Preparation and characterizations of solvent soluble dyes based on dimerized diketo-pyrrolo-pyrrole pigment

Article abstract of DOI:10.1080/15421406.2015.1075831

We have prepared three pigments and six soluble dyes with thermal stability derived from diketo-pyrrolo-pyrrole (DPP) pigment by N-alkylation and dimerization. Synthesized dyes and pigments were measured by an absorption maximum (λ_max) and thermal stability using a UV-VIS spectrophotometer and thermogravimetric analysis (TGA) respectively, comparing with C.I. Pigment Red 254. These dyes exhibited superior solubility to the organic solvents by introducing the linking group (n-octyl). DPP pigments have inferior thermal and solvent stability, which has so far inhibited their commercial adoption in color filter fabrication. The thermal stability of the N-alkylated dyes can be highly contributed by both the carbon number and their shapes of N-alkyl group in DPP ring.

Full text of DOI:10.1080/15421406.2015.1075831

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Preparation and Characterizations of Solvent
Soluble Dyes Based on Dimerized Diketo-pyrrolo-
pyrrole Pigment
Hyun-Young Lee, Jae-Sung Yoo, Hye-Seon Kwon, Jin-Kyu Oh & Jae-Hong Choi
To cite this article: Hyun-Young Lee, Jae-Sung Yoo, Hye-Seon Kwon, Jin-Kyu Oh & Jae-Hong
Choi (2015) Preparation and Characterizations of Solvent Soluble Dyes Based on Dimerized
Diketo-pyrrolo-pyrrole Pigment, Molecular Crystals and Liquid Crystals, 617:1, 73-81, DOI:
10.1080/15421406.2015.1075831
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1075831
Published online: 07 Oct 2015.
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Date: 11 October 2015, At: 07:32

Mol. Cryst. Liq. Cryst., Vol. 617: pp. 73–81, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1075831
Preparation and Characterizations of Solvent
Soluble Dyes Based on Dimerized
Diketo-pyrrolo-pyrrole Pigment
HYUN-YOUNG LEE,1 JAE-SUNG YOO,2 HYE-SEON KWON,2
JIN-KYU OH,2 AND JAE-HONG CHOI1,∗
1Department of Functional Material Engineering, Kyungpook National
University, Daegu, Korea
2Department of Textile System Engineering, Kyungpook National University,
Daegu, Korea
We have prepared three pigments and six soluble dyes with thermal stability derived
from diketo-pyrrolo-pyrrole (DPP) pigment by N-alkylation and dimerization. Synthe-
sized dyes and pigments were measured by an absorption maximum (λmax) and thermal
stability using a UV-VIS spectrophotometer and thermogravimetric analysis (TGA)
respectively, comparing with C.I. Pigment Red 254. These dyes exhibited superior sol-
ubility to the organic solvents by introducing the linking group (n-octyl). DPP pigments
have inferior thermal and solvent stability, which has so far inhibited their commercial
adoption in color filter fabrication. The thermal stability of the N-alkylated dyes can
be highly contributed by both the carbon number and their shapes of N-alkyl group in
DPP ring.
Keywords Diketo-pyrrolo-pyrrole (DPP); solubility; absorption maxima; thermal
stability; dimerization.
1. Introduction
Liquid crystal display (LCD) modules, which are widespread media in today’s information
age, are composed of a backlight unit, a color filter (CF), and a thin-film transistor (TFT)
board, with a liquid crystal between CF and TFT board. The color filter, which converts
the white backlight into various colored lights, consists of RGB (red, green, blue) pixels, a
black matrix (BM) for prevention of light leakage, an overcoat for improving the flatness
of the pixel surface, and a column spacer to control the gap between cells1–2
.
Currently, various dye molecules are under extensive investigation for the manufac-
turing of RGB pixels and BM to improve the performance of LCD. The pigments, being
used for the main colorants of RGB pixels, show superior thermal and photo chemical
stabilities but cause both reflection and scattering of the light due to the presence of
∗Address correspondence to Prof. Choi Jae Hong, Department of Textile System Engineering,
Kyungpook National University, Sangyuk-dong, Buk-gu, Daegu 702–701, Korea (ROK). E-mail:
Jaehong@knu.ac.kr
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
73

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H.-Y. Lee et al.
insoluble pigment particles patterned in the each sub-pixels causing the reduced transmit-
tance of the light, thus gives rise to lower contrast ratio. Dyes can be attractive alternatives
to overcome this limitation due to the reduced light scattering resulting from the fact that
they can be dissolved in the media and exist in molecular form. Recently, their superior
optical properties for higher brightness have outweighed the inferior thermal stabilities
because the LCD manufacturing process temperature has been decreasing3–5
.
In terms of manufacturing process for color filter with pigment based photo resists,
there have been a number of patents describing the use of ink jet printing in the fabrication
of color filter as a means to reduce the cost of color filter manufacture by avoiding the
need for lengthy process and repetition in conventional photolithography. As the ink jet
printing method requires only one-third the number of manufacturing steps comparing with
a photolithography, the soluble dyes for primary colors should be used rather than insoluble
pigments.
As a red color, a symmetrical molecule (C. I. Pigment Red 254) based on DPP, which
was first synthesized by Ciba-Geigy, has been usually used due to its excellent thermal
stability and bright red shade. DPP derivatives are known for being poorly soluble in
common organic solvents due to their tendency to form strong π − π interactions and
hydrogen bondings and many efforts have been carried out on their functionalization to
improve their solubility6–7.As a consequence, during the last few years, several studies
have been reported in the literature on the use of these soluble DPP derivatives in various
application fields.
Herein, we describe efficient synthesis of several DPP dimmers through dimerization
reaction of synthesized DPP pigments precursors as well as their spectroscopic characteri-
zation. In order to satisfy solubility requirements, alkyl chain was introduced between two
N-alkylated dyes molecules.
2. Experimental
Synthesis of Pigments
Synthesis of 3,6-bis(4-chlorophenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (pigment 1).
Under nitrogen condition, potassium tert-butoxide (2.25 g, 0.02 mol) was dissolved in
150 mL of tert-amyl alcohol for 15 min and the mixture was heated to 110◦. Iron(III)
chloride (s.a) was added and stirred for 30 min then 4-chlorobenzonitrile (1.5 g, 0.01 mol)
was added and stirred for 1 hr at the same temperature. Diisopropyl succinate (1.01g,
0.005 mol) was dissolved in 30 mL of tert-amyl alcohol and dropwised to the mixture
for 3 hr. Then 60 mL of water was poured into mixture and stirred for 20 hr at the same
temperature. After reaction, the mixture was cooled to room temperature. The mixture
was filtered off and washed by water, methanol. The red pigment was purified by 50 mL
methanol and stirred for 30 min at 60◦. After stirring, the solution was filtered off and
dried under vacuum to obtain Pigment 1 in 43% yield. C18H10Cl2N2O2, Found C: 60.21 H:
2.84 N: 7.73, MS (m/z) 356(M+).
Other pigments were obtained by same procedure.
Pigment 2 Yield : 15%, C20H16N2O4, Found C: 67.46 H: 4.22 N: 8.04 O: 18.75, (m/z)
348(M+).
Pigment 3 Yield : 38%, C30H20N2O2, Found C: 81.28 H: 4.54 N: 6.17 O: 7.26, (m/z)
440(M+).

Solvent Soluble Dyes by Dimerized DPP Pigment
75
Synthesis of Dyes
Synthesis of 3,6-bis(4-chlorophenyl)-2-octylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (Dye
1). Under nitrogen condition, 6.27g (14.2 mmol) of 3,6-bis(4-chlorophenyl)pyrrolo[3,4-
c]pyrrole-1,4(2H,5H)-dione was suspended in 90 mL of 1-methyl-2-pyrrolidinone for 2 hr
at room temperature. 3.87 g (34.5 mmol) of potassium tert.-butoxide was added to this
slurry under a nitrogen atmosphere. After stirring for one hour, 5.79 g (30 mmol) of 1-
bromooctane was added to the reaction mixture and then the mixture is stirred additionally
for two hours. The mixture was poured into 150 mL of water and the red solid was filtered
off. Subsequent isolation of the mono-substituted component was carried out by adding the
crude product to a mixture of n-hexane (30 mL) and chloroform (3 mL). After stirring for
further 5 min in the same condition, the precipitated solid was filtered and subsequently
washed with a mixture of n-hexane (10 mL) and chloroform (3 mL). The filtered solid was
dried under vacuum to obtain Dye 1 was obtained.
Other dyes were obtained by same procedure.
Dye 1 Yield : 45%, C26H26Cl2N2N2O2, Found C: 67.25 H: 5.64 N: 6.47: 6.2 MS (m/z)
469(M+).
Dye 2 Yield : 38%, C28H32N2O4, Found C: 73.21 H: 7.00 N: 6.01 O: 13.77, MS (m/z)
460(M+).
Dye 3 Yield : 40%, C38H36N2O2, Found C: 82.31 H: 6.51 N: 5.03 O: 5.79, MS (m/z)
552(M+).
Synthesis of 5,5ꢀ-(octane-1,8-diyl)bis(3,6-bis(4-chlorophenyl)-2-octylpyrrolo[3,4-c]pyrro
le-1,4(2H,5H)-dione) (Dye 4). Under nitrogen condition, obtained 3,6-bis(4-
chlorophenyl)-2-octylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione(5 g, 0.01 mol) was slurried
in 70 mL of 1-methyl-2-pyrrolidinone for 15 min at room temperature. 1.673 g (1.5 mmol)
of potassium tert-butoxide was added to the slurry under a nitrogen atmosphere. After
stirring for one hour, 2.04 g (0.6 mmol) of 1,8-diiodooctane was added to the reaction
mixture and then the mixture stirred additionally for three hours. The mixture was poured
into 100 mL of water and the red solid filtered off and purified by column chromatography
(silica gel, dichloromethane as an eluent). After drying, 0.98 g of a red solid of Dye 4 was
obtained. Other dyes were obtained by the same procedure.
Dye 4 Yield : 37%, C60H66Cl4N4O4, Found C:69.11 H:5.67 N:5.21 O:5.90, MS(m/z)
1048(100)
Dye 5 Yield : 20%, C56H62N4O8, Found C: 73.11 H: 6.45 N: 6.10 O: 13.78, MS (m/z)
919(100).
Dye 6 Yield : 25%, C76H70N4O4, Found C: 82.13 H: 6.52 N: 5.08 O: 5.91, MS (m/z)
1103(100).
Structural Analysis
Structural analyses of synthesized dyes were measured using an EA 1108 (Elemental
Analysis) and a Voyager-DE STR Biospectrometry (MALDI-TOF). UV-visible absorption
spectra were obtained from a Shimadzu UV-2100. Thermogravimetric analysis (TGA) was
conducted under nitrogen at a heating rate of 10◦C min−1 with a Seiko TG/DTA 320 & a
SSC 5200H Disk Station.

76
H.-Y. Lee et al.
Figure 1. Synthetic scheme and the structures of pigments 1–3
3. Results and Discussion
Synthesis
To synthesize the dyes, three steps are required. The first step is to prepare pigments.
Pigments 1-3 were synthesized through the succinic ester route. In this route, succinic ester
is condensed in a Pseudo-Stobbe condensation with an aromatic nitrile in the presence of
strong base. It is shown in Figure 1. As in shown Figure 2 and Figure 3, dyes 1-3 were
prepared derived from synthesize DPP pigment through typical SN2 reaction of alkylation.
The N-alkylated and dimerization DPP derivatives can be easily separated by column
chromatography due to their different polarities. Yields of dyes 4-6 were 20% in average.
Spectral Properties of Dyes
The absorption maxima of synthesized pigments and dyes were determined in N-methyl-
2-pyrrolidone (NMP), as listed in Table 1. The absorption maxima of DPP pigments
were observed in the ranged of 516 – 534 nm. It apparently indicated the effects of
substituents presented in DPP phenyl ring. Therefore, Pigment 1, substituted by chlorines
which acted as an electron-withdrawing group observed at 516 nm. In contrast, Pigment 2

Solvent Soluble Dyes by Dimerized DPP Pigment
77
Figure 2. Synthetic scheme and the structures of dyes 1–3
exhibited a hypsochromic shift of 2 nm that caused by electron-donating substituent(-OMe).
Correspondingly, Pigment 3 showed a bathochromic shift in 18 nm comparing to that of
Pigment 1 which caused by extension of conjugation between DPP ring. The absorption
maximum of N-alkylated dyes (4–6) showed a hypochromic shift by 21 – 42 nm. It can
be best explained by the decreased planarity and conjugation length between DPP rings
caused by the introduction of alkyl group8. Contrastively, Dye 4 absorbed at 527 nm which
was red shifted by 22 nm compared to Dye 1. It is mainly due to the extension of the
π-conjugated systems through dimerization of DPP. Two series of dye substituted methoxy
group or benzene ring respectively, were observed in different tendency that was more
Figure 3. Synthetic scheme and the structures of dyes 4–6

78
H.-Y. Lee et al.
Table 1. The absorption maxima (λmax) of the synthesized pigments and dyes
Name
λmax (nm) Absorption
εmax (L mol−1 cm−1
)
Pigment1
2
3
516
514
534
505
485
492
527
489
494
—
—
—
15,100
16,000
11,000
16,200
18,300
11,000
Dye1
2
3
4
5
6
Determined in N-methyl-2-pyrrolidone (NMP)
hypochromic shift than dye substituted chlorine. It is effect of small decrease in planarity
and conjugation length between DPP rings9–12
.
Solubility of Dyes
For dyes to be successfully applied as colorants for LCD color filter, they should exhibit
good solubility in industrial solvents. Regardless of the compatibility with binders, dye
molecules should dissolve more than 5wt% in industrial solvents to have the desired optical
properties. Especially, to prevent the aggregation behavior between the dye molecules after
post baking process, bulky substituents should be introduced and indeed such molecules
are therefore normally also better soluble in organic solvents. The solubility data of the
synthesized dyes in organic solvent, PGMEA, are listed in Table 2. The dyes 4-6 with
linking group between the two N-alkylated dyes showed that solubility in the range of
9–14%. Especially, dye 5 was observed to have the highest solubility at 14.4%. The alkyl
chain substituents of dimerized dyes caused by the steric hindrance to reduce intermolecular
stacking and ether linkages in its structure enhanced its solubility in industrial solvents.
The introduction of alkyl groups considerably effective on hindering the intermolecular
π − π stacking between DPP chromophores. N-alkylated dyes 1-3 have smaller solubility
compared with dyes 4-6 which caused by introduction of bulky alkyl chain and planner
structure (Figure 4). The increased solubility is advantageous for good optical properties
Table 2. Solubility of synthesized dyes in PGMEA
Name
Solubility(g/100ml)
Dye1
9.4
8.4
6.5
12.8
14.4
9.5
2
3
4
5
6

Solvent Soluble Dyes by Dimerized DPP Pigment
79
Figure 4. Weight reduction of some pigments and dyes by TGA.
Figure 5. Geometry-optimized structures of the synthesized pigment 2 and dye 2.

80
H.-Y. Lee et al.
Table 3. Weight reduction of synthesized dyes pigments at different temperature measured
by TGA
Weight (%)
Name
200◦
250◦
Pigment 1
2
3
99.43
98.3
99.08
98.17
99.31
99.57
94.89
95.99
97.28
93.77
94.48
99.24
99.62
96.17
97.57
98.48
95.26
96.83
Dye 1
2
3
4
5
6
as the colorant materials for LCD color filters. Therefore, two series of dye were sufficient
colorants expected high transmittance and contrast ratio for LCD.
Thermal Stability of Dyes
Thermal stability of the colorants used for the fabrication of color filters should be required
to fulfill the post baking process which is generally carried out at 220◦, therefore the weight
reduction of the colorant should be small as possible at 230 – 250◦ by TGA12–14. The thermal
stability of synthesized pigments and dyes are shown in Table 3, and corresponding data
shown in Figure 4. Weight reductions of N-alkylated dyes 1-3 were in the range of 0.43 -
5.11%, whereas the dyes 4-6 showed slightly higher reductions of 2.72 – 6.23% at 250◦.
Dye 1 and 3 substituted by a phenyl ring at ρ-position of DPP ring were to be most stable
analogues in these series which mainly due to substituted phenyl ring at ρ-position that
increased intermolecular π − π stacking.
Geometry Optimization of Dyes
The synthesized pigment 2 is almost planar structure as shown Figure 5 observed structure
from the side. Dye 2 substituted long alkyl chain, is smaller twisted structure than Pigment
2 that is planner structure. However, Dye 5, linked between DPP rings by octyl group, is
not planer structure, and twisted structure especially positioned at DPP ring. It indicates
that dye structure has steric hindrance and decrease intermolecular π −π stacking between
DPP chromophores. Therefore, it is effect on solubility and thermal stability of dyes.
4. Conclusions
In summary, we have successfully synthesized dyes 4-6 by three-step reaction. N-alkylated
dyes 1-3 showed hypsochromic shift due to the decrease in planarity and conjugation
between DPP ring and adjacent phenyl group caused by the introduction of alkyl group.
However the dimerized dyes 4-6 exhibited a bathochomic shift which can be attributable
to the extension of the π-conjugated systems through dimerization of DPP. These dyes
exhibited enhanced solubility of 6 ∼ 14% in PGMEA by introducing alkyl substituents.

Solvent Soluble Dyes by Dimerized DPP Pigment
81
The synthesized dyes are less stable than DPP pigments because alkyl substituents readily
provide decreased intermolecular π − π stacking. However, most of dyes showed less than
5% weight reduction by TGA. In conclusion, these dyes can be suitable for use in LCD
color filter as a red component.
Funding
This work was supported by the National Research Foundation of Korea Grant funded by
the Korean Government [NRF-2012001638].
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