New 1,2,4-triazole-based azo-azomethine dyes. Part I: Synthesis, characterization and spectroscopic studies

Article abstract of DOI:10.1016/j.saa.2011.09.053

Four new 1,2,4-triazole-based azo-azomethine dyes were synthesized via condensation of 3,5-diamino-1,2,4-triazole with azo-coupled o-vanillin precursors. The prepared dyes were characterized by IR, UV-vis and ¹H NMR spectroscopic methods as wel

Full text of DOI:10.1016/j.saa.2011.09.053

Spectrochimica Acta Part A 86 (2012) 39–43
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
New 1,2,4-triazole-based azo-azomethine dyes. Part I:
Synthesis, characterization and spectroscopic studies
Hamid Khanmohammadi∗, Malihe Erfantalab
Faculty of Science, Department of Chemistry, Arak University, Arak 38156-8-8349, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new 1,2,4-triazole-based azo-azomethine dyes were synthesized via condensation of 3,5-diamino-
1,2,4-triazole with azo-coupled o-vanillin precursors. The prepared dyes were characterized by IR, UV–vis
and 1H NMR spectroscopic methods as well as elemental analyses. Thermal properties of the prepared
dyes were examined by thermogravimetric analysis. Results indicated that the framework of the dyes
was stable up to 225 ◦C. Also, the influence of various factors including time and mixed DMSO/EtOH
solution on UV–vis spectra of the dyes were investigated.
Received 15 July 2011
Received in revised form
19 September 2011
Accepted 27 September 2011
Keywords:
© 2011 Elsevier B.V. All rights reserved.
Azo-azomethine
Azo–hydrazone tautomer
Solvatochromism
1,2,4-Triazole
Thermal properties
1. Introduction
In our ongoing interest in the synthesis and spectral studies of
heterocycle-based Schiff base compounds [23,24] and with regard
In recent years, much attention has been devoted to azo dyes
in research areas due to their interesting electronic properties
and applications [1,2]. Azo dyes have been extensively used as
dyestuffs for wool, leather and synthetic fabrics because of their
extraordinary coloring properties and in photonic devices, electro-
optic modulators, components of optical communication systems
because of their second order nonlinear optical properties [3–8].
Heterocycle moieties, such as pyridazine, pyridine and triazoles,
have been used as significant components in new synthetic azo
dyes to enhance their color strength and brilliant shades during
recent years [9–11].
to the considerable recent interest in azo-azomethine compounds
[25,26], that are known to be particularly suited to produce new
dyes, we decided to investigate the possibility of incorporating
the 1,2,4-triazole moiety into azo-coupled o-vanillin derivatives.
We report here the synthesis and characterization of new family
of acyclic azo-containing 1,2,4-triazole-based Schiff base com-
pounds, Ib–IVb, by condensation reaction of azo-coupled o-vanillin
derivatives with 3,5-diamino-1,2,4-triazole (Fig. 1). The thermal
properties of the prepared azo dyes were examined by thermo-
gravimetric analysis which indicated that all dyes were stable up
to 225 ◦C.
Among the plethora of new synthetic azo dyes those contain-
ing 1,2,4-triazoles are of particular interest for several reasons.
1,2,4-Triazoles are stable compounds with rich and well-known
chemistry [12,13]. The incorporation of 1,2,4-triazole into macroa-
cyclic ligands has facilitated the isolation of a wide range of
transition metal complexes with interesting electrochemical and
magnetic properties [14–20]. Also, their electronic properties make
them suitable for surface enhanced resonance Raman scattering
(SERRS) studies [21,22]. These diversified applications are at least in
part a consequence of the numerous approaches that are available
for the insertion of specific functionalities into the triazole nucleus.
2. Experimental
2.1. Materials
All of the reagents and solvents involved in synthesis were of
analytically grade and used as received without further purifi-
cation. o-Vanillin, 2-nitroaniline, 4-chloroaniline, 4-ethylaniline,
2,4-dichloroaniline and 3,5-diamino-1,2,4-triazole were obtained
from Aldrich and Merck.
2.2. Instrumentation
The structures of all synthesized compounds were confirmed
by 1H NMR spectra, recorded on a Bruker AV 300 MHz spec-
trometer. FT-IR spectra were recorded as pressed KBr discs, using
∗
Corresponding author. Tel.: +98 861 2777401; fax: +98 861 4173406.
E-mail address: h-khanmohammadi@araku.ac.ir (H. Khanmohammadi).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.09.053

40
H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 86 (2012) 39–43
1H), 11.16 (br, 1H). IR (KBr, cm−1); 3100–3380 (OH), 1668 (CHO),
1609, 1591, 1487, 1427 (N N), 1265 (C–O), 941 (N N bending),
831.
2.3.1.4. 2-Hydroxy-3-methoxy-5-(2,4-dichlorophenylazo)
benzaldehyde, IVa. Brown solid. Yield: 86%, m.p. = 184–185 ◦C.
1H NMR (d6-DMSO, ppm): ı 3.84 (s, 3H), 7.49 (m, 1H), 7.52 (d,
1H), 7.68 (d, 1H, J = 2.50 Hz), 7.79 (s, 1H), 7.81 (s, 1H), 10.30 (s, 1H).
IR (KBr, cm−1); 3250–3500 (OH), 1701, 1651 (CHO), 1609, 1574,
1487, 1456 (N N), 1246 (C–O), 968 (N N bending), 881.
2.3.2. General procedure for the synthesis of azo-azomethine
dyes, Ib–IVb
A suspension of 3,5-diamino-1,2,4-triazole (0.1 g, 1 mmol) in
absolute EtOH (10 mL) was added to a stirring solution of azo-
coupled precursors, Ia–IVa, (2 mmol) in absolute EtOH (50 mL)
during a period of 10 min at 60–70 ◦C. The mixture was heated in
water bath for 15 h at 80 ◦C with stirring. The mixture was filtered
whilst hot and the obtained solid was washed with hot ethanol
(three time) and then with diethylether. The resulted product was
dried in air.
Fig. 1. 1,2,4-Triazole-based azo-azomethine dyes.
2.3.2.1. 3,5-Bis(4-((2-nitro-phenylazo)-phenol)-2-
methoxyhydroxyphenyl)-1H-1,2,4-triazole, Ib. Red solid. Yield:
53.41%, m.p. = 273–274 ◦C. 1H NMR (d6-DMSO, ppm): ı 3.92 (s, 6H),
7.51 (s, 2H), 7.71 (m, 4H), 7.84 (m, 2H), 8.09 (d, 2H, J = 8.02 Hz), 8.15
(s, 2H), 9.61 (s, 2H), 14.31 (br, 1H). IR (KBr, cm−1); 3286 (NH), 1607
(C N), 1576, 1532, 1464 (N N), 1404, 1344 (NO2), 1273 (C–O),
1128 and 744. Anal. Calcd. for C30H23N11O8. 0.5 C2H5OH: C, 54.07;
N, 22.36; H, 3.77. Found: C, 53.86; N, 22.60; H, 3.71%.
Unicom Galaxy Series FT-IR 5000 spectrophotometer in the region
of 400–4000 cm−1. Melting points were determined on Electrother-
mal 9200 apparatus. Thermal analysis was performed on a TGA
V5.1A DuPont 2000 and Perkin-Elmer Thermogravimetric Analyzer
TG/DTA 6300 instruments. C. H. N. analyses were performed on a
Vario EL III elemental analyzer. Electronic spectral measurements
were carried out using Perkin-Elmer Lamda spectrophotometer in
the range 200–700 nm.
2.3.2.2. 3,5-Bis(4-((4-ethyl-phenylazo)-phenol)-2-
methoxyhydroxyphenyl)-1H-1,2,4-triazole, IIb. Yellow solid. Yield:
51.3%, m.p. = 296–297 ◦C. 1H NMR (d6-DMSO, ppm): ı 1.23 (t,
6H, J = 7.56 Hz), 2.69 (q, 4H, J = 7.56 Hz), 3.95 (s, 6H), 7.41 (d, 4H,
J = 8.32 Hz), 7.61 (s, 2H, J = 1.83 Hz), 7.80 (d, 4H, J = 8.32 Hz), 8.11 (d,
2H, J = 1.83 Hz), 9.60 (s, 2H), 14.25 (br, 1H). IR (KBr, cm−1); 3308
(NH), 1605 (C N), 1579, 1528, 1464 (N N), 1398, 1265 (C–O),
1124, and 837. Anal. Calcd. for C34H33N9O4: C, 64.65; N, 19.96; H,
5.27. Found: C, 64.60; N, 20.33; H, 5.32%.
2.3. Synthesis
2.3.1. General procedure for the synthesis of azo-coupled
o-vanillin precursors
Azo-coupled o-vanillin precursors, Ia–IVa, were prepared
according to the well known literature procedure [24,27].
o-Vanillin (1.52 g, 10 mmol) was dissolved in water (20 mL) con-
taining 0.4 g (10 mmol) of sodium hydroxide and 4.24 g (40 mmol)
of sodium carbonate during the period of 30 min at 0 ◦C. The result-
ing solution was added slowly to a solution of diazonium chloride
(10 mmol) in water at 0–5 ◦C. The reaction mixture was stirred for
1 h at 0 ◦C and then allowed to warm slowly to room temperature.
The product was collected by filtration and washed with 100 mL of
NaCl solution (10%) under vacuum. The obtained solid was dried in
air.
2.3.2.3. 3,5-Bis(4-((4-chloro-phenylazo)-phenol)-2-
methoxyhydroxyphenyl)-1H-1,2,4-triazole,
IIIb. Orange
solid.
Yield: 71.4%, m.p. >300 ◦C. 1H NMR (d6-DMSO, ppm): ı 3.75 (s),
3.92 (s), 4.05 (s), 6.37 (s), 7.31 (br), 7.52 (m), 7.58 (m), 7.71 (m), 7.83
(m), 8.06 (s), 8.15 (s), 9.30 (s), 9.63 (s), 10.18 (s). IR (KBr, cm−1);
3304 (NH), 1709, 1606 (C N), 1577, 1533, 1465 (N N), 1398, 1334,
1267 (C–O), 1128 and 833. Anal. Calcd. for C30H23Cl2N9O4. 0.5
C2H5OH: C, 55.78; N, 18.89; H, 3.89. Found: C, 55.64; N, 18.91; H,
3.74%.
2.3.1.1. 2-Hydroxy-3-methoxy-5-(2-nitrophenylazo)benzaldehyde,
Ia. Orange solid. Yield: 90%, m.p. = 168–170 ◦C. 1H NMR (d6-DMSO,
ppm): ı 3.93 (s, 3H), 7.57 (d, 1H, J = 2.00 Hz), 7.70 (m, 2H), 7.83
(m, 2H), 8.08 (d, 1H, J = 7.94 Hz), 10.37 (s, 1H), 11.29 (br, 1H). IR
(KBr, cm−1); 3250–3470 (OH), 1653 (CHO), 1610, 1585, 1516, 1456
(N N), 1340 (NO2), 1267 (C–O), 958 (N N bending).
2.3.2.4. 3,5-Bis(4-((2,4-dichloro-phenylazo)-phenol)-2-
methoxyhydroxyphenyl)-1H-1,2,4-triazole,
IVb. Orange
solid.
Yield: 59.03%, m.p. >300 ◦C. 1H NMR (d6-DMSO, ppm): ı 3.91 (s),
6.29 (s), 7.55 (br), 7.66 (m), 7.85 (m, br), 8.07 (s), 9.28 (s), 10.36
(s), 12.23 (br). IR (KBr, cm−1); 3304 (NH), 1709 (C O), 1607 (C N),
1578, 1549, 1465 (N N), 1402, 1365, 1267 (C–O), 1136, 1099 and
875. Anal. Calcd. for C30H21Cl4N9O4: C, 50.51; N, 17.67; H, 2.97.
Found: C, 50.81; N, 17.48; H, 2.92%.
2.3.1.2. 2-Hydroxy-3-methoxy-5-(4-ethylphenylazo)benzaldehyde,
IIa. Yellow solid. Yield: 88%, m.p. = 114–115 ◦C. 1H NMR (d6-DMSO,
ppm): ı 1.22 (t, 3H, J = 7.53 Hz), 2.70 (q, 2H, J = 7.53 Hz), 3.97 (s, 3H),
7.41 (d, 2H, J = 8.16 Hz), 7.84 (d, 1H, J = 2.08 Hz), 7.82 (m, 3H), 10.38
(s, 1H), 10.99 (br, 1H). IR (KBr, cm−1); 3050–3400 (OH), 1663 (CHO),
1611, 1589, 1485, 1441 (N N), 1265 (C–O), 941 (N N bending).
3. Result and discussion
2.3.1.3. 2-Hydroxy-3-methoxy-5-(4-chlorophenylazo)benzaldehyde,
IIIa. Yellow solid. Yield: 83%, m.p. = 169–170 ◦C. 1H NMR (d6-
DMSO, ppm): ı 3.97 (s, 3H), 7.67 (m, 3H), 7.88 (m, 3H), 10.39 (s,
The condensation reaction of 3,5-diamino-1,2,4-triazole with
substituted azo-coupled o-vanillin precursors in ethanol, gave good
yield of new 1,2,4-triazole-based azo-azomethine compounds,

H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 86 (2012) 39–43
41
Fig. 3. 1H NMR of IVb at 25 ◦C.
are presented in Section 2. The slightly broad signal at ıH
10.34–11.40 ppm in the 1H NMR spectra of Ia–IIIa is assigned to
the –OH group, as was confirmed by deuterium exchange when
D2O was added to d6-DMSO solution. The disappearance of –OH
signal in the 1H NMR spectrum of IVa indicates that the keto form
stable than enol form in d6-DMSO solution. In the 1H NMR spectra
of Ib and IIb the broad resonance at 14.25–14.33 ppm ascribable to
the N–H proton of heterocycle ring. Also, the 1H NMR spectra of Ib
and IIb show a singlet at ıH 8.10–9.80 ppm assigned to the CH
N
imine protons.
The 1H NMR spectra of the IIIb and IVb contain two sets of
signals with the relative proportions of the two species varying
with temperature which indicate the equilibrium between azo
and hydrazone and/or enaminone tautomers in solution, Fig. 3.
The corresponding HC N-signals in 1H NMR spectra of IVb are at
10.35 ppm, 9.28 ppm.
Fig. 2. Structures of azo, hydrazone and enaminone tautomers.
Ib–IVb. All prepared compounds, are air stable solids, intensely
colored, soluble in DMSO and exhibited dyes character since the
molar extinction coefficients (ε) were over 30,000 M−1 cm−1 [28].
Also, the presences of various coordination sites make Ib–IVb as
very versatile species, which could potentially act as multidentate
ligands.
3.3. Thermal properties
In order to give more insight into the structure of the pre-
pared dyes, the thermal studies of Ib–IVb have been carried out
using thermogravimetry techniques. In the present investigation,
the heating rates were suitably controlled at 10 ◦C min−1 under N2
atmosphere and the weight loss was measured from 25 ◦C up to
700 ◦C.
3.1. Infrared spectra
The assignment of the bands in IR spectra of Ia–IVa and Ib–IVb
were achieved as described before [29]. A strong band observed
in the IR spectra of Ia–IVa in the region of 1650–1670 cm−1 can be
assigned to the ꢀ(C O) group. The IR spectrum of IVa contain addi-
tional band at 1701 cm−1 associated with the presene of keto and
enol forms in solid state. The total absence of ꢀ(C O) absorption
in the IR spectra of Ib–IVb together with the appearance of new
strong ꢀ(C N) absorption in the range of 1604–1610 cm−1 clearly
indicated that a new Schiff-base compound had formed in each
case. Characteristic band at 1709 cm−1 in the IR spectrum of IVb
is an indication of the presence of hydrazone and/or enaminone
tautomer, Fig. 2, which is further supported by 1H NMR studies.
The symmetric N N stretching mode, in the IR spectra of Ib–IVb,
leads to a strong band at ca. 1465 cm−1, while the ꢀ(C–O) stretching
mode gives an intense band within the range 1275–1265 cm−1. The
The initial decomposition and inflection temperatures have
been used as an indication on the thermal stability of the pre-
pared compounds. The TG results indicate that the framework of
IIIb is stable up to 331 ◦C, Fig. 4, whereas others are stable up to
255 ◦C, Table 1. The comparison of Td (decomposition tempera-
ture) showed that the thermal stability of the azo dyes increases
in the order IVb < Ib < IIb < IIIb. This fact should be related with the
structure of the compounds and indicate the 4-chloro-substituted
compound is stable than others. Above 350 ◦C, the TG curves of the
prepared dyes show continuous significant weight loss up to 700 ◦C.
The presence of water or solvent molecules in the lattice of the
dyes is supported by the results of thermogravimetric analysis. The
first stage of mass loss of Ib and IIb is observed between 25 and
250 ◦C which, along with the endothermic peak in the DTA curve, is
in agreement with the calculated values for the removal of ethanol
molecules from the compounds.
IR spectra of Ib–IVb exhibited a medium band at 3308–3285 cm−1
which can be assigned to the ꢀ(NH) of the heterocycle ring. The IR
spectra of Ia and related azo-azomethine dye, Ib, show strong bonds
at 1516–1530 and 1340–1345 cm−1 assigned to the NO2 group [30].
,
3.2. 1H NMR spectra
3.4. The electronic absorption spectra and substituent effects
The 1H NMR spectra of the Ia–IVa and Ib–IVb, obtained at
ambient temperature in d6-DMSO, display a group of signals
corresponding to the hydrogen’s of each molecule. The results
Generally, azobenzene derivatives exhibit
a
low-intensity
*
n → ␲ band in visible, and/or low energy UV region, and
*
a high-intensity ␲ → ␲ band in the UV region [31]. Strong

42
H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 86 (2012) 39–43
Fig. 4. TGA/DTA of IIIb.
Table 1
Thermal analyses data for Ib–IVb.
Compound, M.F. (M. Wt.)
Dissociation
stages
Temperature range
in TG (◦C)
Weight loss, found
(calculated) (%)
Proposed decomposition
assignment
Td (◦C)
Stage I
Stage II
Stage III
Stage I
Stage II
Stage I
Stage II
Stage III
Stage I
Stage II
Stage III
50–210
210–270
280–700
260–310
310–700
50–250
310–353
353–550
180–210
210–370
370–700
3.1 (3.34)
18.4 (18.2)
26.7 (27.4)
21.7 (21.84)
32.6 (32.25)
3.2 (3.44)
25.5 (25.90)
30.3 (30.6)
10.2 (9.95)
16.6 (16.77)
11.0 (11.38)
The loss of 0.5 C2H5OH
The loss of C4H3N5
The loss of C6H4N3O2
The loss of C4H4N5O
The loss of C9H8N2O
The loss of 0.5 C2H5OH
The loss of C5H5N5O2
The loss of C6H4Cl2
The loss of Cl2
255
Ib, C31H26N11O8.5 (688.57)
280
331
IIb, C34
H33N9O4 (631.68)
IIIb, C30
H23Cl2N9O4 (644.47)
225
IVb, C30H21Cl4N9O4 (713.36)
The loss of C3H2N5
The loss of C2H5O2
electron acceptor groups, such as –NO2, effectively reduce the
charge transfer transitional energy of the ␲ → ␲* transition [31,32].
Consequently, the corresponding band is bathochromically shifted
*
and overlap with the n → ␲ band in low energy UV region.
*
The ␲ → ␲ band of azo groups of azo-azomethine compounds
is also more red shifted than related azobenzenes because of
extended ␲-resonance system. This in o-hydroxyl substituted
azo-azomethine compounds, may be inferred from (i) selective
solvation, (ii) the ability of the solvent to form stronger inter-
molecular H-bonds with a particular tautomeric form, and (iii) the
intramolecular hydrogen bonding between C N or N N and OH
groups [25,26].
The electronic absorption spectra of Ib–IVb were recorded at
in DMSO at room temperature, Fig. 5, display mainly two bands
*
arising from the ␲ → ␲* and n → ␲* transitions in addition of ␲–␲
aromatic rings transitions at short wavelengths, Table 2. The broad
band observed in the range of 365–458 nm for all compounds can
Fig. 5. The electronic absorption spectra of Ib–IVb in DMSO (1.0 × 10−5 mol L−1).
*
be assigned to an n → ␲ electronic transition of azo group [25]
and intramolecular charge transfer interaction involving the whole
molecule of dye.
suffers a small shift on changing the substituent attached to the
phenyl moiety.
It can be found that the maximum absorption at 360–455
and 510–615 nm shifts bathochromically along with the increase
in the electron-accepting ability of substituent in the sequence
of IIb < IIIb < Ib < IVb consistent with an increase in molecular
donor–acceptor polarization. The shorter wavelength UV band
3.5. Effect of time on absorption spectra of Ib–IVb
The effect of time on color intensity of Ib–IVb was investigated.
This was determined by monitoring of increase and/or decrease of

H. Khanmohammadi, M. Erfantalab / Spectrochimica Acta Part A 86 (2012) 39–43
43
Table 2
solvent. The absorbance of the other band at 537 nm was increased
upon increasing solvent polarity and was also shifted to lower
wavelength, negative solvatochromism. The recorded spectra show
fine isobestic point which indicate the establishment of equilibrium
between various forms of the molecule.
Absorption spectral data of Ib–IVb in DMSO.
Compounds
ꢁmax/nm (ε × 103/
Compounds
ꢁmax/nm (ε × 103/
M−1 cm−1
)
M−1 cm−1
)
279 (34.0)
393 (36.8)
537 (51.9)
365 (75.8)
514 (27.6)
365(33.4)
458 (26.9)
521(36.8)
263(32.3)
393(27.2)
604(80.7)
Ib
IIIb
4. Conclusion
IIb
IVb
New 1,2,4-triazole-based azo-azomethine dyes were prepared
via condensation of 3,5-diamino-1,2,4-triazole with azo-coupled
o-vanillin precursors. All prepared compounds are thermal stable
solids and exhibit dyes character. The order of thermal stability
found is IVb < Ib < IIb < IIIb. This fact should be related with the
structure of the ligands and indicate the 4-chloro and 4-ethyl sub-
stituted dyes are stable than others. Azo–hydrazone–enaminone
tautomerism was studied on the basis of UV–vis spectra. The result
indicated that all tautomers of para substituted dyes are reasonably
stable in DMSO solution at room temperature.
Acknowledgment
We are grateful to the Arak University for financial support of
this work.
Fig. 6. The absorption spectra of IVb in DMSO at different times.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2011.09.053.
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˙
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