The synthesis and characterizations of thermally-stable yellow metal complex dyes for LCD color filters

Article abstract of DOI:10.1080/15421406.2013.844883

Eight kinds of yellow metal (III) complex azo dyes were synthesized using different diazo component, coupling component and metal ion. The effect on the thermal stability of the synthesized dyes by azo ligand with two different functional groups, alcohol and carboxylic acid, was studied. The characteristics of the synthesized dyes were examined by their absorption maxima, thermal stability by a thermogravimetric analysis (TGA), and the chromatic characteristics of the fabricated films were measured. It was also explained that the effect of functional groups on the thermal stability of the dyes as well as their chromaticity values. Copyright Taylor and Francis Group, LLC.

Full text of DOI:10.1080/15421406.2013.844883

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The Synthesis and Characterizations of
Thermally-Stable Yellow Metal Complex
Dyes for LCD Color Filters
Jae-Yong Lee a , Ji-Hye Kim a , Ji-Hye Bae a , Chun Yoon b , Jae-Pil
Kim c & Jae-Hong Choi a
a Department of Textile System Engineering , Kyungpook National
University , Daegu , 702-701 , Korea
b Department of Chemistry , Sejong University , Seoul , 143-747 ,
Korea
c School of Materials Science and Engineering , Seoul National
University , Seoul , 151-742 , Korea
Published online: 16 Dec 2013.
To cite this article: Jae-Yong Lee , Ji-Hye Kim , Ji-Hye Bae , Chun Yoon , Jae-Pil Kim & Jae-
Hong Choi (2013) The Synthesis and Characterizations of Thermally-Stable Yellow Metal
Complex Dyes for LCD Color Filters, Molecular Crystals and Liquid Crystals, 583:1, 60-69, DOI:
10.1080/15421406.2013.844883
To link to this article: http://dx.doi.org/10.1080/15421406.2013.844883
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Mol. Cryst. Liq. Cryst., Vol. 583: pp. 60–69, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.844883
The Synthesis and Characterizations
of Thermally-Stable Yellow Metal Complex Dyes
for LCD Color Filters
JAE-YONG LEE,1 JI-HYE KIM,1 JI-HYE BAE,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
Eight kinds of yellow metal (III) complex azo dyes were synthesized using different diazo
component, coupling component and metal ion. The effect on the thermal stability of
the synthesized dyes by azo ligand with two different functional groups, alcohol and
carboxylic acid, was studied.
The characteristics of the synthesized dyes were examined by their absorption
maxima, thermal stability by a thermogravimetric analysis (TGA), and the chromatic
characteristics of the fabricated films were measured. It was also explained that the effect
of functional groups on the thermal stability of the dyes as well as their chromaticity
values.
Keywords Absorption maximum; chromaticity diagram; color filter; metal complex
dye; thermal stability
Introduction
The metal complexes of azo compounds containing two coordinating groups present in
ortho positions in such a way as to form two annelated chelate rings with the metal ions
(especially the complexes with copper(III) cobalt(III) and chromium(III)), have been used
as pigments, paints and other coloring materials [1]. Yellow-orange dyes are used in color
filters most widely, both as absorbing agents in cutting filters and as additives to blue,
green, and red dyes for suppressing “parasitic” transmission in the blue range. Such dyes
include monoazo dyes derived from five-membered heterocyclic rings: such as pyrazole,
thiophene, furan, etc [2–5].
The LCD has become an important display device in mobiles, monitors and high-
definition televisions, which requires high resolution and color properties for big screen
size [6–9]. The LCD color filter consists of red, green, and blue color pixels that can
∗Address correspondence to Prof. Jae-Hong Choi, Department of Textile System Engineering,
Kyungpook National University, 1370 Sangyuk-dong, Buk-gu, Daegu 702-701, Korea (ROK). Tel.:
(+82)53-950-5644; Fax: (+82)53-950-6617. E-mail: Jaehong@knu.ac.kr
60

Synthesis of Thermally-Stable Yellow Dyes
61
be manufactured by a photolithographic process using photo resists containing polymeric
binder, dispersant and corresponding pigments.
In these days, ink-jet printing method using conventional pigment-based inks has been
studied to simplify the complicated process and to reduce manufacturing cost of LCD color
filters. However, this method has the limitation due to the blocking of ink-jet nozzles as
well as low chromatic properties of pigment particles. In order to solve these problems,
an ink-jet printing method where dyes are used for higher solubility and superior color
saturation property has been proposed as an alternative way.
In particular, metal-azo complexes are one of the important molecules can improve
thermal stability of organic dyes, therefore have attracted much attention in both academic
and applied research [10–11].
This paper builds on the aforementioned study, reporting work in which eight new
dyes have been synthesized (see Fig. 1,2) and characterized using an elemental analysis.
The absorption characteristics and chromatic values of the synthesized dyes have been also
investigated. Thermal stability of the dyes has been determined by a thermogravimetric
analysis (TGA), comparing with that of C. I. Pigment Yellow 150 (see Fig. 3).
Experimental
Dye Synthesis
The diazo component (4-aminoantipyrine, 0.0049 mol) was added to a solution of concen-
trated hydrochloric acid (35%, 20 ml). The mixture was stirred for 5 hr at room temperature
then was cooled to 5◦C. Sodium nitrite (97%, 0.0049 mol) was added to the mixture above
portionwise at 5–10◦C and then stirred for 2 hours at the same temperature with regular
checking by Thin Layer Chromatography (TLC) (stationary phase : silica gel, mobile phase
: n-hexane/EtOAc = 1/2). The resulting solution was used immediately in the following
coupling reaction. The coupling component (3-aminophenol, 0.0049 mol) was dissolved in
water (30 ml) and sodium hydroxide solution (25%, 10 ml) and then cooled to 0–5◦C by
the addition of ice. The diazonium solution previously prepared was added dropwise over
30 min at the same temperature. The mixture was stirred for a further 3 hours at 5–10◦C
and then aqueous sodium hydroxide solution (25%) was added slowly below 10◦C until
the pH rose to within the range of 7–8. The precipitated product was filtered off, washed
with water until the filtrate was pH 6–7 and dried at 70–75◦C. The crude dyes were puri-
fied by recrystallization from ethanol. The resulting ligand (0.0031 mol) was dissolved in
20ml of ethanol together with sodium carbonate(0.0047 mol). After heating up to reflux,
the chromium(III) acetate hydroxide (0.00156 mol) was added portionwise under vigorous
stirring, whereupon a suspension of the chromium(III) complex dye formed. The precip-
itated solid was collected by filtration, then washed with water and then vacuum dried to
obtain the chromium(III)-azo complex. The crude dyes were purified by recrystallization
from ethanol. The filtered solid was dried under vacuum to obtain dye 1a in 48% yield.
Other dyes were obtained by same procedure except reaction time (2hours for dyes
3a–3d).
Dye 1a 48% Yield : 48%, C34H34CrN10O4 1H NMR(CDCl3), δ: 2.26(s, 3H), 3.11(s,
3H), 5.04(s, 1H), 5.59(s, 2H), 6.14(s, 1H), 6.19(d, 1H), 6.90(t, 1H), 6.91(d, 1H), 7.12(d,1H),
7.37(t, 1H). Found C:59.26 H:5.02 Cr:7.87 N:21.59 O:9.21 Calculated C: 58.45, H: 4.90,
Cr:7.44, N:20.05, O: 9.16, MS 670.69(M+).
Dye 1b Yield : 51%, C38H42CrN10O4 1H NMR(CDCl3), δ: 2.26(s, 3H), 3.06(s, 3H),
3.11(s, 3H), 5.04(s, 1H), 6.27(s, 1H), 6.32(d, 1H), 6.90(t, 1H), 6.91(d, 1H), 7.12(d,1H),

62
J.-Y. Lee et al.
Figure 1. Synthetic scheme of the dye 1a-2.
7.37(t, 1H). Found C:60.87 H:6.11 Cr:6.99 N:19.14 O:9.07 Calculated C, 60.47, H:5.61,
Cr:6.89, N:18.56; O, 8.48, MS 755.8 (M+).
Dye 1c Yield : 43%, C42H36CrN18O4 1H NMR(CDCl3), δ: 2.26(s, 3H), 3.11(s, 3H),
5.04(s, 1H), 6.90(t, 1H), 6.98(d, 1H), 7.12(d,1H), 7.37(t, 1H), 7.37(t, 1H), 7.44(t, 1H),
7.53(t, 1H), 8.04(d,1H), 8.07(d,1H). Found C:65.77 H:4.21 Cr:7.31 N:13.98 O:9.54 Cal-
culated C: 65.62, H: 4.72 Cr:6.76, N: 14.58, O: 8.32, MS 767.7 (M+).

Synthesis of Thermally-Stable Yellow Dyes
63
Figure 2. Synthetic scheme of the dye 3a-3d.
Dye 2 Yield : 42%, C28H26CrN6O4 1H NMR(CDCl3), δ: 3.06(s, 3H), 6.47(s, 1H),
6.52(d, 1H), 7.15(d, 1H), 7.21(t, 1H), 7.48(t, 1H), 7.83(d, 1H). Found C:60.02 H:4.85
Cr:10.35 N:15.14 O:12.04 Calculated C: 59.78, H: 4.66, Cr:9.24, N: 14.94, O:11.38, MS
563.5 (M+).
Dye 3a Yield : 39%, C34H26CrN8O6 1H NMR(CDCl3), δ: 2.34(s, 3H), 7.45(t, 1H),
7.54(d,1H), 7.58(t, 1H), 7.62(d, 1H), 7.79(t, 1H), 8.09(t, 1H), 8.21(d, 1H). Found C:59.23
H:3.96 Cr:7.84 N:16.84 O:14.52 Calculated C:58.79, H: 3.77, Cr:7.49, N: 16.13, O: 13.82,
MS 695.6 (M+).
Dye 3b Yield : 38%, C36H30CrN8O6 1H NMR(CDCl3), δ: 2.34(s, 3H), 7.50(d, 1H),
7.54(d,1H), 7.79(t, 1H), 8.09(t, 1H), 8.21(d, 1H). Found C:60.13 H:4.81 Cr:7.54 N:15.41
O:13.97 Calculated C: 59.83, H: 4.18, Cr:7.20, N: 15.51, O: 13.28, MS 723.6 (M+).
Dye 3c Yield : 38%, C38H34CrN8O6 1H NMR(CDCl3), δ: 2.34(s, 3H), 7.40(t, 1H),
7.45(t,1H), 7.57(d, 1H), 7.58(t, 1H), 7.62(d, 1H), 8.02(d, 1H). Found C:60.34 H:4.51
Figure 3. Structure of C. I. Pigment Yellow 150.

64
J.-Y. Lee et al.
Cr:7.48 N:15.74 O:14.02 Calculated C: 59.83, H: 4.18, Cr:7.20,N: 15.51, O: 13.28, MS
723.1 (M+).
Dye 3d Yield : 41%, C38H34CrN8O6 1H NMR(CDCl3), δ: 2.34(s, 3H), 7.40(t, 1H),
7.42(d,1H), 7.50(d, 1H), 7.57(d, 1H), 8.02(d, 1H). Found C:60.93 H:5.01 Cr:7.17 N:15.18
O:13.37 Calculated C: 60.80, H: 4.56, Cr:6.93, N: 14.93, O: 12.79, MS 751.7 (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. 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
100◦C for 5 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 showninFigs. 1, 2, eight dyes havebeenpreparedbydiazotizationand coupling reactions
and followed by complexation of ligands. All diazo components were sufficiently basic to
enable straightforward diazotization using by aqueous sodium nitrite in dilute hydrochloric
acid. Coupling reaction was carried out by the addition of the diazonium salt previously
prepared to coupler solutions at 0–5◦C. Thereafter the resulting ligand was dissolved in
ethanol together with sodium carbonate. After heating up to reflux, the chromium(III)
acetate hydroxide was added portionwise under vigorous stirring, whereupon a suspension
of the chromium(III) complex dye results.
The precipitated solid was collected by filtration, washed with water and then vacuum
dried to obtain the chromium(III)-azo complex. After separation, the final yield of products
ranged in 38 ∼ 51%. The general synthetic procedure is described in Experimental Section.
UV-VIS spectra of azo dyes are generally affected by their chemical structures such as
chromophores, substituted groups, number of azo groups, metal ions, pH values, solvents
and so on [12–15]. Comparisons of absorption maximum (λmax) of C. I. Pigment Yellow
150, precursor dyes, and synthesized dyes 1a–3d are listed in Table 1. The spectra of
synthesized dyes 1a–3d were shown in Figs. 4–5. All synthesized dyes showed similar
or higher wavelength compared with C. I. Pigment Yellow 150 that was measured to be

Synthesis of Thermally-Stable Yellow Dyes
65
Table 1. Absorption maxima (λmax) of the synthesized dyes 1a–3d and C. I. Pigment Yellow
150
Dye number
Precursor dye λmax (nm)
Metal complex dye λmax (nm)
C. I. Pigment Yellow 150
—
430
482
517
535
537
441
442
434
433
1a
1b
1c
2
3a
3b
3c
3d
435
455
452
471
391
389
388
385
Determined in N-Methyl-2-pyrrolidone (NMP)
430 nm in 1-methyl-2-pyrrolidone. The precursor dyes 1a–1c and dye 2 that contain a
hydroxy group in the ortho-positions of azo bond exhibited λmax in the range of 435 ∼
471 nm, whereas corresponding precursor dyes 3a–3d containing a carboxylic acid group in
the ortho-position exhibited λmax in the range of 385 ∼ 391 nm in 1-methyl-2-pyrrolidone.
Absorption maxima (λmax) of precursor dyes 1a–2 containing a hydroxy group are
much higher than those of the corresponding precursor dyes 3a–3d containing a carboxylic
acid group, which can be mainly attributable for the electron donating effect of hydroxy
group in ortho or para position of azo group leading to π electrons are easily moveable so
that π→π∗ transition exerted easier and absorption maximum shifts to higher wavelength.
After metal complexation reaction, Some dyes 1a–1c and dye 2 that contain a hydroxy
group in the ortho-positions of azo bond exhibited λmax in the range of 482∼537 nm,
Figure 4. UV-VIS Spectra of the synthesized dyes 1a-2 in NMP.

66
J.-Y. Lee et al.
Figure 5. UV-VIS Spectra of the synthesized dyes 3a-3d in NMP.
whereas the corresponding dyes 3a–3d containing a carboxyl group in the ortho-position
exhibited λmax in the range of 433 ∼ 442 nm in 1-methyl-2-pyrrolidone.
These results canbeexplainedbytheformationof ligands bymetal ionwhichconnected
with the oxygen atom of hydroxy group led to the oxygen atom to be easier to give
nonbonded electrons to π -electron system, which attributes to the red shift of absorption
maximum of the azo ligand. This can be increased with the increasing of the positive
electricity ability of metal ion. It can also probably arose from the energy change of the
intense π→π∗ transition of the conjugated chromophore due to the chelation between
metal ions and azo ligand [16–17].
Table 2. Chromaticity diagram (x,y) of the synthesized dyes 1a–3d and C. I. Pigment
Yellow 150
Chromaticity diagram
Dye number
x
y
C. I. Pigment Yellow 150
0.421
0.401
0.435
0.448
0.412
0.406
0.347
0.407
0.412
0.494
0.289
0.273
0.345
0.263
0.489
0.372
0.426
0.452
1a
1b
1c
2
3a
3b
3c
3d
Determined in N-Methyl-2-pyrrolidone (NMP).

Synthesis of Thermally-Stable Yellow Dyes
67
Table 3. Weight reduction of dyes 1a–3d and C. I. Pigment Yellow 150 at different tem-
perature measured by TGA
Weight (%)
Dye number
200◦C
250◦C
C. I. Pigment Yellow 150
97.97
94.25
97.83
97.09
95.11
99.50
98.87
98.62
98.97
97.01
92.81
97.45
94.85
93.62
99.24
98.57
98.32
98.61
1a
1b
1c
2
3a
3b
3c
3d
In terms of chromaticity diagram of the synthesized dyes, all dyes exhibited lower y
values than that of C. I. Pigment Yellow 150, as shown in Table 2, where the chromaticity
diagram of dyes 3a and 3d showed comparatively closer y values to that of C. I. Pigment
Yellow 150 compared to other dyes. The same trend was also found in x values of dyes
3a and 3d. By comparing the y values between dyes containing a hydroxy group and
corresponding dyes containing a carboxylic acid group, higher values were observed with
dye 3a and dye 3d. Therefore, it can be concluded that, in this series, dye 3a features in
similar color properties compared to that of the conventional C. I. Pigment Yellow 150.
Thermal stability of the colorants used for the fabrication of color filter 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 Yellow 150 was 2.99% at
250◦C, whereas synthesized dyes 1a–3d reduced in the range of 0.76∼6.38% at the same
temperature.
Table 4. Color difference in ꢀEab of synthesized dyes after post-baking at 250◦C for 60 min
Dye number
ꢀEab
C. I. Pigment Yellow 150
<3
28.1
26.7
9.4
21.4
2.5
2.1
4.2
3.8
1a
1b
1c
2
3a
3b
3c
3d

68
J.-Y. Lee et al.
Dye 3a–3d containing a carboxyl group exhibited the range of 0.76∼1.68% of weight
reduction which seemed to be slightly higher than that of C. I. Pigment Yellow 150.
As an alternative method to determine the thermal stability of dyes, the color difference
measured in ꢀEab value after fabrication process was carried out, as shown in Table 4. In
terms of ꢀEab value of synthesized dyes, higher color difference (9.4∼28.1) was found
with dyes 1a- 2 containing a hydroxy group in metal complex dye those also showed higher
weight reduction by TGA, whereas much smaller ꢀEab values (2.1∼3.8) observed for the
dyes 3a-3d containing a carboxyl group in metal complex dye those also showed lower
weight reduction by TGA, as mentioned above.
The resultant higher stability of the metal complex dyes based on carboxylic acid
groups over those of corresponding hydroxy groups is probably due to the existence of
resonance forms in which the negative charge is delocalized over two equivalent oxygen
atoms. Since a carboxylate ion is more stable than a hydroxide ion, it is lower in energy
and more highly favored in the formation of metal complex.
However, in commercial study, the compatibility of dyes with other additives used for
the formulation of photo-resist systems should be optimized by a trial error considering
with photo-initiator, monomer, cell stabilizer, binders.
Conclusions
Eight metal complex dyes were prepared via diazotization and coupling reactions followed
by a metal complexation reaction. Absorption maxima (λmax) of dyes containing a hy-
droxy group in the ortho-positions of azo bond consistently exhibited a bathochromic shift
compared to the corresponding dyes containing a carboxyl group in the ortho-position.
The thermal stability of synthesized dyes were examined by both TGA analysis and
ꢀEab values after fabrication process. Except three dyes, by TGA, smaller weight reduction
was observed with other five dyes compared to that of C. I. Pigment Yellow 150. The
synthesized dyes prepared from precursor dyes containing a carboxylic acid group exhibited
higher thermal stability in ꢀEab values in comparison with that of the corresponding dyes
containing hydroxy groups.
Therefore, it can be clearly concluded that some dyes 3a–3d could be used as an
alternative component to replace the conventional yellow pigment to increase the contrast
ratio and the transmittance for the fabrication of LCD color filter.
Acknowledgment
This work was supported by the National Research Foundation of Korea Grant funded by
the Korean Government [NRF-2012001638].
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