Rational design of a colorimetric and fluorescence turn-on chemosensor with benzothiazolium moiety for cyanide detection in aqueous solution

Article abstract of DOI:10.1016/j.saa.2019.117409

A novel colorimetric and fluorescence turn-on chemosensor TBB with benzothiazolium moiety has been explored, which exhibited the high selectivity for cyanide ion (CN^?) in THF–H₂O (2:8, v/v) mixture. The aqueous solution of sensor TBB was scarcely emissive. In the presence of CN^? ion, the nucleophilic addition of CN^? with the benzothiazolium C[dbnd]N bond of TBB produced the new species TBB–CN, consequently resulting in the intense orange–red emission by aggregation-induced emission (AIE) effect. Meanwhile, the color of solution was changed from orange-yellow to light yellow. The sensing mechanism was verified by Mass spectrometry, NMR analysis and DFT calculations.

Full text of DOI:10.1016/j.saa.2019.117409

Accepted Manuscript
Rational design of a colorimetric and fluorescence turn-on
chemosensor with benzothiazolium moiety for cyanide detection
in aqueous solution
Beibei Zhai, Ziwei Hu, Chun Peng, Bing Liu, Wei Li, Chao Gao
PII:
S1386-1425(19)30799-1
DOI:
https://doi.org/10.1016/j.saa.2019.117409
Article
Number:
Reference:
117409
SAA 117409
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
15 February 2019
19 July 2019
20 July 2019
Please cite this article as: B. Zhai, Z. Hu, C. Peng, et al., Rational design of a colorimetric
and fluorescence turn-on chemosensor with benzothiazolium moiety for cyanide detection
in aqueous solution, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy, https://doi.org/10.1016/j.saa.2019.117409
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ACCEPTED MANUSCRIPT
Title: Rational design of a colorimetric and fluorescence turn-on
chemosensor with benzothiazolium moiety for cyanide detection
in aqueous solution
Author names and affiliations:
Name
Country
Affiliations
E-mail
Wuhan Textile University,
Wuhan 430073, Hubei, PR
China
Beibei Zhai
China
1379962526@qq.com
Wuhan Textile University,
Wuhan 430073, Hubei, PR
China
Ziwei Hu
Chun Peng
Bing Liu
Wei Li
China
China
China
China
China
771704605@qq.com
374397673@qq.com
1520352351@qq.com
liwei_whu@sohu.com
chgao@wtu.edu.cn
Wuhan Textile University,
Wuhan 430073, Hubei, PR
China
Wuhan Textile University,
Wuhan 430073, Hubei, PR
China
Wuhan Textile University,
Wuhan 430073, Hubei, PR
China
Wuhan Textile University,
Wuhan 430073, Hubei, PR
China
Chao Gao
Corresponding author:
Name: Chao Gao; Email: chgao@wtu.edu.cn
Permanent address of the corresponding author:
Hubei Key Laboratory of Biomass Fibers and Eco-dyeing & Finishing,
department of Chemistry and Chemical Engineering, Wuhan Textile
University, Yangguang Avenue, Wuhan 430073, Hubei, PR China.

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Abstract: A novel colorimetric and fluorescence turn-on chemosensor
TBB with benzothiazolium moiety has been explored, which exhibited
the high selectivity for cyanide ion (CN−) in THF−H2O (2:8, v/v) mixture.
The aqueous solution of sensor TBB was scarcely emissive. In the
presence of CN− ion, the nucleophilic addition of CN− with the
benzothiazolium C=N bond of TBB produced the new species TBB–CN,
consequently resulting in the intense orange−red emission by
aggregation-induced emission (AIE) effect. Meanwhile, the color of
solution was changed from orange-yellow to light yellow. The sensing
mechanism was verified by Mass spectrometry, NMR analysis and DFT
calculations.
Keywords:
Benzothiazolium;
Cyanide
ion;
Chemosensor;
Aggregation-induced emission.
1. Introduction
Cyanide salts, as a kind of extremely useful materials, have been
widely utilized in many fields including the electroplating, metallurgy,
tanning and plastics production [1-3]. But cyanide ion (CN−) was highly
toxic to mammals, because it could impede the oxidative metabolism
process, destroy the mitochondrial electron transport chain and injure the
central nervous system. Furthermore, it directly caused the death of
human at the concentration of 0.7–3.5 mg/kg of body weight [4,5]. In
accordance with the World Health Organization (WHO), the
concentration of CN− ion in drinking water should be less than 1.9 μM [6].
Therefore, the development of a simple and efficient method for

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monitoring the concentration of CN− ion in environmental and biological
systems was particularly significant.
Among the various detection technologies for CN− ion, fluorescence
analysis has attracted more attentions owing to the advantages of low cost,
convenient use and simple preparation. So far, a plenty of fluorescent
chemosensors with various mechanisms for CN− ion detection have been
reported [7,8]. It was well known that CN− ion was an excellent
nucleophile, and the sensors based on the nucleophilic addition reaction
commonly displayed the high selectivity to CN− ion. Comparing with the
carbonyl groups and electron-deficient alkenes, the benzothiazolium C=N
group was easier attacked by the nucleophilic CN− ion, which endowed
the benzothiazolium-based sensors with the faster response toward CN−
ion [9]. In addition, these sensors could detect CN− ion directly by
naked−eyes due to the color change. In spite of this, they were still
confronted with the problems. Firstly, some of them exhibited the
fluorescence turn-off response, because the nucleophilic addition reaction
of CN− ion with the benzothiazolium moiety interrupted the conjugation
of sensors, and blocked the Intramolecular Charge Transfer (ICT)
processes, consequently resulting in the fluorescence quenching [9−12].
The “turn off” sensors were regarded as unreliable because many other
factors could cause fluorescence quenching [13,14]. Then, the other
sensors
were
based
on
the
pyrene-benzothiazolium,

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naphthopyran-benzothiazolium
or
naphthalimide-benzothiazolium
conjugates. The detection of CN− ion was generally conducted in the
aqueous solutions with 50% volume fraction of volatile and toxic organic
solvents [15−17]. Notably, the above shortcomings significantly restricted
their practical application in CN− ion sensing.
Aggregation-induced emission (AIE) effect was widely used in the
design of fluorescence “turn on” chemosensors [18–20]. The D–A type
triphenylamine–benzothiadiazole conjugates exhibited the long
wavelength emission owing to the push-pull electron process. In which,
the triphenylamine (TPA) group served as the electron donor (D), and the
benzothiadiazole (BT) unit was the electron acceptor (A). In addition,
these conjugates generally possessed the combined twist-intramolecular
charge transfer (TICT) and AIE effects, that the emission intensity was
decreased along with the increase of solvent polarity, but enhanced in the
aggregate state [21]. The cationic benzothiazolium moiety was
hydrophilic, and its introduction into the TPA−BT based luminophors
could improve the water solubility. These new constructed luminophors
were scarcely or non emissive in high polar solvents. In the presence of
CN− ion, its nucleophilic addition reaction to benzothiazolium moiety
destroyed the cationic structure and decreased the water-solubility of
these luminophors. Consequently, the aggregation of new additive species
resulted in the intense emission by AIE mechanism.

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Herein, a novel triphenylamine–benzothiadiazole based sensor TBB
for CN− ion has been developed, which utilized the benzothiazolium
moiety as the recognition group. Sensor TBB was barely emissive in the
THF−H2O (2:8, v/v) mixture. On treatment with CN− ions, the solution
exhibited the intense orange−red emission by AIE effect, and its color
was changed from orange-yellow to light yellow. Comparing with other
similar works, sensor TBB possessed the several advantages (Table S1).
Firstly, TBB displayed the fluorescence turn-on response toward CN– ion.
Secondly, the detection medium contained small amount of organic
solvent (20 vol %). Thirdly, the detection limit of TBB to CN– ion (137
nM) was lower than most other benzothiazolium-based sensors and the
WHO guideline (1.9 μM). Finally, the response time of 3 min was far
shorter than some other similar sensors.
2. Experimental section
2.1. Materials and Instruments
Unless the otherwise stated, the reagents and solvents were purchased
from Energy Chemical (China) CO., Ltd with the guarantee reagent (GR)
grade, and they were used directly without the further purifications.
4-Bromo-7-[4-(diphenylamino)phenyl]-2,1,3-benzothiadiazole
and
3-ethyl-2-methylbenzo[d]thiazol-3-ium bromide were obtained according
to the reported methods [22,23]. The synthetic procedure of compound 1

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was described in the supporting information. Double distilled water was
used in all the experiments. NMR spectra were obtained on the Bruker
1
Ascend 400 MHz Nuclear Magnetic Resonance Spectrometer. The H
NMR (400 MHz) chemical shifts were given using CDCl3 as the internal
13
reference (CDCl3: δ = 7.26 ppm). The C NMR (100 MHz) chemical
shifts were recorded using CDCl3 as the internal standard (CDCl3: δ =
77.16 ppm). High-resolution mass spectra (HRMS) were taken on with a
Bruker-Q-TOF mass spectrometer. Melting points were measured with
SGW X-4 instrument. Absorption spectra were tested on TU-1901
spectrometer. The fluorescence spectra were got with the HITACHI
F-2500 fluorescence spectrometer. The pH measurements were prepared
by Sartorius PB-10 meter. Dynamic light scattering (DLS) experiments
were investigated with Malvern ZS90 particle sizer.
2.2. Synthesis of sensor TBB
A mixture of compound 1 (242 mg, 0.5 mmol) and
3-ethyl-2-methylbenzo[d]thiazol-3-ium bromide (154 mg, 0.6 mmol) in
EtOH (4 mL) was refluxed overnight under N2. After that, the mixture
was concentrated using rotavapor, and the residue was purified by a silica
gel column (DCM/MeOH = 6/1, v/v) to provide TBB as a claret red solid
(191 mg, yield: 53%). m.p. 214.6-216.2 °C. 1H NMR (400 MHz, CDCl3):
δ (ppm) 8.47 (d, J = 15.6 Hz, 1H, vinyl H), 8.26-8.27 (m, 2H, -ArH), 8.17

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(d, J = 15.6 Hz, 1H, vinyl H), 8.04 (d, J = 7.6 Hz, 2H, -ArH),7.88 (d, J =
8.4 Hz, 2H, -ArH), 7.72-7.76 (m, 3H, -ArH), 7.47-7.55 (m, 3H, -ArH),
7.29-7.33 (m, 4H, -ArH), 7.18-7.22 (m, 2H, -ArH), 7.09 (t, J = 7.4 Hz,
13
2H, -ArH), 5.32-5.39 (m, 2H, -CH2), 1.51 (t, J = 7.0 Hz, 3H, -CH3); C
NMR (100 MHz, CDCl3): δ (ppm) 171.93, 154.27, 153.89, 148.57,
147.52, 141.59, 140.89, 134.10, 131.24, 130.51, 130.40, 130.16, 129.87,
129.76, 129.56, 128.91, 128.56, 127.20, 125.25, 125.13, 124.49, 123.69,
122.69, 115.95, 114.94, 113.50, 52.57, 14.98. HRMS (ESI+): m/z calcd.
for C41H31N4S2 [M-Br]+ 643.1985; found 643.1987.
2.3. Photophysical measurements
−
−
−
The sodium salts of anions (F−, Cl−, Br−, I−, H2PO4 , AcO−, N3 , NO3 ,
SO42− and SCN−) and the tetrabutylammonium salt of CN− were used for
the UV-vis absorption spectra and fluorescence spectra measurements.
The selectivity experiments of sensor TBB toward CN− were performed
with the same concentration of various anions. About the titration
experiments, different concentrations of CN− were added into the aqueous
solution of TBB, and then measured the absorption spectra and
fluorescence spectra respectively. The fluorescence intensity was
recorded by fluorescence spectrometer that the slit width was kept at 10
nm.
2.4. The calculation of detection limit

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The detection limit (DL) was calculated according to the common
equation “DL = 3σ/k” that the value of signal-to-noise ratio (S/N) was
regulated at “3” [24]. In which, “σ” was the standard deviation of blank
measurements (8 times detection), and “k” represented the slope between
fluorescence intensity versus the concentration of CN−.
2.5. Theoretical calculations
All the calculations were performed with Gaussian 16 program suite.
The first singlet excited states geometries were optimized by TDDFT
approaches at TD−M062X level, which used H2O as the solvent molecule.
The frontier molecular orbitals and energy levels were calculated by the
TD−B3LYP level.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic route of sensor TBB was showed in Scheme 1. Briefly,
the
condensation
reaction
of
compound
1
with
3-ethyl-2-methylbenzo[d]thiazol-3-ium bromide gave TBB with a
1
moderate yield. The chemical structures of TBB were confirmed by H
NMR, 13C NMR and HRMS.
Scheme 1. The synthetic route of sensor TBB

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3.2. The fluorescence properties of sensor TBB
The reported TPA-BT based conjugates generally possessed the
combined twisted intramolecular charge transfer (TICT) and AIE
characteristics. Their emission intensity was weakened along with the
increase of solvent polarity. In large polar solvents, these compounds
tended to form the twisted charge-separated conformations through the
intramolecular rotation, whose excited-state energy was consumed
non-radioactively, consequently leading to the weak or even no emission.
While in the aggregate state, the intramolecular rotation was restricted,
and the highly twisted molecular conformations hampered the
intermolecular π–π stacking interactions. Thus, the combined effects
resulted in the intense emission. Notably, the TICT possess caused the
weak or no emission in large polar solvents, and AIE effect was
responsible for the intense emission in the aggregate state [25,26]. In
view of this, the fluorescence properties of TBB (10 μM) in various polar
solvents were firstly studied. As shown in Fig. 1, TBB exhibited the
extremely weak emission in all the solvents, including the low polar
toluene, DCM and THF, which was distinctly different from some
reported TPA-BT based derivatives [21,27,28].

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Fig. 1. The fluorescence spectra of sensor TBB (10 μM) in different polar solvents,
and the solutions were excited at their own maxima excitation. Inset: the photographs
of TBB in various solvents were taken under 365 nm light illumination.
3.3. The AIE property of sensor TBB
Subsequently, the AIE property of sensor TBB (10 μM) was
investigated in THF/water mixed solutions with different volume
fractions of water (fw). Similarly with the preceding results, TBB (10 μM)
was scarcely emissive in the pure THF. Upon the increase of fw, the
maximum emission peak had a slight red-shift and the fluorescence
intensity exhibited a mild decrease (Fig. 2), which were caused by the
improvement of solvent polarity [29]. Sensor TBB was able to aggregate
in THF/water mixture until fw was up to 99 vol %. The DLS results
illustrated that the average size of formed nanoaggregates in this solution
was around 350.7 nm (Fig. S1). But the solution still exhibited the
extremely weak emission (Fig. 2). Apparently, TBB didn’t possess the

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AIE activity that exhibited the scarce emission both in dilute solution and
in the aggregate state. Moreover, TBB displayed the faint emission in
TLC plate and solid state, which further certified the non-AIE activity of
TBB (Fig. S2). When the anion of TBB was exchanged to be
-
tetrafluoroborate ion (BF4 ), the obtained solid was also scarcely emissive
(Fig. S2b), thus the scare emission of TBB was not caused by the heavy
atom effect of Br- [28]. In sensor TBB, the BT unit and benzothiazolium
moiety both showed the strong electron−withdrawing ability, which
would cause the intense intramolecular charge transfer and dipole–dipole
interactions, thus leading to the emission quenching in essence [30].
Fig. 2. The fluorescence spectra of sensor TBB (10 μM) in THF/water mixture with
different volume fractions of water (fw). Insets: the photographs of TBB in THF (a)
and THF/water mixture (fw = 99 vol %, b) under 365 nm light illumination.
3.4. Spectroscopic Studies of TBB toward CN−
The sensing behavior of sensor TBB (10 μM) toward CN− in
THF−H2O (2:8, v/v) mixed solution was studied through the UV-vis

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absorption spectra and fluorescence spectra. TBB was well dissolved in
THF−H2O (2:8, v/v) mixture, because the solution had no signal in DLS
test. As shown in Fig. 3, the aqueous solution of TBB exhibited two
absorption peaks at 403 nm and 469 nm, respectively. The DFT
calculations suggested that these two peaks were caused by the π−π*
electronic transition. In the presence of CN− ion (100 μM), the absorption
peaks at 403 nm and 469 nm were disappeared, and a new peak at 431 nm
via π−σ* electronic transition was arisen, implying that the reaction of
CN− ion with TBB was occurred. Meanwhile, the color of solution was
visibly changed from orange-yellow to light yellow. Gratifyingly, the
−
−
−
−
addition of other anions (H2PO4 , AcO−, F−, Cl−, Br−, I−, N3 , NO3 , SO4
and SCN−) caused the negligible change in absorption spectra. Therefore,
sensor TBB was capable of detecting CN− ion by the naked-eyes.
Fig. 3. The UV–vis absorption spectra of sensor TBB (10 μM) in THF−H2O (2:8, v/v)
−
mixed solution with the treatment of 100 μM various anions (CN−, H2PO4 , AcO−, F−,
−
−
−
Cl−, Br−, I−, N3 , NO3 , SO4 and SCN−). The solutions were stirred for 3 min at 32 °C

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before the measurements; Inset: The photographs of TBB in THF−H2O (2:8, v/v)
mixed solution with various anions that taken under sunlight.
Subsequently, the fluorescence response of sensor TBB toward CN−
ion was researched. Sensor TBB (10 μM) exhibited the faint emission in
THF−H2O (2:8, v/v) mixture. Upon the addition of CN− ion (100 μM), an
intense emission at 589 nm was emerged, and the fluorescence intensity
was enhanced almost 7-fold (Fig. 4). Moreover, the DLS data indicated
the aggregation of solution that the average size of nanoaggregates was
around 257.9 nm (Fig. S3). Obviously, the enhancement of fluorescence
intensity was caused by the AIE mechanism. The nucleophilic addition of
CN− ion with TBB produced the new species TBB–CN, whose
aggregation turned on the fluorescence. The addition of other anions
−
−
−
−
(H2PO4 , AcO−, F−, Cl−, Br−, I−, N3 , NO3 , SO4 and SCN−) gave a small
or even no improvement of fluorescence intensity, thus TBB displayed
the high selectivity to CN− ion.

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Fig. 4. The fluorescence spectra of sensor TBB (10 μM) in THF−H2O (2:8, v/v)
−
mixed solution with the treatment of 100 μM various anions (CN−, H2PO4 , AcO−, F−,
−
−
−
Cl−, Br−, I−, N3 , NO3 , SO4 and SCN−), excited at 450 nm. The solutions were
stirred for 3 min at 32 °C before the measurements; Inset: The photographs of TBB in
THF−H2O (2:8, v/v) mixed solution with various anions that taken under 365 nm
light.
For obtaining the detail information on the response of sensor TBB to
CN− ion, the titration experiments were successively performed by the
absorption and fluorescence spectra. Upon the continuous addition of
CN− ion into the THF/H2O (2:8, v/v) mixed solution of TBB (10 μM), the
absorbance at 403 nm and 469 nm was gradually decreased, subsequently
the absorbance of new peak at 431 nm was slightly increased (Fig S4),
which indicated the generation of additive species TBB-CN. Additionally,
the addition of CN− ion obviously enhanced the fluorescence intensity of
solution. Delightfully, a great linear correlation between the fluorescence
intensity at 589 nm and the concentrations of CN− ion was observed (Fig.
5). This linear relationship was mainly caused by the increasing
nanoaggregate numbers of species TBB-CN. According to the reported
formula “DL = 3σ/k”, the detection limit of (DL) sensor TBB toward CN−
ion was calculated to be 1.34×10−7 M, implying that TBB exhibited a
better sensitivity toward CN− than some other benzothiazolium-based
sensors [9,10,12,15,16]. In addition, this detection limit was far lower
than the safe concentration (1.9 µM) of WHO guideline [6], indicating

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that sensor TBB was potentially used to monitor CN− ion in
environmental system. The Job's plot experiment illustrated the 1:1
stoichiometry for the bonding of TBB with CN− ion, and this result was
consistent with the other reports (Fig. S5).
Fig. 5. The fluorescence spectra of sensor TBB (10 μM) in THF−H2O (2:8, v/v)
mixed solution with different concentrations of CN− ion, excited at 450 nm. The
solutions were stirred for 3 min at 32 °C before the measurements; Inset: The plots of
the fluorescence intensity at 589 nm versus the concentrations of CN− ion. In which,
the linear equation was “y = 63.0463x + 939.045”, and R2 = 0.99841.
After that, the time-dependent experiment of sensor TBB toward CN−
ion was recorded (Fig. S6). The fluorescence intensity was increased
dramatically along with the time elapsing, and reached to the maximum
within 3 min, implying that the reaction of CN− ion with TBB was
completed. This response time was distinctly shorter than some reported
benzothiazolium-based sensors [9,12]. Moreover, the pH-dependent
experiment of TBB toward CN− ion was also studied. The fluorescence

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intensity of TBB was barely changed in different pH value (3–10),
suggesting that sensor TBB was greatly stable for pH value (Fig. S7). On
treatment with CN– ions, the fluorescence intensity was dramatically
improved within the pH value from 5 to 9, thus TBB was able to detect
CN– ion in a broad pH range.
In order to evaluate the sensing selectivity of sensor TBB toward CN–
ion, the competitive experiments were performed with 100 μM of various
−
−
−
−
other anions (H2PO4 , AcO−, F−, Cl−, Br−, I−, N3 , NO3 , SO4 and SCN−)
as well as 100 μM of CN− in THF−H2O (2:8, v/v) mixed solution of TBB
(10 μM) (Fig. 6). The competitive anions barely improved the
fluorescence intensity of TBB aqueous solution. Upon the addition of
CN– ion to the solution containing TBB and other anions, the
fluorescence intensity was remarkably enhanced, which was almost
similar to when other competitive anions were absent (Fig. 6). Therefore,
the other anions were negligibly interfered the fluorescence response of
TBB toward CN– ion. The above results implied that sensor TBB was
capable of sensing CN– ion in aqueous solutions even with other
competitive anions.

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Fig. 6. The Histogram about the fluorescence intensity of sensor TBB (10 μM) in
−
THF−H2O (2:8, v/v) mixed solution with 100 μM other anions (H2PO4 , AcO−, F−,
−
−
−
Cl−, Br−, I−, N3 , NO3 , SO4 and SCN−) that before and after treating with CN− (100
μM), excited at 450 nm. The solutions were stirred for 3 min at 32 °C before the
measurements.
3.5. The sensing mechanism
When the reaction of sensor TBB with CN− ion was performed at a
high concentration in THF−H2O mixture, some light yellow precipitate
was generated. After the simple purification, the precipitate structure was
1
13
analyzed by the MS spectrometry, H NMR and C NMR spectra. As
illustrated in Fig. S8, a peak at m/z 670.26 corresponding to the additive
1
product [TBB−CN + H]+ (cal. 670.21) was observed. In the H NMR
spectra of TBB, the peaks at 8.47 and 8.17 ppm were identified as the
vinylic proton H1 and H2 that showed the large coupling constants [23],
which up-field shifted to 5.76 and 4.97 ppm in product TBB−CN.
Besides this, proton signals of the methyl group attached with N (H3) also
displayed the up field shift from 5.35 to 3.61 ppm (Fig. 7). Moreover, the

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13
signals appearing at 171.93 and 52.57 ppm in the C NMR spectrum of
TBB corresponded to the carbons C1 and C2 respectively. While in
product TBB−CN, these two peaks up-field shifted to 72.36 and 43.65
ppm, accompany with the presence of a new peak at 114.93 ppm, which
was considered to be the carbon atom of cyano group [10]. The above
results clearly illustrated that the nucleophilic addition of CN− ion on the
benzothiazolium C=N bond of TBB yielded the additive species
TBB−CN. Unlike TBB, the film of TBB-CN on the glass plate exhibited
an intense emission (Fig S9). Moreover, species TBB-CN displayed the
different aggregation behavior from sensor TBB. The DLS data indicated
the aggregation of TBB-CN (10 μM) in THF−H2O (2:8, v/v) mixed
solution with the average particle size of 293.2 nm (Fig S10), whereas
TBB (10 μM) was only able to aggregate in THF−H2O (1:99, v/v)
mixture. Finally, the solution of TBB-CN exhibited an intense emission
at 589 nm, which was similar with the solution of TBB in THF−H2O (2:8,
v/v) mxiture with CN– ion (Fig S11). Therefore, the nucleophilic addition
reaction of CN− ion with sensor TBB in THF−H2O mixture produced the
compound TBB-CN, whose aggregation gave rise to the intense emission
(Scheme 2).

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1
Fig. 7. The H NMR spectra of sensor TBB (a) and the product from the reaction of
TBB with CN− (b); The 13C NMR spectra of sensor TBB (c) and the product from the
reaction of TBB with CN− (d). The NMR spectra were tested in CDCl3.
Scheme 2. The proposed sensing mechanism of sensor TBB toward CN−
3.6. The DFT studies
To better understand the fluorescence response of sensor TBB toward
CN− ion, the theoretical calculations were performed using the density
functional theory (DFT) method with the TD−B3LYP level of the
Gaussian 16 program. The frontier molecular orbitals and energy levels

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of compounds TBB and TBB−CN were firstly revealed through
time-dependent DFT (TDDFT) calculations, and the isovalue was set at
0.02 (Fig. 8). For TBB, the HOMO level was mainly localized on the
TPA group, and LUMO level distributed on the acceptor moieties such as
BT unit, the adjacent groups and benzothiazolium moiety, respectively.
Such an orbital distribution implied that TBB had an extremely strong
ICT tendency [31], which could quench the fluorescence in essence.
Similarly with TBB, the HOMO level of TBB–CN also distributed on
TPA group. But its LUMO level was primarily localized on the BT unit,
because the nucleophilic addition of CN− to benzothiazolium moiety
destroyed its cationic structure and weakened the electron-withdrawing
ability. In this case, TBB–CN had a larger HOMO–LUMO band gap
(2.5652 eV) than TBB (2.1454 eV), thus its emission wavelength
exhibited a blue-shift, which was consistent with the experimental results.
In addition, the optional structure of TBB–CN in excited state was
studied that using H2O as the solvent molecule (Fig. S12). The BT unit
and the adjacent benzene ring were almost coplanar, but the TPA group
and the additive product of benzothiazolium with CN– were highly
twisted, which was responsible for the intense emission of TBB–CN in
the aggregate state.

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Fig. 8. Frontier molecular orbitals and the energy level diagram of TBB and
TBB−CN. In the optional conformation, the yellow part represents the “S” atom, the
blue part represents the “N” atom, the gray part represents the “C” atom and the white
part represents the “H” atom.
4. Conclusions
In summary, a novel colorimetric and fluorescence turn-on
chemosensor TBB with benzothiazolium moiety for CN− ion has been
explored. Sensor TBB was scarcely emissive in THF/H2O (2:8, v/v)
mixture due to the strong intramolecular charge transfer (ICT) and
dipole–dipole interaction. In the presence of CN− ion, the
nucleophilic addition of CN− ion with benzothiazolium C=N bond of
TBB produced the additive species TBB−CN, whose aggregation
resulted in the intense orange−red emission. Meanwhile, the color of
solution was visually changed from orange-yellow to light yellow. Sensor
TBB exhibited the high selectivity to CN− ion, and the sensing process

ACCEPTED MANUSCRIPT
was completed within 3 min. Furthermore, the detection limit of 137 nM
was far lower than the WHO guideline (1.9 μM).
Acknowledgments
This work was supported by the school foundation of Wuhan Textile
University [grant number 183010]; and the young people fund of Hubei
Education Department [grant number 184043].
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Graphical abstract:
A novel colorimetric and fluorescence turn-on sensor TBB with
benzothiazolium moiety for cyanide ion (CN−) detection in THF−H2O (2:8,
v/v) mixture has been explored.

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Highlights
• A novel chemosensor TBB with benzothiazolium moeity for CN− ion
has been explored.
• Sensor TBB exhibits the high selectivity to CN− ion in aqueous solution.
• The presence of CN− ion results in the color change and fluorescence
turn-on.
• The sensing mechanism is verified by NMR, MS spectrometry and DFT
calculations.