The synthesis of red dyes based on diketo-pyrrolo-pyrrole chromophore to improve heat stability and solubility for colour filter fabrication

Article abstract of DOI:10.1016/j.dyepig.2019.108053

To replace the conventional red pigment derived from diketo-pyrrolo-pyrrole by new red dyes prepared in this research, an acetylene linkage was introduced between N-alkyl diketo-pyrrolo-pyrrole and naphthalimidyl moiety. TGA analyses and solubility measur

Full text of DOI:10.1016/j.dyepig.2019.108053

Dyes and Pigments xxx (xxxx) xxx
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
The synthesis of red dyes based on diketo-pyrrolo-pyrrole chromophore to
improve heat stability and solubility for colour filter fabrication
Tae-Heon Kim, Byung-Jun Lee, Sung-Ok An, Joo-Hong Lee, Jae-Hong Choi*
Department of Textile System Engineering, Kyungpook National University, Daegu, 41566, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
To replace the conventional red pigment derived from diketo-pyrrolo-pyrrole by new red dyes prepared in this
research, an acetylene linkage was introduced between N-alkyl diketo-pyrrolo-pyrrole and naphthalimidyl
moiety. TGA analyses and solubility measurement were carried out to compare those of conventional red
pigment, C.I. Pigment Red 254, which has been most popular as a red pixel of color filter. Most of the new red
dyes were prepared starting from C.I. Pigment Red 254 by the sequent reactions, N-alkylation, Sonogashira
coupling, hydrolysis and further Sonogashira coupling. Chemical structures of all red dyes were confirmed by 1H
NMR and mass spectral analysis. TGA was carried out at 300 �C, and solubility measurement was carried out
using by several organic solvents which can be used in the color filter fabrications. TGA indicated that the
introduction of an acetylene linkage group into the N-alkyl diketo-pyrrolo-pyrrole ring combined by an N-butyl-
naphthalimidyl system improved its heat stability. The result of weight reduction 1.4% at 300 �C can be identical
to that of C.I. Pigment Red 254. The solubility of red dye prepared was observed around 5 g/L, therefore it can be
used in the color filter fabrication process to eliminate the conventional photo-etching process.
Acetylene linkage
Diketopyrrolopyrrole
Dye
Color filter
Heat stability
Solubility
1. Introduction
of derivatives of triphenylamine connected with 1,8-naphthalimide via
an ethynyl linkage group or up to three groups, 5% weight loss tem-
The fabrication process of color filters has been crucial for the quality
of LCD or QD displays however due to the poor solubility of the
trichromatic pigments, such as C.I. Pigment Red 254, C.I. Pigment Green
36, C.I. Pigment Blue 15:6, these colorants have to be prepared by a
milling and photoresist solution causing an expensive manufacturing
cost. Therefore various attempts to replace the conventional pigments
by soluble dyes have tried especially for red color based on diketo-
pyrrolo-pyrrole, perylene, naphthalene or coronene system, but most
of potential red dyes exhibited insufficient performance in terms of heat
stability during a post-baking stage in comparison of pigments [1–10].
In addition, the optical properties, such as a light transmittance and
contrast ratio, of pigment-based color pixels reaches to maximum level,
therefore most of manufacturing industries require enhanced optical
properties. If soluble dye can afford sufficient heat-stability, required
optical properties mentioned can be readily improved to substitute
conventional pigments.
perature ranged in 421–462 �C [13]. It was also reported that dyes based
on 1,8-naphthalimide substituted at 3-position via imine bond with
�
carbazole exhibited 345–368 C of T5 value (5% weight loss tempera-
ture) and 358–417 �C of T10 value (10% weight loss temperature) [14].
However, there is very limited literature reported on the thermal sta-
bility for diketo-pyrrolo-pyrrole based dyes containing acetylene linkage
group and 1,8-naphthalimide ring.
In this study, the diketo-pyrrolo-pyrrole chromophore was modified
by N-alkylation and incorporation of an acetylene linkage group
substituted by a naphthalimide ring these can contribute the enhanced
heat stability of dye molecule. For heat-stability of red dye, the weight
�
�
reduction % by TGA analysis was tested at both 230 C and 300 C,
�
where the results of 230 C can correspond to the conditions of post-
baking stage for color filter manufacturing. To evaluate the optical
properties of dyes prepared, a spin-coating method using a pure dye
solution was optimized which can be differential approach compared to
previous literatures [3,15] those used extra binder system in the dye
solution.
Judging from the literature, it can be presumed that the presence of
one or acetylene groups in the dye molecule readily increases s-char-
acter by sp-hybridized orbitals resulting in heat stability [11,12]. In case
* Corresponding author.
E-mail address: jaehong@knu.ac.kr (J.-H. Choi).
https://doi.org/10.1016/j.dyepig.2019.108053
Received 1 November 2019; Received in revised form 14 November 2019; Accepted 15 November 2019
Available online 16 November 2019
0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Tae-Heon Kim, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108053

T.-H. Kim et al.
Dyes and Pigments xxx (xxxx) xxx
2. Experimental
product was purified by column chromatography (adsorbent: silica gel,
eluent: ethyl acetate/n-hexane(1:5). Yield: 53.2%, 1H NMR (400 MHz,
DMSO‑d6), δ: 0.88 (t, J ¼ 6.5 Hz, 3H, CH3), 1.29 (m, 10H, CH2), 1.63 (m,
2H, CH2), 3.88 (t, J ¼ 7.7 Hz, 2H, CH2), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH),
7.55 (d, J ¼ 5.5 Hz, 4H, ArH), 8.0 (s, 1H, NH); GC-MS 591(Mþ);
Elemental Analysis: Found: C, 55.93; H, 4.69; N, 5.02; calculated for: C,
55.99; H, 4.72; N, 5.01.
2.1. Materials and instrumentation
C.I. Pigment Red 254 used in experiment was manufactured by ENF
Technology Co., Ltd. 1,8-naphthalic anhydride (99%), bromine (98%),
sodium tert-butoxide (97%), potassium hydroxide (KOH) 2-methyl-3-
butyn-2-ol (99%), ethynyltrimethylsilane (98%), copper iodide
(99.5%), and bis(triphenylphosphine)palladium(II) dichloride (98%)
were purchased from Sigma-Aldrich. n-butylamine (99%), n-bro-
moctane (98%), ethyl 2-bromopropionate (98%), ethyl bromoacetate
(98%), were purchased from TCI. Triphenylphosphine (99%), copper(I)
chloride (99%), and silver sulfate (98.5%) were purchased from Alfa-
Aesar. 1-Methyl-2-pyrrolidone (NMP), trimethylamine (TEA), tetrahy-
drofuran (THF), N,N-dimethylmethanamide (DMF), ethanol, methanol,
n-hexane, ethyl acetate, methyl chloride, and other solvents were pur-
chased from Duksan Pure Chemical. All chemicals were used without
any additional purification.
2.2.4. ethyl 2-(3,6-bis(4-bromophenyl)-1,4-dioxo-4,5-dihydropyrrolo[3,4-
c]pyrrol-2(1H)-yl)acetate (intermediate 4): N-acylation
Intermediate 4 was synthesized following the same procedure for
intermediate 1 using ethyl bromoacetate (2.6 g, 13.4 mmol). The crude
product was purified by column chromatography (adsorbent: silica gel,
eluent: ethyl acetate/n-hexane). Yield: 51.1%, 1H NMR (400 MHz,
DMSO‑d6), δ: 1.29 (t, J ¼ 7.1 Hz, 3H, CH3), 4.13 (m, 2H, CH2), 4.28 (s,
2H, CH2), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼ 5.5 Hz, 4H, ArH),
8.0 (s, 1H, NH); GC-MS 533(Mþ); Elemental Analysis: Found: C, 49.65;
H, 3.03; N, 5.26; calculated for: C, 49.58; H, 3.01; N, 5.20.
1H NMR spectra were measured on a Bruker Avance 500 spectrom-
eter at 500 MHz using dimethyl sulfoxide-d6 and chloroform-d as the
solvent. Tetramethylsilane (TMS) was used as an internal standard.
Elemental analysis was conducted with a ThermoFisher Flash 2000
elemental analyzer. Mass spectrometer was performed by gas chroma-
tography/mass spectrometer (GC-MS; 7890 A-5975C GC/MSD, Agilent,
USA).
2.2.5. ethyl 2-(3,6-bis(4-bromophenyl)-1,4-dioxo-4,5-dihydropyrrolo[3,4-
c]pyrrol-2(1H)-yl)propanoate (intermediate 5): N-acylation
Intermediate 5 was synthesized following the same procedure for
intermediate 1 using 2-bromopropionate (2.4 g, 13.4 mmol). The crude
product was purified by column chromatography (adsorbent: silica gel,
eluent: ethyl acetate/n-hexane). Yield: 57.2%, 1H NMR (400 MHz,
DMSO‑d6), δ: 1.29 (t, J ¼ 7.1 Hz, 3H, CH3),1.61 (s, 3H, CH3) 4.21 (m, 2H,
CH2), 5.88 (s, 1H, CH), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼ 5.5 Hz,
4H, ArH), 8.0 (s, 1H, NH); GC-MS 547(Mþ); Elemental Analysis: Found:
C, 50.58; H, 3.32; N, 5.13; calculated for: C, 50.52; H, 3.29; N, 5.10.
2.2. Syntheses of dyes
2.2.1. 3,6-bis(4-bromophenyl)-2,5-dibutylpyrrolo[3,4-c]pyrrole-1,4
(2H,5H)-dione (intermediate 1): N-alkylation
2.2.6. 3-(4-bromophenyl)-2,5-dibutyl-6-(4-((trimethylsilyl)ethynyl)
C. I. Pigment Red 254 (4 g, 8.9 mmol) was added to DMF (80 mL) and
trimethylamine (4 mL) mixed solvent and stirred at room temperature
for 2 h. After stirring, add the potassium tert-butoxide (3.0 g, 22.3 mmol)
and stir for 1 h. Then, add 1-bromobutane (3 g, 22.3 mmol) and stir for
12 h at room temperature under a nitrogen atmosphere. After reaction,
the reaction solution was poured into water (150 mL), allowing pre-
cipitation of the crude product. The crude product was filtered and
dried. After drying, dissolve the dried crude in 400 mL of ethyl acetate
and filter to remove the unreactive C.I. Pigment Red 254 and evaporate
the solvent to obtain the product. And pouring the obtained product into
the chloroform/n-hexane (1:10) mixture, stir for 30 min to filter the
extracted product using the difference in solubility to obtain N,N0-dia-
lkylated components. The crude product was purified by column chro-
matography (adsorbent: silica gel, eluent: ethyl acetate/n-hexane).
Yield: 54.1%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼ 6.5 Hz, 6H,
CH3), 1.31 (m, 4H, CH2), 1.52 (m, 4H, CH2), 3.88 (t, J ¼ 7.7 Hz, 4H,
CH2), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼ 5.5 Hz, 4H, ArH); GC-
MS 559(Mþ); Elemental Analysis: Found: C, 55.93; H, 4.69; N, 5.02;
calculated for: C, 55.43; H, 4.59; N, 5.12.
phenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-di one (intermediate 6)
A
mixture of intermediate 1 (1.1 g, 2.0 mmol), ethynyl-
trimethylsilane (0.2 g, 2.0 mmol), cuprous iodide (0.008 g, 0.04 mmol),
triphenylphosphine (0.026 g, 0.1 mmol), and bis(triphenylphosphine)
Palladium(II)dichloride (0.04 g, 0.06 mmol) was carefully degassed and
charged with nitrogen. Tetrahydrofuran (15 mL) and triethylamine (10
mL) were then added. The reaction mixture was refluxed for 24 h under
a nitrogen atmosphere. The reaction was terminated with ammonia
water (10 mL), then extracted with ethyl acetate (10 mL), washed with
water (10 mL), dried over anhydrous magnesium sulfate, and evapo-
rated to obtain the crude product. The crude product was purified by
column chromatography (adsorbent: silica gel, eluent: ethyl acetate/n-
1
hexane). Yield: 50.1%, H NMR (400 MHz, DMSO‑d6), δ: 0.08 (s, 9H,
CH3), 0.9 (t, J ¼ 6.5 Hz, 6H, CH3), 1.31 (m, 4H, CH2), 1.52 (m, 4H, CH2),
3.88 (t, J ¼ 7.7 Hz, 4H, CH2), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼
5.5 Hz, 4H, ArH); GC-MS 576(Mþ); Elemental Analysis: Found: C, 64.68;
H, 6.13; N, 4.87; calculated for: C, 64.60; H, 6.11; N, 4.88.
2.2.7. 2-Butyl-5-ethynyl-1H-benzo[de]isoquinoline-1,3(2H)-dione
(Intermediate 7): hydrolysis
2.2.2. 3,6-bis(4-bromophenyl)-2-butylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione (intermediate 2): N-alkylation
2-Butyl-5-(3-hydroxy-3-methylbut-1-ynyl)-1H-benzo[de] isoquino-
line-1,3(2H)-dione (0.34 g, 1 mmol) and potassium hydroxide (0.16 g, 3
mmol) were added to anhydrous toluene (3 mL). The mixture was
refluxed for 6 h under a nitrogen atmosphere and then filtered. The
filtrate was evaporated to obtain the crude product, which was purified
by column chromatography (adsorbent: silica gel, eluent: ethyl acetate/
hexane). Yield: 72.3%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼ 6.5
Hz, 3H, CH3), 1.31 (m, 2H, CH2), 1.56 (m, 2H, CH2), 3.14 (t, J ¼ 7.7 Hz,
2H, CH2), 4.05 (s, 1H, CH), 7.79 (m, 1H, ArH), 8.10 (s, 2H, ArH), 8.41 (d,
J ¼ 1.8 Hz, 2H, ArH); GC-MS 278(Mþ); Elemental Analysis: Found: C,
77.96; H, 5.45; N, 5.05; calculated for: C, 77.90; H, 5.43; N, 5.10.
Intermediate 2 was synthesized following the same procedure for
intermediate 1 using 1-bromobutane (1.8 g, 13.4 mmol). The crude
product was purified by column chromatography (adsorbent: silica gel,
eluent: ethyl acetate/n-hexane). Yield: 64.2%, 1H NMR (400 MHz,
DMSO‑d6), δ: 0.9 (t, J ¼ 6.5 Hz, 3H, CH3), 1.31 (m, 2H, CH2), 1.52 (m,
2H, CH2), 3.88 (t, J ¼ 7.7 Hz, 2H, CH2), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH),
7.55 (d, J ¼ 5.5 Hz, 4H, ArH), 8.0 (s, 1H, NH); GC-MS 535(Mþ);
Elemental Analysis: Found: C, 52.62; H, 3.61; N, 5.58; calculated for: C,
52.42; H, 3.51; N, 5.60.
2.2.3. 3,6-bis(4-bromophenyl)-2-octylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione (intermediate 3): N-alkylation
2.2.8. 2-Butyl-5-ethynyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (Dye
1): hydrolysis
Intermediate 3 was synthesized following the same procedure for
intermediate 1 using 1-bromooctane (2.6 g, 13.4 mmol). The crude
Intermediate 6 (0.6 g, 1 mmol) and potassium hydroxide (0.16 g, 3
2

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mmol) were added to anhydrous toluene (3 mL). The mixture was
refluxed for 1 h under a nitrogen atmosphere and then filtered. The
filtrate was evaporated to obtain the crude product, which was purified
by column chromatography (adsorbent: silica gel, eluent: ethyl acetate/
hexane). Yield: 48.3%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼ 6.5
Hz, 9H, CH3), 1.31 (m, 6H, CH2), 1.56 (m, 6H, CH2), 3.14 (t, J ¼ 7.7 Hz,
6H, CH2), 7.27 (d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼ 5.5 Hz, 4H, ArH),
7.79 (m, 1H, ArH), 8.14 (s, 2H, ArH), 8.41 (s, 2H, ArH); GC-MS 754
(Mþ); Elemental Analysis: Found: C, 66.75; H, 5.43; N, 5.50; calculated
for: C, 66.80; H, 5.41; N, 5.56.
6.5 Hz, 3H, CH3), 1.31 (m, 5H, CH2), 1.56 (m, 2H, CH2), 3.14 (t, J ¼ 7.7
Hz, 2H, CH2), 4.13 (m, 2H, CH2), 4.28 (s, 2H, CH2), 7.27 (d, J ¼ 8.1 Hz,
4H, ArH), 7.55 (d, J ¼ 8.1 Hz, 4H, ArH), 7.79 (m, 1H, ArH), 8.0 (s, 1H,
NH), 8.19 (s, 2H, ArH), 8.41 (d, J ¼ 1.8 Hz, 2H, ArH); GC-MS 729(Mþ);
Elemental Analysis: Found: C, 65.94; H, 4.15; N, 5.77; calculated for: C,
65.89; H, 4.10; N, 5.80.
2.2.13. Ethyl 2-(3-(4-bromophenyl)-6-(4-((2-butyl-1,3-dioxo-2,3-
dihydro-1H-benzo[de]isoquinolin-5-yl)ethynyl) phenyl)-1,4-dioxo-4,5-
dihydropyrrolo[3,4-c]pyrrol-2(1H)-yl)propanoate (Dye 6)
Dye 6 was synthesized following the same procedure for dye 2 using
intermediate 5 (1 g, 1.9 mmol). The crude product was purified by
column chromatography (adsorbent: silica gel, eluent: ethyl acetate/n-
hexane). Yield: 48.9%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼
6.5 Hz, 3H, CH3), 1.31 (m, 5H, CH2), 1.62 (m, 5H, CH3), 3.14 (t, J ¼ 7.7
Hz, 2H, CH2), 4.21 (m, 2H, CH2), 5.58 (m, 1H, CH), 7.27 (d, J ¼ 8.1 Hz,
4H, ArH), 7.55 (d, J ¼ 8.1 Hz, 4H, ArH), 7.79 (m, 1H, ArH), 8.0 (s, 1H,
NH), 8.19 (s, 2H, ArH), 8.41 (d, J ¼ 1.8 Hz, 2H, ArH); GC-MS 743(Mþ);
Elemental Analysis: Found: C, 66.31; H, 4.34; N, 5.66; calculated for: C,
66.29; H, 4.31; N, 5.71.
2.2.9. 5-((4-(4-(4-bromophenyl)-2,5-dibutyl-3,6-dioxo-2,3,5,6-
tetrahydropyrrolo[3,4-c]pyrrol-1-yl)phenyl)ethyn yl)-2-butyl-1H-benzo
[de]isoquinoline-1,3(2H)-dione (Dye 2)
A mixture of intermediate 1 (1.1 g, 2.0 mmol), intermediate 7 (0.6 g,
2.0 mmol), cuprous iodide (0.008 g, 0.04 mmol), triphenylphosphine
(0.026 g, 0.1 mmol), and bis(triphenylphosphine)Palladium(II)dichlor-
ide (0.04 g, 0.06 mmol) was carefully degassed and charged with ni-
trogen. Tetrahydrofuran (15 mL) and triethylamine (10 mL) were then
added. The reaction mixture was refluxed for 24 h under a nitrogen
atmosphere. The reaction was terminated with ammonia water (10 mL),
then extracted with ethyl acetate (10 mL), washed with water (10 mL),
dried over anhydrous magnesium sulfate, and evaporated to obtain the
crude product. The crude product was purified by column chromatog-
raphy (adsorbent: silica gel, eluent: ethyl acetate/n-hexane). Yield:
48.6%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼ 6.5 Hz, 9H, CH3),
1.31 (m, 6H, CH2), 1.56 (m, 6H, CH2), 3.14 (t, J ¼ 7.7 Hz, 6H, CH2), 7.27
(d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼ 5.5 Hz, 4H, ArH), 7.79 (m, 1H,
ArH), 8.14 (s, 2H, ArH), 8.41 (s, 2H, ArH); GC-MS 754(Mþ); Elemental
Analysis: Found: C, 69.98; H, 5.30; N, 5.57; calculated for: C, 70.02; H,
5.34; N, 5.57.
2.3. PPP- MO (pariser–parr–pople molecular orbital) calculations
PPP MO calculations were carried out by using a PISYSYEM Version
6.2.
2.4. Discrete fourier transform (DFT) calculations
DFT calculations were carried out using the Gaussian 09 programs
using the B3LYP, 6-31G as the standard for the geometry optimization of
dyes. The band gaps between highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO), and electron
density were calculated by FMO calculations.
2.2.10. 5-((4-(4-(4-bromophenyl)-5-butyl-3,6-dioxo-2,3,5,6-
tetrahydropyrrolo[3,4-c]pyrrol-1-yl)phenyl)ethynyl) -2-butyl-1H-benzo
[de]isoquinoline-1,3(2H)-dione (Dye 3)
Dye 3 was synthesized following the same procedure for dye 2 using
intermediate 2 (1 g, 2.0 mmol). The crude product was purified by
column chromatography (adsorbent: silica gel, eluent: ethyl acetate/n-
hexane). Yield: 47.6%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼
6.5 Hz, 6H, CH3), 1.31 (m, 4H, CH2), 1.56 (m, 4H, CH2), 3.14 (t, J ¼ 7.7
Hz, 4H, CH2), 7.34 (d, J ¼ 8.1 Hz, 4H, ArH), 7.55 (d, J ¼ 5.5 Hz, 4H,
ArH), 7.79 (m, 1H, ArH), 8.0 (s, 1H, ArH), 8.14 (s, 2H, ArH), 8.41 (d, J ¼
1.8 Hz, 2H, ArH); GC-MS 699(Mþ); Elemental Analysis: Found: C, 68.77;
H, 4.62; N, 6.01; calculated for: C, 68.80; H, 4.60; N, 6.03.
2.5. Spectroscopic properties
UV–Visible spectroscopic analysis, the dyes were dissolved in N,N-
dimethylformamide, and absorbance spectra were collected using a
Thermo Scientific Evolution 300 UV–Visible spectrophotometer. Visible
light transmission was measured using the UV–Vis spectrophotometer
(Varian, Cary 4000). The incident beam and test piece surface are
adjusted to 90�. The test piece was measured five times in the 200–800
nm area and then averaged out three transmittances except the highest
and lowest values.
2.2.11. 5-((4-(4-(4-bromophenyl)-5-octyl-3,6-dioxo-2,3,5,6-
tetrahydropyrrolo[3,4-c]pyrrol-1-yl)phenyl)ethyny l)-2-butyl-1H-benzo
[de]isoquinoline-1,3(2H)-dione (Dye 4)
2.6. Solubility test
Dye 4 was synthesized following the same procedure for dye 2 using
intermediate 3 (1 g, 1.8 mmol). The crude product was purified by
column chromatography (adsorbent: silica gel, eluent: ethyl acetate/n-
hexane). Yield: 43.2%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼
6.5 Hz, 6H, CH3), 1.29 (m, 12H, CH2), 1.63 (m, 4H, CH2), 3.14 (t, J ¼ 7.7
Hz, 2H, CH2), 3.88 (t, J ¼ 7.7 Hz, 2H, CH2), 7.27 (d, J ¼ 8.1 Hz, 4H,
ArH), 7.55 (d, J ¼ 5.5 Hz, 4H, ArH), 7.79 (m, 1H, ArH), 8.0 (s, 1H, ArH),
8.19 (s, 2H, ArH), 8.41 (d, J ¼ 1.8 Hz, 2H, ArH); GC-MS 755(Mþ);
Elemental Analysis: Found: C, 70.12; H, 5.34; N, 5.47; calculated for: C,
70.03; H, 5.32; N, 5.58.
The solubility measurements were carried out according to KS M ISO
7579.
2.7. Thermal stability test
To investigate the thermal stability of dyes 1–6. Thermogravimetric
analyses (TGA) were performed. All samples were heated at a rate 10�C/
min, under flowing nitrogen up to 400 �C.
2.8. Fabrication of color filters using synthesized dyes
2.2.12. Ethyl 2-(3-(4-bromophenyl)-6-(4-((2-butyl-1,3-dioxo-2,3-
dihydro-1H-benzo[de]isoquinolin-5-yl)ethyn yl)phenyl)-1,4-dioxo-4,5-
dihydropyrrolo[3,4-c]pyrrol-2(1H)-yl)acetate (Dye 5)
A coating solution was prepared by mixing the dye 5 or 6, PGMEA (1-
Methoxy-2-propyl acetate) in the weight-ratio of 1–30 to 10. The
resultant mixtures were coated on clean glass substrate sized of 5Х5 cm.
The initial speed of spin coating was 200 rpm for 10 s. Then, it was
increased to 500 rpm for further 20 s. The solution coated glass was pre-
baked at 80 �C for 20 min and finally post-baked at 230 �C for 1 h.
Dye 5 was synthesized following the same procedure for dye 2 using
intermediate 4 (1 g, 1.9 mmol). The crude product was purified by
column chromatography (adsorbent: silica gel, eluent: ethyl acetate/n-
hexane). Yield: 45.2%, 1H NMR (400 MHz, DMSO‑d6), δ: 0.9 (t, J ¼
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2.9. Transmittance test
3-carbon position of naphthalene ring, then it was coupled to a phenyl
ring of the diketo-pyrrolo-pyrrole, as shown in Scheme 1. As N-alkyl-1,
8-naphthalimidyl moiety can afford good stability to heat, the combi-
nation of an ethynyl group and 1,8-naphthalimidyl system was
attempted to prepare final dyes.
Transmittance of red lights was measured using the Olympus
Colorimeter OSP-200. The incident beam and test piece surface are
adjusted to 90�. The test piece was measured five times in the 200–800
nm area and then averaged out three transmittances except the highest
and lowest values.
To provide the red color, in this study, C.I. Pigment Red 254 which is
based on diketo-pyrrolo-pyrrole chromophore was utilized as a starting
material for the preparation of final red dyes 1–6, however the
displacement of the bromine atom by an ethynyl group should affect the
absorption maximum of analogue dye comparing to that of C.I. Pigment
Red 254, an N-substituent of the diketo-pyrrolo-pyrrole ring was care-
fully optimized to achieve absorption maximum to be in the range of
red.
2.10. Contrast ratio
Contrast ratio of color filter prepared was measured by BM-5A of
Topcon SpectroRaidoMeter. The measurement standard was based on
the contrast 1/30,000 (bare scale) of the glass substrate.
It was also reported that the N-alkylation of diketo-pyrrolo-pyrrole
ring to be attributable to reduce strong intermolecular interactions
caused by inherent stacking between red pigment molecules resulting in
considerably increased solubility of 6.5–14.4 g/L in PGMEA (1-
methoxy-2-propyl acetate) [1]. However, the requirement of the solu-
bility for color filter fabrications to be compromised with the chemical
stability of dyes, which should give rise to an opposite effect to solubi-
lity, thus the target of solubility was set to be around 5 g/L.
3. Results and discussion
3.1. Dye structure design of target molecules
In the previous studies, 1,8-naphthalimidyl based dyes containing an
ethynyl linkage group exhibited improved thermal-stability compared
to that of corresponding dye which can due to the higher optical band
gap calculated by TD-DFT method (B3LYP/6-31G) [11,16]. In case of
dyes based on triphenylamine core group connected with 1 or 2–3
multiple 1,8-naphthalimidyl ethynyl linkage, the resultant thermal
properties (T5%) found to be 421–462 �C [13]. Therefore, to improve the
thermal-stability of the final dyes, an ethynyl group was substituted at
3.2. Dye synthesis
As described in Scheme 1, six red dyes 1–6 were prepared starting
from C.I. Pigment Red 254, which contained two bromo substituents at
Scheme 1. Synthetic routes of the red dyes 1–6.
4

T.-H. Kim et al.
Dyes and Pigments xxx (xxxx) xxx
the phenyl ring, by the sequent reactions, N-monoalkylation or dia-
lkylation, Sonogashira coupling, hydrolysis and Sonogashira coupling.
In the alkylation process, a mixture of mono-alkylated intermediate and
di-alkylated analogue was inevitably formed, therefore two products
were simply separated by adding the reaction mixture into chloroform
and n-hexane (1:10) solvent system resulting in a precipitation of mono-
alkylated product, which was less soluble, leaving di-alkylated analogue
in the solution. The reaction yields for intermediates 1–5 varied between
50–64% depending on an alkyl group that was due to the formation of a
mixture, as mentioned above. For the preparation of final dyes 2–6, a
direct substitution reaction using Sonogashira coupling conditions
starting from N-alkylated intermediates 1–5 with intermediate 7,
respectively, provided a reasonable reactivity where the resultant yields
were obtained in the range of 43–48% after purification. The structure of
six red dyes prepared shown in Fig. 1.
Table 1
Absorption maxima and molar extinction coefficient of dye 1–6 in DMF solution.
Dye
Absorption maximum (nm)
εmax (L molÀ 5 cmÀ 1
)
C.I. Pigment Red 254
516 [1]
494
–
Dye 1
Dye 2
Dye 3
Dye 4
Dye 5
Dye 6
31,600
39,800
38,400
40,900
41,700
43,200
501
498
504
502
508
508 nm whereas n-butyl substituted analogue (dye 3) showed 498 nm of
absorption maximum which can be explained that an ethyl ester group
of propionate exerted smaller hypsochromic shift due to its electron-
withdrawing inductive effect. In a direct comparison between n-butyl
substituted dye 3 and n-octyl analogue 4, it was observed that a longer
carbon chain gave rise to a small bathochromic shift of 6 nm, thus dye 4
absorbed maximally at 504 nm whereas dye 3 was 498 nm in λmax. In the
case of di-n-butyl substituted dye 2 the difference of absorption
maximum was found to be only 3 nm in comparison with that of mono-n-
butyl substituted analogue 3, therefore it can be assumed that intro-
duction of a second alkyl substituent into the nitrogen of diketo-pyrrolo-
pyrrole system exerted very small effect to the energy gap between
ground state (HOMO) and excited state (LUMO).
3.3. Spectroscopic properties
All six dyes blue-shifted in absorption maximum measured in DMF
comparing to that of C.I. Pigment Red 254, as summarized in Table 1.
The smallest deviation in hypsochromic shift was observed with dye 6
relative to absorption maximum of C.I. Pigment Red 254, whereas dye 3
containing an N-n-octyl substituent provided most blue shifted absorp-
tion maximum. Overall, the introduction of an electron donating group
(R) into the nitrogen of diketo-pyrrolo-pyrrole ring gave rise to a hyp-
sochromic shift, where the most red shifted dye was observed by N-
isopropyl substituent comparing to that by normal alkyl analogues. By
comparing the effects of N-substituent, dye 6 absorbed at a maximum of
In terms of molar extinction coefficient values, both dyes 5 and 6
provided relatively stronger color strength, 41,770 and 43,200 of εmax
,
respectively, thus it can be explained by the contribution of compara-
tively weaker electron-donating groups, n-propionate and iso-
Fig. 1. Chemical structure of red dyes 1–6 based on diketo-pyrrolo-pyrrole.
5

T.-H. Kim et al.
Dyes and Pigments xxx (xxxx) xxx
propionate, relative to n-butyl or n-octyl analogues. The significant in-
creases in molar extinction coefficient value were observed with the
introduction of a naphthalimidyl system to the diketo-pyrrolo-pyrrole
ring. All dyes 2–6 containing a naphthalimidyl ring showed εmax
higher than 38,400 up to 43,200, whereas dye 1 only gave 31,600 of
εmax value. Therefore it can also conclude that the extended conjugation
of
π orbitals exerted by naphthalimidyl ring readily contributed
increased molar extinction coefficient value.
3.4. PPP-MO calculations
The PPP-MO method aims at a reliable theory to predict absorption
maxima and intensities for coloured molecules by virtue of the suc-
cessful combination of theoretical Roothaan equations with a semi-
empirical theory. In this study, two analogue dyes 2 & 3 were calcu-
lated to compare the effects of mono-N-butyl substituent and N,N0-
dibutyl substituents, as well as the calculations for dye 1 that only
contains an acetylene group. Judging from the calculated band gaps
between HOMO-LUMO for dyes 1–3, the absorption maximum to be
bathochromic shifted by the order of dye 1 → dye 3 → dye 2. Similarly,
the HOMO-LUMO band gap was predicted that dye 1 had the largest
band gap (2.31 eV) compared with that of the dyes 2–3 (2.26–2.28 eV),
therefore it was accordant with the observed absorption maximum, as
shown Table 1 for dye 1. However, the values of absorption maximum
for all three dyes 1–3 seem to be not accurate by a PPP-MO method since
the difference in absorption maximum between calculated and observed
values was found with 42 nm, 46 nm, and 44 nm, respectively. Never-
theless, PPP-MO calculation can be useful to predict the tendency of
absorption maxima for the analogue dyes.
Fig. 2. The frontier orbitals of synthesized dyes 3 & 5.
3.6. Thermal-stability properties
Thermal-stability was evaluated by the weight reduction (%), as
mentioned in the experimental part. As summarized in Table 3, weight
�
�
�
reduction (%) was checked at 230 C and 270 C, where 230 C was
corresponding to the post-baking temperature in the color filter fabri-
cation process, and weight loss results at 270 �C was intended to
discriminate relative thermal-stability of dyes 1–6 at a more severe
temperature. Two dyes 1 & 2 substituted by N-di-alkyl groups in the
diketo-pyrrolo-pyrrole ring showed comparatively inferior thermal-
stability compared to that of other dyes 3–6 containing an N-mono-
alkyl substituent. In case of dye 1, the weight reduction was observed by
15.8% and 26.2% at 230 �C, 270 �C, respectively, and dye 2 also
The HOMO/LUMO band gap of the dyes 1–3 calculated with a PPP-
MO method and observed absorption maximum data are shown in
Table 2.
3.5. Discrete fourier transform (DFT) calculations
�
�
To compare the expected geometries of the highest occupied mo-
lecular (HOMO) and lowest unoccupied molecular orbital (LUMO) were
calculated by density functional theory (DFT) at B3LYP/6-31G(d) for
two dyes 3 and 5. As shown in Fig. 2, the calculated HOMO levels of dye
3 and 5 were obtained by À 5.43, À 5.42 eV, respectively. The HOMO
energy levels of the synthesized dyes were similar. In the HOMO, the
electron density distributions were delocalised due to the electron being
localised in the DPP moiety and acetylene linkage, which act as electron-
donating groups. The calculated LUMO levels of dyes 3 and 5 were also
obtained by À 2.61, À 2.88 eV, respectively. In the case of dye 3, the
electron density distributions in the LUMO were localised due to the
distortion of phenyl ring of DPP and naphthalimidyl group around the
ethylene linkage. As a result, LUMO level of dye 3 was calculated to be
higher than that of dye 5. In contrast, dye 5 the LUMO energy value was
lowered because benzene ring of DPP and naphthalimidyl were delo-
calised on the same side with respect to acetylene linkage. Also, it was
confirmed that dye 5 exerted a small bathochromic shift in absorption
maximum compared to that of dye 3 because the acyl group, which
provided electron-withdrawing effect, was delocalised at the LUMO
energy level.
exhibited 14.7% and 21.0% at 230 C, 270 C, respectively. It can be
explained that di-alkyl groups of nitrogen atoms readily diminished
strong intermolecular interactions existing between dye molecules as
usually formed pigment molecules, resulting in significantly decreased
stability to heat. In contrast, four other dyes 3–6 provided much higher
stability which ranged 0.6–4.4% at 230 �C and 0.9–5.6% at 270 �C,
particularly two dyes 5 & 6 performed similar properties to that of C.I.
Pigment Red 254. As it was clearly illustrated, the introduction of a
naphthalimidyl ring via an ethynyl linkage group apparently contrib-
uted the enhancement of thermal-stability which can be attributable to
the increased
π orbital (sp) conjugation throughout dye molecule.
Although the gaps in weight reduction (%) were not too large, however
it can be also revealed that N-alkyl substituents containing ethyl ester
moiety in dye 5 & 6 contributed improved stability further in compar-
ison with N-n-butyl or N-n-octyl groups. The results observed by dye 5,
weight reduction of 0.6% and 0.94% at 230 and 270 �C, respectively, to
be extremely stable to heating conditions for simulating to color filter
Table 3
The weight loss (%) of dye 1–6 by TGA analysis measured at 230 �C and 270 �C.
Dye
Weight loss (%)
Table 2
230 �
C
270 �
C
The HOMO-LUMO band gap, absorption maxima and color strength of the
synthesized dyes 1–3 calculated by PPP-MO calculations.
C.I. Pigment Red 254
0.37
15.8
14.7
2.1
0.43
26.2
21.0
4.9
Dye
Band gap (eV)
Absorption maxima (nm)
Calculated
Dye 1
Dye 2
Dye 3
Dye 4
Dye 5
Dye 6
Observed
Dye 1
Dye 2
Dye 3
2.31
2.26
2.28
536
547
542
494
501
498
4.4
5.6
0.6
0.94
1.4
1.8
6

T.-H. Kim et al.
Dyes and Pigments xxx (xxxx) xxx
fabrication. It was interesting that dye 4 exerted comparatively less
weight reduction over 300 �C which may be associated with a long
carbon chain, n-octyl group, presented in the nitrogen atom of diketo-
pyrrolo-pyrrole ring, as shown in Fig. 3.
Table 4
Solubility of dye 3–6 in methylene chloride.
Dye
Solubility (g/L)
C.I. Pigment Red 254
insoluble
5.3
Judging from these results mentioned, two dyes 5 & 6 exerted
comparatively similar thermal-stability to that of C.I. Pigment Red 254,
therefore these were selected as potential red dye for the fabrication
process of color filter.
Dye 3
Dye 4
Dye 5
Dye 6
5.6
4.4
5.0
3.7. Solubility properties
property to provide improved quality of LED displays, particularly as the
panel size increased the contrast ratio to be paid attentions. As higher
the contrast ratio value, the black and white colours can be highly
distinguished leading to improved quality of displays. Thus color filter
and final panel manufacturers have been in pursuit of improving
contrast ratio particularly for red color to meet the requirements in the
market. Among trichromat pigments, the contrast ratio of red color has
been lowest, around 10000 for C.I. Pigment Red 254, whereas 20000 for
blue and green pigments. As Table 6 illustrates, the contrast ratio of dye
6 was significantly improved by 14300 that was 43% higher than C.I.
Pigment Red 254. Therefore, the improved contrast ratio reported in this
study, can contribute the overall enhancement of the quality for display
panels.
It was reported that N-substitution by an alkyl group in the DPP
(diketo-pyrrolo-pyrrole) based pigment molecule afforded solubility in
common organic solvents, such as PGMEA [1]. Two dyes 1 & 2 exhibited
inferior thermal-stability, as discussed previously, were excluded for the
solubility test, thus Table 3 shows the results of solubility measured in
methylene chloride for dyes 3–6. All four dyes provided similar solu-
bility irrespective to their structure in the range of 4.4% for dye 5 up to
5.6% for dye 4. From the industrial point of view, the required solubility
for a spin-coating process using a dye solution eliminating any
dispersing agent or binder to be around 4–5 g/L, therefore the results
illustrated in Table 4 can sufficient for the industrial requirement.
Nevertheless, only two dyes 5 & 6, featured as more stable dye to heat,
were carried out to examine their transmittance property and contrast
ratio after a spin-coating & backing treatments.
4. Conclusion
Six red dyes were prepared starting from C.I. Pigment Red 254,
which contained two bromo substituents at the phenyl ring, by the
sequent reactions, N-monoalkylation or dialkylation, Sonogashira
coupling, hydrolysis and Sonogashira coupling. By comparing the effects
of N-substituent, dye 6 absorbed with a maximum at 508 nm whereas n-
butyl substituted analogue (dye 3) showed 498 nm of absorption
maximum which can be explained that an ethyl ester group of propio-
nate exerted smaller hypsochromic shift due to its electron-withdrawing
inductive effect. In terms of molar extinction coefficient, all dyes 2–6
containing a naphthalimidyl ring showed εmax higher than 38,400 up to
43,200, whereas dye 1 only gave 31,600 of εmax value. For dyes 1–3, the
absorption maximum to be bathochromic shifted in the order of dye 1 →
dye 3 → dye 2 which was exactly accordant to the calculated order by a
PPP-MO method. In terms of weight reduction (%) by TGA analyses, two
dyes 5 & 6 performed similarly to that of C.I. Pigment Red 254 that
indicated that the introduction of a naphthalimidyl ring via an ethynyl
linkage group apparently contributed the enhancement of thermal-
3.8. Transmittance and contrast ratio properties
Transmittance of red lights after post-baking process of red dye on a
glass substrate directly relate to the efficiency of back-light of LED dis-
plays, therefore it will be beneficial that this value close to 100%.
However irrespective of dye or pigment, some parts of red lights have to
be absorbed and reflected when lights pass through the glass substrate,
thus it is generally acceptable that transmittance value is over 90% in
the industrial aspects. In this study, two dyes 5 & 6 those exerted
comparatively higher thermal-stability were examined by their trans-
mittance, as shown in Fig. 4 and Table 5. In the range of 550–580 nm, it
was observed that two dyes 5 & 6 absorbed some lights compared to C.I.
Pigment Red 254 which may be associated with their shorter λmax than
pigment analogue. Although the difference seems to be small, dye 6
exhibited higher transmittance than dye 5, where 94.5% of trans-
mittance for dye 6 was equivalent to that of C.I. Pigment Red 254.
Therefore, dye 6 was chosen for the contrast ratio test.
stability which can be attributable to the increased
π orbital (sp)
The contrast ratio, that is the ratio of the luminance of the brightest
color (white) to that of the darkest color (black), becomes a crucial
conjugation throughout dye molecule. Four dyes 3–6 provided similar
solubility in CH2Cl2 irrespective to their structure in the range of 4.4%
for dye 5 up to 5.6% for dye 4, these values can be sufficient for the
preparation of a dye solution using PGMEA to evaluate their contrast
ratio and transmittance properties. When dye solution was spin-coated
on a glass substrate then post-baking treated, dye 6 exhibited higher
transmittance than dye 5 which was equivalent to that of C.I. Pigment
Red 254. The contrast ratio of dye 6 was significantly improved by
14300 which was 43% higher than C.I. Pigment Red 254.
It should be emphasized that the enhancement of contrast ratio after
post-baking by new red dye containing an ethynyl linkage group con-
necting diketo-pyrrolo-pyrrole chromophore with a naphthalimidyl
group can enable red dye to be prepared as a dye solution without any
binder or dispersing agent, then to be stable to post-baking temperature
of the color filter fabrications.
Declaration of competing interest
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.
Fig. 3. The weight reduction curves of dyes 1–6 determined by TGA at the
heating rate of 10 �C minÀ 1
.
7

T.-H. Kim et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 4. Transmittance of synthesized dyes 5 and 6 compared to C.I. Pigment Red 254 after post-baking at 230 �C.
[3] Kim JY, Hwang TG, Kim SH, Namgoong JW, Kim JP. Synthesis of high-soluble and
non-fluorescent perylene derivatives and their effect on the contrast ratio of LCD
color filters. Dyes Pigments 2017;137:836–45.
Table 5
Transmittance of synthesized dyes 5 and 6 at 630 nm compared to C.I.
[4] Choi J, Kim SH, Lee WS, Chang JB, Kim JP. The influence of aggregation behavior
of novel quinophthalone dyes on optical and thermal properties of LCD color
filters. Dyes Pigments 2014;101:186–95.
Pigment Red 254 after post-baking at 230 �C.
Dye
Maximum transmittance (%)
[5] Choi J, Sakong C, Choi JH, Yoon C, Kim JP. Synthesis and characterization of some
perylene dyes for dye-based LCD color filters. Dyes Pigments 2011;90(1):82–8.
[6] Sakong C, Kim YD, Choi JH, Yoon C, Kim JP. The synthesis of thermally-stable red
dyes for LCD color filters and analysis of their aggregation and spectral properties.
Dyes Pigments 2011;88(2):166–73.
C.I. Pigment Red 254
94.9
92.1
94.5
Dye 5
Dye 6
[7] Kim YD, Cho JH, Park CR, Choi JH, Kim JP. Synthesis, application and
investigation of structure-thermal stability relationships of thermally stable water-
soluble azo naphthalene dyes for LCD red color filters. Dyes Pigments 2011;89(1):
1–8.
Table 6
Contrast ratio of synthesized dye 6 compared to that of C.I.
[8] Kim YD, Kim JP, Kwon OS, Choi IH. The synthesis and application of thermally-
stable dyes for ink-jet printed LCD color filters. Dyes Pigments 2009;81(1):45–52.
[9] Choi J, Lee WS, Sakong C, Yuk SB, Park JS, Kim JP. Facile synthesis and
characterization of novel coronene chromophores and their application to LCD
color filters. Dyes Pigments 2012;94(1):34–9.
Pigment Red 254.
Dye
Contrast ratio
C.I. Pigment Red 254
10000
14300
Dye 6
[10] Stas S, Balandier JY, et al. Straightforward access to diketopyrrolopyrrole (DPP)
dimers. Dyes Pigments 2013;97:198–208.
[11] Kim KW, Kim GH, Kwon SH, Yoon HI, Son JE, Choi JH. Synthesis and
photophysical properties of blue-emitting fluorescence dyes derived from
naphthalimide derivatives containing a diacetylene linkage group. Dyes Pigments
2018;158:353–61.
Acknowledgements
This work was supported by the Technology Innovation Program
funded by the Ministry of Trade, Industry and Energy (KOREA), Fund ID:
20000453.
[12] Reddy TS, Hwang JY, Choi MS. Aggregation induced emission properties of
naphthalimide–coumarin conjugates with various intermolecular linkages. Dyes
Pigments 2018;158:412–9.
[13] Gudeika D, Grazulevicius JV, Volyniuk D, Juska G, Jankauskas V, Sini G. Effect of
ethynyl linkages on the properties of triphenylamine and 1,8-naphthalimide.
J Phys Chem 2015;119:28335–8.
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