Calix[4]arene-based 1,3,4-Oxadiazoles: Novel fluorescent chemosensors for specific recognition of Cu2+

Article abstract of DOI:10.1002/cplu.201200004

All chemicals were of analytical grade, except where stated otherwise. Dichloromethane and acetonitrile used for photophysical studies were of spectrometric grade. The solutions of metal ions were all prepared from their perchlorate salts. UV/Vis spectra were recorded on a Cary 300 spectrophotometer. Steady-state emission spectra were recorded on a Cary Eclipse spectrophotometer. The emission quantum yields were determined by using the method of Demas and Crosby with quinine sulfate as the standard (φ=0.546 in 1N H₂SO₄).[23] All photophysical measurements, except were stated otherwise, were carried out after the solution was stored at RT for 8 h. The fluorescence spectra were obtained by excitation at λ=315 nm, and both the excitation and emission slits were 3 nm. The thermal behavior were investigated by a polarized optical microscopy (OLYMPUS BX51) and a differential scanning calorimeter (NETZSCH DSC 204) with a heating and cooling rate of 58°Cmin^-1.

Full text of DOI:10.1002/cplu.201200004

DOI: 10.1002/cplu.201200004
Calix[4]arene-Based 1,3,4-Oxadiazoles: Novel Fluorescent Chemosensors for
Specific Recognition of Cu2+
Jie Han,*[a] Fu-Li Wang,[a] Yu-Xin Liu,[a] Feng-Yan Zhang,[a] Ji-Ben Meng,[a] and Zheng-Jie He[a, b]
During the past two decades, there has been a continuous in-
terest in design and synthesis of fluorescent chemosensors ca-
pable of selectively recognizing the divalent copper ion owing
to its environmental and biological importance.[1] A fluorescent
chemosensor usually consists of an ion recognition unit (iono-
phore) and a signaling fluorogenic unit (fluorophore) that con-
verts the binding ion event into an easily monitored light
signal.[2] But sometimes the binding event is difficult to detect
because the fluorophore does not directly contact the bound
metal ion. A feasible method to solve this problem is to devise
fluorescent chemosensors with a fluorophore directly involving
in the interaction with metal ions. However, the examples of
such kinds of fluorescent chemosensors are still scarce.[3] On
the other hand, to date there are many fluorescent chemosen-
sors prepared for Cu2+ recognition, but most of them also re-
spond simultaneously to other competitive metal ions such as
ture the 1,3,4-oxadiazole unit as both a fluorophore and an
ionophore. These sensors display a specific selectivity for Cu2+
recognition over other transition-metal ions including Mn2+
,
Cd2+, Ni2+, Pb2+, and Zn2+ by the fluorescence quenching.
This study also discloses that the different configurations of
the calix[4]arene skeleton of the chemosensors have significant
influence on the recognition of Cu2+
.
The syntheses of 1,3-alt-6 and cone-6 are summarized in
Scheme 1. Reaction of basic calix[4]arene 1 with n-iododecane
afforded 2, which was selectively brominated on two free phe-
nols to yield compound 3 in 85% yield. The remaining phenol-
ic groups of 3 were then alkylated with n-iododecane in the
presence of Cs2CO3 in CH3CN or NaH in DMF to give isomeric
1,3-alt-4 and cone-4, respectively. Subsequently Suzuki cross-
coupling reactions of 1,3-alt-4 and cone-4 with 4-cyanophenyl-
boronic acid gave the corresponding 1,3-alt-5 and cone-5 in
good yields. Compounds 1,3-alt-5 and cone-5 were treated
with sodium azide and NH4Cl and readily provided the corre-
sponding tetrazole intermediates, which were used in the next
step without purification. These intermediates were treated
with 4-methoxybenzoyl chloride and afforded the fluorescent
chemosensors 1,3-alt-6 and cone-6 in total yields of 46% and
62%, respectively. The structures of 1,3-alt-6 and cone-6 were
well established by 1H NMR, 13C NMR, high-resolution mass
spectrometry, and satisfactory elemental analysis. The configu-
ration assignments for the intermediates (4 and 5) and final
Fe3+ [4] Zn2+ [5] Ni2+ [6] Pb2+ [7] and Hg2+ [8]
, , , , . In this context, devel-
opment of fluorescent chemosensors with highly specific selec-
tivity for Cu2+ ion remains an important and challenging
task.[9]
Calixarenes are one class of the most ideal frameworks or
building blocks in the development of fluorescent receptors
for ions and neutral molecules because of their preorganized
binding sites, easy derivatization, and stable three-dimensional
structures.[10] Many calixarene-based fluorescent chemosensors
have been developed with remarkable selectivity for alkali
metal cations.[11] Such kinds of chemosensors for transition-
1
compounds 6 were consistent by typical H NMR and 13C NMR
metal cations such as Cu2+ [3a,12]
,
Zn2+ [13]
,
Pb2+ [14]
,
Hg2+ [15]
,
and
data. In addition, compounds cone-4 (for details, see Figure S1
in the Supporting Information) and 1,3-alt-6 were further con-
firmed unambiguously by single-crystal X-ray analysis.[20] As
shown in Figure 1, two 1,3,4-oxadiazole rings in 1,3-alt-6 adopt
nearly an antiparallel orientation, which makes it is possible
that associated coordination interactions occur between two
1,3,4-oxadiazole units and the transition-metal ion.
Ag+[16] have also attracted special attention in recent years.
1,3,4-Oxadiazole is an efficient fluorescent unit and has been
extensively exploited in organic light-emitting devices[17] and
liquid crystals[18] because of its high photoluminescence quan-
tum yield, excellent thermal and chemical stability, visible exci-
tation, and emission wavelength. Recent studies have revealed
that the 1,3,4-oxadiazole moiety could be also used as a signal-
ing component in fluorescent chemosensors because of its po-
tential coordination sites (N and O) with metal ions.[19] Herein,
we report a novel type of fluorescent chemosensors which
consist of calix[4]arene and 1,3,4-oxadiazole moieties and fea-
The absorption and photoluminescence spectra of 1,3-alt-6
and cone-6 in CH3CN/CH2Cl2 (v/v=4:1) were measured, and
the photophysical data are summarized in Table 1. Similar ab-
sorption patterns were observed with intense absorption
bands (eꢀ69 000 molÀ1 cmÀ1) peaking at 315 and 317 nm, re-
spectively. Both of them displayed a strong blue emission
(lem.max. =403 nm) with photoluminescence quantum yields of
0.73 and 0.63, respectively. The largest absorption band wave-
length for the cone conformer is red-shifted by approximately
2 nm from that of the 1,3-alternate conformer. Meanwhile the
1,3-alternate and cone conformers of the fluorescent chemo-
sensors almost have the same largest absorption and emission
peak wavelengths. Notably, 1,3-alt-6 shows an appreciable in-
crease in photoluminescence quantum yield compared to
cone-6.
[a] Dr. J. Han, F.-L. Wang, Y.-X. Liu, F.-Y. Zhang, Prof. Dr. J.-B. Meng,
Prof. Dr. Z.-J. He
Department of Chemistry
Nankai University, Tianjin 300071 (P. R. China)
Fax: (+86)22-23509933
E-mail: hanjie@nankai.edu.cn
[b] Prof. Dr. Z.-J. He
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201200004.
196
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ChemPlusChem 2012, 77, 196 – 200

To study the optical sensor
properties of 1,3-alt-6 for metal
ions, the representative metal
ions including Cu2+, Mn2+, Cd2+
Ni2+ Pb2+ and Zn2+ were
,
,
,
added to a solution of 1,3-alt-6,
respectively. As seen in Figure 2,
addition of Cu2+ to a solution of
1,3-alt-6 in CH3CN/CH2Cl2 (v/v=
4:1) resulted in a decrease in the
absorbance and a small blue-
shift of approximately 5 nm in
the UV/Vis absorption spectra. In
addition, the luminescence re-
sponse of 1,3-alt-6 toward Cu2+
was found to be more pro-
nounced. Significant emission
quenching with a concomitant
blue-shift
of
approximately
10 nm was observed (Figure 2).
This fact was consistent with its
blue-shift observed in the UV/Vis
absorption spectra. In sharp con-
trast, additions of the other
aforementioned metal ions to
the solution of 1,3-alt-6 in
CH3CN/CH2Cl2 only resulted in
negligible changes in the ab-
sorption spectrum under identi-
cal conditions. The specific re-
sponses of 1,3-alt-6 in absorp-
tion and emission spectroscopic
properties toward Cu2+ ion may
indicate that there is a biased in-
teraction between 1,3,4-oxadia-
Scheme 1. Synthesis of 1,3-alt-6 and cone-6. Reagents and conditions: i) K2CO3, 1-iododecane, CH3CN, reflux, 36 h;
ii) Br2, CHCl3; iii) NaH, 1-iododecane, DMF, RT, 48 h; iv) 1-iododecane, Cs2CO3, CH3CN, reflux, 48 h; v) 4-cyanophenyl-
boronic acid, [PdCl2(PPh3)2], K2CO3, dioxane/H2O (3:1), reflux, 8 h; vi) sodium azide, NH4Cl, DMF, 1158C, 72 h; vii) 4-
methoxybenzoyl chloride, pyridine, reflux, 12 h. DMF=N,N-dimethylformamide.
zole moieties of 1,3-alt-6 and Cu2+ ion over the other metal
ions. This bias of interaction could be rationalized by Irving–
Williams order stability[21] in terms of complex formation. Be-
cause Cu2+ is a well-known paramagnetic ion with an unfilled
d shell and could strongly quench the fluorescence of a fluoro-
phore through the electron or energy transfer between metal
cation and the fluorophore.[22]
To confirm 1,3-alt-6 as an ion-selective fluorescence chemo-
sensor for Cu2+, its fluorescence emission spectra in the pres-
ence of Cu2+ (15 equiv) and a competitive metal ion (30 equiv)
were measured. As shown in Figure 3, no substantial variation
in fluorescence intensity was observed with or without any
competitive metal ion. This result means that 1,3-alt-6 has
Figure 1. The solid-state structure of 1,3-alt-6, where the ellipsoids are
drawn at 30% probability.
highly specific selectivity for Cu2+
.
Table 1. Photophysical properties of 1,3-alt-6 and cone-6 in CH3CN/
CH2Cl2 (v/v=4:1).
The emission spectra of compound 1,3-alt-6 at variable con-
centrations of Cu(ClO4)2 are shown in Figure 4. It can be seen
that the fluorescence intensity of 1,3-alt-6 at the emission
maxima is decreases as the loading amount of Cu2+ increases
and gradually achieves a plateau with 24% fluorescence inten-
sity of 1,3-alt-6 at the concentration of 150 equivalents of
Cu2+. Under the current conditions, the stoichiometry between
Cu2+ and 1,3-alt-6 could not be deduced through a fluores-
Compd[a] labs.max [nm]
(e [LmolÀ1 cmÀ1])
lem.max [nm][b] Stocks’ shift [cmÀ1
]
F[c]
1,3-alt-6
cone-6
317 (7.8ꢁ104)
315 (9.5ꢁ104)
412
412
3366
3466
0.73
0.63
[a] Concentration=5ꢁ10À6 molLÀ1
.
[b] lexc =315 nm.
[c] Determined
using quinine sulfate as the standard (F=0.546 in 1N H2SO4).
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197

cence signal of 1,3-alt-6 was completely revived. This
result demonstrates that the binding of the sensor
and Cu2+ ion is chemically reversible. In addition,
after treated with EDTA, 1,3-alt-6 was recovered from
the solution of 1,3-alt-6 and Cu2+ salt and was
proven to be identical to the original 1,3-alt-6 sample
1
by H NMR spectroscopy.
Unlike the 1,3-alt-6 conformer, the UV spectra
(Figure 5) of cone-6 in CH3CN/CH2Cl2 (v/v=4:1) did
not exhibit considerable changes upon addition of
metal cations including Cu2+, Mn2+, Cd2+, Ni2+, Pb2+
,
and Zn2+. As for its emission spectra shown in
Figure 5, the fluorescence intensity of cone-6 did not
suffer any appreciable changes from addition of the
above metal cations, except for in the case of added
Cu2+. A significant fluorescence intensity decrease of
cone-6 was observed upon addition of Cu2+, thus im-
plying that cone-6 only fluorescently responds to
Cu2+ ion under the experimental conditions.
Figure 2. Absorption spectra and fluorescent emission (lex =315 nm) of 1,3-alt-6 (5 mm)
upon addition of 90 equivalents of Cu2+, Mn2+, Cd2+, Ni2+, Pb2+, and Zn2+ in CH3CN/
CH2Cl2 (v/v=4:1).
Figure 3. Fluorescence emission spectra (lex =315 nm) of 1,3-alt-6 (5 mm)
upon addition of Cu2+ (15 equiv) and other competitive ions (30 equiv).
Figure 4. Fluorescence emission spectra (lex =315 nm) of 1,3-alt-6 (5 mm)
upon addition of variable amounts of Cu2+
.
cence titration experiment because the complexation
process between 1,3-alt-6 and Cu2+ ion is slow. To
confirm this speculation, a solution of 1,3-alt-6 and
90 equivalents of Cu2+ in CH3CN/CH2Cl2 (v/v=4:1)
was stored for seven days at room temperature and
then tested for its photoluminescence emission spec-
trum. As seen in Figure 4 (the dotted line), the fluo-
rescence intensity was almost totally quenched, thus
indicating that the fluorescent response was affected
greatly by the interaction time between the chemo-
sensor 1,3-alt-6 and Cu2+
.
To examine the reversibility of the recognition pro-
cess between sensor 1,3-alt-6 and Cu2+ ion, an aque-
ous solution of ethylenediaminetetraacetic acid
(EDTA, disodium salt; 15 equiv) was added to the so-
lution of 1,3-alt-6 and 15 equivalents of Cu2+ in
Figure 5. Absorption and fluorescent emission spectra of cone-6 (5 mm) upon addition of
CH3CN/CH2Cl2 (v/v=4:1). As expected, the fluores- various metal ions (90 equiv) in CH3CN/CH2Cl2 (4:1; lex =315 nm).
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ChemPlusChem 2012, 77, 196 – 200

To shed light on the effect of the three-dimension structure
of the calix[4]arene on recognition for Cu2+, a reference com-
pound 7, which is considered as a proximate monomer of 1,3-
alt-6 and cone-6, was prepared in 92% yield by the Suzuki
cross-coupling reaction as shown in Scheme 2. Its fluorescent
and S5). In contrast, both1,3-alt-6 and cone-6 do not show any
liquid crystalline behavior as evidenced by the POM observa-
tions. The absence of mesophases in 1,3-alt-6 and cone-6 may
be due to the three-dimensional configuration of the calix[4]ar-
ene skeleton, which produces a large deviation of the whole
molecular shape from linearity.
In summary, a novel type of fluorescent chemosensors were
designed and synthesized by combining the unique features
of calix[4]arene and 1,3,4-oxadiazole units. Both isomers show
specific selectivity for Cu2+ through the fluorescent emission
intensity quenching. The configurations of the chemosensors
have significant effects on the fluorescent recognition for Cu2+
, and the 1,3-alternate conformer showed much better sensitiv-
ity for Cu2+ than the cone counterpart.
Scheme 2. Synthesis of compound 7.
recognition for Cu2+ was also investigated in this study. The
monomer 7 displays a strong blue emission with a lmax value
at 392 nm and quantum yield of 0.76 (for details, see Fig-
ure S3). Addition of 90 equivalents of Cu2+ caused a slight de-
crease of emission intensity of a solution of 7. The relative fluo-
rescence ratios (I/I0) for 1,3-alt-6, cone-6, and 7 in the presence
of 90 equivalents of Cu2+ compared to in the absence of Cu2+
are summarized in Figure 6. Compared to 7, both 1,3-alt-6 and
cone-6 exhibits a better fluorescent response toward addition
of Cu2+. Moreover, the 1,3-alt-6 conformer showed a better
sensitivity for Cu2+ than its cone counterpart.
Experimental Section
All chemicals were of analytical grade, except where stated other-
wise. Dichloromethane and acetonitrile used for photophysical
studies were of spectrometric grade. The solutions of metal ions
were all prepared from their perchlorate salts. UV/Vis spectra were
recorded on a Cary 300 spectrophotometer. Steady-state emission
spectra were recorded on a Cary Eclipse spectrophotometer. The
emission quantum yields were determined by using the method of
Demas and Crosby with quinine sulfate as the standard (F=0.546
in 1N H2SO4).[23] All photophysical measurements, except were
stated otherwise, were carried out after the solution was stored at
RT for 8 h. The fluorescence spectra were obtained by excitation at
l=315 nm, and both the excitation and emission slits were 3 nm.
The thermal behavior were investigated by a polarized optical mi-
croscopy (OLYMPUS BX51) and a differential scanning calorimeter
(NETZSCH DSC 204) with a heating and cooling rate of 58CminÀ1
.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (grant no. 20772064) is gratefully acknowledged.
Keywords: calixarenes · configuration · copper · fluorescent
chemosensors · 1,3,4-oxadiazoles
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Published online on February 9, 2012
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