Design, synthesis, characterization of water-soluble indophenine dyes and their application for dyeing of wool, silk and nylon fabrics

Article abstract of DOI:10.1016/j.dyepig.2020.108385

In view of our continuing interests in application of quinoidal heterocyclic structure into textile dyestuffs, we report here the design and synthesis of four new water-soluble dyes D1-D4 that based on indophenine structure and bearing alkyl chains with different lengths. Their properties on geometry, electronic structure, solubility, thermal stability and absorption spectra have been well investigated. Then, the dyeing ability of D1-D4 is checked by application into dyeing of wool, silk and nylon fabrics and is compared with three commercial acid dyes. The effect of quinoidal coplanarity of indophenine backbone and alkyl substituents on dyeing performance has been revealed. Our study clearly demonstrates that the introduction of rigid quinoidal heterocyclic structure with high coplanarity as the chromophore backbone is an efficient method for the design of high-performance acid dyestuffs in terms of exhaustion, coloring strength and wet fastness.

Full text of DOI:10.1016/j.dyepig.2020.108385

Journal Pre-proof
Design, synthesis, characterization of water-soluble indophenine dyes and their
application for dyeing of wool, silk and nylon fabrics
Jinfang Cai, Hua Jiang, Weiguo Chen, Zhihua Cui
PII:
S0143-7208(19)32569-0
DOI:
https://doi.org/10.1016/j.dyepig.2020.108385
DYPI 108385
Reference:
To appear in: Dyes and Pigments
Received Date: 1 November 2019
Revised Date: 22 March 2020
Accepted Date: 22 March 2020
Please cite this article as: Cai J, Jiang H, Chen W, Cui Z, Design, synthesis, characterization of water-
soluble indophenine dyes and their application for dyeing of wool, silk and nylon fabrics, Dyes and
Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108385.
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CRediT authorship contribution statement
Jinfang Cai: Methodology; Validation; Investigation; Visualization; Data Curation;
Writing-Original Draft;
Hua Jiang: Conceptualization; Methodology; Software; Writing-Review & Editing
Weiguo Chen: Supervision; Writing-Review & Editing
Zhihua Cui: Writing-Review & Editing

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Design, synthesis, characterization of water-soluble indophenine dyes and their application for
dyeing of wool, silk and nylon fabrics
a
a,b,
a,b
a,b
3
Jinfang Cai , Hua Jiang *,Weiguo Chen , Zhihua Cui
a
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Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University,
Hangzhou, 310018, China
b
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Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Hangzhou 310018,
China
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* Corresponding author.
E-mail address: jh@zstu.edu.cn (H. Jiang).
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Abstract: In view of our continuing interests in application of quinoidal heterocyclic structure into
textile dyestuffs, we report here the design and synthesis of four new water-soluble dyes D1-D4 that
based on indophenine structure and bearing alkyl chains with different lengths. Their properties on
geometry, electronic structure, solubility, thermal stability and absorption spectra have been well
investigated. Then, the dyeing ability of D1-D4 is checked by application into dyeing of wool, silk
and nylon fabrics and is compared with three commercial acid dyes. The effect of quinoidal
coplanarity of indophenine backbone and alkyl substituents on dyeing performance has been
revealed. Our study clearly demonstrates that the introduction of rigid quinoidal heterocyclic
structure with high coplanarity as the chromophore backbone is an efficient method for the design of
high-performance acid dyestuffs in terms of exhaustion, coloring strength and wet fastness.
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Keywords: Indophenine; Sulfonation; Absorption; Acid dyes; Dyeing properties

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1. Introduction
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Conventionally, acid dyes are widely applied on dyeing of wool, silk, nylon fabrics under acidic
conditions, and coloration of leather, paper, cosmetics, ink, etc. as well [1, 2]. The acid dyes can be
structurally classified as azo dyes [3-8], anthraquinone dyes [9, 10], triphenylmethane dyes [11, 12],
nitro dyes [13], etc. Among them, acid dyes that containing heterocyclic moiety are acknowledged
with high molar extinction coefficient, high color strength, and good dyeing performance. The
common design strategy for heterocyclic acid dyes is to construct azo chromophore by using
heterocyclic units as diazo or coupling components [14-16]. Direct adoption of heterocyclic
structures with large π-conjugation system as chromophore meets the difficulty of complex multistep
synthesis [17, 18].
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Fabrics can be dyed with acid dyes through ionic bond as well as Van der Waals' force and
hydrogen bond. Unfortunately, these bonds are easily broken in hot wet conditions leading to
relatively poor wet fastness [19, 20]. The invention of acid mordant dyes and pre-metalized acid dyes
indeed largely improves the wet fastness [21-23]. However, the potential heavy-metal pollution
existed in dyed fabrics and waste water has made this dyeing technology limited socially, politically
and legally [24, 25]. Thus, the development of environmentally friendly and high-performance acid
dyestuffs becomes the common goal in this research field.
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Judicious molecular design methods for acid dyes to improve the wet fastness include following
aspects. The first way is to decrease the dyes’ solubility and increase the affinity of dyes-to-fiber
through hydrophobic interaction by introduction of alkyl chain [26, 27]. The sulfonic acid groups on
the dye backbone provide the possibility to form electrostatic interaction with amino groups in fibers

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but the hydrophilicity of sulfonic acid groups might migrate the dye molecules out of the fiber under
wet treatment. Thus, it is expected that reducing the weight ratio of sulfonic acid groups in molecule
ought to improve the wet fastness. Introduction of coplanar structure into dye molecule is another
efficient method. The stronger intermolecular interactions between dye and fiber generated by the
high molecular coplanarity are thought to be helpful for obtaining better wet fastness. Usually, the
construction of chromophore’s structure with high coplanarity can be achieved either by directly
using the planar synthetic building block [17] or by ring closure with intramolecular weak
interactions [5].
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In the effort of searching for new chromophores for high-performance textile dyes, quinoidal
heterocyclic compounds have attracted our attention because of their unique characteristics [28, 29].
The quinoidal heterocyclic structure possesses a highly rigid backbone that all atoms on the
quinoidal π-conjugation system are in the same plane [30]. Besides, most of the quinoidal hetercyclic
5
compounds are highly colored and the highest molar extinction coefficient can be up to 2×10
-1
-1
L∙mol cm [31, 32]. In this context, we envisage that the introduction of quinoidal heterocyclic
backbone should be an effective way for the design of high-performance textile dyestuffs. In 2018,
we reported the application of dicyanomethylene or (alkoxy)acylcyanomethylene endcapped
quinoidal bithiophenes (QBT series) into dyeing poly(ethylene terephthalate) (PET) fabrics [33]. As
a result, the pink/red dyed fabrics exhibited high exhaustion up to 97% and good to excellent fastness
to washing, rubbing and sublimation. Very recently, we also presented the use of indophenine
derivatives for dyeing of PET fabrics and achieved even better anti-thermomigration fastness than
conventional azo or anthraquinone dyes [34]. As confirmed by our experimental and theoretical
studies, there existed large dye-fiber interactions between these quinoidal heterocyclic molecules and

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PET fibers, which could be regarded as the reason for high fastness level. However, the attempt of
designing hydrophilic dyestuffs with quinoidal heterocyclic moiety and the study of their dyeing
application have not been reported.
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As the extension of our work, we report in the present study the design and synthesis of four new
water-soluble indophenine dyes D1-D4 (Fig. 1) and the feasibility of dyeing on several fibers. The
following principles are considered in the design of indophenine dyes molecules. (i) The two fused
benzene rings in indophenine backbone are electron-rich moieties, which are suitable for the
introduction of sulfonic acid groups by sulfonation reaction. (ii) Alkyl chains can be easily
introduced at the nitrogen atoms, hence making it possible to easily fine-tune the intermolecular
interaction between dye molecule and fiber. (iii) The facile synthesis of indophenine dyes by
so-called “indophenine reaction” provides the convenience for scale-up production [35], therefore
facilitating the application research. Our laboratory scale tests demonstrated that these dyes had the
ability of dyeing wool, silk and nylon with deep blue shade. As predicted, the fabrics dyed with
indophenine dyes have even better wet fastness properties than that dyed with selected commercial
acid dyes. Subsequently, structure-property relationships of indophenine quinoidal backbone and
alkyl substituents on dyes’ absorption, exhaustion, dyeing rate, build-up and fastness properties have
been systematically studied.
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2. Experimental section
2.1. Materials and instrumentation
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The chemicals for synthesis, including isatin, ethyl bromide, hexyl bromide, decyl bromide,
thiophene, etc. were purchased from Adamas Reagent Co., Ltd., and used without further purification.

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1-Ethylindoline-2,3-dione [34], 1-hexylindoline-2,3-dione [36], and 1-decylindoline-2,3-dione [37]
were synthesized according to reported papers. Anhydrous toluene for synthesis were pre-treated by
2
2
distillation and stored under nitrogen. Twilled wool fabrics (200 g/m ), plain silk fabrics (40 g/m )
2
and knitted nylon 66 fabrics (230 g/m ) were obtained commercially. The three blue commercial acid
dyes, D5-D7, which represented azo, anthraquinone and triphenylmethane dyes respectively, were
purified by DMF-ether method [38] and used as reference dyes. The chemical structures of D1-D4 as
well as the reference dyes D5-D7 were shown in Fig. 1.
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Fig. 1 Molecular structures of designed dyes D1-D4 and reference dyes D5-D7
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H NMR spectra were measured with a Bruker DMX400 MHz NMR spectrometer at room

−
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temperature in DMSO-d . Infrared (IR) spectra (4000-400 cm ) were recorded using a Nicolet
6
FT-IR 170X spectrophotometer on KBr disks. Electrospray ionization mass spectra (ESI-MS) were
recorded on a Thermo Lcq Fleet mass spectrometer in a scan range of 200–2000 amu. Accurate mass
data were obtained by high resolution mass spectrometry performed on a SYNAPT-G2-S HDMS
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spectrometer. The thermal stability of all the dye samples was investigated using TGA under N . The
2
degradation temperature (T ) was obtained by determining the slope of the portion where the weight
d
percentage decreased sharply on the TGA graph. UV–vis spectra were measured by using Shimadzu
UV-2600 dual-beam spectrophotometer. Dyeing of wool/silk/nylon fabrics was operated on
Chain-Lih DYE-24 adjustable dyeing machine. The K/S value as well as the color parameters of the
dyed fabrics was measured by Datacolor SF 600X spectrophotometer.
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2.2 Synthesis of compounds 5-8
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Concentrated sulfuric acid (15 mL, 98wt%) was added dropwisely to a solution of isatins (20
mmol) and thiophene (40 mmol, 3.37 g) in toluene (40 mL) at 0 °C. The color of the mixture became
dark blue. The mixture continued to react at 0 °C for 3 h until the isatin derivative was totally
consumed. For the synthesis of compound 5, the transparent toluene was removed and the residue
was poured into icy water (200 mL). After filtration, the crude solid was washed with an excess of
water, ethanol, hexane and dichloromethane successively, and dried in vacuum.
3,3'-([2,2′-Bithiophenylidene]-5,5′-diylidene)bis(indolin-2-one) (5), dark blue solid [34], 3.33 g, 78%.
-
Mp: >300 °C. ESI-MS (100%, negative) m/z = 425 ([M−H] ); ESI-HRMS (100%, positive) Calcd.
+
−1
for C H N O S : 426.0497 ([M] ), found: 426.0497; IR (KBr) ν = 1670 cm (C=O).
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2 2
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For compounds 6-8, the reaction mixture was added into icy water (150 mL) and extracted with

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CH Cl (100 mL×3). The organic layer was combined, and washed with saturated brine. The crude
2 2
product was obtained by evaporation of the solvent under reduced pressure, and could be further
purified by column chromatography (eluent: petroleum/EtOAc = 10/1 then CH Cl ).
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3,3'-([2,2′-Bithiophenylidene]-5,5′-diylidene)bis(1-ethylindolin-2-one) (6), dark blue solid [34],
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3.19 g, 66%. Mp: 252–254 °C. ESI-MS (100%, negative) m/z =482 ([M] ); ESI-HRMS (100%,
+
−1
positive) Calcd. for C H N O S : 482.1123 ([M] ), found: 482.1132; IR (KBr) ν =1669 cm
2
8
22
2
2 2
(C=O).
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3,3'-([2,2′-Bithiophenylidene]-5,5′-diylidene)bis(1-hexylindolin-2-one) (7), dark blue solid, 5.06 g,
+
85%. Mp: 202–204 °C. ESI-MS (100%, positive) m/z = 594 ([M] ); ESI-HRMS (100%, positive)
+
−1
Calcd. for C H N O S : 594.2369 ([M] ); found: 594.2370. IR (KBr) ν = 1670 cm (C=O).
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2 2
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3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(1-decylindolin-2-one) (8), dark blue solid, 6.29 g,
+
89%. Mp: 180–182 °C. ESI-MS (100%, positive) m/z = 706 ([M] ); ESI-HRMS (100%, positive)
+
−1
Calcd. for C H N O S : 706.3639 ([M] ); found: 706.3631. IR (KBr) ν = 1669 cm (C=O).
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2.3 Synthesis of dyes D1-D4
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The indophenine intermediates 5-8 (2.56 mmol) was carefully added in batch to conc. H SO (15.6
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mL, 0.256 mol, 98wt%) under room temperature. The reaction was stirred at 60 ºC for 4 h. After the
reaction was completed, the solution was poured onto ice, and sodium chloride was added to
promote the precipitation of target compounds. Finally, the precipitation was filtered, washed with
saturated brine and dried.
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D1, dark blue solid, 1.44 g，96%. Mp: >300 °C. ESI-MS (100%, negative) m/z = 292

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(1/2[M-2H] ), 585 ([M-H] ); ESI-HRMS (100%, negative) Calcd. for C H N O S 584.9549
24 13 2 8 4
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([M-H] ), found: 584.9630; IR (KBr) ν = 1622 cm (C=O).
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D2, dark blue solid, 1.60 g, 97%. Mp: >300 °C. ESI-MS (100%, negative) m/z = 320
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(1/2[M-2H] ), 641 ([M-H] ); ESI-HRMS (100%, negative) Calcd. for C H N O S : 641.0176
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([M-H] ), found: 641.0232; IR (KBr) ν = 1637 cm (C=O).
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D3, dark blue solid, 1.87 g, 97%. Mp: >300 °C. ESI-MS (100%, negative) m/z = 376
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(1/2[M-2H] ); ESI-HRMS (100%, negative) Calcd. for C H N O S : 753.1427 ([M-H] ), found:
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753.1487; IR (KBr) ν = 1637 cm (C=O).
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D4, dark blue solid, 2.04 g, 92%. Mp: >300 °C. ESI-MS (100%, negative) m/z = 432
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(1/2[M-2H] ), 887 ([M-H] , in the form of mono sodium salt); ESI-HRMS (100%, negative) Calcd.
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for C H N O S : 865.2685 ([M-H] ), found: 865.2855; IR (KBr) ν = 1639 cm (C=O).
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2.4 Synthesis of compounds 9-12
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Conc. H SO (1 mL) was added to isatin derivative (2 mmol) under room temperature. The
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reaction was stirred at 60 ºC for 3 h. After the reaction was completed, the solution was poured onto
crushed ice, followed by filtration and washing with icy saturated brine. The crude product could be
further purified by column chromatography (eluent: CH Cl then CH Cl /MeOH (5/1)).
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2,3-Dioxoindoline-5-sulfonic acid (9), orange-red solid, 0.14 g, 31% [37]. Mp: >300 °C. H NMR
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(400 MHz, DMSO-d ) δ 11.14(br, s, 1H), 7.83 (dd, J = 1.4 Hz, J = 8.1 Hz, 1H), 7.64 (d, J = 1.4 Hz,
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1H), 6.90 (d, J = 8.1 Hz, 1H); C NMR (100 MHz, DMSO-d ) δ 184.64, 160.13, 151.34, 142.95,
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136.03, 122.12, 117.45, 112.18. ESI-MS (100%, negative) m/z = 226 ([M-H] ). IR (KBr) ν = 1750

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cm (C=O), 1621 cm (C=O).
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1-Ethyl-2,3-dioxoindoline-5-sulfonic acid (10), orange-red solid, 0.19 g, 37%. Mp: >300 °C. H
NMR (400 MHz, DMSO-d ) δ 7.87 (d, J = 8.0 Hz, 1H), 7.65 (s, 1H), 7.16 (d, J = 8.0 Hz, 1H), 3.70
6
1
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(q, J = 6.8 Hz, 2H), 1.17 (t, J = 6.8 Hz, 3H); C NMR (100 MHz, DMSO-d6) δ 183.43, 158.11,
150.34, 143.34, 135.15, 121.38, 116.79, 109.99, 34.49, 12.26. ESI-MS (100%, negative) m/z = 254
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−1
−1
([M-H] ). IR (KBr) ν = 1747 cm (C=O), 1612 cm (C=O).
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1-Hexyl-2,3-dioxoindoline-5-sulfonic acid (11), orange-red solid, 0.37 g, 59%. Mp: >300 °C. H
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NMR (400 MHz, DMSO-d ) δ 7.86 (dd, J = 1.6 Hz, J = 8.0 Hz, 1H), 7.64 (d, J = 1.6 Hz, 1H), 7.15
6
13
(d, J = 8.0 Hz, 1H), 3.64 (t, J = 7.2 Hz, 2H), 1.59 (m, 2H), 1.27 (m, 6H), 0.84 (t, J = 7.2 Hz, 3H). C
NMR (100 MHz, DMSO-d ) δ 183.80, 158.84, 151.12, 143.95, 135.59, 121.78, 117.17, 110.51,
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31.35, 27.15, 26.32, 22.45, 14.34. ESI-MS (100%, negative) m/z = 310 ([M-H] ). IR (KBr) ν = 1729
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cm (C=O), 1614 cm (C=O).
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1
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1-Decyl-2,3-dioxoindoline-5-sulfonic acid (12), orange-red solid, 0.44 g, 60%. Mp: >300 °C. H
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NMR (400 MHz, DMSO-d ) δ 7.85 (dd, J = 1.6 Hz, J = 8.0 Hz, 1H), 7.62 (d, J = 1.6 Hz, 1H), 7.13
6
(d, J = 8.0 Hz, 1H), 3.64 (t, J = 7.2 Hz, 2H), 1.58 (m, 2H), 1.27-1.24 (m, 14H), 0.84 (t, J = 7.2 Hz,
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3H). C NMR (100 MHz, DMSO-d ) δ 188.56, 163.59, 155.85, 148.83, 140.32, 126.50, 121.92,
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115.23, 36.48, 34.14, 33.89, 31.94, 31.43, 27.30, 19.16. ESI-MS (100%, negative) m/z = 366
-
-1
-1
([M-H] ). IR (KBr) ν = 1731 cm (C=O), 1614 cm (C=O).
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2.5 Synthesis of dyes D1'-D4' by using 9-12 as starting materials
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Concentrated sulfuric acid (15 mL, 98wt%) was added dropwisely to a solution of sulfonated
isatins (2 mmol) and thiophene (4 mmol, 337 mg) at 0°C and stirred for 3 h until the isatin derivative

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was totally consumed. After the reaction was completed, the solution was poured onto ice, and
sodium chloride was added to promote the precipitation of target compounds. Finally, the
precipitation was filtered, washed with saturated brine and dried.
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2.6 General procedure for dyeing of wool, silk and nylon fabrics
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The fabric (1.0 g) was dyed at a liquor ratio of 50:1 with the dye amount at 1% owf level. Dyeing
process was started at 40 °C and heated up by 1 °C/min to certain temperature (98 °C for wool and
nylon, 85 °C for silk). The pH value of dye bath was controlled by AcOH/AcONa buffer solution
(pH 4 for wool and silk, pH 6 for nylon). After dyeing, fabric was picked out from the dye bath,
rinsed thoroughly with tap water and air-dried at room temperature.
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2.7 Measurement for the dyeing properties
2.7.1 Dye exhaustion
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The exhaustion of dyes was determined by spectrophotometric techniques. The dye exhaustion was
calculated based on the following equation:
ꢃꢄ
ꢀ
ꢁ%ꢂ = ꢁ1 − ꢂ × 100
ꢃꢅ
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94
where A and A are the absorbance at λ of the dye solution before and after dyeing, respectively.
0
1
max
2.7.2 Color assessment
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Color yield of dyed fabrics was obtained from dyed samples at maximum absorption wavelength.
Each sample was measured three times in a different area and an average value was used. The “K/S”
value was calculated based on the Kubelka-Munk equation:

ꢁ
1 − ꢈꢂꢉ
ꢆ/ꢇ =
2ꢈ
1
1
2
2
2
2
98
99
00
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03
where K is the adsorption coefficient, S is the scattering coefficient, and R is the reflectance of the
dyed sample. The color coordinates of indophenine dyes were determined on CE-7000A
Gretag-Macbeth computer color matching system. The color values were expressed by using CIE
1976 Color Space method. The coordinates used to determine color values are “L*” for lightness,
“a*” for greenness (negative value) and redness (positive value), “b*” for blueness (negative value)
and yellowness (positive value), “C*” for chroma and “hº” for hue angle.
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2.7.3 Color fastness
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The color fastnesses to washing, rubbing and light were measured in accordance with the ISO
105-C10: 2006, ISO 105-X12: 2016 and ISO 105-B02: 2014, respectively.
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3. Results and discussion
3.1 Synthesis and characterization of dyes D1-D4
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The synthetic routes for water-soluble indophenine dyes are shown in Scheme 1. At first,
hydrophobic indophenine derivatives 5-8 were synthesized by “indophenine reaction”. Subsequent
sulfonation of 5-8 by using concentrated sulfuric acid gave the target compounds D1-D4. Mass
2
-
spectra clearly showed the peak of 1/2[M-2H] for all the four dyes, confirming the introduction of
two sulfonic acid groups (Figure 2). However, it is not easy to identify the exact structures of the
four dyes by NMR, because of the E/Z isomerization of indophenine backbone [35].
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To further determine the position of sulfonic acid groups in benzene rings, another synthetic route
was also carried out that the sulfonated isatins 9-12, prepared by sulfonation of isatins 1-4, were used

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as the intermediates for “indophenine reaction”. Since the intermediates 9-12 were structurally
characterized [39], the position of -SO H in D1'-D4' was definitely confirmed at the para-position to
3
the N atom. Although the mass spectra of D1'-D4' were the same as that of D1-D4, proton NMR
spectra showed big difference between D1'-D4' and D1-D4 (Figure S1 and S2). Considering the
difference of ground-state electron cloud density distributions (Figure S4), the position of -SO H in
3
D1-D4 were suggested mainly located at the meta-position rather than para-position to the N atom
(detailed analysis is in S.I.).
2
2
2
2
2
2
2
24
25
26
27
28
29
30
The synthesized four dyes exhibited good hydrophilicity. D1-D4 could not dissolve in CHCl3,
Et O, toluene, etc., but had moderate to good solubility in water, MeOH, EtOH, DMSO and DMF. As
2
shown in Table 1, the solubility of dyes in water accorded with the weight ratio of sulfonic acid
groups. The introduction of alkyl chains in D2-D4 largely decreased the weight ratio of sulfonic acid
groups. As a result, the solubility decreased from 135 g/L for D1 to less than 20 g/L for D4. In fact,
we found that solution of D4 in water was very instable that dyes would gradually precipitate in
several hours.

2
2
31
32
Scheme 1 Synthetic routes for dyes D1-D4 and D1'-D4'
2
33
2
2
34
35
Figure 2 Mass spectra of D1-D4

2
2
2
36
37
38
Table 1 Weight ratio of -SO H in dye molecule, solubility and T data of D1-D7
3
d
D1
D2
D3
754.96
0.21
85
D4
866.27
0.19
D5
D6
451.45
0.18
<50
D7
669.21
0.24
120
Molecular weight/(g/mol) 586.64
642.74
0.25
130
637.68
a
Weight ratio of -SO H
0.28
135
331
0.25
3
b
Solubility/(g/L)
< 20
290
135
456
Td/ºC
323
306
220
318
a
2
2
39
40
commercial dyes are usually in the form of sodium salt, data here are calculated in the form of sulfonic acid group;
in water at 25 ºC.
b
2
41
2
2
2
2
2
2
2
2
42
43
44
45
46
47
48
49
The thermal stabilities of synthesized dyes were evaluated by TGA technique. Generally, planar
molecular structure enhances π-π stacked interaction leading to high thermal stability for the dyes,
and is likely affected by electronic and steric effects. Figure 3 shows the TGA thermograms of
D1-D4 as well as three reference dyes D5-D7, and the corresponding data are listed in Table 1. The
synthesized indophenine dyes exhibited thermal degradation temperature within the range of
290 °C~331 °C. The T values of the four synthesized dyes were in the order of D1>D2>D3>D4,
d
clearly indicated the important role of alkyl chains. The alkyl chains could rotate out of the plane of
the quinoidal backbone, leading to reduced tendency toward self-aggregation. As a result, T was
d

2
2
2
2
2
2
2
2
2
2
50
51
52
53
54
55
56
57
58
59
lowered accordingly, which was more obvious in D4 with two decyl groups. When compared with
the commercial dyes, the T values of D1-D4 were lower than that of C.I. Acid Blue 113 (D5). This
d
might be caused by the larger π-conjugation system along the bis-azo linkage possessed by D5.
Besides, the tendency to be diradicaloid state under high temperature for indophenine quinoidal
heterocyclic structure might also have negative effect on the T values of D1-D4 [29]. In contrast, the
d
higher T values of D1-D4 than that of C.I. Acid Blue 40 (D6) should be caused by the relatively low
d
molecular weight and rotatable phenyl group in D6 which might depress the tendency to aggregate.
Nevertheless, since the temperature in the dyeing and finishing process was usually less than 300 °C
[2], the synthesized dyes were confirmed to have enough thermal stability for use as textile dyes [8,
18].
2
60
2
2
61
62
Figure 3 Thermogravimetric analysis of D1-D7
2
2
63
64

2
65
3.2 DFT calculations
2
2
2
2
2
66
67
68
69
70
Density functional theory (DFT) calculations were performed to further well understand the
molecular geometries and electronic structures for the four designed dyestuffs [40]. Their optimized
ground state geometry and the energy information were calculated by DFT/B3LYP method with
6-311g (d, p) basis set. To simplify the discussion, only all-trans conformation for D1-D4 was used
here. The results are shown in Figure 4 and the collected data are summarized in Table 2.
2
2
2
2
2
2
2
2
2
2
2
2
71
72
73
74
75
76
77
78
79
80
81
82
The side view of the optimized ground state geometry showed highly planar quinoidal backbone
for indophenine dyes D1-D4, as observed. The high coplanarity could be further concluded from the
calculation for the sum of associated three bond angles or for the dihedral angles (Table S1). Hence,
large intermolecular interactions between dye and fiber would generate, and better wet fastness
property for the designed dyes was expected. The alkyl chains in D2-D4 extended outward
accompanied by a certain degree of inclination. Thus, intermolecular interactions could be fine-tuned
by controlling the length of alkyl chain. The HOMO/LUMO electron cloud density distributions for
D1-D4 were mainly delocalized over the quinoidal π-conjugation system with little contribution from
alkyl chains or sulfonic acid groups, which was very similar with non-sulfonated indophenine
backbone [34]. For D2-D4, the HOMO/LUMO energy levels increased simultaneously as the
extension of alkyl chains. As a result, the bandgap had almost no change and the calculated
absorption maxima were the same (Figure S5).
2
83
Side view
HOMO orbital
LUMO orbital

D1
D2
D3
D4
2
2
2
84
85
86
Figure 4 Optimized molecular geometries and HOMO/LUMO electron cloud density distributions for D1-D4
Table 2 Summarized calculated data for D1-D4
a
b
Dye
D1
D2
D3
D4
E/a.u.
Dipole moment/Debye
0.0006
HOMO/eV
-6.2028
-6.1133
LUMO/eV
-4.2548
-4.1579
-4.1343
-4.1250
Bandgap/eV
1.9480
λmax/nm
-3227.3174
-3384.6046
-3699.2111
-4013.7909
615
0.0006
1.9554
601
0.0004
-6.0891
-6.0795
1.9591
601
0.0004
1.9545
601
a
b
2
2
2
87
88
89
Bandgap = LUMO - HOMO; in vacuum.
3.3 Absorption properties

2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
90
91
92
93
94
95
96
97
98
99
00
01
02
03
04
The absorption spectra of D1-D4 were recorded in DMSO, DMF, EtOH, MeOH and H O at room
2
temperature. As shown in Figure 5, only one major absorption peak was recognized in the visible
region with full width at half maximum (FWHM) values of 127-178 nm for all dyes (Table 3). The
absorption maxima of D1-D4 were in the range of 634~652 nm and were very close in the same
organic solvent, evidently proving that the quinoidal π-conjugation backbone played a leading role in
UV–vis absorption properties. Moreover, the absorption spectra in water for dyes D1-D4 displayed
large hypsochromic shift with respect to that in organic solvents, associated with their purplish blue
color in water other than pure blue in organic solvent. Besides, the molar extinction coefficient in
water was relatively lower than that in organic solvent. This result indicated that these dyes might
aggregate in water, similar with the situation observed for D5 with relatively large π-conjugation
system among the three reference dyes (Table S2) [5, 6]. The alkyl chains had little effect on the
absorption maxima of D1-D4 in organic solvents which matched the theoretical calculation results.
However, it was found that the absorption maxima in water blue-shifted from 569 nm to 554 nm as
the increase of the alkyl chain length from ethyl to decyl, as a result of the increased aggregation
tendency.
3
3
3
3
3
3
05
06
07
08
09
10
As shown in Figure S7, the absorption spectra of D1-D4 in water with different pH values were
also measured. It was found that the four indophenine dyes showed good stability at weak acid to
basic conditions, thus making them suitable for dyeing of fabrics under acid or neutral conditions.
The decreased absorbance and slightly blue-shifted maximum absorption wavelength for D1-D3 at
strong acid condition (pH < 2) might be caused by further aggregation resulted from the protonation
of nitrogen atoms.
3
11

3
12
3
3
13
14
Figure 5 Absorption spectra of dyes D1 (a), D2 (b), D3 (c) and D4 (d)
3
3
3
3
15
16
17
18
Table 3 Summarized absorption data for D1-D4
a
b
Dye
Entry
DMSO
652
DMF
648
EtOH
638
MeOH
635
MeOH/H O
H O
2
2
c
λmax/nm
627
-
569
-1
-1 d
D1
ε/(L∙mol cm )
24384
19735
22604
24511
18788

e
FWHM/nm
158
652
148
648
143
638
143
634
160
624
-
127
569
λmax/nm
-1
-1
D2
D3
D4
ε/(L∙mol cm )
FWHM/nm
λmax/nm
29873
163
35563
161
32010
146
34735
149
24935
121
163
629
-
651
648
639
634
567
-1
-1
ε/(L∙mol cm )
FWHM/nm
λmax/nm
30559
161
35308
168
38014
153
36437
152
25634
134
160
631
-
649
647
638
636
554
-1
-1
ε/(L∙mol cm )
30577
160
35208
162
38827
151
40420
25717
145
FWHM/nm
146
178
a
-5
b
c
d
3
3
19
20
the concentration of the dye solution is 2 × 10 mol/L; 1/1 in volume ratio; maximum absorption wavelength;
e
molar extinction coefficient; full widths at half maximum.
3
3
3
21
22
23
3.4 Dyeing properties
3.4.1 Dye exhaustion and color assessment
3
3
3
24
25
26
The synthesized dyes D1-D4 were applied to colorate wool, silk and nylon fabrics according to the
procedure depicted in experimental section and compared with the three reference dyes D5-D7. To
obtain the best dyeing performance, several dyeing parameters including dyeing temperature, pH

3
3
3
3
3
3
27
28
29
30
31
32
value of dye bath, liquor ratio and salt effect, had been screened at first (Table S3-S10). It was found
that the dyeing temperature and pH value of dye bath for these dyes were mainly decided by the fiber
other than dyestuffs. And the variation of liquor ratio and addition of salt seemed to have little effect
on the dyeing performance of D1-D3. For D4, however, the low liquor ratio value and the addition of
salt would bring about several problems, such as decreased levelness, low exhaustion, which might
be caused by its low solubility.
3
3
33
34
a
Table 4 Dye exhaustion and color parameters of dyed fabrics for dyes D1-D7
b
c
E/%
λmax/nm
K/S
8.1
L*
a*
b*
C*
hº
D1
D2
D3
D4
D5
D6
D7
D1
D2
D3
D4
D5
D6
>99
>99
>99
>99
>99
>99
>99
>99
>99
98.5
98.7
97.1
98.7
590
590
590
590
590
630
640
590
600
620
610
590
630
35.53
26.78
26.29
25.39
18.66
32.28
47.09
37.82
32.19
31.16
32.42
33.45
50.71
-4.93
4.06
-11.29
-25.14
-27.66
-22.10
-13.88
-26.22
-21.42
-22.95
-35.22
-35.25
-33.96
-24.84
-27.18
12.32
25.47
28.33
22.62
14.40
29.07
43.87
23.25
36.87
35.80
34.51
25.00
31.15
246.39
279.16
282.44
282.28
285.38
244.42
209.23
279.11
287.17
280.06
280.26
276.62
240.74
20.0
19.7
13.8
20.4
17.1
26.5
8.8
6.10
Wool
4.81
3.82
-12.55
-38.28
3.68
12.7
10.2
10.8
8.3
10.88
6.25
Silk
6.15
2.88
5.3
-15.22

D7
D1
D2
D3
D4
D5
D6
99.0
93.4
>99
>99
>99
98.3
87.8
>99
640
650
650
650
650
620
640
630
13.0
21.3
24.9
25.0
25.9
21.9
15.2
25.4
59.61
25.48
24.66
26.65
24.89
22.66
37.21
47.25
-37.02
-1.40
5.17
-26.23
-21.57
-29.23
-35.68
-33.55
-22.06
-30.21
-31.93
45.37
21.61
29.68
36.58
34.31
22.08
33.33
44.69
215.31
266.29
280.02
282.76
282.12
272.21
245.00
225.61
8.08
Nylon
7.21
0.85
-14.08
-31.26
d
D7
a
3
3
35
36
Dyeing condition: for wool, 1% owf, pH 4, 98 °C; for silk, 1% owf, pH 4, 85 °C; for nylon, 1% owf, pH 6, 98 °C;
b
c
d
Exhaustion; Data collected from K/S curves; Dyeing at pH 5.
3
37
3
3
3
3
3
3
3
3
3
3
3
38
39
40
41
42
43
44
45
46
47
48
As shown in Table 5, most of the fabrics dyed with D1-D4 at the optimized dyeing conditions
displayed blue shade with satisfactory levelness, except for the dull gray wool fabric dyed with D1.
The data for the dye exhaustions and color parameters of dyed fabrics were collected in Table 4. In
term of dye exhaustion, all the indophenine dyes showed good to excellent exhaustion values,
indicated that the synthesized dyes based on indophenine backbone had good affinity with wool, silk
and nylon fibers. However, there existed remarkable differences in color depth value for the dyed
fabrics. Generally, fabrics dyed with D1 showed the lowest K/S values, which might due to the low
coloring strength of D1, as demonstrated in the K/S curves (Figure S8). Fabrics dyed with D4 also
showed relatively lower K/S values than that dyed with D2 and D3, probably due to the aggregation
of D4 on fibers. This meant that suitable alkyl chains were significant for improving the color depth.
Outstandingly, K/S values of fabrics dyed with D2 and D3 were as high as that dyed with commercial

3
3
3
49
50
51
dyes, indicating their potential practical application as acid dyes.
Table 5 Images of fabric samples dyed with D1-D7 at 1% owf level
D1
D2
D3
D4
D5
D6
D7
Wool
Silk
Nylon
3
3
52
53
3.4.2 Dyeing rate
3
3
3
3
3
3
3
3
54
55
56
57
58
59
60
61
As shown in Figure 6, the dyeing rate curves of indophenine dyes D1-D4 as well as the
commercial dyes D5-D7 for dyeing of wool, silk and nylon fabrics were described by collecting the
exhaustion data at certain dyeing time. In all cases, the synthesized indophenine dyes D1-D4 showed
much lower dyeing rates than the commercial dyes D5-D7. This meant that the high molecular
planarity of indophenine backbone might cause large dye-dye interactions so that more energy was
needed to permit the dye molecules transfer to fiber surface and inside. The relatively low dyeing
rate allowed the good levelness performance without addition of extra leveling agents [2]. Among
the four indophenine dyes, D1 and D4 showed lower dyeing rates than D2 and D3. For dye D1

3
3
3
3
3
3
62
63
64
65
66
67
without substituents, this dyestuff might form inner-salt because of the coexistence of secondary
amino groups and sulfonic acid groups. And for D4, the too long alkyl chains resulting in low
water-solubility should be responsible for the low dyeing rate. Combined with aforementioned data
of dyeing performance, alkyl chains with suitable length seemed to be necessary to fine tune the
dye-dye and dye-fiber interaction. Specifically, D3 with two hexyl groups exhibited the highest
dyeing rate among the four synthesized dyes.
3
3
3
3
68
69
70
71
3
72
3
73

3
3
3
3
74
75
76
77
Dyeing condition: for wool, pH 4, 98 °C; for silk, pH 4, 85 °C; for nylon, pH 6, 98 °C (D7, pH 5)
Figure 6 Dyeing rate curves for wool (a), silk (b) and nylon (c) with D1-D7 at 1% owf level
3.4.3 Effect of dye concentration
3
3
3
3
3
3
3
3
3
3
78
79
80
81
82
83
84
85
86
87
The effects of dye concentration on the K/S values at the maximum absorption wavelength and dye
exhaustions were studied and the results were shown in Figure 7 and Table S11-S13. Generally, the
dyes exhibited good build-up properties with low dye concentration of less than 1% owf. As the dye
concentration increasing, the dye exhaustion could be observed with a gradual decline, indicating the
saturation of dye molecules on the fibers. As a result, K/S values no longer continued to increase
substantially. In all cases, D2 and D3 displayed much better build-up properties than D1 and D4,
especially in dyeing of wool fabrics where D1 and D4 showed low coloring strength and dye
exhaustions. To our delight, D2 and D3 also showed good build-up properties that were comparable
with commercial dyes. In case of dyeing of nylon, fabrics dyed with D2 and D3 could have higher
K/S values than that dyed with D5-D7, making it possible for deep dyeing with indophenine dyes.
3
88
3
89

3
3
90
91
Figure 7 Effect of dye concentration on K/S value for D1-D7 dyeing of wool (a), silk (b) and nylon (c) fabrics
3
3
92
93
3.4.4 Color fastness
3
3
3
3
3
3
4
4
4
4
4
4
94
95
96
97
98
99
00
01
02
03
04
05
The color fastnesses of fabrics dyed with the indophenine dyes D1-D4 were evaluated and
compared with the commercial dyes D5-D7. The fastness data were summarized in Table 6. All
fabrics dyed with the indophenine dyes D1-D4 showed moderate to excellent washing and rubbing
fastnesses, exactly indicating the positive influence of the quinoidal coplanarity of indophenine
backbone. Fabrics dyed with D4 were found to have relatively lower washing and rubbing fastnesses
than that dyed with the other three indophenine dyes. This might be due to the large dye molecular
size and low ratio of sulfonic acid groups for D4 that making massive dye molecules aggregated on
the surface of fiber. Besides, it was noted that the staining on cotton could be remarkably eliminated
by increasing the alkyl chain length. The substantivity of indophenine dyes to cotton fiber also
promoted us to investigate the dyeing performance on cotton fabrics. However, all the low dye
exhaustions, low K/S values and wet fastness of dyed fabrics indicated the unfeasibility of
indophenine dyes as the dyeing materials for cotton fabrics (Table S14 and S15). This phenomenon

4
4
4
4
4
4
4
06
07
08
09
10
11
12
was similar with triphendioxazine-based phosphorus containing acid dye that also had large and
planar π-conjugation system, as reported in literature [17]. Among the four indophenine dyes, D2 and
D3 exhibited the best fastness properties, and even better fastness level than the commercial dyes in
term of washing fastness. The light fastness of fabrics dyed with the indophenine dyes showed
relatively poor degrees, which were equal to the level for the triphenylmethane dye D7 [41]. The
poor light fastnesses for indophenine dyes must be due to the instability of indophenine backbone
under irradiation [34].
4
4
13
14
Table 6 Color fastness data of wool/silk/nylon fabrics dyed with D1-D7
Washing
Staining
Wool/Silk/Nylon
Rubbing
Dye
Light
Change
Dry
Wet
Cotton
a
D1
D2
D3
4/3/5
3-4/2-3/4-5
3-4/3/5
5/4/5
5/5/5
5/5/5
4-5/4/5
4-5/4/5
4/3/5
4/3/5
3/2-3/1-2
2-3/2-3/1-2
2-3/2/1-2
2/2/1-2
4-5/3-4/5
5/4/5
5/4-5/4-5
5/4-5/5
5/3/3-4
5/4/5
4-5/4-5/5
4-5/5/5
3-4/4-5/3
3/4/3
4/3-4/5
3-4/4-5/5
2/3/2
b
D3
5/4-5/5
4-5/3/4-5
4/3-4/4-5
4-5/3/4
1-2/1/2-3
3/1/4
5/5/5
c
D3
3-4/3-4/4
5/5/5
4/3-4/2
D4
D5
D6
D7
2-3/4/3
4/4/5
2-3/2/1
4/2-3/5
2/3/4
5/2-3/3
1-2/2-3/2
2-3/1/3-4
4-5/5/5
4-5/4-5/5
4/5/4-5
6-7/5-6/6-7
6-7/6-7/6-7
1-2/2/1
4/4/4-5
3/3-4/4
3/4/3-4
a
b
c
4
15
The data listed in table are in the order of wool/silk/nylon; Fabrics dyed with D3 at 0.5% owf level; Fabrics