An effective fluorescent optical sensor: Thiazolo-thiazole based dye exhibiting anion/cation sensitivities and acidochromism

Article abstract of DOI:10.1016/j.jphotochem.2021.113456

Fluorescent 2,5-bis(4-hydroxyphenyl)thiazolo[5,4-d]thiazole (HPhTT) sensor molecule consisting of thiazolo-thiazole (TTz) binding central unit and phenolic signalling side groups was synthesized with a symmetric structure. The selectivity, sensitivity and

Full text of DOI:10.1016/j.jphotochem.2021.113456

Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
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Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
An effective fluorescent optical sensor: Thiazolo-thiazole based dye
exhibiting anion/cation sensitivities and acidochromism
,
*
Zeynep Dikmena, Onur Turhanb, Müjgan Yamanc, Vural Bütünc
a Faculty of Engineering, Department of Biomedical Engineering, Eskisehir Osmangazi University, 26040 Eskisehir, Turkey
b Institute of Science, Polymer Science and Technology Department, Eskisehir Osmangazi University, 26040 Eskisehir, Turkey
c Facullty of Science and Letters, Department of Chemistry, Eskisehir Osmangazi University, 26040 Eskisehir, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fluorescent 2,5-bis(4-hydroxyphenyl)thiazolo[5,4-d]thiazole (HPhTT) sensor molecule consisting of thiazolo-
thiazole (TTz) binding central unit and phenolic signalling side groups was synthesized with a symmetric
structure. The selectivity, sensitivity and reversibility properties of optical sensor towards cations and anions
were investigated. TTz unit of the sensor was responsible from cation and proton interactions resulting with
fluorescent quenching, while phenolic–OH groups were sensitive to anions with a deprotonation mechanism. The
fluorescence quenching in the presence of Au3+ with high selectivity, sensitivity and fast response, making it
clear that the sensor was very effective in detecting Au3+. Fluorescent sensor (HPhTT) was also found to be able
to recognize AcO-, F- and CN– anions accompanied by obvious UV–vis absorption change and fluorescence
switch-on response, and exhibited enhanced intramolecular charge transfer (ICT) strongly depending on the
forming conjugated acid after addition of anions. Ratiometric detection and reversible usage of the sensor for
AcO-, F- and CN– anions were also possible. The sensor showed remarkable detection ability in a pH > 9 media
and it has also been demonstrated that it can be used successfully as a pH sensor in an aqueous media. A colour
calculator study using F- anion to estimate colour change, colour purity and colour coordinates under UV light
excitation with I550/I444 ratio indicated that HPhTT dye had a promising potential for fluorescent sensor studies.
Fluorescent sensor
Thiazolo-thiazole
Dyes
Gold sensor
ICT
1. Introduction
Small molecules and macromolecules containing thiazolo[5,4-d]
thiazole (TTz) unit are promising structures for many applications such
Fluorescence spectroscopy has become one of the most popular and
challenging analytical methods in the last two decades with the
demonstration of its highly functional use in biological systems as
enzyme substrates, biomolecular labels, environmental indicators and
cellular stains [1], biochemistry, biomedical engineering, biophysics
and optical sensors. Determination of an analyte by fluorescence enables
high sensitivity and biochemical measurements without radioactive
agents [2]. After the improvement of this highly convenient method in
bioimaging systems, fluorescent sensor dyes exhibiting selective optical
response towards the analyte have been one of the major interest in the
literature. Selective detections of cations [3–13], anions [14–20], acids –
bases [21–23], solvents and more specific chemicals [24–29] by using
fluorescent sensor dyes have major interest in the literature. These smart
molecules can be creatively used in the determination of calcification in
soft tissue [30], fluorescent sensors for detection of analyte in live cells
[31–35], optical detection of nerve agent [36].
as solar cells [37–40], organic light emitting diodes [41], nonlinear
optics [42], photocatalysis [43,44], fluorescent metal–organic frame-
works [45], electrochromic devices [46], and even for biological ap-
plications [47]. These wide range applications result from superior
photo luminescent, solvatochromic [48], electrochromic [49,50] prop-
erties of TTz unit which has remarkable photo-chemical stability
[51–53]. Eventhough the superior properties, reports on usage of TTz
containing molecules as fluorescent sensor is very limited in the litera-
ture [54–56].
Live cell imaging of Au3+ ions may present novel information about
the binding mechanism to the biological molecules, because Au3+ ions
can bound to proteins and DNA resulting with broken down and damage
of the liver, kidney, and nervous system [57,58]. Eventhough there are
lots of studies on Au3+ sensing by using photoluminescent BODIPY and
rhodamine based fluorescent sensors in the literature, any study has not
been found on thiazolo[5,4-d]thiazole containing fluorescent sensor for
* Corresponding author at: Facullty of Science and Letters, Department of Chemistry, Eskisehir Osmangazi University, 26040 Eskisehir, Turkey.
E-mail address: vbutun@ogu.edu.tr (V. Bütün).
https://doi.org/10.1016/j.jphotochem.2021.113456
Received 6 May 2021; Received in revised form 28 June 2021; Accepted 15 July 2021
Available online 21 July 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

Z. Dikmen et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
Au3+ sensing.
DMF was used for all spectral analysis. First, 1 µM HPhTT solution in
DMSO was prepared as a fluorophore stock solution for further UV–vis
titration studies and 0.1 µM HPhTT stock solution was prepared for
fluorescent studies.
Eventhough synthesis of HPhTT molecule has been reported before,
there has not been found a view on the optical and fluorescent sensor
properties of the fluorophore [59,60]. As first reported herein, HPhTT is
a TTz containing small molecule that exhibits a multiple optical
response. Due to HPhTT has many advantage such as one step synthesis,
easy purification, high photo–chemical stability, determination of su-
perior properties of the dye would bring new usage areas such as sen-
sors, bioimaging etc. Here in, we report synthesis and usage of a dye as
multi – responsive fluorescent sensor for the detection of both Au3+ and
Fe3+ cations via fluorescent quenching and piperidine, AcO-, CN– and F-
anions via emission wavelength shifts. This dye also exhibits acid-
ochromic property. TTz unit of this molecule serve as ligand for metal
cations and electron serving cite for protons while phenolic–OH groups
act as anion and base sensitive part of the molecule. As a result of these
multiple stimuli responsive parts of the HPhTT, the dye can be used for
the detection of different kinds of analytes. Herein, for the first time, we
report an optical view of the photoluminescent sensing properties of
HPhTT dye to determine its possible usage areas.
An appropriate amount of corresponding cation and anion salts were
dissolved in DMSO to prepare 10 mM stock solutions of the metal ions. A
quartz cuvette of 10 µM HPhTT solution was titrated by related metal
stock solutions via addition of desired equivalent of cation solutions till a
plateau is reached in the collected spectra. Anion and piperidine titra-
tion studies were carried out as the same method. All measurements
were performed at room temperature.
2.4. Anion selectivity study
In order to determine the selectivity of different anions towards
HPhTT dye in DMSO, UV–vis and fluorescence spectroscopic studies
were carried out by using the tetrabutylammonium (TBA) salts of anions
(TBAX where X = F-, Cl-, I-, Br-, AcO-, CN–, HSO-4 and H2PO-4). Optical
response to titration of the HPhTT with anion salt solutions was also
examined by UV–vis absorption and fluorescent spectra in DMSO.
2. Experimental
2.5. Cation selectivity study
2.1. Materials and instrumentation
Cation selectivity of the HPhTT dye was determined by using metal
salt solutions prepared in DMSO. UV–vis and fluorescence spectra were
recorded to determine its selectivity on various cations. HPhTT dye was
also titrated with the studied cations in DMSO by recording the optical
response via UV–vis and fluorescence spectral analysis. Cations used for
All chemical solvents, reagents, cationic salts and anionic salts were
supplied commercially and used without any further purification. 4-Hy-
droxy benzaldehyde (98%), dithiooxamide (97%), trifluoroacetic acid
(TFA, 99%), hydrochloric acid (HCl, 37%), H3BO3 (99.5%), KOH (85%),
pyridine (ACS Reag.), piperidine (99%), cyclohexanone (CHp, ACS
Reag.) were purchased from Sigma-Aldrich. AgNO3 (99.5%), AlCl3
(99%), ZnCl2 (99%), FeCl3 (99%), SnCl2 (98%), NiCl2 (99.5%), CuCl2
(99%), Hg(NO3)2 (98.5%), CdCl2 (99.9%), dimethyl formamid (DMF,
99.8%), acetonitrille (ACN, 99.8%) and tetrabutylammonium salts
(TBAX): F- (75%, in H2O), Cl- (97%), I- (99%), AcO- (99%), CN– (95%),
HSO-4 (97%), H2PO-4 (99%) were purchased from Sigma-Aldrich.
KAuCl4.3H2O (98%) and TBABr (99%) were purchased from Acros Or-
ganics. MgCl2 (99%), Hg2(NO3)2 (98%), CoCl2 (98%), HCl (37%) were
purchased from Merck. Acetic acid (AcOH, 100%), KH2PO4 (99%) was
supplied from Reidel-de Haen. Dichloromethane (DCM, 99.9%), dime-
thylsulfoxide (DMSO, 99.9%) were supplied from Carlo Erba and
tetrahydrofuran (THF, 99.9%) was from Honeywell. 1H NMR spectrum
was recorded with Jeol ECZ 500R spectrometer at 80 ◦C. UV–vis spectra
were recorded on Perkin-Elmer Lambda 35. Photoluminescence (PL)
spectra were recorded with Perkin-Elmer LS-55 spectrophotometer in
the solid state by using a permeable filter due to high fluorescent in-
tensities of samples.
all of the measurements were obtained as chloride or nitrate salts (Au3+
Ag+, Al3+, Zn2+, Fe3+, Mg2+, Sn2+, Ni2+, Cu2+, Hg2+, Hg+, Cd2+, Co2+
Fe3+) and their solvents were prepared in DMSO.
,
,
2.6. Acidochromic property of the HPhTT
UV–vis and fluorescence spectra of HPhTT solutions in different pH
buffer and acid-base solutions (pH = 1, 3, 5, 7, 9, 11, 12, 13) were
recorded to show the acidochromic behaviour. Buffer solutions of
AcOH/AcOK for pH = 5; KH2PO4/ K2HPO4 for pH = 7; H3BO3/KH2BO3
for pH = 9 were used for the experiments. Buffer solutions were pre-
pared with procedures well described by using Perrin’s desciription
[62]. HPhTT solution in piperidine was also examined to observe the
effect of an organic base.
3. Results and discussions
A fluorescence sensor was designed to generate signal changes
resulting from substrate interaction with the thiazolo-thiazole core
conjugated to phenolic –OH. The design of such thiazole based dye is
important because of the relatively more sensitive fluorescent switches
and electron or charge transfer (CT) of the TTz core. This fluorescence
sensor acts as donor–acceptor-donor (D-A-D) type sensor consisting
para-substituted phenols as donors (D) and TTz chromophore as
acceptor (A), thus, this structure significantly affects the switching
process, absorption and emission wavelengths. The synthesized fluo-
rescence sensor is a thiazolo-thiazole dye substituted symmetrically with
the receptor sensitive to analytes on the chromophore part. TTz based
sensor dye that is containing cation–acid sensitive part composing from
thiazolo-thiazole bicyclic ring, and anion–base sensitive phenolic–OH
part was designed and examined towards various analytes to exhibit its
multiresponsive properties. The sensitivity to the cationic analyte is
based on the nitrogen atoms of the TTz groups while the phenol groups
act as receptors. In order to test the possible applications of TTz chro-
mophoric derivatives having strong and analytically valuable effects on
the absorption and emission behaviour, the photophysical changes of
HPhTT dye were examined as a fluorescence sensor upon various anion
interaction and complexation of various metal cations in DMSO. TTz
2.2. Synthesis
HPhTT dye was synthesized according to the literature by modifi-
cations [60,61]. A solution prepared with 4-hydroxybenzaldehyde
(120.0 mmol, 15.0 g) and dithiooxamide (41.6 mmol, 5.0 g) was
refluxed in 10 mL pyridine for 5 h. The mixture was then cooled to room
temperature and precipitated with ethanol. HPhTT compound was
filtered and recrystallized in cyclohexanone as yellow needle like crys-
tals (yield: 74%). HPhTT product was characterized with 1H NMR in
DMSO‑d6: d = 10.16 ppm (–OH), 7.79–6.88 (8H-m, 3J = 9.00 Hz)
(Fig. S1).
2.3. Photophysical characterization and titrations of HPhTT
The photophysical properties of HPhTT solutions were determined
via UV–vis and fluorescent spectroscopies in four different solvents
(THF, DMF, DMSO, CHp) having different polarities. It is also worth to
mention that the solvents to study were very limited because of solu-
bility limitation of the HPhTT. Due to the better solubility in DMSO,
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Z. Dikmen et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
ring in the structure is acceptor group while phenol ring is donor group
of the dye. Optical properties of an intramolecular charge transfer (ICT)
fluorophore is strongly dependent on the nature of the donor and
acceptor groups.Protonation of nitrogen atoms in the TTz ring by
weakening the donor capacity of side phenol groups and causing an
increase of acceptor strength of central rings results with fluorescent
quenching as depicted in Fig. 1 [63]. In our designed D–A–D dye,
thiazolo-thiazole acceptor unit is responsive to acid and metal cations
while phenolic–OH group acting as donor is sensitive to anions and base.
The base interacted form of the molecule has oxygen anions that is a
stronger donor than the phenolic group causing red shift of emission
wavelength maxima [64]. Possible mechanisms for the HPhTT dye is
depicted in Fig. 1.
interaction of metal cations with DMSO as we investigated from spec-
troscopic titration of Au3+ cation in DMSO in absence of HPhTT
(Fig. S3).
In absorption spectra of HPhTT dye after addition of Au3+, Cu2+ and
Hg1+ cations, an intense band in short wavelength region that are
related to DMSO and cation interaction was observed while all other
metals have almost the same absorbance in the presence of HPhTT
(Fig. 2a). Au3+ addition resulted with an absorption band at 382 nm
related with interaction between thiazolo[5,4-d]thiazole ring and Au3+
cation. The complexation-induced effects of metal cations on emission
spectra are shown in Fig. 2b. Eventhough Fe3+ cations causes fluorescent
quenching of the dye, HPhTT dye exhibits more sensitive fluorescent
response towards Au3+ as shown in fluorescence intensity comparison
graph (Fig. 2c). From the photo daylight is given in Fig. 2d and the
digital image in Fig. 2e, it can be concluded that the complexations of
dye with both Au3+ and Fe3+ ions are fluorescently quenched; it is seen
that this dye shows much higher selectivity to Au3+ than other metals.
Fluorometric response of HPhTT towards all other cations were almost
the same; except Fe3+ and Au3+ ions.
Eventhough TTz functional backbone containing molecules are used
in
a large variety of studies, mostly based on optoelectronics
[37,39–42], there are very limited studies about their sensing properties
[54–56]. Herein we report a D-A-D type ICT fluorophore containing TTz
ring which is sensitive to multi-analytes. The optical response of the
fluorophore against Au3+, makes it a promising candidate as sensor
probe with remarkable (54.7
μM) LOD value. The probe also gives op-
Moreover, the absorption and fluorescent titration profiles of HPhTT
dye with increasing concentrations of Au3+ and Fe3+ ions were also
examined. The intensities of all absorption bands decreased gradually
with increasing concentrations of both Au3+ and Fe3+ ions (Fig. 3a-d).
The absorption intensities attained saturation after addition of 38 equiv.
of Au3+ and 23 equiv. of Fe3+ ions and did not change. Fluorescent in-
tensities of HPhTT obtained via titration with increasing Au3+ and Fe3+
ions decreased that indicated fluorescent quenching. The fluorescence
intensities attained saturation after addition of 76 equiv. of Au3+ and 25
equiv. of Fe3+ ions and did not change. There was downward curvature
between the fluorescence intensities and concentrations for Au3+ while
linear curve good fitting for Fe3+ over a large range of concentrations as
given in Fig. 3c-f. Stern–Volmer plot [(F0/F) versus concentration] was
an upward curvature for Au3+ (Fig. 3c) [61]. The limit of detection of
portunity to detect anions in diluted and concentrated mediums. We also
determined linear color change dependence of fluorophore with
increasing anion amounts. In this context, white light emission can also
be obtained by changing amount of additive anions. This property might
be used in optoelectronics.
3.1. Effect of cations on photophysical properties of HPhTT
Effects of solvents with different polarities (DMSO, DCM, THF, ACN
and CHp) in the dye (1.0 µM for absorption and 0.1 µM for fluorescence
emission) at room temperature were examined via absorption and
fluorescence spectroscopies (Fig. S2). In a low polarity solvent such as
cyclohexanone, the emission occurs only from an locally excited (LE)
state, whereas in a more polar solvent such as DMSO, an ultra-fast
excited state charge transfer reaction occurs to the hydroxyl donating
basic chromophore. This results in strong quenching of the LE emission
and the emergence of a batchromically shifted emission from a lower CT
state. In complexation with the cation, it inhibits the CT process and
reopens the LE emission; ultimately, it can be used to generate a very
efficient fluorescence key with very large fluorescence enhancement
factors. Since the thiazolo-thiazole ICT sensor is essentially
donor–acceptor-substituted fluorophores with strong solvent-dependent
behaviour, DMSO was used as the solvent for all spectrometric mea-
surements. The UV–vis absorption and emission behaviour of towards
variety of metal ions has been investigated in DMSO at room tempera-
ture, absorption (1.0 µM) and emission (0.1 µM) spectra are given in
Fig. 2a-b. HPhTT dye has an absorption wavelength region centered at
382 nm and emission wavelenght maxima (λmax) of 446 nm.
Au3+ was determined as 54.7
μ
M. Stern–Volmer plot for Fe3+ titration
was a linear curve. Titration curves of HPhTT dye towards other metals
without a remarkable change are given in Figs. S4 and S5. The limit of
detection of Fe3+ was determined as 57.3
μM. Photoinduced electron
transfer (PET) is offered for the quenching of the fluorophore resulting
from the interaction between the fluorophore and Au3+, Fe3+ ions in
which electron density diminished upon metal binding [65].
3.2. Effect of anions on photophysical properties of HPhTT
Spectrophotometric and spectrofluorometric titrations were carried
out to investigate anion sensing ability of HPhTT (1.0 µM in DMSO) by
titrating dye with tetrabutylammonium (TBAX) salts (X: AcO-, Br-, CN–,
Cl-, F-, H2PO4- , HSO-4 and I-, 10 mM in DMSO). The titration results were
given in Fig. 4. A new absorption band with 460 nm λmax after addition
of F-, CN– and AcO- anions were occured originated from stronger ICT
mechanism as a result of deprotonation of phenol segment of the dye.
Any remarkable change in the absorbance and fluorescence spectra has
not been observed after addition of other anions (Figs. S6 and S7). Anion
sensitivity properties were also determined by performing emission
titration studies of HPhTT (0.1 µM in DMSO) with the anions. The
emission spectra observed by titration of HPhTT with AcO-, CN– and F- is
shown in Fig. 4b-c. In the emission spectra, it is clearly seen that the
Fig. 2a shows the absorption spectrum of free ligand complexes of
Al3+, Au3+, Cd2+, Co2+, Fe3+, Ag+, Hg+, Mg2+, Sn2+, Hg2+, Zn2+ (10
mM) to the HPhTT dye in DMSO. The figure shows blue shift accom-
panying with a fluorescent quenching through the binding of the Au3+
metal ion. There is no shift when interacting with Al3+, Cd2+, Co2+
,
Fe3+, Ag+, Hg2+, Mg2+, Sn2+, Zn2+cations, whereas complexing with
Au3+, Cu2+, Hg+ have new absorption band formation at 327, 296 nm
and 279 nm respectively. These absorption peaks are related with
Fig. 1. Schematic illustration of ICT mechanisms of HPhTT in various media.
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Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
Fig. 2. UV–vis spectra of HPhTT (a), fluorescent spectra of HPhTT (b) and fluorescent intentisies of HPhTT at λmax (c) before and after addition of metal salt
solutions. Digital images of HPhTT (c = 10 μM in DMSO) after addition of 25 equiv. (c = 250 μM) of various metal cation solutions under day-light (d) and under UV-
light (e).
Fig. 3. UV–vis spectra of HPhTT dye–Au3+ solutions (a), fluorescent spectra of HPhTT dye–Au3+ solutions (b), C–I curve and Stern–Volmer plot of HPhTT dye–Au3+
solutions (c), UV–vis spectra of HPhTT dye–Fe3+ solutions (d), fluorescent spectra of HPhTT dye–Fe3+ solutions (e), C–I curve and Stern–Volmer plot of HPhTT
dye–Fe3+ solutions (f).
fluorescence intensity changes with the addition of AcO-, CN– and F- to
the medium. This remarkable change shows that the fluorophore in-
teracts with these anions, resulting in a new emission band at a longer
wavelength (550 nm). Moreover, the color of the solutions were slightly
changed from bright into yellowish color after addition of F-, CN– and
AcO- anions.
absorption and fluorescent intensities attained saturation. AcO- , CN–
and F- addition resulted with a new band at 550 nm with increasing
intensity and a decrease on the peak at 444 nm due to the concentration
increases. Moreover, while there were yellow color alteration under the
daylight in Fig. 4d., the emission color became white under UV light in
Fig. 4e. Deprotonation of HPhTT is observed in two steps. After diluted
TBAX (X: AcO- , CN– and F-) addition, anion-phenolic-OH group in-
teractions were observed [66] while deprotonation of phenolic-OH
groups was observed after the concentrated TBAX addition. 1H NMR
spectra of HPhTT with increasing TBAF concentration (concentrated
TBAF was used) were given in Figure S11. The spectrophotometric
titration studies with concentrated the AcO- , CN– and F- anions are given
in Fig. S8. The spectrophotometric analysis indicates possible deproto-
nation of the phenolic-OH group through dipole-anion interactions
A new shoulder peak at 550 nm λmax after addition of AcO-, F- and
CN– anions was occured and this shoulder emission peak was observed
after increasing excitation wavelenght from 375 nm to 420 nm as given
in Fig. 4c. After F- addition a broader sholuder occured at 485 nm.
Bathochromic shift of absorption and emission wavelenghts maxima in
the spectra originates from enhanced ICT. Anion addition results with
deprotonation of phenolic–OH group causing formation of oxygene
anion as stronger donor. Titration of HPhTT with anions was studied till
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Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
Fig. 4. UV–vis spectra of HPhTT dye–anion solutions (a), fluorescent spectra of HPhTT dye–anion solutions, λexc = 375 nm (b), fluorescent spectra of HPhTT
dye–anion solutions, λexc = 420 nm (c), digital images of anion–HPhTT dye solution under daylight (d), and under UV-light (e).
between Ph-OH and F- ion, forming the intramolecular charge transfer
(ICT) state in the conjugated system of phenoxide anion with thiazolo-
thiazole.
potential to be used as a fluorescence sensor due to its interaction with
AcO-, CN– and F- anions.
The plots of intensity ratios (I550/I444) versus concentration showed
ratiometric fluorescent response as given in Fig. 5a-f. The dye interacts
with AcO-, CN– and F- anions through mechanism of deprotonation of
the phenolic OH following dipole-anion interactions between Ph-OH
and anions. While the emission peak (λmax = 444 nm) of the dye in
DMSO is 460 nm, it is observed that the fluorescence intensity at 444 nm
decreases as a result of the F- addition while a new emission band is
formed at long wavelength (550 nm). It has shown that the dye has a
3.2.1. Reversibility
Reversibility and reusability of the sensor are particularly important
for applications in real environments. To test the applicability of
HPhTT, titration studies were performed by the addition of trifluoro-
acetic acid (TFA) via using absorption and fluorescence spectroscopy
techniques. After adding 10 equiv. of the anions (only AcO-, F- and CN–)
to the HPhTT solution, a new band was observed at 464 nm. When 10
equiv. of TFA was added to the mixture, the absorption maxima of the
Fig. 5. Fluorescent spectra of F- anion titration curves of HPhTT dye solution (a), fluorescent spectra of CN– anion titration curves of HPhTT dye solution (b),
fluorescent spectra of AcO- anion titration curves of HPhTT dye solution, λexc = 375 nm (c), I550/I444-concentration plot of F- titration (d), I550/I444-concentration plot
of CN– titration (e), I550/I444-concentration plot of AcO- titration (f).
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Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
dye had hypochromic effect and the dye immediately shifted to the
original maxima (Fig. 6a-f). Color switching of HPhTT dye under UV-
light excitation from blue to yellowish and then back to blue with
addition of TFA shows the change of the fluorescence behaviour as
shown in Fig. 6a-c. Additionally, the repeated display of the visual
colour change shows the precise recycling and reusability of the dye.
bathochromic shift was obtained instead of a new band formation with
the addition of base, that results from different optical properties in
aqueous media. UV–vis and fluorescence spectra of HPhTT after H2O
addition measurements and digital images of the solutions under
daylight and UV-light are given in Fig. S9. Importantly, the dye display
different optical response against OH– anions in aqueous media.
An increase of I550/I444 ratio resulted with more obvious color
change under UV light excitation, as expected. After addition of diluted
bases (piperidine, F-, CN–, AcO-, OH–), the color changes were deter-
mined in blue region as shown in Fig. 7e, resulting from the dipole-ion
interractions between –OH and anions. In concentrated basic media,
the color changes were determined in yellow-green region as shown in
Fig. 7e, resulting from phexoide ion formation leading to enhanced ICT.
Eventhough the chromaticity diagrams are commonly used to estimate
color purity and color coordinates for LED researches, usage of such
softwares in fluorescent sensor studies may help to estimate usability of
the fluorescent sensor under UV excitation. We demonstrate usability of
color calculator software after titration of HPhTT with F- anion as given
in Fig. 8a-b. The color change of the HPhTT solutions titrated with F-
anion and calculated colors are given in Fig. 8a. The images shows good
estimation of color by the Color Calculator Software. Increasing F- anion
concentration resulted with linear color change in the chromaticity di-
agram (Fig. 8b). The x and y values calculated for each concentration of
F- was graphed and a linear curve was obtained (Fig. 8c). The related
concentrations of F- were also graphed with increasing I550/I444 ratio
and given in Fig. 8d. After addition of 25 equivalent AcO-, F- and CN–
anions and piperidine in HPhTT solution, the I550/I444 ratios were
calculated as 0.172, 0.208, 0.301, 2.169 respectively. Moreover,
obvious color change under UV light excitation was observed with
concentrated piperidine (2 M). UV–vis spectra and fluorescent spectra of
the HPhTT dye before and after titration with both concentrated and
diluted piperidine were also recorded and digital images of the solutions
under daylight UV-light are given in see, FigS10. Spectroscopic results
indicated a linear change on I550/I444 ratio with concentration.
3.3. Acidochromic properties of HPhTT
In order to understand better the effect of acidic and basic environ-
ments on the HPhTT dye, photophysical response of the dye in different
pH buffer solutions was studied. The pH dependent spectral properties
of the dye were evaluated in Perrin’s description buffer solutions with
pH values ranging from 1, 3, 5, 7, 9, 11 and 13. The acid and base effect
on photophysical properties of HPhTT dye was observed with the ab-
sorption and emission spectra as shown in Fig. 7a-b.
The absorption spectra of the HPhTT dye showed the maxima
wavelength at 330 nm between pH 1 and 7, while the wavelength
maxima shifted to 415 nm between pH 9 and 11. When the pH range
changed from 7 to 9, it was noticed with the naked eye in daylight that
the absorption color of the dye changed from transparent to yellow with
significant color change. The pH dependent spectroscopic study
appeared to give a clear response in absorption. It was observed that the
change of pH had a great influence on the absorbance of the dye and
caused an absorption maximum at 68 nm between pH 7–10. According
to the emission spectra, the fluorescence intensity of the dye increased
significantly with increasing pH from 7 to 10. Turn on mechanism of the
dye from acidic pH ranges to pH 10 in aqueous medium can be clearly
seen in Fig. 7a-b. In particular, it is clearly seen that the HPhTT dye is
yellow in daylight in a basic media and colourless in an acidic media
(Fig. 7c). The fluorescence intensity increased dramatically and conse-
quently indicated a charge transfer (CT) at pH 10. As it can be seen in
Fig. 7d, deprotonation of hydroxyl group in the basic media (pH > 9)
caused enhanced ICT while protonation of N atoms in the TTz ring and
weakening of electron donating ability of phenolic-OH at acidic medium
resulted with quenching of fluorescence. In aqueous basic media,
bathochromic shift in both the absorption wavelength maxima at 412
nm λmax and the emission wavelenght at 498 nm was observed. The
Filter papers were interacted with HPhTT/PVA solution in 0.01 M
NaOH and in 0.1 M NaOH to prepare sensor papers (Fig. 9). The sensor
papers emitting blue light (prepared with 0.01 M NaOH) were used to
determine anion sensitivity, they are not sensitive against cations as
Fig. 6. Before and after TFA addition: UV–vis spectra of AcO-–HPhTT solution (a), UV–vis spectra of CN––HPhTT solution (b), UV–vis spectra of F-–HPhTT solution
(c), fluorescent spectra of AcO-–HPhTT solution (d), fluorescent spectra of CN––HPhTT solution (e), fluorescent spectra of F-–HPhTT solution (f).
6

Z. Dikmen et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
Fig. 7. UV–vis spectra of HPhTT dye in different pH values (a), fluorescent spectra of HPhTT dye in different pH values (b), digital images of HPhTT dye solutions in
different pHs under daylight (c), under UV-light (d), and ICE 2015 chromaticity diagram of the fluorescent sensor dye before and after addition of analytes.
Fig. 8. Digital images of HPhTT dye titrated with different equivalents of F- and good fitting calculated colors in CIE 2015 chromaticity diagram (a), CIE co-
ordinations of HPhTT dye with increasing equivalent of F- (b), x-y coordinates of CIE 2015 chromaticity diagram with increasing [F-] (c), I550/I444 versus [F-] (d).
shown in Fig. 9. Interestingly, the paper was observed selectively
interacted with CN– anion. The sensor papers emitting green light
(prepared with 0. 1 M NaOH) were used to detect cations. Au3+ (in
DMSO) treatment resulted in blue shift, while Au3+ (in H2O) treatment
resulted in quenching of fluorescence.
(HPhTT), is synthesized and its selectivity and sensitivity to multiple
analytes has been investigated. The synthesis of fluorescent sensor dyes
with superior properties reported in the literature involves stepwise
reactions or long syntheses in many studies [4,21,67,68]. One–step
synthesis, easy purification and superior optical properties of HPhTT is
reported in here, that makes the dye an attractive candidate for many
sensing applications.
4. Conclusion
The selectivity and sensitivity of HPhTT to cations and anions and its
photophysical behaviour in aqueous, acidic and basic mediums were
determined. Ratiometric sensing of both Au3+ and Fe3+ cations is
In this study, a fluorescence symmetric sensor with thiazolo-thiazole
(TTz) core, namelly 2,5-bis(4-hydroxyphenyl)thiazolo[5,4-d]thiazole
7

Z. Dikmen et al.
Journal of Photochemistry & Photobiology, A: Chemistry 419 (2021) 113456
Fig. 9. Digital images of HPhTT dye modified filter papers after interacted with anions and cations.
phosphane, A new fluorescent dye for imaging of low pH regions and lipid
membranes in living cells, Dyes Pigments 184 (2021) 108771–108785.
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possible with HPhTT, while ratiometric anion sensing is also possible via
enhanced ICT mechanism even in concentrate mediums. We also pro-
posed gradually increasing ICT enhancement to explain various I550/I444
ratios depending on dipole-ion interactions between Ph-OH and ions and
followed by deprotonation of phenolic-OH. The chemosensor dye is
responsive against AcOꢀ , CN– and Fꢀ anions in both diluted and
concentrated media. We also showed usability of color calculator to
estimate color change of the sensor after addition of analyte. Moreover,
the fluorescence of the dye showing remarkable changes in acidic and
basic aqueous pH buffer solutions can be used as a potential pH sensor. It
has been found that it is a Turn-ON / Turn-OFF type sensor with its
acidochromic effect in an aqueous media. As a result, in the context of
fluorescence and colorimetric sensor development and application
focussed on the detection of multiple analytes, the thiazolo-thiazole
(TTz) dye systems can be used selectively to identify both cations and
anions as dual sensor.
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CRediT authorship contribution statement
[9] O. Hanmeng, N. Chailek, A. Charoenpanich, P. Phuekvilai, N. Yookongkaew,
N. Sanmanee, J. Sirirak, P. Swanglap, N. Wanichacheva, Cu2+selective NIR
fluorescence sensor based on heptamethine cyanine in aqueous media and its
application, Spectrochim. Acta A 240 (2020) 118606–118612.
Zeynep Dikmen: Conceptualization, Methodology, Writing - orig-
inal draft, Visualization, Validation. Onur Turhan: Methodology, Data
curation, Formal analysis. Müjgan Yaman: Methodology, Investigation,
Writing - original draft. Vural Bütün: Supervision, Writing - review &
editing, Funding acquisition.
[10] C.Q. Li, L. Xiao, Q.Y. Zhang, X.J. Cheng, Reaction-based highly selective and
sensitive monomer/polymer probes with Schiff base groups for the detection of
Hg2+ and Fe3+ ions, Spectrochim. Acta A 243 (2020) 118763–118773.
[11] G.J. Li, Y.H. Guan, F.Y. Ye, S.H. Liu, J. Yin, Cyanine-based fluorescent indicator for
mercury ion and bioimaging application in living cells, Spectrochim. Acta A 239
(2020) 118465–118471.
Declaration of Competing Interest
¨
¨ ¨
˘
[12] O. S¸ahin, Ü.O. Ozdemir, N. Seferoglu, Z.K. Genc, K. Kaya, B. Aydıner, S. Tekin,
˘
Z. Seferoglu, New platinum (II) and palladium (II) complexes of coumarin-thiazole
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.
Schiff base with a fluorescent chemosensor properties: Synthesis, spectroscopic
characterization, X-ray structure determination, in vitro anticancer activity on
various human carcinoma cell lines and computational studies, J. Photochem.
Photobiol. B 178 (2018) 428–439.
¨
¨ ¨
˘
[13] B. Aydıner, O. S¸ahin, D. Çakmaz, G. Kaplan, K. Kaya, Ü.O. Ozdemir, N. Seferoglu,
˘
Z. Seferoglu, A highly sensitive and selective fluorescent turn-on chemosensor
Acknowledgments
bearing a 7-diethylaminocoumarin moiety for the detection of cyanide in organic
and aqueous solutions, New J. Chem. 44 (2020) 19155–19165.
[14] G.E. Fernandes, Y.W. Chang, A. Sharma, S. Tutt, One-Step Assembly of
Fluorescence-Based Cyanide Sensors from Inexpensive, Off-The-Shelf Materials,
Sensors 20 (2020) 4488–4501.
This work was supported by the Scientific and Technological
Research Council of Turkey (TUBITAK), GN: 119Z519 and ESOGU-BAP
projects, GN: 201619010. Z.D. acknowledges TUBITAK for the financial
support through BIDEB 2211-A program.
[15] Q. Li, B.B. Guan, W. Zhu, T.H. Liu, L.H. Chen, Y. Wang, D.X. Xue, Highly selective
and sensitive dual-fluorescent probe for cationic Pb2+ and anionic Cr2O27-, CrO24-
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Appendix A. Supplementary data
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9