Structural effects of ditopic azoprobe-cyclodextrin complexes on the selectivity of guest-induced supramolecular chirality

Article abstract of DOI:10.1039/c8cc02242a

Benzo-15-crown-5 and dipicolylamine are contained as the binding sites in a ditopic azoprobe (15C5-Azo-n-dpa). However, the selectivities of guest-induced supramolecular chirality for cations and anions were dramatically altered by a slight change in the spacer length of (15C5-Azo-n-dpa)₂-γ-CyD complexes in water.

Full text of DOI:10.1039/c8cc02242a

Showcasing research from the analytical chemistry group of
Professor Takashi Hayashita at Department of Materials and Life
Sciences, Faculty of Science and Technology, Sophia University,
Tokyo, Japan.
As featured in:
Structural effects of ditopic azoprobe–cyclodextrin complexes
on the selectivity of guest-induced supramolecular chirality
The selectivities of guest-induced supramolecular chirality for
cations and anions were dramatically altered by a slight change
in the spacer length of (15C5-Azo-n-dpa) –γ-cyclodextrin
2
complexes in water.
See Takashi Hayashita et al.,
Chem. Commun., 2018, 54, 12690.
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Structural effects of ditopic azoprobe–
cyclodextrin complexes on the selectivity of
guest-induced supramolecular chirality†
Cite this: Chem. Commun., 2018,
4, 12690
5
Received 20th March 2018,
Accepted 16th June 2018
ab
b
b
b
Shoji Fujiwara,
Takashi Hayashita
Kentaro Nonaka, Mai Yamaguchi, Takeshi Hashimoto
*
and
b
DOI: 10.1039/c8cc02242a
rsc.li/chemcomm
Benzo-15-crown-5 and dipicolylamine are contained as the binding structure of the azoprobe dimer inside the g-CyD cavity, only
+
2+
2ꢀ 5
sites in a ditopic azoprobe (15C5-Azo-n-dpa). However, the selectivities when it recognizes K and Zn in the presence of CO3 . From
of guest-induced supramolecular chirality for cations and anions the ICD spectral change, we can estimate the spatial changes of
were dramatically altered by a slight change in the spacer length of the azoprobe dimer inside g-CyD, which induce a change in the
6
(
2
15C5-Azo-n-dpa) –c-CyD complexes in water.
UV-Vis spectra. Therefore each guest ion can be selectively
detected in the presence of the other guest ions by measuring
Chirality control by supramolecular assemblies and helical the spectral changes.
polymers based on chiral templates has attracted much atten-
tion in recent years. Especially guest-induced chirality control the selectivity of guest-induced supramolecular chirality. The
Herein we report how the 15C5-Azo-n-dpa structure affects
1
is expected to apply for the development of versatile chiral dramatic selectivity changes of supramolecular chirality were
2
switching and sensing systems. To obtain the supramolecular found to be noted for (15C5-Azo-n-dpa) –g-CyD complexes by
2
chirality function, cyclodextrins (CyDs) are quite attractive host controlling the spacer length of 15C5-Azo-n-dpa from ethylene
3
molecules. Optically inert CyDs have a chiral nature in their (n = 2) to propylene (n = 3) to butylene (n = 4) units (Fig. 1). Also,
cavities and can be efficiently combined with various achiral while 15C5-Azo-n-dpa has a dpa binding site for heavy metal
chromoionophores and fluoroionophores to induce chiral nature ions, the selectivity of guest-induced supramolecular chirality
4
2+
by forming an inclusion complex with CyDs. In the previous changed from Zn for the (15C5-Azo-2-dpa)
2
–g-CyD complex to
study, we have reported a ditopic azoprobe (15C5-Azo-2-dpa)
bearing benzo-15-crown-5 (B15C5) and dipicolylamine (dpa)
as recognition sites. 15C5-Azo-2-dpa was found to form a 2 : 1
complex with g-CyD and show a unique response function based
5
on guest-induced supramolecular chirality in water. By allowing
ditopic azoprobes to be incorporated into g-CyD, we revealed the
response behavior of the supramolecular (15C5-Azo-2-dpa) –g-
2
CyD complex in the presence of each cationic and anionic
species by measuring induced circular dichroism (ICD) spectra
and UV-visible (Vis) absorption spectra. We confirmed that only
+
2+
2ꢀ
when K , Zn , and CO
were all present, a large split-type Cotton
3
effect appeared in the measured ICD spectra and a significant
short-wavelength shift took place in the measured UV-Vis spectra.
The result clearly demonstrates that the (15C5-Azo-2-dpa) –g-CyD
2
complex can exhibit supramolecular chirality due to the twisted
a
Department of Current Legal Studies, Faculty of Law, Meiji Gakuin University,
1
518 Kamikurata-cho, Totsuka-ku, Yokohama, Kanagawa 244-8539, Japan
b
Department of Materials and Life Sciences, Faculty of Science and Technology,
Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan.
E-mail: ta-hayas@sophia.ac.jp
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8cc02242a
Fig. 1 Structure of 15C5-Azo-n-dpa and the twisted structures.
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2
+
2
Cu for the (15C5-Azo-4-dpa) –g-CyD complex. However, most
2
+
2+
dpa-based chemosensors display Zn and/or Cd selectivity in
7
water. Thus this is a unique example where the dpa-based
supramolecular sensor exhibits a selectivity change from Zn to
Cu caused by a change in the spacer length. We also report the
specific selectivity changes of guest-induced supramolecular chirality
for alkali-metal cations and anions based on the change in the
2+
2+
2
spacer length of (15C5-Azo-n-dpa) –g-CyD complexes in water.
The synthesis of 15C5-Azo-n-dpa was carried out by the
0
azocoupling of 4 -aminobenzo-15-crown-5 with phenol, followed
by the introduction of bromoethylene, bromopropylene, and
8
bromobutylene spacers using the Williamson ether synthesis.
Then a dpa moiety was introduced under basic conditions with
K CO , and the obtained products were purified using silica gel
2 3
column chromatography. The structures of 15C5-Azo-n-dpa were
1
confirmed via H NMR and combustion analyses. Details of the
synthesis are available in the ESI.†
Job’s plot analyses clearly revealed that 15C5-Azo-n-dpa
formed a 2 : 1 inclusion complex with g-CyD in 4% DMSO–96%
water (v/v) (Fig. S6, ESI†). The ICD spectra of (15C5-Azo-n-dpa)2–
g-CyD complexes are depicted in Fig. 2. For 15C5-Azo-2-dpa
2
+
(
Fig. 2a), the selective ICD response was only noted for Zn over
2
+
3+
2+
2+
2+
2+
other metal ions (Mg , Fe , Ni , Cu , Cd , and Pb , as
nitrate salts) in the presence of 50 mM K CO . The split ICD
2
3
from the negative peak at 351 nm to the positive peak at 394 nm
indicates the clockwise twisted structure of the two azoprobes
6
inside the g-CyD cavity. For 15C5-Azo-3-dpa, however, the ICD
2
+
response was found to be observed not only for Zn but also
2
+
for Cu in the presence of 50 mM K CO . Interestingly the ICD
2
3
2
+
2+
peak shape for Cu was opposite compared with that for Zn ,
2
+
indicating that the Cu complex formed an anti-clockwise
twisted structure inside the g-CyD cavity. This change in the
twisted structure may be due to the difference in the coordination
configuration; Cu was capable of forming a coordination bond
with the phenoxy ether oxygen in the dpa complexes, whereas
Fig. 2 ICD spectra of 15C5-Azo-n-dpa/g-CyD sensors in 4% DMSO aq.:
(
a) [15C5-Azo-2-dpa]; (b) [15C5-Azo-3-dpa]; (c) [15C5-Azo-4-dpa] =
2+
0
5
.04 mM, [Zn(NO
3 2 3 2
) ] = 0.04 mM, [Cu(NO ) ] = 0.04 mM, [g-CyD] =
9
mM, [K CO ] = 50 mM.
2
3
only a few coordination bonds with the phenoxy ether oxygen
2
+
10
were noted for the Zn –dpa complexes. For 15C5-Azo-4-dpa, the
2+
selective ICD response was only noted for Cu over other metal z-axis of the g-CyD cavity, the abovementioned factors make
2+
3+
2+
2+
2+
2+
ions (Mg , Fe , Ni , Zn , Cd , and Pb , as nitrate salts) in the the detailed understanding of guest-induced ICD responses
presence of 50 mM K CO . Although 15C5-Azo-n-dpa possesses difficult. To obtain further evidence for the guest-induced ICD
the same dpa binding site for heavy metal ions, the selectivity was responses, additional analysis based on molecular mechanics
dramatically changed from Zn for the (15C5-Azo-2-dpa)
complex to Cu for the (15C5-Azo-4-dpa) –g-CyD complex in are to be conducted.
the presence of K CO . The (15C5-Azo-3-dpa) –g-CyD complex
exhibited selectivity for both Zn and Cu , indicating an inter- Azo-n-dpa (20 mM), and 50 mM CO
mediate selectivity between 15C5-Azo-2-dpa and 15C5-Azo-4-dpa.
2
3
2+
2
–g-CyD and TD-DFT calculations as well as X-ray crystallography analysis
2+
2
2
+
In the presence of equivalent amounts of Zn with 15C5-
2
3
2
2+
2+
2ꢀ
3
, the ICD intensities at the
maximum wavelength are plotted against the alkali metal ion
11
It is evident that the spacer length of 15C5-Azo-n-dpa played an diameter (Fig. 3a). As we reported previously, the (15C5-Azo-2-
+
important role in controlling the selectivity of guest-induced supra- dpa)
2
–g-CyD complex exhibited high K ion selectivity over
2
+
2ꢀ
molecular chirality. As shown in Fig. 2, it should be noted that other alkali metal ions in the presence of Zn and CO
3
.
the shapes of split Cotton effects based on p–p* transition are This selectivity is consistent with the selectivity of sandwich
not symmetric, indicating the overlap of the Cotton effect based complex formation of the two benzo-15-crown-5 derivatives
8
,12
on n–p* transition at the longer wavelength. In addition, the with alkali metal ions.
However, for the (15C5-Azo-3-dpa)2–
location of azobenzenes along the z-axis of CyD is known to g-CyD complex, the alkali metal ion selectivity was significantly
6
strongly affect the sign and intensity of the Cotton effect.
reduced (Fig. 3a). This indicates that the formation of the
Although we consider that the bulky and hydrophobic B15C5 clockwise twisted structure is dominated only by the bridge
2
ꢀ
2+
moieties restrict the movement of azobenzenes along the formation of CO
3
with the two dpa–Zn complexes in the
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Fig. 3 Selectivity of 15C5-Azo-n-dpa/g-CyD sensors toward alkali metal
ions in 4% DMSO aq., [g-CyD] = 5 mM, [alkali metal ion] = 50 mM: (a) [15C5-
2+
Fig. 5 Selectivity of (15C5-Azo-n-dpa-Cu
2
) –g-CyD sensors toward anions
in 4% DMSO aq.: (a) [15C5-Azo-3-dpa] = 0.04 mM; (b) [15C5-Azo-4-dpa] =
Azo-2-dpa], [15C5-Azo-3-dpa]
=
0.04 mM, [Zn(NO
3 2
) ]
=
0.04 mM;
ꢀ
ꢀ
ꢀ
3
],
0
.04 mM, [Cu(NO
3
2ꢀ
)
2
] = 0.04 mM, [g-CyD] = 5 mM, [Cl ], [OH ], [NO
(
b) [15C5-Azo-3dpa], [15C5-Azo-4-dpa] = 0.04 mM, [Cu(NO
3
)
2
] = 0.04 mM.
ꢀ
[
CH
3
CO
2
], [CO ] = 50 mM.
3
(
1
15C5-Azo-3-dpa)
5C5-Azo-n-dpa upon changing the spacer from ethylene (n = 2) to
propylene (n = 3) should be the reason of this selectivity change. On
2
–g-CyD complex. The enhanced flexibility of
2+
On the other hand, for the Cu complex system of (15C5-Azo-n-
dpa) –g-CyD complexes (n = 3 and 4), the (15C5-Azo-n-dpa) –g-
2
2
2ꢀ
ꢀ
CyD complexes showed both CO3 and OH selectivity (Fig. 5).
This indicates that hydroxo-bridging between the two Cu –dpa
binding sites induced the anti-clockwise twisted structure of the
azoprobe dimer in the (15C5-Azo-n-dpa)
by changing the metal species of the dpa binding sites, the anion
selectivity of the (15C5-Azo-3-dpa)
selectivity with the Zn system to both
and OH selectivity with the Cu system in water. These are
apparently a unique function of guest-induced supramolecular
chirality based on (15C5-Azo-n-dpa) –g-CyD complexes.
In conclusion, we have shown a novel guest-induced supra-
molecular chirality induced by twisted structural switching of
the two 15C5-Azo-n-dpa molecules inside the g-CyD chiral cavity
due to multi-point recognition of guest ions by the ditopic
azoprobes in water. Although the two recognition sites are the
same, a slight change in the spacer length of 15C5-Azo-n-dpa
was found to significantly affect the ICD response selectivity
2+
the other hand, in the presence of equivalent amounts of Cu with
1
g-CyD complex exhibited high K ion selectivity similar to the Zn
system of the (15C5-Azo-2-dpa) –g-CyD complex (Fig. 3b). However,
2+
2
ꢀ
5C5-Azo-n-dpa (20 mM), and 50 mM CO
3
2
, the (15C5-Azo-3-dpa) –
+
2+
2
–g-CyD complex. Thus,
2
for the (15C5-Azo-4-dpa) –g-CyD complex, no alkali metal ion selec-
2
2
–g-CyD complex can be easily
tivity was observed in the anti-clockwise twisted structure of the two
azoprobes inside the g-CyD cavity (Fig. 3b). This is also ascribed to
the enhanced flexibility of 15C5-Azo-n-dpa upon changing the spacer
from propylene (n = 3) to butylene (n = 4).
2ꢀ
2+
controlled from the CO
3
2ꢀ
ꢀ
2+
CO
3
2
The effect of anion species on the ICD responses was also
2
+
examined for (15C5-Azo-n-dpa) –g-CyD complexes. For the Zn
complex system of (15C5-Azo-n-dpa) –g-CyD complexes (n = 2, 3),
the effects of salt species on ICD responses were examined in the
presence of 50 mM KX (X = NO
results are depicted in Fig. 4. Similar to the (15C5-Azo-2-dpa)
CyD complex, the (15C5-Azo-3-dpa) –g-CyD complex exhibited
2
2
ꢀ
ꢀ
ꢀ
3
, CH CO
3 2
, and OH ). The
2
–g-
2
2ꢀ
2ꢀ
high CO
3
+
selectivity, indicating that CO
3
bridging with the
of (15C5-Azo-n-dpa) –g-CyD complexes. To the best of our
2
2
two Zn –dpa binding sites induced the clockwise twisted struc-
ture of the azoprobe dimer inside the g-CyD cavity. In addition,
direct evidence of the relative orientation of the azoprobes and
the macrocyclic ring was obtained via NOESY experiments.
Cross-peaks between H3 protons inside the CyD cavity and
protons of the azoprobes were clearly observed (Fig. S7, ESI†).
knowledge, this is a novel selectivity control based on guest-
induced supramolecular chirality which completely differs
from the design strategy of conventional molecular recognition
systems.
This work was supported by Grants-in-Aid for Scientific
Research (A) (Grant No. JP26248038) and Scientific Research
(C) (Grant No. JP15K05548) from Japan Society for the Promotion
of Science (JSPS). We also thank Prof. Tony D. James (University
of Bath) for his helpful advice.
Conflicts of interest
There are no conflicts to declare.
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Fig. 4 Selectivity of (15C5-Azo-n-dpa-Zn
2
) –g-CyD sensors toward
anions in 4% DMSO aq.: (a) [15C5-Azo-2-dpa] = 0.04 mM; (b) [15C5-
ꢀ
Azo-3-dpa] = 0.04 mM, [Zn(NO
3
2ꢀ
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] = 0.04 mM, [g-CyD] = 5 mM, [Cl ],
ꢀ
ꢀ
ꢀ
[
OH ], [NO
3
], [CH
3
CO
2
], [CO ] = 50 mM.
3
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3
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