Synthesis, structural characterization and tautomeric properties of some novel bis-azo dyes derived from 5-arylidene-2,4-thiazolidinone

Article abstract of DOI:10.1016/j.saa.2014.02.010

Nine new bis-azo dyes derived from 5-arylidene-2,4-thiazolidinone have been synthesized in two steps using Knoevenagel condensation and diazotization-coupling reaction. The structures of the compounds were confirmed by UV-vis, IR, ¹H NMR and ¹³C NMR spectroscopic techniques. The spectral characterizations demonstrate that there is an equilibrium between the azo (T1) and hydrazine (T2 and T3) tautomers for all prepared dyes in solutions. In addition, the solvatochromic behavior of the prepared dyes was evaluated using polarity/polarizability parameter (π^*) in various solvents. The UV-vis absorption spectra of dyes show a bathochromic shift with increasing polarity and base strength of the solvents. Finally, the effects of acid and base on the UV-vis absorption spectra of the dyes with different substituent in diazo component are reported.

Full text of DOI:10.1016/j.saa.2014.02.010

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, structural characterization and tautomeric properties of some
novel bis-azo dyes derived from 5-arylidene-2,4-thiazolidinone
⇑
Asadollah Mohammadi , Mastaneh Safarnejad
Department of Chemistry, Faculty of Sciences, University of Guilan, PO Box 41335-1914, Rasht, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
The partial 1H NMR spectrum of synthesized dye in DMSO.
ꢀ
ꢀ
Nine novel bis-azo dyes were
synthesized and fully characterized.
Spectroscopic properties were
examined by UV–vis, FT-IR and NMR
techniques.
ꢀ
ꢀ
The azo-hydrazone tautomeric forms
of prepared dyes were investigated.
Effects of pH on the absorption
spectrum of dyes were studied.
a r t i c l e i n f o
a b s t r a c t
Article history:
Nine new bis-azo dyes derived from 5-arylidene-2,4-thiazolidinone have been synthesized in two steps
using Knoevenagel condensation and diazotization-coupling reaction. The structures of the compounds
were confirmed by UV–vis, IR, 1H NMR and 13C NMR spectroscopic techniques. The spectral characteriza-
tions demonstrate that there is an equilibrium between the azo (T1) and hydrazine (T2 and T3) tautomers
for all prepared dyes in solutions. In addition, the solvatochromic behavior of the prepared dyes was eval-
uated using polarity/polarizability parameter (pÃ) in various solvents. The UV–vis absorption spectra of
dyes show a bathochromic shift with increasing polarity and base strength of the solvents. Finally, the
effects of acid and base on the UV–vis absorption spectra of the dyes with different substituent in diazo
component are reported.
Received 10 September 2013
Received in revised form 4 January 2014
Accepted 2 February 2014
Available online 15 February 2014
Keywords:
Bis-azo dye
2,4-Thiazolidinonedione
Spectroscopy
Tautomerism
Ó 2014 Elsevier B.V. All rights reserved.
Solvent effect
Substituent effect
Introduction
more (polyazo) azo groups. Since their discovery in the 19th
century, azo compounds have been extensively used as colorants
Azo chromophores are a group of colorant materials character-
ized by the presence of an azo group (AN@NA) but can contain
two (disazo), three (trisazo), or, more rarely, four (tetrakisazo) or
and accounting for over 50% of all commercial dyes [1,2].
In addition to their use as colorants, they have been employed
for many applications, such as in ink jet printing [3], thermal trans-
fer printing [4], photography [5], color additives [6], biomedical
area [7], molecular recognition [8], light controlled polymers [9],
liquid crystal industry [10].
⇑
Corresponding author. Tel.: +98 131 7723677; fax: +98 131 7720733.
E-mail addresses: a_mohammadi@guilan.ac.ir, babak.mohamadiphd@gmail.com
(A. Mohammadi).
http://dx.doi.org/10.1016/j.saa.2014.02.010
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

106
A. Mohammadi, M. Safarnejad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
In addition, bis-azo dyes are a well known group of organic col-
were recorded on a Shimadzu 8400 FT-IR spectrophotometer. The
Proton nuclear magnetic resonance (1H NMR) spectra were ob-
tained on a FT-NMR (400 MHz) Brucker apparatus spectrometer,
and the chemical shifts are expressed in d ppm using TMS as an
internal standard. The visible spectra were measured using a Phar-
macia Biotech Spectrophotometer. The purity determination of the
substrates and reaction monitoring were accompanied by TLC
using silica gel SIL G/UV 254 plates.
orants, due to their versatile applications in various fields. Several
synthetic ways have been developed to synthesis bis-azo deriva-
tives. However, the common methods for synthesis of these classes
of dyes are the using of diazotization-coupling reactions by aro-
matic diamines or phenols [11–13].
On the other hand, azo dyes with hydroxyl group at ortho to azo
band display tautomerism depending on the proton transfer and
azo-hydrazone tautomers of these compounds have been exten-
sively studied. Determination and characterization of azo-hydra-
zone tautomers is interesting, since the tautomers have different
technical properties [14–16].
Thiazolidine and its derivatives as bioactive heterocycles play a
key role in medicinal chemistry and they have been extensively
used as scaffolds for drug development [17–19]. Thiazolidinedi-
ones show a wide variety of biological activities such as antifungal,
antibacterial, antiviral, antitumor, and antidiabetic potential
[20–25]. Furthermore, thiazolidine derivatives represent useful
synthetic building blocks in organic chemistry. However, there
are very few reports on the synthesis of bisazo dyes bearing thia-
zolidinedione moiety in the literature. In addition, synthesis and
application of such dyes is important in pharmaceutical, food, color
and other industries and a foremost task for chemists. According to
the above potent usefulness of the thiazolidinediones and in con-
tinuation of our studies on the synthesis of azo dyes [26], we report
herein the synthesis of some novel bis-azo dyes bearing thiazoli-
dine moiety in order to evaluate structure, solvatochorism and
tautomeric properties (Scheme 1).
Synthesis of 5-(3-hydroxybenzylidene)-2,4-thiazolidinedione (1)
A mixture of m-hydroxy benzaldehyde (0.336 g, 3 mmol) and
2,4-thiazolidinonedione (0.351 g, 3 mmol) with catalytic quantity
of piperidine was refluxed in ethanol for 4–5 h and then cooled
to 0 °C in an ice bath. After that HCl (0.5 M) and water are added
and the precipitate is filtered and washed with water and petro-
leum ether. The solid product was isolated by recrystallization
from EtOH/H2O.
Light Yellow solid; yield 96%; m.p. 273–274 °C; IR (KBr)
m :
cmÀ1
3300 (OH), 3180 (NH), 3063 (@CAH), 1742 (C@O), 1686 (C@O),
1610 (C@C); 1H NMR (400 MHz, DMSO-d6), d (ppm): 6.88 (d, 1H,
J = 8.0 Hz, ArAH), 6.99 (s, 1H, ArAH), 7.04 (d, 1H, J = 8.0 Hz, ArAH),
7.34 (t, 1H, J = 8.0 Hz, ArAH), 7.70 (s, 1H, CH@C), 9.85 (s, 1H, OH),
12.61 (s, 1H, NH) ppm.
Synthesis of bis-azo dyes (3a–i)
Diazotization
The aryl diazonium salts were prepared in good yield from equi-
molar mixture of aniline derivatives (1a–i) (6 mmol) and sodium
nitrite (0.414 g, 6 mmol) according to previously described method
[26].
Experimental
Materials and apparatus
All compounds used in this study were obtained from Merck
and Aldrich Chemical Companies and were used without further
purification. All melting points were determined on an Electrother-
mal melting point apparatus and are uncorrected. Infrared spectra
Coupling
The prepared diazonium salts solution was slowly added to a
stirred solution of 5-(3 hydroxybenzylidene)-2,4-thiazolidinedione
(1) (0.663 g, 3 mmol) as coupling component in alkali medium by
Scheme 1. General synthetic route for synthesis of dyes 3a–3i.

A. Mohammadi, M. Safarnejad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
107
adjusting the pH at 8–9 and the temperature was maintained at 0–
Dye 3f. Dark Red solid; yield 83%; m.p. 244–245 °C; IR (KBr)
m
cmÀ1: 3300 (OH), 3112 (NH), 3040 (@CAH), 1744 (C@O), 1700
(C@O), 1680 (C@O), 1610 (C@C), 1504 (N@N); 1H NMR (400 MHz,
CDCl3), d (ppm): 2.39 (s, 3H, CH3), 2.48 (s, 3H, CH3), 6.75 (d, 1H,
J = 9.6 Hz, ArAH, HA), 7.25 (d, 2H, J = 8.4 Hz, ArAH, HD), 7.36 (m,
4H, ArAH, HC and HE), 7.45 (d, 2H, J = 8.0 Hz, ArAH, HF), 7.73 (d,
1H, J = 9.6 Hz, ArAH, HB), 12.26 (s, 1H, NH), 16.08 (br s, 0.67H,
NANH). 13C NMR (CDCl3), d (ppm): 26.7, 26.9, 119.8, 129.4,
130.0, 131.9, 133.5, 136.0, 141.4, 145.6, 161.4, 162.6, 164.3,
170.5, 171.9, 178.2, 178.7.
5 °C. The resulting solution was stirred for 2–3 h in an ice bath then
allowed to reach room temperature. After 1 h, the pH value of the
diazo liquor was adjusted to pH 4–4.5 by adding a diluted hydro-
chloric acid solution (0.5 M). The precipitated colored solids were
filtered with suction, washed several times with cold water and
purified by further recrystallization from DMF/H2O, and finally
dried in desiccator over anhydrous CaCl2 to give the corresponding
bis-azo dyes 3a–i (Scheme 1). However, the amounts of formed by-
products are trace, and cannot be isolated in the purification proce-
dure. The purity and structure of prepared dyes was confirmed by
physical and spectroscopic data.
Dye 3g. Dark Red solid; yield 90%; m.p. 249–250 °C; IR (KBr)
m
cmÀ1: 3300 (OH), 3155 (NH), 3052 (@CAH), 1742 (C@O), 1715
(C@O), 1700 (C@O), 1680 (C@O), 1610 (C@C), 1502 (N@N); 1H
NMR (400 MHz, DMSO-d6), d (ppm): 2.08 (s, 3H, CH3), 2.12 (s,
3H, CH3), 6.75 (d, 1H, J = 9.2 Hz, ArAH, HA), 7.48 (m, 3H, ArAH,
HD and CH@C), 7.70 (d, 2H, J = 8.8 Hz, ArAH, HF), 7.84 (m, 4H,
ArAH, HC and HE), 9.86 (s, 0.2H, OH), 10.14 (s, 1H, CONH), 10.31
(s, 1H, CONH), 12.12 (br s, 0.42H, CONHCO), 12.34 (br s, 0.50H,
CONHCO), 15.77 (br s, 0.35H, NANH), 16.36 (br s, 0.35H, NANH).
13C NMR (DMSO-d6), d (ppm): 30.3, 31.2, 36.2, 117.0, 125.5,
126.0, 127.9, 130.3, 131.1, 134.4, 143.6, 147.5, 148.0, 162.7,
172.4, 182.6.
Dye 3a. Red solid; yield 78%; m.p. 266–268 °C; IR (KBr)
3350 (OH), 3200 (NH), 3080 (@CAH), 1744 (C@O), 1690 (C@O),
1610 (C@C), 1505 (N@N); 1H NMR (400 MHz, DMSO-d6),
(ppm): 6.70 (dd, 1H, J = 5.2 Hz, ArAH, HA), 7.29 (m, 2H, ArAH,
HD), 7.47 (m, 2H, ArAH, HF), 7.63 (m, 4H, ArAH, HC and HE), 7.87
(d, 1H, J = 9.6 Hz, ArAH, HB), 8.32 (s, 1H, CH@C), 9.86 (s, 0.44H,
OH), 12.23 (s, 1H, NH), 15.73 (br s, 0.56H, NANH). 13C NMR
(DMSO-d6), d (ppm): 46.5, 116.8, 117.0, 119.6, 129.0, 129.1,
129.7, 131.4, 132.8, 134.9, 139.2, 144.6, 159.2, 161.6, 161.7,
164.1, 171.3, 172.6, 178.7, 179.0.
m :
cmÀ1
d
Dye 3b. Red solid; yield 82%; m.p. 239–240 °C; IR (KBr)
3300 (OH), 3184 (NH), 3060 (@CAH), 1746 (C@O), 1684 (C@O),
1610 (C@C), 1503 (N@N); 1H NMR (400 MHz, DMSO-d6),
(ppm): 6.72 (dd, 1H, J = 4.8 Hz, ArAH, HA), 7.44 (d, 2H, J = 8.8 Hz,
ArAH, HD), 7.59 (d, 4H, J = 8.4 Hz, ArAH, HC and HE), 7.70 (d, 2H,
J = 8.4 Hz, ArAH, HF), 7.88 (d, 1H, J = 9.6 Hz, ArAH, HB), 7.96 (s,
1H, CH@C), 9.86 (s, 0.21H, OH), 12.25 (s, 1H, NH), 15.63 (br s,
0.79H, NANH). 13C NMR (DMSO-d6), d (ppm): 116.3, 119.1, 119.5,
129.4, 129.9, 130.4, 132.0, 134.8, 140.6, 144.2, 158.8, 160.2,
161.9, 164.1, 167.6, 170.3, 178.2, 179.1.
m :
cmÀ1
Dye 3h. Red solid; yield 88%; m.p. 253–254 °C; IR (KBr)
3300 (OH), 3118 (NH), 3050 (@CAH), 1744 (C@O), 1684 (C@O),
1610 (C@C), 1502 (N@N); 1H NMR (400 MHz, DMSO-d6),
(ppm): 2.58 (s, 3H, CH3), 2.68 (s, 3H, CH3), 6.71 (d, 1H, J = 10 Hz,
ArAH, HA), 7.66–7.75 (m, 4H, ArAH, HC and HE), 7.93 (d, 1H,
J = 9.6 Hz, ArAH, HB), 8.01(d, 2H, J = 8.4 Hz, ArAH, HD), 8.19 (d,
2H, J = 7.6 Hz, ArAH, HF), 8.34 (s, 1H, CH@C), 9.85 (s, 0.26H, OH),
12.38 (br s, 1H, NH), 15.53 (br s, 0.57H, NANH). 13C NMR
(DMSO-d6), d (ppm): 37.0, 37.7, 121.4, 124.8, 128.0, 129.1, 130.8,
131.5, 136.4, 143.5, 147.9, 148.7, 161.0, 174.9, 181.3, 193.2, 193.6.
m :
cmÀ1
d
d
Dye 3c. Red solid; yield 86%; m.p. 233–234 °C; IR (KBr)
m :
cmÀ1
Dye 3i. Light Red solid; yield 79%; m.p. 299–300 °C; IR (KBr)
m
3114 (NH), 3045 (@CAH), 1744 (C@O), 1704 (C@O), 1608 (C@C),
1506 (N@N); 1H NMR (400 MHz, DMSO-d6), d (ppm): 6.72 (dd,
1H, J = 4.4 Hz, ArAH, HA), 7.52 (d, 4H, J = 8.4 Hz, ArAH, HC and
HE), 7.59 (d, 2H, J = 8.4 Hz, ArAH, HD), 7.84 (d, 2H, J = 8.4 Hz, ArAH,
HF), 7.90 (d, 1H, J = 10 Hz, ArAH, HB), 9.86 (s, 0.37H, OH), 12.29 (s,
1H, NH), 15.66 (br s, 0.63H, NANH). 13C NMR (DMSO-d6), d (ppm):
115.9, 117.4, 118.6, 129.2, 129.3, 129.8, 130.9, 132.8, 135.1, 141.6,
143.6, 157.0, 157.6, 159.4, 161.1, 166.2, 170.9, 174.9, 178.5.
cmÀ1: 3200–3440 (OH, NH), 3066 (@CAH), 1740 (C@O), 1700
(C@O), 1680 (C@O), 1598 (C@C), 1510 (N@N); 1H NMR (400 MHz,
DMSO-d6), d (ppm): 6.70 (d, 1H, J = 9.6 Hz, ArAH, HA), 7.70 (d,
2H, J = 8.0 Hz, ArAH, HC), 7.88 (d, 2H, J = 8.8 Hz, ArAH, HE), 7.93
(d, 1H, J = 9.6 Hz, ArAH, HB), 7.96 (s, 1H, CH@C), 8.24 (d, 2H,
J = 8.8 Hz, ArAH, HD), 8.49 (d, 2H, J = 8.8 Hz, ArAH, HF), 9.85 (s,
0.12H, OH), 12.39 (br s, 1H, NH), 15.23 (br s, 0.74H, NANH). 13C
NMR (DMSO-d6), d (ppm): 31.2, 36.2, 121.8, 122.7, 123.2, 124.8,
126.6, 127.2, 127.6, 133.1, 147.3, 150.1, 162.7.
Dye 3d. Dark Red solid; yield 76%; m.p. 260–261 °C; IR (KBr)
m
cmÀ1: 3190 (NH), 3052 (@CAH), 1744 (C@O), 1700 (C@O), 1612
(C@C), 1512 (N@N); 1H NMR (400 MHz, DMSO-d6), d (ppm): 6.70
(dd, 1H, J = 4.4 Hz, ArAH, HA), 7.52 (d, 4H, J = 8.4 Hz, ArAH, HC
and HE), 7.58 (d, 2H, J = 8.4 Hz, ArAH, HD), 7.70 (s, 1H, CH@C),
7.84 (d, 2H, J = 8.0 Hz, ArAH, HF), 7.89 (d, 1H, J = 9.6 Hz, ArAH,
HB), 9.86 (s, 0.39H, OH), 12.28 (s, 1H, NH), 15.62 (br s, 0.61H,
NANH). 13C NMR (DMSO-d6), d (ppm): 110.3, 110.7, 116.0, 116.5,
118.2, 128.5, 129.8, 132.8, 134.2, 138.8, 144.6, 159.5, 166.3,
169.4, 169.9, 178.1, 178.7.
Results and discussion
Synthesis
Knoevenagel condensation reaction of commercially available
2,4-thiazolidinedione as acidic component with 3-hydroxybenzal-
dehyde in refluxing ethanol in the presence of piperidine catalyst
affords excellent yield of the 5-(3-hydroxybenzylidene)thiazoli-
dine-2,4-dione (1) according to the literature methods [27]
(Scheme 1, step 1).
Dye 3e. Dark Red solid; yield 86%; m.p. 264–265 °C; IR (KBr)
m
cmÀ1: 3112 (NH), 3048 (@CAH), 1744 (C@O), 1700 (C@O), 1680
(C@O), 1605 (C@C), 1508 (N@N); 1H NMR (400 MHz, DMSO-d6), d
(ppm): 2.33 (s, 3H, OCH3), 2.43 (s, 3H, OCH3), 6.72 (dd, 1H,
J = 5.2 Hz, ArAH, HA), 7.25 (d, 2H, J = 7.6 Hz, ArAH, HD), 7.42 (br s,
6H, ArAH, HC, HE and HF), 7.86 (d, 1H, J = 9.6 Hz, ArAH, HB),
12.23 (s, 1H, NH), 15.72 (br s, 0.44H, NANH), 16.25 (br s, 0.56H,
NANH). 13C NMR (DMSO-d6), d (ppm): 59.1, 59.8, 119.1, 127.7,
128.2, 130.4, 132.2, 132.9, 136.9, 140.3, 145.4, 161.2, 161.6,
161.8, 164.8, 170.1, 178.0, 178.7.
The treatment of prepared different aryl diazonium salts with
5-(3-hydroxybenzylidene) thiazolidine-2,4-dione as key interme-
diate in alkaline medium provided novel bis-azo dyes 3a–i in good
yields (Scheme 1, step 2).
The structure of the synthesized intermediate 1 was confirmed
by the IR, 1H NMR spectroscopy. The infrared spectra of intermediate
1 showed the presence two carbonyl groups around 1722 cmÀ1 and
1686 cmÀ1 along with NAH and OAH groups at 3180–3300 cmÀ1
region as overlapped. The 1H NMR spectra of compound 1 displayed

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A. Mohammadi, M. Safarnejad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
the presence of vinyl, hydroxyl and amino protons around d 7.70 (s,
1H, CH@C), 9.85 (br s, 1H, OH) and 12.61 (br s, 1H, NH) ppm,
respectively.
The UV–visible spectra and solvatochromic studies
In order to study of solvent effects on spectral features of the
The structures of the bis-azo dyes 3a–i were confirmed by the
IR, 1H and 13C NMR spectroscopy. The 1H NMR spectra of all the
dyes exhibited significant signals at d 15.23–16.36 and around
9.85 ppm, which can be attributed to the imine NAH proton reso-
nance (hydrazone form) and the hydroxyl OAH proton resonance
(azo form), respectively. In addition, 13C NMR spectra showed three
and/or four C@O signals around at 170 ppm as a proof for the pres-
ence of a hydrazo group for all dyes in the solution. These data are
in agreement with those previously reported for similar com-
pounds [28]. Typical 1H NMR spectra of dye 3a is shown in Fig. 1.
On the other hand, IR and NMR spectra and UV–visible absorp-
tion spectroscopy indicated that the prepared bis-azo compounds
existed in three tautomeric forms namely, azo-enol-azo (T1), hy-
drazo-keto-azo (T2) and azo-keto-hydrazo (T3), but the other tau-
tomeric forms are possible. The tautomeric forms of dyes are
shown in Scheme 2. Examination of all spectroscopic data reveals
that the equilibrium between T1 and T2 or T3 tautomeric forms
is predominantly shifted to the T2 and/or T3. For example, the
dye 3h exists in 65% in hydrazo form and 35% in azo form in solu-
tion of DMSO (Fig. 2).
bis-azo dyes 3a–i, we recorded their absorption spectra in twelve
solvents with different polarity at
a –
concentration of 10À4
10À5 M in the range of 200–800 nm at room temperature. The re-
sults are shown in Table 1. Typical UV–vis absorption spectra of
dyes 3a and 3h in selected solvents are shown in Figs. 3 and 4,
respectively.
As seen in Table 1, the two absorption maxima (kmax) were ob-
served in the range of 322–336 and 478–512 nm, in which attrib-
uted to T1 (azo) and T2 and/or T3 (hydrazone) tautomeric forms.
However, the shape and intensity of both the azo and hydrazo tau-
tomeric forms in all the dyes depend on the solvent–solute interac-
tions and solvent nature as well as on the substituent type. As
shown in Fig. 3, the UV–vis absorption spectra of the dye 3a in po-
lar and highly polar solvents such as DMF and DMSO showed a
main band at about 280–380 nm, assigned to the azo tautomeric
form and a very weak band at 440–540 nm, in which attributed
to hydrazone tautomeric form. However, in protic and chlorinated
solvents, the two main bands at about 280–380 nm and 440–
540 nm were observed, assigned to the tautomeric azo-hydrazo
equilibria.
The UV–vis absorption spectra of all the synthesized dyes
showed two strong bands at about 280–360 and 450–550 nm, in
The absorption spectra of all the investigated dyes except 3a
showed two main bands in the protic and non protic solvents as
for the dye 3h (Fig. 4). However, the intensity of bands in absorp-
tion spectra changes in relation to the type of substituent on the
diazo component.
In addition, the UV–vis absorption spectra (kmax) of the synthe-
sized dyes are influenced by the solvents with different solvato-
chromic effects, which is due to the interaction between the
which attributed to n ? pà and/or
p ?
pà electronic transitions
of azo (T1) and hydrazo (T2 and T3) tautomeric forms, respectively.
The UV–vis, IR, 1H and 13C NMR spectra also showed the important
signals at their expected positions for all prepared bis-azo dyes.
Thus, the chemical structures of bis-azo dyes were confirmed as
shown in Scheme 1.
Fig. 1. 1H NMR spectra of dye 3a.

A. Mohammadi, M. Safarnejad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
109
Scheme 2. The equilibrium between the azo form (T1) and the hydrazone forms (T2 and T3) of prepared dyes.
Fig. 2. The partial 1H NMR spectrum of dye 3h in DMSO.
solute and solvents. Therefore an increase in the polarity of se-
lected solvents has caused a slightly positive bathochromic shift
in the absorption maxima of dyes 3a–3i. For example, in the
absorption spectrum of dye 3a, the kmax is shifted from kmax = 478 -
In order to study of acid and base effects on the spectral charac-
teristics, dyes 3e with electron-donating group (R@OCH3), and 3h
including electron-withdrawing group (R@COCH3) were selected.
The pH values of the selected dyes were also determined spectro-
photometrically in 80% (v/v) methanol–water mixtures according
to the previously described method [26a,29]. The absorption max-
ima and spectra recorded for dye 3e and 3h are given in Table 2
and Figs. 5 and 6. As shown in Table 2 and Fig. 5, there were no sig-
nificant changes in shapes and wavelengths of dye 3e with elec-
nm to kmax = 494 nm (Dkmax = 10 nm) by changing the solvent from
diethyl ether (pà = 0.27) to DMSO (pà = 1.00). Besides the effects of
the substituents and solvent polarity on the absorption spectra,
azo-hydrazone tautomerism was studied and it was concluded that
equilibrium depends on the substituents as well as the solvents.

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A. Mohammadi, M. Safarnejad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
Table 1
Experimental electronic absorption maxima for dyes 3a–i and solvent dipolarity/polarizability parameter (pÃ) [28].
Solvents
pÃ
3a
3b
3c
3d
3e
3f
3g
3h
3i
Diethyl ether
Butanol
Ethyl acetate
Ethanol
0.27
0.47
0.54
0.54
0.55
0.58
0.60
0.64
0.71
0.82
0.87
1.00
322,478
326,484
322,482
326,488
324,484
328,486
326,486
326,488
324,492
326,484
328,488
326,494
320,486
326,490
324,486
326,490
324,488
328,490
324,490
326,490
324,500
328,490
322,498
322,498
322,486
332,486
332,480
330,484
332,484
334,486
324,488
332,488
326,478
336,484
326,494
332,484
322,490
326,494
324,492
326,494
326,492
330,494
324,492
326,496
326,504
328,492
328,496
332,504
326,492
336,498
328,492
334,502
326,496
330,494
332,498
336,510
334,508
332,498
332,500
332,504
324,486
328,488
324,486
328,490
326,490
330,490
326,490
330,494
328,484
328,488
328,494
328,496
324,500
326,500
324,502
326,504
324,502
326,504
326,502
330,506
328,504
328,504
330,512
328,512
330,490
332,494
330,488
330,494
332,492
334,492
330,492
332,492
330,490
334,496
334,498
334,498
332,488
330,486
334,486
334,490
332,484
332,492
332,488
334,488
330,490
334,492
330,492
326,498
Dioxane
Chloroform
Methanol
Glacial acetic acid
Acetone
Dichloromethane
DMF
DMSO
Fig. 3. Absorption spectra of dye 3a in selected solvents.
Fig. 5. Absorption spectra of dye 3e at different pH values.
Fig. 4. Absorption spectra of dye 3h in selected solvents.
Fig. 6. Absorption spectra of dye 3h at different pH values.
8.0. Therefore, the prepared bis-azo dyes with electron-withdraw-
ing substituent such as 3h can be used as new pH indicators.
As seen in Table 1 and Fig. 7, the absorption spectra of dyes 3e
and 3g with electron-donating groups (R@OCH3, NHCOCH3) show a
bathochromic shift in all used solvents when compared with dye 3i
including electron-withdrawing group (R@NO2). This result may be
explained by the fact that the maximum absorption (kmax) values
of the hydrazone tautomeric form of azo dyes with electron-with-
drawing groups show shorter wavelengths in relation to hydrazone
tron-donating group within the pH range of 6.0–13.0. However,
changes were observed in the intensity of bands in very acidic
solutions. The shape, intensity and wavelengths of spectra for
dye 3h with electron-withdrawing group changed upon increasing
the pH from 7.0 to 13.0, while no significant change in shape and
absorption maxima of the spectra occurred from pH 1.0 to 7.0 (Ta-
ble 2 and Fig. 6). As a result, significant changes were observed for
dye 3h including electron-withdrawing group between pH 7.0 and
Table 2
Absorption maxima (kmax) of dyes 3e and 3h at different pH values.
Dye No.
kmax (nm)
pH = 2
pH = 4
pH = 6
pH = 7
pH = 8
pH = 9
pH = 10
pH = 11
pH = 12
pH = 13
3e
3h
322,500
330,490
318,500
330,490
334,498
330,492
332,506
330,492
334,502
330,494
334,504
326,480
332,504
324,476
328,506
322,472
330,504
322,470
332,504
320,456

A. Mohammadi, M. Safarnejad / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 126 (2014) 105–111
111
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.02.010.