Part 1: Synthesis and visible absorption spectra of some new monoazo dyes derived from ethyl 2-amino-4-(4′-substitutedphenyl)thiophenes

Article abstract of DOI:10.1016/j.saa.2014.04.042

Series of monoazo dyes from some ethyl 2-amino-4-(4′- substitutedphenyl) thiophenes were prepared and characterized. The structure of the substances was confirmed by FT-IR, ¹H NMR and mass spectroscopic techniques. The relationship among the structure of the dyes, their absorption characteristics and the solvatochromic and halochromic behaviors of the dyes were investigated. Introduction of electron-accepting substituent into the diazo moiety results in large bathochromic shifts in all solvents used. The dyes exhibited positive solvatochromism and their solvatochromic properties were discussed in relation to tautomerism.

Full text of DOI:10.1016/j.saa.2014.04.042

Accepted Manuscript
Part 1: Synthesis and Visible Absorption Spectra of Some New Monoazo Dyes
Derived from Ethyl 2-amino-4-(4’-substitutedphenyl)thiophenes
Banu Babür, Nermin Ertan
PII:
S1386-1425(14)00615-5
http://dx.doi.org/10.1016/j.saa.2014.04.042
SAA 12006
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
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Accepted Date:
13 January 2014
7 April 2014
Please cite this article as: B. Babür, N. Ertan, Part 1: Synthesis and Visible Absorption Spectra of Some New
Monoazo Dyes Derived from Ethyl 2-amino-4-(4’-substitutedphenyl)thiophenes, Spectrochimica Acta Part A:
Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.042
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Part 1: Synthesis and Visible Absorption Spectra of Some New Monoazo Dyes Derived
from Ethyl 2-amino-4-(4’-substitutedphenyl)thiophenes
Banu BABÜR, Nermin ERTAN
Gazi University, Faculty of Science, Department of Chemistry, 06500 Teknikokullar, Ankara,
Turkey
Abstract
Series of monoazo dyes from some ethyl 2-amino-4-(4’-substitutedphenyl) thiophenes were
1
prepared and characterized. The structure of the substances was confirmed by FT-IR, H-
NMR and mass spectroscopic techniques. The relationship among the structure of the dyes,
their absorption characteristics and the solvatochromic and halochromic behaviors of the dyes
were investigated. Introduction of electron-accepting substituent into the diazo moiety results
in large bathochromic shifts in all solvents used. The dyes exhibited positive solvatochromism
and their solvatochromic properties were discussed in relation to tautomerism.
Keywords: Azo dyes, Thiophene, Absorption spectra, Solvent effect, Substituent effect,
Halochromism
*Corresponding author Tel.: +90 312 202 11 05; Fax: +90 312 212 22 79
*E-mail address: nertan@gazi.edu.tr
†Part of this work was presented at 25. National Chemistry Congress, 27 June- 02 July 2011,
Erzurum, TURKEY.
1

Introduction
Arylazo heterocycles are a versatile class of colored organic compounds that have recently
attracted the interest of many research groups due to their diverse applications, not only as
conventional synthetic dyes and pigments but also as solvatochromic probes and thermally
stable organic second-order nonlinear optical (NLO) chromophores [1-3]. They also play a
significant role in many applications such as optical data storage devices, liquid crystals [2-5],
dye-sensitized solar cells (DSCC), photovoltaic and fluorescent devices, acid-base and metal
sensors [6,7] and so on.
Numerous heterocyclic dyes are now marketed as disperse azo dyes based on heteroaromatic
diazo or coupling components. Thus, the shade especially blues and greens have become
obtainable more economically and properties such as high tinctorial strength and excellent
brightness have become attainable and are commercially competitive with the more expensive
anthraquinone dyes. [8, 9]. 2-Aminothiophene derivatives have been used as diazo
components for the synthesis of disperse azo dyes for a long time because of their
bathochromic effect [10-14]. In addition, it has been reported that thiophene rings lead to
excellent sublimation fastness on fibers and better dye ability because of their small molecular
structure [14, 15].
Numerous reports on the synthesis of hetarylazo dyes based on thiophene have appeared.
Most of these dyes have been prepared from carbocyclic coupling components and 2-
aminothiophene derivatives as diazo components [14-24]. Although thiophene derivatives
have rarely been reported as coupling components for disperse dyes, some examples obtained
from the coupling moieties of 2-aminothiophenes have been reported [25]. We have
previously reported the synthesis of new heterocyclic azo dyes containing pyridone [26,27],
8-hydroxyquinoline [28], and indol [29-31] moiety with hetarylazo or phenylazo groups.
These new π-conjugated systems exhibit remarkable solvatochromic and halochromic
behaviors. In recent years a number of new thienylazo disperse dyes have been prepared from
2-N-substituted aminothiophenes as coupling components and these dyes exhibit different
spectroscopic properties with substitution of 3- and/or 4-position of the thiophene ring [32-
35]. Although the hetarylazo dyes based on thiophene have been known for many years,
relatively little is known about the dyes prepared from a primary amino group consisting
thiophene ring as coupling components [36-38]. Also, influence of substituent on visible
2

absorption spectra and solvatochromic and halochromic behaviours of these dyes have not
been discussed in depth yet. In addition, amino group have a contribution the affinity of the
dye molecules onto fabric with intermolecular H-bond between the fabric and the dye
molecule [39,40].
Consequently, we reported here the synthesis of three series of new heterocyclic monoazo
dyes derived from 2-amino-3-carboetoxy-4-(4’-substitutedphenyl)thiophenes as coupling
components and discussed the correlation between the dye structure and spectral properties.
Azo-hydrazone tautomerism, solvatochromic and halochromic properties of the dyes were
also investigated.
3

Experimental
Materials and instrumentation
The chemical used in the synthesis of all dyes were obtained from Aldrich Chemical
Company (USA) and were used without further purification. Benzene was freshly distillated
and the other solvents were used spectroscopic grade.
IR spectra were recorded on a Mattson 1000 FT-IR spectrophotometer (Gazi University
1
Department of Chemistry, Turkey) in KBr (ν, in cm-1). H NMR spectra were recorded on a
Bruker-Spectrospin Avance DPX 400 MHz Ultra-Shield (Hacettepe University Department of
Chemistry, Turkey) in DMSO-d6 with TMS as the internal reference. Chemical shifts were
expressed in δ units (ppm). Ultraviolet-visible (UV-vis) absorption spectra were recorded on
an Analytikjena Specord 200 Spectrophotometer (Gazi University Department of Chemistry,
Turkey) at the wavelength of maximum absorption (λmax, in nm) in range of solvents, i.e.
Dimethylsulphoxide (DMSO, ε, 46.45), methanol (ε, 32.66), acetic acid (ε 6.17) and
o
chloroform (ε, 4.81). All melting points were uncorrected and in C (Electrothermal 9200
melting point apparatus). Mass analyses were performed on a Waters LCT Premier XE (TOF
MS) (Gazi University Laboratories, Department Pharmacological Sciences).
Synthesis of coupling components
Ethyl 2-cyano-3-phenylbut-2-enoate derivatives (I-a, I-b and I-c) were synthesized by
Knoevenagel condensation with 4-substituted acetophenones and ethyl cyanoacetate as
starting materials [41]. Ethyl 2-amino-4-phenylthiophene-3-carboxylate derivatives (II-a, II-b
and II-c) were synthesized according to the method described in the literature by Gewald
reaction [42, 43] (Scheme 1). Compound II-a was synthesized from compound I-a and sulphur
at presence of triethylamine according to the literature [43]. It was recrystallized from ethanol
to give yellow crystals (65 % yield, m.p: 98-99 oC ; Lit. values m.p.: 98 oC, 62 % yield).
Compound II-b was synthesized from I-b in a similar manner described above for compound
o
II-a and recrystallized from ethanol to give dark yellow crystals (41 % yield, m.p.:81 C;
melting point and yield of II-b was not found in literature) [44].
4

Similarly, compound II-c was synthesized from I-c, purified from column chromatography
(Ethyl acetate/n-Heksane, 1:1 (v/v)) and recrystallized from ethanol to give light yellow
crystals ( 48 % yield , m.p. : 98 oC; Lit. values m.p.: 98-100 oC, 60 % yield) [45].
Scheme 1 is here
Preparation of ethyl 2-amino-5-(4-substitutedphenyldiazenyl)-4-(4'-substitutedphenyl)
thiophene-3-carboxylate derivatives (1-15)
Diazotization of various carbocyclic amines was affected with HCl and NaNO2. A typical
procedure was described below and all compounds (1-15) were prepared in a similar manner
(Scheme 2). The yields of the dyes were in the range of 56-97% and these yields were
obtained after two times recrystallization from ethanol. The purity of the compounds was
confirmed by 1H-NMR data and the reaction was controlled by TLC. Characterization data
were given below, melting points and nomenclature of compounds were given in Table 1.
Scheme 2 and Table 1 are here
General procedure for preparation of substituted thienylazo dyes [46]
P-Substituted aniline (2.0 mmole) was dissolved in hydrochloric acid (1.5 mL conc. HCl in 4
o
mL water). The solution was then cooled to 0-5 C with stirring. Sodium nitrite (0.15 g, 2.0
o
mmole) in water (3 mL) was gradually added to this solution over 15 min period at 0-5 C
with stirring. The mixture was stirred for an additional 1 hour while maintaining at
temperature of 0-5 oC. Excess nitrous acid was destroyed by addition of urea, and the solution
was filtered. The resulting clear diazonium salt was used immediately in the coupling
reaction. Ethyl 2-amino-4-(4'-substitutedphenyl)thiophene-3-carboxylate (2.0 mmole) was
dissolved in acetic acid/propionic acid (4 mL, ratio 3:1) and cooled to 0-5 oC in a salt/ice bath.
The cold diazonium salt solution was added to this cooled solution over 20 min with vigorous
stirring in a drop wise manner, while maintaining the pH at 4-6 by addition of saturated
o
sodium carbonate solution. The mixture was further stirred for 1 hour at 0-5 C and resulting
solid was filtered, washed with cold water, dried, and recrystallized two times in ethanol.
5

Preparation of ethyl 2-amino-5-(phenyldiazenyl)-4-phenylthiophene-3-carboxylate (dye 1)
This dye was obtained from aniline and ethyl 2-amino-4-phenylthiophene-3-carboxylate as
brick red crystals. IR (KBr) νmax (cm-1) : 3371, 3254 (N-H), 3049 (aromatic C-H), 2980
1
(aliphatic C-H), 1664 (C=O), 1272 (C-O), 1574 (C=C), 503 (C-S); H NMR (DMSO-d6): δ
8.55 (br, 2H, –NH2), 7.28-7.44 (m, 10H), 3.90 (q, 2H, -COOCH2CH3, J= 7.10 Hz), 0.85 (t,
3H, -COOCH2CH3, J= 7.10 Hz). TOF MS (m/z) : 352.1 (100%) (M-H)+.
Preparation of ethyl 2-amino-5-(4-methylphenyldiazenyl)-4-phenylthiophene-3-carboxylate
(dye 2)
This dye was obtained from 4-methylaniline and ethyl 2-amino-4-phenylthiophene-3-
carboxylate as orange crystals. IR (KBr) νmax (cm-1) : 3380, 3261 (N-H), 3021 (aromatic C-
1
H), 2978 (aliphatic C-H), 1659 (C=O), 1272 (C-O), 1572 (C=C), 505 (C-S); H NMR
(DMSO-d6): δ 8.48 (br, 2H, –NH2), 7.40 (s, 5H), 7.33 (d, 2H, J= 8.29 Hz), 7.21 (d, 2H, J=
8.29 Hz), 3.95 (q, 2H, -COOCH2CH3, J= 7.12 Hz), 2.30 (s, 3H, -CH3), 0,85 (t, 3H, -
COOCH2CH3, J= 7.08 Hz). TOF MS (m/z) : 366.1 (100%) (M-H)+.
Preparation of ethyl 2-amino-5-(4-methoxyphenyldiazenyl)-4-phenylthiophene-3-carboxylate
(dye 3)
This dye was obtained from 4-methoxylaniline and ethyl 2-amino-4-phenylthiophene-3-
carboxylate as brown crystals. IR (KBr) νmax (cm-1) : 3366, 3245 (N-H), 3066 (aromatic C-H),
2978, 2935 (aliphatic C-H), 1662 (C=O), 1241 (C-O), 1599, 1578 (C=C), 547 (C-S); 1H NMR
(DMSO-d6): δ 8.40 (br, 2H, –NH2), 7.40 (d, 2H, J= 8.98 Hz), 7.37 (s, 5H), 6.95 (d, 2H, J=
9.02 Hz), 3.95 (q, 2H, -COOCH2CH3, J= 7.10 Hz), 3.75 (s, 3H, -OCH3), 0.80 (t, 3H, -
COOCH2CH3, J= 7.10 Hz). TOF MS (m/z): 382.2 (100%) (M-H)+.
Preparation of ethyl 2-amino-5-(4-cyanophenyldiazenyl)-4-phenylthiophene-3-carboxylate
(dye 4)
This dye was obtained from 4-cyanoaniline and ethyl 2-amino-4-phenylthiophene-3-
carboxylate as red crystals. IR (KBr) νmax (cm-1) : 3391, 3286, 3251 (N-H), 3055 (aromatic C-
6

1
-O), 1608 (C=C), 553 (C-S); H
≡N), 1678 (C=O), 1255 (C
H), 2985 (aliphatic C-H), 2220 (C
NMR (DMSO-d6): δ 8.85 (br, 2H, –NH2), 7.80 (d, 2H, J= 8.59 Hz), 7.48 (d, 2H, J= 8.59 Hz),
7.40 (s, 5H), 3.95 (q, 2H, -COOCH2CH3, J= 7.09 Hz), 0.82 (t, 3H, -COOCH2CH3, J= 7.09
Hz). TOF MS (m/z) : 377.1 (100%) (M-H)+.
Preparation of ethyl 2-amino-5-(4-nitrophenyldiazenyl)-4-phenylthiophene-3-carboxylate
(dye 5)
This dye was obtained from 4-nitroaniline and ethyl 2-amino-4-phenylthiophene-3-
carboxylate as black crystals. IR (KBr) νmax (cm-1) : 3447, 3305, 3247 (N-H), 3052 (aromatic
C-H), 2985 (aliphatic C-H), 1669 (C=O), 1594 (C=C), 1507,1373 (NO2), 1244 (C-O), 490 (C-
1
S); H NMR (DMSO-d6): δ 8.96 (br, 2H, –NH2), 8.23 (d, 2H, J= 9.09 Hz), 7.53 (d, 2H, J=
9.07 Hz), 7.43 (s, 5H), 4.00 (q, 2H, -COOCH2CH3, J= 7.07 Hz), 0.85 (t, 3H, -COOCH2CH3,
J= 7.08 Hz). TOF MS (m/z): 395.0 (100%) (M-H)-.
Preparation of ethyl 2-amino-5-(phenyldiazenyl)-4-(4'-nitrophenyl) thiophene-3-carboxylate
(dye 6)
This dye was obtained from aniline and ethyl 2-amino-4-(4'-nitrophenyl)thiophene-3-
carboxylate as light red crystals. IR (KBr) νmax (cm-1) : 3411, 3298, 3250 (N-H), 3061
(aromatic C-H), 2963 (aliphatic C-H), 1671 (C=O), 1589 (C=C), 1509, 1337 (NO2), 1233 (C-
1
O), 543 (C-S); H NMR (DMSO-d6): δ 8.63 (br, 2H, –NH2), 8.27 (d, 2H, J= 8.77 Hz), 7.73
(d, 2H, J= 8.77 Hz), 7.39-7.46 (m, 4H), 7.31-7.36 (m, 1H), 4.00 (q, 2H, -COOCH2CH3, J=
7.07 Hz), 0.90 (t, 3H, -COOCH2CH3, J= 7.07 Hz). TOF MS (m/z): 395.0 (100%) (M-H)-.
Preparation of ethyl 2-amino-5-(4-methylphenyldiazenyl)-4-(4'-nitrophenyl) thiophene-3-
carboxylate (dye 7)
This dye was obtained from 4-methylaniline and ethyl 2-amino-4-(4'-nitrophenyl) thiophene-
3-carboxylate as dark red crystals. IR (KBr) νmax (cm-1) : 3416, 3296, 3253 (N-H), 3026
(aromatic C-H), 2984 (aliphatic C-H), 1672 (C=O), 1593 (C=C), 1503, 1336 (NO2), 1236 (C-
1
O), 600 (C-S); H NMR (DMSO-d6): δ 8.56 (br, 2H, –NH2), 8.26 (d, 2H, J= 8.79 Hz), 7.71
(d, 2H, J= 8.79 Hz), 7.35 (d, 2H, J= 8.28 Hz), 7.22 (d, 2H, J= 8.24 Hz), 4.00 (q, 2H, -
7

COOCH2CH3, J= 7.09 Hz), 2.30 (s, 3H, -CH3), 0.90 (t, 3H, -COOCH2CH3, J= 7.10 Hz). TOF
MS (m/z) : 409.1 (100%) (M-H)-.
Preparation of ethyl 2-amino-5-(4-methoxyphenyldiazenyl)-4-(4'-nitrophenyl) thiophene-3-
carboxylate (dye 8)
This dye was obtained from 4-methoxylaniline and ethyl 2-amino-4-(4'-nitrophenyl)
thiophene-3-carboxylate as brown crystals. IR (KBr) νmax (cm-1) : 3453, 3323 (N-H), 3103
(aromatic C-H), 2983 (aliphatic C-H), 1673 (C=O), 1586 (C=C), 1504, 1337 (NO2), 1103 (C-
1
O), 556 (C-S); H NMR (DMSO-d6): δ 8.49 (br, 2H, –NH2), 8.25 (d, 2H, J= 8.77 Hz), 7.70
(d, 2H, J= 8.74 Hz), 7.43 (d, 2H, J= 8.98 Hz), 6.98 (d, 2H, J= 9.01 Hz), 4.00 (q, 2H, -
COOCH2CH3, J= 7.08 Hz), 3.80 (s, 3H, -OCH3), 0.85 (t, 3H, -COOCH2CH3, J= 7.09 Hz).
TOF MS (m/z) : 425.0 (100%) (M-H)-.
Preparation of ethyl 2-amino-5-(4-cyanophenyldiazenyl)-4-(4'-nitrophenyl) thiophene-3-
carboxylate (dye 9)
This dye was obtained from 4-cyanoaniline and ethyl 2-amino-4-(4'-nitrophenyl) thiophene-3-
carboxylate as red crystals. IR (KBr) νmax (cm-1) : 3487, 3413, 3370 (N-H), 3099 (aromatic C-
H), 2985 (aliphatic C-H), 2221 (C≡N), 1692 (C=O), 1588 (C=C), 1513, 1346 (NO2), 1190 (C-
1
O), 557 (C-S); H NMR (DMSO-d6): δ 8.93 (br, 2H, –NH2), 8.28 (d, 2H, J= 8.78 Hz), 7.84
(d, 2H, J= 8.59 Hz), 7.73 (d, 2H, J= 8.76 Hz), 7.51 (d, 2H, J= 8.58 Hz), 4.00 (q, 2H, -
COOCH2CH3, J= 7.08 Hz), 0.90 (t, 3H, -COOCH2CH3, J= 7.07 Hz). TOF MS (m/z) : 422.1
(100%) (M-H)-.
Preparation of ethyl 2-amino-5-(4-nitrophenyldiazenyl)-4-(4'-nitrophenyl) thiophene-3-
carboxylate (dye 10)
This dye was obtained from 4-nitroaniline and ethyl 2-amino-4-(4'-nitrophenyl) thiophene-3-
carboxylate as black crystals. IR (KBr) νmax (cm-1) : 3453, 3370, 3311 (N-H), 3079 (aromatic
C-H), 2979 (aliphatic C-H), 1671 (C=O), 1585 (C=C), 1513, 1346 (NO2), 1252 (C-O), 584
(C-S); 1H NMR (DMSO-d6): δ 9.03 (br, 2H, –NH2), 8.29 (d, 2H, J= 8.64 Hz), 8.24 (d, 2H, J=
8.98 Hz), 7.75 (d, 2H, J= 8.63 Hz), 7.57 (d, 2H, J=8.96 Hz), 4.00 (q, 2H, -COOCH2CH3, J=
7.10 Hz), 0.90 (t, 3H, -COOCH2CH3, J= 7.09 Hz). TOF MS (m/z) : 440.0 (100%) (M-H)-.
8

Preparation of ethyl 2-amino-5-(phenyldiazenyl)-4-(4'-methoxyphenyl) thiophene-3-
carboxylate (dye 11)
This dye was obtained from aniline and ethyl 2-amino-4-(4'-methoxyphenyl) thiophene-3-
carboxylate as orange crystals. IR (KBr) νmax (cm-1) : 3361, 3211 (N-H), 3054 (aromatic C-
1
H), 2975 (aliphatic C-H), 1667 (C=O), 1574 (C=C), 1248 (C-O), 509 (C-S); H NMR
(DMSO-d6): δ 8.52 (br, 2H, –NH2), 7.35 (d, 2H, J= 8.70 Hz), 7.36-7.47 (m, 5H), 6.96 (d, 2H,
J= 8.72 Hz), 4.00 (q, 2H, -COOCH2CH3, J= 7.10 Hz), 3.80 (s, 3H, -OCH3), 0.95 (t, 3H, -
COOCH2CH3, J= 7.07 Hz). TOF MS (m/z) : 382.1 (100%) (M-H)+.
Preparation of ethyl 2-amino-5-(4-methylphenyldiazenyl)-4-(4'-methoxyphenyl) thiophene-3-
carboxylate (dye 12)
This dye was obtained from 4-methylaniline and ethyl 2-amino-4-(4'-methoxyphenyl)
thiophene-3-carboxylate as orange crystals. IR (KBr) νmax (cm-1) : 3368, 3276 (N-H), 3018
(aromatic C-H), 2981 (aliphatic C-H), 1665 (C=O), 1574 (C=C), 1238 (C-O), 584 (C-S); 1H
NMR (DMSO-d6): δ 8.45 (br, 2H, –NH2), 7.40 (d, 2H, J= 8.16 Hz), 7.35 (d, 2H, J= 8.60 Hz),
7.22 (d, 2H, J= 8.41 Hz), 6.95 (d, 2H, J= 8.74 Hz), 4.00 (q, 2H, -COOCH2CH3, J= 7.14 Hz),
3.80 (s, 3H, -OCH3), 2.30 (s, 3H, -CH3), 0.95 (t, 3H, -COOCH2CH3, J= 7.11 Hz). TOF MS
(m/z) : 396.1 (100%) (M-H)+.
Preparation of ethyl 2-amino-5-(4-methoxyphenyldiazenyl)-4-(4'-methoxyphenyl) thiophene-
3-carboxylate (dye 13)
This dye was obtained from 4-methoxyaniline and ethyl 2-amino-4-(4'-methoxyphenyl)
thiophene-3-carboxylate as orange crystals. IR (KBr) νmax (cm-1) : 3368, 3250 (N-H), 3000
(aromatic C-H), 2928 (aliphatic C-H), 1663 (C=O), 1577 (C=C), 1242 (C-O), 548 (C-S); 1H
NMR (DMSO-d6): δ 8.38 (br, 2H, –NH2), 7.43 (d, 2H, J= 8.97 Hz), 7.34 (d, 2H, J= 8.68 Hz),
7.00 (d, 2H, J=9.00 Hz), 6.94 (d, 2H, J= 8.74 Hz), 4.00 (q, 2H, -COOCH2CH3, J= 7.13 Hz),
3.53 (s, 3H, -OCH3), 3.54 (s, 3H, -OCH3), 0.93 (t, 3H, -COOCH2CH3, J= 7.11 Hz). TOF MS
(m/z) : 410.1 (100%) (M-H)-.
9

Preparation of ethyl 2-amino-5-(4-cyanophenyldiazenyl)-4-(4'-methoxyphenyl) thiophene-3-
carboxylate (dye 14)
This
methoxyphenyl)thiophene-3-carboxylate as black crystals. IR (KBr) νmax (cm-1) : 3434, 3285,
≡N), 1668 (C=O), 1591
dye
was
obtained
from
4-cyanoaniline
and
ethyl
2-amino-4-(4'-
3240 (N-H), 3000 (aromatic C-H), 2932 (aliphatic C-H), 2217 (C
(C=C), 1248 (C-O), 553 (C-S); 1H NMR (DMSO-d6): δ 8.85 (br, 2H, –NH2), 7.80 (d, 2H, J=
8.51 Hz), 7.50 (d, 2H, J= 8.52 Hz), 7.35 (d, 2H, J= 8.62 Hz), 6.95 (d, 2H, J= 8.67 Hz), 4.00
(q, 2H, -COOCH2CH3, J= 7.10 Hz), 3.80 (s, 3H, -OCH3), 0.95 (t, 3H, -COOCH2CH3, J= 7.10
Hz). TOF MS (m/z) : 405.1 (100%) (M-H)-.
Preparation of ethyl 2-amino-5-(4-nitrophenyldiazenyl)-4-(4'-methoxyphenyl) thiophene-3-
carboxylate (dye 15)
This dye was obtained from 4-nitroaniline and ethyl 2-amino-4-(4'-methoxyphenyl)
thiophene-3-carboxylate as black crystals. IR (KBr) νmax (cm-1) : 3444, 3296, 3247 (N-H),
3099 (aromatic C-H), 2984 (aliphatic C-H), 1670 (C=O), 1596 (C=C), 1507, 1374 (NO2),
1
1242 (C-O), 588 (C-S); H NMR (DMSO-d6): δ 8.91 (br, 2H, –NH2), 8.22 (d, 2H, J= 8.97
Hz), 7.52 (d, 2H, J= 8.96 Hz), 7.37 (d, 2H, J= 8.64 Hz), 6.95 (d, 2H, J= 8.67 Hz), 4.00 (q, 2H,
-COOCH2CH3, J= 7.10 Hz), 3.80 (s, 3H, -OCH3), 0.95 (t, 3H, -COOCH2CH3, J= 7.10 Hz).
TOF MS (m/z) : 425.1 (100%) (M-H)-.
Results and Discussion
Ethyl 2-amino-4'-substitutedphenylthiophene-3-carboxylate derivatives (II a-c) were
synthesized by Gewald reaction according to the method described in the literature [43-45]
with 4-substituted acetophenones and ethyl cyanoacetate as starting materials, as shown in
Scheme 1.
As shown in Scheme 2, three series of some new thienylazo dyes (1-15) based on 4-
substituted anilines as diazo components with coupling components, ethyl 2-amino-4-
phenylthiophene-3-carboxylate, ethyl 2-amino-4-(4'-nitrophenyl)thiophene-3-carboxylate and
ethyl 2-amino-4-(4'-methoxyphenyl)thiophene-3-carboxylate compounds were prepared by
10

the same general procedure as described in the Experimental section. Dyes 1-15 were
obtained generally in high yields (56-97 %). The characterization of these thienylazo dyes (1-
1
15) was made by FT-IR, H-NMR, UV-Vis spectra and TOF-MS measurements (see Exp.
Sec.). Physical data of the dyes are given in Table 1.
Scheme 3 is here
Azo-hydrazone tautomerism in disperse azo dyes is one of the most interesting research fields
because it is very important for their optical stability and fastness properties. Hence, we
considered the possible tautomeric structures of the dyes while discussing the spectroscopic
properties of the dyes. The dyes can exist in three possible tautomeric forms, but two of them
may relatively stable, namely amino-azo and imine-hydrazone forms as shown in Scheme 3.
The imineazo-enol form of the dyes was not existed in both solid state and solution because
OH stretching band was not observed in FT-IR spectra and there was no signal of OH proton
in 1H-NMR spectra of the dyes. Additionally, the other forms were stabilized with
intramolecular H-bonding. The deprotonation of the tautomers leads to common anion, and
protonation of the tautomers leads to common cation (Scheme 4 and Scheme 5).
Scheme 4 and 5 are here
Characterization of the dyes by 1H NMR and FT-IR (1-15)
The IR spectra of dyes 1-15 (in KBr) showed two strong broad band within the range 3487-
3361 cm-1 and 3413-3211 cm-1 due to NH2 stretching vibration and a characteristic absorption
band of ester carbonyl group at 1692-1659 cm-1. It can be suggested that these dyes exist in
amino-azo form or imine-hydrazone form in the solid state. And also, it showed that the dyes
prepared are not in diazoamino structures because it is expected that only one NH stretching
band are observed for diazoamino structers. Other νmax values at 3103-3000 cm-1 (aromatic C-
H) and at 2985-2928 cm-1 (aliphatic C-H) were recorded. For dyes 4, 9 and 14 an intense band
within the range 2217-2221 cm-1 assignable to the CN stretching was recorded. In addition,
dyes 5, 10 and 15 showed absorption at 1507, 1513, 1504 and 1373, 1346, 1337cm-1 for the
NO2 group, respectively.
11

1
The H-NMR spectra (DMSO-d6) of dyes 1-15 showed a broad singlet at δ 9.03-8.38 ppm
(2H) assigned to the NH2 group, a triplet at δ 0.80-0.95 ppm (3H) and a quartet at δ 3.90-4.00
ppm (2H) for the ethoxy group. The dyes also showed signals at δ 6.94-8.29 ppm which were
assigned to the aromatic protons of phenyl rings. Detailed spectral data are given in the
Experimental section.
1H-NMR spectra gave an evidence of the presence of the amino-azo tautomeric form. A broad
singlet (2H) at 9.03-8.38 ppm was observed due to the NH2 group and there was no hydrazone
NH signal. It is well known that the hydrazide protons of the hydrazone form for azo dyes are
observed at ~ 14-15 ppm [47, 48]. The observed value for the dyes in DMSO-d6 is a clear
indication of the NH2 protons, which was confirmed by the two stretching frequency values
3487-3361 cm-1 and 3413-3211 cm-1 in the FT-IR spectra. The shift in the second stretching
band and its broadening provide further evidence for the H-bonding. These spectral results
suggested that dyes 1-15 exist as the amino-azo form both the solid state and in the solution.
Substituent effect on absorption spectra of dyes
The λmax values of the dyes (1-15) in chloroform, acetic acid, methanol and DMSO were
summarized in Table 2. As far as absorption maxima are concerned, λmax values are directly
proportional to the electronic effect of the substituent in a benzenoid system.
Table 2 is here
A comparison of the position of the absorption maxima in the three series in relation to
substitution on the diazo component (dyes 1-5, dyes 6-10 and dyes 11-15) generally shows
the same decreasing order NO2 > CN > OCH3 > CH3 > H in each solvent, indicating the more
conjugation of the 4-nitrosubstituted dyes except dye 1 in acetic acid. Also, λmax values of
some dyes did not changed with substitution, for example dye 11 and dye 12 in DMSO. In
particular, these shifts are greatest in DMSO (Fig. 1).
Figure 1 is here
12

From the viewpoint of absorption maxima, electronic spectral data for the variously
substituted thienylazo dyes show that there is a general tendency for the visible absorption
band to move bathochromically with increasing of the electron-withdrawing strength of the
substituent in the 4-position of the diazo component, in accordance with the donor-acceptor
chromogens, as known for azo dyes [7]. These results also confirmed that the dyes prepared
exist in one tautomeric form (amino-azo) in solution.
The introduction of the nitro group in the 4-position into the diazo component results in large
bathochromic shifts in all solvents used (for dye 5 ∆λmax = 61 nm in DMSO, ∆λmax = 56 nm in
methanol, ∆λmax = 35 nm in acetic acid, ∆λmax = 67 nm in chloroform relative to λmax of dye 1
in the same solvents). This bathochromic shift is attributed to the stronger electron-acceptor
character of the nitro group, thus enhancing electron delocalization in the dye molecule. In
contrast, introduction of the nitro group in the 4-position into the phenyl ring of the thienyl
group (dye 6) did not significantly change the λmax values of the dyes (for dye 6 ∆λmax = 1 nm
in DMSO, ∆λmax = 3 nm in methanol, ∆λmax = 10 nm in acetic acid, ∆λmax = 14 nm in
chloroform relative to the λmax of dye 1 in the same solvents). Moreover, the introduction of
electron-donor or electron-acceptor substituent into the phenyl ring at the coupling component
showed nearly the same effect and the spectra obtained showed little, if any, shift compared to
unsubstituted dye 1 in all solvents used. Dye 1 absorbs at 468 nm, dye 6 (X: NO2) and dye 11
(X: OCH3) absorb at 469 nm in DMSO. Hence, the influence of substitution in this position is
negligible. The failure of 4-substituents on the phenyl ring of the coupling component effect
to the color shifts in thienylazo dyes may result from the inability of these substituents to
enter effectively into the resonance of the donor-acceptor part of the dye molecule. In
addition, introduction of nitro groups in both positions of the dye (dye 10) did not
significantly change the λmax values of the dyes either (for dye 10 ∆λmax = 61 nm in DMSO,
∆λmax = 49 nm in methanol, ∆λmax = 37 nm in acetic acid and ∆λmax = 66 nm in chloroform
relative to the λmax of dye 1 in the same solvents) and these observed results do not differ
significantly from that of dye 5. A similar behavior was apparent in 4-CN-substituted dye
(dye 4) which resulted in bathochromic shifts of 33 nm in DMSO, 34 nm in methanol, 18 nm
in acetic acid and 39 nm in chloroform compared to dye 1. Furthermore, it was found that
electron-donor substituents (methyl and methoxy groups) in the 4-position of the diazo
component had only a slight influence on the absorption maxima as compared to those of
electron-acceptor substituents. For example, the ∆λmax values are +1 nm in DMSO, +3 nm in
13

methanol, -6 nm in acetic acid and +23 nm in chloroform for dye 2 relative to dye 1. The
same effect is also observed for dye 3 (Table 2). The largest bathochromic shift in comparison
with the visible spectra of the three series is observed for dye 15 (X: OCH3, R: NO2). This
dye absorbs at 535 nm in DMSO, 511 nm in methanol, 501 nm in acetic acid and 505 nm in
chloroform.
Judging from these results, it can be concluded that the color of the dyes prepared is not
affected by substituents in the phenyl ring in 3-position of the thienyl ring but the largest
bathochromic effect is obtained when a stronger electron-donor substituent in 4-position of
the phenyl ring of the coupling component and a stronger electron-acceptor substituent in 4-
position of the phenyl ring of the diazo component are present in the dye molecule. These
results are in agreement with resonance theory.
Solvent effect on absorption spectra of dyes
Since the tautomeric equilibrium strongly depend on the nature of the media, the
solvatochromic properties of the dyes in solvents of different polarities were studied. For this
purpose, the absorption spectra of the dyes were recorded in solvents of different polarities at
a concentration of approximately 10-6 – 10-8 M. These are run at different concentrations
because of solubility problems and the results are given in Table 2. The absorption spectra of
dye 9 in solvents with different polarities are shown in Fig. 2 as typical example of the three
series investigated. Their visible absorption spectra in all solvents used revealed in each case
one characteristic absorption maximum in the region 430-535 nm. Such an absorption pattern
is similar to that reported for the azo compounds that exist in a single tautomeric form. It can
be seen in Fig. 2 and Table 2 that the dyes exist in one tautomeric form, namely the amino-
azo form since the absorption difference pattern of the dyes was not changed with solvents.
The dyes generally show bathochromic shifts as the polarity of the solvents is increased in the
order DMSO > methanol ~ acetic acid > chloroform. The observed positive solvatochromism
is consistent with a transition having a significant charge transfer character.
Fig. 2 is here
To provide conclusive evidence for the tautomeric structure of the compounds (1-15), the
effects of acid and base on their absorption spectra were investigated. For this purpose, the
14

spectra of the dyes were recorded in acidic and basic media. It was observed that the
absorption curves of the dyes were not sensitive to bases. The absorption maxima of the dyes
did not change when various amounts of 0.1 M KOH was added to their methanolic solutions,
except dyes 5, 10 and 15 (Table 3). This result indicates that these compounds exist in one
tautomeric form in alkaline media and the absence of any dissociation.
Fig. 3 and Table 3 are here
As shown in Table 3 and Fig. 3, in the case of dyes 5, 10 and 15, with the addition of 0.1 M
KOH to their methanolic solutions, the intensity of the absorption band decreased and a new
band appeared at longer wavelengths which were assigned to the common anion form
relatable to resonance stabilization. This indicates that these dyes exist in a partly dissociated
state in methanolic KOH solutions. This is confirmed by the observation of one isosbestic
point in the different basic solutions (Fig. 3). The relative ease of the formation of the
common anions from these dyes may be attributed to the additional resonance contribution of
the nitro groups. Also, to determine the base effect on absorption spectra of the dyes in other
solvents, a small amount of piperidine was added to solutions of dyes 1-5 in DMSO. As
shown in Table 4, the similar effect was observed with their methanolic KOH solutions.
When piperidine was added to the dye solutions in DMSO, the λmax of dyes 1-3 did not
change whereas the λmax of dyes 4 and 5 showed second maximums at longer wavelengths
(for dye 4 λmax: 578 nm and for dye 5 λmax: 644 nm) which was attributed to a common anion
form.
Table 4 is here
The λmax values of some dyes showed large bathochromic shifts when various amounts of 0.1
M HCl were added to their methanolic solutions. As can be seen in Table 3, the largest
bathochromic shift of the absorption maxima was observed for dye 8 (∆λmax = + 86 nm),
which possesses the methoxy group in diazo component and the nitro group in coupling
component. As an example of these dyes, the absorption spectra of dye 7 are given in Fig. 4.
In contrast, the dyes 4, 5, 10, 14 and 15, having an electron accepting group in diazo
component exhibited hypsochromic shifts. The largest hypsochromic shift of the λmax was
observed for dye 15 (∆λmax = -42 nm) which possesses the strong electron-accepting nitro
group in diazo component and the methoxy group in coupling component. Additionally, in the
case of dye 9 (X: NO2; R: CN) the λmax value did not changed and dye 8 exhibited small
bathochromic shift. As apparent in Table 3, the introduction of electron-donating substituent
15

in the p-position of diazo component results in bathochromic shifts whereas the introduction
of electron-accepting substituent in the same position of diazo component results in
hypsochromic shifts. From this view, it can be seen that the absorption maxima of the cationic
form of dyes prepared were affected with substitution of the diazo component.
Fig. 4 is here
In acidic media, protonation of the dyes can occur either at the terminal amino group to give
the ammonium tautomer or at the β-nitrogen atom of the azo group to give the azonium
tautomer [50-52]. The protonation of terminal -NH2 group prevents mesomeric interaction of
the lone pair of electrons with π-electronic system, thus its absorption is expected at shorter
wavelength than the neutral dye. We observed that in acidic media the some dyes prepared
absorbed at longer wavelength than the neutral dyes. This showed that dyes observed at
longer wavelength existed in azonium tautomer which contains a delocalized positive charge.
In contrast, the dyes containing electron accepting substituents such as dye 14 and dye 15
showed hypsochromic shifts (dye 14: ∆λmax = -18 nm; dye 15: ∆λmax =-42 nm in methanolic
HCl solutions). Also, absorption maximum of dye 9 did not change in acidic medium. To
show the relationship between Hammett substituent constants and the wavelength difference,
∆λmax values of the dyes in acidic methanolic solutions relative to neutral methanolic solutions
of the dyes are graphed. ∆λmax values of the dyes showed nearly linear correlation with
Hammett substituent constants, except Fig. 6 (Fig. 5-7). These result also suggested that the
dyes exist one tautomeric form, namely azonium tautomer in acidic medium.
Fig. 5-7 is here
Conclusion
Three series of monoazo dyes consisting primer amino group were synthesized from ethyl 2-
amino-4-(4'-substitutedphenyl) thiophenes and their UV-Vis absorption spectral properties
were investigated. It was observed that the dyes exist in one tautomeric form (azo-amino) in
all solvents used. The largest bathochromic shift was observed by the nitro substituted dyes in
the diazo component in DMSO.
16

Acknowledgement
We are grateful to Gazi University Research Fund for providing financial support for this
study.
References
[1] M.M.M. Raposo, A.M.C.F. Fonseca, M.C.R Castro, M. Belsley, M.F.S. Cordoso, L.M.
Carvalho, P.J. Coelho, Dyes Pigm. 91 (2011) 62-73 and references cited therein.
[2] A. Natansohn, P. Rochon, Chem. Rev. 102 (2002) 4139-4175.
[3] S. Xie, A. Natansohn, P. Rochon, Chem. Mater. 5 (1993) 403-411.
[4] A. Ying-Guey Fuh, Y.D. Chen, C.K. Liu, K.T. Cheng, Dyes Pigm. 92 (2012) 949-953.
[5] X. Tong, Y. Zhao, Chem. Mater. 21 (2009) 4047–4054.
[6] H.S. Freeman, A.T. Peters, Modern colorants: synthesis and structure, Blackie Academic
& Professional, London, 1995, pp. 40-61, 63-71.
[7] H. Zollinger, Color Chemistry, Wiley VCH, Switzerland, 2003, pp. 127, 166-220.
[8] K. Hunger, Industrial Dyes, Wiley-VCH, Verlag, 2002, pp. 1, 14, 34.
[9] R. Hamprecht, A.Westerkamp, Dispers Dyes, Wiley-VCH, Verlag, Weinheim, 2005, pp.
1-4, 135, 139.
[10] A.D. Towns, Developments in azo disperse dyes derived from heterocyclic diazo
components, Dyes Pigm. 42 (1999) 3-28.
[11] M.A. Weaver, L. Shuttleworth, Heterocyclic diazo components, Dyes Pigm. 3 (1982) 81-
121.
[12] P.F. Gordon, P. Gregory, Organic Chemistry in Colour, Springer-Verlag, New York,
1983, pp. 95, 108-111.
[13] R.M. Christie, Colour Chemistry, The Royal Society of Chemistry, RSC Paperbacks,
2001, pp. 1-11.
[14] G. Hallas, A.D. Towns, Dyes Pigm. 33 (1997) 215-28.
[15] G. Hallas, A.D. Towns, Dyes Pigm. 35 (1997) 45-55.
[16] G. Hallas, A.D. Towns, Dyes Pigm. 32, (1996) 135-149.
[17] G. Hallas, A.D. Towns, Dyes Pigm. 33 (1997) 205-213.
[18] G. Hallas, A.D. Towns, Dyes Pigm. 33 (1997) 319-336.
17

[19] G. Hallas, A.D. Towns, Dyes Pigm. 34 (1997) 133-146.
[20] J. Choi, S. Hong, A.D. Towns, J. Soc. Dyers Col. 115 (1999) 32-37.
[21] M.G. Hutchings, A. Mills, Dyes Pigm. 47 (2000) 23-31.
[22] M.S.Yen, I.J.Wang, Dyes Pigm. 62 (2004) 173-180.
[23] M.S. Yen, I.J.Wang, Dyes Pigm. 67 (2005) 183-188.
[24] A.S. Abd-El-Aziz, A.H. Tarek, Dyes Pigm. 70 (2006) 8-17.
[25] M.S. Yen, I.J.Wang, Dyes Pigm 61(2004) 243-250.
[26] N. Ertan, F. Eyduran, Dyes Pigm. 27 (1995) 313-320.
[27] F. Karcı, F. Karcı, Dyes Pigm. 76 (2008) 147-157.
[28] A. Saylam , Z. Seferoğlu, N. Ertan, Dyes Pigm. 76 (2008) 470-476.
[29] A. Saylam, Z. Seferoğlu, N. Ertan, Russ. J. Org. Chem. 44 (2008) 587-594.
[30] Z. Seferoğlu, N. Ertan, Russ. J. Org. Chem. 43 (2007) 1035-1041.
[31] Z. Seferoğlu, N. Ertan, Phosphorous Sulfur Silicon Relat. Elem. 183 (2008) 1236-1251.
[32] G. Hallas, J. Choi, Dyes Pigm. 40 (1999) 119-129.
[33] G, Hallas, J. Choi, Dyes Pigm. 42 (1999) 249-265.
[34] H. Hartmann, I. Zug, J. Chem. Soc. Perkin Trans. 1 (2000) 4316-4320.
[35] K. Gewald, M. Hentschel, J. Morgenstern, Patent 102 397 , 1973.
[36] K. Gewald, M. Gruner, U. Hain, G. Sueptitz, Monatsh. Chem. 119 (1988) 985-92.
[37] H.M. Gaber, M.C. Bagley, Eur. J. Chem. 1(2010) 115-123.
[38] H.M. Gaber, M.C. Bagley, Eur. J. Chem. 2 (2011) 214-222.
[39] R.H. Peters, Textile chemistry, vol 3. Reinhold, 1979.
[40] T.H. Afifi, A.Z. Sayed, J. Soc. Dyers Col. 113 (1997) 256–258.
[41] A.C. Cope, C.M. Hofmann, C. Wyckoff, E. Hardenbergh, J. Am. Chem. Soc. 63 (1941)
3452-3456.
[42] K. Gewald, Chem. Het. Compd. 12 (1976) 1077-1090.
18

[43] K. Gewald, E. Schinke, H. Böttcher, Chem. Ber. 99 (1966) 94-100.
[44] a- C.N. Hodge, W.P. Janzen, K.P. Williams, L.A. Cheatham, Patent WO 2005/033102-
A2 , 2005. b- D.A. Antonetti, P. Titchenell Patent WO 2012/149452-A1, 2012.
[45] M.K. Kathiravan, C.J. Shishoo, T.S. Chitre, K.R. Mahadik, K.S. Jain, Synth. Commun.
37 (2007) 4273-4279.
[46] H. Zollinger, Diazo Chemistry I, VCH, Weinheim, New York, Tokyo, Basel, Cambridge
1994, pp. 11-14, 51.
[47] A. Lycka, Dyes Pigm. 43 (1999) 27-32.
[48] A.S. Shawali, N.M. Harb, K.O. J. Heterocycl. Chem. 22 (1985) 1397-1403.
[49] T.H. Lowry, K.S. Richardson, Mechanism and theory in organic chemistry, Harper &
Row Publishers, New York, Hagerstown, San Francisco, London, 1976, pp. 60-71.
[50] T. Stoyanova, S. Stoyanov, L. Antonov, V. Petrova, Dyes Pigm. 31 (1996)1-12.
[51] S. Stoyanov, L. Antonov, T. Stoyanova, V. Petrova, Dyes Pigm. 32 (1996)171-185.
[52] A.A.M. Prabhu, G. Venkatesh, R.K. Sankaranarayanan, S. Siva, N. Rajendiran, Indian J.
Chem. Sec. A 49A (2010) 407-417.
Figure captions
Fig. 1. Absorption spectra of dyes 6-10 in DMSO.
Fig. 2. Absorption spectra of dye 9 in various solvents.
Fig. 3. Absorption spectra of dye10 in basic solutions of methanol.
Fig. 4. Absorption spectra of dye 7 in acidic solutions of methanol.
Fig. 5. Relationship between Hammett substituent constants with ∆λmax values of the dyes 1-5
(∆λmax:The difference between the azonium and neutral forms of the dyes in methanol).
Fig. 6. Relationship between Hammett substituent constants with ∆λmax values of the dyes
6-10 (∆λmax: The difference between the azonium and neutral forms of the dyes in
methanol).
Fig. 7. Relationship between Hammett substituent constants with ∆λmax values of the dyes
11-15 (∆λmax: The difference between the azonium and neutral forms of the dyes in
methanol).
19

Scheme captions
Scheme 1. Synthesis of coupling components (II a-c) by Gewald reaction.
Scheme 2. Synthesis of dyes (1-15).
Scheme 3. Possible tautomeric forms of dyes in different solvents.
Scheme 4. Possible anionic forms in basic media.
Scheme 5. Possible cationic forms in acidic media.
Table captions
Table 1 Structures of dyes (1-15).
Table 2 Influence of solvent on λmax (nm) of dyes (1-15).
Table 3 Hammett substituent constants (σp) and absorption maxima of dyes (1-15) in
methanolic solution.
Table 4 Absorption maxima of dyes (1-5) with piperidine in DMSO.
20

Fig. 1. Absorption spectra of dyes 6-10 in DMSO.
21

Fig. 2. Absorption spectra of dye 9 in various solvents.
22

Fig. 3. Absorption spectra of dye10 in basic solutions of methanol.
23

Fig. 4. Absorption spectra of dye 7 in acidic solutions of methanol.
24

Fig. 5. Relationship between Hammett substituent constants with ∆λmax values of the dyes 1-5
(∆λmax:The difference between the azonium and neutral forms of the dyes in methanol).
25

Fig. 6. Relationship between Hammett substituent constants with ∆λmax values of the dyes
6-10 (∆λmax: The difference between the azonium and neutral forms of the dyes in
methanol).
26

Fig. 7. Relationship between Hammett substituent constants with ∆λmax values of the dyes
11-15 (∆λmax: The difference between the azonium and neutral forms of the dyes in
methanol).
27

X
X
O
O
NC
OCH2CH3
OCH2CH3
CH3COOH/CH3COO-, Benzene
H3C
O
H3C
CN
I a-c
I-a (X:-H), I-b (X:-NO2), I-c (X:-OCH3)
X
CH3CH2O
X
O
O
OCH2CH3
S8, Ethanol
Triethylamine
NH2
S
H3C
CN
II a-c
I a-c
II-a (X:-H), II-b (X:-NO2), II-c (X:-OCH3)
Scheme 1. Synthesis of coupling components (II a-c) by Gewald reaction.
CH3CH2O
X
NH2
O
(i) NaNO2, HCl, 0-5 °C
NH2
(ii) II a-c
S
N
N
R
X: -H, -NO2, -OCH3
R: -H, -CH3, -OCH3, -CN, -NO2
R
Scheme 2. Synthesis of dyes (1-15).
28

CH3CH2O
CH3CH2O
X
X
CH3CH2O
X
O
O
OH
N
H
H
H
N
N
H
H
S
S
S
N
N
N
N
N
N
Iminazo-enol
Imine-hydrazone
Amino-azo
R
R
R
X: -H, -NO2, -OCH3
R: -H, -CH3, -OCH3, -CN, -NO2
Scheme 3. Possible tautomeric forms of dyes in different solvents.
29