New diaminomaleonitrile-based azo-azomethine dyes; Synthesis, characterization and spectral properties

Article abstract of DOI:10.1016/j.dyepig.2011.12.013

New diaminomaleonitrile-based azo-azomethine dyes were synthesized via a condensation between 2,6-bis(N-(2-amino-1,2-dicyanoetheneimino))-4-methylphenol and 5-(4-substituted-phenyl)-azo-salicyladehyde. FT-IR and ¹H NMR spectroscopy and mass spe

Full text of DOI:10.1016/j.dyepig.2011.12.013

Dyes and Pigments 94 (2012) 163e168
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
New diaminomaleonitrile-based azo-azomethine dyes; synthesis,
characterization and spectral properties
*
Hamid Khanmohammadi , Alieh Abdollahi
Department of Chemistry, Faculty of Science, 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:
New diaminomaleonitrile-based azo-azomethine dyes were synthesized via a condensation between
2,6-bis(N-(2-amino-1,2-dicyanoetheneimino))-4-methylphenol and 5-(4-substituted-phenyl)-azo-sali-
cyladehyde. FT-IR and 1H NMR spectroscopy and mass spectrometry studies as well as elemental analysis
data revealed the symmetrical bis-iminated structures of the dyes. The electrochemical behavior of the
dyes has been investigated by cyclic voltammetry. The effect of various solvents with different polarities
on the UVeVis spectra of the dyes was also studied. The thermal analysis data indicated that the
framework of the dyes was stable up to 285 ꢀC.
Received 14 October 2011
Received in revised form
21 December 2011
Accepted 28 December 2011
Available online 2 January 2012
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
Azo-azomethine
Cyclic voltammetry
Dye
Diaminomaleonitrile
Solvatochromism
Thermal studies
1. Introduction
developed a synthetic route to obtain such dyes containing azo,
azomethine, DAMN and phenol moieties. The choice of these
The use of 2,3-diaminomaleonitrile (DAMN) derivatives in
synthetic dye chemistry has gained great attention in the past decade
due to their impressive and useful chemical and physical properties
[1e4]. DAMNhas low basicityand nucleophilicityduetoits impressive
strong electron-withdrawing effect of two nitrile groups conjugated
with the amino groups. However, DAMN has generally been found to
react with substituted aromatic and/or aliphatic aldehydes, in the
presence of an acid catalyst, to produce mono- and/or bis-iminated
compounds [5e8]. Also, DAMN is a very attractive precursor for the
formation of symmetrical Robson-type coordination compounds [9].
Furthermore, reaction of DAMN with dicarbonyl compounds to
produce [1 þ 2] condensed diamines is also attractive (Fig. 1) [10,11].
The presence of cyano and other electron donor and/or acceptor
functions in the structure of organic compounds affects the elec-
tronic properties of such compounds [12,13] and may provide
enormous potential for the synthesis of new dyes [1e4], phthalo-
cyanines [14,15], and polydentate ligands [16,17]. This combined
with the fact that azo and azomethine groups are relatively robust
and chemically stable has prompted extensive study of azo-
azomethine compounds as dyes and colorants [1,18]. In order to
improve the spectral properties of DAMN-based azo dyes, we
compounds was motivated by two objectives: (i) to provide new
DAMN-based azo-azomethine dyes and (ii) to study their sol-
vatochromic behavior, electrochemical properties and thermal
stability. We report here the synthesis of new family of acyclic
DAMN-based azo-azomethine dyes, 3a and 3b, by condensation
reaction of previously prepared diamine (1) [19] with azo-coupled
salicylaldehyde derivatives, 5-(4-substituted-phenyl)azo-salicyla-
dehyde, Fig. 2. The new dyes have been characterized by spectro-
scopic methods (1H NMR, UV.Vis., IR and Mass) as well as elemental
analysis. Cyclic voltammograms were obtained for 3a and 3b in
DMSO in order to study of electrochemical properties of the dyes.
Also, the solvatochromic behavior and substituent effects of the
prepared dyes in various solvents were evaluated. Furthermore, the
thermal properties of the prepared dyes were examined by ther-
mogravimetric analysis. The results indicated that the framework of
the prepared dyes is stable up to 280 ꢀC.
2. Experimental
2.1. Materials
All of the reagents and solvents involved in synthesis were of
analytical grade and used as received without further purification.
* Corresponding author. Tel.: þ98 861 2777401; fax: þ98 861 4173406.
E-mail address: h-khanmohammadi@araku.ac.ir (H. Khanmohammadi).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.12.013

164
H. Khanmohammadi, A. Abdollahi / Dyes and Pigments 94 (2012) 163e168
H
X
were taken using three electrode potentiostatic systems in DMSO
H
with 0.1 mol cmꢁ3 tetrabutylammonium perchlorate (electro-
chemical grade) as supporting electrolyte. A three electrode system
was used for CV measurements in DMSO and consisted of a platinum
working electrode, a platinum auxiliary electrode, and a calomel
reference electrode.
NC
CN
H
X
H
NC
N
N
CN
2
+
O
O
H2N
NH2
NC
NH2
H2N
CN
2.3. Synthesis
R
,
,
,
X=
2.3.1. General procedure for the synthesis of azo-azomethine dyes,
3a and 3b
N
O
N
H
OH
A solution of diamine, 1, (0.17 g, 0.5 mmol) in 1:1 methanol:
acetic acid (30 mL) was slowly added to a stirring solution of azo-
coupled precursors, 2a and 2b, (1 mmol) in acetic acid (30 mL)
during a period of 10 min at 80e90 ꢀC. After heating under reflux
for 3 h the solution was filtered whilst hot. The appropriate product
was washed with hot acetonitrile and dioxan and dried in air.
Fig. 1. Reaction of dicarbonyls with DAMN to production of diamines.
2,3-Diaminomaleonitrile, 4-nitroaniline and 4-ethylaniline were
obtained from either Aldrich or Merck. 2,6-Diformyl-4-methyl
phenol
and
related
diamine,
2,6-bis(N-(2-amino-1,2-
dicyanoetheneimino))-4-methylphenol, 1, were prepared as
described previously [19,20]. Azo-coupled precursors, 2a and 2b,
were prepared as described previously [21,22].
2.3.2. 2,6-bis((4-(4-nitro-phenylazo)-phenol)-1,2-
dicyanoetheneimino)-4-methylphenol (3a)
Yield: 53%, m.p. ¼ 272e276 ꢀC. 1H NMR (d6-DMSO, 300 MHz):
d
2.29(s, 3H), 7.13(d, 2H, J ¼ 8.78 Hz), 7.91(m, 2H), 8.01(d, 4H,
2.2. Instrumentation
J ¼ 8.35 Hz), 8.06(br, 2H), 8.42(d, 4H, J ¼ 8.35 Hz), 8.53(s, 2H),
8.62(s, 2H), 8.78(s, 2H), 11.20(s, 1H), 11.54(br, 2H). IR (KBr, cmꢁ1);
3416, 3347, 3308, 3210(NeH), 2209, 2238(CN), 1734 (C]O), 1624
(C]N), 1609 (C]C), 1562 (phenoþl ring), 1523 (NO2), 1460 (N]N),
1347(NO2), 1285(CeO), 868. [M]ꢂ ¼ 850.2 molecular ion peak is
not observed. m/z ¼ 57.1, 93.1, 120.0, 146.0, 211.1, 236.2, 265.0, 313.2,
368.3. Anal. Calcd. for C43H26N14O7: C, 60.71; N, 23.05; H, 3.08.
The structure of all synthesized compounds was confirmed by 1H
NMR spectra, recorded on a Bruker AV 300 MHz spectrometer. FT-IR
spectra were recorded as pressed KBr discs, using Unicom Galaxy
Series FT-IR 5000 spectrophotometer in the region of
400e4000 cmꢁ1. Melting points were determined on Electrothermal
9200 apparatus. C. H. N. analysis was performed on a Vario EL III
elemental analyzer. Thermal analyses were performed on a Per-
kineElmer Thermogravimetric Analyzer TG/DTA 6300 instrument.
Electronic spectral measurements were carried out using
PerkineElmer Lambda spectrophotometer in the range 200e700 nm.
Massspectra ofthe dyeswereobtained onAjilentMass5973 Network
spectrometer. Cyclic voltammograms (CV) were performed using an
Autolab model PGSTAT 30V potentiostat/galvanostat. All readings
Found: C, 60.64; N, 23.31; H, 3.16%. lmax (nm) (
(Mꢁ1 cmꢁ1)): 260
3
(34,100), 374 (77,200), 426 (44,600) and 599 (7800) in DMSO.
2.3.3. 2,6-bis((4-(4-ethyl-phenylazo)-phenol)-1,2-
dicyanoetheneimino)-4-methylphenol (3b)
Yield; 44%, m.p. ¼ 294e297 ꢀC. 1H NMR (d6-DMSO, 300 MHz):
d
1.21(t, 6H, J ¼ 7.60 Hz), 2.29(s, 3H), 2.69(q, 4H, J ¼ 7.60 Hz), 7.09(d,
2H, J ¼ 8.85 Hz), 7.41(d, 4H, J ¼ 7.81 Hz), 7.78(d, 4H, J ¼ 7.81 Hz),
Fig. 2. Reaction of DAMN-based diamine with azo precursors.

H. Khanmohammadi, A. Abdollahi / Dyes and Pigments 94 (2012) 163e168
165
Table 1
Tentative assignments of some selected IRa frequencies (cmꢁ1) of the prepared compounds.
Compound
y(C]N, cyanide)
y(NeH)
y(C]N/C]C)
y(C]O)
y(CeO)
y(N]N)
y(NO2)
2a
2b
3a
e
e
1605, 1478
1604, 1479
1624, 1609
1657
1653
1283
1277
1285
1460
1173
1460
1524,1320
e
e
e
2209, 2238
3347, 3308, 3210
Hydrazone and/or enaminone
tautomer 1734
1523, 1347
3b
2210, 2237
3347, 3300, 3198
1626, 1607
Hydrazone and/or enaminone
tautomer 1698
1280
1460
e
a
KBr discs.
7.85(d, 2H, J ¼ 8.85 Hz), 7.93(s, 2H), 8.65(s, 2H), 8.67(s, 2H), 8.68(s,
2H), 11.22(s, 3H). IR (KBr, cmꢁ1); 3405, 3347, 3300, 3198 (NeH),
2210, 2237 (CN), 1698 (C]O), 1626 (C]N), 1607 (C]C), 1562
total absence of
together with the appearance of new
y(C]O) absorption in the IR spectra of 3a and 3b
y
(C]N) absorption in the
range of 1624e1626 cmꢁ1 clearly indicated that a new Schiff-base
compound had formed in each case. The symmetric N]N
stretching mode, in the IR spectra of 3a and 3b, leads to a medium
þ
(phenol ring), 1460 (N]N), 1369, 1280(CeO), 839. [M]ꢂ ¼ 816.3
molecular ion peak is not observed. m/z ¼ 57.1, 105.1, 129.1, 157.0,
211.1, 239.1, 344.1, 368.3. Anal. Calcd. for C47H36N12O3: C, 69.11; N,
intensity band at 1460 cmꢁ1, while the
y(CeO) stretching mode
20.85; H, 4.44. Found: C, 68.86; N, 21.02; H, 4.61%. lmax (nm) (
(Mꢁ1 cmꢁ1)): 255 (35,700), 360 (83,200), 372 (77,400) and 415
(46,700) in DMSO.
3
gives an intense band within the range 1280e1285 cmꢁ1. Strong
bands observed in the IR spectra of 3a and 3b in the region of
3416e3198 cmꢁ1 are due to hydrogen bonded
y(OH) in the azo
tautomer or y(NH) in the hydrazone and/or enaminone tautomers
[1], Fig. 2. The band observed at 1698e1730 cmꢁ1 may be due to C]
O groups of the hydrazone and/or enaminone tautomers, 4 and 5, in
solid state. The IR spectra of 2a and related dye, 3a, show strong
bonds at 1347 and 1523 cmꢁ1 assigned to the NO2 group [24].
3. Result and discussion
New DAMN-drived azo-azomethines, 3a and 3b, have been
synthesized via a condensation reaction of 2,6-bis(N-(2-amino-1,2-
dicyanoetheneimino))-4-methylphenol, 1, with 5-(4-substituted-
phenyl)-azo-salicyladehyde in acetic acid. All prepared compounds,
are thermally stable solids, intensely colored, soluble in DMF and
DMSO and exhibited dyes characteristics. The presence of various
coordination sites in bis-iminated compounds makes them very
versatile species, which could potentially act as multidentate
ligands.
3.2. 1H NMR spectra
The 1H NMR spectra of 3a and 3b, obtained at ambient
temperature in d6-DMSO, display signals that are consistent with
the proposed structures. The slightly broad signal at dH 11.54 and
11.22 ppm, in the spectra of 3a and 3b, respectively, is assigned to
the OH protons, as was confirmed by deuterium exchange when
D2O was added to d6-DMSO solution and this indicates that the azo
form is more stable than other forms in DMSO solution. The 1H
NMR spectra are dominated by intense features at 7.41e7.78 ppm,
for 3a, and 8.01e8.42 ppm, for 3b, which belongs to the AAi’XX’
spin system of the para-disubstituted benzene ring. The ratio Dd/J is
3.1. Infrared spectra
The positions of some of the prominent bands in the IR spectra
of 3a and 3b and their assignments based on extensive data
available for related compounds [1,18,23] are given in Table 1. The
Fig. 3. The fragments observed in the mass spectra of 3a and 3b.

166
H. Khanmohammadi, A. Abdollahi / Dyes and Pigments 94 (2012) 163e168
3b each displays two well-defined irreversible reduction waves.
The first wave is observed at ca. ꢁ1 V. This reduction process may be
due to the partial reduction of the imine bonds to secondary amines
(and subsequent protonation by phenol) [25]. This process may also
be associated with the reduction of the azo bonds. The second wave
was observed at ꢁ1.6 V and ꢁ1.1 V for 3a and 3b, respectively. In
general, the reduction of imines is believed to occur via two well
separated steps and can follow one of two mechanisms [26], with
the first one electron reduction commonly occurring anywhere in
the range from ꢁ1.3 to ꢁ1.8 V versus SCE for conjugated organic
compounds in DMSO. On scanning from 0.0 V to positive potentials
there is a multi-electron irreversible oxidation process which
indicates that oxidation of the original the dyes followed by
a chemical rearrangement or generation of a new chemical species.
3.5. Thermal properties
In order to give more insight into the structure of the prepared
dyes, the thermal studies of 3a and 3b have been carried out using
thermogravimetry techniques. In the present investigation, the
heating rates were suitably controlled at 10 ꢀC minꢁ1 under N2
atmosphere and the weight loss was measured from 25 ꢀC up to
900 ꢀC.
The initial decomposition and inflection temperatures have
been used as an indication of the thermal stability of the prepared
compounds. The TG results indicate that the framework of dyes is
stable up to 280 ꢀC, Table 2. The comparison of Td (decomposition
temperature) showed that the thermal stability of 3b is greater
than 3a, Table 2. Above decomposition temperature the TG curves
of the dyes show continuous significant weight loss up to 900 ꢀC,
Fig. 5. (For 3a see Supporting information).
The first stage of mass loss below 300 ꢀC, which along with the
endothermic peak in the DTA curve, is in agreement with the
calculated values for the removal of HCN and NO2 molecules from
the compounds.
Fig. 4. Cyclic voltammogram for 1 ꢃ 10ꢁ5 M of 3a and 3b in DMSO containing 0.1 M
TBAP.
15.1 and 14.3 for 3a and 3b, respectively. Also, doublet pattern of
high field signal is simplified after irradiation of low field doublet.
The different CH]N protons of 3a and 3b exhibit two set of singlet
resonance at dH 8.55e8.68 ppm.
3.6. The electronic absorption spectra and substituent effects
The electronic absorption spectra of the azo-coupled precursors
and their azo-azomethine derivatives, 3a and 3b, recorded in DMSO
at room temperature are given in Table 3.
Theelectronic absorption spectra of 3a and3b displaymainly four
bands, Fig. 6. The first UV band located at 255e260 nm can be
3.3. Mass spectra
Some of the fragments observed in the mass spectra of the dyes
are shown in Fig. 3. The peaks observed at m/z ¼ 368.3, 313.3 and
265.1 confirm the formation of bis-iminated compounds. The peaks
observed at m/z ¼ 344.1 in the mass spectra of 3b can be assigned to
the fragment containing an ethyl group, Fig. 3.
assigned to the moderate energy (
rings while the second band at 360e374 nm is due to low energy
*) transition involving the -electrons of the azo and azo-
methine groups [27]. It can be found that the maximum absorption
band shifts bathochromicallyalong with the increase in the electron-
accepting ability of substituents in the sequence of 3b < 3a, consis-
tent with an increase in molecular donor-acceptor polarization.
p / p*) transition of the aromatic
(p
/
p
p
3.4. Electrochemistry
The
also more red shifted than that of the related azo precursors
because of extended -resonance system. This may be inferred
from (i) selective solvation, (ii) the ability of the solvent to form
stronger intermolecular H-bonds with a particular tautomeric form
p / p* band of o-hydroxy substituted azo-azomethines is
The cyclic voltammograms (CVs) of 3a and 3b (1 ꢃ10ꢁ5 mol Lꢁ1
)
in the potential range ꢁ2 V to 1.50 V (versus Ag/AgCl) and for scan
rates of 0.01 V sꢁ1 in DMSO containing 0.1 M TBAP supporting
electrolyte are shown in Fig. 4. As can be seen from Figure 4, 3a and
p
Table 2
Thermal analyses data for 3a and 3b.
Compound, M.F. (M. Wt.)
Dissociation Stages
Temperature range
in TG (ꢀC)
Weight loss, found
(calculated) (%)
Proposed decomposition assignment
Td (ꢀC)
3a, C43H26N14O7 (850.76)
Stage I
Stage II
Stage III
Stage I
Stage II
275e300
300e390
390e900
265e330
330e900
17.22 (17.16)
6.12(6.08)
The loss of NO2 þ 2HCN
The loss of NO2
The loss of remaining organic part
The loss of 4 HCN
286
e
3b, C47H36N12O3 (816.87)
13.22 (13.78)
e
318
The loss of remaining organic part

H. Khanmohammadi, A. Abdollahi / Dyes and Pigments 94 (2012) 163e168
167
Fig. 5. TGeTGA curve of 3b.
DMSO, DMF, THF and CH3CN at a concentration of 5 ꢃ 10ꢁ5 M, Fig. 7
(See Supporting information). The polarity and hydrogen bonding
properties of the solvents used in the present investigation, indi-
Table 3
Absorption spectral data of 3a, 3b and their azo precursors in DMSO.
Compounds
lmax/nm (
3
ꢃ 104/dm
Compounds
lmax/nm (
3
ꢃ 104/dm
molꢁ1 cmꢁ1
)
molꢁ1 cmꢁ1
)
cated by the KamleteTaft parameters (
summarized in Table 4. is a scale of solvent hydrogen bond
acceptor (HBA) basicities which refers 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.
a, b, and p*) [28], are
3a
260 (3.41)
374 (7.72)
426 (4.46)
599(0.78)
246 (4.5)
382 (9.7)
551 (1.85)
3b
255 (3.57)
360 (8.32)
372 (7.74)
415 (4.67)
260 (6.36)
b
a
p*
2a
2b
is a scale of solvent dipolarity/polarizability which refer to the
ability of solvent to stabilize a charge or a dipole by virtue of its
347 (11.314)
447 (3.12)
dielectric effect. The
behavior values. The solvatochromic behavior of a dye is the shift of
the * band wavelength due to the presence of solvent with
p* value is correlated with the solvatochromic
and (iii) the intramolecular and intermolecular hydrogen bonding
between C]N or N]N and OH groups.
p
/ p
different polarity, which is due to the interaction between the
solute and solvent molecules [29]. It was found that the absorption
band at 360e374 nm generally shows bathochromic shift (positive
solvatochromism) as the polarity of solvent was increased. The
influence of solvents for the prepared dyes increases in the order
DMSO > DMF > CH3CN > THF. 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
On the other hand, the absorption spectra of 3a and 3b display
a broad band at 415e426 nm assigned to (p / p*) transition
involving the whole electronic system of the compounds with
a considerable charge-transfer (CT) character. Such a CT originates
mainly from the phenolic moiety to the maleonitrile section which
is characterized by high electron-accepting character.
3.7. Solvent effect on the electronic spectra
bonding strength in polar solvents (The b, m/D and p* values for
DMSO and DMF are higher than other solvents). Furthermore, the
absorption spectra of 3a in DMSO and DMF comprise a new band
appearing at longer wavelength (599 and 597 nm, respectively)
which exceeds by far the usual solvent shift. This behaviour may be
interpreted on the basis that the solute molecules are liable to form
a solvated complex with DMF and DMSO molecules, with higher
The behavior of the dyes in various solvents was studied. For this
purpose, the absorption spectra of 3a and 3b were measured in
Fig. 6. The absorption spectra of 3a and 3b in DMSO.
Fig. 7. The absorption spectra of 3a in solvents with different polarities.

168
H. Khanmohammadi, A. Abdollahi / Dyes and Pigments 94 (2012) 163e168
Table 4
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Solvatochromic parameters (a, b,
p*) of used solvents.
a
Solvent
a
b
p
*
3
m/D
THF
DMSO
DMF
0.00
0.00
0.00
0.19
0.55
0.76
0.69
0.40
0.55
1.00
0.88
0.66
7.58
1.74
4.05
3.81
3.94
46.45
36.71
35.49
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b
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4. Conclusion
In the present work two new bis-iminated diaminomaleonitrile-
based azo-azomethine dyes were synthesized via condensation
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2,6-bis(N-(2-amino-1,2-dicyanoetheneimino))-4-
methylphenol with 5-(4-substituted-phenyl)-azo-salicyladehyde.
The solvatochromic behaviour and substituent effects of the
prepared dyes in DMSO, DMF, CH3CN and THF were evaluated. The
results indicated that the
360e374 nm, shows bathochromic shift as the polarity of solvent
was increased.
p / p* band of these compounds, at
The prepared compounds are stable up to 285 ꢀC. According to
the thermal stability and absorption spectra we can conclude that
these compounds may be suitable for use as recording dyes.
Furthermore, the presence of various coordination sites in the
prepared compounds makes them very versatile species, which
could potentially act as multidentate ligands.
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Acknowledgments
We are grateful to the Arak University for financial support of
this work.
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The UV-Vis, 1H NMR, IR spectra and alsoTG/TGA curves of 3a and
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