Synthesis, application and AIE properties of novel fluorescent tetraoxocalix[2]arene[2]triazine: The detection of a hazardous anion, cyanate

Article abstract of DOI:10.1016/j.tet.2020.131647

A highly effective, new heterocalixarene fluorescent receptor comprised of 2-(2-aminophenyl)benzothiazole and tetraoxacalix[2]arene[2]triazine was designed and synthesized by one-step reaction. The sensor candidate exhibiting aggregation induced emission (AIE) was tested for its photophysical behaviour towards detection of various anions. The results showed that our receptor undergo AIE in >40% H₂O-DMSO along with large pseudo Stokes shift (Δλ = 219 nm) and exhibit selective and sensitive detection towards hazardous cyanide's oxidation product, cyanate (CNO^-) ion over other tested anions. The blue-shifted fluorescence emission (λ_em = 492 nm) enhancement with large Stokes shift (Δλ = 144 nm) was observed with the increase in cyanate concentration. The synthesized turn-on sensor towards cyanate detection could be applied in real sample analyses as an improvement to the method currently carried out by international standards and hereby a different approach has been made for the detection of cyanide through its oxidation form, cyanate.

Full text of DOI:10.1016/j.tet.2020.131647

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Synthesis, Application and AIE Properties of Novel Fluorescent
Tetraoxocalix[2]arene[2]triazine: The Detection of a Hazardous Anion, Cyanate
Selahattin Bozkurt, Erkan Halay
PII:
S0040-4020(20)30854-1
https://doi.org/10.1016/j.tet.2020.131647
TET 131647
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 17 July 2020
Revised Date: 19 September 2020
Accepted Date: 21 September 2020
Please cite this article as: Bozkurt S, Halay E, Synthesis, Application and AIE Properties of Novel
Fluorescent Tetraoxocalix[2]arene[2]triazine: The Detection of a Hazardous Anion, Cyanate,
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Synthesis, Application and AIE Properties of Novel Fluorescent
Tetraoxocalix[2]arene[2]triazine: The Detection of a Hazardous Anion,
Cyanate
Selahattin Bozkurta, b, *, Erkan Halaya,c
aScientific Analysis Technological Application and Research Center, Usak University, 64200 Usak, Turkey
bVocational School of Health Services, Usak University, 64200 Usak, Turkey
cDepartment of Chemistry and Chemical Processing Technologies, Banaz Vocational School, Usak University, 64500 Usak,
Turkey
Abstract- A highly effective, new heterocalixarene fluorescent receptor comprised of 2-(2-
aminophenyl)benzothiazole and tetraoxacalix[2]arene[2]triazine was designed and
synthesized by one-step reaction. The sensor candidate exhibiting aggregation induced
emission (AIE) was tested for its photophysical behaviour towards detection of various
anions. The results showed that our receptor undergo AIE in > 40% H2O-DMSO along with
large pseudo Stokes shift (∆λ = 219 nm) and exhibit selective and sensitive detection towards
hazardous cyanide’s oxidation product, cyanate (CNO¯ ) ion over other tested anions. The
blue-shifted fluorescence emission (λem = 492 nm) enhancement with large Stokes shift (∆λ =
144 nm) was observed with the increase in cyanate concentration. The synthesized turn-on
sensor towards cyanate detection could be applied in real sample analyses as an improvement
to the method currently carried out by international standards and hereby a different approach
has been made for the detection of cyanide through its oxidation form, cyanate.
Keywords: Thiazole derivatives, calix[2]arene[2]triazine, aggregation-induced emission
(AIE), anion recognition, cyanide, cyanate, fluorescence spectroscopy.
____________________________________________________________________________________
*Corresponding author. Tel.: +90 276 2212121/4800.
E-mail address: selahattin.bozkurt@usak.edu.tr
1

1. Introduction
There are huge amount of chemical species contributing to pollution of soil and water
of industrial region while many of these have toxic effects on the environment [1, 2]. Among
these species, imaging of bio-relevant ions like cyanide (CN¯ ) has gained increasing
importance because of being one of the most toxic/hazardous chemical species to human
health and the ecosystem even at very low concentrations established by World Health
Organization (WHO) as a permissible concentration of 1.9 µM [3–6]. CN¯ has the ability of
disordering some important biological processes like inhibition of cellular respiration by
binding to the active site of cytochrome oxidase enzyme and reducing the oxidative
metabolism [5, 7–9]. Although the detection of CN¯ is vital, it poses a variety of risks during
its detection due to its extreme toxicity. Instead, it is safer to detect the less toxic cyanate
(CNO¯ ) which is the oxidized product of CN¯ [10, 11].
As a whole structure, the chemical sensor function via different recognition processes,
and among them, chelation enhanced fluorescence (CHEF), excited-state intramolecular
proton transfer (ESIPT) and in particular aggregation induced emission (AIE) have the merit
of wide spectrum range analysis for sensing [12–8]. Since such anionic species are involved
in a variety of chemical processes along with biological and environmental monitoring,
interaction mechanisms between recognition and reporting units and/or host and guests have
been attracting much interest for the anion specific receptor development [19, 20].
Concordantly, some interactions especially noncovalent ones (NCI) related to aromatic rings
such as anion–π and lone-pair electron–π are of importance for describing the supramolecular
architectures and relevant interactions [21–24].
Based on these facts; various detection techniques for cyanide have been in use till
now such as voltammetric, amperometric, titrimetric, chromatographic and
spectrophotometric methods [25]. Among which, the development of fluorescence
spectrophotometry have had a lot of promise in light of the sensing advantages like fast
response, real-time visualization and much higher sensitivity [3, 8, 26]. Within this context,
fluorescent chemosensors/receptors are the molecules that can interact with the guest targets
like hazardous species mentioned above and convert this recognition event into the observable
optical signal outputs at very low concentrations as well. Among the studies performed in this
direction, designing of highly selective and sensitive sensors for ion recognition along with
high light emitting ability in the aggregate form has been of importance for the
2

supramolecular chemistry researchers working with spectrofluorometric technique [27–30].
Therefore, compounds containing pyrene, bodipy, fluorescein, benzimidazole, benzoxazole or
benzothiazole moiety have been studied extensively due to their excellent fluorescent property
being useful in fluorescent detection [31–36]. Amongst them, by means of having both proton
donor and acceptor groups in close proximity, such heterocyclic compounds as the entitled
benzothiazole derivatives show efficient fluorescence emission through excited-state
intramolecular proton transfer (ESIPT) character and provide wide spectrum range analysis
for sensing [37]. In other respects, for ESIPT, which is known to have an abnormal (large)
Stokes shift feature and also known to emit different wavelengths of enol and keto tautomers
separately, it is clear that there should be hydrogen transfer in the structure [15, 38, 39]. In
this context, since the benzothiazole moiety conjugated to the triazine-based heterocalixarene
skeleton contains hydrogen acceptor and donor groups, it is obvious that it will have ESIPT
characteristics by forming enol-keto tautomerism and reveal two seperate emission bands
with low and high energy [28, 40].
With the support of this information, supramolecular chemosensors have been
designed by systematic optimization of their electronic structure by addition of functional
groups to achieve desired properties, which in turn are favourable for the detection of ions. In
this regard, calixarene macrocyclic compounds have been commonly employed as structural
frameworks to construct these receptors thanks to their easy modifiability and changeable
binding core providing such opportunities as tuning the ring size, conformation and binding
properties of the macrocycle [30, 41–45].
In order to prepare new heterocyclic system with fluorescent applications and keeping
all the facts mentioned above in our observance, in the present work, we report a new
heterocalixarene backbone with benzothiazole group for the fluorescence based detection of
cyanate anion via ESIPT mechanism and with the support of AIE phenomenon. We hope that
our findings will be of great value for the researchers working on the design of new calixarene
sensor candidates. Additionally, considering the fact that our results are reasonable for the
real sample analyses, we believe that it is inevitable for the synthesized fluoroionophore to
find application in this field.
3

2. Result and discussion
2.1 Design, synthesis and characterization of novel macrocyclic sensor
In this work, considering the fact that our heterocalixarene macrocycle plays a role as
a pair of tweezers in the anion recognition process through electrostatic interactions along
with benzothiazole moiety, our approach about the fluorescent chemosensor construction is
based on the direct combination of receptor and fluorescence active signalling units without
any linkers by one-step reaction. This is because, it is known that the high-sensitive
compounds that would be used to determine anions and/or cations need to be fluorescent and
carry hydrogen donor and acceptor groups together in close proximity within their structures.
In this sense, our sensor candidate compound 3 features two separate effects on itself for the
anion recognition works in terms of providing hydrogen acceptor and donor groups via
triazine units in heterocalixarene skeleton and also showing fluorescence with benzothiazole
reporter groups. Moreover, to the best of our knowledge, calix[2]arene[2]triazine skeleton
will be the first in the fluorescence recognition studies.
As part of benzothiazole appended novel tetraoxacalix[2]arene[2]triazine derivative,
synthesis of the heterocalixarene scaffold by the reaction of cyanuric chloride with resorcinol
has previously been described [26]. Following that, in order to obtain our sensor candidate,
substitution reaction between heterocalixarene scaffold (as a receptor part) and 2-(2-
aminophenyl) benzothiazole fluorophore (as a signalling part) was carried out using
diisopropylethylamine (DIPEA) as base in acetone under reflux and the target compound 3
was synthesized with moderate yield (65%) (Scheme 1).
4

Scheme 1. Synthetic pathway of our sensor candidate as compound 3; i: DIPEA, acetone, r.t.;
ii: DIPEA, acetone, reflux.
The structure of the synthesized fluorescence compound 3 was characterized by a
1
1
combination of FT-IR, H, 13C NMR and mass spectroscopic techniques. The H NMR
spectrum of 3 exposed signals at 12.63 ppm for NH protons, while aromatic protons
distributed between 6.71 and 8.92 ppm. Additionally, the 13C NMR signals of those aromatic
carbons were observed between 172.5 and 116.4 ppm.
Photophysical properties of compound 3 were studied through absorption and
emission spectroscopic techniques. First of all, in order to examine photophysical properties
in the most appropriate way, we tried to determine the fluorescence responses of our
chemosensor candidate 3 in different solvents. In this sense, it was designated that while the
compound 3 soluble in dimethyl sulphoxide (DMSO) or tetrahydrofuran (THF), it was
insoluble in water and convenient organic solvents such as hexane etc. Additionally, although
compound 3 did not emit florescence in DMSO or THF (1.88 × 10-5 M), turbidity was
observed along with increasing emission when % fraction of water was increased step by step
(Fig. 1). As seen, the compound 3 persisted its low emission up to 60% water, while this low
emission was maintained up to 40% water in DMSO. After the incremental addition of H2O
5

ratio up to 95%, fluorescent intensity was increased. During the work, with the assistance of
spectrometry, we determined absorption maxima of compound 3 (1.88 x 10-5 M in
DMSO:H2O 5:95) at 348 nm. And upon excitation of this sensor candidate 3 at 348 nm, a
broad emission at 567 nm was exhibited with large pseudo Stokes shift of 219 nm (Fig. 2a).
Moreover, it was specified that the fluorescence intensity was increased by 31 fold (for
DMSO) and 21 fold (for THF) at the 95% fraction of water when compared to the non-
aqueous solution as shown in Fig. 2b. Consequently, as the similar works in literature [46–
49], these types of compounds having the properties mentioned above show AIE
(aggregation-induced emission) characteristics and so our sensor candidate 3 is identified as
an AIE compound.
Fig. 1. Photographs of compound 3 in (a) THF–H2O and (b) DMSO–H2O mixtures with different water
fractions (fw) taken under fluorescence lamb
6

a)
b)
Fig. 2. a) Normalized absorption and fluorescence spectra of compound 3 in 95% water b) Emission spectra of
compound 3 in DMSO–H2O mixtures
2.2 Fluorescence behaviour in the precense of various anions
Fluorescence behavior of compound 3 has investigated for various anions such as
¯
¯
fluoride (F¯ ), chloride (Cl¯ ), bromide (Br¯ ), iodide (I¯ ), chlorate (ClO3 ), perchlorate (ClO4 ),
¯
¯
nitrate (NO3 ), bromate (BrO3 ), cyanide (CN¯ ) and cyanate (CNO¯ ). When 5 eq of those ions
are added to compound 3, it shows very strong emission with cyanate, although no
fluorescence emission was observed with other anions. The solution of the sensor 3 and
cyanate showed green emission with emission maxima at 492 nm having large Stokes shift of
144 nm (Fig. 3).
7

Fig. 3. Fluorescence emission responses of compound 3 containing cyanate and other anions
In order to examine the detection e ciency between the sensor 3 and cyanate anion
(CNO¯ ), the titration of 3 with an increasing amount of cyanate (0–14 equivalents) was
monitored by fluorescence spectrophotometry (Fig. 4a). As a matter of fact, the fluorescent
titration curve of the sensor 3 towards cyanate showed a good linear relationship between the
values of 0-75 µM.
The limit of detection (LOD) was determined from the fluorescence titration data [10,
20, 50–53]. On the purpose of gaining the slope, the fluorescent intensity data at 492 nm was
plotted as cyanate concentration (Fig. 4b) Thus the detection limit was calculated through the
following equation:
Limit of Detection = 3σ / K
8

where σ is the standard deviation of blank measurement, and K is the slope between the
fluorescence intensity versus cyanate concentration. The obtained LOD for cyanate with the
value of as low as 0.42 µM has revealed a good sensitivity.
a)
b)
Fig. 4. a) Fluorescent spectra of Compound 3 (1.88 × 10−5 M in DMSO:H2O; 40:60) solution in the presence of
di erent equivalents of cyanate b) a plot of fluorescent intensity of compound 3 in the presence of
di erent equivalents of cyanate (0–75 µM)
As it is included in the literature, while high emission band is belonging to the low-
energy keto form, low emission band formation belonging to high-energy enol form
demonstrates a typical ESIPT character [54–57] (Fig. 5).
Fig. 5. Demonstration of keto/enol dual emissive tautomers with hypsochromic shift
9

1
The H NMR titrations in DMSO-d6 were carried out for further investigation of
cyanate sensing mechanism of compound 3. As seen in Fig. 6, along with the addition of 2
equiv of CNO¯
, the intensity of signal at 11.74 ppm belonging to the NH proton of compound
3 decreased first and the peak shifted to lower field (11.81 ppm). Followingly, upon
CNO¯
incremental addition of
ion (10 equiv), this NH signal disappeared obviously. In
addition to these, meanwhile, the proton signals in aromatic area such as 8.39, 8.20 and 7.66
ppm shifted to the lower field slightly with the chemical shift value of 8.49, 8.26 and 7.72,
respectively. It was easy to find out that the disappearance of the NH signal along with these
signal changes demonstrated the deprotonation of compound 3.
Fig. 6. 1H NMR spectra of compound 3 (1x10-2 M) upon addition of (a) 0 equiv (b) 2 equiv (c) 10 equiv of
CNO¯ ions in DMSO-d6
In order to evaluate the detection of the cyanate ion in the presence of competing
anions, cyanate was interacted with sensor 3 in the presence of numerous anions such as
¯
¯
fluoride (F¯ ), chloride (Cl¯ ), bromide (Br¯ ), iodide (I¯ ), chlorate (ClO3 ), perchlorate (ClO4 ),
10

¯
¯
nitrate (NO3 ), bromate (BrO3 ). As shown in Fig. 7, significant changes in fluorescence
intensity in the presence of related competing ions were not seen, thus, it was understood that
those ions did not interfere in the detection process of cyanate. In this context, compound 3
allows the fluorometric detection of cyanate anion which is less harmfull than cyanide, and it
has appeared to be a good candidate for the analytical detection of cyanate, hence cyanide.
Fig. 7. Fluorescent spectra of compound 3 (1.88 × 10−5 M in DMSO:H2O; 40:60) solution with other anions in
the presence cyanate anion
3. Conclusion
In summary, in the development of this work, the synthesis of benzothiazole derived, a
novel oxo-bridged calix[2]arene[2]triazine compound 3 was carried out in moderate yield,
which could be used as an efficient AIE fluorescent sensor in DMSO-H2O solution. In the
fluorescence experiments conducted, it is observed that the compound 3 has a large pseudo
Stokes shift (219 nm), besides, the fluorescence intensity increased by 31 fold for the DMSO
solution with highly water content (95%). Although compound 3 does not detect one of the
most dangerous anions, cyanide, it has been shown that it can be used to detect cyanate anion
which is an oxidation form of cyanide. Besides, it was determined that all competing anions
11

found abundantly in the water did not make any significant interference in the detection
process. Simultaneously, upon addition of cyanate in DMSO-H2O medium, the compound 3
showed an immediate enhanced fluorescence response with a clear color change from yellow
emission to stronger green emission.
4. Experimental Section
4.1 General Procedures
Melting points were determined using a Gallenkamp Electrothermal melting point
apparatus in open glass capilleries and are uncorrected. Infrared spectroscopy was performed
on a Perkin-Elmer Spectrum Two FT-IR spectrophotometer equipped with ATR apparatus.
Nuclear magnetic resonance experiments were performed on an Varian 400 MHz
spectrometer in deuterated chloroform (CDCl3). Chemical shifts (δ) are reported in parts per
million using tetramethylsilane (TMS) as the standart. Fluorescence spectra were recorded on
an Agilent Cary Eclipse Fluorescence Spectrophotometer. Emission spectra were corrected by
the spectrometer software. Quartz cuvettes with path length of 1 cm were used. While
precoated silica gel 60 F254 aluminium plates were used for thin layer chromatography (TLC),
230 × 400 mesh silica gel was used for flash chromatographic studies. Commercially
available chemicals were reagent grade and purchased from Merck and Sigma Aldrich.
4.2. Synthesis of compound 3
Oxo-bridged calix[2]arene[2]triazine 1 was prepared according to the described
methods [26, 58]. Oxo-bridged calix[2]arene[2]triazine
1
(1 mmol) and
diisopropyl(ethyl)amine (DIPEA) (2.2 mmol) were dissolved in THF (60 mL) under nitrogen
atmosphere, the solution was cooled to 0 °C and 2-(2-aminophenyl)benzothiazole 2 (2.2
equiv) in THF (20 mL) was added dropwise. The reaction mixture was allowed to stir at 0 °C
for 6 hours. It was then refluxed for 48 h. The reaction mixture was filtered and the solvent
was removed in vacuo to afford crude residue. After that, the residue redissolved in CH2Cl2,
washed successively with 0.5 M HCl (50 mL), brine (50 mL) and water. The organic layer
was dried over MgSO4 and the solvent evaporated under reduced pressure. The crude product
was purified by flash chromatography on silica gel (Hexane:EtOAc) to afford Compound 3 as
solids. White solid, Yield 65%, mp: 268–271 °C, FTIR (ATR)
ν
cm–1: 1122 (C-O-C), 1361
12

(C-N), 1539 (C=C), 1611 (C=N); 1H-NMR (400 MHz, CDCl3): 1H NMR (400 MHz, CDCl3)
δH (ppm) 12.65 (s, 2H, NH), 9.02 – 8.82 (m, 2H), 8.25 (d, J = 8.8 Hz, 2H), 7.98 – 7.86 (m,
4H), 7.62 – 7.49 (m, 4H), 7.49 – 7.41 (m, 2H), 7.33 – 7.15 (m, 4H), 6.98 – 6.80 (m, 4H), 6.72
(d, J = 2.2 Hz, 2H); 13C-NMR (100 MHz, CDCl3) δC (ppm) 172.6, 167.6, 152.4, 151.4,
137.2, 133.4, 131.7, 130.3, 126.6, 125.8, 123.5, 123.4, 121.8, 121.2, 120.3, 118.6, 116.5;
HRMS (ESI-TOF) calcd for C44H27N10O4S2 [M+H]+ 823,1658, found 823.1676.
4.3 Preparation of solutions of anions
The stock solutions of various anions such as fluoride (F¯ ), chloride (Cl¯ ), bromide
¯
¯
¯
¯
(Br¯ ), iodide (I¯ ), chlorate (ClO3 ), perchlorate (ClO4 ), nitrate (NO3 ), bromate (BrO3 ) of
concentration 1.5 × 10−3 M were prepared in water from their corresponding inorganic salts as
tetra butyl ammonium or sodium. The stock solutions were diluted to desired concentrations
with the respective solvent system when needed.
For the preparation of CNO¯
anion solution, three separate solutions (Chloramine T,
Phosphate Buffer and Cyanide) are needed to prepare according to the standart method (S.M.)
4500-CN [59].
chloramine T solution as first, 84 mg of chloramine T was
In order to prepare
dissolved in 100 mL of water. Likewise, 1.38 g of NaH2PO4.H2O was dissolved in 100 mL of
water as the second solution. And lastly, 0.251 g KCN and 0.2 g KOH were dissolved in 100
mL of water as the third solution. Followingly, 3.9 mL of cyanide (third solution) was
pippetted and diluted to 100 mL with 0.04M NaOH to obtain approximately 1.5 x 10-3 M
cyanide solution. Subsequently, to this solution, 0.1 mL chloramine T and 0.1 mL phosphate
buffer were added to get the desired concentration of
¯ . In this way, in the presence of
CNO
¯
¯
chloramine T in alkaline medium,
CN is available in the form of its oxidizing product, CNO
.
Acknowledgements
The authors are grateful to the Usak University Research Foundation (2018/SB002)
for the financial support. The authors also gratefully acknowledge UBATAM (Usak
University, Scientific Analysis Technological Application and Research Center) for analyses.
13

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Highlights
V Novel fluorescent sensor based on heterocalixarene skeleton was developed.
V The fluorescent detection fulfilled with ESIPT mechanism and AIE phenomenon
V Detection of cyanate with high sensitivity and selectivity in aqueous solution.
V A new and different approach was made for the safer detection of cyanide.

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: