Synthesis and dyeing properties of indophenine dyes for polyester fabrics

Article abstract of DOI:10.1016/j.dyepig.2019.03.025

In this study, a series of indophenine dyes (D1-D6) that show high molecular planarity, were synthesized and applied as dyeing materials for poly (ethylene terephthalate) fabrics. The relationship between the dye molecular structure and dyeing properties, such as dyeing rate and color fastness properties, was investigated. Compared with conventional azo or anthraquinone dyes, the indophenine dyes had a much lower dyeing rate but very good anti-thermomigration properties. This phenomenon indicated that there exists much larger intermolecular interaction between indophenine dye molecules, which was also confirmed by the results of quantum chemical density functional theory (DFT) calculations. The substituent effect of alkyl groups and halogen atoms (F and Cl) was also investigated. The steric hindrance of linked ethyl groups could efficiently promote dyeing performance, while halogen atoms lowered the color acquisition. As a result, D4 with only two N-ethyl groups incorporated on the indophenine backbone showed the best overall dyeing performance.

Full text of DOI:10.1016/j.dyepig.2019.03.025

Accepted Manuscript
Synthesis and dyeing properties of indophenine dyes for polyester fabrics
Hua Jiang, Qian Hu, Jinfang Cai, Zhihua Cui, Jinhuan Zheng, Weiguo Chen
PII:
S0143-7208(18)32546-4
https://doi.org/10.1016/j.dyepig.2019.03.025
DYPI 7414
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 19 November 2018
Revised Date: 17 January 2019
Accepted Date: 12 March 2019
Please cite this article as: Jiang H, Hu Q, Cai J, Cui Z, Zheng J, Chen W, Synthesis and dyeing
properties of indophenine dyes for polyester fabrics, Dyes and Pigments (2019), doi: https://
doi.org/10.1016/j.dyepig.2019.03.025.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
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Synthesis and dyeing properties of indophenine dyes for polyester fabrics
a, b
a
a
a, b,
a, b
a, b,
Hua Jiang , Qian Hu , Jinfang Cai , Zhihua Cui *, Jinhuan Zheng , Weiguo Chen
*
a
Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University,
Hangzhou, 310018, China
b
Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of China, Hangzhou 310018,
China
* Corresponding author.
E-mail address: zhhcui@zstu.edu.cn (Z. Cui); wgchen_62@126.com (W. Chen).
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Abstract: In this study, a series of indophenine dyes (D1-D6) that show high molecular planarity,
were synthesized and applied as dyeing materials for poly(ethylene terephthalate) fabrics. The
relationship between the dye molecular structure and dyeing properties, such as dyeing rate and color
fastness properties, was investigated. Compared with conventional azo or anthraquinone dyes, the
indophenine dyes had a much lower dyeing rate but very good anti-thermomigration properties. This
phenomenon indicated that there exists much larger intermolecular interaction between indophenine
dye molecules, which was also confirmed by the results of quantum chemical density functional
theory (DFT) calculations. The substituent effect of alkyl groups and halogen atoms (F and Cl) was
also investigated. The steric hindrance of linked ethyl groups could efficiently promote dyeing
performance, while halogen atoms lowered the color acquisition. As a result, D4 with only two
N-ethyl groups incorporated on the indophenine backbone showed the best overall dyeing
performance.
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Keywords: Indophenine; Quinoidal structure; Disperse dye; Dyeing property; Interaction energy;
DFT calculation.
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1. Introduction
With the polyester industry flourishing, the demand for a variety of high-performance disperse
dyes is increasingly sought. In finishing procedures, when dyed polyester fabrics are treated at
temperatures higher than 130 ºC (e.g., heat setting or baking with resin), disperse dyes inside the
fiber can migrate to the surface, resulting in poor washing and rubbing fastness, especially in the case
of deeply dyed fabrics. In addition, when dyeing and finishing polyester/spandex or polyester/viscose
blend fabrics, heavy staining occurs on spandex or viscose fibers [1, 2], causing serious color
fastness problems.
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In the past decades, many efforts have been devoted to develop improved high-fastness disperse
dyes [3-11]. One feasible strategy is to develop disperse dyes with increased molecular size,
including preparation of disazo disperse dyes [3, 4], introduction of bulkier groups into dye
molecules [5], tethering two dye molecules with diester/diurethane groups [6, 7], and polymerizing
dyes [8]. However, large dye molecules are more suitable for dyeing polyurethane (PU) fibers than
poly(ethylene terephthalate) (PET) fibers, since the free volume in the soft region of a PET fiber is
smaller than that of a PU fiber, making it difficult for large dye molecules to penetrate the PET fiber.
Recently, Wang et al. demonstrated that introduction of a planar ring moiety (e.g., phthalimide or
benzodifuranone) into the dye molecule is an effective approach to restrain the thermal migration of
dye molecules [12-14]. The increased molecular coplanarity brings about a larger intermolecular
interaction energy, which increases the dye-dye and dye-fiber forces, leading to better fastness
properties.
In our efforts to search for new chromophores for designing high-performance disperse dyes, we
noticed that there emerged thiophene-based quinoidal compounds used in the field of opto-electronic
materials [15, 16]. In particular, quinoidal thiophene derivatives display very high rigidity and

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planarity owing to the double bond present between each cyclic unit. Such molecular planarity is
useful for enhancing dye-dye and dye-fiber interactions, favoring better fastness [13]. Furthermore,
quinoidal molecules usually exhibit low energy bandgaps with very large molar extinction
coefficients [17, 18]. Therefore, it is promising to apply quinoidal thiophene as a chromophore into
textile dyestuffs. Very recently, we reported the structure-property relationships of quinoidal
bithiophene dyes that were end-capped with cyano or ester groups for dyeing PET fabric [19]. As
expected, the high planarity of these dyes produced excellent washing, rubbing, and sublimation
fastness. The challenge of forming single dye molecules in the dyeing process was overcome by
introduction of suitable alkyl groups into the quinoidal backbone. Hence, quinoidal thiophene
derivatives were demonstrated as potential disperse dyes.
Indophenine, a well-known blue dyestuff based on the quinoidal bithiophene backbone, can be
readily synthesized by isatin and thiophene in the presence of concentrated sulfuric acid [20]. A
remarkable color change occurs, termed the “indophenine reaction”, which has mainly been used as a
visual colorimetric indicator of trace amounts of thiophene. However, because of its poor solubility
originated from strong π-π interaction, the direct application of indophenine was very limited.
Recently, indophenine derivatives with long alkyl chains (up to 12 carbons) were designed and
synthesized, resulting in good solubility in common organic solvents, and subsequent use as
semiconductor materials [21, 22]. On the other hand, indophenine derivatives with short alkyl chains
remain poor solubility [20], and their properties and application have not been fully explored.
In this study, we devised a PET dyeing method using six readily synthesized indophenine
derivatives as disperse dyes. There are three reasons for this molecular design. (i) The larger
quinoidal backbone of the indophenine molecule enhances the dye-dye and dye-fiber forces, which

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provides better fastness on dyed fabrics, especially related to anti-thermomigration. (ii) The nitrogen
of the indolin-2-one moiety in the indophenine molecule provides a convenient site to introduce an
alkyl chain. Thus, fine-tuning intermolecular interactions becomes possible for facilitating the dyeing
process, as we previously reported [19]. (iii) The synthesis of indophenine dyes is quite facile and
scale-up is feasible, which favors disperse dye development for industrial dyeing applications.
Although indophenine is reported to have six cis-trans isomers [22], no discernable evidence of
decreased dyeing performance has been attributed to the coexistence of these isomers. Actually, our
dyeing experiments with derivatized indophenine dyes produced bright blue shades on PET fabrics
with excellent wet fastness. N-ethyl groups were found to promote the dyeing performance, while
halogens (i.e., F and Cl atoms) decreased color acquisition. To gain greater insight into
structure-property relationships of derivatized indophenine dyes, the effect of quinoidal planarity on
dyeing performance and the halogen atom effect have also been studied by calculating the
intermolecular interaction energy.
2. Experimental section
2.1 Materials and instruments
Unless otherwise specified, all chemicals and solvents were purchased commercially (Adamas
Reagent Co., Ltd.). Two commercial dyes (D7, C.I. disperse blue 291; D8, C.I. disperse blue 56)
were provided by Jihua Jiangdong Chemical Co., Ltd. These two dyes had been purified by column
chromatography before use. All reactions were carried out in dried glassware and nitrogen
atmosphere. Solvents were previously dried by conventional methods and stored under nitrogen. Air-
and water-sensitive solutions were transferred with hypodermic syringes.
NMR spectra were recorded with a Bruker DMX 400 MHz NMR spectrometer at room

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temperature in CDCl or DMSO-d . Electrospray ionization mass spectra (ESI-MS) were recorded on
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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 solanX 70 FT-MS spectrometer.
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Infrared (IR) spectra (4000-400 cm ) were recorded using a Nicolet FT-IR 170X spectrophotometer
on KBr disks. Melting points were measured on Mettler-Toledo melting point apparatus and were
uncorrected. UV-Vis spectra were recorded with a Shimadzu UV-2600 double-beam
spectrophotometer using a quartz glass cell with a path length of 1.0 cm. Dyeing of polyester fabrics
was operated by using DYE-24 adjustable dyeing machine (ShangHai Chain-Lih, China). The color
yield of the dyed fabrics was measured by Datacolor SF 600X spectrophotometer (Datacolor,
Switzerland).
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2.2 Preparation of indophenine dyes
2.2.1 3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(indolin-2-one) (D1)
Concentrated sulfuric acid (15 mL) was added to a solution of isatin (20 mmol, 2.95 g) and
thiophene (44 mmol, 3.70 g) in toluene (40 mL) at 0 ºC. It was found that all colored solutes were
involved into sulfuric acid layer, and the color became dark blue. The mixture continued to react at 0
ºC for 3 h. After the reaction was completed, transparent toluene solvent was removed and the
residue was poured into icy water (100 mL) carefully. The crude product was obtained by filtration,
followed by washing with an excess of water, ethanol, hexane and dichloromethane successively,
until each liquid became clear. Finally, a drying process in vacuum gave the target compound as a
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dark blue solid, 3.52 g, 82%. Mp: >350 ºC. ESI-MS (100%, negative) m/z = 425 ([M-H] );
+
ESI-HRMS (m/z) Calcd. for C H N O S : 426.0497 ([M] ), found: 426.0497; IR (KBr) ν = 1676
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2.2.2 3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(5-fluoroindolin-2-one) (D2)
The same procedure as in 2.2.1 was used, except that 5-fluoroindoline-2,3-dione (20 mmol) was
used as starting material. Dark blue solid, 3.30 g, 71%. Mp: >350 ºC. ESI-MS (100%, negative) m/z
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= 461 ([M-H] ); ESI-HRMS (m/z) Calcd. for C H F N O S : 462.0308 ([M] ), found: 462.0312; IR
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(KBr) ν = 1685 cm (C=O).
2.2.3 3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(5-chloroindolin-2-one) (D3)
The same procedure as in 2.2.1 was used, except that 5-chloroindoline-2,3-dione (20 mmol) was
used as starting material. Dark blue solid, 2.96 g, 59%. Mp: >350 ºC. ESI-MS (100%, negative) m/z
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= 493 ([M-H] ); ESI-HRMS (m/z) Calcd. for C H Cl N O S : 516.9609 ([M+Na] ), found:
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516.9628; IR (KBr) ν = 1681 cm (C=O).
2.2.4 1-Ethylindoline-2,3-dione (4)
A solution of sodium hydride (22 mmol, 0.88 g, 60% wt) in anhydrous DMF (30 mL) was added
to a solution of isatin (20 mmol, 2.94 g) in anhydrous DMF (20 mL) at 0 ºC under nitrogen
atmosphere. The solution became dark purple. After stirring for 30 min, iodoethane (21 mmol, 3.28 g)
was added, and the reaction mixture was stirred for another 5 h at room temperature. Water was
added to quench the reaction and it was extracted with dichloromethane. The organic layer was
washed with water and dried over anhydrous magnesium sulphate. The crude product was purified
by column chromatography (petroleum ether:CH Cl = 1:1) to afford compound 4 as orange solid
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[23]. 2.62 g, 75%. Mp: 88-89 ºC. H NMR (400 MHz, CDCl ) δ 7.60–7.56 (m, 2H), 7.10 (t, J = 7.6
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Hz, 1H), 6.90 (d, J = 8.2 Hz, 1H), 3.77 (q, J = 7.2 Hz, 2H), 1.30 (t, J = 7.6 Hz, 3H). C NMR (100
MHz, CDCl ) 183.18, 157.28, 150.08, 138.08, 124.61, 123.08, 116.90, 109.79, 34.40, 11.98; IR (KBr)
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2.2.5 3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(1-ethylindolin-2-one) (D4)
The same procedure as in 2.2.1 was used, except that 1-ethylindoline-2,3-dione (10 mmol) was
used as starting material. Dark blue solid, 1.18 g, 49%. Mp: 252-254 ºC. ESI-MS (100%, negative)
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(KBr) ν = 1681 cm (C=O).
2.2.6 1-Ethyl-5-fluoroindoline-2,3-dione (5)
The same procedure as in 2.2.4 was used, except that 5-fluoroindoline-2,3-dione (20 mmol) was
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used as starting material. Orange red solid, 3.08 g, 80%. Mp: 126-128 ºC. H NMR (400 MHz,
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CDCl ) δ 7.33-7.28 (m, 2H), 6.87 (dd, J = 9.2 Hz, J = 3.6 Hz, 1H), 3.78 (q, J = 7.2 Hz, 2H), 1.31 (t,
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J = 7.2 Hz, 3H). C NMR (100 MHz, CDCl ) δ 182.90, 160.12, 157.51 (d, J = 33.4 Hz), 146.49,
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2.2.7 3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(1-ethyl-5-fluoroindolin-2-one) (D5)
The same procedure as in 2.2.1 was used, except that 1-ethyl-5-fluoroindoline-2,3-dione (10 mmol)
was used as starting material. Dark blue solid, 0.89 g, 34%. Mp: decomposition before melting.
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ESI-MS (100%, negative) m/z = 518 ([M] ); ESI-HRMS (m/z) Calcd. for C H F N O S : 518.0934
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2.2.8 5-Chloro-1-ethylindoline-2,3-dione (6)
The same procedure as in 2.2.4 was used, except that 5-chloroindoline-2,3-dione (20 mmol) was
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used as starting material [24]. Orange red solid, 3.23 g, 77%. Mp: 130-132 ºC. H NMR (400 MHz,
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118.09, 111.34, 34.89, 12.16; IR (KBr) ν = 1735 (C=O), 1608 cm (C=O).
2.2.9 3,3'-([2,2'-Bithiophenylidene]-5,5'-diylidene)bis(5-chloro-1-ethylindolin-2-one) (D6)
The same procedure as in 2.2.1 was used, except that 5-chloro-1-ethylindoline-2,3-dione (10 mmol)
was used as starting material. Dark blue solid, 1.06 g, 38%. Mp: >350 ºC. ESI-MS (100%, negative)
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IR (KBr) ν = 1678 cm (C=O).
2.3 Dyeing procedure
Dye dispersion solution was prepared by grinding the mixture of indophenine dye (0.5 g),
dispersant NNO (Naphthalenesulfonic acid/formaldehyde condensation product, 1.0 g), Zirconium
beads (Φ 0.2 mm, 50 g) and water (50 mL) in a laboratory miniature quartz tube equipped with
mechanical stirrer. After the average particle size decreased to lower than 500 nm checked by laser
particle size analyzer (zetasizer Nano S, UK), the mixture was filtered into volumetric flask. Then, a
solution with certain concentration was prepared after additional water was added.
The polyester fabric (PET 100%, 188 dtex, 2/2 twill) was dyed with dye amount of 1.0% owf
(weight of dye to fabric), kept at a liquor ratio of 1:50 and pH at 5.0 (using acetic acid). The dyeing
temperature started at 30 °C, then raised to 130 °C with heating rate of 2.0 °C/min and maintained at
130 °C for 1 h. After cooling, the dyed fabric was picked out and treated with a solution of sodium
dithionite (2 g/L) and sodium hydroxide (2 g/L) at 80 °C for 5 min. Finally, the fabric was rinsed
with clean water and dried at 60 °C.
2.4 Color assessment
Color depths of dyed PET fabrics were obtained from dyed samples at maximum absorption
wavelength. Each sample was measured three times in a different area and an average value was used.

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The “K/S” value was calculated based on the Kubelka-Munk equation:
(
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where K is the adsorption coefficient, S is the scattering coefficient, and R is the reflectance of the
dyed sample.
In the study of dyeing rate curve, the color intensity (%) was calculated as follows [13]:
Colorꢄdepthꢅꢆꢇꢈꢉꢄꢊꢆꢋꢌꢍꢎ
Colorꢄintensityꢄ(%) =ꢄ
Colorꢄdepthꢊꢇꢆꢏꢐꢆꢑꢐꢄꢊꢆꢋꢌꢍꢎ
ꢗꢘꢘꢄꢏꢋ
Colorꢄdepth =ꢄꢒ
ꢓ(ꢔ)ꢕ/ꢖdꢔ
ꢙꢘꢘꢄꢏꢋ
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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 redness
(positive value) and greenness (negative value), “b*” for yellowness (positive value) and blueness
(negative value), “C*” for chroma and “hº” for hue angle.
2.5 Measurement of migration degree
A piece of dyed PET fabric (sample A) at 1% owf color shade was sewed with another piece of
un-dyed PET fabric (sample B) having equal weight. The combined fabrics were put into a dyeing
bath (liquor ratio 1:30, pH 5.0), then heated from 30 °C to 130 °C with heating rate of 2.0 °C/min
and maintained at 130 °C for 1 h. After cooling, the two samples were picked up and dried. The
migration degree (%) was calculated as follows:
(
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ꢀ/ꢁ)ꢛ
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where (K/S) is the K/S value of sample A after treatment, (K/S) is the K/S value of sample B after
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2.6 Colorfastness
The color fastnesses to washing, rubbing, sublimation and light were measured in accordance with
the ISO 105-C06: 2010 (C1S), ISO 105-X16: 2016, ISO 105-X11: 1994 and ISO 105-B02: 2014,
respectively.
2.7 Density functional theory calculation
All theoretical calculations were performed with Gaussian09 programs [13, 25]. Optimized ground
state geometry of the lowest energy conformation for D1-D6 were calculated at the B3LYP/6-311G
(d, p) level. HOMO/LUMO energy and electron density distribution were calculated at
B3LYP/6-311G ++ (d, p) level. Intermolecular interaction energy was computed by DFT/ωB97XD
method with 6-311G (d, p) basis set for indophenine dyes D1-D3. To simplify the calculation, only
trans isomer (E,E,E isomerization) of indophenine dyes was computed.
3. Results and discussion
3.1 Synthesis and photophysical properties
The six indophenine dyes were synthesized according to a reported method [20], with the synthetic
route shown in Scheme 1. Among the derivatives, D1, D2, and D3 could easily be obtained by the
“indophenine reaction” using isatin, 5-fluoroisatin, or 5-chloroisatin as starting materials,
respectively. For D4-D6, ethyl groups were firstly introduced by N-alkylation (of isatin,
5-fluoroisatin, or 5-chloroisatin) to give N-ethyl isatin derivatives, which subsequently underwent the
“indophenine reaction” to afford the desired products smoothly (D4, D5, and D6, respectively). As
expected, all six dyes exhibited very poor solubility in common organic solvents (e.g., chloroform,
toluene, and tetrahydrofuran). Thus, we purified the synthesized dyes by washing with plenty of
water, ethanol, hexane, and dichloromethane to remove residual sulfuric acid, thiophene, and other

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possible soluble byproducts. The poor solubility also brought serious complications for structural
characterization, in which only mass spectral characterization of the six dyes was possible.
Indophenine dyes are reported to have six structural conformations due to cis-trans isomerism [22].
Nevertheless, our previous results demonstrated that a mixture of isomers had a negligible effect on
dye effectiveness [19]. In addition, the properties of two purified commercial blue disperse dyes,
namely C.I. disperse blue 291 (D7) and C.I. disperse blue 56 (D8; Fig. 1), are also presented here for
comparison.
2
2
25
26
2
27
Scheme 1. Molecular structures and synthetic route for indophenine dyes D1-D6.
2
2
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D7 (C.I. disperse blue 291)
D8 (C.I. disperse blue 56)
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Fig. 1. Structures of two commercially available blue disperse dyes.

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The UV-vis absorption spectra of both indophenine dyes and commercial dyes were measured in
order to understand their optical properties. Thanks to their moderate solubility in DMF, dilute
absorption spectra of indophenine dyes could be recorded, as shown in Fig. 2. All the quinoidal
molecules D1-D6 exhibited broad absorption bands from 400 nm to 800 nm with λmax of
approximately 631 nm to 642 nm, similar to conventional dyes D7 and D8. The maximum absorption
wavelengths of indophenine dyes D1-D3 were slightly red-shifted compared with D4-D6, while the
latter showed higher extinction coefficients.
2
2
39
40
Fig. 2. UV-vis spectra of indophenine dyes D1-D8 in DMF.
Table 1. UV-vis absorption data of dyes D1-D8 in DMF.
2
41
D1
636
D2
640
D3
642
D4
633
D5
631
D6
632
D7
598
D8
636
λmax/nm
-
1
-1
ε/L·mol cm
26434
141
23303
171
33095
151
39021
150
40497
173
52665
172
39048
123
18509
100
a
∆
ν /nm
1/2
a
2
2
2
2
42
43
44
45
half-band width.
3.2 Dyeing properties
3.2.1 Color assessment

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The synthesized indophenine dyes were applied to PET fabrics at 1.0% owf color shade by using
the high temperature exhaustion dyeing method at 130 ºC [19]. The dye exhaustion and color
parameters of dyed PET fabrics are summarized in Table 2. Our initial dyeing attempt of polyester
fabrics resulted in blue shades with excellent color uniformity (Fig. 3). It can be seen that the six
indophenine dyes exhibited large differences in their dyeing performance in terms of dye exhaustion
and color depth. Among them, D4 showed acceptable dye exhaustion and color depth, indicating the
positive effect of N-ethyl groups. The introduction of halogen atoms (D2, D3, D5 and D6) was found
to give a negative influence on the overall dyeing performance. The low exhaustion and precipitation
of some dye particles at the bottom of the bath after dyeing suggest difficulty in formation of single
indophenine dye molecules, indicating the existence of large dye-dye interactions.
Table 2. Dye exhaustion and color parameters of dyed PET fabrics for dyes D1-D8.
a
Exhaustion/% λmax/nm
K/S
11.7
7.8
L*
a*
b*
C*
hº
D1
D2
D3
D4
D5
D6
D7
D8
74.4
61.6
54.2
85.0
44.3
55.6
97.7
93.3
650
650
650
650
650
650
610
630
34.81
37.28
34.76
30.13
35.03
44.41
23.08
28.94
-0.84
1.19
2.58
6.89
5.99
2.77
-1.91
8.01
-25.89
-23.64
-22.73
-33.11
-34.06
-29.00
-21.65
-39.66
25.91 268.15
23.67 272.87
22.87 276.28
33.82 281.76
34.58 279.98
29.13 275.46
21.73 264.95
40.46 281.41
8.8
17.7
9.8
4.8
19.5
20.3
a
2
2
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58
Data collected from K/S curves.

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D1
D2
D3
D4
D5
D6
D7
D8
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Fig. 3. Digital photos of the dyed PET fabrics under D illuminant.
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3.2.2 Dyeing rate curve
As shown in Fig. 4, the dyeing rate curves for indophenine dyes D1-D6 as well as commercial
dyes D7-D8 are depicted by collecting K/S data every 10 min during the dyeing process. It emerged
that the indophenine dyes D1-D6 started dyeing at 100–110 ºC, but most of them needed much more
time than D7 and D8 to reach dyeing equilibrium. The dyeing rate of synthesized indophenine dyes
was much lower than that of conventional azo or anthraquinone dyes. This suggests that there are
much larger dye-dye interactions between indophenine dye molecules and more energy necessary to
weaken the intermolecular forces compared to conventional dyes. Specifically, D4, with two N-ethyl
groups, reduced the dye-dye interactions such that it exhibited a comparable dyeing rate to D7.
Halogen atom substituted dyes (such as D3, D5, and D6) showed much lower dyeing rates. This
suggests that halogen atoms provide larger intermolecular interactions.

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Fig. 4. Dyeing rate curves for indophenine dyes D1-D6 and commercial dyes D7-D8.
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3.2.3 Effect of dyeing temperature
We examined the effect of dyeing temperature using D4 (Fig. 5). The dyeing procedure was
similar to that described in 2.3, except that the temperatures were set at 125 ºC, 130 ºC, 135 ºC and
138 ºC. The resulting K/S values in one batch experiment were 11.4, 16.5, 19.1 and 19.6, respectively.
Compared with D7 and D8, it is encouraging that a competitive color depth using indophenine dyes
could be obtained at higher temperature. The fact that the K/S value increases remarkably with
increasing temperature confirmed the prediction of large dye-dye interactions for indophenine dyes.
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Fig. 5. K/S values of PET fabrics dyed with D4 at different temperatures.
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3.2.3 Color fastness

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The results of color fastness testing for PET fabrics dyed with indophenine dyes D1-D6 at 1.0%
owf color shade are shown in Table 3. All these indophenine dyes exhibited excellent washing,
rubbing, and sublimation fastness on PET fabrics. It should be noted that the tested samples had
undergone a reductive cleaning process with sodium dithionite (2 g/L) and sodium hydroxide (2 g/L)
at 80 ºC for 5 min before testing. On one hand, the reducing conditions might convert the quinoidal
structure into the colorless aromatic state [15]. On the other hand, amide bonds in indophenine dyes
are presumably hydrolyzed to soluble groups. Thus, dyes on the fabric surface could be readily
removed and good wet fastness properties were obtained. Because of the good thermal stability at
high temperature [16], indophenine dyes showed less color staining of PET or cotton than D7 or D8,
as well as better sublimation fastness. We attribute these effects to strong dye-dye and dye-fiber
interaction energies, which can prevent the dye from both transferring to an un-dyed fabric and
undergoing sublimation upon heating. However, the light fastness of indophenine dyes is generally
poor, likely due to their ease of decay to colorless aromatic states under a Xenon light source (see
Fig. S1) [26].
The anti-thermomigration properties of indophenine dyes on PET fabrics were also investigated.
After the dyed PET fabrics (1% owf color shade) were treated with un-dyed fabrics under high
temperature and pressure, the degree of migration was calculated and shown in Fig. 6. Compared
with D7 and D8, the indophenine dyes showed remarkably reduced dye migration properties. This
result clearly indicates that larger dye-fiber interactions exist in fabrics dyed with indophenine dyes.
F/Cl substituted dyes had relatively lower degrees of migration, again suggesting that F/Cl atoms
assist in increasing intermolecular interactions.

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Table 3. Fastness properties of dyes D1-D8 on PET fabrics.
Rubbing
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Wash fastness
Sublimation fastness
fastness
Light
Dye
Stain
Stain
fastness
Change
Dry Wet
Change
PET Cotton
PET Cotton
D1
D2
D3
D4
D5
D6
D7
D8
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4-5
4-5
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4-5
5
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3
4-5
4-4
4-5
4-5
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5
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3
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5
4-5
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5
2-3
3
4-5
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5
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5
2-3
6-7
6-7
4
4
4-5
4-5
4
3-4
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4-5
4
3-4
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Fig. 6. Migration of dyes D1-D8 from dyed PET fabrics to un-dyed PET fabrics.
3
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3.3 Quantum chemical calculations
3.3.1 Molecular geometry and energy
To better understand the structure-property relationships of indophenine dyes, density functional
theory (DFT) calculations were carried out based on the indophenine backbone. The six indophenine
dye molecules were firstly optimized at B3LYP/6-311G (d, p) level. Then, energy information of
these dyes was calculated at B3LYP/6-311G++ (d, p) level based on the optimized molecular
geometry configuration. The results are shown in Fig. 7 and summarized in Table 4.
Side views of the six molecular geometry configurations intuitively showed a very planar
quinoidal backbone; this planarity is the origin of large π-π stacking interactions, which lead to
formation of strong dye-dye and dye-fiber forces. For D4-D6, the N-ethyl groups are found above
and below the quinoidal plane due to rotation of the C-C single bond. In this case, steric hindrance of
N-ethyl groups should favor the generation of more single dye molecules by disrupting π-π stacking
interactions, as demonstrated in our previous study [19]. For all studied indophenine dyes, both the
HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital)
electron cloud densities were evenly distributed across the entire conjugated indophenine backbone,
which presumably influences intermolecular electrostatic interactions.

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Side view
HOMO orbital
LUMO orbital
D1
D2
D3
D4
D5
D6
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Fig. 7. DFT calculations for optimized molecular geometries and HOMO/LUMO orbital diagrams of D1-D6.

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Table 4. Energy information of optimized molecular geometry configuration for indophenine dyes.
Dipole moment
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41
E (a.u.)
LUMO (eV)
HOMO (eV)
∆E (eV)
(Debye)
D1
D2
D3
D4
D5
D6
-1979.67294
-2178.19876
-2898.91476
-2136.97061
-2335.49527
-3056.21168
0.0001
0.0001
0.0001
0.0055
0.0011
0.0001
-3.4287
-3.6168
-3.6815
-3.3773
-3.5577
-3.6192
-5.2948
-5.4757
-5.5342
-5.2428
-5.4170
-5.4869
1.8661
1.8589
1.8527
1.8655
1.8593
1.8677
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3
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3
3
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3.3.2 Interaction energy
The inter-ring interaction from π-π stacking between two dye molecules and the van der Waals'
forces between dye molecules and PET fibers were focused on to determine interaction energies.
When PET fabrics are dyed with conventional monoazo dyes, benzene-benzene interactions
represent both dye-dye and dye-fiber forces. For indophenine dyes, the dye-fiber force is an
indophenine-benzene interaction while the dye-dye force is an indophenine-indophenine interaction.
As reported by Wang et al., face-to-face configurations are more stable than edge-to-face or
point-to-face configurations [12, 13]. Here, face-to-face configurations of indophenine dyes, in
which the dashed line indicates the ring overlap center, were modeled (Fig. 8). The dimer model a1
represents the benzene-benzene interaction, a2-a4 represent the D1-benzene interaction, and a5-a8
indicate possible D1-D1 interactions.

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a6
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a8
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Fig. 8. Molecule interaction energy models for D1.
The intermolecular interaction energy was obtained with the DFT/ωB97XD method in 6-311G (d,
p) level based on the above dimer model, with the results shown in Fig. 9. Both the D1-benzene
interaction energy and D1-D1 interaction energy initially fell to minimum values at 3.5-3.75 Å, and
then slowly increased with increasing inter-ring distance. This suggests that such interaction energies
belong to long-range interactions. D1-benzene interaction energies for a2 (-32.57 kJ/mol), a3 (-33.38
kJ/mol) and a4 (-22.74 kJ/mol) at 3.5 Å were much larger than the benzene-benzene interaction
energy (-10.97 kJ/mol, minimum at 3.75 Å), indicating that a higher interaction energy exists
between D1 and the PET fiber. Interestingly, a2 and a3 configurations displayed much larger
interaction energies than a4, possibly due to favorable electrostatic attractive forces from the
presence of electronegative heteroatoms [13]. For the D1-D1 interaction, we constructed the models
by translating the indophenine backbone overlap along the quinoidal π-conjugation pathway from a5

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to a8. Among these models, the a5 configuration, where all atoms completely overlap, showed very
strong electrostatic repulsion at close distance (d < 3.5 Å). For the remaining D1-D1 configurations,
a6 with three groups of interacting rings appears to be the most stable configuration. The lowest
interaction energies for a6, a7, and a8 at 3.5 Å were -119.89 kJ/mol, -73.35 kJ/mol, and -52.24
kJ/mol, respectively.
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Fig. 9. Benzene-benzene, D1-benzene and D1-D1 interaction energies.
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The electrostatic potential energy diagrams for D1 and a2-a8 configurations are shown in Fig. 10.
The total density diagram of D1 indicates that the most electronegative (O atom) and electropositive
(N-H bond) poles are located at the edge of the pyrrol-2-one moiety. The S atoms, quinoidal olefinic
bond, and benzene rings remain with very little differences in electronegativity. The Mulliken
charges of D1 (Fig. S2) exhibit much larger differences among each atom on the pyrrol-2-one and
quinoidal thiophene moieties than those of the benzene moiety. This charge differentiation benefits
molecular attraction, thus the a2 and a3 configurations exhibit larger interaction energies than the a4
configuration. The D1-D1 interaction models of the a5 configuration clearly showed an entirely

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electrostatic repulsive force, which reduced the configuration stability. In comparison, partly
overlapped a6-a8 configurations possessed much more electrostatic attractive interaction. Since the
dipole moments of the calculated indophenine molecules are close to zero (see Table 4), dispersion
forces are also believed to have a large effect on the D1-D1 interaction. The larger contact surface in
the a6 configuration therefore provides greater dispersion forces, which contribute to the highest
interaction energy among a6-a8.
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a2
a3
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a4
a5
a6
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a7
a8
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Note: Red indicates the highest electronegativity; blue indicates the highest electropositivity.
Fig. 10. Electrostatic potential energy diagrams of D1 (front view) and a2-a8 (side view).

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The effect of halogen atoms on the interaction energy was also studied by comparing dyes D1-D3.
For indophenine-benzene interactions, initial interaction models for D2 and D3 were obtained by
adopting the a3 configuration as shown in Fig. S3, though little difference was found (Fig. S4).
Since the F/Cl substituted sites were much closer to the ring overlapping center in the a4
configuration than that of a2 or a3, the configurations of D2 and D3 were rebuilt by adopting the a4
configurations of D1 (Fig. 11). As shown in Fig. 12, the lowest interaction energy values for b1
(-26.28 kJ/mol) and c1 (-28.77 kJ/mol) configurations were lower than the a4 configuration (-22.74
kJ/mol). For indophenine-indophenine interactions, models were constructed similarly to the a6
configuration. The lowest interaction energy values were -121.94 kJ/mol for the b2 configuration and
-126.36 kJ/mol for the c2 configuration. This suggests that the introduction of fluorine or chlorine
atoms effectively enhances the intermolecular interaction. While the relatively large interaction
energy for halogen atom substituted indophenine dyes could help to obtain better dye
anti-thermomigration, dye-dye forces that are large also promote dimer or oligomer formation. Thus,
the difficulty to form single dye molecules resulted in experimentally low color acquisition for F/Cl
atom substituted dyes.

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b1
b2
c1
c2
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Fig. 11. Molecular interaction energy models for D2 and D3.
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Fig. 12. Indophenine-benzene and indophenine-indophenine interaction energy for D1-D3.
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4. Conclusions
In summary, a series of indophenine dyes D1-D6 that contain N-ethyl groups or F/Cl atoms have
been facilely synthesized through the “indophenine reaction” with isatin derivatives as starting
materials. Although these indophenine dyes showed very low solubility in common organic solvents,
we have revealed a promising application that provides blue shades on polyester fabrics.
The large and planar indophenine backbone produces both strong dye-dye and dye-fiber forces.
On one hand, this is beneficial for preventing dye molecules from leaving dyed fabrics, leading to

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improved washing, rubbing, and sublimation fastness. On the other hand, the lower dyeing rate of
indophenine dyes compared to that of conventional azo/anthraquinone dyes results in higher dyeing
temperature necessary to form single dye molecules in order to facilitate the dyeing process.
Study of the substituent effect demonstrated that N-ethyl groups could improve the dye uptake,
while F/Cl atoms would inhibit the dyeing performance. As a result, D4 with only two N-ethyl
groups showed the best overall dyeing performance. DFT calculations demonstrated that there exists
both a large indophenine-benzene interaction energy and a indophenine-indophenine interaction
energy for indophenine dyes. F/Cl atom substituted indophenine dyes were found to have larger
interaction energies, leading to better anti-thermomigration properties. This study provides a
preliminary suggestion for the rational design of new disperse dyes based on indophenine structure.
Further study of improving dyeing performance by extension of this family of indophenine dyes is
now on going in our group.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 21808210,
51673176), Public Welfare Technology Research Project of Zhejiang Province (LGG18B060003),
Zhejiang Provincial Natural Science Foundation of China (LY16B060006), Zhejiang Provincial Top
Key Academic Discipline of Chemical Engineering and Technology of Zhejiang Sci-Tech University
(CETT2016002) and Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No.
16012087-Y.

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Appendix A. Supplementary data
For supplementary data related to this article, see supplementary information.
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