2,2′-[Benzene-1,2-diylbis(iminomethanediyl)]diphenol derivative bearing two amine and hydroxyl groups as fluorescent receptor for Zinc(II) ion

Article abstract of DOI:10.1016/j.saa.2014.02.003

A new non-natural receptor 2,2′-[benzene-1,2- diylbis(iminomethanediyl)]diphenol (4) was synthesized, and it is fluorogenic behaviour toward various metal ions were investigated. Receptor 4 exhibited pronounced fluorescence enhancement in the presence of

Full text of DOI:10.1016/j.saa.2014.02.003

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 312–316
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
2,20-[Benzene-1,2-diylbis(iminomethanediyl)]diphenol derivative
bearing two amine and hydroxyl groups as fluorescent receptor
for Zinc(II) ion
Kundan Tayade a,b, Suban K. Sahoo c, Rahul Patil a, Narinder Singh d, Sanjay Attarde b, Anil Kuwar a,
⇑
a School of Chemical Sciences, North Maharashtra University, Jalgaon 425001, MS, India
b School of Environmental and Earth Sciences, North Maharashtra University, Jalgaon 425001, MS, India
c Department of Applied Chemistry, SV National Institute Technology, Surat 395007, Gujrat, India
d Department of Chemistry, Indian Institute of Technology, Ropar 140001, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Preparation of a novel fluorescent
sensor for sensitive and selective
determination of Zn2+ ions.
Highly selective, sensitive and rapid
recognition of Zn2+ ion.
ꢀ
ꢀ
Zn2+ ion detection limit as low as
1.74 mM.
a r t i c l e i n f o
a b s t r a c t
A new non-natural receptor 2,20-[benzene-1,2-diylbis(iminomethanediyl)]diphenol (4) was synthesized,
and it is fluorogenic behaviour toward various metal ions were investigated. Receptor 4 exhibited pro-
nounced fluorescence enhancement in the presence of Zn2+, which can visually be discernible by an
orchid fluorescence in compared to other metal ions. The receptor 4 shows high sensitivity and selectivity
for Zn2+ through changes in the fluorescence intensity based on chelation-enhanced fluorescence (CHEF).
The binding modes of the complexes were investigated by Job’s plot and density functional theory (DFT).
Ó 2014 Elsevier B.V. All rights reserved.
Article history:
Received 12 December 2013
Received in revised form 27 January 2014
Accepted 2 February 2014
Available online 19 February 2014
Keywords:
Fluorescent probe
Zn2+
Dipodal aminophenol
Binding constant
DFT
Introduction
sciences [1–5]. In fact, a diversity of effective chemosensors for
transition metals ions has already been developed [6–9]. The
The recognition and sensing of cations via non-natural recep-
tors are of present curiosity in supramolecular chemistry because
of their large importance in chemical, biological and environmental
development of chemosensors for soft transition metal ions
predominantly requires because of their toxic impact on our
environments [10–14].
Along with various biologically important cations, zinc cation
has received growing attention for its established roles in a
variety of fundamental processes such as energy transduction,
⇑
Corresponding author. Tel.: +91 2572257403.
E-mail address: kuwaras@gmail.com (A. Kuwar).
http://dx.doi.org/10.1016/j.saa.2014.02.003
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

K. Tayade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 312–316
313
metalloenzyme function among all of the biologically important
metals, and also it is the second most abundant transition metal
ion in the human body after iron [15–18]. Zinc(II) is required for
human growth and development as human serum contains around
prepared in DMSO/H2O (80:20, v/v) and the corresponding working
solutions (0.1 mM) were prepared by appropriate dilution. The sol-
vent ratio was kept constant throughout the experiment.
19 l
M. Deficiency of Zn2+ may lead to many diseases ranging from
UV visible analysis
Alzheimer’s disease [19] to prostate cancer [20] and diabetes [21].
It is also a demanding problem for chemists to develop a chemo-
sensor that can discriminate Zn2+ from Cd2+ because of the fact that
cadmium and zinc are in the identical group of the periodic table
and have similar coordination properties. Thus, it is become a
target of current research area for chemists to design sensitive
chemosensor which can be selectively and sensitively applied for
the detection of Zn2+ in the presence of other cations including
The UV–visible spectrophotometer experiments were carried in
mixed DMSO/H2O solvent system at room temperatures (298 K) to
determine the cations selectivity of the receptor 4 in the ground
state. The experiment was performed for showing satisfactory
linear relationship between the concentrations and absorbance
intensity and for correlation coefficient. These titration experi-
ments were accomplished through a stepwise addition of cation
to a solution of receptor 4 in 10 ml volumetric flasks. The absor-
bance spectra were recorded in the range of 200–400 nm.
Cd2+
.
Herein, we report the synthesis, characterization and Zinc(II)
fluorosensing properties in DMSO/H2O (80:20, v/v) of receptor 4
(Scheme 1). The receptor 4 showed remarkable enhancement in
the emission intensities in the presence of Zn2+ ion. However, the
emission intensities of the receptor 4 either decreases or remain
unaltered in the presence of others metal ions. Thus, the receptor
4 can be applied for the selective detection of Zn2+ using the
turn-on fluorescence response in the presence of other metal ions.
Fluorescence analysis
The fluorescence experiments were carried out in mixed DMSO/
H2O solvent system at room temperature (298 K) with the aim to
determine the fluorosensing ability of the receptor 4 towards var-
ious cations. The titration experiments were accomplished through
a successive addition of Zn2+ solution (1 mM) to a solution of
receptor 4 (0.1 mM) in a 10 ml volumetric flask. The fluorescence
intensity was recorded at kex/kem = 275 nm/651 nm. The excitation
and emission slits were set to 5.0 nm. After each aliquot addition of
Zn2+ to the receptor solution, enough time was given to attain the
equilibrium. Then, the fluorescence data were collected and ana-
lyzed using the Benesi–Hildebrand equation to calculate the asso-
Experimental
Materials and measurements
All commercial grade chemicals and solvents were used with-
out further purification. The fluorescence and UV–Visible spectra
were recorded respectively on a Fluoromax-4 spectrofluorometer
and a Shimadzu UV-24500 spectrophotometer. The chloride salts
of the metal ions were used in this study. Ultrapure water with a
Millipore Purification System (Milli-Q water) was used throughout
the analytical experiments. The 1H NMR and 13C NMR spectra were
recorded on Varian NMR mercury System 300 spectrometer oper-
ating at 300 MHz and 75 MHz in CDCl3.
ciation constant (K) with the Zn2+
.
Synthesis of receptor 4
Compound 3 was synthesized by reacting one mole of o-pheny-
lene-1,2-diamine (0.108 g, 1 mmol) with two moles of 2-hydroxy-
benzaldehyde (0.244 g, 2 mmol) in ethanolic medium (50 ml) for
1 h over magnetically stirred and refluxed condition. Compound
3 was obtained with good yield and appears as a yellow crystalline
powder with 87% yield, mp > 250 °C. Further, the receptor 4 was
obtained from compound 3 by reduction under NaBH4 in methano-
lic medium with good yield. Yield 80%, mp P 250 °C. 1H NMR
(300 MHz, d, ppm, CDCl3): 1.42–1.46 (d, 6H, 2-CH3), 1.67–1.75
(bs, 2H, 2-OH), 2.72–2.80 (t, 4H, 2-CH2-), 3.80–3.89 (q, 2H,
2 = CH-), 6.74–7.17 (m, 8H, Ar–H), 11.38 (s, 2H, NH). 13C NMR
(75 MHz, d, ppm, CDCl3): 37.0, 115.3, 115.1, 118.4, 121.1,
Samples preparation
All stock and working solutions were prepared in ultrapure
water and spectroscopic grade DMSO. A stock solution of probe 4
(10 mM) in DMSO/H2O (80:20, v/v) was prepared. Receptor 4 is
freely soluble in DMSO/H2O at 25 °C, and the corresponding work-
ing solutions (1 mM) were prepared simply by diluting with
DMSO/H2O (80:20, v/v). Stock solutions of cations (1 mM) were
Scheme 1. Synthesis route for receptor 4.

314
K. Tayade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 312–316
128.2, 128.4, 129.0, 131.0, 154.2. LC-MS (M + H+) calcd. for
20H20N2O2 = 321.4, found for C20H20N2O2 = 321.07.
14
C
12
10
8
Results and discussion
6
4
The receptor 4 was obtained from compound 3 by reduction un-
der NaBH4 with good yield (Scheme 1) and characterized by melt-
ing point, IR, 1H NMR, 13C NMR, mass spectroscopy. The spectral
investigations gave confirmation of the structure of the receptor 4.
The fluorescence spectra (kex = 275 nm) of receptor 4 in the ab-
sence and presence of different metal cations are shown in Fig. 1.
Without cation, receptor 4 revealed no fluorescence. However,
addition of Zn2+ caused a remarkable fluorescence enhancement
at 651 nm. Under the same conditions, the fluorescence response
2
0
Various Metal Ions
Fig. 2. Fluorescence spectrophotometric response of receptor 4 (0.1 mM) upon the
addition of 0.5 equivalent of different metal ions (1 mM) in DMSO containing 20%
water.
of receptor 4 was checked for other metal ions such as Cr3+, Fe3+
,
Mn2+, Cd2+, Co2+, Ni2+, Cu2+, Hg2+, Bi3+, Ce4+ and Pb2+ but no obvious
changes on fluorescence intensities could be observed (Figs. 1 and
2). Further, the ratiometric fluorescence responses (Fig. 4) of recep-
tor 4 indicate the selectivity for the Zn2+ among the other tested
metal ions. The binding of Zn2+ close to fluorophore may have
influenced the hydroxyl functional groups which resulted in the
modulation of the charge transfer (CT) band. Thus, the observed
fluorescence response is the consequence of the chelation-en-
hanced fluorescence (CHEF) [18,22,23] (see Fig. 2).
Competating Ions
Competating Ions + Zn(II) Ions
5.00E+06
4.00E+06
3.00E+06
2.00E+06
1.00E+06
0.00E+00
The practical applicability of receptor 4 as selective fluorescence
sensor for Zn2+ was investigated by performing a competitive
experiment for the estimation of Zn2+ (1 equiv.) in the presence
Various Metal Ions
of other cations such as Cr3+, Fe3+, Mn2+, Cd2+, Co2+, Ni2+, Cu2+
,
Fig. 3. Interference of respective metals (2 equiv.) in Zn2+ ion (0.5 equiv.) detection
Hg2+, Bi3+, Ce4+ and Pb2+ (2 equiv.). No significant difference in
the fluorescence responses of receptor 4 for Zn2+ was observed
by comparing the intensity with the co-existence of other metal
ions (Fig. 3). The unique binding for Zn2+ suggests an interesting
view to develop receptor 4 as a fluorosensor for Zn2+even in the
presence of other metal ions.
with receptor 4 (2000 lL).
6.00E+06
5.00E+06
4.00E+06
3.00E+06
2.00E+06
1.00E+06
The fluorescence titration of receptor 4 upon successive addi-
tion of Zn2+ in DMSO/H2O (80:20, v/v) was shown in Fig. 4. As seen
in Fig. 4, the fluorescence intensity gradually increased by over
14-fold, with the addition of Zn2+. The fluorescence enhancement
with the successive addition of Zn2+ was used to calculate the
detection limit of receptor 4, which responds to Zn2+ linearly in
the micromolar level in its concentration range (Fig. 5) with the
0.00E+00
630
635
640
645
650
655
660
665
670
detection limit down to 1.74 lM.
The binding stoichiometry between receptor 4 and Zn2+ ion was
determined by the Job’s plot method (Fig. 6a) and normalised plot
obtain from the fluorescence titration (Fig. 6b) [24]. The results
indicate the formation of a 1:1 complex and the association con-
stant for the 4-Zn2+ complex was calculated from the fluorescence
titration experiments by Benesi–Hildebrand plot [25] was found to
be 2.0 ꢁ 104 Mꢂ1 (Fig. 7).
Wavelength (nm)
Receptor 4
40 µL
80 µL
10 µL
50 µL
90 µL
160 µL
20 µL
60 µL
100 µL
180 µL
30 µL
70 µL
120 µL
200 µL
140 µL
Fig. 4. Fluorescence titration spectra of 4 (0.1 mM) in response to the presence of
Zn2+ (1 mM) ions in DMSO: H2O (80:20, v/v); kex = 275 nm, kem = 651 nm.
6.00E+06
6.00E+06
5.00E+06
4.00E+06
3.00E+06
2.00E+06
1.00E+06
0.00E+00
y = 1E+11x + 697195
5.00E+06
4.00E+06
3.00E+06
2.00E+06
1.00E+06
0.00E+00
R² = 0.9661
630
635
640
645
650
655
660
665
670
Wavelength (nm)
Receptor 4
Ni(II)
Cr(III)
Mn(II)
Zn(II)
Pb(II)
Fe(III)
Cd(II)
Co(II)
Hg(II)
Cu(II)
0
0.00001
0.00002
0.00003
0.00004
0.00005
Bi(III)
Ce(IV)
[G]
Fig. 1. Fluorescence response of receptor 4 (0.1 mM) under the optimum conditions
towards surveyed ions (1 mM) (kex = 275 nm).
Fig. 5. Linear fitting of normalised plot from fluorescence spectroscopy.

K. Tayade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 312–316
315
3.00E-03
2.50E-03
2.00E-03
1.50E-03
1.00E-03
5.00E-04
0.00E+00
0
0.2
0.4
0.6
0.8
1
Xi
Fig. 8. Changes in absorbance spectrum of receptor 4 (0.1 mM) upon the addition of
Zn2+ metal (1 mM) in DMSO/H2O (80:20, v/v), where red line represents response of
Fig. 6a. Job’s plot revealed 1:1 Stoichiometry of the 4-Zn2+ complexation.
R while violet line indicates response of 4, after addition of 100 lL (0.5 l
L) of Zn2+
ion solution. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
1.20E+00
y = 0.7125x + 3.8841
1.00E+00
R² = 0.9371
state and the receptor 4 need to be excited for binding in the mixed
DMSO/H2O medium [26].
8.00E-01
6.00E-01
4.00E-01
2.00E-01
0.00E+00
The quantum mechanical calculations of the receptor 4 and its
complex with Zn2+ ion were performed at the DFT level by using
the computer program Gaussian 09W [27,28] to propose the possi-
ble 3D structure and also to complement the experimental evi-
dences. All the DFT calculations were performed in the gas phase
with a hybrid functional B3LYP (Becke’s three parameter hybrid
functional using the LYP correlation functional) using the 6-
31G(d,p) basis set. The optimized structures of the receptor and
its two possible complexes with Zn2+ (4- and 6-coordinated) are
shown in Fig. 9. As can be seen (Fig. 9a), the receptor 4 is stabilized
with the presence of the intramolecular hydrogen bond between
the phenolic-OH and the amine-N of length 1.941 Å. In four-coor-
dinated complex (Fig. 9b), the receptor 4 provides a tetrahedral
environment to encapsulate the Zn2+ ion with the average Zn–N
and Zn–O bond length of 2.109 Å and 1.887 Å, respectively.
Whereas under octahedral environment coordinated with two
water molecules, the receptor 4 preferred a trans-configuration
with the average Zn–N and Zn–O bond length of 2.108 Å and
-6
-5
-4
-3
-2
-1
0
LOG [G]
Fig. 6b. Normalised plot obtained for 4.Zn2+ ion (kem = 651 nm).
The infrared spectrum of free receptor 4 showed a broad band
at 3400 cmꢂ1 which can be assigned for OH group. On complexa-
tion with Zn2+, deprotonation of OH group took place on coordina-
tion that resulted in the disappearance of OH band. Further, the
MALDI/TOF-MS data confirmed the formation 1:1 complex be-
tween two deprotonated ligand (receptor 4) and Zn2+ ion [(MS
(ESI): m/z requires (calculated)
384.05.
C20H18ZnN2O2: 383.80, found
2.014 Å, respectively. However, the calculated interaction energy
2þ
(Eint ¼ Ecomplex ꢂ Ereceptor ꢂ E ) of 68.36 kcal/mol for tetrahedral
Zn
The absorption spectrum of receptor 4 with Zn2+ ion is shown in
Fig. 8. Receptor 4 exhibits peak maxima at 248 and 275 nm in
DMSO/H2O. The longer wavelength band at 275 nm may be as-
signed to transition associated with the phenol ring. The addition
of Zn2+ ion into receptor 4 caused no change in the spectral of
receptor 4. The UV–Visible spectra of receptor 4 before and after
the addition of 0.5 equivalent of Zn2+ ion remain relatively un-
changed illustrated that it did not bound with Zn2+ ion in ground
0.0000014
y = 5E-12x + 1E-07
0.0000012
R² = 0.9811
0.000001
0.0000008
0.0000006
0.0000004
0.0000002
0
0
50000
100000
150000
200000
250000
1/[G]
Fig. 9. Optimized structure of (a) receptor 4 and its possible 4-Zn2+ complexes [(b)
Fig. 7. Benesi–Hildebrand plot for receptor 4, 1/
D
F vs 1/[G] (Ka = Intercept/Slope).
tetrahedral and (c) octahedral] through DFT using B3LYP/6-31G(d,p) method.

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K. Tayade et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 312–316
ranging from 0 to 200 lL with the detection limit down to 1.74 lM
in mixed DMSO/H2O medium.
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complex was increased to 415.67 kcal/mol for octahedral complex
which indicates favorably the 4-coordinated complexation. Further
analyses of the plots of frontier molecular orbitals of the receptor
and its Zn2+ complex (Fig. 10) inferred that the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbi-
tal (LUMO) of the receptor 4 are localized on whole molecular sys-
tem. However, the electron density of HOMO and LUMO are
localized only on the phenolic and benzenediamine ring respec-
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which also responsible for the turn-on fluorescence response.
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In conclusion, we have develop an ‘‘off–on’’ responsive and
selective receptor 4 as a probe to monitor the Zn2+ concentration
through changes in fluorescence intensity based on chelation-
enhanced fluorescence (CHEF) mechanism. The receptor showed
high sensitivity and selectivity for Zn2+ detection at concentrations
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