Photoswitching of the association of a permethylated α-cyclodextrin-azobenzene dyad forming a Janus [2]pseudorotaxane

Article abstract of DOI:10.1016/S0040-4039(01)01563-5

For achieving the on-off operation of the superstructure formation of hermaphrodite cyclodextrin derivatives, we synthesized azobenzene-modified permethylated α-cyclodextrins, 1 and 2. These compounds formed Janus [2]pseudorotaxanes in CD₃OD-D<

Full text of DOI:10.1016/S0040-4039(01)01563-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7987–7989
Photoswitching of the association of a permethylated
a-cyclodextrin-azobenzene dyad forming a Janus
[2]pseudorotaxane†
Tatsuhiko Fujimoto,a Asao Nakamura,b Yoshihisa Inoue,b Yoshiteru Sakataa and
Takahiro Kanedaa,*
aThe Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
bInoue Photochirogenesis Project, ERATO, JST, 4-6-3 Kamishinden, Toyonaka, Osaka 560-0085, Japan
Received 13 June 2001; revised 17 August 2001; accepted 21 August 2001
Abstract—For achieving the on–off operation of the superstructure formation of hermaphrodite cyclodextrin derivatives, we
synthesized azobenzene-modified permethylated a-cyclodextrins, 1 and 2. These compounds formed Janus [2]pseudorotaxanes in
CD3OD–D2O mixtures. The E-Z photoisomerization of 1 by UV light irradiation resulted in the dissociation of the superstruc-
ture, but, by boiling the solution, the superstructure was recovered through the thermal isomerization of (Z)-1 to (E)-1. These
processes could be repeated many times without any side reactions. This is the first example of the dynamic control of the
[2]pseudorotaxane formation of modified cyclodextrins by external stimuli. © 2001 Elsevier Science Ltd. All rights reserved.
Superstructures such as daisy chains formed by the
self-association of hermaphrodite molecules have
attracted considerable interest from the viewpoint of
the assembly of molecular machines.1 Cyclodextrins
(CDs) are particularly convenient scaffolds for con-
structing the hermaphrodite molecules. Recently, sev-
eral cyclic n-mers of modified cyclodextrins,
[n]supercyclodextrins2b ([n]SCDs), or [n]pseudoro-
taxanes,2 and [n]rotaxanes,3 have been reported, but
hitherto no dynamic control of such superstructures by
external stimuli has been attempted.4 Here, we report
the photodissociation and reassociation of a Janus3a
[2]pseudorotaxane.
An NMR spectrum of a solution of 1 in CD3OD is
shown in Fig. 2(a). The series of permethylated a-
cyclodextrin-azobenzene dyads tested so far have
always given NMR signals for the cyclic oligomeric
species separated from those for the monomeric species
because of very slow association–dissociation equi-
librium.2 The absence of any additional peaks in the
NMR spectrum of 1 in CD3OD means that this com-
pound does not form any oligomers in this solvent.
Upon photoirradiation of the solution by UV light7 the
photoisomerization of (E)-1 to (Z)-1 was observed by
the changes in the UV–vis absorption spectrum (Fig. 1)
and in the NMR spectrum (Fig. 2(b)), where five sets of
new signals (a¦–e¦) were observed after photoirradia-
tion. The product (Z)-1 also absorbs the incident light
with a smaller molar absorption coefficient. Therefore,
the change in the molar ratio of (Z)-1 to (E)-1 stopped
As the photoreactive hermaphrodite compounds we
synthesized 1 and 2. The desired compounds 1 and 2
were obtained in 76 and 40% yields, respectively, by the
reactions of 6-O-monotosyl permethylated a-CD5 with
an excess of 4-hydroxy- and 4,4%-dihydroxy-azoben-
zenes, in N,N-dimethylformamide at 80°C for 24 h
(Scheme 1).6
OTs
HO
N N
R
α-CD
(OMe)17
K2CO3 / DMF, 80 ºC, 24 h
O
N N
R
Keywords:
azo
compounds;
cyclodextrins;
photochemistry;
supramolecular chemistry.
* Corresponding author. Fax: +81-6-6879-8479; e-mail: kaneda@
α-CD
(OMe)17
1: R = H
2: R = OH
sanken.osaka-u.ac.jp
This paper is dedicated to Emeritus Professor Masazumi Nakagawa
†
on the occasion of his 85th birthday.
Scheme 1.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01563-5

7988
T. Fujimoto et al. / Tetrahedron Letters 42 (2001) 7987–7989
(Z)-1 was easily isomerized back to (E)-1 by heating
the solution nearly up to boiling point for 10 minutes.
In the CD3OD–D2O (4:1) solution the recovery of (E)-1
by heating immediately induced the formation of the
compound 3. Thus, the heating triggered the re-associa-
Figure 1. Absorption spectra of 1 (1.08×10−4 M) in CD3OD–
D2O (1:1) in the dark (solid line) and after photoirradiation
(dotted line).
within 15 min, when a photostationary state was
reached. At the photostationary state 33% of 1 was
transformed into the Z form (Table 1). The Z form was
stable at room temperature for over 24 h. Unfortu-
nately, for compound 2 which showed better solubility
to polar solvents and better self-associating ability com-
pared to 1, the formation of the Z isomer upon pho-
toirradiation was not observed. This is probably
because the thermal back-reaction starting from (Z)-2
to (E)-2 is very fast.
On using a solvent mixture of higher polarity, CD3OD–
D2O (4:1), four sets of new doublets were observed in
the NMR spectrum in the dark (a%–d% in Fig. 2(c)),
indicating the formation of a single, symmetrical n-mer
as a minor species. To determine the association num-
ber for the new species, a more polar medium CD3OD–
D2O (2:1) was used because of a harmonious balance
between the solubility and the intensity ratio of the
species to (E)-1. We obtained the values of n=1.98
0.27 and K2=2600 210,8 and therefore we identified
the new species with
a
[2]SCD or
a
Janus
[2]pseudorotaxane 3.
Upon photoirradiation of UV light, the signal peaks for
[2]SCD disappeared from the NMR spectrum (Fig.
2(d)), and the pattern of the remaining signals was very
much similar to that obtained for the photoirradiated
CD3OD solution. The population of each species con-
tained in the solution after photoirradiation was esti-
mated with the NMR peak areas. The results are listed
in Table 1. The population of (E)-1 decreased from 70
to 50% upon photoirradiation. This leads to a drastic
decrease in the concentration of 3. (Z)-1 populated 50%
after photoirradiation, but no formation of a new
associated species was observed in the NMR spectra.
This means that (Z)-1 does not form any complex even
in CD3OD–D2O (4:1). This is probably because the
bent, bulky structure of the (Z)-azobenzene moiety
prevents the axle from passing through the wheel.
1
As the solvent polarity became much higher, the Z to E
ratio in the photostationary state became larger. How-
ever, as the stability of 3 rose with increasing solvent
polarity, the population of 3 increased accordingly.
Figure 2. The aromatic region of 270 MHz H NMR spectra
of 1. 5.08 mM in CD3OD: (a) in the dark; (b) after photoirra-
diation, 4.07 mM in CD3OD–D2O (4:1); (c) in the dark; (d)
after photoirradiation.

T. Fujimoto et al. / Tetrahedron Letters 42 (2001) 7987–7989
7989
O
N
O
N
N
N
∆
N
O
N
α-CD
(OMe)17
N
O
N
hν
(Z)-1
3
(E)-1
Scheme 2.
Table 1. Population of (E)-, (Z)-1 and 3 under various
Jime´nez, M. C.; Dietrich-Buchecker, C.; Sauvage, J.-P.
Angew. Chem., Int. Ed. Engl. 2000, 39, 3284–3287.
conditionsa
2. (a) Fujimoto, T.; Uejima, Y.; Imaki, H.; Jung, J. H.;
Sakata, Y.; Kaneda, T. Chem. Lett. 2000, 564–565; (b)
Fujimoto, T.; Sakata, Y.; Kaneda, T. Chem. Lett. 2000,
764–765.
3. (a) Fujimoto, T.; Sakata, Y.; Kaneda, T. Chem. Commun.
2000, 2143–2144; (b) Hoshino, T.; Miyauchi, M.;
Kawaguchi, Y.; Yamaguchi, H.; Harada, A. J. Am. Chem.
Soc. 2000, 122, 9876–9877; (c) Onagi, H.; Easton, C. J.;
Lincoln, S. F. Org. Lett. 2001, 3, 1041–1044.
4. Ueno et al. have reported the on–off switching of the
dimer formation of a b-cyclodextrin-azobenzene dyad:
Fukushima, M.; Osa, T.; Ueno, A. J. Chem. Soc., Chem.
Commun. 1991, 15–17. However, the proposed structure of
the dimer whose secondary faces are close to each other is
not the pseudorotaxane.
Conditions
(E)-1 (%)
(Z)-1 (%)
3 (%)
CDCl3
Dark
PSb
Dark
PS
100
79
100
67
0
21
0
33
0
0
0
0
0
30
CD3OD (5.08 mM)
CD3OD–D2O (4:1)
(4.07 mM)
Dark
70
PS
Dark
50
50
50
0
Trace
50
CD3OD–D2O (2:1)c
(B3.39 mM)
PS
Dark
35
27
55
0
9
73
CD3OD–D2O (1:1)c
(B2.54 mM)
PS
17
60
23
a Estimated by the NMR method.
b PS, photostationary state.
5. Jung, J. H.; Takehisa, C.; Sakata, Y.; Kaneda, T. Chem.
Lett. 1996, 147–148.
c The exact concentration is not known, because of precipitates
formed during the measurement.
6. Compound 1: yellow solid, mp 238–240°C. Anal. found:
C, 55.55; H, 7.86; N, 2.01%. Calcd for C65H102O30N2·H2O:
C, 55.38; H, 7.44; N, 1.98%. MALDI-TOF-MS (m/z):
1
1415 [M+Na]+. H NMR (270 MHz, CDCl3, 23°C): l 7.92
tion of 1. By repeating photoirradiation and heating
alternately the dissociation and re-association of 1 was
achieved nearly endlessly without any side reactions
(Scheme 2).
(d, J=9.18 Hz, 2H, Hb), 7.86 (d, J=6.75 Hz, 2H, Hc),
7.49–7.43 (m, 3H, Hd, e), 7.07 (d, J=9.18 Hz, Ha),
5.11–4.99 (m, 6H, CD-H1), 4.47–3.05 (m, CD-H). Com-
pound 2: orange solid, mp 144–146°C. Anal. found: C,
53.21; H, 7.08; N, 2.13%. Calcd for C65H102O31N2·3H2O:
C, 53.40; H, 7.08; N, 2.13%. MALDI-TOF-MS (m/z):
In conclusion, the compounds 1 and 2 in CD3OD–D2O
mixtures formed Janus [2]pseudorotaxanes or [2]SCDs.
A substantial portion of the [2]SCD 3 was destroyed
upon irradiation of UV light. The [2]SCD was recov-
ered by heating the solution. This finding will con-
tribute to development of ways to control the
supramolecular structures by external stimuli.
1
1431 [M+Na]+. H NMR (270 MHz, CDCl3, 23°C): l 7.95
(s, 1H, Ar-OH), 7.78 (d, J=8.90 Hz, 2H, Hb), 7.71 (d,
J=8.90 Hz, 2H, Hc), 6.97 (d, J=8.90 Hz, Ha), 6.87 (d,
J=8.90 Hz, 2H, Hb), 5.05–4.92 (m, 6H, CD-H1), 4.43–
4.30, 4.01–3.99, 3.80–3.05 (m, CD-H).
7. Photoirradiation was performed with a high-pressure mer-
cury arc lamp equipped with a CuSO4 aqueous solution
filter for isolating the light of 366 nm.
8. Association constant Kn was determined assuming that
monomers M associate to form a single n-mer Mn.
Acknowledgements
K
n
This work was supported by a Grant-in-Aid for Scien-
tific Research (12440199) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology of Japan.
nM X Mn
Kn=([M]0−[M])/n[M]n
where n is the association number, [M] is the monomer
concentration, and [M]0 is the initial concentration of the
monomer. The following three sets of the parameters
(C0/mM, Ix, Iy) reported in Ref. 2b were used: (1.2, 1.38,
2.86), (0.6, 9.1, 11.5), and (0.3, 6.8, 6.0).
References
1. (a) Rowan, S. J.; Cantrill, S. J.; Stoddart, J. F.; White, A.
J. P.; Williams, D. J. Org. Lett. 2000, 2, 759–762; (b)