Synthesis and properties of some novel pyrazolone-based heterocyclic azo disperse dyes containing a fluorosulfonyl group

Article abstract of DOI:10.1016/j.dyepig.2012.06.009

A series of heteroarylazo disperse dyes derived from pyrazolones and fluorosulfonyl anilines were synthesized, and their thermal and spectral properties were investigated with respect to the effect of substituents on absorption spectra, halochromism, and

Full text of DOI:10.1016/j.dyepig.2012.06.009

Dyes and Pigments 95 (2012) 580e586
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and properties of some novel pyrazolone-based heterocyclic azo
disperse dyes containing a fluorosulfonyl group
*
Ajay Singh, Ran Choi, Byunghun Choi, Joonseok Koh
Department of Textile Engineering, Konkuk University, Seoul 143-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of heteroarylazo disperse dyes derived from pyrazolones and fluorosulfonyl anilines were
synthesized, and their thermal and spectral properties were investigated with respect to the effect of
substituents on absorption spectra, halochromism, and solvatochromism. Heteroarylazo disperse dyes,
which contain a nitro group instead of a fluorosulfonyl group para to the azo group in the diazo
component, were also synthesized for comparative purpose. The fluorosulfonylarylazopyrazolone dyes
had absorption maxima at shorter wavelength and showed lower extinction coefficients than the
nitroarylazopyrazolone dyes. The synthesized dyes also exhibited positive halochromism and sol-
vatochromism, so the absorption bands of the dyes moved toward longer wavelengths as the acidity or
polarity of the solvent increased. 4-Fluorosulfonyl-substituted dyes generally showed lower thermal
stability than their 4-nitro-substituted analogs, because the lower electron-withdrawing power in the
diazo components tended to decrease the polarizability and dipole interactions.
Received 22 September 2011
Received in revised form
14 June 2012
Accepted 18 June 2012
Available online 3 July 2012
Keywords:
Heteroarylazo disperse dyes
Pyrazolones
Alkali-clearable azo disperse dye
Fluorosulfonyl group
Spectral properties
Thermal stability
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
first-order kinetics were determined by analysis of the dye hydro-
lysis using HPLC [6e8].
Environmental issues have been gaining importance in all
aspects of industrial production [1], and various legislative
requirements have emerged with increasing regularity to reduce
the impact of dyeing processes on the environment. In response,
the industry has been forced to become increasingly innovative in
order to develop new products and practices that are more envi-
ronmentally friendly than existing ones [2]. Therefore, innovation
and developments in color chemistry and dyeing will allow the
colorist to meet ever-increasing environmental restrictions,
produce novel effects, and reduce processing costs.
The demand for environmentally friendly dyes with high wet
fastness on polyester is increasing, and the so-called alkali-clear-
able disperse dyes suggest a promising new direction [3e5]. These
alkali-clearable disperse dyes obviate the need for sodium hydro-
sulfite and significantly reduce the cost of effluent treatment.
Previous studies in our group have investigated the use of novel
alkali-clearable azo disperse dyes containing a fluorosulfonyl
group. Azo disperse dyes of this type were hydrolyzed under
alkaline conditions by an SN2 mechanism (Scheme 1), and pseudo-
Dyeing with such fluorosulfonyl-containing disperse dyes
showed a reasonable level of build-up, excellent wash fastness, and
the option of alkali-clearance to achieve high wash fastness,
replacing reductive clearing and particularly the use of sodium
hydrosulfite, which places a very high biological oxygen demand on
conventional disperse dyeing effluent and generates toxic aromatic
amines [9e11].
The 4-(N,N-diethylamino)-40-fluorosulfonylazobenzene dyes
that have been synthesized in previous studies cover a whole
gamut of colors (red to greenish blue, lmax ¼ 469e620 nm in
ethanol) except the yellow shade area [9]. Recently, we have sug-
gested arylazopyridone disperse dyes for yellow colors because
they have good spectral and dyeing properties [12]. The use of
heterocyclic coupling components and diazo components in the
synthesis of azo disperse dyes is well established, and the resultant
dyes exhibit good tinctorial strength and brighter dyeing than those
derived from aniline-based components [13e24]. Pyridones and
pyrazolones as coupling components have been shown to be
especially important colorants for yellow to orange dyes in indus-
trial applications [25].
In this present study, the synthesis of heteroarylazopyrazolone
disperse dyes containing fluorosulfonyl groups is reported. Pyr-
azolone derivatives (1, 1-alkyl-3-methyl-5-pyrazolone) were used
* Corresponding author. Tel.: þ82 2 450 3527; fax: þ82 2 458 4131.
E-mail addresses: ccdjko@konkuk.ac.kr, ccdjko@hanmail.net (J. Koh).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.06.009

A. Singh et al. / Dyes and Pigments 95 (2012) 580e586
581
X
Y
X
R1
R1
O
S
O
S
R3
R4
R3
R4
OH
F
N
O
N
N
N
N
N
O
O
Y
R2
R2
Water-insoluble
Substantivity for PET
Water-soluble
Low substantivity for PET
Scheme 1. Alkali hydrolysis of 4-amino-40-fluorosulfonylazobenzene disperse dyes.
as coupling components. The thermal and spectral properties of
heteroarylazopyrazolone disperse dyes containing fluorosulfonyl
groups and their nitro-substituted analogs have also been investi-
gated for comparison.
obtained were purified by combination of column chromatography
using dichloromethane: hexane (1:10) as eluting solvents and
recrystallization from methanol.
4a 95% yield, mp 230.5 ꢀC. nmax (KBr)/cmꢁ1 3300 (NH),1670 (C]O),
1604 (C]C, aromatic); 1H NMR (DMSO-d6, 500 MHz, ppm).
2. Experimental
d
2.16 (3H, s, pyrazolone-CH3), 7.84 (2H, d, Ar-H, J ¼ 8.5 Hz), 8.12
(2H, d, Ar-H, J ¼ 9 Hz), 11.70 (1H, s, pyrazolone-NH), 13.16 (1H, s,
NH). Anal. Calcd for C10H9FN4O3S: C 42.25, H 3.19, N 19.71, S
11.28; found: C 42.49, H 3.13, N 20.18, S 10.91.
2.1. General
N-Acetylsulfanilyl chloride, p-dioxane, ethylcyanoacetate, eth-
ylacetoacetate, ammonium hydroxide, piperidine, methylamine,
ethylamine, propionic acid, and nitrosylsulfuric acid were
purchased from Aldrich and used without further purification. All
other chemicals used in the synthesis and characterization were of
laboratory-reagent grade.
4b 95% yield, mp 163.2 ꢀC. nmax (KBr)/cmꢁ1 1670 (C]O), 1604 (C]
C, aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm).
d 2.30 (3H, s,
pyrazolone-CH3), 7.23 (1H, t, PhH of pyrazolone, J ¼ 7.4 Hz), 7.46
(2H, t, PhH of pyrazolone, J ¼ 7.9 Hz), 7.88 (2H, d, Ar-H, J ¼ 7.
7 Hz), 7.91 (1H, t, PhH of pyrazolone, J ¼ 8.9 Hz), 8.13 (2H, d,
The 1H NMR spectra were measured in deuterated DMSO using
an Avance 500 (Bruker, 500 MHz). Melting points were determined
using a DSC 7 PerkineElmer Differential Scanning Calorimeter (USA,
heating rate 5 ꢀC/min, N2 gas). The absorption spectra were
measured in 1 cm quartz cells on an Agilent 8453 spectrophotom-
eter (USA, HP). All crude products were isolated as solids and puri-
fied a combination of column chromatography and recrystallization.
Ar-H,
J
¼
8.95 Hz), 13.22 (1H, s, NH). Anal. Calcd for
C16H13FN4O3S: C 53.33, H 3.64, N 15.55, S 8.90; found: C 53.61, H
3.73, N 15.65, S 8.95.
4c 94% yield, mp 205 ꢀC. nmax (KBr)/cmꢁ1 1670 (C]O). 1H NMR
(DMSO-d6, 500 MHz, ppm).
d 2.30 (s, 3H), 7.50 (2H, d, PhH of
pyrazolone, J ¼ 9 Hz), 7.92 (2H, d, PhH of pyrazolone, J ¼ 9 Hz),
7.94 (2H, d, Ar-H, J ¼ 9 Hz), 8.14 (2H, d, Ar-H, J ¼ 9 Hz), 13.21 (1H,
s, NH). Anal. Calcd for C16H12ClFN4O3S: C 48.67, H 3.06, N 14.19, S
8.12; found: C 48.89, H 3.11, N 14.16, S 8.15.
2.2. Synthesis of diazo components (2)
4d 90% yield, mp 208 ꢀC. nmax (KBr)/cmꢁ1 3310 (NH), 1695 (C]O),
1615 (C]C, aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
Diazo components 2 were prepared using previously described
d
2.22 (3H, s, pyrazolone-CH3), 8.27 (1H, d, Ar-H, J ¼ 9.1 Hz), 8.57
procedures [9].
(1H, d, Ar-H, J ¼ 9 Hz), 8.94 (1H, s, Ar-H), 11.95 (1H, s,
pyrazolone-NH), 14.50 (1H, s, NH). Anal. Calcd for C10H8FN5O5S:
C 36.48, H 2.45, N 21.27, S 9.74; found: C 37.05, H 2.48, N 20.78, S
9.63.
2.3. Synthesis of dyes
2.3.1. Diazotization
4e 93% yield, mp 180 ꢀC. nmax (KBr)/cmꢁ1 1670 (C]O), 1615 (C]C,
4-Fluorosulfonylaniline (2a) (0.02 mol) was diazotized in conc.
hydrochloric acid (35% w/w, 6.9 mL) and water (70 mL) by adding
aqueous sodium nitrite solution (2N, 10 mL) at a temperature of
0e5 ꢀC. In the case of 2-nitro-4-fluorosulfonylaniline (2b), nitro-
sylsulfuric acid was used for the diazotization. 3.8 mL of 40wt%
nitrosylsulfuric acid in sulfuric acid (0.022 mol) was added to
a mixture of diazo component (0.02 mol) in acetic acid/propionic
acid (4:1, 50 mL) at 0e5 ꢀC. After 4e5 h, the completion of diazo-
tization was checked by checking for the presence of excess nitrous
acid using starch-iodide paper.
aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d 2.30 (3H, s,
pyrazolone-CH3), 7.27 (1H, t, PhH of pyrazolone, J ¼ 7.5 Hz), 7.50
(2H, t, PhH of pyrazolone, J ¼ 7.5 Hz), 7.89 (2H, d, PhH of
pyrazolone, J ¼ 7.5 Hz), 8.33 (1H, d, Ar-H, J ¼ 9.1 Hz), 8.61 (1H, d,
Ar-H, J ¼ 9.1 Hz), 8.96 (1H, s, Ar-H), 14.55 (1H, s, NH). Anal. Calcd
for C16H12FN5O5S: C 47.41, H 2.98, N 17.28, S 7.91; found: C 47.61,
H 3.06, N 16.78, S 7.51.
4f 95% yield, mp 214 ꢀC. nmax (KBr)/cmꢁ1 1670 (C]O), 1615 (C]C,
aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d 2.36 (3H, s,
pyrazolone-CH3), 7.52 (2H, d, PhH of pyrazolone, J ¼ 7.5 Hz), 7.92
(2H, d, PhH of pyrazolone, J ¼ 9 Hz), 8.29 (1H, d, Ar-H, J ¼ 9 Hz),
8.59 (1H, d, Ar-H, J ¼ 9 Hz), 8.93 (1H, s, Ar-H), 14.48(1H, s, NH).
Anal. Calcd for C16H11ClFN5O5S: C 43.69, H 2.52, N 15.92, S 7.29;
found: C 43.87, H 2.59, N 15.98, S 6.83.
2.3.2. Coupling
The prepared diazonium salt (3) solution was added to the
corresponding coupling component (1, 0.02 mol) dispersed in
acetone (100 mL) for better homogeneous dispersion (rather than
the conventional aqueous system) and the temperature was
maintained at 0e5 ꢀC. After 30 min, ice (130 g) was added and the
solution was stirred for 2 h then allowed to reach room tempera-
ture. After 4e5 h, the pH value of the diazo liquor was adjusted to
pH 5e6 by the addition of sodium acetate. The precipitated dye (4)
was filtered, washed with water, and dried (Scheme 2). The dyes
In order to synthesize heteroarylazopyrazolone disperse dyes
containing a nitro group (7), the same procedure was repeated
except that substituted amines containing nitro groups (5) were
used in place of substituted amines containing fluorosulfonyl
groups as diazo components (Scheme 3).

582
A. Singh et al. / Dyes and Pigments 95 (2012) 580e586
Scheme 2. Synthesis of heteroarylazopyrazolone disperse dyes containing a fluorosulfonyl group (4).
7a 95% yield, mp 269.6 ꢀC. nmax (KBr)/cmꢁ1 3310 (NH), 1676 (C]O).
1H NMR (DMSO-d6, 500 MHz, ppm):
2.16 (3H, s, pyrazolone-
Anal. Calcd for C10H9N5O3: C 48.58, H 3.67, N 28.33; found: C
48.56, H 3.58, N 28.37.
d
CH3), 7.72 (2H, d, Ar-H, J ¼ 9.15 Hz), 8.26 (2H, d, Ar-H,
7b 95% yield, mp 199.8 ꢀC. nmax (KBr)/cmꢁ1 1676 (C]O), 1608 (C]
J ¼ 9.15 Hz), 11.69 (1H, s, pyrazolone-NH), 13.16 (1H, s, NH).
C, aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d 2.32 (3H, s,
Scheme 3. Synthesis of heteroarylazopyrazolone disperse dyes containing a nitro group (7).

A. Singh et al. / Dyes and Pigments 95 (2012) 580e586
583
The dyes may exist in four possible tautomeric forms, namely
two azo-keto forms (a) and (b), the azo-enol form (c), and the
hydrazone-keto form (d), as shown in Scheme 4. The deprotonation
of the four tautomers leads to a common anion [26]. Numerous
investigations have been carried out to establish the tautomeric
structures of arylazo-5-pyrazolones, both in the solid state and in
solution, and a variety of spectroscopic techniques have been used.
The spectral data generally lead to the conclusion that the tauto-
meric equilibrium of the phenylazopyrazolone dyes is in favor of
the hydrazone form (d) in the solid state and in polar solvents
(Scheme 4) [21,27e33]. In this study, the hydrogen-bonded NH
group at the b
-nitrogen appeared at 13.16e14.55 ppm in the 1H
NMR spectra of the dyes, which confirms that the dyes prepared
exist in the hydrazone form.
3.2. Thermal properties
Although several dyes, such as dyes 4d, 4e, 4f, 7d, 7e, and 7f,
decomposed before they reached their melting points, all of the
synthesized dyes exhibited higher melting points than 4-
aminoazobenzene dyes (Table 1) [9]. These properties can be
attributed to intramolecular hydrogen bonds; the hydrogen atom in
the hydrazone group and the carbonyl oxygen in the pyrazolone
ring take part in intramolecular hydrogen bonding, which holds the
molecule in a more or less planar configuration, and van der Waals
forces help to achieve compact packing of the molecules.
While the relative values of the melting points cannot be easily
explained in detail due to their complex dependence on a number
of factors, a few general trends can be accounted for. It is well
known that the thermal properties in a given series of dyes is
related roughly to their molecular weights; dyes having higher
molecular weight showed higher melting points because more van
der Waals attractions are possible for the larger molecules, due to
the higher surface area over which these forces can operate. It is
also presumed that the more polar dyes, which contain more
powerful electron-withdrawing groups, tend to have better
thermal stability because of the increased polarizability and dipole
interactions; the 4-nitro-substituted dyes (7) generally exhibited
higher thermal stability than the 4-fluorosulfonyl-substituted dyes
(4), and the 2-nitro-4-fluoro(or nitro)-substituted dyes (4d, 4e, 4f,
7d, 7e, and 7f) showed higher thermal stability than the 4-
fluorosulfonyl(or nitro)-substituted dyes (4a, 4b, 4c, 7a, 7b, and 7c).
Compounds 4a, 4d, 7a, and 7d tend to have even higher melting
points than the others because of the formation of intermolecular
Scheme 4. The expected tautomers of the synthesized hetarylazo-5-pyrazolone dyes.
pyrazolone-CH3), 7.22 (1H, t, PhH of pyrazolone, J ¼ 7.5 Hz), 7.46
(2H, t, PhH of pyrazolone, J ¼ 8 Hz), 7.8 (2H, d, Ar-H, J ¼ 9 Hz),
7.89 (2H, d, PhH of pyrazolone, J ¼ 7.5 Hz), 8.28 (2H, d, Ar-H,
J ¼ 7 Hz), 13.22 (1H, s, NH). Anal. Calcd for C16H13N5O3: C
59.44, H 4.05, N 21.66; found: C 59.37, H 4.02, N 21.74.
7c 95% yield, mp 243.4 ꢀC. nmax (KBr)/cmꢁ1 1676 (C]O), 1608 (C]
C, aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d 2.50 (3H, s,
pyrazolone-CH3), 7.53 (2H, d, Ar-H, J ¼ 9 Hz), 7.85 (2H, d, PhH of
pyrazolone, J ¼ 9 Hz), 7.93 (2H, d, PhH of pyrazolone, J ¼ 9 Hz),
8.30 (2H, d, Ar-H, J ¼ 9.5 Hz), 13.20 (1H, s, NH). Anal. Calcd for
C16H12ClN5O3: C 53.72, H 3.38, N 19.58; found: C 53.66, H 3.28, N
19.56.
7d 90% yield, mp 280 ꢀC. nmax (KBr)/cmꢁ1 3310 (NH), 1690 (C]O),
1615 (C]C, aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d
2.22 (3H, s, pyrazolone-CH3), 8.26 (1H, d, Ar-H, J ¼ 9.4 Hz),
8.58 (1H, d, Ar-H, J ¼ 9.3 Hz), 8.92 (1H, s, Ar-H), 11.99 (1H, s,
pyrazolone-NH), 14.51 (1H, s, NH). Anal. Calcd for C10H8N6O5: C
41.10, H 2.76, N 28.76; found: C 41.00, H 2.71, N 28.75.
7e 92% yield, mp 210 ꢀC. nmax (KBr)/cmꢁ1 1676 (C]O), 1608 (C]C,
aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d 2.20 (3H, s,
pyrazolone-CH3), 7.26 (1H, t, PhH of pyrazolone, J ¼ 7.2 Hz), 7.50
(2H, t, PhH of pyrazolone, J ¼ 8.1 Hz), 7.90 (1H, d, Ar-H,
J ¼ 7.8 Hz), 8.14 (2H, d, PhH of pyrazolone, J ¼ 9 Hz), 8.47 (1H,
d, Ar-H, J ¼ 8.1 Hz), 8.80 (1H, s, Ar-H), 14.55 (1H, s, NH). Anal.
Calcd for C16H12N6O5: C 52.18, H 3.28, N 22.82; found: C 52.14, H
3.25, N 22.80.
Table 1
Yields and melting points of the dyes 4 and 7.
O
7f 94% yield, mp 250 ꢀC. nmax (KBr)/cmꢁ1 1676 (C]O), 1608 (C]C,
R
Y
aromatic). 1H NMR (DMSO-d6, 500 MHz, ppm):
d
2.35 (3H, s,
N
N
pyrazolone-CH3), 7.51 (2H, d, PhH of pyrazolone, J ¼ 9 Hz), 7.9
(2H, d, PhH of pyrazolone, J ¼ 8.7 Hz), 8.12 (1H, d, Ar-H,
J ¼ 9.3 Hz), 8.41 (1H, d, Ar-H, J ¼ 9.3 Hz), 8.79 (1H, s, Ar-H),
14.48(1H, s, NH). Anal. Calcd for C16H11ClN6O5: C 47.71, H 2.75,
N 20.87; found: C 47.81, H 2.74, N 20.76.
N
X
N
H3C
Dye
X
Y
R
M.W.
Appearance
4a
4b
4c
4d
4e
4f
7a
7b
7c
7d
7e
7f
SO2F
SO2F
SO2F
SO2F
SO2F
SO2F
NO2
NO2
NO2
NO2
NO2
NO2
H
H
H
NO2
NO2
NO2
H
H
H
H
Ph
Ph-pCl
H
Ph
Ph-pCl
H
Ph
Ph-pCl
H
Ph
284.27
360.36
394.81
329.26
405.36
439.81
247.21
323.31
357.75
292.21
368.30
402.75
Yellow
Orange-yellow
Orange-yellow
Yellow
Orange-yellow
Orange-yellow
Yellow
Orange-yellow
Orange-yellow
Yellow
Orange-yellow
Orange-yellow
3. Results and discussion
3.1. Characterization
The hetarylazopyrazolone dyes 4 and 7 were prepared by
coupling pyrazolone derivatives (1) with diazotized fluo-
rosulfonylanilines, and their structures have been confirmed by 1H
NMR and FT-IR spectral data.
NO2
NO2
NO2
Ph-pCl

584
A. Singh et al. / Dyes and Pigments 95 (2012) 580e586
Table 2
hydrogen bonds (Fig.1) [34]. They are stereochemically able to have
not only intramolecular hydrogen bonds but also intermolecular
hydrogen bonding between adjacent molecules, whereas the other
dyes (4b, 4c, 4e, 4f, 7b, 7c, 7e, and 7f) have only intramolecular
hydrogen bonding.
Spectral data of dyes 4 and 7 in EtOH.
Dye
lmax (nm)
3
max
(I molꢁ1 cmꢁ1
)
Dl1/2(nm)
4a
4b
4c
4d
4e
4f
7a
7b
7c
7d
7e
7f
354
382
386
346
393
393
410
399
401
409
405
405
18,000
26,000
26,000
11,000
11,000
13,000
23,000
31,000
29,000
20,000
10,000
11,000
126
86
88
139
>130
>130
114
77
3.3. Spectral properties
3.3.1. Absorption spectra
Details of the visible absorption spectra of pyrazolone-based
heteroarylazo disperse dyes containing fluorosulfonyl groups and
their nitro analogs are summarized in Table 2. The synthesized dyes
79
114
143
132
developed
a color ranging from yellow to orange-yellow
(354e410 nm in ethanol).
Absorption maxima (lmax) values tend to be related to the
strength of the electronic withdrawing or donating power in the
benzenoid system [35]. Since the electronic transitions in these
compounds involves a general migration of electron density from
the donor group toward the azo group, the greatest effect in terms
of longer wavelength is achieved by placing the substituents in the
positions ortho or para to the azo group for effective conjugation
[36,37].
The 4-fluorosulfonyl-substituted derivatives absorb maximally
at shorter wavelengths, by 12e63 nm in ethanol, than the 4-nitro-
substituted analogs, although the Hammett constant for p-fluo-
rosulfonyl substitution (0.91) is greater compared to p-nitro
substitution (0.78). However, this result is consistent with that of
previous research [7,12].
The introduction of an electron-accepting substituent into the
diazo component ring produced a bathochromic shift; the lmax
values of 2-nitro-4-fluorosulfonyl substituted azo dyes showed
larger bathochromic shifts than those of 4-fluorosulfonyl
substituted azo dyes by 4e11 nm in ethanol. However, the intro-
duction of a 2-nitro group into diazo components containing 3-
methyl-1H-pyrazol-5(4H)-one (4a vs. 4d and 7a vs. 7d) does not
show any remarkable inductive effects, probably because of a steric
Half-band widths (Dl1/2) of the absorption bands in ethanol
were also determined (Table 2). In addition to the effect on lmax
,
substituents also caused a change in the half-band width values.
The value of Dl1/2 is a convenient criterion for the evaluation of the
hue brightness of dyes; dyes with low Dl1/2 show bright hues while
those with high values of Dl1/2 show dull hues. The higher Dl1/2
values are shown by the 2,4-dinitro derivatives (7d, 7e, and 7f) or 2-
nitro-4-fluorosulfonyl derivatives (4d, 4e, and 4f), which is caused
as a result of the broadening the range of energy levels of the
orbitals involved in the transitions associated with light absorption.
3.3.2. Halochromic effects
Protonation of a substituted pyrazolone derivative gives azo-
nium tautomers (Scheme 5), which provide a red-shifted absorp-
tion band and more intense colors than the parent dyes. Compared
to the neutral dye, the ground and excited states in the azonium
species are much closer together in energy, so a bathochromic shift
of the first absorption band is observed on protonation, which is
termed positive halochromism [39,40].
The direction of charge migration that accompanies electronic
excitation in the azonium form is opposite that found in the neutral
dye, and this difference suggests that substituents in the diazo
component or coupling component should have opposite effects in
neutral and in protonated dyes. Indeed, it can be seen that the
bathochromic shift decreases steadily with the general electron-
withdrawing capacity of the substituents in the diazo component
ring and with the general electron-donating capacity of the
substituents in the coupling component ring, and it in fact becomes
negligible when two powerful electron acceptor groups (2,4-dinitro
or 2-fluorosulfonyl-4-nitro) are present in the diazo component
ring (Table 3). Most synthesized dyes containing two electron-
withdrawing groups in the diazo component ring and/or
electron-donating groups in the coupling component ring showed
less positive halochromism due to the strong electronic effect,
while dyes containing just one electron-accepting group in the
diazo component and less powerful electron-donating groups in
the coupling component ring (4a and 7a) showed more positive
effect between nitro groups and b-nitrogen atom in azo group, as
shown in conformation (a) (Fig. 2); such crowding can be relieved
by rotation into conformation (b), so that a single substituent will
not normally exert much of steric effect. However, when the
hydrazone form is predominant over the azo form, both confor-
mations are hindered and non-planarity is the likely outcome [38].
It is well known that, as a rule of thumb, the molar extinction
coefficient (
m3 ax) increases as lmax increases. For the series of
synthesized dyes,
3
max
values tend to increase with increasing
electron-withdrawing capacity in the acceptor ring; the 4-nitro-
substituted dyes had a higher lmax, while the 4-fluorosulfonyl-
substituted analogs showed higher
3
max
values. However, the
introduction of a nitro group at the 2- position of 4-nitro- or 4-
fluorosulfonyl-substituted arylazo dyes caused a decrease in the
extinction coefficient, probably because the steric hindrance effect
is dominant over the inductive effect.
CH3
Y
N
N
O
H
N
O
H
N
N
N
X
H
H
H
Y
X
N
X
N
N
N
N
N
H
O
O
H
N
Y
H
X
N
Y
N
N
H3C
H3C
H3C
a
b
Fig. 1. Inter- and intra-molecular hydrogen bonding of heteroarylazo dye molecules
containing 3-methyl-1H-pyrazol-5(4H)-one (4a, 4d, 7a, and 7d).
Fig. 2. Alternative conformations for an ortho-substituted pyrazolone azobenzene
dyes.

A. Singh et al. / Dyes and Pigments 95 (2012) 580e586
585
Table 4
Y
Y
Solvatochromic effects of the dyes 4 and 7.
O
O
R
R
Dla lmax (nm) Dlb lmax (nm) Dlc
lmax
(Cyclohexane) (nm) (Toluene) (nm) (Ethanol) (nm) (nm) (DMF)
H
H
X
N
X
X
N
Dye lmax (nm)
N
N
NH
N
N
N
4a
4b 381
4c 384
4d 359
357
þ27 384
þ7 388
þ6 390
ꢁ9 350
þ12 396
þ15 400
þ7 412
þ7 403
þ6 404
þ18 416
þ21 414
þ18 416
ꢁ30 354
ꢁ6 382
ꢁ4 386
ꢁ4 346
ꢁ3 393
ꢁ7 393
ꢁ2 410
ꢁ4 399
ꢁ2 402
ꢁ7 409
ꢁ9 405
ꢁ10 406
þ132 486
þ101 483
þ91 476
H3C
H3C
O
(a)
(b)
þ153 499
þ102 495
þ103 496
þ143 553
þ139 538
þ131 533
þ142 551
þ136 541
þ133 539
Y
4e
4f
7a
384
385
405
R
NH
N
N
N
7b 396
7c 398
7d 398
H3C
7e
7f
393
398
(c)
Dla
Dlc
¼
¼
lmax (Toluene)ꢁlmax (CycloHexane); Dlb
¼
lmax (Ethanol)ꢁlmax (Toluene);
Scheme 5. Protonation equilibrium of pyrazolone azo benzene dyes; (a) neutral dye
(b) ammonium tautomer (c) azonium tautomer.
lmax (DMF)ꢁlmax (Ethanol).
state grows; the amount of stabilization of the excited state relative
to the ground state with increasing solvent polarity is raised, and
thus the solvatochromism becomes more marked.
halochromism. In the case of dyes 4a and 7a, the easier protonation
of 3-methyl-1H-pyrazol-5(4H)-one derivatives produced more
positive halochromism than the other dyes.
However, in a few cases, negative solvatochromism between
ethanol and cyclohexane was observed within the general positive
trend; for example, dyes 4a and 4d, which contain 3-methyl-1H-
pyrazol-5(4H)-one in the coupling component ring and a 4-
fluorosulfonyl group in the diazo component ring. Most of their
ethanol solutions were slightly hypsochromically shifted compared
to those in toluene, although they showed overall positive
shifts between cyclohexane and ethanol (Table 4). Solvent polarity
factors are, therefore, more limited in heteroarylazo disperse
dyes containing a 3-methyl-1H-pyrazol-5(4H)-one; intramolecular
hydrogen bonding between the hydroxyl group at the pyrazolone
and azo nitrogen atoms and/or fluoro atoms in these dyes appears
to be the dominant influence. Also, the relatively hypsochromic
shifts in ethanol, compared with the less polar toluene, strongly
indicate the existence of intermolecular hydrogen bonding
between the dye molecules and the ethanol. Thus, the resultant
stabilization of the ground state by these two kinds of hydrogen-
bonding contribution could account for the hypsochromic shifts
3.3.3. Solvatochromic effects
Generally, in many dye molecules, the ground state is less polar
than the excited state, so a polar solvent tends to stabilize the
excited state more than the ground state. This leads to a bath-
ochromic shift in the absorption maximum, which is termed
positive solvatochromism. The interaction of a solvent with a dye
molecule is greater in polar solvents, for example DMF, and it is
most pronounced with a solute molecule that contains a perma-
nent dipole. It seems highly likely that the dyes have ionized in this
solvent, becoming anionic and it is not unusual for dyes in
a hydrazone form to behave in this way in a basic aprotic solvent
such as DMF. As the difference between the polarity of the ground
and excited states is increased by the successive introduction of
stronger electron-withdrawing groups into the diazo component
ring of heteroarylazo disperse dyes, more marked positive sol-
vatochromism is observed [36].
The spectral characterization of the dyes was evaluated with
respect to visible absorption properties in various solvents, and the
solvatochromic effects of dyes 4 and 7 are shown in Table 4. It is
clear that most synthesized dyes exhibit positive solvatochromism
in accordance with most donoreacceptor chromogens, so the
absorption bands of the dyes move toward longer wavelengths as
the polarity of the solvent increases. For example, the introduction
of stronger electron acceptors into the diazo component ring led to
greater differences between the lmax values for their cyclohexane
and DMF solutions. As the electron-withdrawing groups are
introduced, the gap between the polarity of the ground and excited
because the energy difference (DE) for the transition is increased.
4. Conclusions
In this study, a series of arylazopyrazolone disperse dyes con-
taining fluorosulfonyl groups in their structures were prepared
from the coupling reaction between pyrazolone couplers and 4-
fluorosulfonylanilines, and their thermal and spectral properties
were investigated and compared.
The fluorosulfonyl-substituted derivatives developed yellow to
orange-yellow colors that are hypsochromically shifted compared
with nitro-substituted analogs. The trends in the half-band width
and the molar extinction coefficient values of the synthesized dyes
can be explained by the mesomeric effect and by inductive effects
of the substituents.
Table 3
Halochromic effects of the dyes 4 and 7.
Dye
lmax (nm) (EtOH)
lmax (nm) (HCl/EtOH)
Dl (nm)
Most of the dyes show relatively high melting points for their
class that could be due to intramolecular hydrogen bonds, because
they hold the dye molecules in a plane configuration and van der
Waals forces help to achieve compact packing of the molecules. The
dyes that have not only intramolecular hydrogen bonding but also
intermolecular hydrogen bonding between adjacent molecules
showed higher melting points than those having only intra-
molecular hydrogen bonding. The more polar dyes that contain
more powerful electron-withdrawing groups tend to have higher
melting properties because of the increased polarizability and
dipole interactions; the 4-nitro-substituted dyes generally
4a
4b
4c
4d
4e
4f
7a
7b
7c
7d
7e
7f
354
382
385
346
393
393
408
398
402
409
411
412
403
384
386
347
395
399
418
403
404
415
413
413
þ49
þ2
þ1
þ1
þ2
þ6
þ10
þ5
þ2
þ6
þ2
þ1

586
A. Singh et al. / Dyes and Pigments 95 (2012) 580e586
[14] Sayed AZ, El-Gaby MSA. Synthesis of novel dyestuffs containing sulphonamido
moieties and their application on wool and polyamide fibres. Color Technol
2001;117(5):293e7.
[15] Towns AD. Developments in azo disperse dyes derived from heterocyclic
diazo components. Dyes Pigments 1999;42(1):3e28.
[16] Hallas G, Choi J-H. Synthesis and spectral properties of azo dyes derived from
2-aminothiophenes and 2-aminothiazoles. Dyes Pigments 1999;42(3):
249e65.
exhibited higher melting points than the 4-fluorosulfonyl-
substituted dyes and the 2-nitro-4-fluorosulfonyl(or nitro)-
substituted dyes showed higher melting points than the 4-
fluorosulfonyl(or nitro)-substituted dyes.
The substituents of the synthesized dyes affect the halochrom-
ism and solvatochromism to an extent that depends upon their
electron-accepting power. The synthesized dyes generally exhibi-
ted positive halochromism and positive solvatochromism, so the
absorption bands of the dyes moved toward longer wavelengths as
the acidity or polarity of the solvent increased. However, in a few
cases, negative solvatochromism was observed within the general
positive trend, such as in the dyes containing a hydroxyl substituent
at the pyrazolone unit; this feature has been attributed to intra-
molecular hydrogen bonding.
[17] Annen O, Egli R, Hasler R, Henzi B, Jakob H, Matzinger P. Replacement of
disperse anthraquinone dyes. Rev Prog Color 1987;17(1):72e85.
ꢀ
[18] Seferoglu Z, Ertan N. Synthesis and spectral properties of new hetarylazo
indole dyes. Russ J Org Chem 2007;43(7):1035e41.
ꢀ
[19] Seferoglu Z, Ertan N. Synthesis of some novel bis(hetaryl)azo disperse dyes
and investigation of their absorption spectra. Heteroatom Chem 2007;18(6):
622e30.
[20] Ertan N, Eyduran F. The synthesis of some hetarylazopyridone dyes and
solvent effects on their absorption spectra. Dyes Pigments 1995;27(4):
313e20.
[21] Ertan N. Synthesis of some hetarylazopyrazolone dyes and solvent effects on
their absorption spectra. Dyes Pigments 2000;44(1):41e8.
[22] Yen MS, Wang IJ. Synthesis and absorption spectra of hetarylazo dyes derived
from coupler 4-aryl-3-cyano-2-aminothiophenes. Dyes Pigments 2004;61(3):
243e50.
[23] Emandi A, Serban I, Bandula R. Synthesis of some new solvatochromic 1(4)-
substituted pyrazol-5-one azo derivatives. Dyes Pigments 1999;41(1e2):
63e77.
[24] Yen MS, Wang IJ. Synthesis and solvent characteristics of bishetaryl monoazo
dyes derived from polysubstituted-2-aminothiophene derivatives. Dyes
Pigments 2005;67(3):183e8.
[25] Schwander HR. Heterocyclic azo coupling components. Dyes Pigments 1982;
3(2e3):133e60.
[26] Basaif SA, Hassan MA, Gobouri AA. AlCl3-Catalyzed diazocoupling of 1-(aryl/
hetaryl)-3-phenyl-1H-pyrazol-2-in-5-ones in aqueous medium. Synthesis of
hetaryl-azopyrazolones and their application as disperse dyes. Dyes Pigments
2007;72(3):387e91.
[27] Yasuda H, Midorikawa H. The structure of 2-pyrazolin-5-one dyes. J Org Chem
1966;31(6):1722e5.
Acknowledgments
This paper was written as part of Konkuk University’s research
support program for its faculty on sabbatical leave in 2010.
This work was financially supported by the Ministry of Knowl-
edge Economy (MKE) and Korea Institute for Advancement in
Technology (KIAT) through the Workforce Development Program in
Strategic Technology.
This work was supported by National Research Foundation of
Korea Grant funded by the Korean Government (2009-0066635).
References
[1] McCarthy BJ, Burdett BC. Eco-labelling and textile eco-labelling. Rev Prog
Color 1998;28:61e70.
[2] Lewis DM. Coloration in the next century. Rev Prog Color 1999;29:23e8.
[3] Koh JS, Kim JP. Synthesis of phthalimide-based alkali-dischargeable azo
disperse dyes and analysis of their alkaliehydrolysis mechanism. Dyes
Pigments 1998;37(3):265e72.
[4] KohJS,KimJP. Applicationofphthalimide-basedalkali-clearableazodispersedyes
on polyester and polyester/cotton blends. J Soc Dyers Colour 1998;114(4):121e4.
[5] Koh J, Kim JD, Kim JP. Synthesis and application of a temporarily solubilised
alkali-clearable azo disperse dye and analysis of its conversion and hydrolysis
behaviour. Dyes Pigments 2003;56(1):17e26.
[6] Koh J, Greaves AJ. Synthesis and application of an alkali-clearable azo disperse
dye containing a fluorosulfonyl group and analysis of its alkali-hydrolysis
kinetics. Dyes Pigments 2001;50(2):117e26.
[28] Snavely FA, Yoder CH. Study of tautomerism in arylazopyrazolones and
related heterocycles with nuclear magnetic resonance spectroscopy. J Org
Chem 1968;33(2):513e6.
[29] Lycka A, Mustroph H. 15N- and 13C-N.M.R. study of azo-hydrazone tautom-
ꢁ
erism of 3-methyl-1-phenylpyrazole-4,5-dione 4-phenylhydrazone in
dimethyl sulphoxide and pyridine. J Prak Chem 1989;331(1):11e4.
[30] Lestina GJ, Regan TH. Determination of the azo-hydrazono tautomerism of
some 2-pyrazolin-5-one dyes by means of nuclear magnetic resonance
spectroscopy and 15N-labeled compounds. J Org Chem 1969;34(6):1685e6.
[31] Karci F, Ertan N. Hetarylazo disperse dyes derived from 3-methyl-1-(30,50-
dipiperidino-s-triazinyl)-5-pyrazolone as coupling component. Dyes
Pigments 2002;55(2e3):99e108.
[32] Whitaker A. Crystal structures of azo colorants derived from pyrazolone:
a review. J Soc Dyers Colour 1995;111(3):66e72.
[33] Cheon K-P, Park YS, Kazmaier PM, Buncel E. Studies of azoehydrazone
tautomerism and H-bonding in azo-functionalized dendrimers and model
compounds. Dyes Pigments 2002;53(1):3e14.
[7] Koh J, Ph. D. Thesis. Synthesis and application of alkali-clearable azo disperse
dyes containing a fluorosulfonyl group and analysis of their alkali-hydrolysis
kinetics. Seoul National University. 2002.
[8] Koh J, Kim JP. Alkali-hydrolysis kinetics of 4-amino-40-fluorosulfonylazo
benzene disperse dyes and their dyeing fastness properties. Dyes Pigments
2004;60(2):155e65.
[34] Hunger K. The effect of crystal structure on colour application properties of
organic pigments. Rev Prog Color 1999;29(1):71e84.
[35] Peters AT. Substituent effects on the colour, dyeing and fastness properties of
[9] Koh J, Greaves AJ, Kim JP. Synthesis and spectral properties of alkali-clearable
azo disperse dyes containing a fluorosulfonyl group. Dyes Pigments 2003;
56(1):69e81.
[10] Koh J, Kim JP. Colour fastness properties of alkali-clearable azo disperse dyes
containing a fluoro-sulphonyl group. Color Technol 2004;120(2):56e60.
[11] Koh J, Cho DH. Dyeing properties of 4-amino-40-fluorosulfonylazobenzene
dispersedyesonpoly(ethyleneterephthalate). FibersPolymers2004;5(2):134e8.
[12] Koh J. Alkali-clearable disperse dyeing of poly(ethylene terepthalate) with
azohydroxypyridone disperse dyes containing a fluorosulfonyl group. Dyes
Pigments 2006;69(3):233e8.
trisubstituted 4eNe
beHydroxyethyleNebeCyanoethylaminoazobenzenes.
J Soc Dyers Colour 1985;101(11):361e7.
[36] Griffiths J. Colour and constitution of organic molecules. London: Academic
Press; 1976.
[37] Sawicki E. Physical properties of the aminoazobenzene dyes. IV. the position
of proton addition. J Org Chem 1957;22(4):365e7.
[38] Gordon PF, Gregory P. Organic chemistry in colour. New York: Springer-
Verlag; 1983.
[39] Griffiths J, Hill J, Fishwick B. The application of PPP-MO theory to the hal-
ochromism of 4-aminoazobenzene dyes. Dyes Pigments 1991;15(4):307e17.
[40] Bello KA, Griffiths J. Violet to cyan azo dyes derived from 4-amino-3-nitto-
benzaldehyde as diazo component. Dyes Pigments 1989;11(1):65e76.
[13] Karci F, Karci F. Synthesis and absorption spectra of some novel heterocyclic
disazo dyes derived from pyridone and pyrazolone derivatives. Dyes Pigments
2008;76(1):147e57.