd) measurements were carried out. Fig. 2(a)
shows the cd spectra of 2 in aqueous solution and in methanol. A
After synthesizing cinnamamide-a-CDs (1,2 and 3), we
investigated the formation of supramolecular complex in aqueous
solutions. Cinnamamide-a-CDs (1,2 and 3) showed concentration
dependent peak shifts by 1H NMR studies, indicating the
formation of supramolecular complexes in aqueous solutions.
After detailed characterization by 2D NMR studies, it was found
that compound 1 forms linear poly-pseudo-[2]rotaxane.13 To
prepare a stable supramolecular complex, poly[2]rotaxane was
synthesized by the reaction of preorganized poly-pseudo-[2]rotax-
ane with adamantane carboxylic acid as a sterically hindered group
as shown in Method 1 (Scheme 1). The MALDI-TOF mass
spectrum of poly[2]rotaxane showed polymeric species up to
approximately 10,000 (Fig. 1). The interval between signals
Department of Macromolecular Science, Graduate School of Science,
Osaka University, Toyonaka, Osaka 560-0043, Japan.
E-mail: harada@chem.sci.osaka-u.ac.jp; Fax: +81 6 6850 5445;
Tel: +81 6 6850 5445
{ Electronic supplementary information (ESI) available: MALDI-TOF
MS, 1H NMR, diffusion coefficients and detailed synthetic procedures.
See DOI: 10.1039/b713588b
Fig. 1 MALDI-TOF mass spectrum of poly[2]rotaxane.
456 | Chem. Commun., 2008, 456–458
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Fig. 3 Partial 2D ROESY NMR spectrum of 2 in D2O at 30 uC
(600 MHz, mixing time 200 ms).
(Fig. 3). These observations indicate that the adamantyl group is
shallowly included from the wider rim of a-CD cavity. These
results are in agreement with the data from cd spectrometry
measurement.
To estimate the hydrodynamic volume of the supramolecular
assembly, self-diffusion coefficients (Ds) were determined by the
pulsed field gradient spin-echo NMR measurement.14 Compound
3, which has no aromatic group, was used as a reference. Although
the Ds of 3 slightly decreased with an increase in the concentrations
and reached 2.37 6 1026 cm2 s21 at 40 mM, that of 2 significantly
decreased and reached 1.36 6 1026 cm2 s21 at 40 mM. It should
be noted that the hydrodynamic radius (Rh) of 2 is larger than that
of 3 over the whole concentration range. It is thought that the
formation of the larger supramolecular complex from 2 is caused
by not only the host–guest interaction between the adamantyl
group and the a-CD cavity but also the p–p stacking interaction
between the cinnamamide group. The Ds of 2-cinnamoyl-a-CD
(2-CiO-a-CD), which formed a double threaded dimer determined
by single crystal X-ray analysis showed saturation and reached
2.30 6 1026 cm2 s21 (Rh = 0.96 nm) in the lower concentration
region (10–30 mM).{ On the other hand, the Ds of 2 continuously
decreased with the concentrations, showing 1.36 6 1026 cm2 s21
(Rh = 1.67 nm) at 40 mM. These results suggest that compound 2
does not form dimeric assembly but oligomeric assembly in an
aqueous solution. The electrospray ionization (ESI) mass spectrum
provides direct evidence to support the formation of supramole-
cular assembly (Fig. 4). It shows polymeric species up to
approximately 5 mer. Intervals between signals corresponding to
the molecular weight of 2 as a monomer unit are observed.
Fig. 2 Circular dichroism spectra (a) and UV-vis spectra (b) of 2 in
water (blue), addition of b-CD (red) and in methanol (black).
negative–positive Cotton band around 300 nm corresponding to
the 1La transition band of the cinnamamide moiety was observed
in aqueous solution (blue line). This Cotton band is assigned to the
exciton–coupling interaction. It is noted that two or more excitons
are located in relatively close distance, indicating the formation of
helical supramolecular assembly with negative chirality. On the
other hand, no significant Cotton band was observed in methanol
(black line) because of the dissociation of this supramolecular
assembly. On addition of b-CD as a competitive host for an
adamantyl group9 to the aqueous solution of 2, the Cotton band
disappeared as shown by the red line (Fig. 2 (a)) and the
absorption band around 300 nm showed a hyperchromic effect
(Fig. 2(b)). We suppose that the cooperativity effect between
hydrophobic host–guest interaction and p–p stacking interaction
plays an important role in the formation of the supramolecular
assembly of 2.
The 1H NMR spectra of 2 showed that the protons of the
adamantyl group and the cinnamamide group shifted to upfield
with an increase in the concentrations. After addition of b-CD, the
protons of an adamantyl group shifted to upfield (protons a and c)
and downfield (proton b), and the protons of the cinnamamide
group significantly shifted to downfield. These resonance shifts
suggest that the adamantyl group is included in the b-CD cavity
and that p–p stacking interaction between cinnamamide moieties
disappeared. The ROESY NMR spectrum indicated the forma-
tion of a supramolecular assembly. The inner protons C3(H) of
a-CD, which are located in the wider rim, showed the ROEs
correlations to protons a, b and c of the adamantyl group, whereas
the inner protons C5(H) of a-CD, which are located in the
narrower rim, showed the ROEs correlations to protons a and b
Fig. 4 ESI-TOF mass spectrum of 2 (positive mode).
Chem. Commun., 2008, 456–458 | 457
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Fig. 5 Proposed supramolecular structures formed by cinnamamide
a-CD bearing an adamantyl group prepared by Method 1 and Method 2.
Consequently, the proposed supramolecular structures formed
by cinnamamide-a-CD bearing an adamantyl group prepared by
Method 1 or Method 2 are illustrated in Fig. 5.
In conclusion, we have prepared poly[2]rotaxane and supramo-
lecular assemblies formed from the same building blocks using
different preparation methods in an aqueous medium. Although
each unit of poly[2]rotaxane and supramolecular assembly is the
same building block, the structure of each supramolecular complex
was revealed to be quite different. This methodology should be
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formation of supramolecular structures.
The authors thank Dr Akihito Hashidzume, Mr Seiji Adachi
(Osaka University) and JASCO INTERNATIONAL for helpful
advice, 2D-NMR experiments and ESI-TOF MS measurements.
This work has been partially supported by Grant in-Aid No.
A19205014 for Scientific Research and has been conducted with
financial support from the ‘‘Stress and Symbiosis on
Supramolecules’’ program of the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
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Notes and references
{ The Rh value was calculated using the Stokes–Einstein equation. The D
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concentration powder crystal was formed. The D value corrected by
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458 | Chem. Commun., 2008, 456–458
This journal is ß The Royal Society of Chemistry 2008