Synthesis, Characterization, Absorption and Fastness Properties of Novel Monoazo Dyes Derived from 1-Phenyl-3-amino-4-(2-thiazolilazo)pyrazol-5-one

Monosubstitute thiazolyl amines were diazotized in acetic acid, coupled to 1-phenyl3-aminopyrazol-5-one (1a-1c) and acetylated to obtain the 2a-2c dyes. The dyes were characterized by elemental analysis and spectroscopic (Fourier transform infrared (FTIR), nuclear magnetic resonance (NMR), ultraviolet (UV)) methods. The effects on the visible absorption spectra of the substituents present, solvents, pH, concentration and temperature were investigated in detail. Tautomerism of the dyes were investigated by spectroscopic methods. Fastness properties of dyes were studied using the standard method for the assessment of color fastness of textile.


Introduction
Azo dyes are most widely used to dye polyester due to versatility. Among these azo dyes, heterocyclic moieties, e.g., pyrazolone, indole, pyrimidine, imidazole, pyridone, quinolone, coumarin, etc., provide higher tinctorial strength and brighter texture as compared to dyes based on phenylic components. [1][2][3][4][5] Hetarylazo dyes based on heterocyclic diazo and coupling components have been attracted the attention of scientists in recent years. [6][7][8][9][10][11][12] Among these, the hetarylazopyrazolone dyes have high fluorescence, high quantum yield, superior photostability in the visible region, excellent fastness properties, [13][14][15][16] tautomeric structures 6,7,10,17 in addition to their bathochromic effects in the absorption spectra. 6,7,10,17 The absorption and emission properties of the pyrazolones can be tuned according to the properties of the solvent, especially where electron-withdrawing and electron-donating substituents are attached at the 1, 3 and 4-positions of pyrazolone ring. These pyrazolones have shown numerous properties such as non-linear optical chromophores, optical brighteners, as well as being used in solar cells, 18,19 laser printing systems, laser optical recording systems. 12,20 Some of the pyrazolones were found as biologically active and acted as medicine. 21,22 2-Aminothiazoles are widely used in the dye industry as a coupling and diazo components in addition to their important pharmacological and biological activities. 23,24 Hetarylazo disperse dyes obtained from the coupling of heterocyclic diazo components have shown better rubbing, light and washing fastnesses properties than carbocyclic components. [25][26][27] Herein we report the synthesis and derivatization of 3-amino-1-phenyl-4-(2-thiazolilazo)pyrazol-5-one (dyes 1a-1c) and 3-acetamido-1-phenyl-4-(2-thiazolilazo) pyrazol-5-one (dyes 2a-2c) (Scheme 1). Synthesized dyes were characterized by elemental analysis, Fourier transform infrared (FTIR) and 1 H nuclear magnetic resonance (NMR) spectroscopies. The influence of substituent, solvents, pH, concentration, and temperature on their visible absorption spectra were also investigated. Tautomerism of the synthesized dyes were investigated using spectroscopic methods. Fastness properties of dyes were obtained by standard method that has been used for the color fastness of textiles. This study also contributes to the physical, spectral and tautomeric properties of hetarylazopyrazolones. In addition, the effect of oxochrome −NH 2 (dyes 1a-1c) and chromophor −NH−CO−CH 3 (dyes 2a-2c) substituents at the 3-position of pyrazolone ring, electron withdrawing −NO 2 and electron donating −CH 3 substituents on the thiazol ring were also investigated on the color fastness (washing, rubbing, light fastness and increase/diversify the color range) of the novel hetarylazopyrazolone dyes (Scheme 1).
Chemical Company (Darmstadt, Germany) and used without further purification. The solvents used were of highperformance liquid chromatography (HPLC) grade. FTIR spectra were determined on a KBr disc using a MATTSON 1000 spectrophotometer Fourier Transform-Infrared (FT-IR). 1

Dyeing method
Polyester fabric was dyed according to the carrier dyeing method in the laboratory. The carrier swells the polyester fibers, increasing interstitial space to accept more dye molecules into the polymer system.
The dyed fabric was taken out of the bath and thoroughly washed with cold then hot distilled water. Reduction cleaning was done with 1% soup solution at boiling temperature for 15 min to improve the wash fastness. The fabric was again rinsed with distilled water and dried in the air.

Assessment of fastness
The light, washing and rubbing fastness tests were carried out according to the ISO 105-B04, 29 ISO 105-C06 30 and ISO 105-X12 31 (staining of cotton rubbing fabric). The light fastness tests were determined using the international blue scale (1-8), the rubbing and washing fastness tests were determined using the international grey scale (1)(2)(3)(4)(5), where the maximum was ranked the best while least was the inferior. 32,33

Tautomerism
Tautomerism is important not only for chemical properties, but also for different properties as colors and fastness properties. For this reason, the possible tautomeric forms of examined dyes were evaluated in detail. The possible tautomeric forms of the dyes (T 1 -T 6 ) are shown in Scheme 3.
The synthesized new dyes may exist in five possible tautomeric forms, i.e., keto-azo (T 1 and T 5 ), keto-hydrazo (T 2 and T 4 ) and enol-azo (T 3 ). According to the calculation results in the literature, 17,35-40 the most stable tautomeric form is keto-hydrazo form (T 2 ) for hetarylazopyrazolone dyes. This conclusion may occur from the intramolecular O−H bond.

Solvent effects
The absorption spectra of the synthesized dyes were measured in a range of 10 −6 -10 −8 mol L −1 in various solvents. The dielectric constants of the solvents were found in the following order: chloroform (CHCI 3 ) > acetic acid > methanol > acetonitrile > DMF > DMSO. Experimental λ max values of dyes are listed in Table 3.
The absorption spectra of 1a, 1b, 2a and 2b showed only one λ max in all solvents. Additionally, 1a and 1b showed a shoulder at shorter wavelength in methanol, DMF and methanol, DMF, DMSO while 2a and 2b showed it at shorter wavelength in chloroform only. These shoulders indicate the interchange of tautomeric or anionic forms in these solvents. These results as well as 1 H NMR data suggests that 1a and 1b may be in keto-hydrazo (T 2 ) and anionic form (A 1 ) in methanol, DMSO and DMF while 2a and 2b may be in the keto-hydrazo (T 4 ) and keto-azo form (T 1 or T 5 ) in chloroform while in the remaining solvents, these dyes may exist in only one tautomeric or ionic form.
Dyes 1c and 2c showed two maxima in chloroform, methanol, acetonitrile, DMF and DMSO which may have arisen either from the mixture of tautomeric forms or a mixture of a tautomeric and anionic form. 7,35 1 H NMR data of these dyes suggest that 1c and 2c may be in the keto-hydrazo (T 2 ) and anionic form (A 1 ) or may be in the keto-hydrazo (T 4 ) and one of keto-azo forms (T 1 or T 5 ) in all solvents, except acetic acid. All the synthesized dyes have one maximum that may belong to the cationic form in acetic acid (one of C 1 -C 6 , Scheme 4). The absorption maximum of dye 1a reflected bathochromic shift in methanol, DMSO and DMF as compared to the acetic acid, chloroform and acetonitrile.
As an example, 1a shows a λ max at 388, 394 and 393 nm in acetic acid, chloroform and acetonitrile, respectively, those has shifted to 424, 434 and 437 nm in methanol, DMSO and DMF, respectively (Figure 2). 17 These results indicate that 1a may be either tautomerizing or ionizing in basic solvents such as methanol, DMSO and DMF. Dye 1b shifted bathochromically in chloroform (λ max : 444 nm) besides methanol (λ max : 439 nm), DMSO (λ max : 444 nm) and DMF (λ max : 443 nm). 8 The absorbance of  These results reflect that the absorption behaviors of the synthesized dyes are not related to the polarity of solvents but instead to the proton-donor and acceptor properties of solvents, 41 which reinforces the ionization rather than tautomerism. To confirm this thesis, the effect of acid and base was also investigated on the absorbance. Absorbances of the synthesized dye solutions in methanol (HCl and KOH) and chloroform (trifloroacetic acid (TFAA) and piperidine) are provided in Table 4.
The absorbance peaks of 1a-1c were more sensitive to acid in both chloroform and methanol. As an example, λ max of 1a shifted +55 and +23 nm in chloroform + TFAA and methanol + HCI, respectively, while it shifted +49 and +10 nm in chloroform + piperidine and methanol + KOH, respectively ( Figure 3). Absorbances of 1b and 1c changed insignificantly when basic solution was added to their solutions in both chloroform and methanol as compared to 1a. Dye 1b displayed slight bathochromic shifts in the λ max , i.e., +4 and  +1 nm in chloroform + piperidine and methanol + KOH, respectively. Additionally, 1a and 1b also showed a shoulder at shorter wavelength (at 380 and 390 nm) in all acidic-basic solutions. On the other hand, 1c shifted +35 and +54 nm in chloroform + TFAA and methanol + HCI while it showed a shoulder at 472 and 470 nm, respectively. However, λ max of the same dye did not change significantly upon addition of piperidine to its chloroform solution or addition of KOH to its methanol solution. These results are similar to the literaure for hetarylazopyrazolones, 6,7,17 hetarylazocalixarenes, 11 hetarylazopyridones, 3,42 hetarylazocoumarines, 9 hetarylazoquinolines 5 and hetarylazoindoles. 10 These results suggest that 1a-1c may be in a mixture of one tautomeric and cationic form in acidic solutions, while they may be in a mixture of one tautomeric and anionic form (A 1 ) in basic solutions. 8,10 Dyes 2a-2c showed similar hypsochromic shift (−23 nm for 2a, −18 nm for 2b, −20 nm for 2c) with shoulders at 395 and 420 nm in chloroform + TFAA and chloroform + piperidine, respectively. These values sugget that 2a-2c may be in a mixture of two tautomeric form in both chlorofom solutions. By contrast, the λ max of 2a-2c reflected slight bathochromic shift after the addition of HCI or KOH to their methanol solutions (+8, +6 nm for 2a, +13, +9 nm for 2b, +15, +10 nm for 2c in acidic and basic solutions). Dyes 2a-2c showed a shoulder at 400 nm in both acidic and basic methanol. Thus, these dyes may be in a mixture of tautomeric and cationic form in methanol + HCI, while they may be in a mixture of tautomeric and anionic form (A 1 ) in methanol + KOH. 8,10 Absorbances of all the examined dyes in acidic and basic solutions are listed in Table 4, while Table 5 contains the effects of dye concentration and temperature on the absorbance. Results showed that absorbances changed insignificantly with concentration and temperature.

Substituent effects
Absorbance maxima of 1b (−CH 3 on thiazole ring at 4-position) shifted bathochromically in acetic acid, methanol, acetonitrile, DMF and DMSO (6-17 nm) relative to 1a. 6,7,8,10,17 This slight change resulted from weak electron donating effect of −CH 3 . Highest effect of −CH 3 was observed in least polar solvent, i.e., chloroform (+50 nm). 17 On the other hand, 1c (−NO 2 group on thiazole ring at 5-position) reflected both hypsochromic and bathochromical shifts in all solvents except acetic acid. The strong electron-accepting substituent (−NO 2  on thiazole ring of diazo component) showed the highest bathochromical shift in acetonitrile (+141 nm); probably due to resonance stability of the aromatic system. 43 The λ max of 2b and 2c did not change in chloroform and acetic acid as compared to 2a. The λ max of 2b and 2c changed as follows: +3, +20 nm in methanol, +10, +10 nm in acetonitrile, +18, +8 nm in DMF, and +13, +3 nm in DMSO, respectively. Additionally, dye 2c showed a second peak at 315 or 324 nm in all solvents, except acetic acid. These data indicate that 2a-2c, which were obtained by the acetylation of 1a-1c, are less sensitive to effect of substituent on diazo component from 1a-1c. This conclusion may be related to the weak electrondonating ability of acetylamido group (−NH−CO−CH 3 ; chromophore group) than amino group (−NH 2 ; auxochrome group). Acetylation of amino group on coupling component was more effective on the absorbance of 1a-1c from −CH 3 substituent on diazo component. The largest change in the λ max was reflected by 2a-2c in acetic acid as compared to 1a-1c. The λ max values of 1a-1c in acetic acid are: 388, 401, and 402 nm, whereas 2a-2c absorbed at 480 nm in the same solvent. In other solvents, the absorbances of 2a and 2b shifted bathocromically in respect to their corresponding dyes 1a and 1b. The shoulders at 380, 390 (in methanol, DMF) (1a) and 390 nm (in methanol, DMF, DMSO) (1b) disappeared while new shoulders appeared at 313 (2a) and 315 nm (2b) in chloroform. On the other hand, λ max values of 2c showed larger hypsochromical shifts in all solvents as compared to their corresponding dye 1c.

Fastness properties of dyes
The colors of dyes on polyester fabric are shown in Figure 4.
Fastness tests results of polyester fabrics dyed with 1a-1c (3-aminopyrazolone derivatives) were generally higher than those of 2a-2c (3-acetamidopyrazolone derivatives). This may have occured due to the stronger binding of amino group (−NH 2 ) with polyester fibers as compared to the binding of acetamido group (−NH−CO−CH 3 ).

Conclusions
Two series of amino-and acetamido-based monoazo dyes were synthesized from 1-phenyl-3-aminopyrazol-5-on and forwarded to their characterization, UV absorption, tautomeric forms and fastness properties. Variation in the absorbance of the synthesized dyes in acidic and basic media showed that they were more sensitive towards acids. The dyes generally demonstrated bathocromic shifts in polar solvents. Nitro-substituted dye (1c) showed the highest bathochromic shift in DMSO and DMF. λ max values of the dyes 2a-2c either did not change or changed slightly while the dyes 1a-1c were more sensitive to effect of substituent. In adition, keto-hydrazone (T 2 ) tautomer of all dyes predominantly existed in both solid state and in solution. The colors of the dyes 2a-2c (−NH−CO−CH 3 ; chromophor) on polyester fabric were darker than colors of dyes 1a-1c (−NH 2 ; oxochrome). This may have occured due to the absorbances of 2a-2c were more bathocromic in respect to their corresponding dyes 1a-1c. Fastness values of the synthesized dyes on polyester fabric demonstrated that the binding of amino group (−NH 2 ) with polyester fibers is stronger than binding of acetilamido group (−NH−CO−CH 3 ). Fastness tests as a whole were satisfactory in comparision with the literature. We suggest to use these compounds in the dye/color industry and to explore their physicochemical and biological properties.

Supplementary Information
Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.