Simple benzothiazole chemosensor for detection of cyanide anions via nucleophilic addition

Article abstract of DOI:10.1007/s10593-017-2019-7

[Figure not available: see fulltext.] A benzothiazolium salt containing carbazole group has been synthesized in a good yield via simple condensation reaction. Its photophysical properties and affinity for cyanide anion (tetrabutylammonium cyanide) have been investigated. The compound exhibited quick and well visible UV/Vis absorption and fluorescence responses to cyanide anion in MeCN, as the yellow color faded and the orange fluorescence disappeared. The in situ analysis by 1H NMR indicated that the cyanide anion was added to the C=N bond in the benzothiazolyl group.

Full text of DOI:10.1007/s10593-017-2019-7

DOI 10.1007/s10593-017-2019-7
Chemistry of Heterocyclic Compounds 2017, 53(1), 42–45
Simple benzothiazole chemosensor for detection
of cyanide anions via nucleophilic addition
Xueying Yu1, Kangnan Wang1, Duxia Cao1*, Qianqian Wu1,
Ruifang Guan1, Yongxiao Xu1, Yatong Sun1, Zhiqiang Liu2
1
School of Material Science and Engineering, Shandong Provincial Key Laboratory
of Preparation and Measurement of Building Materials, University of Jinan,
336 Nanxinzhuang West St., Jinan 250022, China; e-mail: duxiacao@ujn.edu.cn
2
State Key Laboratory of Crystal Materials, Shandong University,
27 Shanda South St., Jinan 250100, China; e-mail: zqliu@sdu.edu.cn
Published in Khimiya Geterotsiklicheskikh Soedinenii,
2017, 53(1), 42–45
Submitted July 2, 2016
Accepted after revision September 8, 2016
A benzothiazolium salt containing carbazole group has been synthesized in a good yield via simple condensation reaction. Its photo-
physical properties and affinity for cyanide anion (tetrabutylammonium cyanide) have been investigated. The compound exhibited quick
and well visible UV/Vis absorption and fluorescence responses to cyanide anion in MeCN, as the yellow color faded and the orange
1
fluorescence disappeared. The in situ analysis by H NMR indicated that the cyanide anion was added to the C=N bond in the
benzothiazolyl group.
Keywords: benzothiazole, carbazole, cyanide, chemosensor, nucleophilic reaction.
Fluorescent chemosensors have been developed to
identify important anions, with the aim of achieving high
sensitivity and selectivity.1–5 Among the numerous
important anions, cyanide (CN−) has attracted much
attention because of its high toxicity. Cyanide traces can
cause acute symptoms of poisoning, such as vomiting,
convulsion, loss of consciousness, and even death.6–8
Despite the toxicity, cyanides are important raw materials
for the manufacturing of synthetic resins,9 pharma-
ceuticals,10 pesticides,11 and in other industrial areas.12–16
Meanwhile, cyanides are also used to dissolve gold and
silver ores in the mining industry,17 used in electroplating,18
production of plastics,19 and other industrial processes.20
Thus, it has been an important task of chemists to develop
highly selective chemosensors for cyanide anions.21–25
In our earlier work, series of fluorescent chemosensors
for cyanide anions based on the ability of cyanide anion to
undergo nucleophilic addition was reported.26–31 Here we
describe a structurally simple and easily synthesized
benzothiazole-based chemosensor for the detection of CN−
based on nucleophilic addition. This chemosensor exhibits
remarkable response to CN− with obvious color and
fluorescence changes, enabling simple visual detection.
The title compound 1 was synthesized by straight-
forward approach using a condensation reaction between
N,2-dimethylbenzothiazolium iodide and N-ethylcarbazole-
3-aldehyde (Scheme 1) with a yield of 87%. N,2-dimethyl-
benzothiazolium iodide was synthesized by the reaction
between N,2-dimethylbenzothiazole and iodomethane with
MeOH as solvent.31
Scheme 1
Linear absorption and steady-state fluorescence emission
spectra of compound 1 in various solvents of different
polarity are shown in Figure 1. The compound exhibited
strong absorption in the blue region and strong orange
fluorescence with high quantum yield (Φ 0.12 in MeCN).
As shown in Figure 1, the UV/Vis absorption peaks
showed little dependence on the solvent polarity, but the
choice of solvents visibly affected the fluorescence
emission intensity of compound 1, which was clearly
enhanced by increase in solvent polarity.
0009-3122/17/53(01)-0042©2017 Springer Science+Business Media New York
42

Chemistry of Heterocyclic Compounds 2017, 53(1), 42–45
Figure 1. UV/Vis absorption and fluorescence emission spectra
of compound 1 in various solvents.
First, the response of compound 1 to the presence of
CN− ions (tetrabutylammonium cyanide, TBACN) was
measured as changes in UV/Vis absorption, which were
fast (<5 s). Compound 1 showed one major absorption band
at 469 nm, which was attributed to intramolecular charge
transfer within the chemosensor from carbazolyl group to
the benzothiazolium group. When cyanide anions were added,
the absorption peak at 469 nm (ε 5.3 × 10−4 M−1 × cm−1)
gradually decreased and disappeared, while new absorption
peaks appeared in the shorter wavelength region, with the
isosbestic point at 326 nm. The color change is shown in
the insert of Figure 2a. The changes in the absorption
spectrum of solution were accompanied by disappearance
of the yellow color, which allowed purely visual detection
of CN−. Compound 1 also exhibited fluorescence spectral
response to cyanide anions. As shown in Figure 2b, the
fluorescence intensity at 600 nm decreased and orange
fluorescence was nearly completely quenched when
3.5 equiv of cyanide anions were added. The color and
fluorescence changes occurred immediately upon the
addition of CN−. The reaction between compound 1 and
CN− was reversible, based on the fact that more than 1 equiv of
cyanide anions were needed for complete absorption
change and fluorescence quenching. The binding constant
was calculated to be 7.53 × 105 M−1 based on absorbance
titration data.32 The detection limit of compound 1 for CN−
ions with the detection of fluorescence and absorbance
were determined according to references.33,34 The detection
limits in MeCN were 4.6 × 10−5 μM (fluorescence) and
0.046 μM (absorbance), respectively. Compared to pre-
vious reports, this chemosensor possesses the following
advantages. First, the synthesis and purification were very
simple. Second, the response time was short (<5 s). Third,
a visible color change from yellow to colourless with
almost complete quenching of fluorescence was observed,
which allowed visual detection of CN−.
Figure 2. a) UV/Vis absorption and b) fluorescence emission
spectra of compound 1 (c 1.0 × 10−5 M) in MeCN upon addition
of different concentrations of CN−.
Figure 3. Job plots for the receptor-cyanide complex in MeCN
upon the addition of CN−.
1H NMR spectra of compound 1 before and after the
addition of cyanide anions were investigated. As shown in
Figure 4, after the addition of cyanide anions, the olefinic
proton signals of compound 1 at 8.40 ppm (Ha, d, J = 15.6 Hz)
and 8.04 ppm (Hb, d, J = 15.6 Hz) were shifted to 4.93 ppm
The stoichiometry of the interaction between receptor 1
and cyanide anion was determined using UV/Vis absorption
spectra. A maximum was observed in the Job plots (Fig. 3)
at 0.5 fraction, indicating the formation of a 1:1 complex.
43

Chemistry of Heterocyclic Compounds 2017, 53(1), 42–45
1
Figure 4. H NMR spectra of compound 1 a) before and b) after the addition of cyanide
anion in DMSO-d6.
(Ha, d, J = 8.8 Hz) and 4.89 ppm (Hb, d, J = 8.8 Hz),
respectively. The signal corresponding to NMe protons was
shifted upfield from 4.37 to 3.21 ppm. Based on the small
coupling constants of olefinic protons (J = 8.8 Hz), it is
probable that compound 2 assumes a cis configuration
upon the addition of cyanide anions.35
The electrophilicity of sp2 carbon atoms toward CN− can
be enhanced by the presence of positively charged nitrogen
atom in the benzothiazolium unit.36 The addition of cyanide
disrupted the intramolecular charge transfer chain in the
compound and resulted in a change of color and
1
fluorescence emission. The obtained H NMR spectrum
and literature data related to analogous compounds36
together allowed us to propose the mechanism for nucleo-
philic addition of cyanide ion to the positively polarized
carbon atom in benzothiazole group.
The selectivity and competitive binding experiments of
compound 1 to other anions and some small peptides as
glutathione (GSH) and amino acids such as cysteine (Cys)
and homocysteine (Hcy) were carried out. The addition of
other anions such as SO42−, SO32−, SCN−, F−, Cl−, Br−, I−,
−
H2PO4 , HS−, and GSH, Cys, and Hcy caused only slight
changes in UV/Vis absorption and fluorescence spectra
(Fig. 5), which indicates good selectivity of compound 1 to
CN−. The aforementioned competing anions and molecules
also did not interfere with the recognition reaction between
compound 1 and CN−.
In conclusion, a new fluorescent chemosensor with high
selectivity and sensitivity for CN− ion was synthesized. The
sensing mechanism may be typical nucleophilic addition
reaction. This chemosensor possesses short responding
time. Visible color change from yellow to colourless with
almost complete fluorescence quenching is observed. We
expect this chemosensor to be useful for detection of
cyanide ion in a range of chemical industry and environ-
mental samples.
Figure 5. The changes of UV/Vis absorption (a, 469 nm) and
fluorescence (b, 600 nm) of compound 1 (c 1.0 × 10−5 M) in the
presence of various anions in response to CN− ions in MeCN.
Experimental
1H and 13C NMR spectra were recorded on a Bruker
Avance III 400 spectrometer (400 and 100 MHz, respecti-
vely) in DMSO-d6 at room temperature with TMS as
internal standard. 1H NMR of compound 1 before and after
44

Chemistry of Heterocyclic Compounds 2017, 53(1), 42–45
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1
Firstly, H NMR spectrum of compound 1 in DMSO-d6
was measured. After 3 equiv TBACN were added, 1H NMR
spectrum of the solution was measured again. UV/Vis
absorption and fluorescence spectra of compound 1 in
MeCN were recorded at room temperature on a Shimadzu
UV2550 spectrophotometer and a Horiba FluoroMax-4
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butylammonium cyanide as CN− source in MeCN at room
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This work was supported by the National Natural
Science Foundation of China (21672130, 51373069) and
the Natural Science Foundation of Shandong Province
(ZR2013BM005, ZR2015BL011), Colleges and Univer-
sities Science and Technology Foundation of Shandong
Province (J16LA08), the Fund of Graduate Innovation
Foundation of University of Jinan, GIFUJN (S1504).
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