Synthesis and tautomeric structures of some novel thiophene-based bis-heterocyclic monoazo dyes

Article abstract of DOI:10.1016/j.molstruc.2012.05.021

In this study, ethyl 2-amino-5-methyl-4-(phenylcarbamoyl)thiophene-3- carboxylate (1) was prepared using Gewald's methodology. This 2-aminothiophene derivative was diazotised and coupled with, 3-methyl-1H-pyrazolin-5-one, 3-methyl-1-phenylpyrazolin-5-one,

Full text of DOI:10.1016/j.molstruc.2012.05.021

Journal of Molecular Structure 1024 (2012) 117–122
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and tautomeric structures of some novel thiophene-based bis-heterocyclic
monoazo dyes
b
Fikret Karcı a, , Fati Karcı
⇑
a Pamukkale University, Faculty of Science and Arts, Department of Chemistry, Denizli, Turkey
b Pamukkale University, Higher Vocational School of Denizli, Chemical Programme, Denizli, Turkey
h i g h l i g h t s
"
"
"
A series of novel monoazo dyes based on thiophene ring was synthesised.
The solvatochromic behaviours and tautomeric structures of dyes were evaluated in detail.
Acid and base effects on kmax of the dyes were also examined in detail.
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, ethyl 2-amino-5-methyl-4-(phenylcarbamoyl)thiophene-3-carboxylate (1) was prepared
using Gewald’s methodology. This 2-aminothiophene derivative was diazotised and coupled with,
3-methyl-1H-pyrazolin-5-one, 3-methyl-1-phenylpyrazolin-5-one, 3-amino-5-hydroxy-1H-pyrazole,
3-amino-5-hydroxy-1-phenylpyrazole, 3-cyano-6-hydroxy-4-methyl-2-pyridone, barbituric acid and
4-hydroxycoumarin, respectively (2–8). The newly synthesized bis-heterocyclic monoazo dyes based
on thiophene ring were characterised by elemental analysis and spectral methods. The solvatochromic
behaviour and tautomeric structures of these bis-heterocyclic monoazo dyes in various solvents was
evaluated. Acid and base effects on the visible absorption maxima of the dyes are also reported.
Ó 2012 Elsevier B.V. All rights reserved.
Received 10 April 2012
Received in revised form 4 May 2012
Accepted 7 May 2012
Available online 17 May 2012
Keywords:
Aminothiophene
Gewald’s methodology
Azo dyes
Tautomerism
Absorption spectra
1. Introduction
electron sink and the thiophene ring system has less resonance
stabilisation energy to lose on excitation than benzenoid system
In the last four decades, innovations in azo dye chemistry
based on heterocyclic systems have been made as a result of
intensive studies stimulated by the mounting need for bright blue
dyes. Generally speaking, many of the heterocyclic azo dyes show
bathochromic shifts combined with brilliance of shade and high
tinctorial strength compared with conventional anthraquionone
dyes and aminobenzene azo dyes [1–3]. Of most importance are
azo dyes derived from 2-aminothiophenes as diazo components.
For instance, Hallas et al. [4–8] reported the synthesis of azo dyes
derived from 2-aminothiophene derivatives and various heterocy-
clic coupling components, and their application on polyester fi-
bres gave excellent results. The deep colours of these dyes can
be explained by resonance theory; the sulphur atom acts as an
[9]. Also, the electron-withdrawing substituents on the thiophene
ring resulted in bathochromic shifts [10–12]. Aminothiophenes
used as diazo components in the synthesis of thiophene based
heterocyclic azo dyes were easily obtained using Gewald’s meth-
odology [13,14]. Many papers describe the synthesis, dyeing
properties of thiophene based azo dyes for synthetic fibres [15–
21] and for blended polyester/wool fibres [22–24]. Also, some
dyes derived from thiophene are used in solar cell [25,26].
Although, many papers describe the synthesis and properties
of thiophene based monoazo dyes, investigations involved in
bis-heterocyclic monoazo dyes based on thiophene ring have
been limited [27,28]. In conjunction with our interest in this class
of compounds, we have previously reported the synthesis and
spectral properties of some heterocyclic azo dyes [29–35]. The
purpose of this study is to investigate some bis-heterocyclic
monoazo dyes based on thiophene ring. Absorption abilities and
tautomeric structures of these dyes were also examined in detail.
⇑
Corresponding author.
E-mail address: fkarci@pau.edu.tr (F. Karcı).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.05.021

118
F. Karcı, F. Karcı / Journal of Molecular Structure 1024 (2012) 117–122
with water several times and dried in air. Recrystallisation from
aqueous DMF gave red crystals of the product dye 2 (yield 2.95 g,
71%), mp 306–307 °C. IR (KBr):
(cmꢁ1) = 3259, 3242, 3230 (3
2. Experimental
2.1. General
m
NH), 3034 (ArAH), 2982 (AlAH), 1715, 1673, 1645 (3 C@O); 1H-
NMR (DMSO-d6): d = 1.34 (t, 3H, J = 7.05 Hz, CH3), 1.89 (s, 3H,
CH3), 2.09 (s, 3H, CH3), 4.31 (q, 2H, J = 7.04 Hz, OCH2), 7.01–7.69
(m, 5H, ArH), 9.69 (br, tautomeric NH or OH), 9.95 (br, 1H, pyrazole
NH), 10.13 (br, tautomeric NH or OH), 10.75 (br, 1H, PhANH); Anal.
Calcd. for C19H19N5O4S: C: 55.19, H: 4.63, N: 16.94, S: 7.76. Found:
C: 55.43, H: 4.59, N: 16.83, S: 7.65. The above procedure was also
used to synthesize dye 3–8 (Scheme 2).
The chemical used for the synthesis of the compounds were ob-
tained from Aldrich and Merck Chemical Company without further
purification. The solvents used were of spectroscopic grade.
IR spectra were determined using a Mattson 1000 Fourier
Transform-infrared (FT-IR) spectrophotometer on a KBr disc. Nu-
clear magnetic resonance (1H NMR) spectra were recorded on a
Bruker-Spectrospin Avance DPX 400 Ultra-Shield in deuterated
dimethylsulphoxide (DMSO-d6) using tetramethylsilane (TMS) as
the internal reference; chemical shifts was (d) given in ppm. Ultra-
violet–visible (UV–vis) absorption spectra were recorded on a Schi-
madzu UV-1601 double beam spectrophotometer at the
wavelength of maximum absorption (kmax) in a range of solvents,
i.e. DMSO, DMF, acetonitrile, methanol, acetic acid and chloroform
at the various concentrations (1 ꢀ 10ꢁ6–10ꢁ8). Melting points were
determined on an Electrothermal 9100 melting point apparatus
and are uncorrected. Elemental analysis was done on a Leco
CHNS-932 analyser.
2.4. 4-[Ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-30-
carboxylate-20-ylazo]-3-methyl-1-phenylpyrazolin-5-one (3)
Red crystals; yield 67%; mp. 263–264 °C (DMF-H2O); IR (KBr):
m
(cmꢁ1) = 3261, 3228 (2NH), 3031 (ArAH), 2976 (AlAH), 1711,
1681, 1636 (3 C@O); 1H-NMR (DMSO-d6): d = 1.42 (t, 3H,
J = 6.91 Hz, CH3), 2.33 (s, 3H, CH3), 2.60 (s, 3H, CH3), 4.44 (q, 2H,
J = 6.90 Hz, OCH2), 7.10–7.92 (m, 10H, ArH), 9.82 (br, tautomeric
NH or OH), 10.19 (br, 1H, PhANH), 10.36 (br, tautomeric NH or
OH); Anal. Calcd. for C25H23N5O4S: C: 61.34, H: 4.74, N: 14.31, S:
6.55. Found: C: 61.47, H: 4.62, N: 14.18, S: 6.67.
2.2. Synthesis of ethyl 2-amino-4-methyl-5-
(phenylcarbamoyl)thiophene-3-carboxylate (1)
2.5. 4-[Ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-30-
carboxylate-20-ylazo]-3-amino-5-hydroxy-1H-pyrazole (4)
Acetoacetanilide (1.80 g, 0.01 mol), ethyl cyanoacetate (1.19 g,
0.01 mol) and sulphur (0.33 g, 0.01 mol) were refluxed in ethanol
for 3 h using morpholine (0.9 g, 0.01 mol) (Scheme 1). The result-
ing dark solution was cooled and stored overnight in a refrigerator,
followed by filtration, washing with a small amount of ethanol and
then ethanol/water (1:1) mixture and dried in air. Recrystallisation
from ethanol gave white crystals of the product 1 (Scheme 1) [12–
Dark brown crystals; yield 78%; mp. 281–282 °C (DMF-H2O); IR
(KBr):
m
(cmꢁ1) = 3477 (OH), 3382 (NH2), 3266, 3230 (2 NH), 3061
(ArAH), 2988 (AlAH), 1720, 1638 (2 C@O); 1H-NMR (DMSO-d6):
d = 1.38 (t, 3H, J = 7.03 Hz, CH3), 2.57 (s, 3H, CH3), 4.38 (q, 2H,
J = 7.04 Hz, OCH2), 5.99 (br, 2H, NH2), 7.08–7.67 (m, 5H, ArH),
10.13 (br, 1H, OH), 10.80 (br, 1H, pyrazole NH), 14.19 (br, 1H,
PhANH); Anal. Calcd. for C18H18N6O4S: C: 52.17, H: 4.38, N:
20.28, S: 7.74. Found: C: 52.29, H: 4.21, N: 20.42, S: 7.53.
14] (yield 2.19 g, 72%), mp 176–177 °C. IR (KBr):
m
(cmꢁ1) = 3381,
3316 (NH2), 3251 (NH), 3036 (ArAH), 2973 (AlAH), 1647, 1631
(2 C@O); 1H-NMR (DMSO-d6): d = 1.35 (t, 3H, J = 7.02 Hz, CH3),
2.58 (s, 3H, CH3), 4.38 (q, 2H, J = 7.00 Hz, OCH2), 7.12–7.51 (m,
5H, ArH), 7.72 (b, 2H, NH2), 9.78 (br, 1H, PhANH); Anal. Calcd.
for C15H16N2O3S: C: 59.19, H: 5.30, N: 9.20, S: 10.54. Found: C:
59.41, H: 5.14, N: 9.07, S: 10.29.
2.6. 4-[Ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-30-
carboxylate-20-ylazo]-3-amino-5-hydroxy-1-phenylpyrazole (5)
Brown crystals; yield 73%; mp. 253–254 °C (DMF-H2O); IR
(KBr):
m
(cmꢁ1) = 3464 (OH), 3391 (NH2), 3266 (NH), 3055 (ArAH),
2.3. Synthesis of 4-[ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-
2988 (AlAH), 1673, 1640 (2 C@O); 1H-NMR (DMSO-d6): d = 1.42 (t,
3H, J = 7.02 Hz, CH3), 2.58 (s, 3H, CH3), 4.43 (q, 2H, J = 7.13 Hz,
OCH2), 6.58 (br, 2H, NH2), 7.09–7.93 (m, 10H, ArH), 10.18 (br, 1H,
OH), 14.31 (br, 1H, PhANH); Anal. Calcd. for C24H22N6O4S: C:
58.76, H: 4.52, N: 17.13, S: 6.54. Found: C: 58.54, H: 4.62, N:
16.92, S: 6.67.
30-carboxylate-20-ylazo]-3-methyl-1H-pyrazolin-5-one (2)
Ethyl 2-amino-5-methyl-4-(phenylcarbamoyl)thiophene-3-car-
boxylate (3.04 g, 0.01 mol) was dissolved in a mixture of glacial ace-
tic acid and concentrated hydrochloric acid (20 ml, ratio 1:1) and
the solution was then cooled to 0–5 °C. Sodium nitrite (0.69 g,
0.01 mol) in water (10 ml) was then added to this solution dropwise
with vigorous stirring, during about 1 h, while cooling at 0–5 °C.
Then the resulting diazonium solution was added in portions over
30 min to a vigorously stirred solution of 3-methyl-1H-pyrazolin-
5-one (0.98 g, 0.01 mol) in KOH (0.56 g, 0.01 mol) and water
(10 ml) at between 0–5 °C, maintaining the pH at 7–8 by simulta-
neous addition of 10% sodium acetate solution. The mixture was
then stirred for 2 h. at between 0–5 °C. The precipitated product
separated upon dilution with water (50 ml) was filtered off, washed
2.7. 5-[Ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-30-
carboxylate-20-ylazo]-3-cyano-6-hydroxy-4-methyl-2-pyridone (6)
Red crystals; yield 86%; mp. 320–321 °C (DMF-H2O); IR (KBr):
m
(cmꢁ1) = 3266, 3254, 3247 (3 NH), 3055 (ArAH), 2988 (AlAH),
2222 (CN), 1716, 1698, 1670, 1645 (4 C@O); 1H-NMR (DMSO-d6):
d = 1.39 (t, 3H, J = 7.13 Hz, CH3), 2.58 (s, 3H, CH3), 2.74 (s, 3H,
CH3), 4.42 (q, 2H, J = 7.02 Hz, OCH2), 7.10–7.96 (m, 5H, ArH),
H3C
COOC2H5
NH2
H3C
COOC2H5
morpholine
C
H2C
O
+
N
H
C
+
S
S
CN
N
H
C
O
CH2
O
(1)
Scheme 1. Synthesis of thiophene derivative (1).

F. Karcı, F. Karcı / Journal of Molecular Structure 1024 (2012) 117–122
119
H3C
COOC2H5
H3C
COOC2H5
N
N
CH3
N
H
C
S
(3)
O
N
N
CH3
N
N
H
C
O
S
(2)
N
O
N
O
N
H
H3C
COOC2H5
N
N
NH2
N
H
C
O
S
(4)
N
HO
N
H
H3C
COOC2H5
H3C
COOC2H5
H3C
COOC2H5
NH2
N
N
NH2
NaNO2
+
N
H
C
O
S
(5)
N2
N
H
C
O
S
N
H
C
HCl/HAc
S
N
HO
N
O
(1)
H3C
COOC2H5
CH3
N
N
N
H
C
O
CN
O
S
(6)
HO
N
H
H3C
COOC2H5
H3C
COOC2H5
O
OH
O
N
N
N
H
C
H
S
(7)
N
N
N
C
N
S
O
O
(8)
H
O
N
H
O
O
Scheme 2. Synthesis of dyes 2–8.
10.21 (br, 1H, OH), 12.26 (br, 1H, pyridone NH), 15.32 (br, 1H,
PhANH); Anal. Calcd. for C22H19N5O5S: C: 56.77, H: 4.11, N:
15.05, S: 6.89. Found: C: 56.91, H: 4.05, N: 15.18, S: 6.69.
3. Results and discussion
3.1. Spectral characteristics and tautomerism
Dyes 2–8 can exist in three possible tautomeric forms, namely
the azo-keto forms (T1, T4, T7, T10), the azo-enol forms (T2, T5,
T8, T11) and the hydrazo-keto forms (T3, T6, T9, T12) as shown
in Schemes 3–6. The FT-IR spectra of dyes 2 and 3 showed three
imino bands (NH) and two imino (NH) bands, respectively. Also,
FT-IR spectra of dyes 2 and 3 did not show any broad band for hy-
droxyl group. These suggest that dyes 2 and 3 are predominantly in
hydrazo-keto form (T3) as opposed to azo-keto form (T1) and azo-
enol form (T2), in the solid state (Scheme 3).
The FT-IR spectra of dyes 4 and 5 showed one broad hydroxyl
(OH) band and intense two carbonyl (C@O) bands. These suggest
that dyes 4 and 5 are predominantly in azo-enol form (T2) as op-
posed to azo-keto form (T1) and hydrazo-keto form (T3), in the so-
lid state (Scheme 3).
2.8. 5-[ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-30-
carboxylate-20-ylazo] barbituric acid (7)
Orange crystals; yield 81%; mp. 323–324 °C (DMF-H2O); IR
(KBr):
m
(cmꢁ1) = 3288, 3246, 3163 (3 NH), 3075 (ArAH), 2990
(AlAH), 1733, 1705, 1696, 1664, 1636 (5 C@O); 1H-NMR (DMSO-
d6): d = 1.39 (t, 3H, J = 7.14 Hz, CH3), 2.59 (s, 3H, CH3), 4.41 (q,
2H, J = 7.01 Hz, OCH2), 7.09–7.96 (m, 5H, ArH), 10.16 (br, 1H,
OH), 11.48 (br, 1H, pyrimidone NH), 11.73 (br, 1H, pyrimidone
NH), 15.02 (br, 1H, PhANH); Anal. Calcd. for C19H17N5O6S: C:
51.46, H: 3.86, N: 15.79, S: 7.23. Found: C: 51.22, H: 3.67, N:
15.53, S: 7.08.
The FT-IR spectra of dyes 6 and 7 showed three imino (NH)
bands. Also, FT-IR spectra of dyes 6 and 7 did not show any broad
band for hydroxyl group. These suggest that dye 6 is predomi-
nantly in hydrazo-keto form (T6) as opposed to azo-keto form
(T4) and azo-enol form (T5) and dye 7 is predominantly in azo-keto
form (T7) as opposed to azo-enol form (T8) and hydrazo-keto form
(T9) in the solid state (Schemes 4 and 5).
The FT-IR spectra of dye 8 showed one broad hydroxyl (OH)
band and one imino (NH) band. These suggest that dye 8 is pre-
dominantly in azo-enol form (T11) as opposed to azo-keto form
(T10) and hydrazo-keto form (T12), in the solid state (Scheme 6).
2.9. 3-[Ethyl 40-methyl-50-(phenylcarbamoyl)thiophene-30-
carboxylate-20-ylazo]-4- hydroxycoumarin (8)
Red crystals; yield 63%; mp. 271–272 °C (DMF-H2O); IR (KBr):
m
(cmꢁ1) = 3482 (OH), 3278 (NH), 3081 (ArAH), 2989 (AlAH), 1749,
1688, 1653 (3 C@O); 1H-NMR (DMSO-d6): d = 1.46 (t, 3H,
J = 6.88 Hz, CH3), 2.61 (s, 3H, CH3), 4.47 (q, 2H, J = 6.80 Hz, OCH2),
7.11–8.05 (m, 9H, ArH), 10.26 (br, 1H, OH), 16.17 (br, 1H, PhANH);
Anal. Calcd. for C24H19N3O6S: C: 60.37, H: 4.01, N: 8.80, S: 6.72.
Found: C: 60.44, H: 3.83, N: 8.64, S: 6.87.

120
F. Karcı, F. Karcı / Journal of Molecular Structure 1024 (2012) 117–122
H3C
COOC2H5
H3C
COOC2H5
H3C
COOC2H5
R'
R'
N
N
N
H
C
N
N
R'
N
H
C
S
N
N
N
H
S
C
S
H
O
KT
O
KT
N
O
N
O
N
N
R
HO
O
N
R
N
R
(hydrazo-keto)
(azo-keto)
(azo-enol)
(T2)
(T3)
(T1)
2, R: H,
3, R: C6H5 , R': CH3
R': CH3
4, R: H,
R': NH2
5, R: C6H5 , R': NH2
Scheme 3. Tautomeric equilibriums of dyes 2–5.
H3C
COOC2H5
H3C
COOC2H5
H3C
COOC2H5
CH3
CH3
CH3
N
N
N
H
C
N
N
CN
O
CN
N
H
S
C
KT
N N
N
H
KT
C
CN
O
S
S
O
H
O
O
HO
N
H
O
N
H
O
O
N
H
(hydrazo-keto)
(azo-enol)
(T5)
(azo-keto)
(T6)
(T4)
Scheme 4. Tautomeric equilibriums of dye 6.
H3C
COOC2H5
H3C
COOC2H5
H3C
COOC2H5
O
O
O
N
N
N
C
N
N
N
H
S
C
KT
N N
N
KT
C
S
S
NH
NH
NH
O
H
H
O
O
H
HO
N
H
O
O
N
H
O
O
N
H
O
(azo-enol)
(T8)
(azo-keto)
(hydrazo-keto)
(T9)
(T7)
Scheme 5. Tautomeric equilibriums of dye 7.
H3C
COOC2H5
H3C
COOC2H5
H3C
COOC2H5
O
O
OH
O
O
O
N
N
N
C
N
N
O
S
N
H
C
N
N
N
O
C
S
KT
S
KT
O
H
H
O
O
H
O
(hydrazo-keto)
(azo-keto)
(azo-enol)
(T12)
(T11)
(T10)
Scheme 6. Tautomeric equilibriums of dye 8.
1H NMR spectra of dyes 2 showed two broad peaks at 9.69 ppm
variety of solvents in concentrations (10ꢁ6–10ꢁ8 M). Because of
solubility problems, these were run at different concentrations
and the results are summarised in Table 1. The visible absorption
spectra of the dyes did not correlate with the polarity of solvent.
Dyes 2–8 gave a maximum absorption peak in all used solvents.
This result suggests that dyes 2–8 are present in a single tauto-
meric form in all used solvents.
(NH or OH) and 10.13 ppm (NH or OH). Also, 1H NMR spectra of
dyes 3 showed two broad peaks at 9.82 ppm (NH or OH) and
10.36 ppm (NH or OH). These results suggest that dyes 2 and 3
were present as a mixture of two tautomeric forms (T2 and T3)
in DMSO. 1H NMR spectra of dyes 4–8 showed one broad peak
within the range of 10.13–10.26 ppm because of OH proton. Also,
1H NMR spectra of dyes 4–8 did not show any broad band for tau-
tomeric NH proton. These suggest that dyes 4–8 are predominantly
in azo-enol form (T2, T5, T8 and T11, respectively) as opposed to
azo-keto form (T1, T4, T7 and T10, respectively) and hydrazo-keto
form (T3, T6, T9 and T12, respectively) in DMSO.
Table 1
Influence of solvent on kmax (nm) of dyes 2–8.
Dye
no
DMSO DMF Acetonitrile Methanol Acetic
acid
Chloroform
2
3
4
5
6
7
8
514
489
471
452
499
431
471
521
514
500
500
515
433
466
452
440
410
430
476
432
468
467
445
427
442
479
442
466
452
438
412
434
479
441
468
452
440
410
433
443
445
470
3.2. Solvent effects on UV–vis. spectra
As the tautomeric equilibria strongly depend on the nature of
the media, the behaviour of dyes in various solvents was studied.
For this purpose, the UV–vis absorption spectra of dyes 2–8 were
recorded over the range of k between 300 and 700 nm, using a

F. Karcı, F. Karcı / Journal of Molecular Structure 1024 (2012) 117–122
121
2.000
1.500
1.000
0.500
0.000
2.000
1: DMSO
2: DMF
3: Acetonitile
4: Methanol
5: Acetic acid
6: Chloroform
1: Methanol
2: Methanol + HC l
3: Methanol + KOH
1.500
1.000
0.500
0.000
2
5
4
1
1
6
3
2
3
300
400
500
700
600
300
400
500
Wavelength (nm)
700
600
Wavelength (nm)
Fig. 3. Absorption spectra of dye 2 in acidic and basic solutions.
Fig. 1. Absorption spectra of dye 2 in various solvents.
499 nm in DMSO, 515 nm in DMF, 476 nm in acetonitrile and
479 nm in methanol and acetic acid) (Fig. 2). Although, kmax of
dye 7 in DMSO, DMF, and acetonitrile shifted hypsochromically
with respect to the kmax in chloroform, kmax values of dye 7 in meth-
anol, acetic acid and chloroform are similar. kmax values of dye 8 in
all used solvents are similar.
2.000
1.500
1.000
0.500
0.000
1: DMSO
2: DMF
3: Acetonitile
4: Methanol
5: Acetic acid
6: Chloroform
3.3. Acid and base effects on UV–vis. spectra
The effects of acid and base on the absorption of dye solutions
were investigated and the results are shown in Table 2. The absorp-
tion spectra of the dyes 2–6 in methanol was quite sensitive to the
addition of base (potassium hydroxide, 0.1 M), with kmax of dyes 2–
6 showing bathochromic shifts and absorption curves of the dyes
resembled those in DMSO and DMF (e.g. for dye 2 kmax is 467 nm
in methanol, 492 nm in methanol + KOH) (Fig. 3). This result sug-
gests that dyes 2–6 are present in a different tautomeric form in
methanol + KOH than that in methanol and this tautomeric form
resembled those in DMSO and DMF. When base (potassium
hydroxide, 0.1 M) was added to dye solutions in methanol,
kmax of dye 7 showed slight hypsochromic shift with respect to
the kmax in methanol and the absorption spectra of dyes resembled
those in DMSO and DMF. It was also observed that when base
(potassium hydroxide, 0.1 M) was added to dye solutions in meth-
anol, kmax of dye 8 did not change significantly and absorption
curves of the dyes resembled those in DMSO and DMF.
When hydrochloric acid (0.1 M) was added to dye solutions in
methanol, kmax of dyes 2–5 showed slight hypsochromic shifts with
respect to the kmax in methanol and the absorption spectra of dyes
resembled those in acetic acid (e.g. for dye 2 kmax is 467 nm in
methanol, 452 nm in methanol + HCl) (Fig. 3). Probably, these
slight hypsochromic shifts are occured due to solubility of dyes
2–5 are different in methanol + HCl than that in methanol and
3
4
5
2
1
6
300
400
500
700
600
Wavelength (nm)
Fig. 2. Absorption spectra of dye 6 in various solvents.
It was observed that the kmax of dyes 2–5 in DMSO, DMF and
methanol shifted bathochromically with respect to the kmax in
chloroform (e.g. for dye 2 kmax is 452 nm in chloroform, 514 nm
in DMSO, 521 nm in DMF and 467 nm in methanol) (Fig. 1). But,
bathochromic shifts of kmax of dyes 2–5 in methanol are less than
bathochromic shifts of kmax of dyes 2–5 in DMSO and DMF. On
the other hand, kmax values of dyes 2–5 in acetonitrile and acetic
acid did not change significantly with respect to the kmax in chloro-
form. It was also observed that kmax of dye 6 in DMSO, DMF, aceto-
nitrile, methanol and acetic acid shifted bathochromically with
respect to the kmax in chloroform (kmax is 443 nm in chloroform,
Table 2
Absorption maxima of dyes 2–8 in acidic and basic solutions.
Dye no
kmax (nm)
Methanol
Methanol + KOH
Methanol + HCl
Chloroform
Chloroform + piperidine
Acetic acid
2
3
4
5
6
7
8
467
445
427
442
479
442
466
492
492
489
486
523
430
468
452
437
410
430
479
438
469
452
440
410
433
443
445
470
500
501
490
497
502
431
470
452
438
412
434
479
441
468

122
F. Karcı, F. Karcı / Journal of Molecular Structure 1024 (2012) 117–122
2.000
1.500
1.000
0.500
0.000
and DMF. It was also observed that the absorption spectra of dyes
2–6 in methanol and chloroform were quite sensitive to the addi-
tion of base. These results suggest that dyes 2–6 are present in a
different tautomeric form in basic mediums and DMSO, DMF than
that in acetonitrile, methanol, acetic acid and chloroform. Tauto-
meric forms of dyes 2–6 are identical in methanol + KOH, chloro-
form + piperidine, DMSO and DMF. Also, solubility of dyes 2–5 is
different in methanol + HCl than that in methanol and solubility
resembled those in acetic acid.
1: Chloroform
2: Acetic acid
3: Chloroform + piperidine
2
3
1
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Fig. 4. Absorption spectra of dye 2 in acidic and basic solutions.
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when hydrochloric acid (0.1 M) was added to dye solutions in
methanol, kmax of dyes 6–8 did not change significantly.
When piperidine was added to dye solutions in chloroform, kmax
of dyes 2–6 showed bathochromic shifts and absorption curves of
the dyes resembled those in DMSO and DMF (e.g. for dye 2 kmax is
452 nm in chloroform, 500 nm in chloroform + piperidine) (Fig. 4).
This result suggests that dyes 2–6 are present in a different tauto-
meric form in chloroform + piperidine than that in chloroform and
this tautomeric form resembled those in DMSO and DMF.
Although, kmax of the dye 7 showed slight hypsochromic shift when
a small amount of piperidine was added to dye 7 solution in chlo-
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4. Conclusions
The solvatochromic behaviour and tautomeric structures of
newly synthesized bis-heterocyclic monoazo dyes in various sol-
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of maximum absorption of these dyes have been studied. Dyes 2–6
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_
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