Synthesis and spectral characteristics of disazo dyes containing heterocyclic diazo components

Article abstract of DOI:10.1080/15421406.2013.844272

Disazo dyes were derived from heterocyclic amines by diazotizing 2-Amino-6-nitrobenzothiazole, 2-Amino-5-nitrothiazole, and 2-Amino-3,5- dinitrothiophene and coupling with Aniline as middle components, and 1-Naphthylamine, N,N-Dimethyl-1-naphthylamine, N,

Full text of DOI:10.1080/15421406.2013.844272

Mol. Cryst. Liq. Cryst., Vol. 583: pp. 70–76, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.844272
Synthesis and Spectral Characteristics of Disazo
Dyes Containing Heterocyclic Diazo Components
JI-HYE KIM, JAE-YONG LEE, JI-HYE BAE,
AND JAE-HONG CHOI∗
Department of Textile System Engineering, Kyungpook National University,
Deagu 702-701, Korea
Disazo dyes were derived from heterocyclic amines by diazotizing 2-Amino-6-
nitrobenzothiazole, 2-Amino-5-nitrothiazole, and 2-Amino-3,5-dinitrothiophene and
coupling with Aniline as middle components, and 1-Naphthylamine, N,N-Dimethyl-
1-naphthylamine, N,N-Dibutylaniline and N,N-Diethylaniline, Aniline as final compo-
nents. Disazo red, violet, blue dyes were synthesized and the absorption bands of these
compounds were observed in the range of 512–668 nm. The effects of diazo components
and final coupling components on the visible absorption maxima of the dyes were also
discussed.
Keywords Absorption maximum; benzothiazole; disazo dyes; solvatochromism; thi-
azole; thiophene
Introduction
Azo compounds are very important in the fields of dyes, pigments and advanced materials.
It has been known for many years that azo compounds are the most widely used class of
dyes because of their versatile application in various fields, such as the dyeing of textile
fibres and the coloring materials in the food colorants, cosmetic, printing, pharmaceuticals,
plastics, biological-medical studies as well as for advanced applications in organic synthesis
[1, 2].
There are monoazo dyes derived from heterocycles, such as pyridine, pyrazolone,
indoles, quinoline and thiophene derivatives. The use of heterocyclic coupling components
and diazo components in the synthesis of disperse dyes is well established, and the dyes
exhibit good tinctorial strength and brighter dyeing than those derived from aniline-based
components [2–4]. Although, many researchers have described the synthesis and dyeing
properties of monoazo dyes, only a few disazo dyes have been synthesized and their
properties investigated in recent years [2, 5, 6].
In this study, the synthesis of seven disazo dyes derived from various electron do-
nating coupling components and the heterocyclic diazo components, such as 2-amino-6-
nitrobenzothiazole, 2-amino-5-nitrothiazole, and 2-amino-3,5-dinitrothiophene is reported.
The visible absorption spectra in various solvents of these dyes were also discussed. The
∗Address correspondence to Prof. Jae-Hong Choi, Department of Textile Engineering, Kyung-
pook National University, 1370 Sangyuk-dong, Buk-gu, Daegu 702-701, Korea (ROK). Tel.: (+82)53-
950-5644; Fax: (+82)53-950-6617. E-mail: jaehong@knu.ac.kr
70

Synthesis of Disazo Dyes
71
synthetic method involved in the preparation of monoazo dyes 1–3 by coupling of diazo-
tized heterocyclic amines with coupler (aniline) containing diazotizable amino groups, as
well as the subsequent synthesis of final disazo dyes 4–10.
Experimental
Synthesis of Dyes
Synthesis of monoazo dye 2. As presented in Scheme 1, sodium nitrite (97%, 0.46 g,
0.34 mmol) was added to 95% sulfuric acid (5ml) to make a nitrosylsulfuric acid and the
mixture was heated to 60–70◦C for 1 hr and then cooled to 5◦C. 2-Amino-5-nitrothiazole
(1g, 0.34 mmol) was portionwised to an acetic acid and propionic acid mixture (1:1, 20 ml)
under 10◦C, and then the mixture was cooled below 5◦C while a nitrosylsulfuric acid was
being dropped. Diazotization was checked by TLC. The resulting diazonium solution was
added slowly to a vigorously stirred solution of aniline (0.64 ml, 0.34 mmol) dissolved in
99% acetic acid with crushed ice and then the mixture was stirred for 1 hr at 5◦C. After the
reaction was completed, the mixture was neutralized by using aqueous sodium hydroxide
solution (25%). The crude monoazo dye was filtered, and washed with water several times,
then dried and re-crystallized by ethanol [2].
Scheme 1. Synthetic scheme for the preparation of monoazo dye precursors and disazo dyes derived
from thiophene and thiazole diazo components.
This procedure was also used for the synthesis of monoazo dye 1 and 3.
1
Dye 1 Yield: 45%. C10H7N5O4S H NMR(CDCl3), δ: 8.39 (s, 1H, 2-thiophene),
6.63–7.40 (m, 4H, Ar-H), 5.33 (s, 2H, NH2). Found C: 40.65; H: 2.48; N: 23.95; S: 10.86
Calculated C: 40.96; H: 2.41; N: 23.88; S: 10.93, MS 293.26(M+)
1
Dye 2 Yield: 57%. C9H7N5O2S H NMR(CDCl3), δ: 9.03 (s, 1H, thiazole) 7.34 (m,
4H, Ar-H), 3.45 (m, 2H, NH2-). Found C: 43.01; H: 2.86; N: 28.31; S: 12.54 Calculated
C: 43.37; H: 2.83; N: 28.10; S: 12.86, MS 249.25(M+)
Dye 3 Yield: 63%. 1H NMR(CDCl3), δ: 8.05–8.64 (m, 3H, benzothiazole), 6.65–7.25
(m, 4H, Ar-H), 5.35 (s, 2H, -NH2). C13H9N5O2S Found C: 52.45; H: 3.12; N: 23.12; S:
10.41 Calculated C: 52.17; H: 3.03; N: 23.40; S: 10.71, MS 229.31(M+)
Synthesis of disazo dye 5. As presented in Scheme 1, sodium nitrite (97%, 0.28 g, 0.40
mmol) was added to 95% sulfuric acid (3 ml) and the mixture was heated to 60–70◦C
for 1 hr and then cooled to 5◦C. Monoazo dye (1 g, 0.40 mmol) prepared in advance was

72
J.-H. Kim et al.
portionwised to an acetic acid-propionic acid mixed solution (1:1, 20 ml) under 10◦C, and
then the mixture was cooled less than 5◦C while a nitrosylsulfuric acid was being dropped.
Diazotization was checked by TLC. The resulting diazonium solution was added slowly to
a vigorously stirred solution of N,N-diethylaniline (0.60ml, 0.40mmol) dissolved in 99%
acetic acid with crushed ice, and then the mixture was stirred for 1hr at 5◦C. After the
reaction was completed, the mixture was neutralized by using aqueous sodium hydroxide
solution (25%) [2]. The resulting disazo dye was filtered, and washed with water several
times, then dried and purified by column chromatography with n-hexane (95%) : ethyl
acetate (99.5%) (SiO2, n-hexane:ethyl acetate = 5:1).
All other disazo dyes were prepared in a similar procedure.
Dye 4 Yield: 41%. C20H19N7O4S 1H NMR(CDCl3), δ: 6.93–8.37 (m, 8H, Ar-H), 8.38
(s, 1H, 2-thiophene), 3.35 (m, 4H, N-CH2-), 1.12 (t, 6H, -CH3). Found C: 52.69; H: 4.25;
N: 21.78; S: 7.25 Calculated C: 52.97; H: 4.22; N: 21.62; S: 7.07, MS 453.47(M+)
Dye 5 Yield: 56%. C19H19N7O2S 1H NMR(CDCl3), δ: 9.05 (s, 1H, thiazole) 6.99–8.44
(m, 8H, Ar-H), 3.45 (m, 4H, N-CH2-), 1.19 (t, 6H, -CH3). Found C: 55.62; H: 4.59; N:
23.65; S: 7.98 Calculated C: 55.73; H: 4.68; N: 23.95; S: 7.83, MS 409.96(M+)
Dye 6 Yield: 63%. C19H13N7O2S 1H NMR(CDCl3), δ: 8.08–8.64 (m, 3H, benzothia-
zole), 6.85–8.11 (m, 8H, Ar-H), 5.35 (s, 2H, -NH2). Found C: 56.35; H: 3.35; N: 24.50; S:
7.54 Calculated C: 56.57; H: 3.25; N: 24.30; S: 7.95, MS 403.42(M+)
1
Dye 7 Yield: 67%. C23H21N7O2S H NMR(CDCl3), δ: 8.10–8.67 (m, 3H, benzoth-
iazole), 6.99–8.45 (m, 8H, Ar-H), 3.41 (m, 4H, N-CH2-), 1.20 (t, 6H, -CH3). Found C:
60.01; H: 4.64; N: 21.21; S: 6.98 Calculated C: 60.12; H: 4.61; N: 21.34; S: 6.93, MS
459.52(M+)
1
Dye 8 Yield: 65%. C27H29N7O2S H NMR(CDCl3), δ: 8.10–8.69 (m, 3H, benzoth-
iazole), 6.98–8.47 (m, 8H, Ar-H), 3.78 (m, 4H, N-CH2-), 1.56 (m, 4H, -CH2-), 1.38 (m,
4H, -CH2-), 0.98 (t, 6H, -CH3). Found C: 62.56; H: 5.56; N: 19.31; S: 6.15 Calculated C:
62.89; H: 5.67; N: 19.01; S: 6.22, MS 515.63(M+)
Dye 9 Yield: 60%. C23H15N7O2S 1H NMR(CDCl3), δ: 8.08–8.67 (m, 3H, benzothia-
zole), 7.16–8.19(m, 10H, Ar-H), 5.83 (s, 2H, -NH2). Found C: 62.56; H: 3.87; N: 10.51;
S: 6.45 Calculated C: 62.36; H: 3.98; N: 10.36; S: 6.66, MS 453.48(M+)
Dye 10 Yield: 62%. C25H19N7O2S 1H NMR(CDCl3), δ: 7.60–8.26 (m, 3H, benzoth-
iazole), 7.18–8.16(m, 10H, Ar-H), 3.08 (s, 4H, -CH3). Found C: 40.54; H: 2.48; N: 23.95;
S: 10.86 Calculated C: 40.96; H: 2.41; N: 23.88; S: 10.93, MS 481.53(M+)
Structural Analysis and Measurement of Absorption Spectra
Structural analysis of synthesized dyes were measured using an EA 1108 (E.A.) and an HP
6890 & Agilent 5973N MSD (GC-Mass).1H-NMR data were obtained with a Bruker 400
spectrometer using CDCl3 as solvent and TMS as internal standard. UV-visible absorption
spectra were obtained from Shimadzu UV-2100.
Results and Discussion
Calculations by the PPP-MO Method
The PPP-MO procedure has been used to calculate the anticipated absorption spectra in
the visible range as well as the fluorescence absorption maxima of the dyes [7–11], and
PI SYSTEMTM for windows is a commercial program based on a modified PPP SCF-CI-
MO method for rapid and convenient calculations. All bonding and nonbonding interactions

Synthesis of Disazo Dyes
73
Table 1. Calculated and observed absorption maxima of disazo dyes 4–10
Dye number
λCal.max (nm)
λObs.
(nm)∗
max
4
5
6
7
8
9
10
669
631
537
605
605
590
629
668
628
552
600
607
597
615
∗Determined in DMSO.
between the π atoms are calculated by means of Roothaan’s formula for p-p overlap integral
[13, 14]. Table 1 shows calculated and observed absorption maxima of the synthesized dyes.
The most reliable results were observed with dyes 4, 5, 7 and 8 where the difference of
absorption maximum between calculated and observed was less than 5 nm. In this case, the
calculation of target color would be efficient by eliminating the actual synthetic procedures.
For other dyes 6, 9 and 10, the gabs in λmax between two methods were found to be smaller
than 15 nm which could be also sufficient for the simulation of dye structure to achieve
desired colors.
Effects of Heterocyclic Ring on the Absorption Maximum
As shown in Fig. 1, comparison of the absorption maximum for disazo dyes 4, 5 and 7,
these contain N,N-diethylaniline group in the coupling moiety, revealed more bathochromic
1.4
Dye 4
Dye 5
Dye 7
1.2
1.0
0.8
0.6
0.4
0.2
350
400
450
500
550
600
650
700
750
Wavelength, nm
Figure 1. Absorption spectra of dyes 4, 5 and 7 derived from three types of heterocyclic rings.

74
J.-H. Kim et al.
shift by the heterocyclic ring in the diazo moiety ordered from benzothiazole, thiazole to
thiophene. Therefore the most bathochromic disazo dye, in this series, was found with dye
4 that derived from 2-amino-3,5-dinitrothiophene which absorbs maximally at 668 nm.
By comparing with that of corresponding disazo dye 5 contains 2-amino-5-nitrothiazole,
a large bathochromic shift of 40 nm was observed which is mostly attributable to the
strong withdrawing effect exerted by two nitro groups of the thiophene ring. In contrast,
dye 7 derived from 2-amino-6-nitrobenzothiazole diazo moiety showed a hypsochromic
shift compared to that of corresponding dye 5 which was 28 nm. In this case, the diene
effect arisen by a 5-membered heterocyclic ring, such as thiophenes and thiazoles, could be
mainly contributed to the red shift observed [14–18]. Therefore, all disazo dyes prepared
could afford blue to greenish-blue shades, irrespective of heterocyclic diazo component. It
can be concluded that the bathochromic effect depending on heterocyclic rings as a diazo
component increases in the following order: 2-amino-3,5-dinitrothiophene > 2-amino-5-
nitrothiazole > 2-amino-6-nitrobenzothiazole.
Effects of Coupler Ring (Donor) on the Absorption Maximum
In Table 1, the absorption maxima of disazo dyes 6–10 derived from 2-amino-6-
nitrobenzothiazole, as a diazo moiety, coupled with five different electron donors
were summarized. The most bathochromic dye was dye 10 which contains a N,N-
dimethylnaphthalene ring absorbing maximally at 615nm, whereas the dye 9 containing
N,N-unsubstituted naphthalene analogue exhibited absorption maximum of 597 nm that
to be hypsochromically shifted by 18 nm. This result can be explained by the presence
of stronger electron donating substituents, N,N-dimethyl group, in the coupling moiety
of dye 10 leading to lower energy level in the excited state. In similar, dye 7 containing
an N,N-diethylaniline group exhibited a bathochromic shift of 8nm in comparison with
corresponding dye 6 substituted by aniline ring.
In comparison of aniline ring and aminonaphthalene analogue, λmax of dye 9 was
measured to be 597 mn which is 45 mn longer than corresponding dye 6 due to the longer
conjugation of π–electrons arisen by naphthalene ring.
Furthermore, N,N-Dibutyl group in the coupling moiety of the dye 8 contributed to
the red shift by 7nm compared with diethyl analogue (dye 7) that could suggest the longer
carbon chain of N,N-dialkyl group results in a bathochromic shift.
Comparison of the Absorption Maximum Between Disazo Dyes
and Monoazo Precursors
As shown in Table 2, all disazo dyes prepared absorbed maximally at much longer wave-
length compared with that of corresponding monoazo dyes. The most difference in λmax
was observed with disazo dye 7 which exhibited 600nm of λmax, whereas the monoazo
analogue (dye 3) absorbed maximally at 531 nm, leading to a bathochromic shift of 69 nm.
The other disazo dyes 4–6 also showed longer λmax values in the range of 21∼51 nm. Thus,
it is clearly explained that the extension of azo linkage from monoazo to disazo leads to
the longer conjugation of π-systems of the dye molecules affording smaller energy gab
between the ground state and excited state.
Solvatochromism
Solvatochromism can be best described as the effect of a solvent on the color of a dye.
Solvent-solute interactions have various influences on the absorption spectrum, such as

Synthesis of Disazo Dyes
75
Table 2. Comparison of absorption maxima of disazo dyes and their corresponding
precursors
Disazo dye
λmax(nm)
Monoazo dye
λmax(nm)
∗
No.
No.
ꢀ λmax (nm)
4
5
6
7
668
628
552
600
1
2
3
3
638
577
531
531
30
51
21
69
∗Determined in DMSO.
the λmax value and the intensity of a band. Generally in many neutral dye molecules,
the ground state is less polar than the excited state so that a polar solvent will tend to
stabilize the excited state more than the ground state, leading to a bathochromic shift in
the absorption maximum. This effect is termed positive solvatochromism. In contrast with
positive solvatochromism, negative solvatochromism is relatively rare and is most often
observed with colorants which contain a delocalized charge.
The dielectric constant of the solvent reflects its polarity, as shown in Table 3. For
example, water (very polar) has a dielectric constant of 80.1 at 20◦C while n-hexane (very
non-polar) has a dielectric constant of 1.89 at 20◦C [19]. The dielectric constant of DMSO
is 46.7 which is highest polar among three solvents used, and ethyl acetate has the lowest
dielectric constant.
As summarized in Table 3, a positive solvatochromism was observed for all disazo
dyes when these dyes were dissolved in four different solvents, where significant changes
in absorption maxima toward longer wavelengths were consistently observed. The most
change was found in dye 5 which shifted its absorption maximum bathochromically by
64 nm in DMSO compared with in ethyl acetate. In contrast, the smallest value was observed
with dye 9.
Table 3. Dielectric constants of solvents used and absorption maxima of disazo dyes 4–10
in various solvents
Dielectric constant
46.7
21
6.02
DMSO
Acetone
Ethyl acetate
Dye no.
λmax(nm)
4
5
6
7
8
9
10
668
628
552
600
607
597
615
631
586
522
574
578
576
589
624
564
517
567
569
568
566

76
J.-H. Kim et al.
Conclusions
A series of seven disazo dyes based on heterocyclic rings, as a diazo moiety, has been
prepared by the diazotization of monoazo precursor with a subsequent coupling reaction.
The absorption maxima of the disazo dyes prepared varied from 552 nm to 668 nm
depending on the heterocyclic rings and N,N-disubstituents in the coupler moiety. The
most bathochromic shift was observed with dye containing a thiophene ring as a diazotizer,
whereas benzothiazole analogues were comparatively blue-shifted. However, the presence
of naphthalene ring affords a bathochromic shift compared to corresponding phenyl group.
A significant positive solvatochromism was found when dissolved disazo dyes in three
different solvents responding to the conventional neutral dye molecules.
Therefore, these disazo dyes can be used for blue dichroic dyes where suitable affinity
is confirmed with substrates.
Acknowledgment
This work was supported by the National Research Foundation of Korea Grant funded by
the Korean Government [NRF-2012001638]
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