A novel barbituric acid-based azo dye and its derived polyamides: Synthesis, spectroscopic investigation and computational calculations

Article abstract of DOI:10.1016/j.dyepig.2012.05.028

5-[(3,5-dicarboxyphenyl)azo]-barbituric acid, an azo-based aromatic diacid was prepared and fully characterized by FT IR, ¹H and ¹³C NMR along with UV-vis absorption spectroscopy. The effects of varying pH and solvent upon the absorp

Full text of DOI:10.1016/j.dyepig.2012.05.028

Dyes and Pigments 95 (2012) 587e599
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A novel barbituric acid-based azo dye and its derived polyamides: Synthesis,
spectroscopic investigation and computational calculations
*
Mohammad Reza Zamanloo , Amir nasser Shamkhali, Masoumeh Alizadeh, Yagoub Mansoori,
Golamhassan Imanzadeh
Department of Applied Chemistry, Faculty of Basic Science, University of Mohaghegh Ardabili, Ardabil, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
5-[(3,5-dicarboxyphenyl)azo]-barbituric acid, an azo-based aromatic diacid was prepared and fully
characterized by FT IR, 1H and 13C NMR along with UVevis absorption spectroscopy. The effects of
varying pH and solvent upon the absorption spectrum of the azo dye were investigated. The optimized
molecular structures of possible tautomeric forms and HOMOeLUMO energies of expecting preferred
forms of the synthesized heterocyclic azo dye have been calculated using density functional theory,
(B3LYP) methods with TZVP basis set level and Polarizable Continuum Model (PCM) model for solvation
effect. The calculations showed that the most probably preferred form of the azo dye in gas phase and
solution was a hydrazone-keto form. FT IR and UVevis spectroscopy, DSC, TGA and XRD techniques were
used to obtain the characteristics and structural features of the monomer-synthesized polyamides. The
influence of organic solvents on the absorption spectra of the polymers along with viscosity measure-
ments (0.19e0.44 dl/g), solubility and film formability of the synthesized polyamides were also
investigated.
Received 6 November 2011
Received in revised form
6 May 2012
Accepted 14 May 2012
Available online 2 July 2012
Keywords:
Azo dye
Barbituric acid
Computational calculations
Tautomerism
Polyamides
UVevis spectroscopy
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
tinctorial strength and brighter dyeing than those derived from
benzenoids [16e19].
Azo dyes derived from heterocyclic systems exhibit modified
dying and optical properties relative to the azo benzenoid
compounds. The subject is clarified in higher sensitivity of their
absorption spectroscopic properties to structural changes. Direct
influence of heteroatomes on the molecular orbitals, low aroma-
ticity and hyperpolarizibility along with dependence of energy
levels to substituent groups are the most important characteristics
of heterocyclic compounds to access the diversity of optical
absorption properties. Bathochromic shift in the absorption spectra
of hetarylazo dyes is the most important features of their optical
properties [1e7]. Azo dyes derived from various heterocyclic
coupling components such as thiazole, thiophene and pyrazole are
the instances of desired materials [8e15]. On the other hand, the
possibility of tautomerism in hetarylazo compounds which induce
light fastness and moreover coordination ability to metal cations to
forming complexes can verify their application fields. It is well
known that azo dyes with heterocyclic components exhibit good
Azobarbituric acid derivatives are another class of hetarylazo
compounds that characterize to show various tautomeric struc-
tures in their free acid form. They have versatile utilities as their
salts, metal complexes, solid solutions, inclusion and intercalation
compounds [20,21]. These organic pigments are distinguished by
high tinctorial strength, great hiding power, very good light fast-
ness properties, and excellent solvent fastness properties as well as
heat resistance which all indebt to more tautomeric structures.
Hence, they are applied as pigments in printing inks, distempers or
emulsion paints. Some of the spectroscopic properties and tauto-
meric forms of azobarbituric acid derivatives have been described
in recent publications [22e25].
Technical applications of azo-dye compounds mainly depend on
the oxidative, thermal and solvent resistance, along with lightness
strength and optical properties of the azo chromophore. More
superior properties can be offered in the polymeric form in view of
higher processability, free-standing film formability and photo-
activity [26,27]. Three main procedures have developed to intro-
duce an azo chromophore into a polymeric structure, including:
blending of the organic molecular pigment with the polymeric
matrix, chemical bonding of the pigment to the polymer structure
and polymerizing the pigment-containing monomers resulted to
* Corresponding author. Tel.: þ98 4515514702; fax: þ98 4515514701.
E-mail
addresses:
zamanlou@yahoo.com,
mrzamanloo@uma.ac.ir
(M.R. Zamanloo).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.05.028

588
M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
the polymers with the chromophore in side chains or in the main
chain. Non-leaching behavior combined with uniform dispersion of
colorants in polymer matrix without aggregation is the major
advantage of the last method. Hence, the design and synthesis of
functional colorants with polymerization ability have received
considerable attention [28e35]. Polyamides, as the most applicable
engineering plastics, are of great importance in this regard for their
applications as self-coloured materials. Their usages as fiber form in
DSC traces. Thermogravimetric analysis (TGA) was performed with
a PL 1500-TGA at a heating rate of 10 ꢀC/min under nitrogen
atmosphere. Inherent viscosities (hinh) were obtained on 0.5% (w/v)
polyamide solutions in H2SO4 at 30 ꢀC by an Ostwald Routine
Viscometer (Germany). Wide-angle x-ray diffraction (XRD)
measurements were performed at room temperature on a Siemens
D500 x-ray diffractometer (Germany) using Ni-filtered CoeKa
radiation.
man-made textiles take
a special attention because of the
requirement to stability and improved thermal properties espe-
cially at colored state [36e40].
2.3. Synthesis
According to our literature survey there are a few publications
on barbituric acid-based azo dye polymers and their metal
complexes. In this research, a new azo dye monomer was synthe-
sized from barbituric acid and utilized to prepare a series of
aromatic polyamides. The optimized molecular geometries of seven
tautomeric forms of the synthesized hetarylazo monomer were
calculated to distinguish the most stable configuration playing an
important contribution in controlling spectroscopic properties. The
monomer and polymers with the chromophore in side chains were
evaluated for absorption spectroscopic aspects in aqueous and
organic media using UVevis measurements.
2.3.1. 5-[(3,5-dicarboxyphenyl)azo]-barbituric acid (monomer) (IV)
5-aminoisophthalic acid (1.00 g, 5.52 mmol) was dissolved in
12.0 ml of a mixture of DMF and concentrated hydrochloric acid, (2/
1 V). The solution was then cooled to ꢁ5e0 ꢀC and sodium nitrite
(0.427 g, 6.18 mmol, 14.2% aq) was added dropwise with vigorous
stirring. The resultant mixture was stirred for 30 min at 0 ꢀC, and
then 0.707 g (5.52 mmol) of barbituric acid dissolved in 5 ml of
aqueous solution of sodium acetate 5% (w/v) was added portion-
wise to the diazonium solution. The resulting mixture was stirred
for 3.5 h at that temperature. A yellow solid product separated upon
dilution with cold water which was filtered off and washed several
times to reach neutral state. The crude dye was purified by repre-
cipitating of DMF solution to a large amount of water to afford
a bright yellow powder (1.66 g, 94%, vacuum dried at 100 ꢀC for 3 h).
d.t. 319e326 ꢀC, FT IR(KBr, cmꢁ1): 3450 (m), 3200 (m), 3079 (m),
2848 (w), 1720 (s), 1670 (s), 1643 (m), 1527 (s), 1458 (s), 1434 (s),
2. Experimental
2.1. Materials
The used chemicals were purchased from Merck Chemical Co.,
Fluka Chemical Co., and Aldrich Chemical Co. 5-aminoisophethalic
acid was recrystallized from a mixture of N,N’-dimethylformamide
(DMF)/water (3/1 V). 4,4’-diaminodiphenyl ether, 4,4’-dia-
minodiphenyl sulfone, 3,3’-diaminodiphenyl sulfone, 4,4’-dia-
1397 (s), 1278 (s), 1218 (s), 1085 (w). 1H NMR (DMSOed6,
8.25 (s, 2H), 8.34 (m, 1H), 11.34 (s, 1H), 11.53 (s, br, 1H), 14.06 (s, br,
1H). 13C NMR (DMSOed6,
, ppm): 166.5 (C11, 12), 162.1 (C4), 160.2
d, ppm):
d
(C2), 150.2 (C1), 142.9 (C6, 10), 133.1 (C8), 126.9 (C7, 9), 121.7 (C5), 119.4
(C3). Elemental analysis calculated for C12H8N4O7: C, 45.00%; H
2.52%; N, 17.50%; Found: C, 44.12%; H, 3.20%; N, 16.95%.
minodiphenylmethane
and
1,5-diaminonaphtalene
were
recrystallized from ethanol/water (3/1 V). 1,4-diphenylenediamine,
1,3-diphenylenediamine and 4-methyl-1,3-phenylenediamine
were sublimed under reduced pressure. N,N’-dimethylacetamide
(DMAC), N-methyl pyrrolidone (NMP) and pyridine were stirred
over powdered calcium hydride or KOH overnight and then
distilled under reduced pressure. Triphenylphosphite (TPP) was
used without further purification. Anhydrous calcium chloride was
dried under vacuum at 120 ꢀC for 5 h.
2.3.2. Synthesis of model compound
In a 50 ml round-bottom flask equipped with a magnetic stirrer
and reflux condenser, were placed 0.500 g (1.56 mmol) of dicar-
boxylic acid (IV), 0.335 g (3.12 mmol) of p-toluidine, 1.938 g
(6.24 mmol, 1.64 ml) of TPP, 4.0 ml of DMAC, 0.8 ml of pyridine and
0.78 g of CaCl2. The mixture was heated with stirring at 110 ꢀC for
6 h under argon atmosphere. After cooling, 100 ml of methanol was
poured into the resulting mixture and the yellowish precipitate was
filtered off, washed thoroughly with methanol and dried in vacuum
at 100 ꢀC for 4 h. The purification of the product was performed by
reprecipitating from DMF to methanol (0.51 g, 65.4%), d.t.
335e341 ꢀC. FT IR (KBr, cmꢁ1): 3425 (s), 3133 (m), 3067 (m), 2920
(m), 2852 (m), 1741 (s), 1722 (s), 1665 (s), 1650 (s), 1597 (s), 1533 (s),
1518 (m), 1451 (w), 1420 (m) 1395 (s), 1317 (s), 1279 (m). 1H NMR
2.2. Instruments and measurements
Melting points were determined on a Stuart SMP-3 melting
point apparatus with a heating rate of 2 ꢀC/min and not corrected.
Fourier transform infrared (FT IR) spectra (in KBr pellets) were
recorded on a perkin-Elmer RX-I spectrometer in the region of
4000e400 cmꢁ1. Vibration transition frequencies are reported in
wave-number (cmꢁ1). Band intensities are assigned as weak (w),
medium (m), shoulder (sh), strong (s) and broad (br). 1H NMR and
13C NMR spectra were recorded on a Bruker 300 MHz and 75 MHz
spectrometer respectively, in DMSOed6 using tetramethylsilane
(TMS) as an internal reference. Multiplicity of proton resonances
was designated as singlet (s), broad (br) and multiplet (m). A double
beam shimadzu UV-1650 PC spectrophotometer was used to record
the absorption spectra over a wavelength range of 200e600 nm.
Quartz cuvettes were used for measurements in solution. The
diffuse reflectance spectra were recorded on powder samples with
a Color GrapgTM spectrophotometer (Miton Roy Co). Differential
Scanning Calorimetry (DSC) was done using an NETSCH DSC 200 F3.
The measurements were performed in the range of 25e400 ꢀC at
a heating rate of 10 ꢀC/min under a nitrogen atmosphere as cycles
consisting of 1st heatingecoolinge2nd heating scans. Glass tran-
sition temperatures (Tg) were read at the middle of inflections in
(DMSOed6,
J ¼ 7.2 Hz, 4H), 8.25 (m, 2H), 8.32 (s, 1H), 10.42 (t, J ¼ 14.8 2H), 10.88
(s, 1H), 11.39 (s, br, 1H), 14.23 (s, br, 1H). 13C NMR (DMSOed6,
d
, ppm): 2.28 (s, 6H), 7.17 (d, J ¼ 7.2 Hz, 4H), 7.67 (d,
d,
ppm): 165.3 (C11), 164.7 (C4), 150.2 (C2), 142.3 (C1), 137.2 (C7, 9), 137.0
(C15), 133.4 (C12), 133.37 (C6, 10), 129.5 (C14, 16), 124.0 (C5), 120.9 (C8),
119.4 (C13, 17), 118.8 (C3), 21.0 (C18) ppm.
2.3.3. Synthesis of polyamides (PAa-h)
The phosphorylation polycondensation method was used to
prepare the polyamides. As a typical case: In a 50 ml round-bottom
flask equipped with a magnetic stirrer and reflux condenser, were
placed 0.500 g (1.56 mmol) of dicarboxylic acid (IV), 0.308 g
(1.56 mmol) of 4,4’-diamino diphenyl methane, 1.938 g (6.2 mmol,
1.64 ml) of TPP, 5.0 ml of DMAC, 0.50 ml of pyridine and 0.780 g of
CaCl2. The reaction mixture was heated with stirring at 120 ꢀC for
8 h under argon atmosphere. As the polycondensation proceeded,
the solution gradually became viscous. After cooling, 100 ml of

M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
589
Table 1
Table 3
Calculated energies (E/hartree) of the azo and hydrazone tautomers of azo-dye (IV).
Selected interatomic distances and dihedral and torsion angles for the azo and
hydrazone tautomeric forms (2) and (5) of the azo dye (IV) calculated at TZVP basis
set level.
Tautomer
E/Hartree
Tautomer
E/Hartree
Azo-Keto (1)
Azo-Enol (2)
Azo-Enol (3)
Azo-Enol (4)
ꢁ1207.7102694
ꢁ1207.7413862
ꢁ1207.7101748
ꢁ1207.6987158
Selected bond distances (Ǻ)
Hydrazone-Keto (5)
Hydrazone-Keto (6)
Hydrazone-Keto (7)
ꢁ1207.7665129
ꢁ1207.7313351
ꢁ1207.7019972
Azo (2)a
Hydrazone (5)
N18-N19
N19-C20
C20-C22
C22-O27
O27-H29
H29-N18
N18-C1
1.27
1.38
1.39
1.34
0.68
1.67
1.42
1.32
1.33
1.46
1.27
1.84
1.02
1.41
methanol was poured into the resulting viscous mixture with rapid
stirring and the yellow precipitate was filtered off, washed thor-
oughly with methanol and dried in vacuum at 100 ꢀC for 4 h
(0.352 g, 93.6%), d.t. > 350 ꢀC. The inherent viscosity of the resulting
polymer, PAb, was 0.42 dl/g (Conc. H2SO4, 30 ꢀC).
Selected angles (ꢀ)
C20-N19-N18
C1-N18-N19
C20-C22-O27
C22-C20-N19-N18
C6-C1-N18-N19
C21-C20-N19-N18
C2-C1-N18-N19
116.90
116.27
123.10
0.00
0.00
180.00
179.98
121.63
120.80
123.00
0.08
0.20
179.96
179.81
2.3.4. Polymer film preparation
An approximately 8 wt% clear solution of each of the polymers
in NMP, DMAC and DMF was transferred on a glass plate according
to the drop coating method. The subject was placed at RT in a low
humidity air chamber for 2 weeks to evaporate the solvent. The thin
films were removed from the glass plate by immersing in water.
a
Calculations were performed on the fixed structure (2).
exist in some tautomeric forms as shown in Scheme 2 for azo dye
(IV) [22e25].
2.3.5. Geometry optimization of tautomeric forms of the azo dye
All of the tautomeric structures of the azo dye were considered
for geometry optimization using Density Functional Theory (DFT)
method by B3LYP hybrid functional at TZVP basis set level [41e43].
These DFT calculations were performed using Firefly Ab initio
package [44,45]. Important bond lengths and angles for two
probably preferred tautomeric structures (2 and 5) were given in
Table 6. Since during the optimization calculations on azo-enol
structure (2), the enolic hydrogen showed more tendency to join
to N atom, the OeH bond length was held fixed. Polarizable
Continuum Model (PCM) model was used to describe the solvent
effect on tautomeric stabilization [46]. Moreover, in order to eval-
uate the UVevis spectra, the HOMO and LUMO energies of the
preferred tautomeric forms were also calculated.
To have a better understanding and describe the experimental
results DFT calculations have been performed for seven tautomers
of compound IV. The optimized molecular structures including azo-
enol and hydrazone-keto tautomers are depicted in Fig. 1.
Their calculated energies represented in Table 1 illustrate the
hydrazone-keto tautomers are more stable than their azo-enol
counterparts. This is in accordance with other literature data for
similar compounds indicating a larger stability of hydrazone
tautomers over azo forms [47,48].
On the other hand, two tautomeic structures (5) and (2) possess
minimal energy between their partner tautomers. The relative
energy difference and Gibbs free energies of these two comparable
tautomeric forms are depicted in Table 2.
The tautomeric equilibrium constant, KT, between azo and
hydrazone tautomers can be calculated using the following equa-
tion [47]:
3. Results and discussion
3.1. Structural identification and verification of monomer
KT ¼ exp ꢁ
D
G=RT
(1)
The synthetic pathway of the monomer is shown in Scheme 1.
The key step of the synthesis is in situ formation of the diazonium
salt of 5-amino isophthalic acid. Limited solubility of the amine
compound and sensitivity of the process to the reaction pH are
parameters should be considered in reaction controlling. The first
step is carried out in the combined solvent (DMF/H2O) and the
second one is take placed in the presence of sodium acetate.
Thin layer chromatography (TLC) and CHN analysis were used to
confirm the purity of the monomer and FT IR, 1H and 13C NMR along
with UVevis spectroscopic techniques as well as quantum chemical
calculations were used to evaluate the chemical structure and
structural composition of the compound. It is well known that azo
dyes having barbituric acid moieties as prepared in this study can
where the gas constant R is 8.3145 ꢂ 10ꢁ3 kJ molꢁ1 Kꢁ1 and the
temperature T is 298 K. The quantity DG is the difference between
the Gibbs free energies of the given tautomers which is defined as
Hydrazone(5)eGAzo(2). The mole fractions of individual tautomers are
calculated using the following equations [49]:
G
NAzo ¼ 1=ð1 þ KTÞ; NHydrazone ¼ KT=ð1 þ KTÞ
(2)
giving more than 96% of hydrazone-Keto form at room
temperature.
The relative energies of the tautomeric structures (2) and (5)
were also calculated (PCM) in two polar protic and aprotic media
(MeOH and DMSO) to evaluate the solvent effects on their stability
Table 2
Calculated relative energies (
Table 4
D
E/kJ molꢁ1), Gibbs free energies (G/Hartreea), tauto-
Influence of solvent on the absorption properties of azo dye (IV).a
meric equilibrium constant (KT) and mole fractions of the azo and hydrazone
tautomeric forms (NAzo and NHydrazone) of azo dye (IV) at 296 K.
Solvent
(nm) εmax
/p
*
εmax
(Lmolꢁ1 cmꢁ1
/p* εmax
max
(Lmolꢁ1 cmꢁ1
)
)
(Lmolꢁ1 cmꢁ1
)
D
EAzo (2)
/
D
EHydrazone (5)
/
GAzo (2)
(Hartree)
/
GHydrazone (5)/
(Hartree)
DMAC 382
DMSO 383
27,640
26,360
18,170
22,350
26,230
334
332 18,900
328
329
330
8260
270 10,180
262 12,240
(kJ molꢁ1
65.970
)
(kJ molꢁ1
)
00.000
KT
27.388
ꢁ1207.581039
NAzo (2)
0.0352
ꢁ1207.606986
NHydrazone (5)
0.9648
THF
HOAc
376
380
6900
7020
8680
254
8960
D
G/kJ molꢁ1
250 16,320
260 15,320
ꢁ67.860296
a
MeOH 388
1 Hartree ¼ 2625.50 kJ molꢁ1
.
Measured at concentration of 5 ꢂ 10ꢁ5 M.
a

590
M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
Table 5
HOOC
COOH
HOOC
COOH
Influence of different pH conditions on lmax (nm) of the azo dyea.
1) DMF/H O/HCl
pH
lmax (nm)
εmax (Lmolꢁ1 cmꢁ1
)
_
+
2) NaNO (eq)
-5 - 0 o
3.0
5.0
7.0
9.0
11.0
380.0
380.0
384.0
380.0
368
80,140
95,120
83,290
75,340
84,360
NH
N
Cl
C
HOOC
COOH
O
O
a
Measured at concentration of 10ꢁ5 M.
HN
NH
N
N
and absorption properties. As seem from Fig. 2, hydrazone
tautomer shows solvation stability slightly more than azo form
(0.8 kJ/mol), although this energy decrease is substantial for both
structures in respect to gas phase. On the other hand, the deference
of the relative energies has been greater in protic methanol
compared with aprotic DMSO. Calculated dipole moment for two
isomers (2) and (5) being 3.69 and 2.54 D respectively probably
define the result.
O
O
O
CH3COONa (aq, 5%)
-5 - 0 o
HN
NH
C
O
IV
Scheme 1. The synthetic route of the monomer (compound IV).
Table 3 provides DFT results for selected interatomic distances
and angles of the two tautomeric forms (2) and (5) based on the
following structural labeling.
In order to find some clues about the absorption spectra, the
highest occupied molecular orbital (HOMO) as well as HOMOe1
and lowest unoccupied molecular orbital (LUMO) energy levels
diagrams of two comparative tautomers are calculated both in gas
phase and solution, Fig. 3. According to this schematic, the elec-
tronic transitions in hydrazone form (5) take place in slightly
smaller wavelengths than the azo form. On the other hand, charge
distribution in HOMO and LUMO diagrams of both tautomers are
not considerably different which indicate electron transfer
between the two components of the chromophore, namely
hyperpolarizibility, should not perform during the electronic
transition. In addition, since localization and distribution of the
electron density of HOMO and LUMO on all of a system are
measures of light fastness, the finding orbital diagrams propose
a good photostability for azo dye (IV) [50,51]. Furthermore, the
azoehydrazone tautomerism may also improve light fastness due
to reducing in electron density at azo group by intra-molecular
resonance-assisted hydrogen bonding which results in lowered
sensitivity towards photochemical oxidation [52,53].
According to the findings of quantum chemical calculations the
synthesized azo dye should mostly exist in the hydrazone form (5)
at room temperature both as a separated molecule or solvent-
interacted. Since the investigated solvents were proton donor-
acceptor types, solvation stability could indicate the stabilization
effect of such interactions acting also in concentrated media like
solid state.
Structure (2) used for interatomic distance and angle calculations.
Both tautomers show planar geometry with torsion and dihe-
dral angles of zero and 180ꢀ respectively between barbituric unit
and the aromatic ring. Major variations in geometry are at bonds
participating in azoehydrazone tautomerism including azo bond
N18eN19, hydrogen bonds at N19 and O27 atoms (N19eH29 and
O27eH29) as well as N19eC20 and C22eO27 bonds. The remainder
of the bonds and angles in both tautomers do not change signifi-
cantly. On the other hand, bond distance N18eC1 in both azo and
hydrazone forms is in the range of a single bond length, indicating
no substantial resonance character between the chromophore and
the aromatic ring being in contrast with prediction of the planar
arrangement of both tautomers.
UVevisible absorption spectra of the azo dye were recorded over
the wavelength range of 200e800 nm, in different organic solvents
at concentration of 5 ꢂ10ꢁ5 M, Table 4. Three absorption peaks were
recorded in all spectra, Fig. 4. The shortest wavelength peak
observed in the range of 250e270 nm may be attributed to a tran-
sition of the heterocyclic moiety of the molecule with n / p*
characteristic. The n / p* assignment is confirmed by disappearing
in acidic medium where the excitation of n-electrons is expected to
be hindered by protonation. The second weak and shoulder-like
peak located around 328e334 nm can be assigned to the n / p*
electronic transition of eN]Ne group [25]. Finally, the lmax peak
must be related to a
p / p-
p* transition contributing the whole
electronic system of the compound in conjugation with the chro-
mophore. There is no any correlation between the polarity and also
donor-acceptor number of the used solvents with the main
absorption wavelength. Due to the heteroatomic structure as well as
the substantial strength of the intra and intermolecular hydrogen
bondings, electron delocalization and solvent interactions are
limited which is confirmed from the theoretical calculations.
Table 6
Solubility of azo dye (IV).a
Solvent
H2O MeOH THF Acetone HOAc Py DMF DMSO DMAc
Solubility
ꢃ
ꢃ
ꢃ
ꢁ
ꢃ
ꢃ
þ
þ
þ
a
5% (w/v); þ, Soluble at room temperature; ꢃ, Partially soluble on heating; ꢁ,
Insoluble.

M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
591
COOH
O
N
O
HN
N
COOH
O
N
H
(1)
COOH
COOH
COOH
H
O
H
O
O
N
O
N
O
N
N
HN
N
H
COOH
N
N
H
COOH
N
COOH
H
O
N
H
HO
N
O
N
H
O
(3)
(4)
(2)
COOH
COOH
COOH
H
H
O
O
O
N
O
N
O
N
O
HN
N
H
COOH
N
H
N
N
H
COOH
N
COOH
O
N
H
HO
N
O
N
H
(7)
(5)
(6)
Scheme 2. Tautomeric structures of barbituric acid-based azo dye (IV).
Absorption spectrum of the azo dye was also recorded in
aqueous media with different pHs, Fig. 5 and Table 5. The general
pattern of the spectra in acidic media was similar to the spectra in
organic solvents because the azo dye is as non-dissociated mole-
cules. Of course, as mentioned above, protonation of the more basic
site of the compound restricts the first absorption band around
256 nm. On the other hand, the second band appearing at 324 nm is
disappeared with increasing pH. It seems completely dissociated
molecules to be the preferred species in these conditions without
absorption at the referred wavelength. The principle band dis-
played a blue shift in both alkaline and acidic solution relative to
the neutral conditions. The plausible reason may be dissociation of
the acidic functionalities and so weakening the electron accepting
power of these electronegative segments of the molecule in alka-
line solutions especially after pH 9. Whereas, the observed
phenomenon in acidic solutions can arise from the protonation of
the basic sites of the molecule which results to lower the HOMO
energy level. Along with the increasing of hydronium concentration
in the dye solution, azo dye molecules get more likely aggregated to
form micellars [54]. The event was observed in our investigation as
appearing flocculates and producing a colloidal solution with
decreasing the pH to one. These results postulate that, the wave-
length changes is believed to be a substituent effect than a tauto-
meric equilibration change between the azo and hydrazone
configurations which might occur by changing pH.
the azo dye in both aqueous and non aqueous solutions begins above
500 nm exhibiting good optical transparency.
The solid state spectrum (DRS) of the azo dye was also consid-
ered. It consisted of a band with a maximum around 317 nm which
included a structureless red tail upto 448 nm. Since to stabilizing
the hydrazone form with increasing intermolecular interactions,
based on theoretical predictions, the red tail might show existence
of associated forms of the dye molecule in that tautomeric form.
The FT IR spectrum of the compound could giving information of
compact structure showed a high frequency peak assigned to an H-
bonded proton and two imidic stretching peaks appeared at 3450,
3200 and 3079 cmꢁ1 respectively. The other characteristic peaks
comprised the stretching vibration bands of the carbonyl groups at
1720, 1670 and 1648 cmꢁ1(as a shoulder), two intense sharp bands
at 1527 and 1397 cmꢁ1 and a medium one at 1085 cmꢁ1 assigned to
the C]N, CeN and NeN stretching vibrations respectively, as well
as two CeO stretching vibration bands at 1278 and 1218 cmꢁ1
.
According to these results and considering higher decomposition
melting point and thermal stability of the azo dye, greater than
300 ꢀC from TGA/DTA, the compound should be predominantly
existed as a hydrazone-keto form (5) in the solid state [55].
In 1H NMR spectrum of the azo dye, Fig. 6, a high deshielded peak
attributable to a strongly intramolecular H-bonded proton was found
at 14.05 ppm. Regarding the resonance of hydrazonic H-bonded OH
protons appearing at a lower field, the predicted structure of the
compound in solution polar media such as DMSO should be a tauto-
meric form containing intramolecular H-bonding like hydrazone-
keto form (5). Furthermore, for NeH protons of pyrimidine-trione
ring in barbituric segment, two peaks at 11.34 and 11.53 ppm are
observed displaying their similar nature and an unsymmetrical
structure for the molecule. The signal areas in the spectrum are in
The electron-poor heterocyclic pyrimidine and carboxy-
functionalized benzene rings as the formative components of the
azo chromophore strongly reduce the electron polarizibilty of the
molecule. Thus the inter and intramolecular charge transfer
between the components is diminished and the absorption maxima
are significantly blue-shifted. In addition the cut-off wavelength of

592
M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
Fig. 1. Geometry-optimized structures of tautomeric forms of azo-dye (IV).
accord with the molecular structure prediction. It seems that
excepting the H-bonded proton, the molecule lacks free phenolic
protons, because possible exchanging of such protons with solvent
could alter effectively the signal area ratios. It is previously well
known that hydroxy pyrimidine can exist in the keto structure as
barbituric acid [56]. Therefore, we suppose the structure (5), Scheme
2, to be the preferred form of the monomer in solution. The 13C NMR
spectrum, Fig. 7, also approves the above mentioned results.
point emanating from strong intermolecular hydrogen bondings.
The solubility of azo dye (IV) was evaluated in different solvents.
The results showed limited solubility in acidic or basic solvents
such as acetic acid and pyridine, Table 6.
3.2. Model compound
A model reaction was conducted to evaluate the monomer
reactivity and polymerization conditions using the azo dye and an
aromatic amine, Scheme 3. The homogeneous reaction mixture
Azo dyes having barbituric acid moieties display slight solubility
in most organic solvents, high thermal stability and high melting
COOH
O
N
HN
N
H
COOH
O
N
H
O
COOH
(2)
E
65.97
gas
O
N
O
103.6 (101.5)
HN
N
H
COOH
O
N
H
104.4 (102.8)
(5)
66.75 (67.25)
E
sol
Fig. 2. Relative stability of azo (2) and hydrazone (5) tautomers predicted by DFT (B3LYP) calculations for gas phase and PCM for solution in terms of kJ/mol. The solvation stability
values for DMSO are out of parentheses and for Methanol are inside.

M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
593
Fig. 3. Molecular orbital surface and energies of the main frontier molecular orbitals of azo (2) and hydrazone (5) tautomers (The out of parenthesis values are for gas phase and the
inside ones are for solution).
Fig. 5. Electronic absorption spectra of 10ꢁ5 M aqueous solutions of the azo dye with
different pHs.
Fig. 4. Electronic absorption spectra of the azo dye in 5 ꢂ 10ꢁ5 M organic solutions and
in solid state.
Fig. 6. 1H NMR spectrum (300 MHz, DMSO-d6) of azo dye (IV).

594
M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
produced compound (VI) which was not soluble in 5 and 10%
NaHCO3 and NaOH aqueous solutions.
The chemical structure and purity of the reaction product were
confirmed by FT IR, 1H NMR and 13C NMR spectroscopic techniques
together with thin layer chromatography (TLC). The characteristic
peaks of the FT IR spectrum appeared at 3425 cmꢁ1 (stretching
vibration of the amide NH), 1741, 1722 and 1665 cmꢁ1 (C]O of the
barbituric acid ring) as well as 1597 cmꢁ1 (eC]Ne), 1650 cmꢁ1
(amide C]O) and 1395 cmꢁ1 (eCeNe) were indications of the
azoetriketo form in the solid state.
1H and 13C NMR spectra of the model compound showed the
same features as the monomer spectra, Figs. 8 and 9: a distinct
hydrazonic peak at 14.23 ppm and two different NeHs peak’s of
barbituric ring which have been appeared at 10.88 and 11.39 ppm in
1H NMR spectrum. The other signals are simply attributed to the
compound. Here again, the prominence of the hydrazone-keto
form in tautomeric equilibrium in solution is evidenced.
3.3. Azo-barbituric polyamides (PAa-h)
3.3.1. Synthetic inspection
A series of aromatic polyamides with pendant azoebarbituric
groups were synthesized by the YamazakieHigashi phosphorylation
Fig. 7. 13C NMR (75 MHz, DMSO-d6) spectrum of azo dye (IV).
HOOC
COOH
CH
H C
3
3
O
O
CH
N
H
N
H
3
N
TPP/Py/CaCl /NMP
N
2
2
+
O
O
N
110 oC/6h/Argon
N
HN
NH
NH
O
O
2
O
HN
NH
V
O
IV
VI
Scheme 3. The synthetic route of the model compound.
Fig. 8. 1H NMR spectrum (300 MHz, DMSO-d6) of the model compound. The asterisked peaks relate to the solvent.

M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
595
Fig. 9. 13C NMR spectrum (75 MHz, DMSO-d6) of the model compound.
reaction from different aromatic diamines (VIIaeh) and azo dye (IV),
Scheme 4. The reaction parameters such as reaction time, tempera-
ture, solvent and monomer concentration were optimized. Surpris-
ingly, higher yield and polymer viscosity were obtained in DMAc than
NMP. The reaction mixture was heterogeneous in the latter solvent.
Fig. 10 represents the results of temperature and reaction time
dependence of polymerization yield and product viscosities. The
curves simply show the trends observed in polycondensation
reactions.
All polymerizations proceeded as homogeneous and trans-
parent solutions throughout the reaction. The final viscous
mixtures were poured into the methanol to produce the poly-
amides with the yields in the range of 90e96%. The results of some
measurements on PAs are summarized in Table 7.
The polymer formation was confirmed by infrared spectroscopy
and chemical solubility tests in 5% aqueous solutions of NaHCO3
and NaOH. A typical FT-IR spectrum of the polyamides is shown in
Fig. 11. The presence of the characteristic peaks of the amide groups
HOOC
COOH
O
O
Ar
N
H
N
H
n
TPP/Py/CaCl /DMAc
2
N
N
N
+
HN
Ar
HN
2
O
O
2
120 oC/ 8h/ Argon
N
O
O
HN
NH
VIIa-h
HN
NH
O
O
PAa-h
IV
O
Ar:
a
e
H
2
C
b
c
d
f
O
S
O
g
O
S
O
h
Scheme 4. The synthetic route of the azo-polyamides.

596
M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
Fig. 10. Temperature and reaction time dependence of polymerization yield and product viscosities.
and the absence of the original peaks arising from the carboxylic
functions in the corresponding diacid (IV) confirm the polymer
synthesis.
transparent layers. Seemingly, aggregation of polymer chains were
occurred in the former solvents whereas the more polar one, NMP,
suppressed the proposed phenomenon and offered a better film
formability. The resulting films exhibited various degrees of shat-
tering, as presented in Table 7. High boiling point and low diffusion
rate of the used solvents having efficient interactions with the
polymer matrix are determinant factors in the film forming
process.
DMAc solutions of the synthesized polymers were exposed to
UVevis spectroscopy to monitor the absorption characteristics
compared with the monomer as a molecular azo-dye. As Fig. 12
implies, the main chromophore absorption of the polymers
remained virtually unaltered, whereas the lower band related to
barbituric segment appeared at higher wavelengths than to the
monomer. The azo chromophore is located inside the molecule, far
from effective solvent interactions both in polymeric and molecular
structure, whereas barbituric unit can be better accessible to
solvent molecules in a molecular form than a polymeric one.
Probably the excited state of n / p* transition of barbituric unit is
unstablized with increasing solvation by a polar aprotic solvent.
3.3.2. Polymer properties
The solubility of PAaeh was tested in various organic solvents at
the concentration of 0.05 g/dl, Table 8. Polyamides PAaed showed
excellent solubility in aprotic polar solvents such as NMP, DMSO,
DMF and DMAC at room temperature. The enhanced solubility of
these polyamides is believed to be related to the presence of flex-
ible linkages in the polymer backbone. The solubility process of the
polyamides was very slow because of the presence of the polar
protic barbituric groups. The lower mobility of the solvent in
polymer matrix could also influence their film forming ability.
The azo-polyamide films were prepared from their 8% solutions
in NMP, DMAc and DMF by casting onto glass plates based on drop
coating method. The layered solutions dried in a low humidity air
environment at ambient temperature for two weeks. Some of the
resulting films from DMAC or DMF solution (PAa,b,c,d,f) were
opaque, whereas their NMP solutions and the others produced
Table 7
Some of the properties of the synthesized polyamides (PAaeh).
Polymer code
Yield (%)
96.8
hinh (dl/g) a
TD (ꢀC) b
>350
Filmc
Characteristic IR absorptions (cmꢁ1
)
PAa
0.29
Shattered to brittle large pieces
3285 (m), 3058 (w), 2924 (m), 1734 (s), 1670 (s), 1602 (s),
1499 (s), 1466 (s), 1235 (s), 1170 (s)
PAb
PAc
93.6
96.7
0.42
0.20
>350
>350
Shattered to brittle large pieces
Shattered to brittle large pieces
3315 (m), 3059 (w), 2830 (m), 1734 (s), 1702 (s), 1669 (s),
1598 (s), 1514 (s), 1466 (s), 1320 (s), 1256 (s), 1127 (m)
3252 (m), 3096 (w), 2853 (m), 1734 (s), 1672 (s), 1590 (s),
1522 (s), 1466 (m), 1420 (m), 1400 (s), 1319 (s), 1251 (m),
1185 (m)
PAd
95.7
0.18
>350
Shattered to brittle large pieces
3250 (m), 3076 (w), 2825 (m), 1734 (s), 1673 (s), 1595 (s),
1525 (s), 1479 (s), 1421 (m), 1302 (m), 1254 (m), 1150 (s),
1100 (m)
PAe
PAf
PAg
PAh
90.9
90.0
90.5
95.0
0.44
0.24
0.38
0.31
>350
>350
>350
>350
Shattered to brittle small pieces
Shattered to brittle small pieces
Shattered to brittle large pieces
Shattered to brittle small pieces
3216 (m), 3060 (w), 2851 (m), 1734 (s), 1670 (s), 1560 (w),
1515 (s), 1458 (w), 1404 (w), 1314 (s), 1248 (m), 1131 (s)
3251 (m), 3056 (w), 2818 (m), 1733 (s), 1702 (s), 1664 (s),
1609 (s), 1542 (s), 1483 (m), 1417 (m), 1350 (w), 1249 (s)
3198 (m), 3062 (w), 2819 (m), 1734 (s), 1670 (w), 1599 (s),
1522 (s), 1412 (m), 1340 (w), 1248 (s), 1136 (w)
3224 (m), 3068 (w), 2850 (m), 1734 (s), 1670 (s), 1601 (m),
1523 (s), 1458 (w), 1413 (m), 1338 (s), 1267 (s), 1132 (w)
a
Measured at a concentration of 0.5 g/dl in H2SO4 (98%) at 30 ꢀC.
Onset decomposition temperature measured by melting point apparatus.
Solution (8% w/v in NMP, DMAc and DMF) casting film via drop coating technique, ability and quality of films.
b
c

M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
597
120
PAa
PAb
PAe
100
80
60
40
20
Azo dye (IV)
0
100
200
300
400
500
600
o
Temperature ( C)
Fig. 11. FT IR spectrum of the polyamide PAc in KBr.
Fig. 13. TGA thermograms of polyamides PAa, PAb, PAe and azo dye (IV).
Table 8
The solubility behavior of the polyamides (PAaeh).a
1
PAa
PAb
Solvents PAa PAb PAc PAd PAe
PAf
PAg
PAh
NaOH (aq 5%) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ)
0.7
PAe
MeOH
HOAc
THF
DMAc
DMF
NMP
DMSO
H2SO4
e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ)
e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ)
e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ) e (ꢁ)
þ (þ) þ (þ) þ (þ) þ (þ) ꢃ (þ) ꢃ (ꢃ) þ (þ) þ (þ)
þ (þ) þ (þ) þ (þ) þ (þ) ꢃ (ꢃ) ꢃ (ꢃ) þ (þ) ꢃ (ꢃ)
þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ)
þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ)
þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ) þ (þ)
0.4
0.1
a
Solubility concentration: 5 mg/ml; ꢃ, partially soluble; ꢁ, insoluble; þ, soluble;
symbol out of parenthesis is at room temperature; symbol into the parenthesis is on
heating at 70 ꢀC; DMAc, N,N-dimethylacetamide; DMF, N,N-dimethylformamide;
DMSO, dimethylsulfoxide; NMP, N-methyl-2-pyrrolidone; THF, tetrahydrofuran.
-0.2
-0.5
The thermal stability of the azo-PAs was investigated by TGA
analysis at a heating rate of 10 ꢀC/min under a nitrogen atmosphere
and compared with the monomer thermogram. Typical thermal
traces of PAa,b,e are shown in Fig. 13. The polymers were stable up
to 250 ꢀC and their main decomposition process including azo
degradation was initiated above 280 ꢀC. Furthermore, the TGA
25
75
125
175
225
275
325
375
o
Temperature ( C)
Fig. 14. DSC Traces of polyamidse PAa, PAb and PAe.
profiles represented high carbonized residue and limiting oxygen
index at 600 ꢀC, indicating good fire retardant property of the azo-
polymers. Differential scanning calorimetry (DSC) of the selected
polymers (PAa,b,e), Fig. 14, showed high glass transition tempera-
tures. Numerous H-bonding between side barbituric groups
weaken the thermal effects on segmental motions of the polymer
chains. Numerical data related to the thermal properties of poly-
amides PAa, PAb and PAe were depicted in Table 9. In DSC curves,
no other thermal changes, except glass transition were observed
Table 9
Thermal properties data of polyamides PAb, PAc and PAd.
Polymer code
Td (ꢀC)a
Tmax (ꢀC)b
Tg (ꢀC)
Char yield (%)c
LOId
PAa
PAb
PAe
320
280
285
360
320
325
270
260
262
58.8
51.3
47.2
41.0
38.0
36.4
a
Onset of decomposition temperature.
Maximum rate of decomposition temperature.
Carbonized residue at 600 ꢀC.
Limiting oxygen index, LOI ¼ 17.5 þ 0.4CY, CY ¼ Char Yield.
b
c
Fig. 12. Electronic absorption spectra of 10ꢁ5 g/ml solution of azo-dye (IV), PAa and
PAc in DMAc.
d

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M.R. Zamanloo et al. / Dyes and Pigments 95 (2012) 587e599
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