Selective detection of Hg(II) with benzothiazole-based fluorescent organic cation and the resultant complex as a ratiometric sensor for bromide in water

Article abstract of DOI:10.1016/j.tet.2016.04.082

A benzothiazole-based receptor conjugated with an imidazolium cation, receptor N1, was synthesized. Receptor N1 is capable of selectively binding Hg(II) in the presence of other metal ions in water, with a large enhancement in fluorescence intensity at 38

Full text of DOI:10.1016/j.tet.2016.04.082

Accepted Manuscript
Selective detection of Hg(II) with benzothiazole-based fluorescent organic cation and
the resultant complex as a ratiometric sensor for bromide in water
Amanpreet Singh, Ajnesh Singh, Narinder Singh, Doo Ok Jang
PII:
S0040-4020(16)30371-4
10.1016/j.tet.2016.04.082
TET 27725
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 23 March 2016
Revised Date: 29 April 2016
Accepted Date: 30 April 2016
Please cite this article as: Singh A, Singh A, Singh N, Jang DO, Selective detection of Hg(II) with
benzothiazole-based fluorescent organic cation and the resultant complex as a ratiometric sensor for
bromide in water, Tetrahedron (2016), doi: 10.1016/j.tet.2016.04.082.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Selective detection of Hg(II) with benzothiazole-based fluorescent organic cation and
the resultant complex as a ratiometric sensor for bromide in water
Amanpreet Singh, Ajnesh Singh, Narinder Singh* and Doo Ok Jang*

ACCEPTED MANUSCRIPT
Selective detection of Hg(II) with benzothiazole-based fluorescent
organic cation and the resultant complex as a ratiometric sensor for
bromide in water
a
b
a,*
c,*
Amanpreet Singh, Ajnesh Singh, Narinder Singh, and Doo Ok Jang
Email: nsingh@iitrpr.ac.in; dojang@yonsei.ac.kr
a
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
Department of Applied Science & Humanities, Jawaharlal Nehru Govt. Eng. College, Sundernagar, India
b
c
Department of Chemistry, Yonsei University, Wonju 220−710, Republic of Korea
Abstract: A benzothiazole−based receptor conjugated with an imidazolium cation, receptor N1, was
synthesized. Receptor N1 is capable of selectively binding Hg(II) in the presence of other metal ions in
water, with a large enhancement in fluorescence intensity at 380 nm via a PET mechanism. The resulting
-
Hg(II) complex of receptor N1 showed a selective ratiometric response upon addition of Br ions. The
-
Hg(II) complex of receptor N1 was used to determine the concentration of Br ions in the presence of
1
other anions, with a detection limit of 22 nM. H NMR studies showed that the imidazolium hydrogen in
-
the Hg(II) complex of receptor N1 participates in the recognition of Br ions. In the absence of Hg(II),
-
receptor N1 did not recognize Br ions.
Keywords: fluorescence, ratiometric, receptor, recognition, bromide, water
2
9–32
Introduction
receptors have been reported for recognition of anions.
However, only a few of these receptors were developed for
detection of Br due to the lack of suitable binding sites, low
charge density, and a weak tendency to form hydrogen
Due to these challenging properties, the design of
sensors for Br has been difficult. Moreover, receptors for Br
suffer from competition with water molecules in an aqueous
medium.
Inspired by previous reports on imidazolium−conjugated
fluorescent sensors and in continuation of our research
The design and synthesis of heteroditopic receptors with
binding sites for simultaneously complexation with cationic
and anionic species have attracted a great deal of attention in
supramolecular chemistry. These receptors can interact with
a single heteroditopic guest such as amino acids and
−
33–35
bonds.
1
–4
−
−
5
,6
nucleotides or can bind with non−identical guest molecules.
Such systems are interesting due to their direct applications as
molecular sensors and membrane transport agents for
extracting ion pairs and enhancing the solubility of insoluble
36–
activities with benzimidazole/benzothiazole−based receptors,
7
–9
ions. Despite a number of practical applications, sensors for
40
we developed a simple organic receptor N1 that has
1
0–13
ion-pairs are rare, particularly in water.
Due to the diverse range of anion shapes and sizes, their
recognition using an organic receptor is a difficult task.
The recognition of anions is problematic in aqueous medium,
particularly when using a receptor relying purely on hydrogen
bonding, because of competition between anions and polar
solvent molecules for binding sites on the receptor. Ion−pair
recognition may improve the binding affinity of an anion by
providing extra electrostatic interactions from the cation, as
electrostatic interactions are generally less influenced by
proficient binding sites for cation recognition, as shown in
Scheme 1. The distal part of the receptor is tailored with an
imidazolium group that is well-known to provide anion
recognition. Another advantage of this approach lies in the fact
that the imidazolium moiety may impart solubility of the
receptor in an aqueous medium. Sensor activity in aqueous
media is mandatory if the targeted analyte is biologically or
1
4–16
41,42
environmentally important.
1
7–19
solvent than hydrogen bonding.
Another advantage of extra
electrostatic interactions is the cooperative effect. The ion-pair
receptor has sites for a cation and an anion. Upon binding of
one guest molecule (cation), the affinity of the receptor for the
2
0–22
second guest (anion) is increased.
Ion-pair recognitions have been attained by conjugating a
cation receptor with some hydrogen bond donor substitute
2
3–26
−
groups.
Thordarson et al. examined the binding of Cl with
2
7
a tetratopic ion-pair host in the presence of Ca(II). The
tetratopic receptor underwent conformational changes that
allowed the receptor to bind Cl . Smith et al. found that
Scheme 1. Synthesis route of receptor N1.
−
Experimental
General
association of an alkali metal ion with a receptor enhances the
binding affinity of an anion. In addition, a number of
2
8
1

ACCEPTED MANUSCRIPT
o
All chemicals were purchased from Aldrich Chemical Co. and
were used as−received without further purification. H NMR
yield semi−solid compound 3 (3.68 g, 88%). Mp: 87−88 C;
FT−IR ν 3125 (=CH), 3049 (=CH), 1675 (C=N), 1607, 1561,
1
1
spectra were recorded with a JEOL spectrometer operated at
1317 (C−O), 1164 (C−N), 1004, 805, 729; H NMR (400 MHz,
DMSO−d ) 3.79 (s, 3H, CH ), 4.72−4.78 (m, 4H, CH ), 7.17
1
13
4
00 MHz for H NMR and 100 MHz for C NMR. CHN
6
3
2
analysis was performed using a Perkin Elmer 2400 CHN
Elemental Analyzer, while pH measurements were carried out
with a ME/962P instrument. A PerkinElmer L55 fluorescence
spectrophotometer equipped with quartz cuvettes (path length =
(t, 1H, J = 8.0 Hz, Ar−H), 7.30 (d, 1H, J = 16 Hz, Ar−H), 7.43
(t, 1H, J = 16 Hz, Ar−H), 7.49−7.55 (m, 2H, Ar−H), 7.70 (t,
1H, J = 8.0 Hz, Ar−H), 7.94 (t, 1H, J = 8.0 Hz, Ar−H), 8.01 (d,
1H, J = 16 Hz, Ar−H), 8.07 (d, 1H, J = 16 Hz, Ar−H), 8.35 (d,
1
3
1
cm) was employed for fluorescence measurements, with a
1H, J = 16 Hz, Ar−H), 9.34 (s, 1H, imdazolium−H); C NMR
xenon lamp as the excitation source. X−ray diffraction data for
N1 were collected on a Bruker X8 APEX II KAPPA CCD
diffractometer at 293 K using graphite monochromatized
Mo−K radiation ( = 0.71073 Å). IR spectra were recorded
using a Bruker Tensor 27 spectrometer. Binding constant was
calculated using nonlinear regression analysis using Hyperspec
program. Atomic absorption spectroscopy experiments were
performed using Perkin Elmer AAS. Concentration of bromide
was measured using Inductively Coupled Plasma.
(100 MHz, DMSO−d ) δ 36.2, 49.0, 67.3, 114.0, 121.9, 122.2,
6
123.0, 123.2, 124.3, 124.5, 126.8, 129.6, 132.8, 135.6, 137.7,
152.1, 155.8, 162.5. Anal. Calcd for C H BrN OS: C, 54.81;
1
9
18
3
H, 4.36; N, 10.09. Found: C, 54.67; H, 4.25; N, 9.99.
4
4
Synthesis of receptor N1: Sodium perchlorate (0.85 g, 7.0
mmol) was added to a solution of compound 3 (2.10 g, 5.0
mmol) in 25 mL of H O:MeOH (1:1, v/v). The reaction
2
mixture was stirred at room temperature for 2 h. Volatiles were
removed under reduced pressure, and CH CN (20 mL) and
3
4
3
Synthesis of compound 1: A solution of salicylaldehyde
10.6 mL, 100 mmole) and anhydrous K CO (13.8 g, 100
MgSO (1.0 g) were added to the remaining suspension. After
4
(
standing for 1 h, the suspension was filtered. Volatiles were
evaporated under reduced pressure, and the resulting light
yellow powder was recrystallized from MeOH to produce
yellow crystals of N1 (1.86 g, 85%). Mp: 104−106 °C; FT−IR ν
3079 (=CH), 2873 (−CH), 1680 (C=N), 1591, 1340 (C−O),
2
3
mmole) in CH CN (20 mL) was heated to reflux for 1 h, and
3
then 1,2−dibromoethane (86.17 mL, 1000 mmole) was added.
The reaction mixture was refluxed for 10 h under argon. The
reaction mixture was cooled to room temperature and filtered,
and the volatiles were evaporated under vacuum. The resulting
crude product was purified using column chromatography on
−1
1
1179 (C−N), 1103, 920, 775, 706 cm ; H NMR (400 MHz,
DMSO−d ) 3.79 (s, 3H, CH ), 4.72−4.76 (m, 4H, CH ), 7.13
6
3
2
1
silica gel to produce compound 1 (21.8 g, 95%). H NMR (400
(t, 1H, J = 16 Hz, Ar−H), 7.26 (d, 1H, J = 16 Hz, Ar−H), 7.42
(t, 1H, J = 16 Hz, Ar−H), 7.48−7.51 (m, 2H, Ar−H), 7.68 (t,
1H, J = 8.0 Hz, Ar−H), 7.93 (t, 1H, J = 8.0 Hz, Ar−H), 7.98 (d,
1H, J = 16 Hz, Ar−H), 8.04 (d, 1H, J = 16 Hz, Ar−H), 8.35 (d,
MHz, CDCl ) 3.70 (t, J = 8.0 Hz, 2H, CH ), 4.41 (t, J = 8.0
3
2
Hz, 2H, CH ), 6.94 (d, J = 16 Hz, 1H, Ar−H), 7.52 (t, J = 16
2
Hz, 1H, Ar−H), 7.84 (d, J = 16 Hz, 1H, Ar−H), 10.53 (s, 1H,
1
3
13
CHO); C NMR (100 MHz, DMSO−d ) δ 28.8, 68.3, 112.7,
1H, J = 16 Hz, Ar−H), 9.36 (s, 1H, imdazolium−H); C NMR
6
1
21.6, 125.3, 128.6, 136.0, 160.7, 189.7.
Synthesis of compound 2: A solution of compound 1 (4.52 g,
0 mmol) and N−methyl imidazole (1.68 g, 20 mmol) in
(100 MHz, DMSO−d ) 36.3, 49.0, 67.3, 114.2, 121.9, 122.3,
122.4, 123.1, 123.2, 124.3, 125.7, 127.0, 129.59, 133.0, 135.7,
6
2
137.8, 132.1, 155.9, 162.5. Anal. Calcd for C H ClN O S: C,
1
9
18
3
5
CH CN (40 mL) was heated to reflux for 10 h under argon.
After completion of the reaction, the solvent was evaporated
using a rotatory evaporator. The product was recrystallized
52.35; H, 4.16; N, 9.64. Found: C, 52.26; H, 4.10; N, 9.48.
Binding studies: Binding studies with receptor N1 were
performed using fluorescence spectroscopy in an aqueous
3
o
from EtOH to yield compound 2 (6.01 g, 96%). Mp: 106−108
medium. Binding studies were performed at 25±1 C, and the
o
C; FT−IR ν 3147 (=CH), 2994 (−CH), 2969 (−CH ), 1723
solutions were shaken for a sufficient time before recording the
spectra. UV−vis absorption spectra of receptor N1 exhibited a
maximum absorption peak at 260 nm. In fluorescence
spectroscopy, receptor N1 exhibited a weak emission at 380 nm
when excited at 260 nm. For cation binding studies, a 20 M
solution of receptor N1 was prepared in water. Fluorescence
spectra were recorded upon addition of 0.6 equivalents (12 M)
of metal ions. HEPES buffer solution was used to maintain a
pH value of 7.4 in all experiments.
3
(
C=O), 1591, 1508, 1378 (C−O), 1153 (C−N), 1119, 952, 841
−1 1
cm ; H NMR (400 MHz, CDCl ) 4.02 (s, 3H, CH ), 4.54 (t,
3
3
J = 8.0 Hz, 2H, CH ), 5.05 (t, J = 8.0 Hz, 2H, CH ), 7.01 (d, J
2
2
=
8.0 Hz, 1H, ArH), 7.14-7.20 (m, 2H, ArH), 7.20 (s, 1H,
ArH), 7.58 (t, J = 8.0 Hz, 1H, ArH), 7.72 (d, J = 8.0 Hz, 1H,
imdazolium−H), 8.12 (s, 1H, imdazolium−H), 10.12 (s, 1H,
1
3
imdazolium−H), 10.52 (s, 1H, CHO); C NMR (100 MHz,
DMSO−d ) δ 36.8, 49.3, 67.0, 112.8, 121.8, 123.0, 124.3,
6
1
24.7, 132.7, 136.4, 138.0, 158.7, 190.4. Anal. Calcd for
Results and Discussion
Syntheses
C H BrN O : C, 50.18; H, 4.86; N, 9.00. Found: C 50.11; H,
4
1
3
15
2
2
.80; N, 8.91.
Synthesis of compound 3: In a two−neck round-bottom flask
equipped with a reflux condenser, 2−aminothiophenol (1.25 g,
Receptor N1 was synthesized as shown in Scheme 1.
Salicylaldehyde was converted to compound 1 using our
previously developed method. An imidazolium ring was
1
0 mmol) was dissolved in dry MeOH (20 mL). A solution of
43
aldehyde 2 (3.13 g, 10 mmol) in dry MeOH (10 mL) was added
to the above reaction mixture. A balloon filled with argon was
affixed to the condenser, and the reaction mixture was heated to
reflux for 6 h. Reaction progress was monitored using TLC.
After completion of the reaction, the solvent was evaporated to
conjugated to compound 1 through a substitution reaction of
compound 1 with N−methylimidazole. A benzothiazole moiety
was introduced through reaction of compound
2
with
was
−
2
−aminothiophenol. The Br ion in compound 3
2

ACCEPTED MANUSCRIPT
−
exchanged with ClO using a reported method to produce
receptor N1.
sulfur atom. The selectivity of receptor N1 for Hg(II) can be
explained on the basis of cavity size and Pearson’s hard-soft
4
4
4
4
5
acid-base theory. While sulfur is a soft center that interacts
with soft acids, the receptor did not show any significant
changes in fluorescence with other soft metal ions such as
Ag(I), Cu(II), Th(II), and Cd(II). This result might be due to the
size of the receptor cavity.
Structure characterization
Receptor N1 was crystallized in an orthorhombic crystal
system with the Pbca space group (Table S1). The asymmetric
unit contained one cationic moiety of receptor N1 and one
−
ClO4 as counter ion. The ORTEP diagram along with the atom
N1
N1+ Ni(II)
−
numbering scheme is shown in Figure 1 (disordered ClO4 ion
is not shown for clarity). The benzothiazole and phenyl
moieties are coplanar, while the plane containing the
N−methylimidazole has a dihedral angle of 75.46 with the
plane containing the benzothiazole and phenyl moieties.
Selected bond lengths and bond angles are given in Table S2.
1
000
800
600
400
N1 + Ag(I)
N1 + Ca(II)
N1 + Cu(II)
N1 + Na(I)
N1 + K(I)
N1 + Sr(II)
N1 + Th(II)
N1 + Fe(III)
N1 + Cd(II)
N1 + Cr(II)
N1 + Mn(II)
N1 + Co(II)
N1 + Hg(II)
o
The packing of receptor N1 was not determined due to the
−
disordered nature of ClO . Packing diagram is shown in Figure
4
2
.
2
00
0
350
400
450
500
550
600
Wavelength (nm)
Figure 3. Changes in emission spectra upon addition of 0.6 equivalents
of different metal ions to a 20 M solution of receptor N1 in water
(HEPES 10 mM, pH = 7.4) excited at ex = 260 nm.
4
6
To determine the binding stoichiometry, a Job’s plot was
constructed using fluorescence enhancement data, as shown in
Figure S1, confirming that Hg(II) ions bind to receptor N1 in a
ratio of 1:2, i.e., two molecules of receptor N1 bind to one
Hg(II) ion. This result was confirmed by mass spectrometry. To
take mass spectra, Hg(II) complex of ligand N1 was prepared
by dissolving 324 mg of Hg(NO3)2 and 872 mg of N1 in
methanol, resulting mixture was stirred for 60 minutes. Dark
yellow precipitate was collected after filtration of mixture.
Mass analysis of the complex of Hg(II) with receptor N1
Figure 1. ORTEP view of receptor N1 (CCDC 1408239) with 40%
probability of thermal ellipsoids and atom numbering scheme
−
(disordered ClO
4
ion is not shown for clarity).
showed
m/z
=
499,
which
corresponds
to
−
[
2N1Hg(II)(NO ) −2ClO ] (Figure S2). The stoichiometry of
3
2
4
the complex was also confirmed using elemental analysis, the
results of which were in good agreement with the chemical
formula
of
the
complex:
2N1Hg(II)(NO3)2
(
C H Cl HgN O S ) (Table S3). The data revealed that a
3
9
42
2
8
16 2
Figure 2. Packing diagram of receptor N1.
-
mercury ion was attached to two N1 receptors and two NO
ions with two ClO
3
−
Binding properties of receptor N1
counter ions to compensate for the charges
4
on the imidazolium rings.
Cation recognition studies were performed using fluorescence
spectroscopy. Fluorescence spectra were recorded upon
addition of 0.6 equivalents of different cations including Ni(II),
Ag(I), Ca(II), Cu(II), Na(I), K(I), Sr(II), Th(II), Fe(III), Cd(II),
Cr(II), Mn(II), Co(II), and Hg(II). Upon addition of various
cations to a 20 M solution of receptor N1, no changes in the
emission profile were observed with the exception of Hg(II).
When Hg(II) was added, an approximately three−fold
enhancement in the emission profile of receptor N1 was
observed, as shown in Figure 3. The large enhancement in
fluorescence intensity is attributed to cancelation of
photo−induced electron transfer (PET) by the benzothiazole
Upon successive addition of Hg(II) 0 to 12 M (0−0.6 equiv) to
a 20 M solution of receptor N1, a gradual increase in emission
intensity was observed (Figure 4). Different species formed on
different concentration of Hg(II) was find out using Hyperspec
software. A plot of the concentration of Hg(II) and the
fluorescence intensity at 380 nm showed a linear incremental
increase in fluorescence intensity in the range of 0−12 M. As
shown in Figure 5, the Y-axis on the left represents the
fluorescence intensity, whereas the Y-axis on the right
represents the percentage of different species present at
different concentrations of Hg(II). Binding constants for the
N1+Hg(II) complex and 2N1+Hg(II) were determined using
4
7
3

ACCEPTED MANUSCRIPT
nonlinear least-squares regression analysis, which was found to
be 9.53 x 10 and 3.54 x 10 , respectively. The detection limit
N1 + Different metal ions
N1 + Hg(II) + Different metal ions
3
4
1
000
was calculated to be 48 nM using the 3σ method (black dotted
4
8
curve in Fig. 5).
8
6
4
2
00
00
00
00
0
1
000
0
.6 equivalent
8
6
4
2
00
00
00
00
0
Hg(II)
0
equivalent
I)
Sr(II) Th(I
)
)
K(I)
Ni(II) Ag(I) Ca(II) Cu(II) Na(I)
Cr(II)
Mn(II Co(II) Hg(II)
Fe(III Cd(II)
Figure 6. Comparison of the fluorescence intensity of receptor N1 (20
μM) at 380 nm in the presence of Hg(II) (0.6 equiv) and other analytes
350
400
450
500
550
600
Wavelength (nm)
(0.6 equiv) in water (HEPES 10 mM, pH = 7.4) with excitation at ex
=
2
60 nm.
Figure 4. Effect of gradual addition of Hg(II) (0−0.6 equiv) on
the emission profile of receptor N1 (20 M) in water (HEPES
Binding properties of the Hg(II) complex of receptor N1
1
0 mM, pH = 7.4).
The Hg(II) complex of receptor N1 was further subjected to
anion recognition using fluorescence spectroscopy. A solution
of the complex was screened with tetrabutyl ammonium salts of
1
00
1000
90
80
−
−
−
−
−
−
−
several anions (CH CO , ClO , HSO , NO , Cl , Br , I ,
3
2
4
4
3
Free Hg(II)
Hg(II) + N1
Hg(II) + 2N1
−
and F ). None of the anions caused any changes in the intensity
or wavelength of the parent band of the complex. However,
upon addition of Br ions, the parent band at 380 nm was
800
600
400
200
0
70
60
50
40
30
20
10
0
−
shifted to 450 nm, as shown in Figure 7. The bathochromic
−
shift was attributed to Br ion stabilization of the mercury
complex of receptor N1, which leads to a decrease in the band
gap, resulting in a shift of the fluorescence band to higher
wavelength.
−
To study the interaction of Br ions with the Hg(II) complex of
-6
-6
-6
-5
receptor N1, titration was performed by adding small aliquots
0.0
3.0x10
6.0x10
9.0x10
1.2x10
−
of Br ions to a solution of receptor N1 (20 M) and Hg(II) 12
Hg(II), M

M (0.6 equiv). The intensity of the band at 380 nm decreased
−
on addition of Br ions, and a new band at 450 nm emerged, as
shown in Figure 9. An isosbestic point at 412 nm implied that
there was an interaction of Br ions with the Hg(II) complex of
Figure 5. Linear relationship between Hg(II) concentration and
fluorescence intensity at 380 nm with excitation at ex = 260 nm (left
Y-axis) and the percentage of species formed at different
concentrations of Hg(II) (right Y-axis).
−
receptor N1. To obtain a calibration curve, the ratio of
−
FFwas plotted against the concentration of Br (Figure
To examine possible interference from different metal ions
with Hg(II) recognition of receptor N1, competitive binding
studies were performed by adding Hg(II) ions to the solution of
receptor N1 in the presence of other metal ions. The results are
presented in Figure 6. There were no significant changes in
fluorescence intensity at 380 nm, indicating that the
coexistence of other metal ions had no significant interference
on Hg(II) recognition.
1
0), which showed that the Hg(II) complex of receptor N1
−
could be used for selective determination of Br ions in aqueous
media, with a detection limit of 22 nM.
4

ACCEPTED MANUSCRIPT
-
N1 + Hg(II) + Nitrate
no significant interference on Br recognition (Figure 11).
1
200
000
N1 + Hg(II) + Acetate
N1 + Hg(II) + Perchlorate
N1 + Hg(II) + Cyanide
N1 + Hg(II) + Iodide
N1 + Hg(II) + Bromide
N1 + Hg(II) + Fluoride
N1 + Hg(II) + Phosphate
N1 + Hg(II) + Chloride
1000
1
0.75 equivalent
800
800
600
400
200
0
600
400
200
0
-
Br
0
equivalent
600
350
400
450
500
550
600
Wavelength (nm)
350
400
450
500
550
Wavelength (nm)
Figure 7. Changes in fluorescence spectra of a solution of receptor N1
20 M) and Hg(II) (0.6 equiv) upon addition of different anions
(
Figure 9. Changes in fluorescence spectra of a solution of receptor N1
(0.75equiv, 15 M).
−
(20 M) and Hg(II) (0.6 equiv, 12 M) upon successive addition of Br
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
ions (0.75equiv, 15 M).
1.6
2
R =0.98492
Slope = 72547.1
1.4
1.2
1.0
0.8
0.6
0.4
te
e
0.3
0.6
0.9
1.2
1.5
ide
hate
oride
Flu
Nitrate
)
Iod
-
Aceta
Cyanide
) +
Bromide
) +
Chlorid
Br , M
)
+
+
) +
Phosp
Perchlorate
) +
) +
)
+
)
+
N1 + Hg(II
N1 + Hg(II
N1 + Hg(II
N1 + Hg(II
Figure 10. A linear plot of FF of a solution of N1 (20 M) and
Hg(II) (0.6 equiv, 12 M) upon successive addition of Br ions.
N1 + Hg(II
N1 + Hg(II
N1 + Hg(II
N1 + Hg(II
−
N1 + Hg(II
1
The H NMR spectra of receptor N1 were recorded in
Figure 8. Comparison of the fluorescence intensity (F450/F380) in the
presence of receptor N1 (20 M), Hg(II) 12 M (0.6 equiv) and 0.75
equiv of different anions in water (HEPES 10 mM, pH = 7.4) with
excitation at ex = 260 nm.
DMSO−d :D O (95:5, v/v) in the presence and absence of
6
2
Hg(II) and TBA-Br (Figure 12). Upon addition of 2 equiv of
−
Br ions to a solution of receptor N1, no significant changes
were observed. When 2 equiv of Hg(II) was added to a solution
of receptor N1, small shifts of the aliphatic protons were
observed. A shift of the imidazolium hydrogen from 9.26 to
A Job’s plot analysis revealed that the binding stoichiometry of
the Hg(II) complex of receptor N1 to Br ions was 1:1 (Figure
−
S3a). This stoichiometry was confirmed using elemental
analysis, which indicated the formation of a new complex with
9
.72 ppm indicated an interaction of receptor N1 with Hg(II).
1
The H NMR spectrum of receptor N1 was recorded in the
presence of both Hg(II) and TBA-Br, which showed changes in
the aliphatic and aromatic regions. Two triplets at 4.73 and 4.77
ppm shifted to 4.64 and 5.01 ppm, respectively. The
imidazolium proton shifted from 9.26 ppm to 10.11 ppm,
-
chemical formula 2N1Hg(NO )(Br ) (C H BrCl HgN O S )
3
38 36
2
7
13 2
(
Table S3). The association constant calculated with a
4
−1
Benesi−Hildebrand plot was K = 4.42 x 10
S3b).
To examine interference from different anions with Br ion
recognition of the Hg(II) complex of receptor N1, competitive
binding studies were performed by recording fluorescence
spectra in presence of receptor N1 (20 M), Hg(II) (12 M, 0.6
M
(Figure
a
4
8
−
−
confirming that Br ions interact with receptor N1 only in the
presence of Hg(II).
-
equiv), different anions (15 M, 0.75 equiv) and Br ions (0.75
equiv). There were no significant changes in fluorescence
intensity ratio (F450/F380) of receptor N1, indicating that there is
5

ACCEPTED MANUSCRIPT
N1 + Hg(II) + Different anions
N1 + Hg(II) + Different anions + Bromide
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
te
e
ide
hate
oride
Nitrate
Iod
Aceta
Cyanide
Bromide
Flu
Chlorid
Phosp
Perchlorate
Figure 11. Fluorescence intensity ratio (F450/F380) of receptor N1 (20
-
M) in the presence of Hg(II) (0.6 equiv) and Br ions (15 M, 0.75
Scheme 2. Plausible binding modes of receptor N1 to Hg(II)
-
equiv) along with other anions (0.75 equiv) in water (HEPES 10 mM,
and Br ions.
pH = 7.4) with excitation at ex = 260 nm.
1
Figure 12. H NMR spectra in DMSO−d
6
:D
2
O (95:5, v/v): (a) N1 only,
(b) N1+TBA-Br (2.0 equiv), (c) N1+Hg(II) (2.0 equiv), and (d)
N1+Hg(II) (2.0 equiv)+TBA-Br (2.0 equiv).
Absorption and fluorescence spectra were also recorded
-
with/without Hg(II) and/or Br ions. The results indicate that
-
Hg(II) ion is essential for recognizing Br ions with receptor N1
in water (Figures S4 and S5). The binding modes of receptor
N1 involving Hg(II) and Br ions are shown in Scheme 2.
3 2,
Figure 13. DFT computed optimized structure of N1, 2N1+ Hg(NO )
-
-
3 2
and 2N1+ Hg(NO ) + Br .
Receptor N1 selectively binds to Hg(II) to form
In order to explain the electronic spectra of the complexes,
theoretical calculations were carried out by using the DMol 3
package with density functional theory (DFT). All calculations
were done via generalized gradient approximations (GGA) with
double numeric plus polarization (DNP). The geometry of the
structures was minimized to calculate the HOMO-LUMO gap.
From the optimized structures, it was observed that two ligand
molecules coordinated with Hg(II), along with two nitrate ions
2
N1Hg(II)(NO ) , which was confirmed using elemental
3 2
analysis. The Hg(II) complex of receptor N1 selectively
-
-
recognizes Br ions to produce 2N1Hg(NO )(Br ). In the
3
-
absence of Hg(II), receptor N1 does not recognize Br ions.
4
9
(Figure 13). The benzothiazole ligand coordinates to Hg(II) in
such a way that cationic moieties are opposite to each other.
However, on substitution of the nitrate with bromide ions, both
imidazolium cations rearrange to form a cavity for the bromide
ion. Here, the bromide ion interacts with Hg(II) through ionic
6

ACCEPTED MANUSCRIPT
interaction, whereas the positive charge of the imidazolium ring
provides the cavity for the anion. The HOMO-LUMO diagram
of N1 shows that the HOMO is spread over the benzothiazole
moiety, whereas the LUMO is on the imidazolium cation
(Figure 14). On complexation with Hg(II) and bromide, the
HOMO shifts toward the metal and bromide ion, whereas the
LUMO spreads over the aromatic ring (benzothiazole). The
change in the HOMO-LUMO orbitals causes the change in the
emission profile of the receptor.
To evaluate the present sensor in real time analysis, water
samples were collected from different resources and used for
preparation of the N1 solution (20 M). Different
concentrations of Hg(II) were spiked to this solution, and the
fluorescence intensity was recorded at 380 nm. By comparing
fluorescence intensity with the calibration curve in Figure 10,
the amount of Hg(II) was determined. The high percentage of
recovery for Hg(II) indicates the authentication of receptor N1
for real time analysis. Similarly, for real time evaluation of
bromide ions in the river water sample, an N1 solution (20 M)
also containing 12 M of Hg(II) was prepared. Different
concentrations of bromide ions were spiked into the samples 1b,
Figure 14. Energy correlation of HOMO-LUMO gap between ligand
N1 and N1 complex of Hg(II)-Br.
To authenticate the experiments, the found concentrations of
Hg(II) and bromide were also determined using AAS (atomic
absorption spectroscopy) and ICP (inductively coupled plasma),
respectively, which was in agreement with the presented
methods (Table 1).
2
b, 3b, 1b’, 2b’ and 3b’, and the fluorescence spectra were
recorded. By comparing the fluorescence intensity at 450 nm
with the calibration curve in Figure 10, the amount of bromide
was determined.
Table 1. Result of Hg(II) and bromide sensing in real samples.
Sample
Hg(II)
added (M)
Hg(II) found (M)
Recovery
(%)
Sample
Bromide
added (M)
Bromide found (M)
Recovery
(%)
Present
method
AAS
Present
Method
ICP
Tap water
1
2
3
a
a
a
0
5
10
-
-
1b
2b
3b
0
10
15
0.9
10.3
15.7
0.91
10.2
15.7
4.97
9.95
4.92
9.93
99.4
99.5
94.5
98.7
River water
1
2
3
a’
a’
a’
0
5
10
-
-
1b’
2b’
3b’
0
10
15
0.5
10.1
15.4
0.5
10.2
15.3
4.97
9.96
4.95
9.96
99.4
99.6
96.2
96.4
-
Conclusion
Br ions. In the absence of Hg(II), receptor N1 did not
-
recognize Br ions.
We synthesized a novel benzothiazole-based receptor N1 that
incorporated an imidazolium cation. Receptor N1 was
investigated for sensing cations and anions in water. Receptor
N1 selectively recognized Hg(II), with a large enhancement in
emission intensity at 380 nm. The Hg(II) complex of receptor
Acknowledgements
A.S. is thankful to CSIR−New Delhi, India, for his fellowship.
Authors are thankful to S. Kaur of Panjab University
Chandigarh for help in evaluation of binding constant.
-
N1 selectively recognized Br ions in water in a ratiometric
manner. The ratiometric plot of FFand interference
studiesshowed that the Hg(II) complex of receptor N1 could be
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