Synthesis and Dyeing Properties of Three Hemicyanine-Based Fluorescent Polymeric Dyes

Abstract

This research deals with the synthesis of hemicyanine-based fluorescent polymeric dyes and their use in acrylic dyeing. Three new hemicyanine dyes were synthesized by the Knoevenagel reaction, and grafted to carboxymethylchitosan (O-CMC) to form polymeric dyes. The resulting dyes were characterized using ¹H-NMR and ¹³C-NMR spectroscopy, indicating that these polymeric dyes were successfully synthesized. The practical utility of these polymeric dyes was demonstrated by their successful application on acrylic fabrics to give highly fluorescent colors. The fluorescence, color characteristics, and photostability of these dyed fabrics were tested using a fluorescence spectrometer, a spectrophotometer, and a lightfastness tester. Results showed that acrylic fabrics dyed with these polymeric dyes were fluorescent and had better photostability than the parent dyes.

Keywords

Fluorescence Fluorescent Dyes Hemicyanine Dyes Knoevenagel Reaction Photostability Polymeric Dyes

Introduction

Fluorescent dyes absorb light energy of a specific wavelength and re-emit it at a different wavelength. Fabrics dyed with fluorescent dyes have unusually brighter colors and higher visibility than those of non-fluorescent dyes.¹Because of their intensified perceptibility, textiles dyed with fluorescent dyes can be used in manufacturing articles requiring high visibility such as safety vests for policemen, firefighters, sports clothes, and signage.² Many different kinds of fluorescent dyes have been used in textiles (e.g., derivatives of pyrene, coumarin, anthraquinone, 1,8-naph-thalimide, and hemicyanine).³

Hemicyanine dyes are an important group of fluorescent dyes and have applications in optical power limiting,⁴frequency-upconverted lasing,⁵ fluorescence probes,⁶molecular electronics,⁷ and elemental detection.⁸ Studies on hemicyanine dyes have been largely focused on optical power limiting, two-photon pumped upconversion intracavity lasing, and fluorescence probes.⁹ Our research group has synthesized a series of hemicyanine fluorescent dyes, and investigated their application in dyeing wool, silk, and acrylic fabrics to produce fluorescent fabrics.^10–13Despite their good dyeing properties, these hemicyanine derivatives do not have adequate photostability for more demanding applications where prolonged exposure to light is involved. Fortunately, there are many ways to improve the photostability of dyes (e.g., embedding in sol-gel coatings,¹⁴ using UV absorbers,¹⁵ controlling the weaving process,¹⁶ and introducing a polymer skeleton to create polymeric dyes).¹⁷

In recent years, our research group has mainly focused on methods to improve the photostability of fluorescent dyes by introducing a polymer skeleton, which can be used as a channel to transfer the energy absorbed by dye moieties resulting in enhanced photostability of synthesized dyes. This study is aimed at designing and synthesizing hemi-cyanine-based fluorescent polymeric dyes with improved photostability, their application in dyeing of acrylic fabrics, and evaluation of their color characteristics, fluorescence, and photostability.

Experimental

Materials

Piperidine, 4-methylpyridine (1),

4-N,N-diethylaminobenzaldehyde, 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM), and carboxymethylchitosan (O-CMC, viscosity: 200–400 mPa•s) were purchased from Sino Pharm Chemical Reagent Co. Ltd. 2-Methylbenzothiazole (5a), 2,3,3-trimethyl-3-H-indole (5b), and 6-bromocaproic acid were purchased from Aladdin. Solvents were purchased from Enox. All reagents were used without further purification.

Synthesis 1-(5-Carboxypentyl)-4-methylpyridin-1-ium bromide (2)

4-Methylpyridine (1) (1.86 g, 20.00 mmol) and 6-bromo-caproic acid (4.29 g, 22.00 mmol) were dissolved in 50 mL ethanol in a round-bottom flask equipped with a magnetic stirring apparatus. The reaction mixture was stirred under reflux for 24 h. Ten, ethanol was removed under reduced pressure and the resultant solid product was washed with dichloromethane to give 1-(5-carboxypentyl)-4-methyl-pyridin-1-ium bromide (2) (Scheme 1) in 89.2% yield. ¹H-NMR (DMSO-d₆, 600 MHz): δ 8.96 (d, 2H, J = 6.6 Hz, Py-CH), 7.99 (d, 2H, J = 6.3 Hz, Py-CH), 4.54 (t, 2H, J = 7.4 Hz, ⁺N-CH₂-), 2.61 (s, 3H, -CH₃), 2.15 (t, 2H, J = 7.3 Hz, -CH₂-), 1.92–1.86 (m, 2H, -CH₂-), 1.56–1.48 (m, 2H, -CH₂-), 1.26 (dt,, 2H, J = 7.7 Hz, -CH₂-). ¹³C-NMR (DMSO-d₆, 151 MHz): δ 175.87 (s, -COOH), 159.17 (s, Ph-H), 144.17 (s, Ph-H), 128.76 (s, Ph-H), 60.14 (s, ⁺N-CH₂-), 34.64 (s, -CH₂-), 30.76 (s, -CH₂-), 25.45 (s, -CH₂-), 24.49 (s, -CH₂-), 21.80 (s, -CH₃).

Scheme 1

Synthesis of polymeric dye 4.

(E)-1-(5-Carboxypentyl)-4-(4-(diethylamino)styryl) pyridin-1-ium bromide (3)

1-(5-Carboxypentyl)-4-methylpyridin-1-ium bromide (2) (4.31 g, 15.00 mmol) and 4-diethylaminobenzaldehyde (3.20 g, 16.50 mmol) were dissolved in 50 mL ethanol in a round-bottom flask equipped with a magnetic stirring apparatus and 0.1 mL piperidine was added dropwise.¹⁸ The Knoevenagel reaction was carried out at 80 °C for 12 h. In the next step, ethanol was removed under reduced pressure, the crude red solid was obtained, and purified by silica gel column chromatography (ethyl acetate/methanol, 6:1 v/v eluent) to give (E)-1-(5-carboxypentyl)-4-(4-(diethylamino)styryl) pyridin-1-ium bromide (3) in 72.0% yield. ¹H-NMR (D₂O, 400 MHz): δ 8.35 (d, 2H, Py-H), 7.78 (d, 2H, Py-H), 7.64 (d, 1H, -CH=CH-) 7.56 (d, 2H, J = 8.2 Hz, Ph-H), 6.93 (d, 1H, -CH=CH-), 6.81 (d, 2H, J = 7.5 Hz, Ph-H), 4.27 (m, 2H, ⁺N-CH₂-), 3.40 (dd, 4H, J = 7.2 Hz, -N-CH₂-), 2.12 (m, 2H, J = 7.3 Hz, -CH₂-CO-), 1.89 (s, 2H, -CH₂-), 1.54 (m, 2H, -CH₂-), 1.27 (m, 2H, -CH₂-), 1.18 (m, 6H, J = 7.1 Hz, -CH₃). FTIR (cm^–1) 1716 (C=O, stretch), 1642 (C=C), 1583 (Ph), 1524 (Py), 1081 (C-O-C, asymmetric).

2-Methylbenzothiazole (5a) and 2,3,3-Trimethyl-3H-indole (5b) Based Intermediates 6a and 6b

Synthesis of compounds 6a and 6b are shown in Scheme 2. Compounds 5a or 5b (20.00 mmol) and 6-bromo-caproic acid (22.00 mmol) were dissolved in 40 mL o-dichlorobenzene in a round-bottom flask equipped with a magnetic stirring apparatus. The reaction mixture was stirred under 120 °C for 48 h. The mixture was allowed to cool and was filtered. The crude product was washed with ethyl acetate several times¹⁹ to give 3-(5-carboxypentyl)-2-methylbenzo[d]thiazol-3-ium bromide (6a) from 5a in 63.0% yield. ¹H-NMR (DMSO-d₆, 600 MHz): δ 12.03 (s, 1H, -COOH), 8.45 (d, 1H, J = 8.1 Hz, Ph-H), 8.34 (d, 1H, J = 8.5 Hz, Ph-H), 7.89 (t, 1H, J = 7.9 Hz, Ph-H), 7.81 (t, 1H, J = 7.7 Hz, Ph-H), 4.73–4.69 (m, 2H, ⁺N-CH₂-), 3.36 (s, 3H, -CH₃), 2.23 (t, 2H, J = 7.3 Hz, -CH₂-), 1.88–1.83 (m, 2H, -CH₂-), 1.60–1.54 (m, 2H, -CH₂-), 1.48–1.43 (m, 2H, -CH₂-). ¹³C-NMR (DMSO-d₆, 151 MHz): δ 177.56 (s, -COOH), 174.74 (s, -S-C-CH₃), 141.26 (s, Ph), 129.81 (s, Ph), 128.54 (s, Ph), 125.06 (s, Ph), 117.31 (s, Ph), 109.98 (s, Ph), 49.46, 40.11 (s, ⁺N-CH₂-), 33.87 (s, -CH₂-), 27.93 (s, -CH₂-), 25.88 (s, -CH₂-), 24.45 (s, -CH₂-), 17.23 (s, -CH₃) and 1-(5-carboxypentyl)-2,3,3-trimethyl-3H-indol-1-ium bromide (6b) was synthesized from 5b in 89.2% yield. ¹H-NMR (DMSO-d₆, 600 MHz): δ 12.02 (s, 1H, -COOH), 7.96 (s, 1H, Ph-H), 7.84 (d, 1H, Ph-H), 7.66–7.60 (m, 2H, Ph-H), 4.45 (t, 2H, J = 7.6 Hz, ⁺N-CH₂-), 2.83 (s, 2H, -CH₂-), 2.24 (d, 2H, J = 7.3 Hz, -CH₂-), 1.84 (s, 2H, -CH₂-), 1.57 (s, 6H, -CH₃), 1.47 (m, 3H, -CH₃), 1.25 (d, 2H, J = 7.2 Hz, -CH₂-).

Scheme 2

Synthesis of polymeric dyes 8a and 8b.

Hemicyanine Dye Synthesis

Compounds 6a or 6b (12.00 mmol) and 4-diethylaminoben-zaldehyde (13.20 mmol) were dissolved in 50 mL ethanol in a round-bottom flask equipped with a magnetic stirring apparatus and 0.1 mL piperidine was added dropwise. The Knoevenagel reaction was carried out under reflux for 12 h,²⁰ the crude product was obtained, and purified by silica gel column chromatography (ethyl acetate/methanol, 15:1, v/v) to give (E)-3-(5-carboxypentyl)-2-(4-(diethylamino) styryl)benzo[d]thiazol-3-ium bromide (7a) in 80.8% yield. ¹H-NMR (DMSO-d₆, 600 MHz): δ 8.33 (d, 1H, J = 8.2 Hz, Ph-H), 8.13 (s, 1H, Ph-H), 7.94 (m, 2H, Ph-H), 7.8 (m, 2H, Ph-H), 7.71 (d, 1H, J = 7.8 Hz, Ph-H), 7.61 (d, 1H, J = 15.1 Hz, -CH=CH-), 6.87 (d, 2H, J = 8.9 Hz, -CH=CH-), 4.82 (s, 2H, ⁺N-CH₂-), 4.17 (m, 4H, -N-CH₂-), 3.36 (s, 2H, -CH₂-), 2.34 (m, 2H, J = 7.4 Hz, -CH₂-), 1.8 (s, 2H, -CH₂-), 1.62 (m,2H, -CH₂-), 1.37 (m, 2H, J = 8.0 Hz, -CH₂-), 1.20 (d, 6H, J = 7.0 Hz, -CH₃). FTIR (cm^–1) 1711 (C=O stretching), 1644 (C=C), 1620 (C=N), 1569 (Ph), 1093 (C-O-C, asymmetric) and (E)-1-(5-carboxypentyl)-2-(4-(diethylamino)styryl)-3,3-dimethyl-3H-indol-1-ium bromide (7b) was synthesized in 73.1% yield. ¹H-NMR (DMSO-d₆, 600 MHz): δ 8.32 (d, 1H, Ph-H), 8.05 (s, 2H, Ph-H), 7.70 (d, 1H, J = 8.3 Hz, Ph-H), 7.54 (s, 1H, Ph-H), 7.47 (d, 2H, Ph-H), 7.21 (d, 1H, J = 15.7 Hz, -CH=CH-), 6.88 (d, 1H, J = 9.1 Hz, -CH=CH-), 6.52 (s, 1H, Ph-H), 4.49 (s, 2H, ⁺N-CH₂-), 3.56 (d, 4H, J = 7.0 Hz, -N-CH₂-), 2.61 (s, 2H, -CH₂-), 2.38 (s, 2H, -CH₂-), 2.29 (t, 2H, J = 7.3 Hz, -CH₂-), 1.75 (m, 6H, -CH₃), 1.58 (d, 2H, -CH₂-), 1.18 (t, 6H, J = 7.1 Hz, -CH₃). FTIR (cm^–1) 1711 (C=O, stretching), 1644 (C=C), 1620 (C=N), 1570 (Ph), 1080 (C-O-C, asymmetric).

Graf Polymeric Dye Synthesis

Hemicyanine dye (3, 7a, or 7b, 5.00 mmol), DMTMM (0.5826 g, 2.50 mmol), and O-CMC (2.51 g) were dissolved in 100 mL water in a round-bottom flask equipped with a magnetic stirring apparatus. The reaction mixture was stirred at room temperature for 3–4 h. The mixture was dialyzed for several days until the water was transparent and colorless to give products 4, 8a, or 8b.

Polymeric Dye Characterization

The distinction between Fourier transform infrared (FTIR) spectra of O-CMC and polymeric dyes can be seen in Fig. 1. Compared to the FTIR spectra of O-CMC, the bands at 1150 cm^–1 (C-N stretch in Ph-NH₂) and 899 cm^–1 (N-H amino deformation) were not present in the IR spectra of the polymeric dyes. In contrast, amide N-H stretching peaks are seen for 4 (3497 cm^–1), 8a (3482 cm^–1), and 8b (3476 cm^–1) in IR spectra of the polymeric dyes.

Fig. 1

FTIR spectra of the polymeric dyes and polymer skeleton.

The various amide characteristic absorption peaks of polymeric dyes 4, 8a, and 8b can be seen in Fig. 1. For amide I, the absorption peaks were 1664, 1664, and 1679 cm^–1, respectively. For amide II, the absorption peaks were 1541, 1540, and 1543 cm^–1, respectively. Amide III absorption peaks were found at 1340, 1340, and 1337 cm^–1, respectively. The ¹³C-NMR data is in Table I.

Table I

¹³C-NMR of Polymeric Dyes

Dye	¹³C-NMR (101 MHz, δ, ppm)
Polymeric dye- 4	174.19 (s, -CO-NH-), 149.48 (s, Py-C), 147.14 (s, Ph-C), 141.39 (s, Py-C), 126.58 (s, Ph-C), 123.59 (s, -CH=CH-), 121.33 (s, Py-C), 119.89 (s, Ph-C), 112.85(s, Ph-C), 87.43-62.85 (m, O-CMC), 58.82 (s, -N-CH₂-), 54.58 (s, O-CMC), 47.58 (s, O-CMC) 43.47–40.86 (m, O-CMC), 39.3 (s, -N-CH₂), 23.59–20.35 (m, CH₂), 10.99 (s, -CH₃)
Polymeric dye-8a	174.97 (s, -CO-NH-), 169 (s, N=C-S), 149.49 (s, Ph), 133.96 (s, -CH=CH-), 127.27–116.93 (m, Ph-C), 111.41 (s, -CH=CH-), 105–94.48 (m, Ph-C), 78.78–62.02 (m, O-CMC), 44.89 (s, O-CMC), 38.74–29.07 (m, -CH₂-), 23.22–16.23 (m, -CH₂-), 10.10 (m, -CH₃)
Polymeric dye-8b	175.39 (s, -CO-NH-), 170.77 (s, -N=C-), 154.00 (s, -CH=CH-), 129.36– 125.17 (m, Ph-C), 122.96–118.82 (s, Ph-C), 114.62 (s, -CH=CH-), 99.59–96.31 (m, Ph-C), 85.68–61.02 (m, O-CMC), 58.61–47.25 (m, O-CMC and -CH₂-), 38.96–30.82 (m, -CH₂-), 24.07–19.91 (m, -CH₂- and -CH₃), 16.12–12.25 (m, -CH₃)

Dyeing

The hemicyanine dyes (3, 7a, and 7b) and their corresponding polymeric dyes (4, 8a, and 8b) were used to dye acrylic fabrics (2 g/piece, woven, from Shanghai Textile Industry Institute of Technical Supervision). The fabrics were dyed in an X-5 Dyeing machine (Foshan Huangju). The dye solutions were prepared with the required amount of each dye (200% owf), sodium sulfate (3 g/L, Sinopharm Chemical Reagent Co. Ltd.), and levelling agent O (0.5 g/L). The pH of the dyebath was maintained at 4.5-5.0 by an acetic acid-sodium acetate (both supplied by Sinopharm Chemical Reagent Co. Ltd.) buffer solution. The liquor-to-goods ratio was kept at 50:1. The schematic representation of this dyeing procedure is given in Fig. 2. After immersing acrylic fabric in the dye solutions at room temperature (RT), the temperature was increased to 100 °C at the rate of 1 °C/min, with a holding time of 60 min, and finally cooled to 70 °C at 1.25 °C/min. At the end of dyeing, the dyed acrylic fabric was rinsed thoroughly in distilled water and allowed to dry in the open air.

Fig. 2

Time-temperature profile of dyeing process.

Analysis

¹H-NMR spectra were recorded with a GCT-TOF NMR spectrometer (Varian) at RT using tetramethylsilane (TMS) as the internal standard. ¹³C-NMR spectra of the solid was recorded with a GCT-TOF NMR spectrometer at 101 MHz. The reflectance spectra, chromaticity (x, y coordinates), and apparent color depth (K/S) of the dyed fabrics were measured using a HunterLab UltraScan Pro reflectance spectrophotometer (standard illuminant D₆₅, 10° standard observer). The emission spectra of the dyed fabrics were recorded using a Horib-Fluoroma-4 fluorescence spectrophotometer (Horiba).

Results and Discussion

Color Characteristics

Digital photographs of dyed acrylic fabrics and aqueous polymeric dye solutions are shown in Fig. 3. Polymeric dyes 4, 8a, and 8b yielded bright brown, pink, and purple shades, respectively. From the color coordinates (L*, a*, and b*) shown in Table II, the color parameters of acrylic fabrics dyed with dyes 3, 4, 7a, and 8a were in the yellow-red section of color space, whereas color parameters of dyes 7b and 8b were in the red-blue section of color space. The a*–b* data was in good agreement with the visual appearance of the dyed fabrics. A small darkening effect was seen for the polymeric dyes compared to their respective parent hemicy-anine dye, which is evident from the slightly lower L* values of fabrics dyed with polymeric dyes. This darkening effect could be due to better fixation of polymeric dye.²¹

Fig. 3

Chemical structure and images of dyed acrylic fabric and aqueous solution of polymeric dyes.

Table II

Color Characteristics of Dyed Acrylic Fabrics

Fabric Dye	L*	a*	b*
Polymeric dye-4	89.245	25.5	36.69
Dye-3	91.62	19.36	24.34
Polymeric dye-8a	76.77	55.52	12.22
Dye-7a	77.6	51.4	7.795
Polymeric dye-8b	69.425	57.61	-9.36
Dye-7b	75.59	49.03	-7.66

Fluorescence Properties

Reflectance and fluorescence spectra are important for characterizing fabric dyed with fluorescent dyes. The visual effects of fluorescent dyes involve both the color when dyes absorb visible light selectively and the resulting emission fluorescence. In Fig. 4, the absorption spectra were in the range of 400 to 550 nm. The reflectance of the undyed acrylic fabrics was around 80%. The maximum reflectivity of acrylic fabrics dyed with three polymeric dyes was around 120%. Furthermore, there were obvious emission peaks in emission spectra of fabrics dyed with the three polymeric dyes (Fig. 5). The maximum emission wavelengths of three polymeric dyes were 558 (4), 578 (8a), and 580 nm (8b), and the excitation wavelengths were 480 (4), 500 (8a), and 494 nm (8b). The reflectance and the emission spectra confirmed that the acrylic fabrics dyed with the three polymeric dyes were fluorescent. The very similar reflectance curves and emission spectra of fabrics dyed with 8a and 8b are due to the closely related chemical structures of these two polymeric dyes.

Fig. 4

Reflectance spectra of acrylic fabrics dyed with polymeric dyes 4, 8a, and 8b.

Fig. 5

Emission spectra of acrylic fabrics dyed with polymeric dyes 4, 8a, and 8b.

Lightfastness

Fig. 6 shows the effect of light exposure on the K/S values of fabrics dyed with dyes 3 and 4. After 4 h of light exposure, K/S values for the fabric dyed with 3 decreased by 73.8%, whereas a 64.7% decline was observed for the fabric dyed with polymeric dye 4. Similarly, K/S values of fabrics dyed with polymeric dyes 8a and 8b were less affected than fabrics dyed with parent hemicyanine dyes 7a and 7b after the dyed fabrics were exposed to light for 4 h (Figs. 7 and 8). The K/S value of fabric dyed with polymeric dye 8b decreased by 7.09%, and the K/S value of fabric dyed with 7b decreased by 8.11% after light exposure (Fig. 7). This indicates that the fabrics dyed with polymeric dyes had better lightfastness than fabrics dyed with hemicyanine dyes. The better lightfastness of the fabrics dyed with polymeric dyes may be due to molecular modifications promoting the consumption of excess energy not converted into emission that might otherwise lead to early dye degeneration.¹⁷

Fig. 6

K/S values of fabric dyed with polymeric dyes 3 and 4 before and after 4 h light exposure.

Fig. 7

K/S values of fabric dyed with polymeric dyes 7b and 8b before and after 4 h light exposure.

Fig. 8

K/S values of fabric dyed with polymeric dyes 7a and 8a before and after 4 h light exposure.

Fig. 9 shows the effect of light exposure duration on relative change in K/S values of fabrics dyed with polymeric dyes and parent hemicyanine dyes. Overall, the K/S values of fabrics dyed with the three polymeric dyes changed less than K/S values of the fabrics dyed with hemicyanine dyes, indicating a greater photostability of the polymeric dyes on the fabric. This is due to less photoinduced degradation of the polymeric dye molecules attached to the fabric surface,²² and due to improved consumption of spare energy, as previously noted.¹⁷

Fig. 9

Effect of light exposure duration on changes in K/S values.

Crockfastness

Wet and dry crockfastness of fabric dyed by three polymeric dyes were tested according to the GB/T 3920-2008 standard and evaluated by ISO 105/A03-1993 (Table III). For the fabrics dyed with polymeric dyes, dry crock ratings of 4–5 and wet crock ratings of 4 were attained. Due to the polymeric dyes being ionically-bonded to the fiber, the fabric dyed by the polymeric dyes had similar crockfastness values.

Table III

Crockfastness of Acrylic Fabric Dyed with Polymeric Dyes

Fabric Dye	Crockfastness
Fabric Dye	Dry Crockfastness	Wet Crockfastness
Polymeric dye-4	4–5	4
Polymeric dye-8a	4–5	4
Polymeric dye-8b	4–5	4

Conclusions

Tree hemicyanine based fluorescent polymeric dyes (4, 8a, and 8b) were synthesized and successfully applied to the dyeing of acrylic fabrics. The dyed acrylic fabrics exhibited bright colors. The reflectivity of the dyed acrylic fabrics was around 120%, and the range of the emission wavelength was from 550 to 650 nm; fabrics dyed with the polymeric dyes had good fluorescence properties. The photostability of fabrics dyed with polymeric dyes were better than fabrics dyed with parent hemicyanine dyes, especially for the acrylic fabric dyed with polymeric dye 8b.

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