Novel and Sustainable Colorants Developed via Incorporating Azo Chromophores into Dopamine Molecules.

Inspired by the application of dopamine as an "anchor" and UV absorber, novel sustainable colorants with biscatecholic structure were synthesized through a simple incorporation of simple azo chromophores with dopamine. Their structures were confirmed using MS and NMR analyses, and their application on textile materials was investigated. Compared to the simple azo chromophores with almost no coloring ability on fabrics, the biscatecholic colorants could color different fabrics effectively, mainly through self-polymerization only in the presence of a trace amount of organic base at room temperature, which is environmentally friendly in terms of saving resources and alleviating chemical pollution. Meanwhile, the UV resistance of colored fabrics was enhanced significantly, showing the advantage of protecting wearers from UV damage.

Introduction

3,4-Dihydroxy-l-phenylalanine (DOPA) is an amino acid found in adhesive proteins secreted by marine mussels, which is one organism in nature and exhibits unique adhesion on rocks.[1−3] It is believed that DOPA plays an important role in the adhesion mechanism of marine mussel through cross-linking and different physicochemical interactions.[4−6] In much research, dopamine is frequently used as an alternative for DOPA to mimic adhesive protein because of its similar structure to DOPA and more wide distribution.[7] Dopamine could generate surface-adherent films on diverse substrates through self-polymerization, but the mechanism is complex and still elusive. Many researchers have suggested that it is attributed to the oxidation of catechol under alkaline conditions.[6,8−11] Based on the adhesive property, dopamine is widely used as a sustainable coating material through codeposition with other materials including inorganic materials, polymers, and nanoparticles.[10,12−15]

With the benefits of having catechol and amine groups, dopamine could be used as the starting points for covalent modification with molecules, and therefore it is often used to conjugate with other molecules or material surfaces to achieve functional applications, such as bioadhesives or magnetic nanoparticles.[4,16−22] However, few studies have been focused on its application on coloration. Furthermore, dopamine has an obvious absorption peak at ca. 280 nm, and its UV absorbance can be increased significantly via the self-polymerization into polydopamine (pDa), which is also known as a major pigment of natural melanin.[9,23,24] Hence, dopamine–melanin has already been used as an effective UV absorber to enhance the UV-shielding performance of nanoparticles and films.[25,26] The fabric treated with dopamine has been reported to show better UV resistance.[27]

Therefore, the effective utilization of functional properties of dopamine on textiles will be a useful and potential strategy. In our previous studies, dopamine was applied to color various natural and synthetic textile materials through different methods including polydopamine-assisted dyeing and chemical modification, and all the studies showed positive results.[28−30] To continue our interest, herein we bonded covalently simple azo chromophores with dopamine molecules, successfully synthesizing a kind of novel and sustainable colorant. In the structure of the colorants, the azo groups were used as the chromophores and catechols on dopamine as the anchoring points. Compared with the dyes used in previous studies, the azo molecules selected in this work are just simple and common models containing chromophores, with inferior coloring ability on materials. The synthesized biscatecholic colorants demonstrated enhanced coloring performance and good UV-shielding properties. They could be regarded as the products of “evolution” by growing the arms holding anchors (as shown in Figure ). This study provides a more comprehensive extension for the application of dopamine in textile fields.

Figure 1

Illustration of the evolutionary process from an azo chromophore to a colorant.

Experimental Section

Materials and Instruments

Aniline-4-sulfonic acid, 2-methoxyaniline-5-sulfonic acid, sodium hypochlorite solution with 6–14% active chlorine and 98% oxalyl chloride, 1.0 M tetrabutylammonium fluoride solution in THF, 4-dimethylaminopyridine (DMAP), and other reagent-grade chemicals were purchased from Shenzhen Dieckmann Technology Co., Ltd. and used without further purification.

High-resolution mass spectra were obtained on a Waters mass spectrometer. Nuclear magnetic resonance (NMR) spectra were recorded on a Jeol ECZ500R (500 MHz) spectrophotometer. The absorption spectra were recorded on a UH5300 UV–vis spectrophotometer.

Colorimetric analyses of the fabrics were conducted using a Datacolor 650 spectrophotometer, and the results were recorded in the form of CIE, L, a, and b color space, and K/S values. Wash fastness, crocking fastness, and light fastness were assessed using the same method used in a previous report.[29]

The surface morphology of fibers was characterized using a Hitachi TM3000 Benchtop Scanning Electron Microscope (SEM) from Japan, and cross-section images were investigated on an optical microscope: Nikon Optiphot-POL from Germany; UV–vis transmittance spectra and the ultraviolet protection factor (UPF) of colored fabrics were measured by a Varian Cary 300 UV–visible spectrophotometer.

Synthesis of Biscatechol Azobenzene Compounds

The synthesis route of biscatecholic colorants is shown in Scheme , and the preparation of sample 1d is described in detail.

Scheme 1

Synthesis Route and Chemical Structures of the Biscatecholic Colorants

(i) NaClO/ice bath; (ii) (ClCO)2/reflux; (iii) TBDMS-dopamine/K2CO3; (iv) TBAF/THF.

Synthesis of Azobenzene-4,4′-disulfonic Acid (1b)

1a, aniline-4-sulfonic acid (0.35 g, 2 mmol), and anhydrous sodium carbonate (0.42 g, 4 mmol) were added to 20 mL of water under stirring. The resultant solution was cooled in an ice-salt bath, and 25 mL of sodium hypochlorite solution with about 6–14% activated chlorine was dropwise added. The mixture was further stirred for 3 h in the bath. Then, the solution was neutralized with diluted hydrogen chloride to pH 7.0, and the precipitate was collected by filtration. After washing with DI water and drying, the target product was obtained as orange solids. 1H NMR (D2O): δ 7.85–7.87 (d, 4H, ArH), 7.82–7.83 (d, 4H, ArH). HRMS (TOF, ES–): m/z = 340.9735 (M – H) –, calcd for C12H10N2O6S2 342.35.

Synthesis of Azobenzene-4,4′-disulfonyl Chloride (1c)

1b, azobenzene-4,4′-disulfonic acid (0.34 g, 1 mmol), was added to 25 mL of oxalyl chloride and refluxed overnight. After the reaction, the solution was poured into 500 mL of ice water under stirring. Then, the mixture was filtrated and washed with ice water until the filtrate became neutral. The obtained solid was dried under a vacuum at room temperature. 1H NMR (chloroform-D): δ 8.27–8.29 (d, 4H, ArH), 8.18–8.21 (d, 4H, ArH).

Synthesis of Catechol Derivative: TBDMS-dopamine

The solution of dopamine hydrochloride (0.95 g, 5 mmol) in 45 mL of dichloromethane was stirred in a water bath, and tert-butyldimethylsilyl chloride (2.26 g, 15 mmol) in 20 mL of dichloromethane was added dropwise to the solution over 15 min. The water bath was then replaced by an ice bath, and triethylamine (1.52 g, 15 mmol) was added to the mixture. After stirring in the bath for 1 h, the resultant reaction solution was further stirred for about 20 h at room temperature. Then, 50 mL of DI water was added into the solution under stirring, and the bottom layer was collected with a separation funnel. The collected solution was evaporated under vacuum to remove the solvent, and the target product was obtained as brown oil. HRMS (TOF, ES+): m/z = 382.2596 (M + H)+, calcd for C20H39NO2Si2 381.70.

Synthesis of Biscatechol Colorant (1d)

1c, azobenzene-4,4′-disulfonyl chloride (0.15 g, 0.4 mmol), TBDMS-dopamine (0.38 g, 1 mmol), anhydrous sodium carbonate (0.16 g, 1.5 mmol), and anhydrous sodium sulfate (0.21 g, 1.5 mmol) were added into 25 mL of anhydrous acetone. After stirring for 5 h at room temperature, the solvent was removed under vacuum, and the residue was added into 25 mL of tetrahydrofuran. An amount of 1 mL of TBAF solution was added, and the mixture was stirred for another 30 min at room temperature. Finally, the residue was obtained after removing THF and was purified by silica gel column chromatography using methanol/ethyl acetate/1,2-dichloroethane (1:1:5, v/v) as eluent to obtain 1d. Yield: 49.5%. 1H NMR (DMSO-d6): δ 8.79 (s, 2H, OH), 8.72 (s, 2H, OH), 8.09 (d, 4H, ArH), 8.00 (d, 4H, ArH), 7.89 (t, 2H, ArH), 6.60 (d, 2H, ArH), 6.52 (s, 2H, NH), 6.38 (d, 2H, ArH), 2.93 (m, 4H, CH2), 2.50 (m, 4H, CH2). 13C NMR (DMSO-d6): δ 153.96, 145.48, 144.14, 143.49, 129.73, 128.47, 123.95, 119.73, 116.44, 115.33, 45.00, 35.24. HRMS (TOF, ES–): m/z = 611.1278 [M – H]−, calcd for C28H28N4O8S2 612.67.

2d was synthesized with similar procedures. Yield: 45.2%. 1H NMR (DMSO-d6): δ 8.75 (m, 4H, OH), 7.50 (d, 2H, ArH), 7.47 (d, 2H, ArH), 6.99 (d, 2H, ArH), 6.96 (s, 2H, ArH), 6.64 (d, 2H, ArH), 6.52 (s, 2H, ArH), 6.37 (t, 2H, NH), 3.72 (s, 6H, CH3), 2.57 (t, 4H, CH2), 2.43 (t, 4H, CH2). 13C NMR (DMSO-d6): δ 151.22, 145.46, 144.10, 143.52, 132.13, 129.88, 128.25, 119.82, 117.15, 116.54, 115.94, 112.82, 56.60, 44.77, 35.04. HRMS (TOF, ES–): m/z [M – H]−, calcd for C30H32N4O10S2 672.73, found 671.1491.

Coloration

Textile fabrics were washed with 5 g/L of sodium dodecyl sulfate solution at 60 °C for 20 min to remove surface impurities and then washed with DI water and dried in air. Then, fabrics were immersed in 0.05% (w/v) colorant solution in methanol (liquor ratio 50:1) with and without 0.2 mg/mL of DMAP, respectively, at room temperature for 24 h and subsequently rinsed with water. After drying, the fabrics were washed with 0.5% detergent at 60 °C for 10 min to remove the unfastened colorant and then air-dried. For comparative purposes, both azo chromophores (1b and 2b) and commercial dye acid yellow 11 (AY11) were used to color fabrics in water with and without 0.2 mg/mL of DMAP, respectively.

Results and Discussion

Synthesis and Characterization

Azo compounds are usually synthesized through a diazotization coupling reaction, with asymmetric structures[31−34] (as shown in Scheme a). However, the novel colorants developed in this study are biscatecholic azobenzene compounds with a symmetric structure, and their precursors, the azo chromophores, are suitable to be synthesized via an oxidization–coupling reaction,[35−37] where aromatic amines can self-couple to form azo bonds (as shown in Scheme b). Based on the symmetric structure, two dopamine molecules could be simply incorporated into the azo chromophore through a one-step reaction. For the oxidization–coupling reaction, air can be used as an oxidation agent in the presence of metal catalysts, such as Ag, Pt, Au, and Pd.[38−42] Comparably, sodium hypochlorite offers the advantages of low cost, easy availability, and proper oxidization ability.[37] Therefore, sodium hypochlorite was employed in this study. The obtained sulfonic acid of azo compounds can be easily converted to chlorosulfonyl derivatives through a chlorination process in the presence of chlorination reagents, such as chlorosulfonic acid, phosphorus oxychloride, and thionyl chloride.[43,44] In this work, oxalyl chloride was used as the chlorination reagent because of its mild reactivity and easy post-treatment involved.[45] In order to protect the catecholic groups of dopamine molecules during the synthesis process, tert-butyldimethylsilyl chloride was used since it has moderate stability and can easily be removed in the presence of TBAF under mild conditions at the final stage.[46−49]

Scheme 2

Comparison of the Diazotization Coupling Reaction and Oxidization Coupling Reaction

Characterization of Biscatecholic Colorants

The structures of colorants 1d and 2d were deduced with the aid of MS and NMR data. One thing worth noting is that the peaks for H6 at ca. δ 2.50 ppm obviously interfered with the solvent residual peak, as shown in Figure (1HNMR spectrum in DMSO-d6) for 1d. Therefore, the peaks were further verified with methanol-d4 as the solvent. It could be observed from the inset image in Figure that two clear triplets with a correct ratio of the integral area confirmed the presence of H7 and H6. More spectra could be found in the Supporting Information.

Figure 2

1H NMR spectrum of 1d in DMSO-d6. Inset: partial 1H NMR spectrum in methanol-d4.

UV–vis Absorption Spectra

The absorption spectra of three compounds 1d, 2d, and dopamine in methanol with the concentration of 0.01 mg/mL were shown in Figure . Compared to the absorption peak of dopamine at ca. 280 nm, the phenolic hydroxyl absorption appeared in the spectrum of the as-synthesized compounds, and the wavelengths of maximum absorbance (λmax) of 1d and 2d were located at ca. 326 and 372 nm, respectively. Two compounds had the same chromophore of azobenzene, but the electron-donating methoxy group on 2d contributed to the increase of electron cloud density, resulting in bathochromic shifts of λmax. The molar attenuation coefficients of 1d and 2d were shown in Table .

Figure 3

UV–vis spectra of 1d, 2d, and dopamine in methanol.

Table 1

Molar attenuation Coefficient (ε) of the Biscatecholic Colorants in Methanol

compounds	1d	2d
ε (L·mol–1·cm–1)	5.91 × 104	1.33 × 104

The λmax values of the biscatecholic colorants in different solvents were also studied. As listed in Table , the λmax of these compounds was similar in polar and less polar solvents, which indicated that their absorptions showed no obvious dependence on solvent polarity.

Table 2

λmax of the Biscatecholic Colorants in Different Solventsa

compounds	methanol/nm	ethanol/nm	DMF/nm	acetone/nm
1d	326	325	330	332
2d	372	370	376	374

DMF: N,N-dimethylformamide.

Colorimetric Analysis

Fabrics colored with 1b, 1d, 2b, and 2d colorants in solvent with and without the presence of DMAP (organic base) were evaluated by colorimetric analysis. As shown in Figure , the appearance of fabrics colored by 1d and 2d without DMAP is deeper than that of 1b and 2b, but the enhanced effects were not distinct, especially on silk and cotton. Compared to commercial dye AY11, the coloration effect of as-synthesized colorants on textile materials also had no obvious advantage. However, the coloring ability of 1d and 2d was enhanced significantly with the presence of DMAP, and all colored fabrics showed more distinct color appearance than that of 1b and 2b as well as acid yellow 11.

Figure 4

Photographic images of the coloration results of common textile materials. 1b, 2b, and AY11 colored fabrics in water; 1d and 2d colored fabrics in methanol; with and without organic base: DMAP (4-dimethylaminopyridine).

As the simple models contained only chromophore, 1b and 2b could hardly color fabrics either with or without DMAP. One of the possible reasons was that these molecules were small, making them difficult to be trapped between fibers. More importantly, their interaction with fabric might only involve weak Van der Waals forces. These could be attributed to their little coloring ability on fabrics and make them useless as colorants though they have a chromophoric group.

Without DMAP, the coloring ability of 1d and 2d was mainly attributed to hydrogen bonding between their hydroxyl groups and the hydroxyl and amino groups on fibers. Different from other commercial dyes, as-synthesized compounds have a longer molecular chain and special symmetrical structure; hence the intramolecular hydrogen bonding could be easily formed. This would cause the decrease in the interaction between hydroxyl groups on colorants and fabrics, leading to poor coloring performance. The existence of intramolecular hydrogen bonds could be revealed from the FTIR spectra of the solution of as-synthesized compounds in Figure , of which there was a wide and strong peak between 3650 and 3050 cm–1. However, these fabrics could be colored with 1d and 2d effectively in the presence of DMAP. Researchers have shown that organic base could catalyze the formation of a polydopamine (pDA) coating on substrates in aqueous conditions,[50−53] and the presence of organic base is important for the deprotonation of dopamine and also the subsequent oxidative polymerization.[54] In this study, organic base (DMAP; pKb is about 4.4 under the standard state) also played an important role in the coloration of 1d and 2d on fabrics, improving their coloring ability significantly. We assumed that in the presence of DMAP the polymerization of 1d and 2d could also take place, and then the formed polymers coated the fabrics, which is similar to the formation principle of dopamine-based adhesive coatings on various materials. This explanation could be supported by the coating of a metal substrate with as-synthesized colorants at a higher concentration (as shown in Figure a–d). After collection and evaluation with a scanning electron microscope, spherical morphologies of the deposited particles formed through self-polymerization of colorant molecules were observed clearly (as shown in Figure e–g). Compared to the particles formed by dopamine, the diameter of particles formed by as-synthesized colorants was larger. This could be reasonably explained by the size expansion of combining a large azo chromophore molecule with two dopamine molecules. It is worth mentioning that both the surface and interior part of the fibers could be “coated” because the molecules of 1d and 2d were small and could easily enter the inside of the fibers before polymerization. The colored cross section of fibers could be used to support the proposed idea (as shown in Figure ). Compared with the bases used in other studies such as piperidine and trimethylamine, DMAP has a higher nucleophilicity which is beneficial for the polymerization[54−56] and slighter small for application. The chemical interactions (Michael addition or Schiff base reaction) suggested in many studies[6,16,57−59] were also likely involved in the process of coloring (Figure S1). The addition of DMAP had an adverse impact on the coloring performance of AY11, which had no functional catechol group.

Figure 5

FTIR spectra of a solution of 1d and 2d in methanol.

Figure 6

Photographic images of a metal substrate coated by 5 mg/mL of dopamine and biscatecholic colorants, respectively, in methanol with 5 mg/mL of DMAP: (A) blank metal substrate, (B) dopamine, (C) 1d, (D) 2d. The SEM images of polymerized particles: (E) polydopamine (pDa), (F) poly-1d, and (G) poly-2d.

Figure 7

Microscopic images of the cross-section view of fibers before (far left) and after coloration with 1d and 2d with DMAP (blue fibers as background).

The surface morphology of colored fibers was also investigated, as shown in Figure . The surface morphologies of colored silk and wool were rougher than that of blank fabrics, while the roughness of cotton before and after coloration had a negligible difference. This result was probably attributed to the better resistance of cotton fabric to alkali than that of protein fabrics (silk and wool). On different fiber surfaces, microscaled particles originating from the colorants could be observed to various extents. Microscaled particles on wool were the most obvious, while those on cotton were the least obvious. The result was in line with the K/S value of three colored fabrics found, where the wool had the highest value and the cotton had the lowest value (as shown in Table ). The possible cause for this was that wool had a tighter and rougher structure than silk and cotton, contributing to the trapping and aggregation of particles. In addition, wool had more thiol groups which were beneficial for chemical interactions between the wool fabric and the as-synthesized colorants. By contrast, the cotton fabric had a smoother surface, and it also had no effective functional groups supporting chemical interactions; therefore, the number of particles on its surface was fewer.

Figure 8

Surface morphology of fabrics before (far left) and after coloration with 1d and 2d with DMAP.

Table 3

Colorimetric Analysis and Fastness of the Colored Fabrics (1d and 2d with DMAP and AY11 without DMAP)

						wash fastness
							staining						crocking fastness
fabrics	colorant	L*	a*	b*	K/S	color change	wool	acrylic	polyester	nylon	cotton	acetate	dry	wet	light fastness
silk	blank	90.47	1.44	2.9	--	--	--	--	--	--	--	--	--	--	--
	1d	78.91	6.69	27.83	1.44	4	4–5	4–5	4–5	4–5	4–5	4–5	4–5	4	2–3
	2d	69.54	6.22	33.06	5.19	3–4	4–5	4–5	4–5	4–5	4–5	4–5	3–4	3	3
	AY11	87.41	4.92	46.73	2.51	1	3	4–5	4–5	3–4	4	4	4–5	4	3–4
wool	blank	87.42	3.02	0.75	--	--	--	--	--	--	--	--	--	--	--
	1d	73.9	7.19	18.88	3.38	3–4	4–5	4–5	4–5	4–5	3–4	4–5	4	4	2–3
	2d	71.4	5.61	23.7	5.47	3–4	4–5	4–5	4–5	4–5	3	4–5	3–4	3–4	3
	AY11	81.85	1.39	58.3	9.35	1	3	4–5	4–5	3–4	2–3	3–4	4–5	4–5	3–4
cotton	blank	95.25	3.63	11.78	--	--	--	--	--	--	--	--	--	--	--
	1d	79.76	3.78	13.23	0.73	4–5	4–5	4–5	4–5	4–5	4–5	4–5	4	4	2
	2d	80.12	2.95	23.77	1.79	3–4	4–5	4–5	4–5	4–5	4–5	4–5	4–5	3–4	3
	AY11	93.2	0.39	7.7	0.06	--	--	--	--	--	--	--	--	--	--

From the result of colorimetric analyses, as shown in Table , the color depth (K/S value) of fabrics colored with the as-synthesized colorants with DMAP was better than that without DMAP. The crocking fastness of fabrics colored with 2d was poorer than that of 1d, which was probably because 2d had a bigger molecular volume and lower planarity, weakening the adhesive force between the polymers and the fibers.

UV Absorber

The transmittance spectra of the fabrics colored with 1d and 2d in the presence of DMAP were investigated. As shown in Figure , the UV transmittance of the silk and cotton colored with 1d and 2d was reduced significantly, when compared to the spectra of blank fabrics. They also had lower transmittance than the fabrics colored with commercial AY11. Accordingly, their UPF values showed an obvious increase. This meant that their ability to protect wearers from UV damage was improved. This result would be positive and significant because silk and cotton are used frequently in summer outfits, but the UV protection property of these blank fabrics is generally insufficient. For wool fabric, the employment of as-synthesized colorants could also reduce its UV transmittance, and this reduction was relatively less distinct than that of silk and cotton because of its tighter fabric construction. Also, owing to this special structure, even for blank wool, its UV protection is good; therefore, its UPF increase made little sense, and there is no discussion about that. Nevertheless, the results proved that 1d and 2d could endow textile materials with UV-shielding functionalities.

Figure 9

Transmittance spectra of colored fabrics with 1d, 2d, and AY11 and the UPF values of cotton and silk.

Conclusions

Two novel biscatecholic compounds were synthesized, which demonstrated how simple azo chromophores with little coloring ability “evolved” into effective colorants. Compared to some common commercial dyes, the novel colorants could color fabrics easily and effectively at room temperature without any chemical auxiliaries. The employment of them would greatly reduce energy and water consumption as well as reduce chemical pollution, which is environmentally friendly. Meanwhile, the fabrics colored with these compounds had an enhancement in terms of UV shielding effect and showed a significant increase in UV protection, an issue people often care about. Furthermore, by virtue of the obvious adhesive features, the colorants have a promising application on other materials such as synthetic substrates or glasses.