Ion-sensing properties of thiophene-based oxazol-5-ones

Article abstract of DOI:10.1080/00387010.2021.1881128

Seven fluorescent thiophene-based oxazol-5-one derivatives were prepared via Erlenmeyer reaction of 2-thiophenecarboxaldehyde and 3-thiophenecarboxaldehyde with different aryl glycine intermediates. Their absorption and emission properties were investigat

Full text of DOI:10.1080/00387010.2021.1881128

Spectroscopy Letters
An International Journal for Rapid Communication
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Ion-sensing properties of thiophene-based
oxazol-5-ones
Aslı Çıkıt Şener, Derya Topkaya, Serap Alp & Gülsiye Öztürk Ürüt
To cite this article: Aslı Çıkıt Şener, Derya Topkaya, Serap Alp & Gülsiye Öztürk Ürüt (2021) Ion-
sensing properties of thiophene-based oxazol-5-ones, Spectroscopy Letters, 54:3, 171-179, DOI:
10.1080/00387010.2021.1881128
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SPECTROSCOPY LETTERS
2021, VOL. 54, NO. 3, 171–179
https://doi.org/10.1080/00387010.2021.1881128
Ion-sensing properties of thiophene-based oxazol-5-ones
a
b
b
b
€
€
€
€
€
Aslı C¸ıkıt S¸ener , Derya Topkaya , Serap Alp and Gulsiye Ozturk Urut
a
b
€
Graduate School of Natural and Applied Science, Dokuz Eylul University, Izmir, Turkey; Department of Chemistry, Faculty of Science,
Dokuz Eylul University, Izmir, Turkey
ABSTRACT
ARTICLE HISTORY
Received 15 October 2020
Accepted 19 January 2021
Seven fluorescent thiophene-based oxazol-5-one derivatives were prepared via Erlenmeyer
reaction of 2-thiophenecarboxaldehyde and 3-thiophenecarboxaldehyde with different aryl
glycine intermediates. Their absorption and emission properties were investigated in solu-
tion environment and in the solid film matrix of polyvinylchloride. Moreover, the fluores-
cence sensing abilities to iron (III), cobalt (II), nickel (II), zinc (II) and copper (II) ions were
examined in polyvinylchloride matrix. Thiophene-based oxazol-5-one derivatives revealed
high sensitivity and selectivity toward iron (III). A good linearity with a correlation coefficient
of 0.99 was observed in the concentration range of 1.0 ꢀ 10ꢁ7 to 1.0 ꢀ 10ꢁ3 M iron (III) for
KEYWORDS
Fluorescence sensing; iron
(III); oxazol-5-one;
polyvinylchloride
film; thiophene
2-phenyl-4-(2-thienylmethylene)oxazol-5-one,
2-(3-thienyl)-4-(2-thienylmethylene)oxazol-5-
one and 2-(3-thienyl)-4-(3-thienylmethylene)oxazol-5-one.
Introduction
the performance of the sensing system such as
sensitivity, stability and response time.[23,25]
Many immobilization methods like covalent,
physical and electrostatic immobilization have
been developed. Polyvinylchloride (PVC) doped
state of the organic molecules provides them high
stability and permeability for the ions, which
gives opportunity for wide range of applications
in sensors and electronics.[13,22,25] By using PVC
immobilized solid state, as all the advantages
pointed out by literature, it was aimed to develop
sensing system with high sensitivity and stability.
The development of sensors for target ion detec-
tion have a great potential for different chemical
and biological applications.[3,4,6,15,16,22,24–31] One of
the most abundant and necessary transition metal
ion in human body due to being involved in many
biological processes such as oxygen delivery, cell
metabolism, electron transfer and synthesis of
DNA and RNA is the trivalent form of iron
(Fe3þ).[22–42] As much as lack of iron, its overdose
is also detrimental for health, namely its abnormal
values can cause diseases such as Alzheimer’s,
Parkinson’s, anemia, heart failure, liver and kidney
damage, intelligence decline, neurodegenerative
Because of their favorable physicochemical prop-
erties, heterocyclic compounds are promising
candidates for developing functional organic and
biochemical
materials,
agrochemicals
and
dyes.[1–6] One of the most extensively studied
electron donating heterocyclic structure is thio-
phene and its derivatives. They are reported to be
used in preparing fluorescent materials with per-
fect properties by lowering the energy of conju-
gated structures for diversity of applications.[1–21]
Due to its favorable properties reported in the
literature, the thiophene moiety was chosen in
order to develop fluorescent ion-sensing
derivatives.
Due to their applicability in sensors, organic
light-emitting diodes, field effect transistors and
solid state lasers, the fluorescent organic conju-
gated structures in solid, immobilized and fluor-
escent
doped
film
state
are
under
interest.[12,13,22–25] In comparison to solution
phase, immobilized phase provides many advan-
tages like giving opportunity to perform study in
fully aqueous media. Immobilization step of the
fluorescent molecule is important because it affect
CONTACT Serap Alp
serap.alp@deu.edu.tr
Department of Chemistry, Faculty of Sciences, Dokuz Eylul University, Tinaztepe Campus, Buca, 35160,
Izmir, Turkey.
ß 2021 Taylor & Francis Group, LLC

172
A. ÇIKIT ŞENER ET AL.
diseases, diabetes, loss of motor skills, the burden
of hemochromatosis, etc.; thus, its detection is very
important.[23–29,31–35,37–42] Despite the fact that
many techniques were tried to be developed for
Fe3þ detection, there are nevertheless some restric-
tions of sensitivity and selectivity thus preparing an
efficient sensor for Fe3þ in this respect is import-
ant.[25–28,30–34,37–42] This was our motivation to
synthesize sensitive, selective and efficient fluores-
cent derivatives for Fe3þ sensing.
points apparatus. Infrared spectra were recorded
on a Perkin ELMER FT-IR infrared spectrometer
(spectrum BX-II). H-NMR were obtained on a
1
high resolution Fourier transform Bruker WH-
400 1H NMR spectrometer with chloroform
(CDCl3) as solvent. Analytical and preparative
thin layer chromatography (TLC) as carried out
using silica gel 60 HF254 (Merck). Column chro-
matography was carried out by using 70-230
mesh silica gel (0.063-0.2 mm, Merck) UV/visible
absorption spectra were recorded with Schimadzu
UV-1601 spectrophotometer. All fluorescence
measurements were undertaken by using Varian-
Carry Eclipse spectrofluorimeter.
In this work, seven thiophene bearing oxazol-5-
one derivatives, 2-phenyl-4-(2-thienylmethyle-
ne)oxazol-5-one(PTO-I),
2-(4-nitrophenyl)-4-(2-
thienylmethylene)oxazol-5-one (PTO-II), 2-(4-me
thylphenyl)-4-(2-thienylmethylene)oxazol-5-one
(PTO-III), 2-(2-thienyl)-4-(2-thienylmethylene)oxa-
zol-5-one (PTO-IV), 2-(2-thienyl)-4-(3-thienylme-
thylene)oxazol-5-one (PTO-V), 2-(3-thienyl)-4-(2-
thienylmethylene)oxazol-5-one (PTO-VI) and 2-(3-
thienyl)-4-(3-thienylmethylene)oxazol-5-one (PTO-
VII), were synthesized and characterized structur-
ally. Their absorption and emission properties
were detected in the solvents of dichloromethane
(DCM), tetrahydrofuran (THF) and acetonitrile
(ACN) and in the solid film matrix of PVC. In
our previous studies we have reported on Fe3þ
detection with different oxazol-5-one derivative.[43]
Due to their promising results, the sensing proper-
ties of the synthesized derivatives toward Fe3þ
were also investigated in PVC matrix and reported
in this paper.
Absorption and fluorescence emission spectral
data of polymer films were acquired in quartz
cells which were filled with sample solution and
polymer films were placed in diagonal position.
The advantage of this kind of a placement was to
improve the reproducibility of the measurement.
General synthesis of oxazol-5-one derivatives
The synthesis of the oxazol-5-one derivatives
(Fig. 1) starting from aromatic aldehyde and N-
aryloylglycine were carried out by Erlenmeyer
[44]
€
Ploch method.
A mixture of aromatic alde-
hyde and sodium acetate was heated with acetic
anhydride until the mixture just liquefied, and
then heating was continued for 4 h. After comple-
tion of the reaction ethanol was added while
cooling and reaction mixture was allowed to
stand overnight. The crystalline product was col-
lected by filtration, washed with cold ethanol and
recrystallized from ethanol. After recrystallization
process, purity of the products was controlled by
TLC method and applied column chromatog-
raphy techniques if it is necessary.
Materials and methods
Chemicals and equipment
The membrane components, PVC (high molecu-
lar weight) and the plasticizer bis-(2-ethylhex-
yl)phthalate (DOP) were supplied from Fluka.
Lipophilic anionic additive reagent potassium tet-
rakis-(4-chlorophenyl)borate (PTCPB), tetra-
hydrofuran (THF) and acetonitrile acetonitrile
(ACN) were obtained from Aldrich. All the other
chemicals were obtained from Fluka and Merck.
The polyester support (Mylar type) was provided
from DuPont, Switzerland. Bidistilled ultra-pure
water was used throughout the studies.
2-Phenyl-4-(2-thienylmethylene)oxazol-5-one (PTO-I)
0.560 g (5.0 mmol) 2-thiophenenecarboxaldehyde,
0.95 mL (10.0 mmol) acetic anhydride, 0.9 g
(5.0 mmol) benzoylglycine and 0.68 g (5.0 mmol)
sodium acetate (yield 80ꢁ%1, 1.01 g). Mp
159–160 ꢂC. FT-IR (KBr, cm ): 1789 (C¼O),
1
1632 (C¼N), 1152 (C–O), 696 (C–S). H NMR
(400 MHz, CDCl3, d (ppm)): 8.19 ꢁ 8.16 (d, 2H,
J ¼ 12 Hz), 7.71 ꢁ 7.73 (d, 1H, J ¼ 8 Hz),
7.65 ꢁ 7.63 (d, 1H, J ¼ 8 Hz), 7.63 ꢁ 7.58 (t, 1H,
All melting points were measured in sealed
tubes using an electrothermal digital melting

SPECTROSCOPY LETTERS
173
Figure 1. Molecular structure of thiophene-based oxazol-5-one derivatives proposed for ion-sensing. Seven derivatives with
different aryl and thienyl groups have been synthesized. PTO-I: 2-phenyl-4-(2-thienylmethylene)oxazol-5-one; PTO-II: 2-(4-nitro-
phenyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-III: 2-(4-methylphenyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-IV: 2-(2-thienyl)-4-
(2-thienylmethylene)oxazol-5-one; PTO-V: 2-(2-thienyl)-4-(3-thienylmethylene)oxazol-5-one; PTO-VI: 2-(3-thienyl)-4-(2-thienylmethyle-
’
’
ne)oxazol-5-one; PTO-VII: 2-(3-thienyl)-4-(3-thienylmethylene)oxazol-5-one; Ar1 : aryl group; Ar2 : thienyl group.
J ¼ 10 Hz), 7.57 ꢁ 7.50 (m, 2H), 7.48 (s, 1H),
7.17 ꢁ 7.16 (t, 1H, J ¼ 2 Hz).
sodium acetate (yield 60ꢁ%1, 1.05 g). Mp
219–220 ꢂC. FT-IR (KBr, cm ): 1792 (C¼O),
1
1643 (C¼N), 1154 (C–O), 714 (C–S). H NMR
2-(4-Nitrophenyl)-4-(2-thienylmethylene)oxazol-5-
one (PTO-II)
0.47 g (0.0042 mol), 2-thiophenenecarboxalde-
hyde, 0.8 mL (0.0084 mol) acetic anhydride, 0.85 g
(400 MHz, CDCl3, d (ppm)): 8.07 ꢁ 8.05 (d, 2H,
J ¼ 8 Hz), 7.71 ꢁ 7.69 (d, 1H, J ¼ 8 Hz)
,
7.63 ꢁ 7.62 (d, 1H, J ¼ 4 Hz), 7.45 (s, 1H),
7.33 ꢁ 7.31 (d, 2H, J ¼ 8 Hz), 7.17 ꢁ 7.15 (dd, 1H,
J ¼ 8 Hz, J ¼ 4 Hz), 2.45 (s, 3H).
(0.0042 mol)
4-nitroglycine
and
0.57 g
(0.0042 mol) sodium acetate (yield 65ꢁ%1, 0.81 g).
Mp 170–171 ꢂC. FT-IR (KBr, cm ): 1789
2-(2-Thienyl)-4-(2-thienylmethylene)oxazol-5-one
(PTO-IV)
0.224g (0.002 mol) 2-thiophenenecarboxaldehyde,
0.38 mL (0.04 mol) acetic anhydride, 0.45 g
1
(C¼O), 1648 (C¼N), 1155 (C–O), 705 (C–S). H
NMR (400 MHz, CDCl3, d (ppm)): 8.40 ꢁ 8.32
(m, 4H), 7.81 ꢁ 7.79 (d, 1H, J ¼ 8 Hz), 7.69 ꢁ 7.68
(d, 1H, J ¼ 4 Hz), 7.60 (s, 1H), 7.21 ꢁ 7.19 (t,
1H, J ¼ 4 Hz).
(0.002 mol)
2-thienylglycine
and
0.272 g
(0.002mol) sodium acetate (yield 82%, 0.43 g).
Mp 168–170 ꢂC. FT-IR (KBr, cmꢁ1): 1793
1
(C¼O), 1640 (C¼N), 1151 (C–O), 711 (C–S). H
2-(4-Methylphenyl)-4-(2-thienylmethylene)oxazol-5-
one (PTO-III)
0.728 g (0.0065 mol) 2-thiophenenecarboxalde-
hyde, 2.38 mL (0.013 mol) acetic anhydride, 1.1 g
(0.0065 mol) tolylglycine and 0.884 g (0.0065 mol)
NMR (400 MHz, CDCl3, d (ppm)): 7.88–7.87 (d,
1H, J ¼ 4 Hz), 7.70–7.69 (d, 1H, J ¼ 4 Hz),
7.67–7.66 (d, 1H, J ¼ 4 Hz), 7.63–7.62 (d, 1H,
J ¼ 4 Hz), 7.43 (s, 1H), 7.21–7.19 (dd, 1H,

174
A. ÇIKIT ŞENER ET AL.
J ¼ 8 Hz,
J ¼ 4 Hz),
7.16–7.14
(dd,
1H,
(PTCPB) to oxazol-5-one derivative in 1.5 mL of
dried THF. The resulting cocktails were spread
onto a 125 mm polyester support (Mylar TM
type). The polymer support is optically fully
transparent, ion impermeable and exhibits good
adhesion to PVC. Sensor slides were kept in a
desiccator therefore the damage from the ambient
air of laboratory was avoided. Each sensor PVC
film was cut with a size of 13 ꢀ 50 mm.
J ¼ 8 Hz, J ¼ 4 Hz).
2-(2-Thienyl)-4-(3-thienylmethylene)oxazol-5-one
(PTO-V)
0.738g (0.006 mol) 3-thiophenenecarboxaldehyde,
1.142 mL (0.012 mol) acetic anhydride, 1.222 g
(0.006 mol) 2-thienylglycine and 0.9 g (0.006 mol)
sodium acetate (yield 48ꢁ%1, 0.75 g). Mp
181–182 ꢂC. FT-IR (KBr, cm ): 1788 (C¼O),
1
1648 (C¼N), 1149 (C–O), 712 (C–S). H NMR
Results and discussion
(400 MHz, CDCl3, d (ppm)): 8.17 (s, 1H),
7.89–7.87 (m, 2H), 7.68–7.67 (d, 1H, J ¼ 4 Hz),
7.40 (s, 1H), 7.26–7.25 (d, 1H, J ¼ 4 Hz),
7.21–7.19 (dd, 1H, J ¼ 8 Hz, J ¼ 4 Hz).
Structural characterization
The oxazole-5-one derivatives (Fig. 1) have been
€
successfully synthesized by Erlenmeyer Ploch
method[44] with high yields up to 82%. In general,
the compounds bearing 2-thienylmethylene moiety
were obtained in higher yields in comparison to
the ones which contain 3-thienylmethylene moiety.
The presence of electron withdrawing group in the
position 2 makes the formation of the oxazolone
ring difficult, thus reducing the efficiency. The
structures of the synthesized derivatives were iden-
2-(3-Thienyl)-4-(2-thienylmethylene)oxazol-5-one
(PTO-VI)
0.112 g (0.001 mol) 2-thiophenenecarboxaldehyde,
0.190 mL (0.002 mol) acetic anhydride, 0.187 g
(0.001 mol)
3-thienylglicine
and
0.136 g
(0.001 mol) sodium acetate (yield 72%, 0.19 g).
Mp 178–180 ꢂC. (KBr, cmꢁ1): 1786 (C¼O), 1644
(C¼N), 1153 (C–O), 714 (C–S). 1H NMR
(400 MHz, CDCl3, d (ppm)): 8.19 (s, 1H),
7.75–7.74 (d, 1H, J ¼ 4 Hz), 7.71–7.70 (d, 1H,
J ¼ 4 Hz), 7.63–7.62 (d, 1H, J ¼ 4 Hz), 7.46 (s,
1H), 7.45–7.44 (d, 1H, J ¼ 4 Hz), 7.17–7.14 (dd,
1H, J ¼ 12 Hz, J ¼ 4 Hz).
1
tified by using FT-IR and H-NMR spectroscopy
techniques. The results of FT-IR technique showed
that the oxazole-5-one derivatives which have been
synthesized have the characteristic bands of the
oxazole-5-one ring, thiophene ring and amide
group (C¼Ostretch at 1786–1793 cmꢁ1, C¼Nstretch at
1632–1648 cmꢁ1 and C–Sstretch at 696–715 cmꢁ1).
1
2-(3-Thienyl)-4-(3-thienylmethylene)oxazol-5-one
(PTO-VII)
0.112 g (0.001 mol) 3-thiophenenecarboxaldehyde,
0.190 mL (0.002 mol) acetic anhydride, 0.22 g
The results of the H-NMR technique proved
that oxazole-5-one derivatives have the expected
characteristic peaks. The arylmethylene proton
which indicates the formation of oxazol-5-one
derivatives has a singlet signal between 7.40 and
7.60 ppm for all the derivatives. The protons on
aromatic ring have multiplet and doublet peaks.
Protons on the thienyl groups have shown doub-
let signal between 7.13 and 8.19 ppm as expected
in all the derivatives.
(0.001 mol)
3-thienylglicine
and
0.136 g
(0.001 mol) sodium acetate (yield 55%, 0.14 g).
Mp 185–186 ꢂC. (KBr, cmꢁ1): 1789 (C¼O), 1648
(C¼N), 1146 (C–O), 715 (C–S). 1H NMR
(400 MHz, CDCl3, d (ppm)): 8.19 (s, 1H), 8.17 (s,
1H), 7.91–7.90 (d, 1H, J ¼ 4 Hz), 7.74–7.73 (d,
1H, J ¼ 4 Hz), 7.46–7.44 (d, 1H, J ¼ 8 Hz), 7.41 (s,
1H), 7.27–7.26 (d, 1H, J ¼ 4 Hz).
Absorption and emission properties
All derivatives’ maximum absorption wavelengths
were detected by using UV-vis absorption spec-
trophotometer in the solution of acetonitrile
(ACN), tetrahydrofuran (THF), DCM (Table 1
and Fig. 2) and solid matrix of PVC (Table 2). In
general, the longest absorption maxima for all
Preparation of polyvinylchloride membranes
A mixture for membrane preparation was
obtained by dissolving 120 mg of polyvinyl chlor-
ide (PVC), 240 mg of plasticizer and equimolar
potassium tetrakis-(4- chlorophenil) borate

SPECTROSCOPY LETTERS
175
Table 1. Absorption and emission properties of thiophene-based oxazol-5-one derivatives proposed for ion-sensing in the sol-
vents of dichloromethane, tetrahydrofuran and acetonitrile: absorption maxima; molar extinction coefficients; emission maxima;
Stokes’ shift and singlet energy.
Solvent
DCM
THF
ACN
abs
f
abs
f
abs
f
PTO
kmax
emaxꢀ 104
kmax
Dk
Es
kmax
emaxꢀ 104
kmax
Dk
Es
kmax
emaxꢀ 104
kmax
Dk
Es
PTO-I
392
305
393
403
380
390
364
2.90
1.90
3.40
3.10
1.60
2.00
1.10
452
538
465
553
444
442
410
60
118
55
150
64
52
46
73.6
93.4
72.5
70.7
75.0
73.0
78.2
390
310
390
402
380
387
362
4.50
1.90
3.50
3.31
2.23
3.28
0.77
438
530
465
528
436
435
413
48
110
55
126
56
48
51
73.0
91.9
73.0
70.9
75.0
73.6
78.7
387
309
388
398
376
385
362
4.00
1.70
3.70
3.36
2.33
2.86
0.70
458
583
465
551
438
437
412
71
163
55
153
62
52
50
73.6
92.2
73.4
71.6
75.7
74.0
78.7
PTO-II
PTO-III
PTO-IV
PTO-V
PTO-VI
PTO-VII
As seen from the table the derivatives synthesized have large Stokes’ Shift values up to 163 nanometers.
kmaxabs: absorption maxima (nanometer); emax: molar extinction coefficient (cantimeterꢁ1molꢁ1Liter); kmaxf: emission maxima (nanometer); Dk: Stokes’
Shift (nanometer); Es: singlet energy (kilocalorie/mol); PTO-thiophene-based oxazol-5-ones; PTO-I: 2-phenyl-4-(2-thienylmethylene)oxazol-5-one; PTO-II:
2-(4-nitrophenyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-III: 2-(4-methylphenyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-IV: 2-(2-thienyl)-4-(2-thienylme-
thylene)oxazol-5-one; PTO-V: 2-(2-thienyl)-4-(3-thienylmethylene)oxazol-5-one; PTO-VI: 2-(3-thienyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-VII: 2-(3-
thienyl)-4-(3-thienylmethylene)oxazol-5-one; DCM: dichloromethane; THF: tetrahydrofuran; ACN: acetonitrile.
group which prolongs the conjugation pathway.
Besides, in general for all the derivatives the
highest Stokes’ shift values were observed in
ACN, which has the highest polarity. Also in
PVC matrix the longest emission maximum and
largest Stokes’ shift value was observed for
PTO-II.
Fluorescence response to iron (III) ion
Fluorescence response to Fe3þ, Co2þ, Ni2þ, Zn2þ
and Cu2þ was tested by incorporating PTO fluo-
Figure 2. Absorption spectra of thiophene-based oxazol-5-one
derivatives derivatives proposed for ion-sensing in polyvinyl-
chloride. The absorption maxima of the derivatives
roionophores in plasticized polyvinyl chloride
matrix bearing the lipophilic anionic additive of
potassium tetrakis(4-chlorophenyl) borate. One of
the most important feature of an ion sensor is its
selectivity. The prepared sensor membranes
revealed negligible response to the ions under
interest except for Fe3þ (Figs. 3 and 4). Among
the studied ions iron (III) has the smallest ionic
radius and this could be the reason for the select-
ive emission quenching of it but not the other
transition metal ions. The fluorescence quenching
of PTO derivatives by Fe3þ ions in aqueous solu-
tions of 1.0 ꢀ 10ꢁ2 M CH3COOH/CH3COONa
buffer with a pH of 5 was utilized as sensor
response (Fig. 3).
varies between 366 and 417 nanometers. I: 2-phenyl-4-(2-thie-
nylmethylene)oxazol-5-one; II: 2-(4-nitrophenyl)-4-(2-thienylme-
thylene)oxazol-5-one; III: 2-(4-methylphenyl)-4-(2-thienylmet
hylene)oxazol-5-one; IV: 2-(2-thienyl)-4-(2-thienylmethylene)oxa-
zol-5-one; V: 2-(2-thienyl)-4-(3-thienylmethylene)oxazol-5-one;
VI: 2-(3-thienyl)-4-(2-thienylmethylene)oxazol-5-one; VII: 2-(3-
thienyl)-4-(3-thienylmethylene)oxazol-5-one.
derivatives was observed in DCM solution.
Among PTO derivatives, PTO-IV which has 2-
thienyl moiety on both sides, had the longest
absorption maximum wavelength. In PVC matrix,
the derivatives of PTO-II and PTO-IV exhibited
higher bathochromic shift.
Moreover, their emission spectra were also col-
lected by exciting the molecules at their absorp-
tion maximum in both solution and PVC matrix
and the results obtained are given in Tables 1
and 2. PTO-II and PTO-IV exhibited the longest
emission maxima in all the solvents studied. The
longest emission maximum and largest Stokes’
shift value was observed for PTO-II in ACN, pre-
sumably due to strong electron withdrawing nitro
The sensor response toward Fe3þ was both
tested in plain aqueous solutions and aqueous
solutions
of
1.0 ꢀ 10ꢁ2
M
CH3COOH/
CH3COONa buffer with a pH of 5. The results
obtained revealed that the PTO derivatives
response toward Fe3þ ions were more stable and
reproducible in aqueous solutions of 1.0 ꢀ 10ꢁ2
M CH3COOH/CH3COONa buffer with a pH of

176
A. ÇIKIT ŞENER ET AL.
Table 2. Absorption and emission properties of thiophene-based oxazol-5-one derivatives derivatives proposed for ion-sensing in
polyvinylchloride matrix (absorption maxima; molar extinction coefficients; emission maxima; Stokes’ Shift; singlet energy and
fluorescence response upon addition of iron (III) in polyvinylchloride (concentration range; the relative standard deviation;
correlation coefficient; regeneration).
abs
f
PTO
kmax
emax ꢀ 104
kmax
Dk
Es
Concentration range
RSD
R2
Regeneration
PTO-I
379
417
398
405
383
392
366
0.0020
0.0014
0.0006
0.0038
0.0029
0.0021
0.0025
423
518
448
468
449
453
423
44
101
50
63
66
61
44
75.3
68.4
71.6
70.4
74.4
72.7
75.3
1 ꢀ 10-7 to 1 ꢀ 10-3
1 ꢀ 10-7 to 1 ꢀ 10-3
1 ꢀ 10-7 to 1 ꢀ 10-3
1 ꢀ 10-7 to 1 ꢀ 10-3
1 ꢀ 10-7 to 1 ꢀ 10-3
1 ꢀ 10-7 to 1 ꢀ 10-3
1 ꢀ 10-7 to 1 ꢀ 10-3
5.0 ꢀ 10-2 (n ¼ 5)
3.0 ꢀ 10-3 (n ¼ 5)
8.5 ꢀ 10-5 (n ¼ 5)
1.6 ꢀ 10-2 (n ¼ 5)
7.3 ꢀ 10-3 (n ¼ 5)
9.6 ꢀ 10-5 (n ¼ 5)
1.2 ꢀ 10-2 (n ¼ 5)
0.9943
0.9686
0.9748
0.9710
0.9700
0.9947
0.9870
80.6
90.1
87.5
78.7
77.5
86.8
87.0
PTO-II
PTO-III
PTO-IV
PTO-V
PTO-VI
PTO-VII
The table reveals that the derivatives have a response to iron at wide concentration range and good regeneration.
kmaxabs: absorption maxima (nanometer); emax: molar extinction coefficient (cantimeterꢁ1molꢁ1Liter); kmaxf: emission maxima (nanometer); Dk: Stokes’ shift (nano-
meter); Es: singlet energy (kilocalorie/mol); concentration range (Molarity); RSD: the relative standard deviation (percent); R2: correlation coefficient; regeneration
(percent); PTO-thiophene-based oxazol-5-ones; PTO-I: 2-phenyl-4-(2-thienylmethylene)oxazol-5-one; PTO-II: 2-(4-nitrophenyl)-4-(2-thienylmethylene)oxazol-5-one;
PTO-III: 2-(4-methylphenyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-IV: 2-(2-thienyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-V: 2-(2-thienyl)-4-(3-thienylmethyle-
ne)oxazol-5-one; PTO-VI: 2-(3-thienyl)-4-(2-thienylmethylene)oxazol-5-one; PTO-VII: 2-(3-thienyl)-4-(3-thienylmethylene)oxazol-5-one.
Figure 3. Fluorescence spectra and Stern–Volmer plot of 2-(3-thienyl)-4-(2-thienylmethylene)oxazol-5-one derivative doped in poly-
vinylchloride for ion-sensing after addition of different concentrations of iron (III) (Molar) (a: buffer, b: 1 ꢀ 10ꢁ7, c: 1 ꢀ 10ꢁ6, d:
1 ꢀ 10ꢁ5, e: 2 ꢀ 10ꢁ5, f: 4 ꢀ 10ꢁ5, g: 6 ꢀ 10ꢁ5, h: 8 ꢀ 10ꢁ5, j: 2 ꢀ 10ꢁ4, k: 4 ꢀ 10–4, l: 6 ꢀ 10ꢁ4, m: 8 ꢀ 10ꢁ4, n: 1 ꢀ 10ꢁ3). The
spectra and Stern–Volmer plot reveal good linearity in a wide concentration range for iron (III). I0: Fluorescence signal in the
absence of iron (III); I: Fluorescence signal in the presence of iron (III); R2: correlation coefficient; y ¼ 1E–04x þ 1.07: refers to
Stern–Volmer equation, (I0/I) ¼ KSV[Q] þ 1; KSV: Stern–Volmer constant; [Q]: concentration of the quencher (iron (III)); y: (I0/I); x: [Q].
5 than in plain aqueous solutions thus further
in more efficient interaction with Fe (III). Fe3þ
ions selective fluorescence PTO-VI bearing sensor
membranes optical response at different Fe3þ
concentrations are shown in Fig. 3. Sensor mem-
branes exhibited calibration curves in a wide con-
centration range between 1.0ꢀ 10ꢁ7ꢁ1.0 ꢀ 10ꢁ3
M (Fig. 3 and Table 2).
Moreover, the regeneration studies proved that
the sensor membranes reversibility is satisfactory,
which is also an important feature for optical
sensors (Table 2). Another important characteris-
tic for defining the compatibility of an optical
sensor membrane for selective detection of an
ion under interest is reproducibility. By exposing
the same sensor membranes of PTO derivatives
to fixed concentrations of Fe3þ in the range of
8.5 ꢀ 10ꢁ5 and 5.0 ꢀ 10ꢁ2 M the relative standard
experiments were performed in 1.0 ꢀ 10ꢁ2
M
CH3COOH/CH3COONa buffer with a pH of 5.
By replacing the aryl group on the oxazolone
(as seen in PTO 1, II and III) with a thienyl
group (as seen in the other four) resulted in
slightly greater emission quenching. With the
introduction of sulfur atom into the structure, the
interaction of electrons with iron (III) increased
and hence the molecular rigidity decreased and as
a result, the aromaticity decreased, ending up in
more quenching. Although all the seven deriva-
tives exhibited similar emission quenching, among
them, 2-(3-thienyl)-4-(2-thienylmethylene)oxazol-
5-one derivative displayed the greatest emission
quenching presumably due to the position of
both thienyl moieties which could have resulted

SPECTROSCOPY LETTERS
177
Disclosure statement
No potential conflict of interest was reported by
the author(s).
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