Synthesis, characterization and crystal structure of a new fluorescent probe based on Schiff Base for the detection of Zinc (II)

Article abstract of DOI:10.1016/j.inoche.2011.05.015

A Schiff base compound (L) was synthesized using o-vanillin and 4, 4′-diaminodiphenylmethane and characterized by elemental analysis, ¹H NMR, ¹³C NMR, IR, electronic absorption spectra and X-ray diffraction single crystal analysis. T

Full text of DOI:10.1016/j.inoche.2011.05.015

Inorganic Chemistry Communications 14 (2011) 1297–1301
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization and crystal structure of a new fluorescent probe based on
Schiff Base for the detection of Zinc (II)
⁎
Shan-bin Liu, Cai-feng Bi , Yu-hua Fan, Yu Zhao, Peng-fei Zhang, Qing-dan Luo, Dong-mei Zhang
College of Chemistry and Chemical Engineering, Ocean University of China, Shandong 266100, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A Schiff base compound (L) was synthesized using o-vanillin and 4, 4′-diaminodiphenylmethane and
characterized by elemental analysis, 1H NMR, 13C NMR, IR, electronic absorption spectra and X-ray diffraction
single crystal analysis. The title compound(L) has no fluorescence intensity in a range of 450–650 nm, but its
fluorescence spectrum shows enhancement in the intensity of the signal at 372 nm on binding with the Zn(II)
cation from pH=6 to 14. No such significant change was observed for other metal ions. Fluorescence intensity
was linear with concentration of Zn (II) cation in a range from 1×10−7 mol·L−1 to 1.2×10−5 mol·L−1. This
Schiff base compound is a promising system for the development of new fluorescent probes for the detection
of Zn (II) cation.
Received 9 April 2011
Accepted 16 May 2011
Available online 23 May 2011
Keywords:
Fluorescent probe
Zn (II)
Schiff base
© 2011 Elsevier B.V. All rights reserved.
4, 4′-diaminodiphenylmethane
Zinc is the second most abundant transition metal in the human
body, which plays an important role in biological functions, such
as gene expression, apoptosis, enzyme regulation, and neurotransmis-
sion [1].The estimation of the extent of zinc deficiency which prevails
upon the young children in many parts of Africa and Asia is a challenging
problem because of the lack of suitable biochemical probes, which are
specific for Zn (II) [2]. The design and synthesis of fluorescent sensors
with high selectivity and sensitivity to zinc ions is a vibrant field [3]. In
despite of having many commercial Zn(II) sensors such as variety of
probes based on quinoline, dansyl, anthracene, and fluorescein for the
sensing of Zn(II) [4] and most of them are excellent Zn(II) sensors,
some of the reported synthesis methods are always too complicated
or inadequate selectivity, insufficient sensitivity, depending on fluores-
cence upon the dye concentration. So it is important to design new
ones to improve their sensitivity, selectivity, reliability and easily
synthesized.
Schiff bases (imines) are known as the good ligands for metal
ions [5]. Schiff bases and their transition metal complexes are well
known in the antibacterial, antifungal, anticancer [6], clinical, analytical
and pharmacological area [7] and as magnetic materials [8], etc.
In this paper, we synthesized fluorescent probe (Scheme 1) based on
Schiff Base (L) [9] for the detection of Zn (II), which has high selective
response to Zn (II). The IR, electronic spectra, selectivity, and the effect of
Zn (II) ion concentration, pH and solution of the fluorescent probe (L)
were studied. It shows that this Schiff base is a promising system for the
development of new fluorescent probe for the detection of Zn (II) cation.
The structure of title compound (L) consists of C29H26N2O4 (Fig. 1).
The bond lengths of N (1)\C (8) is 1.287 , shorter than other bond
lengths of C\N (1.47 ), near C_N double bond (1.30 ). So we
consider that N (1) and C (8) as C_N double bond. The distance
´
˚
between N (1) and H (1) is 1.878 , which is shorter than 2.63 A,
illuminating a generating hydrogen bond. The torsion of C (10)\
C (9)\C (8)\N (1) is 2.5° and the torsion of O(1)\H(1)\N(1)\C(8)
is 1.5°, showing that ring C(9)C(10)C(11)C(12)C(13)C(14) and C(8),
N(1), O(1), H(1) generate a conjugated effect by hydrogen bond O(1)\
H(1).The angle of C(4)\C(7)\C(4A) is 110.36°, which makes the whole
molecule seem as “V” shaped. Selected bond distances and bond angles
for L were summarized in Table 1.
In the IR spectrum of the title compound (L), the presence of a
broad band at 3386 cm−1 was assigned to v (O\H) of the hydroxyl
group. The absorption band at 1617.9 cm−1 was attributed to v(C_N)
of the Schiff base ligand [10]. The peak at 1201.8 cm−1 indicated the
v(C\OH) of the hydroxyl group. Fig. 2 showed the electronic spectra
of the title compound with (b) and without (a) Zn (II) ion solution in
DMF. There is one absorption peak at 323 nm in a range of 300–
550 nm in the spectrum of L. The peak is assigned to the n–π*
transition of conjugation between a lone pair electron of the N atom
in the C_N group and П bond of the benzene ring. Two peaks
appeared at 320 nm and 411 nm when the Zn (II) ion was added. The
first peak is assigned to the n–π* transition of conjugation between a
lone pair electron of the N atom in the C_N group and П bond of the
benzene ring. The peak at 411 nm is attributed to L to metal charge
transfer (LMCT), which indicates that the L and Zn (II) have formed a
complex.
Visual and fluorescence responses of L with and without Zn (II)
ions (1.0 equiv) in DMF solution were shown in Fig. 3. The two photos
were obtained in sunlight and upon excitation at 365 nm using a UV
⁎
Corresponding author.
E-mail address: bicaifeng301@163.com (C. Bi).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.015

1298
S. Liu et al. / Inorganic Chemistry Communications 14 (2011) 1297–1301
1.00
0.75
0.50
0.25
0.00
CHO
H2
C
OH
O
+
a
CH3
H2N
NH2
4,4′-diaminodiphenylmethane
o-vanillin
b
N
N
OH
CH3
HO
H3C
O
O
L
300
350
400
450
500
550
Scheme 1. Synthesis of Schiff base compound L.
Wavelength(nm)
Fig. 2. Electronic spectra of L (a) and L added with Zn (II) (b) (solution in DMF).
of a new extended π-electron conjugation system between L and
Zn (II). Furthermore, with the coordination to metal ion, a selective
chelation-enhanced fluorescence effect is observed, which is a sig-
nificant enhancement of the fluorescence intensity of Schiff-base
emerged quickly [12]. In order to investigate the effect of other
metal ions on the fluorescence spectra of L, Na(I), Mg(II), Ca(II),
Al(III), Cu(II), Co(II), Ni(II), Cd(II), Mn(II), Sn(IV), Pb(II), Cr(III), Fe(III),
La(III), Yb(III), Er(III), and Pr(III) were used to evaluate the metal ion
binding properties to L. As shown in Fig. 3 (b), no such significant
changes in fluorescence spectra were observed when L is exposed to
other ions under the same experimental conditions. This interesting
feature reveals that L can serve as an excellent selective fluorescent
probe for Zn (II).
In order to explore the practical utility of L as an ion-selective
fluorescence probe for Zn (II), competitive experiments were carried
out in the presence of other metal ions by titrating Zn (II) and other
metal ions [100 equivalent of Zn (II)] to solution of L. The concen-
tration of L was 1×10−3 mol·L−1, and the concentration of Zn (II)
was 1×10−5 mol·L−1. The results of experiment, as shown in Fig. 5,
indicated that at high concentration of L, there is no significant change
in the fluorescence intensity even at 100 equivalent additions of other
metal ions, suggesting that the system formed by Zn (II) and L is stable
when other metal ion are existed. Thus it may be concluded that L is a
Zn (II) selective receptor even in the presence of other interference
metal ions.
Fig. 1. A molecular structure of L.
lamp, respectively. To evaluate the fluorescence selective ability of L
binding metal ion, the changes in fluorescence intensity of L upon
addition of different metal salts were recorded. The experiment was
carried out by additions of the metal ion to equivalent concentration
of the solution of L, recording the emission spectra at λex =372 nm in
the range 450–650 nm. The fluorescence spectra of L with Na(I),
Mg(II), Ca(II), Al(III), Cu(II), Co(II), Ni(II), Cd(II), Mn(II), Sn(IV),
Pb(II), Cr(III), Fe(III), La(III), Yb(III), Er(III), and Pr(III) compared
to L with Zn(II) ion were shown in Fig. 4.
Fig. 4 clearly shows that in the absence of Zn(II), the fluorescence
emission of L (λem =538 nm) is very weak, due to isomerization of
the C_N double bond and effect of intramolecular charge transfer
(ICT) consists in Schiff base L. Compounds with an unbridged C_N
structure are often nonfluorescent due to the C_N isomerization.
But it may be inhibited by complexation with special ion [11]. Upon
addition of Zn (II) to the solution of L, there is a significant enhance-
ment in fluorescence intensity at 538 nm (Fig. 4a). The large increase
in fluorescence intensity with red shift is attributed to the reduction
of intramolecular charge transfer (ICT) effect in L and the formation
To learn more about the properties of L as a receptor for zinc,
fluorescence titration was carried out, recording the emission spectra
Table 1
Selected bond lengths (Å) and angles (°) for the L.
Bond names
Bond lengths
Bond angles
Angle
C(1)\N(1)
C(10)\O(1)
C(8)\C(9)
C(8)\N(1)
C(11)\O(2)
C(15)\O(2)
C(4)\C(7)
1.418
1.354
1.454
1.287
1.377
1.424
1.512
C4\C7\C4A
C1\N1\C8
N1\C8\C9
C(10)\O1\H(1)
C(9)\C(10)\O1
C(11)\O(2)\C(15)
110.36
121.49
121.83
109.46
122.94
117.12
Fig. 3. Visual (a) and fluorescence responses (b) of L (1.0×10−4 mol·L−1) with (right)
and without (left) Zn (II) ions (1.0 equiv) in DMF solution, fluorescence response was
excitation at 365 nm using a UV lamp.
Symmetry code: −x+1, −y, z.

S. Liu et al. / Inorganic Chemistry Communications 14 (2011) 1297–1301
1299
0 mol·L-1
a
4000
3000
2000
1000
0
a
0.1x10-6 mol·L-1
0.5x10-6 mol·L-1
1x10-6 mol·L-1
2x10-6 mol·L-1
5x10-6 mol·L-1
8x10-6 mol·L-1
10x10-6 mol·L-1
12x10-6 mol·L-1
15x10-6 mol·L-1
20x10-6 mol·L-1
30x10-6 mol·L-1
50x10-6 mol·L-1
100x10-6 mol·L-1
800
Zn (II) ion + L
L
600
400
200
450
500
550
600
650
Wavelength(nm)
0
450
500
550
600
650
Wavelength(nm)
b
4000
3000
2000
1000
0
b
Zn(II)
800
600
400
200
0
Other ion diffrent from Zn(II)
450
500
550
600
650
Wavelength(nm)
0
10
20
30
40
50
60
70
80
90 100
[Zn]x10-6
Fig. 4. (a) Fluorescence spectra of
(1.0×10−3 mol·L−1
L
(1.0 × 10− 3 mol·L− 1
)
and L–Zn (II)
)
with an excitation at 372 nm. (b) Fluorescence spectra of
L (1.0×10−3 mol·L−1) with different ions (1.0×10−3 mol·L−1) with an excitation at
372 nm. Other ion include Na(I), Mg(II), Ca(II), Al(III), Cu(II), Co(II), Ni(II), Cd(II), Mn(II),
Sn(II), Pb(II), Cr(III), Fe(III), La(III), Yb(III), Er(III) and Pr(III) ion.
c
1.6
1.2
0.8
0.4
Fig. 6. (a) Fluorescence spectra change of L (1×10−5 mol·L−1) upon addition of zinc
salt (0 mol·L− 1–100×10− 6mol·L−1) in mixture solution of DMF and ethanol (8:2,
v/v). (b) Changes in the fluorescence intensity of L upon titration of Zn(II) in DMF and
ethanol (8:2, v/v), plots denote the fluorescence intensity of L at 538 nm (excited at
273 nm) after the addition of various concentrations of Zn (II);(c) Linear regression
0.0
Zn Na Mg Ca Al Sn Pb Cr Fe Cu Co Ni Cd Mn La Yb Er Pr
Metal ion
equation of
L ) –
(1 × 10− 5mol·L− 1 upon addition of zinc salt (0 mol·L− 1
Fig. 5. Effect of competitive metal ion ([L]=1×10−3 mol·L−1, [Zn]=1×10−5 mol·L−1),
other ions are 100 equivalents of [Zn].
1.2×10−5 mol·L−1) in mixture solution of DMF and ethanol (8:2, v/v), data obtained
from the emission spectra of displayed in Fig. 6(a).

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S. Liu et al. / Inorganic Chemistry Communications 14 (2011) 1297–1301
1000
750
500
250
0
hard to be coordinated for the lack of easy-coordinated atom [11].
When dissolved in THF, toluene and chloroform, the L–Zn (II) system
displayed fluorescence property. So we can speculate that no solution
molecule contributes to the coordination and L could be used for deter-
mining Zn (II) in other solutions such as N, N-dimethylformamide
(DMF), dimethylsulfoxide (DMSO), tetrahydrofuran (THF), toluene,
chloroform.
In conclusion, we have synthesized a novel Schiff base fluores-
cent probe for Zn (II) derived from o-vanillin and 4, 4′-diaminodi-
phenylmethane. This fluorescent probe is based on a significant
enhancement in fluorescence intensity after adding Zn (II) in DMF,
DMSO, THF, toluene and chloroform. This fluorescent probe has high
selectivity for binding Zn (II). This is reflected in competitive
binding experiments with a range of metal cations. This L–Zn (II)
system is stable at pH 6 to 14. A very good linear relationship of
fluorescence intensity is observed in a Zn (II) concentration ranging
from 1×10−7 mol·L−1 to 1.2×10−5 mol·L− 1 in DMF solution.
3
6
9
pH
12
15
Fig. 7. Fluorescence intensity (at 538 nm) of L with Zn (II) (both in 1.0×10−4 mol·L−1
)
under different pH values with an excitation at 273 nm.
Acknowledgements
This work was supported by the National Science Foundation of
China (No. 21071134 and No. 20971115).
in the range 450–650 nm (λex =372 nm), maintaining the concen-
tration of L 1×10− 5 mol·L−1 with the increase in concentration of
Zn(II). With the increase of the concentration of Zn (II) ion, the
fluorescence intensity has enhanced, while the shape of the emission
band is not changed too much (Fig. 6). The title compound (L)
exhibits a high sensitivity toward zinc. Fluorescence intensity is linear
with concentration of Zn (II) cation in a range from 0.1×10−6 mol·L−1
to 1.2×10−5 mol·L−1. Linear regression equation is I=45.86c+18.12,
R2=0.9991.
Appendix A. Supplementary material
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.05.015.
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