A new selective fluorescent sensor for Fe3+ based on a pyrazoline derivative

Article abstract of DOI:10.1016/j.saa.2013.04.118

(Graph Presented) A new pyrazoline derivative was designed and synthesized. The structure of the pyrazoline was confirmed by single crystal X-ray diffraction and its photophysical properties were studied by absorption and fluorescence spectra. This compound can be used to determine Fe³⁺ ion with high selectivity among a series of cations in tetrahydrofuran and even in aqueous tetrahydrofuran. This sensor forms a 1:1 complex with Fe³⁺ and displays fluorescent quenching.

Full text of DOI:10.1016/j.saa.2013.04.118

Accepted Manuscript
A new selective fluorescent sensor for Fe3+ based on a pyrazoline derivative
Shengli Hu, Shushu Zhang, Chan Gao, Caihua Xu, Qing Gao
PII:
S1386-1425(13)00467-8
http://dx.doi.org/10.1016/j.saa.2013.04.118
SAA 10509
DOI:
Reference:
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Spectrochimica Acta Part A: Molecular and Biomo‐
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Accepted Date:
12 December 2012
11 April 2013
25 April 2013
Please cite this article as: S. Hu, S. Zhang, C. Gao, C. Xu, Q. Gao, A new selective fluorescent sensor for Fe3+ based
on a pyrazoline derivative, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2013), doi:
http://dx.doi.org/10.1016/j.saa.2013.04.118
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A new selective fluorescent sensor for Fe3+ based on a
pyrazoline derivative
Shengli Hua,*, Shushu Zhang a, Chan Gaoa, Caihua Xu a, Qing Gaob,*
a Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, Department of
Chemical and Enviromental Engineering, Hubei Normal University, Huangshi
435002, PR China
b Faculty of materials science & engineering, Hubei University,Wuhan 430062, PR
China
Abstract
A new pyrazoline derivative was designed and synthesized. The structure of the
pyrazoline was confirmed by single crystal X-ray diffraction and its photophysical
properties were studied by absorption and fluorescence spectra. This compound can
be used to determine Fe3+ ion with high selectivity among a series of cations in
tetrahydrofuran and even in aqueous tetrahydrofuran. This sensor forms a 1:1
complex with Fe3+ and displays fluorescent quenching.
Key words
*Corresponding author: E-mail：hushengli168@126.com Tel.: +86 0714 6515602 Fax: +86 0714
6515602
*Corresponding author: E-mail：gaoqing@126.com. Tel.: +86 027 67867773 Fax: +86 027
67867773
1

Pyrazoline, Fluorescent sensor, Iron(III) ions, Selective
1. Introdution
Selective signalling of chemically and biologically important metal ions has
always gained much research attention in supramolecular chemistry[1]. Of particular
interest is the development of chemosensors for the heavy and transition metal ions
because of their toxic impacts on our environment[2] and many important roles in
living system[3]. As an important physiologically relevant metal ion, Fe3+ ion plays
an indispensable role in many biochemical processes at the cellular level[4], and both
its deficiency and excess can induce a variety of diseases[5]. Therefore, the
determination of traces of iron ion in clinical, medicinal, environmental and different
industrial samples has been an important topic in environmental and biological
analysis. However, the examples of Fe3+-selective fluorescent sensors are still
scarce[6-15] owing to its easy to be interfered by other transition-metal ions such as
Cu2+ and Co2+.Therefore, designing a highly selective sensor for Fe3+ is still a
challenge.
1,3,5-Triaryl-2-pyrazolines, with their rigid but only partly unsaturated central
pyrazoline ring, are well-known fluorescent compounds widely used in fluorescent
dyes emitting blue fluorescence with high fluorescence quantum yield [16-20] and
electroluminescence fields [21–27], However, to the best of our knowledge, only a
few examples have been reported on the interactions between pyrazoline derivatives
and zinc ion [28–30]. Receently, we had reported a selective pyrazoline-based
fluorescent chemosensor for Cu2+[31]. In continuing our research to develop new
2

fluorescent chemsensors based on pyrazoline chromophore, we design a new
pyrazoline derivative 1, in which a pyridine ring and a benzothiazole moiety are
bonded to the pyrazoline ring, and hope this molecule to adopt a rigid Y-shaped
conformation, which might be able to bind selectively with a metal ion. The primary
test showed that 1 possesses a highly selective response of fluorescence quenching
toward Fe3+ in THF and even in THF/H2O (95:5, v/v) solution.
2. Experimental
2.1. Reagents
All the reagents were purchased from commercial suppliers and used without
further purification. The salts used in stock solutions of metal ions were CoCl2·6H2O,
ZnCl2, NaCl, CuCl2·2H2O, NiCl2·6H2O, KCl, CdCl2·2H2O, HgCl2, FeCl3·6H2O,
Mg(ClO4)2, Al(NO3)3·9H2O.
2.2. Apparatus
NMR spectra were measured on a Varian Mercury 300 spectrometer operating at
300 MHz for 1H and 75 MHz for 13C relative to tetramethylsilane as internal standard.
MS spectra were obtained on a Finnigan Trace MS spectrometer. IR spectra were
recorded on a Perkin-Elmer PE-983 infrared spectrometer as KBr pellets with
absorption reported in cm-1. Absorption spectra were determined on UV-2501 PC
spectrophotometer. Fluorescence spectra measurements were performed on a
FluoroMax-P spectrofluorimeter equipped with a xenon discharge lamp, 1 cm quartz
cells at room temperature (about 298K).
2.3. Synthesis of 2-(5-(4-Chlorophenyl)-3-(pyridin-2-yl)-4,5-dihydropyrazol-1-yl)
3

benzo[d]thiazole (1)
The synthetic route of the proposed compound 1 is shown in Scheme 1. Starting
materials chalcone (2)[32] and 2-hydrazinobenzothiazole (3)[33] were prepared
according to literatures. To a stirred solution of chalcone 2 (0.243 g, 1.0 mmol) in
AcOH (15 mL) was added 2-hydrazinylpyridine (3) (0.165 g, 1.0 mmol). The reaction
mixture was refluxed for 4 h. The progress of the reaction was monitored by TLC.
After completion, the reaction mixture was cooled to room temperature and the
solvent was evaporated in vacuo, and the crude product was washed with water, and
consequently recrystallized from ethanol to afford pure compound 1 as a yellow solid,
yield 62%; M.p.:220–221 °C; IR (νmax, KBr, cm-1): 3053, 2987, 2921, 1755, 1724,
1459, 1421, 1261, 1150, 757, 692; 1H NMR (CDCl3), δ (ppm): 8.58–8.60(m, 1H,
Ar-H), 8.17–8.19(m,1H, Ar–H), 7.76(d,1H, 1.5Hz), 7.54(d,1H, J = 8.1 Hz, Ar–H),
7.65(m,1H, J = 8.1 Hz, Ar–H), 7.25-7.32(m, 6H,Ar–H), 7.09-7.14(m, 1H,Ar–H),
5.80(dd, 1H, , J = 5.4, 5.4 Hz, pyrazoline-H) , 4.05(dd,1H, J = 12, 12 Hz,
pyrazoline-H) , 3.49(dd, 1H, J = 5.4, 5.4 Hz, pyrazoline-H). 13C NMR (CDCl3), δ
(ppm): 43.29, 63.41, 120.30, 120.80, 121.36, 122.17, 124.09, 125.76, 127.49, 128.98,
131.69, 133.46, 136.25,139.59, 149.37, 150.41, 152.31, 153.93, 162.89. ESI mass
spectrometry: m/z 391.1 (100% [M+H]+); M+ calculated 390.1.
（Scheme 1）
2.4. X-ray crystallography of compound 1
Suitable single crystals of 1 for X-ray structural analysis were obtained by slow
evaporation of a solution of 1 in CHCl3-CH3OH (20:1, v/v) mixture at room
temperature. The diffraction data was collected with a Bruker SMART CCD
4

diffractometer using a graphite monochromated MoKα radiation (λ = 0.71073Å) at
296(2)K. The structures were solved by direct methods with SHELXS-97 program
and refinements on F2 were performed with SHELXL-97 program by full-matrix
least-squares techniques with anisotropic thermal parameters for the non-hydrogen
atoms. All H atoms were initially located in a difference Fourier map. All H atoms
were placed in geometrically idealized positions and constrained to ride on their
parent atoms, with C–H = 0.93Å and Uiso(H) = 1.2Ueq(C).
2.5. Analytical procedure and Quantum yield
The stock solutions of 1(1.0×10-5 M) were prepared by dissolving 1 in THF and in
THF/water(95:5, v/v) containing HEPES buffer(10mM, pH= 7.4). The cationic stocks
were all in EtOH with a concentration of 3.0×10-3 M for UV–vis absorption and
fluorescence spectra analysis. For metal ion absorption and fluorescence titration
experiments, each time 3 mL solution of 1 filled in a quartz cell of 1 cm optical path
length, and we increased concentrations of metal ions by stepwise addition of
different equivalents using a micro-syringe. After each addition of Fe3+ ion, the
solution was stirred for 3 min. The volume of cationic stock solution added was less
than 100 μL with the purpose of keeping the total volume of testing solution without
obvious change. For all measurements of fluorescence spectra of 1, the excitation was
at 352 nm.
3. Results and discussion
3.1. Synthesis and structural characteristics of 1
The 3, 5-diaryl pyrazoline derivative 1 was obtained by the reaction of chalcone (2)
5

with 2-hydrazinobenzothiazole (3) in AcOH under reflux. The yield of 1 was 62%.
The structures of 1 were identified by using 1H NMR, IR and MS. The structure of 1
was further comfirmed by X-ray diffraction analysis.
The molecular view of 1 is shown in Fig. 1. A summary of crystallographic data
collection parameters and refinement parameters for 1 are compiled in Table 1.
The structure of compound 1 is crystallized in Orthorhombic space group Pbcn. One
benzene moiety, one pyridine ring and a 2-hydrazinobenzothiazole moiety are bonded
to the pyrazoline ring at the atoms of C8, C10 and N2, respectively. Consistent with a
pronounced electronic interaction, the bond lengths of C10–C11, N2–C7 are
significantly shorter as would be expected for a single bond. Moreover, the bond
lengths of N2–N3, N3–C10, C8–C9 agree well with the equivalent ones in similar
structures [34]. In the crystal of 1, torsion angle C7–N2–C8–C16 of 66.85°shows C7
in the pyridazine moiety adopts an antiperiplanar conformation with respect to the
C16 atom of the benzene ring. In the asymmetry unit, the pyrazoline ring, pyridine
ring and 2-hydrazinobenzothiazole are almost coplanar. And the pyrazoline ring
makes dihedral angles with pyridine and 2-hydrazinobenzothiazole of 4.07(1)° and
5.29(1)o, respectively, while the dihedral angle between pyrazoline and benzene
moiety is 85.42(1)°.
（Fig. 1）.
（Table 1）
The packing diagram of the compound 1 is shown in Fig. 2. The molecules are
6

connected by weak π· · ·π interactions and C–H· · ·π interaction. C6 · ·Cg3 3.512(2)Å
C15· · ·Cg5 3.789(2)Å and C21 · ·Cg1 3.6800(18)Å.
（Fig. 2）
3.2. Spectral characteristics
The absorption spectrum of compound 1 exhibits a broad band at 354 nm at room
temperature in THF. Binding affinities of compound 1 toward various metal ions were
evaluated by UV–vis spectroscopy measurements. Upon addition of these metal ions,
the absorption spectrum changes in different manner as shown in Fig. 3. In the case of
K+, Na+, Cr3+, Zn2+, Cd2+, Co2+, Cu2+, Hg2+, Mg2+, Mn2+, Ni2+, Al3+, and Pb2+, the
absorption curve did not obvious change, whereas in the case of Fe3+, the addition of
Fe3+ ion caused a increase of absorption intensity at 354 nm, accompanied by the
obvious hypsochromic shift of the absorption peak (from 354 to 337 nm). indicating
the formation of a new complex between compound 1 and Fe3+.
（Fig. 3.）
The UV–vis absorption spectra of 1 (1×10-5 M) in THF in the presence of various
concentrations of Fe3+ ion (0–20×10-5 M) are shown in Fig. 4, and the inset shows the
plots of changes as a function of increasing concentrations of Fe3+. As shown in Fig. 4,
the UV absorbance of 1 at 354 nm enhanced from 0.24 to 1.44 when increasing
concentration of Fe3+ from 0 to 20×10-5 M. A satisfactory linear relationship between
UV–vis absorbance and Fe3+ concentration was observed with the correlation
7

coefficient as high as 0.998.
（Fig. 4）.
The fluorescence titration spectra of 1 in THF with Fe3+ shows an emission
maximum peak at 462 nm (Fig. 5). The fluorescence quantum yield of compound 1 in
the absence of Fe3+ was calculated to be 0.48 with respect to quinine sulphate in 0.1 N
H2SO4 solution (Φs =0.54) [35]. As Fe3+ ion was gradually titrated, the fluorescence
intensity of compound 1 gradually decreased and when the amount of Fe3+ ion added
was about 20×10-5M, the fluorescence intensity almost reached minimum. The
quantum yield of 1 was calculated to be 0.02 in the presence of Fe3+ ion (20×10-5M)
and almost reduced to 4.2% of the initial one. To determine the stoichiometry of
compound 1 and Fe3+ ion in the complex, Job’s method [36] was employed by using
the emission changes at 462 nm as a function of molar fraction of Fe3+. A maximum
emission was observed when the molar fraction of Fe3+ reached 0.5 (Fig. 6),
indicating that Fe3+ ions form a 1:1 complex with the sensing compound. Based on
the above fluorescence titration of 1 with Fe3+, the Stern-Volmer constants (Ksv) was
calculated to be 1.2×104 M-1by a modified Stern-Volmer plot[37] (Fig. 5, inset). The
detection limit, based on the definition by IUPAC (CDL =3Sb/m) [38], was found to
be 1.4×10-6 M from 10 blank solutions.
（Fig. 5）
8

（Fig. 6）
The selectivity and tolerance of compound 1 in THF for Fe3+ ion over other
metal cations such as K+, Na+, Cr3+, Mg2+, Hg2+, Cd2+, Zn2+, Co2+, Ni2+, Pb2+, Mn2+,
Cu2+, and Al3+ ions were investigated by adding metal cations (10×10-5 M) to the
solution of compound 1(10×10-5M). As depicted in Fig. 7, Fe3+ produced significant
quenching in the fluorescent emission of 1, the other tested metals only show
relatively insignificant changes, this means that sensor 1 has a high selectivity to Fe3+
ion.
（Fig. 7）
To further gauge selectivity for Fe3+ ion over other metal ions, competition
experiments of Fe3+ ion mixed with other metal ions were carried out from
fluorescence spectra and the results are shown in Fig.8. The fluorescence intensity of
1 (1×10-5 M) in the presence of 10 equiv of the Fe3+ ion was almost unaffected by the
addition of 10 equiv of competing metal ions (Cd2+, Cr3+, Cu2+, Hg2+, Mg2+, Mn2+,
Zn2+, Co2+, Ni2+, K+, Al3+, and Na+). These results suggested that molecule 1 could be
used as Fe3+ selective fluorescent chemosensor.
（Fig. 8）
In order to achieve a real application in infield monitoring. the photophysical
properties of sensor 1 were also examined in THF/water(95:5, v/v) containing HEPES
9

buffer(10mM, pH=7.4) at a concentration level of 1×10-5 M. Changes of the UV-vis
absorption spectra and fluorescence properties of 1 caused by various metal ions were
shown in Fig. 9 and 10, respectively. The result showed Fe3+ also produced significant
enhancement in the absorption spectra and quenching in the fluorescent emission of 1,
the other tested metals only show relatively insignificant changes. So it can be
concluded that 1 also has higher selectivity for recognition of Fe3+ in anquous THF.
（Fig. 9）.
（Fig. 10）
The UV–vis absorption of 1 (1×10-5 M) in THF/water(95:5, v/v) containing
HEPES buffer(10mM, pH= 7.4) in the presence of various concentrations of Fe3+ ion
(0–20×10-5 M) are shown in Fig. 11, and the inset shows the plots of changes as a
function of increasing concentrations of Fe3+. As shown in Fig. 11, compound 1
exhibits a broad band at 354 nm. Coordination of Fe3+ to 1 resulted in the formation
of a new absorption band at 318 nm and synchronous increase in the absorption band
at 354 nm, accompanied by the obvious hypsochromic shift of the absorption peak
(from 354 to 362 nm).A satisfactory linear relationship between UV–vis absorbance
and Fe3+ concentration was observed with the correlation coefficient as high as 0.999.
（Fig. 11）
10

Similarly, fluorescence titration of 1 with Fe3+ was carried out in THF/water(95:5,
v/v) containing HEPES buffer(10mM, pH=7.4) at a concentration level of 1×10-5 M.
Addition of Fe3+ to the solition of 1 causes a quenching in the fluorescent emission of
1(Fig. 12).When the amount of Fe3+ ion added was about 20×10-5M, the fluorescence
intensity reduced to 38.5% of the initial one. The calculated quantum yield of 1 with
respect to quinine sulphate in 0.1 N H2SO4 solution (Φs =0.54) was reduced from 0.28
of the initial to 0.08. Job’s plot of 1 with Fe3+ ions also indicating the formation of a
1:1 complex (Fig. 13). The Stern-Volmer constants (Ksv) was calculated in
THF/water(95:5, v/v) containing HEPES buffer (10mM, pH=7.4) was 2.3×103 M-1
(Fig. 12, inset). The detection limit was found to be 3.0×10-6 M from 10 blank
solutions.
（Fig. 12）
（Fig. 13）
To investigate the mechanism of the fluorescence quenching for 1 by comparing
with our previous report[31], Fe3+ may be easily establish coordinative interactions
with the pyridine, pyrazoline, and benzo[d]thiazole moiety than other metal ions
examined, the capture of Fe3+ resulted in the electron or energy transfer from 1 to Fe3+;
thus, 1 showed quenching of the fluorescence for Fe3+ and provided a high selectivity
for Fe3+ over the other tested metal ions.
11

4. Conclusion
In summary, a new selective fluorescent sensor based on a pyrazoline unit was
synthesized and used for the determination of Fe3+ ion with high selectivity in THF and
in THF/water(95:5, v/v) containing HEPES buffer(10mM, pH= 7.4). This sensor
formed a 1:1 complex with Fe3+ and showed a fluorescent quenching.
Supplementary material
The crystallographic data (excluding structure factors) of 1 have been deposited
with the Cambridge Crystallographic Center as supplementary publication no. 894133.
Copy of this information may be obtained free of charge viawww: http://
www.ccdc.cam.ac.uk or from The Director, CCDC, 12 Union Road, Cambridge
CB221EZ, UK (fax: t44 1223/336 033; email: deposite@ccdc.cam.ac.uk). Structural
factors are available on request from the authors.
Acknowledgments
We thank the National Natural Science Foundation of China (Grant No.
20872042), and the Science Thchnology Foundation for Creative Research Group of
HBDE(2013), and the Hubei Province Education Ministry Foundation of China
(Grant No. D20112507) for financial support.
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O
O
Cl
KOH
+
OHC
Cl
N
N
Cl
2
HAc
S
N
S
reflux
N
.
N
S
HN
NH2
NH2NH2 H2O
N
S
N
HAc
Br2
NHNH2
N
NH2
37%HCl
1
3
Scheme 1
1

Fig. 1
2

Empirical formua
Formula weigh
Temperature
C21H15ClN4S
390.88
298(2) K
0.71073
Wavelength (Å)
Crystal system
Orthorhombic
Space group
Pbcn
a (Å)
22.723(2)
b (Å)
11.0052(10)
c (Å)
14.6753(13)
α
90°
β
90°
γ
90°
V(Å3)
3669.9(6)
Z
8
Density (calculated)
Index ranges
1.415 Mg/m3
-22≤h≤28, -13≤k≤13, -18≤l≤18
1616
F(000)
Crystal size
0.20×0.10×0.10 mm
1.79 to 25.99°
23756
θ range for data collection
Reflections collected
Independent reflections
Max.and min.transmission
Data/restraints/parameters
3599 [R(int) = 0.0268]
0.9672 and 0.9359
3599 / 0 / 245
1.092
Goodness-of-fit on F2
Absorption correction
Final R indices (I >2σ(I))
R indices (all data)
None
R1 =0.0307, wR2 = 0.0885
R1 =0.0395, wR2 = 0.0930
0.230 and -0.214 e Å-3
Largest diff. peak and hole
Table 1.
3

Fig. 2
4

1.2
1.0
0.8
0.6
0.4
0.2
0.0
337nm
Fe3+
1, K+,Na+, Cr3+, Zn2+, Cd2+
,
Co2+, Cu2+, Hg2+,Mg2+
Mn2+, Ni2+, Al3+, Pb2+
,
355nm
275
300
325
350
375
400
425
450
475
Wavelength (nm)
Fig. 3
5

2.0
1.5
1.0
0.5
0.0
2.0
1.5
1.0
0.5
0.0
Y = 0.24124 + 0.06171 * X
R = 0.998
20eq
Fe3+
0
0
2
4
6
8
10 12 14 16 18 20
[Fe3+](10-5M)
275 300 325 350 375 400 425 450 475 500
Wavelength (nm)
Fig. 4
6

1400
1200
1000
800
600
400
200
0
2.2
2.0
1.8
1.6
1.4
1.2
1.0
462nm
y = 0.88923 + 0.11844x
R = 0.99263
Ksv = 1.2*104M-1
0
Fe3+
20 eq
0
2
4
6
8
10
[ Fe3+
]
/ 10-5
M
375
400
425
450
475
500
525
550
575
600
Wavelength(nm)
Fig. 5
7

140
120
100
80
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
X = [ 1] /{ [ 1] +[ Fe3+] }
Fig. 6
8

1400
1200
1000
800
600
400
200
0
1 only, K+, Na+,Cr3+, Mg2+,
Hg2+, Cd2+, Zn2+, Co2+, Ni2+,
Pb2+, Mn2+, Cu2+, Al3+
Fe3+
375
400
425
450
475
500
525
550
575
600
Wavelength (nm)
Fig. 7
9

1
1.02 1.01
1.01
0.98
0.95 0.98 0.99
0.99
0.97
0.96
0.97
0.94
0.98
1.0
0.8
0.6
0.4
0.2
0.0
3+
3+l
A
2+
2+
2+
2+
g
M
2+
2+
2+
+
+
2+
ly
2+
r
d
u
g
n
n
o
n
Z
i
a
b
P
C
+
C
+
C
H
C
+K
o
M
+
N
N
3+
3+
+
+
+
3+
+
3+
+
+
+
+
+
3+
3+
3+
3+
3+
3+
3+
3+
3+
3+
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fe
Fig. 8
10

0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
362 nm
Fe3+
1, K+, Na+, Ni2+,Pb2+, Zn2+,
Mn2+, Mg2+, Cd2+, Hg2+,
Cu2+, Cr3+, Co2+ , Al3+
354 nm
275
300
325
350
375
400
425
450
475
Wavelength(nm)
Fig. 9
11

1400
1200
1000
800
600
400
200
0
1, K+, Na+, Ni2+,Pb2+, Zn2+,
Mn2+, Mg2+, Cd2+, Hg2+,
Cu2+, Cr3+, Co2+ , Al3+
Fe3+
375
400
425
450
475
500
525
550
575
600
Wavelength(nm)
Fig. 10
12

2.0
1.5
1.0
0.5
0.0
1.5
1.0
0.5
0.0
Y = 0.29241 + 0.05084 * X
R = 0.99996
20eq
Fe3+
362nm
0
318nm
0
2
4
6
8
10
12
14
16
18
20
[Fe3+](10-5M)
300
350
400
450
500
Wavelength (nm)
Fig. 11
13

0.25
0.20
0.15
0.10
0.05
0.00
462nm
y = 0.0021+ 0.02339x
R = 0.99723
1400
1200
1000
800
600
400
200
0
Ksv = 2.3*103 M-1
0
Fe3+
20eq
0
2
4
6
8
10
[Fe3+]*10-5
M
400
425
450
475
500
525
550
575
600
Wavelength (nm)
Fig. 12
14