Exploiting the INHIBIT-ESIPT mechanism for the design of fluorescent chemosensor with a large blue-shift in emission

Article abstract of DOI:10.1016/j.jphotochem.2016.11.018

A set of two novel fluoroscent receptors 2a and 2b with sulfonamide binding site and 2-(2'-aminophenyl)benzothiazole scaffolding as a signaling unit have been synthesized by condensation approach, which can undergo excited-state intramolecular proton tran

Full text of DOI:10.1016/j.jphotochem.2016.11.018

Accepted Manuscript
Title: Exploiting the INHIBIT-ESIPT Mechanism for the
Design of Fluorescent Chemosensor with a Large Blue-Shift
in Emission
Author: Gargi Dhaka Navneet Kaur Jasvinder Singh
PII:
S1010-6030(16)30818-8
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2016.11.018
JPC 10442
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
27-9-2016
11-11-2016
13-11-2016
Please cite this article as: Gargi Dhaka, Navneet Kaur, Jasvinder Singh, Exploiting the
INHIBIT-ESIPT Mechanism for the Design of Fluorescent Chemosensor with a Large
Blue-Shift in Emission, Journal of Photochemistry and Photobiology A: Chemistry
http://dx.doi.org/10.1016/j.jphotochem.2016.11.018
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Exploiting the INHIBIT-ESIPT Mechanism for the Design of Fluorescent
Chemosensor with a Large Blue-Shift in Emission
Gargi Dhaka, Navneet Kaur* and Jasvinder Singh
Department of Chemistry, Panjab University, Chandigarh 160014, India
 Corresponding author. Tel.: +91 172 2534430; fax: +91 172 2545074;
e-mail: neet_chem@yahoo.co.in; neet_chem@pu.ac.in

2
Graphical Abstract
S
S
N
S
N
ESIPT
H N
O
H N
O
N
Zn2+
O
S
H
O
S
N
O
Zn2+
S
O
Tautomer
CH3
CH3

em = 551 nm
abs = 324 nm

CH3
em = 456 nm

A-
S
S
N
S
N
N
N
N
N
A-
H
O
O
O
S
H
S
O
S
H
O
A-
O
A-
CH3
abs = 378 nm
em = 468 nm


CH3
CH3
A- = F-, AcO-, H2PO4
-

Highlights
 Benzothiazole based sulfonamide possessing chemosensopr for Zn2+, F-, AcO- and
-
H2PO4 ions.
 Anions addition resulted in formation of new band at longer wavelength (Δλ = 54 nm)
in UV-vis spectra
 Zn2+ and anions addition caused substantial blue shift (Δλ = 83 nm) along with
fluorescent enhancement in fluorescence spectra.
 Inhibition of ESIPT responsible for absorption and fluorescence changes.

4
Abstract:
A set of two novel fluoroscent receptors 2a and 2b with sulfonamide binding site and 2-(2’-
aminophenyl)benzothiazole scaffolding as a signaling unit have been synthesized by
condensation approach, which can undergo excited-state intramolecular proton transfer
(ESIPT) upon excitation. In CH3CN solution of 2a, this ESIPT phenomenon was perturbed
and showed a remarkable hypsochromic shift (Δλ ~ 83 nm), by capturing of Zn2+ metal ion
selectively out of other interfering metal ions including Na+, K+, Mg2+, Al3+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, Cd2+ and Hg2+. Similarly, the deprotonation of the sulfonamide proton of
-
this acidic receptor 2a by basic anions such as F-, AcO-, and H2PO4 also resulted in a
substantial blue shift due to disruption of ESIPT. This blue shift was accompanied by
enhancement of emission intensity and fluorescent color change from dark blue to light blue.
Keywords: Benzothiazole; ESIPT; Anion sensing; Zn2+ sensor
1. Introduction
Currently, chemosensors that translate molecular recognition into highly sensitive and simply
detected signals have been keenly investigated. Fluorescence-based probes provide high
sensitivity and are therefore on the whole well suited for the visualization of cations and
anions in the media contaminated with them [1]. In particular, the development of a
fluorescent probe for Zn2+ ion in the presence of a variety of other metal ions has fetched
great consideration [2-3]. These efforts are motivated by the fact that Zn2+ is the second most
abundant transition-metal in living organisms and performing significant roles in
physiological and pathological processes such as neural signal transmission, regulators of
enzymes, structural cofactors in metalloproteins, and gene expression [4-5]. Amusingly, to
compare with other tissues, high concentrations of Zn2+ are present in pancreatic islets, which
act critically in insulin biosynthesis, storage, and secretion [6]. It is also reported that Zn2+ ion
is a potent killer of neurons via oxidative stress, the cause of neurodegenerative disorders
such as Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), Guam ALS-
parkinsonism dementia, Parkinson’s disease, hypoxiaischemia, and epilepsy [7-8].
Likewise, there exists a great interest in the development of synthetic receptors for
anions too, due to their medicinal and environment potential applications [9-10]. In
particular, the carboxylate and fluoride anions are critical components of numerous metabolic
processes and also exhibit specific biochemical behaviors in the enzymes and antibodies [11].

Arrays of Zn2+ selective fluorescent sensors employing fluorescein [12], quinolone
[13], indole [14], coumarin [15] and other fluorophores [16] or proteins [17] have been
developed, but a bulk of these, translates the binding event into either an increase or decrease
of the emission intensity [18]. However, probes that exhibit a spectral shift upon binding to
the analyte offer an elegant way out to this problem [19]. Moreover, excited-state
intramolecular proton transfer (ESIPT) is a renowned photophysical process responsible for
unusually large stoke shifts in benzazole. Inhibition of the ESIPT process by cation binding
yields a significant hypsochromic shift of the fluorescence emission maximum [20-22].
Although inhibition of the ESIPT has been greatly exploited for cation sensing, it was
scantily employed for the anion sensing [23].
To prepare an ESIPT-based chemosensor, the sensor moiety should form an
intramolecular hydrogen bond in the ground state with the adjacent hydrogen-bond acceptor
[24-25]. Therefore we introduced sulfonamide group of moderate acidity, as a binding
moiety, which is a good hydrogen-bonding donor and can bind efficiently with both the
cations and anions. However, use of heteroaromatic ring system such as thiazole, which
contains soft heteroatoms N and S, as electron donors to metal cations, particularly Zn2+, is
very rare in literature [26]. To date, application of benzofused heteroaromatic ring system eg.
benzothiazole, having substitution at 2 and 2’ position, as a Zn2+ chemosensor based on
INHIBIT-ESIPT mechanism has not been reported to the best of our knowledge.
2. Materials and Methods
All solvents and reagents were purchased from Aldrich. All other chemicals were used as
received without further purification. Deionized water was used throughout the experiments.
Acetonitrile (CH3CN) was of HPLC grade. The UV-Vis spectra were recorded on a Jasco v –
530 UV/ Vis spectrophotometer. All fluorescence spectra were recorded on Hitachi F-7000
spectrophotometer. All the metal ions such as Na+, K+, Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+,
Zn2+, Hg2+ and Cd2+ were added as their perchlorate salts; whereas all the anions such as F-,
-
-
2-
Cl-, Br-, I-, AcO-, H2PO4 , CN-, HSO4 and SO4 were added as their tetrabutylammonium
salts for the different UV-Vis and fluorescence spectroscopic experiments.
1
Melting point was determined in capillary and is uncorrected. H and 13C NMR spectra were
recorded on a BRUKER AVANCE 400 and 100 MHz instrument using tetramethylsilane as
an internal standard. The chemical shifts are reported in  values relative to TMS and
coupling constants (J) are expressed in Hz. 13C NMR spectra were recorded at 100 MHz and
values are reported relative to CDCl3 signal at  77.0.

6
2.1 General procedure for fluorescence experiments
Fluorescence spectra were recorded using HITACHI-7000 spectrophotometer with quartz cell
of 1 cm width and 3.5 cm height. The excitation was carried out at 340 nm for probe 2a and
2b with 5 nm excitation and emission slit widths in fluorometer. Stock solution of the probe
2a and 2b (1 x 10-2 M) was prepared in DMSO. This stock solution was diluted with CH3CN
and used further for different spectroscopic experiments. All absorption scans were saved as
ACS II files and further processed in Excel(tm) to produce all graphs shown.
2.2 General procedure for 1H NMR experiments
1
The H NMR titrations were carried out in DMSO-d6 using TMS as an internal reference
standard. To the 3.0 x 10-3 M solution of 2a in DMSO, the varying equivalents of TBA+F-
were added and 1H NMR spectra were recorded after each addition.
3. Experimental
3.1 Synthesis of 2-(Benzothiazol-2-yl)-aniline (1)
Absolute ethanol was added to mixture of previously synthesized 2-(2-
nitrophenyl)benzothiazole (250 mg, 1mmol) and SnCl2.2H2O (439 mg, 5 mmol) and refluxed
o
at 70 C for 5 hrs [27]. After completion of reaction (TLC), the crude product was separated
and recrystallized to yield light yellow reduced product 1. Yellow solid, mp (oC) 184, Yield
(%) 70; IR (cm-1) 3453.87 and 3284.13 (two sharp peaks, νNH2), 1603.05 (νC=Nstr) 1487.55 (νC-
Nstr.); 1H NMR (DMSO, 400 MHz) δ (ppm): 6.32 (s, 2H, NH2), 6.67 (t, J1 = J2 = 8.0 Hz, 1H,
ArH), 6.71 (d, J = 8.0 Hz, 1H, ArH), 7.15 (t, J1 = J2 = 8.0 Hz, 1H, ArH), 7.27 (t, J1 = J2 = 8.0
Hz, 1H, ArH), 7.38 (t, J1 = J2 = 8.0 Hz, 1H, ArH), 7.63 (d, J = 8.0 Hz, 1H, ArH), 7.80 (d, J =
8.0 Hz, 1H, ArH), 7.89 (d, J = 8.0 Hz, 1H, ArH); 13C NMR (CDCl3, 75 MHz) δ (ppm):
169.24, 159.74, 146.78, 133.27, 131.82, 130.98, 126.04, 124.87, 122.48, 121.20, 116.91,
116.81, 115.28; ESI-MS: m/z (relative abundance (%), assignment) = 227.2 [(M+1)+].
3.2 Synthesis of N-(2-(Benzothiazol-2-ylphenyl)-4-methylbenzenesulfonamide (2a)
The mixture of 1 (250 mg, 1 mmol) and 4-methylbenzenesulfonyl chloride (202 mg, 1.3
0
mmol) in the presence of K2CO3 (183 mg, 1.2 mmol) was refluxed at 80 C in dry CH3CN
-
with a catalytic amount of salt TBA+HSO4 . After completion of reaction (TLC), the reaction
mixture was allowed to cool down and then filtered off. The filtrate was kept under high
vacuum to obtain the solid compound which was further purified by recrystallisation method
to yield pure 2a. Light Brown solid, mp (oC) 150, yield (%) 78; IR (cm-1) 3648 (νN-Hstr), 3064
1
(νAr-Hstr), 1583 (νC=Nstr), 1483 (νC-Nstr); H NMR (CDCl3, 400 MHz) δ (ppm): 2.24 (s, 3H, -
CH3), 7.19 (d, 2H, J = 8.0 Hz, ArH), 7.26 (t, 1H, J1 = J2 = 8.0 Hz, ArH), 7.37 (t, 1H, J1 = J2

= 8.0 Hz, ArH), 7.50 (t, 1H, J1 = J2 = 8.0 Hz, ArH), 7.55 (t, 1H, J1 = J2 = 8.0 Hz, ArH), 7.58
(d, 2H, J = 8.0 Hz, ArH), 7.63 (d, 1H, J = 8.0 Hz, ArH), 7.91 (d, 1H, J = 8.0 Hz, ArH), 8.10
(d, 1H, J = 8.0 Hz, ArH), 8.18 (d, 1H, J = 8.0 Hz, ArH), 11.8 (s, 1H, NH); 13C NMR (CDCl3,
100 MHz) δ (ppm): 167.9, 152.6, 143.5, 136.9, 136.6, 133.2, 131.7, 129.9, 129.4, 127.1,
126.8, 125.9, 123.7, 123.1, 121.3, 120.4, 120.3, 21.5; ESI-MS: m/z (relative abundance (%),
assignment) = 381.2 [100, (M+1)+].
3.3 Synthesis of N-(-6-(Benzothiazol-2(3H)-ylidene)cyclohexa-2,4-dienylidene)-4-nitro-
benzenesulfonamide (2b)
The reaction of 1 with 4-nitrobenzenesulfonyl chloride in similar reaction conditions as for
2a, yielded 2b. Pale yellow solid, mp (oC) 210, Yield (%) 79; IR (cm-1) 3080 (νAr-Hstr), 1583
1
(νC=Nstr), 1487 (νC-Nstr); H NMR (DMSO-d6, 400 MHz) δ (ppm): 7.06 (t, 1H, J1 = J2 = 8.0
Hz, ArH), 7.21 (s, 1H, NH, exchanges with D2O), 7.31 (d, 1H, J = 8.0 Hz, ArH), 7.35 (t, 1H,
J1 = J2 = 8.0 Hz, ArH), 7.43 (t, 1H, J1 = J2 = 8.0 Hz, ArH), 7.74 (d, 1H, J = 8.0 Hz, ArH),
7.85 (d, 1H, J = 8.0 Hz, ArH), 7.93 (d, 3H, J = 8.0 Hz, ArH), 8.07 (d, 2H, J = 8.0 Hz, ArH);
13C NMR (CDCl3, 100 MHz) δ (ppm): 167.8, 152.9, 147.9, 139.6, 131.6, 130.1, 127.1, 126.4,
125.5, 123.7, 122.6, 121.8, 121.3, 120.5, 119.1; ESI-MS: m/z (relative abundance (%),
assignment) = 492.4 [100, (M+2)+ +2K+]. Due to poor solubility and for getting better D2O
exchange NMR, 1H NMR of 2b was carried out in DMSO-d6.
4. Results and Discussions
Scheme 1 outlines the synthesis of sulfonamide possessing benzothiazole derivatives, 2a and
2b. The refluxing of 1 with different sulfonyl chloride, in dry CH3CN, gave respective
derivatives, 2a and 2b, containing sulfonamide linkage. The desired products were explicitly
characterized by NMR spectra (1H NMR, 13C NMR), IR and Mass spectrum (Figs. S1 – S6).

8
Dry CH3CN,
Reflux
S
S
N
S
N
SnCl2.2H2O
EtOH
N
O
S
HN
O
O2N
H2N
O
Cl
CH3
S
1
O
Dry CH3CN,
Reflux
O
Cl
S
NO2
2a
CH3
O
S
N
H
N
O
S
O
NO2
2b
Scheme 1. Synthetic procedure for receptors 2a and 2b.
4.1 UV-Vis spectral studies of receptors 2a and 2b
The metal ion binding affinity of 2a (30 µM) was determined by both UV-Vis absorption
spectra and fluorescence emission spectra in the absence and presence of various cations such
as Na+, K+, Mg2+, Al3+, Ni2+, Cu2+, Fe2+, Fe3+, Zn2+, Cd2+, Hg2+, Co2+, Pb2+ (as their
-
-
perchlorate salts) and anions such as AcO-, F-, Cl-, Br-, I-, H2PO4 , and HSO4 (as their
tetrabutylammonium salts), in CH3CN. In the absence of mentioned ions, the UV-Vis
absorption spectrum of 2a showed two characteristic absorption maxima λmax at 294 nm and
324 nm (Fig. 1). The low energy band (324 nm) was anticipated to intramolecular charge
transfer (ICT) transition while the second high energy band at 294 nm corresponds to ππ*
electronic transitions. The addition of 100 equiv. of Na+, K+, Mg2+, Al3+, Ni2+, Cu2+, Fe2+,
-
Fe3+, Cd2+, Hg2+, Co2+, Cl-, Br-, I- and HSO4 did not result in any color or spectrum changes
(Fig. S7). Addition of 100 equiv. of Pb2+ resulted in the precipitation of the solution. The
increased absorbance due to Cu2+ ions at 294 and 324 nm was due to their own absorbance
near 300 nm (Fig. S8) and for the same reason, any actual changes observed with Cu2+ cannot
be identified. However, it was observed that upon addition of F- and AcO- ions, receptor 2a
resulted in dramatic absorption changes.
Upon addition of F- to 2a, the absorption band at 324 nm reduced gradually and new
band appeared at 378 nm, with concomitant development of an isosbestic point at 348 nm.
The formation of isosbestic point showed the presence of two ionic species in the medium.
This new band appeared with considerable red shift of 54 nm may be attributed to increased

intramolecular charge transfer (ICT) transition within the whole molecule arising due to F-
ion binding (Fig. 1) [28].
Fig. 1. Family of UV-Vis spectra taken in the course of the titration of 2a (30 µM in CH3CN) with F-
ions; Inset: Plot of absorbances at 324 and 378 nm vs. the concentration of F- added.
Similar type of absorption changes i.e. decreased absorbance at 324 nm and
emergence of new band at 378 nm, were observed with addition of AcO- ions to CH3CN
solution of 2a (Fig. S9). To know the stoichiometry between the guest (F- /AcO-) and host
(2a) in CH3CN solution, Job’s plot (Fig. S10) has been drawn that showed 1:1 stoichiometry.
The association constants of 2a with F- and AcO- were calculated based on the UV-Vis
titration through the Benesi-Hildebrand equation [29-30] and found to be 7.20 x 106 M-1
(LOD=4.7 µM) and 1.5 x 105 M-1 (LOD=5 µM), respectively.
The UV-Vis spectrum of other receptor 2b possessing –NO2 group did not show
changes upon addition of any of cations and anions (Fig. S11).
4.2 Fluorescence spectral studies of receptors 2a and 2b
4.2.1 Metal ion sensing properties
When excited at 340 nm, 2a which contains an intramolecular hydrogen bond, underwent
ESIPT and yielded a highly stoke’s shifted low-intensity emission at 551 nm. As depicted in
Fig. 2, among 100 equiv. of various metal ions such as Na+, K+, Mg2+, Al3+, Mn2+, Fe2+, Fe3+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+ and Pb2+, only Zn2+ yielded a bright, blue-shifted
fluorescence emission when added to a CH3CN solution of 2a (5 µM; λex 340 nm). Addition
of 100 equiv. of Pb2+ resulted in the precipitation of the solution. Recently, Badiei et al
synthesized benzimidazole based chemosensor that showed a quenched red-shifted
fluorescent peak in the presence of Zn2+ ion [31]. However, its -NO2 analogue, 2b, was
weakly fluorescent and thus, effect of addition of cations and anions cannot be studied [32].

10
Fig. 2. Fluorescence response of 2a (5 µM) to various metal ions (500 µM) in CH3CN. Bars represent
the fluorescence inetnsity at 456 nm. Inset: Fluorescence color change observed upon addition of
various metal ions.
In the fluorescence titration spectra of 2a (Fig. 3), at the initial stage of Zn2+ addition (0-0.2
equiv.) emission band at 551 nm underwent a hypsochromic shift to 456 nm gradually. This
was ascribed as the Zn2+ coordinated with sulfonamide and benzothiazole groups and
disrupted the ESIPT, thus causing emission with a normal Stoke’s shift. On further addition
of Zn2+ (0.2-40 equiv.) this new emission band underwent fluorescent enhancement around
456 nm. The binding mode of 2a with Zn2+ from the results of fluorescence titration spectra
and Job’s plot (Fig. S12) showed to be 1:1 with binding affinity (Ka) of 2.4 x 104 M-1
(LOD=9.7 µM).
Fig. 3. Family of fluorescence spectra taken in the course of the titration of 2a (5 µM in CH3CN; λex
340 nm) with Zn2+ ions; Inset: Plot of absorbances at 456 nm vs. the concentration of Zn2+ added.

The selectivity and tolerance of 2a for Zn2+ ion over other metal cations such as Na+, K+,
Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Cd2+ were investigated. The hypsochromic shift and
fluorescent enhancement induced by Zn2+ remained almost unperturbed and did not get any
interference by the coexistence of other interfering cations (Fig. 4a). Earlier literature reports
also point to the selectivity of chemosensors towards Zn2+ ions in presence of other
interfering cations such as Ni2+ and Co2+ [33-35]. However, upon addition of more than two
times of other metal ions, the paramagnetic transition-metal cations Fe2+, Fe3+ and Cu2+
partially or completely quenched the fluorescence emission due to non-radiative decay of the
excited states [36]. Also addition of excess of Hg2+ ions quenched the fluorescence due to
heavy atom effect.
Fig. 4. Metal ion selectivity profile of 2a (5 µM) via changes observed in fluorescence emission
spectra with addition of 40 equiv. of Zn2+ with (a) 40 equiv. and (b) 100 equiv. of other interfering
cations.
4.2.2 Anion sensing properties
The qualitative anion sensing behavior of 2a (5 µM) was determined by recording the
fluorescence spectra in CH3CN (λex 340 nm). The low intensity emission band was observed
-
at 551 nm. Upon interaction with F-, AcO- and H2PO4 ions, enhancement of fluorescence
intensity has been observed at 468 nm. In the case of weak basic ions such as Cl-, Br-, I- and
-
-
HSO4 , ClO4 only nominal changes were observed in the fluorescence spectra (Fig. 5).

12
Fig. 5. Fluorescence response of 2a (5 µM) to various anions (500 µM) in CH3CN. Bars represent the
fluorescence intensity at 468 nm. Inset: Fluorescence color change observed upon addition of various
anions.
Fluorescence titrations were next carried out with increasing concentrations of F-, AcO- and
-
H2PO4 anions. Upon gradual addition of F- (0-20 equiv.) to solution of 2a, the low-intensity
tautomeric emission band (λem 551 nm) underwent hypsochromic shift (λem 468 nm), along
with a great fluorescence enhancement induced by the deprotonation (Fig. 6). This
fluorescence enhancement was accompanied by color change from dark blue to light blue,
when irradiated with light of wavelength 365 nm (Fig. 5 inset).
Fig. 6. Family of fluorescence spectra taken in the course of the titration of 2a (5 µM in CH3CN; λex
340 nm) with F- ions; Inset: Plot of fluorescence intensity at 468 nm vs. the concentration of F- added.
Similar effects were observed in the fluorescence spectra of 2a upon the addition of
-
AcO- and H2PO4 ions (Fig. S13-S14). Addition of these two ions also resulted in appearance
of new blue shifted band at 468 nm. Thus, addition of more basic anions might have induced

deprotonation of sulfonamide proton, which was earlier responsible for causing ESIPT
phenomenon. Consequently, the fluorescence anion titrations experienced the disappearance
of the tautomer emission (λem 551 nm) in favor of the deprotonation emission (λem 468 nm).
-
For F-, AcO- and H2PO4 titrations, Job’s plot gave 1:1 stoichiometry (Fig. S14), for which
binding constants were found to be in order of 9.4 x 106 M-1 (LOD=2.9 µM), 2.1 x 105 M-1
(LOD=16 µM) and 9.6 x 104 M-1 (LOD=20 µM), respectively.
-
The selectivity of 2a for F-/AcO-/H2PO4 ions over other anions such as Cl-, Br-, I- and
-
-
-
HSO4 , ClO4 were investigated by taking 30 equiv. of F-/AcO-/H2PO4 and 30 equiv. of other
interfering anions. (Fig. 7).The anions induced hypsochromic shift and fluorescent
enhancement remained unperturbed and did not get any interference by the coexistence of
other interfering anions (Fig. 7).
Fig. 7. Anion selectivity profile of 2a (5 µM) via changes observed in fluorescence emission spectra with
-
addition of 30 equiv. of (a) F-; (b) AcO- and (c) H2PO4 ions in the presence of 30 equiv. of other
interfering anions.
4.3 1H NMR titrations
To further elucidate the nature of the intermolecular interactions between anions and receptor
2a, 1H NMR titration experiments of 2a (3 x 10-2 mol l-1) with F- were performed in DMSO-
d6. The systematic alterations in 1H NMR signals of 2a upon addition of progressive amount
of F- ion have been depicted in Fig. 8. The hydrogen bond interaction (deprotonation in

14
extreme cases) between the binding sites of the host and the anion may give rise to two
1
effects: (i) through-bond effects which results in the upfield shifting of protons in H NMR
spectrum due to increased electron density of the benzene ring and (ii) through-space effects,
which, polarize C–H bond in proximity to hydrogen bond, create the partial positive charge
on the proton and cause downfield shifts [37]. In Fig. 8, the NH of receptor 2a appeared as
sharp singlet at δ 11.8. Addition of 0.5 equiv. of F- broadened this signal and finally, NH
signal disappeared after addition of 1.0 equiv. of F- ions. Moreover, all the aromatic protons
shifted upfield, indicative of the increase in the electron density of the phenyl ring owing to
the through-bond effects. This increased electron density caused the remarkable
bathochromic shift in UV-Vis and emission enhancement in fluorescence spectra of the 2a.
Fig. 8. Partial 1H NMR titration spectra of 2a (3.0 x 10-2 M) upon increasing addition of F- (TBA+F-)
ion (0 – 1 Equiv.) in DMSO-d6.
4.4 Plausible sensing model
In CH3CN, 2a existed in its tautomeric form displaying weak emission at 551 nm. The
-
addition of F-, AcO- and H2PO4 inhibited the intramolecular proton transfer from
sulfonamide NH to benzothiazole nitrogen and caused the intermolecular proton transfer
1
from sulfonamide NH to F-, which has been shown by the H NMR titration. As

electronegativity of fluoride is greater compared to nitrogen as well as strong hydrogen bond
interaction of fluoride with sulfonamide NH rather than benzothiazole nitrogen, there is more
stabilization of negative charge on F- ion compared to the negative charge on benzothiazole
nitrogen, which made the probability of proton transfer to benzothiazole nitrogen
comparatively less. The same type of intramolecular proton transfer has been shown by Aslan
et al recently [38]. Moreover, emission enhancement observed in fluorescence spectra of 2a
upon addition of Zn2+ might be attributed to the inhibition of ESIPT phenomenon, which
favored the intramolecular charge transfer (ICT) transition within the whole molecule
(Scheme 2).
On the other hand, 1H NMR spectrum of 2b revealed that NH proton appeared at much
upfield value i.e. at δ 7.21 in comparison to that of 2a which showed NH proton at δ 11.8.
This implied that sulfonamide proton completely moved to the benzothiazole moiety in the
2b, making it less acidic to bind with any of anions.
S
S
S
N
ESIPT
N
H N
O
H N
O
N
Zn2+
O
O
S
H
O
S
N
Zn2+
S
O
Tautomer
em = 551 nm
CH3
CH3
abs = 324 nm


CH3
em = 456 nm

A-
S
S
N
S
N
N
N
N
N
A-
H
O
O
O
S
H
S
O
S
H
O
A-
O
A-
CH3
abs = 378 nm
em = 468 nm


CH3
CH3
A- = F-, AcO-, H2PO4
-
Scheme 2. Possible sensing mechanism of 2a with analytes in the ground and excited states in
CH3CN.
Conclusions
To sum up, condensation of 2-(2’-aminophenyl)benzothiazole with p-toluenesulfonyl
chloride yielded 2a that has been utilized to detect Zn2+ selectively amongst various
-
interfering cations in CH3CN solvent. Addition of F-, AcO- and H2PO4 to solution of 2a
resulted in formation of new band at longer wavength (Δλ = 54 nm) in UV-Vis spectra and
substantial blue shift (Δλ = 83 nm) along with fluorescent enhancement in fluorescence

16
spectra. These changes could be attributed to the inhibition of excited state intramolecular
proton transfer (ESIPT) due to deprotonation of sulfonamide proton in the presence of F-,
-
AcO- and H2PO4 ions.
Acknowledgements
The authors are greatly thankful to SAIF, Panjab University Chandigarh for recording the
NMR and Mass spectra and are grateful to DST (Grant no. SR/FT/CS-36/2011), UGC (Grant
no. AB2/12/3115) and DST PURSE-II (Grant no. 48/RPC) for the financial assistance.
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