UNIVERZITET U BEOGRADU TEHNOLOŠKO-METALURŠKI FAKULTET SINTEZA, STRUKTURA I SOLVATOHROMIZAM NOVIH 5-(4-SUPSTITUISANIH FENILAZO)-4-(4-SUPSTITUISANIH FENIL)- -6-HIDROKSI-3-CIJANO-2-PIRIDONA Doktorska disertacija Mr ADEL. S. ALIMMARI Beograd, 2012 UNIVERSITY OF BELGRADE FACULTY OF TECHNOLOGY AND METALLURGY SYNTHESIS, STRUCTURE AND SOLVATOCHROMISM OF NEW 5-(4-SUBSTITUTED PHENYLAZO)-4-(4-SUBSTITUTED PHENYL)- -6-HYDROXY-3-CYANO-2-PYRIDONES Doctoral dissertation Mr ADEL. S. ALIMMARI Belgrade, 2012 UNIVERSITY OF BELGRADE FACULTY OF TECHNOLOGY AND METALLURGY Final Examing Committee _______________________________ Dr Gordana Ušćumlić, full professor of TMF, Thesis Supervisor _______________________________ Dr Dušan Mijin, full professor of TMF _______________________________ Dr Nataša Valentić, assistant professor of TMF _______________________________ Dr Vesna Vitnik, scientific adviser of IHTM, university of Belgrade Candidate _______________________________ Mr Adel. S. Alimmari CONTENTS ABSTRACT ........................................................................................................................1 ABSTRACT IN SERBIAN ...............................................................................................2 1. INTRODUCTION..........................................................................................................3 2. THEORETICAL PART ................................................................................................5 2.1. SYNTHESIS, STRUCTURE AND PROPERTIES OF AZO PYRIDONE DYES ............................5 2.1.1. General synthesis .......................................................................................................................... 6 2.1.1.1. Monoazo dyes ..............................................................................................7 2.1.1.2. Disazo dyes ................................................................................................18 2.1.1.3. Trisazo dyes ...............................................................................................22 2.1.2. Properties of azo pyridone dyes .............................................................................................. 23 2.1.3. Azo – hydrazone tautomerism of azo pyridone dyes ....................................................... 24 2.1.3.1. UV spectroscopic study of tautomerism ....................................................27 2.1.3.2. NMR spectroscopic study of tautomerism .................................................29 2.1.3.3. Analysis of solvent influence on azo – hydrazone tautomerism using ab initio quantum chemical calculations......................................................................31 2.2. CORRELATION ANALYSIS IN ORGANIC CHEMISTRY ..................................................33 2.2.1. Substitution effects and linear free energy relationships ................................................. 33 2.2.2. The Hammett equation and its extension ............................................................................. 38 2.2.3. Separation of electronic effects............................................................................................... 47 2.3. SOLVENT EFFECTS ....................................................................................................51 2.3.1. Solvent effects on the keto / enol equlibria ......................................................................... 53 2.3.2. Solvent effects on the other tautomeric equlibria .............................................................. 54 2.3.3. Solvent effects on the rates of homogenous chemical reactions ................................... 55 2.3.3.1. The Grunwald – Winstein equation ...........................................................56 2.3.4. Koppel – Palm solvatochromic treatment ............................................................................ 58 2.3.5. Kamlet – Taft solvatochromic treatment ............................................................................. 58 2.3.6. Correlation analysis of solvent effects by means of substituent constants................. 60 3. EXPERIMENTAL PART ...........................................................................................61 3.1. PREPARATION OF 5-ARYLAZO-6-HYDROXY-4-PHENYL-3-CYANO-2-PYRIDONE DYES (A1–A12) .......................................................................................................................61 3.2. PREPARATION OF 5-ARYLAZO-6-HYDROXY-4-(4-METHOXYPHENYL)-3-CYANO-2- PYRIDONE DYES (A13–A23) ............................................................................................65 3.3. PREPARATION OF 5-ARYLAZO-6-HYDROXY-4-(4-NITROPHENYL)-3-CYANO-2- PYRIDONE DYES (A24–A33) ............................................................................................69 4. RESULTATES AND DISCUSSION ..........................................................................73 4.1. SOLVENT AND STRUCTURAL EFFECTS ON THE UV-VIS ABSORPTION SPECTRA OF 5- -ARYLAZO-6-HYDROXY-4-PHENYL-3-CYANO-2-PYRIDONE DYES ....................................73 4.1.1. Spectral characteristics and tautomerism .................................................................73 4.1.2. Solvent effects on the azo–hydrazone tautomerism .................................................74 4.1.3. Substituent effects on the azo–hydrazone tautomerism ...........................................82 4.1.4. Quantum chemical calculations ...............................................................................84 4.2. SOLVENT AND STRUCTURAL EFFECTS ON THE UV-VIS ABSORPTION SPECTRA OF 5- -ARYLAZO-6-HYDOXY-4-(4-METHOXYPHENYL)-3-CYANO-2-PYRIDONE DYES ................85 4.2.1. Spectral characteristics of arylazo pyridone dyes ....................................................87 4.2.2. Solvent effects on the UV-vis absorption spectra ....................................................87 4.2.3. Absorption maxima of hydrazone form as a function of dispersive interaction ......92 4.2.4. Variation of absorption maxima with ET N ...............................................................94 4.2.5. Correlation with multiparameter solvent polarity scales .........................................95 4.2.6. Substituents effects on the UV-vis absorption spectra ..........................................102 4.3. SOLVENT AND STRUCTURAL EFFECTS ON THE UV-VIS ABSORPTION SPECTRA OF 5- ARYLAZO-6-HYDOXY-4-(4-NITROPHENYL)-3-CYANO-2-PYRIDONE DYES ......................104 4.3.1. Spectral characteristics and solvatochromism .......................................................105 5. CONCLUSIONS ........................................................................................................115 6. REFERENCES ..........................................................................................................117 7. APPENDIX .................................................................................................................125 LIST OF SCHEMES Scheme 2.1. Preparation of the arylazo pyridone dyes from pyridone ..................................... 6 Scheme 2.2. Preparation of the arylazo pyridone dyes from arylazo intermediate .............7 Scheme 2.3. Disperse dyes with pyridone moiety substituted by Me or Ph group in position 4 and with OH or C6H5NH in position 6 ..............................................................10 Scheme 2.4. The equilibrium between hydrazone form and azo form ..............................27 Scheme 2.5. The equilibrium between azo (34a) and hydrozone (34b) tautomeric forms of 1-phenylazo-4-naftol ..........................................................................................................31 Scheme 2.6. Inductive effect of -N + Me3 substituent ..........................................................35 Scheme 2.7. Resonance effect ...........................................................................................37 Scheme 2.8. Influence charge at the starred carbon...........................................................43 Scheme 2.9. Duality of substituent constants ....................................................................44 Scheme 2.10. Ionization of p-substituted phenols .............................................................48 Scheme 2.11. Resonance in p-nitrophenolate ....................................................................48 Scheme 2.12. Heterolysis reaction of p-substituted phenyldimethyl chloromethanes ......48 Scheme 2.13. Resonance in p-aminobenzylic cation .........................................................49 Scheme 2.14. Resonance effect at meta-position...............................................................50 Scheme 2.15. Keto – enol tautomeric form .......................................................................53 Scheme 2.16. Lactam – lactim tautomeric forms ..............................................................54 Scheme 4.1. The equilibrium between azo form (A) and hydrazone form (B) of 5- arylazo-6-hydoxy-4-phenyl-3-cyano-2-pyridones (X = H (A1), OH (A2), OCH3 (A3), CH3 (A4), Cl (A5), Br (A6), I (A7), F(A8), CN (A9), COOH (A10), COCH3 (A11), NO2 (A12)). ................................................................................................................................74 Scheme 4.2. The equilibrium between azo form (A) and hydrazone form (B) of 5- arylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes (X = H (A13), F (A14), Cl (A15), Br (A16), I (A17), OH (A18), CH3 (A19), OCH3 (A20), COCH3 (A21), CN (A22), NO2 (A23)). Resonance effect of electron-accepting (structure C) and electron-donating (structure D) substituents of the arylazo component on the hydrazone tautome ..............................................................................................................................86 Scheme 4.3. The equilibrium between azo form (A) and hydrazone form (B) of 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes: R = H (A24), 4-F (A25), 3-Cl (A26), 4-Br (A27), 4-I (A28), 4-CN (A29), 4-COCH3 (A30), 4-CH3 (A31), 4-NO2 (A32), 4-OH (A33). ................................................................................................................105 LIST OF TABLES Table 2.1. Absorption spectral data for the dye (33) (λmax/nm; εmax /10 4 M -1 cm -1 ) ..............27 Table 2.2. Values of the energies of the H (hydrazo) and A (azo) forms (HF/6-31G level) in different solvents calculated according to the Onsager model ........................................7 Table 2.3. σ values for several commonly encountered substituents.................................42 Table 2.4. Inductive and resonance values ........................................................................50 Table 2.5. Dielectric constant of some common solvents .................................................53 Table 2.6. Selected values of Kamlet-Taft parameters ......................................................59 Table 4.1. Absorption maxima of the hydrazone tautomer (B) of arylazo pyridone dyes (A1– A12) in protic solvents..............................................................................................76 Table 4.2. Absorption maxima of hydrazone tautomer (B) of arylazo pyridone dyes (A1– A12) in aprotic solvents. ....................................................................................................77 Table 4.3. Solvent parameters ............................................................................................79 Table 4.4. Regression fits to the solvatochromic parameters ............................................80 Table 4.5. Percentage contribution of the solvatochromic parameters ..............................82 Table 4.6. The relative energies and the statistical Boltzmann distribution weighted values of the most stable azo-hydrazone tautomers of dye A1 ..........................................84 Table 4.7. The absorption frequencies of the investigated compounds (A13–A23) in selected solvents.................................................................................................................89 Table 4.8. The physical parameters of the solvents ...........................................................91 Table 4.9. The results of the correlation between υmax and the solvent disperzive function f(n)......................................................................................................................................93 Table 4.10. Regression fits to the solvatochromic parameters (Eq. 4.1) ...........................96 Table 4.11. Percentage contribution of solvatochromic parameters (Eq. 4.1) ...................97 Table 4.12. Regression fits to solvatochromic parameters (Eq. 4.5) .................................98 Table 4.13. Percentage contribution of solvatochromic parameters (Eq. 4.5). ..................99 Table 4.14. The absorption frequencies of the investigated compounds (A24–A33) in selected solvents...............................................................................................................108 Table 4.15. Regression fits to the solvatochromic parameters (Eq. 4.1) .........................109 Table 4.16. Percentage contribution of solvatochromic parameters (Eq. 4.1) .................110 Table 4.17. Regression fits to solvatochromic parameters (Eq. 4.5) ...............................111 Table 4.18. Percentage contribution of solvatochromic parameters (Eq. 4.5) .................112 LIST OF FIGURES Figure 2.1. Disperse dyes obtained from (disulfoanilino)dihalotriazines ............................9 Figure 2.2. Disperse dyes prepared from phthalimidyl and pyridone moieties .................10 Figure 2.3. Azo dyes prepared from 3-cyano-4-hydroxy-2-phenyl-2-pyridone (4) and 3- cyano-6-hydroxy-4-phenyl-2-pyridone (5) ........................................................................11 Figure 2.4. Cyanodimethyl(phenylhydrazono)pyridinethione (6) and its chloropyridine derivative (7) ......................................................................................................................11 Figure 2.5. Yellow reactive azo dyes obtained from cyanuric fluoride .............................12 Figure 2.6. Basic azo pyridone dyes used to dye acrylic fibbers .......................................13 Figure 2.7. Cyano hydroxy pyridone dyes for liquid-crystal displays ...............................14 Figure 2.8. Pyridone azo dyes used in the production of colored plastics .........................14 Figure 2.9. Azo dyes used in jet and hot-melt printing ......................................................15 Figure 2.10. Yellow dyes derived from 1-substituted 3-cyano-6-hydroxy-4-methyl-2- pyridones which are used in inks for thermal-transfer recording ......................................15 Figure 2.11. Azo pyridone dyes used in a thermal-transfer printing material ...................16 Azo pyridone dyes from 6-hydroxy-2-pyridone and isoxazole (16), 1,2,4-triazole (17) or pyrazole (18) ......................................................................................................................17 Figure 2.13. 5-(4-Arylazophenyl)azo-3-cyano-6-hydroxy-4-methyl-2-pyridones ............18 Figure 2.14. Reactive disazo pyridone dyes ......................................................................19 Figure 2.15. Disazo pyridone dyes used in phase change inks ..........................................19 Figure 2.16. Disazo dyes obtained from bispyridone derivative .......................................20 Figure 2.17. Disazo yellow pyridone dyes used in the production of colored plastics......20 Figure 2.18. Disazo dyes from substituted 6-hydroxy-2-pyridones (24) and 4-methyl-3- cyano-6-hydroxy-2-pyridones (25) which are used as electrophotographic photoconductors .................................................................................................................21 Figure 2.19. Trisazo pyridone dyes used to dye nylon 6 ...................................................22 Figure 2.20. Azopyridone moieties used for the synthesis of trisazo colorants ................23 Figure 2.21. Azo pyridone dyes with good photostability .................................................24 Figure 2.22. Azo (A) - hydrazone (H) tautomerism ..........................................................25 Figure 2.23. Structure of C.I. Disperse Yellow 119 (30), D.I. Disperse Yellow 211 (31) and C.I. Disperse Yellow 114 (32) ....................................................................................26 Figure 2.24. The chemical structure of the azo dye synthesized from 1-ethyl-3-cyano-6- -hydroxy-4-methyl-5-amino-2-pyridone............................................................................28 Figure 2.25. Absorption spectra of dye 33 in different solvents: (1. Acetone, 2. Ethanol, 3. Chloroform, 4. DMSO, 5. DMF) ...................................................................................28 Figure 2.26. Absorption of dye 33 in different volume ratios (a. ; b. ; c. ; d. ) of chloroform / DMSO ...........................................................................................................29 Figure 2.27. The chemical structures of the azo pyrodone dyes investigated by Q. Peng at al. ........................................................................................................................................29 Figure 2.28. The 13C NMR spectra of azo pyridone dyes (Figure 2.27, substituent 4’- SO3K) (at 90 o C). (A) in DMSO-d6; (B) 20ml piperidine was added to (A); (C) Na2CO3was added to (A) ...................................................................................................30 Figure 2.29. The most stable azo (A) and hydrazone (B) tautomer optimized structures of dye A1, at B3LYP/6-31G(d) level of theory .....................................................................33 Figure 2.30. Hammett plot for the hydrolysis of ethyl benzoates ......................................40 Figure 2.31. Hammett plots for phenylacetic acid and benzoic acid ionization in ethenol47 Figure 2.32. One dimensional Gibbs energy diagram for a chemical reaction in three different solvents I, II and III .............................................................................................56 Figure 4.1. Absorption spectra of dyes A1–A12 in methanol ...........................................75 Figure 4.2. Absorption spectra of dyes A1–A12 in dimethyl sulfoxide ............................75 Figure 4.3. Resonance effect of electron-accepting (structure C) and electron-donating (structure D) substituents of the arylazo component on the hydrazone tautomer ..............81 Figure 4.4. Relationship between νmax and σp+ for arylazo pyridone dyes A1–A12 in methanol .............................................................................................................................83 Figure 4.5. The most stable hydrazone tautomer (a) and some other azo-hydrazone tautomers of 5-phenylazo-6-hydrohy-4-phenyl-3-cyano-2-pyridone. (The geometries correspond to the energy minimum in vacuo). ..................................................................85 Figure 4.6. The UV-vis absorption spectra of 5-arylazo-6-hydroxy-4-(4-methoxyphenyl)- 3-cyano-2-pyridone dyes in ethanol (a) and dimethyl sulfoxide (b)..................................90 Figure 4.7. The correlation of υmax of the dye A18 with ET N (excluded ethanol) ..............94 Figure 4.8. Experimental versus calculated values of νmax from Eq. 4.1 (A) and Eq. 4.5 (B) ....................................................................................................................................101 Figure 4.9. Relationship between υmax and σp+ for arylazo pyridone dyes in ethanol (excluding A14 and A19).................................................................................................104 Figure 4.10. UV-vis absorption spectra of azo dyes in different solvents (ethanol, ethyl acetate, dimethylformamide) ...........................................................................................106 Figure 4.11. The plot of υmax calculated against υmax observed for Kamlet-Taft (I) and Catalan (II) equation in different solvents .......................................................................113 1 ABSTRACT Three series of some novel arylazo pyridone dyes, 5-arylazo-6-hydroxy-4-phenyl- 3-cyano-2-pyridone dyes, 5-arylazo-6-hydroxy-4-(4-metoxyphenyl)-3-cyano-2-pyridone dyes and 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes have been synthetized. The structure of the dyes was confirmed by UV-Vis, FTIR, 1 H NMR and 13 C NMR spectroscopy and elemental analysis. The solvatochromic behavior of the dyes was evaluated with respect to their visible absorption properties in various solvents. The azo-hydrazone tautomeric equilibration was found to depend on the substituents as well as on the solvent. These dyes exist in the hydrazone form in the solid state and in solvent DMSO-d6 and there was an equilibrium between hydrazone form and azo form in the different solvents. The Kamlet-Taft and Catalan parameters were used for describing the solute- solvent interactions and solvatochromic shifts of the visible absorption band. It was found that the solute dipolarity / polarizability (especially polarizability by Catalan equation) play an important role in the description of the pronounced solvatochromism in the studied solutions. The Catalan solvent scales were found to be more suitable for describing the solvatochromic shifts. The geometry data of the investigated dyes were obtained using DFT quantum- chemical calculations. The obtained calculational results are in very good agreement with the experimental data. Keywords: Arylazo pyridone dyes, Tautomerism, Solvent Effects, Substituent Effects, Solvatochromism Scientific field: Chemistry Specific scientific field: Organic Chemistry 2 IZVOD U okviru proučavanja strukture i karakteristika azo piridonskih boja, u čvrstom stanju i u rastvoru, su sintetisane tri nove serije arilazo piridonskih boja koje do sada nisu poznate u literaturi: 5-arilazo-6-hidroksi-4-fenil-6-cijano-2-piridonske boje, 5-arilazo-6- hidroksi-4-(4-metoksifenil)-6-cijano-2-piridonske boje i 5-arilazo-6-hidroksi-4-(4- nitrofenil)-6-cijano-2-piridonske boje. Sve azo boje su sintetisane diazotovanjem odgovarajućih 4-supstituisanih anilina i kuplovanjem dobijenih diazo soli sa odgovarajućim piridonima, prethodno dobijenim iz odgovarajućih etil-4-supstituisanih benzoilacetata i cijanoacetamida. Struktura sintetisanih azo boja je potrvđena na osnovu podataka iz UV-vis, FTIR, 1 H NMR i 13 C NMR spektara. Solvatohromna svojstva su određena u odnosu na njihovu apsorpciju u vidljivom delu UV-Vis spektra u različitim rastvaračima. Efekti polarnosti rastvarača, proton-donorske i proton-akceptorske karakteristike interakcije rastvarač-rastvorena supstanca su kvantitativno procenjene Kamlet-Taft-ovim i Catalan-ovim solvatohromnim modelima. Proučavanje uticaja supstituenata na arilazo komponenti na azo-hidrazon tautomeriju sintetisanih boja izvršeno je korelacijom UV-Vis apsorpcionih frekvenci Hammett-ovom jednačinom. Azo boje sintetizovane u ovoj disertaciji mogu postojati u azo i hidrazon tautomernim oblicima. Na azo-hidrazon ravnotežu veliki uticaj imaju rastvarači i supstituenti prisutni u arilazo komponenti. Rezultati ostvareni u ovom radu ukazuju na dominantan hidrazonski tautomerni oblik, kako u čvrstom stanju tako i u rastvorima azo boja u ratsvaračima različitih svojstava. Hidrazonska struktura proučavanih azo boja je potvrđena kvantno-hemijskim izračunavanjima korištenjem DFT/B3LYP metode. Ključne reči: Arilazo piridonske boje, Tautomerija, Uticaj rastvarača, Uticaj supstituenata, Solvatohromizam. Naučna oblast: Hemija Uža naučna oblast: Organska hemija 3 1. INTRODUCTION Azo compounds are very important in the field of dyes, pigments and advanced materials. Over 50% of all colorants are azo dyes and they are most widely used compounds in various areas, such as dyeing textile fibres, coloring of different materials, in biological-medical studies and advanced applications in organic synthesis. The success of azo colorants is in the simplicity of their synthesis, in almost innumerable possibilities presented by variation on the diazo compounds and coupling components, to the generally high molar extinction coefficient, as well as to the medium to high light and wet fastness properties. In recent years, azo dyes have attracted wide interest and found many uses in materials for optical applications and in analysis. Due to their properties, including optical storage capacity, optical swithching, holography and non-linear optical properties, polymers with azo units represent promising candidates for photoactive materials. Pyridone derivatives are heterocyclic intermediates for the preparation of azo dyes. The azo pyridone dyes give bright hues and are suitable for dyeing of polyester fabrics. The physico-chemical properties of arylazo pyridones are closely related to their tautomerism. Determination of azo-hydrazone tautomers in the solid state and in solution is quite interesting both from theoretical and practical standpoints, since the tautomers have different technical properties and dying performances. The UV-vis absorption spectroscopy is widely used method for investigation of the intermolecular interaction and solvatochromism. Several investigations on substituted arylazo pyridones, with respect to their visible absorption spectra in various solvents, have been carried out and reviewed. It has been concluded that the equiliubium between the two tautomers is influenced by the structure of the compounds and the solvent used. The introduction of the electron-attracting or electron-donating subsitutents into the para or ortho positions of the diazo components of an azo pyridone dye, resulted in additive or substractive color shifts and fading rates, depending on the nature and the orientation of the subsitutents. In this thesis three series of arylazo pyridone dyes, 5-arylazo-6-hydroxy-4-phenyl- 3-cyano-2-pyridone dyes, 5-arylazo-6-hydroxy-4-(4-metoxyphenyl)-3-cyano-2-pyridone dyes and 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes have been synthetized. The structure of the dyes was confirmed by UV-vis, FTIR, 1 H NMR and 4 13 C NMR spectroscopy and elemental analysis. The solvatochromic properties have been studied in a set of twenty solvents of different properties. Different solvent parameters, such as microsopic solvent polarity ET N , relative permitivity r, refractive index n, the Kamlet-Taft and the Catalan parameters were used for describing the solvent-solute interactions and the solvatochromic shifts of the UV-vis absorption band of the investigated arylazo dyes. For the quantitive assessment of the substituent effects, the simple Hammett equation was used. The geometry data of the investigated dyes were obtained using DFT quantum- chemical calculations. The obtained results are in very good agreement with the experimental data. 5 2. THEORETICAL PART 2.1. Synthesis, structure and properties of azo pyridone dyes Azo dyes are synthetic organic colorants bearing chromophoric azo group (-N=N-). On one side azo group is attached to an aromatic or heterocyclic nucleus and on the other, to an unsaturated molecule of the carbocyclic, heterocyclic, or aliphatic type [1]. IUPAC defines azo compounds as: "Derivatives of diazene (diimide), HN=NH, wherein both hydrogens are substituted by hydrocarbyl groups, e.g. PhN=NPh azobenzene or diphenyldiazene" [2]. Azo group is not presented in natural dyes. Commercially, these colorants are the largest and most versatile class of organic dyestuffs. As published in Kirk-Othmer Encyclopedia of Chemical Technology [1] in 2003, there were more than 10,000 Colour Index (CI) generic names assigned to commercial colorants. Approximately 4,500 of them are in use, and over 50% of these belong to the azo compounds. Azo dyes can be divided according to the number of azo groups to monoazo, disazo, trisazo and polyazo, and also further subdivision can be achieved according to the solubility or according to the types of component used. The widest range of usage of azo dyes is because of number of variations in chemical structure and methods of application which are generally not complex. Cotton, paper, silk, leather, and wool can be dyed by azo dyes. Also, there are azo dyes for dying polyamides, polyesters, acrylics, polyolefins, viscose rayon, and cellulose acetate. They can be used for the coloring of paints, varnishes, plastics, printing inks, rubber, foods, drugs, and cosmetics. Azo colorants are also used in diazo printing and color photography. The shades of azo dyes cover the whole spectrum [1]. Among azo colorants arylazo pyridone dyes have become important in last several decades. The high molar extinction coefficient, and the medium to high light and wet fastness properties are very favourable [3]. They find application generaly as disperse dyes. Disperse dyes are characterized by low aqueous solubility and are applied to hydrophobic fibers from an aqueous system, in which the dye is present in a highly dispersed state. The importance of disperse dyes increased in the 1970s and 1980s due to the use of polyester and nylon as the main synthetic fibers. Also, disperse dyes were used rapidly since 1970 in inks for the heat-transfer printing of polyester [1]. 6 2.1.1. General synthesis The main synthetic route for the preparation of azo dyes is coupling reaction between an aromatic diazo compound and a coupling component. Of all dyes manufactured, about 60% are produced by this reaction [1]. The success of azo colorants is due to the simplicity of their synthesis by diazotization and azo coupling, and to the almost innumerable possibilities presented by variation on the diazo compounds and coupling components [3]. All coupling components used to prepare azo dyes have the common feature of an active hydrogen atom bound to a carbon atom. Generally, arylazo pyridone dyes can be prepared from pyridone moiety as a coupling component and variuos diazonium salts using well known reaction [3] (Scheme 2.1). R is usualy methyl, R´ ethoxy and R´´ hydroxy group. Nitrogen can be substituted, while diazonim salts can be derived from diferent substituted anilines or other heterocyclic derivatives. Pyridones can be prepared using known procedures [4-7]. N N N O R H R'' CN R R' O O NH 2 O NC N 2 N O R H R'' CN + + Scheme 2.1. Preparation of the arylazo pyridone dyes from pyridone. Arylazo dyes containing pyridone ring can be also prepared from arylazo diketones or arylazo ketoesters (obtained by coupling β-diketones or β-ketoesters with 7 diazonim salts) by condensation with cyanoacetamide (Scheme 2.2) [8-10]. And here, R is usualy methyl, R´ ethoxy or methyl and R´´ hydroxy or methyl group. N N N O R H R'' CN R R' O O NH 2 O NC N 2 R R' O O N N+ + Scheme 2.2. Preparation of the arylazo pyridone dyes from arylazo intermediate. Lately, a novel protocol for the rapid synthesis of pyridone colorants under controlled microwave irradiation in a dedicated reactor is described according to Scheme 2.2. Short reaction times, high isolated yields, and versatility for different substrates are the advantages of the reported method [11]. All other procedures are mostly variants given in Schemes 2.1 or 2.2, where different diazo salts and pyridones were used. 2.1.1.1. Monoazo dyes Disperse azo dyes useful for dyeing polyester fibers fast brilliant yellow shades were described in 1972 by Burkhard et al. These dyes were prepared by coupling diazotized anilines with 3-cyano-6-hydroxy-4-methyl-2-pyridones in HOAc at 0-5 o C and pH 4.5 [12]. Starting from different anilines disperse dyes were prepared and used in dyeing or printing polyester fibers with fast, yellow to greenish yellow shades [13]. In addition, disperse azo dyes were manufactured and used to dye synthetic fibers fast 8 greenish yellow to red shades, by further modification of amino component (e.g. 5- amino-4,6-dicyanoindan) [14]. Lightfast yellow shades were obtained by dyeing poly(ethylene terephthalate) (PTT) fibers with azo dyes where molecules like decyl 4- aminobenzoate was diazotized and coupled with 3-cyano-6-hydroxy-4-methyl-2-pyridone [15]. Besides yellow shades, polyester fibers were dyed fast orange shades [16]. It was shown that dye prepared from p-toluidine and 3-cyano-6-hydroxy-4-methyl-2-pyridone is useful for dyeing and printing hydrophobic synthetic fibers, e.g., polyester, in mixture with Disperse Yellow 54 and/or Disperse Yellow 64 [17]. By coupling diazotized 2,4,3,5-(NC)2Me2C6HNH2 with 3-cyano-6-hydroxy-1,4- dimethyl-2-pyridone in aqueous NaOH, the yellow azo dye was prepared and used for dyeing polyester fibers light- and sublimation fast yellow shades [18]. Another azo dye was prepared by coupling diazotized 3-H2NC6H4O3SPh with 3-cyano-6-hydroxy-1,4- dimethyl-2-pyridone and used for dyeing polyester fibers a fast greenish yellow shade [19]. In addition, azo dyes prepared by coupling diazotized 2,4-O2N(RO)C6H3NH2 (R = Me(CH2)3CHEtCH2, Me(OCH2CH2)2, Me(CH2)9) with the same pyridone, were used to dye polyester fibers fast orange shades from aqueous dispersions and from tetrachloroethylene [20]. An improvement in azo dye synthesis in mean of yield, purity, and production efficiency, was achieved by using NO2 in pentane at –8°C for diazotization followed by coupling in water at 0-5°C [21]. Brilliant yellow color was obtained by coupling of diazotized aniline 2,4-R1O(N O2)C6H3NH2 (R1 = C1-4) with two 1-substituted alkyl (R2 = C1-4, (CH2)nOR3, R3 = C1-4 alkyl; n = 1-3) cyano(methyl)hydroxypyridones when used to dye polyester fabric [22]. Also, disperse dyes were prepared from 1-alkyl substituted cyano 2-pyridones where alkyl groups had 1-4 carbon atoms [23]. A fast greenish yellow shade on polyester fibers was obtained by using an azo dye prepared by coupling diazotized 4-ClC6H4OCH2CH2O2CC6H4NH2 with 3-cyano-1-ethyl- 2-hydroxy-4-methyl-6-pyridone [24]. When 1-butyl-3-cyano-6-hydroxy-4-methyl-2-pyridone was used as a coupling component and 4-H2NC6H4SO2NHCH2CHEtBu was diazotized, disperse azo dye was obtained and used for dyeing polyester fibers light-, wet-, and heat fast greenish yellow shades [25]. 9 Other 1-substituted 3-cyano-6-hydroxy-4-methyl-2-pyridones were used as coupling components. When 1-substituents were: (un)substituted Ph, C3-4 alkenyloxy, C3-4 alkynyloxy, PhO and (un)substituted C1-8 alkoxy, azo dyes were obtained with different substituents in arylazo part of dye. Thus, 4-H2NC6H4CO2CH2CO2CH2Ph was diazotized and coupled with 1-butyl-3-cyano-6-hydroxy-4-methyl-2-pyridone to give dye that gave brilliant greenish yellow on polyester fibers, both in polyester fabrics and in polyester-cotton blends [26]. By coupling diazotized 2-nitroaniline with 3-cyano-1-(2- ethylhexyl)-6-hydroxy-4-methyl-2-pyridone an azo dye insoluble in water was obtained which produce heat- and wet fast greenish yellow on polyester fibers [27]. When such dye was heated at 80-85 °C for 2 h, an azo dye in a specified crystal form was obtained and used for dyeing of textured polyester yarns, giving rub-fast dyeing [28]. The water-soluble disperse azo dyes including light yellow, yellow, orange, red, violet and blue colours were obtained from 3-cyano-6-hydroxy-4-methyl-2-pyridone or 1- substituted 3-cyano-6-hydroxy-4-methyl-2-pyridones (substituents Me, OMe, SO2CH2COOH) [29]. Dyes having formula 1 (Figure 2.1; R = H or optionally substituted alkyl; Q = CN, CONH2; X = halogen), were obtained from (disulfoanilino)dihalotriazines and the appropriate azo pyridine, were useful in dyeing or printing of hydroxy- or nitrogen- contaning substrates [30]. N N N N N H N SO 3 HX N H SO 3 H N Me O R OH Q HO 3 S 1 Figure 2.1. Disperse dyes obtained from (disulfoanilino)dihalotriazines. Disperse azo dyes prepared from phthalimidyl and pyridone moieties showed improved washfastness in dyeing of hydrophobic fibers (Figure 2.2, R 1 = C1-12 alkyl or CnH2n(OCH2CH2)mOR3, n = 2-8, m = 0-4, R3 = C1-12 alkyl, C6-24 aryl, or C6-24 aralkyl, R 2 = Me, Et, Pr, Bu, 2-methoxyethyl or 2-ethoxyethyl, X = halo, Y = H, Cl, or Br) [31]. 10 N N N N Me O OH O O Y X CN 2 R2 R1 Figure 2.2. Disperse dyes prepared from phthalimidyl and pyridone moieties. Azo dyes for dyeing of polyester fabrics were in addition prepared by coupling diazotized p-substituted anilines (H, Me, NO2) with 1-substituted 5-cyano-6-hydroxy-4- methyl-2-pyridones (N-substituents = H, Me, HOCH2CH2, Me2NCH2CH2, C12H25) [32]. Recently a synthesis of new azo dyes was reported. These dyes were prepared by diazotisation, coupling and cyclization reactions, starting from various aryldiazonium salts and different ß-diketoesters followed by condensation with cyanoacetamide, as given in Scheme 2.2. The pyridone moiety was substituted by Me or Ph group in position 4 and with OH or C6H5NH in position 6 [33]. NH 2 O CN X 1 Y 1 O O N N H Ar N NAr N O H X 2 Y 2 CN + 3 Scheme 2.3. Disperse dyes with pyridone moiety substituted by Me or Ph group in position 4 and with OH or C6H5NH in position 6. Azo dyes given in Figure 2.3 were prepared from 3-cyano-4-hydroxy-2-phenyl-2- pyridone (4) as well as from 3-cyano-6-hydroxy-4-phenyl-2-pyridone (5) and coupling with aniline and p-substituted anilines (Me, OMe, Cl) [34]. 11 N N N OH O CN H X N N N OH CN H X O 4 5 Figure 2.3. Azo dyes prepared from 3-cyano-4-hydroxy-2-phenyl-2-pyridone (4) and 3- cyano-6-hydroxy-4-phenyl-2-pyridone (5). Thiopyridones (Figure 2.4) were prepared by the reaction of thiocyanoacetamides with sodium ethoxide and then with MeCOC(COMe)=NNHPh to give cyanodimethyl- -(phenylhydrazono)-pyridinethione (6). Alternatively, 6 was chlorinated with Cl2 in CHCl3 to give chloropyridine derivative (7) [8]. S Cl N N N Me H Me CN N N N Me Me CN 6 7 Figure 2.4. Cyanodimethyl(phenylhydrazono)pyridinethione (6) and its chloropyridine derivative (7). It should be mentioned that disperse dyes were also prepared from 2- aminothiophene derivatives (2-amino-4,5,6,7-tetrahydro[b]thiophene-3-carbonitrile and ethyl 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate) and 5-cyano-6- hydroxy-4-methyl-2-pyridone and 5-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone. The dyes had generally good coloration and fastness properties on polyester [35,36]. 12 Reactive dyes Five reactive azo dyes for cotton were prepared from 1-(2-aminoethyl)-3-cyano-2- hydroxy-4-methyl-6-pyridone. Also, a cationic azo dye for polyacrylonitrile fiber, a disperse azo dye for hydrophobic fibers, and an acid azo dye for nylon were prepared [37]. More yellow reactive azo dyes (Figure 2.5; M = H, alkali metal; R 1 , R 2 = H, optionally substituted C1-4-alkyl; R 3 = H, optionally substituted C1-4-alkyl or -alkoxy, halogen, carboxy, Ph; R 4 = C1-4-alkyl, Ph; R 5 = carbonamido, cyano, sulfomethyl; R 6 = C1-4-alkyl; X = vinyl or vinyl-forming group; n = 0-2) were obtained from cyanuric fluoride, the requisite aromatic amines, and a pyridone derivate coupling component [38]. N N N R OOH R R N NN N R F N SO 2 X R (SO 3 M)n 1 2 3 4 5 6 R 8 Figure 2.5. Yellow reactive azo dyes obtained from cyanuric fluoride. Basic dyes Nine basic dyes, based on 2-pyridone moiety, were manufactured and used to dye polyacrylonitrile fast yellow shades. Thus, [α-(4- aminobenzenesulfonamido)ethyl]pyridinium chloride was diazotized and coupled with 1- ethyl-3-cyano-2-hydroxy-4-methyl-6-pyridone to give an azo dye. A number of variations were applied in arylazo part as well as in substituent in position 1 of pyridone part [39]. Basic dyes, given in Figure 2.6, were prepared in one step by coupling diazotized aromatic primary amines with 6-hydroxy-2-pyridones and used to dye acrylic fibers fast yellow shades. 3-Pyridinium (9) or 3-cyano-2-pyridone (10) dyes were 1-substituted with at least one alkyl group with more than 5 C-atoms [40]. 13 O N N N Me C 10 H 21 OH N MeO NO 2 O N N N Me Et OH CO CN (C 8 H 17 ) 2 N MeCH 2 + Cl _ Cl _ + 9 10 Figure 2.6. Basic azo pyridone dyes used to dye acrylic fibbers. Monoazo dyes for other uses Azo pyridone dyes were further used for paints and printing inks with yellow to blue shades. This printing inks were based on a monoazo dye e.g. 1-ethyl-3-cyano-4- methyl-5-[(3-nitrophenyl)azo]-6-hydroxy-2-pyridone. Besides a dye, a film-forming binder and a solvent were used [41]. Azo dyes obtained from 1-substituted pyridones (C1-8 alkyl, allyl) e.g. 1-butyl-3- cyano-2-hydroxy-4-methyl-6-pyridone, were used do dye coating components, organic solvents and mineral oil products in yellow shades. Different substituted anilines were used like 3,4-Me(H2N)C6H3SO2NBu2 [42]. A yellow dye derived from substituted 3-cyano-6-hydroxy-2-pyridones with different groups in positions 1 and 4 were used for the preparation of a powder toner composition comprised a thermoplastic polymer blended with 0.5-10 % of a yellow dye. The dyes can be used where images are fixed by thermal fusion [43]. Cyano hydroxy pyridones were also used as colorants to prepare color filters for liquid-crystal displays. The dyes have the structure presented in Figure 2.7 [M = H, cation; R 1 , R 3 = H, (un)substituted C1-8 organic group, A; R 2 = (un)substituted C1-8 organic group; R 4 , R 5 , X, Y, Z = H, CN, substituent; c = 2-6; m, n = 0-2], containing ≥ 1 SO3M or PO3M2 group, with certain specified exclusions [44]. 14 N R OH N O N R CO 2 M N NN N RR R (CH 2 )c 1 2 Z (X)m (Y)n A= 3 4 5 11 Figure 2.7. Cyano hydroxy pyridone dyes for liquid-crystal displays. The pyridone azo dyes given in Figure 2.8 (A = diazo component residue; R 1 = H, optionally hydroxyl- or phenyl-substituted C1-6-alkyl, azo pyridone derivative, ester, amide, keto; R 2 = azo pyridone derivative, ester, amide, keto; R 1 R 2 N may form a heterocycle; Y = cyano, CONH2, CH2SO3H; n = 2-6) were used in the production of colored plastics or polymeric color particles [45]. N R R N Me Y O (CH 2 )n OH NN A 1 2 12 Figure 2.8. Pyridone azo dyes used in the production of colored plastics. Azo dyes presented in Figure 2.9 (X = H, Cl, Br, CN, SO2Me, OH, OMe, NO2; Y = H, Cl, Br, Cl; Z = coupling component group like 1-butyl-3-cyano-6-hydroxy-4- methyl-2-pyridone) were prepared, and used in dyeing and in jet and hot-melt printing. These dyes are suited for application to hydrophobic and synthetic textiles with good fastness in yellow shade [46]. 15 OO Me Y N N X Z 13 Figure 2.9. Azo dyes used in jet and hot-melt printing. The yellow dyes [Figure 2.10, R 1 = C3-8 branched or cyclic (substituted) alkyl; R 2 = C5-10 branched or cyclic (substituted) alkoxyalkyl] were derived from 1-substituted 4-methyl-3-cyano-6-hydroxy-2-pyridones, and used to produce ink which contained a binder resin, and an organic solvent and/or H2O. The ink was used for thermal-transfer recording. The dyes gave images with good storage stability [47]. N N N Me O OH CN R OCO CH 2 CO 2 R 1 2 14 Figure 2.10. Yellow dyes derived from 1-substituted 3-cyano-6-hydroxy-4-methyl-2- pyridones which are used in inks for thermal-transfer recording. Similarly, dyes were prepared having formula AN=NR [A = halogenated phenyl; R = 1-C2-6 alkyl(or alkoxy)-5-cyano-2-hydroxy-3-methyl-6-pyridon-3-yl group] and used to get inks for a transfer sheet which was used with a thermal receiving sheet [48]. In another patent pyridone-based yellow monoazo dyes were used to obtain ink by dissolving 2-8% dye and 2-8% binder in 84-96% organic solvent for thermal transfer printing [49]. Azo pyridone dyes, which structure is given in Figure 2.11 [A = (substituted) aromatic ring; B = single bond or divalent linking group; X = N or CY 1 ; Y 1-4 = H, halo, straight-chain or branched alkyl which may be substituted, aryl, heteroaryl, alkoxy, cycloalkyl, alkenyl, allyl, aralkyl, dialkylamino, alkylamino, the plural groups of Y 1-4 may be condensed to form a 5- to 7-membered ring; R 1 = straight-chain or branched alkyl which may be substituted; R 2 = straight-chain or branched alkyl which may be substituted, cycloalkyl, alkenyl, aryl, aralkyl, allyl] were prepared and used in a thermal- 16 transfer printing material comprises the dye-donating material and a dye image-receiving material possessing a image-receiving layer containing a dye-fixing material [50]. N R OH O R N N CN N X B Y Y Y 3 1 2 4 A 2 15 Figure 2.11. Azo pyridone dyes used in a thermal-transfer printing material. Monoazo dyes derived from 1-substituted 3-cyano-6-hydroxy-4-methyl-2- pyridones were used in thermal-transfer printing. Different substituted anilines were used with substituents like (un)substituted succinimido or maleimido or glutarimido; halo, C1-4 alkyl, alkoxy groups [51]. It is interesting to mention that dyes derived from different anilines and 3-cyano- 6-hydroxy-2-pyridones were used in a media useful for recording and/or reading information by using blue laser [52]. Azo pyridones are also useful as colorants in phase change inks. Different derivatives of 3-cyano-6-hydroxy-2-pyridones were used [53,54]. Other inks were produced from azo pyridone dyes, as given in Japan patent [55], where was clamed that ink shows good storage stability while image formed from the ink showed optical density 1.1 and good water and light resistance. The dyes obtained from 1-substituted 4-alkyl (C1-10 alkyl, methoxymethyl, trifluoromethyl)-3-cyano-6-hydroxy-2-pyridones were used for ink-jet inks, liquid crystal displays, plasma display panels, and solid-state image sensors (e.g., CCD, CMOS) [56]. Tanaka et al. [57] claimed yellow toners prepared from azo pyridone dyes obtained from substituted 6-hydroxy-2-pyridones. As an example the following preparation can be given: o-nitrobenzoic acid and SOCl2 were heated at 60 0 C for 1 h, and then Et3N and di(2-ethylhexyl)amine were added therein and reacted at 80 0 C for 2 h; the resulting 2-nitrobenzoyl 2-diethylhexylamide was reduced, diazotized, and coupled with 1,2-dihydro-6-hydroxy-4-methyl-2-oxo-3-pyridinecarbonitrile to give a pyridone derivative. 17 Metal complexed dyes Azo pyridone dyes were used in order to provide an optical recording medium. This medium had a recording layer improved in light stability capable of recording and regeneration of high-density optical information by short-wavelength laser beams. The recording layer contained a metal complex of pyridone azo compounds as given in Figure 2.12 (R 1-10 = H, monovalent functional group). The metal complex was obtained from a 6-hydroxy-2-pyridone structure as a coupler component and an isoxazole, 1,2,4-triazole or pyrazole structure as a diazo component and an ion of bivalent metal, such as Ni, Co, Fe, Zn, Cu or Mn [58]. N O RR OH R N N O N R R N O RR OH R N N N N N R R N O RR OH R N N N N R R R 1 23 4 5 6 7 8 9 10 1 23 1 23 16 17 18 Figure 2.12. Azo pyridone dyes from 6-hydroxy-2-pyridone and isoxazole (16), 1,2,4- triazole (17) or pyrazole (18). Also, Cr-complex azo dyes were obtained from o,o'-dihydroxyphenylazopyridone intermediates and ammonium chromium sulfate [59]. Pigments Except dyes and their metal complexes, azo pyridone colorants can be prepared as pigments. So, by treatment of azo pyridone dye, obtained from 2-pyridones (e.g. 3-cyano- 6-hydroxy-4-methyl-2-pyridone) with BaCl2 yellow azo pigments were obtained and used in printing inks [60]. Thermally stable yellow pigments obtained by coupling diazotized 2,5- R 1 CO(R 2 CO)C6H3NH2 (R 1 , R 2 = MeO, NH2) with 5-cyano-2-hydroxy-4-methyl-6- pyridone as an aqueous solution of Na or K salt or aqueous dispersion at pH 3-10 and 0- 30 0 C were dried at 105 0 C and grounded. The pigments were stable up to 350 0 C and resistant to organic solvents and suitable for plastics and baking varnishes [61]. 18 2.1.1.2. Disazo dyes Disperse dyes 5-(4-Arylazophenyl)azo-3-cyano-6-hydroxy-4-methyl-2-pyridones (Figure 2.13, X = H, Me, Cl, O2N; Y = H, Me, MeO, O2N; Z = H, MeO) were prepared (57-87% yield) by coupling the appropriate amino azo compound with 3-cyano-6-hydroxy-4-methyl-2- pyridone. When used on polyester yellow, orange, brown, and red shades with pick-up poor to excellent were obtained and light fastness was fair to excellent, and sublimation fastness was fair to very good [62]. N H OH O Me X Y N N N N CN Z 19 Figure 2.13. 5-(4-Arylazophenyl)azo-3-cyano-6-hydroxy-4-methyl-2-pyridones. Reactive dyes Disazo reactive dyes, which dyed cotton in yellow shades, containing hydroxyl pyridone and fluorotriazine groups were prepared. Thus, pyridone dye was obtained from 5-[(3-amino-6-sulfophenyl)azo]-1-ethyl-6-hydroxy-4-methyl-3-(sulfomethyl)-2-pyridone and p-phenylenediamine [63]. Reactive disazo pyridone dyes (Figure 2.14, R 1 = H, optionally substituted C1-4- alkyl; R 2 = H, optionally substituted C1-4-alkyl with substituents including fiber reactive groups; R 3 , R 4 = diazo component group optionally containing fiber-reactive group; X = SO3H, SO3Na, OH; Z = C2-4-alkylene, arylene, heterocyclic diradical) with improved solubility were obtained. In this preparations, e.g. a yellow disazo reactive dye was obtained by condensation of 2 mol 1-ethyl-6-hydroxy-2-pyridone with 1 mol glutaraldehyde-NaHSO3 to provide a bis-pyridone coupling component which was then treated with 2 mol of diazotized 4-(2-sulfatoethylsulfonyl)aniline [64]. 19 N R OH O R R N N X X N R OHO R RNN 3 Z 4 1 2 1 2 20 Figure 2.14. Reactive disazo pyridone dyes. Disazo dyes for other uses Disazo dyes were prepared by tetrazotizing a dianiline and coupling it with a pyridone or by diazotizing aniline and coupling it with a dipyridone. Obtained colorants (Figure 2.15, R 1 = alkylene, arylene, arylalkylene, alkylarylene, alkyleneoxy, aryleneoxy, arylalkyleneoxy, alkylaryleneoxy, polyalkyleneoxy, polyaryleneoxy, polyarylalkyleneoxy, polyalkyl-aryleneoxy, heterocyclic, silylene, siloxane, polysilylene, polysiloxane group; R 2 , R 2' = alkyl, aryl arylalkyl, alkylaryl, alkoxy,aryloxy, arylalkyloxy, alkylaryloxy, polyalkyleneoxy, polyaryleneoxy, polyarylalkyleneoxy, polyalkylaryleneoxy, heterocyclic, silyl, siloxane, polysilylene, polysiloxane group, etc.; R 3 , R 3' = alkyl, aryl, arylalkyl, alkylaryl group; X, X' = direct bond, O, S, N-containing linking group, alkylidene group; Z, Z' = H, halogen, nitro, alkyl, aryl, arylalkyl, alkylaryl, etc.) can be used in phase change inks [65-67]. Diazopyridone colorants were also prepared by coupling substituted pyridone with a diazonium salt to form diazopyridone compounds [68]. N R OH N O N R CN X ORN R OH N O N R NC XO 3 2 Z 3 1 2 Z , , , , 21 Figure 2.15. Disazo pyridone dyes used in phase change inks. 20 Another example of disazo dyes which are useful as colorants for phase change inks is given in Figure 2.16 (R 1 , R 2 = hydrocarbyl; R 3 , R 6 = H, halogen, nitro, org. group; R 4 , R 5 = organic group; X, X' = direct bond, O, S, imino, optionally substituted methylene; Z = unsaturated alkylene; m, n are integers). In this synthesis, bispyridone derivative was obtained first and then coupled with diazotized moiety [69,70]. NR O (CH 2 )m N OH N X O R NC N R O (CH 2 )n NOH N X O R CN 4 56 Z 4 6 Z R , , , , 22 Figure 2.16. Disazo dyes obtained from bispyridone derivative. Disazo yellow pyridone dyes given in Figure 2.17 (Y = CN, CONH2, CH2SO3H; R = organic group) were described and used in the production of colored plastics or polymeric color particles. Dyes had good heat resistance, migration resistance, tinctorial strength, and fastness when used for bulk coloration [71]. N N Me O ROH Y N SO 3 N N Me O R OH Y N SO 3 Me Me 23 Figure 2.17. Disazo yellow pyridone dyes used in the production of colored plastics. 21 Dyes presented in Figure 2.18 (R = H, alkyl, alkenyl, cycloalkyl, aryl, or aromatic heterocyclyl; R 1 = alkyl, aryl, or aromatic heterocyclyl; R 2 = H, cyano, carbamoyl, carboxyl, ester, or acyl; Z = a divalent linking from an aromatic or heterocyclic moiety) were used as electrophotographic photoconductors having a high photosensitivity and excellent characteristics in repeated use [72]. N N N R OOH R R NN N R O OH R R N N N Me OOH H CNNN N Me O OH H NC 1 2 Z 1 2 24 25 Figure 2.18. Disazo dyes from substituted 6-hydroxy-2-pyridones (24) and 4-methyl-3- cyano-6-hydroxy-2-pyridones (25) which are used as electrophotographic photoconductors. Some new disazo dyes were synthesized by diazotization of 5-amino-3-methyl-4- hetarylazo-1H-pyrazoles and 5-amino-3-methyl-4-hetarylazo-1-phenylpyrazoles and coupling with 3-cyano-6-hydroxy-4-methyl-2-pyridone and 3-methyl-1H-pyrazole-5-one [73]. 22 2.1.1.3. Trisazo dyes Except mono and disazo pyridone dyes, trisazo pyridone dyes were prepared. So, pyridone dyes shown in Figure 2.19 (in a free acid form; R = H, C1-6-alkyl; X = H, aminocarbonyl, sulfomethyl, sulfo; Y = H, Me, Et, CF3, Cl, NO2; m = 0-2) were obtained and used to dye nylon 6 [74]. (HO 3 S)m (SO 3 H)m (SO 3 H)m N Me O R OH NN X NH N N N N H N H N N N N O O X R OHMe OH R X Me Y Y Y 26 Figure 2.19. Trisazo pyridone dyes used to dye nylon 6. In another example of trisazo colorants, compounds having 3 azopyridone moieties (Figure 2.20) bonded to a central atom, a monomeric group of atoms, an oligomer, or a polymer were produced for use as yellow dyes for hot-melt inks. Typicaly, a dye was manufactured by reaction of dodecylamine with ethyl cyanoacetate, cyclization of the intermediate with ethyl acetoacetate, and reaction of the resulting pyridone derivative with dipentaerythritol hexaanthranilate [75]. 23 N R OH N O N R CN R X O 3 1 2 Z 27 Figure 2.20. Azopyridone moieties used for the synthesis of trisazo colorants. 2.1.2. Properties of azo pyridone dyes Pyrydone azo dyes generally produce yellow shades on fabrics but other shades were also reported [12-20,25-27,29]. Dye structural effects on the intensity of color (yellow, green, and orange) and fastness properties on nylon 66 and polyester knits were also discussed [76]. Many authors have reported the fastness properties [31,35,36,46,62,71] of azo pyridone dyes and some have studied the photofading kinetics of 3-(p- and o-substituted arylazo)-5-cyano-2-hydroxy-4-methyl-6-pyridone dyes in amide solvents (DMF, HCONH2, and AcNMe2) and n-hexane. It was established that a fair linear correlation existed between the observed rate constant and the free energies of transfer, suggesting the possibility that the photofading rate increased with increasing solvation of dyes. The rate was increased by the presence of two electron-withdrawing substituents (NO2 and Cl) on the benzene ring [77]. The same authors have also studied photostability of 3- (mono- and di-substituted arylazo)-5-cyano-2-hydroxy-4-methyl-6-pyridones in N,N- dimethylformamide. Photodegradation was observed when dyes were irradiated by 254 nm light. It was established that the primary photochemical reaction with pyridone azo dyes involved hydrogen abstraction from the amide solvent. Also, it was found out that the simultaneous presence of two electron-withdrawing substituents in diazo component of dyes caused a bathochromic effect and an increase of fading rate, while introduction of an alkylol group to coupling component resulted in hypsochromic shifts and in decrease of fading rate [78]. Besides direct photodegradation, photocatalytic degradation of 5-(4- sulpho phenylazo)-3-cyano-6-hydroxy-4-methyl-2-pyridone in the presence of commercial TiO2, in aqueous solutions by simulated sunlight was studied [79]. 24 In another work, yellow disperse azo dye was synthesized from 2,6-dichloro-4- nitroaniline and 3-cyano-6-hydroxy-1,4-dimethyl-2-pyridone and its dyeing, fastness, and photodegradation behavior on polyester fabric was investigated. It was found that the build-up and lightfastness of dye derived from pyridone was not good [80]. Introduction of various substituents can have different impact on dye properties. So when in dyes presented in Figure 2.21 (R 1 = H, Me, C3H7, C4H9, C6H13, C8H17, Ph; R 2 = Me, CF3; R 3 = CF 3 , C4F9, C6F13, C8F17) a long perfluoroalkyl group was introduced a lowered film- forming ability and sensitivity but good photostability was achieved [81]. When 5-(2- benzothiazolylazo)-3-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone was compared to other unsymmetrical and symmetrical bis(hetaryl)azo dyes it was found that only pyridone derivative showed remarkable difference of decomposition temperature [82]. N N N R R OH O R CN 1 23 28 Figure 2.21. Azo pyridone dyes with good photostability. 2.1.3. Azo – hydrozone tautomerism of azo pyridone dyes A number of studies can be found in literature in which series of pyridones were coupled with diazotized substituted anilines and than substituent and/or solvents effects were discussed. So dyes derived from 3-cyano-6-hydroxy-4-methyl-2-pyridone [83,84], 3-amino-5-cyano-1-ethyl-6-hydroxy-4-methyl-2-pyridone [85], 4-amino-6-hydroxy-2- pyridones, 4,6-diamino-2-pyridone-3-carbonitrile and 2,4-diamino-6-pyridone-3- carbonitrile [86], 5-(2-pyrido-5-yl)azo-thiophene derivatives [87], 4-(p-substituted) phenyl-2-(2-pyrido-5-yl)azo-thiazole derivatives [88], 5-(arylazo)-3-cyano-4-methyl-6- methyl/phenyl-2-pyridinones [76] and 1-butyl-3-cyano-6-hydroxy-4-methyl-2-pyridone [89] were studied among others. In these studies, often azo-hydrazone tautomerism was investigated. Thus, three series of dyes were prepared by coupling diazonium salts to 2- (ethylthio)- and 2-(butylthio)-4,6-diaminopyrimidine as well as to 3-cyano-6-hydroxy- 25 1,4-dimethyl-2-pyridone. IR spectra and visible absorption spectroscopy indicated that the arylazopyrimidines existed in the azo tautomeric form, while the pyridone dyes existed as hydrazones [90]. Also, absorption spectra of ten 5-(4-substituted arylazo)-3- cyano-6-hydroxy-4-methyl-2-pyridones have been recorded in fifteen solvents in the range 200-600 nm. Besides the effects of the substituents on the absorption spectra and the effects of solvent polarity and solvent/solute hydrogen bonding interactions, azo- hydrazone tautomerism (Figure 2.22, X = OH, OCH3, CH3, C2H5, H, Cl, Br, I, COOH, NO2) was studied and it was concluded that equilibrium depends on the substituents as well as the solvents [91]. In addition, twelve pyridone-based disperse dyes were synthesized from a variety of substituted pyridones and 3-amino-5-nitrobenzoisothiazole, 2-amino-6-nitrobenzothiazole and 2-chloro-4-nitroaniline. Here azo-quinohydrazone isomerism was discussed [92]. N N N O Me H OH CN N N N O Me H CN H O X X (A) (H)29 Figure 2.22. Azo (A) - hydrazone (H) tautomerism. Azo-hydrazone tautomerism was also studied by crystallography. So 2-(2- methoxyethoxy)ethyl 4-[(5-cyano-1-ethyl-4-methyl-2,6-dioxo-1,2,3,6-tetrahydropyridin- 3-ylidene)hydrazino]benzoate crystallizes in the hydrazone form [93]. The same conclusion was obtained for C.I. Disperse Yellow 114 (5-cyano-2-hydroxy-1,4-dimethyl- 6-pyridone component) [94]. C.I. Disperse Yellow 119 and D.I. Disperse Yellow 211 (pyridine-1-ethyl-3-cyano-4-methyl-2,6-dione backbone) (Figure 2.23) also crystallize in the hydrazone form [95]. 26 N N O O N H N O Et O Me N N N O O N H N O Et O Me N Cl N N H N O Me O Me N S O O 30 31 32 + - + - Figure 2.23. Structure of C.I. Disperse Yellow 119 (30), D.I. Disperse Yellow 211 (31) and C.I. Disperse Yellow 114 (32). 5-(3- and 4-Substituted arylazo)-3-cyano-4,6-dimethyl-2-pyridones and 3-cyano- 4,6-diphenyl-5-(3- and 4-substituted phenylazo)-2-pyridones were prepared and their absorption spectra were recorded in different protic and aprotic solvents in the range 200– 600 nm. The 2-pyridone/2-hydroxypiridine tautomeric equilibration was studied and it was found that it depends on the substituents as well as on the solvents [9,96,97]. 5-(3- and 4-substituted arylazo)-3-cyano-4,6-dimethyl-2-pyridones were also studied by RP C18 TLC in other to determine parameters such as lipophilicity which is important for dye application on fibers [98]. On the basis of above results, it may be concluded that the azo colourants containing hydroxyl and amino substituents ortho or para to the azo groups can in principle exist as mixture of azo and hydazone tautomers. While azo – hydrazone tautomerism is quite interesting from a theoretical view point, it is also important from a practical stand point because the two tautomers have different technical properties and dyeing performances [99]. This phenomenon, called tautomerism involves the removal of a hydrogen from one part of the molecule, and the addition of a hydrogen to a different part of the molecule (Scheme 2.4). Tautomeric forms can be identified from their characteristic spectra. Ketohydrozones are normally bathochromic compared to their counter part hydroxyazo forms. Ketohydrozones also have higher molar extinction coefficient. However, not all azo dyes show tautomerism, and some tautomeric forms are more stable than others. 27 Since the work published by Zincke and Bindewald [100] the azo-hydrazone tautomerism has been investigated by numerous workers with a view to: (i) prove the existence of an equilibrium, if any (almost all) the available spectroscopic methods have been used; (ii) investigate the influence of factors such as: substituent, solvent, temperature; (iii) to gain an understanding of the (non) existence of such a thermodynamic equilibrium. The intention of this part of theoretical study of azo-hydrazone tautomerism is to focus mainly in recent publications dealing with new aspect of tautomerism of pyridone arylazo derivatives, using 1 H NMR, 13 C NMR and UV-Vis spectroscopic methods and using quantum chemical ab initio methods. N O O N CH 3 N H Ar N O OH N CH 3 NAr R1 R2 R1 R2 Scheme 2.4. The equilibrium between hydrazone form and azo form 2.1.3.1. UV-Vis spectroscopic study of tautomerism The use of UV-Vis spectroscopy for study of the tautomeric equilibrium between azo and hydrazone forms requires a knowledge of the molar extinction coefficient (εmax) of the individual forms and the values of λmax. Visible absorption spectral data of the azo dye 33 (Figure 2.24) is shown in the Table 2.1. and Figure 2.25 [85]. The visible spectra of the dye 33 were found to exhibit a strong solvent dependence. Table 2.1. Absorption spectral data for the dye 33 (λmax/nm; εmax /10 4 M -1 cm -1 ) Solvent Ethanol Acetone Chloroform DMF DMSO λmax 488 486 482 535/492 535 εmax 2.55 3.75 2.29 1.49/1.53 0.83 28 N N N CH 3 O C 2 H 5 OH NNC N CH 3 OH 33 Figure 2.24. The chemical structure of the azo dye synthesized from 1-ethyl-3-cyano-6- -hydroxy-4-methyl-5-amino-2-pyridone. Figure 2.25. Absorption spectra of dye 33 in different solvents: (1. Acetone, 2. Ethanol, 3. Chloroform, 4. DMSO, 5. DMF). It was observed that although in ethanol, acetone and chloroform the absorption spectra of the dye 33 does not change significantly, λmax of the dye shifted considerably in DMF and DMSO (Table 2.1.). The absorption spectra of dye 33 at different volume ratios of the mixture solvents of chloroform/DMSO are shown in Figure 2.26. From it one can see that there is an isobestic point in it, and with the increase of the volume content of DMSO, the absorption of the azo form increase, while that of the hydrazone form decreases. 29 Figure 2.26. Absorption of dye 33 in different volume ratios (a. ; b. ; c. ; d. ) of chloroform/DMSO. 2.1.3.2. NMR Spectroscopic study of tautomerism Q. Peng et al. [101] studied 1 H NMR and 13 C NMR spectra of 21 pyridone azo dyes in deuterated chloroform and deuterated dimethyl sulfoxide. It is shown that there is hydrazone – azo tautomerism in these dyes (Figure 2.27.). N N H N O CH 3 O YCH 3 X (X = H, SO3K, COOK, NO2, Cl, OCH3, CH3) (Y = CN, CONH2) Figure 2.27. The chemical structures of the azo pyridone dyes investigated by Q. Peng at al. [101]. The chemical shifts of the imino group for investigated dyes (Figure 2.28.) are within the range 14.3 – 16.1 ppm. The colour change of the solutions was obvious when 20 µl of an organic base (piperidine) was added to 0.5 ml of sample solutions (1:25,v/v). The peak for the imino group at lower field disappeared and moved to higher field ( δ 3.6 – 8.0) and overlapped with the NH proton peak of piperidine in both CDCl3 and DMSO- d6. A small amount of anhydrous sodium carbonate had the same effect on the 30 tautomerism as piperidine in DMSO-d6. This phenomenon suggests that perhaps the H atom of the imino group should transfer to a hydroxyl group. The 13 C NMR spectra were determined under the same conditions of concentration and temperatures as those used for the 1 H NMR spectra. The chemical shifts of corresponding carbon atoms were nearly the same when the samples were measured in CDCl3 and DMSO-d6. Dye with substituent 4’- SO3K (Figure 2.27.) may be used as an example, as shown in Figure 2.28. When the sample is in DMSO-d6 (0.4 M), the chemical shifts of the carbon atoms on the pyridone ring are similar to those reported by Cee at al. [102] and from the 1 H NMR spectra, the dye exists in the hydrazone form (Figure 2.28.A). After adding piperidine, the spectrum transfers to B. If Na2CO3 is added to the solution, the spectrum would transfer to C. According to the above discussion, the tautomer would be azo form in C. Figure 2.28. The 13 C NMR spectra of azo pyridone dyes (Figure 2.27, substituent 4’- SO3K) (at 90 o C). (A) in DMSO-d6; (B) 20ml piperidine was added to (A); (C) Na2CO3was added to (A). 31 2.1.3.3. Analysis of solvent influence on the azo – hydrazone tautomerism using ab initio quantum chemical calculations Since 92% of mono-azo dyes published in the Colour Index [103] are potentially tautomeric characters, and their spectral behaviour in solution (determinated by the tautomeric ratio) is very relevant to industrial applications, the investigations over the solvent effect on the tautomeric equilibrium position are of present interest as in theoretical as well as in practical aspect. Modeling of the solvent effect on the azo – hydrazone tautomerism by using quantum – chemical method is valuable, because it could shed light on the root of the phenomenon. L. Antonov et al. [104] reported results of the modeling of the solvent effect on the azo – hydrazone tautomeric equilibrium of 1-phenylazo-4-naftol (34a) (Scheme 2.5) by using ab initio quantum - chemical calculations. N N OH N N O H 34a 34b Scheme 2.5. The equilibrium between azo (34a) and hydrazone (34b) tautomeric forms of 1-phenylazo-4-naftol Quantum – chemical calculations have usually been carried out in the gas phase, but while gas phase predictions are suitable for many purposes, they are inadequate for describing ten azo – hydrazone tautomeric equilibrium in solution, because properties of the tautomers differs considerably between the gas phase and solution. Dye – solvent interactions were firstly modeled using the Gaussian ’94 Onsager reaction field model [105] where the solvent is viewed as a continuous medium of uniform dielectric constant ε and the solvent – solute interaction is the only dipole. The results from such modeling are presented in Table 2.2. As can be seen from Table 2.2 there is no correlation between the relative energies (hydrazo – azo gap) and the tautomeric constants in different 32 solvents, and these calculational results confirm the experimental fact that the interactions between tautomer and solvent are specific. Table 2.2. Values of the energies of the H (hydrazo) and A (azo) forms (HF/6-31G level) in different solvents calculated according to the Onsager model [105] Solvent ε ERHF ERHF H-A KT H A gap (a.u.) (a.u.) (kcal/mol) Gas phase -796.630 -796.632 1.531 - i-Octane 1.96 -796.630 -796.632 1.317 0.116 CH2Cl2 9.08 -796.631 -796.632 0.947 1.570 CH3OH 32.63 -796.631 -796.632 0.834 0.254 Water 80.37 -796.631 -796.632 0.803 1.934 The results of Antonov et al. [104] show that in methanol, methylene chloride and water there exists a strong hydrogen bonding between the particular tautomer and solvents, as well as dipole – dipole dye – solvent interactions. The results show that the hydrazo form is more stable in water and methylene chloride, while methanol and i-octan stabilize the azo form. In this thesis DFT calculation was performed for different azo – hydrazone tautomers of 5-phenylazo-4-phenyl-6-hydroxy-3-cyano-2-pyridone dye (A1). The structure was preliminary optimized by semi – empirical PM3 method and the most stable geometries in vacuum were reoptimized at B3LYP/6-31G(d) level of theory [106,107]. The Gaussian 03 program package was used [108]. The results show that hydrazone tautomeric form (B) (Figure 2.29) of this azo pyridone dye is dominant. 33 Figure 2.29. The most stable azo (A) and hydrazone (B) tautomer optimized structures of dye A1, at B3LYP/6-31G(d) level of theory. 2.2. Correlation analysis in organic chemistry 2.2.1. Substitution effects and linear free energy relationships Two types of experiments for the study of reaction mechanisms are usually applied. First is an analysis of kinetics, which can tell us which molecules are involved in a mechanism prior to and/or during the rate-determining step. Kinetic results can be used to exclude mechanisms that are found experimentally not to conform to the predicted rate law. However, the kinetic results do not tell us the nature of any intermediates, nor do they indicate which bonds have been broken or formed during the reaction. To get this kind of information, we need to turn to second tools of physical organic chemistry, such as linear free energy relationships; this latter experiment gives us some limited structural information about the activated complex. More indepth analysis of the structural of the transition state is obtained from studies of substituent effects. A substituent is any group on a molecule, such as a methyl, nitro, hydroxy, etc. A substituent effect is the manner in which the reactivity of the molecule changes when substituents are changed. The studies of substituent effects represent a key pillar of physical organic chemistry. When they are carefully applied, substituent effects are used to determine how the free energies of reaction and activation vary as a function of chemical structure. 34 In the context of pharmaceutical studies, where activity, bioavailability, and other medicinally related data are collected as a function of the chemical structure of the drug, the substituent effect studies are referred to as structure-activity relationships (SAR). Most important for physical organic chemistry, the nature of the structure-reactivity relationship is often informative about the mechanism of the reaction. The logic of conventional structure-function relationship should be familiar. Experiment can be designed to test for changes in charges along a reaction coordinate by interchanging functional group, such as switching an electron donating group to an electron withdrawing group. If a positive charge is being created in a rate- determining step, then adjacent electron donating groups should stabilize the transition state and the reaction should speed up. Conversely, adjacent electron withdrawing groups should destabilize the transition state and therefore retard the reaction .Similar effects can be observed for equilibria. As we will see later, there are linear relationship (called linear free energy relationship or LFER) between the free energy of activation or reaction free energy change induced by a substituent and parameter that describes the electron donating or electron withdrawing characteristics of the substituent. Changing substituent in the reactants or solvent should also influence the steric congestion, solvation, leaving group ability, nucleophilicity, acidity or basicity and a variety of other chemical attributes. The origin of substituent effects [109] There are two different aspects of bonding valence bond theory (VBT) and molecular orbital theory (MOT). In MO theory orbitals that are spread out over all atoms in a molecule. When adding or changing substituents, new molecule orbitals are created that involve the atomic orbitals on the substituents. Although this theory can be used successfully to analyze substituent effects, the concept of delocalized molecule orbitals presents difficulties in visualizing localized changes in a molecule brought about by a substituent change. 35 Inductive effects An inductive effect (І), results from the ability of an atom or group of atom to withdraw or donate electrons through ζ bonds [109]. + І means electron donating, - І electron withdrawing inductive effects [110,111]. Perturbation evident from substituent constant results in the effect at meta position greater than that at para. This is clearly observable in substituent such as –N+Me3 in Scheme 2.6. Scheme 2.6. Inductive effect of -N + Me3 substituent The basis of this electronic displacement is probably complex but originates in part from difference in electronegativity which cause polarization of both ζ – and π bonds and also from electrostatic effects experienced at the reaction canter due to the changes and dipoles resident on the substituent .Two mechanisms for inductive polarization may be considered. The classical inductive effect is a polarization through bonds, both of ζ and π types, becoming progressively attenuated. The other, known as a filed effect, is propagated through space and depends more for its intensity on proximity than on the number of bonds separating source and receptor. In practice it is difficult to separate the two, which may both components of the I effect [110]. Strongly electronegative atoms or groups are best at drawing electrons to themselves. Conversely, a group can donate electrons via the ζ bond framework [112]. The further away the group from the site of reaction, the lower its ability to effect the reaction via induction. For example, chloroacetic acid is substantially more acidic than acetic acid. 36 However, the increase in acidity induced by the electronegativity chlorine diminishes the further away from the carboxy1 group. Resonance effect A resonance effect reflects the ability of an atom or group of atoms to withdraw or donate electron through π bonds. This is also sometimes referred to as mesomeric effect in older literature [109]. Resonance effect is denoted R. Many substituents give rise to a perturbation that is greater when they are located para than when they are meta; this suggests the transmission mechanism is of a conjugative nature in which charge is relayed to alternate atoms. This effect is described as a ‘+R effect‘ if it results in donation of electrons from substituent to reaction center and as a ‘-R effect‘ if a withdrawal of electrons results [112]. In order to exercise a resonance effect, a substituent must posses a p- or π-orbital which is available to conjugate whit the π-MOS of the system. Two situations are important. a) X – is a donor group and typically possesses an unshared electron pair or π- electrons on an atom directly attached to the ring. Examples are: -NR2, - OR, - SR, -PR2, -HAL, - CH=CH2 These groups are all capable of exerting a + R effect which stabilizes an acceptor centre. The extreme situation is depicted by the structures (І, ІІ) in Scheme 2.7. 37 Scheme 2.7. Resonance effect Little interaction between donor and acceptor centres will occur if they are located meta since quinoid resonance structure analogous to Scheme 2.7 (ІІ, V) cannot be drawn and analogous structure (VІ) in Scheme 2.7 is therefore of high energy and less important. b) Substituent Z have a π-acceptor centre adjacent to the ring , since no commonly encountered substituents possess vacant bonding p-orbitals, this means in practice groups which can act as electron acceptors by simultaneously releasing π electrons to adjacent hetero atoms and whose contributing structures have a positively charged atom attached to the ring in Scheme 2.7 (ІІІ,V). Common examples are: All such groups tend to accept electronic charge and stabilize donor centers (e.g. carbon bring some degree of carbanion character adjacent to the ring), illustrated by contributing structure (V) in Scheme 2.7. Again the stronger –R interaction occur when substituent and reaction center and located ortho or para (Scheme 2.7, V). It may be observed that there is no fundamental difference between these two situation; there is in each case a transfer of charge between two centers by conjugation and the differentiation 38 into +R and –R effects merely depends upon which part is designated as a substituent and which as the reaction centre . Steric effect Steric effects can also have a dramatic influence on the rate of a reaction, as well as conformation. Large atom or groups influence the manner in which molecules collide, often deflecting the reactants away from the angle or depth of collision necessary for the reaction to occur. For example, the SN2 reaction becomes slower due to steric effects as the carbon with the leaving group is more highly substituted with alkyl groups. The nucleophile cannot penetrate to the carbon with the leaving group when larger groups are attached [109]. 2.2.2. The Hammett equation and its extension The most important and the most widely applied structure reactivity relationship is due to L.P. Hammett which relates rates and the equilibria for many reactions of compounds containing substituted pheny1 groups [113]. He dealt with systems of type: R is a reaction site in the side chain attached to a benzene ring and X is a meta or para substituent. Hammett excluded ortho substituents on grounds that there would be specific steric interaction between reaction site and substituent which would not be amenable to a regular quantitative treatment. Aliphatic compounds are also not correlated with Hammett relationship [113,114]. 39 It the early 1930s, Hammett [115,116] and Burkhard [117], discovered linear relationship involving logk or logK for a number of systems. This work led to the formulation of the Hammett equation, which describes the influence of polar meta- or para substituents on the side-chain reactions of benzene derivation [118]. This relation has become known as Hammett equation, and is widely applied in the form: log (k/k0) = ρ ζ (2.1) log (K/K0) = ρ ζ (2.2) Here k or K is the rate or equilibrium constant respectively, for a side-chain reaction of a meta- or para- substituted benzene derivates, and k0 or K0 are the rate or equilibrium constants respectively, for unsubstituted “parent’’ compound. ζ is the substituent constant, which depends solely on the nature and position of the substituent X, and ρ is the reaction constant, which depends on the reaction conditions, such as the reaction medium and temperature, under which it takes place, and also on the nature of the side chain R. The ionization of benzoic acids in water at 25 0 C was chosen as the reference system with ρ equal to 1.00. This resulted in electron withdrawing substituents constants being positive and electron-releasing substituent constants being negative. The validity of equations 2.1 and 2.2 is restricted to substituents in the meta- and para- positive of the benzene ring [119,120]. The relationship may be exemplified by the rates of hydrolysis of substituted ethy1 benzoates. The correlation is illustrated graphically in Figure 2.30, which show log k/ k0. 40 Figure 2.30. Hammett plot for the hydrolysis of ethyl benzoates. where k0 is the rate constant for hydrolysis of ethy1 benzoate and k is the rate constant for the substituted esters, plotted against log K/K0 where K and K0 are the corresponding acid dissociation constants. The Hammett equation is an example of linear free energy relationship. Equilibrium constants and rate constants are related to free energy relationship to free energy differences by the equation [113,121]: log K = -∆GΘ/2.3RT (2.3) Where ∆GΘ is the standard free energy change of reaction. For chemical rate processes: log k = log(kBT/h) - ∆G # /2.3 RT (2.4) Where ∆G# is the standard free energy of activation (T is the Kelvin scale temperature; R is the gas constant, kB is the Boltzmann constant, and h is the Plank constant). log (k/k◦) = log k - log k0 =∆∆G/2.3RT=ρ ζ (2.5) where the expression ∆∆G is the second difference: the difference between ∆G values for the substituted and unsubstituted reaction. 41 Substitutent constant (ζ) To show how reaction mechanisms vary as a function of the electronic changes induced by substituents, chemists use Hammett plots. Hammett defined a scale that measured the ability of substituents to influence the acidity of benzoic acid. The substituents are placed meta or para to the carboxylic group to eliminate any possible steric effects associated with an ortho substituent, and therefore only, field polarizability, inductive and resonance effects should be operative [109]. Equations 2.1 and 2.2 were used to define a substituent parameter ζx for each substituent X. Hydrogen is the reference substituent. Thus, all acidity equilibrium constants for the substituted benzoic acids are compared to the equilibrium constant for benzoic acid itself (ζH = 0 by definition). Table 2.3 gives a number of ζ value: 42 Table 2.3. ζ Values for Several Commonly Encountered Substituents [122]. Substituents ζ m ζ p ζ p + ζ p – -NH2 -0.09 -0.30 -1.3 - -OH 013 -0.38 -092 - -OCH3 0.10 -0.12 -0.78 - -(CH3)3 -0.09 -0.15 -0.26 - -CH3 -0.06 -0.14 -031 - -C6H6 0.05 0.05 -0.18 -0.08 -I 0.35 0.18 0.13 - -Br 0.37 0.26 0.15 - -Cl 0.37 0.24 0.11 - -F 0.34 0.15 -0.07 - -COCH3 0.36 0.47 - 0.82 -COOH 0.35 0.44 - 0.73 -OCOCH3 0.39 0.31 0.18 - -NO2 0.71 0.81 - 1.23 Where, ζm, ζp Hammett's substituent constant are defined by using the dissociation constants of para- and meta- substituted benzoic acids, which are composite substituent constants as both resonance and inductive / field effects are present. ζ+ are composite substituent constants applied in cases when the reaction site is electron withdrawing, dissociation of dimethylpheny-carbinyl chlorides. ζ- are composite substituent constants applied in cases when the reaction site is electron-donating, dissociation of phenols and anilines [123]. A different set of ζ values is necessary for each different position on the benzoic acid, because the ability of substituent to influence the acidity of benzoic acid depends upon position relative to the carboxy group. When ζ is negative, the substituted benzoic acid is less acidic than benzoic acid itself, and when ζ is positive, the substituted benzoic acid is more acidic. Note that electron donating groups have negative ζ values and electron withdrawing groups have positive ζ values. This trend is exactly as predicted, 43 because electron withdrawing groups should stabilize the negative charge of carboxy ion and electron donating groups should destabilize this charge. One interesting feature about the ionization of benzoic acid becomes apparent upon studying table 2.3. The ζP value generally reflect a larger influence of the substituent at this position than do the ζm values (the absolute value of ζP > ζm ) even though the meta position is closer to the ionizing group than is the para position. This difference in part reflects the ability of the para position to influence charge at the starred carbon (Scheme 2.8) via resonance, an influence that is not possible for the meta position. This difference is clearly evident with the hydroxy and methoxy groups. Scheme 2.8. Influence charge at the starred carbon In the meta position these groups are found to be electron withdrawing to ware the starred carbon, an inductive effect. In the para position, these groups are electron donating, a resonance effect. Note that while this is a resonance effect, it is not resonance with the carboxylate anion. The negative charge of benzoate anion is not in conjugation with the aromatic ring and so can not be stabilized by resonance [109]. Duality of substituent constant: σ+ and σ- An early refinement was the suggestion of duality of substituents. Marked deviation from the Hammett equation were particularly noted for para substituents with important resonance effects, both electron–withdrawing + R and electron–releasing – R. These give rise to exalted resonance effect when such substituents are engaged in crose – conjugation with reaction sites of the opposite type, i.e. electron rich or electron deficient. This is exemplified by the structure (І) in Scheme 2.9. 44 Y X + N + O O -O CMe 2 MeO (I) + (II) (III) Scheme 2.9. Duality of substituent constants These para substituents given exalted electron withdrawal are assigned ζ- values [124]. These are based on the ionization of meta– substituted phenols in water at 25 °C, giving a defining ρ values for this reaction. This ρ value is then used to derive exalted or ζ- values for para substituents such as NO2, COMe, CN or CF3. The importance of structure such as (ІІ) in (Scheme 2.9), for ionization of p-nitrophenol is thus estimated. Para substituents giving exalted electron release are assigned ζ+ value [125]. The reaction now used for determining such constants is SN1 solvolysis of substituted phenyl dimethylcarbiny (t-cumyl) chlorides in 90% aqueous acetone at 25 °C, using meta substituents to define ρ. The exalted or ζ+ values for para substituents such as OMe, Me, NH2 and Cl can be calculated and demonstrate the importance of structures such as (ІІІ) in Scheme 2.9. It should be pointed out that, in defining ρ for use in estimating ζ-, strongly + R meta substituents are not used and for ζ+ strongly – R meta substituents are not used. This precludes any relayed or secondary effects. Normal or unexelted substituent constant: σn and σ0 The duality of ζ value was strongly criticized by Van Bekkum et al [126] who consider cross – conjugation or through - conjugation as a continuous process. A sliding extent of such interaction was considered to be present, depending on the reaction studied. Eight ‘‘primary’’ ζ value of meta conjugation, including H, were considered to be of general applicability, together with p-COMe and p-NO2 where cross – conjugation effect can be ruled out. All other para conjugation were excluded, as were –R meta substituents such as OMe and NH2 in case of relayed effects. These primary values were used to calculate ζ values relevant that particular reaction for all other substituents. This analysis did indicate a sliding scales for –R substituents and +R centers for +R substituents, average unexalted or ‘ normal ‘substituents were calculated and denoted ζn. Large difference between ζ and ζn arise for substituents such as p-COMe, p-NH2 and 45 p-OH, where cross-conjugation between the substituents and CO2H occur in the original reference reaction the ionization of benzoic acids. Ionization of para– substituted phenylacetic and 3-phenylpropionic acids and the alkaline hydrolysis of the corresponding ethy1 phenylacetates and benzy1 acetates. All these systems have substituents which are unaffected by cross-conjugation due to the insulating CH2 group. These reaction series were then used to generate a series of ζ0 values. Thus, meta and para substituents with + R effects have ζ0 and ζ values which are almost identical, whereas the substituents with – R effect deviate appreciably. The ζn and ζ0 scales are almost the same. Reaction constant (ρ) Now that a scale for substituent effects has been established, it was possible to determine if other reactions respond to substituents the way benzoic acid does. The goal is to use benzoic acid ionization as a reference reaction that creates a negative charge and compare other reactions to it as a means to see if they also creates a negative charge, or conversely, a positive charge. Furthermore, it was necessary to determine if different reactions are more or less sensitive to the substituents than are the acidities of benzoic acid derivatives more used. To do this the Hammett relationships given in equations 2.7 and 2.8 for thermodynamic and kinetic analyses, respectively. To determine ρ, plot log(K/ K 0) or log (k/ k o) versus ζ for the new reaction under study. Rho (ρ) is simply the slope of this plot. log (K/ K 0) = ρ ζ (2.7) log (k/ k o) = ρ ζ (2.8) ρ describes the sensitivity of the new reaction to substituents effects relative to the influence of the substituent on the ionization of benzoic acids. It is called the reaction constant or sensitivity constant for each new reaction under study. The following values of ρ lead to the associated conclusions: 1. When ρ > 1, the reaction under study is more sensitive to substituents than benzoic acids and negative charge is building during the reaction. 46 2. When 0 < ρ < 1, the reaction is less sensitive to substituents than benzoic acids, but negative charge is still building. 3. When ρ is equal to or close to 0 , the reaction shows no substituent effects This can mean no change in charge occurs in the equilibrium or rate - determining step. 4. When ρ is negative, the reaction is creating positive charge. For example, Figure 2.31 shows a plot of the log data of ionization constant of substituted phenylacetic acids in water, and the ionization of substituted benzoic acids in ethanol. The ρ for phenylacetic acid derivatives is 0.56, while that for benzoic acids derivatives in ethanol is 2.25. These indicate much lower and higher sensitivities of acidities to the substituent effects, respectively [109]. The smaller ρ for phenylacetic acid derives form the more remote position of the substituents to the carboxyl group relative to that in benzoic acid. The larger ρ value for the ionization of benzoic acid in ethanol reflects reduced stabilization of the negative charge in the carboxylate product in ethanol relative to that in water (remember ρ = 1 in H2O). Hence, the substituents become more important in stabilizing or destabilizing the negative charge in the product. Interestingly, the lines are linear even though the ρ value is derived ζ- value of benzoic acid ionization in water. 47 Figure 2.31. Hammett plots for phenylacetic acid and benzoic acid ionization in ethenol [127,128]. Very often the magnitude of ρ is used as a guide to the amount of charge that has been developed in transition state or in the product. Such an interpretation must be made with caution, because ρ really only relates the sensitivity of ionization to the substituents. In the examples just discussed, the amount of charge on the products is the same in all three reactions (phenylacetic acid, and benzoic acid in ethanol or water), but the ρ value are significantly different. 2.2.3. Separation of electronic effects An important development was the concept that substituent electronic effects could be considered to be separable and additive. Substituent effects come in inductive resonance, and steric effects. Inductive and resonance are considered as electronic effects, whereas steric effects largely depend upon the size of the substituent. However, even steric effects are electronic in origin. They are repulsions brought about by atoms approaching within their respective Van der Waals contact distances where the electronic 48 clouds of the groups involved repel each other. Most chemists, however, separate the concepts of sterics and electronics [109]. Separation of inductive and resonance effects Beside the ζp and ζm concept, tables of substituent constants usually list ζ- and ζ+ values for the use of the classical Hammett equation. As it was already mentioned, two new substituents effects scales were produced, one for groups that stabilize negative charges via resonance (ζ-), and the other for groups that stabilize positive charges via resonance (ζ+). The ζ- scale is based upon the ionization of para- substituted phenols (Scheme 2.10), for which groups like the nitro group can stabilize the negative charges via resonance (Scheme 2.11). Scheme 2.10. Ionization of p-substituted phenols Scheme 2.11. Resonance in p-nitrophenolate The ζ+ scale is based upon the heterolysis reaction of para- substituted phenyldimethy1 chloromethanes (Scheme 2.12), in which groups like amino can stabilize the positive charge via resonance (Scheme 2.13). Scheme 2.12. Heterolysis reaction of p-substituted phenyldimethyl chloromethanes 49 Scheme 2.13. Resonance in p-aminobenzylic cation Several ζ+ and ζ- values are given in Table 2.3. Note that the ζ+ values are defined so that negative ρ values correspond to the creation of positive charge, just as with normal Hammett plots. The electron withdrawing groups have positive ζ + values, and the electron donating groups have negative ζ values, just as with ζ values. Hammett ζ values measure the resultant of inductive and resonance effect. Taft and Lewis [129,130] suggested that the resultant effects should be quantitatively separable in the inductive and resonance contributions through equations 2.10 and 2.11. ζP = ζ1 + ζ or ζp – ζI = ζR or ζp – ζR = ζ1 (2.10) ζm = ζI + αζR (2.11) The inductive effect, given by ζI, is assumed to operate equally form the meta– and the para- position. The resonance effect, given by ζR, contributes to ζm indirectly, α being the relay conefficient. Values ζm, ζP and ζI are sufficient to obtain value of ζR and α. This assumes that the polar effect has the same force at the meta- and para- positions. The resonance (mesomeric) effect, on the other hand, is fully at the para position, but it is attenuated (α < 1) in transmission to the meta- position (Scheme 2.14) [113]. 50 Scheme 2.14. Resonance effect at meta-position Taft and Lewis set up a scale based on alicyclic and aliphatic reactivities. For oridinary Hammett ζ values, based on the ionization of benzoic acid, a value for α of 0.33 was suggested. Selected value of ζI and ζR are in Table 2.4. Table 2.4. Inductive and resonance values [124]. 51 Taft [131] has obtained ζR 0 value by an extension of his F NMR method [113]. The analysis into resonance effect may also be performed for ζ0 constant, giving ζR 0 value (α = 0.5) or with ζ+ and ζ- canstants (giving ζR + and ζR, respectively ) [132]. The importance of the separation of sigma parameters into ζI and ζR contribution is that it suggests the possibility of a ‘dual subsistent – parameter‘ DSP treatment for reaction series through equation 2.12. log (k/k0) = ρi ζI+ ρR ζR (2.12) Provided that the various ζR type scales distinguished above are linearly related to each other, it should be satisfactory to characterize each substituent by ζI and say ζR 0 , and apply equation 2.12 to meta and para reaction series separately. With the Hammett equation we have for each substituent, position – dependent sigma value, ζm and ζP (arbitrarily becoming ζ- or ζ+ on occasion) but a single ρ value for each reaction series . In Taft's treatment, each subsistent is characterized by position–independent ζI and ζR value, but the susceptibility to inductive and to resonance effect (variability of cross– conjugation ) is to be expressed separately through position–dependent ρR and ρI values. Taft and his colleagues use f = SD/RMS as measure of the success of correlation. SD is the root mean square of deviations and RMS is the root mean square of the ρi. If the f < 0.1 the correlation is considered of good precision. The ratio of ρR and ρI in the λ = ρR/ρI can be related to the importance of through – conjugation and can be used to detect the inhibition of the transmission of the resonance effects. Furthermore, ζR and ζi can be of opposite sing so the λ will be negative detecting that the opposing stabilization which can be afforded by inductive and resonance effect. 2.3. Solvent effects One of the most important features for the success of the planned reaction is the selection of a suitable solvent. Since solvent effects on chemical reactivity have been known for more than a century, most chemists are familiar with the face that solvents may have a strong influence on reaction rates and equilibria. The number of solvent systems and their associated solvent effects examined is so enormous that a complete description of aspects would fill several volumes. 52 The influence of solvents on the rates of chemical reactions [133] was first noted by Berthelot and Pean de Saint–Gilles in 1862 in connection with their studies on esterification of acetic acid and ethanol [134]. After thorough studies on the reaction of trialklamines with haloalkanes, Menschutkin in 1890 concluded that a reaction cannot be separated from the medium in which it is performed [135]. Menschutkin also discovered that, in reactions between liquids, one of the reaction partners may constitute an unfavourable solvent. Thus, in the preparation of acetanilide, it is not without importantce whether aniline is added to an excess of acetic, or vice versa, since aniline in this case is an unfavourable reaction medium, Menschutkin related the influence of solvents primarily to their chemical, not their physical properties. The influence of solvents on chemical equilibria was discovered in 1896, simultaneously with the discovery of keto-enol tautomerism in 1,3-dicarbony1 compounds and the nitro-isonitro tautomerism of primarily and secondary nitro compounds [136]. The study of the keto-enol equilibrium of ethy1 formylphenylacetate in eight solvents, led Wislicenus to the conclusion that the keto-form predominates in alcoholic solutions, the enol-form in chloroform or benzenes. He stated that the final ratio in which the two tautomeric forms coexist, must depend on the nature of the solvent and on its dissociating power, whereby he suggested that the dielectric constant were a possible measure of this ’power’. Stobbe was the first to review these results [137]. He divided the solvents into two groups according to their ability to isomerize tautomeric compounds. His classification reflects, to some extent, the modern division into protic and aprotic solvents on constitutional and tautomeric isomerisation equilibria was later studied in detail by Dimroth [138] and Meyer [139]. The similarities in the relationships between solvent effects on reaction rate, equilibrium position, and absorption spectra has been related to the general solvating ability of the solvent in a fundamental paper by Scheibe et al [140]. Chemists have classified solvents according to [141]:  Structure, comprising: protic solvents: solvents that contain relatively mobile protons, such as those bonded to oxygen, nitrogen, or sulphur (attached to an electronegative atom ); and aprotic solvents, in which all hydrogen's are bonded to carbon. 53  Dielectric constant, comprising; polar solvents, those that have high dielectric constant (high polarizability); and non-polar solvents, are compound that have low dielectric constant (low polarizability), these solvents are not miscible with water: Some of the more common solvents can be roughly classified as in table 2.5 on the basis of their structure and dielectric constant. Table 2.5. Dielectric Constant of some common solvents a Aprotic solvents Protic solvents Nonpolar Polar Hexane 1.9 Pyridine 12 Acetic acid 6.1 Dioxane 2.2 Acetone 21 Tert-Butanol 12.5 Benzene 2.3 Nitromethane 36 Ammonia 22 Diethy1 ether 4.3 Dimethylformamide 37 Ethanol 24.5 Chloroform 8.4 Acetonitrile 38 Methanol 32.7 Tetrahydofuran 7.6 Dimethy1 sulfoxide 47 Water 78 Carbon tetrachloride 2.2 Hexamethy1 phosphoramide 30 Triflouro acetic acid 8.6 a Dielectric Constant data are abstracted from the compilation of solvent properties in J.A Riddick and W. B .Bunger (eds.), Organic solvent , Vol . ІІ of Techniques of Organic chemistry, Third Edition, Wiley- Interscience New York, 1970. 2.3.1. Solvent effects on the keto / enol equilibria [142] In general, 1,3-dicarbony1 compounds, which include β-dialdehydes, β-ketoaldehydes, β-diketones and β-ketocarboxylic esters, may exist in solution or as pure compound in three tautomeric forms: the diketo form (Scheme 2.15 a), the cis-enolic (Scheme 2.15 b) , and the trans-enolic form (scheme 2.15 c). Scheme 2.15. Keto / enol tautomeric form 54 Open–chain 1,3- dicarbony1 compounds are observed in the trans-enolic form only in rare cases [143]. When the trans-enolic form is excluded, the keto/enol equilibrium constant KT (equilibrium constant of a tautomeric equilibrium) is given by equation 2.13. KT = [enol] / [ diketo] (2.13) In solution, open–chain 1,3-dicarbony1 compounds enolize practically exclusively to the cis-enolic form (scheme 2.15), which is stabilized by intermolecular hydrogen bonding. In contrast, cyclic 1,3-dicarbony1 compounds (e.g. cycloalkane-1,3-diones), can give either trans-enols (for small rings) or cis-enolic (for large rings). As the diketo form usually is more dipolar than the chelated cis-enolic form, the keto/enol ratio often depends on solvent polarity. 2.3.2. Solvent effects on the other tautomeric equilibria [143] Solvent effects similar to those described for the keto/enol equilibrium can also be found for other tautomerisms, e.g. lactam/lactim, azo/hydrazone, ring/chain equilibria, etc [144,145] One of the classic studies of lactam/lactim tautomerisms is the determination of the 2-hydroxypyridine (scheme 2.16 a) 2-pyridone (scheme 2.16 b) equilibrium [144,145] N OH N O H N O H .. .. + (a) (b) (c) Scheme 2.16. Lactam – lactim tautomeric forms By considering the lactam–lactim equilibrium (Scheme 2.16 a) (Scheme 2.16 b) in solvents of varying polarity it has been found that increasing solvent polarity shifts the equilibrium towards the pyridone-form. This form is more dipolar than the hydroxyl-form due to the contribution of the charge –separated mesomeric form (Scheme 16 c). Furthermore, the hydrogen-bonding ability of the solvent plays an important role since hydrogen-bond acceptors stabilize the enol-form. 55 2.3.3. Solvent effects on the rates of homogenous chemical reactions [142] A change of solvent can considerably change both the rate and order of homogeneous chemical reactions. Already in 1890, Menschutkin demonstrated in his classical study on the quaternization of triethylamine with iodoethane in 23 solvents, that the rate of reaction varied remarkably depending on the choice of solvent. In ditehy1 ether, the rate was four times faster than in n-hexane, 36 times faster in benzene, 280 times faster in methanol, and 742 times faster in benzy1 alcohol [146]. Thus, by means of a proper choice of solvent, decisive acceleration or deceleration of a chemical reaction can be achieved. In solution, ions are produced by the heterolysis of covalent bonds in ionogens. This ionization reaction is favoured by solvents due to their cooperative EPD (electron pair donor) and EPA (electron pair acceptor) properties, and hence the behaviour of ions and molecules is dictated mainly by the solvent and only to a lesser extent by their intrinsic properties. Consider a reaction between the starting compounds A and B, and suppose that during the course of the reaction these two first form an activated complex, which than decomposes to the end products, C and D. The reaction can then be described as follows A+B (AB) C+D Reactants Activated complex products If the reaction takes place in solution, then the initial reactants, as well as the activated complex, will be solvated to a different extent, according to the solvating power of the solvent used. This differential solvation can retard or accelerate a reaction in the manner described below in Figure 2.32. 56 Figure 2.32. One dimensional Gibbs energy diagram for a chemical reaction in three different solvents I, II and III. ∆GI represents the Gibbs energy of activation for a given chemical reaction an ideal solvent I. In such a case neither the reactants nor the activated complex are solvated. If in another solvent ІІ only the activated complex is solvated, then∆ G≠ІІ results. The Gibbs energy of activation is reduced by the Gibbs energy of transfer, ∆ G≠І to ∆ G ≠ ІІ with consequent rate acceleration. If on the other hand, only the initial reactants are solvated as happens in solvent ІІІ, then ∆ G≠ ІІІ results. Gibbs energy of activation is increased by the Gibbs energy of transfer ∆ G≠І to ∆ G ≠ ІІІ with a consequent rate deceleration. The solvation of the products does not have any influence on the reaction rate. Because in reality the initial reactants as well as the activated complex are solvated, but usually to a different extent, the difference of both Gibbs transfer energies determines the reaction rate in solution. 2.3.3.1. The Grunwald – Winstein equation The majority of these are based either on linear free energy relationships for chemical processes or on solvatochromic shifts in electronic spectra, for example in the former category. 57 Grunwald and Winstein [147] are suggested treating solvent effects on rates in terms of equation 2.14, similar in form to the Hammett equation. log (к /к0) = mY (2.14) or logк = logкo + mY (2.15) Log k refers to a given reaction in a given solvent, log k 0 to the same reaction in 80% v/v aqueous ethanol as a standard solvent, Y is a parameter characteristic of the given solvent and m is a parameter characteristic of the given reaction, which measures its susceptibility to changes in solvent; the analogy of Y and m to ζ and ρ respectively is obvious. Scales of Y and m were established by taking Y = 0 for 80 % ethanol, and selecting the solvolysis of t-buty1 chloride at 25 °C as a standard reaction, for which m is defined as 1.0. Y values are known for various one-component solvents (mainly alcohols) and for various mixtures of organic solvents with water or second organic solvents. Grunwald and Winstein [148] found that the SN1 solvolysis of 2-chloro-2- methy1propane (tert-buti1chloride) is strongly accelerated by polar solvents, and is 335000 times faster in water than in the less polar solvent ethanol [147]. The authors defined a solvent ‘’ ionizing power’’ parameter Y using equation 2.16, Y = log kt-BuCl – logk 0 t-BuCl (2.16) Where log k 0 t-BuCL is the first order rate constant for the solvolysis of tert-buty1 chloride at 25 ˚C in (80 % ethanol and 20% water, Y=0) as reference solvent. Grunwald – Winstein equation 2.14 is fairly successful in a large number of cases. Good linear relationships between logk and Y are shown by the solvolysis of various tertiary halides and secondary alky1 sulphonates, i.e. reactions which proceed by an SN1 mechanism, like the standard reaction. The situation for SN2 reactions (e.g. solvolysis of primary haloalkanes) or for reaction borderline mechanism (e.g. solvolysis of secondary haloalkanes) is less satisfactory. 58 2.3.4. Koppel – Palm solvatochromic treatment [149] Solvent – solute interactions are classified as non-specific. The former are divided into polarization and polarizability effects, to be characterized by the parameters Y and P respectively. Specific interactions concern donor –acceptor interaction of solvent with solute. A solvent may function as a Lewis base (electron donor) capable of nucleophilic solvation, or as a Lewis acid (electron acceptor) capable of electrophilic solvation. These phenomena are characterized by the different parameters and may then be used in the correlation analysis of solvent effects by means of equation 2.17. A = A˚ + yY + pP + bB + eE (2.17) A is the value of the solvent – dependent property (log K, v, etc.) in a given solvent, A0 is the statistical quantity (intercept term) corresponding to the value of the property in the gas-phase as reference ‘solvent’; y, p, b and e are the regression coefficients. The setting–up of scales for Y and P depends on the assumption that capability of solvents for polarization and polarizability interactions is validly measured by dielectric constant and refractive index respectively. Thus Y is defined either as (ε-1)/ (2ε +1), the Kirkwood function, or as (ε -1)/(2ε +1), a function based on the expression for molar polarization. 2.3.5. Kamlet – Taft solvatochromic treatment [150] An interesting approach, called the solvatochromic comparison method, used to evaluate a β-scale of solvent hydrogen-bond acceptor (HBA) basicities (corresponding to the Koppel-Palm B scale), and an α-scale of solvent hydrogen-bond donor (HBD) acidities (corresponding to the Koppel-Palm E scale), and π*-scale of solvent dipolarity/polarizability using UV-Vis spectral data of solvatochromic compounds. A selection values of β, α and π* are recorded in Table 2.6. 59 Table 2.6. Selected values of Kamlet-Taft parameters [151] Solvent β α π * solvent β α π * n-Pentane 0.0 0.0 -0.08 n-Hexanol 0.84 0.80 0.40 n-Hexane 0.0 0.0 -0.04 c-Hexanol 0.84 0.66 0.45 n-Heptane 0.0 0.0 -0.08 Benzy1 alcohol 0.52 0.60 0.98 c-Hexane 0.0 0.0 0.0 THF 0.55 0.0 0.58 Benzene 0.1 0.0 0.59 Acetonitrile 0.40 0.19 0.75 Methanol 0.66 0.98 0.60 Acetone 0.43 0.08 0.71 Ethanol 0.75 0.86 0.54 Formamide 0.48 0.71 0.97 n-Propanol 0.90 0.84 0.82 N-Methylformamide 0.80 0.62 0.90 i-Propanol 0.84 0.76 0.48 N,N-Dimethy1formamide 0.69 0.0 0.88 n- Butanol 0.84 0.84 0.47 Dimethy1 sulfoxide 0.76 0.0 1.0 i- Butanol 0.84 0.79 0.40 Dimethy1 sulfide 0.34 0.0 0.57 s- Butanol 0.80 0.69 0.40 Formic acid 0.38 1.23 0.65 t- Butanol 0.93 0.42 0.41 Acetic acid 0.45 1.12 0.64 n-Pentanol 0.86 0.84 0.40 Propanoic acid 0.45 1.12 0.58 i- Pentanol 0.86 0.84 0.40 Acetic anhydride 0.29 0.0 0.76 t- Pentanol 0.93 0.28 0.40 Water 0.47 1.17 1.09 Kamlet and Taft have shown β, α and π * may be applied in the correlation analysis by multiple regressions of reaction rates and equilibria, and of spectroscopic data; equation 2.18 applies: XYZ = XYZ0 + s π * + α α + b β (2.18) where XYZ0, a, b and s are (solvent-independent) coefficients characteristic of the process and indicative of the sensitivity to the accompanying solvent properties. Kamlet and Taft’s solvatochromic parameters have been used in one-, two- and three-parameter correlation involving different combination of these parameters which are called linear solvation energy relationships (LSER's) [142]. 60 2.3.6. Correlation analysis of solvent effects by means of substituent constants It is sometimes possible to express aspects of solvent polarity in a series of structurally related solvents by means of appropriate substituent constants. Thus providing a link between solvent effects and Hammett or Taft equations. In a general way this may not be entirely straightforward, since substituent constants concern effects transmitted to a localized reaction centre, while solvent effects, in principle, involve the interaction of the reacting species with the solvent molecule in entirely. In some series of solvents, however, a particular centre in the solvent molecule is likely to dominate solvent-solute interaction and in such cases the influence of substituents on that centre may play important role in solvent effects. Electronic or steric substituent constants may then be relevant as solvent parameters, usually in connection with multiple regression. Chapman et. al. [152] have established that the solvent effects is best interpreted in terms of the following properties: (a) the behaviour of the solvent as a dielectric in facilitating the separation of opposite charges in the formation of the activated complex; (b) the ability of the medium to solvate the carboxylic proton and thus stabilize the initial state relative to the transition state; and (c) the ability of protic solvents to form hydrogen bonds with the negative ends of the ion-pair, and thus stabilize the transition state relative to the initial state. Multiple linear regression of log k with f (ε),ζ* and nγH gives equation 2.19. log k = log k0 + af(ε) + b ζ* + cnγH (2.19) where f(ε) is the Kirkwood function [(ε-1)/( 2ε+1)] of dielectric constant, ζ* is the Taft constant for the alkyl group of the alcohol, and nγH is the number of γ-hydrogen atoms in the alcohol. 61 3. EXPERIMENTAL PART General All starting materials were obtained from Aldrich and Fluka, and were used without further purification. The IR spectra were determined using a Bomem Fourier Transform-infrared (FT- IR) spectrophotometer, MB-Series in the form of the KBr pallets. The 13 C and 1 H NMR spectral measurements were performed on a Varian Gemini 2000 (200 MHz). The spectra were recorded at room temperature in deuterated dimethyl sulfoxide (DMSO-d6). The chemical shifts are expressed in ppm values referenced to TMS. The ultraviolet-visible (UV-vis) absorption spectra were recorded on a Schimadzu 1700 spectrophotometer in the region 200–600 nm. The spectra were run in spectroquality solvents (Fluka) using concentration of 1×10 –5 M. All melting points were uncorrected and are in degree Celsius. Elemental analyses were performed using a VARIO EL III elemental analyzer. 3.1. Preparation of 5-arylazo-6-hydroxy-4-phenyl-3-cyano-2-pyridone dyes (A1-A12) All the investigated arylazo pyridone dyes were synthesized from the corresponding diazonium salts and 4-phenyl-6-hydroxy-3-cyano-2-pyridone using classical reaction for the synthesis of the azo compounds [32]. 4-Phenyl-6-hydroxy-3- cyano-2-pyridone was prepared from ethyl benzoylacetate and cyanoacetamide using a modified literature procedure [153]. General Procedure for the preparation of 4-phenyl-6-hydroxy-3-cyano-2-pyridone 4-Phenyl-6-hydroxy-3-cyano-2-pyridone was prepared from ethyl benzoylacetate and cyanoacetamide using a modified literature procedure. Equimolar amounts (10 mmol) of ethyl benzoylacetate and cyanoacetamide were heated under reflux in absolute ethanol (10 mL) in the presence of potassium hydroxide (10 mmol) as catalyst for 20 h. The product was isolated by filtration and purified by crystallization from ethanol. Characterization data are given below. 62 4-Phenyl-6-hydroxy-3-cyano-2-pyridone. White crystalline solid; m.p.: 279–280 °C (lit. m.p.: 280 °C [153]); yield: 62 %; IR (KBr, ν/cm–1): 3419 (OH), 3321 (NH), 2227 (CN), 1654 (C=O); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 6.26 (1H, s, PyrH); 7.60–7.40 (5H, m, PhH); 11.74 (1H, s, OH). General Procedure for the preparation of 5-arylazo-6-hydroxy-4-phenyl-3-cyano-2- pyridone dyes (A1–A12) All the investigated arylazo pyridone dyes were synthesized from the corresponding diazonium salts and 4-phenyl-6-hydroxy-3-cyano-2-pyridone using classical reaction for the synthesis of the azo compounds. The obtained compounds were purified by crystallization from acetone and then analyzed. Characterization data are given below. 5-Phenylazo-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A1). Orange crystalline solid; m.p.: 257–260 °C; yield: 75 %; anal. calcd. for C18H12N4O2: C, 68.35; H, 3.82; N, 17.71; found: C, 68.51; H, 3.75; N, 17.57; IR (KBr, ν/cm–1): 3390 (NH of hydrazone form), 3153 (NH on heterocyclic), 2229 (CN), 1654, 1630 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.42–7.10 (5H, m, ArH); 7.70–7.42 (5H, m, ArH); 12.22 (1H, s, NH on heterocyclic); 14.61 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.24 (C2), 96.80 (C3), 162.02 (C4), 117.26 (C5), 161.29 (C6). 5-(4-Hydroxyphenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A2). Dark red crystalline solid; m.p.: 272–274 °C; yield: 58 %; anal. calcd. for C18H12N4O3: C, 65.06; H, 3.64; N, 16.86; found: C, 65.22; H, 3.72; N, 16.72; IR (KBr, ν/cm–1): 3385 (NH of hydrazone form), 3153 (NH on heterocyclic), 2230 (CN), 1654, 1637 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.27 (2H, d, J = 9 Hz, ArH); 7.60–7.40 (5H, m, ArH); 8.77 (2H, d, J = 9.6 Hz, ArH); 10.04 (1H, s, OH substituent); 12.12 (1H, s, NH on heterocyclic); 14.93 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.40 (C2), 97.04 (C3), 162.26 (C4), 118.98 (C5), 161.53 (C6). 5-(4-Methoxyphenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A3). Brown crystalline solid; m.p.: 268–270 °C; yield: 60 %; anal. calcd. for C19H14N4O3: C, 65.89; H, 4.07; N, 16.18; found: C, 66.06; H, 3.96; N, 16.72; IR (KBr, ν/cm–1): 3386 (NH of 63 hydrazone form), 3152 (NH on heterocyclic), 2231 (CN), 1664, 1642 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 3.72 (3H, s, OCH3 substituent); 7.60–7.45 (5H, m, ArH); 7.75–7.60 (4H, m, ArH); 12.15 (1H, s, NH on heterocyclic); 14.82 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.31 (C2), 96.87 (C3), 162.11 (C4), 119.05 (C5), 161.44 (C6). 5-(4-Methylphenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A4). Red crystalline solid; m.p.: 263–267 °C; yield: 54 %; anal. calcd. for C19H14N4O2: C, 69.08; H, 4.27; N, 16.96; found: C, 69.24; H, 4.13; N, 16.02; IR (KBr, ν/cm–1): 3387 (NH of hydrazone form), 3135 (NH on heterocyclic), 2224 (CN), 1661, 1640 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 2.30 (3H, s, CH3 substituent); 7.60–7.42 (7H, m, ArH); 7.66 (2H, d, J = 9.6 Hz, ArH); 12.19 (1H, s, NH on heterocyclic); 14.68 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.28 (C2), 96.82 (C3), 162.04 (C4), 117.27 (C5), 161.31 (C6). 5-(4-Chlorophenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A5). Light orange crystalline solid; m.p.: 272–274 °C; yield: 72 %; anal. calcd. for C18H11ClN4O2: C, 61.64; H, 3.16; N, 15.97; found: C, 61.48; H, 3.08; N, 15.76; IR (KBr, ν/cm–1): 3406 (NH of hydrazone form), 3110 (NH on heterocyclic), 2217 (CN), 1660, 1631 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.26 (2H, d, J = 9, ArH); 7.38 (2H, d, J = 8.4 Hz, ArH); 7.70–7.44 (5H, m, ArH); 12.26 (1H, s, NH on heterocyclic); 14.52 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 162.97 (C2), 96.80 (C3), 161.82 (C4), 118.77 (C5), 161.20 (C6). 5-(4-Bromophenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A6). Yellow crystalline solid; m.p.: 269–271 °C; yield: 68 %; anal. calcd. for C18H11BrN4O2: C, 54.70; H, 2.81; N, 14.18; found: C, 54.82; H, 2.72; N, 14.06; IR (KBr, ν/cm–1): 3385 (NH of hydrazone form), 3141 (NH on heterocyclic), 2223 (CN), 1672, 1654 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.16 (2H, d, J = 8.4, ArH); 7.70–7.44 (7H, m, ArH); 12.26 (1H, s, NH on heterocyclic); 14.48 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.06 (C2), 96.82 (C3), 161.88 (C4), 118.92 (C5), 161.23 (C6). 5-(4-Iodophenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A7). Dark red crystalline solid; m.p.: 267–270 °C; yield: 52 %; anal. calcd. for C18H11IN4O2: C, 48.89; H, 2.51; N, 64 12.67; found: C, 48.66; H, 2.38; N, 12.48; IR (KBr, ν/cm–1): 3398 (NH of hydrazone form), 3217 (NH on heterocyclic), 2225 (CN), 1684, 1651 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.04 (2H, d, J = 9, ArH); 7.70–7.44 (5H, m, ArH); 7.67 (2H, d, J = 8.4 Hz, ArH); 12.24 (1H, s, NH on heterocyclic); 14.49 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.24 (C2), 96.80 (C3), 161.89 (C4), 119.23 (C5), 161.24 (C6). 5-(4-Fluorophenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A8). Brown crystalline solid; m.p.: 262–264 °C; yield: 50 %; anal. calcd. for C18H11FN4O2: C, 64.67; H, 3.32; N, 16.76; found: C, 64.42; H, 3.24; N, 16.58; IR (KBr, ν/cm–1): 3385 (NH of hydrazone form), 3153 (NH on heterocyclic), 2224 (CN), 1654, 1628 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.20 (2H, d, J = 9, ArH); 7.62–7.40 (5H, m, ArH); 7.69 (2H, d, J = 8.4 Hz, ArH); 12.22 (1H, s, NH on heterocyclic); 14.59 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.27 (C2), 96.85 (C3), 161.89 (C4), 119.28 (C5), 161.15 (C6). 5-(4-Cyanophenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A9). Dark red crystalline solid; m.p.: 270–272 °C; yield: 48 %; anal. calcd. for C19H11N5O2: C, 66.86; H, 3.25; N, 20.52; found: C, 66.71; H, 3.18; N, 20.38; IR (KBr, ν/cm–1): 3386 (NH of hydrazone form), 3152 (NH on heterocyclic), 2231 (CN), 1668, 1652 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 8.03–7.40 (9H, m, ArH); 12.33 (1H, s, NH on heterocyclic); 14.29 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.28 (C2), 96.85 (C3), 161.70 (C4), 117.48 (C5), 161.13 (C6). 5-(4-Carboxyphenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A10). Dark red crystalline solid; m.p.: 260–262 °C; yield: 54 %; anal. calcd. for C19H12N4O4: C, 63.33; H, 3.36; N, 15.55; found: C, 63.52; H, 3.22; N, 15.38; IR (KBr, ν/cm–1): 3382 (NH of hydrazone form), 3168 (NH on heterocyclic), 2231 (CN), 1683, 1654 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.60–7.40 (7H, m, ArH); 7.66 (2H, d, J = 9.6 Hz, ArH); 12.30 (1H, s, NH on heterocyclic); 14.27 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.33 (C2), 96.83 (C3), 161.95 (C4), 116.96 (C5), 161.24 (C6). 5-(4-Acetylphenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A11). Dark red crystalline solid; m.p.: 264–266 °C; yield: 58 %; anal. calcd. for C20H14N4O3: C, 67.03; 65 H, 3.94; N, 15.63; found: C, 67.16; H, 3.68; N, 15.47; IR (KBr, ν/cm–1): 3385 (NH of hydrazone form), 3145 (NH on heterocyclic), 2231 (CN), 1667, 1635 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 2.50 (3H, s, CH3CO substituent); 7.32 (2H, d, J = 8.4 Hz, ArH); 7.65–7.43 (5H, m, ArH); 7.90 (2H, d, J = 8.6 Hz, ArH); 12.34 (1H, s, NH on heterocyclic); 14.46 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 163.27 (C2), 96.84 (C3), 161.80 (C4), 116.88 (C5), 161.22 (C6). 5-(4-Nitrophenylazo)-6-hydroxy-4-phenyl-3-cyano-2-pyridone (A12). Dark yellow crystalline solid; m.p.: 268–270 °C; yield: 60 %; anal. calcd. for C18H11N5O4: C, 59.84; H, 3.07; N, 19.38; found: C, 59.97; H, 2.98; N, 19.21; IR (KBr, ν/cm–1): 3414 (NH of hydrazone form), 3112 (NH on heterocyclic), 2227 (CN), 1671, 1655 (C=O on heterocyclic); 1 H NMR (200 MHz, DMSO-d6, δ/ppm): 7.40 (2H, d, J = 9, ArH); 7.65– 7.45 (5H, m, ArH); 8.19 (2H, d, J = 9 Hz, ArH); 12.37 (1H, s, NH on heterocyclic); 14.37 (1H, s, NH of hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 161.58 (C2), 103.14 (C3), 161.09 (C4), 117.30 (C5), 161.78 (C6). 3.2. Preparation of 5-arylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes (A13 – A23) All the investigated arylazo pyridone dyes were synthesized from the corresponding diazonium salts and 6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone using classical reaction for the synthesis of the azo compounds [32]. 6-Hydroxy-4-(4- methoxyphenyl)-3-cyano-2-pyridone was prepared from ethyl 4-methoxyphenyl benzoylacetate and cyanoacetamide in absolute ethanol in the presence of potassium hydroxide using modified literature procedure [154]. All starting materials were obtained from Aldrich and Fluka and were used without further purification. The obtained compounds were purified by crystallization from acetone and then analyzed. Characterization data are given below. 5-Phenylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A13). Orange crystalline solid; m.p.: 235–238 °C, yield 50%, anal. calcd. for C19H14N4O3: C, 65.89; H , 4.07; N , 16.18; found; C, 65.94; H, 3.89; N , 16.08; IR (KBr, ν/cm–1): 3432 66 (NH of hydrazone form), 3137 (NH on heterocyclic), 2223 (CN), 1663, 1641 (C=O on heterocyclic); ¹H NMR (200 MHz, DMSO-d6, δ/ppm): 3.88 (3H , s, OCH3); 7.12 (2H, d, J=8.4 Hz, ArOCH3); 7.28–7.42 (5H, m, ArH); 7.53 (2H, d, J=9 Hz, ArOCH3); 12.18 (1H, s, NH on heterocyclic); 14.69 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6 , δ/ppm): 162.12 (C2), 99.82 (C3), 161.48 (C4), 117.25 (C5), 160.64 (C6). 5-(4-Fluorophenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A14). Yellow crystalline solid; m.p.: 262–265 °C, yield 41%; anal.calcd. for C19H13F N4O3: C, 62.64; H, 3.60; N, 15.38; found: C, 62.48; H, 3.48; N, 15.34; IR (KBr, ν/cm–1): 3425 (NH of hydrazone form), 3206 (HN on heterocyclic), 2221 (CN), 1697, 1659 (C=O on heterocyclic); ¹HNMR (200 MHz , DMSO-d6, δ/ppm): 3.87 (3H, s, OCH3); 7.12 (2H, d, J=9Hz, ArF); 7.20–7.33 (4H, m, ArF + ArOCH3); 7.52 (2H, d, J=9Hz , ArOCH3); 12.17 (1H, s, NH on heterocyclic); 14.46 (1H, s, NH on hydrazone form); ¹³C NMR (50 MHz , DMSO-d6 , δ/ppm): 161.98 (C2), 99.84 (C3), 161.40 (C4), 119.30 (C5), 160.65 (C6). 5-(4-Chlorophenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A15). Light orange crystalline solid; m.p.: 265–268 °C, yield 44%; anal.calcd. for C19H13Cl N4O3: C, 59.93; H, 3.44; N, 14.71; found: C, 60.08; H, 3.34; N, 14.58; IR (KBr, ν/cm –1 ): 3435 (NH of hydrazone form), 3213 (HN on heterocyclic), 2221 (CN), 1697 , 1660 (C=O on heterocyclic); ¹HNMR (200 MHz , DMSO-d6, δ/ppm): 3.88 (3H, s, OCH3); 7.12 (2H , d , J=8.8 Hz , ArOCH3) ; 7.35 (2H, d, J=8.8 Hz, ArCl) ; 7.45 (2H, d, J=7.6 Hz, ArCl); 7.52 (2H, d, J=7.4 Hz, ArOCH3); 12.19 (1H, s, NH on heterocyclic); 14.58 (1H, s, NH on heterocyclic); ¹³C NMR (50 MHz , DMSO-d6, δ/ppm): 161.93 (C2), 100.31 (C3), 161.32 (C4), 118.83 (C5), 160.49 (C6). 5-(4-Bromophenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A16). Dark yellow crystalline solid; m.p.: 270–273 °C, yield 40%; anal.calcd. for C19H13Br N4O3: C, 53.67; H, 3.08; N, 13.18; found: C, 54.06; H, 3.02; N, 13.08; IR (KBr, ν/cm –1 ): 3435 (NH of hydrazone form), 3206 (HN on heterocyclic), 2221 (CN), 1695, 1659 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.88 (3H, s, OCH3); 7.12 (2H, d, J=8.4 Hz , ArBr); 7.28 (2H, d, J=8.4 Hz , ArOCH3); 7.51 (2H, d, J=8.4 Hz, ArBr); 7.58 (2H, d, J=9 Hz, ArOCH3); 12.20 (1H, s, NH on heterocyclic); 14.54 (1H, s, NH on hydrazone form); 13 C NMR (50 MHz, DMSO-d6, δ/ppm): 161.91 (C2), 100.33 (C3), 161.31 (C4), 119.10 (C5), 160.45 (C6). 67 5-(4-Iodophenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A17). Dark orange crystalline solid; m.p.: 257-260 °C, yield 25%; anal.calcd. for C19H13I N4O3: C, 48.32; H, 2.77; N, 11.86; found: C, 48.36; H, 2.58; N, 11.68; IR (KBr, ν/cm–1) : 3438 (NH of hydrazone form), 3202 (HN on heterocyclic), 2215 (CN), 1699, 1660 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.88 (3H, s, OCH3); 7.05–7.17 (4H, m, ArOCH3 + ArI); 7.52 (2H, d, J=9 Hz, ArI); 7.74 (2H, d, J=8.4 Hz, ArOCH3); 12.19 (1H, s, NH on heterocyclic); 14.52 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.96 (C2), 100.28 (C3), 161.51 (C4), 119.27 (C5), 160.49 (C6). 5-(4-Hydroxyphenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A18). Dark red crystalline solid; m.p.: 233-236 °C, yield 37%; anal.calcd. for C19H14N4O4: C, 62.98; H, 3.89; N, 15.46; found: C, 62.76; H, 3.66; N , 15.32; IR (KBr, ν/cm–1): 3412 (NH of hydrazone form) , 3218 (HN on heterocyclic) , 2214 (CN), 1691, 1643 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.87 (3H, s, OCH3); 6.28 (2H, d, J=8.4 Hz, ArOH); 7.05 (2H, d, J=9 Hz, ArOH); 7.21 (2H, d, J=9 Hz, ArOCH3); 7.50 (2H, d, J=9 Hz, ArOCH3); 9.96 (1H, s, ArOH); 12.08 (1H, s, NH on heterocyclic); 14.69 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz , DMSO-d6, δ/ppm): 162.27 (C2), 97.85 (C3), 161.85 (C4), 119.30 (C5), 160.89 (C6). 5-(4-Methylphenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A19). Dark orange crystalline solid; m.p.: 233-236 °C, yield 42%; anal.calcd. for C20H16N4O3: C, 66.66; H, 4.48; N, 15.55; found: C, 66.52; H, 4.32; N, 15.35; IR (KBr, ν/cm–1): 3425 (NH of hydrazone form), 3201 (NH on heterocyclic), 2214 (CN), 1654, 1643 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 2.30 (3H, s, CH3); 3.88 (3H, s, CH3); 3.88 (3H , s, OCH3); 7.54 (2H, d, J=9 Hz, ArCH3); 7.21 (4H, s, ArCH3 + ArOCH3); 7.75 (2H, d, J=8.4 Hz, ArOCH3); 12.14 (1H, s, NH on heterocyclic); 14.52 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz , DMSO-d6, δ/ppm): 162.13 (C2), 99.18 (C3), 161.40 (C4) , 117.26 (C5), 160.98 (C6). 5-(4-Methoxyphenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A20). Dark brown crystalline solid; m.p.: 232-235 °C, yield 27%; anal.calcd. for C20H16N4O4: C, 63.82; H, 4.28; N, 14.89; found: C, 63.78; H, 4.32; N, 14.54; IR (KBr, ν/cm–1): 3347 (NH of hydrazone form), 3225 (NH on heterocyclic), 2212 (CN), 1662 , 1648 (C=O on 68 heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.76 (3H, s, N–ArOCH3); 3.88 (3H, s, OCH3); 6.59 (4H, m, N–ArOCH3 + ArOCH3); 7.45 (2H, d, J=8.4 Hz , N– ArOCH3); 7.50 (2H, d, J=8.4 Hz, ArOCH3); 12.10 (1H, s, NH on heterocyclic); 14.63 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm) : 161.75 (C2), 99.02 (C3), 161.75 (C4), 119.01 (C5), 160.78 (C6). 5-(4-Acetylphenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A21). Dark yellow crystalline solid; m.p.: 267-270 °C, yield 22%; anal.calcd. for C21H16N4O4: C, 64.94; H, 4.15; N, 14.43; found: C, 64.78; H, 3.98; N, 14.36; IR (KBr, ν/cm–1): 3438 (NH of hydrazone form), 3206 (NH on heterocyclic), 2215 (CN), 1698, 1675 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.76 (3H, s, N–ArOCH3); 3.88 (3H, s, OCH3); 6.59 (4H, m, N–ArOCH3 + ArOCH3); 7.45 (2H, d, J=8.4 Hz, N– ArOCH3); 7.50 (2H, d, J=8.4 Hz, ArOCH3); 12.10 (1H, s, NH on heterocyclic); 14.63 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.86 (C2), 101.22 (C3), 161.27 (C4), 116.81 (C5), 160.40 (C6). 5-(4-Cyanophenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A22). Yellow crystalline solid; m.p.: 279-282 °C, yield 25%; anal.calcd. for C20H13N5O3: C, 64.69; H, 3.53; N, 18.86; found: C, 64.48; H, 3.32; N, 18.56; IR (KBr, ν/cm–1): 3425 (NH of hydrazone form), 3221 (NH on heterocyclic), 2227 (CN), 1678, 1654 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.88 (3H, s, OCH3); 7.05 (2H, d, J=9 Hz, ArCN); 7.13 (2H, d, J=9 Hz, ArOCH3); 7.21 (2H, d, J=9 Hz, ArCN); 7.84 (2H, d, J=8.4 Hz ArOCH3); 12.28 (1H, s, NH on heterocyclic); 14.41 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz , DMSO-d6, δ/ppm): 161.75 (C2), 101.86 (C3), 161.24 (C4), 118.88 (C5), 160.75 (C6). 5-(4-Nitrophenylazo)-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone (A23). Dark yellow crystalline solid; m.p.: 275-278 °C , yield 23% ; anal.calcd. for C19H13N5O5: C, 58.31; H, 3.35; N, 17.90; found: C, 58.22; H, 3.24; N, 17.72; IR (KBr, ν/cm–1): 3431 (NH of hydrazone form), 3213 (NH on heterocyclic), 2214 (CN), 1689, 1675 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 3.89 (3H, s, OCH3); 7.06 (2H, d, J=8.8 Hz, ArOCH3); 7.15 (2H, d, J=9 Hz, ArNO2); 7.52 (2H, d, J=8.4 Hz, ArCH3); 8.24 (2H, d, J=9 Hz, ArNO2); 12.31 (1H, s, NH on heterocyclic); 14.34 (1H, s, NH of 69 hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.68 (C2),102.28 (C3), 161.22 (C4), 119.85 (C5), 160.33 (C6). 3.3. Preparation of 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes (A24–A33) All then investigated arylazo pyridone dyes were synthesized from the corresponding diazonium salts and 6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone using classical reaction for the synthesis of the azo compounds [32]. 6-Hydroxy-4-(4- nitrophenyl)-3-cyano-2-pyridone was prepared from ethyl-4-nitrophenyl benzoylacetate and cyanoacetamide in absolute ethanol in the presence of potassium hydroxide using modified literature procedure [154]. All starting materials were obtained from Aldrich and Fluka, and were used without further purification. The obtained compounds were purified by crystallization from acetone and then analyzed. Characterization data are given below. 5-Phenylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A24). Brown crystalline solid; m.p.: 260–263 °C; yield 50%; anal. calcd. for C18H11N5O4: C, 59.84; H, 3.07; N, 19.38; found: C, 59.62; H, 2.98; N, 19.18; IR (KBr, ν/cm–1): 3439 (NH of hydrazone form), 3185 (NH on heterocyclic), 2223 (CN), 1692, 1668 (C=O on heterocyclic); ¹HNMR (200 MHz DMSO-d6, δ/ppm): 6.71 (1H, m, Ar); 7.00–7.60 (4H, m, Ar); 7.85 (2H, d, J=9 Hz, ArNO2); 8.45 (2H, d, J=9 Hz , ArNO2); 12.34 ( 1H, s, NH on theterocylic); 14.69 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.64 (C2), 113.60 (C3), 161.02 (C4), 121.84 (C5), 159.16 (C6). 5-(4-Methylpherylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A25). Light red crystalline solid; m.p.: 222-225 °C; yield 43%; anal. calcd. for C19H13N5O4: C, 60.80; H,3.49; N, 18.66; found: C, 60.42; H, 3.27; N, 18.44; IR (KBr, ν/cm–1): 3458 (NH of hydrazone form), 3157 (NH on heterocyclic), 2226 (CN), 1674, 1658 (C=O on heterocyclic); ¹HNMR (200 MHz, DMSO-d6, δ/ppm): 2.26 (3H, s, CH3); 7.85 (2H, d, J=9 Hz, ArCH3); 8.18 (2H, d, J=9 Hz, ArCH3); 8.35 (2H, d, J=8.4 Hz, ArNO2); 8.42 (2H, d, J=9 Hz, ArNO2); 12.31 (1H, s, NH on heterocyclic); 14.69 ( 1H, s, NH on hydrozone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.80 (C2), 115.15 (C3), 160.95 (C4), 123.35 (C5), 159.07 (C6). 70 5-(4-Hydroxyphenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A26). Dark red crystalline solid; m.p.: 285-287 °C, yield 37%; anal.calcd. for C18H11N5O5: C, 57.30; H, 2.94; N, 18.56; found: C, 55.25; H, 2.78; N, 18.33; IR (KBr, ν/cm–1): 3426 (NH of hydrazone form), 3203 (HN on heterocyclic); ¹HNMR (200 MHz, DMSO-d6, δ/ppm): 6.78 (2H, d, J=9 Hz, ArOH); 7.17 (2H, d, J=9 Hz , ArOH); 8.18 (2H, d, J=9 Hz, ArNO2); 8.43 (2H, d, J=9 Hz , ArNO2); 9.96 (1H, s, ArOH); 12.23 (1H, s, NH on hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.97 (C2), 116.68 (C3), 161.04 (C4), 123.98 (C5), 158.92 (C6). 5-(4-Chlorophenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A27). Light brown crystalline solid; m.p.: 220-223 °C, yield 44%; anal. calcd. for C18H10ClN5O4: C, 54.63; H, 2.55; N, 17.70; found: C, 54.38; H, 2.32; N, 17.56; IR (KBr , ν/cm–1): 3459 (NH of hydrazone form), 3170 (NH on heterocyclic), 2223 (CN), 1684, 1652 (C=O on heterocyclic); ¹HNMR (200 MHz , DMSO-d6, δ/ppm): 7.14–7.54 (4H , m , ArCl); 8.16 (2H, d, J=8.4 Hz, ArNO2); 8.43 (2H, d, J=9 Hz, ArNO2); 12.13(1H, s, NH on heterocyclic); 14.58 (1H, s, NH of hydrazone form), ¹³CNMR (50 MHz, DMSO-d6, δ/ppm): 162.98 (C2), 116.28 (C3), 161.04 (C4), 124.06 (C5), 159.09 (C6). 5-(4-Bromophenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A28). Red crystalline solid; m.p.: 242-245 °C; yield 42%, anal. calcd. for C18H10BrN5O4: C, 49.11; H, 2.29; N, 15.91; found: C, 48.88; H, 2.11; N, 15.78; IR (KBr , ν/cm–1) : 3433 ( NH of hydrazone form), 3157 (NH on heterocyclic), 2206 (CN), 1683, 1662 (C=O on teterocyclic); ¹HNMR (200 MHz , DMSO-d6, δ/ppm): 7.20 (2H, d, J=8.8 Hz , ArBr); 7.54 (2H, d, J=8.4 Hz, ArBr); 8.16 (2H, d, J=9 Hz, ArNO2); 8.42 (2H, d, J=9 Hz, ArNO2); 12.35 (1H, s, NH on heterocyclic); 14.54 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 162.91 (C2), 114.99 (C3), 161.60 (C4), 123.45 (C5), 160.89 (C6). 5-(4-Iodophenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A29). Dark violet crystalline solid; m.p.: 218-221 °C, yield 35%, anal. calcd. for C18H10IN5O4: C, 44.37; H, 2.07; N, 14.37; found; C, 44.18; H, 1.98; N, 14.28; IR (KBr , ν/cm–1): 3439 (NH of hydrazone form), 3145 (NH on heterocyclic), 2225 (CN), 1673, 1658 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 7.05 (2H , d , J=7.8 Hz, ArI); 7.69 (2H, d, J=7.8 Hz, ArI); 7.80 (2H, d, J=8.8 Hz, Ar NO2); 8.42 (2H, d, J=8 Hz, 71 ArNO2); 12.34 (1H, s, NH on heterocyclic); 14.52 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 162.69 (C2), 114.98 (C3), 161.60 (C4), 123.44 (C5), 160.88 (C6). 5-(4-Florophenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A30). Dark brown crystalline solid; m.p.: 232-235 °C, yield 40%, anal. calcd. for C18H10FN5O4: C, 57.00; H , 2.66; N, 18.46; found; C, 48.36; H, 2.48; N, 18.23; IR (KBr , ν/cm–1): 3439 (NH of hydrazone form), 3196 (NH on heterocyclic), 2225 (CN), 1697, 1656 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 7.22 (2H, d, J=9 Hz, ArF); 7.77 (2H, d, J=9 Hz, ArF); 8.17 (2H, d, J=9 Hz, ArNO2); 8.40 (2H, d, J=9 Hz, ArNO2); 12.20 (1H, s, NH on heterocyclic); 14.46 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 162.67 (C2), 115.00 (C3), 161.67 (C4), 123.31 (C5), 160.58 (C6). 5-(4-Acetylphenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A31). Brown crystalline solid; m.p.: 235-238 °C, yield 25%, anal. calcd. for C20H13N5O5: C, 59.56; H, 3.25; N, 17.36; found; C, 59.14; H, 3.06; N, 17.34; IR (KBr , ν/cm–1): 3452 (NH of hydrazone form), 3209 (NH on heterocyclic), 2224 (CN), 1680, 1658 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 2.52 (3H, s, COCH3); 7.30 (2H, d, J=9 Hz, ArCOCH3) ; 7.83 (2H, d, J= 9 Hz, ArNO2); 7.95 (2H, d, J=8.8 Hz, ArCOCH3); 8.45 (2H, d, J=8.8 Hz, ArNO2); 12.39 (1H, s, NH of heterocyclic); 14.41 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 161.47 (C2), 112.67 (C3), 160.85 (C4), 123.49 (C5), 159.05 (C6). 5-(4-Cyanophenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A32). Orange crystalline solid; m.p.: 255-258 °C, yield 30%, anal. calcd. for C19H10N6O4: C, 59.07; H, 2.61; N, 21.75; found; C, 58.88; H, 2.36; N, 21.64; IR (KBr , ν/cm–1): 3446 (NH of hydrazone form), 3229 (NH on heterocyclic), 2222 (CN), 1684, 1651 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 7.40 (2H, d, J=9 Hz, ArCN); 8.20 (2H, d, J=9 Hz, ArCN); 8.35 (2H, d, J= 8.4 Hz, ArNO2); 8.45 (2H, d, J=9 Hz, ArNO2); 12.46 (1H, s, NH on heterocyclic); 14.38 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 162.91 (C2), 113.69 (C3), 161.40 (C4), 123.51 (C5), 160.75 (C6). 72 5-(4-Nitrophenylazo)-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone (A33). Light brown crystalline solid; m.p.: 265-268 °C, yield 26%, anal. calcd. for C18H10N6O6: C, 53.21; H, 2.48; N, 20.68; found; C, 52.88; H, 2.32; N, 20.49; IR (KBr, ν/cm–1): 3446 (NH of hydrazone form), 3268 (NH on heterocyclic), 2218 (CN), 1688, 1653 (C=O on heterocyclic); ¹H NMR (200 MHz , DMSO-d6, δ/ppm): 7.41 (2H, d, J=9.4 Hz, N– ArNO2); 7.80 (2H, d, J=8.8 Hz, ArNO2); 8.32 (2H, d, J= 8.4 Hz, N–ArNO2); 8.45 (2H, d, J=9 Hz, ArNO2); 12.46 (1H, s, NH on heterocyclic); 14.38 (1H, s, NH of hydrazone form); ¹³C NMR (50 MHz, DMSO-d6, δ/ppm): 162.88 (C2), 112.62 (C3), 161.31 (C4), 123.60 (C5), 160.76 (C6). 73 4. RESULTS AND DISCUSSION 4.1. Solvent and structural effects on the UV-vis absorption spectra of 5-arylazo-6- hydroxy-4-phenyl-3-cyano-2-pyridone dyes The arylazo pyridone dyes prepared in this work may exist in two main tautomeric forms (Scheme 4.1). Generally, tautomers not only have different colors, but also have different tinctorial strength and different properties, e.g., light fastness [78]. Due to the commercial importance of arylazo pyridone dyes, the azo-hydrazone tautomerism has been intensively studied [99–101]. It was concluded that the equilibrium between the two tautomers is influenced by the structure of the compound and the solvent used [99–101]. 4.1.1. Spectral characteristics and tautomerism The infrared spectra of all the synthesized dyes showed two intense carbonyl bands at about 1630 and 1684 cm –1 , which were assigned to the diketohydrazone form. The FT-IR spectra also showed a band at 3110–3217 cm–1, assigned to the imino group (N–H) of the heterocyclic (pyridine) ring and a band at 3382–3414 cm–1 that was assigned to the N–H of hydrazo tautomeric form. The 1 H NMR spectra of the dyes exhibited a broad signal near 14.27–14.93 ppm. This signal corresponds to the imine N–H proton resonance of the hydrazone form (Scheme 4.1, Structure B). N. Ertan et al. [83] reported the 1 H NMR spectra of some azo pyridone dyes in CF3COOD / CDCl3 and showed that these dyes existed in the hydrazone form with the N–H peak in the range 15.10–15.60 ppm. Q. Peng et al. [101] also reported the 1H NMR spectra of azo pyridone dyes in CDCl3 and concluded that the azo pyridone dyes exist in the hydrazone form and with the N–H peaks appearing within the range 14.30–16.09 ppm. Lucka and Machacek [155] and Cee et al. [102] concluded from 13 C NMR studies of some N-alkyl derivatives of azopyridones that pyridone azo dyes in CDCl3 and DMSO-d6 exist in the hydrazone form. Our results are in agreement with these results. 74 N H OH O H 5 C 6 N NC N X N H O H 5 C 6 N NC N X O H 2 3 4 5 6 2 3 4 5 6 A B Scheme 4.1. The equilibrium between azo form (A) and hydrazone form (B) of 5-arylazo- 6-hydoxy-4-phenyl-3-cyano-2-pyridones (X = H (A1), OH (A2), OCH3 (A3), CH3 (A4), Cl (A5), Br (A6), I (A7), F(A8), CN (A9), COOH (A10), COCH3 (A11), NO2 (A12)). 4.1.2. Solvent effects on the azo–hydrazone tautomerism Since the tautomeric equilibria strongly depend on the nature of the media, the behavior of selected arylazo pyridone dyes in thirteen protic and aprotic solvents was studied. For this purpose, the absorption spectra of the pyridone dyes (A1–A12) at a concentration 1×10 –5 mol dm –3 were recorded over the λ range between 200 and 600 nm in the selected solvent set. The characteristic absorption spectra of the investigated azo dyes in methanol and dimethyl sulfoxide are shown in Figures 4.1 and 4.2. 75 Figure 4.1. Absorption spectra of dyes A1–A12 in methanol. Figure 4.2. Absorption spectra of dyes A1–A12 in dimethyl sulfoxide. 76 The UV-vis absorption spectra of all the dyes showed a weak band at about 260– 370 nm, assigned to the azo tautomeric form and a strong band at 375–550 nm, which was assigned to hydrazone tautomeric form. The absorption maxima, which correspond to a transition in which electron density is transferred from the hydrazone –NH group to the pyridone carbonyl group (lower energy band), are presented in Tables 4.1 and 4.2. It was observed that, although slightly positive solvatochromism is evident, the absorption spectra of dyes A1–A12 did not change significantly in all the employed solvents and the absorption maxima did not correlate with the polarity of the solvent. Table 4.1. Absorption maxima of the hydrazone tautomer (B) of arylazo pyridone dyes (A1– A12) in protic solvents. Dye No. λmax (nm) Methanol Ethanol Propan-1-ol Propan-2-ol Butan-1-ol 2-Methyl- -2-propanol A1 433 434 434 433 433 431 A2 460 470 476 472 470 474 A3 460 458 458 455 448 454 A4 445 446 447 445 446 445 A5 437 435 438 439 439 439 A6 437 438 438 438 442 440 A7 440 442 443 442 442 442 A8 433 436 436 434 436 436 A9 424 426 425 425 425 424 A10 432 432 433 433 433 433 A11 437 436 436 435 435 435 A12 429 428 429 428 428 427 77 Table 4.2. Absorption maxima of hydrazone tautomer (B) of arylazo pyridone dyes (A1– A12) in aprotic solvents. Dye No. λmax (nm) Tetrahy- drofuran Dioxane Methyl acetate Ethyl acetate N,N-Dimethyl- formamide N,N-Dimethyl- acetamide Dimethyl sulfoxide A1 431 430 430 429 415 428 437 A2 466 461 460 460 470 470 473 A3 455 455 454 454 449 456 459 A4 442 442 441 440 423 441 449 A5 434 437 434 433 417 427 440 A6 436 437 434 433 421 425 441 A7 439 441 437 437 425 432 447 A8 432 432 431 429 410 422 437 A9 426 426 423 423 433 434 430 A10 431 432 429 429 431 433 437 A11 434 436 433 432 436 439 442 A12 429 430 427 427 451 452 437 Additional evidence for the solvent effect on the structure-property relationship of arylazo pyridone dyes was obtained from the correlation of the absorption frequencies (ν = 1 / λ in cm–1) for the hydrazone tautomeric form (Tables 4.1 and 4.2) with the Kamlet- Taft Solvatochromic Equation 4.1 [150] of the following form: ν = ν0 + sπ* + bβ + aα (4.1) where π* is an index of the solvent dipolarity / polarizability, β is a measure of the solvent hydrogen-bonding acceptor (HBA) basicity, α is a measure of the solvent hydrogen-bonding donor (HBD) acidity and ν0 is the regression value of the solute property in cyclohexane as the reference solvent. The regression coefficients s, b and a in Eq. 4.1 are a measure the relative susceptibilities of the absorption frequencies to the indicated solvent parameters. The linear solvation energy relationship (LSER) concept 78 developed by Kamlet and Taft is one of the most ambitions and successful quantitative treatments of solvation effects. This treatment assumes attractive interactions between a solute and its environment and enables an estimation of the ability of the investigated compounds to form hydrogen bonds. The solvent parameters [151] are given in Table 4.3. The correlations of the absorption frequencies νmax for hydrazone tautomer were realized by means of multiple linear regression analysis. It was found that νmax in the selected solvent sets showed satisfactory correlation with the π*, β and α parameters. The results of the multiple regressions are presented in Tables 4.4 and 4.5, and the coefficients ν0, s, b and a fitted at the 95 % confidence level are presented in Table 4.4. The negative sign of the a coefficient (Table 4.4) for all dyes (excluding the H, CN, COOH and NO2 substituents) and the s and b coefficients for strong electron-donating substituents and strong electron-accepting substituents indicate a bathochromic shifts with increasing solvent dipolarity / polarizability and solvent hydrogen bond acidity and basicity. This suggests stabilization of the electron excited state relative to the ground state. The positive sign of the a coefficient for strong electron-accepting substituents and the s and b coefficients for moderate electron-donating and electron-accepting substituents indicate hypsochromic shifts with increasing solvent dipolarity / polarizability and both types of hydrogen bonding effects. These results showed that the solvent effect on the UV-vis absorption spectra of the investigated azo pyridone dyes is very complex and strongly dependent on the nature of the substituent on the arylazo component. They also indicated that the electronic behavior of the nitrogen atoms of hydrazone group are somewhat different between derivatives with electron-donating and electron-accepting substituents (Figure 4.3, Structures C and D). This phenomenon is caused by the difference in the conjugational or migrating ability of the electron lone pairs on the nitrogen atoms of the pyridone azo dyes. The strong electron-donating substituents in the phenyl group produce extensive delocalization in the arylazo group (Figure 4.3, Structure D), while the influence of the strong electron-accepting substituents are opposite, due to the positive charge on the nitrogen atom in the hydrazone tautomer (Figure 4.3, Structure C). 79 Table 4.3. Solvent parameters [151] No. Solvent π* β α 1 Methanol 0.60 0.62 0.93 2 Ethanol 0.54 0.77 0.83 3 Propan-1-ol 0.52 0.83 0.8 4 Propan-2-ol 0.48 0.95 0.76 5 Butan-1-ol 0.47 0.88 0.79 6 2-Methyl-2-propanol 0.41 0.11 0.68 7 Tetrahydrofuran 0.58 0.55 0 8 Dioxane 0.55 0.37 0 9 Methyl acetate 0.60 0.42 0 10 Ethyl acetate 0.55 0.45 0 11 N,N-Dimethylformamide 0.88 0.69 0 12 N,N-Dimethylacetamide 0.88 0.76 0 13 Dimethyl sulfoxide 1.00 0.76 0 80 Table 4.4. Regression fits to the solvatochromic parameters (Eq. 4.1) No. Substi- tuent ν0 x 10 –3 (cm –1 ) s x 10 –3 (cm –1 ) b x 10 –3 (cm –1 ) a x 10 –3 (cm –1 ) r a s b F c Solvents used in the calculation d A1 H 21.92 (±0.180) 1.81 (±0.240) 0.80 (±0.170) 0.52 (±0.100) 0.9789 0.078 46 1–11 A2 OH 21.34 (±0.155) –0.53 (±0.269) –0.86 (±0.280) –0.20 (±0.191) 0.9417 0.099 21 2–13 A3 OCH3 23.02 (±0.159) –0.66 (±0.089) 0.22 (±0.078) –0.31 (±0.104) 0.9614 0.037 24 1–10,13 A4 CH3 23.02 (±0.159) –0.44 (±0.222) –0.19 (±0.197) –0.24 (±0.118) 0.8128 0.099 5 1–10,12,13 A5 Cl 21.28 (±0.183) 2.57 (±0.246) 0.61 (±0.176) –0.31 (±0.098) 0.9802 0.081 57 1–11 A6 Br 21.61 (±0.194) 2.11 (±0.260) 0.38 (±0.186) –0.27 (±0.104) 0.9703 0.085 38 1–11 A7 I 21.39 (±0.164) 2.01 (±0.210) 0.50 (±0.147) –0.25 (±0.083) 0.9812 0.067 52 1–9,11 A8 F 21.14 (±0.256) 3.02 (±0.344) 0.79 (±0.246) –0.43 (±0.138) 0.9738 0.113 43 1–11 A9 CN 24.50 (±0.102) –1.21 (±0.114) –0.51 (±0.097) 0.10 (±0.051) 0.9862 0.042 71 1–7,10–12 A10 COOH 24.58 (±0.097) –1.25 (±0.121) –0.57 (±0.098) 0.09 (±0.055) 0.9823 0.046 64 1–7, 9–12 A11 COCH3 23.73 (±0.050) –0.92 (±0.062) –0.23 (±0.057) –0.16 (±0.033) 0.9878 0.027 94 1–12 A12 NO2 27.02 (±0.398) –5.14 (±0.533) –1.71 (±0.382) 0.47 (±0.214) 0.9735 0.176 42 1–7,9,10, 12,13 a Correlation coefficient; b Standard error of the estimate; c Fisher's test; d Solvent number as given in Table 4.3. 81 N X N N H O O H CN + + N NX N H O O H CN - - C D Figure 4.3. Resonance effect of electron-accepting (structure C) and electron-donating (structure D) substituents of the arylazo component on the hydrazone tautomer. The percentage contributions of the solvatochromic parameters (Table 4.5) for the azo dyes with strong and moderate electron-accepting substituents on the arylazo group, showed that the most of the solvatochromism is due to the solvent dipolarity / polarizability rather than to the solvent acidity and basicity. These results could be explained by the effect of the positive charge on the nitrogen atom in the hydrazone tautomer (Figure 4.3, Structure C) and stabilization of this form mostly due to the solvent dipolarity / polarizability (non-specific solute-solvent interactions) than by hydrogen bond donating and hydrogen bond accepting properties (specific solute-solvent interactions). 82 Table 4.5. Percentage contribution of the solvatochromic parameters Substituent Pπ* (%) Pβ (%) Pα (%) H 58 25 17 OH 33 54 13 OCH3 55 19 26 CH3 50 22 28 Cl 74 17 9 Br 76 14 40 I 73 18 9 F 71 19 10 CN 66 28 6 COOH 65 30 5 COCH3 70 18 12 NO2 70 23 7 4.1.3. Substituent effects on the azo–hydrazone tautomerism As seen in Tables 4.1 and 4.2, the absorption spectra of the p-nitro derivative (dye A12) were shifted hypsochromically in all used solvents (excluding DMSO and DMF) when compared with dye A1. Moreover, the absorption spectra of the p-hydroxy and p-methoxy derivatives (dyes A2 and A3) were shifted bathochromically in all used solvents when compared with dye A1. It is well known that the λmax values of the hydrazone tautomeric form of an azo dyes will show a general shift to shorter wavelengths when substituents of increasing electron withdrawing strength are introduced into the ring of the diazo component. In contrast, electron donor substituents produce strong bathochromic shifts [156]. The results presented in Tables 4.1 and 4.2 are in agreement with these conclusions. Thus, the observed relationship between the substituent constants and the λmax values strongly suggests that the lower energy absorption maxima of the investigated azo dyes originate from hydrazones (Scheme 4.1, Structure B). 83 In order to explain these results, the absorption frequencies were correlated by the Hammett Equation 4.2 using σp or σp+ substituent constants [116]: ν = ν0 + ρσp (4.2) where ρ is a proportionality constant reflecting the sensitivity of the absorption frequencies to the substituent effects. The substituent σp or σp+ constants measure the electronic effect of the substituents. The plot νmax vs. the σp substituent constants gave a correlation which showed deviations from the Hammet Equation in all dipolar aprotic solvents. However, a linear Hammett correlation was obtained in protic solvents. A better correlation of νmax was obtained with the σp+ substituent constants [125] than with the σp constants in all solvents, which indicates extensive delocalization in the arylazo group. The existence of the linear correlation with positive slope presented in Figure 4.4 and Equation 4.3 was interpreted as evidence of the diketohydrazone structure. Figure 4.4. Relationship between νmax and ζp+ for arylazo pyridone dyes A1–A12 in methanol. νmax = 0.974 σp+ + 22.744 (4.3) (r = 0.9114, s = 0.25, F = 49, n = 12) 84 4.1.4. Quantum chemical calculations DFT calculations were performed for different azo-hydrazone tautomers of dye A1. The structures were preliminary optimized by the semi-empirical PM3 method and the most stable geometries in vacuum were reoptimized at the B3LYP/6-31G(d) level of theory [106,107]. The Gaussian 03 program package was used [108]. The DFT calculations suggested that the diketohydrazone form (a) (Figure 4.5) is the most stabile tautomer of dye A1. The relative energies and the statistical Boltzmann distribution weighted values of the most stable azo-hydrazone tautomers of dye A1 are given in Table 4.6 and Figure 4.5. Table 4.6. The relative energies and the statistical Boltzmann distribution weighted values of the most stable azo-hydrazone tautomers of dye A1 Tautomer of dye A1 Relative energies [kcal mol –1 ] Statistical Boltzmann distribution weighted values [%] (a) 0.000 100.00 (b) 21.729 0.00 (a) Relative energy = 0.000 kcal mol –1 Equilibrium population = 100.00% (b) Relative energy = 21.729 kcal mol –1 Equilibrium population = 0.00% 85 (c) Relative energy = 39.599 kcal mol –1 Equilibrium population = 0.00% (d) Relative energy = 34.371 kcal mol –1 Equilibrium population = 0.00% Figure 4.5. The most stable hydrazone tautomer (a) and some other azo-hydrazone tautomers of 5-phenylazo-6-hydrohy-4-phenyl-3-cyano-2-pyridone. (The geometries correspond to the energy minimum in vacuo). 4.2. Solvent and structural effects on the UV-vis absorption spectra of 5-arylazo-6- -hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes In continuous of our investigations of novel arylazo pyridone dyes, eleven new 5- arylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes (Scheme. 4.2) were synthesized in order to characterize azo-hydrazone tautomerism, as well as to study the solvent and substituent effects on the electronic absorption spectra. The absorption spectra were recorded in the range from 300 to 600 nm in twenty solvents of different properties. Different solvent parameters, such as microscopic solvent polarity, ET N , relative permittivity, εr, refractive index, n, the Kamlet-Taft and the Catalan parameters were used for describing the solute-solvent interactions and solvatochromic shifts of the UV-vis absorption band of the investigated arylazo pyridone dyes. For quantitative assessment of the substituent effects on the absorption frequencies, the simple Hammett equation was used. 86 Scheme 4.2. The equilibrium between azo form (A) and hydrazone form (B) of 5- arylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes (X = H (A13), F (A14), Cl (A15), Br (A16), I (A17), OH (A18), CH3 (A19), OCH3 (A20), COCH3 (A21), CN (A22), NO2 (A23)). Resonance effect of electron-accepting (structure C) and electron-donating (structure D) substituents of the arylazo component on the hydrazone tautomer. 87 4.2.1. Spectral characteristics of arylazo pyridone dyes The 5-arylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes can exist in two tautomeric forms (Scheme 4.2). The infrared spectra of all synthesized dyes showed two intense carbonyl bands at about 1642 and 1699 cm -1 , which were assigned to the diketohydrazone form. The spectra exhibited broad bands in the region 3137– 3225 cm -1 and 3421–3438 cm-1 which were assigned to the N–H group from pyridone ring and hydrazone form, respectively. The 1 HNMR spectra of the dyes exhibit a broad signal near 14.34–14.69 ppm. This signal corresponds to imine NH proton resonance of the hydrazone form (Scheme 4.2, Structure B). Ertan and Gurkan [83] have reported the 1H NMR spectra of some azo pyridone dyes in CF3COOD/CDCl3 and showed that these dyes exist in the hydrazone form with NH peaks in the range of 15.1–15.6 ppm. 1 3C NMR studies of some N-alkyl derivatives of azopyridones in solutions in CDCl3 and DMSO-d6 by Lucka and Machacek [155] and Cee et al. [102] led to conclusion that pyridone azo dyes exist in the hydrazone form. Our experimental results are in agreement with these results. Physical properties of the solvents including the Kamlet-Taft solvatochromic parameters α, β and π* were taken from Ref. [150], whereas the Catalan SP, SdP, SA and SB solvent from Ref. [157], and the relative permittivity, εr, refractive index, n, and the ET N solvent polarity from Ref. [142]. Substituent constants, ζp and ζp+, were taken from Ref. [125]. The correlation analysis was carried out using Microsoft Excel software, which considers the 95% confidence level. The goodness of the fit is discussed using the correlation coefficient (R), the standard error of the estimate (s) and Fischer's significance test (F). 4.2.2. Solvent effects on the UV-vis absorption spectra The electronic absorption spectra of 5-arylazo-6-hydroxy-4-(4-methoxyphenyl)-3- cyano-2-pyridone dyes (A13–A23) were measured at room temperature in twenty solvents in the range 300–600 nm and the characteristic spectra in representative solvents are shown in Fig. 4.6. The UV-vis spectra of all dyes showed a weak band at about 330– 88 360 nm and a strong band at 420–480 nm which was assigned to azo and hydrazone tautomeric form, respectively. The absorption maxima, which correspond to a transition in which electron density is transferred from the hydrazone –NH group to the pyridone carbonyl group (lower energy band), is presented in Table 4.7 and were studied. The physical parameters of the solvents used are listed in Table 4.8: relative permittivity, εr, refractive index, n, the ET N and corresponding solvent parameters used for the Kamlet-Taft and Catalan parameters. The solvents are arranged with increasing their relative permittivity. To explain the effects of solvents and substituents on electronic absorption spectra of azo pyridone dyes, the absorption maxima of the lower energy band of the unsubstituted dye (A13) in different solvents, was taken as the reference. The data from Table 4.7 confirm that the positions of the UV-vis absorption frequencies depend on the nature of the used solvent and substituent on the benzene ring of the coupling component. The introduction of electron-donating substituents in the benzene ring produced bathochromic shift of absorption maxima as compared to that of the unsubstituted azo dye in all solvents (Fig. 4.6). Electron-attracting substituents caused bathochromic or hypsochromic shifts when there is a change from polar to non-polar solvents, respectively. Also, on going from aprotic to protic solvents, a bathochromic shift of the low- energy absorption band is observed. Azo pyridone dyes show positive solvatochromism i.e. the position of the absorption maxima of the lower energy band is shifted to a lower energy with increasing the solvent polatity / dipolarity. This behavior indicates that the azo pyridone dyes are more polar in the excited state than in their ground state. 89 Table 4.7. The absorption frequencies of the investigated compounds (A13–A23) in selected solvents. Solvent / No. υmax × 10 -3 (cm -1 ) A13 A14 A15 A16 A17 A18 A19 A20 A21 A22 A23 Dioxane 23.20 23.22 23.02 22.90 22.79 21.73 22.88 22.12 23.18 23.34 23.18 Disopropyl ether 23.32 23.26 23.10 23.06 22.90 21.70 22.80 22.00 23.24 23.59 23.54 Diethyl ether 23.28 23.27 23.13 23.12 22.92 21.70 22.94 22.22 23.19 23.54 23.51 Chloroform 22.78 22.78 22.52 22.57 22.32 21.57 22.40 21.48 22.73 23.09 22.96 Ethyl acetate 23.27 23.26 23.14 23.08 22.89 21.77 23.06 22.21 23.01 23.54 23.34 Methyl acetate 23.25 23.15 23.08 22.90 22.91 21.73 22.95 22.10 23.16 23.52 23.32 Tetrahydrofuran 23.18 23.15 23.07 23.00 22.84 21.56 22.95 22.14 23.02 23.40 23.22 Cyclohexanone 22.81 23.00 22.97 22.98 22.70 21.44 22.75 21.98 22.85 23.30 23.13 Butan-2-ol 22.96 22.92 22.92 22.83 22.60 21.03 22.73 22.37 23.08 23.24 22.88 Butan-1-ol 22.97 22.96 22.89 22.78 22.70 21.00 22.67 22.37 23.18 23.28 22.87 Propan-2-ol 23.06 23.10 22.90 22.87 22.90 21.12 22.98 22.94 22.86 23.32 23.42 Propan-1-ol 22.98 23.06 22.88 22.85 22.93 21.16 22.84 22.55 22.84 23.26 23.28 Acetone 23.21 23.12 23.10 23.14 22.84 21.65 22.86 22.16 23.23 23.47 23.35 Ethanol 23.07 23.12 22.92 22.90 22.97 21.74 23.05 22.63 22.93 23.42 23.39 Ethylene glycol 22.73 22.73 22.68 22.57 22.35 21.11 22.53 22.22 22.57 23.09 22.83 Methanol 23.09 23.00 22.89 22.84 22.85 21.28 22.79 22.50 22.95 23.38 23.25 Acetonitrile 23.18 23.13 23.00 23.00 22.73 21.68 22.77 22.18 22.95 23.37 23.18 N,N-Dimethylformamide 23.15 23.24 23.09 23.46 22.68 21.28 23.42 22.04 22.82 23.25 22.71 N,N-Dimethylacetamide 23.07 23.26 22.99 22.96 22.65 21.25 22.70 22.08 22.78 23.23 23.04 Dimethyl sulfoxide 22.95 23.04 22.77 22.84 22.43 21.14 22.65 21.83 22.65 23.04 22.78 90 Figure 4.6. The UV-vis absorption spectra of 5-arylazo-6-hydroxy-4-(4-methoxyphenyl)- 3-cyano-2-pyridone dyes in ethanol (a) and dimethyl sulfoxide (b). 91 Table 4.8. The physical parameters of the solvents. No. Solvent εr n ET N Kamlet-Taft Catalan π* β α SP SdP SB SA 1 Dioxane 2.21 1.4224 0.164 0.49 0.37 0.00 0.737 0.312 0.444 0.000 2 Disopropyl ether 3.80 1.3680 0.105 0.27 0.49 0.00 0.625 0.324 0.657 0.000 3 Diethyl ether 4.20 1.3524 0.117 0.24 0.47 0.00 0.617 0.385 0.562 0.000 4 Chloroform 4.81 1.4459 0.259 0.53 0.10 0.20 0.783 0.614 0.071 0.047 5 Ethyl acetate 6.02 1.3724 0.228 0.45 0.45 0.00 0.656 0.603 0.542 0.000 6 Methyl acetate 6.68 1.3614 0.253 0.60 0.42 0.00 0.645 0.637 0.527 0.000 7 Tetrahydrofuran 7.58 1.4072 0.207 0.55 0.55 0.00 0.714 0.634 0.591 0.000 8 Cyclohexanone 15.00 1.4648 0.281 0.68 0.53 0.00 0.760 0.745 0.482 0.000 9 Butan-2-ol 16.56 1.3971 0.506 0.40 0.80 0.69 0.656 0.706 0.888 0.221 10 Butan-1-ol 17.51 1.3993 0.586 0.47 0.84 0.84 0.674 0.655 0.809 0.341 11 Propan-2-ol 19.92 1.3772 0.546 0.48 0.84 0.76 0.633 0.808 0.830 0.283 12 Propan-1-ol 20.45 1.3856 0.617 0.52 0.90 0.84 0.658 0.748 0.782 0.367 13 Acetone 20.56 1.3587 0.355 0.62 0.48 0.08 0.651 0.907 0.475 0.000 14 Ethanol 24.55 1.3614 0.654 0.54 0.75 0.86 0.633 0.783 0.658 0.400 15 Ethylene glycol 31.69 1.4475 0.790 0.92 0.52 0.90 0.777 0.910 0.534 0.717 16 Methanol 32.66 1.3284 0.762 0.60 0.66 0.98 0.608 0.904 0.545 0.605 17 Acetonitrile 35.94 1.3441 0.460 0.66 0.40 0.19 0.645 0.974 0.286 0.044 18 N,N-Dimethylformamide 36.71 1.4305 0.386 0.88 0.69 0.00 0.759 0.977 0.613 0.031 19 N,N-Dimethylacetamide 37.78 1.4384 0.377 0.88 0.76 0.00 0.763 0.987 0.650 0.028 20 Dimethyl sulfoxide 46.45 1.4793 0.444 1.00 0.76 0.00 0.830 1.000 0.647 0.072 92 4.2.3. Absorption maxima of hydrazone form as a function of dispersive interaction In order to explain the solvatochromic behavior of azo pyridone dyes, their spectral properties are correlated with different solvent polarity scales. The dispersive interaction function, f(n) gives an indication of the atomic and electronic polarization part of the intermolecular interaction occurring between solutes dissolved in solutions. The dispersive function is given by the relation: 1n 1n f(n) 2 2    (4.4) where n is refractive index of solvent. The correlation between υmax of the studied compounds and the solvent dispersive function, f(n), proves that dispersion forces can strongly influence the position of the azo pyridone dyes visible absorption band. Instead, in this case for all studied samples the plots υmax as a function of n or f(n) deviate from linearity for all used solvents. The linear relations are obtained for two isolated solvent classes (Table 4.9). The plot of υmax for all studied dyes versus relative permittivity gives correlation which shows deviation from linearity in all investigated solvents indicating that the relative permittivity is not the sole parameter governing the solvent shift (data not shown). The nonlinear character of υmax as a function of f(n) show that specific solvent effects have an important influence on solvatochromism and is examined. 93 Table 4.9. The results of the correlation between υmax and the solvent disperzive function f(n). Dyes υmax vs. f(n) Solvent excluded from the calculation a A13 I. υmax = -4.04 f(n) + 24.50 (R = 0.935, n = 9) II. υmax = -5.09 f(n) + 24.61 (R = 0.920, n = 11) A14 I. υmax = -3.50 f(n) + 24.31 (R = 0.976, n = 6) II. υmax = -6.50 f(n) + 25.06 (R = 0.969, n = 10) (1, 16, 18, 19) A15 I. υmax = -4.70 f(n) + 24.61 (R = 0.906, n = 6) II. υmax = -8.14 f(n) + 25.49 (R = 0.923, n = 11) (16, 17, 18) A16 I. υmax = -3.41 f(n) + 24.11 (R = 0.955, n = 7) II. υmax = -5.83 f(n) + 24.62 (R = 0.807, n = 10) (8, 16, 18) A17 I. υmax = -5.35 f(n) + 24.55 (R = 0.919, n = 13) II. υmax = -6.60 f(n) + 24.71 (R = 0.932, n = 7) A18 I. υmax = -8.35 f(n) + 24.19 (R = 0.943, n = 12) II. υmax = -5.33 f(n) + 22.77 (R = 0.923, n = 5) (1, 4, 8) A19 I. υmax = -5.56 f(n) + 24.73 (R = 0.954, n = 8) II. υmax = -7.78 f(n) + 25.22 (R = 0.954, n = 9) (16, 17, 18) A20 I. υmax = -10.26 f(n) + 25.67 (R = 0.921, n = 11) II. υmax = -12.23 f(n) + 25.80 (R = 0.955, n = 8) (11) A21 I. υmax = -5.09 f(n) + 24.73 (R = 0.916, n = 8) II. υmax = -3.35 f(n) + 23.92 (R = 0.916, n = 9) (1, 15, 18) A22 I. υmax = -4.12 f(n) + 24.78 (R = 0.940, n = 7) II. υmax = -3.52 f(n) + 24.38 (R = 0.921, n = 11) (13, 14) A23 I. υmax = -7.70 f(n) + 25.72 (R = 0.905, n = 15) II. υmax = -8.19 f(n) + 25.52 (R = 0.999, n = 5) a Solvent number as given in Table 4.8. 94 4.2.4. Variation of absorption maxima with ET N The empirical solvent polarity index, ET N is also used to study the solvent-solute interaction influence on electronic transitions for the all studied azo pyridone dyes. The plot of the absorption maxima of the dyes against the solvent empirical polarity scale gives nonlinear correlation for all used solvents. The linear relations are obtained for two separated solvent classes, each with good linearity but different slopes (Fig. 4.7). The deviation from linear correlation between υmax and ET N values can be explained by taking into account that ET N represent a dipolarity / polarizability and acidity contributions. These results indicated that basicity contributions of solvent play an important role on absorption spectra of the studied dyes and should be included in correlation. Figure 4.7. The correlation of υmax of the dye A18 with ET N (excluded ethanol). 95 4.2.5. Correlation with multiparameter solvent polarity scales The effect of solvent dipolarity / polarizability and hydrogen bonding on the absorption spectra are interpreted by means of linear solvation energy relationship (LSER) using a Kamlet–Taft solvatochromic equation 4.1 (Kamlet et al., 1981) of the following form: ν = ν0 + sπ* + bβ + aα (4.1) The solvent parameters are given in Table 4.3. The correlations of the absorption frequencies νmax for hydrazone tautomer were carried out by means of multiple linear regression analysis. The results of the multiple regressions are presented in Tables 4.10 and 4.11, and coefficients ν0, s, b and a (Table 10) fit at the 95% confidence level. 96 Table 4.10. Regression fits to the solvatochromic parameters (Eq. 4.1). No. Substituent ν0·10 -3 (cm -1 ) s·10 -3 (cm -1 ) b·10 -3 (cm -1 ) a·10 -3 (cm -1 ) R a s b F c n d A13 H 23.30 (±0.064) -0.32 (±0,063) 0.21 (±0.100) -0.40 (±0.046) 0.9787 0.041 61 12 A14 F 23.30 (±0.063) -0.41 (±0.065) 0.22 (±0.105) -0.31 (±0.048) 0.9654 0.047 37 12 A15 Cl 23.41 (±0.061) -0.47 (±0.060) -0.20 (±0.088) -0.15 (±0.036) 0.9593 0.042 38 14 A16 Br 23.05 (±0.060) -0.36 (±0.062) 0.27 (±0.101) -0.34 (±0.045) 0.9505 0.050 31 14 A17 I 22.87 (±0.074) -0.69 (±0.077) 0.58 (±0.124) -0.16 (±0.053) 0.9643 0.055 31 11 A18 OH 22.22 (±0.127) -0.39 (±0.125) -0.78 (±0.198) -0.35 (±0.092) 0.9728 0.081 47 12 A19 CH3 23.34 (±0.073) -0.51 (±0.077) -0.24 (±0.101) -0.28 (±0.046) 0.9714 0.048 33 10 A20 OCH3 22.39 (±0.084) -0.69 (±0.111) 0.28 (±0.131) 0.37 (±0.050) 0.9670 0.065 48 14 A21 COCH3 23.39 (±0.045) -0.72 (±0.158) / e -0.16 (±0.040) 0.9605 0.057 65 14 A22 CN 23.85 (±0.064) -0.55 (±0.064) -0.23 (±0.104) -0.12 (±0.045) 0.9587 0.053 38 14 A23 NO2 23.93 (±0.065) -1.12 (±0.084) 0.13 (±0.046) -0.17 (±0.065) 0.9683 0.070 60 16 a Correlation coefficient. b Standard error of the estimate. c Fisher's test. d Number of solvents included in correlation. e Negligible value with high standard error. 97 Table 4.11. Percentage contribution of solvatochromic parameters (Eq. 4.1). No. Substituent Pπ* (%) Pβ (%) Pα (%) A13 H 34.4 22.6 43.0 A14 F 43.6 23.4 33.0 A15 Cl 57.3 24.4 18.3 A16 Br 37.1 27.8 35.1 A17 I 48.2 40.6 11.2 A18 OH 25.7 51.3 23.0 A19 CH3 49.5 23.3 27.2 A20 OCH3 51.4 20.9 27.7 A21 COCH3 81.8 0.0 18.2 A22 CN 61.1 25.6 13.3 A23 NO2 78.9 9.2 11.9 The influence of solvent characteristics on the shift of υmax is additionally analyzed using the linear solvation energy relationship (LSER) model of Catalan [157], given by Eq. 4.5: ν = ν0 + aSA + bSB + cSP + dSdP (4.5) where SA, SB, SP and SdP characterize solvent acidity, basicity, polarizability and dipolarity of a solvent, respectively; and a–d are the regression coefficients describing the sensitivity of the absorption maxima to the different types of the solvent–solute interactions. The advantage of this concept in regard to Kamlet–Taft solvatochromic model is that it gives possibility to separate non-specific solvent effects into two terms: dipolarity and polarizability. The results of the multiple regressions are presented in Tables 4.12 and 4.13. 98 Table 4.12. Regression fits to solvatochromic parameters (Eq. 4.5). No. Substi- tuent ν0·10 -3 (cm -1 ) d·10 -3 (cm -1 ) c·10 -3 (cm -1 ) b·10 -3 (cm -1 ) a·10 -3 (cm -1 ) R a s b F c n d A13 H 24.03 (±0.132) -0.11 (±0.038) -0.95 (±0.169) -0.16 (±0.070) -0.53 (±0.041) 0.9936 0.024 136 12 A14 F 23.66 (±0.175) 0.22 (±0.094) -1.31 (±0.263) 0.51 (±0.096) -0.57 (±0.0.074) 0.9542 0.059 25 15 A15 Cl 23.92 (±0.105) -0.29 (±0.074) -0.83 (±0.158) -0.20 (±0.063) -0.29 (±0.047) 0.9776 0.033 48 14 A16 Br 23.70 (±0.159) 0.15 (±0.069) -1.47 (±0.215) 0.44 (±0.094) -0.56 (±0.067) 0.9674 0.054 33 14 A17 I 24.35 (±0.167) -0.25 (±0.081) -2.25 (±0.225) 0.40 (±0.114) -0.25 (±0.067) 0.9664 0.055 35 15 A18 OH 23.69 (±0.418) -0.27 (±0.111) -1.61 (±0.525) -1.46 (±0.216) -0.46 (±0.125) 0.9814 0.071 46 12 A19 CH3 23.97 (±0.211) -0.31 (±0.103) -1.72 (±0.268) 0.64 (±0.110) -0.25 (±0.083) 0.9580 0.067 25 14 A20 OCH3 23.51 (±0.260) 0.29 (±0.140) -2.81 (±0.382) 0.58 (±0.122) 0.45 (±0.111) 0.9638 0.084 36 16 A21 COCH3 24.09 (±0.165) -0.46 (±0.080) -1.11 (±0.264) / e -0.32 (±0.079) 0.9633 0.059 47 15 A22 CN 24.61 (±0.159) -0.28 (±0.082) -1.30 (±0.252) -0.21 (±0.104) -0.25 (±0.070) 0.9684 0.049 34 14 A23 NO2 25.02 (±0.147) -0.24 (±0.065) -2.50 (±0.197) 0.23 (±0.078) -0.21 (±0.062) 0.9792 0.051 70 17 a Correlation coefficient. b Standard error of the estimate. c Fisher's test. d Number of solvents included in correlation. e Negligible value with high standard error. 99 Table 4.13. Percentage contribution of solvatochromic parameters (Eq. 4.5). No. Substituent PSdP (%) PSP (%) PB (%) PA (%) A13 H 6.3 54.3 9.1 30.3 A14 F 8.4 50.2 19.5 21.9 A15 Cl 18.0 51.6 12.4 18.0 A16 Br 5.7 56.1 16.9 21.3 A17 I 7.9 71.4 12.8 7.9 A18 OH 7.1 42.4 38.4 12.1 A19 CH3 10.6 58.9 21.9 8.6 A20 OCH3 7.0 68.0 14.0 11.0 A21 COCH3 24.3 58.7 0.0 17.0 A22 CN 13.7 63.7 10.3 12.3 A23 NO2 7.5 78.6 7.2 6.7 It was found that absorption frequencies of hydrazone form for azo dyes in selected solvents show satisfactory correlation with π*, β and α as well as with SdP, SP, SB and SA parameters. However, the multiple regression analysis of the νmax data using Kamlet-Taft model in which non-specific solvent effects are included in single parameter π*, leads to a smaller correlation quality (R) and / or smaller number of solvents (n) which are included in correlations. The advantage of Catalan solvatochromic model stems from separation of non-specific interaction on polarity and polarizability solvent effects. As it can see from Tables 4.11 and 4.13 the solvent polarizability is the main factor that influences on the spectral shifts of all investigated compounds. From the analysis of absorption frequencies according to Kamlet-Taft equation 4.1 it was found that the negative sign of a coefficient (excluding CH3O substituent) and s coefficient for all arylazo dyes (Table 4.10) indicate a bathochromic shifts with both increasing solvent hydrogen-bond acidity and solvent polarity. This suggests stabilization of the electron excited state relative to the ground state. The positive sign of b coefficient for all investigated dyes (excluding A15, A18, A19 and A22) indicate a hypsochromic shifts with increasing of solvent hydrogen-bond acceptor basicity which suggests stabilization of the ground state relative to the electronic excited state. 100 The percentage contribution of solvatochromic parameters (Table 4.11) examined separately, for all azo dyes showed (excluding A13 and A18) that the most of the solvatochromism is due to solvent dipolarity / polarizability rather than on the hydrogen- bond acidity or basicity. The percentage contribution of solvatochromic parameters for A18 where hydrogen-bond basicity is the most important parameter can be explained by the effect of the positive charge on oxygen atom (from hydroxyl group in arylazo component) in the hydrazone tautomer (Scheme 4.2, Structure D) and stabilization of this form rather due to the solvent hydrogen bond acceptor basicity than through hydrogen bond donating acidity and solvent polarity. Moreover, the multiparameter regression analysis according to Catalan equation 4.5 showed negative sign of a coefficient (excluding CH3O substituent) and c coefficient for all arylazo dyes (Table 4.12) indicate a bathochromic shifts with increasing of solvent hydrogen-bond acidity and solvent polarizability. The positive sign of b coefficient for all investigated dyes (excluding A13, A15, A19 and A22) indicate a hypsochromic shifts with increasing solvent hydrogen-bond acceptor basicity. The negative sign of d coefficient (excluding A14, A16 and A20) indicate a bathochromic shifts with increasing solvent dipolarity. The percentage contribution of solvatochromic parameters (Table 4.13) for all azo dyes showed that solvent polarizability is the most important parameter which influences the absorption frequencies shifts. Solvent hydrogen-bond acidity and basicity have a moderate influence on solvatochromism, whereby the effect of solvent acidity has a more significant impact compared with the solvent basicity. Solvent dipolarity (Table 4.13) has negligible impact on solvatochromism (excluding A21). All the results obtained using Kamlet-Taft and Catalan model indicate that the solvent effects on azo-hydrazone equilibrium and UV-vis absorption spectra of the investigated azo pyridone dyes are very complex and strongly dependable on the nature of the substituent on the arylazo component. This also indicated that the electronic behavior of the nitrogen atoms of hydrazone group is somewhat different between derivatives with electron-donating and electron-accepting substituents (Scheme 4.2, Structures C and D). In all cases, the use of Catalan scale gives better regressions than the Kamlet-Taft model. The advantage of Catalan model derived from the division of non- specific solvent characteristics on solvent polarizability and dipolarity which allows 101 better insight into influence of these interactions on solvatochromism and as shown in Tables 4.11 and 4.13 non-specific solvent interactions have a major impact on the absorption maxima shifts. The degree of success of Eq. 4.1 and Eq. 4.5 are shown in Fig. 4.8 for all investigated compounds, and excellent linear relationships between the experimental values of νmax and the predicted absorption maxima calculated with Eq. 4.1 and Eq. 4.5 are observed. 102 Figure 4.8. Experimental versus calculated values of νmax from Eq. 4.1 (A) and Eq. 4.5 (B). 4.2.6. Substituents effects on the UV-vis absorption spectra As seen in Table 4.7, the absorption spectra of A15–A20 dyes were shifted batochromically in all used solvents when compared to dye A13. The absorption spectra of the dyes with electron-accepting substituents were generaly shifted hypsochromically in all used solvents (excluding dipolar aprotic solvents) when compared to dye A13. It is well known that the absorption maxima of the hydrazone tautomeric form of an azo dyes shift to higher wavelenghts when substituents with electron donating characteristics are introduced into the ring of the diazo component. In contrast, electron-accepting substituents generaly produce hypsochromic shift. The results presented in Table 4.7 are in agreement with this conclusion and observed relationship strongly suggests that the absorption maxima of the lower energy band of investigated azo dyes originates from hydrazone tautomeric form (Scheme. 4.2, B). The linear free energy relationship (LFER) methodology was applied to the υmax of the studied arylazo pyridone dyes with the aim to get an insight into factors 103 determining the absorption maxima shifts. The transmission of electronic substituent effects was studied using the Hammett Equation, Eq. 4.2: s = ρ·σ + h (4.2) where s is a substituent-dependent value: absorption frequencies (υmax), ρ is the proportionality constant reflecting the sensitivity of the υmax to the substituent effects, σ is the corresponding substituent constant (measure the electronic effect of the substituents), and h is the intercept (i.e., describes the unsubstituted member of the series). The plot υmax vs. the ζp and ζp+ substituent constants gave a correlation which showed deviations from the Hammett Equation in all dipolar aprotic solvents. However, a linear Hammett correlation was obtained in protic and non-dipolar aprotic solvents (excluding A14 and A19) (Fig. 4.8). A better correlation of υmax was obtained with the ζp+ substituent constants than with the ζp constants in the solvents used with exception of dipolar aprotic solvents, which indicates extensive delocalization in the azo group (– N=N–). The existence of these correlations was interpreted as an evidence of significant effect of substituent on azo-hydrazone tautomerism. The azo group (–N=N–) is an electron-acceptor group and the imino group (–NH–) is an electron-donor, so that azo group is stabilized by the more electron-donating substituents, while an electron- accepting group stabilizes the hydrazone form. Satisfactory linear dependence with positive slope presented in Fig. 4.9 and Eq. 4.6 confirms the presence of a hydrazone form in observed solvent. 104 Figure 4.9. Relationship between υmax and ζp+ for arylazo pyridone dyes in ethanol (excluding A14 and A19). νmax = 0.776 σp+ + 22.794 (4.6) (r = 0.910, s = 0.22, F = 33, n = 9) 4.3. Solvent and structural effects on the UV-vis absorption spectra of 5-arylazo-6- -hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes In the third part of this thesis ten new 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3- -cyano-2-pyridone dyes (Scheme 4.3) have been synthesized and their solvatochromic properties have been studied in a set of twenty solvents of different properties. In order to describe the spectral changes and the solute - solvent interactions multiparameter Kamlet- Taft and Catalan solvent scales were used. The effects of the solvent and substituent on the azo - hydrazone tautomeric equilibrium were studied and evaluated. 105 N N N O H CN OH NO 2 R N N N O H CN H O NO 2 R A B Scheme 4.3. The equilibrium between azo form (A) and hydrazone form (B) of 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes: R = H (A24), 4-F (A25), 3-Cl (A26), 4-Br (A27), 4-I (A28), 4-CN (A29), 4-COCH3 (A30), 4-CH3 (A31), 4-NO2 (A32), 4-OH (A33). 4.3.1. Spectral characteristics and solvatochromism The infrared spectra of all the synthesized dyes showed two intense carbonyl bands at about 1643 and 1702 cm -1 , which were assigned to the diketohydrazone form. The FT-IR spectra also showed a band at 3045 – 3268 cm-1, assigned to the imino group (N–H) of the heterocyclic (pyridine) ring and a band at 3432–3458 cm-1 that was assigned to the N–H of hydrazone tautomeric form. The 1 HNMR spectra of the dyes exibited a broad signal near 14.38–14.69 ppm. This signal corresponds to the imine N–H proton resonance of the hydrazone form (Scheme 4.3, Structure B). The UV-vis absorption spectra of 5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3- cyano-2-pyridone dyes (A24–A33) were measured at room temperature in twenty solvents in the range 300–700 nm and the characteristic spectra in representative solvents are shown in Figure 4.10 (ethanol, ethyl acetate, dimethylformamide). 106 107 Figure 4.10. UV-vis absorption spectra of azo dyes in different solvents (ethanol, ethyl acetate, dimethylformamide). The absorption maxima, which correspond to a transition in which electron density is transfered from the hydrazone –NH group to the pyridone carbonyl group (lower energy band) are presented in Table 4.14. It was observed that, although slightly positive solvatochromism is evident, the absorption spectra of dyes A24–A33 did not change significantly in all the employed solvents and the absorption maxima did not correlate with the polarity of the solvent. 108 Table 4.14. The absorption frequencies of the investigated compounds (A24–A33) in selected solvents. Solvent / No. υmax × 10 -3 (cm -1 ) A24 A25 A26 A27 A28 A29 A30 A31 A32 A33 Methanol 23.20 22.88 23.47 22.73 22.68 23.36 22.94 22.27 23.09 21.01 Ethanol 24.10 23.09 24.51 22.57 22.83 22.94 23.09 22.22 23.53 20.88 Propan-1-ol 23.81 23.26 24.21 22.94 24.33 23.58 22.68 25.00 23.09 20.75 Propan-2-ol 23.42 22.94 23.53 22.57 22.52 22.73 23.15 22.22 23.36 20.70 Butan-1-ol 23.04 22.62 23.26 22.52 22.42 22.78 22.88 22.17 23.26 20.66 2-Methylpropan-1-ol 22.88 22.68 23.09 22.57 22.42 23.09 22.78 22.17 23.26 20.70 2-Methylpropan-2-ol 22.94 22.68 23.09 22.62 22.37 22.73 22.88 22.12 23.26 20.66 Acetonitrile 23.04 22.83 23.20 22.73 22.52 23.26 22.83 22.32 23.04 21.37 Acetone 22.94 22.88 23.31 22.78 22.57 23.36 22.83 22.37 23.09 21.28 1.2-Ethanediol 22.88 22.57 23.81 22.68 22.42 22.57 22.78 21.98 22.94 20.83 Dichloromethane 22.57 22.42 22.83 22.22 22.03 22.88 22.47 21.93 22.83 21.23 1.4-Dioxane 22.68 22.62 22.94 22.42 22.22 23.15 22.62 21.98 22.94 20.75 Tetrahydrofuran 22.99 22.83 24.39 22.73 22.57 23.26 22.83 22.37 23.09 20.66 Diisopropyl ether 22.62 22.99 23.42 22.83 22.68 23.31 23.04 22.52 24.04 21.23 Cyclohexanone 22.83 22.68 23.04 22.62 22.47 23.15 22.73 22.27 22.99 21.05 Ethyl acetate 23.09 22.88 23.36 22.78 22.57 23.36 22.83 22.42 23.20 21.32 Methyl acetate 23.04 22.91 23.31 22.75 22.57 23.39 22.88 22.42 23.20 21.34 Dimethyl sulfoxide 22.70 22.65 23.34 22.45 22.37 22.91 22.57 22.08 22.57 20.77 N,N-Dimethylformamide 25.25 25.19 24.57 24.81 24.88 25.00 23.58 22.03 22.32 21.10 N,N-Dimethylacetamide 23.39 22.88 23.26 22.57 22.60 23.15 22.78 22.25 22.86 20.88 109 The effect of solvent dipolarity / polarizability and hydrogen bonding on the absorption spectra are intrepreted by means of LSER using Kamlet-Taft solvatochromic equation 4.1. The solvent parameters are given in Table 4.3. The correlations of the absorption frequencies υmax for hydrazone tautomer were carried out by means of multiple linear regression analysis. The results are presented in Tables 4.15 and 4.16 and coefficients υ0. s. b. a fit (Tab 4.15) at the 95% confidence level. Table 4.15. Regression fits to the solvatochromic parameters (Eq. 4.1). No. Substituent ν0·10 -3 (cm -1 ) s·10 -3 (cm -1 ) b·10 -3 (cm -1 ) a·10 -3 (cm -1 ) R a s b F c n d A24 H 19.95 (±0.452) 3.19 (±0.593) 3.12 (±0.545) -0.89 (±0.309) 0.928 0.300 19 13 A25 F 20.20 (±0.463) 2.45 (±0.536) 3.63 (±0.569) -1.83 (±0.320) 0.920 0.312 16 13 A26 Cl 23.06 (±0.180) -0.60 (±0.23) 1.01 (±0.243) 0.89 (±0.139) 0.957 0.160 33 13 A27 Br 19.40 (±0.445) 4.03 (±0.588) 2.37 (±0.387) -0.93 (±0.229) 0.932 0.240 24 15 A28 I 18.90 (±0.552) 4.26 (±0.757) 2.86 (±0.542) -0.93 (±0.305) 0.929 0.295 19 13 A29 CN 21.39 (±0.368) 1.86 (±0.423) 2.38 (±0.446) -1.81 (±0.241) 0.928 0.247 21 14 A30 COCH3 22.99 (±0.085) -0.69 (±0.103) 0.45 (±0.112) 0.14 (±0.054) 0.939 0.072 25 14 A31 CH3 22.89 (±0.075) -0.61 (±0.076) -0.37 (±0.112) -0.10 (±0.043) 0.950 0.054 34 15 A32 NO2 24.88 (±0.137) -2.64 (±0.171) -0.33 (±0.147) -0.04 (±0.083) 0.982 0.082 81 13 A33 OH 21.90 (±0.116) -0.23 (±0.116) -1.08 (±0.151) -0.21 (±0.073) 0.953 0.091 39 16 a Correlation coefficient. b Standard error of the estimate. c Fisher's test. d Number of solvents included in correlation. 110 Table 4.16. Percentage contribution of solvatochromic parameters (Eq. 4.1). No. Substituent Pπ* (%) Pβ (%) Pα (%) A24 H 44.31 43.33 12.36 A25 F 30.97 45.89 23.14 A26 Cl 24.00 40.40 35.60 A27 Br 54.98 32.33 12.69 A28 I 52.92 35.53 11.55 A29 CN 30.74 39.34 29.92 A30 COCH3 53.91 35.16 10.94 A31 CH3 56.48 34.26 9.26 A32 NO2 87.71 10.96 1.33 A33 OH 15.13 71.05 13.82 The influence of solvent characteristics on the shift of υmax was additionally analyzed using the linear solvation energy relationship model of Catalan given by Eq. 4.5. The results of the of multiple linear regression analysis are presented in Tables 4.17 and 4.18. 111 Table 4.17. Regression fits to solvatochromic parameters (Eq. 4.5). No. Substituent ν0·10 -3 (cm -1 ) d·10 -3 (cm -1 ) c·10 -3 (cm -1 ) b·10 -3 (cm -1 ) a·10 -3 (cm -1 ) R a s b F c n d A24 H 24.69 (±0.406) 0.02 (±0.177) -2.99 (±0.545) 0.59 (±0.242) 1.99 (±0.293) 0.985 0.105 49 11 A25 F 23.69 (±0.218) 0.195 (±0.100) -2.13 (±0.296) 0.94 (±0.133) -0.17 (±0.089) 0.957 0.072 28 15 A26 Cl 23.98 (±0.335) -0.29 (±0.137) -2.27 (±0.467) 1.23 (±0.185) 1.06 (±0.147) 0.978 0.097 39 12 A27 Br 23.87 (±0.217) 0.25 (±0.091) -2.40 (±0.314) 0.59 (±0.125) -1.04 (±0.162) 0.949 0.064 21 14 A28 I 23.19 (±0.196) 0.28 (±0.090) -1.98 (±0.268) 0.97 (±0.131) -0.05 (±0.081) 0.963 0.065 29 14 A29 CN 24.33 (±0.341) -0.05 (±0.143) -1.10 (±0.460) -0.54 (±0.218) -0.88 (±0.150) 0.944 0.104 19 14 A30 COCH3 23.75 (±0.187) -0.04 (±0.109) -1.75 (±0.266) 0.58 (±0.111) 0.10 (±0.078) 0.960 0.062 27 14 A31 CH3 23.59 (±0.158) -0.40 (±0.096) -1.12 (±0.234) -0.37 (±0.109) -0.22 (±0.066) 0.959 0.051 29 15 A32 NO2 27.12 (±0.667) -1.47 (±0.281) -4.14 (±0.922) -0.12 (±0.269) -0.11 (±0.289) 0.949 0.145 18 13 A33 OH 22.89 (±0.197) -0.36 (±0.119) -1.07 (±0.292) -1.35 (±0.111) -0.33 (±0.079) 0.977 0.065 59 16 a Correlation coefficient. b Standard error of the estimate. c Fisher's test. d Number of solvents included in correlation. 112 Table 4.18. Percentage contribution of solvatochromic parameters (Eq. 4.5). No. Substituent PSdP (%) PSP (%) PB (%) PA (%) A24 H 0.36 53.49 10.55 35.6 A25 F 5.68 62.01 27.37 4.95 A26 Cl 5.98 46.8 25.36 21.86 A27 Br 5.84 56.07 13.79 24.3 A28 I 8.54 60.37 29.57 1.52 A29 CN 2.01 44.18 21.69 32.13 A30 COCH3 1.62 70.85 23.48 4.05 A31 CH3 18.96 53.08 17.54 10.43 A32 NO2 25.17 70.89 2.05 1.88 A33 OH 11.58 34.41 43.41 10.61 All the results obtained using Kamlet-Taft and Catalan model indicate that the solvent effects on azo–hydrazone equlibrium and UV-vis absorption spectra of the investigated arylazo pyridone dyes are very complex and strongly dependable on the nature of the substituent on the arylazo component. In all cases. the use of Catalan scale gives better regressions than the Kamlet-Taft model. The degree of success of of Eq. 4.1 and Eq. 4.5 are shown in Fig. 4.11 for all investigated compounds. and excellent linear relationships between the experimental values of υmax and the predicted absorption maxima calculated with Eq. 4.1 and Eq. 4.5 are observed. The Catalan solvent scales were found to be more suitable for describing the solvatochromism of investigated arylazo pyridone dyes. 113 Figure 4.11. The plot of υmax calculated against υmax observed for Kamlet-Taft (I) and Catalan (II) equation in different solvents. (I) (II) 114 The obtained results show that arylazo dyes with nitro group on the benzene ring in position 4 of pyridone nucleus have stronger batochromic shifts than other two series of azo dyes. in all used solvents. The percentage of contribution of solvatochromic parameters for all investigated dyes shows that most of the solvatochromism is due to solvent dipolarity / polarizability rather than to the solvent acidity and basicity. These results can be explained by the effect of the positive charge on nitrogen atom in the hydrazone tautomer and stabilization of this form mostly due to the solvent dipolarity / polarizability (non-specific solute-solvent interactions) than due to hydrogen bond donor and hydrogen bond acceptor properties. 115 5. CONCLUSIONS In this thesis three series of 33 novel arylazo pyridone dyes were synthesized:  5-arylazo-6-hydroxy-4-phenyl-3-cyano-2-pyridone dyes;  5-arylazo-6-hydroxy-4-(4-methoxyphenyl)-3-cyano-2-pyridone dyes;  5-arylazo-6-hydroxy-4-(4-nitrophenyl)-3-cyano-2-pyridone dyes. The structure of the dyes was confirmed by UV-Vis, FT-IR, 1 H NMR and 13 C NMR spectroscopy end elemental analysis. Characterization and the absorption ability of the dyes were studied. The results showed that the solvent effect on UV-vis absorption spectra of the investigated arylazo pyridone dyes is very complex and strongly depends on the nature of the substituent on the arylazo component. The introduction of the electron-donating substituents into the arylazo ring results in strong batochromic shifts in all solvents. These solvatochromic properties are evident for the hydrazone tautomeric form. The introduction of electron-attracting substituents into the arylazo ring produces slight batochromic or hypsochromic shift. These dyes exist in the hydrazone form in the solid state and in solvent DMSO-d6 and there was an equilibrium between hydrazone form and azo form in the different solvents. The Kamlet-Taft and Catalan parameters were used for describing the solute- solvent interactions and solvatochromic shifts of the visible absorption band. The satisfactory correlation of the ultraviolet absorption frequencies of the investigated pyridone arylazo dyes with equations 4.1. and 4.5. indicates that the correct models were selected. This means that these models give a correct interpretation of the linear solvation energy relationships of the complex system of the azo dyes in the solvents used. In the case, where both solvents and substrates, are hydrogen bond donors and acceptors, it was proven to be quite difficult to untagle solvent dipolarity / polarizability and hydrogen bonding interactions. For these resons it has been demonstrated that solvatochromic equations 4.1. and 4.5. can be used to evaluate the effects of both types of hydrogen bonding and of solvent dipolarity and polarizability effects. It was found that the solute dipolarity / polarizability (especially polarizability by Catalan equation) play an important role in the description of the pronounced solvatochromism in the studied solutions. The Catalan solvent scales were found to be more siutable for describing the solvatochromic shifts. 116 On the basis of the results presented in this thesis, it may be concluded that all the synthetized dyes exist in the hydrazone tautomeric form in the solid state. and dyes were predominantly as hydrazones in all the applied solvents. The calculational results of the geometry data using DFT quantum-chemical calculations. were in very good agreement with the experimental data. The obtained results show that arylazo dyes with nitro group on the benzene ring in position 4 of pyridone nucleus have stronger batochromic shifts than other two series of azo dyes. in all used solvents. The percentage of contribution of solvatochromic parameters for all investigated dyes shows that most of the solvatochromism is due to solvent dipolarity / polarizability rather than to the solvent acidity and basicity. 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Ušćumlić, "Solvent and Structural Effects on the UV-Vis Absoprpiton spectra of Some 4,6-Disubstituted-3- cyano-2-pyridones" J. Sol. Chem. 41 (10) 2012, 1825-1836, ISSN: 0095-9782, IF (2011) = 1.415 Proceedings [1] B. Božić, A. Alimmari, D. Mijin, G. Ušćumlić, "Synthesis, Structure and Solvatochromic Properties of Novel Dyes Derived from 4-Nitrophenyl-6-hydroxy- 3-cyano-2-pyridone", 17 th ESOC, Hersonissos, Greece 2011, pp. 146. [2] A. Alimmari, B. Božić, D. Mijin, G. Ušćumlić, "Solvent and Strucutral Effects on the Azo-Hydrazone Tautomerism of 5-(4-Substituted Phenylazo)-4-(4- Methoxyphenyl)-6-hydroxy-3-cyano-2-pyridone", 17 th ESOC, Hersonissos, Greece 2011, pp. 149. [3] Adel S. A. Alimmari, Aleksandar D. Marinković, Dušan Ž. Mijin, Nataša V. Valentić and Gordana S. Ušćumlić, "Solvent and substituent effect on the UV-Vis absorption spectra of 5-(3- and 4-subsituted phenylazo)-4,6-diphenyl-3-cyano-2- pyridones", 47 th Meeting of the Serbian Chemical Society, Belgrade, Serbia, March 21 st 2009., Book of Abstracts pp.136. [4] Radovan M. Vukićević, Adel S. A. Alimmari, Dušan Ž. Mijin, Nataša V. Valentić and Gordana S. Ušćumlić, "Solvent and substituent effect on the UV-Vis absorption spectra of 5-(3- and 4-subsituted phenylazo)-4-phenyl-6-hydroxy-3-cyano-2- pyridones", 47 th Meeting of the Serbian Chemical Society, Belgrade, Serbia, March 21 st 2009., Book of Abstracts, pp. 137. 126 [5] Adel S. A. Alimmari, Aleksandar D. Marinković, Nenad Ž. Jovanović, Dušan Ž. Mijin, Nataša V. Valentić and Gordana S. Ušćumlić, "New azo dyes from 4-(4- metoxyphenyl)-6-hydroxy-3-cyano-2-pyridone", 48 th Meeting of the Serbian Chemical Society, Novi Sad, Serbia, April 17 th -18 th 2010., Book of Abstracts, pp. 150. [6] Bojan Đ. Božić, Aleksandar D. Marinković, Adel S. A. Alimmari, Jasmina S. Đukanović, Dušan Ž. Mijin and Gordana S. Ušćumlić, "Solvent and substituent effect on the UV-Vis absorption spectra of 4,6-disupstituted pyridones", 48 th Meeting of the Serbian Chemical Society, Novi Sad, Serbia, April 17 th -18 th 2010., Book of Abstracts pp,. 189. 127 Curriculum vitae Personal Information Name: Adel. S. A. ALIMMARI Place of birth: G KEAR/ LIBYA Date of birth: 10/09/ 1972 Address: Perve pruge 3/35 Belgrade 11080 Phone #: +381600912355 Education Bachelor degree in Chemistry faculty of Science Misurata University (1996) MSc degree in Organic Chemistry Faculty of Technology and Metallurgy University of Belgrade (2005) Professional Experience 1999-2006 The quality control department in liquid fuel plant in the Research center and Development Tripoli, Libya 2006-2008 Department of Chemistry, Faculty of Science University of Al Khums, Libya, Assistant professor 131 1. Autorstvo - Dozvoljavate umnožavanje, distribuciju i javno saopštavanje dela, i prerade, ako se navede ime autora na način određen od strane autora ili davaoca licence, čak i u komercijalne svrhe. Ovo je najslobodnija od svih licenci. 2. Autorstvo – nekomercijalno. Dozvoljavate umnožavanje, distribuciju i javno saopštavanje dela, i prerade, ako se navede ime autora na način određen od strane autora ili davaoca licence. Ova licenca ne dozvoljava komercijalnu upotrebu dela. 3. Autorstvo - nekomercijalno – bez prerade. Dozvoljavate umnožavanje, distribuciju i javno saopštavanje dela, bez promena, preoblikovanja ili upotrebe dela u svom delu, ako se navede ime autora na način određen od strane autora ili davaoca licence. Ova licenca ne dozvoljava komercijalnu upotrebu dela. U odnosu na sve ostale licence, ovom licencom se ograničava najveći obim prava korišćenja dela. 4. Autorstvo - nekomercijalno – deliti pod istim uslovima. Dozvoljavate umnožavanje, distribuciju i javno saopštavanje dela, i prerade, ako se navede ime autora na način određen od strane autora ili davaoca licence i ako se prerada distribuira pod istom ili sličnom licencom. Ova licenca ne dozvoljava komercijalnu upotrebu dela i prerada. 5. Autorstvo – bez prerade. Dozvoljavate umnožavanje, distribuciju i javno saopštavanje dela, bez promena, preoblikovanja ili upotrebe dela u svom delu, ako se navede ime autora na način određen od strane autora ili davaoca licence. Ova licenca dozvoljava komercijalnu upotrebu dela. 6. Autorstvo - deliti pod istim uslovima. Dozvoljavate umnožavanje, distribuciju i javno saopštavanje dela, i prerade, ako se navede ime autora na način određen od strane autora ili davaoca licence i ako se prerada distribuira pod istom ili sličnom licencom. Ova licenca dozvoljava komercijalnu upotrebu dela i prerada. Slična je softverskim licencama, odnosno licencama otvorenog koda.