The synergy of CHEF and ICT toward fluorescence ‘turn-on’ probes based on push-pull benzothiazoles for selective detection of Cu2+ in acetonitrile/water mixture

Article abstract of DOI:10.1016/j.jphotochem.2021.113318

New push-pull schiff base ligands based on benzothiazole (BZ) unit were developed for the selective detection of Cu²⁺ through fluorescence ‘turn-on’ mechanism. These derivatives with electron withdrawing trifluoromethyl (-CF₃) and cy

Full text of DOI:10.1016/j.jphotochem.2021.113318

Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
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Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
The synergy of CHEF and ICT toward fluorescence ‘turn-on’ probes based
on push-pull benzothiazoles for selective detection of Cu2+ in acetonitrile/
water mixture
Jukkrit Nootema, Rathawat Daengngernb, Chanchai Sattayanong, Worawat Wattanathanac,
,f,
Suttipong Wannapaiboond, Paitoon Rashatasakhone,f, Kantapat Chansaenpaka
*
a National Nanotechnology Center, National Science and Technology Development Agency, Thailand Science Park, Pathum Thani, 12120, Thailand
b Integrated Applied Chemistry Research Unit, School of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok, 10520, Thailand
c Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Ladyao, Chatuchak, Bangkok, 10900, Thailand
d Synchrotron Light Research Institute, 111 University Avenue, Suranaree, Muang, Nakhon, Ratchasima, 30000, Thailand
e Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand
f Research Network of Nanotec-CU on Nanotechnology for Food and Agriculture, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok,
10330, Thailand
g School of Liberal Arts, King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand
A R T I C L E I N F O
A B S T R A C T
Keywords:
New push-pull schiff base ligands based on benzothiazole (BZ) unit were developed for the selective detection of
Cu2+ through fluorescence ‘turn-on’ mechanism. These derivatives with electron withdrawing trifluoromethyl
(-CF3) and cyano (-CN) substituents (BZ2 and BZ3) demonstrated a prominent fluorescence enhancement upon
copper ion binding which could be the results from the synergistic effect between the chelation-enhanced
fluorescence (CHEF) and the intramolecular charge transfer (ICT) processes. In addition, these compounds dis-
Fluorescence
Sensor
Copper
Benzothiazole
Drinking water
played 1:1 binding with Cu2+ with low limits of detection of 0.77
μM and 0.64 μM for BZ2 and BZ3, respectively,
in acetonitrile-water (3:1 v/v) media. The electronic and photophysical properties of these BZ ligands and the
copper ion complexes were modelled by the density functional theory (DFT) and the time-dependent density
functional theory (TD-DFT) calculations, respectively. Analysis of X-ray absorption spectra probed at Cu K-edge
of Cu2+-BZ mixtures revealed the complex formation of BZ ligands with the targeted Cu2+ and confirmed the
non-centrosymetric structures of the complexes as predicted by the DFT calculation. The electron density dis-
tributions of the HOMO-LUMOs in the computational results as well as large stokes shifts of the ligand-metal
complexes in the experimental data confirmed the strong ICT effect after Cu2+ binding which is a key process
promoting fluorescence ‘turn-on’ mechanism.
1. Introduction
as well as gastrointestinal disturbance [3,4]. Due to these detrimental
effects to human health, fluorescent sensors for Cu2+ have gained much
Copper ion (Cu2+) is the third most abundant transition metal ion in
human body after ferrous (Fe2+) and zinc (Zn2+) ions, respectively [1]. It
plays essential roles in several biological processes, such as a cofactor for
electron transfers in many proteins or a catalyst for redox reactions in
biological pathways. However, excessive uptake of copper ions in
human can cause severe neurodegenerative diseases, including, Alz-
heimer, Parkinson, Huntington and Prion diseases [2]. In addition,
long-term exposure to copper ion can give rise to liver or kidney damage
attention and a number of them have been developed to detect copper
contaminants in environments, waters, and cellular matrices [5–8].
Cu2+ ion is well known as a fluorescence quencher due to its para-
magnetic effect, so that, most of fluorescent chemosensors detected Cu2+
via fluorescence quenching processes. [9–14] However, chemosensors
possessing fluorescence enhancement are more desirable as they can be
simply monitored and more applicable for biological studies, such as,
cellular or small animal imaging [7,15,16]. The fluorescence turn-on
* Corresponding author at: National Nanotechnology Center, National Science and Technology Development Agency, Thailand Science Park, Pathum Thani, 12120,
Thailand.
E-mail address: kantapat.cha@nanotec.or.th (K. Chansaenpak).
https://doi.org/10.1016/j.jphotochem.2021.113318
Received 26 January 2021; Received in revised form 3 April 2021; Accepted 21 April 2021
Available online 25 April 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
amount of these stock solutions.
2.2.2. UV–vis absorption measurement
The suitable amount of stock solution of BZ was added to acetonitrile
: water (3 : 1) solution (3 mL) in a 3.5 mL quartz cuvette (final
concentration = 25 μM). The UV–vis absorption spectra were recorded
Fig. 1. The proposed push-pull benzothiazole molecules in this work.
before and after 15 min incubation time with appropriate amount of Cu
(NO3)2.3H2O.
probes can be constructed based on various mechanisms, such as,
chelation-induced enhanced fluorescence (CHEF) [17], intramolecular
charge transfer (ICT) [18], excited-state intramolecular proton transfer
(ESIPT) [19], and fluorescence resonance energy transfer (FRET) [20].
Among these, CHEF plays a vital role in metal sensing as most metal ions
tend to chelate with the organic donor ligands. Indeed, CHEF phenom-
enon leads to an increased conjugation in organic molecule upon metal
binding which could facilitate ICT process throughout the pi-systems
yielding fluorescence enhancement [21]. However, the fluorescence
probes relying on the synergy of CHEF and ICT are scarcely reported in
literature, therefore, there is still a large room for improvement.
As part of our investigations on fluorescent sensors, we pinned our
interest to the benzothiazole (BZ) backbone due to its ease of prepara-
tion and functionalization as well as its intrinsic optical properties.
[22–30] Similar to other fluorescence sensors, most of benzothiazole
derivatives detected Cu2+ through fluorescence turn-off processes. [3,
31–35] Only a few examples can sense Cu2+ via fluorescence turn-on
pathway. [36] In this work, we have prepared novel push-pull benzo-
thiazole derivatives employing a thiazole unit as an electron donor
group and electron-withdrawing (EWG) functionalized phenylene as an
electron acceptor moiety to acquire ICT effect (Fig. 1). The two nitrogen
atoms in the proposed molecules are aimed to chelate Cu2+ ion leading
to fluorescence enhancement via CHEF and ICT processes. The experi-
mental details as well as density functional theory (DFT) calculation
results regarding Cu2+ sensing were presented in this report.
2.2.3. Fluorescence measurement
The suitable amount of stock solution of BZ was added to acetonitrile
: water (3 : 1) solution (3 mL) in a 3.5 mL quartz cuvette (final
concentration = 25 μM). The fluorescence spectra were recorded before
and after 15 min incubation time with appropriate amounts of Cu
(NO3)2.3H2O, using the following parameters: excitation wavelength
=332 nm for BZ1, 343 nm for BZ2, 375 nm for BZ3, and 367 nm for BZ4,
excitation slit =10 nm, and emission slit =10 nm.
2.3. Determination of fluorescence quantum yield
Fluorescence quantum yields of the BZ-Cu2+ complexes were
measured in water using quinine sulfate in 0.1 M H2SO4 as a standard
(Qstd = 0.58) and were calculated based on the Eq. (1) [37]:
(
)
(
)
(
)
2
ŋsample
ŋstd
Asample
Astd
Istd
Isample
Q = Qstd
×
×
×
(1)
where Q denotes the fluorescence quantum yields, A is the peak area of
emission spectra, Istands for absorption intensities at the excitation
wavelength and ŋ is the solvent reflective index.
2.4. Binding stoichiometry and binding constant determination
The binding stoichiometry of the complexes between benzothiazole
derivatives (BZ2 and BZ3) and Cu2+ was determined by Job’s plot ex-
periments. [38] The samples for Job’s plot were prepared by mixing BZ
compounds (BZ2 or BZ3) with Cu2+ at different ratios of BZ over total
2. Experimental section
2.1. Materials and instruments
concentration of BZ and Cu2+ ([BZ]/([BZ]
+
[Cu2+])) in
acetonitrile-water (3:1 v/v) solution, while maintaining the overall
2-Hydrazinobenzothiazole, benzaldehyde, 4-(trifluoromethyl)benz-
aldehyde, 4-formylbenzonitrile, 4-(diethylamino)benzaldehyde, and
glacial acetic acid were purchased from TCI Chemicals; all metal nitrate
salts were purchased from Sigma Aldrich; ethanol (ACS grade) were
purchased from Honeywell. Standard buffer solutions pH 1–12 were
purchased from Merck (glycine/NaCl/HCl for pH 1, Citric acid/NaOH/
HCl for pH 2–6, Na2HPO4/KH2PO4 for pH 7, boric acid/ KCl/NaOH for
pH 8–11, Na3PO4/NaOH for pH 12). All chemicals were used without
further purification. Electrospray mass spectra were obtained from a
Bruker micrOTOF spectrometer. NMR spectra were recorded on a Bruker
NMR 400 and 500 MHz spectrometer at ambient temperature. Chemical
shifts are given in ppm, and are referenced to residual 1H and 13C solvent
signals. The crystallographic measurement was performed using a
Bruker APEX-II CCD area detector diffractometer. UV–vis absorption
and fluorescence spectra were acquired from a Cary Series UV–vis-NIR
spectrophotometer (Agilent Tech, Santa Clara, CA, USA) and a Perkin
Elmer LS55 fluorescence spectrometer, respectively.
concentration of BZ and Cu2+ ([BZ] + [Cu2+]) at 50
μM. The emission
intensities of the samples were recorded at 455 nm for BZ2 and 490 for
BZ3 (excitation wavelength =343 nm for BZ2 and 375 nm for BZ3).
Then, the emission intensities at 455 and 490 nm were plotted against
molar fractions of BZ2 and BZ3 ([BZ2]/([BZ2] + [Cu2+]) and
[BZ3]/([BZ3] + [Cu2+]), respectively.
The binding constant values were determined from emission in-
tensities of BZ2 and BZ3 (25
μ
M) upon the constantly addition of Cu2+
from 0.1 molar equivalent (2.5
(0.1 molar equivalent or 2.5
modified Benesi-Hildebrand Eq. (2) [39]:
μM) to 1 molar equivalent (25 μM)
μM increment). The data was fitted to the
1
1
1
[
]
=
+
(2)
K(Fmax ꢀ Fmin) Cu2+
F ꢀ Fmin
Fmax ꢀ Fmin
where Fmin denotes the emission intensities of benzothiazole-based dyes
in the absence of copper ions, F is the emission intensities of the dyes at
intermediate copper concentrations, Fmax stands for the emission in-
tensities of the dyes at the complete interaction with copper ions, and K
is the binding constant values.
2.2. General details for UV–vis and fluorescence measurements
2.2.1. Preparation of the stock solutions
The stock solutions of BZ compounds was prepared by dissolving
30 mg of BZ with 50 mL of acetonitrile : water (3 : 1) solution in standard
volumetric flasks (~ 2 mM). The stock solution of Cu(NO3)2.3H2O was
prepared by dissolving 30 mg of Cu(NO3)2.3H2O with 25 mL acetonitrile
: water (3 : 1) solution in standard volumetric flasks (5 mM). UV–vis and
fluorescence measurements were performed by taking appropriate
2.5. Theoretical calculation
The ground-state geometry optimization of BZ1, BZ2, and BZ3 and
their complexes with Cu2+ (BZ-Cu2þ) was performed without symmetry
constraints. The Becke’s three-parameter hybrid exchange functional
with the Lee-Yang-Parr gradient-corrected correlation (B3LYP) [40,41]
2

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
Scheme 1. The synthesis of BZ1, BZ2, BZ3, and BZ4.
Fig. 2. The molecular structures of A) BZ2 (CCD No. 2014836) and B) BZ3 (CCD No. 2014835) showing the atom-labeling scheme. Displacement ellipsoids are
drawn at the 50 % probability level. The non-IUPAC atom labeling is for the convenience of discussion. Views of the crystal packing which illustrate C) the part of C
2
–
–
(4) N H⋯N hydrogen bonds (red dash lines) in the extended structure of BZ2 and D) the R2(8) N H⋯N hydrogen bonding loop (red dash lines) between two
inversion related molecules of BZ3.
and the triple-ζ valence quality with one set of polarization functions
(TZVP) [42] were applied for all atoms except Cu atom. To reduce
computational cost, the LANL2DZ effective core potential was applied
for the Cu atom to describe the Cu core electrons. The optimized
structures and electronic property were then examined in implicit
acetonitrile using conductor-like polarized continuum model (C-PCM)
framework [43,44]. The frontier molecular orbitals (FMOs) and
HOMO-LUMO energy diagrams of BZs and BZ-Cu2þ complexes were
also calculated at B3LYP/TZVP level of theory. Simulated UV–vis ab-
sorption spectra and vertical excitation energy (Eex) were carried out
using the time-dependent PBE0 function (TD-PBE0) [45] with the same
basis set in implicit acetonitrile. This selected method was elucidated in
our previous work [46]. All calculations were performed by the
Gaussian 16 program package [47].
ray absorption near edge structure (XANES) and extended X-ray ab-
sorption fine structure (EXAFS) analyses. The XAS measurements pro-
bed at Cu K-edge were performed at Beamline 1.1 W: Multiple X-ray
Techniques, Synchrotron Light Research Institute (SLRI), Thailand. The
XAS measurements were conducted in both transmission and fluores-
cence modes at ambient temperature and pressure by simultaneously
measuring the solutions together with the Cu foil as standard reference
for an in-line alignment of the energy shift during the synchrotron-
operating time. The XAS measurements of Cu(NO3)2.3H2O solution in
acetonitrile : water 3:1 v/v was carried out as standard references for
comparison with the Cu2+-BZ mixtures. The obtained data were
analyzed using ATHENA and ARTEMIS software.
3. Results and discussion
3.1. Synthesis and crystal structures of benzothiazole derivatives
2.6. X-ray absorption spectroscopy (XAS) of the mixtures between Cu2+
and BZ ligands
Novel benzothiazole derivatives featuring electron withdrawing
groups (EWG), including, trifluoromethyl (BZ2) and cyano (BZ3) and
electron donating group, such as, diethylamino (BZ4) were synthesized
by a straightforward and environmental-friendly method as shown in
Scheme 1. The known benzothiozole containing phenyl ring (BZ1) was
To examine the structural information of the binding between the BZ
ligands and the target Cu2+ ions as well as their complexes, the mixtures
between BZ ligands and Cu(NO3)2.3H2O in acetonitrile : water 3:1 v/v
were characterized by X-ray absorption spectroscopy (XAS) in both X-
3

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
Fig. 3. A-C) Absorption spectra of BZ derivatives (25
μ
M) before and after addition of 1 molar equivalent of Cu2+ (25
μM). D-F) Emission spectra of BZ derivatives (25
μ
M) before and after addition of 1 molar equivalent of Cu2+ (25
μM).
also synthesized to use as a reference compound to study the effect of
electron tuning substituents toward photophysical properties of the
dyes. [48] Their NMR characterizations as well as mass spectrometry
results are shown in the supporting information (SI) section.
3.2. Photophysical properties
Firstly, the photophysical properties of BZ1¡3 upon copper ion
complexations were investigated to probe the effect of electron with-
drawing substituents. As seen in Fig. 3A-C, the absorption spectra of all
Furthermore, the structures of BZ2 and BZ3 were confirmed by
single crystal X-ray diffraction. Crystal data and their corresponding
refinement details of the compounds BZ2 and BZ3, are tabulated in
Table S1 and the structural models referring their crystal structures are
displayed in Fig. 2A-B. The intermolecular interactions within the
crystals of BZ2 and BZ3 are displayed in Fig. 2C-D and listed in
BZ derivatives (25 μM) exhibited the decrease in absorption intensities
at 333 nm for BZ1, 344 nm for BZ2 and 360 nm for BZ3 upon the
addition of Cu2+ (25
μM, 1 molar equivalent) which were similar to
those of the pyrene analogue reported in the literature. [36] They also
showed the bathochromic shifts of 10 and 5 nm for BZ1 and BZ2 and no
Table S2. The N H⋯N hydrogen bonds were observed in the crystal
shift for BZ3 upon Cu2+ binding (25
μM, 1 molar equivalent). These
–
–
structures of both compounds. The freely-refined N H bond distances
results initially confirmed the copper binding for all BZ derivatives. It is
also important to note that the absorption maxima of free BZ2 and BZ3
demonstrated bathochromic shifts of 11 nm and 26 nm compared with
that of BZ1, respectively (Figure S20) due to electron-withdrawing ef-
fects from the –CF3 and –CN groups. These results indicated that the
electronic transition energy levels of these BZ and BZ-Cu2þ complexes
could be tuned by varying electron-withdrawing substituents.
are 0.89(2) Å and 0.85(2) Å for BZ2 and BZ3, respectively. Although the
crystal system and space group of both compound crystals were similar,
BZ2 and BZ3 possessed the different hydrogen bonding fashions. In the
–
case of BZ3, the N H⋯N hydrogen bonds linked two molecules related
by an inversion center into a dimer creating an R22(8)-loop motif
–
(Fig. 2D). Unlike BZ3, the molecules of BZ2 were joined by the N H⋯N
hydrogen bonds creating an infinite C(4) chains propagating along the
[010] direction (Fig. 2C and Figure S15). In addition, the molecular
In the emission spectra, BZ1, BZ2, and BZ3 displayed increased
fluorescent intensities upon the addition of Cu2+ (Fig. 3D-F). Similar to
the observation in absorption spectra, the emission maxima of BZ2-
Cu2þ and BZ3- Cu2þ showed red shifts of 23 nm and 58 nm from that of
BZ1-Cu2þ resulting blue and green fluorescence emission, consecutively
planarity, the π-π interactions, and the Hirshfeld surface (HS) analysis of
the crystal structures are further explained in the SI section.
Fig. 4. Proposed sensing mechanism for electron-withdrawing substituted benzothiazoles upon copper ion binding.
4

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
Fig. 5. Absorption, emission spectra, and stokes shifts of BZ-Cu2+ complexes.
Fig. 6. A) Copper ion complexation reaction of BZ4, B) Absorption spectra of BZ4 (25
μ
M) before and after addition of 1 molar equivalent of Cu2+ (25
μM), C)
Emission spectra of BZ4 (25
μ
M) before and after addition of 1 molar equivalent of Cu2+ (25
μM).
(Figure S21). These fluorescence turn-on phenomena could be the re-
sults from chelation-induced enhanced fluorescence (CHEF) effect and
inhibited C = N isomerization leading to the rigid backbone (Fig. 4). In
addition, the intramolecular charge transfer (ICT) process is facilitated
over the molecules causing large stokes’ shifts in BZ-Cu2þ complexes
(Fig. 5) suggesting strong ICT effect as seen in other ICT-based fluores-
cent molecules in the literature. [49–51]
3.3. Sensing properties toward copper ion
The selectivity of BZ2 and BZ3 toward various metal cations was
investigated by mixing them with 5 equivalents of Cu2+, Ca2+, Mg2+
,
,
Co2+, Ni2+, Mn2+, Zn2+, Cd2+, Pb2+, Al3+, Fe3+, Cr3+, Ag+, and Hg2+
respectively. Their selective fluorescence enhancements by copper ions
were observed with a strong blue and green fluorescence emissions
(Fig. 7A). The fluorescence spectra of BZ2 and BZ3 in the presence of
various metal ions were shown in Figure S22 and S23 in the SI section.
Subsequently, the effects of interference of the above-mentioned cations
on Cu2+ sensing were also studied. The emission intensities in Fig. 7B
suggested that the co-existence of these metal ions does not interfere
with Cu2+detection in this solvent system. It is also interesting to note
that the co-presence of Hg2+ with Cu2+ could slightly enhance fluores-
cence signals in both BZ2 and BZ3.
To prove that the ICT mechanism plays a vital role in these fluores-
cence ‘turn-on’ processes, the copper ion binding of non-ICT benzo-
thiazole containing electron-donating diethylamino substituent (BZ4)
was investigated. As displayed in Fig. 6, the BZ4 underwent fluorescence
quenching phenomena upon copper complexation due to paramagnetic
effect. The BZ4-Cu2þ showed the lowest stokes’ shifts in this series
(76 nm) indicating weak ICT upon copper complexation. These results
confirmed that the fluorescence ‘turn-on’ processes in BZ1¡3 de-
rivatives upon Cu2+ binding were caused by the synergistic effect be-
tween CHEF and ICT processes. As BZ2 and BZ3 are new compounds
that possess superior ICT phenomena, they were selected for further
studies as copper ion fluorescent sensors.
Next, Cu2+ titrations with BZ2 and BZ3 (25
μM) were investigated by
both absorption and emission techniques. As demonstrated in Fig. 8A-B,
when Cu2+ was constantly added from 0.1 molar equivalent (2.5
μM) to
1 molar equivalent (25
μ
M) of Cu2+ into the solution of 1 molar
equivalent of BZ compounds (25 μM) (0.1 molar equivalent or 2.5 μM
5

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
Fig. 7. A) Images of BZ2 and BZ3 (25 μM) before and after addition of various metal ions (5 equiv., 125 μM) in acetonitrile-water (v/v = 3:1). B) Bar chart
demonstrating fluorescence intensities at 455 nm for BZ2 and 490 nm for BZ3 in the presence of different metal ions (selectivity test) and in the presence of Cu2+
(125 μM) co-existing with other metal ions (125 μM) (interfering test).
Fig. 8. Changes in the absorption spectra of BZ2 (A), BZ3 (B) and emission spectra of BZ2 (C), BZ3 (D) (excitation wavelength =343 nm for BZ2 and 375 nm for
BZ3) in the presence of various molar equivalents of Cu2+ in acetonitrile-water (3:1 v/v) solutions (final concentration of BZ compounds = 25
μM).
increment), the absorption intensities at 344 and 360 nm gradually
decreased while the emission intensities at 455 nm and 490 nm gently
increased for BZ2 and BZ3, respectively. There were no significant
changes in absorption and emission intensities observed after the addi-
between these BZ ligands and Cu2+. The photophysical properties,
including, extinction coefficients (ε), fluorescence quantum yields (Φf),
of BZ2 and BZ3 before and after addition of Cu2+ were summarized in
Table 1.
tions of 2–5 molar equivalents (50–125
μ
M) of Cu2+ to the solution of
The binding stoichiometry of BZ-Cu2+ complexes was further
confirmed by Job’s plot analysis. As display in Fig. 9A-B, the emission
1 molar equivalent of BZ compounds suggesting 1:1 binding ratios
6

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
intensities at 455 nm and 490 nm were plotted against molar fractions of
BZ2 and BZ3 ([BZ2]/([BZ2] + [Cu2+]) and [BZ3]/([BZ3] + [Cu2+]),
while maintaining the total concentrations of BZ compound and Cu2+
Table 1
Photophysical properties of BZ2 and BZ3 in in acetonitrile-water (3:1 v/v) so-
lution (1
Cpd.
μ
M) before and after addition of Cu2+
.
([BZ] + [Cu2+]) at 50
μM. The maximum emission intensities of both
Absorption
Emission
Emission with Cu2+[a]
compounds were observed at the mole fractions of 0.5, indicating 1:1
[b]
λabs
ε
(Mꢀ 1 cmꢀ 1
)
λemiss
Φf
λemiss
Φf[c] (n = 3)
binding ratios of BZ2-Cu2+ and BZ3-Cu2+ complexes. [38] The de-
BZ2
BZ3
344
360
3.4 × 104
NF[d]
455
490
0.40 (± 0.025)
0.42 (± 0.03)
tections of molecular ions at m/z
= 481.0509 {[(BZ2)Cu(NO3)
3.1 × 104
NF
(H2O)2]+} and 411.0895 {[(BZ3-H)Cu(H2O)4]+} from electrospray
ionization mass spectra (ESI-MS) of the complexes served as the addition
proofs of the 1:1 binding stoichiometry of BZ2 and BZ3 with Cu2+
(Figure S13-S14).
[a]
[b]
1.5
μM of Cu(NO3)2.3H2O was added.
excitation wavelength =343 nm for BZ2 and 375 nm for BZ3.
[c]
fluorescence quantum yields estimated by using quinine sulfate in 0.1 M
H2SO4 as a standard (Φf = 0.58).
The binding constants (K) of BZ2 and BZ3 were determined based on
Benesi-Hildebrand Eq. (2) by plotting 1/(F-Fmin) vs. 1/[Cu2+] as depic-
ted in Fig. 10A-B. The data was linearly fitted, and the K values were
calculated as 2.18 × 104 and 3.30 × 104 Mꢀ 1 for BZ2 and BZ3,
respectively. These K values are comparable to that of compound IV and
[d]
NF = non-fluorescence.
Fig. 9. Job plots for BZ2 (A) and BZ3 (B) in acetonitrile-water (3:1 v/v).
Fig. 10. Binding plots generated from the analysis of fluorescence enhancements of BZ2 (A) and BZ3 (B) upon addition of Cu2+, and fitted in accordance with Eq.(2),
and standard calibration curves demonstrating the fluorescence intensity increment at 455 nm for BZ2 (C) and 490 nm for BZ3 (D) as a function of copper con-
centrations. Fmin = Emission intensities of the benzothiazole-based dyes in the absence of copper ions.
7

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
compounds in acetonitrile/water media.
Table 2
Comparison of BZ2 and BZ3 with other reported benzothiazole-based sensors for
copper ion detection.
Sensor
3.4. Theoretical calculations
Solvent
system
LOD
Binding
Sensing
Reference
(ppb)
Constant (K)
(binding mode:
Cu2+: ligand)
mechanism
To further understand the electronic and photophysical properties of
BZ and BZ-Cu2þ complexes, the DFT calculations were preformed using
B3LYP/TZVP level of theory in the implicit solvent of acetonitrile and
the LANL2DZ basis set for Cu2+. The optimized structures as well as the
calculated highest occupied molecular orbital (HOMO), lowest unoc-
cupied molecular orbital (LUMO) energy levels, and the HOMO-LUMO
energy gap of the BZ1¡3 and BZ1¡3-Cu2þ are presented in Fig. 12.
Comparing among free BZ ligands, the HOMO-LUMO energy gaps
decreased from BZ1 to BZ3 (ΔE = 3.96, 3.76, and 3.54 eV, for BZ1, BZ2,
and BZ3, respectively), as the electron withdrawing groups (-CF3, and
-CN) could stabilize the LUMO levels effectively. The LUMO isosurface
diagrams of BZ2 and BZ3 demonstrated larger electron distributions on
CF3- and CN-substituted phenylene indicating weak ICT in these mole-
cules. When Cu2+ interacted with nitrogen donor atoms in the ICT-based
compounds, such as BZ2 and BZ3, the ground states of BZ2-Cu2þ and
BZ3-Cu2þ are more stabilized than the excited state leading to larger
energy gaps of BZ2-Cu2þ and BZ3-Cu2þ comparing to those of BZ2 and
BZ3 [49]. These computational results are consistent with the experi-
mental outcomes that the BZ1 displayed the highest red shift upon Cu2+
binding (Fig. 3A-C). The changes of HOMO and LUMO diagrams in
BZ2-Cu2þ and BZ3-Cu2þ showed distinct electron density moves
distributing from benzothiazole units in the HOMOs to CF3- and
CN-connected phenylene groups in the LUMOs, respectively, suggesting
a strong ICT effect in these complexes.
I
Acetonitrile-
water (3:1 v/
v)
2.73
5.00 × 108 Mꢀ 2
Turn-on
[36]
μM
(1:2)
II
Acetonitrile
0.014
N.R.
Turn-off
Turn-off
[32]
[34]
μ
M
III
Acetonitrile-
water (4:1 v/
v)
1.05
2.92 × 1010
μM
Mꢀ 2 (2:1)
IV
V
DMSO-water
(2:98v/v)
Acetonitrile-
water (1:1 v/
v)
2.4 nM
7.78 × 104 Mꢀ 1
(1:1)
Turn-off
Turn-off
[31]
[33]
0.36
5.70 × 106 Mꢀ 1
(1:1)
μM
VI
Acetonitrile-
water (9:1 v/
v)
N.R.
5.06 × 106 Mꢀ 1
Turn-off
Turn-on
Turn-on
[35]
(1:1)
BZ2
BZ3
Acetonitrile-
water (3:1 v/
v)
0.77
2.18 × 104 Mꢀ 1
This work
This work
μM
(1:1)
Acetonitrile-
water (3:1 v/
v)
0.64
3.30 × 104 Mꢀ 1
μM
(1:1)
N.R. = Not Report.
slightly lower than those of compound V and VI in the literature
(Table 2). Next, the limits of detection (LOD) of these chemosensors
were determined from the plots of emission intensities (F-Fmin)/Fmin and
Next, the UV–vis spectra of BZ1, BZ2, BZ3, BZ1-Cu2þ, BZ2-Cu2þ
,
and BZ3-Cu2þ were calculated using time-dependent density functional
theory (TD-DFT) calculations in an acetonitrile medium. The simulated
absorption peaks obtained from TD-PBE0/TZVP level of theory
(Figure S24-S29) agreed well with the experimentally obtained peaks as
seen in Table 3. The simulated results by this TD-PBE0/TZVP method
indicated that the electronic transitions of BZ1¡3 are predominantly
contributed from HOMO-LUMO transitions with up to 99 % contribu-
tions while the electronic transitions of BZ1-Cu2þ, BZ2-Cu2þ, and BZ3-
Cu2þ complexes are mainly contributed from the HOMO-LUMO and the
HOMO-LUMO+1 transitions.
Cu2+ concentrations as demonstrated in Fig. 10C-D by employing 3
σ/m
formula, where
σ is the standard deviation of fluorescence signals of
sensors in the absence of Cu2+ and m is the slopes of the plots. The LODs
of BZ2 and BZ3 were calculated as 0.77 and 0.64 μM, respectively.
Compared with the LODs of other benzothiazole-based sensors in the
literature, the LODs of BZ2 and BZ3 were lower than those of pyrene-
linked benzothiazole (sensor I) and calix [4]arene appended benzo-
thiazole (Sensor III) (Table 2).
The effect of pH toward Cu2+ binding of BZ2 and BZ3 was investi-
gated in acetonitrile-standard buffer solution (3:1 v/v) in the pH range
of 1ꢀ 12. As displayed in Fig. 11, these BZ compounds worked well in
neutral pH condition (pH = 7) as they demonstrated distinct fluorescent
intensity change upon Cu2+ binding. This result suggested the suitable
pH value for the Cu2+ ion detection in real water samples using these
probes. There were no florescence signals observed at the acidic pHs (pH
1–6) with and without the presence of copper ion which could be the
results from the protonations at the imine and the nitrogen atom in the
five-membered ring as seen in the literature. [28] These protonations
could block both electron conjugation and metal binding of BZ
3.5. Revealing structural binding of BZ ligands with targeted Cu2+ by XAS
XAS measurements probed at the Cu K-edge were carried out in both
XANES and EXAFS regions to identify the structural information of the
binding between the BZ ligands and the targeted Cu2+ ions. XANES
spectra (Fig. 13A) reveals the significantly different fingerprint patterns
of the equimolar BZ1-Cu2þ, BZ2-Cu2þ, and BZ3-Cu2þ mixtures in
acetonitrile : water of 3:1 v/v from the one of Cu(NO3)2.3H2O solution in
the same solvent system. Moreover, EXAFS data of the BZ-Cu2þ
Fig. 11. Fluorescence intensity variation at maximum emission of 455 nm for BZ2 (A) and 490 nm for BZ3 (B) at media alternation of pH from 1 to 12.
8

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
Fig. 12. Frontier molecular orbitals (HOMO and LUMO) of BZ1, BZ1-Cu2þ, BZ2, BZ2-Cu2þ, BZ3, and BZ3-Cu2þ and energy gaps (ΔE in eV) computed by B3LYP/
TZVP level of theory. The HOMO and LUMO orbitals are plotted with isovalue of 0.02.
complex consisting of 6 H2O ligands coordinated to Cu2+ in octahedral
Table 3
geometry. According to the structural optimization based on DFT
calculation, the 4-coordinated non-centrosymmetric BZ-Cu2þ com-
Comparison of calculated UV-vis absorption wavelength (λcal), experimental UV-
vis absorption wavelength (λexp), excitation energies (Eex), oscillator strengths
plexes are suggested. The EXAFS fittings using these structural models
(f), and molecular orbital contributions (MOs) of BZ1, BZ2, BZ3, BZ1-Cu2þ
,
indicate the well-matched of the 4-coordinated non-centrosymmetric
BZ-Cu2þ structural models with the experimental data (Figure S30
and Table S3). In all, XANES and EXAFS data of the BZ-Cu2þ mixtures
exhibit the binding between Cu2+ and the BZ ligands with a 1:1 ratio in a
4-coordinated non-centrosymmetric fashion.
BZ2-Cu2þ, and BZ3-Cu2þ. The calculation obtained from TD-PBE0/TZVP level
of theory in implicit acetonitrile.
Electronic and photophysical properties
Compound
λcal
λexp
(nm)
Eex
f
MOs (% Contributions)
(nm)
(eV)
BZ1
339
355
376
350
333
344
360
343
3.65
3.49
3.30
3.54
1.0466
1.0040
1.0565
0.5752
HOMO→LUMO (99%)
HOMO→LUMO (99%)
HOMO→LUMO (99%)
HOMO→LUMO (47 %),
HOMO→LUMO+1 (49 %)
HOMO→LUMO (49 %),
HOMO→LUMO+1 (46 %)
HOMO→LUMO (37 %),
HOMO→LUMO+1 (41 %)
4. Conclusions
BZ2
BZ3
In summary, we have synthesized new Schiff base ligands containing
the benzothiazolyl hydrazone acting as a metal ion binding site and an
electron donor moiety and the electron withdrawing trifluoromethyl
and cyano substituted phenylene as an electron acceptor. These de-
rivatives exhibited selective fluorescence ‘turn-on’ responses toward
copper ion binding due to the synergistic phenomena between the
chelation-enahnced fluorescence (CHEF) and the intramolecular charge
transfer (ICT) processes. The job plots, Benesi-Hildebrand analysis and
the XAS analysis confirmed 1:1 binding mode between copper ion and
the ligands with the binding constants of 2.18 × 104 Mꢀ 1 and 3.30 × 104
BZ1-Cu2þ
BZ2-Cu2þ
BZ3-Cu2þ
357
372
349
360
3.48
3.33
0.5222
0.4397
mixtures shown in k-space (Fig. 13B) and R-space (Fig. 13C) exhibit the
deviation of oscillation pattern and the radial distance distribution from
the aqua-based Cu2+ complex. This observation clearly confirms that the
targeted Cu2+ are bound to the BZ ligands and form the BZ-Cu2þ
complexes. To get insight into the structural information of the BZ1-
Cu2þ, BZ2-Cu2þ, and BZ3-Cu2þ complexes, the detailed analysis of
XANES and EXAFS fitting using the structural model obtained from DFT
calculation are further examined (Figure S30 and Table S3). The pro-
nounced pre-edge peak at about 8980 eV reflects the quadrupole
allowed 1s-3d transitions, which reveals a non-centrosymmetric envi-
ronment of Cu2+ due to the 3d-4p orbital mixing. [52,53] Note that, the
pre-edge at 8980 eV is not observed in the case of aqua-based Cu2+
M-1 for BZ2 and BZ3, respectively. The low limits of detections (0.77
μM
for BZ2 and 0.64
μ
M for BZ3), their Cu2+ binding capability at the
physiological pH and their interference studies suggested the potential
of these compounds for Cu2+ analysis in real water samples, such as, tap
water, drinking water, and natural water. The density functional theory
(DFT) and the time-dependent density functional theory (TD-DFT) cal-
culations indicated strong accepting ability of trifluoromethyl and cyano
group leading to lower HOMO-LUMO energy gaps and bathochromic
shifts in BZ2 and BZ3 compared with the non-substituted phenylene
derivative (BZ1). The electron density movements from the HOMOs to
9

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
Fig. 13. XANES spectra probed at Cu K-edge of the mixtures of BZ1-Cu2þ, BZ2-Cu2þ, and BZ3-Cu2þ in acetonitrile : water 3:1 v/v comparing with Cu(NO3)2.3H2O
solution and the standard Cu foil (A) and the corresponding EXAFS data plotted in k-space (B) and the corresponding Fourier-transformed R-space from the k range of
3 – 7.5 Åꢀ 1 (C).
the LUMOs in BZ2-Cu2þ and BZ3- Cu2þ as well as their large stokes
[5] M. Saleem, M. Rafiq, M. Hanif, M.A. Shaheen, S.-Y. Seo, A brief review on
shifts confirmed the strong ICT process after Cu2+ binding which play
fluorescent copper sensor based on conjugated organic dyes, J. Fluoresc. 28 (1)
(2018) 97–165.
vital role in fluorescence ‘turn-on’ mechanism.
[6] S. Chowdhury, B. Rooj, A. Dutta, U. Mandal, Review on recent advances in metal
ions sensing using different fluorescent probes, J. Fluoresc. 28 (4) (2018)
999–1021.
CRediT authorship contribution statement
[7] K.P. Carter, A.M. Young, A.E. Palmer, Fluorescent sensors for measuring metal ions
in living systems, Chem. Rev. 114 (8) (2014) 4564–4601.
[8] E.V. Antina, N.A. Bumagina, A.I. V’Yugin, A.V. Solomonov, Fluorescent indicators
of metal ions based on dipyrrоmethene platform, Dye. Pigment. 136 (2017)
368–381.
Jukkrit Nootem: Methodology, Investigation, Validation, Writing -
original draft. Rathawat Daengngern: Investigation, Validation,
Writing - review & editing. Chanchai Sattayanon: Formal analysis,
Writing - review & editing. Worawat Wattanathana: Formal analysis,
Writing - review & editing. Suttipong Wannapaiboon: Formal analysis,
Writing - review & editing. Paitoon Rashatasakhon: Resources, Su-
pervision. Kantapat Chansaenpak: Conceptualization, Methodology,
Resources, Supervision, Project administration, Funding acquisition,
Writing - review & editing.
[9] A. Torrado, G.K. Walkup, B. Imperiali, Exploiting polypeptide motifs for the design
of selective Cu(II) ion chemosensors, J. Am. Chem. Soc. 120 (3) (1998) 609–610.
[10] G. Klein, D. Kaufmann, S. Schürch, J.-L. Reymond, A fluorescent metal sensor
based on macrocyclic chelation, Chem. Commun. (6) (2001) 561–562.
´
[11] M. Boiocchi, L. Fabbrizzi, M. Licchelli, D. Sacchi, M. Vazquez, C. Zampa, A two-
channel molecular dosimeter for the optical detection of copper(ii), Chem.
Commun. (15) (2003) 1812–1813.
[12] Y. Zheng, J. Orbulescu, X. Ji, F.M. Andreopoulos, S.M. Pham, R.M. Leblanc,
Development of fluorescent film sensors for the detection of divalent copper, J. Am.
Chem. Soc. 125 (9) (2003) 2680–2686.
[13] B.C. Roy, B. Chandra, D. Hromas, S. Mallik, Synthesis of new, pyrene-containing,
metal-chelating lipids and sensing of cupric ions, Org. Lett. 5 (1) (2003) 11–14.
[14] S.H. Kim, J.S. Kim, S.M. Park, S.-K. Chang, Hg2+-Selective OFFꢀ ON and Cu2+-
Selective ONꢀ OFF type fluoroionophore based upon cyclam, Org. Lett. 8 (3)
(2006) 371–374.
Declaration of Competing Interest
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.
[15] J. Chan, S.C. Dodani, C.J. Chang, Reaction-based small-molecule fluorescent
probes for chemoselective bioimaging, Nat. Chem. 4 (12) (2012) 973–984.
[16] K. Chansaenpak, A. Kamkaew, O. Weeranantanapan, K. Suttisintong,
G. Tumcharern, Coumarin probe for selective detection of fluoride ions in aqueous
solution and its bioimaging in live cells, Sensors 18 (7) (2018).
Acknowledgments
[17] Y. Dong, R. Fan, W. Chen, P. Wang, Y. Yang, A simple quinolone Schiff-base
containing CHEF based fluorescence ‘turn-on’ chemosensor for distinguishing Zn2
+ and Hg2+ with high sensitivity, selectivity and reversibility, J. Chem. Soc.
Dalton Trans. 46 (20) (2017) 6769–6775.
This work has been financially supported by the Research Network of
NANOTEC (RNN) program of the National Nanotechnology Center
(NANOTEC), NSTDA, Ministry of Higher Education, Science, Research,
and Innovation, Thailand (Grant No. P1851658) and Thailand Devel-
opment and Promotion of Science and Technology Talents Project (DPST
Research Grant 007/2559). Synchrotron Light Research Institute,
Thailand is also acknowledged for the provision of XAS beamtime at the
Beamline 1.1 W (Multiple X-ray Techniques, MXT).
[18] S.R. Bhatta, B. Mondal, G. Vijaykumar, A. Thakur, ICT–isomerization-Induced turn-
on fluorescence probe with a large emission shift for mercury ion: application in
combinational molecular logic, Inorg. Chem. 56 (19) (2017) 11577–11590.
[19] V. Kumar, A. Kumar, U. Diwan, Ramesh Shweta, S.K. Srivastava, et al.,
Salicylideneimines as efficient dual channel emissive probes for Al3+: harnessing
ESIPT and ICT processes, Sens. Actuators B Chem. 207 (2015) 650–657.
[20] P. Sontisiri, P. Yingyuad, P. Thongyoo, A highly selective “Turn On” fluorescent
probe based on FRET mechanism for hydrogen sulfide detection in living cells,
J. Photochem. Photobiol. A: Chem. 391 (2020), 112401.
[21] S. Goswami, K. Aich, S. Das, C. Das Mukhopadhyay, D. Sarkar, T.K. Mondal, A new
visible-light-excitable ICT-CHEF-mediated fluorescence ‘turn-on’ probe for the
selective detection of Cd2+ in a mixed aqueous system with live-cell imaging,
J. Chem. Soc. Dalton Trans. 44 (12) (2015) 5763–5770.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2021.
113318.
[22] C. Chang, F. Wang, J. Qiang, Z. Zhang, Y. Chen, W. Zhang, et al., Benzothiazole-
based fluorescent sensor for hypochlorite detection and its application for
biological imaging, Sens. Actuators B Chem. 243 (2017) 22–28.
[23] X. Gong, X. Ding, N. Jiang, T. Zhong, G. Wang, Benzothiazole-based fluorescence
chemosensors for rapid recognition and “turn-off” fluorescence detection of Fe3+
ions in aqueous solution and in living cells, Microchem. J. 152 (2020), 104351.
[24] D. Musib, M.K. Raza, S.S. Devi, M. Roy, A reversible, benzothiazole-based “Turn-
on” fluorescence sensor for selective detection of Zn2+ ions in vitro, J. Chem. Sci.
132 (1) (2020) 43.
References
[1] J.A. Cowan, Inorganic Biochemistry: An Introduction, Wiley-VCH, New York,
1997.
[2] V. Desai, S.G. Kaler, Role of copper in human neurological disorders, Am. J. Clin.
Nutr. 88 (3) (2008), 855S-8S.
[25] Y. Chen, T. Wei, Z. Zhang, T. Chen, J. Li, J. Qiang, et al., A benzothiazole-based
fluorescent probe for ratiometric detection of Al3+ in aqueous medium and living
cells, Ind. Eng. Chem. Res. 56 (43) (2017) 12267–12275.
[3] K. Ghosh, D. Kar, Anthraquinone coupled benzothiazole-based receptor for
selective sensing of Cu2+, J. Incl. Phenom. Macrocycl. Chem. 77 (1) (2013) 67–74.
[26] C. Chang, F. Wang, T. Wei, X. Chen, Benzothiazole-based fluorescent sensor for
ratiometric detection of Zn(II) ions and secondary sensing PPi and its applications
´
[4] Y. Zheng, K.M. Gattas-Asfura, V. Konka, R.M. Leblanc, A dansylated peptide for the
selective detection of copper ions, Chem. Commun. (20) (2002) 2350–2351.
10

J. Nootem et al.
Journal of Photochemistry & Photobiology, A: Chemistry 415 (2021) 113318
for biological imaging and PPase catalysis assays, Ind. Eng. Chem. Res. 56 (31)
[40] A.D. Becke, Density-functional thermochemistry. III. The role of exact exchange,
J. Chem. Phys. 98 (7) (1993) 5648–5652.
(2017) 8797–8805.
[27] L. Wang, X. Chen, Q. Xia, R. Liu, J. Qu, Deep-red AIE-Active fluorophore for
hypochlorite detection and bioimaging in live cells, Ind. Eng. Chem. Res. 57 (23)
(2018) 7735–7741.
[41] P.J. Stephens, F.J. Devlin, C.F. Chabalowski, M.J. Frisch, Ab initio calculation of
vibrational absorption and circular dichroism spectra using density functional
force fields, J. Phys. Chem. 98 (45) (1994) 11623–11627.
¨
[28] V. Kachwal, I.S. Vamsi Krishna, L. Fageria, J. Chaudhary, R. Kinkar Roy,
R. Chowdhury, et al., Exploring the hidden potential of a benzothiazole-based
Schiff-base exhibiting AIE and ESIPT and its activity in pH sensing, intracellular
imaging and ultrasensitive & selective detection of aluminium (Al3+), Analyst 143
(15) (2018) 3741–3748.
[42] A. Schafer, H. Horn, R. Ahlrichs, Fully optimized contracted Gaussian basis sets for
atoms Li to Kr, J. Chem. Phys. 97 (4) (1992) 2571–2577.
[43] M. Cossi, V. Barone, Time-dependent density functional theory for molecules in
liquid solutions, J. Chem. Phys. 115 (10) (2001) 4708–4717.
[44] P. Ordon, A. Tachibana, Investigation of the role of the C-PCM solvent effect in
reactivity indices, J. Chem. Sci. Bangalore (Bangalore) 117 (5) (2005) 583–589.
[45] C. Adamo, V. Barone, Toward reliable density functional methods without
adjustable parameters: the PBE0 model, J. Chem. Phys. 110 (13) (1999)
6158–6170.
[29] K. Chauhan, P. Singh, B. Kumari, R.K. Singhal, Synthesis of new benzothiazole
Schiff base as selective and sensitive colorimetric sensor for arsenic on-site
detection at ppb level, Anal. Methods 9 (11) (2017) 1779–1785.
[30] C. Sravani, A. Sivaramakrishna, Benzothiazole-based sensors for protons and
chromium(III) ions, ChemistrySelect. 2 (20) (2017) 5688–5694.
[31] B. Gu, L. Huang, W. Su, X. Duan, H. Li, S. Yao, A benzothiazole-based fluorescent
probe for distinguishing and bioimaging of Hg2+ and Cu2+, Anal. Chim. Acta 954
(2017) 97–104.
[46] J. Nootem, C. Sattayanon, S. Namuangruk, P. Rashatasakhon, W. Wattanathana,
G. Tumcharern, et al., Solvatochromic triazaborolopyridinium probes toward ultra-
sensitive trace water detection in organic solvents, Dye. Pigment. 181 (2020),
108554.
[32] K. Wang, W. Feng, Y. Wang, D. Cao, R. Guan, X. Yu, et al., A coumarin derivative
with benzothiazole Schiff’s base structure as chemosensor for cyanide and copper
ions, Inorg. Chem. Commun. 71 (2016) 102–104.
[47] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
et al., Gaussian 16 Rev. B.01 (2016). Wallingford CT.
[48] A.C. Pinheiro, M.V.N. de Souza, M.C.S. Lourenço, C.F. da Costa, T.C. Baddeley, J.
N. Low, et al., Synthesis, potent anti-TB activity against M. Tuberculosis ATTC
27294, crystal structures and complex formation of selected 2-arylidenehydrazi-
nylbenzothiazole derivatives, J. Mol. Struct. 1178 (2019) 655–668.
[49] S. Sahana, G. Mishra, S. Sivakumar, P.K. Bharadwaj, 2-(2′-Hydroxyphenyl)-
benzothiazole (HBT)-terpyridine conjugate: a highly specific ICT based fluorescent
probe for Zn2+ ions and its application in confocal cell imaging, J. Photochem.
Photobiol. A: Chem. 351 (2018) 231–239.
[33] Y.B. Wagh, A. Kuwar, S.K. Sahoo, J. Gallucci, D.S. Dalal, Highly selective
fluorimetric sensor for Cu2+ and Hg2+ using a benzothiazole-based receptor in
semi-aqueous media and molecular docking studies, RSC Adv. 5 (56) (2015)
45528–45534.
[34] S. Erdemir, B. Tabakci, M. Tabakci, A highly selective fluorescent sensor based on
calix[4]arene appended benzothiazole units for Cu2+, S2ꢀ and HSO4ꢀ ions in
aqueous solution, Sens. Actuators B Chem. 228 (2016) 109–116.
[35] N. Kaur, J. Singh, G. Dhaka, R. Rani, V. Luxami, Benzothiazole-based chemosensor
for CNꢀ and Cu2+: multi-logic operations within a single molecule, Supramol.
Chem. 27 (7-8) (2015) 453–459.
[50] M. Zhu, Y. Xu, L. Sang, Z. Zhao, L. Wang, X. Wu, et al., An ICT-based fluorescent
probe with a large Stokes shift for measuring hydrazine in biological and water
samples, Environ. Pollut. 256 (2020), 113427.
[36] H.-F. Wang, S.-P. Wu, A pyrene-based highly selective turn-on fluorescent sensor
for copper(II) ions and its application in living cell imaging, Sens. Actuators B
Chem. 181 (2013) 743–748.
[51] X. Ren, F. Zhang, H. Luo, L. Liao, X. Song, W. Chen, Red-emitting boron difluoride
complexes with a mega-large stokes shift and unexpectedly high fluorescence
quantum yield, Chem. Commun. 56 (14) (2020) 2159–2162.
[37] K. Chansaenpak, H. Wang, M. Wang, B. Giglio, X. Ma, H. Yuan, et al., Synthesis and
evaluation of [18F]-Ammonium BODIPY dyes as potential positron emission
tomography agents for myocardial perfusion imaging, Chem. Eur. J. 22 (34) (2016)
12122–12129.
[52] G. Aquilanti, M. Giorgetti, M. Minicucci, G. Papini, M. Pellei, M. Tegoni, et al.,
A study on the coordinative versatility of new N,S-donor macrocyclic ligands:
XAFS, and Cu2+ complexation thermodynamics in solution, J. Chem. Soc. Dalton
Trans. 40 (12) (2011) 2764–2777.
[38] Y. Li, X. Han, Y. Song, An azo-phenol derivative probe: colorimetric and “turn-on”
fluorescent detection of copper(ii) ions and pH value in aqueous solution, RSC Adv.
7 (33) (2017) 20537–20541.
[53] J.E. Hahn, R.A. Scott, K.O. Hodgson, S. Doniach, S.R. Desjardins, E.I. Solomon,
Observation of an electric quadrupole transition in the X-ray absorption spectrum
of a Cu(II) complex, Chem. Phys. Lett. 88 (6) (1982) 595–598.
[39] Li C-r, Qin J-c, Wang G-q, Wang B-d, Z.-y. Yang, A novel pyrazine derivative as a
“turn on” fluorescent sensor for the highly selective and sensitive detection of Al3
+, Anal. Methods 7 (8) (2015) 3500–3505.
11