New 1,2,4-triazole-based azo-azomethine dyes. Part II: Synthesis, characterization, electrochemical properties and computational studies

Article abstract of DOI:10.1016/j.saa.2012.07.041

A new series of monoiminated 1,2,4-triazole-based azo-azomethine dyes have been synthesized via condensation reaction of 4-amino-3-methyl-5-mercapto-1,2,4- triazole with various substituted azo-coupled salicylaldehyde. The dyes have been characterized by using FT-IR, UV-Vis and ¹H NMR spectroscopic methods as well as elemental analysis. The electrochemical behavior of the dyes has been investigated by cyclic voltammetry in DMSO at five different scan rates. Solvatochromic behavior of the dyes has been also investigated in four organic solvents with different polarities. Furthermore, the ¹H chemical shielding of the dyes were studied by the gauge independent atomic orbital (GIAO) method at the level of density functional theory (DFT).

Full text of DOI:10.1016/j.saa.2012.07.041

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
Contents lists available at SciVerse ScienceDirect
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 II: Synthesis,
characterization, electrochemical properties and computational studies
⇑
Hamid Khanmohammadi , Malihe Erfantalab, Atena Bayat, Ali Babaei, Masoud Sohrabi
Faculty of Science, Department of Chemistry, Arak University, Arak 38156-8-8349, Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
New monoiminated 1,2,4-triazole-based azo–azomethine dyes were synthesized via condensation of 4-
amino-3-methyl-5-mercapto-1,2,4-triazole with azo-coupled salicylaldehyde. Electrochemical behavior
of the dyes has been investigated by cyclic voltammetry in DMSO at five different scan rates.
"
"
"
New monoiminated 1,2,4-triazole-
based azo–azomethine dyes have
been synthesized.
Electrochemical behavior of the dyes
has been investigated by cyclic
voltammetry.
The 1H chemical shielding of the
dyes were studied by computational
methods.
a r t i c l e i n f o
a b s t r a c t
Article history:
A new series of monoiminated 1,2,4-triazole-based azo–azomethine dyes have been synthesized via con-
densation reaction of 4-amino-3-methyl-5-mercapto-1,2,4-triazole with various substituted azo-coupled
salicylaldehyde. The dyes have been characterized by using FT-IR, UV–Vis and 1H NMR spectroscopic
methods as well as elemental analysis. The electrochemical behavior of the dyes has been investigated
by cyclic voltammetry in DMSO at five different scan rates. Solvatochromic behavior of the dyes has been
also investigated in four organic solvents with different polarities. Furthermore, the 1H chemical shield-
ing of the dyes were studied by the gauge independent atomic orbital (GIAO) method at the level of den-
sity functional theory (DFT).
Received 28 March 2012
Received in revised form 7 July 2012
Accepted 10 July 2012
Available online 31 July 2012
Keywords:
Azo–Azomethine
Cyclic voltammetry
1,2,4-Triazole
Tautomers
Ó 2012 Elsevier B.V. All rights reserved.
Solvatochromism
GIAO
Introduction
reactions [5,6], molecular materials [7,8] and also complexing abil-
ity towards some toxic metals [9]. On the other hand, considerable
During the past two decades, considerable attention has been
paid to the chemistry of azomethine compounds and their metal
complexes [1–4]. This may be attributed to their stability and po-
tential applications in many fields such as catalysis of chemical
attention has been paid to the chemistry of azo compounds as
thermally and optically stable dyes [10,11]. Azo compounds have
been extensively used as dyestuffs for wool, leather and synthetic
fabrics because of their extraordinary coloring properties and in
photonic devices, electrooptic modulators, components of optical
communication systems of their second order nonlinear optical
properties [12–15]. It has been generally accepted that the strong
⇑
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 Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.07.041

H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
877
H
R
NH2
O
1
2
3
4
5
6
7
8
9
10
4-F
1) NaNO
2 , HCl
4-Br
0-5
C
N
N
OH
4-Et
2)
Salicylaldehyde
4-Cl
R
R
4-NO2
2,4-Cl
3,4-Cl
3,5-Cl
3-NO2
2-NO2
N
N
H3C
SH
N
NH2
H
N
N
S
H3C
N
N
H
OH
N
N
R
1b 2b
3b
4b
5b
6b
7b
8b
9b
10b
R
4-F 4-Br 4-Et 4-Cl 4-NO2 2,4-Cl 3,4-Cl 3,5-Cl 3-NO2 2-NO2
Scheme 1. General synthesis of 1,2,4-triazole-based azo–azomethine dyes.
electronic absorption maximum of azo compounds can be tailored
by the presence of various chromophores [16]. This, combined with
the fact that azo groups are relatively robust and chemically stable,
has prompted extensive study of azo–azomethine compounds as
dyes and colorants. Indeed, in order to improve the spectral prop-
erties of azo–azomethine dyes heteroaromatic moieties such as
pyridazine-, pyridine-, tetrazine- and triazoles- have attracted
continuing interest over the past years [16–18]. This tendency
can be consequence of the numerous researches that are available
for diversified applications of these compounds [19].
Among the plethora of new synthetic azo–azomethine dyes
those containing 1,2,4-triazole are of particular interest for several
reasons. Most of 1,2,4-triazole-based compounds are thermally
stable compounds with rich and well-known chemistry [20–22].
On the other hand, the electronic properties of triazole-base com-
pounds make them suitable for surface enhanced resonance Ra-
man scattering (SERRS) studies [23,24]. Also, the incorporation of
1,2,4-triazole into macroacyclic ligands has facilitated the isolation
of a wide range of transition metal complexes with interesting
electrochemical and magnetic properties [25–29].
studies of 1,2,4-triazole-based dyes. The prepared dyes have been
characterized by spectroscopic methods (1H NMR, UV–Vis and IR)
as well as elemental analysis data. In order to study of substituted
groups on voltammetric behavior of the dyes cyclic voltammo-
grams of 1b–10b in DMSO solution is reported at five different scan
rates. Solvatochromic behavior of the dyes in various solvents with
different polarities was also evaluated. Moreover, in order to com-
parison between experimental and theoretical NMR data, which
may be helpful in making correct assignments and understanding
the relationship between chemical shielding and molecular struc-
ture, complete analyses of the 1H NMR spectra of the dyes is re-
ported. For this reason, the 1H NMR isotropic chemical shieldings
of the dyes was systematically studied by the GIAO method at
the level of density functional theory (DFT) with 6-31G⁄ and 6-
311 + G⁄ basis sets.
Experimental
Materials
Up to now, the electrochemical behavior of the synthetic azo
and also azomethine compounds have been extensively reported
in the literature [30–33]. However, based on our knowledge no
example of electrochemical studies on 1,2,4-triazole-based azo–
azomethine dyes were reported to date. Therefore, we report here
the design, synthesis, and electrochemical study of new series of
1,2,4-triazole-based monoiminated azo–azomethine dyes, 1b–
10b (Scheme 1). The dyes have been prepared by condensation
reaction of 4-amino-3-methyl-5-mercapto-1,2,4-triazole with var-
ious substituted azo-coupled salicylaldehyde, 1–10 [13,17]. The
choice of these compounds was motivated by two objectives: (i)
to provide new monoiminated 1,2,4-triazole-based azo–azome-
thine dyes and (ii) to enrich the electrochemical and spectral
All of the reagents and solvents involved in synthesis were of
analytically grade and used as received without further purifica-
tion. Salicylaldehyde, various substituted of aniline, disulfide car-
bon, acetic acid glacial and hydrazine (80%) were obtained from
Aldrich and Merck. 4-amino-3-methyl-5-mercapto-1,2,4-triazole
and azo-coupled salicylaldehyde precursors, 1–10, were prepared
according to the well-known literature procedure [13,17,34].
Instrumentation
1H NMR spectra were obtained with a Bruker Avance 300 MHz
spectrometer. Infrared spectra were recorded as pressed KBr discs,
using a Unicom Galaxy Series FT-IR 5000 spectrophotometer

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H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
(400–4000 cmꢀ1). Melting points were determined on Electrother-
mal 9200 apparatus. Electronic spectral measurements were car-
ried out using Analytik Jena Specord S600 spectrophotometer in
the range 250–900 nm. C.H.N. analyses were performed on a Vario
EL III elemental analyzer. Electrochemical measurements were re-
corded on an Autolab 30 V potentiostat/galvanostat for Eco Chem-
ical Company. All readings were taken using three electrode
potentiostatic systems in DMSO with 0.1 mol cmꢀ3 tetrabutylam-
monium perchlorate (TBAP, electrochemical grade) as supporting
electrolyte. A three-electrode assembly composed of a platinum
working electrode, a platinum auxiliary electrode, and calomel ref-
erence electrode was used with sample concentrations of
1 ꢁ 10ꢀ3 M.
4-Imino-4-(4-ethyle-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (3b)
Yellow solid, yield: 73%, m.p. 197–8 °C. 1H NMR(d6–DMSO,
ppm): dH 13.78(br, s, 1H), 10.34(s, 1H), 8.44(s, 1H), 7.98(d, 1H,
J = 8.78 Hz), 7.79(d, 2H, J = 8.00 Hz), 7.40(d, 2H, J = 8.00 Hz),
7.17(d, 1H, J = 8.78 Hz), 2.68(q, 2H, J = 7.10 Hz), 2.40(s, 3H),
1.22(t, 3H, J = 7.10 Hz). IR (KBr, cmꢀ1); 3097, 1703, 1622(C=N),
1601(C=C), 1564, 1485(N=N), 1408, 1271(C–O), 1172, 1107,
842(C=S), 642 and 445. Anal. Calcd. for C18H18N6OS: C, 59.00; N,
22.93; H, 4.95; S, 8.75. Found: C, 58.8; N, 23.0; H, 4.9; S, 7.7%.
4-Imino-4-(4-chloro-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (4b)
Yellow solid, yield: 87%, m.p. 275–6 °C. 1H NMR(d6–DMSO,
ppm): dH 13.73(s, 1H), 11.46(br, s, 1H), 10.37(s, 1H), 8.47(d, 1H,
J = 2.30 Hz), 8.02(dd, 1H, J = 2.30 Hz, J = 8.84 Hz), 7.99(d, 2H,
J = 8.63 Hz), 7.63(d, 2H, J = 8.63 Hz), 7.18(d, 1H, J = 8.84 Hz),
2.39(s, 3H). IR (KBr, cmꢀ1): 3107, 1622(C=N), 1604(C=C), 1585,
1568, 1496, 1487(N=N), 1340, 1273(C–O), 1172, 1082, 856(C=S)
and 677. Anal. Calcd. for C16H13ClN6OS: C, 51.54; N, 22.54; H,
3.51; S, 8.60. Found: C, 51.6; N, 22.7; H, 3.5; S, 7.9%.
Theoretical calculations
The structures and NMR chemical shifts of the compounds were
calculated using Gaussian-03 [35] series of programs. The geome-
try of all compounds were fully optimized at the B3LYP/6-31G⁄ le-
vel. Vibrational frequency analyzes, calculated at the same level of
theory, indicate that the optimized structures are at the stationary
points corresponding to local minima without any imaginary fre-
quency. The optimized structure in gas phase was used for NMR
calculations. The calculations of 1H NMR chemical shielding in
DMSO for all dyes were performed using GIAO/DFT [36,37] method
at 6-31G(d) and 6-311 + G(d) basis sets. In all these calculations,
the impact of the solvent was taken into account using the PCM
of Tomasi et al. [38,39] with the explicit treatment of hydrogens
(radii taken from the UFF force field [39]) as implemented in the
Gaussian program. A general expression dCal = dTMS ꢀ d was used
to convert calculated magnetic isotropic shielding tensors to chem-
ical shifts dCal (ppm), where dTMS stands for shielding constant of
tetramethylsilane hydrogens.
4-Imino-4-(4-nitro-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (5b)
Orange solid, yield: 88%, m.p. 285–6 °C. 1H NMR(d6–DMSO,
ppm): dH 13.77(s, 1H), 11.61(s, br, 1H), 10.40(s, 1H), 8.50(s, 1H),
8.38(m, 2H), 8.03(m, 3H), 7.18(d, 1H, J = 8.30 Hz), 2.39(s, 3H). IR
(KBr, cmꢀ1); 3119, 1620(C=N), 1606(C=C), 1591, 1523,
1487(N=N), 1340, 1275(C–O), 1153, 1107, 871(C=S), 684 and 597.
Anal. Calcd. for C16H13N7O3S: C, 50.12; N, 25.57; H, 3.42; S, 8.36.
Found: C, 50.7; N, 26.0; H, 3.6; S, 7.9%.
4-Imino-4-(2,4-di-chloro-phenylazo)-phenol-3-methyl-5-mercapto-
1,2,4-triazole (6b)
Orange solid, yield: 78%, m.p. 239–40 °C. 1H NMR(d6–DMSO,
ppm): dH 13.76(s, 1H), 11.52(s, 1H), 10.42(s, 1H), 8.49(s, 1H),
8.01(d, 1H, J = 9.00 Hz), 7.87(s, 1H), 7.69(d, 1H, J = 6.00 Hz),
7.53(d, 1H, J = 6.00 Hz), 7.19(d, 1H, J = 9.00 Hz), 2.39(s, 3H). IR
(KBr, cmꢀ1); 3105, 1602(C=N), 1575, 1485(N=N), 1332, 1288,
1271(C–O), 1141, 1095, 742, 839(C=S), 717 and 632. Anal. Calcd.
for C16H12Cl2N6OS: C, 47.18; N, 20.63; H, 2.97; S, 7.87. Found: C,
48.9; N, 20.5; H, 3.0; S, 7.2%.
Synthesis
General procedure for the synthesis of the dyes
To
a solution of 4-amino-3-methyl-5-mercapto-1,2,4-tria-
zole(0.130 gr, 1 mmol) in acetic acid (10 mL) was added a solution
of azo-coupled salicylaldehyde precursors (1 mmol) in DMF (5 mL).
The reaction mixture was heated under reflux for 15 h. The solid
product were filtrated and washed with hot ethanol.
4-Imino-4-(3,4-di-chloro-phenylazo)-phenol-3-methyl-5-mercapto-
1,2,4-triazole (7b)
4-Imino-4-(4-fluoro-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (1b)
Recrystallization of the product from DMF yielded 7b as a dark
yellow powder. Yield: 78%, m.p. 250–1 °C. 1H NMR(d6–DMSO,
ppm): dH 13.76(s, 1H), 11.52(s, br, 1H), 10.38(s, 1H), 8.48(s, 1H),
8.02(m, 2H), 7.85(s, br, 3H), 7.18(d, 1H, J = 9.00 Hz), 2.39(s, 3H).
IR (KBr, cmꢀ1); 3099, 1660, 1622(C=N), 1606(C=C), 1589, 1500,
1487(N=N), 1342, 1271(C–O), 1172, 823(C=S), 673 and 609. Anal.
Calcd. for C16H12Cl2N6OS.1DMF: C, 47.51; N, 20.41; H, 3.99; S,
6.68. Found: C, 48.3; N, 19.8; H, 4.0; S, 6.6%.
Yellow solid, yield: 67%, m.p. 244–5 °C. 1H NMR (d6–DMSO,
ppm): dH 13.75(s, br, 1H), 11.36(s, br, 1H), 10.36(s, 1H), 8.45(s,
1H), 7.93(m, br, 3H), 7.40(m, br, 2H), 7.17(m, br, 1H), 2.39(s, 3H).
IR (KBr, cmꢀ1); 3101, 1658, 1622(C=N), 1606(C=C), 1589, 1496,
1485(N=N), 1344, 1222(C–O), 846 (C=S), 671 and 466. Anal. Calcd.
for C16H13FN6OS: C, 53.92; N, 23.58; H, 3.68; S, 9.00. Found: C, 53.8;
N, 23.4; H, 3.3; S, 8.0%.
4-Imino-4-(4-bromo-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (2b)
4-Imino-4-(3,5-di-chloro-phenylazo)-phenol-3-methyl-5-mercapto-
1,2,4-triazole (8b)
Dark brown solid, yield: 69%, m.p. 256–7 °C. 1H NMR (d6–DMSO,
ppm): dH 13.75(s, br, 1H), 11.45(s, br, 1H), 10.37(s, 1H), 8.45(s, 1H),
7.99(d, 1H, J = 6.00 Hz), 7.77(s, br, 4H), 7.17(d, 1H, J = 6.00 Hz),
2.38(s, 3H). IR (KBr, cmꢀ1); 3119, 1667, 1651, 1620(C=N),
1604(C=C), 1587, 1487(N=N), 1273(C–O), 1170, 1099, 844(C=S),
831, 669 and 597. Anal. Calcd. for C16H13BrN6OS: C, 46.05; N,
20.14; H, 3.14; S, 7.68. Found: C, 46.2; N, 19.9; H, 3.8; S, 7.8%.
Orange solid, yield: 72%, m.p. 242–3 °C. 1H NMR(d6–DMSO,
ppm): dH 13.76(s, 1H), 11.55(s, 1H), 10.39(s, 1H), 8.49(s, 1H),
8.01(d, 1H, J = 9.00 Hz), 7.85(s, br, 2H), 7.77(s, br, 1H), 7.19(d, 1H,
J = 9.00 Hz), 2.39(s, 3H). IR (KBr, cmꢀ1); 3115, 1624(C=N),
1604(C=C), 1593, 1572, 1487(N=N), 1269(C–O), 1176, 1101,
862(C=S), 802 and 671. Anal. Calcd. for C16H12Cl2N6OS: C, 47.18;
N, 20.63; H, 2.97; S, 7.87. Found: C, 47.5; N, 20.3; H, 2.6; S, 7.9%.

H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
879
Table 1
Result and discussion
Absorption spectral data of 1b–10b in various organic solvents; kmax, nm (
e
ꢁ 103,
Mꢀ1 cmꢀ1).
New 1,2,4-triazole-base azo–azomethines, 1b–10b, have been
synthesized via condensation reaction of 4-amino-3-methyl-5-
mercapto-1,2,4-triazole with various substituted azo-coupled sali-
cylaldehyde. The condensation reaction provides the expected
monoiminated azo–azomethine in good yields, ranging from 67%
to 91%. All prepared compounds, are air stable solids, intensely col-
ored, soluble in DMSO and exhibited dyes character since the mo-
Compound
1b
DMSO
CH3CN
CHCl3
CH3COOH
349(52.5)
438(13.5)
356(23.8)
469(9.2)
346(41.9)
442(20.2)
347(22.6)
–
378(17.2)
533(9.9)
373(25.4)
480(20.4)
357(11.2)
478(5.4)
392(15.9)
479(21.7)
356(15.9)
478(13.2)
363(16.6)
486(13.3)
339(64.7)
–
343(35.0)
–
340(58.4)
–
336(25.9)
–
359(26.4)
–
350(37.5)
–
344(18.8)
–
341(32.0)
–
341(71.4)
–
345(36.5)
–
342(59.7)
–
339(12.5)
–
362(28.4)
–
349(46.3)
–
344(19.3)
–
343(32.3)
–
342(63.8)
–
342(34.5)
–
339(53.4)
–
334(26.6)
–
356(20.4)
–
350(44.9)
–
345(18.9)
–
341(32.4)
–
2b
3b
4b
5b
6b
7b
8b
9b
10b
lar extinction coefficients (e
) were over 30,000 Mꢀ1 cmꢀ1 [40].
Electronic absorption spectra of the dyes
Electronic absorption spectra of the dyes, 1b–10b, recorded in
DMSO between 300–700 nm, display mainly two bands, Table 1
and Fig. 1. The first band located at 345–393 nm corresponds to
low energy (p– p-electrons of the azo
p⁄) transition involving the
341(29.3)
–
338(28.7)
–
345(38.8)
–
338(38.8)
–
341(30.7)
–
338(30.8)
–
and azomethine groups [40]. The broad band observed in the range
448–527 nm can be assigned to an intramolecular charge transfer
interaction involving the whole molecule. The intramolecular CT
band can be assigned to the existence of tautomeric equilibrium
originating from hydroxyl group in o-position to the C=N bond
[41], Fig. 2. Dramatic differences in energy of low energy band oc-
cur upon arylazo substitution of the dyes. The strength influence of
the acceptor group is demonstrated by comparison of the absorp-
tion maxima of compounds 3b and 5b as the longest wavelength
transition is shifted from 444 nm in 3b to 527 nm for 5b (Table
1). This effect has been attributed to the stabilization of LUMO
by the electron-withdrawing groups [41]. We found that the lon-
gest wave number absorption band of para-arylazo substituted
dyes, 1b–5b, at 448–527 nm, shifts bathochromically along with
the increase in the electron-accepting ability of substituent in the
sequence of 3b < 4b < 1b < 2b < 5b consistent with an increase in
molecular donor–acceptor polarization, Fig. 3. The similar trend
was observed for di-chloro-substituted dyes, 7b < 8b < 6b.
4-Imino-4-(3-nitro-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (9b)
Orange solid, yield: 91%, m.p. 267–8 °C. 1H NMR(d6–DMSO,
ppm): dH 13.75(s, 1H), 11.51(s, br, 1H), 10.39(s, 1H), 8.48(s, 2H),
8.32(m, 1H), 8.27(d, 1H, J = 6.00 Hz), 8.02(m, 1H, J = 9.00 Hz),
7.94(s, 1H), 7.85(m, 1H), 7.17(d, 1H, J = 9.00 Hz), 2.40(s, 3H). IR
(KBr, cmꢀ1); 3045, 1649, 1622(C=N), 1606(C=C), 1593, 1531,
1487(N=N), 1340, 1271(C–O), 1105, 868(C=S), 794, 738, 671 and
476. Anal. Calcd. for C16H13N7O3S: C, 50.12; N, 25.57; H, 3.42; S,
8.36. Found: C, 50.2; N, 25.5; H, 3.8; S, 7.6%.
4-Imino-4-(2-nitro-phenylazo)-phenol-3-methyl-5-mercapto-1,2,4-
triazole (10b)
Brown solid, yield: 83%, m.p. 235–6 °C. 1H NMR(d6–DMSO,
ppm): dH 13.76(s, br, 1H), 11.56(s, br, 1H), 10.44(s, 1H), 8.45(s,
1H), 8.07(m, 1H), 7.80(m, 1H, J = 6.00 Hz), 7.72(s, br, 3H), 7.19(d,
1H, J = 9.00 Hz), 2.39(s, 3H). IR (KBr, cmꢀ1); 3128, 1666,
1624(C=N), 1602(C=C), 1525, 1485(N=N), 1348, 1255(C–O), 1080,
842(C=S), 781, 669 and 623. Anal. Calcd. for C16H13N7O3S: C,
50.12; N, 25.57; H, 3.42; S, 8.36. Found: C, 49.6; N, 25.6; H, 3.4;
S, 7.6%.
Solvent effect on absorption spectra of the dyes
It has long been known that UV–Vis absorption spectra of
chemical compounds, especially azo dyes, may be influenced
by the surrounding medium and that the solvents can bring
about a change in the position, intensity, and shape of absorp-
tion bands [42]. This phenomenon was termed as solvatochro-
mism [42,43] and is due to the interaction between the solute
Fig. 1. The absorption spectra of 1b–10b in DMSO.

880
H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
H
H
H
N
N
N
N
N
N
S
S
S
H3C
H3C
N
N
H3C
N
N
N
N
H
H
H
H
O
O
OH
H
N
N
N
N
N
N
R
R
R
(b)
(c)
(a)
Fig. 2. (a) Azo, (b) hydrazone and (c) enaminone tautomers.
Fig. 3. Effect of para substituents on absorption spectra of the prepared compounds 1b–5b in DMSO.
Fig. 4. Absorption spectra of 5b in various organic solvents (See also supporting information).
and solvent molecules [43]. To evaluate the intermolecular forces
between solvents and solute molecules, we carried out a preli-
minary study of the absorption spectra for all dyes in selected
solvents of different solvation character (DMSO, CH3CN, CHCl3,
and acetic acid) at room temperature, Fig. 4. The results are gi-
ven in Table 1.

H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
881
Table 2
Solvatochromic parameters (
Table 3
a
, b, and p⁄) of used solvents.
Electrochemical dataa of prepared dyes (1b-10b) in DMSO.
Solvent
a
b
p⁄
e
l/D
Dyes
Eox(1)
Eox(2)
Eox(3)
Eox(4)
Ered(1)
Ered(2)
DMSO
CHCl3
CH3CN
CH3COOH
0.00
0.20
0.19
1.12
0.76
0.10
0.40
0.45
1.00
0.69
0.66
0.64
46.45
4.89
35.94
6.17
4.05
1.15
3.90
1.68
1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
ꢀ1.83b
ꢀ1.75b
ꢀ1.86b
ꢀ1.72b
ꢀ1.64b
ꢀ1.58b
ꢀ1.66b
ꢀ1.55b
–
+0.29
ꢀ1.56b
+0.27
+0.28
ꢀ0.96
+0.28
+0.28
+0.31
ꢀ1.04b
ꢀ1.24
+0.54
+0.62
+0.49
+0.55
+0.36
+0.58
+0.58
+0.59
+0.36
+0.32
–
–
–
–
–
–
+0.91b
–
ꢀ0.47
+0.84
+0.82
+0.98
ꢀ0.75c
ꢀ0.79c
ꢀ0.21
+1.03
ꢀ0.92
+1.03
+0.65b
+0.98b
+1.02b
+1.02b
+0.63b
+0.62
ꢀ0.84
ꢀ1.77c
ꢀ1.72c
ꢀ0.80
ꢀ0.86
–
The polarity and hydrogen bonding properties of the solvents
used in the present investigation, indicated by the Kamlet–Taft
parameters (
scale of solvent hydrogen-bond acceptor (HBA) basicity which re-
fers to the acceptance of the proton of a hydrogen bond. is a scale
of solvent hydrogen bond donor (HBD) acidities which refer to the
donation of the proton. p⁄ is a scale of solvent dipolarity/polariz-
ability which refer to the ability of solvent to stabilize a charge
or a dipole by virtue of its dielectric effect. The p⁄ value is corre-
lated with the solvatochromic behavior values.
ꢀ1.71b
a
, b, and p⁄) [33], are summarized in Table 2. b is a
a
b
c
Potentials quoted in V vs. calomel in DMSO;
Reversible wave.
Shoulder.
t
= 50 mV sꢀ1; TBAP (0.1 mol Lꢀ1).
a
forms of the compound [24]. Representative UV–Vis spectrum of
7b is shown in Fig. 5 (See also supporting information).
It was found that the absorption band at 345–393 nm generally
shows bathochromic shift (positive solvatochromism) as the polar-
ity of solvent was increased. The influence of solvents for the pre-
pared dyes increases in the order DMSO > CHCl3>CH3CN >
CH3COOH. This positive solvatochromism exhibited by the dyes
may be due to the effect of dipole moment changes of the excited
state and/or changes in the hydrogen bonding strength in polar
Electrochemistry of azo dyes
To obtain a deeper insight into the ground state properties and
more specifically the mutual donor–acceptor electronic influence,
we studied the redox properties of the dyes, 1b–10b, by cyclic vol-
tammetry. These results are presented in Table 3.
All azo dyes presented similar cyclic voltammograms illustrated
by the voltammogram of 3b (Fig. 6), in both positive and negative
scans. For all dyes, four oxidation peaks were observed, except 1b,
3b and 9b. In the anodic scan, the first irreversible anodic peak,
IIIa, in the potential range of +0.62 to +1.03 V (vs. Calomel) was ob-
served. The dyes display oxidations at more positive potentials as a
consequence of the destabilizing effect of the electron-withdraw-
ing group on the arylazo moiety [30]. The second anodic feature,
IIa, may be assigned to the irreversible oxidation of the triazole
moiety [45]. To confirm this cyclic voltagrams of the pure azo-cou-
pled precursor’s were performed separately (See supporting infor-
mation). Also, all dyes displayed a reversible reduction peak, IIr,
in the range of ꢀ1.86 to ꢀ1.04 V (vs Calomel). This peak can be
attributed to the reduction of the azo compounds to hydrazone
derivatives [46]. In the volammograms of all dyes an apparently
irreversible reduction wave, Ir, was detected. The reductive wave
did not appear in the low scan rates. These reductive peaks appear
to be associated with the formation of unstable amine products,
which were reduced in the potential range of 0.85 to ꢀ1.40 V.
The cyclic voltammograms of all dyes were recorded at five
different scan rates (see supporting information). The reduction
solvents (The b, l
/D and p⁄ values for DMSO are higher than other
solvents).
Furthermore, the absorption spectra of the dyes in DMSO com-
prise a new band appearing at longer wavelength (438–533 nm)
which exceeds by far the usual solvent shift. This behavior may
be interpreted on the basis that the solute molecules are liable to
form a solvated complex with DMSO molecules, with higher b val-
ues in comparison to other solvents, through an intermolecular H-
bonding [44].
Effect of DMSO/CHCl3 mixture on absorption spectra of the dyes
In order to study the effect of CHCl3 on solvent–solute interac-
tions and tautomeric equilibrium in solution, the UV–Vis spectra of
the dyes were recorded in various ratios of DMSO/ CHCl3 media.
Generally, the intensity of absorption bands in pure DMSO was de-
creased upon addition of CHCl3. The position of all absorption
bands were also shifted to lower wavelength during the addition
of CHCl3.The recorded spectra show fine isobestic point which indi-
cate the establishment of equilibrium between various tautomeric
Fig. 5. The electronic absorption spectra of 7b (3.0 ꢁ 10ꢀ5 M) in mixture of DMSO/CH3Cl.

882
H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
Fig. 6. Cyclic voltammogram for 3b (1.0 ꢁ 10ꢀ3 M) in DMSO solution with various scan rates.
Table 4
Hydrogen chemical shifts (d, ppm) in d6–DMSO for 1,2,4-triazole-based azo–azomethine dyes (1b–10b).
11
H
N
N
S
1
H3C
N
N
12
OH
H
2
3
4
5
N
N
6
7
10
R
9
8
Dyes
1b
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
H11
H12
2.39a
2.720b
2.908c
2.38
2.709
2.651
2.40
10.36
8.45
7.93
7.17
7.93
7.40
–
–
–
–
–
7.40
7.777
715
7.93
13.75
10.290
10.427
13.75
10.288
10.446
–
11.36
9.972
9.198
11.45
9.962
9.195
–
10.019
11.297
10.37
10.015
11.297
10.34
8.575
8.915
8.45
8.569
8.926
8.44
7.905
7.969
7.99
7.902
7.991
7.98
7.813
8.047
7.17
6.924
7.687
7.17
8.086
8.741
7.77
8.312
8.001
7.79
7.771
7.715
7.77
7.784
8.156
7.40
8.086
8.741
7.77
8.312
8.001
7.79
2b
3b
7.77
7.784
8.156
7.40
2.68,
2.737
2.904
10.003
11.393
8.553
8.892
7.879
7.955
7.413
8.055
8.142
8.274
8.080
8.112
1.22
3.037,
8.080
8.112
8.142
8.274
10.237
10.378
9.973
9.201
1.561
2.442,
1.161
4b
5b
6b
7b
2.39
10.37
9.998
8.47
8.02
7.18
7.99
7.63
–
–
–
–
–
–
–
–
–
–
–
–
7.63
7.754
7.645
8.38
8.328
8.591
7.53
8.234
7.803
–
7.99
7.833
8.363
8.03
8.500
8.770
–
13.73
11.46
9.171
9.180
11.61
9.945
9.255
11.52
9.972
9.193
11.52
9.956
9.186
2.728
2.845
2.39
2.746
2.899
2.39
2.720
2.845
2.39
2.982
2.854
8.521
8.838
8.49
8.584
8.495
8.49
8.603
8.860
8.48
8.605
8.856
7.663
7.927
8.03
7.966
7.997
8.01
8.166
7.950
7.85
7.994
8.773
7.643
7.484
7.18
7.482
7.336
7.19
7.453
8.465
7.18
7.428
7.970
7.833
8.363
8.03
8.500
8.770
7.69
8.174
8.698
7.85
8.239
8.129
7.754
7.645
8.38
8.328
8.591
7.87
7.913
7.688
8.02
7.813
8.596
10.713
10.329
13.77
10.371
10.494
13.76
10.347
10.409
13.76
10.741
10.472
10.906
10.40
10.075
11.491
10.42
10.021
11.846
10.38
–
–
7.85
8.285
8.337
9.919
11.199
–
–

H. Khanmohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 876–884
883
Table 4 (continued)
Dyes
8b
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
H11
H12
2.40
10.39
8.49
8.01
7.19
7.85
–
–
–
7.85
7.583
7.764
7.728.362
7.856
7.77
–
–
–
–
–
–
7.85
7.761
8.454
8.47
8.371
8.785
–
13.76
11.55
9.972
9.200
11.51
9.972
9.902
11.56
9.904
9.153
2.972
2.855
2.40
2.735
2.877
2.39
9.959
10.845
10.39
10.486
11.119
10.44
8.783
8.891
8.47
8.791
9.048
8.45
8.055
7.973
8.02
8.172
7.994
8.07
7.440
7.383
7.16
7.822
7.369
7.19
8.014
8.065
8.32
8.672
8.784
7.72
7.787
7.707
8.28
8.375
8.454
7.72
10.774
10.475
13.75
8.298
10.516
13.75
9b
10b
7.80
8.369
7.229
2.693
2.846
9.806
10.665
8.475
8.619
8.463
7.992
7.625
7.178
8.228
7.777
7.869
7.937
–
–
10.367
10.490
a
b
c
Experimental.
Calculated with B3LYP/6-31G(d).
Calculated with B3LYP/6-311 + G(d).
The following conclusions can be straightforwardly derived
based on presented data. On an absolute scale, the computed
hydrogen chemical shifts for aliphatic region, methyl group, at
DFT level of theory are in good agreement with the experimental
data. Whereas, correlation between experimental and DFT calcu-
lated chemical shifts of aromatic hydrogens is less. Differences be-
tween the calculated and measured values, particularly in the case
of hydrogen-bonding atoms (OH groups), may be a result of solvent
interactions, intermolecular and/or intramolecular interactions be-
tween atoms.
Conclusion
New 2,4-triazole-based azo–azomethine dyes are prepared by
condensation reaction of 4-amino-3-methyl-5-mercapto-1,2,4-tri-
azole with various substituted azo-coupled salicylaldehyde. UV–
Vis spectra of the dyes were studied in different solvents with var-
ious polarities. The result indicated that the absorption band at
345–393 nm generally shows bathochromic shift (positive solvato-
chromism) as the polarity of solvent was increased. Cyclic voltam-
mograms of the dyes show reversible azo reduction wave which
was shift to negative potential by increasing scan rate.
Fig. 7. Plot of experimental chemical shifts vs. calculation chemical shifts from the
GIAO B3LYP/6-31G(d) and B3LYP/6-311 + G(d) in DMSO.
The 1H NMR chemical shifts of all dyes, 1b–10b, in DMSO solu-
tion were systematically studied by GIAO/DFT method. The com-
puted chemical shifts correlate with the experimental ones in the
GIAO method, especially for aliphatic hydrogens. The proposed ap-
proach can be potentially useful in an extended way to predict the
chemical shifts of the 1,2,4-triazole-based azo–azomethine dyes.
and oxidation peaks are mostly dependent upon the scan rate. As
the scan rate is increased new cathodic waves are observed, the
anodic waves are eliminated and theirs wave height increased
and shift to positive and/or negative potential. Moreover, the
reversible nature of the reduction processes of azo groups, for
all prepared compounds, was confirmed from the shift of the
wave potentials to more negative values on increasing the scan
rate [30].
Acknowledgment
We are grateful to the Arak University for financial support of
this work.
Computational studies
The 1H NMR chemical shifts of all hydrogens were calculated on
the optimized structures of compounds using GIAO/DFT method
with 6-311 + G⁄ and 6-311 + G⁄ basis set for all atoms. Calculated
and measured 1H chemical shifts of all hydrogens are tabulated
in Table 4. The correlations between experimental and computed
chemical shifts of hydrogens are shown in Fig. 8. The correlations
are linear and they are described by the equation:
Appendix A. Supplementary material
The 1H NMR, IR and UV–Vis. spectra and also CV of all prepared
compounds can be obtained free of charge via http://www.araku.
ac.ir/~h_khanmohammadi/Supporting information-doc-7.rar.
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