Hetarylazoindoles 2. Spectroscopic and structural investigation of new benzothiazolylazo indole dyes

Article abstract of DOI:10.1016/j.molstruc.2013.04.020

Synthesis of new benzothiazolylazo indole dyes were carried out by diazotization of 2-aminobenzothiazole, 6- and 5,6-disubstituted 2-aminobenzothiazoles using with nitrosyl sulfuric acid and coupling with various 2- and 1,2-disubstituted indole derivative

Full text of DOI:10.1016/j.molstruc.2013.04.020

Journal of Molecular Structure 1047 (2013) 22–30
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Hetarylazoindoles 2. Spectroscopic and structural investigation of new
benzothiazolylazo indole dyes
b
a
b
Zeynel Seferog˘lu a, , Filiz Betül Kaynak , Nermin Ertan , Süheyla Özbey
⇑
a Gazi University, Faculty of Science, Department of Chemistry, 06500 Teknikokullar, Ankara, Turkey
b Hacettepe University, Faculty of Engineering, Department of Physics Engineering, 06800 Beytepe, Ankara, Turkey
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
ꢀ
Benzothiazolylazo indole dyes were
synthesized.
The most stable tautomeric form of
the dyes was determined by using
model compounds.
ꢀ
The crystal structures of the dye
which can be shown potantially
tautomeric forms, were obtained
from different solvents.
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis of new benzothiazolylazo indole dyes were carried out by diazotization of 2-aminobenzothia-
zole, 6- and 5,6-disubstituted 2-aminobenzothiazoles using with nitrosyl sulfuric acid and coupling with
various 2- and 1,2-disubstituted indole derivatives. The structures of the dyes were characterized by
UV–vis, FT-IR, 1H NMR, mass spectroscopic techniques and elemental analysis. Their solvatochromic
properties in different solvents were investigated and their absorption spectra were found to be solvent
dependent. The azo-hydrazone tautomerism was discussed in relation to solvent and substituent effects.
In addition, acid–base effect on this equilibrium was also evaluated. To determine the most tautomeric
form of the prepared dyes in solid state, X-ray data for 2-Methyl-3-(6-methoxybenzothiazol-2-yldiaze-
nyl)-1H-indole were recorded in different solvents. The X-ray results showed that the dye was stable as
azo tautomer in the solid state.
Received 3 January 2013
Received in revised form 4 April 2013
Accepted 9 April 2013
Available online 29 April 2013
Keywords:
Hetarylazo
Benzothiazolylazo indole dyes
Solvent effects
Azo-hydrazone
Model compounds
Crystal structure
Published by Elsevier B.V.
1. Introduction
The majority of commercially important azo colorants contain a
single azo bridge. Thus monoazo dyes are the most important type
Although, azo dyes are able to split off genotoxic and carcino-
genic amines, the dyes and derivatives still constitue in the most
important chemical class of commercial organic colorants until
now. They account for around 50% of the dyes used in traditional
textile applications. Furthermore, they are also used in the fields
of non-linear optics (NLO), optical data storage, advanced applica-
tions in organic synthesis, biological–medicinal studies and in
analytical chemistry as acid–base, redox and metallochromic
indicator and sensor [1–8].
of azo dyes. Generally the dyes based on heterocyclic amines have
been developed and the resultant dyes have higher tinctorial
strength and give brighter color than those derived fromaniline-
based diazo components [1]. For instance, amino-substituted
thiazole, benzothiazole, isothiazole, thiadiazole and thiophene
compounds afford very electronegative diazo components and
consequently, provide
a
pronounced bathochromic effect
compared to the benzenoid compounds. In addition, heterocyclic
coupling components such as pyridone, pyrazolone, pyrimidine,
thiophene, quinoline and indole derivatives are also very impor-
tant for industrial and other advanced applications [1,9–11].
Therefore, the synthesis and investigation of spectroscopic and
⇑
Corresponding author. Tel.: +90 312 2021525; fax: +90 312 2122279.
E-mail address: znseferoglu@gazi.edu.tr (Z. Seferog˘lu).
0022-2860/$ - see front matter Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.molstruc.2013.04.020

Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
23
tautomeric properties of many monoazo dyes containing one or
two heterocyclic ring in molecule have been studied in the past
decades [12–15]. The dyes prepared from enol or enamine type
coupling components are known to exhibit azo-hydrazone tautom-
erism which is quite interesting from a theoretical and practical
viewpoint. Because the tautomers have different technical proper-
ties and dyeing performance [16,17].
for 2-aminobenzothiazole; all other dyes were prepared in a simi-
lar manner. Characterization data are shown in below.
2.1.1. 2-Methyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-1H-indole
(1a)
2-Aminobenzothiazole (0.30 g, 2.0 mmol) was dissolved in hot
glacial acetic acid/propionic acid mixture (2:1, 6.0 ml) and was
rapidly cooled in an ice–salt bath to ꢁ5 °C. The mixture was then
stirred for 2 h. at 0–5 °C. The liquor was then added in portions
during 30 min to a cold solution of nitrosyl sulfuric acid [prepared
from sodium nitrite (0.15 g) and concentrated sulfuric acid (3 ml at
50 °C)]. The mixture was stirred for an additional 2 h at 0 °C. Excess
nitrous acid was destroyed by the addition of urea. The resulting
diazonium salt was cooled in salt/ice mixture. 2-methlyindole
(0.26 g, 2.0 mmol) was dissolved in mixture of acetic acid/propi-
onic acid solution (3:1, 8.0 ml) and cooled in salt/ice bath and then
cold diazonium solution was added to this cooled solution by stir-
ring in a dropwise manner. The solution was stirred at 0–5 °C for
2 h. and pH of the reaction mixture was maintained at 4–6 by
simultaneous addition of saturated sodium carbonate solution.
The mixture was stirred for a further 1 h. at 5 °C. The resulting
product was filtered, washed with water, dried and crystallized
from ethanol mixture gave brown powder (yield: 0.29 g, 51%;
m.p: 168–170 °C), FT-IR (KBr) ˆmax: 3345 (indole ANH), 3059 (aro-
The quantitative evaluation of the azo-hydrazone tautomeric
equilibria can be done by UV–vis and NMR spectroscopy in solu-
tion. However, these methods have key limitations. In the case of
UV–vis spectroscopy, the equilibrium constant (KT) cannot gener-
ally be obtained directly because the molar extinction coefficient
of the individual tautomers cannot be determined, due to overlap-
ping peaks of the two forms [18,19]. In addition, the 1H NMR spec-
troscopy is not suitable for determination of azo-hydrazone
tautomeric equilibria. Because the equilibrium between azo and
hydrazone tautomers is rapidly established on the NMR time scale
[20,21]. However, 15N, 14N and 13C NMR chemical shift data can
readily be employed to study quantitatively the azo-hydrazone
tautomeric equilibrium [22–24]. Furthermore this method is not
employed for all azo dyes. Among the different methods, UV–vis
and NMR spectroscopy are still the best methods for qualitative
studies of tautomeric behavior in solutions. Also, the useful system
for determination of the azo-hydrazone tautomerism is to use
model compounds [25–28].
matic CAH), 2969, 2920 (aliphatic CAH), 1605 (C@C) cmꢁ1 1H
;
The main purpose of the present work is to determine the
most dominant tautomeric form of hetarylazo indole dyes by
using model compounds. For this purpose, we synthesized here
a new series of benzothiazolylazo indole dyes using 2-methyl
and 2-phenyl indoles as coupling components which may involve
azo-hydrazone tautomerism. For comparison of spectroscopic
properties and determination of dominant tautomeric structure
of the synthesized dyes from 2-methyl- and 2-phenylindoles,
we prepared also their model compounds which exist solely in
azo form.
NMR (DMSO-d6/CDCl3): d 9.71 (b, indole ANH), 7.98 (1H,m), 7.37
(1H,m), 7.30–7.15 (4H,m), 6.82 (2H,m), 2.36 (3H,s).
Anal. Calcd. For C16H12N4S: C, 65.73; H, 4.14; N, 19.16; S, 10.97
Found: C, 65.68; H, 4.19; N, 19.11; S, 10.92%.
MS (m/z, 70 eV): 292.0 (M+), 277.0, 158.0, 130.0.
2.1.2. 1,2-Dimethyl-3-(benzothiazol-2-yldiazenyl)-1H-indole (1b)
This dye was obtained from 2-aminobenzothiazole and 1,2-
dimethylindole as red crystals (yield: 0.48 g, 79%; m.p: 209–
211 °C), FT-IR (KBr) ˆmax: 3063 (aromatic CAH), 2917 (aliphatic
CAH), 1506 (C@C) cmꢁ1 1H NMR (DMSO-d6): d 8.38 (1H,m), 7.98
;
2. Experimental
(2H,m), 7.68 (1H,m), 7.49 (1H,m), 7.42 (3H,m), 3.98 (3H,s ANCH3),
2.82 (3H,s).
Anal. Calcd. For C17H14N4S: C, 66.64; H, 4.61; N, 18.29; S, 10.47
Found: C, 66.58; H, 4.66; N, 18.22; S, 10.48%.
MS (m/z, 100 eV): 307.1 (M + 1)+, 172.1, 144.1.
The chemical used in the synthesis of all dyes were obtained
from Aldrich Chemical Company and were used without further
purification. The spectroscopic grade solvents were used.
IR spectra were recorded on a Mattson 1000 FT-IR spectro-
photometer in KBr. 1H NMR spectra were recorded on a Bru-
ker-Spectrospin Avance DPX 400 MHz Ultra-Shield in DMSO-d6
and CDCl3 with TMS as internal reference. Chemical shifts are
expressed in d units (ppm). Ultraviolet–visible (UV–vis) absorp-
tion spectra were recorded on analytikjena specord 200 spectro-
photometer at the wavelength of maximum absorption (kmax) in
range of solvents, i.e. Dimethylsulfoxide (DMSO), dimethylform-
amide (DMF), acetonitrile, methanol, acetic acid and chloroform.
Change of kmax was investigated when 0.1 ml piperidine was
added to 1 ml dye solutions in chloroform, DMSO and DMF, sim-
ilarly, when 0.1 ml base (potassium hydroxide, 0.1 M) or 0.1 ml
acid (hydrochloric acid, 0.1 M) was added to 1 ml dye solutions
in methanol. Mass spectra were recorded on Agilent 5973 Net-
work Mass Selective Detector, SIS (Direct Insertion Probe), elec-
tron impact 70 or 100 eV and on AGILENT 1100 MSD by
Turkish Research Council Laboratories (Center of Science and
Technology Research of Turkey). All melting points were uncor-
rected and in °C.
2.1.3. 2-Phenyl-3-(benzothiazol-2-yldiazenyl)-1H-indole (1c)
This dye was obtained from 2-aminobenzothiazole and 2-phen-
ylindole as red crystals (yield: 0.49 g, 69%; m.p: 233–235 °C), FT-IR
(KBr) ˆmax: 3414 (indole ANH), 3161, 3063 (aromatic CAH) 1406
(C@C) cmꢁ1 1H NMR (DMSO-d6/CDCl3): d 12.51 (b, indole ANH),
;
8.64 (1H,m), 8.30 (1H,m), 8.20 (2H,m), 8.0 (1H,m), 7.86 (1H,m),
7.63–7.28 (7H,m).
Anal. Calcd. For C21H14N4S: C, 71.16; H, 3.98; N, 15.81; S, 9.05
Found: C, 71.09; H, 4.01; N, 15.90; S, 9.11%.
MS (m/z, 70 eV): 354.0 (M+), 222.0, 193.0.
2.1.4. 1-Methyl-2-phenyl-3-(benzothiazol-2-yldiazenyl)-1H-indole
(1d)
This dye was obtained from 2-aminobenzothiazole and 1-
methyl-2-phenylindole as brown powder (yield: 0.57 g, 77%;
m.p: 244–245 °C), FT-IR (KBr) ˆmax: 3056, 3031 (aromatic CAH),
2940 (aliphatic CAH), 1471 (C@C) cmꢁ1 1H NMR (CDCl3): d 8.34
;
(1H,m), 7.88 (1H,m), 7.63 (1H,m), 7.65 (3H,m), 7.55–7.40 (5H,m),
7.28 (1H,m), 3.89 (3H,s ANCH3).
2.1. Preparation of hetarylazoindole dyes
Anal. Calcd. For C22H16N4S: C, 71.71; H, 4.38; N, 15.21; S, 8.70
Found: C, 71.71; H, 4.36; N, 15.13; S, 8.66%.
Diazotization of various heterocyclic amines was done by nitro-
syl sulfuric acid. A typical procedure is that described below used
MS (m/z, 70 eV): 368.0 (M+), 222.0, 193.0.

24
Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
2.1.5. 2-Methyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-1H-indole
(2a)
2.1.10. 1,2-Dimethyl-3-(5,6-dimethylbenzothiazol-2-yldiazenyl)-1H-
indole (3b)
This dye was obtained from 2-amino-6-methoxybenzothiazole
and 2-methylindole as red crystals (yield: 0.57 g, 88%; m.p: 266–
267 °C), FT-IR (KBr) ˆmax: 3197 (indole ANH), 3070, 3005 (aromatic
This dye was obtained from 2-amino-5,6-dimethylbenzothiaz-
ole and 1,2-dimethylindole as red crystals (yield: 0.52 g, 78%;
m.p: 253–254 °C), FT-IR (KBr) ˆmax: 3011 (aromatic CAH), 2917
CAH), 2966, 2921 (aliphatic CAH), 1599 (C@C), 1234 (CAO) cmꢁ1
;
(aliphatic CAH), 1523 (C@C) cmꢁ1 1H NMR (DMSO-d6): d 8.36
;
1H NMR (DMSO-d6/CDCl3): d 11.69 (b, indole ANH), 8.04 (1H,m),
7.46 (1H,d), 6.96 (1H,m), 6.90 (1H,m), 6.82 (2H,m), 6.61 (1H,m),
3.48 (3H,s AOCH3), 2.41 (3H,s).
(1H,m), 7.76 (1H,s), 7.74 (1H,s), 7.68 (1H,m), 7.38 (2H,m), 3.88
(3H,s ANCH3), 2.83 (3H,s), 2.33 (3H,s), 2.31 (3H,s).
Anal. Calcd. For C19H18N4S: C, 68.23; H, 5.42; N, 16.75; S, 9.59
Found: C, 68.36; H, 5.39; N, 16.82; S, 9.48%.
Anal. Calcd. For C17H14N4OS: C, 63.33; H, 4.38; N, 17.38; S, 9.95
Found: C, 63.39; H, 4.27; N, 17.22; S, 10.01%.
MS (m/z, 100 eV): 335.1 (M + 1)+, 172.1, 144.1.
MS (m/z, 70 eV): 321.1 (M ꢁ 1)+, 158.1, 130.0.
2.1.11. 2-Phenyl-3-(5,6-dimethylbenzothiazol-2-yldiazenyl)-1H-
indole (3c)
2.1.6. 1,2-Dimethyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-1H-
indole (2b)
This dye was obtained from 2-amino-5,6-dimethylbenzothiaz-
ole and 2-phenylindole as red crystals (yield: 0.54 g, 71%; m.p:
This dye was obtained from 2-amino-6-methoxybenzothiazole
and 1,2-dimethylindole as red powder (yield: 0.54 g, 80%; m.p:
259–260 °C), FT-IR (KBr) ˆmax: 3059 (aromatic CAH), 2935 (ali-
169 °C decomposition), FT-IR (KBr) ˆmax
ANH), 3011 (aromatic CAH), 2917 (aliphatic CAH), 1523 (C@C)
cmꢁ1 1H NMR (DMSO-d6): d 13.44 (b, indole ANH), 8.48 (1H,m),
8.27 (2H,m), 8,17 (1H,m), 7,79 (1H,s), 7,76 (1H,s), 7.68–7.32
(5H,m), 2.34 (3H,s), 2.31 (3H,s).
: 3426–3215 (indole
;
phatic CAH), 1598 (C@C), 1218 (CAO) cmꢁ1
;
1H NMR (DMSO-d6):
d 8.36 (1H,m), 7.87 (1H,d), 7.66 (1H,m), 7.58 (1H,m), 7.38 (2H,m),
7.04 (1H,m), 3.89 (s, 3H, ANCH3), 3.87 (s, 3H, AOCH3), 2.80 (s, 3H).
Anal. Calcd. For C18H16N4OS: C, 64.26; H, 4.79; N, 16.65; S, 9.53
Found: C, 64.20; H, 4.67; N, 16.69; S, 9.48%.
Anal. Calcd. For C23H18N4S: C, 72.22; H, 4.74; N, 14.65; S, 8.38
Found: C, 72.31; H, 4.76; N, 14.70; S, 8.36%.
MS (m/z, 150 eV): 383.1 (M + 1)+, 220.1, 192.1.
MS (m/z, 100 eV): 337.1 (M + 1)+, 172.1, 144.1.
2.1.12. 1-Methyl-2-phenyl-3-(5,6-dimethylbenzothiazol-2-
yldiazenyl)-1H-indole (3d)
2.1.7. 2-Phenyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-1H-indole
(2c)
This dye was obtained from 2-amino-5,6-dimethylbenzothiaz-
ole and 1-methyl-2-phenylindole as red crystals (yield: 0.61 g,
77%; m.p: 270–271 °C), FT-IR (KBr) ˆmax: 3056, 3018 (aromatic
This dye was obtained from 2-amino-6-methoxybenzothiazole
and 2-phenylindole as red crystals (yield: 0.77 g, 76%; m.p:
240 °C decomposition), FT-IR (KBr) ˆmax: 3414 (indole ANH),
3049 (aromatic CAH), 2967 (aliphatic CAH), 1600 (C@C), 1218
CAH), 2966, 2915 (aliphatic CAH), 1496 (C@C) cmꢁ1 1H NMR
;
(DMSO-d6): d 8.48 (1H,m), 7.79 (4H,m), 7.66 (4H,m), 7.50 (2H,m),
3.91 (3H,s ANCH3), 2.32 (3H,s), 2.30 (3H,s).
(CAO) cmꢁ1
;
1H NMR (CDCl3): d 9.21 (b, indole ANH), 8.72
Anal. Calcd. For C24H20N4S: C, 72.70; H, 5.08; N, 14.13; S, 8.09
Found: C, 72.54; H, 5.06; N, 14.01; S, 8.08%.
(1H,m), 8.18 (2H,m), 8.02 (1H,m), 7.79–7.28 (8H,m), 7.04 (1H,m),
3.88 (3H,s AOCH3).
MS (m/z, 150 eV): 397.2 (M + 1)+, 234.1, 206.1.
Anal. Calcd. For C22H16N4OS: C, 68.73; H, 4.19; N, 14.57; S, 8.34
Found: C, 68.90; H, 4.16; N, 14.50; S, 8.33%.
MS (m/z, 70 eV): 384.0 (M+), 222.0, 193.0.
2.1.13. 1,2-Dimethyl-3-(6-nitrobenzothiazol-2-yldiazenyl)-1H-indole
(4a)
This dye was obtained from 2-amino-6-nitrobenzothiazole and
1,2-dimethylindole as red powder (yield: 0.46 g, 66%; m.p: 255 °C
decomposition), FT-IR (KBr) ˆmax: 3059 (aromatic CAH), 2919 (ali-
2.1.8. 1-Methyl-2-phenyl-3-(6-methoxybenzothiazol-2-yldiazenyl)-
1H-indole (2d)
phatic CAH), 1511 (C@C) cmꢁ1 1H NMR (DMSO-d6): d 9.0 (1H,s),
;
This dye was obtained from 2-amino-6-methoxybenzothiazole
and 1-methyl-2-phenylindole as red crystals (yield: 0.55 g, 69%;
m.p: 230–232 °C), FT-IR (KBr) ˆmax: 3051 (aromatic CAH), 2941
8,38 (1H,m), 8,30 (1H,m), 8,08 (1H,m), 7,71 (1H,m), 6,94 (2H,m),
3.90 (3H,s ANCH3), 2.83 (3H,s).
Anal. Calcd. For C17H13N5O2S: C, 58.11; H, 3.73; N, 19.93; S, 9.13
Found: C, 58.0; H, 3.67; N, 20.02; S, 9.08%.
(aliphatic CAH), 1527 (C@C), 1227 (CAO) cmꢁ1 1H NMR (DMSO-
;
d6): d 8.44 (1H,m), 8.26 (1H,m), 8.19 (2H,m), 7.90 (1H,m), 7.79–
7.28 (8H,m), 7.04 (1H,m) 3.70 (3H,s ANCH3), 3.67 (3H,s AOCH3).
Anal. Calcd. For C24H18N4OS: C, 69.33; H, 4.55; N, 14.06; S, 8.05
Found: C, 69.46; H, 4.60; N, 14.11; S, 8.08%.
MS (m/z, 100 eV): 352.05 (M + 1)+, 172.0, 146.0.
2.1.14. 2-Phenyl-3-(6-nitrobenzothiazol-2-yldiazenyl)-1H-indole (4b)
This dye was obtained from 2-amino-6-nitrobenzothiazole and
2-phenylindole as red powder (yield: 0.49 g, 61%; m.p: 253 °C
decomposition), FT-IR (KBr) ˆmax: 3424 (indole ANH), 3059 (aro-
matic CAH), 1524 (C@C) cmꢁ1; 1H NMR (DMSO-d6): d 10.73 (b, in-
dole ANH), 7.73 (2H,m), 7.48 (2H,m), 7.35–7.22 (5H,m), 7.08
(1H,m), 6.97 (1H,m), 6.73 (1H,m).
2.10. MS (m/z, 70 eV): 398.0 (M+), 221.0, 206.0.
2.1.9. 2-Methyl-3-(5,6-dimethylbenzothiazol-2-yldiazenyl)-1H-indole
(3a)
This dye was obtained from 2-amino-5,6-dimethylbenzothiaz-
ole and 2-methylindole as red crystals (yield: 0.43 g, 68%; m.p:
287 °C decomposition), FT-IR (KBr) ˆmax: 3427 (indole ANH),
3063, 3024 (aromatic CAH), 2966, 2928 (aliphatic CAH), 1496
Anal. Calcd. For C21H13N5O2S: C, 63.15; H, 3.28; N, 17.53; S, 8.03
Found: C, 62.97; H, 3.22; N, 17.36; S, 8.08%.
MS (m/z, 150 eV): 220.1 (M ꢁ 177)+, 205 (M ꢁ 194)+, 195.1
(M ꢁ 204)+.
(C@C) cmꢁ1 1H NMR (DMSO-d6): d 12.70 (b, indole ANH), 8.31
;
(1H,m), 7.77 (1H,s), 7.74 (1H,s), 7.48 (1H,m), 7.31 (2H,m), 2.78
(3H,s), 2.37 (3H,s), 2.35 (3H,s).
2.1.15. 1-Methyl-2-phenyl-3-(6-nitrobenzothiazol-2-yldiazenyl)-1H-
indole (4c)
Anal. Calcd. For C18H16N4S: C, 67.47; H, 5.03; N, 17.49; S, 10.01
Found: C, 67.36; H, 4.99; N, 17.55; S, 10.11%.
This dye was obtained from 2-amino-6-nitrobenzothiazole and
1-methyl-2-phenylindole as red powder (yield: 0.55 g, 67%; m.p:
145 °C decomposition), FT-IR (KBr) ˆmax: 3052 (aromatic CAH),
MS (m/z, 150 eV): 321.1(M + 1)+, 158.1, 130.1.

Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
25
2942 (aliphatic CAH), 1466 (C@C) cmꢁ1; 1H NMR (DMSO-d6): d 9,0
(1H,s), 8.38 (1H,m), 8.30 (1H,m), 8.10 (1H,m), 7.82 (2H,m), 7,70
(3H,m), 7,53 (3H,m), 3.83 (3H,s ANCH3).
Table 2
Selected bond lengths (Å), bond angles (°) and main dihedral angles (°) of the
compounds 2a-ethanol and 2a-acetamide.
2a-ethanol
2a-acetamide
Anal. Calcd. For C22H15N5O2S: C, 63.91; H, 3.66; N, 16.94; S, 7.76
Found: C, 63.78; H, 3.60; N, 16.90; S, 7.88%.
C1AN1
C8AN1
C3AC8
C2AC3
C1AC2
C1AC9
C2AN2
N2AN3
C10AN3
C10AN4
C11AN4
C16AC11
C16AS1
C10AS1
C14AO1
C17AO1
1.334(4)
1.386(4)
1.403(4)
1.440(4)
1.398(4)
1.486(4)
1.348(4)
1.294(3)
1.393(4)
1.302(4)
1.391(4)
1.396(4)
1.739(3)
1.751(3)
1.365(4)
1.417(4)
1.381(12)
1.451(10)
1.418(16)
1.463(13)
–
128.5(3)
133.3(3)
116.8(3)
108.7(3)
121.1(2)
124.5(3)
118.2(3)
–
108.3(13)
–
1.0(5)
179.0(3)
ꢁ179.7(3)
ꢁ179.3(3)
1.334(4)
1.385(4)
1.402(4)
1.448(4)
1.411(4)
1.476(4)
1.354(4)
1.285(3)
1.379(4)
1.307(4)
1.390(4)
1.394(4)
1.733(3)
1.746(3)
1.370(4)
1.419(4)
1.209(4)
–
MS (m/z, 150 eV): 414.1 (M + 1)+, 221.1, 206.1.
2.2. X-ray analysis
The X-ray structure analysis was carried out for 2a-ethanol and
2a-acetamide. Intensity data were collected on Enraf–Nonius
CAD4 four circle diffractometer equipped with graphite monochro-
mator Mo Ka radiation [29]. Structures were solved by direct
methods using SHELXS-97 [30] and refined by full-matrix least
squares on F2 (SHELXL-97) [31]. All non-hydrogen atom parame-
ters were refined anisotropically. The hydrogen atoms were lo-
cated on perspective difference Fourier maps or placed at
calculated positions and refined in the riding model with Ueq(-
H) = 1.3Ueq(C), and fixed distances of CAH = 0.93 Å (aromatic),
CAH = 0.96 Å (methyl) and NAH = 0.86 Å (aromatic). The OH
hydrogen of ethanol solvent was placed from difference Fourier
map. The geometric calculations were performed using the pro-
grams PLATON [32] and PARST [33]. In compound 2a-ethanol,
the ethanol moiety was disordered, with occupancy factors of
0.563(7) for C18, C19 and of 0.437(7) for C180, C190 atoms. All crys-
tallographic data and experimental details are listed in Table 1. Se-
lected bond lengths, bond angles and main dihedral angles of the
compounds are given in Table 2 and ORTEP drawings [34] of the
structures with atomic numbering are shown in Figs. 1 and 2,
respectively.
C18AO2
C18AO20
C18AC19
C180AC190
C18AN5
1.501(5)
–
1.294(4)
128.3(3)
132.1(3)
114.1(2)
111.5(3)
123.8(2)
124.5(3)
118.0(3)
123.3(3)
124.0(3)
112.7(3)
0.0(5)
C2AC1AC9
N2AC2AC3
N3AN2AC2
N2AN3AC10
N3AC10AS1
O1AC14AC15
C14AO1AC17
O2AC18AN5
O2AC18AC19
N5AC18AC19
C3AC2AN2AN3
C2AN2AN3AC10
N4AC10AN3AN2
C13AC14AO1AC17
179.0(2)
179.1(3)
176.8(3)
Crystallographic data (with the exception of structure factors)
for the structures in this paper were deposited with the Cambridge
Crystallographic Data Centre as supplementary publication num-
bers CCDC 876358 and 876359 for compounds 2a-ethanol and
2a-acetamide, respectively. Copies of the data can be obtained, free
of charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).
Table 1
Crystal data and details of the structure determination of the compounds 2a-ethanol
and 2a-acetamide.
3. Results and discussion
2a-ethanol
17H14N4OS, C2H6O
368.45
Monoclinic
P21/n
2a-acetamide
3.1. Single crystal X-ray structure determinations
Formula
C
C17H14N4OS, C2H5NO
381.45
Formula weight
Crystal system
Space group
Cell constants
a (Å)
To determine the most tautomeric form of the dyes in solid
state single crystal X-ray structure determinations have been car-
ried out on the complexes of ethanol and acetamide of 2a. Suitable
single crystals of 2a were obtained by slow evaporation from eth-
anol and acetonitrile in several days. The structures predicted from
chemical and spectral analysis are confirmed by X-ray diffraction.
The introduction of the solvent molecules into the crystal structure
did not affect the geometry of the main molecules although the
two compounds are crystallized in different crystal systems. The
interatomic distances and angles are very similar in both com-
pounds. The indole moieties of the compounds are nearly planar
and the dihedral angles between the best planes of the pyrrole
and phenyl rings are 0.8(1)° and 1.67(11)° for compounds 2a-eth-
anol and 2a-acetamide, respectively. Planarity of the indole ring
system is usually observed and these values are comparable with
those of N0[(1E)-1-methylpropylidene]-5-methyl-3-phenyl-1H-in-
dole-2-carbohydrazide [1.89(6)°] and N’-cyclopentylidene- 5-
methyl-3-phenyl-1H-indole-2-arbohydrazide [0.81(9)°] [35] and
2-phenyl-3-(5-methyl-isoxazol-3-yldiazenyl)-1H-indole [0.4(4)°]
Triclinic
ꢀ
P1
11.3482(10)
9.6047(10)
17.614(3)
90
90.353(12)
90
1919.8(4)
4; 1.275
0.189
9.081(3)
b (Å)
c (Å)
10.072(2)
10.931(4)
91.51(2)
105.83(3)
102.31(2)
936.0(5)
2; 1.353
0.198
0.30 ꢂ 0.24 ꢂ 0.18
a
(°)
b (°)
c
(°)
V (Å3)
Z; Dcalc (g cmꢁ3
)
l
(Mo K
a
) (mmꢁ1
)
Crystal size (mm)
Radiation
Temp (K)
h limits (°)
Index ranges
0.42 ꢂ 0.18 ꢂ 0.12
Mo Ka (k = 0.71073 Å) Mo Ka (k = 0.71073 Å)
293(2)
293(2)
2.42–25.65
ꢁ13 6 h P 13;
ꢁ11 6 k P 11;
0 6 l P 21
7230
2.39–26.29
ꢁ11 6 h P 0;
ꢁ12 6 k P 12;
ꢁ13 6 l P 13
4045
3795 [I > 2r(I)]
245
0.0631/0.155
0.0
1.048
Reflections collected
Reflns. used in refinement 3627 [I > 2
r(I)]
No. of refined parameters
R/Rw values
Rint
GOF
Final shift
246
0.0506/0.1275
0.0961
1.027
0.0
and
1-methyl-2-phenyl-3-(pyridin-3-yldiazenyl)-1H-indole
[4.5(4)°] [36].
0.0
The 6-methoxybenzothiazole heterocyclic system is also nearly
planar for both compounds. The dihedral angles between the best
(D
q
)min, (
D
q)
(e Åꢁ3
)
ꢁ0.236, 0.286
ꢁ0.829, 0.566
max

26
Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
C18
N5
O2
O2
C18
C19
C5
C12
C19
N3
C6
N4
C5
C4
C3
C11
C13
C14
C12
C4
C6
C7
N4
C11
C16
C7
C13
C14
C10
C2
O1
N3
C3
C2
C8
C16
N2
C15
S1
C10
C8
O1
C1
C9
N1
C17
N2
C15
S1
N1
C1
C17
C9
Fig. 1. ORTEP-3 view of the compound 2a-ethanol showing the atom-numbering
scheme. Displacement ellipsoids are drawn at the 50% probability level (top). Part of
the crystal structure of compound showing the hydrogen bonds (bottom).
planes of thiazole and the phenyl rings are 0.91(10)° and 2.06(9)°
for compounds 2a-ethanol and 2a-acetamide, respectively. The
indole and the 6-methoxybenzothiazole heterocyclic systems are
coplanar. The planarity may be attributed to the presence of an
intramolecular H-bond between N3 and C4 atom in both
compounds. The maximum deviation of C9 atom from the indole
heterocyclic system is 0.077(4)° for 2a-ethanol (0.062(3)° for 2a-
acetamide). N2 atom deviates from the indole heterocyle as
ꢁ0.083(3)° and N3 atom deviates from the 6-methoxybenzothiaz-
ole heterocyclic system as 1.151(3)° for 2a-acetamide.
The bond lengths N2@N3 are 1.294(3) Å and 1.285(3) Å for two
complexes of 2a indicates that the structures crystallized in azo
form. In compound 2a-acetamide, the amide carbonyl bond length
CO (C18AO2) is normal (1.209 Å). The CAN and C@O bonds of the
acetamide in the structure of 2a-acetamide, are similar to that
observed in [37,38]. In 2a-acetamide, it is observed that there is
a unique contact of O2ꢃ ꢃ ꢃC9 (bond length = 3.570 Å) between the
solvent and the main molecule. The orientation of the amide group
is nearly parallel to the main skeleton and the dihedral angle be-
tween the main skeleton and the acetamide is 11.01(11)°.
Fig. 2. ORTEP-3 view of the compound 2a-acetamide, showing the atom-number-
ing scheme. Displacement ellipsoids are drawn at the 50% probability level (top).
Part of the crystal structure of compound showing the hydrogen bonds (bottom).
O2AH2ꢃ ꢃ ꢃN4] lead to an infinite one-dimensional chain along the
base vector [10 ꢁ1]. These chains are linked by means of CAHꢃ ꢃ ꢃ
p
bond and
pꢃ ꢃ ꢃ
p
interactions into layers parallel to the (301) plane.
Furthermore, there are face-to-face stacking
p p
ꢃ ꢃ ꢃ
interactions be-
tween the indole rings. In, 2a-acetamide, the N5 and O2 atoms of
the solvent are involved in two hydrogen bonds [N1AH1ꢃ ꢃ ꢃO2 and
N5AH5ꢃ ꢃ ꢃN4] which form infinite one dimensional chains along
the [110] base vector. Also, there are three CAHꢃ ꢃ ꢃ
p interactions
between the layers as given in Table 4.
3.2. Solvent effect on absorption spectra of the dyes
To investigate of solvatochromic behavior of dyes, the absorp-
tion spectra of benzothiazolylazoindole dyes (1a,c, 2a,c, 3a,c and
4c in Scheme 1) were measured in various solvents at a concentra-
tion of approximately 10ꢁ6–10ꢁ7 M and were compared with those
of corresponding model compounds (1b,d, 2b,d, 3b,d, 4b,d) which
contain 1,2-disubstituted indole ring. These compounds can be
used as reference for the structures which can be existed azo-
hydrazone tautomers. The results are given in Table 5. It was found
that the absorption maxima of benzothiazolylazoindole dyes
showed bathochromic shifts as the polarity of solvent was in-
creased except in acetic acid.
Hydrogen bonding may play a significant role in stabilizing
crystal structure (Table 3). The packing arrangements of two com-
pounds are governed by NAHꢃ ꢃ ꢃO/N and OAHꢃ ꢃ ꢃN interactions be-
tween the cocrystal solvent molecules and the main molecules. In
the compounds, the solvent molecules act as donor and acceptor of
two hydrogen bonds. In 2a-ethanol, these two hydrogen bonds
[one is between the ethanol and the indole moiety, N1AH1ꢃ ꢃ ꢃO2
and the other is between the ethanol and the thiazole part,

Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
27
Table 3
Hydrogen bonds.
DAHꢃ ꢃ ꢃA
Sym. operation
DAH (Å)
Hꢃ ꢃ ꢃA (Å)
Dꢃ ꢃ ꢃA (Å)
DAHꢃ ꢃ ꢃA (°)
2a ethanol
N1AH1ꢃ ꢃ ꢃO2
O2AH20ꢃ ꢃ ꢃN4
C4AH4ꢃ ꢃ ꢃN3
1/2 ꢁ x, ꢁ1/2 + y, 1/2 ꢁ z
1 ꢁ x, 1 ꢁ y, ꢁ z
0.86
0.80(4)
0.93
1.90
1.99(4)
2.76
2.754(5)
2.784(4)
3.158(4)
177
169(5)
111
2a acetamide
N1AH1ꢃ ꢃ ꢃO2
N5AH5Aꢃ ꢃ ꢃN4
C4AH4ꢃ ꢃ ꢃN3
1 ꢁ x, ꢁy, 1 ꢁ z
0.86
0.86
0.93
2.01
1.87
2.62
2.866(4)
2.724(3)
3.070(5)
172
176
110
2 ꢁ x, 1 ꢁ y, 1 ꢁ z
Table 4
CAHꢃ ꢃ ꢃ
p and pꢃ ꢃ ꢃp interactions.
CAHꢃ ꢃ ꢃCga
Sym. operation
Hꢃ ꢃ ꢃCg (Å)
c
b(°)
Cꢃ ꢃ ꢃCg (Å)
CAHꢃ ꢃ ꢃCg (°)
2a ethanol
C9AH9Bꢃ ꢃ ꢃCg(C3 ? C8)
1 ꢁ x, ꢁy, 1 ꢁ z
x, y, z
2.72
2.90
5.5
7.5
3.549(4)
3.563(5)
144
127
C19AH19Bꢃ ꢃ ꢃCg(S1 ? C16)
2a acetamide
C6AH6ꢃ ꢃ ꢃCg(N1 ? C8)
3/2 ꢁ x, 1/2 + y, 1/2 ꢁ z
2.93
2.85
2.91
9.3
7.1
6.5
ac (°)
3.450(5)
3.645(5)
3.725(15)
bd (°)
116
140
144
C17AH17Cꢃ ꢃ ꢃCg(C3 ? C8)
1 ꢁ x, ꢁy, ꢁz
1 + x, 1 + y, z
C190AH19Dꢃ ꢃ ꢃCg(C11 ? C16)
pꢃ ꢃ ꢃ
p
contacts
Cgꢃ ꢃ ꢃCg (Å)
Cgꢃ ꢃ ꢃplaneCg(2) (Å)
2a ethanol
Cg(N1 ? C8)ꢃ ꢃ ꢃCg(N1 ? C8)
1 ꢁ x, ꢁy, 1 ꢁ z
3.515(2)
0.0
13.46
3.419
a
b
c
Cg = center of gravity of the aromatic ring.
c
= angle defined by a line connecting center of gravity of the aromatic ring with Y atom and the normal to the aromatic ring.
a
= angle between the planes of two aromatic rings.
d
b = angle between Cgꢃ ꢃ ꢃCg line and normal to the plane of the first aromatic ring.
R3
R4
S
N
N
N
N
R1
R2
1a.R1= H, R2= CH3, R3, R4=H
b.R1= CH3, R2= CH3, R3, R4=H
c.R1= H, R2= Ph, R3, R4=H
d.R1= CH3, R2= Ph, R3, R4=H
3a.R1=H, R2= CH3, R3, R4= CH3
b.R1= CH3, R2= CH3, R3, R4= CH3
c.R1= H, R2= Ph, R3, R4= CH3
d.R1= CH3, R2= Ph, R3, R4= CH3
2a.R1= H, R2= CH3, R3= OCH3, R4= H
b.R1= CH3, R2= CH3, R3= OCH3, R4= H
c.R1= H, R2= Ph, R3= OCH3, R4= H d.R1=
CH3, R2=Ph, R3= OCH3, R4= H
4b.R1= CH3, R2= CH3, R3= NO2, R4= H
c.R1= H, R2=Ph, R3= NO2, R4= H
d.R1= CH3, R2= Ph, R3= NO2, R4= H
Scheme 1. Structures of dyes.
The absorption curves of dyes 2a and 2b in various solvents are
The synthesized benzothiazolylazo indole dyes theoretically
may be involved in azo-hydrazone tautomers (Scheme 2). Thus,
the role of the azo-hydrazone tautomerism in relation to the visible
absorption spectra and solvatochromic behavior of the dyes have
been investigated. The visible absorption spectra of the prepared
dyes showed one absorption maximum in all solvents used, with
the exception of the dye 4c in DMSO and DMF. It is known that
two maxima are observed for compounds exhibiting azo-hydra-
zone tautomeric equilibria or anion [16,17,25]. In addition, when
a comparison between kmax values of the dyes (1a,c, 2a,c, 3a,c,
4c) and their corresponding model compounds (1b,d, 2b,d, 3b,d,
4b,d), the dyes are in favor of the predominantly azo form except
3c and 4c in all solvents used. The small hypsochromic and batho-
chromic shifts in kmax of the dyes in these solvents are due to sol-
ute–solvent interactions. The electronic absorption spectra of the
dye 4c in all solvent used solutions differ significantly from its
shown in Fig 3. Dye 2a showed absorption maxima at 467 nm in
DMSO, 466 nm in DMF, 455 nm in acetonitrile, 464 nm in metha-
nol, 474 nm in acetic acid, and 459 nm in chloroform. The same
trend was observed for dye 2b [e.g. 473 nm in DMSO, 469 nm in
DMF, 463 nm in acetonitrile, 467 nm in methanol, 476 nm in acetic
acid, and 465 nm in chloroform]. As can be seen in Table 5, similar
results were observed in
Dkmax values of the other benzothiazoly-
lazoindole dyes when compared with their model compounds (ex-
cept 3c and 4b in DMSO and DMF). In proton-donating solvent such
as acetic acid, the kmax values of the dyes prepared (1a,b, 1d, 2a-c,
3b, 3d) showed larger bathochromic shifts than in higher polar sol-
vents, e.g. DMSO and DMF, as shown in Table 5. This is obviously
due to the formation of hydrogen bonds between dye and acetic
acid. This case may be stabilize the excited state of the dyes leading
to a resultant bathochromic shift.

28
Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
Table 5
Absorption maxima of dyes (1a-d, 2a-d, 3a-d, 4b-d) in solution media.
Dye
no
kmax (nm)
DMSO
DMSO + piperidine DMF
DMF + piperidine Acetonitrile Methanol Methanol + KOH Methanol + HCl Acetic
acid
Chloroform Chloroform
+ piperidine
1a
1b
1c
1d
2a
2b
2c
2d
3a
3b
3c
3d
4b
4c
458
465
485
476
467
473
492
485
487
468
524
479
494
476,
557s
502
490
464
518
477
496
474
522
486
494
466
524
479
496
561
465
460
489
466
466
469
496
478
462
463
521
475
490
561,
463
498
488
462
515
465
495
470
523
480
495
462
520
477
490
566
443
453
467
461
455
463
478
474
451
460
498
468
482
455
452
460
477
469
464
467
480
477
460
465
483
474
481
460
482
459
507
504
507
533
527
517
519
543
536
513
517
541
531
506
467
465
471
479
481
472
476
493
485
473
477
490
483
487
454
448
454
464
471
459
465
482
475
452
461
477
470
485
459
464
454
477
470
484
466
489
477
459
462
484
471
485
471
463
488
467
514
477
483
464
509
475
483
544,438s
4d
501
499
490
489
488
499
495
493
492
Abbreviation: s,shoulder.
model compounds (4d). The spectra of the dye 4c in DMSO and
DMF showed one absorption maximum with a shoulder at longer
wavelengths and two absorption maxima respectively. Whereas
it showed one absorption maximum in the other solvents used (Ta-
ble 5) (Fig. 4). The position of shoulder is practically closer to their
longer absorption maximum of this dye in DMF which may be indi-
cated in the azo tautomer or anion form. So, the absorption at
shorter wavelengths of this dye in DMSO and DMF may be assigned
to hydrazone form. Because absorption maximum of its model
compound (4d) has different value in DMSO and DMF. These re-
sults suggest that dye 4c may be present in more than one tauto-
meric form (hydrazone) in acetonitrile, methanol, acetic acid and
chloroform.
The effects of the acid and base on the absorption maxima of the
dye solutions were investigated and the results are shown in
Table 5. The absorption maxima of the benzothiazolylazoindole
dyes (except model compounds) in methanol are quite sensitive
to the addition of base (KOH) were observed bathochromic shifts
and one dominant absorption maximum (except dye 4c) (Table 5).
It may be acid–base equilibrium between azo or hydrazone
tautomers. These bathochromic shifts is the largest when inroduc-
tion of strong electron withdrawing group (NO2). For instance, the
dye 4c showed one absorption maximum in methanol. When KOH
was added in methanolic solution of the dye, one absorption max-
imum with a shoulder is observed at shorter wavelength.
When piperidine was added to DMSO and DMF solutions, kmax
values of the dyes (1a, 1c, 2a, 2c, 3a, 4c) showed large bathochro-
mic shifts with the exception dye 3a in DMSO, dye 4c in DMF
and 3c in DMSO and DMF. There was no significant change or
any change when piperidine was added to solutions of the dyes
(3a,c and 4c) in DMSO and DMF. This indicates that dyes 3a,c
and 4c exist common anion form or tautomer-anion mixture in
DMSO and DMF and their more basic solutions. These results are
in aggrement with those obtained for hetraylazoindoles and quin-
olines in our previous works [15–17,25–27]. In the case of the dye
4c, with the addition of piperidine to their DMSO and DMF solu-
tions, the shoulder were converted to maximum in DMSO and kmax
at shorter wavelength was not seen in DMF (Table 5). A typical
example is shown in Fig. 5. It may be an acid–base equilibrium be-
tween hydrazone tautomer and corresponding common anion
Fig. 3. Absorption spectra in various solvents (a) for dye 2a and (b) 2b.

Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
29
H
N
S
N
S
N
N
N
N
N
N
H
CH3
CH3
(Azo-enamine)
(Hydrazone-imine)
A
B
-
-
H
+ H
+ H
H
S
N
S
N
N
N
N
N
N
N
H
H
CH3
CH3
(Cation)
(Anion)
C
D
Scheme 2. Azo-hydrazone tautomeric forms and anionic/cationic form of hetarylazo indole dyes (1a and c, 2a and c, 3a and c, and 4c).
(Scheme 2). In addition, kmax values of the dyes (1a, 1c, 2a, 2c, 3a,
4c) showed generally small bathochromic shifts, when piperidine
was added to chloroform solutions with the exception of dye 2a.
On the other hand, kmax values of the all dyes in methanol
showed large bathochromic shifts when 0.1 M HCl was added with
the exception of nitro substituted benzothiazolylazo dyes (4c and
4d) due to strong electron withdrawing properties of NO2.
3.3. Substituent effect on absorption spectra of the dyes in various
solvents
The effect of substitution, in the benzothiazole ring of the diazo
component and in the side chain of the 2-position of coupling
component on the visible absorption spectra was also investigated.
The visible absorption spectra of the dyes prepared indicated that
1-methyl-2-phenyl substituted dyes (1d, 2d, 3d and 4d) generally
absorb bathochromically compared with the 1,2-dimethyl substi-
tuted dyes (1b, 2b, 3b and 4b) in all solvents used (Table 5). The
kmax values of the dye 4d showed bathochromic shifts in acetoni-
trile and acetic acid when compared with the dye 4b. The small
bathochromic shifts in kmax values were appeared in all solvents
used. Presumably, the resonance effect enhanced by an electron-
rich phenyl group is more favored than the slightly strong field
Fig. 4. Absorption spectra of dye 4b in various solvents.
effect of a methyl group. When
Dkmax value between dyes 1a and
1c, 2a and 2c, 3a and 3c was also compared with those obtained
between dyes 1b and 1d, 2b and 2d, 3b and 3d, respectively, the
resonance effect of phenyl group on the absorption maxima for
dyes (1a,c, 2a,c and 3a,c) is greater than their corresponding model
compounds and resulted in larger bathochromic shifts (e.g.
Dkmax
value is 37 nm for dye 3c relative to dye 3a in DMSO, kmax value
D
is 37 nm for dye 3d relative to dye 3b in DMSO). Also, the reso-
nance effect of phenyl group at 2-position of indole ring has a sig-
nificant effect on color in benzothiazolylazo indole dyes due to its
p-electron cloud. The similar results were observed in our previous
Fig. 5. Absorption spectra of dye 4b in acidic and basic solution.
work on hetarylazo indole dyes [16,17,25–27].

30
Z. Seferog˘lu et al. / Journal of Molecular Structure 1047 (2013) 22–30
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more bathochromically in accordance with the electron accep-
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These compounds involve a migration of electron density from
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of this type of compounds were shifted longer wavelength. The
obtaining absorption maxima for benzothiazolylazo indole dyes
indicated that electron-donating groups (methoxy and methyl) in
benzothiazole ring at 6- or 5,6- position generally show more bath-
ochromic shift when compared with the unsubstituted dyes (1a-d)
in all solvents used. In the case of nitro substituted benzothiazoly-
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of the nitro group. Similar results were observed in the thiazolylaz-
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The new series benzothiazolylazo indole dyes were synthesized
and characterized in this study. The most stable tautomeric form of
these dyes were determined easily with using model compounds
in solution and solid state. All spectroscopic data showed that,
the azo form predominant in all cases. However, the dyes 3c and
4c showed anionic form with increasing solvent polarity. The crys-
tal structures of 2a clearly show that this compound exist as azo
form stabilized via a strong hydrogen bonding between dye and
solvents. The dyes showed sensitivity to acid and base. Generally
the dyes were anionic form in basic solution except model com-
pounds and were cationic form in acidic solution.
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Acknowledgement
We are very grateful to Gazi University Research Fund for pro-
viding financial support for this project.
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