Synthesis and spectral properties of some azo disperse dyes containing a benzothiazole moiety

Article abstract of DOI:10.1016/j.molliq.2013.08.021

A known method was employed for the preparation of four substituted benzothiazole amines I-IV in moderate yields. These heterocyclic amines were diazotized with nitorsyl sulfuric acid and subsequently coupled with N,N-diethyl aniline and N-phenyl-2, 2′-iminodiethanol to afford heteroarylazo amine dyes 1-8 in satisfactory yields. These dyes were characterized by UV-Visible, FT-IR, ¹H NMR, and elemental analysis techniques. The solvatochromism behavior of the azo compounds is investigated by studying their spectra in pure and mixed organic solvents of different characteristics. The color of the dyes is discussed with respect to the nature of the heterocyclic ring and substituent present therein. In addition, effects of acid and base on the visible absorption maxima of the dyes are also reported.

Full text of DOI:10.1016/j.molliq.2013.08.021

Journal of Molecular Liquids 188 (2013) 173–177
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and spectral properties of some azo disperse dyes containing a
☆
benzothiazole moiety
b
a
E.O. Moradi Rufchahi a, , Hessamoddin Yousefi , Mojgan Mohammadinia
⁎
a
Department of Chemistry, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran
b
Department of Chemistry, Faculty of Science, University of Guilan, Rasht, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A known method was employed for the preparation of four substituted benzothiazole amines I–IV in moderate
yields. These heterocyclic amines were diazotized with nitorsyl sulfuric acid and subsequently coupled with N,N-
diethyl aniline and N-phenyl-2, 2′-iminodiethanol to afford heteroarylazo amine dyes 1–8 in satisfactory yields.
These dyes were characterized by UV–Visible, FT-IR, 1H NMR, and elemental analysis techniques. The
solvatochromism behavior of the azo compounds is investigated by studying their spectra in pure and mixed or-
ganic solvents of different characteristics. The color of the dyes is discussed with respect to the nature of the het-
erocyclic ring and substituent present therein. In addition, effects of acid and base on the visible absorption
maxima of the dyes are also reported.
Received 26 June 2013
Accepted 28 August 2013
Available online 16 October 2013
Keywords:
Azo dye
Solvatochromism
Mixed solvent
Heterocyclic amine
Diazotization
Tautomerism
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
substituted 2-aminobenzothiazoles I–IV as diazo components
(Scheme 1) and the subsequent monoazo disperse dyes 1–8, starting
Azo dyes with heterocyclic diazo components have been intensively
investigated to produce bright and strong color shades ranging from red
to greenish blue on synthetic fabrics [1,2]. These results led to commer-
cial products to replace the conventional anthraquinone dyes [3]. Addi-
tionally, particularly bathochromic azo dyes that are structurally
analogous to the azo benzenes may be obtained by replacement of the
carbocyclic acceptor ring with a thiazole or benzothiazole ring [4–6].
These sulfur or sulfur–nitrogen containing heterocyclic azo dyes pro-
vide bright and strong shades that range from red through green and
blue, which complement the yellow-orange colors of the nitrogen het-
erocyclic azo dyes to provide a complete coverage of the entire shade
range [7–14]. It is also well known that azo dyes including thiazolyl
components have been hugely utilized in the fields of nonlinear optics
and optical data storage and have shown excellent optical properties
in comparison with azo dyes derived from substituted anilines
[15–19]. Derivatives of 2-aminobenzothiazole have a long history of
use as heterocyclic diazo components for disperse dyes [20]. Some of
the best-known representatives of coupling components for thiazole
and benzothiazole diazo components are N,N-disubstituted anilines
and their derivatives which are among the most widely used com-
pounds for the synthesis of a vast variety of azo dyes and pigments
[21–24]. In the present study, we report the synthesis of the four
from N,N-diethylaniline and N-phenyl-2,2′-imino diethanol (Scheme 2).
The spectral properties of the prepared dyes were also evaluated in
pure and mixed organic solvents of various polarities. The pH effect is
also included with the aim of elucidating the absorption behaviors of
the compounds under investigation.
2. Experimental
2.1. General
All starting materials were obtained from Merck and Aldrich chem-
ical companies and were used without further purification. IR spectra
were recorded on a Shimadzu 8400 FT-IR spectrophotometer. 1H NMR
spectra were obtained by FT-NMR 400 MHz Brucker apparatus in
DMSO-d6 using TMS as internal standard. The absorption spectra of
the compounds were run on a Cary UV–Vis double-beam spectropho-
tometer (Model 100). Mass spectra were recorded on a Micromass
Agilent Technology (HP) spectrometer, operating at 70eV. The elemen-
tal analysis was determined on a Leco CHNS-900 analyzer. Melting points
were recorded with an Electro-thermal apparatus and uncorrected.
2.2. Synthesis of 2-aminobenzothiazoles I–IV
☆
This paper is dedicated to Prof. M.R. Yazdanbakhsh on the occasion of his 70th
birthday.
Corresponding author. Tel.: +98 1315515849; fax: +98 1412229081.
E-mail addresses: ena_moradi@yahoo.com, moradierufchahi1350@gmail.com,
A solution of bromine (0.26 mL, 10.0 mmol) in acetic acid (5.0 mL)
was added over about 30 min to a mixture of aromatic amine
(10.0 mmol) and potassium thiocyanate (1.07 g, 11.0 mmol) in acetic
acid (20 mL), the temperature being kept between 25 and 35 °C. The
⁎
moradierufchahi@liau.ac.ir (E.O. Moradi Rufchahi).
0167-7322/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molliq.2013.08.021

174
E.O. Moradi Rufchahi et al. / Journal of Molecular Liquids 188 (2013) 173–177
NH2
(400 MHz, DMSO-d6); δ ppm: 8.10 (1H, s), 7.97 (1H, d, J = 8.0 Hz),
X
7.48 (1H, dd, J = 2.1, 8.0 Hz), 5.15 (2H, br, NH2).
S
1) KSCN, CH3COOH
2)Br2, 20 h
NH2
2.2.4. Naphtho [1,2-d]thiazol-2-amine IV
N
Pale brown crystals (1.23 g, 61.5%), mp N 350 °C; FT-IR (KBr): ν
(cm−1): 3414 and 3225 (NH2), 3075 (Aro.-H), 1H NMR (400 MHz,
DMSO-d6); δ ppm: 8.48 (1H, d, J = 8.4 Hz), 8.46 (1H, s), 8.29 (1H, d,
J = 8.4 Hz), 8.05 (1H, s), 7.45 (1H, t, J = 7.0 Hz), 7.70 (1H, t, J =
7.0Hz), 6.25 (2H, br, NH2), MS: Calcd for C11H8N2S (M+) m/e
200.04. Found: 200.30.
I: X= -CH3, II:X= -Br, III:X= -Cl
X
NH2
NH2
N
S
1) KSCN, CH3COOH
2)Br2, 20 h
2.3. Preparation of benzothiazoleazo dyes 1–8
2-Aminobenzothiazoles (2.0 mmol) were dissolved in glacial acetic
acid: propionic acid mixture (2:1, 6.0 mL) and was quickly cooled in
an ice-salt bath to 0–5°C. The liquor was then added in portions during
15min to a cold solution of nitrosyl sulfuric acid (prepared from sodium
nitrite (2.2mmol, 0.15g) and concentrated sulfuric acid (3mL at 60°C)).
The mixture was stirred for an additional 3 h at the same temperature.
After completion of diazotization procedure, the diazonium salt solution
was added dropwise to the solution of coupler compounds (2.0 mmol)
in acetic acid. The resulting solution was vigorously stirred at 0–4 °C
for 2 h, while the pH of the reaction mixture was maintained at 5–6 by
simultaneous addition of sodium hydroxide solution (0.5M). The prog-
ress of the reaction was evaluated by thin layer chromatography (TLC)
and then crude dyes were filtered, washed with cold ethanol and puri-
fied by recrystallization method. The physical and spectral data of the
purified dyes are shown in Table 1.
IV
Scheme 1. Synthetic routes for the prepared 2-aminobenzothiazoles.
slurry was then stirred for 20 h at room temperature. The precipitate
was filtered, washed with a little acetic acid, slurried in water, made
neutral with aqueous ammonia, and filtered again. The residue was
boiled for 20 min with excess of hydrochloric acid (15% v:v) and the
hot mixture was filtered from impurities. The filtrate, cooled to 10 °C,
was made alkaline with aqueous ammonia and the precipitate was fil-
tered, washed, and dried. The crude product was recrystallized from
ethanol–water to afford I–IV, respectively. The physical and spectral
data of the purified dyes are as follows.
3. Results and discussion
2.2.1. 2-Amino-6-methyl benzothiazole I
White pearly crystals (0.80g, 49%), m.p. 118–120°C (reported 137°C
[25]); FT-IR (KBr): ν (cm−1): 3370 and 3212 (NH2), 3075 (Aro.-H),
2950 (Aliph.-H), 1H NMR (400 MHz, DMSO-d6); δ ppm: 7.45 (1H, d,
J = 8.2 Hz), 7.42 (1H, s), 7.15 (1H, dd, J = 1.1, 8.2 Hz), 5.55 (2H, br,
NH2), 2.41 (1H, s, \CH3).
3.1. Synthesis and characterization
As depicted in Scheme 1, a known method was modified and then
employed for the preparation of substituted 2-aminobenzothiazoles I–
IV. The known 2-aminobenzothiazoles (I–III) and the new one (IV)
were obtained by allowing one-pot reaction of the appropriate aniline
derivatives with thiocyanogen generated from bromine and potassium
thiocyanate. The crude products were obtained in the satisfactory yields
and purified by crystallization from ethanol–water. In addition to com-
parison with authentic samples, all of the known and new prepared
compounds were characterized by the FT-IR and 1H NMR spectral anal-
ysis. For compound IV, the Mass spectrum was measured and confirmed
the desired structure. 1H NMR spectra recorded for the prepared
2.2.2. 2-Amino-6-bromo benzothiazole II
Yellow crystals (1.50 g, 65.5%), m.p187–190 °C; FT-IR (KBr): ν
(cm−1): 3375 and 3188 (NH2), 3070 (Aro.-H), 1H NMR (400 MHz,
DMSO-d6); δ ppm: 7.34 (1H, d, J = 7.9 Hz), 7.18 (1H, s), 7.03 (1H,
dd, J = 2.0, 7.9 Hz), 5.45 (2H, br, NH2).
2.2.3. 2-Amino-6-choloro benzothiazole III
White crystals (0.77g, 42%), m.p. 208–211°C (reported 209°C [25]);
FT-IR (KBr): ν (cm−1): 3394 and 3218 (NH2), 3050 (Aro.-H), 1H NMR
H2SO4,0-5 o
C
-
Het-N2+HSO4
Table 1
Het-NH2
+ NaNO2
The physical properties of the dyes prepared in this study.
R
R
R
R
Dye Color mp
(°C)
Recrystallization %C
Calcd.
%H
%N
Yield
(%)
N
N
CH3COOH
pH=5-6, 0-5 oC
-
Het-N2+HSO4
N
+
Calcd.
Found
Calcd.
Found
Het
N
Found
Dye
R
Dye
R
Het
1
2
3
4
5
6
7
8
Purple 170– DMF/H2O
66.63 67.15 6.21 6.19 17.27 17.88 72
52.45 52.11 4.40 4.47 14.39 14.73 75
59.21 59.92 4.97 4.91 16.25 16.87 74
69.97 69.21 5.59 5.69 15.54 15.74 65
60.65 60.89 5.66 5.61 15.72 15.25 69
48.46 48.91 4.07 4.14 13.30 13.02 81
54.18 54.92 4.55 4.58 14.87 15.23 63
64.27 63.92 5.14 5.19 14.28 13.05 71
N
S
171
1
-CH2CH3
-CH2CH3
-CH2CH3
5
-CH2CH2OH
Dark
red
N250 DMF/H2O
H3C
N
Bright 160– DMF/H2O
2
3
6
7
-CH2CH2OH
-CH2CH2OH
red
161
Clear
red
160– DMF/H2O
162
Br
Cl
S
N
Dark
red
187– DMF/H2O
189
S
Dark
red
232– DMF/H2O
234
N
4
-CH2CH3
8
-CH2CH2OH
Clear
red
183– DMF/H2O
184
S
Dark
red
179– DMF/H2O
180
Scheme 2. Synthetic routes for the preparation of azo dyes 1–8.

E.O. Moradi Rufchahi et al. / Journal of Molecular Liquids 188 (2013) 173–177
175
Table 2
Characterization data of azo dyes 1–8.
Dye IR (KBr, cm−1
vO\H vAroC
)
1H NMR (DMSO-d6, ppm)
vC_C vN_N Aro.-H
1590 1495 6.92 (d, 2H, J=8.8Hz), 7.35 (dd, 1H, J=8.2, J=1.6Hz), 7.82 (d, 1H, J=1.6Hz), 7.84 1.20 (t, 6H, CH3, J = 7.2 Hz),
\
vAliphC
\
Alip.-H
O\H
H
H
1
–
3090
2980
-
(d, 2H, J = 8.8 Hz), 7.88 (d, 1H, J = 8.8 Hz)
2.46 (s, 3H, CH3), 3.56 (q, 4H,
CH2, J = 7.2 Hz)
2
3
4
–
–
–
3080
3100
3150
2990
2984
2985
1598 1492 6.95 (d, 2H, J=8.8Hz), 7.54 (dd, 1H, J=8.4, J=2.0Hz), 7.86 (d, 2H, J=8.8Hz), 7.98 1.20 (t, 6H, CH3, J = 7.2 Hz),
(d, 1H, J = 2.0 Hz) 3.58 (q, 4H, CH2, J = 7.2 Hz)
1600 1490 6.94 (d, 2H, J = 9.2 Hz), 7.78 (d, 1H, J = 8.8 Hz),7.82 (d, 2H, J = 9.2 Hz), 7.89 (d, 1H, 1.36 (t, 6H, CH3, J = 7.1 Hz),
J = 8.8 Hz), 8.20 (s, 1H) 3.53 (q, 4H, CH2, J = 7.1 Hz)
–
–
–
1592 1519 7.02 (d, 2H, J = 9.2 Hz), 7.75 (t, 1H, J = 7.2 Hz), 7.84 (t, 1H, Aro.-H, J = 7.2 Hz), 7.90 1.28 (t, 6H, CH3, J = 7.1 Hz),
(d, 2H, J = 9.2 Hz), 8.09 (d, 1H, J = 7.2 Hz), 8.31 (d, 1H, J = 7.2 Hz), 8.48 (s, 1H), 8.78 3.53 (q, 4H, CH2, J = 7.1 Hz)
(s, 1H)
5
6
7
3400 3085
3445 3100
3390 3140
2920
2950
2936
1595 1493 6.90 (d, 2H, J = 8.8 Hz), 7.52 (d, 1H, J = 8.4 Hz), 7.95 (d, 1H, J = 8.4 Hz), 8.01 (d, 2H, 2.50 (s, 3H, CH3), 3.35 (s, 8H, 4.88
J = 8.8 Hz), 8.04 (s, 1H)
CH2)
(br, 2H)
4.94 (br,
2H)
1630 1525 7.01 (d, 2H, Aro.-H, J = 8.8 Hz), 7.66 (dd, 1H, Aro.-H, J = 8.8, J = 2.0 Hz), 7.84 (d, 2H. 3.66 (s, 8H, CH2)
Aro.-H, J = 8.8 Hz), 7.92 (d, 1H, Aro.-H, J = 8.8 Hz), 8.32 (d, 1H, Aro.-H, J = 2.0 Hz)
1597 1498 7.07 (d, 2H, Aro.-H, J = 8.9 Hz), 7.54 (dd, 1H, Aro.- H, J = 8.4, 2.0 Hz),7.89 (d, 2H,
3.55 (t, 4H, N–CH2, J=5.2Hz), 5.10 (t,
Aro.- H, J = 8.9 Hz), 7.95 (d, 1H, Aro.-H, J = 8.4 Hz), 8.26 (d, 1H, Aro.-H, J = 2.0 Hz) 3.66 (t, 4H, OCH2, J = 5.2 Hz) 2H, J =
5.20 Hz)
8
3422 3145
2994
1593 1521 6.97 (d, 2H, Aro.-H, J=8.8Hz), 7.48 (d, 1H, Aro.-H, J=7.2Hz), 7.83 (t, 1H, Aro.-H, J= 3.69 (s, 8H, CH2)
7.2 Hz), 7.86–7.89 (m, 2H, overlapped, Aro.-H), 7.89 (d, 2H, Aro.-H, J = 8.8 Hz), 8.37
(d, 1H, Aro.-H, J = 4.0 Hz), 8.70 (d, 1H, Aro.-H, J = 4.0 Hz)
4.92 (br,
2H)
compounds clearly supported the proposed structures. The protons be-
longing to the aromatic system were observed at the expected chemical
shifts and integral values.
acid, acetonitrile, dimethyl formamide and dimethyl sulfoxide) in the
wavelength rang 350–700 nm at a concentration ~10−5 mol·L−1. The
results are given in Table 3. The dyes have low solubility in some of
the used solvents but are completely soluble in DMSO. Therefore,
stock solutions of each dye with a concentration of ~10−3 mol·L−1
were accurately prepared in DMSO and dilutions of these stocks were
used for absorption measurements.
As an illustrative example, the spectral shifts of dye 6 in various sol-
vents are shown in Fig. 1. The maximum absorption of dye 6 showed a
significant shift in CH3COOH and DMSO with respect to the maximum
absorption in other solvents (e.g. λmax is 542 in CH3COOH, 538 nm in
DMSO, 530 nm in DMF, 520 nm in ethanol, 518 nm in CH3CN, and
506 nm in chloroform). The same trends of absorption shifts in various
solvents were observed for the entire series of dyes 1–8.
As it is apparent in Table 3, the electronic absorption spectra of these
benzothiazole aminoazo dyes don't show a regular variation with the
polarity of solvents. These dyes, apparently, didn't exhibit a strong sol-
vent dependence.
The substituent effects on the visible absorption spectra of the
heterocyclic azo dyes 1–8 were also evaluated. For example, we
found that dyes 3 and 7 that contain an electron acceptor chloro
group on the 6 positions of benzothiazolyl group showed a
bathochromic shift of +10 and +12 nm relative to dyes 1 and 5
in DMSO, respectively. It was also observed that the aminoazo
dyes which have naphtothiazolyl group (4 and 8) show ba-
thochromic shifts in comparison with benzothiazolyl amines. As a
In order to synthesize the aminoazo dyes 1–8, 2-aminobenzothiazoles
I–IV were diazotised with nitrosylsulfuric acid in a mixture of acetic and
propionic acids. Coupling reaction was carried out by adding the diazoni-
um solution to a solution of N,N-disubstituted anilines in acetic acid
(Scheme 2). The chemical structures of these dyes were confirmed by
some spectroscopic methods and elemental analysis. The physical and
spectral data of the dyes are summarized in Tables 1 and 2. Spectral mea-
surements of the synthesized azo dyes revealed good agreement with
proposed structures. FT-IR spectrum of compounds 1–8 showed aromatic
and aliphatic C\H bands at 3080–3150 cm−1 and 2936–2990 cm−1, re-
spectively. The dyes also revealed the aromatic C_C bands at 1590–
1630 cm−1 and the azo N_N peaks at 1490–1525 cm−1 in their IR-
spectral data.
The aromatic protons of the dyes were observed from 8.78 to
6.92 ppm in the 1H NMR spectra measured in DMSO-d6 at 25 °C. Dyes
1–4 indicated a quartet peak at δ = 3.58–3.53 ppm and a triplet peak
about δ=1.36–1.20ppm. These signals were attributed to the adjacent
methylene and methyl protons, respectively. Other significant signals
that were detected in 1H NMR spectrum of dyes 5–8, are two distinct
quartets at 3.69–3.35ppm which can be ascribed to methylene protons
connected to nitrogen and oxygen atoms. In addition, the singlet and
triplet of hydroxyl group were observed at δ = 5.10–4.88 ppm for
these dyes.
3.2. UV–Visible study of the prepared aminoazo dyes
The absorption spectra of aminoazo dyes 1–8 were measured in six
organic solvents with different polarities (chloroform, ethanol, acetic
Table 3
Absorption maxima (in nm) of dyes 1–8 in different solvents.
Dye
DMSO
DMF
CH3CN
CHCl3
EtOH
CH3COOH
1
2
3
4
5
6
7
8
526
540
536
556
528
538
540
556
520
528
528
548
520
530
530
548
512
520
522
540
508
518
520
536
516
518
512
540
498
506
512
528
508
524
528
544
510
520
522
538
566, 602
560, 596
558, 592
560
576, 608
542, 602
538, 604
542
Fig. 1. Absorption spectra of dye 6 in different solvents.

176
E.O. Moradi Rufchahi et al. / Journal of Molecular Liquids 188 (2013) 173–177
X
S
R
N
N
N
N
R
(A)
H+
H+
X
S
X
H
N+
S
R
N
R
N
N+
H
N
N
N
N
(B)
R
R
(C)
H+
H+
X
S
H
N+
R
N
N+
H
N
R
(D)
Scheme 3. Tautomeric changes on protonation of aminoazo dyes.
result, the λmax of dye 4 showed bathochromic shift of +30nm rel-
ative to 1 and λmax of dye 8 is +28nm longer than that of dye 5, due
to the extended resonance system in comparison with other dyes.
Table 1 also illustrates the effect of two different coupling compo-
nents. The dyes 1–4 which are based on the N, N-diethyl aniline
as coupling component showing λmax at the highest wavelength
in the series.
As is well known [26–28] and shown in Scheme 3, in strong proton
donating solvents such as acetic acid, azo dyes with amino groups in
the para position undergo mono-protonation to give the ammonium
(B) and/or the azonium (C) tautomers followed by further protonation
to give (D). The color change for the ammonium isomer is hypsochromic
and weaker (even colorless), while the azonium isomer is strongly
bathochromic and more intense. The di-protonated product (D) is nearly
the same color as the base dye.
C which makes a bathochromic shift in absorption spectrum. Fig. 2
shows the absorption spectra of dyes 1–8 in acetic acid.
Absorption maxima of the prepared dyes 1–8 were also measured in
their 80% water–ethanol solutions with different pH values (Table 4).
The dyes showed relatively good pH stability. The nature of these
changes in solutions with different acidity is the same, as shown for
dye 1 in Fig. 3. Changes in the spectra are observed at acidity higher
than ~10−2 mol·L−1 (pH ≤ 1.5) and the absorption curves are resem-
bled those in high proton donating solvents such as acetic acid. It can
be suggested that a bathochromically shifted band appears as a result
of proton addition and this is attributed to the azonium (C) tautomer
in the possible azonium–ammonium equilibrium. The appearance of
this band only in acetic acid as a solvent can be ascribed to its high acid-
ity as well as its pronounced character as a strong proton donating sol-
vent. Further support is achieved by studying the spectroscopic
behavior of these compounds in DMF containing increasing amounts
of acetic acid. The spectra of the dyes were recorded in the DMF–acetic
acid mixed solvents. All of the dyes except dyes 4 and 8 exhibited an
isosbestic point which indicates the establishment of equilibrium be-
tween the free (azo form A) and the H-bond solvated species (azonium
form C) of the dyes. This behavior indicates that acetic acid molecules
have a greater tendency to form solvated complexes with the dye mol-
ecule than DMF. It is evident that with increasing acetic acid concentra-
tion the absorbance of first band decreases, whereas a new band
appears at a longer wavelength. Fig. 4 shows the absorption spectra of
The absorption maximum of all dyes except for dyes 4 and 8 showed
two bands and bathochromic shift (positive solvatochromism) in acidic
solvent (glacial acetic acid) relative to other investigated solvents in this
study. This shift in absorption maximum can be attributed to the sol-
vent–dye interaction and the presence of these dyes in azonium form
Table 4
Absorption maxima (λmax in nm) of prepared azo dyes in different pH values.
pH
1.50
2.97
3.39
5.45
7.05
7.30
10.00
11.05
Dye
λmax
1
2
3
4
5
6
7
8
528, 608
532, 606
532, 610
548
518, 614
526, 612
526, 602
544
522
532
530
544
516
526
528
542
524
532
532
552
516
526
526
544
522
532
528
546
516
526
528
542
524
530
528
550
516
526
526
542
524
534
530
552
516
526
526
544
522
530
530
554
516
526
526
542
522
532
528
552
516
526
528
542
Fig. 2. Absorption spectra of dyes 1–8 in acetic acid (numbers represent the number of the
dyes).

E.O. Moradi Rufchahi et al. / Journal of Molecular Liquids 188 (2013) 173–177
177
Fig. 4. Visible electronic absorption spectra of 5.0×10−5 mol·L−1 solutions of compound 6
in DMF–CH3COOH mixtures. 1. 1.1 mol·L−1 CH3COOH, 2. 2.18 mol·L−1 CH3COOH, 3.
2.87 mol·L−1 CH3COOH, 4. 3.45 mol·L−1 CH3COOH, 5. 5.24 mol·L−1 CH3COOH, 6.
6.01 mol·L−1 CH3COOH, 7. 8.13 mol·L−1 CH3COOH, 8. 9.25 mol·L−1 CH3COOH, 9.
10.12 mol·L−1 CH3COOH, 10. 11.00mol·L−1 CH3COOH, and 11. 12.11mol·L−1 CH3COOH.
Fig. 3. Absorption spectra of dye 1 in different pH values.
complex with the solute molecules. This is due to the low ionization po-
tential and high hydrogen bond donating character of CH3COOH.
dye 6 in mixed DMF–acetic acid. The spectrum does not show a band at
about 320 nm due to the ammonium tautomer, and is influenced sub-
stantially by the acid concentration. The tautomeric equilibrium in
these dyes is thus shifted almost completely to the azonium form [25].
This is due to the low ionization potential of acetic acid as well as to
its high H-bond donating character. The visible spectra of compounds 4
and 8 in acetic acid are different from those discussed above, the absorp-
tion band being broad and exhibiting only one maximum. However, the
wavelength bands are located within the range of the spectra of the
other discussed dyes (1–3 and 5–7) in the same solvent. It can be con-
cluded that dyes 4 and 8 are liable to form a solvated complex with
acetic acid through an intermolecular H-bonding.
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Four 2-aminobenzothiazole derivatives I–IV have been synthesized
and characterized by elemental analysis, IR, UV–Vis and 1HNMR spec-
troscopy. These compounds were diazotized and successfully coupled
with N,N-diethyl aniline and N-phenyl-2, 2′-iminodiethanol to prepare
corresponding azo dyes 1–8 in moderate to good yields. The solva-
tochromic behavior of these compounds was investigated by studying
their visible spectra in several pure and mixed organic solvents.
The azo compounds displayed one band in their electronic absorp-
tion spectra in all used organic solvents except acetic acid. This band
was attributed to electronic transition in aromatic and heteroaromatic
systems involving the entire electronic system of the compounds.
Whereas in acetic acid a second band at longer wavelengths for some
azo compounds (i.e., 1–3 and 5–7) was observed.
The observation of a special behavior for compounds 1–3 and 5–7 in
the presence of CH3COOH in DMF–CH3COOH mixture is examined and
indicated that CH3COOH has a greater tendency to form a solvated