1. Introduction

Computational Chemistry

2332-5968 2332-5984

Scientific Research Publishing

10.4236/cc.2024.122002

cc-136263

Articles

Chemistry Materials Science

Prediction of the Photofading of Selected Derivatives of 5-(4-X-Phenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone: Theoretical Studies, Comparison of AM1 and PM3 Methods

Krzysztof

Wojciechowski

¹ Lucjan

Szuster

aInstitute of Environmental Engineering and Building Installations, Lodz University of Technology, Łódź, Poland

aInstitute of Leather Industry, Łódź, Poland

26 09 2024

12 02 25 56 14, March 2024 27, March 2024 27, April 2024

2014

This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

We analysed the photooxidation reaction in the electro-( ¹O ₂) and nucleophilic (

O_{2}^{• -}

) reaction of 2-pyridone azo derivatives. First, we calculated the energy (enthalpies) of tautomers formation, which is a measure of durability and the probability of their formation. We performed the light fastness calculations of the monoazopyridone dyes. Using the semi-empirical methods of quantum chemistry AM1 and PM3, the reactivity indicators of superdelocalisability (

S_{r}^{E (N)}

) and the electron density distribution in ground state on the highest occupied HOMO orbital and the lowest unoccupied excited state LUMO in 2-pyridone phenylazo derivatives were calculated. Superdelocalisability coefficients enable the stability to oxidising agents of various chemical molecules depending on the tautomeric forms in which they may occur. The results of the electron density calculations at the HOMO and LUMO boundary orbitals allow to determine the tendency to electrophilic attack with singlet oxygen ¹O ₂ or nucleophilic attack of the superoxide anion

O_{2}^{• -}

on a specific atom in the molecule. The structure of the dyes was optimised with MM+, MD and AM1 or PM3 until a constant energy value was achieved with a convergence criterion of 0.01 kcal/mol.

Photochemical Degradation AM1 and PM3 Methods Boundary Orbitals Electron Densities Electrophilic and Nucleophilic Reaction HOMO and LUMO Orbitals

1. Introduction

Azo dyes are a large group of dyes constituting about 50% of all commercial brands [1] . Their wide application results from their relatively simple synthesis and a wide range of obtained colors. Azo dyes are used for dyeing fabrics, leathers, cellulose, paper as well as in the food and cosmetics industries. In addition, they exhibit antibiotic, antifungal and anti-HIV properties. They are also used in photosensitive photovoltaic cells, biological research on fluorescent probes, photonics, non-linear optical devices, and polarising filters [2] - [4] .

Azo derivatives of 2-pyridone are used as disperse dyes for polyester and nylon [5] and in printing inks. They have been tested in thermal transfer printing processes and in the production of color filters [6] . Their fluorescent behavior has also been described. Dyes based on pyridone groups have been tested as antioxidant, antimicrobial and antitumor agents [3] [7] - [9] .

Azo derivatives of pyridone have been known since around 1960. Arylazopyridone derivatives are a particularly important class of dyes, and their use is considered one of the most important achievements of dye chemistry [10] [11] .

The success of pyridone dyes is due to the simplicity of their synthesis, their ability to couple with many diazo compounds, high coefficients of molar extinction and relatively high lightfastness [7] [12] [13] .

The color of monoazo derivatives is limited to the absorption range from greenish-yellow to orange, in a few cases to red [2] [10] [14] [15] . They are characterised by high tinctorial strength, comparable to some azonaphthols.

Theoretically, 2-pyridone azo dyes can occur in several tautomeric forms, with the hydrazone form predominating [16] - [18] . The hydrazone form of the dye is characterised by a bathochromic shift of the absorption bands in relation to the azo tautomer [19] . Tautomerism [AZO-HYD] significantly affects the rate of decomposition of dyes in the photochemical reaction [20] - [23] . However, due to the existence of the equilibrium [HYD Δ anionAZO] at different pH values, a significant technological problem of discoloration in the dyeing process may occur [24] [25] .

The substituents influence the color of azo and hydrazone forms in various ways. In azo tautomers, the acceptor substituents (E_A) in the diazo compound cause the bathochromic effect, while the electron donating substituents (E_D) cause the hipsochromic effect.

For hydrazone tautomers, the effect is reverse. Owing to this, it is possible to determine the tautomeric form, in which a given dye occurs. From the point of view of the valence bond theory, AZO dyes are characterised by significant differences between the ground and excited states. They usually occur in the form of a neutral aromatic structure, while in the excited singlet state this form is lost and the HOMO-LUMO energy differences are smaller [16] [19] .

In the tautomeric form of AZO, the phenylazo group is a substituent of E_A. Thus, the E_A groups stabilised the excited state, while the E_D groups destabilise it. Photoreduction of azo dyes by UV radiation causes the formation of hydrazine radicals, which in the disproportionation reaction cause the production of appropriate amines and iminoquinones [26] .

Reaction intermediates are unstable, have a short lifetime and are present in low concentrations. These features prevent their accumulation and do not affect the further course of the reaction.

Despite various doubts, the proposed mechanism of reduction by reducing azo bonds is currently considered to be confirmed by the results of experimental studies [27] , and derivatives containing nitro groups undergo photoreduction to a nitroso group and then to an amino group [28] [29] .

As research shows, the oxidation rate of dyes depends on their structure [30] [31] and the nature of the substituents [32] . It has been confirmed, for example, that by oxidation with the Fe³⁺-EDTA-H₂O₂ system (derivatives with -CH₃, -OCH₃, -Cl substituents were tested), Cl derivatives oxidised and discolored the fastest [32] . The analysis was performed using the DFT method [31] . Research results suggest that the photodegradation of o-hydroxyazo dyes takes place in the hydrazone form, and singlet oxygen is the oxidant [33] [34] . This leads to the hyperoxide form, which decomposes by homolysis or heterolysis to quinone and to the ionic form of the diazonium residue (X-p-diazonium ion).

The C-N bond cleavage mechanism was first proposed by Spadano et al. [35] . A mechanism involving bond cleavage has been proposed based on the analysis of the decomposition of azo dyes by hydroxyl radicals (OH^•). In experimental studies, it has been found that the attack of the radical occurs on the carbon atom attached to the azo bond. The main decomposition products in the oxidation process are benzene derivatives [32] [36] - [38] .

Initially, the reaction leads to the formation of a radical, a molecule of phenyldiazene and phenol, followed by its decomposition into a molecule of nitrogen and a phenyl radical.

The above mechanism has been proposed for the Fenton reaction Fe³⁺/H₂O₂, in which the reactant is the hydroxyl radical OH^●, and the reaction products are decay and oxidation products. However, in most cases, the o-hydroxyazo dyes are mainly in the hydrazone form and a different bond cleavage of the C-N mechanism should be proposed.

The light fastness of dyes has been the subject of many studies. The most numerous concerned azo dyes [30] [31] [39] - [55] . The properties of dyes on the dyed material depend on:

It was noticed that the increase in dye resistance results from the presence of E_A substituents in their molecules, which become unstable to oxidation. However, this theory raises several objections [40] .

Figure 1 Figure 1. Tautomeric azo-hydrazone equilibria of 5-(4-X-phenylazo)-3-cyano-1-(H or C2H5)-6-hydroxy-4-methyl-2-pyridone derivatives.

Molecular orbital (MO) calculations provide insight into the photo-oxidation reaction process, depending on the electronic properties of the molecules [58] . Most of them were carried out in dyes containing azo groups [59] [60] . To understand the influence of the dye structure on lightfastness, the identification of reactive sites in a particular dye was addressed first [61] [62] .

The rapid discoloration and degradation into small particles indicate that the reaction may proceed via N=N bond cleavage. The use of Fukui indices, which determine the reactivity of individual atoms of the molecule, is very helpful in explaining this phenomenon. The presence of the hydrazone tautomer seems to be more conducive to the C9-N8 bond cleavage mechanism ( Figure 1 ).

The aim of our research was.to calculate and compare the lightfastness of probable tautomeric forms of azo dyes, NH- and NEt-2-pyridone derivatives using the AM1 and PM3 methods. These two types of dyes exist in an equilibrium of azo (AZO1, AZO2, AZO3)—hydrazone (HYD1, HYD2, HYD3) and lactam (AZO1, AZO2, HYD3)—lactim (AZO3, HYD1, HYD2) tautomeric forms.

Molecular orbital (MO) calculations were performed to gain insight into the process of photo-oxidation with oxygen in electrophilic and nucleophilic reactions. The results of the MO calculations were used to obtain the electron density of orbital in the ground HOMO and excited LUMO orbitals ( $f_{r}^{E (N)}$ ), and determination of the place where the aerobic decomposition of azo dyes took place using singlet oxygen ¹O₂ or superoxide radical anion $O_{2}^{• -}$ [30] [41] [58] [59] [63] . According to the boundary orbital theory, only the interactions between the HOMO of the dye and the LUMO of the oxidant are considered [39] . The probability of a reaction is represented by the value of the superdelocalisation coefficient ( $S_{r}^{E}$ or $S_{r}^{N}$ ) on the appropriate nitrogen or carbon atom.

For our research, we used the AM1 and PM3 quantum-chemical calculation methods. The AM1 method is an enhanced MNDO method and uses the same input data. These are the most accurate methods used in many different calculations. Its advantages include correct mapping of interactions using hydrogen bonds, good prediction of activation energy barriers and molecular formation heat with an error of up to 40% less than in the MNDO method. Changing some of the theoretical assumptions (functions describing the repulsion between atomic cores) and assigning new parameters to them improves the efficiency of the AM1 and PM3 methods.

The main problems are the incorrect length of the oxygen-phosphorus bonds, as the nitro derivatives have excessive positive energies and the bonds in peroxides are too short. Therefore, in many cases, the PM3 method is used, which is an improved version of AM1.

The PM3 method is a reparametrised version of AM1, based on neglecting the diatom overlap approximation (NDDO). The PM3 method differs from AM1 only in the values of input parameters for calculations. The parameters in the PM3 method are derived from a wide variety of experimental data, and their comparison with the calculated properties particles using computer methods. Typical non-bonding interactions are weaker than those resulting from the calculations of the AM1 method. The PM3 is mainly used for the calculation of organic molecules, it also has parameters for the calculation of many major groups of elements. It is also used to test transition metal compounds, such as Ti, Mn, Fe, Co, Ni, Cu, Zr, Mo, Rh, Pd, Hf, Ta and W.

2. Methodology

The molecular structures of all derivatives were fully optimised using semi-empirical AM1 and PM3 quantum chemical computation methods with full optimization of all bond lengths, angles, and torsion angles (HyperChem v.8.0.6, HyperCube Inc.). After obtaining structures optimised in the ground state by the MM+ molecular mechanics method, the geometry of the molecule was completely optimised without any geometrical constraints (RMS gradient 0.01 kcal/Amol), molecular dynamics (MDs; run time 1 ps, step size 0.001 ps, simulation temperature 300 K) and the AM1 or PM3 methods. Finally, the Hartree-Fock Hamiltonian (UMF) was used to calculate the CI configuration interaction in the gas phase at 25˚C. MD and AM1 or PM3 calculations were performed 3 to 5 times until a constant lowest standard enthalpy of formation H_f (kcal mol⁻¹) was obtained (convergence limit 0.01 kcal/mol). Singlet ground state energies were refined by using multielectron configuration interactions that analysed all combinations of six electrons in the three highest occupied molecular orbitals (HOMO) and the three lowest unoccupied molecular orbitals (LUMO).

3. Results and Discussion

All dyes were subjected to MO calculations using the AM1 method [64] [65] and compared with calculations made by PM3 methods [33] [66] . The calculations were conducted under the assumption that the molecules are in a vacuum and did not consider interactions of the dyes with fibers or with the products of their photochemical decomposition.

In the calculations of dye properties, they are not associated, and their photochemical degradation is much faster than the physical disintegration of the fiber [40] [59] . Hence, the latter factor may not be taken into account when interpreting the results [53] [60] .

The calculations show the electron densities on individual atoms in the ground HOMO and the excited LUMO energy level of the dye. Frontier’s molecular orbital theory suggests that the high electron density area in the dye HOMO is the site of singlet oxygen ¹O₂ electrophilic attack. The area of high electron density in the LUMO is the location where the nucleophilic attack by the anion radical $O_{2}^{• -}$ takes place [61] [67] . It is assumed that the attack of the electrophilic factor occurs when the energy difference ΔE between the singlet oxygen LUMO and the HOMO of the dye is less than 6 eV [39] [60] .

The mechanism of the oxidation reaction with C=C singlet oxygen bonds using the MINDO/3 method is described by Dewar and Thiele [68] , and other authors [40] [67] [69] .

In our study, we performed calculations of the electron density of the highest occupied molecular orbital HOMO and the lowest unoccupied molecular orbital LUMO of the dye.

Superdelocalisability ( $S_{r}^{E (N)}$ ) describes the relative propensity of compounds to electrophilic and nucleophilic reactions with oxygen:

$S_{r}^{E (N)} = \frac{f_{r}^{E (N)}}{E_{HOMO (LUMO)}} (- a)$ (1)

where ( $f_{r}^{E (N)}$ ) is a measure of the electron density in the ground/excited state on the r-atom and a is multiplied to make this value positive (−1 eV), this allows to compare the reactivity of the corresponding atoms in various molecules [60] [69] - [71] .

In our analysis, we made the following assumptions [40] :

The larger these values are, the greater the likelihood of a reaction at carbon C using one of the active oxygen species. The results of the electrophilic ( $S_{r}^{E}$ ) and nucleophilic ( $S_{r}^{N}$ ) attack location calculations are presented in the table and in the figures.

The tested monoazo dyes, derivatives of 5-(4-X-phenylazo)-3-cyano-1-(H or C₂H₅)-6-hydroxy-4-methyl-2-pyridone, are obtained in a conventional coupling reaction. Studies of their IR and UV-Vis spectra indicate that they usually occur in the hydrazone form, although in solvents other than DMSO they may also occur in the form of the tautomeric equilibrium [AZO—HYD] [31] [42] [43] .

It is possible to predict the most probable tautomers of the studied dyes (azo-hydrazone or lactam-lactim) by means of quantum-chemical calculations and analysing changes in the energy of their formation (enthalpy) [72] - [78] .

Using the ab initio method of quantum-chemical calculations, greater stability of the lactam form than of the lactim form was predicted [79] - [81] . For example, crystalline 5-amino-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone derivatives have been found to exist as a hydrazone, while in various solvents as an equilibrium [AZO—HYD].

Using UV-Visible spectrophotometric methods, changes in absorption and their intensity (ε_max) are observed in the ranges assigned to individual AZO and/or HYD tautomers. However, absorption bands result from the overlapping of partial bands assigned to all AZO or HYD tautomers. The above situation results from the fact that the bands (AZO, HYD) have very similar values of their absorption maxima λ_max [82] .

AZO1, AZO2 dyes are lactam tautomers of the AZO3 lactim form, HYD1, HYD2 dyes are lactim tautomers, and HYD3 are lactam tautomers. According to the literature [79] , the lactim form with an aromatic structure (e.g. AZO3) should be the most stable ( Figure 1 ).

The results of the calculation enthalpy in the formation [AZO, HYD] tautomers and reactivity coefficient SE for the [ene] reaction are presented in Tables 1(a)-6(b) and a graphical presentation of changes in Figure 2 . The results of the calculation for the most propable location of the electrophilic and nucleophilic reaction are presented in Tables 7(a)-9(b) and their exemplary graphic presentation in Figure 3 and Figure 4 . The diameters of the circles are proportional to the superdelocalisation value. For our research, we chose derivatives with the largest possible difference in the σ_p-Hammett constants of the substituents (σ_p = −0.17 (Me), σ_p = −0.00(H), and σ_p = +0.78 (NO₂) [1] .

4. Tautomerism [AZO-HYD] 4.1. 5-(4-Nitrophenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone (<xref ref-type="table" rid="table1(a)"> Table 1(a) </xref> and <xref ref-type="table" rid="table1(b)"> Table 1(b) </xref>)

[AM1 method] NH_NO2 derivatives exist in the form of the tautomeric equilibrium [AZO—HYD] typical for 2-pyridone derivatives ( Figure 1 ). At the same time, they may occur in the form of a tautomeric lactam-lactim equilibrium.

The HYD3 tautomer have the lowest heat of formation (H = 48.339 kcal/mol) and the highest probability of formation. The remaining tautomers should be formed in the order (ΔH kcal/mol, Table 1(a) ):

HYD3 < AZO1(+5.456) < AZO2(+6.506) < AZO3(+11.036) < HYD2(+16.632) < HYD1(+20.920)

Table 1 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 1(a). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-(4-nitrophenylazo)-3-cyano-(1-H)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the AM1 method for the reaction [ene].

NH_NO2
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3
ΔE[kcal/mol]	53.795	54.845	59.375	69.259	64.971	48.339
C1/C2	0.0202		0.0205
C1/C3	0.0205	0.214		0.202	0.0191	0.0200
C3/C4			0.0207	0.203	0.0206	0.0203

[PM3 method]. In the PM3 method, the order is as follows (AZO1 27,709 kcal/mol; ΔH kcal/mol, Table 1(b) ):

AZO1 < HYD3(+1.538) < AZO2(+2.020) < AZO3(+2.757) < HYD2(+13.913) < HYD1(+19.024)

Table 2 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 1(b). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-(4-nitrophenylazo)-3-cyano-(1-H)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method for the reaction [ene].

NH_NO2
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3
ΔE[kcal/mol]	27.709	29.729	30.466	46.733	41.622	29.247
C2/C6			0.0122
C1/C3			0.0452
C3/C4	0.0116	0.0111
C5/C6(C4)		0.0139 (0.0113)		0.0187 (0.0219)	0.0192 (0.0216)	0.0214 (0.0189)

The NEt derivatives are formed in the following order (H, ΔH kcal/mol):

[AM1, Table 2(a) ] HYD3(49.714) < AZO1(+5.363) < AZO2(+6.317)

[PM3, Table 2(b) ] AZO1(20.042) < HYD3(+8.737) < AZO2(+9.737)

Table 3 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 2(a). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-(4-nitrophenylazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the AM1 method for the reaction [ene].

NEt_NO2
	AZO1	AZO2	HYD3
ΔE [kcal/mol]	55.077	56.031	49.714
C1/C2(C3)	0.0210	0.0217	(0.0203)
C3/C4	0.0213	0.0222	0.206
N8/C9		0.0263
C15/C16	0.0291	0.0297	0.272

Table 4 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 2(b). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-(4-nitrophenylazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method for the reaction [ene].

NEt_NO2
	AZO1	AZO2	HYD3
ΔE [kcal/mol]	20.042	29.779	28.797
C1/C2(C3)	0.0445	(0.0478)
C5/C6(C4)			0.0215 (0.0189)
C15/C16	0.0209	0.0260	0.0238

According to the AM1 method, in the NH derivative, the HYD3 tautomer is the most probable form, followed by the AZO1 tautomer, while the HYD1 tautomer is the least probable.

The PM3 method, however, indicates that the formation of the AZO1 tautomer is the most probable, followed by HYD3. However, the energy differences between these tautomers allow to assume that the state of azo-hydrazone equilibrium with small energy changes should be taken into account. According to literature data, 2-pyridone derivatives should occur mainly in the form of a hydrazone tautomer, however, these considerations do not concern the possibility of occurrence of these derivatives in the form of imido-iminol tautomers.

Spectrophotometric data do not confirm theoretical research. More precise conclusions will be possible using the method of deconvolution bands absorption into their components, described by Antonov and co-workers [82] . This method also allows to identify hidden absorption bands that can be attributed to different tautomers and to determine their percentages and equilibrium constants K_T in the mixture.

NEt derivatives according to AM1 methods ( Table 2(a) ) occur mainly in the form of the HYD3 tautomer, the least in the form of AZO2. In the PM3 method ( Table 2(b) ), the most stable form is the tautomer AZO1, the least stable and probable is AZO2.

4.2. 5-Phenylazo-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone (<xref ref-type="table" rid="table3(a)"> Table 3(a) </xref> and <xref ref-type="table" rid="table3(b)"> Table 3(b) </xref>)

[AM1 method] These derivatives exist in the form of the HYD3 tautomer, its enthalpy of formation is 43,070 kcal/mol.

Table 5 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 3(a). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-phenylazo-3-cyano-1-H-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the AM1 method for the reaction [ene].

NH_H
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3
ΔE[kcal/mol]	48.636	50.620	54.441	64.020	59.842	43.070
C1/C3	0.0273	0.0285	0.0266	0.0269	0.0271	0.0267
C2/C6	0.0270	0.0277	0.0261	0.0290	0.0285	0.0284

Table 6 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 3(b). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-phenylazo-3-cyano-1-H-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method for the reaction [ene].

NH_H
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3
ΔE[kcal/mol]	32.842	38.305	36.572	47.975	47.941	33.006
C2/C6			0.0203
C1/C3(C2)	(0.0211)	0.0224	0.0214	0.0202	0.0202	(0.0201)
C3/C4	0.0204
C5/C6(C4)		0.0186	0.0185	0.0242 (0.0208)	0.0275 (0.0248)	0.0218 (0.0244)

The other forms are less durable and the energy differences are (ΔH [kcal/mol], Table 3(a) ):

HYD3 < AZO1(+5.566) < AZO2(+7.550) < AZO3(+11.371) < HYD2(+16.772) < HYD1(+20.950)

and in the PM3 method, the order is as follows (AZO1 H = 32.842kcal/mol; ΔH kcal/mol, Table 3(b) ):

AZO1 < HYD3(+0.164) < AZO3(+3.730) < AZO2(+5.463) < HYD2(+15.099) ≤ HYD1(+15.133)

In this derivative (PM3 method) a small energy difference between the HYD2 and HYD1 tautomers is noticeable, about 0.2%.

NEt derivatives are probably formed in the following order (H, ΔH kcal/mol):

[AM1, Table 4(a) ] HYD3(44.668) < AZO1(+5.492) < AZO2(+10.139)

Table 7 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 4(a). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-phenylazo-3-cyano-1-ethyl-6-hydroxy-4-methyl-2 pyridone derivatives calculated by the AM1 method for the reaction [ene].

NEt_H
	AZO1	AZO2	HYD3
ΔE [kcal/mol]	50.160	54.807	44.668
C1/C2(C3)	0.0276 (0.0278)	(0.0290)	0.0270
C2/C6	0.0275	0.0281	0.0266
C3/C4	0.0258		0.0287
C15/C16	0.0302	0.0306

[PM3, Table 4(b) ] HYD3(35.363) < AZO1(+0.254) < AZO2(+1.924)

Table 8 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 4(b). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-phenylazo-3-cyano-1-ethyl-6-hydroxy-4-methyl-2 pyridone derivatives calculated by the PM3 method for the reaction [ene].

NEt_H
	AZO1	AZO2	HYD3
	AZO1	AZO2	HYD3
ΔE [kcal/mol]	35.617	37.287	35.363
C1/C2(C3)	0.0217	(0.0226)	0.0201
C2/C6		0.0210	0.0193
C3/C4	0.0218
C5/C6(C4)	(0.0193)	0.0180	(0.0265)
C15/C16	0.0271		0.0248

The HYD3 tautomer should form first, followed by the AZO1. However, the energy difference between these tautomers (PM3) is very small and amounts to only 0.254 kcal/mol.

Figure 2 Figure 2. Graphical presentation of changes in the formation enthalpy for [AZO, HYD] tautomers of selected [NEt or NH] derivatives of 5-phenylazo-3-cyano-6-hydroxy-4-methyl-2-pyridone calculated using the AM1 and PM3 methods (see <xref ref-type="table" rid="table3(a)"> Table 3(a) </xref> and <xref ref-type="table" rid="table3(b)"> Table 3(b) </xref>, <xref ref-type="table" rid="table4(a)"> Table 4(a) </xref> and <xref ref-type="table" rid="table4(b)"> Table 4(b) </xref>).

[PM3 method]. Pyridone dyes containing the NH group occur in the forms AZO1 and HYD3. They are least likely to occur in the form HYD1 ( Figure 2 , Table 3(b) ). This feature is confirmed by their enthalpies of formation and can be ordered as follows (ΔH = H_HYD3 − H_taut, where H_taut—energy of any tautomer, [kcal/mol]).

AZO1 > HYD3(+0.164) > AZO3(+3.730) > AZO2(+5.463) > HYD2(+15.099) ≈ HYD1(+15.134)

For NEt derivatives, the enthalpy differences in ΔH [kcal/mol] are smaller and are as follows ( Table 4(b) ):

AZO1 > HYD3(+0.394) > AZO2(+1.670)

In these derivatives, the dyes should exist mainly in the azo-lactam form, i.e. in the AZO1 or hydrazone HYD3. This is indicated by the very small energy difference between the tautomers.

In the phenylazo derivative (X=H) in the PM3 method, the differences in the enthalpy of formation of AZO1 and HYD3 isomers, both for NH and NEt derivatives, are very small and amount to 0.49% and 0.71%, respectively. These calculations allow us make conclusions about the ease of transition from one tautomeric form to another and the stabilisation of the equilibrium, whose state depends, for example, on external factors. As a result, the dye may change its shape and physicochemical properties, as well as its color or shade, e.g. depending on the conditions of thermal treatment.

4.3. 5-(4-Methylphenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone (<xref ref-type="table" rid="table5(a)"> Table 5(a) </xref> and <xref ref-type="table" rid="table5(b)"> Table 5(b) </xref>)

[AM1 method] The most likely tautomeric form is HYD3 (NH_H), with enthalpy of formation −35.245 kcal/mol. Other forms are less fast and can be ordered according to increasing energy (ΔH [kcal/mol], Table 5(a) ):

HYD3 > AZO1(+5496) > AZO2(+7533) > AZO3(+15,507) > HYD2(+16,768) > HYD1(+20,954)

Table 9 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 5(a). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-(4-methylphenylazo)-3-cyano-1-H-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the AM1 method for the reaction [ene].

NH_H
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3
ΔE[kcal/mol]	40.741	42.778	50.752	56.199	52.013	35.245
C3/C4	0.0270	0.0249	0.0241	0.0263	0.0268	0.0265
C2/C6	0.0253	0.0277	0.0262	0.0291	0.0286	0.0285

[PM3 method] In this method, the dyes are arranged in the following series (the most probable is the AZO1 tautomer with H = 25,690 kcal/mol (ΔH [kcal/mol], Table 5(b) ):

AZO1 > HYD3(+1.661) > AZO2(+3.112) > AZO3(+3.311) > HYD2(+13.261) > HYD1(+18.356)

Table 10 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 5(b). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE of 5-(4-methylphenylazo)-3-cyano-1-H-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method for the reaction [ene].

NH_H
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3
ΔE[kcal/mol]	25.690	28.802	29.001	44.046	38.951	27.351
C2/C6						0.0192
C1/C3(C2)		0.0200	0.0188	0.0173	(0.0168)
C3/C4	0.0194			0.0192		0.0217
C5/C6(C4)	0.0176	0.0194	0.0188	0.0284	0.0255 (0.0283)	(0.0279)

NEt derivatives can exist only in the form of three tautomers, and are as follows (H kcal/mol and ΔH kcal/mol):

[AM1, Table 9(a) ] HYD3(36.857) < AZO1(+5.5452) < AZO2(+7.094)

Table 11 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 6(a). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE 5-(4-methylphenyloazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2 pyridone derivatives calculated by the AM1 method for the reaction [ene].

NEt_H
	AZO1	AZO2	HYD3
ΔE [kcal/mol]	42.309	43.951	36.857
C2/C6	0.0259	0.0255	0.0287
C3/C4	0.0274	0.0280	0.0267
C15/C16	0.0304	0.0310	0.0291

[PM3, Table 5(b) ] AZO1(25.166) < HYD3(+1.125) < AZO2(+3.441)

Table 12 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 6(b). Enthalpy (energy) of formation ΔE [kcal/mol] and reactivity coefficients SE 5-(4-methylphenyloazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2 pyridone derivatives calculated by the PM3 method for the reaction [ene].

NEt_H
	AZO1	AZO2	HYD3
ΔE [kcal/mol]	25.166	28.607	26.291
C1/C2(C3)	0.0189	(0.0201)
C2/C6	0.0199	0.0209	0.0196
C3/C4	0.0207
C5/C6(C4)			(0.0281)
C15/C16	0.0270	0.0274	0.0253

Calculations reveal that the dyes exist mainly as HYD3 (AM1) or AZO1 (PM3) tautomers. It can be assumed that these dyes exist in equilibrium of azo-hydrazone tautomeric forms, their proportions can change with a slight change in the energy of the system. The HYD1 tautomer is the least probable, both in the calculations made by AM1 and PM3 methods.

In the NEt derivatives, there are similar dependencies and tautomeric equilibrium between [HYD3 Δ AZO1], the least probable is AZO2 tautomer.

Derivatives containing the p–CH₃ group occur mainly in the azo-lactam form AZO1 and the lactam-hydrazone form HYD3 ( Table 6(a) and Table 6(b) ).

5. Electrophilic Photooxidation 1O2 (SE) 5.1. 5-(4-Nitrophenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

[AM1 method] Values calculated by AM1 method show, first of all, photooxidation reactions on the N13 nitrogen atom. Further order is as follows (tautomer, S^E, Table 7(a) and Figure 3 ):

HYD3(0.0370) > HYD2(0.0322) > AZO1(0.0304) > > AZO2(0.0303) > HYD1(0.0300) > AZO3(0.0254)

Table 13 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 7(a). Reactivity coefficients S(E/N) of 5-(4-nitrophenylazo)-3-cyano-(1-H or ethyl)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the AM1 method in the electrophilic and nucleophilic oxidation reaction.

	AM1						AM1
	NH_NO2						NEt_NO2
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3	AZO1	AZO2	HYD3
C1	0.0135	0.0139	0.0128	0.0160	0.0158	0.0156	0.0137	0.0131	0.0159
C4	0.0124	0.0119	0.0134	0.0161	0.0160	0.0159	0.0144	0.0140	0.0162
C6	0.0142	0.0142	0.0107	0.0190	0.0183	0.0182	0.0125	0.0102	0.0186
C9	0.0292	0.0219	0.0173	0.0161	0.0249	0.0203	0.0300	0.0220	0.0199
N7	0.0181			0.0218	0.0206	0.0211	0.0189		0.0215
C11	0.0177	0.0247	0.0201	0.0121	0.0103	0.0104	0.0185	0.0258
C16(Et)							0.0248	0.0253	0.0240
C(CH3)	0.0211	0.0216	0.0209	0.0206	0.0213	0.0209	0.0213	0.0217	0.0369
N13	0.0304	0.0303	0.0254	0.0300	0.0322	0.0370	0.0255	0.0253	0.0323

Nucleophilic reaction.

C12	0.1676	0.0974	0.0998	0.1273	0.0850	0.1507	0.1691	0.0955	0.1570
C14	0.0652	0.1993	0.0964	0.0585	0.1172	0.1384	0.0673	0.1913	0.1446

Nitro derivatives should then oxidise on the N7(H) atom of the HYD form or C9 of the AZO form. The reaction should therefore proceed through an N-oxide or nitroso group. The highest reactivity is characteristic of the heterocyclic ring, the reactivity of carbon atoms in the phenylazo residue is up to 2 times lower than the reactivity of the N13 atom in the heterocycle ring.

The NEt derivatives should react in the following order (atom, S^E, Table 7(a) ):

HYD3(C(CH3), 0.0369) > AZO1(C9, 0.0300) > AZO2(C11, 0.0258)

The reaction products will therefore depend on the tautomeric form in which the dye will be present. As in the NH derivative, nitrogen and carbon atoms in the heterocyclic ring are characterised by higher reactivity.

[PM3 method] Calculations made by the PM3 method indicate that the C1 atom of the phenyl ring is the most reactive, followed by the C9 atom of the heterocyclic ring (S^N(C1), S^N(C1)/S^N(C9)) ( Table 7(b) and Figure 3 ):

HYD1(0.0475, 2.04) > HYD2(0.0471, 1.42) > HYD3(0.0468, 1.58) > AZO2(0.0463, 1.66) > AZO1(0.0452) > AZO3(0.0443, 1.97)

Table 14 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 7(b). Reactivity coefficients S(E/H) of 5-(4-nitrophenylazo)-3-cyano-1-(H or ethyl)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method in the electrophilic and nucleophilic oxidation reaction.

	PM3						PM3
	NH_NO2						NEt_NO2
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3	AZO1	AZO2	HYD3
C1	0.0452	0.0463	0.0443	0.0475	0.0471	0.0468	0.0436	0.0463	0.0473
C4	0.0112	0.0100		0.0171	0.0160	0.0136			0.0139
C6	0.0127	0.0126	0.0119	0.0138	0.0136	0.0161	0.0111	0.0122	0.0164
C9	0.0352	0.0278	0.0225	0.0233	0.0331	0.0296	0.0387	0.0286	0.0295
N7	0.0142						0.0163
C11	0.0155	0.0215	0.0168				0.0168	0.0231	0.0103
C16(Et)							0.0139	0.0139	0.0138
C(CH3)	0.0107	0.0110	0.0105				0.0102	0.0123	0.0104
N13	0.0170		0.0188	0.0241	0.0275			0.0167

Nucleophilic reaction.

C12	0.0994		0.1003	0.1169	0.0736	0.1021	0.1236		0.1094
N13(N7)	0.1294	0.1073		(0.2253)	(0.2238)	(0.2331)	0.0837	0.1016	(0.2405)
C14		0.1334	0.0960	0.0456	0.1172	0.0920		0.1327	0.1006

The C1 atom owes its high reactivity to the presence of a strong E_A substituent, which is the nitro group. NEt derivatives (S^N(C1), S_N(C1)/S^N(C9)) react similarly ( Table 7(b) and Figure 3 ):

HYD3(0.0473, 1.60) > AZO2(0.0463) > AZO1(0.0436, 1.13)

Figure 3 The NEt derivatives should react with 1O2 in a similar manner. The most reactive position is on the C1 carbon atom in the aromatic ring (Table 7(b)). The next reaction should take place on the C9 atom (SN(C1), SN(C1)/SN(C9)):--HYD3(0.047, 1.605) > AZO2(0.046, 1.619) > AZO1(0.044, 1.128)--The reaction should lead to heterolytic degradation of bonds in the aromatic ring of diazo compounds. In p-NO2 derivatives, photochemical decomposition is higher than in derivatives with ED groups. In the diazo compounds, the C1 carbon atom is the place where a dye molecule, first, undergoes photochemical oxidation of 1O2 (SE). The reactivity of the C9 atom is also high and comparable to reactivity of the other dyes (p-CH3, p-H), in which the photooxidation takes place on the C9 carbon atom in the 2-pyridone ring (Table 2(a) and Table 2(b), Table 3(a) and Table 3(b)). Only the PM3 method indicates high reactivity of nitro derivatives on the C1 atom. This conclusion is consistent with the results of the research on azo dyes, i.e. it proves the low fastness to oxidation of the phenyl ring [76] [83].--5.2. 5-Phenylazo-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone--In an electrophilic reaction, they undergo photooxidation on the C9 carbon atom of the heterocyclic ring.--[AM1 method] Calculations made by AM1 method indicate that electrophilic oxidation of 1O2 should first occur on the N13 nitrogen atom of the heterocyclic ring. The reactivity can be ranked as follows (SE, Table 8(a) and Figure 4):--HYD3(0.0394) > HYD2(0.0346) > HYD1(0.0332) > AZO1(0.0321) > AZO2(0.0321) > AZO3(0.0272)--Table 8(a). Reactivity coefficients S(E/N) of 5-phenylazo-3-cyano-1-(H or ethyl)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the AM1 method in the electrophilic and nucleophilic oxidation reaction.--AM1 AM1 NH_H NEt_H AZO1 AZO2 AZO3 HYD1 HYD2 HYD3 AZO1 AZO2 HYD3 C1 0.0150 0.0148 0.0147 0.0129 0.0131 0.0150 C2 0.0145 0.0147 0.0156 C3 0.0147 0.0158 0.0146 0.0150 0.0160 C4 0.0145 0.0143 0.0171 C6 0.0125 0.0176 0.0168 0.0169 C9 0.0298 0.0207 0.0248 0.0211 0.0302 0.0256 0.0308 0.0214 0.0253 N7 0.0175 0.0136 0.0211 0.0195 0.0203 0.0181 0.0208 C11 0.0194 0.0253 0.0151 0.0201 0.0263 0.0133 C16(Et) 0.0259 0.0264 0.0254 C(CH3) 0.0220 0.0223 0.0213 0.0219 0.0226 0.0222 0.0222 0.0225 0.0222 N13 0.0321 0.0321 0.0272 0.0332 0.0346 0.0394 0.0270 0.0269 0.0343--Nucleophilic reaction.--C12 0.2703 0.1391 0.1055 0.1648 0.1068 0.2010 0.2903 0.1468 0.2131 C14 0.0877 0.3227 0.1173 0.0710 0.1508 0.1832 0.0926 0.3333 0.1945--

Next, the reaction should take place on the C9 carbon atom in the heterocyclic ring (S^N(N13)/S^N(C9)):

HYD3(1.54) > HYD2(1.15) > HYD1(1.57) > AZO1(1.08) > AZO2(1.55) > AZO3(1.10)

The NEt derivatives react similarly, and the corresponding relationships are as follows (S^N(N13), S^N(N13)/S^N(C9)) ( Table 8(a) ):

HYD3(0.0343, 1.36) > AZO1(0.0270, 0.88) > AZO2(0.0269, 1.02)

[PM3 method] The most reactive is the N13 (AZO3, HYD1) or C9 (AZO1, AZO2, HYD2, HYD3) atom of the heterocyclic ring. The reactivity of these atoms can be arranged as follows (atom, S^N, Table 8(b) and Figure 4 ):

HYD2(C9.0.0392) > HYD3(C9.0.0358) > AZO1(C9.0.0352) > HYD1(N13.0.0288) > AZO3(N13.0.0267) > AZO2(C9.0.0264)

Table 15 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 8(b). Reactivity coefficients SE of 5-phenylazo-3-cyano-1-(H or ethyl)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method in the electrophilic and nucleophilic oxidation reaction.

	PM3						PM3
	NH_H						NEt_H
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3	AZO1	AZO2	HYD3
C1				0.0111	0.0105	0.0108
C2	0.0118						0.0118
C3		0.0130	0.0124					0.0131
C4						0.0127			0.0126
C6					0.0151	0.0117			0.0139
C9				0.0135	0.0125
N7	0.0352	0.0264	0.0214	0.0243	0.0392	0.0358	0.0351	0.0270	0.0357
C11	0.0114
C16(Et)	0.0169	0.0225	0.0110	0.0115	0.0119	0.0127	0.0182	0.0233	0.0129
C(CH3)							0.0144	0.0148	0.0146
N13	0.0112	0.0122	0.0108	0.0101	0.0115	0.0106	0.0125	0.0121	0.0110

Nucleophilic reaction.

C12	0.2004		0.1902	0.1440	0.0929	0.1498	0.1773		0.1531
N13(N7)	0.1151	0.1322		(0.1106)	(0.2762)	(0.2628)	0.2093	0.1451	(0.3119)
C14	0.1123	0.1805	0.1869	0.0439	0.1503	0.1263		0.2115	0.1351

NEt derivatives should react on the C9 carbon atom and their reactivity is as follows ( Table 8(a) and Table 8(b) ):

HYD3(0.0357) > AZO1(0.0351) > AZO2(0.0270)

Figure 4 5.3. 5-(4-Methylphenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone--[AM1 method] In the N-H derivative, in the electrophilic oxidation reaction of 1O2, the most reactive is the N13 nitrogen atoms on the heterocyclic ring in the following order (SE, Table 9(a)):--HYD3(0.0399) > HYD2(0.0351) > HYD1(0.0337) > AZO1(0.0324) = AZO2(0.0324) > AZO3(0.0197)--In the next bond, the reaction should take place on the C9 atom in the pyridone ring:--HYD2(0.0308) > AZO1(0.0299) > HYD3(0.0262) > AZO2(0.0255) > AZO3(0.0249) > HYD1(C(CH3), 0.0221)--Table 9(a). Reactivity coefficients S(E/N) of 5-(4-methyphenylazo)-3-cyjano-1-(H or ethyl)-6-hydroxy-4-methyl-2 pyridone derivatives calculated by the AM1 method in the electrophilic and nucleophilic oxidation reaction.--AM1 AM1 NH_CH3 NEt_CH3 AZO1 AZO2 AZO3 HYD1 HYD2 HYD3 AZO1 AZO2 HYD3 C2 0.0152 0.0154 0.0121 C3 0.0150 0.0162 0.0150 0.0150 0.0161 C4 0.0140 0.0140 C6 0.0113 0.0171 0.0167 C9 0.0299 0.0206 0.0249 0.0216 0.0308 0.0262 0.0309 0.0213 0.0259 N7 0.0174 0.0132 0.0210 0.0194 0.0203 0.0180 0.0207 C11 0.0196 0.0255 0.0221 0.0151 0.0203 0.0264 0.0136 C16(Et) 0.0261 0.0267 0.0257 C(CH3) 0.0222 0.0224 0.0216 0.0221 0.0229 0.0225 0.0224 0.0226 0.0225 PhC(CH3) 0.0206 0.0209 0.0201 0.0202 0.0204 0.0202 0.0208 0.0211 0.0203 N13 0.0324 0.0324 0.0197 0.0337 0.0351 0.0399 0.0272 0.0270 0.0348--Nucleophilic reaction--C12 0.2782 0.1454 0.1106 0.1680 0.1099 0.2053 0.3013 0.1592 0.2175 C14 0.1081 0.3297 0.1170 0.0726 0.1537 0.1866 0.1151 0.3721 0.1986--These calculations show that the pyridone phenyl ring is responsible for the photooxidation fastness in the electrophilic reaction. The reactivity of C1 ÷ C6 carbon atoms in the phenylazo residue is much lower, more than twice as low, than that of the atoms in the heterocycle.--NEt derivatives react similarly to NH, mainly on the N13 nitrogen atom, only in the AZO1 tautomer the reaction should first take place on the C9 carbon atom (atom, SE, Table 9(a)):--

HYD3(N13, 0.0348) > AZO1(C9, 0.0309) > AZO2(N13, 0.0270)

The calculations show that the photochemical stabilities of 2-pyridone derivatives depend on the type of tautomer in which they occur. The general relationship, however, is the conclusion that the electrophilic reaction occurs first in the phenyl ring, whose photooxidation fastness may be even 2.87 times lower than that of the atoms in the pyridone ring (e.g. NEt, HYD3, N13:C2 = 2.87).

[PM3 method] Calculations made by the PM3 method indicate a different course of the reaction. In this method, the C9 carbon atom of the heterocyclic ring is the most reactive followed by the reaction on the C11 atom to which a strong electron withdrawing substituent (E_A) cyano group -CN is attached. The reactivity of these derivatives can be as follows (S^E[C9], S^E[C9]/S^E[C11], Table 9(b) ):

HYD2(0.0399, 3.24) > HYD3(0.0365, 2.68) > AZO1(0.0352, 2.06) > HYD1(0.0301, 2.16) > AZO2(0.0263, 1.18) > AZO3(0.0215, 1.18)

and for NEt derivatives (as above) ( Table 9(b) ):

HYD3(0.0365, 2.63) > AZO1(0.0364, 2.03) > AZO2(0.0269, 1.16).

HYD3(N7.0.3080) > AZO2(C14.0.2124) > AZO1(N7.0.1899)

Table 16 <xref ref-type="bibr" rid="scirp.136263-"></xref>Table 9(b). Reactivity coefficients S(E/N) of 5-(4-methylphenylazo)-3-cyano-1-(H or ethyl)-6-hydroxy-4-methyl-2-pyridone derivatives calculated by the PM3 method in the electrophilic and nucleophilic reaction.

	PM3						PM3
	NH_CH3						NEt_CH3
	AZO1	AZO2	AZO3	HYD1	HYD2	HYD3	AZO1	AZO2	HYD3
C2	0.0121	0.0133	0.0127				0.0127
C3	0.0123	0.0140	0.0134					0.0137
C4									0.0122
C6(C5)				(0.0156)	(0.0166)	(0.0161)			(0.0159)
C9	0.0352	0.0263	0.0215	0.0301	0.0399	0.0365	0.0364	0.0269	0.0365
N7							0.0115
C11	0.0171	0.0223	0.0181	0.0139	0.0123	0.0136	0.0179	0.0232	0.0139
C16(Et)							0.0145	0.0147	0.0147
C(CH3)					0.0116	0.0113
N13			0.0203

Nucleophilic reaction.

C12	0.1525		0.1322	0.1548	0.0955	0.1376	0.1675		0.1484
N13(N7)	0.1844	0.1436		(0.2832)	(0.2820)	(0.2957)	0.1899	0.1401	(0.3080)
C14		0.1895	0.1385	0.0546	0.1535	0.1207		0.2124	0.1325

The performed calculations indicate a different likely reaction mechanism in the AM1 and PM3 methods. In the former, the reaction products may be nitroso or nitro derivatives, then ketones or acids. In the PM3 method, the reaction should take place in the heterocyclic ring on the C9 atom through peroxide structures. There is no high reactivity of the nitrogen in the azo or hydrazone bond. Only in the nucleophilic oxidation reaction, the reaction can take place as a result of the reaction of nitrogen N7(H) or N13, especially in the hydrazone form of the dye.

In the analysis of the reactivity of molecules to electrophilic ¹O₂ oxidation, a high value of the superdelocalisabillity coefficient S^E was not observed on the adjacent atoms of non-ionised molecules, hence they can undergo photochemical decomposition only through an epoxidation mechanism.

6. Nucleophilic Photooxidation <math display="inline" xmlns="http://www.w3.org/1998/Math/MathML"> <mrow> <msubsup> <mi> O </mi> <mn> 2 </mn> <mrow> <mo> • </mo> <mo> − </mo> </mrow> </msubsup> </mrow> </math> (SN) 6.1. 5-(4-Nitrophenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

[AM1 method] In the nucleophilic photooxidation, the reaction should take place on the C12 or C14 atom. The NH derivatives should react in the following order (atom, S^N, Table 7(a) ):

AZO2(C14, 0.1993) > AZO1(C12, 0.1676) > HYD3(C12, 0.1507) > HYD1(C12, 0.1273) > HYD2(C14, 0.1172) > AZO3(C12, 0.0998)

and for NEt (atom, S^N, Table 7(a) ):

AZO2(C14, 0.1913) > AZO1(C12, 0.1691) > HYD3(C12, 0.1570)

i.e. the carbonyl carbon atom is most reactive.

[PM3 method] In the nucleophilic reaction, the hydrazone nitrogen atom N7(H) in the HYD1 ÷ HYD3 tautomers is characterised by high reactivity, the AZO tautomers react almost 1.8 - 2 times slower on the carbonyl atom C12 or C14. For the NH derivative, the reactivity of individual atoms can be arranged as follows (atom, S^N, Table 7(b) ):

HYD3(N7, 0.2331) > HYD1(N7, 0.2253) > HYD2(N7, 0.2238) > AZO2(C14, 0.1334) > AZO1(N13, 0.1294) > AZO3(C12, 0.01003)

and for the derivative NEt (atom, S^N, Table 7(b) ):

HYD3(N7, 0.2405) > AZO2(C14, 0.1327) > AZO1(C12, 0.1236)

The calculations performed using the AM1 and PM3 methods indicate different oxidation mechanisms. The AM1 method indicates that oxidation should take place in the heterocyclic ring on the N13 nitrogen atom, then on the C9 atom, while in the PM3 method, the reaction should take place first on the C1 atom of the phenyl ring, whose reactivity is even 2 times higher than that of the carbon C9 (e.g. HYD1).

6.2. 5-Phenylazo-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

[AM1 method] In the $O_{2}^{• -}$ nucleophilic reaction, the reaction should take place at the C12 or C14 carbonyl atom (atom, S^N, Table 8(a) ):

[NH] AZO2(C14, 0.3227) > AZO1(C12, 0.2703) > HYD3(C12, 0.2010) > HYD1(C12, 0.1648) > HYD2((C14, 0.1508) > AZO3(C14, 0.1173)

[NEt] AZO2(C14, 0.3303) > AZO1(C12, 0.2903) > HYD3(C14, 0.1945)

[PM3 method] In the nucleophilic reaction $O_{2}^{• -}$ hydrazone derivatives react first on the N7(H) nitrogen atom (S^N, Table 8(b) ):

HYD2(N7, 0.2762) > HYD3(N7, 0.2628) > AZO1(C12, 0.2004) > AZO3(C12, 0.1902) > AZO2(C14, 0.1805) > HYD1(C12, 0.1440)

That is, similarly to the previously discussed derivatives, the reaction should take place in a tautomeric form with a carbonyl carbon atom.

The NEt derivatives react as follows (atom, S^N, Table 8(b) ):

HYD3(N7, 0.3119) > AZO2(C14, 0.2115) > AZO1(N13, 0.2093)

The PM3 method differs from AM1 in that it considers the probability of reaction at the N7 hydrazone nitrogen atom, whereas in the AM1 method, only C12 or C14 carbonyl carbon atoms are reactive.

6.3. 5-(4-Methylphenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

[AM1 method] The nucleophilic oxidation reaction of $O_{2}^{• -}$ occurs exclusively on C12 or C14 atoms to which oxygen atoms -OH or =O are attached. The carbon atom in the C=O carbonyl group is characterised by the highest reactivity. The reactivity differences are significant and amount to (atom, S^N[C=O]/S^N[C-OH], Table 9(a) ):

AZO1(C12, 2.57) > HYD1(C12, 2.31) > AZO2(C14, 2.27) > HYD2(C14, 1.39) > AZO3(C14, 1.06) > HYD3(C12, 1.10)

and for NEt derivatives (as above) ( Table 9(a) ):

AZO1(C12, 2.62) > AZO2(C14, 2.34) > HYD3(C12, 1.09)

If the C12 and C14 atoms are similar in nature and form C=O carbonyl groups, their reactivity is similar, differing only by about 6% - 9% (HYD3).

[PM3 method] In the nucleophilic photooxidation, the reaction should first take place on the hydrazone nitrogen atom N7(H). Reactivity on this atom is greater than on C12 or C14 (S^E[N7], S^N[N7]/S^N[C12 or C14], atom, Table 9(b) ):

HYD3(0.2957, 2.15, C12) > HYD1(0.2832, 1.83, C12) > HYD2(0.2820, 1.84, C14)

In this reaction, the azo tautomers AZO1 ÷ AZO3 have low superdelocalisabilitie coefficients and the reaction should take place on the C14(AZO2, AZO3) or N13(AZO1) atom.

The NEt derivatives react as follows (atom, S^E, Table 9(b) ):

HYD3(0.3080, N7) > AZO2(0.2124, C14) > AZO1(0.1899, N7)

7. [ene] Double Bond Reaction

For the studied tautomers, we calculated the reactivity coefficients of double bonds in the electrophilic ¹O₂ oxidation reaction. These reactions may compete with oxidation on the C or N atoms in the molecule. Their values were calculated according to Equation (1), where the electron density on the double bond is the result of the sum of the LUMO level charges of the neighboring atoms.

The analysis of the obtained results leads to the conclusion that the phenyl ring, mainly the C2=C6 or C3=C4 bond, is responsible for the light resistance.

7.1. 5-(4-Nitrophenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

In the AM1 method ( Table 1(a) , Table 2(a) ), the most likely decay mechanism is on C1/C3 atoms in AZO2 (0.0214) tautomer, then C3=C4 in AZO3(0.0207) and HYD2(0.0206) and C1=C3 in AZO1(0.0205).

In the PM3 method ( Table 1(a) and Table 1(b) ) the C1=C3 bond in AZO3 is almost twice as reactive (0.0452) than the bond in HYD1(0.0219, C5=C6) and C5=C6 in HYD2(0.0216).

NEt derivatives, as in the cases of dyes discussed above, the ethyl substituent has the highest reactivity for electrophilic oxidation.

In the AM1 method, in the AZO2 tautomer it is 0.0297, in AZO1 it is 0.0291, while in the PM3 method it is 0.0260 in AZO2 and 0.0238 in HYD3.

Only in these derivatives, in the PM3 method, there is a probability that oxidation occurs on C5=C6(C4) atoms in the [ene] reaction The superdelocalisability coefficients on these bonds: C1=C2(C3) in the AZO2(0.0478) and AZO1(0.0445) tautomers are almost twice as high as in the Et substituent in NEt-2-pyridone.

7.2. 5-Phenylazo-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

Calculations using the AM1 method ( Table 3(a) ) indicate that the most reactive bond in the [ene] reaction is the C2=C6 in the HYD1 tautomer (S^E = 0.0290) in HYD2 (S^E = 0.0285) and AZO2 (S^E = 0.0285, C1=C3).

However, calculations using the PM3 method ( Table 3(b) ) indicate that the [ene] reaction should take place on the C5=C6(C4) bond. The most likely reaction is in the tautomer HYD2(S^E = 0.0275, C5=C6) followed by HYD3 (S^E = 0.0244, C5=C4) and HYD1 (S^E = 0.0242, C5=C4).

The NEt isomer reacts first on the Et group (AZO2, 0.0306 [AM1]) and (AZO1, 0.0271 [PM3], Table 4(a) and Table 4(b) ). Next, the reaction should take place in AZO1(0.0302, Et [AM1], Table 3(a) ) and HYD3(0.0265, C5/C4 [PM3]).

7.3. 5-(4-Methylphenylazo)-3-Cyano-1-(H or Ethyl)-6-Hydroxy-4-Methyl-2-Pyridone

The AM1 method ( Table 5(a) ) of N-H derivatives indicates the following reactivity of tautomers in the reaction [ene] (S^E, bond):

HYD1(0.0291, C2=C6) > HYD2(0.0286, C2=C6) ≈ HYD3(0.0285, C2=C6) > AZO2(0.0277, C2=C6)

In the PM3 method, the order is as follows (S^E, bond, Table 5(b) ):

HYD1(0.0284, C5=C6) ≈ HYD2(0.0283, C5=C4) > HYD3(0.0279, C5=C4) > AZO2(0.0200, C1=C3)

and indicates a different reaction site with the electrophilic agent ¹O₂. The PM3 method indicates the reaction proceeds on the C5=C6 bond adjacent to the N(H)7-N8 hydrazone bond. In the AM1 method, the reaction should take place in the phenyl ring. NEt derivatives should react differently ( Table 6(a) and Table 6(b) ). The presence of an ethyl substituent in the 2-pyridone ring makes it the place most susceptible to ¹O₂ oxidation. Both in the AM1 ( Table 6(a) ) and PM3 ( Table 6(b) ) methods, the C15/C16 substituent (Et) in the AZO2 tautomer should be oxidised first, then in AZO1 and HYD3. Then, the reaction would take place in the AZO2 and AZO1 tautomer at the C3=C4 (AM1) or C2=C6 (PM3) position.

[AM1, Table 6(a) ]

AZO2(0.0310, Et) > AZO1(0.0304, Et) > HYD3(0.0291, Et) > HYD3(0.0287, C2=C6) > AZO2(0.0280, C3=C4) > AZO1(0.0274, C3=C4)

[PM3, Table 6(b) ]

HYD3(0.0281, C5=C6) > AZO2(0.0274, Et) > AZO1(0.0270, Et) > HYD3(0.0253, Et) > AZO2(0.0209, C2=C6) > AZO1(0.0207, C3=C4)

In the calculations of the electronic structure of dyes and their reactivity, the AM1 and PM3 methods were used to predict their electronic properties, geometric structure, total energy and enthalpy of formation. The calculations did not consider the dyes interaction with fibers or with the products of their photochemical decomposition. The calculations were conducted assuming that the molecules are in a vacuum. They made it possible to obtain the HOMO and LUMO frontier electron density orbitals and calculate their superdelocalisibility coefficients based on the known values of the electron densities f^E and f^N, as well as the HOMO and LUMO energies. The higher these values are, the greater the probability of a reaction on a carbon or nitrogen atom using one of the active oxygen species.

By uv-vis spectrophotometric methods, it is possible to observe changes in absorption and their intensity (molar extinction < ε_max) within the ranges assigned to individual AZO, HYD tautomers, which are, however, resulting from the overlapping of many partial bands, assigned to all AZO or HYD tautomers. This is due to the fact that the aformentioned bands (AZO, HYD) maintain almost the same values of their absorption maximum λ_max [80] [81] .

AZO1, AZO2 dyes are lactam tautomers of the AZO3 lactim form, HYD1, HYD2 dyes are lactim tautomers, HYD3 is a lactam tautomer. According to literature data [42] , the lactim form with aromatic structure should be the most (e.g. AZO3).

The performed calculations show that the attack of the oxidising agent can take place on the carbon atom in the pyridone or phenyl ring. The reaction is accompanied by significant changes in the electronic structures of the molecules. We performed calculations of molecular reactivity to photooxidation of probable forms of tautomeric azo dyes, derivatives of NH- and NEt-pyridones-2. These dyes can exist in equilibrium of azo-hydrazone and lactam-lactim tautomeric forms, and their proportions depend on several factors such as the nature of diazo compounds and their substituents, the temperature, or the solvent. They can exist in the form of six tautomers, for NH-pyridone-2 derivatives, and three for NEt-pyridone-2. As the calculations showed, the dyes in question occur mainly in the form of AZO1 and HYD3, while the other tautomers play a less significant role.

Literature data indicate that under certain conditions, this equilibrium may be completely shifted towards one of these tautomers. However, our calculations indicate that a state in which one of the forms predominates and the dye is present in a mixture is more likely. This is evidenced by the small energy difference between the tautomers,

The analysis of the performed calculations concerning the processes of electrophilic (¹O₂) and nucleophilic ( $O_{2}^{• -}$ ) photooxidation of selected azo derivatives of 2-pyridone leads to different conclusions.

Each method, AM1 or PM3, suggests a different reaction course, especially in the presence of a strongly accepting substituent in the phenyl residue, which is the -NO₂ group (σ_p = 0.78). In this case, the PM3 method suggests that the decomposition should occur first in the aromatic ring, only the ethyl substituent (Et) in the Net-2-pyridone residue would undergo the reaction next. In other cases (σ_p = H, CH₃), the ethyl group would undergo the oxidation reaction first, then the next process may take place in the phenyl ring of the phenylazo residue.

The tested dyes, as is also evident from other publications and experimental studies, should be present mainly in the HYD hydrazone form. In the AM1 method, such forms have the lowest enthalpies of formation, while in PM3 such forms should be AZO forms. It is characteristic, however, that in both methods, the form of HYD1 and AZO1 differ slightly in energy, which may suggest that their equilibrium is unstable, and they can easily change one to the another.

None of these calculation methods suggest, however, that the oxidation reaction proceeds according to an oxidation mechanism of unsaturated bonds (the exception is the [ene] reaction of the NO₂ derivative in the PM3 method). Their reactivity is lower than that of the atoms in the phenyl or heterocyclic ring. The results of the calculations allow us to draw a conclusion on the influence of the substituents on the lightfastness of the dyes. The lowest lightfastness should be found in nitro derivatives, which have the highest reactivity in the electrophilic oxidation with singlet oxygen ¹O₂ has the C1 atom, to which the E_A substituent is attached. The spatial structure of the dyes may also be important, a factor that is usually overlooked when interpreting the calculation results.

In the reaction of singlet oxygen ¹O²with an amino group, the reaction can lead to nitroso or nitro derivatives, or possibly to epoxidation or 1,2-addition derivatives to the C=C bond. Consequently, it would be impossible to determine the reactivity or rate of decomposition of a given chemical compound, since the influence of substituents in the ring on the reaction course will vary from one site to another. In addition, the possibility of 1,2-addition to the C=C bond should be taken into account if the values of Σf^N or ΣS^E of adjacent carbon atoms are higher than the value of the superdelocalisability coefficient of a single carbon atom, which would cause them to react according to the epoxidation mechanism. In such situations, the reaction can result in the cleavage of the double bond and the formation of aldehydes or ketones, and—with full mineralisation—carbon dioxide. High values of superdelocalisability on adjacent carbon atoms are observed in some dyes, suggesting the possibility of 1,2-addition.

The probalility of the reaction is indicated by a high value of the S^E (or S^N) coefficient on the relevant atom. The calculations show that the attack of the oxidising agent can also occur on nitrogen atoms. This is accompanied by significant changes in molecules electronic structure.

Lightfastness can be tested for the probability of attack by an electrophilic or nucleophilic oxidising agent. The analysis of changes in ΔE, HOMO/LUMO frontier electron density and respective superdelocalisability can satisfactorily explain the lightfastness of these dyes.

8. Conclusions

Using the AM1 and PM3 methods, we calculated the reactivity of these derivatives in the S^E electrophilic photooxidation with singlet oxygen ¹O₂ and in the S_N nucleophilic reaction of with the anion radical $O_{2}^{• -}$ . The performed calculations indicate that the atom most sensitive to ¹O₂ oxidation in the electrophilic reaction is the C9 carbon of the heterocyclic ring, followed by the C11 carbon atom with the -CN substituent, where a low electron density is observed (except for the p-NO₂ derivative). In the reaction result, the heterocyclic ring of the tautomers AZO1 ÷ AZO3, HYD1 ÷ HYD3 is disintegrated in NH- and NEt-pyridone-2 derivatives, when -CH₃, H substituents are present in the para- position of the diazo compound. In p-NO₂ derivative C1 carbon atoms in the aromatic ring are the most reactive, then the reaction should lead to the decomposition of the heterocyclic ring initiated by the reaction of singlet oxygen ¹O₂ on the C9 carbon atom. However, in no case can one expect the disintegration of double bonds, i.e. [ene] type reactions, due to low S^E reactivity indices in the vicinity of C1, C9 and C11 atoms.

Photooxidation reaction according to the nucleophilic mechanism of S^N by means of the anion-radical $O_{2}^{• -}$ should take place on the carbon atoms of the C=O group (AZO1, AZO2) or C-OH (AZO3) groups, leading to the decomposition of the heterocyclic ring (NH-pyridones) or on the atom -N7(H)- bond in the hydrazone form of dyes (HYD1 ÷ HYD3).

Next, the reaction should take place on the C12/C14 carbons (C=O) of the heterocyclic ring. As a result, the decomposition of the azo bond in the hydrazone form should be expected first, followed by the decomposition of the heterocyclic ring. In the S^N reaction, the course and direction of the reaction are not influenced by the type of substituent in the diazo compound, photooxidation reactions for all derivatives (CH₃, H, NO₂) should occur in a similar way and according to a similar mechanism.

It should be expected that the reactivity of the tested dyes is very similar to the reactivity of azo dyes, where the nitrogen atom of the azo bond undergoes electrophilic reaction by singlet oxygen ¹O₂, which was confirmed by of AM1 and PM3 calculations [36] [67] .

The AM1 method allowed the calculation of ΔE (= E_HOMO[dye] − E_LUMO[¹O₂]) of the dyes studied, which is 2.611 - 3.786 eV, and 2.787 - 4.496 eV for PM3 and is below the dye reactivity limit ~6 eV.

Predicting the properties of dyes and their correlations with the calculated values by quantum-chemical methods has some limitations. For example, calculations are made for molecules in a vacuum, to omit the formation of intermolecular hydrogen bonds (IHB), neglecting the influence of the solvent, environment (e.g. dyed fabric), temperature, etc.

References 1

Racané, L., Mihalić, Z., Cerić, H., Popović, J. and Tralić-Kulenović, V. (2013) Synthesis, Structure and Tautomerism of Two Benzothiazolyl Azo Derivatives of 2-Naphthol: A Crystallographic, NMR and Computational Study. Dyes and Pigments, 96, 672-678. >https://doi.org/10.1016/j.dyepig.2012.10.009

Zollinger, H. (2003) Colour Chemistry: Synthesis, Properties and Application of Organic Dyes and Pigments. Wiley-VCH.

Porobić, S.J., Božić, B.Đ., Dramićanin, M.D., Vitnik, V., Vitnik, Ž., Marinović-Cincović, M., et al. (2020) Absorption and Fluorescence Spectral Properties of Azo Dyes Based on 3-Amido-6-Hydroxy-4-Methyl-2-Pyridone: Solvent and Substituent Effects. Dyes and Pigments, 175, Article ID: 108139. >https://doi.org/10.1016/j.dyepig.2019.108139

Vitnik, V.D., Vitnik, Ž.J., Božić, B.Đ., Valentić, N.V., Dilber, S.P., Mijin, D.Ž., et al. (2017) Experimental and Theoretical Insight into the Electronic Properties of 4-Aryl-5-Arylazo-3-Cyano-6-Hydroxy-2-Pyridone Dyes. Coloration Technology, 133, 223-233. >https://doi.org/10.1111/cote.12271

Abeta, S. (1992) Compatibility of Acid Dyes on Nylon. Dyes and Pigments, 18, 57-68. >https://doi.org/10.1016/0143-7208(92)80006-9

Chudgar, R.J. and Oakes, J. (2014) Dyes, Azo (Kirk-Othmer Encyclopedia of Chemical Technology). Enigma, Inc.

Mijin, D.Z., Baghbanzadeh, M., Reidlinger, C. and Oliver Kappe, C. (2010) The Microwave-Assisted Synthesis of 5-Arylazo-4,6-Disubstituted-3-Cyano-2-Pyridone Dyes. Dyes and Pigments, 85, 73-78. >https://doi.org/10.1016/j.dyepig.2009.10.006

Mirković, J.M., Božić, B.Đ., Mutavdžić, D.R., Ušćumlić, G.S. and Mijin, D.Ž. (2014) Solvent and Structural Effects on the Spectral Shifts of 5-(Substituted Phenylazo)-3-Cyano-6-Hydroxy-1-(2-Hydroxyethyl)-4-Methyl-2-Pyridones. Chemical Physics Letters, 615, 62-68. >https://doi.org/10.1016/j.cplett.2014.09.063

Al-Etaibi, A.M. and El-Apasery, M.A. (2020) A Comprehensive Review on the Synthesis and Versatile Applications of Biologically Active Pyridone-Based Disperse Dyes. International Journal of Environmental Research and Public Health, 17, Article 4714. >https://doi.org/10.3390/ijerph17134714

Lubai, C. (1986) Colour and Constitution of Azo Dyes Derived from 2-Thioalkyl-4,6-Diaminopyrimidines and 3-Cyano-1,4-Dimethyl-6-Hydroxy-2-Pyridone as Coupling Component. Dyes and Pigments, 7, 373-388. >https://doi.org/10.1016/0143-7208(86)80005-5

Schwander, H.R. (1982) Heterocyclic Azo Coupling Components. Dyes and Pigments, 3, 133-160. >https://doi.org/10.1016/0143-7208(82)80018-1

Mijin, D.Ž., Ušćumlić, G.S., Perišić-Janjić, N.U. and Valentić, N.V. (2006) Substituent and Solvent Effects on the UV/vis Absorption Spectra of 5-(3-and 4-Substituted Arylazo)-4, 6-Dimethyl-3-Cyano-2-Pyridones. Chemical Physics Letters, 418, 223-229. >https://doi.org/10.1016/j.cplett.2005.10.130

Karcı, F. and Karcı, F. (2008) Synthesis and Absorption Spectra of Some Novel Heterocyclic Disazo Dyes Derived from Pyridone and Pyrazolone Derivatives. Dyes and Pigments, 76, 147-157. >https://doi.org/10.1016/j.dyepig.2006.07.029

Mijin, D., Uscumlic, G., Perisic-Janjic, N., Trkulja, I., Radetic, M. and Jovancic, P. (2006) Synthesis, Properties and Color Assessment of Some New 5-(3-and 4-Substituted Phenylazo)-4, 6-Dimethyl-3-Cyano-2-Pyridones. Journal of the Serbian Chemical Society, 71, 435-444. >https://doi.org/10.2298/jsc0605435m

Ušćumlić, G.S., Mijin, D.Z., Valentić, N.V., Vajs, V.V. and Sušić, B.M. (2004) Substituent and Solvent Effects on the UV/vis Absorption Spectra of 5-(4-Substituted Arylazo)-6-Hydroxy-4-Methyl-3-Cyano-2-Pyridones. Chemical Physics Letters, 397, 148-153. >https://doi.org/10.1016/j.cplett.2004.07.057

Griffiths, J. (1976) Colour and Constitution of Organic Molecules. Academic Press.

Towns, A.D. (1999) Developments in Azo Disperse Dyes Derived from Heterocyclic Diazo Components. Dyes and Pigments, 42, 3-28. >https://doi.org/10.1016/s0143-7208(99)00005-4

Hallas, G. (1999) Synthesis and Spectral Properties of Azo Dyes Derived from 2-Aminothiophenes and 2-Aminothiazoles. Dyes and Pigments, 42, 249-265. >https://doi.org/10.1016/s0143-7208(99)00031-5

Gordon, P.F. and Gregory, P. (1987) Organic Chemistry in Colour. Springer-Verlag.

Peng, Q. (1992) Hydrazone-Azo Tautomerism of Pyridone Azo Dyes Part III—Effect of Dye Structure and Solvents on the Dissociation of Pyridone Azo Dyes. Dyes and Pigments, 18, 271-286. >https://doi.org/10.1016/0143-7208(92)80017-h

Neevel, J.G., van Beek, H.C.A., den Ouden, H.H.I. and van de Graaf, B. (1990) Photo–oxidation of Azo Dyes in the Presence of Biacetyl and Oxygen. Journal of the Society of Dyers and Colourists, 106, 176-181. >https://doi.org/10.1111/j.1478-4408.1990.tb01255.x

Heijkoop, G. and van Beek, H.C.A. (1977) ESR Investigation of the Radical Intermediates Formed in the Photoreduction of Azo Dyes. Recueil des Travaux Chimiques des Pays-Bas, 96, 83-85. >https://doi.org/10.1002/recl.19770960309

Okada, Y., Hihara, T. and Morita, Z. (2009) Photofading of Phenylazo-Aniline,-Pyridone and-Quinolone Disperse Dyes on a Nylon 6 Substrate. Coloration Technology, 125, 86-98. >https://doi.org/10.1111/j.1478-4408.2009.00179.x

Peng, Q. (1991) Hydrazone-azo Tautomerism of Pyridone Azo Dyes Part II: Relationship between Structure and Ph Values. Dyes and Pigments, 15, 263-274. >https://doi.org/10.1016/0143-7208(91)80011-w

Wang, I.J. and Wang, P.Y. (1990) Photofading of Azo Pyridone Dyes in Solution: Part I: Photo-Fading Kinetics and Thermodynamics of 2-(p-and o-Substituted Aryla-zo)-2-Hydroxy-4-Methyl-5-Cyano-6-Pyridone Dyes in Amide Solvents. Textile Research Journal, 60, 297-300. >https://doi.org/10.1177/004051759006000508

van Beek, H.C.A., Heerjes, P.M. and Rutges, A.A.M. (1973) The Photofading of Mixtures of Azo Dyes and Mixtures of an Azo Dye and Oxygen in Aquous Solutions Containing D, L-Mandelic Acid. Journal of the Society of Dyers and Colourists, 89, 389-396. >https://doi.org/10.1111/j.1478-4408.1973.tb03108.x

Klessinger, M. and Michl, J. (1995) Excited States and Photochemistry of Organic Molecules. VCH.

Görner, H. (2002) Photoreduction of Trans-4-R-4’-Nitrostilbenes with R: Ome, H and NO ₂ by Di-and Trialkylamines in Polar Media. Physical Chemistry Chemical Physics, 4, 482-489. >https://doi.org/10.1039/b108192f

Görner, H. and Döpp, D. (2003) Transients in the Photoreduction of Dinitroarenes by Triethylamine and N, n-Dialkylanilines in Benzene. Journal of Photochemistry and Photobiology A: Chemistry, 159, 219-225. >https://doi.org/10.1016/s1010-6030(03)00187-4

Özen, A.S., Aviyente, V. and Klein, R.A. (2003) Modeling the Oxidative Degradation of Azo Dyes: A Density Functional Theory Study. The Journal of Physical Chemistry A, 107, 4898-4907. >https://doi.org/10.1021/jp026287z

Özen, A.S., Aviyente, V., De Proft, F. and Geerlings, P. (2004) Modeling the Substituent Effect on the Oxidative Degradation of Azo Dyes. The Journal of Physical Chemistry A, 108, 5990-6000. >https://doi.org/10.1021/jp037138z

Nam, S., Renganathan, V. and Tratnyek, P.G. (2001) Substituent Effects on Azo Dye Oxidation by the FeIII-EDTA-H ₂O ₂ System. Chemosphere, 45, 59-65. >https://doi.org/10.1016/s0045-6535(00)00599-3

Griffiths, J. and Hawkins, C. (1977) Oxidation by Singlet Oxygen of Arylazonaphthols Exhibiting Azo-Hydrazone Tautomerism. Journal of the Chemical Society, Perkin Transactions 2, 1977, 747-752. >https://doi.org/10.1039/p29770000747

Fenical, W., Kearns, D.R. and Radlick, P. (1969) Mechanism of the Reaction of ¹δ _g Excited Oxygen with Olefins. II. Elimination of the Concerted “Ene” Mechanism as the Route to Allylic Hydroperoxides. Journal of the American Chemical Society, 91, 7771-7772. >https://doi.org/10.1021/ja50001a057

Spadaro, J.T., Isabelle, L. and Renganathan, V. (1994) Hydroxyl Radical Mediated Degradation of Azo Dyes: Evidence for Benzene Generation. Environmental Science&Technology, 28, 1389-1393. >https://doi.org/10.1021/es00056a031

Das, S., Kamat, P.V., Padmaja, S., Au, V. and Madison, S.A. (1999) Free Radical Induced Oxidation of the Azo Dye Acid Yellow 9. Journal of the Chemical Society, Perkin Transactions 2, 1999, 1219-1224. >https://doi.org/10.1039/a809720h

Nasr, C., Vinodgopal, K., Fisher, L., Hotchandani, S., Chattopadhyay, A.K. and Kamat, P.V. (1996) Environmental Photochemistry on Semiconductor Surfaces. Visible Light Induced Degradation of a Textile Diazo Dye, Naphthol Blue Black, on TiO ₂ Nanoparticles. The Journal of Physical Chemistry, 100, 8436-8442. >https://doi.org/10.1021/jp953556v

Tanaka, K., Padermpole, K. and Hisanaga, T. (2000) Photocatalytic Degradation of Commercial Azo Dyes. Water Research, 34, 327-333. >https://doi.org/10.1016/s0043-1354(99)00093-7

Hihara, T., Okada, Y. and Morita, Z. (2003) Reactivity of Phenylazonaphthol Sulfonates, Their Estimation by Semiempirical Molecular Orbital PM5 Method, and the Relation between Their Reactivity and Azo-Hydrazone Tautomerism. Dyes and Pigments, 59, 201-222. >https://doi.org/10.1016/s0143-7208(03)00109-8

Tripathi, S., Vasudev, I. and Ray, A. (2008) Photofading of Azo Dyes: A Theoretical Study. Coloration Technology, 124, 151-158. >https://doi.org/10.1111/j.1478-4408.2008.00135.x

Morley, J.O., Guy, O.J. and Charlton, M.H. (2004) Molecular Modeling Studies on the Photochemical Stability of Azo Dyes. The Journal of Physical Chemistry A, 108, 10542-10550. >https://doi.org/10.1021/jp047001o

Aksel Eren, H., Avinc, O. and Erismis, B. (2012) Comparison of Different Ultrasound Support Methods during Colour and Chemical Oxygen Demand Removal of Disperse and Reactive Dyebath Solutions by Ozonation. Coloration Technology, 128, 446-453. >https://doi.org/10.1111/j.1478-4408.2012.00398.x

Arslan-Alaton, I., Basar, G. and Olmez-Hanci, T. (2012) Ozonation of the Reactive Dye Intermediate 2-Naphthylamine 3, 6, 8-Trisulphonic Acid (K-Acid): Kinetic Assessment, Ozonation Products and Ecotoxicity. Coloration Technology, 128, 387-394. >https://doi.org/10.1111/j.1478-4408.2012.00392.x

Hihara, T. (2003) Azo-Hydrazone Tautomerism of Phenylazonaphthol Sulfonates and Their Analysis Using the Semiempirical Molecular Orbital PM5 Method. Dyes and Pigments, 59, 25-41. >https://doi.org/10.1016/s0143-7208(03)00093-7

Tauro, S. and Coutinho, E. (2000) Azo, Hydrazone and Other Tautomers of the Azo Dye 7-Amino-4-Hydroxy-3-[(4-Methoxy-2-Sulfophenyl)Azo]-2-Naphthalenesulfonic Acid: A PM3 Study. Journal of Molecular Structure: THEOCHEM, 532, 23-29. >https://doi.org/10.1016/s0166-1280(00)00500-5

Monahan, A.R., Germano, N.J. and Blossey, D.F. (1971) Aggregation of Arylazonaphthols. II. Steric Effects on Dimer Structure in Water. The Journal of Physical Chemistry, 75, 1227-1233. >https://doi.org/10.1021/j100679a009

Hihara, T., Okada, Y. and Morita, Z. (2002) Photo-Oxidation and-Reduction of Vat Dyes on Water-Swollen Cellulose and Their Lightfastness on Dry Cellulose. Dyes and Pigments, 53, 153-177. >https://doi.org/10.1016/s0143-7208(02)00017-7

Kumar, K., Saravana Devi, S., Krishnamurthi, K., Gampawar, S., Mishra, N., Pandya, G.H., et al. (2006) Decolorisation, Biodegradation and Detoxification of Benzidine Based Azo Dye. Bioresource Technology, 97, 407-413. >https://doi.org/10.1016/j.biortech.2005.03.031

Baird, M.S., Hamlin, J.D., O'Sullivan, A. and Whiting, A. (2008) An Insight into the Mechanism of the Cellulose Dyeing Process: Molecular Modelling and Simulations of Cellulose and Its Interactions with Water, Urea, Aromatic Azo-Dyes and Aryl Ammonium Compounds. Dyes and Pigments, 76, 406-416. >https://doi.org/10.1016/j.dyepig.2006.09.011

Flower, K.R., Hamlin, J.D. and Whiting, A. (2002) An Insight into the Mechanism of the Cellulose Dyeing Process, Part 1: Modelling Azo-Systems in Azo-Linked Aromatics and Dyes. Molecular Simulation, 28, 1031-1047. >https://doi.org/10.1080/0892702021000011061

Himeno, K., Okada, Y. and Morita, Z. (2000) Photofading of Monoazo Disperse Dyes on Polyester and Polyamide Substrates. Dyes and Pigments, 45, 109-123. >https://doi.org/10.1016/s0143-7208(00)00009-7

Batchelor, S.N., Carr, D., Coleman, C.E., Fairclough, L. and Jarvis, A. (2003) The Photofading Mechanism of Commercial Reactive Dyes on Cotton. Dyes and Pigments, 59, 269-275. >https://doi.org/10.1016/s0143-7208(03)00118-9

Hihara, T., Okada, Y. and Morita, Z. (2004) Relationship between Photochemical Properties and Colourfastness Due to Light-Related Effects on Monoazo Reactive Dyes Derived from H-Acid, γ-Acid, and Related Naphthalene Sulfonic Acids. Dyes and Pigments, 60, 23-48. >https://doi.org/10.1016/s0143-7208(03)00139-6

Chang, J. and Kuo, T. (2000) Kinetics of Bacterial Decolorization of Azo Dye with Escherichia coli NO3. Bioresource Technology, 75, 107-111. >https://doi.org/10.1016/s0960-8524(00)00049-3

Ince, N.H. (1999) “Critical” Effect of Hydrogen Peroxide in Photochemical Dye Degradation. Water Research, 33, 1080-1084. >https://doi.org/10.1016/s0043-1354(98)00295-4

Giles, C.H. and de Bernat, J. (1976) The Nature of Azo Dye Light Fading in Polyester and the Physical State of the Dye. Textile Research Journal, 46, 673-676. >https://doi.org/10.1177/004051757604600911

Evans, N.A. and Stapleton, I.W. (1978) Structural Factors Affecting the Lightfastness of Dyed Fibers. Antraquinoid Dyes. In: Venkataraman, K., Ed., The Chemistry of Synthetic Dyes, vol. VIII, Academic Press, 247.

Ray, A. (1995) CNDO/II Study on the Fading of Anthraquinone Dyes. Dyes and Pigments, 27, 327-331. >https://doi.org/10.1016/0143-7208(94)00069-e

Guo, L., Meng, F.S., Gong, X.D., Xiao, H.M., Chen, K.C. and Tian, H. (2001) Synthesis and Spectral Properties of Soluble Trimethylsilyl Substituted Metal-Phthalocyanines. Dyes and Pigments, 49, 83-91. >https://doi.org/10.1016/s0143-7208(01)00011-0

Wang, L., Wang, X. and Zhang, H. (2005) A Theoretical Study on Photobleaching Mechanisms of Hypocrellins. Dyes and Pigments, 67, 161-166. >https://doi.org/10.1016/j.dyepig.2004.11.007

Corbett, J.F. (1984) Chemistry of Hair Colorant Process-Science as an Aid to Formulation and Development. Journal of the Society of Cosmetic Chemists, 35, 297-310.

Evans, N.A. and Stapleton, I.W. (1978) Structural Factors Affecting the Lightfastness of Dyed Fibres. Nitrodiphenylamine Dyes. In: Venkataraman, K., Ed., The Chemistry of Synthetic Dyes, Vol. VIII, Academic Press, 256.

Okada, Y., Hihara, T. and Morita, Z. (2008) Analysis of the Photofading of Phenylazo-Aniline and Phenylazo-Pyridone Disperse Dyes on Poly(Ethylene Terephthalate) Substrate Using the Semiempirical Molecular Orbital PM5 Method. Dyes and Pigments, 79, 111-125. >https://doi.org/10.1016/j.dyepig.2008.01.013

Dewar, M.J.S., Zoebisch, E.G., Healy, E.F. and Stewart, J.J.P. (1985) Development and Use of Quantum Mechanical Molecular Models. 76. AM1: A New General Purpose Quantum Mechanical Molecular Model. Journal of the American Chemical Society, 107, 3902-3909. >https://doi.org/10.1021/ja00299a024

Dewar, M.J.S. and Dieter, K.M. (1986) Evaluation of AM1 Calculated Proton Affinities and Deprotonation Enthalpies. Journal of the American Chemical Society, 108, 8075-8086. >https://doi.org/10.1021/ja00285a033

Stewart, J.J.P. (1989) Optimization of Parameters for Semiempirical Methods I. Method. Journal of Computational Chemistry, 10, 209-220. >https://doi.org/10.1002/jcc.540100208

Morita, Z. and Hada, S. (1999) A Semiempirical Molecular Orbital Study on the Reaction of an Aminopyrazolinyl Azo Dye with Singlet Molecular Oxygen. Dyes and Pigments, 41, 1-10. >https://doi.org/10.1016/s0143-7208(98)00038-2

Dewar, M.J.S. and Thiel, W. (1975) Ground States of Molecules. XXX. MINDO/3 Study of Reactions of Singlet (1.δ.g) Oxygen with Carbon-Carbon Double Bonds. Journal of the American Chemical Society, 97, 3978-3986. >https://doi.org/10.1021/ja00847a018

Fukui, K., Kato, H. and Yonezawa, T. (1961) “Frontier Electron Density” in Saturated Hydrocarbons. Bulletin of the Chemical Society of Japan, 34, 442-445. >https://doi.org/10.1246/bcsj.34.442

Fukui, K., Nagata, C. and Yonezawa, T. (1958) Electronic Structure and Auxin Activity of Benzoic Acid Derivatives. Journal of the American Chemical Society, 80, 2267-2270. >https://doi.org/10.1021/ja01542a058

Wojciechowski, K. and Szuster, L. (2020) Prediction of Photofading of n-β-Hydroxyethylamino-o-Nitrophenyl Derivatives: A Theoretical Study. Coloration Technology, 137, 134-144. >https://doi.org/10.1111/cote.12517

Reichardt, C. (1988) Solvent and Solvent Effects in Organic Chemistry. 2nd Edition, VCH.

Katritzky, A.R. and Lagowski, J.M. (1963) Prototropic Tautomerism of Heteroaromatic Compounds. IV Edition, Academic Press.

Kelemen, J. (1981) Azo-hydrazone Tautomerism in Azo Dyes. I. A Comparative Study of 1-Phenylazo-2-Naphthol and 1-Phenylazo-2-Naphthylamine Derivatives by Electronic Spectroscopy. Dyes and Pigments, 2, 73-91. >https://doi.org/10.1016/0143-7208(81)80009-5

Mustroph, H. (1987) Über die Azo-Hydrazon-Tautomerie. Zeitschrift für Chemie, 27, 281-289. >https://doi.org/10.1002/zfch.19870270803

Zollinger, H. (1961) Azo and Diazo Chemistry, Aliphatic and Aromatic Compounds. Interscience.

Antonov, L.M. (1996) Theoretical Investigations of the Tautomerism of Some Phenyla-Zonaphthols. 31 UNESCO Course for Advanced Research in Chemistry and Chemical Engineering, Tokyo Institute of Technology.

Chevrier, M., Bensaude, O., Guillerez, J. and Dubois, J. (1980) Equilibre lactime-lactame des hydroxy-pyridines et des hydroxy-pyrimidines (uraciles) en milieu apolaire: Etude infra-rouge. Tetrahedron Letters, 21, 3359-3362. >https://doi.org/10.1016/s0040-4039(00)78688-6

Scanlan, M.J., Hillier, I.H. and Davies, R.H. (1982) Prediction of Tautomeric Equilibri of Hydroxypyridines by Ab Initio Molecular Orbital Methods. Journal of the Chemical Society, Chemical Communications, 1982, 685-686. >https://doi.org/10.1039/c39820000685

Mason, S.F. (1957) 1002. The Tautomerism of N-Heteroaromatic Hydroxy-Compounds. Part II. Ultraviolet Spectra. Journal of the Chemical Society (Resumed), 1957, 5010-5017. >https://doi.org/10.1039/jr9570005010

Mason, S.F. (1958) 131. the Tautomerism of N-Heteroaromatic Hydroxy-Compounds. Part III. Ionisation Constants. Journal of the Chemical Society (Resumed), 1958, 674-685. >https://doi.org/10.1039/jr9580000674

Antonov, L. and Nedeltcheva, D. (2000) Resolution of Overlapping UV-Vis Absorption Bands and Quantitative Analysis. Chemical Society Reviews, 29, 217-227. >https://doi.org/10.1039/a900007k

Giles, C.H. and McKay, R.B. (1973) The Lightfastness of Dyes: A Review. Textile Research Journal, 33, 528-577. >https://doi.org/10.1177/004051756303300707