Disparate orientation of [1]rotaxanes

Article abstract of DOI:10.1016/j.tetlet.2007.07.209

A novel [1]rotaxane 2 has been synthesized employing a 4-methyl-benzenesulfonyl and an azobenzene modified β-cyclodextrin (β-CyD) at the 2 position, through self-inclusion complexation and Suzuki-coupling capping in aqueous solution. Disparate absorption,

Full text of DOI:10.1016/j.tetlet.2007.07.209

Tetrahedron Letters 48 (2007) 7112–7116
Disparate orientation of [1]rotaxanes
Xiang Ma, Qiaochun Wang and He Tian*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology,
Shanghai 200237, PR China
Received 1 July 2007; revised 28 July 2007; accepted 31 July 2007
Available online 3 August 2007
Abstract—A novel [1]rotaxane 2 has been synthesized employing a 4-methyl-benzenesulfonyl and an azobenzene modified b-cyclo-
dextrin (b-CyD) at the 2 position, through self-inclusion complexation and Suzuki-coupling capping in aqueous solution. Disparate
absorption, induced circular dichroism (ICD) properties from its isomer [1]rotaxane 1, prepared from the isomeric b-CyD modifier
at the 6 position, and the photoisomerization of [1]rotaxane 2 were thoroughly investigated.
ꢀ 2007 Elsevier Ltd. All rights reserved.
The design of preparation of molecular machines with
controlled motions at the molecular level is essential
for the development of future toolbox of nanotechno-
logy. Rotaxanes,1 in which the relative positions of the
interlocked components can be changed in response to
an external input and can be capable of varying physical
properties, have attracted more and more attention be-
cause of their challenging constructions and potential
applications in areas such as molecular switches,2,3
molecular logic gates,4,5 and memory devices.6 Cyclo-
dextrins (CyDs) continue to be attractive wheel compo-
nents in constructing rotaxanes recently.7 The
foundation for the construction of a CyD-based rotax-
ane is the interactions between the hydrophobic cavity
of the CyD and a special hydrophobic unit in the linear
component.
of b-CyD when synthesizing [1]rotaxanes for molecular
shuttles. Herein, a new b-CyD-based [1]rotaxane 2 was
prepared via the self-complementary of the azobenzene
modified b-CyD on the untraditional 2 position and
consequent Suzuki-coupling capping in aqueous solu-
tions for the first time (Fig. 1).11 And its photoisomer-
ization of the novel [1]rotaxane isomer including 1H
NMR, absorption and induced circular dichroism
(ICD) spectra was thoroughly investigated.
The synthesis and purification of 2-OTs-CD were con-
ducted as previously reported,12 which was obtained
with an yield of 41% from the sulfonylation of b-CyD
with 4-methylbenzene-1-sulfonyl chloride (4-OTsCl).
With a considerable variance from the 6-OTs-CD and b-
CyD, with a solubility of 0.04 and 1.89 g/100 mL in
water, respectively, 2-OTs-CD is much more soluble
(S > 35 g/mL).12 Moreover, the absorption and induced
circular dichroism (ICD) spectra of 2-OTs-CD and 6-
OTs-CD in aqueous solution exhibit different signals
as shown in Figure 2. Compared with the kmax of 6-
OTs-CD at 227 nm, 2-OTs-CD shows a similar absorp-
tion peak at 222 nm but a blue shift about 5 nm. The
ICD spectra of these two modified b-CD derivatives ex-
hibit minus Cotton effects. However, the Cotton effect of
6-OTs-CD is much more minus than that of 2-OTs-CD.
This is attributed to the different positions of the Ts moi-
eties relative to the CyD rings in these two molecules,
which coincide with the Kajtar sector rules as
reported.12 Evidently, the disparate solubility in water
and spectrum properties above encode the considerable
differences of these two b-CyD isomers.
Great attention has been given to the CyD-based rotax-
anes. So far, there are few reports concerning unidirec-
tional threading,8 and the reports concerning CyD
based unidirectional rotaxanes are rare.9 Recently, we
have reported a unidirectional [1]rotaxane 1 constructed
by threading from the narrow rim of the b-CyD cavity,10
in which b-CyD was firstly modified by 4-indo-phenyl-
azo-phenol on the 6 position and then capped by a stop-
per after its self-complementary. From this point of
view, we would like to widen the scope of these works
to develop new methods for controlling the orientation
Keywords: Cyclodextrin (CyD); [1]Rotaxane; Induced circular dichro-
ism (ICD).
*
Corresponding author. Tel.: +86 21 64252756; fax: +86 21
64252288; e-mail: hetian@ecust.edu.cn
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.07.209

X. Ma et al. / Tetrahedron Letters 48 (2007) 7112–7116
7113
SO3Na
O
N
O
N
N
O
OTs
OH
O
N
N
I
SO3Na
[1]Rotaxane 1
O
N
O
6-Azo-CD
N
(HO)2B
6-OTs-CD
N
I
DMF, K2CO3
OTs
Pd(OAc)2, Na2CO3, H2O
N
N
I
O
N
O
O
O
N
N
NaO3S
2-OTs-CD
2-Azo-CD
[1]Rotaxane 2
Figure 1. The synthesis of [1]rotaxane 1 and [1]rotaxane 2.
b
a
4
2
1.0
0.8
0
6-OTs-CD
2-OTs-CD
0.6
0.4
0.2
0.0
-2
-4
-6
-8
6-OTs-CD
2-OTs-CD
220
240
260
280
300
200
220
240
260
280
300
Wavelength/nm
Wavelength/nm
Figure 2. (a) UV absorption (2.5 · 10À5 M) and (b) ICD (2.0 · 10À4 M) spectra of 2-OTs-CD and 6-OTs-CD in water at 298 K.
Through the etherification from the different OTs-CD
isomers and 4-indo-phenyl-azo-phenol we obtained
two azobenzene modified b-CyD isomers (6-Azo-CD
and 2-Azo-CD).13 2-Azo-CD was also found to be much
more soluble than its isomer 6-Azo-CD (2-Azo-CD is
very soluble in water while the latter is just ordinarily
soluble in hot water). And their absorption and ICD
spectra in aqueous solution are shown in Figure 3. Dis-
similar optical spectral properties are also obviously
encoding the two isomers. This may be attributed to
the different self-inclusion configuration of azo moieties
in the CyD cavities between 6-Azo-CD and 2-Azo-CD
(the angles between the transition dipole moment of
the azo groups and the axis of b-CyD cavities are
different).
Similar to [1]rotaxane 1,10 [1]rotaxane 2 was also pre-
pared conveniently and directly through self-inclusion
complexation of 2-Azo-CD and subsequent Suzuki-cou-
pling capping in aqueous solution.
However, these two isomers have obvious disparate
physical and structural properties. [1]rotaxane 2 is pro-
vided with a much better water solubility than the other
as well as the two derivatives on the 2 position of b-CyD
(2-OTs-CD and 2-Azo-CD) mentioned above. And,
10
0.8
a
b
8
6-Azo-CD
0.6
6
6-Azo-CD
2-Azo-CD
4
2
2-Azo-CD
0.4
0
0.2
0.0
-2
-4
-6
300
350
400
450
500
550
300
350
400
450
500
550
Wavelength/nm
Wavelength/nm
Figure 3. (a) UV absorption (2.5 · 10À5 M) and (b) ICD (2.0 · 10À4 M) spectra of 2-Azo-CD and 6-Azo-CD in water at 298 K.

7114
X. Ma et al. / Tetrahedron Letters 48 (2007) 7112–7116
moreover, observed resonance difference occurs between
the two [1]rotaxane isomers as indicated (Fig. 4) by H
correspondingly the initial peaks appearing at d = 7.17,
7.85, 7.50 and 7.71 ppm, respectively. It is reasonable
that the signals of aromatic protons of the azobenzene
unit generally shift to upfield upon their isomerization
from trans to cis configuration, as a result of the mag-
netic shielding effect of aromatic rings. Moreover, what
should be noted is that Hd in [1]rotaxane 2 was found in
an obvious upfield shift of about 0.97 ppm after the irra-
diated isomerization from the trans conformation to cis,
shifting more upfield more than Hd in [1]rotaxane 1 is re-
ported in our previous paper, which shifted upfield
about 0.7 ppm.10 It is reasonable that after the irradi-
ated isomerization from trans to cis, the narrow rim of
b-CyD in [1]rotaxane 2 influences much more to the aro-
matic Hd of the thread than that effected by the evase
rim of b-CyD in [1]rotaxane 1. Integrals of the two sig-
nals of He appear with about 1:3 ratio (trans to cis),
which suggests that at the photostationary state about
75% of E-[1]rotaxane 2 was transformed to Z-[1]rotax-
1
NMR spectra in DMSO-d6 (298 K). The shield effects
of CyD rings with reverse orientation engender discrep-
ant chemical shifts to the aromatic protons of the
1
threads. In [1]rotaxane 2, the H NMR signals for Hd,
He, Hf and Ha of the thread arise downfields by about
0.08, 0.22, 0.15 and 0.03 ppm with respect to the corre-
sponding protons in [1]rotaxane 1, respectively. The
UV–vis spectra and ICD spectra of [1]rotaxane 2 and
[1]rotaxane 1 recorded at 298 K in DMSO show similar
signals (see Supplementary data, Fig. S1).
The E/Z photoisomerization reaction of [1]rotaxane 2 in
DMSO-d6 (298 K) is revealed by the corresponding
chemical shift changes of the thread protons in 1H
NMR spectra (Fig. 5). Irradiation at 365 nm for 3 h
leads to several new signals of cis-azobenzene, appearing
at d = 6.86 (Ha), 6.90 (Hd), 7.44 (Hf) and 7.61 (He) ppm,
Figure 4. Partial 1H NMR spectra of [1]rotaxane 1 (top) and [1]rotaxane 2 (bottom) in DMSO-d6 at 298 K.
Figure 5. Partial 1H NMR spectra of [1]rotaxane 2 at the original state (top) and at the photostationary state (bottom) after irradiation on 365 nm
for 3 h in DMSO at 298 K. ( at the bottom stands for the Hd, He, Hf and Ha of E-[1]rotaxane 2 without isomerization.)
*

X. Ma et al. / Tetrahedron Letters 48 (2007) 7112–7116
7115
ane 2 isomer. This isomerization ratio is higher than that
6
5
4
3
2
a
of [1]rotaxane 1 (about 50%),10 which is due to the evase
rim of b-CyD in [1]rotaxane 2, which provides more
space for the isomerized cis-azobenzene easily than the
narrow rim of b-CyD in [1]rotaxane 1.
c
b
a (d)
1
0
-1
-2
-3
-4
On account of the trans–cis photoisomerization of the
azobenzene moiety and the subsequent shuttling move-
ment of the b-CyD ring, the shuttles of [1]rotaxane 2
can be confirmed by the varied absorption and ICD
spectra. The irradiation on the [1]rotaxane 2 solution
at 365 nm for 15 min results in photoisomerization to
give a E/Z mixture, characterized by a slight rise of
the absorption at around 292 nm (DA = 0.21) and a de-
crease in absorption at 348 nm (DA = 0.52), as well as
the presence of an isobestic point at 313 nm (Supple-
mentary data, Fig. S2). Because of good photoreversibil-
ity of azobenzene derivative, the photochemical process
of the [1]rotaxane 2 system is highly reproducible, and as
a result, the photoinduced shuttling motion of the CyD
ring can be carried out repeatedly with reversible
absorption signal changes.
200 250 300 350 400 450 500 550
Wavelength/nm
b
3
a (d)
b
c
2
1
0
-1
-2
-3
Temperature increase can induce isomerization of azo-
benzene from Z form to E form.14 UV measurements
along with temperature variance of [1]rotaxane 2 show
(Supplementary data, Fig. S2) that a gradual increase
of the absorption maxima at about 348 nm exists when
the aqueous [1]rotaxane 2 was heated from room tem-
perature to 60 and 80 ꢁC. The temperature raise in-
creases the proportion of E-[1]rotaxane 2 in the usual
mixture conformation of [1]rotaxane 2. And the vi-
brancy of the active b-CyD ring at higher temperature
affects the absorption spectra of azobenzene moiety lit-
tle. The spectra change can be shifted back by natural
cooling to room temperature.7c
250 300 350 400
500 550
Wavelength/n4m50
Figure 6. (a) Photoisomeric ICD spectra of [1]rotaxane 2 (25 ꢁC,
2.0 · 10À4 M) after irradiation at 365 nm for 0 min (a), 3 min (b),
15 min (c), the spectra change can be shifted back by irradiation at
430 nm for 15 min (d); (b) variable-temperature ICD spectra of
[1]rotaxane 2 (2.0 · 10À4 M) at 25 ꢁC (a), 60 ꢁC (b), 80 ꢁC (c), the
spectra change can be shifted back by natural cooling to 25 ꢁC (d).
phenol, the gained 2-Azo-CD was employed to experi-
ence a self-inclusion complexation and Suzuki-coupling
capping in aqueous solution to obtain this novel
[1]rotaxane 2. The physical, structural and spectral
properties of the b-CyD modifiers and final [1]rotaxane
on the 2 position, 2-OTs-CD, 2-Azo-CD and [1]rotaxane
2, were found disparate from the corresponding isomers
on the 6 position. Specifically, they have a better aque-
ous solubility, different absorption and ICD curves.
The azobenzene based [1]rotaxane 2 can be driven to
isomerize reversibly by both light and heating, encoding
The azobenzene moiety of [1]rotaxane 2 is located in the
b-CyD cavity. The ICD spectra with respect to the
photoisomerization
of aqueous [1]rotaxane
2
(2.0 · 10À4 M) solution (Fig. 6) shows a positive Cotton
effect at 423 nm and a weaker negative Cotton effect at
317 nm. After the prolonged irradiation at 360 nm, these
two Cotton Effects experience a similar sign of ICD sig-
nals but only an intensity increase to the trans-azoben-
zene [1]rotaxane 2. The ICD spectral changes can be
shifted back by irradiation at 430 nm.
1
by obvious chemical shifts in H NMR spectra, UV–vis
absorption spectra and ICD spectra.
Nevertheless, variable-temperature ICD measurements
of [1]rotaxane 2 were observed to exhibit a contrary
trend. Both the ICD signals at 423 and 317 nm de-
creased gradually while the aqueous [1]rotaxane 2 was
heated from room temperature (25 ꢁC) to 80 ꢁC. And
when it was cooled back to room temperature, the
changes of the ICD signals were shifted back. This ther-
mosensitive ICD variance was in accordance with other
azobenzene-based rotaxanes we reported previously.7c
Acknowledgments
This work was funded by the NSFC/ China (50673025,
90401026), National Basic Research 973 Program
(2006CB806205) and partially by the Scientific Commit-
tee of Shanghai.
In conclusion, on the basis of the previous work con-
cerning [1]rotaxane 1, we prepared its isomer, [1]rotax-
ane 2, conveniently and directly. Firstly, b-CyD was
modified by 4-methyl-benzenesulfonyl chloride on the
2 position and then connected with 4-indo-phenyl-azo-
Supplementary data
Synthetic details of the compounds associated with this
article, the compared UV–vis spectrum and ICD spec-

7116
X. Ma et al. / Tetrahedron Letters 48 (2007) 7112–7116
´
Buma, W. J.; Wurpel, G. W. H.; Leon, S.; Zerbetto, F.
trum of [1]rotaxane 1 and [1]rotaxane 2, photoisomeric
and variable-temperature UV absorption spectra of
[1]rotaxane 2 can be found in the PDF format. Supple-
mentary data associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2007.07.209.
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