A novel fluorescent sensor for Fe3+ and Cr3+ based on a calix[4]arene bearing two coumarin units

Article abstract of DOI:10.3184/174751912X13459806897922

A novel turn-on fluorescent sensor based on a calix[4]arene with two coumarin units as fluorophores and iminogroups as ionophores on the upper rim has been synthesised. Its recognition properties towards heavy and transition metal ions have been studied by UV-Vis and fluorescence spectrometry. The compound showed an enhanced fluorescent intensity in the presence of Fe ³⁺ or Cr³⁺ ion, a high selectivity to Fe³⁺ or Cr³⁺ ions and a complexation ratio towards Fe³⁺ or Cr ³⁺ of 1:1. The association constant for Fe³⁺ was 5.37×105 M-1 and for Cr³⁺ was 9.18×105 M-1.

Full text of DOI:10.3184/174751912X13459806897922

JOURNAL OF CHEMICAL RESEARCH 2012
OCTOBER, 587–589
RESEARCH PAPER 587
3
+
3+
A novel fluorescent sensor for Fe and Cr based on a calix[4]arene
bearing two coumarin units
a
b
b
He-wen Wang , Ya-qing Feng * and Shu-xian Meng
a
College of Chemical Engineering, Huanggang Normal University, Huanggang 438000, P. R. China
bSchool of Chemical Engineering andTechnology, Tianjin University, Tianjin, 300072, P. R. China
A novel turn-on fluorescent sensor based on a calix[4]arene with two coumarin units as fluorophores and imino-
groups as ionophores on the upper rim has been synthesised. Its recognition properties towards heavy and transi-
tion metal ions have been studied by UV-Vis and fluorescence spectrometry. The compound showed an enhanced
3
+
3+
3+
3+
fluorescent intensity in the presence of Fe or Cr ion, a high selectivity to Fe or Cr ions and a complexation ratio
3
+
3+
3+
5
−1
3+
5
−1
towards Fe or Cr of 1:1. The association constant for Fe was 5.37×10 M and for Cr was 9.18×10 M .
Keywords: calixarene, coumarin, fluorescent sensor, recognition of metal ions
The design and synthesis of selective fluorescent sensors for
cation detection, especially for heavy metal and transition
metal cations, is of great interest. Compared with divalent
ion, based on a calix[4]arene having, on its upper rim, imine
groups as ionophores and coumarin units as fluorophores
(1, see Scheme 1).
1
transition metal cations, few studies have been devoted to
developing organic probes that are sensitive to triple-charged
Results and discussion
3
+
2–4
3+
5–7
metal cations, such as Fe ion and Cr ion. The trivalent
forms of iron and chromium play important roles in environ-
mental and biological systems and therefore it is worthwhile to
Synthesis of the compound 1 is depicted in Scheme 1. Com-
15
16
17
18
19
20
pounds 2 , 3 , 4 , 5 , 6 and 7 were prepared according to
the literature. The reaction of 5,17-diaminocalix[4]arene (6)
with 5 equiv. of 7-methoxy-2-oxo-2H-chromene-4-carbalde-
hyde (7) in ethanol afforded calix[4]arene 1 in 67% yield.
3
+
3+
develop fluorescent sensors for the detection of Fe and Cr .
Calixarene, a basic molecular scaffold for construction of
selective ionophores, has been incorporated into fluorescent
ion sensors. Calixarene-based fluorescent sensors show
1
13
Compound 1 was fully characterised by H NMR, C NMR,
IR, MS and elemental analysis.
Compound 1 adopts a cone conformation which is con-
firmed by the presence of an AB (J = 13.5 Hz) system charac-
8
great sensitivity and selectivity for alkali metal ions, alkaline
9
10
earth metal ions and several transition metal ions. However,
3
+
3+
only a few fluorescent calixarene derivatives for Fe and Cr
have been reported.
teristic of methylene bridges (Ar–CH –Ar) at 3.23 ppm and
2
1
1,12
1
Due to the paramagnetic nature of
4.50 ppm in its H NMR spectrum and by the position of
iron and chromium ions, their presence is mostly signaled
by fluorescence quenching. In terms of sensitivity, fluoro-
ionophores showing fluorescence enhancement as a result of
metal-ion binding are superior to those exhibiting fluorescence
the carbon atoms of the methylene bridges at 31.4 ppm in its
1
3
C NMR spectrum. Only one signal of the imino-protons
(CH=N at 7.70 ppm) shows that the C=N bonds adopt the
2
1
E-configuration, in accord with the literature.
The complexation behaviour of calix[4]arene 1 towards
1
3,14
quenching. As a continuation of our work
on the design and
2
+
3+
2+
synthesis of fluorogenic chemosensors, we report a novel turn-
different heavy and transition metal cations (Ni , Cr , Pb ,
3
+
3+
2+
2+
2+
+
2+
2+
2+
3+
2+
2+
on fluorescent sensor for selective recognition of Fe and Cr
Cd , Cu , Zn , Ag , Hg , Co , Fe , Fe , Mn , Ca and
Scheme 1 Synthesis of compound 1.
*
Correspondent. E-mail: yqfeng@tju.edu.cn

5
88 JOURNAL OF CHEMICAL RESEARCH 2012
3+
La ) was investigated by UV-Vis absorption spectroscopy.
As shown in Fig. 1, the absorption spectrum of 1 in dichloro-
methane-methanol solution(1:1, v/v) exhibits one absorption
3
+
3+
band at 360nm. Upon addition of Fe or Cr to the solution,
the band at 360nm decreased and a new blue-shifted band at
3+
3+
3
29 nm for Fe or 327 nm for Cr appeared while the other
metal ions gave no changes, which indicates that compound 1
3
+
3+
can selectively recognise Fe and Cr .
Figure 2 shows the fluorescence spectra (λ =360nm) of 1
ex
−5
(
(
10 M) measured in dichloromethane–methanol solution
3+ 3+
−5
1:1, v/v) with Fe and Cr (10 M). Because of the photo-
induced electron transfer (PET) from the lone-pair electron
of the imine nitrogen atom to the coumarin units, the very
weak fluorescent emissions of compound 1 were observed at
3+
3+
4
35 nm. When Fe or Cr were added, the nitrogen atom of an
imino-group donates its lone pair of electrons to an empty
3+
3+
orbital of Fe or Cr and inhibits the PET process, thus the
fluorescent emission intensity was enhanced remarkably (about
3
+
3+
2
8-fold for Fe ; 31-fold for Cr ). As shown in Fig. 3, under
Fig. 3 Change in the fluorescence intensity at 435 nm of
compound 1 (10µM) upon addition of different metal ions
the same conditions, no obvious fluorescence changes were
observed for other tested metal ions except for Cu or Hg ,
which induce a very weak emission enhancement (about 1.88-
fold for Cu ; 1.85-fold for Hg ). The results suggest that the
novel coumarin-calix[4]arene derivative can serve as a turn-on
fluorescent sensor for Fe and Cr .
2
+
2+
2 2 3
(10µM) in CH Cl and CH OH (v/v=1/1) (λex= 360 nm).
2+
2+
The complexation stoichiometric ratio between compound
3
+
3+
3+
3+
1 and Fe or Cr ions was measured by a Job’s plot experi-
ment. As shown in Fig. 4, the Job’s plot for compound 1
3
+
and Cr shows the maximum point at the mole fraction of
0
.5, indicating a typical 1:1 complexation. Similarly, the
3+
complexation ratio of compound 1 and Fe is also 1:1.
3+
The association constants between compound 1 and Fe or
3
+
Cr ions were determined by fluorescent titration. Upon titra-
tion of 1 with Cr exciting at 360 nm, (given in Fig. 5 as an
3
+
example), the fluorescence intensity is enhanced remarkably.
3
+
From linear fitting of the titration curve of the 1-Cr complex,
5
the association constant was determined to be 9.18×10 . Simi-
3
+
larly, from the titration of 1 with Fe , the association constant
5
was calculated to be 5.37×10 . The results indicate a high
3
+
3+
affinity of 1 with Fe and Cr ions.
In conclusion, we have synthesised a novel turn-on fluores-
cent chemosensor 1 based on calix[4]arene, which shows
3
+
3+
sensitive and selective recognition to Fe and Cr .
Experimental
All reagents are commercially available and purified by standard
methods prior to use. The solvents for absorption and fluorescence
measure were of spectrometric grade. Melting points were measured
on an X4 apparatus (uncorrected). The FT-IR spectra (KBr pellets)
Fig. 1 UV-Vis spectra of compound 1 (10µM) upon addition of
various ions (10µM) in CH Cl and CH OH (v/v=1/1).
2 2 3
1
were measured on a BIO-RAD FTS3000 IR spectrometer. The H
Fig. 2 Fluorescence emission spectra of compound 1 (10µM)
3
+
3+
upon addition of Fe and Cr (10µM) in CH
v/v=1/1) (λex= 360 nm).
2 2 3
Cl and CH OH
Fig. 4 Job’s plot for compound 1 and Cr3+ (λex= 360 nm).
(

JOURNAL OF CHEMICAL RESEARCH 2012 589
1
1
1
35.56, 129.08, 127.94, 122.48, 120.78, 116.43, 112.21, 110.22,
01.24, 75.52, 75.20, 55.95, 32.67, 32.34, 31.35, 19.75, 19.43, 14.38,
+
4.24; MS (m/z): 1051.4 (M+H ) (Calcd for C H N O 1050). Anal.
66
70
2
10
Calcd for C H N O : C, 75.42; H, 6.73; N, 2.68. Found: C, 75.40; H,
66
70
2
10
6
.71; N 2.66%.
Job plot experiment
Stock solutions of 1 (10µM) in CH Cl and CH OH (v/v=1/1),
2
2
3
Fe(NO ) ·9H O (10µM) and Cr(NO ) ·9H O (10µM) in CH Cl and
3
3
2
3
3
2
2
2
CH OH (v/v=1/1) were prepared. The concentrations of each CH Cl
3
2
2
and CH OH solution were varied, but the total volume was fixed to be
3
5
.0 mL.
This work was financially supported by the National Natural
Science Foundation of China(No. 21076147), Natural Science
Foundation of Tianjin (No. 10JCZDJC23700) and the
Doctoral Foundation of Huanggang Normal University (No.
0
9CD155).
Fig. 5 Fluoresence emission spectra of 1 (10µM) for Cr3+ ion
titration in CH Cl and CH OH (v/v=1/1) (λex= 360 nm).
Received 6 March 2012; accepted 8 August 2012
Paper 1201199 doi: 10.3184/174751912X13459806897922
Published online: 28 September 2012
2
2
3
13
NMR and C NMR spectra were recorded in CDCl using TMS as an
3
References
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2
8
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−
1
1
44–245 °C (dec.); IR (KBr, cm ): 1728, 1616; H NMR (CDCl ):
3
.44 (d, 2H, J = 9 Hz, cumarin-H), 7.70 (s, 2H, CH=N), 7.01 (d, 4H,
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J = 13.5 Hz, ArCH Ar), 4.05 (t, 4H, J = 8 Hz, OCH CH CH CH ),
2
2
2
2
3
3
3
1
1
1
.89 (s, 6H, cumarin-OCH ), 3.83 (t, 4H, J = 7 Hz, OCH CH CH CH ),
3
2
2
2
3
.23 (d, 4H, J = 13.5 Hz, ArCH Ar), 2.00–1.93 (m, 4H, CH CH CH ),
2
2
2
3
.93-1.87 (m, 4H, CH CH CH ), 1.61–1.53 (m, 4H, CH CH ), 1.43–
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3
3
3
62.86, 161.06, 157.43, 156.50, 156.03, 155.38, 145.03, 136.20,

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