Synthesis and evaluation of a novel fluorescent sensor based on?hexahomotrioxacalix[3]arene for Zn2+and Cd2+

Article abstract of DOI:10.1016/j.tet.2016.06.055

A novel type of selective and sensitive fluorescent sensor having triazole rings as the binding sites on the lower rim of a hexahomotrioxacalix[3]arene scaffold in a cone conformation is reported. This sensor has desirable properties for practical applica

Full text of DOI:10.1016/j.tet.2016.06.055

Tetrahedron 72 (2016) 4854e4858
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Synthesis and evaluation of a novel fluorescent sensor based
on hexahomotrioxacalix[3]arene for Zn2þ and Cd2þ
Xue-Kai Jiang a, Yusuke Ikejiri a, Cheng-Cheng Jin a, Chong Wu a, Jiang-Lin Zhao a,
,
Xin-Long Ni b, Xi Zeng b, Carl Redshawc, Takehiko Yamato a
*
a Department of Applied Chemistry, Faculty of Science and Engineering, Saga University, Honjo-machi 1, Saga, 840-8502, Japan
b Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang, Guizhou, 550025, China
c Department of Chemistry, The University of Hull, Cottingham Road, Hull, Yorkshire, HU6 7RX, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2016
Received in revised form 20 June 2016
Accepted 21 June 2016
Available online 23 June 2016
A novel type of selective and sensitive fluorescent sensor having triazole rings as the binding sites on the
lower rim of a hexahomotrioxacalix[3]arene scaffold in a cone conformation is reported. This sensor has
desirable properties for practical applications, including selectivity for detecting Zn2þ and Cd2þ in the
presence of excess competing metal ions at low ion concentration or as a fluorescence enhancement type
chemosensor due to the cavity of calixarene changing from a ‘flattened-cone’ to a more-upright form and
inhibition of PET. In contrast, the results suggested that receptor 1 is highly sensitive and selective for
Cu2þ and Fe3þ as a fluorescence quenching type chemosensor due to the photoinduced electron transfer
(PET) or heavy atom effect.
Keywords:
Hexahomotrioxacalix[3]arene
Metal ion recognition
Pyrenyl-appended triazole
Fluorescent sensor
Zinc cation
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Calixarenes and their derivatives are attractive compounds for
use in hosteguest and supramolecular chemistry. In particular
Owing to their simplicity, high sensitivity, and low detection
limits for trace chemicals in chemistry, biology, and the environ-
ment,1,2 fluorescent chemosensors have received much attention
in the field of supramolecular chemistry. Generally, an effective
fluorescent chemosensor consists of an ion recognition unit and
a fluorogenic unit, which converts the actuating signal from the
ionophore unit into a light signal. Amongst the different fluoro-
genic units, anthracene derivatives are key species in the design of
fluorescent chemosensors materials, which have found wide uti-
lization in lasers, phosphors, and light-emitting devices.3 Al-
hexahomotrioxacalix[3]arene derivatives with C3-symmetry can
selectively bind ammonium ions which play important roles in
both chemistry and biology.5,6 Furthermore, the incorporation of
two types of recognition sites via the introduction of different
ionophores on the hexahomotrioxacalix[3]arene will create po-
tential hetero-ditopic receptors with the capability of binding cat-
ions and anions, e.g., ammonium ions and halides. Therefore, many
fluorescent chemosensors based on calixarenes, which show highly
selective recognition of metal cations,7 ammonium cations,8 and
fullerene derivatives, have been reported.9
though
a
tremendous number of anthracene-based organic
Additionally, the use of Click chemistry10 has seen a significant
growth in the derivatization of calixarenes owing to its reliability,
specificity, biocompatibility, and efficiency. It has been proven to be
a promising strategy for the chemical modification of calixarenes.
In 2005, Zhao and co-workers11 applied Click chemistry to the
synthesis of water soluble calixarenes, which laid a solid founda-
tion for this methodology. Click chemistry has also been used to
synthesize calixarene conjugates of chromophores and bioactive
molecules such as glycosides,12 sialoclusters,13 and amino acids.14
Because of the highly selective nature of the alkyne-azide cyclo-
addition, the Click reaction is a general method to introduce various
functional groups/moieties at the upper or lower rim of calixarenes.
Therefore, we hypothesized that suitably arranged functionalized
materials have been investigated with the aim of potential appli-
cations as photoluminescence (PL) and/or electroluminescence
(EL) devices in films and the solid state, the practical development
of PL and EL devices is in fact restricted, usually owing to their poor
stability. In contrast, strongly luminescent anthracene-based
inorganiceorganic hybrid materials with higher stability could
be a class of promising candidates for light-emitting as well as EL
applications.4
* Corresponding author. Fax: þ81 952 28 8548; e-mail address: yamatot@cc.
saga-u.ac.jp (T. Yamato).
http://dx.doi.org/10.1016/j.tet.2016.06.055
0040-4020/Ó 2016 Elsevier Ltd. All rights reserved.

X.-K. Jiang et al. / Tetrahedron 72 (2016) 4854e4858
4855
groups containing nitrogen atoms attached to a hexahomotriox-
acalix[3]arene should be a good receptor candidate for cations.
Therefore, with this in mind, we have synthesized chemosensor 1
and studied its cation-binding affinity.
The fluorescence emission of the solutions was measured at an
excitation wavelength of 365 nm, and the emission intensities were
measured at 418 nm. Measurements were repeated a minimum of
three times for each addition. At high concentrations, emission
quenching was observed, suggesting the formation of
inter-molecular associates of 1.
2. Results and discussion
However, under dilute conditions, emission quenching was not
observed. The critical association concentration value determined
from the concentration-variable emission spectra was determined
The synthesis of 1 was carried out as shown in Scheme 1. We
first synthesized 3 in 55% yield by the reaction of hexahomotri-
oxacalix[3]arene and propargyl bromide in the presence of
Cs2CO3 in dry acetone solution. The 1H NMR spectroscopic results
suggested that 3 adopts a partial-cone structure.15 Accordingly,
fluorescent compound 1 can be obtained from the reaction of 3
with 9-azidomethylanthracene under standard conditions for Click
chemistry. The coupling of 3 with 9-azidomethyl-anthracene
afforded cone conformation compound 1 in 75% yield. The 1H NMR
spectrum of 1 shows a singlet for the tert-butyl protons at
to be 10 mM. To remove effects of the inter-molecular associates,
absorption and fluorescence measurements were carried out under
critical association concentration. A similar procedure for the
fluorescence measurements of 2 was conducted. The fluorescence
emission of the solutions was measured at an excitation wave-
length of 365 nm, and the emission intensities were measured at
418 nm. More interestingly, compared to 2, the fluorescence in-
tensity of 1 is obviously different from reference compound 2 at the
concentration (30 mM). The fluorescence spectra of 1 and 2 under
d
0.94 ppm, and a doublet at
the 13C NMR spectrum of 1 exhibits two peaks for the methyl and
the quaternary carbon atoms of the t-Bu groups at 31.32,
d 4.03 ppm for the bridge protons, and
the same solutions are shown in Fig. 1. When 2 was excited at
365 nm, strong emission peaks near 400e500 nm were observed,
which were assigned as emission from a single anthracene moiety,
respectively. In contrast, when 1 was excited at 365 nm, the
emission peaks were weak. These observations indicated that the
d
34.04 ppm, three peaks for methylene carbon at 45.51, 66.94,
68.87 ppm and 14 peaks for aromatic carbon, respectively, in-
dicating a C3-symmetric structure for sensor 1. The same procedure
was also employed in the synthesis of 2 from 4-tert-butyl-2,6-
dimethylphenol (Scheme 1). Compound 1 contains both the calix-
arene and the triazole groups as metal ion binding sites, whereas 2
contains only a triazole for metal ion binding. Compare to com-
pound 1, the 1H NMR spectrum of the reference compound 2 shows
that the protons on the anthracene ring appeared at the lower
formation of
pep stacking between the anthracene moieties
appended on 1 can quench the fluorescence.
magnetic fields at around
the proton on the triazole ring also appeared at the lower magnetic
field at 7.08 ppm (Dd 0.11 ppm) (Fig. S23). These findings strongly
indicate the anthracene moieties appended on 1 were sterically-
d 7.52e8.60 ppm (Dd 0.05e0.2 ppm), and
d
fixed to be in close proximity to allow the formation of
pep
stacking between the anthracene moieties.
Fig. 1. Fluorescence spectra of 1 (10 mM) and 2 (30 mM) in CH3CN.
To get an insight into the binding properties of chemosensor 1
toward metal cations, we first investigated the fluorescence
changes upon addition of a wide range of metal cations including
Kþ, Naþ, Mg2þ, Ca2þ, Mn2þ, Cr3þ, Fe2þ, Ni2þ, Sr2þ, Agþ, Hg2þ, Pb2þ
,
Cu2þ, Fe3þ, Zn2þ and Cd2þ. The fluorescence changes are depicted in
Figs. 2 and 3. Addition of Zn2þ and Cd2þ to the solution of 1 induced
obvious ratiometric changes, where the emission increases. By
contrast, no significant spectral changes were observed upon ad-
dition of most of the other metal cations apart from Cu2þ and Fe3þ
where quenching was observed. These results suggest that com-
plexations between 1 and Cu2þ, Fe3þ, Zn2þ and Cd2þ ions through
inter-molecular interactions might be proposed.
In contrast to chemosensor 1, chemosensor 2 exhibited a strong
emission at 418 nm, and a similar experiment was carried out. The
fluorescence intensity changes of 2 upon addition of different metal
ions are shown in Fig. S3. Addition of Cu2þ and Fe3þ caused a strong
and medium fluorescence quenching, respectively.
Scheme 1. Synthetic pathway for compounds 1 and 2. (a) CuI in THF and water, reflux,
20 h.
Solutions giving concentrations of 1 (10
mM) in CH3CN were
prepared as follows. Test solutions were prepared by taking 70 mL
of the calixarene stock solution in a 10 mL volumetric flask, adding
0, 0.2, 0.4, 0.6, 0.8 and 1.0 mL of stock solution, and making up to the
Upon addition of Zn2þ, the fluorescence intensity of solution 1
increased gradually (Fig. 4). The saturation behavior of the fluo-
rescence intensity after adding 2 equiv of Zn2þ reveals that a 1:1
stoichiometry best describes the binding mode of Zn2þ and 1,
which is also supported by the Job’s plot data (Fig. S6). According to
volume with CH3CN. The fluorescence spectrum of 1 (10
mM) in
CH3CN exhibits a characteristic monomer emission of anthracene.

4856
X.-K. Jiang et al. / Tetrahedron 72 (2016) 4854e4858
m
M) upon titration by Cd2þ
Fig. 2. Fluorescence spectra of 1 (10 mM) in CH3CN in the presence of different metal
Fig. 5. Changes in fluorescence emission spectra of 1 (10
(from 0e30 M) in CH3CN (excitation at 365 nm). Inset is molar ratio plot.
ions (10 equiv). Metal ions include Kþ, Naþ, Liþ, Ca2þ, Cr3þ, Ni2þ, Cu2þ, Zn2þ, Agþ, Cd2þ
Hg2þ, Fe3þ, Fe2þ and Pb2þ. Excitation was performed at 365 nm.
,
m
Cd2þ, the fluorescence intensity of solution 1 increased gradually. A
Job’s plot binding between 1 and Cd2þ ion reveals a 1:1 stoichi-
ometry (Fig. S10), while the association constant (Ka value) for the
1400
1200
complexation with Cd2þ ion by
1 was determined to be
4.06ꢀ104 Mꢁ1 as observed by the fluorescence titration experi-
ments in CH3CN. On the other hand, the fluorescence of 1 was ef-
ficiently quenched by Cu2þ and Fe3þ; over 99% fluorescence
quenching was observed with 10 equiv. A Job’s plot binding be-
tween 1 and Cu2þ ion reveals a 1:1 stoichiometry (Fig. S15), while
the association constant of Cu2þ, calculated from the Bene-
1000
800
600
400
200
sieHildebrand equation,16 was found to be 5.79ꢀ105 Mꢁ1
.
Cu2+Hg2+
Cr3+
Ni2+ Fe3+ Fe2+
The compound 1, 1$Cu2þ and 1$Zn2þ mixtures were also ana-
lyzed by HPLC (Fig. 6). A chromatographic peak of derivative 1
appeared at 8.40 min. After the addition of Cu2þ and Zn2þ, the in-
tensity of the peak at 8.40 min decreased, accompanied by the
emergence of a new peaks at 6.30 min and 14.50 min, respectively;
these results demonstrate clearly the formation of new products
(1$Cu2þ and 1$Zn2þ complexes).
2+ Ag+ Cd2+Zn2+
Pb
Li+
0
Na+ K+ Mg2+ Ca2+
-200
Metal ions
M) to 100 mM various tested ions in
Fig. 3. Fluorescence responses of receptor 1 (10
CH3CN. I0 is the fluorescence emission intensity at 418 nm for free receptor 1, and I is
the fluorescent intensity after adding ions at 298 K. lex¼365 nm.
m
Fig. 6. HPLC chromatograms of derivative 1 and complex (a) 0.3 mM of 1; (b) 0.3 mM
of 1 with 6 mM of Cu2þ is added; (c) 0.3 mM of 1 with 6 mM of Zn2þ is added.
Fig. 4. Changes in fluorescence emission spectra of 1 (10
(from 0e30 M) in CH3CN (excitation at 365 nm). Inset is molar ratio plot.
m
M) upon titration by Zn2þ
m
To confirm the binding mechanism, 1H NMR spectra of 1 and the
1$Zn2þ complex were measured in a mixture of CDCl3/CD3CN (10:1,
v/v). As shown in Fig. 7B, upon gradual addition of Zn2þ salt
(0.5 equiv) to a solution of 1, the resonances corresponding to the
protons of receptor 1 were split into two sets of signals. After ad-
dition of 1 equiv. Zn2þ in receptor 1 the original proton signals
disappeared. This result suggests the presence of the complexed
form between 13Zn2þ and the uncomplexed form of free 1. The
signal for proton Ha on the triazole ring undergoes a downfield shift
the 1:1 model, the association constant of Zn2þ, calculated froꢁm1 the
Benesi-Hildebrand equation,16 was found to be 1.44ꢀ104
M . As
a result, 1 can be regarded as being highly sensitive to the Zn2þ ion,
especially given the large fluorescence dynamic range and the low
detection limit of 3.79ꢀ10ꢁ7 M. The quantum yield of 1 is V¼0.11.
To further study the sensitivity of 1 toward Cd2þ, fluorescence
titration experiments were carried out (Fig. 5). Upon addition of

X.-K. Jiang et al. / Tetrahedron 72 (2016) 4854e4858
4857
3. Conclusions
In summary, we have synthesized a new type of selective and
sensitive fluorescent sensor having triazole rings as the binding
sites at the lower rim of a hexahomotrioxacalix[3]arene scaffold in
a cone conformation. The selective binding behavior of chemo-
sensor 1 has been evaluated by fluorescence spectra and 1H NMR
spectroscopic analysis. This sensor has desirable properties for
practical applications, including selectivity for detecting Zn2þ and
Cd2þ in the presence of excess competing metal ions at low ion
concentration or as a fluorescence enhancement type chemosensor
due to the cavity of calixarene changed from a ‘flattened-cone’ to
a more-upright form and inhibition of photoinduced electron
transfer (PET). In contrast, the results suggested that receptor 1 is
highly sensitive and selective for Cu2þ and Fe3þ as a fluorescence
quenching type chemosensor due to photoinduced electron
transfer (PET) or heavy atom effect.
Further studies on the synthesis of tritopic receptors based on the
hexahomotrioxacalix[3]arene are also underway in our laboratory.
4. Experimental section
4.1. General
All melting points (Yanagimoto MP-S1) are uncorrected. 1H NMR
and 13C NMR spectra were recorded on a Nippon Denshi JEOL FT-
300 NMR spectrometer and Varian-400MR-vnmrs400 with SiMe4
as AQ2OM spectrophotometer. Mass spectra were obtained with
a Nippon Denshi JMS-HX110A Ultrahigh Performance mass spec-
trometer at 75 eV by using a direct-inlet system. UVevis spectra
were recorded using a Shimadzu UV-3150 UVevis-NIR spectro-
photometer. Elemental analyses were performed by a Yanaco MT-5.
Fluorescence quantum yields were recorded in solution (Hama-
matsu Photonics K. K. Quantaurus-QY A10094) using the integrated
sphere absolute PL quantum yield measurement method.
Fig. 7. (A) Binding mode of 1 upon complexation with Zn2þ ion as perchlorate salt. (B)
Partial 1H NMR spectra of 1 (4.0 mM) in CDCl3: CD3CN (10:1, v/v) upon addition of Zn2þ
ion at 298 K. (a) Free 1, (b) 13Zn2þ (0.5 equiv), and (c) 13Zn2þ (1.0 equiv).
from
d
7.16 ppme7.70 ppm (DdH¼0.64 ppm), and the OCH2-triazole
linked proton of Hd is shifted from
d
4.06 ppme5.13 ppm (Fig. S22).
These spectral changes suggested that the Zn2þ ion is selectively
bound by the nitrogen atoms on the triazole rings. Moreover, the
signal for the proton on the anthracene moiety revealed a down-
field shift, which indicated that the anthracene moieties appen-
4.2. Materials
Compounds 3 and 4 were synthesized according to our previous
report.15
ded on 1 were alienated by Zn2þ to prohibit the formation of
pep
stacking between the anthracene moiety. On the other hand, it is
noted that the proton Hb on the phenyl of hexahomotrioxacalix[3]
4.2.1. Synthesis of receptor 1. Copper iodide (20 mg) was added to
a
solution of compound 3 (200 mg, 0.28 mmol) and 9-
arene also experienced a downfield shift from
d
6.60e6.81 ppm,
azidomethylanthracene (210 mg, 0.90 mmol) in 20 mL THF/H2O
(4:1) and the mixture was heated at 65 C for 24 h. The resulting
ꢂ
and the DdH value for Hax and Heq of the ArCH2O methylene protons
changed to 0.58 ppm (Fig. S22), respectively. The large DdH value for
Hax and Heq indicated that the phenol groups in the complex are
positioned in a more-upright form, the calix cavity changed from
a ‘flattened-cone’ to a more-upright form that is similar to the
previously reported examples.17,18 The concept of Zn2þ complexa-
tion by the host chemosensor 1 is shown in Fig. 7A. From the above
discussion, the binding mode of 1$Zn2þ indicated that the phenol
groups in the complex are situated in an upright form and also the
anthracene moieties are far apart from each other to reduce the
solution was cooled and diluted with water and extracted thrice
with CH2Cl2. The organic layer was separated and dried (MgSO4)
and evaporated to give the solid crude product. The residue was
eluted from a column chromatography of silica gel with hexane/
ethyl acetate (v/v¼4:1) to give the desired product cone-1 (290 mg,
75%) as colorless prisms. Mp 154e156 ꢂC. 1H NMR (400 MHz,
CDCl3):
d
0.94 (27H, s, C(CH3)3), 4.03 (12H, d, OCH2O, J¼4.0 Hz), 4.10
(6H, s, eOCH2), 6.29 (6H, s, AneCH2), 6.68 (6H, s, AreH), 7.08 (3H, s,
triazoleeH), 7.39e7.43 (6H, m, AneH), 7.47e7.51 (6H, m, AneH),
7.94 (6H, d, J¼8.4 Hz, AneH), 8.28 (6H, d, J¼8.4 Hz, AneH) and 8.42
pep
stacking in presence of Zn2þ which results the fluorescence
enhancement.
(3H, s, AneH) ppm. 13C NMR (75 MHz, CDCl3):
d 31.32, 34.12, 45.51,
On the other hand, the peaks of Ha, Hb, Hc and Hd completely
disappeared and the signals of the anthracene ring protons and
benzyl protons were blurred, which is attributed to both the con-
formation changes and the paramagnetic effect of the Cu2þ. Once
the Cu2þ was captured by the nitrogen, the protons adjacent to
Cu2þ were strongly affected by the Cu2þ due to inherent para-
magnetism of Cu2þ. Thus, the complexation between the heavy
metal ions and sensor 1 led to the quenching of the fluorescence
emission through the heavy metal ion effect, and/or reversed PET
that is similar to the previously reported examples.19
66.94, 68.87, 122.71, 123.29, 124.42, 125.45, 125.77, 127.64, 129.42,
129.74, 130.77, 130.86, 131.43, 144.27, 146.23 and 151.78 ppm. IR:
nmax (KBr)/cmꢁ1: 3310, 2960, 1575, 1436, 1367, 1268, 1090 and 1002.
FABMS: m/z: 1389.58 (Mþ). Anal. Calcd for C90H87O6N9 (1389.12): C,
77.73; H, 6.31. Found: C, 77.90; H, 6.37.
4.2.2. Synthesis of receptor 2. Copper iodide (20 mg) was added to
a solution of compound 4 (100 mg, 0.47 mmol) and 9-azidome-
thylanthracene (340 mg, 1.45 mmol) in 20 mL THF/H2O (4:1) and
the mixture was heated at 65 ꢂC for 24 h. The resulting solution was

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X.-K. Jiang et al. / Tetrahedron 72 (2016) 4854e4858
(c) Kim, J. S.; Quang, D. T. Chem. Rev. 2007, 107, 3780e3799; (d) Coquiere, D.; Le
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cooled and diluted with water and extracted thrice with CH2Cl2.
The organic layer was separated and dried (MgSO4) and evaporated
to give the solid crude product. The residue was eluted from a col-
umn chromatography of silica gel with hexane/ethyl acetate (v/
v¼4:1) to give the desired product 2 (170 mg, 81%) as colorless
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prisms. Mp 194e196 ꢂC. 1H NMR (300 MHz, CDCl3)
d 1.22 (s, 9H,
C(CH3)3), 2.10 (s, 6H, AreCH3), 4.78 (s, 2H, eOCH2), 6.57 (s, 2H,
AneCH2), 6.89 (s, 2H, AreH), 7.19 (s, 1H, trizoleeH), 7.52e7.64 (m,
4H, AneH), 8.09 (d, 2H, J¼6.0 Hz, AneH), 8.33 (d, 2H, J¼6.0 Hz,
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AneH), 8.60 (s, 1H, AneH). 13C NMR (CDCl3, 75 MHz, CDCl3):
d 31.6,
34.2, 37.4, 46.7, 49.85, 62.2, 114.3, 122.5, 123.1, 123.8, 125.6, 126.3,
127.9, 129.7, 130.1, 130.9, 131.6, 144.0, 144.5, 156.0. IR: nmax (KBr)/
cmꢁ1: 3019, 1966. FABMS: m/z: 449.78 (Mþ). Anal. Calcd for
C
30H31ON3 (449.25): C, 80.14; H, 6.95. Found: C, 80.38; H, 7.03.
Acknowledgements
This work was performed under the Cooperative Research
Program of ‘Network Joint Research Center for Materials and De-
vices (Institute for Materials Chemistry and Engineering, Kyushu
University)’. We would like to thank the OTEC at Saga University
and the International Collaborative Project Fund of Guizhou prov-
ince at Guizhou University for financial support. We also would like
to thank the EPSRC (for a travel grant to C.R.).
Supplementary data
11. Eui, H. R.; Zhao, Y. Org. Lett. 2005, 7, 1035e1037.
Supplementary data (Details of the NMR spectra and titration
experimental data) associated with this article can be found, in the
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