Highly selective turn-on fluorogenic chemosensor for Zn2+ based on chelation enhanced fluorescence

Article abstract of DOI:10.1016/j.inoche.2019.02.020

A new fluorescent zinc sensor was constructed based on cyclic and noncyclic Schiff's bases obtained by the condensation of 4?(diethylamino) salicylaldehyde and 2?aminobenzenethio under two different catalytic conditions such as sulphuric acid yields 2?(be

Full text of DOI:10.1016/j.inoche.2019.02.020

Accepted Manuscript
Highly selective turn-on fluorogenic chemosensor for Zn2+ based
on chelation enhanced fluorescence
Vetriarasu Venkatesan, Selva Kumar R, S.K. Ashok Kumar,
Suban K. Sahoo
PII:
S1387-7003(19)30076-0
DOI:
https://doi.org/10.1016/j.inoche.2019.02.020
INOCHE 7278
Reference:
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
20 January 2019
7 February 2019
15 February 2019
Please cite this article as: V. Venkatesan, S. Kumar R, S.K.A. Kumar, et al., Highly
selective turn-on fluorogenic chemosensor for Zn2+ based on chelation enhanced
fluorescence,
Inorganic
Chemistry
Communications,
https://doi.org/10.1016/
j.inoche.2019.02.020
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ACCEPTED MANUSCRIPT
2+
Highly Selective Turn-On Fluorogenic Chemosensor for Zn Based
on Chelation Enhanced Fluorescence
Vetriarasu Venkatesan , Selva Kumar R , S.K. Ashok Kumar and Suban K. Sahoo b
a
a
a,*
a
Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology, Vellore-
32014, Tamil Nadu, India. E-mail: ashok312002@gmail.com
6
b
Department of Applied Chemistry, S. V. National Institute Technology, Surat-395007, Gujarat, India.
ABSTRACT
A new fluorescent zinc sensor was constructed based on cyclic and noncyclic Schiff’s bases
obtained by the condensation of 4-(diethylamino) salicylaldehyde and 2-aminobenzenethio under
two different catalytic conditions such as sulphuric acid yields 2-(benzo[d]thiazol-2-yl)-5-
(
diethylamino) phenol (L
mercaptophenyl)imino)methyl) phenol (L
CH OH/H O (1:1, v/v) due to excited state intra-molecular change transfer (ESIPT) process but
upon interaction with Zn2+ shows strong fluorescence due to chelation-enhanced fluorescence
CHEF) but with selected metal ions, there is no such a fluorescence change was observed. The
Job’s plot and B-H plot studies were revealed the formation of 2:1 and 1:1 stoichiometry with an
1
)
and acetic acid gives to (E)-5-(diethylamino)-2-(((2-
2
). Both L and L alone exhibits a week fluorescence in
1
2
3
2
(
4
-1
4
-1
estimated association constant (K
a
) of 4.9 x 10 M and 2.1 x 10 M with L
1
and L
2
respectively.
-8
-7
The detection limit of both sensors L
1
and L
2
were found to be 6.7 x 10 M and 3.6 x10 M
2
+
respectively. The fluorescence reversibility study was done by adding Zn and EDTA sequentially
2
+
to L
1
and L . Both sensors were successfully used for the determination of Zn in the different
2
water samples and pharmaceutical multivitamins tablet. The sensing mechanism was studied by
1H NMR, ESI-mass analysis as well as theoretical calculations using the Gaussian 09 program.
Keywords: Schiff’s base, Fluorescence, CHEF, Zinc, DFT study.

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1
. Introduction
Zinc is the second richest transition metal in the human body after iron, and it plays important role
in numerous biological processes including regulation of enzymes, a physical cofactor in
metalloproteins, neural signal transmission and gene expression [1–4]. But higher levels of Zn2+
in humans have been concerned in neurodegenerative syndromes [5–7]. In recent years, the
2
+
development of fluorescent sensors for Zn has become a very active area in the field of chemical
2
+
2+,
2+
biology [8,9]. Recognition of metal ions such as Zn , Cd and Hg is still a challenge since
10
those metals do not show spectroscopic and magnetic responses due to its d system [10]. So,
optical sensors based on ion-induced fluorescence changes afford an optimal choice to detect Zn2+
in biological and environmental samples due to their high simplicity, sensitivity, selectivity,
response time and low detection limit [11–13]. Accordingly, an organic molecules possessing a
Schiff’s base structure has potential applications in the development of colorimetric chemosensors
and fluorogenic sensors used for environmental sample monitoring [14–16].
It has been observed from the literature, molecules having α-hydroxy imine-based structure
provides week fluorescence with large Stokes shift are due to the excited state intramolecular
proton transfer (ESIPT) phenomenon [17–25]. These molecules when it interacts with zinc ions
form a complex either 1:1 or 1:2 ratio, which restricts the ESIPT process and increases the
chelation-enhanced fluorescence (CHEF) process [26-30]. Schiff bases are known to form stable
complexes with transition metal ions and act as ion carriers. The key features of Schiff bases are
providing a proper geometry, cavity control of host-gest complexation, modulation of its
lipophilicity leads to outstanding selectivity, sensitivity and stability for a specific ion. Besides,
the presences of N, O, and S as donor atoms are well known to form a strong complex with
transition metal ions and such systems are used as a ion carrier in making fluorogenic sensors for
Zn2+ [29–33]. In this work, we have a designed and synthesized ligands L
and L using 4-
1
2
diethylamino salicylaldehyde and 2-amino thiophenol in methanol to undergo a simple
condensation reaction to get new Schiff’s bases. The designed ligands in this work are already
synthesized and used for biological applications [34,35] but there is no report on the fluorogenic
2
+
sensor for Zn estimation.

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2
2
. Experimental section
.1. Materials and methods
Metal nitrates, chloride salts, and solvents were purchased from SD fine and Merck chemicals (AR
Grade). 2-aminobenzenethiol and 4-(diethylamino)-2-hydroxybenzaldehyde were purchased from
Sigma Aldrich. The absorption spectra were recorded on a JASCO V-730, UV-visible
spectrophotometer using a 1 cm quartz cell. Fluorescence measurements were carried out on a
JASCO FP-8200 fluorescence spectrophotometer equipped with a xenon lamp and instrumental
parameters were controlled by JASCO Spectra Manager 2. Mass spectra of the prepared
compounds were recorded on a Perkin-Elmer Clarus 680 GC and Clarus 600 MS. NMR spectra
1
were recorded using a 400 MHz Advance Bruker NMR spectrometer. H chemical shift was
recorded in ppm downfield from tetramethylsilane (TMS) as an internal standard. Fluorescence
lifetime analysis was performed using a HORIBA Jobin-Yvon instrument. The geometry of L ,
1
2
+
L
2, and its Zn complexes was optimized by density functional theory (DFT) using the Gaussian
9 program[36]. And the Becke-3-Lee-Yang-Parr (B3LYP) exchange-correlation functional
method with the standard basis set 6-31G(d,p) for C, H, N, O and S atoms and the LANL2DZ
0
2
+
effective core potential for Zn atom[9].
.2 Synthesis and characterization of L
The ligand L was synthesized slightly modified from reported procedure [34,37] by reacting 2-
2
1
and L
2
1
aminobenzenethiol (0.125g, 1 eq) and 4-(Diethylamino) salicylaldehyde (0.19g, 1 eq) were taken
in 20 mL of methanol in the presence of catalytic amount of sulphuric and contents were allowed
to react at room temperature for an overnight. After completing the reaction, the precipitate was
filtered, washed by methanol several times and kept for air drying, after vacuum drying offers a
yellow solid with a yield of 90%. Whereas L
eq) and 4-(diethylamino)-2-hydroxybenzaldehyde (0.77g, 1 eq) in 20 mL methanol solution in
the presence of few drops of acetic acid as a catalyst and reaction mixture was stirred at room
temperature under N atmosphere for overnight. The progress of the reaction was monitored
2
was prepared by reacting 2-aminobenzenethiol (0.5g,
1
2
through by TLC. The product was filtered and washed with methanol offer the yellow solid with
a yield of 85%.

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Scheme 1. Reagents and conditions to synthesize L
1
and L .
2
1
L
1
: Molecular formula C17
H
18
N
2
OS, H NMR (400MHz, CDCl
3
) 1.94-2.23 (t, 6H), 3.37-3.43 (q,
1
3
4
H), 6.27 (d, 2H), 7.422 (s, 1H), 7.45-7.47 (d, 2H), 7.79-7.78 (m, 2H), 12.56 (s, 1H), C NMR
(
100MHz, CDCl ) 12.72, 44.58, 97.91, 104.16, 121.03, 121.23, 124.16, 126.25, 129.87, 131.92,
3
1
51.34, 152.26, 159.82, 169.60. HR-MS: 298.1820.
1
L
2
: Molecular formula C17
H
20
N
2
OS, H NMR (400MHz, CDCl
3
) 1.142 (t, 6H), 3.34-3.41 (q, 4H),
6
1
1
.13 (s, 1H), 6.37 (S, 1H), 7.19-7.30 (t, 1H), 7.39-7.42 (t, 1H), 7.44-7.49 (m, 3H), 8.78 (s, 1H),
1
3
3.04 (s, 1H). C NMR (100MHz, CDCl
3
) 12.73, 44.69, 97.78, 103.9, 109.18, 117.05, 126.16,
26.27, 126.96, 131.22, 134.09, 146.83, 152.04, 160.79, 163.48. HR-MS: 300.2263.
2
+
2
.3 Preparation of pharmaceutical samples for Zn analysis
Zinc-containing commercially available multivitamin pharmaceutical samples contain 1mg of zinc
in 1mg tablet and 1.25 ml syrup form were quantitatively taken in a beaker and digested with 5mL
of HNO
3
on a sand bath till its dryness then added 5 ml of 20% of H
2
SO and continued the heating
4
till the volume was reduced to 1.5 mL then cooled the contents to room temperature [35].
Ammonium hydroxide solution was added to neutralize the solution. Then the solution was
filtrated and transferred to a 10 ml standard flask further solution was diluted using MeOH/H
1:1, v/v) and the total amount of zinc is 1.5 mM. A proper dilution was made to bring the zinc
concentration to 10 μM during the analysis.
2
O
(
3
. Results and discussion
The synthesized benzothiazole based ligand L
1
containing an acidic hydroxy group at o-position
and N, N’-diethyl group at p-position with respect to the -N=C moiety. An essential condition for

ACCEPTED MANUSCRIPT
ESIPT is the presence of H- bond between donor (-OH and NH
2
) and proton acceptor (=N- and –
C=O) groups in close vicinity to each other in a molecule. Whereas in the case of ligand L
2
containing one more acidic thiol group at o’-position. The location of these groups in such that
there is an intra-molecular hydrogen bonding. This leads to the excited state intramolecular proton
transfer (ESIPT) and thus it shows week emission band[34]. According to Pearson’s HASB
2
+
concept[38], L
between L and L
as bidentate and ligand L
are selected as a suitable sensing material for Zn by the spectrofluorometric method.
1
and L
with Zn are characterized by optical spectroscopy, in which ligand L
can at as tridentate coordination with a metal center. Hence, L
2
act as a soft donor while Zn act Lewis acid and host-guest formation
2
+
1
2
1
can act
and L
2
1
2
2
+
3
.1 Sensing ability of L
1
and L
2
towards cations
A preliminary investigation was performed using the spectrophotometric method in solution phase
-5
using solvent media MeOH/H
2
O (1:1, v/v). The recognition ability of L
1
and L (5×10 M) were
2
+
3+
2+
2+
2+
investigated in the presence and absence of different metal ions such as Ag , Cr , Co , Ni , Ca ,
2
+
2+
2+
2+
2+
3+
2+
3+
3+
-4
Zn , Cu , Hg , Cd , Mg , Al , Pb , Fe and In (5x10 M) by recording the UV-visible
spectra (Fig. 1a). Alone L showed two absorption bands appearing at 380 nm and 425 nm while
showed only one absorption band at 380 nm. Upon interacting L with different metal ions, the
peak appeared at 380 nm undergo hypochromic effect and peak appeared at 425 nm undergo
on interacting with some transition metal ions exhibits
a red-shift while and other metal ions exhibit a hypochromic effect (Fig. 1b). Overall from this
study, the quantitative estimation of metal ions using L and L is not possible due to an overall of
spectral bands or at moderate concentration possible interference from each other.
1
L
L
2
1
1
hyperchromic effect. But in the case of L
2
1
2

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Fig 1. UV-Vis spectral responses of (a) L
1
and (b) L
2
with various metal ions in MeOH/H O (1:1, v/v)
2
media
In order to know fluorescence ability of ligands and its complexes, the initially visual test was
conducted by exposing the prepared solutions of L and L with selected metal ions (10 eq.) in
MeOH/H O (1:1, v/v) using wavelength 365 nm in UV-chamber. The results reveal that only
solution containing Zn exhibits fluorescence phenomenon visible to the necked eye and this
evidence used for spectrofluorometric characterization of L and L with intention as a fluorogenic
sensor for Zn (Fig. 2 a,b). In order to know the fluorogenic behavior of L and L with selected
metal ions, the fluorescence spectra were recorded by excitation at 380 nm. The studies revealed
1
2
2
2
+
1
2
2
+
1
2
2
+
that only L
chelation-enhanced fluorescence (CHEF) (Fig. 2 c,d) [26]. The week fluorescence of L
can be explained by the ESIPT process [34]. In the case of L
-Zn2+ complex shows 5-fold
-Zn complex shows 4-fold enhanced
1
and L
2
with Zn exhibits a strong fluorescence enhancement at 440 nm due to strong
1
and L
2
1
2
+
enhanced emission exhibits at 435 nm and similarly in L
2
2
+
emission intensity at 440 nm. Enhancement of fluorescence of ligands and its Zn complexes may
2
+
be attributed to the strong binding of Zn that would impose rigidity and hence decrease the non-
radioactive decay of the excited state of ligand. The transition metal ions with close shell d-orbitals,
2
+
such as Zn do not introduce low-energy metal-centered or charge-separated excited state into the
molecule, so energy and electron-transfer process cannot take place[39]. The fluorescence
2
+
response time and L with Zn are found to be 30 and 60s respectively (Fig. 7S).
2

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Fig 2. Image taken in UV light chamber and emission spectral responses of (a) L
1
(b) L
2
(c) L
1
and (d) L
2
-
4
with various metal ions (2x10 M) in MeOH/H
2
O (1:1, v/v) media.
and L with the addition of Zn were studied
2
+
The steady changes in fluorescence response of L
by spectrofluorimetric titration. Incremental addition of Zn (1 to 50 µM) into L
study the change in emission intensity (Fig 3a,b). The limit of detection (LOD) and limit of
quantification (LOQ) of Zn2+ using L
and L were calculated based on the calibration curve
1
2
2
+
1
and L leads to
2
1
2
obtained from the fluorescence titration. According to IUPAC method, the LOD and LOQ can be
calculated by using 3σ/slope and 10σ/slope respectively, where the slope is obtained from the
calibration curve (Fig 3c,d) and σ is the standard deviation of the emission intensity of L
1
and
2
+
-8
L
2
[40]. Results show that by using L
1
and L
2
, the LOD of Zn down to 6.67 x 10 M and 3.56 x
-7
-7
-6
1
0 M respectively while LOQ found to be 2.23 x 10 M and 1.2 x 10 M respectively.

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-5
-5
2+
-4
Fig 3. Emission spectral titration of probes (a) L
1
(2x10 M), (b) L (2x10 M) with Zn (2x10 M) in
2
2
+
MeOH/H O (1:1, v/v) media. The inserted figure represents a calibration curve of Zn using (c) L and (d)
2
1
2
L .
2
+
The binding stoichiometry of ligands (L
In this method, both ligands (L and L
1:1, v/v) media and recorded their fluorescence spectra (Figure 4a,b) [41]. It reveals that the
maximum fluorescence was noticed at 0.35 and the 0.5 mole-fraction which confirms the
1
and L
2
) with Zn was determined by Job’s plot method.
2
+
1
2
) and Zn taken in a different mole ratio in MeOH/H O
2
(
2
+
formation of 2:1 and 1:1 ligand to Zn complex with L
1
and L
of both ligands with Zn was estimated using the Benesi Hildebrand (B-H) plot[42]. From the
fluorescence titration data, the binding constant was calculated using the equation (i) for the L
2
respectively. The binding constant
2
+
1
2
+
and L with Zn complex (Fig. 4c,d).
2
1
1
1
=
ꢀ
+
ꢂ − − − (푖)
ꢁ
2
(
퐹 − 퐹0)
(퐹 − 퐹푠) (퐾(퐹 − 퐹 )[푍푛 ])
0
Where F is the fluorescence intensity of ligand (L
1
and L
2
), F
0
is fluorescence intensity with the
2
+
incremental addition of Zn and Fs is fluorescence intensity at saturation point. The plot of 1 / [F
2
+
2
–
F
0
] versus 1/ [Zn ] is linear (R of L
1
= 0.9989 and L
2
= 0.9987) and the binding constant (K )
a
4
-1
4
-1
2+
2+
was found to be 4.92 x 10 M and 2.048 x 10 M for L
1
-Zn and L
2
-Zn respectively. Further,
2
+
the stability of L
1
/L
2
with Zn complex in the presence and absence of common interfering metal
2+
2+,
2+
ions such as Ca , Cd and Pb were studied for 6 h by recording the emission intensity at

ACCEPTED MANUSCRIPT
2
+
intervals of 30 min. Results showed that both L
1
and L complex with Zn is stable for 100 min,
2
beyond 100 min incremental decrease in intensity was observed (Fig. 8S).
-Zn (b) and (d) L
-Zn2+
2
+
Fig 4. Job’s plots and BH plots of (a) and (c) L
1
2
3.2 pH studies
2
+
The influence of pH on fluorescence response of L
between pH 1-11 using appropriate buffer solutions (Fig. 6a). The intensity of free ligands (L
2) did not get altered from pH 4.5-10 But the intensity decreases below pH 3.0 due to protonation
of hydroxyl functional groups present in the ligands which reduce the ESIPT process. The same
1
, L2, and their Zn complexes was carried out
1
and
L
2
+
study was extended in the presence of Zn , results reveal that emission intensity remains unaltered
-Zn2+ and
L -Zn which represents the working
2+
in the pH range 4-9 and 5.5-10 with respect to L
1
2
range of the ligands. However, the intensity decreases below the pH 4 with L
-Zn2+ and pH 5 with
1

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L
2
-Zn2+ respectively the decrease in intensity in the acidic region is due to protonation of a
2
+
nitrogen atom, which restricts the ESIPT phenomenon. In case of L
same 5.5-10 whereas the intensity decreases in below pH 5 while above it starts to increases (Fig.
b). The decrease in intensity in acidic region (1.0-4.0) due to protonation of a nitrogen atom
which restricts ESIPT phenomenon. The increasing fluorescence intensity of L and L
-Zn2+
complex in basic pH (˃ 9) may be due to deprotonation at hydroxy (-OH) and thiol (-SH) leads to
2
-Zn intensity remains the
6
2
2
2
+
intra-molecular charge transfer (ICT) process. But in the case of L
1
-Zn at higher pH (˃10) there
is a decrease in the intensity could be due to the formation of Zn(OH)
2
[43]
.
2
+
-Zn2+
Fig 6. Effect of pH on (a) L
1
and L
1
-Zn (b) L
2
and L
2
3
.3 Interference studies
2
+
The specificity of L
1
and L
2
as a fluorogenic sensor for the detection of Zn in the presence of
OH/H O (1:1, v/v) (Fig. 7 a,b). For competitive
(50 μM) were treated with 10 eq of interfering metal ions such as (Ag , Cr ,
various competing cations was explored in CH
3
2
+
3+
studies, L
1
and L
2
2
+
2+
2+
2+
2+
2+
2+
3+
2+
3+,
3+
Co , Ni , Ca , Cu , Hg , Cd , Mg , Al , Pb , Fe and In ) then each solution was excited
at wavelength maximum and emission intensity measured at 440 nm and 435 nm respectively.
2
+
Results show that both L
1
and L exhibits good selectivity with Zn ions.
2

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Fig 7. Bar diagram representing fluorescence intensity of (a) L
, L
1 1
-Zn2+ and L
1
-Zn2+ (b) L
2
, L
-Zn2+
2
and L
.4 Reversibility studies
To check the dynamic response of the L
2
-Zn2+ with various competing metal ions in MeOH/H
2
O (1:1, v/v) media excited at λex=380 nm.
3
2
+
2+
1
-Zn complex, sequentially L contacted with Zn and
1
EDTA in CH
3
OH/H
2
O (1:1 v/v) media were performed. Due to the high stability constant of
2
+
2+
EDTA-Zn complex, binding reversibility L -Zn complex with EDTA was observed. A similar
1
2
+
method was adopted for L
2
-Zn complex. The purpose of this test to show how quickly L and
1
2
L can be recoverable from its complex using EDTA (Fig. 8 a,b).
-Zn2+ (b) L
-Zn with EDTA
2+
Fig 8. Reversibility studies of (a) L
1
2

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3
.5 Quantum yield and fluorescence lifetime analysis
2
+
2+
The fluorescence properties of L
1
, L
and L
were enhanced significantly. Fluorescence quantum yield (??
2
L
1
-Zn and L
2
-Zn were studied in methanol medium.
2
+
2+
-Zn2+
-Zn2+
When Zn ions interacted with L
1
2
, the fluorescence intensity of the L
1
-Zn and L
2
2
+
F
) of L , L , L -Zn and L
1
2
1
2
were studied in methanol solution and calculated by using comparative William’s method [44]
using quinine sulphate as a standard reference [45]. To calculate the quantum yield, initially record
UV-Vis absorption and emission spectra of five different concentrations of standard reference
2
+
2+
(quinine sulphate) and sample (L
1
, L
1
-Zn , L
2
and L -Zn ) were recorded. The integrated
2
fluorescence intensity was plotted against absorbance of reference and sample to calculate the
gradients of reference and sample (Fig. 9S). The quantum yield was calculated and tabulated in
the Table 1. Further, fluorescence lifetime is an average time of the molecules taken from excited
state to ground state of the emitting photon. Fluorescence lifetime parameter is important in
fluorescence studies such as fluorescence resonance energy transfer and fluorescence-lifetime
2
+
2+
imaging. The fluorescence lifetime of L
1
, L
1
-Zn , L
2
and L -Zn complex solution was excited
2
at 370 nm using a standard excitation light emitting diode laser (LED) as source and emission was
2
+
2+
observed at 440 nm for L
shows that the calculated (k
also the non-radioactive decay (knr) of the L
1
and L
1
-Zn and 435 nm for L
2
and L -Zn . (Fig. 5a, b). Table 1
2
2
+
2+
r
) value for the L
1
-Zn and L
2
-Zn is more compare to L
1
and L
2
and
2
+
2+
1
-Zn and L
2
-Zn is decreased compare to L
1
and L
2
shows that the complex emits a strong fluorescence even at a lower cconcentration of the complex.
2
+
2+
Table 1. Photophysical properties of ligands (L
1
, L
2
) and complexes (L
1
-Zn , L -Zn ).
2
Quantum
Lifetime
(ns)
K
r
K
nr
Ligand
7
-1
7 -1
Yield (??
F
)
(x10 s )
(x10 s )
L
1
0.033
1.68
1.99
57.6
L
L
1
2
-Zn2+
0.085
0.023
0.067
2.1
4.05
2.0
43.5
84.2
56.9
L
2
1.16
1.64
-Zn2+
4.1

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2
+
2+
Fig 5. (a) Time-resolved fluorescence decay of (a) L
1
and L
1
-Zn (b) of L
2
and L -Zn complex and
2
prompt (λex = 370 nm)
3
.6 Binding mechanism study
2
+
1
The binding mechanism of L
1
and L
2
with Zn were studied by H NMR and ESI-mass analysis.
1
2+
For this purpose, the H NMR of alone L
1
and L and their complexes with Zn were recorded in
2
DMOS-d
discussed. Accordingly, the ligand L
protons in the range from 6.27 to 7.79 ppm while in the case of L
ppm indicates –OH, at 6.13ppm correspond to -SH and aromatic peaks are resonated in the range
6
(Figure 9a,b,). Only those protons which are influenced during binding events are
shows a peak at 12.56 ppm corresponds to –OH and aromatic
, the peak appeared at 13.04
1
2
2
+
-
of 6.39 ppm to 8.78 ppm. When L
1
and L
2
are binding with Zn then OH peak is disappeared and
other aromatic peaks are experiencing shielding effect which confirms the direct involvement of -
2
+
2+
-
-
OH and –C=N- in the binding process with Zn . In the case of L -Zn , both the OH and SH
2
peaks are disappeared and the other aromatic peaks (7-7.5 and 6.39 ppm) are experienced the
shielding effect which confirms the direct involvement of –OH, -SH and nitrogen of –C=N- in the
2
+
1
2+
binding process with Zn . Overall the H NMR spectral change of free ligand and its Zn complex
2
+
supports the complexation mechanism proposed. Further, the ESI mass spectra of L
Zn2+ complex was studied. Accordingly, the mass spectra of L -Zn2+ shows a mass peak at
60.8450 which corresponds to involvement of two ligands and one zinc (2L +1Zn) with a
-Zn shows a major mass peak at 423.8832
1
-Zn and L
2
-
1
6
1
2
+
calculated mass of 660.1701 whereas in the case of L
2

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which corresponds to one ligand, one zinc and one nitrate (1L
2
+1Zn+1NO
3
) with the calculated
mass of 423.0218.
(
a)
(b)
(d)
(c)
1
2+
2+
-Zn and (d) L
-Zn2+
2+
Fig 9. H NMR and ESI-MS spectra of (a) L
1
, L
1
-Zn (b) L
, L
2 2
-Zn . (c) L
1
2
3
.7 Theoretical studies
2
+
To support experimental photophysical properties of L
1
and L towards Zn were calculated by
2
using density functional theory (DFT) using Gaussian 09 computational code. The geometries of
2
+
optimization of L
1G** basis set in the gas phase. Accordingly, the optimized structure of L
nonplanar due to the strain created by the five-member and six-member rings present in amino
1
and L
2
and their complex towards Zn were carried out using a B3LYP/6-
3
1
shows slightly
2
+
2+
thiophenol and salicylic aldehyde while in L
2
, L
1
-Zn and L
-Zn2+ system, the Zn2+ is at center
-Zn complex due to the formation of semi cavity
2
-Zn exhibits a planar geometry
(
Fig. 10 a,b). The optimized geometry shows that in L
1
2
+
surrounded by 2L1 ligands while in case of L
2

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2
+
created by the L
2
, Hence, Zn is tied up at the center of N, S and O groups of L
and Zn is -2528.9
exhibits more negative energy
2
. The interaction
2
+
energy (Eint=Ecomplex - Ereceptor - Eanalyte) of the complex formed between the L
1
2
+
kcal/mol while L
compare to L due to stable complexation occurred between L
well correlated with association constant values of L
2
with Zn is -1225.9 kcal/mol. It shows that L
1
2
+
2
1
and Zn . Theoretical results are
2
+
4
-1
2+
1
-Zn (4.92 x 10 M ) and L
2
-Zn (2.048 x
4
-1
1
0 M ) estimated by spectrofluorometric. The Mullikan’s population and NBO charge analysis
2
+
2+
show that electron populations are more with L
1
-Zn and L
2
-Zn compared to individual L and
1
2
+
2+
2+
L
2
(Table 1S). The bond length between L
1
towards Zn shows N-Zn is 2.084 Å and O-Zn is
2
+
2+
2+
1
.958 Å while the bond length between L
2
towards Zn shows N-Zn is 2.033 Å, O-Zn is 1.897
Å and S-Zn is 2.274 Å. The frontier molecular orbitals (FMOs) studies show that the L , L
is predominantly present
2
+
1
1
-
2
+
2+
Zn , L
2
and L
2
-Zn were analyzed (Fig. 10S and 11S). HOMO of L
1
on amino thiophenol group while LUMO is on salicylic aldehyde group however in the case of L
1
-
-
2
+
Zn , it is opposite. In the case of L
2
, both HOMO and LUMO present all over the L
2
but in L
2
2
+
2+
Zn , LUMO is maximum appeared over ligand and HOMO is located over Zn . These
observations show there is a ligand to metal charge transfer is observed. Besides, both the L and
1
2
+
L
2
shows a decrease in the energy band gap (ΔE) of L
1
and L
2
after interacting with Zn .
2
+
2+
Fig 10. DFT computed optimized structures of (a) L
1
(b) L
1
-Zn (c) L
2
(d) L -Zn using B3LYP method.
2

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3
.8 Analytical application
The analytical utility of L
1
and L
were assisted by determination of Zn2+ in different water
2
samples and a pharmaceutical multivitamin tablet. The results of various water sample analysis
and multivitamin sample analysis shown in Table 2S. All results were derived from triplicate
measurements with standard error. The recovery percentage is greater than 98 % which shows
2
+
proposed sensor L
1
and L can be utilized for the Zn analysis in real samples. Also, the present
2
sensor was compared with reported literature shown in Table 3S [1,6,12,13,46] and It concludes
that the present sensor highly useful
4
. Conclusion
In conclusion, we have synthesized and characterized L
1
and L
2
by reacting 4-(diethylamino)
and
show week fluorescence upon the complexation with Zn , the increase in fluorescence is due
to CHEF. The spectrofluorometric studies reveal that L and L forms 2:1 and 1:1 ligand to metal
complex with Zn with an estimated association constant of 4.9 x10 M and 2.1 x 10 M and
salicylaldehyde with 2-aminobenzenethio under different catalytic conditions. Both ligands L
1
2
+
L
2
1
2
2
+
4
-1
4
-1
2
+
-8
-7
LOD of Zn was found to be 6.7 x 10 M and 3.6 x10 M respectively. The present sensor was
2
+
used to estimate Zn present in various water samples as well as in the pharmaceutical samples.
1
The sensing mechanism was studied by H NMR, ESI-mass analysis and DFT theoretical studies.
Conflicts of interest
There are no conflicts to declare
Acknowledgments
Authors are grateful to Department of Science and Technology, GOI for supporting the work
through the project grant (EMR/2017/000816) and thankful to DST-VIT-FIST for NMR and
Vellore Institute of Technology, Vellore for other research facilities.

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Graphical abstract

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Highlights

Developed an easy-to-synthesize cyclic and noncyclic Schiff’s bases for the selective
2
+
detection of Zn by spectrofluorometric method.


Both ESIPT and CHEF processes were explored as a key sensing mechanism.
-8
The LOD of the sensor is 6.7 x 10 M which is lower than the WHO guidelines in
drinking water.
1


The sensing mechanism was well supported by H NMR, mass analysis and DFT.
The receptor L can be recyclable after treatment with a EDTA