Synthesis of fluorescent cationic coumarin dyes with rigid molecular structures to improve lightfastness and their related modacrylic dyed fibers

Article abstract of DOI:10.1016/j.dyepig.2021.109294

In this study, three novel cationic, fluorescent, coumarin dyes (6a, 6b and 6c) with rigid molecular structures were synthesized and characterized by spectroscopic studies. The photochemical behavior (UV light) of these dyes was studied in ethanol and DMF

Full text of DOI:10.1016/j.dyepig.2021.109294

Dyes and Pigments 190 (2021) 109294
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthesis of fluorescent cationic coumarin dyes with rigid molecular
structures to improve lightfastness and their related modacrylic dyed fibers
Raju Penthala, Young-A Son*
Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this study, three novel cationic, fluorescent, coumarin dyes (6a, 6b and 6c) with rigid molecular structures
were synthesized and characterized by spectroscopic studies. The photochemical behavior (UV light) of these
dyes was studied in ethanol and DMF solvents. Compared to that of a commercial dye (basic yellow-40), the
synthesized dyes exhibited good photostability in these solvents. A dyeing experiment was performed on mod-
acrylic fibers with these dyes using a traditional cationic dyeing process. The effect of pH on the absorption
spectra and dye bath exhaustion were also studied. The dyeing results showed that all the dyes exhibited obvious
fluorescence effects. Based on the luminance factor (β), and photostability behavior, the synthesized dyes can
hopefully be applied in high-visibility, fluorescent, yellow warning textiles with improved lightfastness.
Coumarin
Cationic fluorescent dyes
Photostability
Modacrylic fibers
Dyeing
1. Introduction
in various biological applications [7–10]. However, one of the major
uses is in the textile industry, and fluorescent dyes have long been used
In the textile industry, synthetic fibers are one of the leading mate-
rials used and exceed the production of natural fibers. Among synthetic
fibers, acrylic fibers (modacrylic, polyacrylonitrile, and polyamide and
m-aramid fibers) are the most important after polyester and nylon fibers
[1,2]. Generally, modacrylic fibers are made from a polymer chain of
acrylonitrile units (less than 85%). These fibers are inherently flame
resistant and exhibit desirable properties, such as chemical resistance,
good elasticity, a soft, wool-like texture, high resistance to outdoor
exposure, good hand-feel, resilience and dimensional stability [3]. Due
to these properties, these fibers are widely used in high-performance
protective cloth, such as firefighting turnout gear, paint-roller covers,
scatter rugs, and carpets [4].
for the dyeing of textiles. These dyes make the dyed textiles highly
perceptible due to significant increase in color brightness, saturation
and visibility [4]. This intensifies the perceptibility of the textiles, which
are mainly apparel for firefighters and policemen, sports clothes, but
further useful applications exist for traffic and safety purposes. Due to
their wide applicability, there is an increasing demand for the devel-
opment of high-visibility textiles [11].
Several cationic fluorescent dyes, including coumarin [12–14],
pyrene [15], acridine [16], xanthene [17], 1,8-naphthalimide [18] and
anthraquinone [19], are available. However, a limited number of
cationic fluorescent dyes have exhibited technical performance char-
acteristics of the dye application class regarding the color index on
acrylic fibers. Among them, coumarins are of special interest and are
commercially the most important dyes, and they produce bright
greenish yellow shades of excellent brilliance [20]. An important com-
mercial product is C.I. Basic yellow-40, which is often applied in the
dyeing of acrylic fibers [21]. However, this commercial fluorescent dye
exhibited poor lightfastness properties [22]. Hence, it did not meet the
standards required for more demanding applications where prolonged
exposure to light is involved (traffic) [20]. Moreover, multiple fused
rings with highly rigid molecular structures exhibit high fluorescent
quantum yields as well as photostability [23]. To improve the light-
fastness of coumarin dyes (basic yellow-40), we designed and
In general, anionic dyes, namely, reactive, acid, and direct dyes, are
not suitable for the dyeing of acrylic fibers. These dyes mainly suffer
from repulsion effects between the anionic groups present in the dyes
and those present in the acrylic fibers [5]. Disperse dyes and cationic
dyes have been used, but disperse dyes exhibit poor affinity for the fibers
and give pale to medium shades. Only cationic dyes produce deep shades
and high tinctorial strength and have good water solubility, but they
exhibit poor lightfastness [6].
Fluorescent dyes generally absorb and emit radiation in most of the
visible spectral range. These dyes are used in several ways, such as in
lasers, solar collector systems, organic light-emitting diodes (LEDs) and
* Corresponding author.
E-mail address: yason@cnu.ac.kr (Y.-A. Son).
https://doi.org/10.1016/j.dyepig.2021.109294
Received 13 December 2020; Received in revised form 23 February 2021; Accepted 4 March 2021
Available online 11 March 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
synthesized novel cationic dyes with rigid molecular structures that
were obtained by performing a condensation reaction between 7-dieth-
ylamino-3-(2-benzimidazolyl)-iminochromene and aromatic aldehydes.
The synthesized cationic, fused coumarin dyes exhibiting shades of
green fluorescent yellow were structurally analyzed, and their tinctorial
properties were tested in the dyeing process of modacrylic fibers.
26000 spectrophotometer was used to record reflectance spectra for all
the dyed fibers. An AB Sciex 4000 QTRAP mass spectrometer was used to
record mass spectral data. Bruker micrOTOF-Q spectrometer was used to
record HRMS spectral data. An ALPHA-P spectrometer was used to re-
cord Fourier transform infrared (FT-IR) spectra. An Electrothermal-
IA9100/OA melting point apparatus was used to visually determine
melting points with the capillary tube method.
2. Experimental
2.3. Synthesis
2.1. Chemicals and materials
A three-step route was involved in the synthesis of the designed
targets (Scheme 1). In the first step, a 2-iminocoumarin derivative (3)
was obtained by performing a condensation reaction followed by a
cyclization reaction between 4-(diethylamino)-2-hydroxybenzaldehyde
(1) and 2-cyano methyl benzimidazole (2) in the presence of piperidine
[24]. Due to the high reactivity of this 2-iminocoumarin derivative, it
readily undergoes reaction with electrophiles and nucleophiles. In this
study, aromatic aldehydes were used to react with these 2-iminocou-
marin derivatives in the presence of catalytic amounts of piperidine
[25]. Hence, in these reaction, rigid molecular structure (5) was ob-
tained by the special fixation of benzimidazolic NH and iminochromene
C=N moiety with a methylene bridge. Furthermore, these rigid struc-
tures reacted with methyl iodide in the presence of dimethylformamide
to yield the cationic fluorescent dyes (6) [26].
We purchased 4-(diethylamino)-2-hydroxybenzaldehyde, 2-(1H-
benzo [d]imidazole-2-yl)acetonitrile, 4-methoxybenzaldehyde, 2-hy-
droxy-3-methoxybenzaldehyde, and 4-formylbenzoic acid from TCI
(Tokyo Chemical Industry Co., LTD.). Methyl iodide was purchased from
Sigma Aldrich, phosphoric acid, acetic acid, boric acid and piperidine
from Alfa Aesar, and sodium hydroxide, anhydrous sodium sulfate
(Na2SO4), ethanol, n-butanol, N,N-dimethylformamide, hexane, ethyl
acetate, and diethyl ether from Samchun Pure Chemical Co., Ltd. Basic
yellow-40 and a levelling agent were supplied by Ukseung Chemical Co.,
Ltd. All the chemicals, reagents and solvents were supplied from the
chemical industries with more than 98% purity and were of HPLC grade.
Modacrylic fiber was supplied from the Korean Textile Development
Institute.
2.2. Characterization
2.4. Experimental details
By using silica gel G60 F254 (Merck) plates, we performed thin layer
chromatography (TLC). Product formation was monitored by visuali-
zation on a TLC plate with a UV lamp (254 and 366 nm). By using an
AVANCE III spectrometer, we recorded 1H NMR spectra at 600 MHz and
13C NMR at 150 MHz using CDCl3 and DMS-d6 solvents. Tetra methyl
silane (TMS) is used as an internal standard, and chemical shift values
are given in ppm. UV–visible absorption spectra were recorded for all
dyes by using an Agilent 8453 spectrophotometer. A Shimadzu UV
2.4.1. General experimental procedure for the synthesis of compounds 5a,
5b and 5c
3-(1H-benzo [d]imidazole-2-yl)-N,N-diethyl-2-imino-2H-chromen-
7-amine (3) (1 g, 3.01 mmol) and aromatic aldehyde (3.01 mmol) were
taken in a round bottom flask and added n-butanol and a catalytic
amount of piperidine (0.15 mmol) and refluxed for 3 h. After completion
of reaction, the reaction mixture was cooled to room temperature and
left for 4–5 h. The solid was formed and filtered and washed with cold
Scheme 1. Synthesis of cationic florescent coumarin dyes.
2

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
ethanol.
1H NMR (DMSO‑d6, 600 MHz): δ 9.58 (s, 1H), 9.04 (s, 1H), 7.95 (d, J =
9.0 Hz, 1H), 7.69 (d, J = 8.1 Hz, 1H), 7.65 (s, 1H), 7.32 (d, J = 7.5 Hz,
1H), 7.26–7.22 (m, 1H), 7.20 (d, J = 7.7 Hz, 1H), 7.16–7.13 (m, 2H),
7.06 (d, J = 8.0 Hz, 1H), 6.91 (t, J = 8.0 Hz, 1H), 3.73 (s, 3H), 3.59 (q, J
= 7.1 Hz, 4H), 3.39 (s, 3H), 1.20 (t, J = 7.1 Hz, 6H); 13C NMR (DMSO‑d6,
600 MHz): δ 155.6, 154.3, 151.2, 148.2, 145.5, 144.4, 133.1, 130.5,
129.5, 126.4, 122.7, 122.4, 119.6, 118.8, 112.0, 111.1, 108.5, 108.2,
105.9, 97.3, 56.2, 44.5, 40.5, 12.8; HRMS: 481.2263 [M]+, calculated
for C29H29N4O+3 : 481.2234.
2.4.2. N,N-diethyl-7-(4-methoxyphenyl)-7H- benzo[4,5]imidazo[1,2-c]
chromeno [3,2-e] pyrimidin-3-amine (5a)
Yellow solid; Yield: 68% (0.88 g); mp: 263–265 ◦C; FT-IR (KBr)
(
νmax/cmꢀ 1): 3055, 2969, 2845, 1674, 1595, 1259, 1175; 1H NMR
(DMSO‑d6, 600 MHz): δ 8.14 (s, 1H), 7.61 (d, J = 7.9 Hz, 1H), 7.48 (d, J
= 8.2 Hz, 1H), 7.32–7.04 (m, 6H), 6.89 (d, J = 7.7 Hz, 2H), 6.62 (d, J =
8.2 Hz, 1H), 6.45 (s, 1H), 3.70 (s, 3H), 3.44 (q, J = 7.3 Hz, 4H), 1.13 (t, J
= 7.3 Hz, 6H); 13C NMR (DMSO‑d6, 150 MHz): δ 155.7, 154.2, 151.2,
148.5, 145.1, 144.7, 132.8, 131.1, 129.7, 126.6, 122.3, 122.1, 119.4,
108.5, 108.1, 105.4, 97.3, 56.2, 44.6, 12.7; HRMS: 451.2122 [M+H]+,
calculated for C28H27N4O2: 451.2134.
2.4.8. 7-(4-Carboxyphenyl)-3-(diethylamino) -6-methyl-7H-benzo [4,5]
imidazo[1,2-c] chromeno[3,2-e]pyrimidin-6-ium (6c)
Yellow orange solid; yield: 60% (0.31 g); mp: 272–274 ◦C; FT-IR
(KBr) (νmax/cmꢀ 1): 3305, 2995, 1719, 1625, 1584, 1509, 1476, 1419,
1354, 1251, 1184, 1132; 1H NMR (DMSO‑d6, 600 MHz): δ 8.56 (s, 1H),
8.25–8.13 (m, 5H), 8.12–8.08 (m, 2H), 7.77–7.74 (m, 2H), 7.66 (d, J =
9.0 Hz, 1H), 6.91 (dd, J = 9.0, 2.2 Hz, 1H), 6.74 (d, J = 2.0 Hz, 1H), 3.56
(q, J = 7.1 Hz, 4H), 3.12 (s, 3H), 1.17 (t, J = 7.1 Hz, 6H); 13C NMR
(DMSO‑d6, 600 MHz): δ 167.6, 155.6, 154.3, 151.4, 145.2, 144.6, 132.9,
130.7, 130.3, 126.7, 123.0, 122.7, 119.0, 111.4, 108.7, 108.1, 104.9,
97.3, 72.8, 44.6, 40.4, 12.8. HRMS: 480.2155 [M+H]+, calculated for
2.4.3. 2-(3-(Diethylamino)-7H-benzo[4,5]imidazo[1,2-c] chromeno[3,2-
e] pyrimidin-7-yl)-6-methoxyphenol (5b)
Yellow orange solid; Yield: 65% (0.91 g); mp: 271–273 ◦C; FT-IR
(KBr) (νmax/cmꢀ 1): 3361, 2989, 2925, 2871, 1651, 1625, 1589, 1249,
1171; 1H NMR (DMSO‑d6, 600 MHz): δ 9.04 (s, 1H), 8.09 (s, 1H), 7.59
(d, J = 7.9 Hz, 1H), 7.47 (s, 1H), 7.45 (s, 1H), 7.18–7.12 (m, 2H),
7.08–7.03 (m, 1H), 6.90–6.86 (m, 1H), 6.76–6.67 (m, 2H), 6.60 (dd, J =
8.8, 2.4 Hz, 1H), 6.43 (d, J = 2.0 Hz, 1H), 3.75 (s, 3H), 3.48–3.38 (m,
4H), 1.12 (t, J = 7.1 Hz, 6H); 13C NMR (DMSO‑d6, 150 MHz): δ 153.3,
150.2, 147.2, 143.7, 132.6, 129.6, 129.5, 126.1, 122.7, 121.8, 119.1,
117.3, 111.7, 111.3, 108.5, 105.2, 96.3, 55.5, 43.5, 12.7; HRMS:
467.2069 [M+H]+, calculated for C28H27N4O3: 467.2083.
C
29H28N4O+3 : 480.2156.
2.5. Dye dispersion preparation
The dye dispersion solutions were prepared with cationic dyes and a
dispersing agent (0.5 g/l). In the dye dispersion process, the required
amount of cationic dyes (1%, 2%, 3%, 4%, 5%, and 7% owf) was first
separately dissolved in ethanol and ethylene glycol (3:1 mol ratio) and
sonicated for 10 min. Further, a 60 ◦C temperature was applied for 5 min
to completely dissolve the dyes. These soluble dyes were properly mixed
with the required molar ratio of the dispersing agent (Polyoxyethylene
alkyl ether AEO-40). The dye dispersion solutions were further stirred at
60 ◦C for 1 h and at room temperature for 1 h. The final dye dispersion
solutions were used for dyeing.
2.4.4. 4-(3-(Diethylamino)-7H-benzo[4,5] imidazo[1,2-c] chromeno[3,2-
e] pyrimidin-7-yl)benzoic acid (5c)
Yellow solid; yield: 64% (0.89 g); mp: 324–326 ◦C; FT-IR (KBr)
(
νmax/cmꢀ 1): 3369, 2948, 2918, 2865, 1655, 1601, 1555, 1522, 1255,
1128; 1H NMR (DMSO‑d6, 600 MHz): δ 13.01 (brs, 1H), 8.19 (s, 1H),
7.92 (d, J = 8.4 Hz, 2H), 7.63 (d, J = 8.0 HZ, 1H), 7.50 (d, J = 8.8 Hz,
1H), 7.46 (d, J = 8.2 Hz, 2H), 7.44 (s, 1H), 7.21–7.16 (m, 1H), 7.07 (d, J
= 3.7 Hz, 2H), 6.64 (dd, J = 8.8, 2.4 Hz, 1H), 6.46 (d, J = 2.2 Hz, 1H),
3.47–3.41 (m, 4H), 1.12 (t, J = 7.1 Hz, 6H); 13C NMR (DMSO‑d6, 150
MHz): δ 164.8, 155.2, 154.7, 151.0, 144.7, 144.1, 133.2, 130.7, 130.5,
129.9, 126.1, 123.5, 123.0, 118.6, 111.7, 108.8, 107.7, 105.1, 96.9,
71.2, 43.8, 12.3. HRMS: 465.1919 [M+H]+, calculated for C28H25N4O3:
465.1927.
2.6. Dyeing
The dyeing experiment was carried out with an ACE-6000 T model
infrared laboratory-scale dyeing instrument by following the traditional
cationic dyeing process. The dye bath pH was maintained with buffer
solutions (phosphoric acid-acetic acid-boric acid/alkaline sodium hy-
droxide). The modacrylic fiber (500 mg samples) were immersed in the
above dye dispersion solutions 1:50 (liquor ratio) at 20 ◦C. The dye bath
temperature was slowly raised to 80 ◦C at a rate of 2 ◦C/min and further
increased to 95 ◦C at a rate of 1 ◦C/min. The dyeing was continued at this
temperature for 60 min (Fig. 1). Furthermore, the dyeing bath was
slowly cooled to room temperature. The dyed modacrylic fibers were
separated and rinsed in distilled water and allowed to dry in the open
air.
2.4.5. General experimental procedure for the synthesis of dyes 6 (a-c)
A solution of compound 5 (1.0 mmol) in dimethylformamide (15 ml)
was added to methyl iodide (2.5 mmol) at room temperature, and the
reaction mixture was stirred at 80 ◦C for 16 h. The reaction mixture was
cooled to room temperature, and dimethylformamide was evaporated in
vacuo. The resulting solid was filtered and washed with diethyl ether:
hexane (1:1, 50 mL) to afford the title compounds.
2.4.6. 3-(Diethylamino)-7-(4-methoxyphenyl)- 6-methyl-7H-benzo[4,5]
imidazo[1,2-c] chromeno[3,2-e] pyrimidin-6-ium (6a)
Yellow orange solid; yield: 66% (0.34 g); mp: 288–290 ◦C; FT-IR
(KBr) (νmax/cmꢀ 1): 2990, 2955, 1649, 1576, 1538, 1509, 1322, 1251,
1170 1009; 1H NMR (DMSO‑d6, 600 MHz): δ 9.59 (s, 1H), 9.05 (s, 1H),
7.95 (d, J = 9.0 Hz, 1H), 7.70 (d, J = 8.0 Hz, 1H), 7.67–7.6 5 (m, 1H),
7.35–7.18 (m, 4H), 7.17–7.12 (m, 2H), 7.07 (d, J = 7.1 Hz, 1H),
6.94–6.89 (m, 1H), 3.74 (s, 3H), 3.60 (q, J = 7.3 Hz, 4H), 3.40 (s, 3H),
1.21 (t, J = 7.3 Hz, 6H); 13C NMR (DMSO‑d6, 600 MHz): δ 155.5, 155.0,
151.3, 146.0, 144.4, 139.9, 132.8, 130.6, 126.6, 125.2, 122.8, 122.6,
119.0, 117.1, 111.0, 108.6, 105.9, 97.3, 44.5, 40.4, 12.8; HRMS:
465.2337 [M]+, calculated for C29H29N4O+2 : 465.2285.
3. Results and discussion
3.1. Absorption spectra of dye solutions at various pH values
In this study, the optical properties of the synthesized dyes were
measured at different pH values to understand the effect of pH on the
dye bath. From the results, the absorption spectra of the three cationic
dyes changed slightly when the pH values were between 4.9 and 7.92,
whereas a large change was observed in the absorption spectra when the
pH values were between 4.03 and 2.08 (Fig. 2a). This phenomenon
could be explained by the reversible change of the molecular structures
of the dyes at different pH values. Due to the presence of a potential
protonation center (3rd position nitrogen atom), at low pH values, these
fluorescent monocationic dyes can be converted to nonfluorescent
dicationic dyes by the abstraction of protons by the potential center (the
2.4.7. 3-(Diethylamino)-7- (2-hydroxy-3-methoxyphenyl)- 6-methyl-7H-
benzo[4,5] imidazo[1,2-c]chromeno[3,2-e]pyrimidin-6-ium (6b)
Yellow orange solid; yield: 58% (0.3 g); mp: 305–307 ◦C; FT-IR (KBr)
(
νmax/cmꢀ 1): 3005, 2989, 1628, 1582, 1380, 1322, 1265, 1174, 1076;
3

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
Fig. 1. Profile of the dyeing experiment.
Fig. 3. Effect of pH on dye exhaustion (dye concentration: 1% owf).
A0 ꢀ Ai
%E = 100 X
(1)
A 0
Where A0 is the absorbance of a dye solution before exhaustion and Ai is
the absorbance of a dye solution after exhaustion. From the results, at pH
2.08, the exhaustion of all the dyes seemed to be slightly low; by
increasing the pH value, the exhaustion was also slightly increased and
almost constant within the pH range of 5.0–7.9. From these results, it
can be concluded that the modacrylic fibers adsorbed both mono- and
dications. Due to the presence of a nonacidic microenvironment in the
inner region of the modacrylic fibers, the dications may have been
converted to monocations [22]. By considering the dye exhaustion re-
sults and the stability of modacrylic fibers, as well as that of the syn-
thesized fluorescent dyes, the dye bath pH could be fixed at 5.5 to obtain
maximum dye exhaustion.
Fig. 2. (a) pH-dependent changes in the absorption spectra of dye 6b and (b)
pH-dependent change in the molecular structure of the dyes.
nitrogen atom) [26,27]. As a result, the electron-donating ability of the
3rd position -amino functional group was reduced, thereby decreasing
the electron transfer capability from the 3rd position -amino group to
the coumarin moiety (Fig. 2b). Hence, the absorbance at the maximum
absorption wavelength of these dyes decreased in the visible range with
decreasing pH.
3.2. Effect of pH on dye exhaustion
By considering the effect of pH on the molecular structures of the
synthesized dyes, it is important to study how the pH values can influ-
ence dye exhaustion. Herein, the percentage of dye exhaustion (%E) was
studied with 1% owf of the dyes at various pH values ranging from 2.05
to 7.92. The results are shown in Fig. 3. The percentage of exhaustion (%
E) was determined by using the following equation.
3.3. Photophysical properties
The UV–visible absorption and fluorescence emission of the syn-
thesized dyes 6a, 6b and 6c were recorded in various solvents (ethanol,
DMSO, ACN, DMF, CHCl3 and toluene) at a concentration of 0.1 × 10ꢀ 4
mol/l. The results are summarized in Table 1. These three compounds
were fluorescently yellow in the solutions and were also observed to be
4

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
Table 1
Spectral properties of the dyes 6a, 6b and 6c in different solvents.
Solvent
6a
6b
6c
ꢀ 1 ꢀ 1
ꢀ 1 ꢀ 1
ꢀ 1 ꢀ 1
λabs
λemi
Ɛ
Φf
λabs
λemi
Ɛ
Φf
λabs
λemi
Ɛ
(M cm)
Φf
(M cm)
(M cm)
Ethanol
DMSO
ACN
435
435
433
433
416
409
493
493
491
493
487
487
31,830
30,406
30,013
26,942
25,125
19,471
0.92
0.84
0.92
0.88
0.94
0.93
501
501
501
501
494
490
539
539
538
538
537
537
47,195
45,424
43,902
37,915
35,601
34,417
0.94
0.86
0.93
0.89
0.96
0.95
435
434
434
431
422
422
492
492
490
490
485
485
44,804
40,989
38,280
38,503
35,187
30,521
0.91
0.85
0.9
DMF
0.87
0.95
0.94
CHCl3
Toluene
sensitive to the solvent polarities. Compound 6a showed intense ab-
sorption between 409 nm and 435 nm in the tested solvents. Similarly,
compound 6b showed an absorption maximum 491–502 nm. Further-
more, compound 6c exhibited absorption maxima at 422 nm–435 nm.
The substituents on the phenyl ring (minor chromophoric unit) have a
very little influence on the absorption spectra. The short wavelength
absorption bands corresponding to these chromophore units are over-
lapped with the most intense absorption bands of the main chromo-
phoric unit. Hence the correct identification was not possible [27].
Compound 6b showed red shifted absorption and emission when
compared to those of compounds 6a and 6c. The red shifted absorption
and emission were mainly due to the presence of hydroxy (–OH) and
methoxy (–OMe) auxochromes. Moreover, there is a possibility to form
an intra-molecular hydrogen bond between the –OH group and the
benzimidazole nitrogen, the molecule 6b may undergo aggregation,
which could be responsible for the observed changes in the nature of the
molecule and the intensity of the absorption and emission spectra. The
excitation wavelength used for the measurement of the fluorescence
properties of the cationic dyes was the absorption maxima of the com-
pounds in the respective solvents. Compound 6a showed blue shifted
emission at approximately 487 nm in less polar solvents (toluene and
CHCl3) and red shifted emission at approximately 493 nm in more polar
solvents (ethanol, DMSO, ACN and DMF). Similarly, compounds 6b and
6c also exhibited blue shifted emissions at 537 and 485 nm and red
shifted emissions at 540 and 492 nm in less polar and more polar sol-
vents, respectively. In addition, compound 6b showed red shifted
emission compared to that of compounds 6a and 6c. The molar
extinction coefficients of the dyes (6a, 6b and 6c) varied from 19,459 to
47,195 molꢀ 1 cmꢀ 1. Fluorescence quantum yields of the synthesized
dyes were evaluated with respect to the known compound as
29]). The three dyes 6a, 6b and 6c exhibited higher quantum yields in
non-polar solvents like CHCl3 and toluene. Whereas, in the polar sol-
vents, like ethanol, DMSO, ACN and DMF, the quantum yields were little
lower. This may be due to destabilization of TICT state (twisted intra-
molecular charge transfer) in non-polar solvents [30].
3.4. Photodegradation
Photodegradation studies of the three synthesized cationic fluores-
cent dyes (6a, 6b and 6c) and basic yellow-40 (commercial dye) were
conducted in ethanol and DMF solutions to understand the photo-
stability of the synthesized dyes (Fig. 4). First, ~5.2 × 10ꢀ 5 mol/l dye
solutions in ethanol and DMF were prepared and kept in quartz vessels
and treated with Ushio SP-7 Ultra violet lamp having UXM-Q256BY light
source (275 mW at 365 nm). The intensity of light absorbed by the so-
lution (~1016 photons cmꢀ 3sꢀ 1) was determined by using uranyl oxalate
actinometry [31]. The percentage of dye degradation was determined by
measuring the absorption spectra of dye solutions at different time in-
tervals. The remaining dye concentration was determined spectropho-
tometrically. From the results, the degradation of all the synthesized
dyes and basic yellow-40 was somewhat enhanced in DMF and retarded
in ethanol. This may be because ethanol (protic) retards photodecom-
position due to participating in hydrogen bonding with the electroneg-
ative atom (nitrogen) of the chromophore, thereby inhibiting radical
formation [32]. Basic yellow-40 displayed less photostability when
compared to the synthesized dyes in ethanol (35%) and DMF (42%)
solutions. Whereas, for the synthesized dyes, after 12 h irradiation,
~9.3–27% degradation was observed in ethanol, and 15–29% degra-
dation was observed in DMF. The photostability order of the dyes was
6b > 6c > 6a > basic yellow-40. Compared to basic yellow 40, syn-
thesized dye molecules have highly rigid chromophore structural unit
coumarin-6 with known quantum yield (ethanol solvent (ϕ = 0.89 [28,
f
Fig. 4. Absorption spectra of dyes 6a, 6b, 6c and Basic yellow-40 in EtOH and DMF solutions subjected to UV irradiation at different time intervals.
5

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
Fig. 5. (a) Rigid coumarin, and (b) Flexible coumarin chromophore structural units.
(Fig. 5), they may undergo more compact molecular packing [33–36].
Hence this kind of molecular packing will enhance the photochemical
stability of the molecules [37,38]. The highest photostability was
observed for the 6b dye in an ethanol solution, which displayed only
~9.3% degradation. Further, this molecule has UV-absorbing unit (-OH)
in the dye chromophores, which may have participated an
intra-molecular hydrogen bonding with the benzimidazole nitrogen. As
a result, an additional six-membered ring was formed. Hence, more light
energy was required to break that bond [39–42]. According to the above
results, to predict the photostability of the synthesized dyes, ethanol was
slightly better than DMF solvent.
aggregated state within the fiber interior [22]. Hence, the fluorescence
quenching effect increased, and the reflectance of the dyed fiber
decreased.
3.6. Chromaticity and color assessment
To determine whether the synthesized, novel, fluorescent, cationic
dyes were applicable in high-visibility textiles, the dyed modacrylic fi-
bers were assessed before and after exposure to a xenon arc lamp and
studied the CIELAB coordinates like lightness (L*), color-opponent di-
mensions (a* and b*) chroma (C*), h (hue angle from 0◦ to 360◦) and
further chromaticity coordinates and luminance factor (β) were also
listed in Tables 2 and 3. According to the L* value, basic yellow-40 was
slightly lighter than the synthesized dyes, and the L* values of the dyes
varied from 87.21 to 89.03. The negative value of a* for all the dyes
indicated that the color hues were shifted toward the green on the red-
green axis, and the positive values of b* indicated that the color hues
were shifted toward yellow on the yellow-blue axis. The chroma (C*)
values varied from 57.08 to 69.65. According to the chroma results,
compound 6c was slightly duller than the other dyes 6a and 6b. Basic
yellow-40 was slightly brighter than the synthesized dyes. From the
chromaticity diagram (Fig. 7), all the synthesized dyes were placed in
the fluorescent yellow region before and after exposure to the xenon arc
lamp. The luminance factor (β) values for all the dyed fibers 0.81-0.75
(before irradiation) and 0.71-0.67 (after irradiation). Basic yellow-40
exhibited a higher (β) value compared to the synthesized dyes. Based
on the chromaticity coordinates (x, y), the degradation percentage of
order is basic yellow-40 (35%) > 6a (27%) > 6c (19%) > 6b (18%).
Among the tested dyes 6b and 6c exhibited high photostability. Hence,
some of the newly synthesized dyes exhibited good photostability when
compared to the basic yellow-40. Based on the above results, the syn-
thesized, novel, fluorescent, cationic dyes could potentially be applied in
high-visibility textiles with improved lightfastness.
3.5. Reflectance spectra of dyed modacrylic fibers
The reflectance spectra of undyed modacrylic fiber were less than
100% reflectance, whereas, for the dyed modacrylic fibers, the reflec-
tance percentage was more than 100%. As the dye concentration
increased, the reflectance maximum value also increased for certain
concentrations of the dyes. However, the reflectance maximum value
decreased for all dyes after reaching a critical concentration. This may
be due to the fluorescence quenching effect. The critical concentrations
were 2.0, 4.0, 3.0 and 2.0% (owf) for dyes 6a, 6b, 6c and basic yellow-
40, respectively (Fig. 6). From the graph, after reaching the critical
concentration for dye 6a, the reflectance maximum values decreased
significantly when compared to the decrease of the other dyes. This
result was attributed to the presence of –OCH3 substitution, which in-
creases the hydrophobic nature of the molecule. But, in remaining two
other dyes 6b and 6c having –OH and –COOH substitutions which are
more favorable for hydrophilic character. Moreover, at high dye con-
centrations, this dye molecule 6a may have existed in a favorable
3.7. Fluorescence emission of dyed modacrylic fibers
For the high visibility warning textiles, fluorescence intensity is very
important. Hence, we recorded the fluorescence emission spectra for all
the dyed modacrylic fibers before and after exposure to the xenon lamp.
The excitation wavelength was fixed at 455 nm. The fluorescence
emission wavelengths for the synthesized dye modacrylic fibers dis-
played a red-shift in comparison with the respective dyes in the solutions
(6a: 508 nm, 6b: 542 nm, 6c: 501 nm and basic yellow 40: 498 nm).
Further, shoulder peaks are observed for the fluorescence emission
spectra of all the dyed fabrics. This may be due to the suppression of
intermolecular rotations of the dye molecules after adsorption on the
fabrics [42]. Meanwhile, the dyed fabrics were exposed to the xenon
lamp and the fluorescence emission wavelengths for all the dyes shifted
to blue-shift and the fluorescence intensity also decreased. Compared to
the basic yellow-40, the synthesized dyes fluorescence intensity was
decreased is small. Therefore, the synthesized dyes can be used for high
Fig. 6. Maximum reflectance values of dyed modacrylic fibers.
6

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
Table 2
Color characteristics of dyed modacrylic fibers before exposure to a xenon lamp.
Dye
L*
a*
b*
C*
h
x
y
β
6a
88.43
87.33
87.21
89.03
ꢀ 14.87
ꢀ 11.02
ꢀ 7.77
60.69
54.05
51.52
63.36
66.50
59.58
57.08
69.65
103.77
101.52
98.57
0.435
0.434
0.385
0.402
0.446
0.417
0.457
0.478
0.75
0.80
0.77
0.81
6b
6c
Basic yellow-40
ꢀ 17.83
105.72
Table 3
Color characteristics of dyed modacrylic fibers after exposure to a xenon lamp.
Dye
L*
a*
b*
C*
h
x
y
β
6a
89.28
88.42
88.81
90.16
ꢀ 10.02
ꢀ 7.75
65.74
59.08
56.73
68.10
62.49
55.16
52.10
65.82
98.67
97.48
96.34
99.65
0.417
0.415
0.394
0.40
0.425
0.401
0.452
0.452
0.67
0.69
0.68
0.71
6b
6c
ꢀ 6.31
Basic yellow-40
ꢀ 11.58
Fig. 9. Fluorescence emission spectra of the dyed modacrylic fibers.
4. Conclusions
Fig. 7. Chromaticity of dyed modacrylic fibers.
In conclusion, three novel, cationic, fluorescent coumarin dyes, 6a,
6b, and 6c were synthesized and evaluated by spectroscopic analysis.
The photophysical and photostability properties of these compounds
were also studied by using UV–visible spectroscopy in different solvents.
Dyeing experiments were performed on modacrylic fibers using the
traditional dyeing method. The dye bath pH should be kept at ~5.5–7.5
visibility textiles. The dyed modacrylic fibers photo images and fluo-
rescence emission spectra are shown in Fig. 8 and Fig. 9 (before and after
exposure to the xenon lamp).
Fig. 8. Photo images of the dyed modacrylic fibers.
7

R. Penthala and Y.-A. Son
Dyes and Pigments 190 (2021) 109294
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Raju Penthala: Conceptualization, Methodology, Investigation,
Formal analysis, Visualization, Writing – original draft. Young-A Son:
Conceptualization, Methodology, Investigation, Formal analysis,
Writing – review & editing, Supervision.
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Declaration of competing interest
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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This work was supported by the Technology Innovation Program
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visibility fluorescent dye materials on modacrylic and its blended fab-
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8