Synthesis, spectroscopic, thermal and electrochemical studies on thiazolyl azo based disperse dyes bearing coumarin

Article abstract of DOI:10.1016/j.molstruc.2015.12.032

In this study, seven novel thiazolyl azo disperse dyes (6a-g) were synthesized and fully characterized by FT-IR, ¹H NMR, ¹³C NMR, and mass spectral techniques. The electronic absorption spectra of the dyes in solvents of different polarities cover a λ_max range of 404-512 nm. The absorption properties of the dyes changed drastically upon acidification. This was due to the protonation of the nitrogen in the thiazole ring, which in turn increased the donor-acceptor interplay of the π system in the dyes, and therefore increased the absorption properties of the prepared dyes. Thermal analysis showed that these dyes are thermal stable up to 269°C. Additionally, the electrochemical behavior of the dyes (6a-g) were investigated using cyclic voltammetric and chronoamperometric techniques, in the presence of 0.10 M tetrabutylammonium tetrafluoroborate, in dimethylsulfoxide, at a glassy carbon electrode. The number of transferred electrons, and the diffusion coefficient were determined by electrochemical methods. The results showed that, for all the dyes, one oxidation peak and two reduction peaks were observed.

Full text of DOI:10.1016/j.molstruc.2015.12.032

Accepted Manuscript
Synthesis, spectroscopic, thermal and electrochemical studies on thiazolyl azo based
disperse dyes bearing coumarin
Müjgan Özkütük, Ezgi İpek, Burcu Aydıner, Serhat Mamaş, Zeynel Seferoğlu
PII:
S0022-2860(15)30520-2
10.1016/j.molstruc.2015.12.032
MOLSTR 22065
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 26 October 2015
Revised Date: 7 December 2015
Accepted Date: 14 December 2015
Please cite this article as: M. Özkütük, E. İpek, B. Aydıner, S. Mamaş, Z. Seferoğlu, Synthesis,
spectroscopic, thermal and electrochemical studies on thiazolyl azo based disperse dyes bearing
coumarin, Journal of Molecular Structure (2016), doi: 10.1016/j.molstruc.2015.12.032.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis, spectroscopic, thermal and electrochemical studies on
thiazolyl azo based disperse dyes bearing coumarin
Müjgan Özkütük a,*, Ezgi İpek a, Burcu Aydıner b, Serhat Mamaşb and
Zeynel Seferoğlub
aEskişehir Osmangazi University, Department of Chemistry, 26480 Eskişehir, Turkey
bGazi University, Department of Chemistry, 06500 Ankara, Turkey
Abstract
In this study, seven novel thiazolyl azo disperse dyes (6a-g) were synthesized and fully
1
characterized by FT-IR, H NMR, 13C NMR, and mass spectral techniques. The electronic
absorption spectra of the dyes in solvents of different polarities cover a λmax range of 404-512
nm. The absorption properties of the dyes changed drastically upon acidification. This was
due to the protonation of the nitrogen in the thiazole ring, which in turn increased the donor-
acceptor interplay of the π system in the dyes, and therefore increased the absorption
properties of the prepared dyes. Thermal analysis showed that these dyes are thermal stable up
o
to 269 C. Additionally, the electrochemical behavior of the dyes (6a-g) were investigated
using cyclic voltammetric and chronoamperometric techniques, in the presence of 0.10 M
tetrabutylammonium tetrafluoroborate, in dimethylsulfoxide, at a glassy carbon electrode. The
number of transferred electrons, and the diffusion coefficient were determined by
electrochemical methods. The results showed that, for all the dyes, one oxidation peak and
two reduction peaks were observed.
Keywords: Azo dyes, Coumarin, 2-aminothiazole, Acidochromism, Spectroscopy,
Electrochemistry.
*Corresponding author Tel.: +90 222 2393750; fax: +90 222 2393578
*E-mail addresses: mujgan@ogu.edu.tr, znseferoglu@gazi.edu.tr
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1. Introduction
Nitrogen-containing heterocyclic compounds constitute an important class of aromatic
heterocyclic structures due to their exceptional pharmacological properties [1]. These
compounds have a variety of functional groups and form the backbone of bioactive
compounds [2-6]. Thiazole ring plays an important role in the chemistry of heterocyclic
compounds and exists in the structure of a number of biologically active molecules [7,8]. It is
found in the structure of some natural products such as Vitamin B1 (thiamine), penicillin and
carboxylase. Furthermore, 2-aminothiazole which is an important derivative of thiazole, is a
leading precursor in the development of drug design [9]. 2-aminothiazole based compounds,
which are derived by attaching different substituents at different positions on the ring, show
some important pharmacological and biological activities [11,12]. Apart from the
pharmacologically important field of use, 2-aminothiazole and its derivatives are widely used
as coupling and diazo components in dye industry [13]. The increase in fiber production in the
textile industry requires the development and synthesis of new disperse dyes. Disperse azo
dyes have attracted considerable attention in having good fastness properties, and due to their
cheap production in recent years. Color, light, and wash fastness of heteroarylazo disperse
dyes obtained by coupling with the diazo component of another heteroaryl component are
better than the carbocyclic analogs, since both the diazo and the coupling components are
heterocyclic compounds [14-16]. Therefore, in recent years, a number of azo dyes, which
were obtained by using 2-aminothiazole as the coupling component and its derivatives as the
diazo components, and their properties were investigated [17-20]. The synthesis of these new
derivatives of 2-aminothiazo will particularly contribute to diversification, and will also lead
to increase in color range.
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Coumarin has a high fluorescence in visible light, a high quantum yield, and superior photo-
stability [21-24]. Moreover, the absorption and emission characteristics, according to the
characteristics of the solvent used, can be adjusted easily in coumarin derivatives, especially
when having electron accepting or donating substituents at the 3- and 7-positions [25,26].
Coumarin derivatives obtained in these forms are potential materials for a wide field of
applications such as optical brighteners, laser dyes, nonlinear optical chromophores (NLO),
solar collectors, monitoring systems in medicine and biology, electroluminescence (EL)
materials, fluorescent labels and chemosensors [27-36]. Furthermore, coumarin ring is readily
available and functionalized. Therefore, we proposed that the integration of coumarin unit
into thiazolyl azo disperse dye based structures, leads to novel dye structures with promising
properties that may be considered for the development of functional dyes. Therefore, in this
study, we report the syntheses of azo dyes bearing coumarin unit as the main structure. Due to
its good photophysical properties we think that the synthesized azo dyes (6a-g) can exhibit
better photophysical activity. Firstly, here we determined the photophysical properties of a
series of newly synthesized thiazolyl azo disperse dyes in different solvents and then
examined the azo-hydrazone tautomeric equilibria in the same solvents. The reduction-
oxidation and thermal behavior of the azo dyes play an important role in biomedical research,
dyeing textile fibers, liquid crystal displays, electro-optical devices, laser-inkjet printers, and
treatment of azo dyes waste [37]. The electrochemical properties of the synthesized azo dyes
were evaluated. The thermal properties of the synthesized azo dyes were also investigated by
using thermogravimetric analysis.
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2. Experimental
2.1. Materials and Methods
All the chemicals used in this study were purchased from Sigma-Aldrich, and were used as
without any further purification. Thin-layer chromatography was carried out using precoated
aluminium-backed plates (Merck Silica Gel 60 F254) and visualised under UV light (λ=254-
362 nm). FT-IR (ATR) Spectra were recorded on a Perkin Elmer 100 FT-IR spectrometer (ν
1
13
are in cm-1). H and C NMR spectra were recorded on a Bruker 300 MHz spectrometer in
DMSO-d6. Coupling constant (J) are given in hertz (Hz). Signals are abbreviated as follows:
singlet, s; doublet, d; doublet-doublet, dd; triplet, t; multiplet, m. Chemical shifts (δ) are given
in parts permillion (ppm) using the residue solvent peaks as reference relative to TMS. ‘J’
values are given in Hertz (Hz). Mass analysis was obtained by using Waters 2695 Alliance
ZQ Micromass LCMS working with ESI apparatus; in m/z (rel. %) (Department of
Pharmacological Sciences, Ankara University Laboratories). The melting points were
measured using Schmelzpunktbestimmer SMP II apparatus and are uncorrected. Thermal
analyses (TG/DTA) were performed with a Perkin Elmer Diamond TG/DTA Thermal
Analyzer at a heating rate of 10 °C min−1 in a static atmosphere, at the temperature of 30–900
°C. Voltametric measurements were carried out using CHI 760d Electrochemical
Workstation. Glassy carbon electrode (BAS MF-2012) and 11 ꢀm-ultramicro carbon
electrode (BAS MF-2007) were used as the working electrodes. The electrodes were polished
with 1 ꢀm and 0.05 ꢀm alumina slurries made from dry Buehler alumina and ultra pure water,
on polishing microcloth, before each use. A platinum wire was used as the auxiliary electrode
(BAS MW-1032). The reference electrode was a silver wire in contact with 0.01 M AgNO3
(BAS MF-2052). All the solutions were deaerated for 15 min. with pure nitrogen. All
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measurements were taken at room temperature, 21 ± 1 °C. In all the voltammetric
measurements, the background currents were automatically subtracted from the original
currents obtained. The number of transferred electrons, and the diffusion coefficients were
determined by ultramicro electrode CV technique of Baranski method [38].
2.2. Preparation of dyes (6a-g)
Diazotization of various heterocyclic amines was affected using H2SO4 and NaNO2. A typical
diazotization and coupling procedures are described below for 2-amino-4-(coumarin-3-
yl)thiazole (3) and 1,2-dimethyl-1H-indole (5a), and all the other dyes were prepared in a
similar manner. All the compounds obtained were purified by crystallization from ethanol,
and then analyzed.
Preparation of (E)-3-(2-((1,2-dimethyl-1H-indol-3-yl)diazenyl)thiazol-4-yl)-2H-chromen-2-
one (6a)
2-amino-4-(coumarin-3-yl)thiazole (3) (0.49 g, 2.0 mmol) was dissolved in glacial acetic
acid-propionic acid mixture (2:1, 6.0 mL) and was rapidly cooled in a salt/ice bath to 0-5 oC.
The solution was placed in an ice-salt bath to cool. After that, cold solution of NaNO2 (0.15 g,
2 mmol) in water (3.0 mL) was added dropwise while stirring. The mixture was stirred for an
additional 1 h at 0 oC. Excess nitrous acid was removed by the addition of urea. The resulting
diazonium salt (4) was cooled in a salt/ice mixture. The 1,2-dimethyl-1H-indole (5a) (0.29 g,
2.0 mmol) was also dissolved in a mixture of glacial acetic acid- propionic acid solution (3:1,
8 mL) and cooled in an ice bath. Cold diazonium solution was added to this cooled solution in
o
a dropwise manner while stirring. The solution was then stirred at 0-5 C for 1 h. The pH of
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the reaction mixture was maintained at 4-6 by the intermitent addition of saturated sodium
carbonate solution. The mixture was further stirred for 1 h at room temperature. The resulting
solid was filtered, washed with cold water, and then dried. Recrystallization of the resulting
dried solid from ethanol gave an orange powder (MW: 400.45 g/mol, C22H16N4O2S yield:
o
0.649 g, 81%; m.p: 281-282 C); FT-IR (ATR): νmax: 3077 (aromatic C-H), 1712 (C=O),
1484, 1475, 1451 (C=C), 1399 (N=N), 1360, 1315, 1251 (C-N), 1169 (C-O-C) cm-1; 1H NMR
(300 MHz, DMSO-d6): δ 8.94 (s, 1H), 8.36 (m, 1H), 8.29 (s, 1H), 8.00 (dd, J = 7.8, 1.3 Hz,
1H), 7.65 (m, 3H), 7.48 – 7.37 (m, 4H), 3.84 (s, 3H,-NCH3), 2.79 (s, 3H,-CH3); 13C NMR (75
MHz, DMSO-d6): δ 179.3, 159.4, 153.0, 151.9, 147.2, 139.7, 138.4, 132.6, 132.6, 132.4,
129.6, 125.0, 124.6, 122.3, 120.7, 119.7, 119.2, 119.1, 118.5, 116.3, 111.7, 31.0, 10.6; LC-
MS: (M+1)+ (m/z): 401.24 (100%).
Preparation
of
(E)-3-(2-((1-methyl-2-phenyl-1H-indol-3-yl)diazenyl)thiazol-4-yl)-2H-
chromen-2-one (6b)
This dye was obtained by coupling diazotized amine (4) with 1-methyl-2-phenyl-1H-indole
(5b), as a tile red powder (MW: 462.52 g/mol, C27H18N4O2S, yield: 0.592 g, 64%; m.p: 261-
262 °C). FT-IR (ATR): νmax: 3055 (aromatic C-H), 1720 (C=O), 1489, 1468(C=C), 1387
1
(N=N), 1323, 1252 (C-N), 1170 (C-O-C) cm-1; H NMR (300 MHz, DMSO-d6): δ 8.97 (s,
1H), 8.52 – 8.46 (m, 1H), 8.29 (s, 1H), 8.02 (dd, J = 7.7, 1.2; Hz, 1H), 7.85 – 7.76 (m, 3H),
7.70-7.60 (m, 4H), 7.55 – 7.39 (m, 5H), 3.91 (s, 3H,-NCH3); 13C NMR (75 MHz, DMSO-d6):
δ 159.4, 153.0, 139.8, 138.4, 132.4, 132.2, 130.4, 129.6, 128.6, 128.3, 125.8, 125.3, 125.2,
122.9, 120.6, 119.8, 119.7, 118.2, 116.3, 112.1, 32.7; LC-MS: (M+1)+ (m/z): 463.39 (100%),
485.19 (M+Na).
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Preparation of (E)-3-(2-((2-phenyl-1H-indol-3-yl)diazenyl)thiazol-4-yl)-2H-chromen-2-one
(6c)
This dye was obtained by coupling diazotized amine (4) with 2-phenyl-1H-indole (5c), as a
red powder (MW: 448.50 g/mol, C26H16N4O2S, yield: 0.762 g, 85%; m.p: 272-273 oC) FT-IR
(ATR): νmax: 3305 (N-H), 3130 (aromatic C-H), 1717 (C=O), , 1463, 1454 (C=C), 1393,
1
1349 (N=N), 1207 (C-N), 1168 (C-O-C) cm-1; H NMR (300 MHz, DMSO-d6): δ 13.00 (s,
1H,-NH), 8.98 (s, 1H), 8.50-8.42 (m, 1H), 8.38 (s, 1H), 8.16 – 8.09 (m, 2H), 8.02 (dd, J = 7.9,
1.4 Hz, 1H), 7.71–7.52 (m, 5H), 7.51–7.35 (m, 5H); 13C NMR (75 MHz, DMSO-d6): δ 179.3,
159.4, 153.0, 147.6, 146.8, 139.9, 137.0, 132.5, 130.7, 130.5, 129.7, 129.3, 126.0, 125.3,
124.7, 123.3, 120.6, 119.9, 119.7, 119.2, 116.3, 113.1; LC-MS: (M+1)+ (m/z): 449.31 (100%).
Preparation of (E)-3-(2-((4-(diethylamino)phenyl)diazenyl)thiazol-4-yl)-2H-chromen-2-one
(6d)
This dye was obtained by coupling diazotized amine (4) with N,N-diethylaniline (5d), as a
o
claret red powder (MW: 404.48 g/mol, C22H20N4O2S yield: 0.421 g, 52%; m.p: 290-291 C).
FT-IR (ATR): νmax: 3148 (aromatic C-H), 1717 (C=O), 1484 (C=C), 1360 (N=N), 1349, 1235
(C-N), 1150 (C-O-C) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 8.95 (s, 1H), 8.38 (s, 1H), 8.00
(d, J = 6.5 Hz, 1H), 7.85 (d, J = 9.2 Hz, 2H), 7.71-7,60 (m, 1H), 7.51 – 7.37 (m, 4H), 6.91 (d,
J = 7.0 Hz, 3H), 3.15 (s, 6H);13C NMR (75 MHz, DMSO-d6): a good spectrum was not
obtained; LC-MS: (M +2)+ (m/z): 406.62 (100%), 363.09 (6.1%, M-44).
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Preparation of (E)-3-(2-((4-(phenylamino)phenyl)diazenyl)thiazol-4-yl)-2H-chromen-2-one
(6e)
This dye was obtained, by coupling diazotized amine (4) with N,N-diphenylamine (5e), as a
o
claret red powder (MW: 424.47 g/mol, C24H16N4O2S, yield: 0.636 g, 75%; m.p: 275-276 C).
FT-IR (ATR): νmax: 3349 (N-H), 3028 (aromatic C-H), 1708 (C=O), 1496, 1481 (C=C), 1387
1
(N=N) 1252, 1200 (C-N), 1142 (C-O-C) cm-1; H NMR (300 MHz, DMSO-d6): δ 9.36 (s,
1H,-NH), 8.92 (s, 1H), 8.39 (s, 1H), 7.96 (dd, J = 7.7, 1.2 Hz, 1H), 7.85 (d, J = 9.0 Hz, 2H),
7.67 – 7.61 (dt, J =8.5 Hz, 1.4 1H), 7.46 – 7.34 (m, 5H), 7.28 (d, J = 7.5 Hz, 2H), 7.17 (d, J
=9.1 Hz, 2H), 7.10 (d, J =7.2 Hz, 1H); 13C NMR (75 MHz, DMSO-d6): a good spectrum was
not obtained; LC-MS: (M+1)+ (m/z): 425.57 (100%).
Preparation
of
(E)-1,3-dimethyl-5-((4-(2-oxo-2H-chromen-3-yl)thiazol-2-
yl)diazenyl)pyrimidine-2,4,6(1H,3H,5H)-trione (6f)
This dye was obtained, by coupling diazotized amine (4) with 1,3-dimethylbarbituric acid
(5f), as a dark caramel powder (MW: 411.39 g/mol, C18H13N5O5S, yield: 0.197 g, 24%; m.p:
o
324 C). FT-IR (ATR): νmax: 3146 (aromatic C-H), 1723 (C=O), 1434 (C=C), 1347 (N=N)
1
1231 (C-N), 1090 (C-O-C) cm-1; H NMR (300 MHz, DMSO-d6): δ 14.50 (b, tautomeric
hydrazone -NH), 8.81 (s, 1H), 8.16 (s, 1H), 7.93 (d, J = 7.0 Hz, 1H), 7.66 (t, J = 7.7 Hz, 1H),
7.51 – 7.37 (m, 3H), 3.23 (s, 6H,-NCH3);13C NMR (75 MHz, DMSO-d6): a good spectrum
was not obtained; LC-MS: (M+1)+ (m/z): 412.38 (32%), 434.13 (100%).
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Preparation
of
(E)-6-amino-1,3-dimethyl-5-((4-(2-oxo-2H-chromen-3-yl)thiazol-2-
yl)diazenyl)pyrimidine-2,4(1H,3H)-dione (6g)
This dye was obtained, by coupling diazotized amine (4) with 6-amino-1,3-dimethyluracil
(5g), as a light brown powder (MW: 410.41 g/mol, C18H14N6O4S yield: 0.263 g, 32%; m.p:
o
308 C). FT-IR (ATR): νmax: 3222-3168 (N-H), 3028 (aromatic C-H), 1713 (C=O), 1447
(C=C), 1328 (N=N) 1252, 1202 (C-N), 1168 (C-O-C) cm-1; 1H-NMR (300 MHz, DMSO-d6):
δ 11.50 (b, tautomeric hydrazone -NH), 9.40 (s, 1H,-NH), 8.88 (s, 1H), 8.25 (s, 1H), 7.99 (d, J
= 6.7 Hz, 1H), 7.70-7,60 (m, 1H), 7.51 – 7.39 (m, 3H), 3.41 (s, 3H,-NCH3), 3.25 (s, 3H, -
NCH3). 13C- NMR (75 MHz, DMSO-d6): a good spectrum was not obtained; LC-MS: (M+1)+
(m/z): 411.26 (100%).
3. Result and Discussion
3.1. The synthetic procedures and characterizations of thiazolyl azo disperse dyes (6a-g)
The azo disperse dyes (6a-g) were preapared by coupling; 1,2-methylindole (5a), 1-methyl-2-
phenylindole (5b), 2-phenylindole (5c), N,N-diethylaniline (5d), N,N-diphenylamine (5e), 1,3-
dimethylbarbituric acid (5f), and 6-amino-1,3-dimethyluracil (5g), with diazotized 2-amino-
(4-coumarinyl)thiazole (4), (Scheme 1). In the synthetic pathway, 2-hydroxybenzaldehyde
was condensed with ethylacetoacetate in ethanol using catalytic amounts of piperidine to
afford 3-acetylcoumarin (1) in excellent yield. The 3-acetylcoumarin (1) was subjected to
α−bromination reaction to obtain 3-(2-bromoacetyl)coumarin (2). The condensation of (2)
with an active thiourea compound under Hantzsch reactions followed by an intramolecular
cyclization, gave 2-amino-4-(coumarin-3-yl)thiazole (3). Compound (3) was diazotized in
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absolute ethanol, containing nitrosylsulphuric acid to afford the diazotized amine (4). The
thiazolyl azo based disperse dyes (6a-g) were obtained from the diazotized amine (4) and
relevant coupling components (5a-g). Dyes 6a and 6b were used as model compounds for dye
6c, for evaluating its photophysical and tautomeric properties. Dyes 6a and 6b can exist only
in azo tautomeric form. However, dye 6c exists in two possible tautomeric forms, namely the
azo and the hydrazone forms. Dye 6d can be evaluated together with 6e because they were
obtained from similar coupling components. Dye 6d can only exist in azo tautomeric form,
but 6e may exist in both azo and hydrazone tautomeric forms, because of solvent-solute
interactions. Dyes 6f and 6g can also exist in two possible tautomeric forms. Thus, in this
study, the role of the azo-hydrazone tautomerism, in relation to the visible absorption spectra
and solvatochromic behavior of the prepared dyes have been investigated deeply. The
thiazolyl azo disperse dyes were obtained in good yields. All the compounds (6a-g) were
1
characterized by spectroscopic methods (FT-IR, H/13C NMR and LC-MS) (1H/13C NMR
and LC-MS spectra can be seen in Figs. S1a-g, Figs. S2a-c and Figs. S3a-g in
Supplementary materials). Physical and spectral data of the dyes prepared are given in
Section 3.
Scheme 1 is here
The existence of typical lactone carbonyl groups in the synthesized dyes (6a-g), confirmed by
the infrared spectra showed bands at 1723-1708 cm-1 region. The spectra also showed the azo
1
(-N=N-) group at 1399-1348 cm-1 region. The H-NMR spectra of the dyes have a signal at
8.94 ppm attributed to the signal of the hydrogen in the thiazole ring, and 8.29 ppm also
attributed to the signal of the hydrogen in the coumarin ring. The azo dye 6c showed a broad
peak at 13.00 ppm for the indole –NH. The –NH peak from N,N-diphenylamine for dye 6e
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appeared at 9.36 ppm. The results from NMR spectra for both dyes showed that the dyes may
1
exist in azo tautomeric form in DMSO. In the H-NMR spectra of dye 6f in DMSO-d6, the
signal at 14.50 ppm can be attributed to the –NH proton of the hydrazone. The tautomeric
hydrazone (–NH) and the tautomeric imine (–NH), for dye 6g, were recorded as broad
singlets at 11.50 and 9.40 ppm in DMSO-d6 respectively. It is expected that the hydrazone –
NH proton resonance appears approximately between 13.0-16.0 ppm. Therefore, the peak
which is at downfield is attributed to hydrazone –NH (Scheme 2). These results show that,
dyes 6f and 6g may exist predominantly as the hydrazone-imine form in DMSO-d6 solution.
Scheme 2 is here
3.2. Absorption Properties
The absorption maxima (λmax) values of dyes can be directly proportional to polarity of their
solvents and the electronic effect of the substituents in the diazo and the coupling components
[39-41]. Therefore, to investigate the influence of polarity on the absorption maxima of the
dyes (6a-g), their absorption spectra were recorded in solvents of different polarities at 20 µM
concentration. The absorption maxima (λmax) of the synthesized dyes (6a-g) are presented in
Table 1.
Table 1 is here
The absorption maxima of the synthesized dyes ranged from 404 to 512 nm. Within the
synthesized thiazolyl azo disperse dyes (6a-g), we investigated the relationship between the
shift observed in the absorption maxima, and the polar characteristics of the coupling
components. All the dyes (6a-g) exhibited solvatochromic properties in all the solvents used,
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i.e. a bathochromic shift of the absorption band was observed with increasing solvent polarity.
It was observed that the λmax values of the preapared dyes (6a-g), showed larger
bathochchromic shifts in highly polar solvents, e.g. DMSO and DMF, than the rest solvents,
e.g. acetonitrile, chloroform and toluene, as shown in Table 1. Interestingly, it was also
observed that the λmax values of all the preapared dyes showed larger bathochromic shifts in
acetic acid than in polar aprotic solvents such as acetonitrile and THF. These larger
bathochromic shifts, in a proton-donating solvent such as acetic acid were considered due to
the intermolecular hydrogen bonding between the dyes and the acetic acid which are capable
of stabilizing the ground state of the dyes leading to a consequential bathochromic shifts (for
dye 6b, 466 nm in acetic acid, 462 nm in acetonitrile, 458 nm in THF; for dye 6f, 441 nm in
acetic acid, 436 nm in acetonitrile, and 439 nm in THF).
A comparison between the λmax values of dyes 6a and 6b, and their corresponding model
compound 5c which is only in azo form in all the solvents used reveals that the additional
substitution of methyl group on the nitrogen atom of the indole ring brings about small
hypsochromic shifts of the λmax values (Table 1). Therefore, we assigned the structures of the
dyes 6a and 6b to the azo form in all the solvents used. However, the λmax values of dye 6c in
DMSO (λmax=500 nm) and in DMF (λmax=496 nm) differ significantly from the other solvents
used. The absorption at longer wavelengths may be attributed to the anionic form. Also, dye
6c (λmax=500 nm in DMSO) absorbs at longer wavelengths than the corresponding model
dyes; 6a (λmax=462 nm in DMSO) and 6b (λmax=471 nm in DMSO) (Figs. 1a and 1b). This
can also be attributed to the anionic form for dye 6c.
Figure 1a is here
Figure 1b is here
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As can be seen in Table 1, λmax values for dyes 6d and 6e showed similar effects in solvents
with different polarities, which is susceptible to positive salvotochromic effect in the solvents
used. Therefore, the absorption at longer wavelengts may be attributed to the azo tautomer.
Similar results were observed, in ∆λmax values, of dye 6d when compared with dye 6e. The
introduction of the N,N-diphenylamine into the diazo component of dye 6e, increased the
bathochromic shift significantly in DMSO (λmax=500 nm) (Fig S4c in Supplementary
materials). The electron donating N,N-diphenylamine moiety enhanced the electron
delacolization from N,N-diphenylamine to the thiazole part, via the azo bridge in dye 6e. With
regards to absorption, compunds 6f and 6g showed only weak solvatochromism properties. In
contrast, hypsochromic effects were observed with solvent polarity, in these dyes (6f and 6g)
(Figs S4d and S4e in Supplementary materials). The hypsochromic shift the most likely is
as a result of the more pronounced donor-acceptor interplay between the donor functionalities
and the electron-poor uracil ring. As can be seen in Scheme 2, the dyes 6f and 6g may be
stabilized in the hydrazone forms because of the intramolecular N-H…N hydrogen bond. The
stabilized hydrazone form for both dyes in 1H NMR spectra in DMSO-d6, can be seen clearly
(Figs. S1f and S1g in Supplementary materials), and they exhibited only one absorption
band in the absorption spectra. These results suggest that the dyes 6f and 6g may exist
predominantly in the hydrazone forms in all the solvents used. The results from spectroscopic
techniques are compatible with our previous study [42,43]. Dye 6g is insoluble in organic
nonpolar solvents such as toluene therefore, we couldn’t not show absorption of dye 6g in
toluene.
13

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3.3. Acidochromic Effects
Application of a relevant dye in monitoring pH changes has attracted considerable attention in
recent years. In our study, the presence of basic nitrogen atom in the thiazole ring, and an
acidic hydrogen in the indole and N,N-diphenylamine rings prompted us to study the
possibility of protonation and deprotonation of these dyes in dichloromethane (CH2Cl2), by
using trifluoroacetic acid (TFA) and piperidine respectively. The effects of the acid and base
on the absorption maxima of the dye solutions were elucidated and the results are shown in
Table 2.
Table 2 is here
The absorption maxima of the thiazolyl azo disperse dyes (6a-e) in CH2Cl2 showed
bathochromic shift due to protonation upon addition of TFA (10 mM) to the solutions of the
dyes. Upon addition of the trifluoroacetic acid (TFA) to the solutions of the dyes (6a-e) in
CH2Cl2, major color changes were observed with the growth of a new absorption band in the
range 512-610 nm and a well-defined isosbestic points.
The azo dyes (6a and 6b) exhibited bathochromic shifts of 58 nm for 6a, and 56 nm for 6b,
relative to the absorption maxima. This might be explained by the initial protonation of the
thiazole ring (Scheme 3). However, dye 6c showed bathochromic shift of 76 nm as compared
with the model compounds 6a and 6b, for which the absorption maximum may be explained
by the initial protonation of the thiazole ring, that led to the increased in donor-acceptor
interplay between the 2-phenylindole nitrogen functionality and the protonated nitrogen on
thiazole ring (Figs. 2a- 2c).
14

ACCEPTED MANUSCRIPT
Figures 2a-c are here
Scheme 3 is here
Upon addition of TFA, the absorption properties of the dyes 6b, 6c, 6d, and 6e also changed
significantly (Table 2). Interestingly, dye 6e showed significant bathochromic shift (108 nm)
relative to the absorption maxima as compared to the other dyes (Fig. 2b). Dye 6e showed
absorption maxima at 470 nm in CH2Cl2, and upon addition of TFA to the solution of 6e, the
wavelength (λmax) value shifted to 542 nm. For the protonated form of dye 6e, the positive
charge on the nitrogen atom in the thiazole moiety was completely delocalized leading to the
experimentally observed bathochromic shift (108 nm) in comparison with the neutral form.
The protonation of the azo group led to an increase in donor-acceptor interplay between the
N,N-diphenylamino functionality and the protonated thiazole ring [44,45] (Scheme 4).
Scheme 4 is here
It was observed that, the thiazolyl azo disperse dyes (6a-e) showed color change in acidic
media (Fig. 3) and these color differences can be observed easily with naked eye. For dyes 6a
and 6b a color change from orange to rose was observed at 512 nm and 520 nm in acidic
media respectively. For dye 6c, a color change from yellow to pinky was observed at 542 nm
in acidic media. Interesingly, for dye 6d a color change was observed from red to blue and
changed the λmax values from 499 nm to 610 absorption in acidic media. Moreover, for this
dye (6d) large bathochromic shift was observed as compared to the other dyes. Dye 6e
showed a color change from orange to brown and a change in λmax value from 484 nm to 592
nm in acidic media.
15

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Figure 3 is here
Dyes 6c and 6e have acidic proton, and their photophysical properties can be change in basic
solution (Figs. 4a,b and Figs. 5a,b). Therefore, the behavior of dyes 6c and 6e, in basic
solution have been investigated and the results are shown in Table 2. The λmax values of dye
6c, especially in DMSO and DMF solutions were sensitive to the addition of piperidine and
showed small bathochromic shifts were observed. When piperidine was addeded to DMSO
and DMF solutions, the λmax value of dye 6c showed small bathochromic shift, but this effect
was not observed in chloroform solution. As shown in Fig. 4a, the addition of piperidine to a
solution of dye 6c in DMF led to a shift of the λmax value from 496 nm to 518 nm (Fig. 4a).
Also, dye 6c showed a yellow color in DMF solution, but upon addition of piperidine, the
color changed from yellow to rose (Fig. 4b). This result shows that, the dye 6c may exhibit
partial deprotonation form in DMSO and DMF. On the other hand, there was no significant
change for the λmax value and color of dye 6e, when piperidine was added to solutions of the
dye in DMSO and DMF. This indicates that dye 6e exists either as in anionic form or
tautomer-anion mixture, in DMSO and DMF, and more basic solutions of DMSO and DMF.
These results are in aggrement with those obtained for hetraylazoindoles in our previous
works [41]. A small bathochromic shift was observed, when piperidine was added to
chloroform solution of dye 6e, which led to a shift from 486 nm to 504 nm (Fig. 5a), and the
same color change was observed (Fig. 5b). The results show that the dye 6e exist in anionic
form in basic chloroform solution.
Figures 4a and 4b are here
Figures 5a and 5b are here
16

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Dyes 6f and 6g are not included in the donor-acceptor system therefore no significant change
in their color and absorption maximum were observed when TFA was added to CH2Cl2
solutions of the dyes 6f and 6g (Figs. S5c and S5d in Supplementary materials).
3.4. Electrochemical properties
The electrochemical behavior of the compounds in DMSO at glassy carbon electrode was
investigated using cyclic voltammetric (CV) and chronoamperometric (CA) techniques. The
CV of the compounds (6a-g), in DMSO, containing 0.1 M TBATFB, on glassy carbon
electrode at 0.01, 0.05, 0.1, 0.5, 1 V s-1 scan rates (vs. Ag/Ag+), are shown in Fig. 6. The
reduction (Epc) and oxidation (Epa) potentials are presented in Table 3.
Figure 6 is here
Table 3 is here
To investigate the donor–acceptor electronic influence the electrochemical properties of the
compounds were investigated by cyclic voltammetry and CA methods. These results are
shown in Table 4. All the dyes showed similar cyclic voltammograms illustrated by the Fig.
6. The CV of all the dyes were recorded at five different scan rates (0.01, 0.05, 0.1, 0.5, 1 V.s-
1).
Table 4 is here
For all the dyes, one oxidation peak was observed. In the anodic scan, the anodic peak, in the
potential range of +0.60 to +1.2 V (vs. Ag/Ag+), was observed. The compounds displayed
17

ACCEPTED MANUSCRIPT
oxidations at more positive potentials as a consequence of the destabilizing effect of the
electron-withdrawing group [46,47]. Also, all the dyes displayed two reduction peaks, in the
range of -0.5 to -2.0 V (vs Ag/Ag+). The first reduction peak appears to be associated with the
formation of unstable amine products, which were reduced in the potential range of +0.45 to -
1.0 V. Moreover, the reversible reduction of azo groups, for all the compounds prepared, was
confirmed from the shift of the reduction potentials, to more negative values, on increasing
the scan rate [46]. The second reduction peak in the range of -1.76 to -2.0 V (vs Ag/Ag+), can
be attributed to the reduction of the azo compounds to hydrazone derivatives [48].
The simplest way to understand whether the investigated compounds derivatives made a
strong adsorption onto the electrode surface was the appearance of another peak at a more
positive or negative potentials than the reduction peak. There were no such pre- or post- peaks
observed for any of the compounds investigated, which indicated the absence of strong
adsorption. Also, the fact that the slopes of log (ipc/ ꢀA) vs. log (v/ V.s-1) were about 0.5 for
both the first and second reduction peaks were a further verification of this situation [49-51].
Therefore, it can conveniently be claimed that the reactions investigated are not controlled by
adsorption phenomenon. At the same time, the decrease of ipc/v1/2 vs. scan rate graphs
indicates that the reactions are diffusion controlled [52]. Since the current function decreases
exponentially towards the higher scan rates, shows that the mechanism of electron transfer is
followed by EC (electrochemical-chemical) type (Fig. 6) [53].
3.5. Determination of the Number of Transferred Electrons and the Diffusion Coefficients
The number of transferred electrons (n) during the reduction of the compounds and their
diffusion coefficients (D) were determined using ultramicro electrode and
chronoamperometry [38]. The n and D values were calculated as follows Eq. 1:
18

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nsS2iscs
Ss2ic
DsSs2i2
Ss2is2
n =
D =
(1)
where; i is the limiting steady-state current, S is the slope of the of the chronoamperometric i
vs t–1/2 plot for compounds.
Diffusion coefficients of the compounds obtained from the Cottrell slopes, are in accordance
with their molecular weights. These are the expected results because the increase in the
carbon chain increases the molecular weight which is inversely related to the diffusion
coefficient in the compounds (6a-g) (Table 4).
3.6. Thermal stabilities
The thermal stabilites of the synthesized thiazolyl azo dyes were investigated by using
thermogravimetric analysis (TGA). The overlay TGA analysis curves for the azo dyes (6a-g)
under nitrogen atmosphere (Fig. 7) showed no weight loss up to 100-150 °C, indicating the
absence of water molecules and any other adsorptive solvent molecules. The TGA curves
indicate that the dyes are having good thermal stability except 6b, and in the TGA curve of
dye 6a, two steep weight loss which was observed from 269 °C to 515°C is included. Dye 6f
bearing 1,3-dimethylbarbituric acid moiety as the coupling component is having good thermal
stability from 305 °C to 511°C.
Figure 7 is here
19

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4. Conclusions
In summary, we have synthesized and characterized a novel series of thiazolyl azo dyes.
1
These dyes were characterized by UV-vis, FT-IR, H/13C NMR, and mass spectroscopic
techniques. Additionally, the solvents effects on the visible absorption spectra of the dyes
were evaluated. The synthesized dyes exhibited solvatochromism, namely the absorption band
was red shifted with increasing solvent polarity. These azo dyes displayed significant changes
in their absorption maxima, and upon addition of TFA, this induced the protonation of the
nitrogen atom on the thiazole ring. Due to the wide range of π conjugation, the thiazolyl azo
dyes showed acidochromic properties. It was observed that, the absorption curves of the dyes
were significantly sensitive to acid. The electrochemical behavior of the dyes were
investigated using CV and CA techniques. The results showed that, for all the dyes, one
oxidation peak, and two reduction peaks were observed. Moreover, the TGA results show that
the dyes possess a good thermal stability for applications.
Acknowledgements
We are grateful to The Scientific and Technological Research Council of TURKEY for
providing financial support (Project Grant No: 115Z016) for this study.
Supplementary material
Supplementary data (copies of 1H NMR, 13C NMR, and LC-MS spectra for compounds Figs.
S1–S14) associated with this article can be found, in the online version, at…..
20

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Scheme Captions
Scheme 1. Synthetic pathway and structure of dyes 6a-g.
Scheme 2. The azo-hydrazone tautomeric forms of 6f and 6g.
Scheme 3. Protonated azo form and its resonance stabilized structures of dye 6a.
Scheme 4. Azo-hydrazone tautomeric and cationic forms of dye 6e.
Figure Captions
Fig 1a. Absorption spectra of dye 6a (20 µΜ) in various solvents.
Fig 1b. Absorption spectra of dye 6c (20 µΜ) in various solvents.
Fig 2a. Spectrophotometric titration of TFA to dye 6a (20 µΜ) in CH2Cl2. Arrows indicate
the increase and decrease of absorbance intensity upon addition 10 mM TFA in CH2Cl2.
Fig 2b. Spectrophotometric titration of TFA to dye 6c (20 µΜ) in CH2Cl2. Arrows indicate
the increase and decrease of absorbance intensity upon addition 10 mM TFA in CH2Cl2.
Fig 2c. Spectrophotometric titration of TFA to dye 6e (20 µΜ) in CH2Cl2. Arrows indicate
the increase and decrease of absorbance intensity upon addition 10 mM TFA in CH2Cl2.
Fig 3. Pictures of absorption color of 6a-g (20 µΜ) in CH2Cl2 solutions in the absence (left)
presence (right) of 10 mM TFA in CH2Cl2.
Fig 4a. Absorption spectra of dye 6c (20 µΜ) in DMF and in addition of piperidine (0.5-2
mL).
Fig 4b. Pictures of absorption color of 6c (20 µΜ) in different solutions in the absence (left)
presence (right) of piperidine.
Fig 5a. Absorption spectra of dye 6e (20 µΜ) in chloroform and in addition of piperidine
(0.5-2 mL).
25

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Fig 5b. Pictures of absorption color of 6e (20 µΜ) in different solutions in the absence (left)
presence (right) of piperidine.
Fig 6. Cyclic voltamogram (CV) of compound 6a-g in DMSO containing 0.1 M TBATFB on
glassy carbon electrode at a scan rate of 0.01-1 Vs–1(vs. Ag/Ag+).
Fig 7. Thermal gravimetric analysis (TGA) curves of dyes 6a-g.
Tables Captions
Table 1. Photophysical data of dyes 6a-g.
Table 2. pH-Dependent absorption properties of dyes 6a–g.
Table 3. Electrochemical data of compounds in DMSO. Ag/Ag+ in DMSO; scan rate = 0.1
V/s; TBATFB (0.1 mol.L-1)
Table 4. Transferred electron numbers (n), limit current (i), cottrell slope (s), diffusion
coefficients of the compounds in DMSO.
26

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Table 1. Photophysical data of dyes 6a-g.
DMSO
DMF
Acetonitrile
Acetic Acid
THF
Chloroform
Toluene
Dye
λabs (nma) (log εb)
453(4.51)
462(4.56)
471(4.46)
500(4.46)
508(4.20)
512(4.52)
411(4.55)
444(4.62)
459(4.55)
467(4.38)
496(4.47)
508(3.95)
506(4.48)
417(4.76)
443(4.52)
453(4.53)
462(4.30)
466(4.41)
500(3.91)
490(4.49)
404(4.22)
436(4.10)
451(4.50)
458(4.36)
467(4.41)
492(3.93)
491(4.51)
404(4.42)
439(4.27)
455(4.53)
465(4.37)
470(4.39)
486(4.05)
486(4.48)
422(4.18)
448(4.17)
452(4.45)
458(4.38)
461(4.28)
485 (3.94)
478 (4.32)
416 (4.42)
-
6a
6b
6c
6d
6e
6f
466(4.31)
473(4.42)
499(4.47)
492(4.50)
412(4.43)
441(4.25)
6g
aLong wavelength absorption maximum, in nm; c=(20 ꢀM);bε = molar absorption coefficient, cm-1M-1.

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Table 2. pH-Dependent absorption properties of dyes 6a–g.
UV/vis (TFA in
DCM) λmax (nm)a
(logεb)
UV/vis (Pip.
in DMSO) λmax
(nm)a (logεb)
UV/vis (Pip. in
DMF)
UV/vis (Pip. in
Chloroform) λmax
(nm)a (logεb)
UV/vis (CH2Cl2)
Dyes
λ
max (nm)a (logεb)
454(4.40)
464(4.11)
466(4.50)
499(4.35)
484(4.44)
417(4.19)
437(3.78)
λmax (nm)a (logεb)
512(4.38)
520(4.04)
542(4.46)
610(4.13)
592 (4.32)
418(4.15)
450(4.28)
-
-
-
6a
6b
6c
6d
6e
6f
-
-
-
519(4.21)
518(4.41)
479(4.18)
-
-
-
513(4.02)
509(4.13)
504(4.10)
-
-
-
-
-
-
6g
aLong wavelength absorption maximum, in nm; c=(20 ꢀM);bε = molar absorption coefficient, cm-1M-1.