A newly synthesized thiazole derivative as a fluoride ion chemosensor: Naked-eye, spectroscopic, electrochemical and NMR studies

Article abstract of DOI:10.1016/j.saa.2014.02.032

2,3-Indoledione 3-thiosemicarbazone (TSCI) and a novel compound 3-(2-(4-(4-phenoxyphenyl)thiazol-2-yl)hydrazono)indolin-2-one (FTHI) were synthesized with high yield and characterized by spectroscopic techniques. The complexation behaviors of TSCI and FTHI for various anionic species (F ^-, Cl^-, Br^-, I^-, NO2-, NO3-, BzO^-, HSO4-, ClO4-) in CH₃CN were investigated and compared by UV-vis spectroscopy, cyclic voltammetry and ¹H NMR titration techniques. FTHI showed high degree of selectivity for fluoride over other anions. This selectivity could be easily observed by the naked eye, indicating that FTHI is potential colorimetric sensor for fluoride anion.

Full text of DOI:10.1016/j.saa.2014.02.032

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 575–582
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A newly synthesized thiazole derivative as a fluoride ion chemosensor:
Naked-eye, spectroscopic, electrochemical and NMR studies
⇑
Ahmet Burak Sarıgüney, Ahmet Özgür Saf, Ahmet Cosßkun
Chemistry Department, Necmettin Erbakan University, 42099 Meram, Konya, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
New thiazole derivative have been
synthesized and characterized.
Complexation behaviors for various
anionic species investigated.
It is a very good naked eye detection
sensor for the fluoride anion.
a r t i c l e i n f o
a b s t r a c t
Article history:
2,3-Indoledione 3-thiosemicarbazone (TSCI) and a novel compound 3-(2-(4-(4-phenoxyphenyl)thiazol-2-
yl)hydrazono)indolin-2-one (FTHI) were synthesized with high yield and characterized by spectroscopꢁic
techniques. The complꢁexation behaviors of TSCI and FTHI for various anionic species (Fꢁ, Clꢁ, Brꢁ, Iꢁ, NO2 ,
NO3ꢁ, BzOꢁ, HSO4ꢁ, ClO4 ) in CH3CN were investigated and compared by UV–vis spectroscopy, cyclic vol-
tammetry and 1H NMR titration techniques. FTHI showed high degree of selectivity for fluoride over other
anions. This selectivity could be easily observed by the naked eye, indicating that FTHI is potential color-
imetric sensor for fluoride anion.
Received 30 November 2013
Received in revised form 4 February 2014
Accepted 7 February 2014
Available online 22 February 2014
Keywords:
Fluoride
Isatin-thiazole ring
Anion sensor
Ó 2014 Elsevier B.V. All rights reserved.
Voltammetric method
1H NMR titration
Introduction
anion is one of the most interest received special attention as it
plays crucial roles in the dental care and treatment of osteoporosis.
Selective recognition and sensing of anions using neutral mole-
cules as receptors has been one of the most important areas in
chemistry due to the importance of anions in biological, chemical,
industrial and environmental processes [1–3]. Considering the par-
ticular importance of anions, it has been important to fabricate new
anion sensors or develop new techniques for sensing anions. A
number of anion sensors containing different moieties have been
developed [4] for fluoride [5], benzoate [6], hydrogen sulfate (bisul-
fate) [7] and pyrophosphate anions [8]. In particular, the fluoride
However, excess fluoride anion causes several serious diseases such
as bone disorder (fluorosis), collagen breakdown, depression of thy-
roid activity and disruption of immune systems [9,10].
The anion binding mainly occurs through weak hydrogen bond-
ing or electrostatic interactions and thus is usually weaker than cat-
ion binding [11]. In general, the receptors must provide one or more
hydrogen-bonding donor groups such as ANH or AOH. Correct ori-
entation of the hydrogen bond donors and/or acceptors can also
provide selective anion recognition and color changes [12–14]. It
should be noted that anions have a wide variety of geometries
and comparatively large sizes, and this have to be taken into ac-
count in the development of selective anion sensors.
⇑
Corresponding author. Tel.: +90 505 659 5313; fax: +90 332 323 8225.
E-mail address: acoskun42@gmail.com (A. Coßskun).
http://dx.doi.org/10.1016/j.saa.2014.02.032
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

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The class of heterocyclic compounds known as thiazoles and
experiments were made on a Shimadzu UV-1800 Double Beam
Spectrophotometer. All of the electrochemical measurements were
carried out with PAR 263/A2 (Princeton Applied Research, USA)
potentiostat/galvanostat combined with BAS C3 electrochemical
cell stand.
their derivatives are found in many natural and synthetic products.
Different thiazole ring bearing compounds are used in colorimetric
sensor [15,16], fluorescent dye [17,18] and pharmacological [19]
studies. Among the reported colorimetric studies, chemosensors
have attracted considerable attention due to specific color change
by complexation with anions which can be detected by the naked
eye [20,21].
Synthesis
The complexation behaviors of TSCI and FTHI with various an-
ions (Fꢁ, Clꢁ, Brꢁ, Iꢁ, NO2ꢁ, NO3ꢁ, BzOꢁ, HSO4ꢁ, ClO4ꢁ) were investi-
gated by naked-eye, spectrophotometric, voltammetric methods
and 1H NMR titration respectively in this study. FTHI exhibited
selective binding property for Fꢁ among other anions.
Compounds were synthesized according to the route shown in
Scheme 1.
Synthesis of 4-(Phenoxy)Phenacyl Bromide (PPB)
PPB was synthesized according to methods described in the lit-
erature [22].
Material and methods
Reagent and apparatus
Synthesis of TSCI
Isatin (2.1 mmol, 0.30 g) was added dropwise to a stirred hot
solution of thiosemicarbazide (2 mmol, 0.182 g) in the etha-
nol:water (25:10 mL), and then heated to 135 °C at reflux for 5 h,
a precipitate was formed. The reaction was monitored and finished
by TLC. No further purification of the product was necessary. The
precipitate was filtered off and washed with ice cold ethanol
(2 ꢂ 15 mL) and dried in vacuum oven [23].
Unless otherwise noted, chemicals were obtained from global
suppliers (Merck or Aldrich) and were used without further purifi-
cation. Solvents were of HPLC or analytical grade and they were
dried with molecular sieves (3 Å). All anions, in the form of tetra-
butylammonium salts [(n-C4H9)4NA, A = Fꢁ, Clꢁ, Brꢁ, Iꢁ, NO2ꢁ,
NOꢁ3 , BzOꢁ, HSO4ꢁ, ClOꢁ4 ] stored in a desiccator under vacuum con-
taining self-indicating silica, were dried prior to use over P2O5.
Ultrapure water was freshly obtained from Direct-Q3 water purifi-
cation system (Millipore, USA).
TSCI: Yield %72 (0.317 g), m.p. 244–247 °C; 1H NMR (400 MHz,
DMSO-d6, 25 °C), (d: ppm): 12.45 (s, 1H, NHAC@O), 11.19 (s,
1H,@NANHA), 9.04 and 8.68 (s, 2H, ANH2), 7.63 (d, J = 7.55, 1H),
7.33 (t, J = 7.7, 1H), 7.06 (t, J = 7.6, 1H), 6.90 (d, J = 7.8, 1H); 13C
NMR (100 MHz DMSO-d6, 25 °C), (d: ppm): 178.50, 162.60,
142.32, 132.00, 130.50, 122.33, 120.93, 119.95, 111.00; calculated
1H NMR titration experiments and 13C NMR spectra were re-
corded on a Varian 400 MHz Spectrometer. Melting point was re-
corded on an EZ Melt-MPA120 capillary apparatus. UV–vis
Scheme 1. Synthesis procedure of PPB, TSCI and FTHI compounds.

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577
Fig. 1. Color changes of TSCI and FTHI (3 ꢂ 10ꢁ5 M) after addition of various anions (9 ꢂ 10ꢁ5 M) in CH3CN.
(%) for C9H8N4OS (220.04); C: 49.08, H: 3.66, N: 25.44, O: 7.26, S:
14.56, found: C: 49.06, H: 3.67, N: 25.39.
DMSO-d6, 25 °C), (d: ppm): 165.70, 163.00, 150.76, 150.59,
141.28, 132.02, 130.05, 129.97, 128.74, 127.46, 123.65, 122.43,
119.81, 118.84, 118.58, 111.16, 105.85; calculated (%) for C23H16-
N4O2S (412,46); C: 66.97, H: 3.91, N: 13.58, O: 7.76, S: 7.77, found:
C: 66.94, H: 3.89, N: 13.54.
Synthesis of FTHI
FTHI synthesized according to method described in literature as
a
novel compound [24]. TSCI (0.68 mmol, 0.15 g) and PPB
(0.61 mmol, 0.178 g) was added to water (5 mL) and stirred 24 h
at room temperature. The reaction was monitored by TLC. The mix-
ture was then extracted with ethyl acetate (2 ꢂ 25 mL). After
removing the ethyl acetate, the resulting product was separated.
The residue was purified by column chromatography on SiO2 using
EtOAc/hexane = 4/3 as the eluent.
Measurements
The binding ability of TSCI and FTHI for anions was investigated
using UV–vis spectroscopic titration with 10 mm path length
quartz cells [25]. The same concentrations of anion solutions were
added to the sample cell and the differential absorption spectra
were obtained directly using the instrument. Stock solutions of
FTHI: Yield %75 (0.1896 g), m.p. decomposed 235 °C; 1H NMR
(400 MHz, DMSO-d6, 25 °C), (d: ppm): 13.33 (s, 1H, NHAC@O),
11.25 (s, 1H, @NANHA), 7.89 (d, J = 8.7, 2H), 7.52 (d, J = 8.8, 2H),
7.40 (t, J = 7.8, 2H), 7.32 (t, J = 7.6, 1H), 7.15 (t, J = 7.4, 1H), 7.03–
7.09 (m, 4H), 7.01 (s, 1H), 6.95 (d, J = 7.8, 1H). 13C NMR (100 MHz
compounds and anions were prepared 3 ꢂ 10ꢁ5 and 9 ꢂ 10ꢁ5
M
in CH3CN, respectively.
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) were performed in glass cell with conventional
a
a
Fig. 2. UV–vis spectral changes of TSCI (3 ꢂ 10ꢁ5) in CH3CN upon edition of various anions (9 ꢂ 10ꢁ5) (a) and UV–vis spectral changes of FTHI (3 ꢂ 10ꢁ5) in CH3CN upon
edition of various anions (9 ꢂ 10ꢁ5) (b).

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Fig. 3. UV–vis absorbtion spectra of FTHI upon addition of Fꢁ (0–10 eq) in CH3CN, inset shows mol fraction.
three-electrode configuration in CH3CN including 0.1 M tetrabu-
thylammonium perchlorate (TBAP, Aldrich) as supporting electro-
lyte. As working electrode,
a glassy carbon electrode (GCE)
having surface diameter of 2 mm was used together with a plati-
num wire as counter electrode. In all cases, the reference electrode
was Ag/Ag+ (0.1 M) in CH3CN and was standardized for the redox
couple, ferrocenium/ferrocene. Prior to each experiment, GCE was
polished with 1.0, 0.3 and 0.05 lm alumina powder (PACE Tech-
nologies, USA). Residual alumina particles were removed by an
ultrasonic cleaner (Sonorex Super RK 106, Germany). Then, the
electrode was dried and washed with pure CH3CN before use.
The electrochemical measurements (CV and DPV) were recorded
anodically at potential range of 0.0–2.0 V.
1H NMR and 13C NMR spectra were obtained in DMSO-d6. For 1H
NMR titration, Tetrabutylammonnium (TBA) salt of fluoride anion
was used and concentration was increased as 0.5, 1.0, 2.0 and 4.0
equivalents. All experiments were performed at room temperature.
Fig. 4. Nonlinear curve fitting of the absorbance at 490 nm against the added
amount of Fꢁ.
Results and discussion
Colorimetric experiments
Table 1
In the naked-eye colorimetric experiments, TSCI solution shows
slightly change in color from light-yellow to light-reddish with the
addition of Fꢁ, but does not show any ꢁchange in the presence of
Clꢁ, Brꢁ, Iꢁ, NOꢁ2 , NO3ꢁ, BzOꢁ, HSO4ꢁ, ClO4 . FTHI solution undergoes
a dramatic color change from light-yellow to orange in the pres-
ence of Fꢁ. Moreover, FTHI solution shows slight color change from
light-yellow to light-orange in the presence of BzOꢁ, while it re-
veals no color changeꢁupon addition of other anions Clꢁ, Brꢁ, Iꢁ,
NOꢁ2 , NOꢁ3 , HSO4ꢁ, ClO4 which can be easily distinguished by the
naked eye as can be seen in Fig. 1.
Stability constants of the FTHI in the presence of some anions in CH3CN.
Anions
Fꢁ
BzOꢁ
Ka
(7.46 0.43) ꢂ 105
(0.31 0.01) ꢂ 105
In the presence of Fꢁ, the maximum absorption peaks of TSCI
slightly shifted to a shorter wavelength, from 364 to 349 nm
(Dk = 15 nm). However, there was absolutely no change in the
absorption spectrum of TSCI in the presence of other anions. Con-
versely, the characteristic absorption peak of FTHI at 410 nm re-
mained stable in the presence of the used anions except Fꢁ and
BzOꢁ. Upon the addition of Fꢁ, there was a prominent change that
the maximum absorption peak at 410 nm disappeared while a new
strong peak at 500 nm appeared. Addition of BzOꢁ resulted in sim-
ilar but weak peaks at 500 nm.
UV–vis titration spectra
In order to evaluate the interactions of TSCI and FTHI towards
the anions, a series of UV–vis experiments were carried out in CH3-
CN (3 ꢂ 10ꢁ5 M) by adding tetrabutylammonium salts of anions
(9 ꢂ 10ꢁ5 M), shown in Fig. 2(a) and (b), respectively.

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579
Fig. 5. The cyclic voltammogram (CV) of FTHI; inset shows the CV of TSCI (first scan).
FTHI plus large excess anion solutions, respectively and C0f is the
concentration of the free FTHI and Ka is the stability constant.
The obtained curve of Eq. (1) for Fꢁ by using Sigma Plot 10 has
been presented in Fig. 4. The calculated stability constants (Ka) for
1:1 stoichiometry between anions and FTHI were given in Table 1.
Consequently, it was observed that FTHI showed selectivity toward
the anions in the order of Fꢁ ꢃ BzOꢁ and was essentially not sen-
sitive to other anions.
The absorption changes at the different concentrations of Fꢁ
and the related isosbestic point at 442 nm that indicates the pres-
ence of two species at equilibrium as shown in Fig. 3 [26,27]. The
stoichiometry between FTHI and Fꢁ was determined as 1:1 com-
plex formation by mole fraction method having value of 1.21 at
490 nm as shown in inset of Fig. 3. On the basis of UV–vis band
changes upon the addition of anions and 1:1 complexation ratio
between FTHI and anions, the stability constant (Ka) can be deter-
mined by the nonlinear curve fitting procedure by using following
equation [28,29]:
Electrochemical measurements
A ꢁ A0f
A ꢁ A0f
The electrochemical properties of the TSCI and FTHI were inves-
tigated by CV and DPV. The obtained cyclic voltammograms for
TSCI and FTHI in CH3CN solution containing 0.1 M TBAP as the sup-
porting electrolyte has been shown in Fig. 5. TSCI shows only one
irreversible oxidation peak at 1.65 V (Epa), while FTHI has two ano-
dic peaks having irreversible nature at 1.31 (Epa1) and 1.77 V (Epa2).
r ¼
þ
ð1Þ
Ac ꢁ A0 C0 xKa xðA0 ꢁ AÞ
f
f
c
where r shows the added equivalents of anions, A0f , A and Ac0 are
absorbance of the free FTHI, FTHI plus r equivalents of anions and
Fig. 6. The DPV of FTHI upon increasing amount of Fꢁ in CH3CN including 0.1 M TBAP; inset shows DPV of TSCI in same conditions.

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Fig. 7. 1H NMR spectra of FTHI upon addition of 0.5, 1.0, 2.0 and 4.0 eq. Fꢁ in DMSO-d6 at 25 °C.
The peaks (Epa and Epa2) at 1.65 and 1.77 V have been attributed to
the oxidation of the isatin ring in the TSCI and FTHI, respectively, as
expected from the known electro activity of isatin rings [30–32].
Moreover, the other oxidation peak (Epa1) at 1.31 V of FTHI has
been expected to belong to the oxidation of the thiazole ring, as
known electro activity of thiazole rings [33].
In order to obtain a deeper insight to the interaction behaviors
between each compound and Fꢁ, amperometric titration experi-
ments were carried out by differential pulse voltammetry (DPV).
The corresponding pulse voltammograms and their changes ob-
tained by adding of Fꢁ were shown in Fig. 6 for TSCI and FTHI. As
can be seen, there is a weak potential shift for TSCI, while the obvi-
ous peak potential shifts were obtained for the first peak as well as
dramatic decrease for current values of the second oxidation peak
for FTHI. This reveals that the presence of thiazole ring in the FTHI
increase the interaction behavior of the isatin ring towards Fꢁ. This
indicates the formation of a complex between compounds (TSCI,
FTHI) and Fꢁ, which is in accordance with spectroscopic data.
1H NMR titration spectra
In order to evaluate the interactions between the FTHI and Fꢁ,
we also carried out 1H NMR studies in DMSO-d6, upon addition
of Fꢁ as its TBA salt to the DMSO-d6 solution of the FTHI (Fig. 7).
The changes at 1H NMR spectra of FTHI after the addition of Fꢁ
were given in Fig. 7. It is noticed that the signal of isatin rings N-Ha
Fig. 8. 13C NMR spectra of FTHI in DMSO-d6 at 25 °C.

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Fig. 9. The possible interaction form of FTHI and Fꢁ.
proton at 13.30 ppm disappeared on addition of Fꢁ. The signal of N-
Hb shifted upfield from 11.25 to 9.80 ppm. The chemical shift of
Hb proton reached a constant at 0.5 eq upon addition of Fꢁ. This
upfield shift can be attributed to the expected increase in the elec-
tronic density caused by interaction of Fꢁ. However, signals of all
the protons in benzene rings slightly shifted to upfield [34]. This
result suggested us that the interaction might take place in the imi-
dole form. To support this, 13C NMR spectra were recorded for free
FTHI and mixed solution of Fꢁ and FTHI. As can be seen in Fig. 8 the
peak of carbonyl carbon shifted from 163.0 ppm to 156.5 ppm [35].
As a result, the interaction between FTHI and Fꢁ takes place in imi-
dole form (Fig. 9). 1H NMR titrations of FTHI with Fꢁ supported the
results obtained from the UV–vis and CV studies.
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Conclusion
We described the synthesis and structural characterization of a
novel compound and its anion binding ability was determined by
spectrophotometric and voltammetric methods. The results of this
studies showed that FTHI has selectivity for fluoride anion over
other anions in acetonitrile. More importantly, FTHI display
naked-eye detection at room temperature. The fluoride anion
detection gives rise to a large bathochromic shift in the absorbtion
spectrum (from yellow to orange). The stoichiometric ratio and the
stability constant were determined spectrophotometrically. The
structural interactions obtained from voltammetric experiments
have confirmed with 1H NMR titrations and binding properties
revealed.
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Acknowledgment
The authors are grateful for the grant sponsored by the Scien-
tific Research Projects (BAP-121210006) of Selcuk University.
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