Guest-induced supramolecular chirality in a ditopic azoprobe-cyclodextrin complex in water

Article abstract of DOI:10.1039/c4cc04227a

We report a novel supramolecular chirality induced by the twisted structural change of two ditopic azoprobes (15C5-Azo-dpa) inside the chiral cavity of γ-cyclodextrin (γ-CyD) due to multi-point recognition of guest ions by 15C5-Azo-dpa molecules in water. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04227a

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Guest-induced supramolecular chirality in a
ditopic azoprobe–cyclodextrin complex in water†
Cite this: Chem. Commun., 2014,
50, 10059
Kentaro Nonaka, Mai Yamaguchi, Masashi Yasui, Shoji Fujiwara, Takeshi Hashimoto
and Takashi Hayashita*
Received 3rd June 2014,
Accepted 9th July 2014
DOI: 10.1039/c4cc04227a
www.rsc.org/chemcomm
We report a novel supramolecular chirality induced by the twisted
structural change of two ditopic azoprobes (15C5-Azo-dpa) inside the
chiral cavity of c-cyclodextrin (c-CyD) due to multi-point recognition
of guest ions by 15C5-Azo-dpa molecules in water.
Chirality control by supramolecular assemblies and helical polymers
based on chiral templates has received much attention in recent
years.1 Those systems have been widely used in chiroptical devices,2
memory devices,3 and chiral catalysis.4 Pioneering work on supra-
molecular chirality was conducted by Yashima’s group, who designed
chiral recognition systems by using helical polymers.5 Recently,
Kumar et al. reported that naphthalene diimide amphiphiles functio-
nalized with the dipicolylethylenediamine motif self-assembled with
tunable chirality upon molecular recognition of various adenosine
phosphates.6 They showed that competitive guest binding induced
the dynamic reversal of the helix assemblies. Such guest-induced
chirality control is expected to have potential application in the
development of versatile chiral switching and sensing devices.7
Here we report a simple system of guest-induced chirality
control in the 2 : 1 inclusion complex of ditopic azoprobes with
g-cyclodextrin (g-CyD) in water. The ditopic azoprobe is a receptor
Fig. 1 Structures of 15C5-azo-dpa and g-CyD.
2ꢀ
possessing two different recognition sites that are expected to show (ICD) response was noted upon the addition of carbonate (CO3
)
excellent recognition function when two guest molecules exist in or acetate (CH3CO2ꢀ) ions in the presence of both K+ and Zn2+
the system. As shown in Fig. 1, we have designed a ditopic azoprobe ions. Such supramolecular responses were successfully used to
(15C5-Azo-dpa) that bears benzo-15-crown-5 (B15C5) and dipicolyl- sense guest anions in water.
amine (dpa) as the recognition sites (Fig. 1).
The synthesis of 15C5-Azo-dpa proceeded with the azocoupling of
The azochromophore moiety can be used as a photosignal 40-aminobenzo-15-crown-5 with phenol, followed by the introduction
transducer.8 Although 15C5-Azo-dpa is almost insoluble in water, of a bromoethylene spacer using the Williamson ether synthesis.9
g-CyD can improve its solubility in water by forming a stable Then, a dpa moiety was introduced under basic conditions with
inclusion complex with it. The 15C5-Azo-dpa–g-CyD complex was K2CO3, and the obtained product was purified by silica column
found to show a unique response based on the guest-induced chromatography. The structure of 15C5-Azo-dpa was confirmed by
supramolecular chirality in water. An induced circular dichroism 1H NMR and combustion analyses (Fig. S1–S3, ESI†). Details of the
synthesis are available in the ESI (Scheme S1, ESI†).
In our previous work, we showed that B15C5 azoprobes
Department of Materials and Life Sciences, Sophia University, 7-1 Kioi-cho,
selectively formed a 2 : 1 inclusion complex with g-CyD in the
Chiyoda-ku, Tokyo 102-8554, Japan. E-mail: ta-hayas@sophia.ac.jp
presence of K+ ions in water.10 Meanwhile, Hamachi’s group
† Electronic supplementary information (ESI) available: Measurement condi-
and other research groups reported that the Zn2+ complexes of
tions, synthesis of azoprobes, and the measurements of NMR spectra. See DOI:
10.1039/c4cc04227a
dpa units interacted with various phosphate anion derivatives
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Fig. 2 ICD and UV-Vis spectra of 15C5-Azo-dpa–g-CyD sensors.
[15C5-Azo-dpa] = 40 mM, [g-CyD] = 5 mM, and (a) K+, (b) Zn2+, (c) K+
+
Zn2+, (d) K+ + Zn2+ + Tri, in 4% DMSO aq., pH = 11.0 at 25 1C; K+ (50 mM
K2CO3), Zn2+ (40 mM Zn(NO3)2), Tri (3.0 mM Na5P3O10); (M = mol dmꢀ3).
Fig. 3 The changes in UV-Vis spectra and the absorbance ratio of
15C5-Azo-dpa–g-CyD sensor as a function of zinc ion concentration.
[15C5-Azo-dpa] = 20 mM, [Zn2+] = 0 – 45 mM, [K2CO3] = 50 mM, [g-CyD] =
5 mM, in 4% aq. DMSO, pH = 11.0, at 25 1C (M = mol dmꢀ3).
through the unoccupied coordination sites in the Zn2+–dpa
complexes.11 Thus, it is interesting to examine how such guest
species affect the response function of the 15C5-Azo-dpa–g-CyD attests the 1 : 1 complex formation of Zn2+ with the dpa binding site
complex in water. Fig. 2 shows the ICD spectra and the UV-Vis in 15C5-Azo-dpa. The sigmoid response observed in Fig. 3 indicates
absorption spectra of 15C5-Azo-dpa (40 mM) in 5.0 mM g-CyD the occurrence of a successive binding reaction of Zn2+ with the two
aqueous solution with K+ ions (a), with Zn2+ ions (b), with K+ and 15C5-Azo-dpa molecules inside g-CyD. The formation of the 2 : 1
Zn2+ ions (c), and with K+, Zn2+, and triphosphate (Tri) ions (d). complex of 15C5-Azo-dpa probes with g-CyD was confirmed by the
Interestingly, the split-type Cotton effect was observed only in the analysis of Job’s plots (Fig. S4, ESI†).
presence of both K+ and Zn2+ ions (spectrum c in the top panel of
To elucidate the mechanism of the split ICD noted for the
Fig. 2). In addition, the UV-Vis absorbance ratio of 366 nm to 425 nm 15C5-Azo-dpa–g-CyD complex, we examined the salt effect by
(A366/A425) exhibited the largest blue shift in the presence of both K+ substituting 100 mM KNO3 and 100 mM KOH for 50 mM K2CO3 in
and Zn2+ ions. The azobenzene chromophores are known to exhibit the presence of 40 mM Zn2+. Interestingly, no split ICD was observed
strong ICD by forming an inclusion complex with CyDs based on the for the KNO3 and KOH systems, indicating the possibility of CO32ꢀ
chiral nature of the CyD cavities.12 We have shown that this ICD bridging between the two Zn2+–dpa binding sites (data not shown).
response was affected by the dimer formation of the azoprobes The effect of CO32ꢀ concentration on the ICD spectra of the 15C5-Azo-
inside g-CyD to induce a split-type Cotton effect.8 In addition, an dpa–g-CyD complex is depicted in Fig. 4. It is evident that the addition
exciton interaction of the azodimer was found to induce clear of CO32ꢀ ions enhanced the split ICD for the 15C5-Azo-dpa–g-CyD
changes in the UV-Vis absorption spectra. Thus, the observed split complex at pH 11.0. The ICD intensity change as a function of CO32ꢀ
ICD in spectrum c in the top panel of Fig. 2 and the largest blue shift concentration at a constant K+ concentration of 0.10 M was well fitted
in the UV-Vis spectra indicated the formation of a clockwise twisted by a 1 : 1 binding isotherm, and the apparent 1 : 1 binding constant of
dimer of 15C5-Azo-dpa probes inside g-CyD in the presence of both CO3 ions was calculated to be 83.5 ꢁ 6.0 Mꢀ1 (Fig. S5, ESI†). The
2ꢀ
K+ and Zn2+ ions. The addition of triphosphate was found to inhibit triphosphate binding may break this CO32ꢀ bridging of the two 15C5-
the twisted structure formation of the 15C5-Azo-dpa–g-CyD complex Azo-dpa molecules inside g-CyD to reduce the split ICD. In addition,
in water, probably due to the coordination of the triphosphate anion direct evidence of the relative orientation of the azoprobes and
with the Zn2+–dpa complex in 15C5-Azo-dpa.
the macrocyclic ring was obtained by NOESY experiments. Cross
Fig. 3 shows an absorbance vs. wavelength plot and the effects of peaks between H3 protons inside the CyD cavity and protons of
Zn2+ concentration on the UV-Vis absorbance ratio (A366/A425) of the the azoprobes were clearly observed (Fig. S8, ESI†).
15C5-Azo-dpa–g-CyD complex in the presence of 50 mM K2CO3 in
If this CO32ꢀ bridging were the key mechanism for the induction
water. It is evident that the absorbance ratio gradually increased with of the split ICD, a similar split ICD induced by the CH3CO2ꢀ bridging
the addition of Zn2+ and reached a plateau near the equivalence would be expected under neutral pH conditions. In fact, we observed
point with 15C5-Azo-dpa (20 mM). The finding that an equivalent the typical split ICD in the presence of 50 mM CH3CO2K at pH 7.0
amount of Zn2+ in excess of 20 mM caused no spectral changes (Fig. 5). However, it should be noted that the split ICD had peak
10060 | Chem. Commun., 2014, 50, 10059--10061
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(24655069 and 26248038). This communication is dedicated to
Professor Shinkai Seiji (Kyushu University, Sojo University) on
the occasion of his 70th birthday.
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40 mM, [Zn2+] = 40 mM, [K2CO3] = 5–40 mM, [g-CyD] = 5 mM, in 4% DMSO
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Fig. 5 ICD spectra of 15C5-Azo-dpa–g-CyD sensors, [15C5-Azo-dpa] =
40 mM, [Zn2+] = 40 mM, [g-CyD] = 5 mM, (a) [CH3CO2Na] = 50 mM, (b)
[CH3CO2K] = 50 mM, (c) [CH3CO2Cs] = 50 mM, in 4% DMSO aq., pH = 7.0
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2ꢀ
shapes that differed from those of the CO3 bridging system,
indicating that a somewhat different mode of the twisted
structure was formed inside g-CyD. In contrast, no split ICD was
noted in the presence of 50 mM CH3CO2Na or 50 mM CH3CO2Cs.
This indicated that the clockwise twisted structure of the two 15C5-
Azo-dpa molecules inside g-CyD was induced only in the presence of
K+, Zn2+, and the bridging anions of the two dpa–Zn2+ binding sites,
such as CO32ꢀ or CH3CO2ꢀ ions. By titration analysis of CH3CO2ꢀ at
a constant K+ concenꢀtration of 0.10 M, the apparent 1 : 1 binding
constant of CH3CO2 for the 15C5-Azo-dpa–g-CyD complex in
the presence of an equivalent amount of Zn2+ was determined to
be 180 ꢁ 27 Mꢀ1 at pH 7.0 (Fig. S6, ESI†).
In conclusion, we have shown a novel supramolecular
chirality induced by the twisted structural change of two
15C5-Azo-dpa molecules inside the g-CyD chiral cavity due to
multi-point recognition of guest ions by the ditopic azoprobes
in water. In this system, all components of the guest species
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This work was supported by Grants-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
1962, 4, 113.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 10059--10061 | 10061