Novel symmetric Schiff-base benzobisthiazole-salicylidene derivative with fluorescence turn-on behavior for detecting Pb2+ ion

Article abstract of DOI:10.1016/j.jphotochem.2019.112190

A novel ligand, N²,N⁶-bis(2-salicylidene)benzobisthiazole (BSBBT), was designed and synthesized with an excellent yield through a facile one-step condensation eco-reaction for fluorescent detection of Pb²⁺ ions. BSBBT was well-characterized through FT-IR, ¹H NMR, ¹³C NMR and XRD for inclusive study of photo-physical behaviors. From the value of aggregation induced enhanced emission (AIEE) and excited-state intramolecular proton transfer (ESIPT) properties with a large Stocks shift (160 nm), BSBBT was characterized highly emissive in DMSO/H₂O (30:70, v/v) solution as well as in solid-state. BSBBT showed a strong fast response and highly selective and sensitive “turn-on” fluorescent sensing behavior for detecting Pb²⁺ ions in phosphate buffer solution (PBS) without any interference from other metal cations at the physiological pH (～7.4). The calculated binding constant and detection limit were 0.41 × 10⁵ M^?1 and 2.23 × 10^-6 M, respectively. Density Functional Theory calculation confirmed the formation of stable BSBBT-Pb²⁺ complex with a lower energy gap compared to BSBBT. BSBBT is the first high-efficient luminescent benzobisthiazole-base fluorescent sensor reported so far for detecting Pb²⁺ ions in PBS at the physiological pH range.

Full text of DOI:10.1016/j.jphotochem.2019.112190

Journal Pre-proof
Novel Symmetric Schiff-Base Benzobisthiazole-Salicylidene Derivative
with Fluorescence Turn-on Behavior for Detecting Pb2 Ions
+
Marjan Rahimi, Abbas Amini, Hossein Behmadi
PII:
S1010-6030(19)31539-4
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112190
JPC 112190
Reference:
To appear in:
Journal of Photochemistry & Photobiology, A: Chemistry
Received Date:
Revised Date:
Accepted Date:
5 September 2019
19 October 2019
21 October 2019
Please cite this article as: Rahimi M, Amini A, Behmadi H, Novel Symmetric Schiff-Base
Benzobisthiazole-Salicylidene Derivative with Fluorescence Turn-on Behavior for Detecting
2+
Pb Ions, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2019),
doi: https://doi.org/10.1016/j.jphotochem.2019.112190
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Novel Symmetric Schiff-Base Benzobisthiazole-Salicylidene Derivative with Fluorescence Turn-on
2+
Behavior for Detecting Pb Ions
a
b,*
a
Marjan Rahimi , Abbas Amini , Hossein Behmadi
aDepartment of Chemistry, Faculty of Science, Islamic Azad University, Mashhad Branch, P.O. Box:
1735-413, Mashhad, Iran
9
bCentre for Infrastructure Engineering, Western Sydney University, Penrith, NSW 2751, Australia
*Corresponding Author.
A. Amini: E-mail: a.amini@westernsydney.edu.au, Fax: +61-247360106, Tel.: +61-402850916
Graphical abstract
Highlights:





Novel benzobisthiazole-salicylidene-base chemo-sensor showed large Stokes shift
The ligand had enhanced emission in solution and solid-state due to aggregation
2+
Probe had fast turn-on Pb ion detection in aqueous medium at physiological pH
-6
5
-1
Detection limit and binding constants were 2.23×10 M and 0.41×10 M
Computational studies confirmed the experimental sensory behavior of new ligand
1

Abstract
2
6
A novel ligand, N ,N -bis(2-salicylidene)benzobisthiazole (BSBBT), was designed and synthesized with
an excellent yield through a facile one-step condensation eco-reaction for fluorescent detection of Pb2+
1
13
ions. BSBBT was well-characterized through FT-IR, H NMR, C NMR, XRD for inclusive study of
photo-physical behaviors. From the value of aggregation induced enhanced emission (AIEE) and excited-
state intramolecular proton transfer (ESIPT) properties with a large Stocks shift (160 nm), BSBBT was
characterized highly emissive in DMSO/H
2
O (30:70, v/v) solution as well as in solid state. BSBBT
showed a strong fast response and highly selective and sensitive “turn-on” fluorescent sensing behavior
2+
for detecting Pb ions in phosphate buffer solution (PBS) without any interference from other metal
cations at the physiological pH (~7). The calculated binding constant and detection limit were 0.41×105
-1
-6
M and 2.23×10 M, respectively. Density Functional Theory calculation confirmed the formation of
2+
stable BSBBT-Pb complex with a lower energy gap compared to BSBBT. BSBBT is the first high-
2
+
efficient luminescent benzobisthiazole-base fluorescent sensor reported so far for detecting Pb ions in
PBS at the physiological pH range.
2
+
Keywords: Fluorescent sensor, Benzobisthiazole-Salicylidene, tridentate ligand, Pb , Schiff-base
derivative
1. Introduction
Lead (Pb) with various usage in industries, including batteries, wires, paints, computers etc., is considered
as the third most abundant toxic heavy metal circulating in soil and water resources [1]. Little intake of
Lead can lead to permanent or serious damage to brain [2], immune system [3], liver and kidney [4],
blood cells and memory [5]. As per its deadly role in ecology, biology and human health, the
2
+
development of highly selective and sensitive sensors for easy, accurate and rapid tracing of Pb ions is
necessitated.
2

2
+
Various lab-on-the-chip chemo-sensors have been proposed for detecting Pb ions on the basis of
absorption and fluorescence spectroscopy through binding moieties and other components, such as
peptide [6], protein [7], DNAzyme [8], polymer [9], carbon quantum dots [10], and supramolecular
complexes of Bis(O
2
S macrocycle) [11]. These setups have serious drawbacks, including sophisticated
2
instruments, complex data analysis, delayed response time, and significant interferences received from
other transition metal ions. Among the probes, the fluorescence-base ones are very encouraging for
2
+
detecting Pb , as per their operational simplicity and reliability, high selectivity and sensitivity, and with
the capability of being monitored by naked eyes.
Chromophore benzobisthiazole (BBTz or trans-BBTz) compounds are promising structures for photo-
transistor equipment [12], photo-voltaic [13] and solar cells [14]. BBTz has delocalized π-expanded
structure with over three fused conjugated aromatic rings made of a symmetric molecule. With high
fluorescence property, thermal stability and electron affinity, BBTz is suitable for engineered
chromophores which need two-photon absorption in the near-IR region [15], as well as hetero-aromatic
systems which require diagnostic agents for 3D fluorescence imaging [16]. Sulfur atom is less
electronegative than the oxygen atom with similar electronegativity to carbon atom. In BBTz, the electron
density is shared more equally between sulfur and carbon, and thus the π-orbitals would be more
delocalized. Empty d orbitals of the sulfur atom can contribute in the molecular π-orbitals, and that
decrease the energy of π-π* transition [17]. Although BBTz molecules (with three conjugated
heterocyclic rings) have been rarely reported in literature [18], many fluorescence probes containing
benzothiazole (BTz) moieties (with two conjugated aromatic rings) have been used for detecting various
bioactive analytes such as anions and cations (Table 1).
2
+
Most of sensitive fluorescent chemo-sensors for detecting Pb are restricted to organic or a mixture of
organic/aqueous solvents [19] due to their poor water solubility, hence, they cannot be used for biological
2
+
media. So, designing a highly selective and sensitive probe for monitoring trace amount of Pb in pure
aqueous systems seems to be very challenging. To address this problem, we introduce and synthesize a
novel Schiff-base chemo-sensor fluorescent probe, BSBBT, through a facile one-step and low-cost
3

condensation reaction. We take the advantage from two fluorophores, trans-BBTz and salicylaldehyde
2+
moieties, to construct a “turn-on” Pb responsive fluoro-sensor in PBS. BSBBT structure acts on the
basis of excited-state intramolecular proton transfer (ESIPT), originated from the keto and enol tautomers
of salicylaldehyde moiety. Tautomerization-base fluorescent sensors are encouraging because of their
dual fluorescence intensity changes at the near infrared zone (700–900 nm) and large Stokes shift. Stokes
shift (Δλ) is the difference in the wavelength of maxima band of absorption and fluorescence emission
spectra of similar electronic transition. In biological contexts, these sensors can act efficiently through
prohibiting photo-damage caused by the self-absorbance and strong interference derived from short
wavelength emission [20]. ESIPT molecules are useful building block materials for photonic devices,
potentially suitable for white-light emission [21].
After synthesizing the BSBBT compound, the structural-induced photo-physical properties of BSBBT
both in solution and solid state, pH-dependency, ESIPT properties, selectivity and sensitivity toward Pb2+
were thoroughly investigated. The Density Functional Theory (DFT) calculations were carried out to
verify the electronic structure and geometry of new ligand, BSBBT. As far as we know, BSBBT is the
2
+
first high-efficient luminescent BBTz-base fluorescent sensor reported so far for detecting Pb ions in
PBS at the physiological pH range. These significances indicate the potential applicability of BSBBT
molecule for physiological and environmental purposes.
Table 1. Recent BTz moieties-base fluoro-sensor probes to detect various cations and anions in different
solutions.
DL1
(M)
K
2
Solvent
Ion
detection
O (1:1) Hg2+
Merits; Drawbacks
b
Ref.
-
1
mixture (v/v)
(M )
3
Turn-on sensor, simple synthesis; Using toxic solvent, 1.59×10-8
pH=8.5
THF /H
2
-
[22]
4
2+
2+
5
DMSO /H
2
O
Cu
Hg
Turn-on sensor for Hg , pH=7.4 , dual-target
detection; Multi-step synthesis, turn-off sensor for
2
+
7.6×10-9
7.78 104
(2:98)
[23]
[24]
2
+
Cu , using toxic solvent
THF/HEPES6
Hg
Ag
2+
Turn-on sensor, pH=7.4, dual-target detection; Using
toxic solvent, complex synthesis
0.27×10
0.45×10
-6
4.04×10
1.22×10
4
4
2
+
-6
(4:6)
7
3+
-9
6
7
7
5
DMF /H
96:4)
2
O
Al
Cr
Multi-target detection, pH=7; Using high quantity of
toxic solvent, multi-step synthesis
27×10
25×10
33×10
67×10
4.85×10
1.99×10
3.09×10
1.23×10
3
+
-9
(
[25]
2
+
-9
Zn
F-
-
9
4

DMSO-PBS
3:1)
Turn-on sensor, fluorescence signal in near-infrared
pH=7.4; Using high quantity of toxic solvent, delayed
response time, multi-step synthesis
(
Pd2+
48×10-9
2.2×10-6
-
-
[26]
[27]
THF/H
2
O (1:9)
Al3+
Al3
Turn-on sensor, pH=7.4; Using toxic solvent, multi-
step synthesis
Multi-target detection, pH=7.4; Using high quantity
of toxic solvent
+
Methanol/H
7:3)
2
O
2
+
0.1×10-6
3.2×10-9
0.32×10-6
(
Zn
Cd
Cu
-
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
2
2
+
+
Acetonitrile/
O (5:5)
DMSO/H
9:1)
DMF/H
Turn-on sensor, large Stokes shift; Using toxic
solvent
Turn-on sensor; Using high quantity of toxic solvent
-
H
2
2
O
Cysteine
-
(
O (9:1) Zn2
+
Dual-target detection, turn-on sensor; Using high
quantity of toxic solvent
pH=7.4, dual-target detection, turn-on sensor;
Complex synthesis
Turn-on sensor, pH=7.2; Multi-step synthesis,
interference from other cations
pH=7.4; Using high quantity of toxic solvent, multi-
step synthesis
0.36×10
-7
-
2
2
+
-6
Cd
1.16×10
2
+
-9
4.53×104
4.04×104
2.40×106
Buffer solution
Ethanol-HEPES
Zn
PPi
7×10
2.58×10-7
3.48×10-7
(
65:35)
Acetonitrile/
O (9:1)
Methanol/H
1:1)
DMSO/H
3:2)
H
2
Zn2+
-8
4
2
O
Turn-on sensor, simple synthesis; Using toxic solvent
6.7×10
4.9×10
2.1×10
9.95×10
-
7
4
(
3.6×10
-
9
5
2
O
pH=7; Using high quantity of toxic solvent, multi-
step synthesis
35×10
(
THF/H
2
O (1:9)
Picric acid
Au3+
Aqueous solution; Using toxic solvent, multi-step
synthesis
-
-
-
[37]
[38]
EtOH/PBS
buffer (3:7)
Acetonitrile,
DMSO,
pH=7.4, dual fluorescence response; Costly
9×10-9
Ratiometric sensor; Turn-off sensor, using high
quantity of toxic solvent
F-
73×10-9
138×10
-
-
[39]
[40]
-
9
Ethanol/H
3:1)
DMSO/HEPES
4:6)
2
O
(
Cu2+
PO
4
Dual-target detection, pH=7.35; Using toxic solvent,
multi-step synthesis
-8
-7
1.018×10
1
3
-
(
.52×10
2
+
1
00% aqueous
Cu
S
Dual-target detection, pH=7.4, 100% aqueous
solution, large Stokes shift; Turn-off sensor,
multi-step synthesis
2
-
63×10-9
solution
-
-
-
[41]
[42]
[43]
DMF/H
2
O (1:9) CN-
Turn-on sensor, pH=7.4; Using toxic solvent
0.15×10-6
2.9×10-9
Aqueous buffer
solution
Thiophenol
Pb2+
Turn-on sensor, large Stokes shift, pH=7.4;
Complex synthesis
1
00% aqueous
Turn-on sensor, fast response, large Stokes shift,
pH=7.4, using 100% aqueous solution, one-step
synthesis, high yield product, AIE-active molecule,
no interference from other metal ions
buffer solution
2.23×10-6
0.41×105
1
2
3
4
5
6
7
DL: Detection Limit
: Binding Constant
THF: Tetrahydrofuran
DMSO: dimethyl sulfoxide
pH=7.4; Physiological pH
HEPES: (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) buffer
DMF: Dimethyformamide
K
b
2. Experimental
5

2
.1. Materials
Para-phenylenediamine (PPDA), 2-hydroxybenzaldehyde (Salicylaldehyde), ammonium thiocyanate
NH SCN), bromine (Br ) and other solvents were obtained from MERCK, Germany, and used as
(
4
2
received without further purifications. BSBBT was synthesized for the first time in our lab. Aqueous
solutions were prepared using double distilled deionized water.
2.2. Characterization setup
Absorption measurements were acquired from a CARY50, VARIAN Spectrometer, Australia, within the
wavelength range of 200–800 nm. Fluorescence measurements were performed on a Varian Cary Eclipse
fluorescence spectrophotometer using 1 cm path quartz cell; emission spectra were recorded in the range
of 300 to 800 nm. All test solutions were examined using the excitation and emission slit of 5 nm. All
spectral investigations were carried out at room temperature. Melting points were recorded by a Stuart
SMP3 melting point apparatus. Fourier transform infrared spectra (FT-IR) were obtained from a Tensor
−
1
2
7 spectrophotometer, Germany, using the KBr pellet mode in the region of 4000-400 cm . The X-Ray
Diffraction (XRD) patterns were collected from a diffractometer, PAN analytical, X' Pert PRO MPD,
1
13
Netherland, using Ni-filtered Cu Kα radiation (k=1.5418 Å). H NMR (300 MHz) and C NMR (75
MHz) spectra were withdrawn from a Bruker Avance DRX-300 spectrophotometer, Germany, with
tetramethylsilane (TMS) as the internal standard and CDCl
3
, DMSO-d
6
and D O as the deuterated
2
solvents. Mass spectrum was acquired from a Varian Mat, CH-7 at 70 eV, Mass Spectrometer, Germany.
1
The following abbreviations were used to describe spin multiplicities in H NMR spectra: s = singlet; d =
doublet; t = triplet; m = multiplet.
2.3. Spectroscopic measurements
2.3.1. Aggregation induced enhanced emission (AIEE) measurements
Due to insolubility in water, the stock solution of BSBBT (1.0 mM) was prepared from DMSO. The test
samples (at fixed concentrations) were prepared by adding 50 μL of stock solution of BSBBT in a 5mL
volumetric flask. To furnish 10 μM solution, the samples were diluted by adding the mixtures of
6

DMSO/water solution with different water contents, f
emission measurements of the resultant solutions were obtained immediately at room temperature.
.3.2. pH measurements
pH values were measured by 827 pH Lab, Metrohm, Switzerland, equipped with a glass electrode
Thermo Scientific Orion 9157BNMD Triode). pH meter was calibrated with a standard PBS at pH 7.0.
w
(varies from 0 to 98% v/v). Afterwards, the
2
(
The desired pH level was obtained by the addition of 0.1 M NaOH and 0.1 M HCl. The test solutions
were prepared by diluting 80 μL of the BSBBT stock solution in 4 mL of PBS. The test solutions were
excited at λex=268 nm.
2.3.3. Cation sensing measurements
3+
2+
2+
2+
2+
2+
3+
2+
2+
2+
2+
2+
2+
Stock solutions of Al , Ba , Ca , Cd , Cr , Cu , Fe , Hg , Mg , Mn , Ni , Pb and, Zn were
prepared in deionized double distilled water from their nitrate and chloride salts (10 mM). The test
samples were prepared by adding 50 μL of stock solution of BSBBT to 50 μL of stock solutions of metal
cations in a 5-mLvolumetric flask. The mixtures were then diluted with PBS at pH=7.4 in order to reach
the desired concentration. DL was obtained from the fluorescence titration experiments and calculated
from DL=3σ/k, where, σ is the standard deviation of blank solution (containing BSBBT in the absence of
2
+
cations) and k is the slope of emission intensity vs. [Pb ]. Blank measurements were performed ten times
using the fluorescence spectrum of blank from which the standard deviation was calculated, while the
2
+
emission intensity was plotted versus [Pb ] to find the slope. Parameter K
b
(binding constant) was
determined from the emission intensity data in the fluorescence titration experiments according to
2
+
Benesi–Hildebrand equation [44], 1/(I-I
0 b
)=1/(K (Imax-I
0
)[Pb ])-1/(Imax-I
0
), where I and I represent the
0
2
+
emission intensity of BSBBT in the presence and absence of Pb , respectively. Imax is the saturated
2
+
2+
emission of BSBBT in the presence of excess amount of Pb ; [Pb ] here is the concentration of added
2
+
Pb ion. All test solutions were excited at 276 nm.
2.4. Syntheses
2.4.1. Synthesis of p-Phenylenebis(thiourea) (PBTU)
7

PBTU was synthesized according to the procedure introduced in [45] with some changes. To a 250-mL
flask, the following items were added: 15.14 g (140 mmol) of PPDA, 126 mL distilled water, 28 mL of
concentrated hydrochloric acid, and 0.98 g of activated charcoal. The mixture was warmed up to 50ºC and
transferred into another 250-mL flask after filtration. NH SCN (47.95 g, 600 mmol) was added and the
4
mixture stirred at a temperature of 90-100ºC for 20-24 h. The yellow, granular product began to
precipitate after 2 h reaction. The mixture was allowed to cool down, and then the product was collected
by filtration, washed by 80 mL hot water, and finally dried at 100ºC under reduced pressure. (C
8
H N
10 4
S
2
):
-1
light yellow granular solid, yield: 29.6 g (93.43 %); m.p.: 214-219°C. FT-IR spectrum (cm , refer to Fig.
-
1
1
1
a): 3344, 3267, and 3173 cm (NH and NH ), 3015, 2061, 1617, 1549, 1511, 1478, 1413, 1314, 1222,
2
-1
-1
069 cm (C=S), 838, 786, 698, 591, 512 cm .
2.4.2. Synthesis of 2,6-Diaminobenzo [1,2-d:4,5-d'] bisthiazole (DABBT)
The compound DABBT was synthesized according to the procedure defined in [45] with some changes.
To a stirred suspension of 22.6 g (100 mmol) of PBTU in 105.6 mL of dry chloroform, 12.5 mL solution
of (6.24 mmol of Br ) of bromine in 19 mL of chloroform was added such that the pot temperature
2
remained <50ºC. The orange slurry was stirred at room temperature overnight and then refluxed for 24 h.
The mixture was allowed to cool down under a slow stream of argon. The granular orange solid was
collected by filtration. The crude product was washed with 45 mL of chloroform, dried in air, and stirred
with aqueous sodium bisulfite (15.09 g NaHSO /114 mL water). The yellow solid was collected by
3
filtration, washed with 38 mL of concentrated ammonium hydroxide, and finally washed with 76 mL of
water. The crude DABBT was recrystallized twice from 120 mL of glacial acetic acid, and dried to a
constant weight at 85ºC under reduced pressure. (C
H
8 6
N
4
S
2
): pale yellow feathery needles, yield: 12.07 g
-
1
-1
(
53.5 %); m.p.>350ºC. FT-IR spectrum (cm , Fig. 1b): 3390, 3270, and 3173 cm υ (NH and NH
2
), 3052
-
1
-1
-1
-1
cm υ (Ar-CH), 1637 cm υ (C=N), 1539 cm υ (C=C), 1436, 1316, 1114, 1068 cm (C=S), 858, 616,
-1
5
04 cm .
8

Scheme 1. (Colorful) Synthesis route of BSBBT
Scheme 2. (Colorful) Optimized spatial view of the enol form of BSBBT using Gaussian 09 software
2.4.3. Synthesis of BSBBT
BSBBT was synthesized in our laboratory for the first time through the following procedure. In a 100 mL
flask equipped with a reflux condenser and magnet, Salicylaldehyde (2.61 mL, 25 mmol), DABBT (2.24
g, 10 mmol) and 35 mL glacial acetic acid were mixed and refluxed for 2 h. The hot content of flask was
separated from the mixture by using a Büchner funnel, washed with hot ethanol and dried.
9

-1
22 14
(C H N
4
O
2
S ): orange fluorescent solid, yield: 3.4618 g (79.8%); m.p.: 320°C. FT-IR spectrum (cm ,
2
-1
-1
-1 1
Fig. 1c): 3442 cm (O-H phenol), 1626 cm υ (C=N), 1520, 1287, 1174, 761, 572, 451 cm . H NMR
300 MHz, DMSO-d , TMS); δ (ppm): 10.25, 10.75 (s, 2H, OH), 7.63 (s, 2H, Ar-CH), 7.65-7.70 (dd, 2H,
Ar-CH), 7.50-7.60 (td, 4H, Ar-CH), 7.0-7.1 (m, 2H, Ar-CH) (Fig. 2). C NMR (75 MHz, DMSO-d
ppm): 192.19, 165.36, 161.19, 148.16, 136.91, 129.67, 122.76, 119.97, 117.69, 110.08 (Fig. 3).
(
6
1
3
6
); δ
(
1
0

(
a) 1.0
(b)
1.0
0.9
0.8
0.9
0.8
0.7
0.6
0.5
0.4
4000 3500 3000 2500 2000 1500 1000
500
4000 3500 3000 2500 2000 1500 1000
500
-
1
Wavenumber (cm )
Wavenumber (cm-1)
(c)
(d)
1.00
0.95
0.90
0.85
0.80
50000
40000
30000
20000
10000
0
4000 3500 3000 2500 2000 1500 1000
500
5
10
15
20
25
30
35
40
-
1
Wavenumber (cm )
2 (degree)
Figure 1. (Colorful) FT-IR spectra of: (a) PBTU, (b) DABBT, (c) BSBBT, and, (d) XRD pattern of
BSBBT.
1
1

1
Figure 2. (Colorful) H NMR (300 MHz, DMSO-d ) spectrum of BSBBT
6
13
Figure 3. (Colorful) C NMR (75 MHz, DMSO-d ) spectrum of BSBBT
6
1
2

3. Results and discussion
A novel luminophore, BSBBT, was facilely synthesized through a simple one-step cost effective Schiff-
base condensation reaction between DABBT and salicylaldehyde (1:2 mole ratio) in an acidic medium
(
Scheme 1); pure orangish precipitate was obtained with an excellent yield. The molecular structure of
Schiff-base BSBBT was well-characterized by several techniques, including FT-IR (Fig. 1a-c), XRD
1
13
patterns (Fig. 1d), H NMR (Fig. 2), and C NMR (Fig. 3). BBTz chromophore with heteroatoms
(
[
Nitrogen and Sulfur) acted as a good chelator for selectively binding heavy and transition metal cations
15]. XRD results of as-prepared BSBBT solid exhibited multiple strong and sharp diffraction peaks,
implying that the sample was in a polymorphous microcrystalline state [46].
.1. UV-Visible absorption and fluorescence emission spectra
3
Optical characteristics of ligand BSBBT (20 μM) were investigated through the normalized absorption
and fluorescence emission spectra both in solutions (DMSO as aprotic solvent, PBS and protic solvent)
and in solid state at room temperature; the results were tabulated in Table 2.
Table 2. Optical characteristics of chromophore BSBBT (20 μM) in solution and solid-state.
State
Color
Solvent
DMSO
PBS
Absorption Fluorescence intensity (nm)
Stokes Shift
(nm)
31
160
176
λ
abs (nm)
λ
ex
λ
em
Solution
yellow
301
276
68
276
332 (Broad), 552 (Weak)
538 (Sharp)
554 (Sharp)
2
yellow-
green
294 (sharp)
378 (broad)
3
50
702 (Sharp), (near
324
infrared region (NIR),
7
00-900)
722 (Sharp), (NIR, 700-
00)
748 (Sharp), (NIR, 700-
00)
762 (Sharp), (NIR, 700-
00)
778 (Sharp), (NIR, 700-
00)
3
3
3
3
60
74
81
89
344
370
384
400
9
9
9
9
Solid-State orange
-
-
-
-
250
370
420 (Broad), 460 (Broad)
415 (Broad)
-
-
BSBBT was not dissolved in some organic solvents such as chloroform, acetone, triethylorthoformate,
,2-dichloromethane, THF and acetonitrile. However, it was readily dissolved in toluene and ethylacetate
1
1
3

but non-emissive in these solvents. Further, BSBBT was solved in DMSO and DMF (Lewis-base aprotic
solvents) after 15 minutes stirring at 70°C, but it was insoluble in water with strong orange emission in its
solid-state (Fig. 4c, and Table 2). As illustrated in Fig. 4a, the observed maximum absorption at 301 nm
in DMSO originated from the π-π* intermolecular H-bonding between O-H of salicylaldehyde and imine-
N atom of ligand. This rigidified the molecular conformation and led to sharp and intense peaks in buffer
solution [47]. To some extent, BSBBT had a fluorescence emission in DMSO because of its enough high
viscosity and aprotic properties to slow down the non-radiative intramolecular rotations around C=N
linkages available in the salicylidene and BBTz moieties, restricting the intramolecular rotation
mechanism in BSBBT through the coplanar association with the process. As a matter of fact, the ESIPT
process is the intramolecular proton transfer process between O-H of salicylaldehyde and imine-N atom
of ligand, which significantly rigidifies the molecular conformation. So, the large Stokes shift (~160 nm)
indicated that when the enol form of BSBBT was rapidly rearranged to the excited keto form, the ESIPT
process occurred. This phenomenon was responsible for the strong red-shifted emission at 538 nm in
buffer and a weak and broad peak at 300-400 nm in DMSO (Fig. 4b).
For ESIPT properties, the fluorescence characterization of BSBBT probe was examined through several
experiments. The fluorescence behavior of BSBBT at varied concentrations in methanol showed slight
increase after dilution in methanol. Also, the emission intensity decreased by the increment of excitation
wavelength in DMSO (Figs. S1-S4, Table A, Supplementary Information). The pH-dependent behavior of
BSBBT was a piece of fact for the existence of different species (both enol- and keto-forms) in the
solution. The DFT calculations confirmed the tautomerization process in the excited state through time
dependent Self-Consistent Field (TD-SCF) semi-empirical AM1 basis set calculations, where the total
energy (E ) of enol- and keto-forms of BSBBT were characterized as 4.2394 and 8.6503 eV, respectively.
T
As expected, the keto-form of BSBBT was energetically more stable than its enol-form in the first excited
singlet state, but in the ground state, the enol-form of BSBBT was energetically more stable than its keto-
form (Fig. S5).
1
4

(a)
(c)
300
DMSO (b) 500
DMSO
Buffer
268, Excitation wavelength (nm)
0
0
0
0
0
.8
.6
.4
.2
.0
Buffer
276
350
360
250
400
300
200
100
0
3
74
381
89
200
3
1
50
00
1
50
0
2
00
300
400
500
600
700
800
300
350
400
450
500
550
550
600
650
700
750
800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
2
2
2
1
1
1
1
40
(d)
(e)
300
250
200
150
100
2
50 nm
370 nm
20
00
80
60
40
20
50
270
300
330
360
390
420
450
390
405
420
435
450
465
480
Wavelength (nm)
Wavelength (nm)
Figure 4. (Colorful) Photo-physical study of BSBBT (20 μM) at room temperature; (a) absorption spectra
in DMSO (olive) and PBS (orange), (b) emission spectra in DMSO (olive) and PBS (pH=7.4) (orange),
λ
ex=268 nm, (c) emission spectra in PBS (pH=7.4) at various excitation wavelengths, (d) solid-state
photo-luminescence (PL) spectra, λex=250 nm, and, (e) solid-state PL spectra, λex=370 nm.
3.2. AIEE behavior of BSBBT
To examine the AIEE activity of BSBBT (10 μM) molecule, we probed the emission behavior in
DMSO/water (solvent/non-solvent) binary mixtures. As revealed in Figs. 5a and 5b, BSBBT exhibited a
distinct fluorescent behavior in DMSO/H
showed weak fluorescence in pure DMSO solution; it became stronger at f
was formed in the DMSO/H O mixture. The fluorescence intensity showed a sudden maximum increase
at f =70%. The solutions emitted bright yellow-green light while the emission intensity of this water
2
O mixture with various water fractions (f
w
). Initially, BSBBT
w
= 40% when the suspension
2
w
content was ~66 times higher than that in pure DMSO solvent, verifying BSBBT as a good AIEE-active
compound. The unique fluorescent behavior of BSBBT can be ascribed to the ESIPT property of
salicylaldehyde Schiff-base derivatives which are responsible for the narrower and higher intense
1
5

emission peaks. This property reduced the intermolecular interactions in aggregate or in the solid-state,
and brought difficulty to the cis-trans isomerization of C=N group [47]. When a large amount of non-
solvent (f >60%) existed in the mixture BSBBT molecules tended to aggregate [48]. As the aggregation
w
directly affected the molecular arrangement, conformational flexibility, and intermolecular interactions,
the novel solid-state properties of fluorophores expected to be correlated to the AIEE phenomenon [48].
PL spectra of BSBBT in Figs. 4d and 4e depict the strong and broad fluorescence emission under 250 and
370 nm excitation wavelengths for the solid-state luminescence of BSBBT [49].
600
500
400
300
200
100
0
(a)
(b)
0
1
2
3
4
5
6
7
8
9
9
, Water fraction (%)
0
0
0
0
0
0
0
0
0
8
5
00
00
4
300
2
00
00
0
1
535
540
545
550
555
560
565
570
0
10 20 30 40 50 60 70 80 90 100
Wavelength (nm)
Water fraction (%)
Figure 5. (Colorful) The AIEE behavior of BSBBT (10 μM) in DMSO/H
2
O mixture through increasing f
w
from 0 to 98%, (v/v), λex = 276 nm: (a) emission spectra, and, (b) bar diagram of Maximum Fluorescence
Intensity (Max. FI) vs. water fraction.
3.3. pH effect
The pH-dependent behavior of BSBBT was investigated by altering the pH value of aqueous solution
within the range of pH=3-11 (Fig. 6). A gradual increase in the fluorescence intensity was observed when
the pH increased from 3 to 7; the maximum emission intensity occurred at pH=7 indicating the efficient
potential of BSBBT for physiological pH range. After pH=7, the emission intensities at pH 5 and 6 were
maximum.
1
6

(a) 250
(b) 250
3, pH
4
5
6
7
8
9
2
1
1
00
50
00
200
150
100
50
10
1
1
5
0
0
0
525
530
535
540
545
550
3
4
5
6
7
8
9
10
11
Wavelength (nm)
pH
Figure 6. (Colorful) Fluorescent pH-dependent behavior of BSBBT (20 μM) at various pHs in PBS at
room temperature; (a) fluorescence spectra, and, (b) Maximum Fluorescence Intensity (Max. FI) vs. pH,
λ
ex = 268 nm.
3.4. Fluorescence-base cation sensing
For the selectivity, the fluorescence response of BSBBT to various metal cations was investigated. In the
absence of metal cations, BSBBT (10 μM) showed very weak emission in PBS (pH=7.4). Fast
isomerization around C=N and photo-electron transfer from the nitrogen of benzobisthizole to
salicylidene moiety fluorophore were the probable cause of weak fluorescence of BSBBT [50]. Fig. 7a
represents the fluorescence emission spectra of BSBBT induced by various metal ions (0.1 mM),
3+
2+
2+
2+
2+
2+
3+
2+
2+
2+
2+
2+
2+
including Al , Ba , Ca , Cd , Cr , Cu , Fe , Hg , Mg , Mn , Ni , Pb and Zn . Among the
2+
examined metal cations, only Pb ion significantly enhanced the fluorescence signal with no color
change (Fig. 7b). No significant response was seen after adding other metal cations. This fluorescence
2
+
enhancement was attributed to the inhibition of non-radiative process by the conjugation of Pb ions with
the C=N linkage available in salicylidene and BBTz moieties [51]. The imine-N was essential for the
chelation with most metal ions (Fig. 9); the involvement of imine-N with benzobisthiazole N/S during the
2+
chelation with Pb ions ceased the isomerization process and increased the fluorescence activity as a
result of chelation enhanced fluorescence emission (CHEF) effect [52]. No color change was observed
1
7

2+
with naked eyes. Pb ions interacted with both hard and soft binding sites. The BSBBT ligand provided
2
+
suitable number of proper binding sites, thus, BSBBT displayed a high selectivity for Pb ions [53]. To
2+
further evaluate the selectivity of BSBBT as an effective fluorescent sensor for Pb , a series of
competitive experiments were performed on BSBBT in the presence of various metal ions (Fig. 7c); there
was no interference by other metal ions. As a result, BSBBT could serve as a selective fluorescent sensor
2+
for the recognition of Pb ions without any interference of other metal ions.
1
000
(b)
(c)
(a)
BSBBT
Pb (II)
Al (III)
Ba (II)
Ca (II)
Cd (II)
Cr (II)
Cu (II)
Fe (III)
Hg (II)
Mg (II)
Mn (II)
Ni (II)
Zn (II)
1
200
800
600
400
200
0
1000
8
00
00
6
400
200
0
535
540
545
550
555
560
565
570
Wavelength (nm)
Metal ion
Figure 7. (Colorful) (a) Fluorescence spectral response of BSBBT (10 μM) to various metal cations (0.1
2
+
mM), (b) BSBBT solution (left) and BSBBT-Pb solution (right), (c) fluorescence intensity of BSBBT
2
+
against various metal ions with (violet bars) and without (colorful bars) Pb in PBS, pH=7.4, λex = 276
nm.
Fluorescence titration experiments were performed in order to have further insight into the sensing
2+
behavior of BSBBT towards Pb . Fig. 8a verified the gradual increase of fluorescence intensity upon the
2+
2+
rise in the concentration of Pb . The perfect linear relationship of fluorescence intensity vs. [Pb ] ions is
-6
2
obvious (Fig. 8b). DL was determined as 2.23×10 M (slope = 28.12573, R = 0.98566). The parameter
2+
5
-1
2
K of BSBBT towards Pb was calculated as 0.41×10 M (R = 0.95663) (Fig. 8c). These results suggest
b
2
+
BSBBT compound as a highly selective and sensitive chemo-sensor for targeting Pb in PBS over a wide
range of metal ions without any interference from other components.
1
8

2
+
In order to determine the binding ratio between BSBBT and Pb , the Job’s plot experiment was
performed (Fig. 8d) where the A /(A-A ) of BSBBT at λAbs=353.9 nm was plotted against the molar
fraction of BSBBT ([BSBBT]/([BSBBT] + [Pb ])). As a result, the concentration of BSBBT-Pb
complex approached the maximum at the BSBBT molar fraction of ~0.31, indicating the binding of
0
0
2
+
2+
2+
BSBBT with Pb at a binding stoichiometry of 1:2.
a) 1000
(b)
(
13
9
50
1
1
1
9
8
7
6
5
4
3
2
1
0
2
1
0
800
600
400
200
0
900
850
800
750
700
, eq. (BSBBT)
Slope=28.12573
2
R =0.98566
650
535
540
545
550
555
560
565
2
3
4
5
6
7
8
9
10 11 12
2
+
Wavelength (nm)
[Pb ], M
(c)
2.25
(d)
0.0020
0.0019
0.0018
0.0017
0.0016
0.0015
0.0014
2.00
1.75
1
.50
.25
1
1.00
0
.75
.50
0
-9
Slope=5.9404210
2
R =0.95663

=0.31
0.25
.00
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
80000
100000
120000
140000
160000
Mole fraction of BSBBT
2
+
-1
1
/[Pb ], M
2
+
Figure 8. (Colorful) (a) Fluorescence spectra of BSBBT (10 μM) upon the addition of Pb at various
concentrations (2-12 μM) in PBS, pH=7.4, λex=276 nm, (b) fluorescence intensity vs. increasing
2
+
-6
: 0.41×105
) vs. molar fraction of BSBBT in the aqueous
concentration of Pb ; calculated DL: 2.23×10 M, (c) Benesi–Hildebrand plot; calculated K
b
-1
2+
M , and, (d) Job’s plot of BSBBT-Pb complex, A
0
/(A-A
0
1
9

2
+
methanolic solution. Job’s plot showed the binding stoichiometry of 1:2 for BSBBT/Pb at the mole ratio
of 0.31.
3.5.DFT Computation
Quantum chemical calculations were carried out to gain further insight into the optical structure,
2+
geometry and energy difference between the frontier orbitals of compounds BSBBT and BSBBT-Pb ;
the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital
(
LUMO). The planar and linear structure and frontier molecular orbitals of BSBBT were explored after
optimizing the molecular structure using the Gaussian 09 quantum chemistry packages, DFT/B3LYP/6–
2
+
3
1G+(d,p) and DFT/B3LYP/LanL2DZ method for BSBBT and BSBBT-Pb , respectively. Visualization
of the optimized geometrical structures, frontier molecular orbitals, and electrostatic potential mapped
onto the electron density surfaces were performed by GaussView 5. The optimized geometries of BSBBT
2+
ligand and BSBBT-Pb complex and their corresponded HOMO-LUMO orbitals are presented in Fig. 9.
2
+
The calculated energy gap (ΔE, eV) of BSBBT and BSBBT-Pb were obtained from HOMO and LUMO
energy eigen values:
ΔE (BSBBT) = ELUMO-EHOMO = -0.09978 - (-0.21202) = 0.11224 (eV)
(1)
(2)
2+
ΔE (BSBBT-Pb ) = ELUMO-EHOMO = -0.12783 - (-0.10454) = -0.02329 (eV)
2
+
Decreased energy gap (ΔE) of BSBBT-Pb to -0.02329 (eV) confirmed the significant stability of
2
+
BSBBT-Pb when compared to BSBBT.
2
0

Figure 9. (Colorful) The optimized geometry and HOMO-LUMO energy gap for BSBBT and BSBBT-
2
+
Pb complex using DFT/B3LYP/6–31G+ (d, p) and DFT/B3LYP/ LanL2DZ methods, respectively by
Gaussian 09 software.
As a result of complex formation, some of the optimized geometrical parameters of ligand, including
bond lengths and bond angles, were changed including the elongation of imine and C=O bond lengths as
2+
a result of complexation with Pb (Table 3). In addition, Molecular Electrostatic Potential (MEP)
mapping was performed to visualize the relative polarity and reactivity of active molecular sites. MEP of
BSBBT (Fig. 10a) shows the nucleophilic regions which are electron rich areas located around the
electronegative atoms; N and O. The nucleophilic sites in the BSBBT molecule are suitable and
2
+
susceptible to react with the electron deficient species (electrophilic sites), such as Pb ions, in order to
2+
tightly create bonded BSBBT-Pb complex. This strong interaction pulls the charge of nucleophile
2+
2+
towards empty orbitals of Pb cation (electrophile) in the sensory process of Pb by BSBBT (Fig. 10b)
[
54].
2
1

2
+
Figure 10. (Colorful) MEP of (a) BSBBT and (b) BSBBT-Pb complex obtained from DFT calculations.
2
+
Table 3. Bond lengths and bond angles of BSBBT and BSBBT-Pb complex obtained from DFT
calculations.
Bond length (Å)
Bond angle (degree)
Bond
Bond
BSBBT
1.37110 1.35043
BSBBT-Pb2+
BSBBT
BSBBT-Pb2+
117.57768
127.46268
104.14449
128.25606
125.99197
C11-N13
C14-C15
C20-O25
N12-C11-N13
C11-N13-C14
C11-N13-H43
S10-C11-N13
N13-C14-C15
121.66858
122.95927
136.61337
123.32050
121.82646
1.43766 1.42708
1.33582 1.35955
O25-Pb43 0.99526 2.07052
N13-Pb43 1.73744 2.29572
2
2

4. Conclusion
A novel symmetric tridentate fast-response turn-on fluorescent sensor was introduced and facilely
synthesized based on the benzobisthiazole-salicylidene Schiff-base derivative with high yield. The
photophysical analysis showed an enhanced emission in the aggregates with excellent AIEE property in
both DMSO/H O solution and solid state. Tautomerization of keto/enol or hydrogen bonding interactions
2
in the molecule originated the large Stokes shifts and ESIPT properties which introduced it as a useful
material for biological photonic devices. BSBBT performed as a highly selective and sensitive
2
+
benzobisthiazole-based fluorescent sensor for detecting Pb ions in PBS at pH=7.4. CHEF effect
-6
5
-1
outreached a good detection limit as low as 2.23×10 M and strong K of 0.41×10 M without any
b
interferences from other metal cations co-existed in the solution. DFT calculation for chemo-sensor
BSBBT were carried out and confirmed the stability of complex with a lower energy gap (ΔE) of
2
+
BSBBT-Pb complex, supporting the experimental sensing results. By emitting light in the yellow-green
region, BSBBT showed an excellent AIEE-activity in DMSO/buffer (30:70, v/v) solution and maximum
fluorescence response at pH=7, which qualify it for biological probes. As far as we know, BSBBT is the
2
+
first multi-responsive benzobisthiazole-base luminescent sensor for the detection of Pb ions in PBS at
the physiological pH range.
Conflicts of interest
The authors declare no competing financial interest.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as
potential competing interests:
Acknowledgement
The authors express their gratitude to Islamic Azad University, Mashhad Branch, for the financial
support. The Australian College of Kuwait is highly acknowledged for providing the Research Grants No:
2
3

IRC-2018/2019-SOE-ME-PR10 and R11 for this study. Also, special thanks go to Mr. Mohammad Akbar
Rahimi and Mrs. Leila Gharaee for their great supports.
2
4

References
1.
Alfonso, M.a., A. Tárraga, and P. Molina, Ferrocene-based heteroditopic receptors displaying
high selectivity toward lead and mercury metal cations through different channels. The Journal of
organic chemistry, 2011. 76(3): p. 939-947.
2
3
4
5
.
.
.
.
Madadrang, C.J., et al., Adsorption behavior of EDTA-graphene oxide for Pb (II) removal. ACS
applied materials & interfaces, 2012. 4(3): p. 1186-1193.
Kim, H.N., et al., Fluorescent and colorimetric sensors for detection of lead, cadmium, and
mercury ions. Chemical Society Reviews, 2012. 41(8): p. 3210-3244.
Hu, J. and M.S. Elioff, Detection of Zn2+, Cd2+, Hg2+, and Pb2+ ions through label-free poly-
L-glutamic acid. Talanta, 2018. 188: p. 552-561.
Li, T., S. Dong, and E. Wang, A lead (II)-driven DNA molecular device for turn-on fluorescence
detection of lead (II) ion with high selectivity and sensitivity. Journal of the American Chemical
Society, 2010. 132(38): p. 13156-13157.
6
7
8
.
.
.
Deo, S. and H.A. Godwin, A selective, ratiometric fluorescent sensor for Pb2+. Journal of the
American Chemical Society, 2000. 122(1): p. 174-175.
Liu, C.-W., C.-C. Huang, and H.-T. Chang, Highly selective DNA-based sensor for lead (II) and
mercury (II) ions. Analytical chemistry, 2009. 81(6): p. 2383-2387.
Chang, I.-H., et al., Miniaturized lead sensor based on lead-specific DNAzyme in a nanocapillary
interconnected microfluidic device. Environmental science & technology, 2005. 39(10): p. 3756-
3761.
9
1
.
Kim, I.-B., et al., Sensing of lead ions by a carboxylate-substituted PPE: multivalency effects.
Macromolecules, 2005. 38(11): p. 4560-4562.
0.
Kumar, V.V., T. Raman, and S.P. Anthony, Fluorescent carbon quantum dots chemosensor for
selective turn-on sensing of Zn 2+ and turn-off sensing of Pb 2+ in aqueous medium and
zebrafish eggs. New Journal of Chemistry, 2017. 41(24): p. 15157-15164.
2
5

11.
Kim, S., et al., Ligand Isomer Effect on the Formations of Supramolecular Lead (II), Mercury
II), and Copper (II)/Mercury (II) Complexes of Bis-O2S2-Macrocycle. Crystal Growth &
(
Design, 2018. 18(4): p. 2424-2431.
1
1
1
2.
3.
4.
Lin, Y., et al., Thiazole‐based organic semiconductors for organic electronics. Advanced
Materials, 2012. 24(23): p. 3087-3106.
Diaz, E., et al., Hybrid photoluminescent materials containing a benzobisthiazole core for liquid
crystal and gel applications. Soft matter, 2017. 13(9): p. 1804-1815.
Intemann, J.J., et al., Synthesis, characterization, and electroluminescence properties of poly
(fluorenevinylene benzobisthiazoles). Journal of Polymer Science Part A: Polymer Chemistry,
2013. 51(4): p. 916-923.
1
1
5.
6.
Hrobꢀrik, P., et al., Quadrupolar benzobisthiazole-cored arylamines as highly efficient two-
photon absorbing fluorophores. Organic letters, 2014. 16(24): p. 6358-6361.
Hrobárik, P., et al., Benzothiazoles with tunable electron-withdrawing strength and reverse
polarity: a route to triphenylamine-based chromophores with enhanced two-photon absorption.
The Journal of organic chemistry, 2011. 76(21): p. 8726-8736.
1
1
1
7.
8.
9.
Hrobárik, P., et al., Molecular engineering of benzothiazolium salts with large quadratic
hyperpolarizabilities: can auxiliary electron-withdrawing groups enhance nonlinear optical
responses? The Journal of Physical Chemistry C, 2010. 114(50): p. 22289-22302.
Papucci, C., et al., Green/Yellow‐Emitting Conjugated Heterocyclic Fluorophores for
Luminescent Solar Concentrators. European Journal of Organic Chemistry, 2018. 2018(20-21): p.
2657-2666.
Mishra, J., N. Kaur, and A.K. Ganguli, Selective and sensitive fluorescence recognition of Pb (II)
in aqueous medium by organic nanoparticles of a urea linker based tetrapodal receptor: Effect of
linker molecules in a sensor on chemosensing. Inorganica Chimica Acta, 2019. 487: p. 214-220.
2
6

2
2
0.
1.
Xu, Y., et al., Switchable and selective detection of Zn 2+ or Cd 2+ in living cells based on 3′-O-
substituted arrangement of benzoxazole-derived fluorescent probes. Chemical Communications,
2014. 50(56): p. 7514-7516.
Serdiuk, I.E., White light from a single fluorophore: a strategy utilizing excited-state
intramolecular proton-transfer phenomenon and its verification. The Journal of Physical
Chemistry C, 2017. 121(9): p. 5277-5286.
2
2
2
2.
3.
4.
Xu, D., et al., A benzothizole-based fluorescent probe for Hg2+ recognition utilizing ESIPT
coupled AIE characteristics. Tetrahedron Letters, 2017. 58(37): p. 3654-3657.
Gu, B., et al., A benzothiazole-based fluorescent probe for distinguishing and bioimaging of
Hg2+ and Cu2+. Analytica chimica acta, 2017. 954: p. 97-104.
Chen, S., et al., 2-Hydroxy benzothiazole modified rhodol: aggregation-induced emission and
dual-channel fluorescence sensing of Hg2+ and Ag+ ions. Sensors and Actuators B: Chemical,
2018. 255: p. 2086-2094.
2
2
2
5.
6.
7.
Zhang, H., et al., A multifunctional tripodal fluorescent probe for the recognition of Cr3+, Al3+,
Zn2+ and F− with controllable ESIPT processes. Dyes and Pigments, 2019. 162: p. 257-265.
Jie, X., et al., A new colorimetric, near-infrared fluorescent probe for rapid detection of
palladium with high sensitivity and selectivity. Talanta, 2018. 183: p. 164-171.
Chen, Y., et al., A benzothiazole-based fluorescent probe for ratiometric detection of Al3+ in
aqueous medium and living cells. Industrial & Engineering Chemistry Research, 2017. 56(43): p.
12267-12275.
2
2
8.
9.
Lu, Z.-N., et al., A selective fluorescent chemosensor for Cd2+ based on 8-hydroxylquinoline-
benzothiazole conjugate and imaging application. Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy, 2019. 213: p. 57-63.
Zhao, Y.-H., et al., A novel ratiometric and reversible fluorescence probe with a large Stokes
shift for Cu2+ based on a new clamp-on unit. Analytica Chimica Acta, 2019.
2
7

3
3
0.
1.
Yu, Y., et al., A novel benzothiazole-based fluorescent probe for cysteine detection and its
application on test paper and in living cells. Talanta, 2018. 176: p. 151-155.
Li, J., et al., A benzothiazole-based fluorescent probe for efficient detection and discrimination of
Zn2+ and Cd2+, using cysteine as an auxiliary reagent. Sensors and Actuators B: Chemical,
2018. 268: p. 446-455.
3
2.
Chang, C., et al., Benzothiazole-based fluorescent sensor for ratiometric detection of Zn (II) ions
and secondary sensing PPi and its applications for biological imaging and PPase catalysis
assays. Industrial & Engineering Chemistry Research, 2017. 56(31): p. 8797-8805.
Tang, Y., et al., Synthesis of a new pyrene-devived fluorescent probe for the detection of Zn2+.
Tetrahedron Letters, 2018. 59(44): p. 3916-3922.
3
3
3
3.
4.
5.
Gong, J., et al., A thiazolo [4, 5-b] pyridine-based fluorescent probe for detection of zinc ions and
application for in vitro and in vivo bioimaging. Talanta, 2018. 185: p. 396-404.
Venkatesan, V., S.A. Kumar, and S.K. Sahoo, Highly selective turn-on fluorogenic chemosensor
for Zn2+ based on chelation enhanced fluorescence. Inorganic Chemistry Communications,
2019. 102: p. 171-179.
3
3
3
3
6.
7.
8.
9.
Sahana, S., et al., 2-(2′-Hydroxyphenyl)-benzothiazole (HBT)-terpyridine conjugate: A highly
specific ICT based fluorescent probe for Zn2+ ions and its application in confocal cell imaging.
Journal of Photochemistry and Photobiology A: Chemistry, 2018. 351: p. 231-239.
Kaur, I., et al., Excitation wavelength based reversible multicolour photoluminescence by a single
chromophore upon aggregation: Detection of picric acid-application in bioimaging. Sensors and
Actuators B: Chemical, 2019. 281: p. 613-622.
Wang, W., et al., Strategically modified highly selective mitochondria-targeted two-photon
fluorescent probe for Au3+ employing Schiff-base: Inhibited C= N isomerization vs. hydrolysis
mechanism. Dyes and Pigments, 2018. 150: p. 241-251.
Wu, X., et al., Highly Sensitive Ratiometric Fluorescent Paper Sensors for the Detection of
Fluoride Ions. ACS Omega, 2019. 4(3): p. 4918-4926.
2
8

4
4
4
4
0.
1.
2.
3.
Kaur, I., P. Kaur, and K. Singh, 2-(4-Amino-2-hydroxyphenyl) benzothiazole based Schiff-base:
Complexation/decomplexation driven photo physical tuning of fluorescence leading to Cu2+ and
PO43− detection. Sensors and Actuators B: Chemical, 2018. 257: p. 1083-1092.
Wang, H., et al., A facile fluorescent probe with a large Stokes shift for sequentially detecting
copper and sulfide in 100% aqueous solution and imaging them in living cells. Sensors and
Actuators B: Chemical, 2018. 256: p. 600-608.
Xu, W., et al., A highly selective turn-on fluorescent and chromogenic probe for CN− and its
applications in imaging of living cells and zebrafish in vivo. Sensors and Actuators B: Chemical,
2017. 251: p. 366-373.
Chen, W., et al., A phenothiazine coumarin-based red emitting fluorescent probe for nanomolar
detection of thiophenol with a large Stokes shift. Sensors and Actuators B: Chemical, 2017. 245:
p. 702-710.
4
4
4.
5.
Singh, R. and G. Das, “Turn-on” Pb 2+ sensing and rapid detection of biothiols in aqueous
medium and real samples. Analyst, 2019. 144(2): p. 567-572.
Wolfe, J.F., B.H. Loo, and F. Arnold, Rigid-rod polymers. 2. Synthesis and thermal properties of
para-aromatic polymers with 2, 6-benzobisthiazole units in the main chain. Macromolecules,
1981. 14(4): p. 915-920.
4
4
4
6.
7.
8.
Shen, X.Y., et al., Effects of substitution with donor–acceptor groups on the properties of
tetraphenylethene trimer: aggregation-induced emission, solvatochromism, and
mechanochromism. The Journal of Physical Chemistry C, 2013. 117(14): p. 7334-7347.
Xu, S., et al., An organic molecule with asymmetric structure exhibiting aggregation‐induced
emission, delayed fluorescence, and mechanoluminescence. Angewandte Chemie International
Edition, 2015. 54(3): p. 874-878.
Liu, D., et al., Rh-Catalyzed C–H Amidation of 2-Arylbenzo [d] thiazoles: An Approach to Single
Organic Molecule White Light Emitters in the Solid State. Organic letters, 2019.
2
9