Synthesis and thermal properties of arylazo pyridone dyes

Article abstract of DOI:10.1016/j.dyepig.2019.107602

Thermal degradation properties of 5-(4-substitutedphenylazo)-3-amido-6-hydroxy-4-methyl-2-pyridones and 5-(4-substitutedphenylazo)-3-cyano-6-hydroxy-4-methyl-2-pyridones dyes, differing in electron withdrawing and electron donating substituents in para-position of diazo components were examined. The structure of the synthesized compounds has been confirmed by ¹H NMR,¹³C NMR, FTIR, UV–Vis and XRD analysis techniques. The results obtained with thermogravimetric analysis (TGA) – derivative thermogravimetry (DTG) and differential thermal analysis (DTA) were combined with GC-mass spectral fragmentation to obtain thermal decomposition mechanism. Non-isothermal kinetics were monitored by application of TGA-DTG-DTA. For Kinetic behavior of the investigated dyes during their degradation in an inert atmosphere, Kissinger, Ozawa, Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) isoconversional (model-free) methods were applied. It was found that different thermal stabilities of investigated dyes are the consequence of their different chemical structures, including diverse substituents.

Full text of DOI:10.1016/j.dyepig.2019.107602

Accepted Manuscript
Synthesis and thermal properties of arylazo pyridone dyes
Slavica J. Porobić, Aleksandar D. Krstić, Dragana J. Jovanović, Jelena M. Lađarević,
Đurica B. Katnić, Dušan Ž. Mijin, Milena Marinović-Cincović
PII:
S0143-7208(19)31055-1
DOI:
https://doi.org/10.1016/j.dyepig.2019.107602
Article Number: 107602
Reference:
DYPI 107602
To appear in: Dyes and Pigments
Received Date: 9 May 2019
Revised Date: 30 May 2019
Accepted Date: 30 May 2019
Please cite this article as: Porobić SJ, Krstić AD, Jovanović DJ, Lađarević JM, Katnić ĐB, Mijin DušŽ,
Marinović-Cincović M, Synthesis and thermal properties of arylazo pyridone dyes, Dyes and Pigments
(2019), doi: https://doi.org/10.1016/j.dyepig.2019.107602.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Synthesis and thermal properties of arylazo pyridone dyes
Slavica J. Porobića, Aleksandar D. Krstićb, Dragana J. Jovanovića, Jelena M. Lađarevićc, Đurica B.
Katnićb, Dušan Ž. Mijinc, Milena Marinović-Cincovića,*
aUniversity of Belgrade, Institute of Nuclear Sciences “Vinča”, Laboratory for Radiation Chemistry and Physics,
“Gamma”, Mike Petrovića Alasa 12-14, P.O. Box 522, 11001 Belgrade, Serbia
bUniversity of Belgrade, Institute of Nuclear Sciences “Vinča”, Department of Physical Chemistry, Mike Petrovića Alasa
12-14, P.O. Box 522, 11001 Belgrade, Serbia
cUniversity of Belgrade, Faculty of Technology and Metallurgy, Department of Organic Chemistry, Karnegijeva Street 4,
11120 Belgrade, Serbia
Abstract
Thermal degradation properties of 5-(4-substitutedphenylazo)-3-amido-6-hydroxy-4-methyl-2-
pyridones and 5-(4-substitutedphenylazo)-3-cyano-6-hydroxy-4-methyl-2-pyridones dyes, differing in
electron withdrawing and electron donating substituents in para- position of diazo components were
1
examined. The structure of the synthesized compounds has been confirmed by H NMR,13C NMR,
FTIR, UV-Vis and XRD analysis techniques. The results obtained with thermogravimetric analysis
(TGA) – derivative thermogravimetry (DTG) and differential thermal analysis (DTA) were combined
with GC-mass spectral fragmentation to obtain thermal decomposition mechanism. Non-isothermal
kinetics were monitored by application of TGA-DTG-DTA. For Kinetic behavior of the investigated
dyes during their degradation in an inert atmosphere, Kissinger, Ozawa, Flynn-Wall-Ozawa (FWO)
and Kissinger-Akahira-Sunose (KAS) isoconversional (model-free) methods were applied. It was
found that different thermal stabilities of investigated dyes are the consequence of their different
chemical structures, including diverse substituents.
Keywords: Azo dyes; Degradation; Thermal stability; Kinetics behavior
*Corresponding Author
E-mail address: milena@vinca.rs (M. Marinović-Cincović).
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1. Introduction
Azo dyes represent a large class of compounds with wide industrial applications such as optical data
storage, dye-sensitized solar cells, laser technology, ink-jet printers, non-linear optics and colorimetric
sensors [1-9]. Also, they have found application in the textile and pharmaceutical industry [9-13].
Heterocyclic azo dyes are preferred over carbocyclic azo dyes due to prominent bathochromic effect.
Heterocyclic azo compounds containing S and N atoms have shown potential applications in a number
of biological reactions. Furthermore, these dyes are environmentally friendly [14-17].
The significance of azo dyes is reflected in a simple preparation by diazotization and azo coupling,
and possibilities of combining many different diazo compounds and coupling components in order to
obtain a large number of dyes of different colors and characteristics [18].
An important group of azo dyes is based on pyridone derivatives as coupling components [18-20].
These dyes have excellent coloration properties and give bright yellow-red hues. The utilization of the
dyes for special uses is associated with their thermal stability. Depending on the nature of substituent
(electron withdrawing or electron donating) that is located on the para- position of the diazo
components of azo pyridone dyes, different thermal stability can be obtained [21].
The results of the thermal analysis of arylazo dyes can be used to obtain useful information about their
thermal stability and also to determine the temperature range in which they can be used without
changes in their composition, color and properties.
The resistance to heat at elevated temperatures is one of the main properties required of dyes
used in high-temperature processes such as dyeing, printing and in high technology areas such as
lasers and electro-optical devices [21-28]. The kinetics of thermal degradation of dyes is an important
factor in describing the impact of experimental parameters included in the characterization procedure
or in the synthesis routes on the parameters which are related to the reactivity of dyes, in given
conditions that would dictate their application [29-34].
In this work, three new 5-(4-substituted phenylazo)-3-amido-6-hydroxy-4-methyl-2-pyridone dyes
were synthesized in order to examine their thermal stability and kinetic behavior, and compare them
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with the same quantities of their 3-cyano analogs that were previously synthesized by Ušćumlić et al.
[35, 36].
The structures of the synthesized compounds have been confirmed by elemental analysis,
FTIR, NMR, UV-Vis and XRD analysis, and thermal stability is examined using TGA-DTG and DTA
techniques. According to our best knowledge, the kinetics behavior from generic standpoints for these
synthesized pyridone dyes with established kinetic parameters (such as activation energy, E, and pre-
exponential factor, Z) is not reported in the open literature. Toward a kinetic analysis of the
synthesized azo dyes, a certain number of selected non-isothermal kinetic methods were applied.
2. Experimental
2.1. Materials
2-Cyanoacetamide (99%), Acros; ethylacetoacetate, Fluka; methanol (>99.99 %), Acros;
potassium hydroxide, Hemoss; hydrochloric acid, Merck; sulfuric acid, Zorka Šabac; aniline (>98%),
Fluka; p-bromoaniline (>98%), Fluka; 4-aminophenol (>97%), Fluka; sodium nitrite, Hemoss;
ethanol, HPLC, Fisher Chemical; DMSO (>97 %), Merck.
2.2. Synthesis
All investigated azo dyes have been synthesized from the corresponding diazonium salts and
3-amido-6-hydroxy-4-methyl-2-pyridone (1a-c) or 3-cyano-6-hydroxy-4-methyl-2-pyridone (2a-c)
using the classical reaction of diazotization and diazo-coupling for the synthesis of the azo compounds
[37]. The crude products were recrystallized from N,N-dimethylformamide.
2.2.1. Synthesis of 3-cyano-6-hydroxy-4-methyl-2-pyridone
3-Cyano-6-hydroxy-4-methyl-2-pyridone has been prepared as published before [38]. The 2-
Cyanoacetamide (5 g, 0.06 mol), ethyl acetoacetate (7.8 g, 0.06 mol) and potassium hydroxide (3.92
g, 0.07 mol) of were dissolved in methanol (120 mL). A homogeneous mixture was heated with
stirring for 8 hours in a flask equipped with an Allihn condenser. By filtration on the Büchner funnel,
the solid was separated, which was then dissolved in distilled water. Concentrated hydrochloric acid
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was added to the solution to give an acidic medium. Then, the solid substance was separated by
filtration on the Büchner funnel, and it was dried in the air.
2.2.2. Synthesis of 3-amido-6-hydroxy-4-methyl-2-pyridone
In order to convert 3-cyano-6-hydroxy-4-methyl-2-pyridone to 3-amido-6-hydroxy-4-methyl-
2-pyridone, acid hydrolysis was done using the following procedure: 3-cyano-6-hydroxy-4-methyl-2-
pyridone (1.0 g, 6.67 mmol) was added carefully to 10 mL of sulphuric acid (with 15% of SO3). The
reaction temperature was maintained below 100 °C. Then, the reaction mixture was heated for 30 min
using a water bath at 95–100 °C and after that the mixture was poured over 200 g of crushed ice. The
obtained crystals were isolated by filtration and dried on air. The structure of 3-amido-6-hydroxy-4-
methyl-2-pyridone is confirmed by the following characteristics: white crystalline substance; m.p.196-
197°C, yield 42%; FT-IR (KBr, ν/cm–1): 3482 (NH), 3419 (OH), 1670, 1623 (C=O), 1557 (NH), 1430
1
(C–N), 708 (NH); H NMR (200 MHz, DMSO-d6, δ/ppm): 2.38 (3H, s, CH3), 5.52 (1H, s, C5);13C
NMR (50 MHz, DMSO-d6, δ/ppm):169.97, 164.70, 159.37, 156.14, 101.74, 99.82, 23.14.
2.2.3. Preparation of the diazonium salt
The corresponding aniline was dissolved in concentrated hydrochloric acid (2.5 mL) and
cooled down to 0 °C. Sodium nitrate (11 mmol) was dissolved in cold water (4 mL) and added
dropwise to the aniline solution. The mixture was stirred for an hour to give the diazonium salt of the
corresponding aniline [39].
2.2.4. Synthesis of 5-(4-substitutedphenylazo)-3-amido-6-hydroxy-4-methyl-2-pyridones (1a-c) and 5-
(4-substitutedphenylazo)-3-cyano-6-hydroxy-4-methyl-2-pyridones (2a-c)
The corresponding pyridone (10 mmol) was dissolved in an aqueous potassium hydroxide
solution and cooled down to 0 °C. Then diazonium salt of selected aniline was added dropwise to a
solution of pyridone which was stirred for half an hour. Additionally, the mixture was stirred for 3
hours while maintaining a low temperature (0 °C). Further, the mixture was left in the fridge
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overnight, after that, it was filtered, washed with water and dried. The compounds were recrystallized
using N,N-dimethylformamide, and the solid substance is separated by filtration on the Büchner
funnel. Obtained dyes were dried in the air. The overall synthesis scheme is presented in Fig. 1.
Fig. 1. The dyes synthesis scheme and their azo-hydrazone tautomerism.
2.3. Methods of characterization
Elemental analysis was performed using a Vario EL III elemental analyzer.
2.3.1. FT-IR, NMR
The IR spectra were recorded using a Bomem (Canada) MB-Series 100 Fourier transform-
1
infrared (FT-IR) spectrophotometer in the form of KBr pellets. The H and 13C NMR data were
performed using a Varian Gemini 2000 (200 Hz and 50 Hz, respectively) in deuterated dimethyl
sulfoxide (DMSO-d6) and trifluoroacetic acid (CF3COOD) with tetramethylsilane (TMS) as an
internal standard. All spectral measurements were carried out at room temperature (25 °C).
2.3.2. UV/Vis spectra
The ultraviolet-visible (UV/Vis) absorption spectra were recorded on a Shimadzu UV-Visible
UV-2600 (Japan) spectrophotometer in the range 200-700nm.
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2.3.3. X-ray powder diffraction (XRD)
Azo dyes were examined by X-ray powder diffraction. XRD measurements were performed
using a Rigaku (Japan)SmartLab diffractometer with X-ray lamp working on 40V/30mA. Diffraction
data were recorded in a 2θ range from 10° to 60° counting 10°min–1 in 0.02° steps.
2.3.4. Thermogravimetric (TG) and derivative thermogravimetry (DTG) studies
The thermal stability of the samples was investigated by the TGA and DTG techniques, as well
as DTA technique, using a SetaramSetsys Evolution 1750 instrument (France). The samples were
heated from 30 to 800 °C in a flow rate of φ = 20 cm3 min–1, under the pure argon (Ar) with a heating
rate of β = 2.5, 5, 10 and 20 °C min–1. For thermal characterization purposes of the studied samples,
thermo-analytical curves at 10 °C min–1 were only displayed. The average mass of the samples was
about 4 mg.
2.3.5. Gas chromatography-mass spectrometry (GC-MS)
For GC/MS analysis Agilent Technologies 7890B gas chromatograph coupled with 5977 MSD
mass detector was used. The gas chromatograph was equipped with a capillary column HP-5 MS Inert
((5% phenyl)-methylpolysiloxane, 30m x 0.25 mm, film thickness 0.25 µm from Agilent
technologies). For data processing MassHunter Qualitative analysis software from Agilent
Technologies was used. The injector was operated at 280 °C. The oven temperature was raised from
150 °C to 305 °C at a heating rate 5 °C per minute and isothermally hold 10 minutes. As a carrier gas
helium at 0.9 ml min–1 was used. Injection volume was 1 µl. Samples were dissolved in
dimethylformamide (DMF) (1:1000) and injected in a splitless mode. Mass detector was operated at
the ionization energy 70 eV in the 40-550 amu range, threshold was 150 amu and speed scanning of
1.56 u/s.
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3. Results and discussion
3.1.FTIR and 1H NMR results
The investigated dyes exhibit azo-hydrazone tautomerism (Figure 1), due to the presence of the
–OH group in the ortho- position to the azo bridge [35].
1
The IR and H NMR data for the synthesized arylazo pyridone dyes are given in Table 1.
13
Elemental analysis and C NMR spectral data of dyes 1a-c are given in Table 2. The IR and NMR
spectra of the dyes clearly show the existence of the hydrazone form in powder form and DMSO-d6,
respectively. The infrared spectra of 1a, 1b and 1c dyes (Table 1) showed characteristic vibrations of
two carbonyl group in the region of νCO 1643–1673 cm−1 indicating the hydrazone tautomeric form.
Also, N–H stretching vibrations from hydrazone group are observed the region 3436-3457 cm−1, while
N–H stretching of pyridone moiety is noted in the region of 3129-3164 cm−1. The infrared spectra of
2a, 2b and 2c dyes (Table 1) also showed the presence of the hydrazone form: two carbonyl stretching
vibrations (1648-1674 cm−1), N–H hydrazone stretching (3385–3390 cm–1), N–H stretching from
pyridone ring (3141–3153 cm–1). Sharp peaks at 2224–2230 cm–1 are attributed to –CN group [36].
1
The H NMR spectra of all dyes (Table 1) exhibit a broad signal near 14.14–14.87 ppm. This
signal corresponds to N–H proton resonance of the hydrazone form. These experiments are in
13
agreement with the previously reported results [35,40]. Also, C NMR spectra of the 1a-1c dyes
confirmed the existence of the hydrazone form. Peaks observed in the region 166.5–166.9 ppm are
ascribed to carbonyl group of 3-amido, while peaks from originating from pyridone ring are observed
at 161.9–162.2 ppm. The signals at 134.1 (1a), 126.0 (1b) and 126.5 (1c) are ascribed to hydrazone
C=N group.
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Table 1. IR and 1H NMR spectral data of synthesized arylazo pyridone dyes
IR (KBr) ν (cm–1)
1H NMR (200 MHz, DMSO-d6, δ (ppm))
Dye
1a
νCN
νNH
νCO
2.23 (3H, s, CH3), 6.85 (2H, d, J = 8.0 Hz, Ar–H), 7.41 (2H, d, J = 8.0 Hz,
Ar–H), 7.48 (1H, s, NH2), 7.68 (1H, s, NH2), 9.68 (H, s, OH), 11.56 (1H, s,
NH on heterocyclic), 14.48 (1H, s, NH of hydrazone form)
/
3436, 3129
1670, 1643
2.23 (3H, s, CH3), 7.20 (1H, t, J = 7.0 Hz, Ar–H), 7.36-7.60 (5H, m,
NH2+Ar–H), 7.71 (1H, s, NH2), 11.68 (1H, s, NH on heterocyclic), 14.25
(1H, s, NH of hydrazone form)
/
/
1673, 1656
1673, 1653
1b
1c
3457, 3164
3445, 3148
2.21 (3H, s, CH3), 7.50 (2H, d, J = 10.0 Hz, Ar–H), 7.51 (1H, s, NH2), 7.60
(2H, d, J = 8.0 Hz, Ar–H), 7.69 (1H, s, NH2), 11.70 (1H, s, NH on
heterocyclic), 14.14 (1H, s, NH of hydrazone form)
2.50 (3H, s, CH3), 6.88 (2H, d, J = 9 Hz, Ar–H), 7.55 (2H, d, J = 9 Hz, Ar–
H), 11.93 (1H, s, N–H on heterocyclic), 14.87 (1H, s, N–H of hydrazone
form)
2224
2230
2227
3385,3153
3390,3153
3385,3141
1668, 1648
1670, 1656
1674, 1665
2a
2b
2c
2.76 (3H, s, CH3), 7.55 (5H, m, Ar–H), 11.89 (1H, s, N–H on heterocyclic),
14.55 (1H, s, N–H of hydrazone form)
2.50 (3H, s, CH3), 7.51 (2H, d, J = 9 Hz, Ar–H), 7.67 (2H, d, J = 9 Hz, Ar–
H), 11.99 (1H, s, N–H on heterocyclic), 14.45 (1H, s, N–H of hydrazone
form)
Table 2. Elemental analysis and 13C NMR spectral data of arylazo pyridone dyes (1a-c)
Dye
1a
Elemental analysis
13C NMR (50 MHz, DMSO-d6, δ/ppm)
C13H12N4O4: C, 54.17; H, 4.20; N, 19.44
Found: C, 54.01; H, 4.10; N, 19.11
166.9 (CONH2), 162.2 (Py), 162.0 (Py), 156.1 (Ar), 146.1 (Py), 134.1
(Ar), 124.0 (C=N), 122.6 (Py), 118.1 (Ar), 116.5 (Ar), 14.5 (CH3)
C13H12N4O3: C, 57.35; H, 4.44; N, 20.58
Found: C, 56.81; H, 4.31; N, 19.77
166.6 (CONH2), 162.2 (Py), 162.0 (Py), 145.6 (Py), 142.0 (Ar), 129.9
(Ar), 126.0 (C=N), 125.5 (Ar), 124.0 (Py), 116.3 (Ar), 14.5 (CH3)
1b
1c
C13H11BrN4O3: C, 44.46; H, 3.16; N, 15.95
Found: C, 44.11; H, 3.20; N, 16.12
166.5 (CONH2), 162.2 (Py), 161.9 (Py), 145.3 (Py), 141.5 (Ar), 132.6
(Ar), 126.5 (C=N), 124.6 (Py), 118.3 (Ar), 117.2 (Ar), 14.5 (CH3)
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3.2.UV-Vis results
The UV-Vis spectral data of arylazo pyridone dyes in ethanol [35] are shown in Table 3. The
obtained data confirm that the positions of the UV-Vis absorption maxima depend on the nature of
the substituents of the diazo component as well as on the nature of the substituents of the pyridone
component. Introduction of a donor substituent (–OH) into the diazo component leads to a
significant bathochromic shift of the long-wavelength absorption maximum, due to reinforced π-
delocalization. From Table 3, it can be noted that introduction of amido group in the position 3 of
the pyridone ring instead of cyano causes a hypsochromic shift of absorption maxima of the
corresponding dyes. It has been established that for 3-cyano pyridone dyes, cyano group represents
the principal acceptor of the charge transfer through the molecule [36,41]. The replacement of this
strong electron-accepting group with weaker amido- group causes the change in the transmission
mode of electron density which is reflected by afore mentioned hypsochromic shift. Also, CIE lab
diagram for dyes 1a-c and 2a-c is shown in Fig. 2.
Table 3. The wavelength maxima λmax (nm) from UV-Vis absorption spectra of arylazo pyridone dyes
in ethanol and the molar extinction coefficients
Dye
1a
1b
1c
2a
2b
2c
λmax (nm)
451.3
4.48
420.1
4.49
420.2
4.56
469.5
4.52
433.1
4.37
438.5
4.32
log ε (mol–1dm3cm–1)
Fig. 2. CIE lab diagram for dyes 1a-c and 2a-c
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3.3. XRD results
The XRD analysis for all synthesized dyes (Figs. 3a and 3b), showed relatively intense narrow
reflection peaks suggesting that the all synthesized dyes are highly crystalline, and no additional
thermal treatment is necessary. Aziz et al. [42] also confirmed the crystalline form of azo dyes through
the XRD spectra analysis. Arylazo pyridone dyes generally crystallize in hydrazone form [43, 44].
The formation of layered structures is the consequence of hydrogen bonds and π-π stacking
interaction. The change of the substituent on the phenyl ring causes a change in the dihedral angle
between the plane of the pyridone and the phenyl ring and leads to a different package of molecules.
Fig. 3. XRD patterns for dyes: a) 1a, 1b and 1c; b) 2a, 2b and 2c
3.4. Thermal analysis results
Thermal stability of azo dyes was analyzed using TGA-DTG and DTA techniques. The
temperature of the beginning of the sample decomposition represents the thermal stability of the
investigated dyes. The simultaneous TGA-DTG curves at the heating rate of 10 °C min–1 are shown in
Figs. 4-9. Based on the mass spectral fragmentation pathways, the degradation of dyes 1a-c and 2a-c
is assumed. Also, the resulting degradation fragments have been confirmed by FT-IR spectra of the
dyes before heating and the residue after heating at specified temperatures.
TGA curve (Fig. 4) of compound 1a shows that thermal degradation occurs in three steps. The
first step (at 50 – 80 °C) is due to evaporation of free adsorbed water (4%) which can be seen on DTA
+
curve (Fig. 10) as a peak at 71°C. The second step (at 277 – 400 °C) is due to loss of -C6H6ON2 , CO
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and NO (Exper. % = 63, Calcd. = 62.5 %) and the third step (at 400 – 800 °C) is due to fragmentation
of residual pyridone C5H5NO+. These mass losses are observed as two peaks in DTG feature (Fig. 4)
at 70 and 305.6 °C, and in DTA curve (Fig. 10) as endothermic peaks at 71 and 305 and 365 °C,
respectively.
Compound 1b shows three degradation steps (Fig. 5): the first step (at 180 – 270 °C) is due to
dehydroxylation of chemically bonded water (4%). The second step (at 270 – 425 °C) is due to loss of
+
–C6H5N2 , CO2, NO (Exper. % = 65.9, Calcd. = 65.8 %) and the third step is due to the fragmentation
of residual pyridone C5H4NO–. These mass losses appeared in DTG feature at 285.8 °C (Fig. 5) and at
283.6 °C as endothermic peak observed in DTA curve (Fig. 10).
Compound 1c shows four degradation steps (Fig. 6): the first step (at 70 – 95 °C) is due to the
loss of free adsorbed water (2%), the second step (at 100 – 120 °C) is due to the loss of –HCN (Exper.
+
% = 7.5, Calcd. = 7.7 %), the third step (at 290 –500 °C) is due to loss of –C6H4BrN2 and CO2
(Exper. % = 71.5, Calcd. = 71.0 %). The fourth step (at 500 –800 °C) is due to residual gaseous
compounds. These mass losses appeared in DTG feature at 101.5 and 299.6 °C (Fig. 6) and at 103.3
and 296.9 °C, as two endothermic peaks in DTA curve (Fig. 10).
The degradation of compound 2a occurs in three steps (Fig. 7): the first step (at about 80 °C) is
due to the loss of free adsorbed water (2%), the second step (at 130 – 195 °C) is due to
dehydroxylation of chemically bonded water (10%). The third step (at 275 – 700 °C) is due to the
+
loss of C6H5N2 , HCN and CH4 (Exper. % = 51, Calcd. = 50 %). These mass losses are observed as
two peaks in DTG feature (Fig. 7) at 192.1 and 291.6 °C, and in DTA curve as an endothermic peak at
80 and 192 and exothermic peak at 291.7 °C, respectively (Fig. 11).
Compound 2b shows three degradation steps (Fig. 8): the first step (at 190 – 240 °C) is due to
dehydroxylation of chemically bonded water (10%), the second step (at 288-350 °C) is due to loss of
C6H5N+ (Exper. % = 36, Calcd. = 35.8 %), the third step (at 350 – 800 °C) is due to the slow
degradation of residual pyridone C7H5N3O+. These mass losses are observed as three peaks in DTG
feature (Fig. 8) at 85 °C (free adsorbed water), 228.7 and 308.2 °C, and in DTA curve as endothermic
peaks at 79.5 and 234.6 °C, respectively, and one exothermic peak at 311.1 °C (Fig. 11).
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Compound 2c shows three degradation steps (Fig. 9): the first step at about 85 °C is due to the
loss of free adsorbed water (3%), the second step (at 260 – 320 °C) is due to loss of HBr, HCN and
·OH (Exper. % = 37, Calcd. = 37.5 %) and the third step (at 320 – 700 °C) is due to the loss of C6H6
(Exper. % = 23, Calcd. = 23.4 %). These mass losses appeared in DTG feature at a single point of
280.7 °C (Fig. 9) and at 282.1 °C as one exothermic peak in DTA curve (Fig. 11).
The appearance of some endothermic peaks in the DTA curves arises from the chemical
reaction of separated gases with residual substance. In all examined dyes, the mass losses linked to the
first step were attributed to the elimination of adsorbed water molecules, that exhibits endothermic
behavior. Initial degradation of dyes molecule structure occurs in the second degradation step (above
250 °C). The remaining of 10-30% represent some formed carbon-based residues.
Fig. 4. The TG/DTG curves of dye 1a at 10 oC min–1
Fig. 5. The TG/DTG curves of dye 1b at 10 oC min–1
Fig. 6. The TG/DTG curves of dye 1c at 10 oC min–1
Fig. 7. The TG/DTG curves of dye 2a at 10 oC min–1
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Fig. 8. The TG/DTG curves of dye 2b at 10 oC min–1
Fig. 9. The TG/DTG curves of dye 2c at 10 oC min–1
Fig. 11. DTA curves of 2a-c dyes at 10 oC min–1
Fig. 10. DTA curves of 1a-c dyes at 10 oC min–1
The thermal characteristics of the synthesized dyes, which include corresponding degradation
temperature intervals (°C), the positions of DTG and DTA peaks (expressed in °C), the mass loss
values, as well as the total mass loss values (expressed in %) were summarized in Table 4.
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Table 4. Thermal characteristics of arylazo pyridone dyes.
DTA peak value (°C)
Degradation
temperature
interval (°C)
DTG peak
Mass
Total mass
loss (%)
Compound
value (°C)
loss (%)
Endo
Exo
50 - 80
70.0
4
63
71.0
305.0
365.0
88
82
90
/
1a
1b
1c
277 - 400
400 - 800
180 - 270
270 - 425
425 - 800
70 - 95
305.6
21
4
285.8
65.9
12.1
2
283.6
294.0
100 - 120
290 - 500
500 - 800
101.5
299.6
7.5
71.5
9
103.3
296.9
303.7
291.7
80
2
130 - 195
275 - 700
192.1
291.6
10
51
63
192.0
2a
/
85.0
228.7
308.2
190 - 240
288 - 350
350 - 800
85
10
36
79.5
72
63
311.1
282.1
2b
2c
234.6
26.4
3
260 - 320
320 - 700
280.7
37
/
23
Based on these results (Fig. 4-11 and Table 4), it can be noticed that thermal stability does not
only depend on the change of the cyano group into the amide on the pyridone ring, but also on the
nature of the substituent at the p- position of the benzene ring of the coupling component. Considering
dyes 1a-c, the compound 1b exhibits the lowest decomposition temperature (270 °C), even though it
has the lowest structural steric hindrance (absence of substituent on p- position on phenyl ring),
whereas the compound 1c is of the highest one (290 °C) and the highest steric hindrance.
14

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Taking into account the somewhat different shape of the thermo-analytical curves for dyes 2a-
c compared to those present in dyes 1a-c, the dye 2c shows the lowest decomposition temperature
(260 °C), while dye 2b exhibits the highest (288 °C) (Table 4). The different thermal stabilities among
various dye groups is probably the consequence of their chemical structure including diverse
substituents that strongly affects this characterization issue. However, the slow and continuous
decomposition of the molecular structure at several degradation steps (i.e. through three to four steps)
with residues about 20% at the end of the process are observed as the general property of the studied
dyes during degradation in an inert atmosphere (Table 4). As a result of these issues, the investigated
dyes exhibit a complex thermo-analytical curves behavior. This probably can be attributed to their
hardly decomposing properties related to corresponding structure (high value of molecular mass ~
300g mol–1). The different ways by which the dyes degrade depends upon the nature of substituent
(hydroxy group or bromine atome). This phenomenon was related to the fact, whether the substituent
is in ortho-, meta-, or para-position on the aromatic ring [45]. It was observed that if the substituent is
in the para-position, the dye may show three steps of mass loss [45]. So, with respect to the
temperature of degradation beginning, the following order can be obtained: dyes 1c > 1a > 1b, and
dyes 2b > 2a > 2c. Different behavior of substituted dyes with -OH and -Br substituent is probably
due to the different strength of the hydrogen bonds between the substituent on the phenyl ring and the
hydrogen of the amide group or of the -NH group (hydrazone form).
Since that the initial degradation temperature after the release of adsorbed water molecules represents
a measure of thermal stability [46], from above-indicated results, we can conclude that dyes 1a-c
exhibits higher thermal stability than dyes 2a-c.
The enhanced thermal stability of dyes 1a-c is supported by the fact that these dyes have more
abilities to create intra-molecular hydrogen bonds between the oxygen of the carbonyl group and the
proton from the amide group [47]. In addition, the thermal resistance was highest for more hindered
dyes (Fig. 1), especially, for Br-substituted dye (about 290 °C).
15

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3.5. Mass spectra
The mass spectra of the obtained dyes are presented in Figures 12 a-f. Some characteristic degradation
fragments are presented in Table 5. Other signals are probably derived from an intermediate or are
formed by the interaction of the fragments. The corresponding chromatograms are presented in Figs.
S1-S6 (Supplementary material).
Table 5. Characteristics m/z values and corresponding fragments.
m/z
m/z
Chemical
formula
Molecular
weight, g/mol
229.24
122.13
95.10
Dye
1a
(experimental) (theoretical)
229.1
123.1
95.0
229.09
123.05
95.04
C12H11N3O2
C6H6N2O+
C5H5NO+
+
77.0
77.04
C6H5
77.11
272.1
256.0
227.1
199.1
105.0
94.1
272.09
256.10
227.09
199.07
105.04
94.03
C13H12N4O3
C13H12N4O2
C12H11N4O–
C11H9N3O
272.26
256.27
227.25
199.21
105.12
94.09
1b
+
C6H5N2
C5H4NO–
281.1
184.9
154.9
97.1
281.0
C11H10BrN3O
280.13
184.02
156.00
97.12
1c
+
184.95
154.95
97.05
C6H4BrN2
C6H4Br+
C5H7NO
240.2
105.0
254.2
91.7
240.10
105.04
254.08
91.04
C13H12N4O
240.1
2a
2b
2c
+
C6H5N2
105.12
254.25
91.11
C13H10N4O2
C6H5N+
254.1
238.1
77.1
254.08
238.09
77.04
C13H10N4O2
C13H10N4O
254.25
238.25
77.11
+
C6H5
94.1
94.03
C5H4NO–
94.09
16

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Fig. 12. Mass spectra fragmentation of dyes: a)1a, b) 1b, c) 1c, d) 2a, e) 2b, f) 2c
17

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3.6. Kinetic analysis
The kinetic parameters at various heating rates (2.5, 5, 10 and 20 °C min–1) for molecular
structure degradation process that occurred at the second steps (250-400 °C) of all investigated dyes
were determined using “model-free” kinetics.
The model-free analysis allows determining the activation energy of the reaction process
without the assumption of a kinetic model for the process [48]. Also, the reaction type is usually not
required to calculate the activation energy. The application of these methods is based on TGA curves
recorded at 4 different heating rates.
Under linear heating condition, apparent activation energy and pre-exponential factor of the
primary decomposition of all dyes are determined by two methods [49, 50]: Kissinger (1) and Ozawa
(2) method. These methods are based on the changes in the exothermic peak temperature Tmax, with
changes in the heating rate β:
ꢃ
ꢄꢅꢈꢆꢇ
ꢉꢊ
ꢋꢆ
ꢀꢁꢂ
= ꢀꢁꢂ
−
(1)
ꢋꢆ 2.303 ∙ ꢊꢄꢅꢆꢇ
ꢍ
ꢀꢁꢂꢃ = ꢀꢁꢂ ꢌꢍꢎ − 2.315 − 0.4567 ꢐꢏꢑ ꢔ (2)
ꢎ
ꢏ
ꢒꢎꢓ
The activation energy and pre-exponential factor are evaluated from the linear dependence of log β
versus 1/Tmax as the slope and intercept, respectively.
To obtain the activation energy at different degrees of conversion, isoconversional integral methods
Flynn-Wall-Ozawa (FWO) and Kissinger-Akahira-Sunose (KAS) were applied, according to the
equation (3) and (4), respectively [51, 52].
ꢌꢍ
ꢎ
ꢍ
ꢎ
ꢀꢕꢃ = ꢀꢕ ꢐꢏꢖ(ꢗ)ꢔ − 5.331 − 1.052 ꢏꢑ (3)
ꢌꢏ
ꢍ
ꢎ
ꢀꢕ ꢐ ꢘ ꢔ = ꢀꢕ ꢐꢍ ꢖ(ꢗ)ꢔ − ꢏꢑ
(4)
ꢙ
ꢑ
ꢎ
β is the heating rate (2.5, 5, 10 and 20 °C min–1), f(α) is the conversion function, Z – pre-exponential
factor, Ea – activation energy (J mol–1), R – gas constant (8.314 J mol–1 K–1), T – sample temperature
in Kelvin.
18

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The values of the activation energies can be derived from the slope lnβ = f(1/T) for FWO method or
from the slope ln(β/T2) = f(1/T) for KAS method, without knowledge of f(α), by taking the values of β
and T for different α values. The conversion degree dependence of temperature at different heating
rates is presented in Fig. 13.
The influence of different temperatures regimes upon the thermal behavior of the investigated
compound can provide kinetic parameters indicating change in the reaction pathway. The complexity
of a stage can be expressed from the activation energy dependence on the conversion degree [33].
When Ea does not depend on α, only single reaction is involved and should be described by a unique
kinetic triplet (Ea, Z, α). If activation energy changes with conversion degree, the process is complex.
In the case of an increase in Ea with α, the process involves parallel reaction. In the case of a decrease
in Ea with α and the concave shape of its evolution, the process has reversible stages. For decreasing
convex shape, the process changes the limiting stage. The calculated Ea(α) dependence is presented in
Fig. 14.
Fig.13. The conversion degree of thermal decomposition vs. temperature for dyes: a)1a, b) 1b, c)1c,
d)2a, e)2b, f)2c.
19

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Fig. 14. Activation energies versus conversion degree for dyes: a)1a, b) 1b, c)1c, d)2a, e)2b, f)2c
using FWO and KAS methods.
For compound 1a (Fig. 14a) increases in Ea value (0.45 < α < 0.65) up to approximately 390 kJ mol–1
(FWO and KAS) can be attributed to the appearance of parallel reactions involving in the degradation
process. These encompasses heterolytic splitting of azo bonds [53], where the phenol derivative in a
+
form of C6H6ON2 is releasing together with other charged chemical species. This is in accordance
with the assumed degradation process (3.4).
Decreasing of Ea values for compounds 1b, 1c, 2a, 2b and 2c with a conversion progression is
attributed to the energy relaxation of this reaction stage, with released products which are instantly
formed. However, this trend has a decreasing convex E-conversion shape (Fig 14b - f), so the
degradation process changes the limiting stage [54]. A small increase in the Ea values in the
conversion range 0.75 < α < 0.9 can be attributed to bond breaking events related to anhydrous dye
structure and gaseous products releasing CO2 and NO. The activation energy obtained by model free
methods like the Kissinger, Ozawa, FWO and KAS are comparable and in good agreement.
20

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Table 6. The value of Ea and Z using Kissinger and Ozawa method for dyes 1a-c and 2a-c
Compound
Kissinger method
Ozawa method
Ea, kJ/mol
Z, min–1
Ea, kJ/mol
Z, min–1
456.06
2.21E+41
442.74
3.85E+40
1a
1b
1c
2a
2b
(R=0.996)
(R= 0.996)
408.33
2.34E+38
5.61E+31
2.11E+55
3.67E+63
397.03
4.72E+37
1.63E+31
6.21E+41
6.77E+49
(R=0.998)
(R=0.998)
346.64
338.57
(R=0.979)
(R=0.979)
474.69
442.46
(R=0.942)
(R=0.940)
580.16
542.48
(R=0.978)
(R=0.978)
483.65
8.78E+56
451.14
2.56E+43
2c
(R=0.980)
(R=0.979)
Table 7. Kinetics and thermodynamics parameters value calculated by FOW and KAS methods for 1a
FWO
KAS
Ea
∆H
∆G
∆S
Ea
∆ H
∆ G
∆ S
(kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol)
α
0.15
0.2
258.18
273.32
291.47
313.79
333.41
351.80
366.40
380.43
380.99
383.04
377.51
361.39
345.94
301.39
253.55
268.67
286.80
309.09
328.71
347.08
361.68
375.70
376.25
378.29
372.76
356.64
341.19
296.64
149.43
149.16
148.85
148.50
148.22
147.96
147.77
147.59
147.58
147.56
147.63
147.83
148.04
148.70
0.182
0.209
0.241
0.281
0.316
0.348
0.374
0.399
0.400
0.403
0.394
0.365
0.338
0.259
262.13
278.02
297.08
320.52
341.14
360.47
375.82
390.56
391.14
393.28
387.46
370.49
354.23
307.35
257.50
273.37
292.41
315.84
336.44
355.76
371.09
385.83
386.40
388.54
382.71
365.74
349.48
302.59
149.36
149.08
148.76
148.40
148.11
147.84
147.65
147.46
147.46
147.43
147.50
147.71
147.93
148.60
0.189
0.217
0.251
0.293
0.329
0.364
0.391
0.417
0.418
0.422
0.412
0.382
0.353
0.269
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
21

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Table 8. Kinetics and thermodynamics parameters value calculated by FOW and KAS methods 2a
FWO
KAS
Ea
∆H
∆G
∆S
Ea
∆H
∆G
∆S
α
(kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol)
0.15
0.2
421.04
415.22
401.73
403.55
392.85
386.06
369.80
341.28
295.39
259.80
234.03
216.36
213.32
227.08
416.45
410.62
397.13
398.95
388.24
381.45
365.19
336.66
290.77
255.16
229.37
211.66
208.56
222.23
142.38
142.45
142.60
142.58
142.70
142.78
142.98
143.35
144.02
144.61
145.09
145.45
145.52
145.23
0.494
0.484
0.459
0.462
0.443
0.430
0.401
0.349
0.265
0.199
0.152
0.119
0.114
0.139
433.57
427.44
413.24
415.15
403.88
396.73
379.62
349.60
301.29
263.81
236.64
217.95
214.63
228.93
428.98
422.84
408.64
410.54
399.27
392.12
375.00
344.98
296.67
259.17
231.97
213.25
209.87
224.09
142.25
142.31
142.47
142.45
142.58
142.66
142.86
143.24
143.93
144.54
145.04
145.42
145.49
145.19
0.517
0.506
0.480
0.483
0.463
0.450
0.419
0.364
0.275
0.207
0.157
0.122
0.116
0.142
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
Enthalpy change, ∆H is the difference in energy level between the reagent and activated complex. The
overall positive enthalpy change affirms that degradation of all obtained dyes is an endothermic
reaction where heat energies readily absorbed by the system for breaking and formation of new
chemical bonds. The difference in free Gibbs energy, ∆G, is the total increase in energy of a system
during the approach of reagents and the formation of the activated complex. Generally, ∆G slightly
grows with the progression of the conversion degree. A positive change in ∆G indicates endergonic
reactions, i.e. non-spontaneous reactions. As degradation continues, more energy is required by the
dyes to undergo conversion. Entropy change, ∆S, is the measure of the degree of disorder of a system
where it explains the closeness of a system to its own thermodynamic equilibrium. As shown in Table
7 and 8, ∆S is high at initial conversional fraction but decreases as the conversion of sample increases.
This indicates that as dyes are being degraded thermally, more reagents is being converted causing the
unreacted reagent to decrease [55].
22

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4. Conclusion
Thermal degradation of newly synthesized 5-(4-substitutedphenylazo)-3-amido-6-hydroxy-4-
methyl-2-pyridone and previously synthesized 5-(4-substitutedphenylazo)-3-cyano-6-hydroxy-4-
methyl-2-pyridone dyes, with different substituents in para-position of diazo components was
investigated. The structures of the synthesized dyes were confirmed by NMR, FTIR, UV-Vis and
XRD techniques. According to FTIR and NMR spectral data, dyes exist in the hydrazone form in the
solid state and in DMSO-d6. By application of simultaneous TGA-DTG technique with additional
DTA analysis, the thermal stability and kinetic analysis of the degradation process of the investigated
dyes were performed. The thermal analysis has shown that these compounds are stable up to 250 °C.
As a result of this behavior, these dyes are suitable for use in high-temperature processes as
photocopying and printing. Considering ultimate criteria related to the initial temperature for
occurring of the primary degradation step, the following orders of thermal stability of dyes were
established: dyes 1c (Br-substituted) > 1a (OH-substituted) > 1b (unsubstituted), and dyes 2b
(unsubstituted) > 2a (OH-substituted) > 2c (Br-substituted). It was established that independently of a
given substituent, the thermal stabilities of synthesized dyes are different. Generally, dyes 1a-c are
more stable than dyes 2a-c.
It was found that different thermal stabilities of various dye groups are the consequence of
their different chemical structures, including diverse substituents that strongly affects thermal analysis
characterization issues. Complicated thermo-analytical curves for all dyes were identified. Results
obtained from TGA, DTG and DTA in combination with GC-mass spectral fragmentation give proper
consent. The dyes degradation occurs due to breaking of C=N bond (hydrazone form), except for dye
2b, which degrade through the loss of N proton, followed by tautomerization to azo form and N=N
bond breaking.
The kinetic parameters indicating change in the reaction pathways were established using
various isoconversional (model-free) methods. Based on these results, it was found that the
degradation mechanism of investigated arylazo pyridone dyes is complex. During the main
23

ACCEPTED MANUSCRIPT
degradation stage, the activation energy decreases with the conversion progress, indicating a complex
degradation process.
Acknowledgments
Authors would like to acknowledge the financial support of Ministry of Education, Science
and Technological Development of the Republic of Serbia under the Projects 45020, 172056 and
172013.
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