Dual-controllable stepwise supramolecular interconversions

Article abstract of DOI:10.1039/b926323c

A new unsymmetric guest compound comprising ferrocene and azobenzene moieties was synthesized and can form a host-guest system with β-cyclodextrin (β-CD) complexed. The complexation stoichiometries and sites of the ensembles can be selectively controlled

Full text of DOI:10.1039/b926323c

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Dual-controllable stepwise supramolecular interconversionsw
Liangliang Zhu, Dong Zhang, Dahui Qu, Qiaochun Wang, Xiang Ma and He Tian*
Received (in Cambridge, UK) 14th December 2009, Accepted 16th January 2010
First published as an Advance Article on the web 2nd February 2010
DOI: 10.1039/b926323c
A new unsymmetric guest compound comprising ferrocene and
azobenzene moieties was synthesized and can form a host–guest
system with b-cyclodextrin (b-CD) complexed. The complexation
stoichiometries and sites of the ensembles can be selectively
controlled by reversible redox and photoirradiation.
sequence if they work sufficiently), which leads to the
disassembly of the two b-CD rings together. Moreover, the
rings can be reassembled to the guest in situ for the full
reversibility of the redox and the photochemical operations,
thus the process of the interconversion for steady states of one
2 : 1 complex, two 1 : 1 complexes and one guest compound can
be exhibited. The demonstration of such a controllable process
might be an important step toward the goal of multi-mode
driven supramolecule-to-supramolecule transformations.
FVAC2CD was prepared from the guest compound FVA
by co-grinding of the guest with 2 equiv. solid b-cyclodextrin
for ca. 15 min. Two reference compounds Ref1 and Ref2 were
also synthesized for control experiments, to study individually
the binding ability and assembly–disassembly process of b-CD
to the ferrocene and the azobenzene moieties (Scheme 1). The
complexation of b-CD to the ferrocene and azobenzene
moieties in FVA was studied with measurement of UV-Vis
spectra and cyclic voltammograms (CV). The maximum
absorption band for FVA in water at around 325 nm was
red-shifted (from Curve a to c, Fig. 1) because of the binding
effect of b-CD to the azobenzene moiety. The absorption peak
decreases remarkably with the trans-to-cis isomerization of
azobenzene. But the near superposition of Curves b and d in
Fig. 1 reveals that the isomerized cis-azobenzene species was
hardly complexed because of a progressive disassembly–
photochemical process.9 Adding excess of b-CD led to a shift
in the peak potential in CV towards more positive values
(Fig. 1, inset), which is indeed connected to the fact that FVA
Chemists have been pursuing machinery systems at the
molecular level for almost two decades,1 since these archi-
tectures are showing an increasing potential for materials and
biological applications.2 As a primary prototype of molecular
3
machines, pseudorotaxane-like supramolecular ensembles
could further undergo many flexible reactions or perform
versatile tasks, on account of their unique host–guest
construction and the characteristic assembly/disassembly
behaviors. Thus far, the replacement of components in an
4
ensemble by competing substances has provided a working
principle for tunable supramolecular interconversions.
However, to the best of our knowledge, control of the host
assembly/disassembly behaviors in host–guest systems with
multiple stimuli, to selectively modulate the complexation
stoichiometries and sites, has rarely been reported. Herein,
we demonstrate that a selective control of the host assembly/
disassembly behaviors, and the stepwise interconversion for
multi-steady states of ensembles can be driven by redox and
photoirradiation.
To modulate the complexation stoichiometries and sites,
an unsymmetric guest compound comprising at least two
different binding stations is expected to be effective. Ferrocene
5
and azobenzene groups are typical electro- and light-active
species, respectively, which can also be recognized with high
selectivity by host cyclodextrins (CDs).6 In this work, they
were harnessed to be covalently linked by a viologen unit as a
hydrophilic electropositive barrier,7 and a 2 : 1 complex
(FVAC2CD) can be formed by encircling of two b-cyclodextrin
(b-CD) rings onto both the binding moieties,8 as shown in
Scheme 1. When one of the binding stations is selectively
translated by the external input, the corresponding assembled
b-CD will depart from the guest but the other ring remains
complexing with the other station, resulting in two different
1 : 1 complex isomers (FVA-oCCD, FVA-pCCD). The 2 : 1
complex can also be converted to a guest structure (FVA-op)
when both the external inputs are present (regardless of the
Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science & Technology,
Shanghai 200237, P.R. China. E-mail: tianhe@ecust.edu.cn;
Fax: (+86) 21-64252288; Tel: (+86) 21-64252756
w Electronic supplementary information (ESI) available: Synthetic
details, NMR/2D NOESY NMR spectra, ESI-MS spectra, figures
and curves mentioned in the text, reference data for Ref1 and Ref2. See
DOI: 10.1039/b926323c
Scheme 1 The interconversion network of the dual-driven (redox-
and light-driven) ensembles between the four states: one 2 : 1 complex
(FVAC2CD), two 1 : 1 complexes (FVA-oCCD, FVA-pCCD), and
one guest structure (FVA-op).
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Fig. 1 Absorption spectra of FVA (1.01 ꢁ 10ꢂ4 M) in water at 298 K
(a) in the initial state, (b) after irradiation at 365 nm for 30 min, (c) in
presence of 20 equiv. b-CD and (d) in presence of 20 equiv. b-CD after
irradiation at 365 nm for 30 min. Inset: Cyclic voltammograms of FVA
(1.01 ꢁ 10ꢂ4 M) in water at 298 K (a) in the initial state and (b) in
presence of 20 equiv. b-CD.
is more difficult to oxidize in the presence of b-CD.5c The trend
of the O1 wave with a sigmoidal shape is slightly turned to be
flat on the forward scan (Curve b, Fig. 1, inset), indicating a
progressive disassembly–electrochemical process according to
the observations of Amatore et al.5d However, the current on
the reverse scan exhibited a peak around the R1 wave rather
than a plateau, which confirms that the electrogenerated
ferrocenium species was much less complexed. Such
assembly/disassembly behaviors can also be illustrated by
the huge difference of the association constants between
b-CD and the pair of redox species (ESI, Fig. S4w).
Fig. 2 1H NMR spectra (400 MHz in D2O at 298 K) of (A) FVA;
(B) FVAC2CD; (C) FVA-oCCD, addition of 1.2 equiv. ferric chloride
to FVAC2CD; (D) FVA-pCCD, irradiation on FVAC2CD at 365 nm
for 2 h; (E) FVA-op, irradiation on FVAC2CD at 365 nm for 2 h, then
addition of 1.2 equiv. ferric chloride in situ.
cluster of new signals corresponding to the protons of the
0
azobenzenyl group in cis-form, with Hj to Hm appearing at
0
d = 7.42–6.95 (Fig. 2D). The protons Ha,b remain split but
0
0
0 0
The results mentioned above indicate that both the stations
in compound FVA can be efficiently complexed with a
b-CD ring in an aqueous environment, but conditions are
unfavourable for complexing when ferrocene is oxidized or the
azobenzene is photoisomerized to the cis-form. The assembly/
disassembly process could be repeated because of the
reversibility of the redox and the photochemical operations
(ESI, Fig. S5w). Based on such a controllable assembly/
disassembly effect, next we turn to study the interconversion
Hk ,l return to being overlapped (d = 6.97, Hk ,l in Fig. 2D),
which indicates the ferrocenyl group was still encapsulated by
b-CD but the cis-formed azobenzene was not in the photo-
stationary state. The assembly/disassembly behaviors of the
two 1 : 1 complexes are also confirmed by finding the overlap
of corresponding proton peaks of the guest compounds
(ESI, Fig. S6w). Oxidation of FVA-pCCD can generate the
disassembled state FVA-op. No distinct b-CD ring induced
splitting was found in the 1H NMR spectroscopy. Fig. 2E
features the guest structure of the disassembled state. These
interconverted geometries are also confirmed by finding of the
corresponding NOEs in 2D NOESY NMR spectra (see ESI,
Fig. S7w). The interconversion of b-CD inclusion complexes
will cause componential molecular weight alterations, because
of the assembly/disassembly process of the b-CD ring to the
guest subunit. In this way, the formation of the four states of
the supramolecular ensembles was also evidenced by ESI-MS
(see ESI, Fig. S8w).
1
of b-CD inclusion complexes via H NMR spectroscopy.
As shown in Fig. 2B, it clearly reveals one group of proton
signals of the guest component in FVAC2CD, which is
different from that of FVA (shown in Fig. 2A) and indicates
a thorough complexation with 2 equiv. b-CD at the test
concentration (ca. 1.0 ꢁ 10ꢂ2 M). Also, the change in the
resonance of the ferrocenyl and the azobenzenyl protons is
profound. Before complexing, the proton peaks of Ha,b or Hk,l
in FVA are overlapped (d = 4.08, Ha,b and d = 7.90, Hk,l in
Fig. 2A). The encapsulation of b-CD caused the splitting of
these proton peaks, with Ha,b appearing at d = 4.22–4.15, Hk
appearing at d = 7.93 and Hl appearing at d = 7.88 (Fig. 2B).
Oxidation of the ferrocenyl moiety generated FVA-oCCD.
Although the paramagnetism of the oxidized ferrocenium
moiety10 makes its proton signals Ha,b greatly shielded in the
1H NMR spectroscopy, the resonances of the protons Hk,l are
still split (Fig. 2C). It can be found that the azobenzenyl group
remained well complexed by b-CD in the oxidation state. On
the other hand, irradiation at 365 nm on the FVAC2CD
caused the trans-to-cis photoisomerization of the azobenzene
unit, which generates FVA-pCCD. The irradiation led to a
Cyclodextrin-based supramolecular complexes are typical
species which are very sensitive to generate induced circular
dichroism (ICD) signals.11 Specifically, the encapsulation of
b-CD to both the ferrocene and the azobenzene stations in
FVAC2CD produces a negative Cotton effect of ICD at about
255 nm, and a positive one at around 330 nm (see Fig. 3, curve
a), respectively. These signals take place at wavelengths
corresponding to their UV-Vis absorptions (ESI, Fig. S9w).
The negative ICD signal is attributed to the electronic transi-
tions of the ferrocenyl moiety, which is located in the cavity of
b-CD and aligned vertical to the axis of the chiral
host, whereas the positive one belongs to transitions of the
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In summary, a novel ensemble system of b-CD inclusion
complexes was prepared. The assembly/disassembly behaviors
of the b-CD ring can be selectively controlled by reversible
redox and photochemical operations, thus the modulation of
the complexation stoichiometries and sites was realized.
Meantime, the ensemble structures are equivalent to the
number of ICD signal peaks. Such stepwise supramolecular
interconversions provide an effective paradigm for design and
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This work was supported by NSFC/China (20972053,
20603009), the National Basic Research 973 Program
(2006CB806200), and the Scientific Committee of Shanghai.
We thank Professor Yitao Long and Mr Wei Song for relevant
advice on the electrochemical study.
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Chem. Commun., 2010, 46, 2587–2589 | 2589