Disperse Dyes Based on 2 -N,N- Dibutylamino-4-phenyl thiazole: Synthesis,Solvatochromism,and Dyeing Studies

Abstract

The coupler 2-N,N-dibutylamino-4-phenyl thiazole was used in the preparation of five novel monoazo disperse dyes. Effects of substituents on the electronic absorption properties of the dyes were studied by UV-Vis spectroscopy. The resulting dyes showed little positive solvatochromism, whereas a bathochromic shift in absorption maxima was observed for all five dyes in highly polar solvents. These dyes showed good discharge ability on polyester fabric when compared to nylon fabric and their fastness properties were evaluated. Being low melting, the new dyes could be used for heat transfer printing at a particular set temperature.

Keywords

2-Aminothiazole Disperse Azo Dyes Heat Transfer Printing Solvatochromism

Introduction

Azo disperse dyes have been studied widely for use as coloring materials, as photo responsive molecular switches due to their photo isomerization,^1-3 materials for reversible optical information storage,⁴ and nonlinear optics.⁵ Ease of synthesis, low-cost efficient production, high color strength, and adjustable wide color range from yellow to blue-green are the advantages of azo disperse dyes.^6-10 Azo dyes derived from thiazole have been reported for polyester fiber with good washfastness.^10-13 Disperse dyes derived from 2-aminothiazoles have been studied, with the 2-amino-5-ni-trothiazole based blue dischargeable monoazo C.I. Disperse Blue 339 being commercially produced.^14-16 2-Aminothia-zoles are also known as intermediates used in various drug syntheses.^17-20 Thus, azo dyes obtained from 2-aminothiazole were investigated in recent years.^18,20-27

We have synthesized five novel heterocyclic azo disperse dyes derived from 2-N,N-dibutylamino-4-phenyl thiazole as a coupler with different para-substituted diazonium salts obtained by the diazotization of the corresponding aniline derivatives. The dyes were characterized by nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy, and the electronic absorption of these dyes in various microenvironment was studied. The dyes were applied on polyester and nylon fabrics and their fastness properties were studied. These dyes show good fastness properties on polyester dyes when compared to nylon. K/S values were also determined for all five disperse dyes. Being low melting dyes with moderate sublimation fastness, these dyes could be good candidates for heat transfer printing as small molecular weight molecules show better transferability onto polyester fabric from the intermediary paper used in heat transfer printing. Thus, our research continues the study of 2-aminothiazole based disperse dyes.

Experimental

Materials and Methods

Aniline, p-nitroaniline, p-chloroaniline, p-methoxyaniline, p-methylaniline, sodium nitrite, sodium hydroxide, Sara-gen SO (dispersing agent), Saragen KDF (leveling agent), and conc. HCl were purchased from S.D. Fine Chemicals Ltd. All reagents used were characterized by melting point or boiling point and used without purification. All solvents were used after distillation. The coupler was produced by a known sequence of reactions.^24,28-32 The synthesized dyes were purified by recrystallization and column chro-matography wherever necessary. The melting point of the synthesized dyes were measured on an instrument from Sunder Industrial Products. The UV-Vis spectra were recorded on Perkin-Elmer Lambda 25 UV-Vis spectro-photometer. An Agilent Technology NMR at 500 MHz and 125 MHz was used to record the ¹H NMR and ¹³C NMR spectra respectively using tetramethylsilane (TMS) as an internal standard. FTIR spectra were recorded on a Jasco 4100 Fourier Transform IR instrument (with attenuated total reflection (ATR) accessories).

Synthesis and Characterization

Dyes 1-5 were synthesized using the strategy shown in Scheme 1. All synthesized dyes were characterized by melting point, FTIR, ¹H NMR, ¹³C NMR, and CHN analysis.

Scheme 1

Synthesis of dyes 1-5.

Synthesis of (E)-N,N-Dibutyl-5-((4-nitrophenyl)diazenyl)-4-phenylthiazol-2-amine (1)

A solution of aryl diazonium chloride was prepared by adding cold (0-5 °C) sodium nitrite solution (7.37 mmol) to a cold (0-5 °C) suspension of p-nitroaniline (4.23 mmol) in 3 mL of conc. HCl with stirring and stirred for 30 min at 0-5 °C. The freshly prepared solution was added with continuous stirring to a cold solution (0-5 °C) of coupler 2-N,N-dibutylamino-4-phenyl thiazole (3.46 mmol) dissolved in 20 mL of 1,4-dioxane and stirred at 0-5 °C. The reaction mass was stirred for 2 h until a thick green solid was seen. The precipitated crude dye was filtered, washed, dried, and recrystallized from a 1:1 ethanol/water solution to give 1 (83%), mp 90-93 °C, ¹H NMR (CDCl₃, δ): 8.35-8.31 (m, 2H), 8.29-8.26 (m, 2H), 7.79-7.75 (m, 2H), 7.52-7.46 (m, 3H), 3.81 (s, 2H), 3.47 (s, 2H), 1.78-1.71 (m, 4H), 1.43 (dq, 4H), 1.00 (t, 6H). ¹³C NMR (CDCl₃, δ): 170.87 (s), 160.19 (s), 157.41 (s), 146.12 (s), 140.77 (s), 133.87 (s), 130.76 (s), 130.24 (s), 128.33 (s), 124.84 (s), 122.09 (s), 20.13 (s), 13.81 (s). Anal. Calcd for C₂₃H₂₇N₅O₂S (437.18 g/mol): C, 63.13; H, 6.22; N, 16.01. Found: C, 63.17; H, 6.21; N, 16.04.

Synthesis of (E)-N,N-Dibutyl-5-((4-chlorophenyl) diazenyl)-4-phenylthiazol-2-amine (2)

A diazonium chloride solution of p-chloroaniline (5.83 mmol) was obtained by adding cold (0-5 °C) sodium nitrite solution (7.98 mmol) to a cold (0-5 °C) suspension of p-chloroaniline in 3 mL of conc. HCl with stirring. Coupler 2-N,N-dibutylamino-4-phenyl thiazole (4.50 mmol) was dissolved in 1,4-dioxane and cooled at 0-5 °C. The diazonium salt solution was added dropwise to the coupler solution with constant stirring at 0-5 °C. After complete addition of diazonium salt, the reaction mixture was stirred for 4 h at 0-5 °C. The orange solid precipitate was filtered, washed with water, and recrystallized from 1:1 ethanol/water solution to give the bright orange color solid dye 2 (87%), mp 117-120 °C, ¹H NMR (CDCl₃, δ): 8.35 (d, 2H), 7.64 (d, 2H), 7.49-7.38 (m, 3H), 7.23 (d, 2H), 3.58 (s, 4H), 1.76-1.68 (m, 4H), 1.45-1.36 (m, 4H), 0.99 (t, 6H). ¹³C NMR (CDCl₃, δ): 168.84 (s), 154.91 (s), 151.21 (s), 140.43 (s), 138.72 (s), 134.52 (s), 130.42 (s), 129.64 (s), 129.11 (s), 128.13 (s), 122.11 (s), 29.53 (s), 21.38 (s), 20.16 (s), 13.89 (s). Anal. Calcd for C₂₃H₂₇ClN₄S (426.16 g/mol): C, 64.69; H, 6.37; N, 13.12. Found: C, 64.68; H, 6.38; N, 13.11.

Synthesis of (E)-N,N-Dibutyl-5-((4-methoxyphenyl) diazenyl)-4-phenylthiazol-2-amine (3)

A solution of p-methoxyaniline diazonium chloride was prepared by adding a cold (0-5 °C) sodium nitrite solution (8.27 mmol) to a cold (0-5 °C) suspension of p-methoxy-aniline (8.11 mmol) in 3 mL of conc. HCl with stirring. Simultaneously, the coupler 2-N,N-dibutylamino-4-phenyl thiazole (4.23 mmol) was dissolved in 1,4-dioxane and stirred until a clear solution was obtained. The diazonium salt solution was added dropwise to the prepared coupler solution at 0-5 °C with constant stirring. The color of the reaction system changed to dark red. After 8 h of stirring at 0-5 °C, the semi-solid product was filtered and purified by column chromatography using alumina with ethyl acetate/ hexane as the eluent. The pure compound was an orange solid, that was dried and collected as 3 (72%), mp 76-78 °C, ¹H NMR (CDCl₃, δ): 8.36-8.31 (m, 2H), 7.71 (d, 2H), 7.43 (dt, 3H), 6.95 (d, 2H), 3.88 (s, 3H), 3.57 (s, 4H), 1.77-1.68 (m, 4H), 1.57 (s, 4H), 1.45-1.36 (m, 4H), 0.99 (t, 6H). ¹³C NMR (CDCl₃,δ): 168.49 (s), 160.12 (s), 154.02 (s), 147.51 (s), 140.52 (s), 134.62 (s), 130.32 (s), 128.94 (s), 128.10 (s), 123.68 (s), 114.20 (s), 55.49 (s), 29.54 (s), 20.16 (s), 13.87 (s). Anal. Calcd for C₂₄H₃₀N₄OS (422.21 g/mol): C, 68.21; H, 7.16; N, 13.26. Found: C, 63.20; H, 7.15; N, 13.27.

Synthesis of (E)-N,N-dibutyl-4-phenyl-5-(p-tolyldiazenyl) thiazole-2-amine (4)

The procedure for synthesis of 4 was the same as 3 using p-methylaniline (9.33 mmol) and 2-N,N-dibutylamino-4-phenyl thiazole (7.10 mmol) coupler. Purification of the compound was done by column chromatography with alumina and ethyl acetate/hexane as eluent. The orange colored solid obtained was dried and collected as 4 (65%), mp 77-80 °C, ¹H NMR (CDCl₃, δ): 8.38-8.32 (m, 2H), 7.64 (d, 2H), 7.50-7.34 (m, 3H), 7.27-7.21 (m, 2H), 3.49 (dd, 4H), 2.39 (s, 3H), 1.78-1.68 (m, 4H), 1.45-1.36 (m, 4H), 0.99 (t, 6H). ¹³C NMR (CDCl₃,δ): 168.85 (s), 154.89 (s), 151.24 (s), 140.46 (s), 138.70 (s), 134.54 (s), 130.42 (s), 129.63 (s), 129.09 (s), 128.36 (s), 128.11 (s), 125.98 (s), 122.11 (s), 51.07 (s), 29.50 (d, J = 7.4 Hz), 21.36 (s), 20.16 (s), 13.87 (s). Anal. Calcd for C₂₄H₃₀N₄S (406.21 g/mol): C, 70.90; H, 7.44; N, 13.78. Found: C, 70.93; H, 7.43; N, 13.79.

Synthesis of (E)-N,N-Dibutyl-4-phenyl-5-(phenyldiazenyl) thiazole-2-amine (5)

Aniline oil (10.95 mmol) was mixed in 2.3 mL of conc. HCl at 0-5 °C. A cold (0-5 °C) solution of sodium nitrate (11.05 mmol) was added dropwise to the amine solution at 0-5 °C and the mixture was stirred for 30 min at 0-5 °C. The 2-N,N-dibutylamino-4-phenyl thiazole (6.45 mmol) coupler was dissolved in 1,4-dioxane and stirred until a clear solution was obtained. The diazonium salt solution was added dropwise to the prepared coupler solution at 0-5 °C with constant stirring. The liquid compound synthesized is purified by column chromatography with alumina and ethyl acetate/hexane as eluent. A viscous, red colored material was obtained as 5 (65%). ¹H NMR (CDCl₃, δ): 8.36 (d, 2H), 7.74 (d, 2H), 7.50-7.39 (m, 5H), 7.30 (dd, 1H), 3.58 (s, 4H), 1.77-1.68 (m, 4H), 1.47-1.37 (m, 4H), 1.00 (t, 6H). ¹³C NMR (CDCl₃, δ): 169.16 (s), 155.74 (s), 153.19 (s), 140.38 (s), 134.44 (s), 130.49 (s), 129.28 (s), 128.96 (s), 128.42 (s), 128.17 (s), 122.18 (s), 29.52 (s), 20.17 (s), 13.89 (s). Anal. Calcd for C₂₃H₂₈N₄S (392.20 g/mol): C, 70.37; H, 7.19; N, 14.27. Found: C, 70.49; H, 7.18; N, 14.28.

Dyeing Procedure

Disperse dyeing of polyester and nylon fabrics with 1.5% synthesized dyes 1-5 (calculated on weight of the fabric, owf) was performed at a liquor ratio (LR) of 1:30 in a Ros-sari Labtech Flexi Dyer machine using the high temperature, high pressure (HTHP) dyeing method. Dispersing agent (5 g/L) and leveling agent (5 g/L) was added during dyeing and the pH of the dyebaths was adjusted to 4-5 with acetic acid. Dyeing was commenced at 35 °C, and ramped up to the dyeing temperature at a rate of 2.5 °C/min; then dyeing was maintained at 130 °C for 1 h. After 1 h, the temperature was cooled to 70 °C at 2.5 °C/min and finally, the dyebath was drained and the dyed samples were rinsed with water at room temperature (RT). Alkaline reduction clearing was carried out in a bath containing 2 g/L of anhydrous sodium carbonate and 0.5 g/L of sodium dithionate at 60 °C for 20 min. After reduction clearing, soaping (1g/L soap) was performed at 70 °C for 10 min, followed by neutralization with 1 mL/L of acetic acid solution for 10 min. The samples were then rinsed with plenty of cold water and dried in air.

Results and Discussion

Synthesis of Azo Dyes

Novel 2-N,N-dibutylamino-4-phenyl thiazole based azo disperse dyes 1-5 were synthesized by diazotizing five para-substituted amines and subsequent coupling with 2-N,N-dibutylamino-4-phenyl thiazole under acidic coupling conditions. The 4-positon of 2-N,N-dibutylamino-4-phenyl thiazole color is active for electrophilic substitution, and therefore can be easily coupled with a diazonium salt. The thiazole moiety in the synthesized azo dyes is responsible for the resulting bright and strong shades of orange. These dyes were low melting, which was attributed to the presence of the long butyl chains on the amino nitrogen of the thiazole core. The dye 2 gave the highest yield (87%), which was attributed to the stability of the diazonium salt due the presence of the chloro group at the para position. The synthesized dyes were well characterized by ¹H and ¹³C NMR spectroscopy. The FTIR spectra of the synthesized dyes showed the formation of azo bonds at 1545 cm⁻¹, thus confirming the structure of the dyes (Fig. 1).

Fig. 1

FTIR spectra of dyes 1-5.

Photophysical Properties

The photophysical properties of 2 μM solutions of disperse dyes 1-5 were measured in different solvent environments and tabulated in Table I. The absorption maxima of all dyes ranged from 470-534 nm and were non-emissive in nature. The absorption plots of dyes 1-5 in different solvents is presented in Fig. 2. The presence of electron withdrawing groups, such as nitro and chloro groups, led to bathochromic shifts in the absorption maxima of the dyes. The bathochromic shift was in the order: 1 (Nitro, 534 nm) < 2 (Chloro, 490 nm) < 3 (Methoxy, 488 nm) < 4 (Methyl, 482 nm) ≍ 5 (Hydrogen, 490 nm) in dimethyl sulfoxide solvent. The same trend was observed in all studied solvents.

Fig. 2

UV-Vis absorption spectra of 2 μM solution of dyes 1-5 in different solvents.

Table I

Photophysical Properties of Dyes 1-5 in Different Solvents

Dye	Solvent ^a	λ^abs^b		∊_max^c	f^d	μ²_ge^e
Dye	Solvent ^a	cm⁻¹	nm	∊_max^c	f^d	μ²_ge^e
1	Toluene	19230.77	520	6950	0.12	1.37023E-35
	DCM	18796.99	532	5700	0.11	1.2112E-35
	Chloroform	19083.97	524	5200	0.90	9.99165E-35
	Acetonitrile	19011.41	526	2600	0.22	2.42829E-35
	Acetone	20408.16	490	4300	0.76	7.97864E-35
	DMSO	18726.59	534	8200	0.15	1.67735E-35
	DMF	18796.99	532	8300	0.15	1.74024E-35
2	Toluene	20833.33	480	8050	0.15	1.50526E-35
	DCM	20661.16	484	9500	0.17	1.70545E-35
	Chloroform	20161.29	496	9600	0.18	1.94301E-35
	Acetonitrile	20833.33	480	8900	0.15	1.56889E-35
	Acetone	20080.32	498	9300	0.17	1.78225E-35
	DMSO	20408.16	490	9000	0.15	1.61418E-35
	DMF	20661.16	484	8850	0.15	1.58918E-35
3	Toluene	20920.50	478	8550	0.17	1.70899E-35
	DCM	20833.33	480	9500	0.17	1.69807E-35
	Chloroform	19083.97	524	6750	0.18	2.01275E-35
	Acetonitrile	20920.50	478	7850	0.13	1.33229E-35
	Acetone	19841.27	504	5950	0.17	1.85845E-35
	DMSO	20491.80	488	9900	0.18	1.82394E-35
	DMF	20661.16	484	9650	0.17	1.77368E-35
4	Toluene	21186.44	472	18400	0.35	3.51734E-35
	DCM	21008.40	476	20050	0.36	3.68527E-35
	Chloroform	19531.25	512	16600	0.35	3.80712E-35
	Acetonitrile	21186.44	472	18600	0.37	3.68151E-35
	Acetone	19841.27	504	20450	0.40	4.24936E-35
	DMSO	20746.89	482	20000	0.36	3.68764E-35
	DMF	20920.50	478	18350	0.33	3.39664E-35
5	Toluene	21276.60	470	24050	0.46	4.59051E-35
	DCM	21097.05	474	24250	0.45	4.54839E-35
	Chloroform	20080.32	498	27150	0.56	5.9887E-35
	Acetonitrile	21276.60	470	19400	0.36	3.61033E-35
	Acetone	20080.32	498	27150	0.47	4.9483E-35
	DMSO	20833.33	480	27500	0.51	5.19994E-35
	DMF	21008.40	476	24350	0.45	4.57518E-35

DCM = dichloromethane, DMSO = dimethylsulfoxide, DMF = dimethylformamide

absorption maxima

molar extinction coefficient

oscillator strength

transition dipole moment square in esu.cm.

All dyes showed less solvatochromism due to the presence of the N,N-dibutyl alkyl group, which created stearic hindrance between the solute-solvent molecules, thus lessening solvent interactions. The absorption maxima was shifted to higher wavelengths, although less so (by 7-10 nm) with increased solvent polarity (Table I). Thus, from the study of electronic absorption spectra of the synthesized dyes, the presence of electron withdrawing substituent groups on the diazo component led to bathochromic shifts in the absorption maxima when compared to electron donor substituents used as the diazo component.

Color Assessment of Polyester and Nylon Fabrics

The color assessment of the dyed samples was performed using a Spectrascan 5100+ instrument, and K/S, L*, a*, and b* values were measured. The K/S value for dye 1 was less when compared to dyes 2-5 for both polyester and nylon dyed fabrics. The K/S value for nylon dyed fabrics was higher than for polyester fabrics, while the color shade for polyester dyed fabrics was brighter than for nylon fabrics (Table II).

Table II

Color Coordinates (CIELAB) for Polyester and Nylon Dyeing

Dye	L*	a*	b*	c*	H*	K/S
Polyester
1	70.728	19.264	-6.101	20.207	342.434	1.8058
2	84.837	16.925	14.151	22.061	39.883	8.0449
3	87.337	13.688	14.969	20.284	47.54	11.1009
4	87.718	11.778	15.66	19.595	53.032	9.5214
5	88.314	11.27	15.478	19.146	53.919	11.5013
Nylon
1	81.781	7.478	-2.518	7.891	341.398	9.771
2	89.081	11.371	10.289	15.335	42.123	15.597
3	91.624	7.72	12.461	14.659	58.197	15.378
4	90.739	10.026	12.671	16.158	51.626	16.626
5	92.656	6.653	12.054	13.768	61.08	14.919

Lightfastness

The lightfastness of the dyed fabrics was evaluated. A sample specimen was cut in half, with one half covered by a black sheet and the remaining half exposed to a Xenon lamp continuously for 17 h on a Q-SUN lightfastness tester. The samples were compared with a Blue Wool Scale to rate the lightfastness. The polyester fabrics showed better lightfastness properties for all dyes when compared to the nylon fabrics (Table III). For polyester fabrics, the sample dyed with dye 1 had the poorest lightfastness rating, whereas the sample dyed with dye 5 has the highest rating among them. The dyed nylon fabrics had very poor lightfastness properties, except for the dye 5 dyed sample. Therefore, dye 1 on both nylon and polyester had poor lightfastness properties, whereas dye 5 had very good to excellent lightfastness properties on both fabrics. Dye 3 dye also showed good lightfastness properties on polyester, while the remaining dyes also showed average to good light-fastness properties on polyester.

Table III

Lightfastness and Washfastness Properties of Dyed Polyester and Nylon Fabric for Dyes 1-5

Dye	Polyester		Nylon
Dye	Lightfastness^*	Washfastness^*	Lightfastness^*	Washfastness^*
1	3	5	1	5
2	5-6	5	1-2	5
3	6-7	5	3-4	4
4	5	4	1-2	4
5	7	5	4-5	4

Ratings. Lightfastness: 1-poor to 8-excellent. Washfastness: 1-poor to 5-excellent.

Washfastness

The dyed polyester and nylon samples using dyes 1-5 were sandwiched between two undyed cloth samples (polyester/nylon) and then treated with 5 g/L soap solution for 30 min at 70 °C and washfastness was rated by comparing with a standard scale after wash treatment. The polyester dyed samples showed excellent washfastness, whereas it ranged from moderate to excellent for nylon samples (Table III).

Fastness to Sublimation

Composite specimen dyed samples were made by sandwiching dyed samples between two white polyester fabrics, and were then exposed to different temperatures (150 °C, 180 °C, and 210 °C) for 30 s, and subsequently evaluated for change in color and staining of adjacent fabric by comparison with a gray scale. Sublimation fastness and staining of the polyester fabric was slightly better than for nylon fabrics (Table IV). The least color change was observed for dyes 1-5 at 150 °C, with a slight change at 180 °C, and the most at 210 °C. Similarly, maximum staining was seen at 210 °C for all dyes. Overall, the dyed polyester fabrics showed moderate sublimation fastness, which was better than the dyed nylon samples.

Table IV

Sublimation Fastness for Dyes 1-5

Dye	Sublimation Fastness (Color Change)^*			Staining on Fabric After Sublimation^*
Dye	150 °C	180 °C	210 °C	150 °C	180 °C	210 °C
Polyester
1	5	5	4-5	4-5	4	3-4
2	5	5	4-5	5	4	3
3	5	4-5	4-5	5	4	3-4
4	5	4-5	4-5	5	4	3
5	5	4-5	3	4-5	3-4	3
Nylon
1	5	3-4	3	5	4	3
2	5	4-5	3-4	4-5	3-4	2
3	4-5	4-5	3-4	4-5	3-4	2-3
4	5	4-5	4	4	3-4	2-3
5	5	4-5	3-4	4	4	2

Rating. Sublimation fastness: 1-poor to 5-excellent.

Conclusion

We have successfully synthesized five novel monoazo thiazole disperse dyes 1-5 by a simple and efficient protocol where the diazonium salts of para-substituted amines were coupled with 2-N,N-dibutylamino-4-phenyl thiazole. All dyes were obtained in good yields, had different shades of orange to purple color, and were characterized by ¹H and ¹³C NMR spectroscopy, and FTIR. The electronic excitation of the dyes was studied based on UV-Vis absorption spectros-copy, where bathochromic shifts were observed for dyes with a strong acceptor such as dye 1, which contained a nitro group, when compared to dye 5, where the nitro group was replaced with a hydrogen atom. Also, bathochromic shifts in absorption maxima were observed for all of these dyes in all solvents tested, from nonpolar to polar solvents.

Dyeing on polyester and nylon fabrics for all dyes tested was carried by the HTHP method under suitable dyeing conditions.

Both dyed polyester and nylon fabric samples dyed with dyes 1-5 showed excellent washfastness. The polyester dyed samples dyed with dyes 1-5 showed lightfastness rated from good to excellent, whereas the dyeings on nylon were rated poor to good. Similarly, the sublimation fastness on polyester was good to excellent, whereas on nylon were moderate to excellent. K/S values were also determined for all five disperse dyes. The long butyl alkyl chain in the synthesized disperse dyes and absence of polar end groups were responsible for the low melting points of these dyes. Dyes 1-5 were sublimable at a set temperature, and therefore can be used as sublimation dyes in heat transfer printing.

Supporting Information

The recorded ¹H and ¹³C NMR spectra for all the synthesized dyes in this research are available from the authors upon request.

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