Preparation of thermally stable red dyes derived from diketo-pyrrolo- pyrrole pigment for LCD color filter

Article abstract of DOI:10.1080/15421406.2011.600222

6 dyes derived from Diketo-pyrrolo-pyrrole pigment were prepared by introducing an Nalkyl group, an N-acyl group or an N-aryl group to the DPP-ring. Absorption maxima, thermal stability by a thermogravimetric analysis (TGA), and the chromatic characterist

Full text of DOI:10.1080/15421406.2011.600222

Mol. Cryst. Liq. Cryst., Vol. 550: pp. 76–82, 2011
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.600222
Preparation of Thermally Stable Red Dyes Derived
from Diketo-Pyrrolo-Pyrrole Pigment
for LCD Color Filter
SE-HWA CHOI,1 GA-HYE KIL,1 NA-RIM KIM,1 CHUN YOON,2
JAE-PIL KIM,3 AND JAE-HONG CHOI1,∗
1Department of Textile System Engineering, Kyungpook National University,
Daegu 702-701, Korea
2Department of Chemistry, Sejong University, Seoul 143-747, Korea
3School of Materials Science and Engineering, Seoul National University,
Seoul 151-742, Korea
6 dyes derived from Diketo-pyrrolo-pyrrole pigment were prepared by introducing an N-
alkyl group, an N-acyl group or an N-aryl group to the DPP-ring. Absorption maxima,
thermal stability by a thermogravimetric analysis (TGA), and the chromatic charac-
teristics of the fabricated films were measured. It was explained that the observed
hypsochromic shift and the effects of N-substituent on thermal stability of the dyes as
well as a chromaticity diagram.
Keywords Diketo-pyrrolo-pyrrole (DPP); absorption maxima; thermal stability; N-
acylation; color filter; chromaticity diagram
Introduction
The following pigments, such as C. I. Pigment Red 254, C. I. Pigment Green 36 and C. I.
Pigment Blue 15:6, have been used as primary colors (red, green and blue) for LCD color
filters, utilizing a photolithographic process [1–4].
C. I. Pigment Red 254, a symmetrical molecule based on DPP, was first developed
by Ciba-Geigy, can be widely used for color filters due to its excellent thermal stability
and bright shade [5–7]. However, this compound is insoluble in the most of common sol-
vents. The lack of solubility is presumed to result from extensive inter-molecular hydrogen
bondings in the solid [8–10], as well as derived in part from π-π stacking, a phenomenon
which is obvious from X-ray crystallography [10]. Therefore, the cancellation of the inter-
molecular hydrogen bondings via N-methylation [11–13] or N-arylation [14–16] of DPP
yields not only more soluble derivatives (in the order DPP ꢀ N-monomethyl-DPP ꢀ
N,N’-dimethyl-DPP), but also less thermal stable analogues.
In general, dyes exhibit superior brightness and contrast ratio compared to those of
pigments, however, dyes are poor in thermal resistance and photo-stability. The authors
reported [17] that soluble DPP derivatives with thermal stability can be prepared upon N-
monoalkylation in the DPP ring. The thermal stability of the dyes can greatly attributable
∗Corresponding author. E-mail: jaehong@knu.ac.kr
76

Red Dyes for Color Filter
77
to both the length of carbon chain and their shapes of N-alkyl groups in DPP. In particular,
dyes substituted by a normal C8 or C9 alkyl group exhibited extremely high stability which
seems to be competitive to C. I. Pigment Red 254.
In this paper, four electron withdrawing groups were introduced into the DPP-ring so
as to compare the absorption maxima, thermal stability and chromaticity values between the
dyes based on N-alkyl groups (dyes 1 and 2) and N-electron withdrawing groups (dyes 3–6).
Synthesized dyes were measured by an absorption maximum (λmax) and thermal stability
using a UV-VIS spectrophotometer and thermogravimetric analysis (TGA), respectively.
The chromaticcharacteristics of thefabricatedfilm withthered dyes prepared wereanalyzed
by a spectrophotometer.
Experimental
Dye Synthesis
Under nitrogen condition, 3,6-bis(4-chlorophenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(3.56 g, 0.01 mol) and sodium tert-butoxide (2.88g, 0.03 mol) were dissolved in DMF
150 ml and stirred for 1 hr at room temperature, then triethylamine (1g, 0.01mol), propionyl
chloride (1.4g, 0.015 mol) and I2 (0.2g, 0.001 mol) were added. The mixture was heated
to 50◦C, further stirred for 15 hrs at the same temperature with regular checking by TLC
(stationary phase : silica gel, mobile phase : n-hexane/EtOAc = 2/1). After completion
of the reaction, iodine was destroyed by adding saturated Na2S2O3 aq. solution (5 ml).
When the reaction completed, cooled to room temperature, then ethyl acetate (150 ml) and
distilled water (200 ml) were added. The ethyl acetate layer was separated from the aqueous
layer, followed by drying with MgSO4, anh. (around 10 g) and filtered. Evaporation of the
ethyl acetate gave a mixture of mono and di-N-substituted products. Subsequent isolation
of mono-substituted component (Dye 3) was carried out by adding the crude product into
a mixture of n-hexane (30 ml) and chloroform (3 ml). After stirring for a further 5 minutes
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
a vacuum to obtain dye 3 in 38% yield. C21H14Cl2N2O3 Found C:60.98 H:3.35 N:6.75
O:11.51, Calculated C:61.03 H:3.41 N:6.78 O:11.61, MS 412.04(M+)
Other dyes were obtained by same procedure except reaction temperature (60◦C for
dyes 4–5; room temperature for dye 6).
Dye 1 & Dye 2 : previously reported in the reference 17.
Dye 4 Yield : 36%, Found C:61.77 H:3.75 N:6.61 O:11.21, Calculated C22H16Cl2N2O3
C:61.84 H:3.77 N:6.56 O:11.23, MS 426.05(M+)
Dye 5 Yield : 35%, Found C:62.50 H:4.08 N:6.40 O:10.91, Calculated C23H18Cl2N2O3
C:62.60 H:4.11 N:6.35 O:10.88, MS 440.07(M+)
Dye 6 Yield : 37%, Found C:54.98 H:2.22 N:10.65 O:18.23, Calculated C24H12Cl2N4O6
C:55.09 H:2.31 N:10.71 O:18.35, MS 522.01(M+)
Structural Analysis
Structural analysis of synthesized dyes were measured using an EA 1108 (E.A.) and an HP
6890 & Agilent 5973N MSD (GC-Mass).
UV-visible absorption spectra were obtained from a Shimadzu UV-2100. The data of
the color properties and transmittance spectra were measured using an MCPD 3700.

78
S.-H. Choi et al.
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.
Fabrication of Color Filter
In order to measure optical characteristics and thermal stability, spin-coating was carried
out onto glass using a MIDAS System SPIN-1200D spin-coater. The glass was spun at a
low to moderate speed of 300 rpm for 10 seconds to evenly spread the solution (dye-based
ink). The solution was prepared with synthesized dyes, a solvent (1-Methyl-2-pyrrolidone)
and a binder based on acrylate. Once spin-coating was completed, the film was placed
quickly onto a hot plate (heated to around 100◦C) for 5 minutes to evaporate the solvent.
Thermal Stability Test as Color Filter
The thermal stability of synthesized dyes after fabrication was measured by pre-baking at
220◦C for 30 min and post-baking at 250◦C for 60 min, then color differences in ꢁEab were
determined using an MCPD 3700.
Results and Discussion
As shown in Figure 1, four dyes have been prepared starting from C. I. Pigment Red 254 by
a typical SN2 type acylation (Dye 3–5) and arylation (Dye 6) reactions using corresponding
acyl/aryl chlorides. Reaction conditions for the formation of N-anion of DPP were optimized
with a strong base, sodium tert-butoxide and a polar aprotic solvent, DMF where insoluble
starting material (C. I. Pigment Red 254) was readily dissolved in the reaction mixture.
The mixture of N-monoacyl/aryl and N,N’-diacyl/aryl derivatives was formed thus need to
Figure 1.

Red Dyes for Color Filter
79
Table 1. Absorption maxima (λmax) of the synthesized dyes 1–6 and C. I.
Pigment Red 254
Dye number
λmax (nm)
Pigment Red 254
516
504
504
461
464
465
476
1
2
3
4
5
6
Determined in 1-Methyl-2-pyrrolidone.
be further separated by a crystallization using with a mixture of n-hexane and chloroform.
After separation, the final yield of N-monoacyl and aryl products ranged in 35 ∼ 38%. A
general synthetic procedure is described in Experimental Section.
Comparisons of absorption maxima of C. I. Pigment Red 254 and synthesized dyes
1–6 are listed in Table 1. All dyes 1–6 containing a N-substituent absorbed maximally at
shorter wavelength compared with that of C. I. Pigment Red 254 that was measured to
be 516 nm in 1-methyl-2-pyrrolidone. Both synthesized dye 1 and dye 2 that contain an
electron donating group (alkyl group) in the DPP ring exhibited λmax of 504 nm, whereas
Dyes 3–6 substituted with an electron withdrawing groups (acyl group for dyes 3–5, aryl
group for dye 6) in the DPP ring exhibited λmax in the range of 461 ∼ 476 nm in 1-
methyl-2-pyrrolidone which shifted to much shorter wavelength than those for dyes 1 and
2. Hypsochromic shift observed with Dyes 1 and 2 can be best explained by the decrease
of planarity and conjugation between DPP ring and adjacent phenyl group caused by the
introduction of alkyl group [20]. The absorption maxima of dyes 3–5 hypsochromically
shifted dramatically in the range of 51 ∼ 55 nm in comparison with that of N-unsubstituted
analogue (C. I. Pigment Red 254). This blue shift could be attributable to the inductive
effect arised from electron withdrawing groups (acyl group) led to the destabilized excited
state. In this case, both effects of the decrease of planarity and conjugation, which was
observed with dyes 1 and 2, and inductive effect resulted in the destabilized excited state
occurred simultaneously. It was also found that the effect of carbon number of acyl group
on the absorption maximum to be minimal.
Dye 6 containing an N-aryl group exhibited smaller shift compared with dyes 3–5
which may be contributed by the increased conjugation of π-electrons of DPP ring with
aryl group leading to smaller energy difference between HOMO and LUMO. Therefore this
effect can compensate the negative inductive effect of strong electron withdrawing group
exerted by 2,4-dintro group.
As shown in Table 2, chromaticity diagram of all synthesized dyes exhibited smaller x
values and higher y values than C. I. Pigment Red 254 indicating the color of synthesized
dyes to be overall yellower. It was also found that x values of dyes containing an N-alkyl
group were higher than those of N-acyl and N-aryl analogues where dye 6 substituted
by stronger electron withdrawing group showed comparatively closer value to dyes 1 and
2. Similar trend was observed in y values, thus dyes 3–6 exhibited lower y values in
comparison with those of dyes 1 and 2. Among this series, dye 6 seemed to be most similar
to that of C. I. Pigment Red 254 in y value.

80
S.-H. Choi et al.
Table 2. Chromaticity diagram (x,y) and transmittance (%) of the synthesized dyes 1–6
and C. I. Pigment Red 254
Chromaticity diagram
Transmittance
Dye number
x
y
(%, at 630 nm)
Pigment Red 254
0.577
0.539
0.565
0.408
0.393
0.456
0.524
0.336
0.448
0.452
0.427
0.419
0.432
0.401
98.9
99.8
99.8
100
99.6
100
1
2
3
4
5
6
99.9
Determined in 1-Methyl-2-pyrrolidone.
In terms of transmittance at 630 nm for synthesized dyes, all materials exhibited over
99.6% up to 100% which indicated more efficient transmittance of red light in comparison
with C. I. Pigment Red 254 that showed 98.9%. In case of color filter, transmittance of the
light greatly contributes the contrast ratio and brightness of the LCD panel, therefore more
efficient performance exerted by the dyes prepared could be highly valuable property. The
relevant values have been summarized in Table 2.
Thermal stability of the colorants used for the fabrication of color filters is one of
the crucial requirements to fulfill the post baking process which is generally carried out
at 220◦C, therefore the weight reduction of colorant should be as small as possible at 200
∼ 250◦C by TGA [18–19]. As shown in Table 3, weight reduction of C. I. Pigment Red
254 was 0.69% at 250◦C, whereas synthesized dye 1–6 reduced in the range of 0.01 ∼
21.53% at the same temperature. As previously reported [17], the dyes 1 and 2 containing
a long chain of straight-alkyl group exhibited higher thermal stability (in the range of 0.01
∼ 0.34%) that seemed to be as equivalent as that of C. I. Pigment Red 254. Comparatively
lower stability was observed with dyes 3–5 substituted by an N-acyl group, where the
stability decreased in order of dye 3 ꢀ dye 4 ꢀ dye 5. However, 2,4-dinitrophenyl group
Table 3. Weight reduction of dyes 1–6 and C. I. Pigment Red 254 at different
temperature measured by TGA
Weight (%)
Dye number
200◦C
250◦C
Pigment Red 254
99.51
99.99
99.85
97.68
92.62
89.76
98.55
99.31
99.99
99.66
89.45
84.50
78.47
97.83
1
2
3
4
5
6

Red Dyes for Color Filter
81
Table 4. Color difference ꢁEab of synthesized dyes
Dye number
ꢁEab
1
2
3
4
5
6
7.7
6.3
3.5
3.9
3.7
3.8
present in DPP ring contributed to the increased thermal stability to compare with N-acyl
groups. Thus the weight reduction of dye 6 was 2.17% which seemed to be very similar
to that of C. I. Pigment Red 254. It can be explained that acyl groups have comparatively
lower bond dissociation energy than that of alkyl or aryl groups. It was found that as carbon
number of N-acyl group increases, the thermal stability of the dyes at 250◦C decreases that
may be due to the decreased intermolecular dispersion forces.
As an alternative method to determine the thermal stability of dyes, the color difference
measured in ꢁEab value after fabrication process was examined, as shown in Table 4. In
terms of ꢁEab value of synthesized dyes, higher color difference (7.7 ∼ 6.3) was found with
dyes 1 and 2 those showed lower weight reduction by TGA, whereas smaller ꢁEab values
(3.5 ∼ 3.9) observed for dyes 3–6 containing an electron withdrawing group in DPP. This
opposite tendency may be derived from the effects by the monomer and/or binder used in
the fabrication process.
Conclusions
In this paper, 6 dyes with N-alkyl group, N-acyl group or N-aryl group in the DPP ring
were prepared derived from C. I. Pigment Red 254. The final yield of N-monoacyl and
aryl products ranged in 35 ∼ 38% after separation from the mixture of mono- and di-
substituted derivatives. dyes 1 and 2 that contain an electron donating group (alkyl group)
exhibited λmax of 504 nm, whereas Dyes 3–6 substituted with an electron withdrawing
groups exhibited λmax in the range of 461 ∼ 476 nm. It can be explained by that both
effects of the decrease of planarity and conjugation, and an inductive effect resulted in the
destabilized excited state. Dye containing an N-aryl group exhibited smaller shift compared
with dyes 3–5 which may be contributed by the increased conjugation of π-electrons of
DPP ring with aryl group. Chromaticity diagram of all synthesized dyes exhibited smaller
x values and higher y values than C. I. Pigment Red 254 indicating the color of synthesized
dyes to be overall yellower. In terms of transmittance at 630 nm for synthesized dyes, all
materials exhibited over 99.6% up to 100% which indicated more efficient transmittance
of red light.
TGA analysis showed that all synthesized dye reduced in the range of 0.01 ∼ 21.53%
at 250◦C where N-alkyl groups exerted higher thermal stability whereas N-acyl groups
exhibited comparatively lower property which can be mainly attributed to the bond dissoci-
ation energy of N-substituent. However smaller ꢁEab values (3.5 ∼ 3.9) observed for dyes
3–6 containing an electron withdrawing group after fabrication of color filter.

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S.-H. Choi et al.
Acknowledgment
This research was supported by a grant from the Fundamental R&D Program for Core
Technology of Materials funded by the Ministry of Knowledge Economy, Republic of
Korea.
References
[1] Ichimura, K., Advanced Technologies for LCD Color Filters, CMC Publishing: Tokyo, pp. 8
(2006).
[2] Takahashi, T. J. Imag. Soc., 41, 68 (2002).
[3] Sugiura, T. J. Print. Sci. Tech., 356 (1996).
[4] Carr, K., Freeman, H. S., and Peters, A. T. (Eds) Colorants for Non-Textile Applications,
Elsevier: London, pp. 28–29 (2000).
[5] Zollinger, H. Color Chemistry, Syntheses, Properties, and Applications of Organic Dyes and
Pigments; Wiley-VCH: Weinheim, pp. 331 (2003).
[6] Iqbal, A., Cassar, L., Rochat, A. C., Pfenninger, L., and Wallquist, O., J. Coating Technol., 60,
37 (1988).
[7] Iqbal, A., Kirchmayr, R., Rochat, A. C., Pfenninger, L., and Wallquist, O. Bull. Soc. Chim.
Belg., 97, 615 (1988).
[8] Hao, Z. and Iqbal, A. Chem. Soc. Rev., 26, 203 (1997).
[9] Iqbal, A., Jost, M., Kirchmayr, R., Pfdnninger, J., Rochat, A., and Wallquist, O. Bull. Soc. Chim.
Belg., 97, 615 (1988).
[10] Mizuguchi, J., Grubenmann, A., Wooden, G., and Rihs, G. Acta Crystallogr., B48, 696 (1992).
[11] Mizuguchi, J. and Rihs, G. Ber. Bunsenges. Phys. Chem., 96(4), 597 (1992).
[12] Mizuguchi, J. and Wooden, G. Ber. Bunsenges. Phys. Chem., 95(10), 1264 (1991).
[13] Ciba-Geigy, WO 08537; 1996.
[14] Riggs, R. L., et al. Tetrahedron, 61, 11230 (2005).
[15] Morton, C. J. H., et al. Tetrahedron, 61, 727 (2005).
[16] Zollinger, H., Color Chemistry, Syntheses, Properties, and Applications of Organic Dyes and
Pigments, Wiley-VCH: Weinheim, pp. 336–339 (2003).
[17] Choi, J. H., et al. Bull. Korean. Chem. Soc., 31(4), 1073 (2010).
[18] Das, S., Basu, R., Minch, M., and Nandy, P. Dyes & Pigments, 29, 191 (1995).
[19] Chunlong, Z., Nianchun, M., and Liyun, L. Dyes & Pigments, 23, 13 (1993).
[20] Martin, V., Martin, W., Jan, V., Petr, T., and Stanislav, L. J. Fluoresc., 18, 1185 (2008).

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