A light-powered stretch-contraction supramolecular system based on cobalt coordinated [1]rotaxane

Article abstract of DOI:10.1039/c0ob00764a

A mechanically switchable bistable [1]rotaxane, constituted of azobenzene modified cyclodextrins (CyDs) and a Schiff base bridged by a metallosalen unit, was designed and synthesized. ¹H NOESY NMR and ICD spectra were investigated to characterize the movement process of this stretch-contraction supramolecular system. The geometries of [1]rotaxane before and after irradiation by UV light were optimized and calculated. Coordinated with cobalt(iii) ion, the rotaxane becomes more rigid and linear, which is seen from the more obvious signals in the induced circular dichroism (ICD) and ¹H NMR spectra. This type of light-powered [1]rotaxane has favourable repeatability and exhibits a novel approach to elaborate the transformation of a light-driven molecular machine.

Full text of DOI:10.1039/c0ob00764a

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PAPER
A light-powered stretch–contraction supramolecular system based on cobalt
coordinated [1]rotaxane†
Chao Gao, Xiang Ma, Qiong Zhang, Qiaochun Wang, Dahui Qu and He Tian*
Received 22nd September 2010, Accepted 19th October 2010
DOI: 10.1039/c0ob00764a
A mechanically switchable bistable [1]rotaxane, constituted of azobenzene modified cyclodextrins
(CyDs) and a Schiff base bridged by a metallosalen unit, was designed and synthesized. 1H NOESY
NMR and ICD spectra were investigated to characterize the movement process of this
stretch–contraction supramolecular system. The geometries of [1]rotaxane before and after irradiation
by UV light were optimized and calculated. Coordinated with cobalt(III) ion, the rotaxane becomes
more rigid and linear, which is seen from the more obvious signals in the induced circular
dichroism (ICD) and 1H NMR spectra. This type of light-powered [1]rotaxane has favourable
repeatability and exhibits a novel approach to elaborate the transformation of a light-driven molecular
machine.
reported that b-CyD derivatives formed self-inclusion complexes
in aqueous solution and focused on their dynamic behavior.
Recently our group reported a fully optical CyD-based [1]rotaxane
via self-complementary and Suzuki-coupling capping.11 Most
[1]rotaxanes contain only one macrocycle and a bulky stopper, till
now it is rare to use symmetrical structures to form [1]rotaxanes
containing two macrocycles. Here we prepare self-assembled b-
CyD derivatives connecting with each other via metal salen unit
to form a novel light-driven [1]rotaxane.
Obviously, photochemical stimulation is one of the valuable
choices to be employed in the design of molecular machines.
Photochemical responses have self-evident advantages: (1) the
systems can be stimulated much more convenient and switched
faster; (2) they can work without generating any chemical waste;
(3) some suitable optical output signals can be used for detecting
(reading) mechanical molecular motions.12 Stilbene and azoben-
zene moieties 13 can be photoisomerized efficiently depending on
photochemical stimulation, and are widely used in self-assembled
Introduction
Supramolecular chemistry, since this basic concept was originated
by J. M. Lehn et al.,1 has become a perfect bridge to tightly
combine biology and chemistry. Profiting from the elegance and
complexity of biological systems,2 a wealth of inspiration is pro-
vided for synthetic chemists. And artificial molecular machines 3
have gradually appeared as one of the emerging fields of chemistry
in the past decade. It has a wide range of potential applications in
many fields such as molecular wires,4 molecular memories and
5
logic gates.6 The construction of molecular engineering attracts
a lot of attention from material chemists devoting themselves to
build their own nano-empires.
Beginning with the first molecular shuttle synthesized by
Stoddart et al. in 1990s,7 molecules or molecular assemblies, for
which a certain part can be set into motion deliberately, have
triggered great interest as “machines” in the course of years.
According to different properties, such as different host–guest
systems, and different stimulated manners, the assemblies are
divided into different categories. Till now various molecular ma-
chines become more complicated and functional.8 Rotaxanes, as
mechanically interlocked molecular architectures, have developed
extremely fast in recent years. The first CyD-based [1]rotaxane
systems. More recently a stilbene rotaxane based on cyclodextrin
13c
driven by UV light was synthesized by Easton
et al. to
realize the contraction and extension of a muscle. On the other
hand, owing to its high hydrophilicity, low toxicity, and selective
14
recognition towards many model substrates, cyclodextrin
as
9
was synthesized by Easton et al. starting from a [2]rotaxane
via an indirect method by fixing a covalent linkage between the
stilbene axis and the CyD rings. Henceforth Harada
a natural macrocycle has a huge range of applications in drug
transportation, biological or pharmic research and biological
catalysts.14b The cyclodextrin encircling some guests such as the
azobenzene moiety can generate induced circular dichroism (ICD)
signals,15 which can be set as an optical output signal to observe the
motions of molecular compounds. ICD spectra can undoubtedly
monitor the molecular-scale motions much faster and more
sensitively when the rotaxane is irradiated by UV light. The
introduction of a cobalt ion strengthens the rigidity and linearity
of this molecule, which amplifies and regularizes the ICD signals.
10
et al.
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science & Technology, Meilong Road 130,
Shanghai 200237, P. R. China. E-mail: tianhe@ecust.edu.cn; Fax: +86-21
64252288
† Electronic supplementary information (ESI) available: Synthetic meth-
ods, 2D NMR spectrum, UV-Vis and ICD spectra details. See DOI:
10.1039/c0ob00764a
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Here we report the synthesis of a symmetric [1]rotaxane
combined with a cobalt(III) ion bridged by Schiff base units 16 as a
light-powered molecular machine. We emphasize the performance
of the azobenzene moiety and the Schiff base unit before and after
UV irradiation in ICD spectra. The [1]rotaxane generates obvious
signal changes after UV irradiation which indicated the motion
of rotaxane through ICD spectra. b-CyD derivatives possessing
functional azobenzene groups and the Schiff units are discussed
from the viewpoints of the size/shape matching and induced-fit
interaction between the host and guest. 1H NOESY NMR spectra
are also employed to characterize the movement process of this
[1]rotaxane. This novel light-driven synthetic molecular actuator
might be constructed as a potential nano-generator.
the CyD cavity, then reacted with two equivalents of ethyl diamine
(EDA) to create the intermediate compound. After this solution
was extracted by CH2Cl2 to remove the 1-adamantanol, the rest
of the procedures for synthesizing compound RASC-Co are the
same as that of ASC-Co.
Both ASC-Co and the reference compound RASC-Co were
1
1
characterized by means of H NMR spectra, H NOESY NMR
spectra and MALDI-TOF mass spectrometry. Fig. 1 shows the
1
partial H NMR spectra (500 MHz) of ASC-Co and RASC-Co
in D2O (298 K). The chemical shifts of Hc, Hd protons and Hf of
the thread decrease by d = 0.5, 0.17 and 0.21 ppm in the ASC-
Co with respect to RASC-Co and the chemical shifts of Ha, a’
,
Hb, b’ and He protons increase by d = 0.30, 0.15 and 0.22 ppm,
respectively. The reasons giving rise to these changes of signals are
multiple, and the influence of the b-CyD cavity on the electron
cloud density of these protons is the most important, besides, the
spatial rotation and twist of the azobenzene moiety plays a role.17
The MALDI-TOF spectra of ASC-Co and RASC-Co are similar
and both exhibit the same signal at m/z 1451.5, which corresponds
to [ASC-Co (or RASC-Co) + Na - H2O]2+ respectively, since the
m/z of RASC-Co and ASC-Co are identical (Figure S1 and S2 in
Supporting Information).
Results and discussion
Synthesis and characterization of [1]rotaxane (ASC-Co)
The synthesis of ASC-Co was started from the compound mono-6-
deoxy-6-(4-aminobenzoyl)-b-CyD (B), as illustrated in Scheme 1.
B was prepared from sodium 4-aminobenzoate and 6-TsO-b-
CyD. 6-Azo-CyD (C) was obtained from B and salicylaldehyde
in 32% yield. C was stirred in aqueous solution for 24 h before
reacting with two equivalents of ethyl diamine (EDA) to create
the [1]rotaxane (ASC). After dialysis through a 2000 MWCO
cellulose membrane with distilled water (5 L), the pure ASC
was coordinated with cobalt ion in aqueous solution for 3 h and
then gave birth to the target product [1]rotaxane-Co (ASC-Co).
C was assembled with 1-adamantanol firstly to form an inclusion
complex after stirring with ten equivalents of 1-adamantanol for
24 h in water solution in order to push the azobenzene unit out of
Fig. 1 Partial 1H NMR spectra of RASC-Co (a) and ASC-Co (b) in D2O
at 25 ◦C.
1
The H NOESY NMR spectra may supply direct evidence to
support the inclusion and response to the relative spatial position.
Some protons located close in space can display NOE (Nuclear
Overhauser Effect) cross-peaks between the relevant protons in 1H
NOESY NMR spectra. The presence of the azobenzene moiety
1
inside the b-CyD cavity in ASC-Co has been confirmed by H
NMR NOESY spectroscopy in aqueous solution. The NOESY
spectrum of ASC-Co (Fig. 2) displays identifiable NOE cross-
peaks between H3, H5, H6 and H6¢ of b-CyD and Hb, Hb’ of
azobenzene moiety; H5, H6 protons of b-CyD and Ha, Ha’ protons
of azobenzene moiety; H3 proton of b-CyD and Hd, He protons
of azobenzene moiety. Whereas, the NOE cross-peak between H2
protons of b-CyD and Hf proton of Schiff base unit also shows
clear correlation. Hf proton belongs to the -C N- group which
used to be outside the cavity and forms no signal in the 1H NOESY
NMR, but the calculated structure below of ASC-Co suggests that
the Hf proton is much closer to the H2 protons of b-CyD which
causes the strong NOE cross-peaks.
Compared with the ASC-Co, there are no obvious NOE cross-
peaks between the H3 and H5 protons on the interior of the b-CyD
annulus and the aromatic protons in the 2D NOESY spectrum
of RASC-Co (Figure S3 in SI), indicating the location of the
Scheme
[1]rotaxane-Co (ASC-Co).
1 The synthesis of RASC-Co, [1]rotaxane (ASC) and
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Fig. 3 Partial 1H NMR spectra of ASC-Co in the original state (a) and
in the photostationary state (b) after irradiation by 365 nm for 1 h in D2O.
7.13 (He) and 7.93 (Hb, b’) ppm, respectively. The signals of Ha* and
Hb* protons are up-field shifted (Dd up to 0.45 and 1.18 ppm) with
respect to the original positions, and other protons are also shifted
compared with their initial positions. It is reasonable that the
aromatic protons signals of the azobenzene moiety are generally
shifted upon their isomerization from trans to cis configuration, as
a result of a complex effect of relative spatial configuration changes
and magnetic shielding effect caused by the aromatic rings in the
other half of azobenzene unit when the two aromatic rings close.
Integrals of the two signals for Hf and Hf* protons appear with a
1 : 2 ratio, which suggests that, at the photostationary state, about
67% of trans-ASC-Co transformed to the cis-ASC-Co. The ratio
of photoisomerization inferred that due to the transformation
of azobenzene moieties from trans to cis, the two rings crowded
together and shorten the whole molecule.
Fig. 2 Partial 1H NOESY NMR spectra of ASC-Co in D2O at 25 ◦C.
azobenzene moiety above the CyD cavity and the exclusion of the
thread out of the CyD cavity, thereby confirming the structures
of ASC-Co and RASC-Co. Besides, NOE cross-peaks between
1
Hg and Hc are found in the H NOESY NMR of ASC (Figure
S4 in SI), however Hg protons of Schiff base unit have no NOE
cross-peak with the Hc proton of the azobenzene moiety in the 1H
NOESY NMR of ASC-Co. The result suggests that after the ASC
coordinated with cobalt(III) ion, the metallosalen-bridge cannot
move freely, and cobalt(III) ion fixes the bridge so tightly that the
whole molecule becomes more rigid. As a consequence the NOE
cross-peaks intensity of He, Hd, Hc protons of azobenzene moiety
becomes weaker after coordination compared with the signals
in Figure S4, and the azobenzene moiety cannot move further
into the cavity of b-CyD. As shown in Figure S5, irradiation of
ASC-Co at 365 nm for 1 h leads to new sets of NOEs between
protons of the azobenzene unit and protons of b-CyD. The cross-
peaks of Ha*, Hd* and Hc* protons change when contrasted with
the signals before irradiation due to the shift of position. These
results indicate that ASC-Co undergoes photoisomerization and
the azobenzene unit changes from trans to cis and generates some
new chemical shifts. The Ha and Hb protons somehow move
outside or near the narrow rim of b-CyD, in the meantime He,
Hd and Hc protons are drawn into the cavity of b-CyD.
ICD spectrum as a notable output signal becomes a convenient
and widely employed method for the elucidation of the absolute
conformation of chiral organic compounds. ICD experiments on
-4
ASC, ASC-Co and RASC-Co (1.2 ¥ 10 mol dm-3) are carried
out in order to contrast and monitor the stretch or contraction of
the supramolecular system in aqueous solution at 25 ◦C.
With knowledge of previous investigations 18 on the ICD proper-
ties of cyclodextrin modified structures, the positive/negative ICD
signal can be observed once the electric transition dipole moment
of the guest inside the host cavity is aligned parallel/perpendicular
to the axis of cyclodextrin. On the other hand, the ICD signal is
dramatically reversed when the electric transition moment of the
guest is outside the host cavity. Whereas the ICD signal turns to
zero if the angle between the transition moment and the CyD axis
is about 54.7◦.18c Above all, the signal of ICD can supply proper
evidence to testify to the spatial position between the host and the
guest.
Contraction motion with photoisomerization
The E/Z photoisomerization process of ASC-Co in D2O is
investigated by monitoring the changes in relative 1H NMR
spectra. The 1H NMR spectra can provide direct evidence to
prove the variety of positions of ASC-Co: the chemical shifts of
the new protons are revealed because of their different chemical
environments. To validate these hypotheses, the azobenzene-based
ASC-Co was irradiated by the UV light to induce its E/Z
photoisomerization.
As expected and shown in Fig. 3b, irradiation by the UV light
at 365 nm for 1 h leads to some new signals for cis-ASC-Co,
appearing at d = 10.26 (Hf*), 7.94 (Ha*, Hc*, Hd*), 6.90 (He*) and
6.75 (Hb*) ppm, corresponding to the initial peaks of trans-ASC-
Co appearing at d = 9.91 (Hf), 8.39 (Ha, a’), 7.75 (Hd), 7.60 (Hc),
Firstly the differences of ICD signals between RASC-Co and
ASC-Co were compared. As seen from Figure S8, the ICD
spectrum of RASC-Co in aqueous solution shows that a wide
negative Cotton effect peak and wide positive Cotton effect peak
rise at 450 nm and 350 nm irregularly. On the contrary, the
ICD signal of ASC-Co is obvious and regular under comparable
conditions. According to Harata, Kajta´r and Nau’s sector rule
on the ICD phenomena of b-CyD complexes, we can deduce that
the azobenzene and Schiff base moieties in these compounds in
the range of 220–550 nm are induced by two allowed transitions
perpendicular to each other. That is to say, the peaks in the ICD
spectra (Fig. 4) respectively belong to four electric transition dipole
moments: p–p*, n–p* of the azobenzene moiety and p–p*, n–p*
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influence the transformation, the force from photoisomerization
also plays a critical role as two strong arms to drive the two
cyclodextrins to be close and far apart.
Fig. 4 ICD spectra changes for ASC-Co (a) and ASC (b) (25 ◦C, 1.2¥
10-4 mol dm-3) in aqueous solution by irradiation at 365 nm. And the
spectra changes shift back to a large extent by irradiation at 430 nm [Line
ASC-Co (10 min)-back and b (10 min)-back].
Fig. 5 Relationship of the angles a1, b1, a2, b2 and positions between
the b-CyD ring and the linear subunit before (a) and after (b) the
photoisomerization of the azobenzene moiety and the Schiff base unit
in H2O about ASC-Co. a1, a2 are the angles between the axis and the
orientation of p→p* transition which are separately belong to azobenzene
moiety and Schiff base unit while b1, b2 are the angles between the axis
and the orientation of n→p* transition.
of the Schiff base unit. In Fig. 4a, the ICD spectrum of ASC-
Co shows a negative Cotton effect peak at 232 nm, and two
positive Cotton effect peaks at around 281 nm and 352 nm. Upon
irradiation with UV light of 365 nm for 10 min, two new strong
positive Cotton effects increase at around 259 nm, 431 nm and a
strong negative cotton effects at 314 nm reveals gradually.
In addition, Fig. 6 shows that azobenzene and Schiff base units
are both encircled by the cavity of cyclodextrin after irradiation at
around 365 nm, while the azobenzene moiety is the only one unit
which is almost in the cavity of b-CyD before irradiation. In the
Fig. 4a, the ICD signals of the Schiff base unit containing n→p*
transition and p→p* transition have red-shifts of about 26 nm
and 34 nm, respectively.
The angles of the four transitions are used to assist with the ICD
spectra analysis. As Fig. 5a shows, b1 is supposed to be the angle
which causes the signal at around 431 nm to be nearly zero. The
p→p* transition of the azobenzene moiety generates a positive
cotton effect peak that is ascribed to a1. On the contrary, the Schiff
base unit is outside the cavity of b-CyD and n→p* the transition
of the Schiff base unit generates a positive Cotton effect peak
at 281 nm, whereas the p→p* transition of the Schiff case unit
gives rise to a negative Cotton effect. As mentioned above, the
azobenzene moieties in ASC-Co are deep inside the b-CyD cavity
with an acclivitous orientation. After irradiation by 365 nm for
10 min, the azobenzene moiety changes from trans-status to cis-
status. As a consequence the Schiff base unit is forced to be dragged
into the cavity to cause the spectrum signal to change substantially.
With the location of the Schiff base unit moving from outside to
The geometries of trans- and cis-ASC-Co (Fig. 6 below) and
RASC-Co shaft (Figure S6) were optimized with the hybrid
19
20
density functional method B3LYP
using the Gaussian 03
program. The calculations were performed by using the effective
core pseudopotentials (ECP) basis set on the Co(III) ion, and
the standard basis set 3-21G for carbon, oxygen, nitrogen, and
hydrogen atoms. The Ne core electrons of Co were replaced by
an ECP and DZ quality Hay and Wadt Los Alamos ECP basis
21
set (LANL2DZ) was used for the valence electrons. In Fig. 6,
when the hydrogen bond exists, it can be seen that after irradiation
by UV light at 365 nm, the azobenzene units photoisomerized
from trans to cis and the two cyclodextrins move closer together,
accompanied by the contraction of the shaft into an “M” type
due to the hydrogen bonding interaction formed between the two
cyclodextrins. By contrast, the conformation of the shaft (Figure
S6 in SI) near the Co(III) coordination center hardly changes due
to the isomerization, which emphasizes the role of hydrogen bonds
between two cyclodextrins to shrink the molecule. The length of
˚
˚
ASC-Co changes from 26.06 A in trans-status to 19.49 A in cis-
˚
status (DL = 6.57 A). Besides the hydrogen bond as a main factor to
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Fig. 6 Calculated structures of ASC-Co from trans- to cis-form after the irradiations.
inside gradually, the signals reverse relatively. From Fig. 5b, the
Schiff base unit enters the cavity to cause a negative Cotton effect
which is larger than the positive one before, and a positive Cotton
effect peak. Meanwhile, azobenzene moieties cause other changes
of spectrum at the wavelength range between 350 nm and 500 nm.
Compared with the ICD spectrum of ASC-Co, the ICD
spectrum of ASC has some discernable differences (Fig. 4). One
of distinct differences is an extra negative cotton effect at around
392 nm. After irradiation by UV light at 365 nm, the negative
cotton effect peak at 392 nm decreases meanwhile a positive cotton
effect arises at 430 nm concomitantly. As shown in Figure S6 in
SI, the structure does not have absolute mirror-symmetry as we
presumed. The angle of the metallosalen somehow twists against
each other due to the steric hindrance. When the azobenzene
moiety of ASC transformed from the trans- to cis-form, the
azobenzene moiety doesn’t go towards the outside firstly but
shrinks into the cavity of b-CyD. Without the ion combining
with the Schiff base, the shaft with which the b-CyD glides along
is longer than ASC-Co. This means it can provide much more
space for b-CyD to move, which makes the azobenzene moiety
near to the narrow rim. Owing to the relative spatial position, the
azobenzene moiety compresses to the center when transforming.
As the bond vibrates, the unit moves and vibrates towards the
outside. The signal changes from a negative cotton effect to a
positive cotton effect and red-shifts about 40 nm. The ICD signals
of the Schiff base unit are similar to that of ASC-Co. The spectrum
of ASC without metal coordination is not as regular as the one of
ASC-Co.
With respect to the UV/Vis spectroscopy experiments, we
compared the absorption spectra of ASC-Co with RASC-Co in
Fig. 7a, which shows that the absorption value of ASC-Co has
a blue shift from 372 nm to 352 nm (Dl = 20 nm) and the
intensity of absorption was weaker after the azobenzene moieties
embedded in the cavity of b-CyD. And Fig. 7b provides some
details to characterize the motion of this machine. For ASC-
Co, after irradiation upon 365 nm, the absorption intensity of
the azobenzene moiety degenerates from 0.41 to 0.27. Whereas,
the absorption peak of the Schiff base unit strengthens from
0.45 to 0.46 with about an 11 nm red-shift concomitantly. It is
attributed to the photoisomerization of the azobenzene moieties.
When ASC-Co reaches to its photostationary state, a decrease
in absorption at around 348 nm (DA = 0.14) and an increase
in absorption at around 240 nm (DA = 0.01) happen. After
irradiation by 430 nm for ten minutes, about 70.2% cis-ASC-Co
transformed back to the trans-ASC-Co according to Fig. 7b, which
Fig. 7 (a) UV/Vis absorption spectra of RASC-Co (dash) and ASC-Co
(solid); (b) the absorption spectra of ASC-Co (1.1 ¥ 10-5 mol dm-3) in H2O
at 298 K before (solid), after (dash) irradiation at 365 nm for 5 min and
after (dot) irradiation at 430 nm for 10 min.
exhibits good reversibility again. Compared with the UV/Vis
spectroscopy spectra between ASC-Co and ASC (Figure S9 in SI),
it can be seen that the UV absorption intensity of the azobenzene
moiety slightly degenerates after combining with cobalt(III) ion,
besides, the absorption peak of Schiff base unit at 239 nm is more
distinct after combining with cobalt(III) ion. All of the spectra
demonstrate the fact that ASC-Co and ASC both have favorable
photoisomerization properties, moreover the greater rigidity and
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linearity of the compounds through combining with the cobalt(III)
ion encourages the regulation of the signals.
increases the rigidity of the whole compound and induces more
regular signals in the ICD spectra. The ASC-Co containing Schiff
base unit, a part of which is a notable catalyst in biochemistry,
will bring more potential applications. A logical development
by replacing the chemically driven redox process with optical
stimulations would contribute to a technological basis for the
production of a new class of molecular scale devices based upon
nanomechanical motion in switchable interlocked molecules.
Since [1]rotaxane is one of the most crucial targets, the quantity
of repeatability is measured. Since the azobenzene derivative has
good photoreversibility, the photochemical process of the ASC-Co
is highly reproducible over more than five cycles (Fig. 8). Moreover,
the molecular machine is amazingly stable even preserved in
aqueous solution for one month, when the repeatability is the
same as initially.
Acknowledgements
C. Gao and X. Ma contributed equally to this work. This
work is financially supported by National Basic Research 973
Program (2011CB808400), NSFC/China (20972053 & 20802019),
X. Ma thanks the support from the Fundamental Research Funds
for the Central Universities (WJ0911001), the Ph.D. Programs
Foundation of Ministry of Education of China (200802511029)
and the ‘Chen Guang’ project supported by Shanghai Municipal
Education Commission and Shanghai Education Development
Foundation. C. G. is grateful for discussions with Ms. Ruyi Sun,
Mr. Liangliang Zhu.
Notes and references
Fig. 8 The Cotton effect peak of ASC-Co (at 390 nm) on conducting five
optical irradiation switching cycles.
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Besides investigating the motion of ASC-Co and ASC, the
optical spectroscopy of compound C is also observed and it
showed large differences in both UV/Vis and ICD spectra
compared with RASC-Co and ASC-Co. The n→p* transition and
p→p* transition give rise separately to a positive cotton effect and
a negative cotton effect at 459 nm and 389 nm in ICD spectrum
(Figure S10 and S11 in SI). Because the biggest absorbance
wavelength is more than 411 nm as shown in the UV/Vis spectrum,
it did not represent any distinctly regular change in either the
intensity or the wavelength after irradiation by 365 nm for even 1 h.
After the aldehyde group reacts with ethyl diamine, the absorption
peak will move to 361 nm which is less than the absorption peak
of C. It shows that the Schiff base unit employs a remarkable role
in this supramolecular system.
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In summary, a novel light-driven [1]rotaxane based on cyclodex-
trins and azobenzene moieties has been constructed and showed
unique movements. H NMR spectroscopic experiments reveal
1
clearly that the transformation of azobenzene moieties from
trans- to cis- can cause the whole [1]rotaxane to contract or
extend along with the thread components when irradiated by
UV light, which causes the azobenzene moiety to move deep
into the cavity of b-CyD and the Schiff unit begin to enter the
cavity of b-CyD. As a consequence, two units shuttle through the
cavity of b-CyD commutatively leaving obvious tracks in ICD
spectra after irradiation by 365 nm and 430 nm, respectively. The
changes of ICD spectra are highly distinct as the output signal to
monitor the motions of the machine, and theoretical calculated
structures vividly describe the movement and the shape changes
(from “linear” type to “M” type). Combining with cobalt(III) ion
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